Single-Photon Emission Computed Tomography Artifacts

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Chapter 5 Single-Photon Emission Computed Tomography Artifacts

E. Gordon Depuey

INTRODUCTION

Single-photon emission computed tomographic (SPECT) imaging of myocardial perfusion provides a sensitive means of detecting and localizing coronary artery disease (CAD) and assessing left ventricular (LV) function. However, the method suffers from the potential for poor specificity due to image artifacts related to both patient and technical factors. By recognizing the sources of such artifacts and understanding the means available to avoid them, the technologist and interpreting physician can substantially improve the specificity of SPECT studies and more meaningfully contribute to appropriate patient treatment.

TECHNICAL ARTIFACTS

Flood Field Nonuniformity

Flood field nonuniformity will result in “ring” artifacts in reconstructed SPECT images. These relatively photon-deficient rings may be apparent in tomographic slices and in severe cases may also appear in polar coordinate maps (Fig. 5-1). Because patients are frequently positioned differently within the camera field for rest and stress scans, flood field artifacts may occur in differents of the myocardium, mimicking reversible or partially reversible defects. Therefore, routine acquisition and inspection of intrinsic and extrinsic flood fields acquired according to vendor recommendations are absolutely critical to avoid such artifacts. Flood fields are routinely acquired the first thing in the morning each working day. However, if ring artifacts appear in clinical SPECT images, it may be necessary to reacquire flood field images in the middle of the day.

Figure 5-1 Flood field nonuniformity resulting in ring artifacts and artifactual myocardial perfusion defects.

(Reproduced with permission from Iskandrian AE, Verani MS [eds]: Nuclear Cardiac Imaging: Principles and Applications, 3rd ed. New York: Oxford University Press, 2003.)

Center of Rotation and Camera-Held Alignment Errors

If the camera center of rotation (COR) is incorrect, filtered backprojection during SPECT reconstruction will result in image misregistration and apparent misalignment of the myocardial walls (Fig. 5-2). Technically, this is similar to the error created by cardiac motion. However, unlike motion artifacts, those due to COR error are usually more systematic and predictable. The severity of the apparent defect is directly proportional to the magnitude of the COR error. An error similar to that produced by the wrong center of rotation is produced when the detector is not aligned perpendicular to the radius of rotation.

Figure 5-2 Schematic representation of camera center of rotation error and resultant artifactual myocardial perfusion defects. Defects are in opposite sides of the “heart,” and “tails” of activity are present extending from the edges of the defects.

(Reproduced with permission from Iskandrian AE, Verani MS [eds]: Nuclear Cardiac Imaging: Principles and Applications, 3rd ed. New York: Oxford University Press, 2003.)

To avoid these artifacts, camera COR checks should be performed according to the camera manufacturer’s guidelines. The resultant plots should be inspected regularly by a knowledgeable technologist and physician before the camera is used for patient studies. With the observation of a myocardial defect suspected of representing such an artifact, the COR determination should be repeated immediately. Likewise for variable-angle multiheaded detector systems, camera head alignment should be assessed periodically, particularly after camera repair and whenever the detectors are impacted by objects such as stretchers.

Errors in Selecting Oblique Cardiac Axes and Subsequent Polar Map Reconstruction

If the long axis of the left ventricle (LV) is defined incorrectly on either the transaxial or midventricular vertical long-axis slice, the geometry of the heart in subsequently reconstructed orthogonal tomographic slices can be distorted. Consequently, the apparent regional count density can be altered in polar maps, resulting in apparent perfusion defects. These may be accentuated by quantitative analyses in which patient data are compared to normal files (Fig. 5-3). In polar map reconstruction, such errors most often occur in basal myocardial regions at the periphery of the bull’s-eye plot, owing to foreshortening of one of the myocardial walls (Fig. 5-4). Also, the apex, which often demonstrates physiologic thinning and decreased count density, is displaced from the exact center of the polar plot. The displaced apex may also consequently result in an artifact. Misregistration of perfusion defects in short-axis slices and polar maps is not infrequently encountered in patients with sizable severe infarcts.

Figure 5-3 If the long axis of the left ventricle is selected inconsistently, in stress and rest transaxial tomographic slices, perfusion defect localization, cavity configuration, and the size of the basal septal defect due to the membranous septum will be represented inconsistently in reconstructed short-axis tomograms.

(Reproduced with permission from Iskandrian AE, Verani MS [eds]: Nuclear Cardiac Imaging: Principles and Applications, 3rd ed. New York: Oxford University Press, 2003.)

Figure 5-4 Stress tomograms (top row) were reconstructed correctly in this normal patient, whereas the long axis of the left ventricle was selected incorrectly for the resting tomograms. As a result, the ventricular cavity in the resting short-axis slices appears elliptical, and there is an artifactual perfusion defect in the basal inferoseptal region.

(Reproduced with permission from DePuey EG, Garcia EG, Berman DS [eds]: Cardiac SPECT Imaging, 2nd ed. New York: Raven, 2001, p 195.)

Selection of Apex and Base for Polar Map Reconstruction

Accurate and reproducible selection of the apex and base of the LV myocardium is necessary in stress and rest images. Positioning limits for slice selection that lie too far apically or basally will result in apparent perfusion defects (Fig. 5-5). In contrast, positioning slice limits too tightly, so that they do not encompass the entire heart, will potentially result in underestimation of the size and extent of a defect. Likewise, in regions of normal myocardium, maximal myocardial count density may not be correctly sampled.

Figure 5-5 When the apical limits for reconstruction are defined distal to the left ventricular myocardium in stress images (A), an artifactual extensive and severe apical ischemic defect is produced in the polar plots (B).

(A, Reproduced with permission from DePuey EG, Garcia EG, Berman DS [eds]: Cardiac SPECT Imaging, 2nd ed. New York: Raven, 2001, p 98. B, Reproduced with permission from Iskandrian AE, Verani MS [eds]: Nuclear Cardiac Imaging: Principles and Applications, 3rd ed. New York: Oxford University Press, 2003.)

Arrhythmias and Gating Errors

If arrhythmia is present or if the heart rate changes during SPECT acquisition, inordinately short or long cardiac cycles will be rejected during an 8- or 16-frame/gated acquisition. If the degree of the regular beat rejection varies during the SPECT acquisition, the number of cardiac cycles acquired for each projection image may vary if each projection image is acquired for the same length of time. Therefore, projection images will vary in count density. When viewed in endless loop cinematic format, the projection images will appear to “flash.” The most serious effect of arrhythmia is a decrease in image count density due to these “discarded” cardiac cycles. However, only with severe arrhythmias associated with atrial fibrillation and frequent premature ventricular contractions are clinically significant perfusion artifacts encountered. Although this problem may be overcome by prolonging SPECT acquisition until a prescribed number of regular cardiac cycles have been acquired, such longer acquisition times increase the probability of patient motion due to discomfort or anxiety. Recently, some manufacturers have established a “9th bin” wherein rejected cardiac cycles are stored temporarily and excluded from reconstruction of the gated tomograms, but added back into the summed, nongated images. Alternately, other manufacturers allow simultaneous acquisition of gated and nongated images so that arrhythmic beat rejection will not result in low count-density ungated images.

If the accepted cardiac cycles vary in length, and because each cycle begins at the R-wave used for gating, relative shorter beats will not fill the entire 8 or 16 “bins” of the cardiac cycle. When all of the acquired cardiac cycles are summed to create an 8- or 16-frame gated image, this will result in data “dropout” in the last few “bins” of the cardiac cycle.

Displays of cardiac image count density from individual planar projection images is now possible using some commercially available software programs (Fig. 5-6). Errors in LV volume and ejection fraction may result from gating errors, but these are beyond the scope of this chapter.¹

Figure 5-6 A, Count density is plotted on the y axis for each of the eight “bins” of an 8-frame per cardiac cycle gated post-stress myocardial perfusion scan for the 64 camera stops in the 180-degree SPECT imaging arc of a 90-degree-angled, two-detector scintillation camera. In this patient with no arrhythmia and a very regular R-R interval, the curves are superimposed for each of the eight gated “bins.” B, In this arrhythmic patient, cardiac cycles are rejected during camera stops ~8 to 24 and simultaneously during stops ~40 to 56 for the other scintillation detector. This is manifested as a decrease in counts acquired/accepted at those stops. C, In this patient, all cardiac cycles were accepted during the SPECT acquisition. However, owing to variation of the R-R interval of the accepted beats, there is a decrease in count density in the eighth gated “bin.”

PATIENT-RELATED ARTIFACTS

Soft-Tissue Attenuation (See Chapters 6 and 7)

The location of the attenuation artifact depends on the position of the soft-tissue attenuator in relation to the left ventricle. The severity of the artifact depends on the size and density of the attenuator in relation to adjacent tissue. Within the resultant SPECT image, the artifact may appear as a fixed or reversible defect or may mimic “reverse redistribution,” depending on whether the attenuator is in a constant or variable position in stress and delayed image acquisitions. The severity of the attenuation artifact also depends on the energy of the incident photon. Attenuation artifacts for technetium (^99mTc)-labeled myocardial perfusion agents will be somewhat less marked than for thallium (²⁰¹Tl).³ However, for either isotope, evaluation of regional wall motion is helpful in differentiating attenuation artifacts from myocardial scarring as a source of fixed perfusion defects.⁴^–⁶

Breast Attenuation

Because breasts vary in size, position, configuration, and density, breast attenuation artifacts are extremely variable in appearance. In addressing the characteristics of breast attenuation, it is always necessary to consider the position and configuration of the breasts with the patient in the supine position, because patients are imaged in this position with most commercially available SPECT systems. In women of average body habitus, the left breast overlies the anterolateral wall of the heart. In women with large, pendulous breasts, the breasts lie adjacent to the lateral chest wall and more often result in a lateral attenuation artifact. In some women with very large, pendulous breasts, the attenuation artifact may create apparent inferior or inferolateral perfusion defects. In women with very large breasts, the breast tissue may overlie the entire LV. In this instance, the resulting attenuation artifact may be diffuse and less discrete or may primarily involve the apex. In addition to breast position, the caudal angulation of the heart within the thorax will affect the appearance of the attenuation artifact.²^–³ Thus, in summary, the severity of breast attenuation artifacts is not necessarily directly proportional to breast size or chest circumference but may vary considerably according to the position, configuration, and density of the breast, body habitus of the patient, and orientation of the heart within the thorax. Additionally, when women are imaged upright or in a semi-upright, “reclining” position, the breasts are usually more pendulous than in the supine position. Therefore, apical and inferior breast attenuation artifacts are more frequent when women are imaged in these positions.

In some laboratories, a binder is used to flatten the breasts against the chest wall. Although this technique can be quite useful to decrease the thickness of the breast, there is often a degree of uncertainty about the exact position of the breast beneath the binder. Moreover, precise repositioning of the breast beneath the binder in stress and rest studies is difficult. Therefore, in most laboratories, breast binders are not used. For similar reasons, taping the breast upward in an attempt to avoid its overlying the heart is associated with uncertainty in reproducing the breast position for stress and rest SPECT acquisitions.

The interpreting physician must also be cognizant of factors that may further alter the position, configuration, and density of the breast. A bra positions the breast anteriorly and increases the amount of soft-tissue attenuation by thickening the breast. For this reason, it is recommended that SPECT imaging be performed with the bra off. Breast implants may be denser than normal breast tissue and may accentuate attenuation artifacts. Therefore, women who are candidates for SPECT should always be questioned about a history of breast implants or augmentation. Patients with previous left mastectomies may wear a breast prosthesis. Such prostheses may be quite dense, and women who have them must always be instructed to remove them before myocardial perfusion imaging.

Quantitative analysis of SPECT myocardial perfusion imaging is helpful, particularly to the novice, in differentiating normal, physiologic variations in radiotracer distribution from true perfusion defects. However, gender-matched normal files are derived from an average population, and normal limits are established on the basis of variations in regional count density within this population. However, this type of analysis does not take into account the marked variability in body habitus observed in the patient population referred for myocardial perfusion imaging. Therefore, quantitative analysis, instead of aiding the physician in identifying soft-tissue attenuation artifacts, may actually decrease the specificity of SPECT testing by incorrectly identifying such artifacts as abnormalities.

Attenuation correction for SPECT is now commercially available but is still a topic of intense interest and research. The methods for accomplishing it have used either scatter correction or attenuation correction using a separate radionuclide or x-ray transmission image. This approach has been demonstrated to significantly reduce breast and other attenuation artifacts in phantom and patient studies.⁷^–¹⁸ However, whereas attenuation correction may minimize attenuation artifacts created by overlying breast tissue, breast artifacts are frequently not totally eliminated.

Certain breast attenuation artifacts are unique to quantitative analysis in cardiac SPECT. The most common is an apparent inferior perfusion defect incorrectly identified by quantitative analysis in women who have had a left mastectomy. After removal of the breast, there is little or no anterior soft-tissue attenuation. In such cases, the anterior and inferior myocardial count densities are nearly the same as they are for males. Since the normal female file anticipates that the inferior wall will be more intense than the anterior wall, and in the patient study the inferior wall has an identical or lower intensity than the anterior wall, the inferior wall will be identified as abnormal (Fig. 5-7). This quantitative error can be circumvented by comparison of the patient’s data to the normal male file instead. Similar inferior artifacts have been observed in quantitative plots in women with very small breasts, and it might be argued that data from such patients should be compared to normal male limits. However, small breasts may be quite dense, and the assumption that they do not produce photon attenuation may be incorrect. Therefore, in most laboratories, the normal male file is used routinely only for women who have undergone left mastectomy.

Figure 5-7 Technetium-99m-sestamibi stress and rest tomograms (A) and quantitative polar plots (B) in a patient with a left mastectomy. Patient data are compared to a normal female file, and quantitatively an inferior defect is identified artifactually. However, when patient data are compared to a normal male file (C), which anticipates no breast attenuation, quantitative analysis demonstrates no blackened pixels in the inferior wall.

(Reproduced with permission from Iskandrian AE, Verani MS [eds]: Nuclear Cardiac Imaging: Principles and Applications, 3rd ed. New York: Oxford University Press, 2003.)

The position of the breast may vary from stress to rest if a woman wears different clothing at the time of the two SPECT acquisitions. Breast position varies considerably with a bra on and off. The position of the breast may also vary depending upon the degree of elevation of the left arm, which is preferably positioned above the head for a SPECT acquisition. A shifting breast attenuation artifact can mimic a reversible perfusion defect (i.e., ischemia). For example, if during stress SPECT acquisition, the breast lies high over the anterior chest wall, an anterior attenuation artifact may result. However, if during the resting acquisition, the breast lies more laterally and inferiorly, the artifact will involve the inferolateral wall of the LV. The resulting attenuation artifacts are therefore different in the stress and rest images, so artifactual defect reversibility as well as artifactual “reverse distribution” may result. In this example, the anterior defect will appear to be reversible, whereas there will appear to be “reverse distribution” in the inferolateral wall (Fig. 5-8).

Figure 5-8 A, Left lateral planar projection images at stress (left) demonstrate a discrete breast shadow “eclipsing” the upper half to two-thirds of the left ventricle (LV). However, in the resting projection images (right), the breast shadow covers the entire LV, indicating a more inferior position of the breast at the time of resting SPECT acquisition. B, Short-axis tomograms, polar plots, and 3D surface images demonstrate an anterior defect in stress images, not apparent at rest. Differentiation of a shifting breast attenuation artifact from anterior ischemia (and lateral “reverse distribution”) is problematical. C, The stress SPECT acquisition was repeated with the patient’s left arm lowered somewhat, not as fully extended above the head. In the repeat study, the breast “shadow” is comparable in the stress (left) and rest (right) planar projection images. D, With the breast repositioned, now eclipsing the entire left ventricle, stress and rest tomograms are both normal.

The astute physician and technologist can recognize breast attenuation artifacts by several means. From the following list, the observer should learn to anticipate artifacts:

1. The patient should always be instructed to remove her bra before every image acquisition. It is useful for the technologist to document that the bra has been removed.

2. A history of mastectomy should be documented, and breast prostheses must be removed if possible. Any history of breast augmentation should be noted.

3. Positioning of the patient on the SPECT imaging table should be reproduced as closely as possible for stress and rest images. Because the position of the elevated left arm greatly influences the position of the left breast, the imaging table’s position should be identical for stress and rest image acquisitions, and the left arm should be positioned identically for each image acquisition. An arm holder is particularly useful to ensure reproducible arm positioning.

4. The planar images from which SPECT data are reconstructed should be viewed in rotating cinematic format. The position of the breast “shadow” should be observed with regard to the portion of the LV myocardium that is eclipsed. Such a shadow is usually noted to extend beyond the heart to overlie the adjacent left hemithorax and can thereby be differentiated from a true myocardial perfusion defect in these rotating planar images. Moreover, the density of the breast can be estimated from the degree of attenuation of photons emanating from structures within the thorax, including the myocardium and lungs. The constancy (or inconstancy) of the position of the left breast in stress and rest images should be ascertained.

5. In reconstructed SPECT images, an area of markedly decreased photon density may be observed anterior or lateral to the heart in patients with very large or dense breasts. Adjacent to this photopenic defect are frequently observed accentuated streak artifacts due to nonisotropic attenuation.³ The position of the breast is best evaluated from transaxial or horizontal long-axis slices. When such a large area of markedly decreased count density is noted, an attenuation artifact that may mimic a myocardial perfusion defect often occurs in the adjacent myocardium.

Gated myocardial perfusion SPECT is helpful in differentiating breast attenuation artifacts from scar, since artifacts will demonstrate normal wall motion and wall thickening, whereas infarcts, if they are transmural and sizable, will be hypokinetic with decreased wall thickening (Fig. 5-9).^4,⁵ However, patients with nontransmural myocardial infarctions might have normal wall motion, depending on the thickness of the infarct zone. For artifacts created by a breast that shifts in position from stress to rest, gating is less helpful to differentiate artifact from ischemia as a cause of the reversible or partially reversible defect.

Figure 5-9 In planar projection images (A) a photopenic, curvilinear, photon-deficient area secondary to breast attenuation, “eclipsing” the superior portion of the left ventricle, is noted to extend to regions adjacent to the thorax. An anterior breast artifact is present in stress and rest ^99mTc-sestamibi SPECT images. The artifact appears essentially fixed in tomographic slices (B) and polar coordinate plots (C). However, by careful inspection, the anterior/anterolateral defect appears somewhat more marked at rest. Gated end-diastolic and end-systolic midventricular short-axis (top) and vertical long-axis (bottom) slices (D) demonstrate normal anterior-wall motion and thickening, thus favoring an attenuation artifact rather than anterior infarction as a cause of the fixed defect.

Lateral Chest-Wall Fat Attenuation

In obese patients, there may be a considerable accumulation of fat in the lateral chest wall. When such a patient lies supine on the imaging palette, the thickness of soft tissue in the lateral chest wall is further accentuated. Although this tissue thickness may be the same as or greater than that of the breast, chest-wall fat is usually more uniformly distributed. Therefore, the resultant attenuation artifact is usually more diffuse, often involving the entire lateral wall of the LV.

Like breast attenuation artifacts, apparent perfusion defects due to lateral chest-wall fat should be anticipated before the actual inspection of tomographic slices, through awareness of the following points:

1. Height, weight, and chest-wall circumference should be recorded for all patients. In obese men and women, lateral chest-wall attenuation artifacts occur frequently, and in general their severity is directly proportional to the chest circumference.

2. In the planar images displayed in rotating cinematic format, the LV will exhibit a markedly decreased count density in lateral and left posterior oblique (LPO) views in patients with excessive lateral chest-wall fat. Although patients with true lateral perfusion defects in the distribution of the circumflex coronary artery may exhibit similar findings in the rotating planar images, a perfusion abnormality in the lateral wall is sometimes more often identifiable in the left anterior oblique (LAO) view in these patients, since lateral chest-wall fat usually lies more posterior when the patient lies supine.

3. As for breast attenuation artifacts, gated SPECT and attenuation correction are useful in differentiating lateral chest-wall attenuation artifacts from true perfusion defects. Normal lateral wall motion and wall thickening strongly favor the former.

Diffuse Depth-Dependent Soft-Tissue Attenuation

In some obese patients, soft-tissue attenuation may be diffuse. Under these circumstances, the entire myocardium may be attenuated. Because attenuation within soft tissue is distance dependent, the more basal portions of the LV myocardium will be attenuated to the greatest degree. Therefore, in obese patients, an apparent circumferential decrease in trace concentration may be present at the base. The artifact may be most pronounced at the base of the inferior and posterolateral walls, which, considering the normal anatomic position of the heart within the thorax, are furthest from the detector (Fig. 5-10). As for other attenuation artifacts, gating may be helpful to differentiate soft-tissue attenuation from scar. Normal wall motion and thickening of fixed basal perfusion defects in an obese patient favor the presence of attenuation artifacts rather than scar. Resolution recovery with or without attenuation correction likewise is helpful in circumventing this artifact.

Figure 5-10 This study was performed on a 320-pound woman with a low clinical likelihood of coronary artery disease. A, A transaxial midventricular tomogram demonstrates apparently decreased technetium (^99mTc)-sestamibi concentration in the base of the inferoposterior wall, which is the region of the left ventricle (LV) most distant from the detector and most attenuated by soft tissue. B, In ^99mTc-sestamibi stress and rest tomograms, tracer concentration appears to be decreased at the base of the LV, particularly the base of the lateral wall. C, Polar plots similarly demonstrate an apparent circumferential decrease in activity at the base.

Diaphragmatic Attenuation

The inferior wall of the LV of both men and women normally exhibits a decreased count density. This is most likely due to the attenuation of photons from the inferior wall of the LV by both the left hemidiaphragm and to a lesser degree the overlying right ventricle and right ventricular blood pool. In normal men, the anterior-to-inferior count-density ratio with ²⁰¹Tl SPECT is 1.2:1.⁶ For ^99mTc, the ratio is approximately 1:1.¹ In women, the ratio is approximately unity with both ²⁰¹Tl and ^99mTc. This has been postulated to be due to the counterbalancing of inferior wall attenuation caused by the left hemidiaphragm in women by breast attenuation. However, it is doubtful whether that alone explains the difference, since the inferior-to-lateral wall ratio is also higher in women.

Left hemidiaphragmatic elevation can result in accentuated attenuation and inferior myocardial perfusion artifacts. Diaphragmatic elevation is common in obese patients. In patients with LV dilation, diaphragmatic attenuation may be accentuated. This may be because the dilated heart “sinks” down below the diaphragm, or possibly because the dilated ventricular blood pool results in enhanced attenuation of photons from the inferoposterior wall, which must pass through the ventricular cavity to reach the scintillation detector.

Attenuation artifacts due to left hemidiaphragmatic elevation are usually constant (fixed) in stress and rest images. However, the interpreting physician must anticipate unusual clinical circumstances in which the degree of diaphragmatic elevation is inconstant. A change in the position of the left hemidiaphragm may occur, for instance, in a patient who is anxious and swallows air during exercise but expels the air prior to subsequent resting images. This will result in more marked diaphragmatic attenuation in the stress images, mimicking a reversible ischemic perfusion defect in the inferior wall.

Infrequently, densities above or below the diaphragm may overlie the LV in planar projections used for SPECT reconstruction and result in attenuation artifacts. A left pleural effusion may attenuate the inferior and lateral walls of the LV. In patients with ascites, such as those with liver disease and those undergoing peritoneal dialysis, ascites fluid may accumulate below the left hemidiaphragm, elevating it and resulting in attenuation of photons from the inferior wall of the LV.¹⁹ Patients with chest pain often undergo a battery of diagnostic tests that may include an upper gastrointestinal series with barium contrast. Occasionally, a loop of bowel containing barium may be superimposed on a portion of the left ventricle in the planar images used for SPECT reconstruction. Although very uncommon, such attenuation artifacts may be localized and mimic inferior wall perfusion defects.

To anticipate and recognize attenuation artifacts caused by the left hemidiaphragm and subdiaphragmatic contents and structures, the following factors should be considered before interpreting SPECT images:

1. The degree of abdominal protuberance should be noted as the patient is lying supine.

2. The hospital chart should be perused, or the patient should be questioned about having conditions predisposing to pleural effusion or free peritoneal fluid (e.g., liver disease, peritoneal dialysis).

3. The patient should be questioned about barium contrast studies within the preceding week. If there is a suspicion of barium contrast creating an attenuation artifact, an abdominal x-ray may help to confirm the presence and location of barium in the bowel.

4. The planar images used for SPECT reconstruction should be reviewed in a rotating cinematic format. The position of the left hemidiaphragm is usually identifiable in the left lateral or LPO views as a curvilinear region of tracer concentration representing the diaphragm itself or as a photopenic defect due to the stomach, which lies immediately below the diaphragm (Fig. 5-11). Attenuation of the inferior or inferoposterior wall of the LV is often evident from these views. To better define the position of the left hemidiaphragm and the associated potential for an attenuation artifact, some laboratories have obtained a separate left lateral planar view (preferably obtained with the patient in the right lateral decubitus position) with a higher count density.²⁰

5. Gated ^99mTc perfusion SPECT is very useful in differentiating diaphragmatic attenuation artifacts from inferior or inferoposterior myocardial infarction. Defects due to attenuation will move and thicken normally, whereas infarcts will exhibit decreased wall motion and wall thickening (Fig. 5-12).⁴ As noted previously, however, some caution in this regard is appropriate; defects related to small subendocardial myocardial infarctions might move and thicken normally, thus mimicking attenuation, whereas some regions of hibernating myocardium that are viable may not move, simulating infarction.

Figure 5-11 Top, Left lateral views demonstrating a curvilinear photopenic “shadow,” indicating the position of the left hemidiaphragm. The arrows in the duplicate images (right) indicate the position of the diaphragm. In this case, the position of the diaphragm is normal, just below the inferior wall of the left ventricle. Bottom, Left hemidiaphragm elevation is indicated by tracer concentration in subdiaphragmatic structures “overlapping” the inferior wall.

Figure 5-12 Technetium-99m-sestamibi stress and rest single-photon emission computed tomography (SPECT) tomographic slices (A) and polar coordinate plots (B) demonstrate an apparent mild relative decrease in tracer concentration in the inferior wall. Gated end-diastolic and end-systolic midventricular short-axis and vertical long-axis tomograms (C) demonstrate normal inferior-wall motion and wall thickening, thus favoring an attenuation artifact rather than inferior infarction as a cause of the fixed defect.

SPECT image acquisitions in which the patient’s position is altered to shift the level of the diaphragm and thereby minimize diaphragmatic attenuation have helped to increase diagnostic specificity. Upright imaging will cause the diaphragm to shift downward. Imaging in the prone position is generally well tolerated by patients.²¹ In the prone position, the heart shifts anteriorly to a slight degree, and the diaphragm and subdiaphragmatic contents are pushed down. There is also generally less motion of the anterior portion of the chest in the prone position and less upward creep.²¹ Furthermore, the depth of respiration decreases, minimizing respiratory motion of the heart. Inferior myocardial perfusion defects noted with the patient supine but that are absent from prone images most likely represent diaphragmatic attenuation artifacts (Fig. 5-13).

Figure 5-13 A, In stress and rest technetium-99m-sestamibi single-photon emission computed tomography (SPECT) tomograms, polar plots, and 3D surface-rendered images, a decreased inferior count density secondary to diaphragmatic attenuation is present when the patient is imaged supinely. B, With repeat prone imaging for both stress and rest acquisitions, the diaphragmatic attenuation artifact is no longer present.

Tracer washout and redistribution occur rapidly in stress ²⁰¹Tl SPECT, making it infeasible to repeat stress images in the prone position. Delayed redistribution images can be repeated with the patient in the prone position without appreciable tracer washout. However, this does not permit the differentiation of an attenuation artifact from a reversible ischemic abnormality. For that reason, some investigators have advocated prone imaging as an alternative to supine imaging in selected patients in whom diaphragmatic attenuation is anticipated.^21,²² However, the physician must be cautious in interpreting routine prone SPECT myocardial perfusion images, because in the prone position, the heart lies more anteriorly than in the supine position, and there may be alterations in the geometric relationships between the detector and the heart. With the more anterior position of the heart, the anterior wall and septum may lie closer to the detector, thereby increasing the apparent anterior and septal count density. Because of this, supine SPECT studies should not be compared to a normal supine SPECT database for purposes of quantitative analysis. Also, artifactual perfusion decrease in the anteroseptal wall, probably secondary to increased sternal attenuation, has been reported.²²

APPARENT WORSENING OF ATTENUATION ARTIFACTS IN LOW-DOSE REST IMAGES

Apart from the systematic approach of inspecting rotating projection images, viewing tomographic slices and polar plots or three-dimensional reconstructions, and observing wall motion and wall thickening in gated tomograms, another clue to a defect being attributable to soft-tissue attenuation is “pseudo–reverse distribution” in low-dose resting images (see Fig. 5-9).²³ The cause of this apparent worsening of attenuation artifacts in lower-dose resting images is most likely attributable to the filters routinely used to process data using filtered backprojection. In obese patients with low count-density images, it has been demonstrated that both breast and diaphragmatic attenuation artifacts may appear more marked in the lower count-density resting images acquired using a rest/stress 1-day protocol employing ^99mTc-myocardial perfusion radiopharmaceuticals.

ADVANTAGES AND POTENTIAL DISADVANTAGES OF ATTENUATION CORRECTION IN RESOLVING ATTENUATION ARTIFACTS (See Chapters 6 and 7)

Attenuation correction using Gd-153 line sources or x-ray transmission sources are beneficial in differentiating attenuation artifacts from true perfusion defects. Currently, however, attenuation correction is in limited use because of the cost of the transmission sources or additional equipment and the fact that the method is presently not reimbursed by third-party payers. Also, there are artifacts unique to attenuation correction. If the emission and transmission scans are misregistered, significant myocardial perfusion artifacts may occur that mimic CAD ^24,²⁵ (Fig 5-14). This is particularly problematical for x-ray attenuation methods where the transmission and emission scans are acquired sequentially as opposed to scanning line sources where the scans are “interleaved.” Therefore, careful quality control of image registration is essential. In addition, scatter from subdiaphragmatic structures (see later) may be accentuated with attenuation correction, further confounding evaluation of the inferior wall of the LV.

Figure 5-14 Severe misregistration of a fused transaxial single-photon emission computed tomography/computed tomography (SPECT/CT) image.

(Reproduced with permission from Goetze S, Wahl WL: Prevalence of misregistration between SPECT and CT for attenuation-corrected myocardial perfusion SPECT. J Nucl Cardiol 14:200–206, 2007.)

SCATTERED ABDOMINAL VISCERAL ACTIVITY

Tracer uptake by the liver or reflux of tracer excreted by the liver into the stomach may degrade SPECT myocardial perfusion images through scatter of photons from the liver or stomach into the inferior wall of the LV. Thereby true inferior perfusion defects may be obscured. If scatter into the inferior wall is more marked at rest than with stress, the relative increase in inferior count density from rest to stress can easily mimic inferior reversibility (i.e., ischemia). Because the gallbladder is spatially separated from the heart, it seldom if ever creates such image artifacts.

To minimize scatter for SPECT with ^99mTc-labeled tracers, a 15% to 20% window over the ^99mTc photopeak is used. This finite window accepts photons scattered through as much as a 40-degree angle directly perpendicular to the detector, passing through the collimator holes. Such Compton scatter degrades ²⁰¹Tl images to an even greater degree because of the wider energy window used to encompass the emitted mercury x-rays.

In polar maps and some visual displays, as well as with quantitative analysis, SPECT images are normalized to the region of the myocardium having the greatest count density. If liver, stomach, or other abdominal visceral activity is superimposed upon the inferior wall, images will be incorrectly normalized to this area, making other areas of the heart appear count deficient. Therefore, visual or quantitative analysis may incorrectly identify defects in areas remote from the superimposed abdominal visceral activity (Fig. 5-15).

Figure 5-15 Dipyridamole stress and rest technetium-99m-sestamibi tomograms (A) and quantitative polar maps (B) in a patient with excessive subdiaphragmatic tracer concentration increasing left ventricular inferior-wall count density by means of Compton scatter. In reconstructing the stress polar map, abdominal visceral activity was included in the radius of search used for circumferential count-rate analysis, thus artifactually augmenting inferior-wall count density. By normalization to the inferior region, artifacts remote from the inferior wall were created in quantitative plots.

(Reproduced with permission from Iskandrian AE, Verani MS [eds]: Nuclear Cardiac Imaging: Principles and Applications, 3rd ed. New York: Oxford University Press, 2003, p 98.)

Recognition of the presence and location of abdominal visceral concentrations of tracer is essential to the accurate interpretation of myocardial perfusion SPECT studies and the avoidance of artifacts. Although abdominal visceral activity can usually be identified on tomographic slices, undeniably the best method to assess abdominal visceral activity is to inspect the multiple planar images used for SPECT reconstruction in a rotating cinematic format at the computer console. To optimize image reconstruction in the presence of abdominal visceral activity, or to determine the need to reimage selected patients, as described later, it is essential that the technologist or physician view these images before the patient leaves the laboratory.

For exercise images, abdominal tracer concentration can be minimized by ensuring that patients perform maximal exercise, resulting in the shunting of blood from the liver to the working skeletal musculature. For adenosine or dipyridamole, the added performance of dynamic, submaximal exercise (walking, pedaling, etc.) has been shown to significantly decrease tracer concentration in the liver in immediate images.²⁶ Exercise is performed during adenosine infusion or immediately after the infusion of dipyridamole, and the radiotracer is injected during exercise, at least 1 minute before its cessation. For ^99mTc-sestamibi imaging, concentration in the liver decreases progressively after tracer injection, and injection-to-imaging times of 40 to 45 minutes for rest studies are recommended with this agent. For exercise ^99mTc-tetrofosmin studies, the liver uptake is less problematic, and imaging at 20 to 30 minutes after injection is possible.

For pharmacologic stress studies with ^99mTc-sestamibi, in which liver uptake of the agent is more marked, an interval of at least 45 minutes may be preferable. An 8-oz glass of milk or a light fatty meal may augment hepatic clearance of ^99mTc-sestamibi and is offered to patients 15 minutes before both stress and rest SPECT imaging. However, this intervention may be counterproductive; tracer from the gallbladder will enter the small bowel and frequently the stomach, which may lie immediately adjacent to the inferior wall of the LV. For ^99mTc-tetrofosmin imaging, injection-to-imaging times may be shortened because of more rapid liver clearance of tracer.

For ^99mTc-sestamibi and ^99mTc-tetrofosmin studies, additional maneuvers are available to decrease or eliminate tracer concentration in the stomach or bowel, after which repeat SPECT imaging can be performed. A large (at least 16-oz) glass of water can be given to clear activity from the stomach and promote bowel motility. If the patient ambulates after drinking water, gastric emptying may be further enhanced. Because the oral administration of water has been demonstrated to be so effective in minimizing gastric tracer concentration, most laboratories now routinely give patients water to drink 10 to 15 minutes before both stress and rest acquisitions using either ^99mTc-sestamibi or tetrofosmin.

In tomographic sections, abdominal visceral activity is usually adequately distinguished from the left ventricular myocardium. However, artifacts are more common in reconstructed polar maps, because the radius of search used for the circumferential profile count-rate analysis of slices commonly extends to include abdominal visceral activity. The technologist must therefore be careful to exclude as much abdominal visceral activity as possible from regions used for reconstructing polar maps. However, care must be taken to be sure that the myocardial limiting region of interest (ROI) encompasses the entire left ventricular myocardium, particularly in basal slices where the epicardial dimensions are the greatest. If a portion of the myocardium is not encompassed, the maximal regional myocardial count density may not be detected in the bull’s-eye radius of search, thereby resulting in an artifactually low regional count density in the polar plot. A deceptively attractive means for eliminating abdominal visceral activity is to place a lead shield or apron over the areas of abdominal uptake. However, because abdominal visceral uptake can closely approximate that in the heart, and an apron may shift during SPECT image acquisition, there is the potential for attenuating the inferior myocardial wall as well.

Compton scatter of photons by overlying soft tissue significantly degrades cardiac image resolution, decreases image contrast, and decreases spatial resolution, thereby potentially decreasing test sensitivity in detecting CAD. Software methods are under development that estimate and correct for the distribution of scatter from the energy spectrum of detected photons. A “triple energy window” is used, selecting not only the ^99mTc photopeak but also two narrow windows above and below that photopeak. This scatter correction method has been demonstrated to improve myocardial perfusion contrast in SPECT scans acquired with and without attenuation correction and processed with either filtered backprojection or maximum-likelihood expectation maximum (MLEM).²⁷^–³⁰

THE RAMP FILTER ARTIFACT

As an inherent component of filtered backprojection, the Ramp filter is used to eliminate the “star artifact” associated with reconstruction of a finite number of projection images. This filtering process minimizes count density adjacent to an intense object, thereby better delineating its borders and increasing its contrast. However, when the Ramp filter is applied to intense tracer concentration adjacent to the heart—frequently present in the liver, stomach, or bowel—there may be an artifactual decrease in count density in the inferior wall of the LV (Fig. 5-16). Because the distribution and intensity of subdiaphragmatic activity often varies between stress and resting images (like the artifact due to scattered activity), the Ramp filter artifact is likewise variable. Therefore, both scatter of subdiaphragmatic activity and the Ramp filter may produce fixed, reversible, or reverse-distribution inferior artifacts.

Figure 5-16 In resting tomographic slices (A), intense tracer concentration is present in the fundus of the stomach, localized adjacent to the inferior wall of the left ventricle (LV) in the x plane. By means of the Ramp filter in the resting scan, counts are subtracted from the adjacent inferior wall of the LV, creating an artifactual resting inferior perfusion defect and creating the impression of “reverse distribution” in both the tomographic slices and the polar plots (B). Oral administration of fluid usually “flushes out” tracer from the stomach. In C, stomach activity produces an inferior artifactual defect due to the Ramp filter (left), which is eliminated after the patient drinks 16 oz of water (right).

Because iterative reconstruction does not incorporate the Ramp filter, it has been proposed as a means to avoid the Ramp filter artifact. Preliminary results have demonstrated, for instance, that ^99mTc-sestamibi images may be acquired early (15 minutes) after resting tracer injection and processed with iterative reconstruction without artifactual inferior-wall defects, despite considerable liver radiotracer concentration.³¹ However, not all nuclear medicine computer systems, particularly older models, are equipped with iterative reconstruction, so at the present time this alternative processing algorithm is not widely available.

MOTION ARTIFACTS (See Chapter 4)

Because of the process of filtered backprojection used for reconstructing SPECT images, cardiac motion relative to the detector can create image misregistration and artifact. Cardiac motion has several sources. As it contracts, the heart rotates physiologically on its axis. It is unlikely that such motion creates artifacts large enough to be recognized or misconstrued as perfusion abnormalities. A reversible ²⁰¹Tl SPECT artifact may result from “upward creep” of the heart.³² A patient’s rate and depth of respiration increase markedly during dynamic exercise, resulting in more marked diaphragmatic excursion, increased lung volume, and a lower position of the diaphragm. If SPECT image acquisition is begun while the rate and depth of respiration are still increased, the position of the diaphragm and thus of the heart will be low. During the acquisition, as the depth of respiration decreases, the height of the diaphragm will progressively rise, with a gradual upward shift of the heart. Such cardiac motion will not be present in delayed resting images, when the rate of respiration is slower and more regular. Diaphragmatic creep artifacts are common with exercise ²⁰¹Tl, in which exercise SPECT images are acquired immediately after dynamic exercise. A 15-minute delay between exercise and ²⁰¹Tl image acquisition helps to allow for the respiratory rate to return to baseline. Diaphragmatic creep artifacts are avoided with pharmacologic stress, which does not significantly increase respiration, and with ^99mTc-sestamibi or tetrofosmin SPECT, for which there is a delay between exercise and stress image acquisition.

The most common source of motion artifact during SPECT image acquisition is patient movement. Such motion is unpredictable and thus most problematic. Patients may move at any time during the acquisition. The motion may be gradual or abrupt, may be vertical (axial), horizontal (sideways), or rotational, and may occur once or multiple times. With newer cameras that allow for upright patient positioning, additional patient motion in the “z-direction” is also possible if the patient slumps or leans forward in the imaging chair.

Artifact location, configuration, and severity will depend on all of these factors.³³^–³⁶ Cooper and colleagues evaluated the effect of patient motion in creating ²⁰¹Tl SPECT image artifacts.³⁴ The visual detectability of artifacts by experienced observers was directly proportional to the magnitude of motion. One-half pixel (3.25 mm) of motion was not visually detectable, 1 pixel of motion was recognized but judged to be clinically insignificant, and 2 pixels of motion resulted in artifacts potentially misconstrued as true perfusion defects. By quantitative analysis, 2 pixels of motion in the axial direction resulted in clinically significant artifacts in 5% of interpretations.

Patient motion is particularly problematic, because the location and severity of the resultant artifact depend not only on the magnitude of motion but also on its direction and the location within the 180-degree imaging arc. Cooper and coworkers observed that motion artifacts are more noticeable when axial motion occurs at the midpoint of the 180-degree acquisition.³⁴ They postulated that this was because the backprojected images were more evenly split between projections of two different distributions of radioactivity, one before and one after movement. These authors also observed that sideways motion results in more marked artifacts when it occurs in the anterior view, when the heart is parallel to the camera, and results in the least artifacts in the lateral projection, when the heart is more oblique or perpendicular to the camera.

The most reliable method for detecting the degree, direction, and frequency of cardiac motion is to inspect the rotating planar images at the computer console. An alternate, somewhat less reliable method is to add the individual planar frames to produce a summed image in which the heart forms a horizontal “stripe” as it moves left (45-degree right anterior oblique [RAO] projection) to right (45-degree LPO projection) across the field of view. Although abrupt downward or upward motion and diaphragmatic creep are reliably detected by this technique, it is difficult to detect sideways motion, z-direction motion, “cardiac bounce,” or erratic up-and-down motion throughout the acquisition. A third method for detecting cardiac motion is by inspection of the cardiac sinogram (Fig. 5-17). The sinogram can be thought of as a “stack” of planar views in which the y axis has been compressed. In the initial 45-degree RAO projection, the vertically compressed cardiac activity is therefore positioned in the lower left corner of the image. As the heart moves rightward across the field of view, subsequent compressed cardiac images are “stacked” in a sinusoidal configuration (reflecting a one-dimensional projection of an object rotating in a circular or elliptical orbit). Discontinuity of the sinogram indicates abrupt patient motion. However, gradual continuous motion is usually not apparent on the sinogram.

Figure 5-17 Sinogram of a 180-degree myocardial perfusion single-photon emission computed tomography (SPECT) acquisition. Discontinuity of the sinogram is indicative of patient motion.

When cardiac motion occurs, misalignment of data by filtered backprojection often results in telltale artifacts in tomographic slices. The anterior and posterior walls of the LV may appear to be misaligned, with curvilinear tails of activity extending from the myocardium into adjacent background regions (Fig. 5-18).

Figure 5-18 A, In a patient with marked downward motion during the 180-degree technetium-99m-sestamibi single-photon emission computed tomography (SPECT) post stress acquisition, reconstructed short-axis tomograms reveal apparent misalignment of the septal and lateral walls of the left ventricle, contralateral anterior and inferior defects, and “rabbit ears” or “tails” of activity streaming from the defects. In the stress vertical and horizontal long-axis tomograms, there appears to be discontinuity of the epicardial border. B, The stress SPECT acquisition was repeated, ensuring that the patient did not move. The resulting stress SPECT images are entirely normal.

It is essential that the physician interpreting a cardiac SPECT scan evaluate the study for motion and potential associated artifacts. It is equally or more important for the technologist to assess cardiac motion. If motion is noted during an acquisition, the acquisition may be terminated and restarted. Although this may be impractical for ²⁰¹Tl stress studies, reimaging is feasible for delayed ²⁰¹Tl studies and with ^99mTc-sestamibi and ^99mTc-tetrofosmin. Likewise, if motion is detected using the quality-control methods described previously, these studies may be reacquired. Therefore, it is important that rotating planar images be viewed before the patient leaves the laboratory to avoid a repeat visit and reinjection.

Motion artifacts can be avoided or minimized by an alert technologist. The patient should be positioned as comfortably as possible on the imaging table and should be observed during the entire SPECT acquisition. Stress on the low back should be minimized by supporting the lumbar spine and the knees. An arm rest is very helpful for maximizing patient comfort and thus minimizing motion. Nevertheless, patients often cannot hold their arms above their heads for the entire acquisition. Since arm movement often results in motion of the torso, the technologist should carefully reposition the patient’s arm(s), avoiding any motion of the chest. The technologist should also encourage the patient to maintain a slow, steady rate of respiration. Similarly, coughing should be prevented if possible. SPECT imaging may be performed with the left arm by the side, providing it is posterior to the heart and does not attenuate the heart in any of the planar projection images, with the possible exception of the very last LPO views. A significant drawback of image acquisition with the left arm down is increased detector-to-heart distance, in turn resulting in significantly degraded tomographic spatial resolution.

One method of correcting studies for motion is to manually shift individual image frames, so that the heart remains within a constant, horizontal plane. However, this technique is relatively tedious and time-consuming and not available on all computer workstations. Automated computer methods to correct cardiac motion are now commercially available.³⁷^–⁴¹ Their applicability may be limited if there is considerable concentration of tracer in the liver, which confounds the detection of myocardial borders.^37,³⁸ In addition, whereas motion in the vertical direction is reliably corrected either by manual or computer methods, horizontal, rotational, and z-direction motion are not reliably corrected. Importantly, studies can often be salvaged with motion correction, obviating the need for repeat acquisition, and in some cases, reinjection (Fig. 5-19). Nevertheless, despite the development of these methods, to date, visual inspection of the rotating planar images remains the most important physician- and technologist-dependent method of quality control in cardiac SPECT.

Figure 5-19 Tomograms acquired with greater than 3 pixels of vertical motion without (bottom) and with (top) motion correction applied.

ARTIFACTS RELATED TO NONCORONARY HEART DISEASE

Myocardial Hypertrophy

Diffuse myocardial hypertrophy often results from systemic hypertension or the increased volume or pressure overload associated with valvular heart disease. Such hypertrophy results in a generalized increase in uptake of the radiopharmaceuticals used for studying myocardial perfusion. Regional myocardial hypertrophy will appear as a localized increase in image count density.⁴² If the tomographic slice or polar map is normalized to this region of increased count density, regions adjacent to and distant from the “hot spot” will appear to have relatively decreased count densities. Papillary muscle hypertrophy may occur in patients with LV hypertrophy, creating localized hot spots in the anterolateral and inferolateral wall. Occasionally there may be localized apical hypertrophy, in which case there may be an apical hot spot with a consequent relative decrease in count density in the more basal aspects of the LV (Fig. 5-20). Additionally, a relative increase in septal-wall count density is common in hypertensive patients.⁴³ It is not clear whether this relative increase in septal count density is due to some degree of selective septal hypertrophy or to alterations in regional blood flow or metabolism in hypertensive patients. In patients with long-standing hypertension, a significant decrease in the lateral-to–septal wall count-density ratio as compared to that in normotensive controls has been reported.⁴³ Thus, in tomographic slices, raw polar maps, and quantitative plots, the lateral wall often displays a relative decrease in count density, simulating CAD in the territory of the circumflex coronary artery (Fig. 5-21). This defect is usually “fixed” in stress and rest images obtained with either ²⁰¹Tl or ^99mTc-labeled agents, mimicking myocardial infarction. However, occasionally the septum may appear slightly less intense in resting images, rendering the defect partly reversible and raising a clinical suspicion of ischemia.

Figure 5-20 In this patient with hypertrophic cardiomyopathy with marked involvement of the apex, there is an increase in apical count density noted in tomographic slices (A). Polar plots, which are normalized to the apex in this particular case, demonstrate a relative decrease in count density in the more peripheral aspects of the left ventricle (B).

(Reproduced with permission from Iskandrian AE, Verani MS [eds]: Nuclear Cardiac Imaging: Principles and Applications, 3rd ed. New York: Oxford University Press, 2003, p 103.)

Figure 5-21 In this patient with longstanding systemic hypertension, there is a relative increase in count density in the septum in technetium-99m-sestamibi stress and rest tomograms, polar maps, and 3D surface-rendered images (viewing the septum of the left ventricle). Because images are normalized to the region of greatest count density (the septum), the remainder of the ventricle, particularly the lateral wall, appears abnormal.

The physician interpreting the scan should anticipate this artifact in patients with hypertension. Therefore, it is important to obtain appropriate historical information. Similarly, the medical record should be examined for historical or echocardiographic evidence of idiopathic hypertrophic subaortic stenosis (IHSS) or asymmetric septal hypertrophy (ASH), which may produce a similar artifact. Moreover, inspection of the electrocardiograph (ECG) for evidence of LV hypertrophy is worthwhile. However, the standard voltage criteria are relatively insensitive to hypertrophy.⁴⁴ In gated SPECT studies in hypertensive patients without CAD, areas of relatively decreased count density, usually involving the lateral wall, will move and thicken vigorously.

Left Bundle Branch Block (See Chapter 16)

In exercise myocardial perfusion studies, reversible septal perfusion defects occur in patients with left bundle branch block (LBBB), mimicking exercise-induced septal ischemia. Such artifacts have been reported to occur in 30% to 90% of cases of LBBB, depending on whether visual or quantitative analysis was applied.^45,⁴⁶ Hirzel and associates demonstrated in dog experiments that increased heart rates associated with right ventricular pacing, thus mimicking LBBB, result in a relative decrease in blood flow to the septum.⁴⁷ This decrease was not seen in experiments using atrial pacing, which mimics the sinus tachycardia of exercise. These investigators postulated that the decrease in septal blood flow is due to asynchronous relaxation of the septum, which is out of phase with diastolic filling of the remainder of the ventricle, during which coronary perfusion is maximal. At higher heart rates, the degree of septal asynchrony relative to the ECG R-R interval is greater than at rest, making septal perfusion defects appear reversible (Fig. 5-22). It has been reported that the severity of reversible perfusion defects is most marked in LBBB patients who achieve very high heart rates (>170 beats/min).⁴⁵

Figure 5-22 In this patient with left bundle branch block (LBBB) and no coronary artery disease, technetium-99m sestamibi short-axis tomograms and polar plots (A) were acquired after tracer injections at rest and during peak treadmill exercise. A reversible septal perfusion defect is present. The study was repeated, substituting dipyridamole pharmacologic stress for exercise (B). The reversible septal defect is no longer present.

(Courtesy of Alan Rozanski MD, St. Luke’s-Roosevelt Hospital, New York.)

It has been observed that perfusion defects associated with LBBB in patients without CAD spare the apex.⁴⁸ Thus, if an apical defect is present in a patient with LBBB, disease of the left anterior descending (LAD) coronary artery should be suspected. Also, quantitative techniques that analyze relative count density in only the more basal aspect of the septum have been reported to increase the specificity of detection of LAD CAD.⁴⁹ Perfusion abnormalities in the territories of the right and circumflex arteries carry a high sensitivity for the presence of CAD, despite the presence of LBBB.⁴⁵ Similar artifacts have not been reported for right bundle branch block (RBBB) or left anterior hemiblock. Thus, to anticipate reversible septal perfusion artifacts, it is imperative that the interpreting physician inspect the ECG tracing or report for the presence of LBBB.

It has been reported that septal artifacts associated with LBBB can be minimized by substituting pharmacologic stress with intravenous dipyridamole for exercise.^50,⁵¹ Because dipyridamole and adenosine have only a slight positive chronotropic effect, increasing the heart rate by only approximately 10 beats/min in most patients, there is little increase in septal asynchrony relative to the R-R interval. Thus, septal perfusion is kept relatively constant with dipyridamole and adenosine stress, and septal artifacts are avoided. In contrast, dobutamine has a positive inotropic effect, so this pharmacologic agent should be avoided in LBBB patients. However, it should be cautioned that patients undergoing dipyridamole occasionally have high resting heart rates due to underlying medical conditions, and that in other individuals there may be a marked increase in heart rate associated with the infusion of pharmaceuticals. Therefore, septal artifacts may nonetheless be expected to occur if such patients also have LBBB. In most patients, it is advisable to perform low-level exercise in conjunction with dipyridamole and adenosine pharmacologic stress to minimize side effects and shunt blood away from the abdominal viscera to the working musculature. However, because of the associated increase in heart rate effected by low-level exercise, it should be avoided in LBBB patients.

SUMMARY

The list of technical and clinical circumstances that can result in SPECT image artifacts is considerable.⁵² To avoid artifacts and optimize test specificity, both the technologist and interpreting physician must be aware of factors potentially contributing to the creation of artifacts.

REFERENCES

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