Image Quality

Factors affecting image quality and characteristics of CTA images include temporal and spatial resolution. Spatial resolution refers to the degree of blurring in the image and the ability to discriminate objects and structures of small size. Axial resolution within the scan plane can be improved by using a small field of view, larger matrix size, smaller focal spot, and smaller detectors. The demand for high spatial resolution to visualize the various coronary segments that course with decreasing diameter to the apex is high. One of the major goals of MDCT technology development has been to obtain similar spatial resolution in all directions, also expressed as isotropic spatial resolution.¹⁹ CTA requires high spatial resolution to enable accurate detection and interrogation of small arterial branch vessels. Both in-plane and through-plane spatial resolutions are important indices that require optimization to provide isotropic source data. Isotropic means that the spatial resolution is approximately equal in all planes, which is necessary to enable multiplanar image reconstruction after acquisition. Isotropic data avoid loss of spatial resolution in one plane and reduce partial volume effects after source data reconstruction.²⁰ The increased scanner detector numbers has allowed increased through-plane spatial resolution as the detector width is reduced with increased numbers (1 to 1.25 mm with 4-detector row CT, 0.5 to 0.625 mm with 64-detector row CT).²¹ The new MDCT scanners have not improved in-plane spatial resolution; this is determined by detector geometry and the convolution kernel (reconstruction algorithm).²⁰ Temporal resolution is defined as the required time for data acquisition per slice. It represents the length of the reconstruction window during each heart cycle, which is determined by the gantry speed.

Noise is the random fluctuation of pixel values in a region that receives the same radiation exposure of another. Noise is an important determinant of CT image quality and limits the visibility of low contrast structures. It is determined primarily by the number of photons used to make an image (quantum mottle). Increasing tube current and voltage can reduce it. It can also be reduced by increasing voxel size (decreasing matrix size, increasing field of view, or increasing slice thickness).

Contrast is the difference of the intensity of one area relative to another. Image contrast is the difference in the intensity of a lesion and that of the surrounding background. CT is superior to conventional radiography for detecting low contrast differences. CT contrast is the difference in the Hounsfield unit (HU) values between tissues. CT contrast increases as tube voltage (kV) decreases and is not affected by tube current (mAs). Adding a contrast medium such as iodine can increase contrast. The displayed image contrast is determined by the CT window (width and level).

Scanner Design

Since the first pencil beam EMI scanner in 1972, there have been numerous generations of CT scanners. The first four generations of scanners all had a single row of detectors (single slice) with evolving x-ray tube and detector configurations. Initially, for these generations, the image data were acquired one slice at a time. This involved scanning a slice and then moving the patient table to the next slice position and scanning again, otherwise known as step-and-shoot. The next advancement was the introduction of helical (spiral) CT, which involved continuous moving of the patient table through the CT gantry, as the tube rotated around the patient. The relationship between the patient and tube motion is called pitch, which is defined as table movement (mm) during each full rotation of the x-ray tube divided by the collimation width (mm). A faster pitch means thicker slices, reduced resolution, but lower scan time and lower patient dose. A pitch of greater than 1 will leave gaps between slabs, and a pitch of less than 1 will allow necessary overlap among slabs.

Electron-beam CT technology, also known as fifth-generation CT, provides excellent temporal resolution that allows freezing of cardiac motion through very short acquisition times. In electron-beam CT, the detectors are stationary. The x-ray source is fixed. It consists of a 210-degree ring of tungsten. This is bombarded by an electromagnetically focused beam of electrons fired from an x-ray gun. The patient is placed between the x-ray source and detector, obviating the need for moving any part of the scanner during the examination. Whereas electron-beam CT has better temporal resolution at present (50 ms), the spatial resolution is not nearly as good as that of MDCT because the collimators are too thick.

Detectors

The next major advancement was the development of MDCT. This incorporated the use of multiple rows of detectors to detect wider fan beams. This configuration makes more efficient use of x-ray tube output and covers a larger area in the patient’s z-axis (long axis) for every tube rotation. Unlike in conventional CT, the slice thickness of MDCT is determined by detector width and not by collimator thickness. The number of rows has increased at a rapid pace during recent years, covering 2, 4, 8, 16, 32, 64, 128, 256, and now 320 slices. The current driving force behind this rapid evolution is coronary artery disease.

Flying focal spot technology improves spatial resolution without decreasing the size of the detector elements by use of two overlapping x-ray beams without a corresponding increase in dose. Detector elements of 0.6 mm can be used to acquire 0.33-mm spatial resolution images.

Diagnostic performance in coronary CTA is primarily determined by temporal resolution, which is the required time for data acquisition per slice. The determinant of this is the gantry rotational speed. For the typical setup of a single tube and detector, half a gantry rotation is necessary to acquire the data for volume reconstruction, that is, temporal resolution is equal to half gantry rotation. The temporal resolution of a 64-slice CT scanner with a gantry rotation of 330 ms is 166 ms. For motion-free images to be obtained at any phase in the cardiac cycle, a temporal resolution of 10 ms is required. Achievement of such high temporal resolution is impossible with CT. As a result, cardiac CT phase reconstruction is centered on the quiescent or low-motion window in end-diastole. The postulated required temporal resolution for reliable cardiac imaging is in the range of 65 ms.²² One manufacturer has developed a 256-slice CT scanner (128 × 0.625) with a rotation time of 270 ms and temporal resolution of 135 ms. However, with automatic multisegmental reconstructions with voxel-based optimization, temporal resolutions up to 68 ms are achieved.

Other platforms use dual-source CT technology, which comprises two x-ray tubes and two corresponding detectors. The two acquisition systems are mounted on the rotating gantry, with an angular offset of 90 degrees. For cardiac imaging, a detector configuration of 64 × 0.6 mm is used, whereby two subsequent 64-slice readings with a flying focal spot are combined into two 128-slice projections, with isotropic spatial resolution of 0.33 mm. With the tube rotation time of 280 ms, data can be sampled over only 90 degrees of a gantry rotation (as opposed to 180 degrees with single-source systems), resulting in the industry’s fastest temporal resolution of 75 ms. The ultrafast tube rotation and table feed enable acquisition of the entire heart in 250 ms within a single diastolic phase without a breath-hold, resulting in reduced dose down to an unprecedented 1 mSv or less.

The newly developed 320-slice CT scanner with 0.5-mm detectors (16-cm coverage along the patient’s z-axis) images the body in a cylindrical fashion and can scan the entire heart prospectively, in one heartbeat without any table movement. This is expected to reduce the likelihood of both cardiac and respiratory motion artifacts.

Dual Energy

One of the more recent advances in CT has been the introduction of dual energy. In dual-energy mode, the two tubes emit x-ray spectra of different energy levels that are simultaneously registered in their corresponding detector array. At any given time point, this provides synchronous dual-energy image formation for the entire anatomy encompassed by the scan range. This enables differentiation of scanned tissues with different spectral properties, allowing the identification and subtraction of vessel wall calcification.

Retrospective Gating

Cardiovascular MDCT imaging is currently predominantly performed in the spiral (helical) mode; data are acquired by constant rotation of the x-ray tube/detector system throughout the entire cardiac cycle (continuous scanning). The data acquired are linked to the ECG tracing, allowing retrospective reconstruction of multiple cardiac phases when the study is complete. A specific phase within the R–R interval can be chosen to create a stack of images.

There are two techniques to retrospectively gate the scan to the ECG for image reconstruction. In one technique, the images are collected from one particular point in time in the cardiac cycle, which is defined as a percentage of the R–R interval. Alternatively, this point in time is defined at an absolute fixed time in milliseconds before or after the R–R interval. This latter method is better for irregular heart rhythms.

Retrospective gating allows faster coverage of the heart than does prospective triggering because images are reconstructed at every heartbeat. Continuous spiral acquisition allows overlapping of image sections and therefore permits 20% greater in-plane spatial resolution than that allowed by the collimator itself, resulting in a resolution of 0.6 mm for a 0.75-mm section and 0.4 mm for a 0.6-mm section.

Continuous acquisition throughout the cardiac cycle also allows retrospective reconstruction at different phases of the cardiac cycle. This permits selection of the best phases for each of the coronary arteries and their segments where there is least motion and best image quality. Retrospectively, individual heartbeats may be deleted, or the reconstruction interval for an individual beat can be shifted manually if there are arrhythmias or variable heart rates. Retrospective triggering is the preferred method of triggering for assessment of cardiac function and valve disease.

Prospective Gating

Prospective ECG triggering is a sequential scan in which data acquisition is prospectively triggered by the ECG signal in diastole. Data are collected only at a predefined cardiac phase, established by the operator before the acquisition.

With this technique, tube current is turned on only at a predefined point after the R wave (a constant cardiac phase). At all other times between each R–R interval, no radiation is emitted. This triggering method requires a regular heart rhythm; otherwise, the image created during each heartbeat will occur at a different part of the cardiac phases, resulting in artifacts. Prospective gating is most often used for calcium scoring. Prospective gating reduces the radiation dose by up to 10 times. However, this occurs at the expense of unavailability of systolic phases for additional image reconstruction (which may be needed if diastolic images are suboptimal). In addition, assessment of cardiac function is not possible.

Hsieh and coworkers²³ have developed a new approach for CTA referred to as prospectively gated axial. This technique uses a combined step-and-shoot axial data acquisition and an incrementally moving table with prospective adaptive ECG triggering. This method takes advantage of the large-volume coverage available with the 64-slice MDCT scanner that enables complete coverage of the heart in two or three steps. With this technique, the table is stationary during the image acquisition. It then moves to the next position for another scan initiated by the subsequent cardiac cycle. The result is very little overlap between the scans, significant (50% to 80%) reduction in radiation dose, and more robust and adaptive ECG gating. Earls and associates²⁴ reported an effective dose for the prospectively gated axial group (mean, 2.8 mSv) that was significantly lower than that for the retrospectively gated helical group (mean, 18.4 mSv). This represents a reduction in mean effective dose to the patient by up to 80% from the retrospectively gated helical to the prospectively gated axial approach.

ECG Dose Modulation

Dose modulation is used to help reduce the radiation exposure. During retrospective scanning, the tube current (mA) is turned on continuously during the examination. The tube current is dialed down by 80% during systole. Although the lower milliamperage may result in a suboptimal quality image, this may not be of clinical consequence. The reduced dose of radiation is therefore applied before 35% and after 80% of the R–R interval, which reduces the radiation dose by up to 40% of the usual dose. With most machines, dose modulation is the default mode and needs to be turned off if it is not desired. With increased heart rates, dose modulation cannot be used.

Reconstruction Techniques

Reconstruction of images for the evaluation of arteries uses thin slices with the smallest field of view (e.g., 18 to 20 cm) that encompasses only the heart. If the entire chest needs to be evaluated, a larger field of view is used with thicker slices. Additional data sets with a larger field of view that includes the entire chest should be reconstructed for the evaluation of extracardiac structures.

Kernel or reconstruction filters determine the balance between image resolution and smoothness (or signal-to-noise ratio). Smooth kernels are used for coronary CTA. Sharper kernels provide better definition of lumen borders and reduce blooming artifacts from high-density structures (e.g., stent) at the expense of higher image noise. The use of smooth kernels provides images with lower noise at the expense of possibly increased blooming of calcium and stents.

Fixed Scan Delay

Few authors currently use the method of setting a standard timed scan delay after the intravenous administration of contrast material to acquire images in the arterial phase.^35,36 By setting the same scan delay for every patient, one is assuming that exactly the same hemodynamic conditions exist in all patients. This method does not accommodate patients with any variation from the normal (i.e., low blood pressure, low cardiac outputs, hypovolemia, high outputs). We would not agree with employing this method of a fixed scan delay time.

Bolus Tracking

To more accurately determine the optimal scan delay after intravenous administration of contrast material in patients with variable hemodynamics, the technique of contrast bolus tracking can be employed. Used by many authors,^37,39 this is an efficient way to optimize arterial opacification. Initially, a single low-dose CT image is obtained, without administration of contrast material, at the level of the common carotid for the carotids, the ascending aorta for the coronary and thoracic aorta, and the celiac axis for the abdominal aorta and runoffs. A 10- to 15-mm² circular region of interest is placed inside the middle of the aortic lumen, and this will subsequently measure the Hounsfield units of the aortic lumen on subsequent scanning. At 8 to 10 seconds after intravenous administration of contrast material, serial low-dose monitoring CT scans are obtained at the same table position at 1-second intervals. When the region of interest detects a preset contrast enhancement level (usually 100 to 150 HU value), there is automatic triggering of the scanner to acquire images in the desired scan range. This time-efficient method ensures optimal arterial enhancement within the region of interest, which can be moved to a different arterial location if desired. It enables lower contrast use and reduces scan-to-scan and patient-to-patient variability in arterial opacification.

Test Bolus

Another method to optimize arterial contrast opacification is that of a test bolus or timing bolus acquisition. Described by a number of authors,^38,40 this technique involves intravenous administration of a small bolus (20 to 30 mL) of contrast material followed by serial CT data acquisitions at one table position, usually at the levels described for bolus tracking. Images are acquired, after a scan delay of 8 to 10 seconds, every 1 to 2 seconds for a predetermined number of images (20 to 40) or until the CT technologist chooses to manually stop the acquisition after the contrast peak within the aorta. A time versus Hounsfield unit (attenuation) curve is then generated by placing a region of interest over the contrast-opacified aorta. The time taken to reach peak opacification is then used as the scan delay for the actual CTA and thus corresponds to the time taken for the contrast material to pass from the intravenous injection site of interest. This method is useful as it detects variable transit times between patients with different hemodynamic states and allows individualization of scan delays.

Adaptive Method

The runoff vessels in the symptomatic limb can be problematic with CTA as a result of proximal/in-flow stenosis or, alternatively, distal hyperemia at the site of arterial ulceration; these can cause arterial flow discrepancy between the two lower limbs. Thus, one limb will have a faster flow rate than the other, resulting in good contrast opacification of the arteries ipsilaterally but failed opacification contralaterally. CT fails at eliciting dynamic information. This problem may be somewhat reduced by use of an adaptive method of contrast detection that was described by Qanadli and associates.⁴¹ This method is similar to that of a test bolus technique in that a small 30-mL contrast bolus is administered intravenously, followed by two low-dose (20 mA, 120 kV) CT acquisitions. The first is at the level of the descending thoracic aorta (at vertebra level T12) acquired 20 seconds after the start of intravenous administration of the contrast material for 10 seconds; the second level is the popliteal arteries below the knee acquired 30 seconds after the start of the intravenous bolus (i.e., after the aortic level is finished). Three time-density curves are created, one from the aortic level and one from each of the popliteal artery levels. The time to peak contrast enhancement is then determined for each level (aortic time = T1; popliteal time = T2). If the right and left popliteal times are different, the symptomatic leg or the longer time is taken to be T2. The aortopopliteal transit time (Tt) is determined from T2 − T1. The CT gantry rotation time and table speed are then set to match Tt in order that the popliteal artery will be imaged at peak enhancement. T1 is the delay time between commencement of the usual intravenous contrast bolus and commencement of data acquisition.⁴¹ Laswed and colleagues⁴² applied this adaptive method clinically in patients with peripheral arterial disease, with DSA correlation, and determined that MDCTA has a sensitivity and specificity of 100% for arterial lesion detection on a per-patient basis. Analyzed on a per-segment basis, MDCTA had a sensitivity and specificity for lesion detection of 91% and 96%, respectively, in the below-knee arteries and 100% and 90%, respectively, in the distal pedal arteries. This method was found to be reproducible, had high image quality, avoided the problem of venous overlay, and resolved the issue of differential peripheral arterial opacification.⁴²

Contrast Media Concentration

A recent review attempted to determine the difference that contrast media iodine concentration makes to image quality of MDCTA in the peripheral vasculature.⁴³ All of the studies reviewed used iodine concentrations of 300 mg/mL and higher; whereas improved arterial enhancement and visualization were demonstrated with higher iodine concentrations, there was no clear evidence of a significant difference in diagnostic efficacy for the different iodine concentrations.⁴³ Iezzi and coworkers,⁴⁴ comparing iodine concentrations of 300 mg/mL with 400 mg/mL on a 4-detector scanner, also determined no significant differences in diagnostic ability of CTA for peripheral arterial disease. This could perhaps be explained by applying findings from the coronary CTA literature, whereby Becker and colleagues⁴⁵ determined that coronary artery attenuation levels of 250 to 300 HU were optimal for the evaluation of coronary artery disease because higher attenuation values may underestimate the amount of atherosclerosis owing to obscuration of vessel wall calcification. In our institution, we use a contrast media iodine concentration of 350 mg/mL.

Dual-Head Power Injectors

To achieve the desired arterial Hounsfield unit, a fast iodinated contrast injection with a tight arterial contrast bolus is necessary. Thus, patients will need to have a well-positioned, large-bore (18-gauge), intravenous cannula within the antecubital fossa. A dual-head power injector permits both contrast material and saline to be administered separately, concurrently, and sequentially. This means that two separate injection phases are possible, the first with 100% iodinated contrast material and the second with a 100% saline flush. The advantages of this method are that the arterial contrast enhancement is improved and prolonged, the contrast dose is reduced because most of the administered contrast material is within the arterial side, and the saline flush at the end clears dense contrast material from the superior vena cava to avoid streak artifact.^14,30,46 The value of a triphasic injection strategy with an extra second phase comprising a contrast-saline mixture could also potentially reduce the requirements for contrast material. The new multislice scanners in combination with dual-head injectors allow marked decrease in the amount of contrast material used for routine CTA. However, the total volume of contrast material administered for a routine abdominal CT study (usually 150 mL of 300 mg/mL concentration of contrast material) should not change if solid organs, such as liver, are evaluated in conjunction with CTA. Lowering of the total volume of contrast material may potentially reduce the sensitivity of lesion detection.^12,32

Many authors have reported the use of gadolinium chelates for CTA in patients who have diminished renal function.^47–50 Although gadolinium is radiodense and may be used as a contrast agent with radiography, many gadolinium-based products have higher osmolality than iodine-based contrast media and are therefore potentially more nephrotoxic. More important, there have been several reports of nephrogenic systemic fibrosis leading to serious physical disability in patients with end-stage renal disease receiving gadolinium-containing contrast agents.^51,52

Cerebral CTA

Intracranial vascular disease can now be identified and characterized noninvasively by visualization of the blood vessels with contiguous thin slices and creation of isotropic maximum intensity projection (MIP) images and three-dimensional volume rendered reformations. The newer generation of MDCT scanners, such as the 320-slice CT scanner, allows dynamic entire-volume imaging, producing CT-DSA in all dimensions (two-, three-, and four-dimensional). The time-resolved data sets can be used to perform three-dimensional perfusion with quantification. This technique is not likely to completely replace conventional diagnostic DSA as it has inferior spatial and temporal resolution.

The scanning volume of CTA is determined on the basis of the location of vascular lesions or suspected vascular lesions on CT, MRI, or magnetic resonance angiography. If the lesions are suspected in the supratentorial or circle of Willis region, the scan volume begins at the level of the sellar floor and extends cranially. If the lesions are multiple or unknown, the scan volume can be extended caudally to the level of the foramen magnum. If required, the extracranial vasculature (carotids and aortic arch) can be evaluated by extending the scan volume farther caudal to the level of the aortic arch.

Automated bolus tracking is the preferred technique to reliably obtain the optimal arterial phase by placing a region of interest on the internal carotid artery close to the skull base. An injection volume of 75 mL of a contrast agent with high iodine concentration (350 mg/mL) at a flow rate of 5 mL/sec is typically used. Images are acquired in the caudal to cranial direction with a slice thickness of 1 mm, slice interval of 0.5 mm, and 0.6 mm of collimation (Fig. 80-1).

FIGURE 80-1 Axial MIP image (A) and volume rendered image (B) of CTA of the intracranial vessels demonstrating normal anatomy.

Carotid CTA

In the United States, stroke is the leading cause of adult disability and the third leading cause of death; only heart disease and cancer cause more deaths annually. About 80% of strokes are ischemic strokes. Ischemic cerebrovascular events are often related to atherosclerotic narrowing at the carotid bifurcation. Conventional DSA is the reference standard for the evaluation of carotid artery disease.^53,54 The advantage of noninvasive CTA is the reduction in cost and procedural risks. With conventional DSA, several studies have demonstrated up to a 4% risk of transient ischemic attack or minor stroke, a 1% risk of major stroke, and a small (<1%) risk of death.⁵⁵ Plaque composition can be evaluated by CTA, which is an important indicator of plaque stability and helps with the assessment of risk factors for thromboembolic events. Noninvasive CTA provides the necessary information required before carotid endarterectomy and endovascular therapies.

The scanning volume of CTA is determined on the basis of the initial anteroposterior and lateral topograms. The volume should include all the vascular structures from the aortic arch to the intracranial circulation at the level of the external auditory meatus.

Automated bolus tracking or test bolus triggering techniques can reliably obtain the optimal arterial phase by placing a region of interest on the aortic arch. An injection volume of 75 mL of a contrast agent with high iodine concentration (350 mg/mL) at a flow rate of 5 mL/sec is typically used. Images are acquired in the caudal to cranial direction with a slice thickness of 1 mm, slice interval of 0.5 mm, and 0.6 mm of collimation (Fig. 80-2).

FIGURE 80-2 Coronal MIP image of the carotid vessels (A) shows calcified and soft plaque causing moderate stenosis of the right internal carotid artery beyond the bulb. The volume rendered image of the right side (B) demonstrates the eccentric calcified plaque causing predominantly positive remodeling. The in-vessel view (C) shows the calcified plaque lying within the wall of the carotid bulb.

Thoracic CTA

The disease processes affecting the aortic root and thoracic aorta include aneurysmal dilation, aortic dissection along with intramural hematoma, and penetrating ulcer. By diagnosis, surveillance, and monitoring, imaging plays an important role in determining the timing and role of endovascular and surgical intervention.

Conventional DSA is regarded as the reference standard for evaluation of the thoracic arterial system. CTA of the thoracic aorta is currently the preferred method, however, for follow-up of patients with thoracic aortic aneurysms.⁵⁶ It can, in addition to evaluating luminal disease, assess the vessel wall for plaque composition, hematoma, and dissections. Three-dimensional images demonstrate the normal anatomic relationship of the branch vessels, the mediastinal structures, and the extent of the disease process.

Faster imaging acquisitions and ECG-triggered gating allow nearly complete suppression of respiratory and cardiac motion. Motion artifact in the region of the aortic root can be misinterpreted as aortic dissection. The main disadvantage of retrospective ECG gating, which is increased patient dose, can be overcome by performing prospective gating instead.

The scanning volume of CTA is determined by the indication. It is typical to start at the lung apices, which provides adequate coverage of the supra-aortic vessels. The end position is typically at the level of the mid-kidney; however, if a dissection is suspected, the coverage can be extended to the groin. ECG gating is usually reserved for the evaluation of aortic dissections along with aortic root and ascending aortic aneurysmal disease.

Automated bolus tracking or test bolus triggering techniques can reliably obtain the optimal arterial phase by placing a region of interest on the ascending aorta. An injection volume of 100 mL of a contrast agent with high iodine concentration (350 mg/mL) at a flow rate of 5 mL/sec is typically used. Images are acquired in the cranial to caudal direction with a slice thickness of 1.5 mm, slice interval of 1.0 mm, and 0.6 mm of collimation (Figs. 80-3 to 80-5).

FIGURE 80-3 Oblique coronal multiplanar reconstruction (A) shows a right-sided aortic arch with the left subclavian artery arising from a diverticulum of Kommerell. A thick MIP image (B) and volume rendered image (C) help demonstrate the vascular relationships.

FIGURE 80-4 Volume rendered image of ECG-gated CTA of the thoracic aorta demonstrates a large fusiform aneurysm of the ascending thoracic aorta.

FIGURE 80-5 Oblique MIP image (A) and volume rendered image (B) of CTA of the thorax demonstrate three arteries arising from the distal descending thoracic aorta to supply the left lower pulmonary lobe, consistent with a sequestration.

Pulmonary CTA

The introduction of MDCT has led to CTA being the technique of choice for the evaluation of most known or suspected pulmonary vascular pathologic processes. Spatial resolution has improved significantly, allowing the detection of very small peripheral pulmonary emboli.⁵⁷ Pulmonary CTA can also be used to evaluate for pulmonary hypertension, systemic arterialization of the lung parenchyma in pulmonary sequestration, pulmonary arteriovenous malformations, and congenital pulmonary vascular anomalies. For the evaluation of pulmonary embolism, conventional DSA is still considered the gold standard, but it is rarely used.^58,59

The scanning volume starts at the lung and extends caudally to below the lung bases. Automated bolus tracking can reliably obtain the optimal arterial phase by placing a region of interest on the main pulmonary trunk. An injection volume of 120 mL of a contrast agent with high iodine concentration (350 mg/mL) at a flow rate of 4 mL/sec is typically used. Images are acquired in the cranial to caudal direction with a slice thickness of 1.5 mm, slice interval of 1.5 mm, and 0.6 mm of collimation (Figs. 80-6 to 80-9).

FIGURE 80-6 Volume rendered axial image of CTA of the pulmonary artery demonstrates a pseudoaneurysm of the right lower lobe pulmonary artery.

FIGURE 80-7 Axial thin MIP image (A) of CTA of the pulmonary artery shows bilateral pulmonary artery stents. The volume rendered image (B) demonstrates the stents and the pulmonary valve prosthesis.

FIGURE 80-8 Coronal thick MIP image of CTA of the thorax shows a left superior pulmonary vein draining vertically into the left brachiocephalic vein, consistent with anomalous pulmonary venous return.

FIGURE 80-9 Axial CTA of the thorax (A) shows a left-sided superior vena cava lying anterior to the left main pulmonary artery. The coronal oblique MIP image (B) and volume rendered image (C) demonstrate the vein draining into the coronary sinus.

Coronary CTA

Invasive coronary angiography remains the gold standard to determine the precise location and degree of stenosis because of its high spatial and temporal resolution (0.13 to 0.30 mm and 20 ms). It also allows therapeutic angioplasty and stenting at the time of the procedure. However, it is an expensive procedure with significant radiation exposure and carries a small risk of serious complications.⁶⁰ Furthermore, only one third of these examinations are performed in conjunction with an interventional therapeutic procedure.⁶¹ Twenty percent of patients have normal or minimal coronary artery disease. Thus, a noninvasive assessment of coronary arteries is highly desirable for the diagnosis of coronary artery disease.

Coronary artery disease is a more systemic, diffuse condition, and the treatment is likewise often systemic. Whereas it is important to identify, to quantify, and to treat discrete, significant obstructive lesions, it may also be just as important to determine whether atherosclerotic plaque is present in the coronary arteries so that systemic therapies and lifestyle modifications can be initiated. Until the development of MDCT, CT in the assessment of coronary artery disease was restricted to the detection and quantification of coronary artery calcium. CT has a number of advantages over invasive coronary angiography. It is a noninvasive, three-dimensional technique that can obtain a calcium score, has a high negative predictive value for coronary artery disease, has the potential to characterize plaque components, provides additional anatomic information, and entails no recovery time after the study.

Patients who are not good candidates for CTA include those with unstable acute coronary syndrome who may need percutaneous coronary intervention, patients in atrial fibrillation, those with renal failure, and those with allergy to iodinated contrast material. Obesity limits the ability to obtain diagnostic examinations because of the increased soft tissue attenuation and image noise. With body mass indices above 30, it has been shown that the accuracy of the test falls below that for patients of normal weight.⁶² Coronary artery calcification also reduces the diagnostic accuracy of CTA.

To conduct a coronary CTA examination, intravenous access is obtained, ideally with an 18-gauge antecubital catheter in the right arm. Next, breath-holding is practiced with the patient, and the patient is instructed to avoid swallowing and movement for avoidance of step artifact in the resultant image. β Blockade is used to achieve slower heart rates, either intravenously or orally, depending on the clinical setting. A sample protocol includes intravenous injection of metoprolol (5 mg) repeated up to three times, depending on the heart rate. Alternatively, oral dosing can be administered the night before and the morning of the examination. Slowing of the heart rate is essential for image quality in 16- and 64-slice CT. A study of dual-source CT demonstrated slightly lower per-segment evaluability for high heart rates without β blockade but no decrease in diagnostic accuracy for the detection of coronary artery stenosis.⁶³ Nitroglycerin (0.4 mg sublingually) is given to vasodilate the coronary arteries and to improve visualization.

The scanning volume starts at the aortic arch and extends caudally to the apex of the heart. Test bolus triggering can reliably obtain the optimal arterial phase by placing a region of interest on the ascending thoracic aorta. An injection volume of 100 mL of a contrast agent with high iodine concentration (350 mg/mL) at a flow rate of 5 mL/sec is typically used. Images are acquired in the cranial to caudal direction with a slice thickness of 0.75 mm, slice interval of 0.6 mm, and 0.6 mm of collimation (Fig. 80-10).

FIGURE 80-10 A, Volume rendered technique shows the relationship of the coronary arteries to the cardiac chambers. B, The vessel-only view of the left anterior descending artery alongside the curved planar reformat. The blue dot on the distal left anterior descending artery corresponds to the central axial image.

A protocol similar to that used to image the coronary arteries can be applied to assess the pulmonary and cardiac veins. To assess the pulmonary vein ostia, a contrast bolus of more than 100 mL is recommended with gating optional. The time to scan will be slightly earlier than for the coronaries, with a scan duration of only 2 seconds necessary. Bolus tracking technique can be used with the region of interest placed within the left atrium to optimize opacification of the pulmonary vein ostia.

A popular protocol for administration of contrast material uses a triphasic injection; the first phase is contrast material, the second phase is an admixture (70% saline/30% contrast material), and the third phase is a bolus chaser of saline alone. The advantage of this triphasic technique is that it will produce enough contrast in the right ventricle to allow visualization of the septum for left ventricular functional assessment. The rate of infusion of contrast material is a critical determinant of the quality of the image. A minimal flow rate of 5 mL/sec is optimal for the general population. For obese patients, higher rates should be attempted.⁶⁴

Personalized, computerized, patient-based dosing of contrast media delivery protocols are showing promise, helping to produce diagnostic quality images more consistently for coronary CTA. They personalize the scan delay, flow rate, and volume for each of the three phases of injection on the basis of the patient’s weight, scan time, time to aorta peak, HU peak number, and concentration of iodine used (300 or 350 mL).

Abdominal CTA

Common abdominal applications of CTA include evaluation of the abdominal aorta, preoperative and postoperative assessment for kidney and liver transplantation, preoperative planning for hepatic segmentectomy and pancreatic surgery, and evaluation of mesenteric ischemia and gastrointestinal hemorrhage. The protocols for each application should be individually tailored to optimize imaging of the targeted vascular structures. For example, planning of a Whipple procedure should include both arterial and portal venous phase images.

Abdominal Aorta

Full assessment of aortic dissection by CTA requires unenhanced CT followed by contrast-enhanced CT. Unenhanced CT is useful for detection of dense intramural hematoma that may be mistaken for chronic thrombus on the enhanced images.

The scanning volume starts at the diaphragm and extends caudally to the symphysis pubis. Bolus tracking triggering can reliably obtain the optimal arterial phase by placing a region of interest on the aorta at the level of the celiac artery. An injection volume of 100 mL of a contrast agent with high iodine concentration (350 mg/mL) at a flow rate of 5 mL/sec is typically used. Images are acquired in the cranial to caudal direction with a slice thickness of 2.0 mm, slice interval of 2.0 mm, and 0.6 mm of collimation (Fig. 80-11).

FIGURE 80-11 A volume rendered image of CTA of the abdominal aorta demonstrates a fusiform aneurysm of the infrarenal abdominal aorta.

Patients treated with endovascular aneurysm repair undergo frequent imaging by CTA to monitor for endoleak, stent migration, or fracture as well as stability of aneurysm size (Fig. 80-12).⁶⁵ Increase in the diameter of an aneurysm is associated with endoleak. Evaluation of an endoleak requires unenhanced CT imaging as well as imaging during arterial and venous phases of contrast enhancement. Unenhanced images are used to detect artifact from calcification or embolization material that may mimic endoleak on the enhanced images. Venous phase images have been shown to enhance the detection rate of endoleaks.

FIGURE 80-12 A volume rendered image of CTA of the abdominal aorta demonstrates bi-limbed endovascular stent grafts within the abdominal aorta, extending into the iliac vessels.

Pancreas

Staging of pancreatic cancer with multiphasic CT is now the standard of care in many institutions.⁶⁶ It requires arterial, parenchymal, and portal venous phase images to evaluate the solid organs and the mesentery.^67–69 Addition of a non–contrast phase allows detection of parenchymal and vascular calcifications. Neutral oral contrast agents help distend the bowel and improve visualization of the bowel wall. Positive oral contrast agents (e.g., iodine-based agents) obscure vascular detail and should be avoided in all abdominal CTA.

Although curved multiplanar reformations of the pancreas improve visualization of the parenchyma and pancreatic duct, MIP and volume rendered images are needed for vascular analysis (Fig. 80-13).^70,71 The newest generation of scanners allows rapid multiplanar and maximum intensity projections on the scanner, thus obviating the need for routine data transfer to an independent image processing workstation.

FIGURE 80-13 A 39-year-old man, status post simultaneous pancreas-kidney transplantation. This coronal MIP image from MDCTA demonstrates widely patent renal transplant and pancreatic transplant arteries anastomosing with both native common iliac arteries.

Renal Imaging

A comprehensive examination includes non–contrast-enhanced images to evaluate for renal calculi and vascular calcification, arterial phase to detect vascular anatomic variants and anomalies, nephrographic phase to detect renal parenchymal abnormalities, and excretory phase to evaluate the collecting systems and ureters.^72–75 Coronal thin-slab MIP images are useful for optimal visualization of the renal arteries (Fig. 80-14).⁷⁶ However, because MIP images do not provide spatial depth, volume rendered images are also beneficial (Fig. 80-15). It is feasible to scan the renal vasculature with 50 mL of nonionic iodinated contrast material and saline chase using a dual injector on a 16- or 64-slice CT scanner with excellent results.¹⁴ However, to improve opacification of the ureters, saline bolus or diuretics may be beneficial.⁷⁷

FIGURE 80-14 Axial oblique thick MIP image demonstrates beading of the distal left renal artery, suggestive of fibromuscular dysplasia.

FIGURE 80-15 Axial thick MIP image (A) of renal CTA of a healthy renal donor demonstrates an accessory renal artery anterior to the main left renal artery. The volume rendered image (B) helps define the anatomic relationships.

Hepatic and Mesenteric Vasculature

CTA allows excellent visualization of the hepatic and mesenteric vasculature (Fig. 80-16). Because of high temporal resolution, it may be necessary to scan the region of interest twice; a “pure” arterial phase does not allow evaluation of the venous structures. This is necessary because incomplete enhancement of the mesenteric venous branches during late arterial phase may mimic thrombosis.

FIGURE 80-16 Volume rendered image of the abdominal aorta demonstrates an accessory left hepatic artery arising from the celiac axis.

Celiac and mesenteric stenoses are best visualized by sagittal thin-slab MIP or volume rendered images (Fig. 80-17). However, a thin-slab coronal MIP image also provides an excellent overview of the mesenteric vascular structures as well as the bowel loops (Fig. 80-18).⁷⁸ The use of neutral oral contrast agents is important in evaluating the mesentery to improve visualization of the bowel mucosa.

FIGURE 80-17 Sagittal MIP image (A) and volume rendered image (B) of the abdominal aorta show a metal endovascular stent placed in the proximal celiac artery.

FIGURE 80-18 Coronal MIP image of CTA of the mesenteric vessels shows a focal mycotic aneurysm of the distal superior mesenteric artery.

Acute mesenteric ischemia is a life-threatening event that may be caused by a variety of factors. These include embolic phenomenon, severe hypoperfusion, thrombosis of stenotic vessels, dissection, hypercoagulable states, and vasculitis. Diagnosis of mesenteric venous thrombosis requires delayed images to avoid early-phase incomplete enhancement. Unenhanced images are also beneficial because they may show a hyperdense thrombus.^7,79

Lower Extremity CTA

Most institutions will initially perform a scout view, from the diaphragms to the feet; an optional non–contrast-enhanced acquisition from the celiac axis to the feet; and an arterial phase–timed acquisition from the celiac axis to the feet, with the option of a second later limited acquisition from the lower thighs to the feet in the event that the contrast material has a slower transit time on the symptomatic side.

Bolus tracking triggering can reliably obtain the optimal arterial phase by placing a region of interest on the aorta at the level of the celiac artery. An injection volume of 150 mL of a contrast agent with high iodine concentration (350 mg/mL) at a flow rate of 5 mL/sec is typically used. Images are acquired in the cranial to caudal direction with a slice thickness of 1.0 mm, slice interval of 1.0 mm, and 0.6 mm of collimation (Figs. 80-19 to 80-24).

FIGURE 80-19 A 76-year-old man with intermittent claudication at 500 m. Left anterior oblique coronal MDCTA demonstrates a localized dissection flap (arrow) with a corresponding moderate stenosis of the left common iliac artery (LCIA).

FIGURE 80-20 A 40-year-old man with peripheral arterial disease, status post left common iliac artery and left external iliac artery stenting through the right common femoral artery approach. Sagittal MIP image from MDCTA (A) demonstrates a right common femoral arterial pseudoaneurysm (arrow). Volume rendered view (B) demonstrates the right common femoral arterial pseudoaneurysm; note the left common and external iliac arterial stents in situ.

FIGURE 80-21 A volume rendered view of CTA of the lower extremities shows occlusion of the right distal superficial femoral artery at the level of Hunter’s canal.

FIGURE 80-22 A 58-year-old woman complaining of intermittent claudication at 200 m. This posterior volume rendered view of MDCTA demonstrates bilateral superficial femoral arterial stents in situ, with evidence of a tight stenosis within the right superficial femoral artery distal to the stent (arrow).

FIGURE 80-23 A 48-year-old man presented to the emergency department after a fall from a stepladder; 5 years previously, he had a posterior left knee dislocation after a road traffic accident. MDCT topogram (A) demonstrates a markedly swollen left thigh. The volume rendered image (B) demonstrates the large hematoma adjacent to the distal left superficial femoral artery displacing the soft tissues medially. Axial contrast-enhanced CT (C) shows a large hematoma medially within the left thigh and a large pseudoaneurysm within the hematoma. Coronal MIP image from MDCTA (D) demonstrates the pseudoaneurysm donor artery, which is the superficial femoral artery.

FIGURE 80-24 A 69-year-old man with a swelling behind the right knee. Sagittal MIP image from MDCTA (A) demonstrates a fusiform aneurysm of the proximal popliteal artery with minimal mural hematoma. Volume rendered posterior view (B) demonstrates the fusiform right popliteal aneurysm. Coronal MIP image from MDCTA (C) shows a popliteal tight stenosis inferior to the fusiform aneurysm.

Because most patients who are having peripheral CTA have known peripheral arterial disease, one would expect them to have some arterial stenoses or occlusions; thus, a fast scan time is not always desirable because one does not want to scan ahead of the contrast bolus. Thus, a table pitch of 1.1, gantry rotation of 0.37 second, table increment of 21.1 mm/360-degree rotation, and table speed of 63 mm/sec will all result in a scan time of 23 seconds and will require a fast contrast injection bolus. However, if one suspects more severe atherosclerosis, a slower contrast injection bolus and correspondingly slower/longer scan time is desirable. Longer scan time can be achieved by reducing the table speed through the scanner (i.e., the pitch) and by slowing the gantry rotation time. Thus, in this scenario, a table pitch of 0.85, gantry rotation of 0.5 second, table increment of 17 mm/360-degree rotation, and table speed of 32 mm/sec will all result in a scan time of 40 seconds. As mentioned previously, different detector numbers and different manufacturers will require different scan protocols.

Artifacts

Streak artifacts can be noted around high-attenuation structures, such as endovascular stents, pacing wires, and coils, and can obscure adjacent structures. These are difficult to avoid and can be reduced with special artifact reduction software developed by the manufacturers.

High-attenuation structures such as stents and calcified plaques can appear enlarged (“bloomed”) because of partial volume averaging effects. This results in overestimation of the size of the calcified plaque. This can obscure the coronary lumen, limiting the estimation of stenosis in the affected segments. Although sharper filters or kernels and thinner slices (0.5 to 0.6 mm) may reduce the artifact with stents, this has little effect on calcification.

Beam hardening is important to recognize. This artifact is a low-density focus in a reconstructed image, appearing similar to noncalcified coronary atherosclerotic plaque. It is a result of low-energy photon absorption as the x-ray beam crosses a high-density structure, such as a surgical clip or calcification. In areas neighboring the dense structure, the high-energy beam passes through with little absorption, resulting in a low-density focus. It occurs in one direction of the scan plane.

Other types of artifacts commonly observed in the thorax are due to incomplete breath-holding, observed on sagittal or coronal views. These are seen as “stair-step” artifacts through the entire data set, including nonmoving structures, such as the bones. Adequate instruction to the patient before imaging is essential to avoid such artifacts.⁸⁰

Many unique artifacts can result from imaging of the rapidly moving heart, the most common of which is the result of cardiac pulsation.¹⁹ This produces an image with horizontal slabs of the image in displaced alignment. The second type is banding artifacts, which result from an increased heart rate during the scan. These are similar in appearance to pulsation artifacts. These artifacts especially occur in patients with high heart rates, in patients with heart rate variability, and in the presence of irregular or ectopic heart beats (e.g., premature ventricular contractions and atrial fibrillation). These can be minimized by scanning with higher temporal resolution on the order of 50 ms or by multisecond reconstruction with MDCT. A β blocker should be used to reduce the heart rate to less than 65 beats/min.⁸¹ Recent data support high diagnostic accuracy for the detection of coronary artery stenosis with the use of dual-source CT without β blockade, attributed to the improved temporal resolution (75 to 83 ms) compared with single-source MDCT (166 ms).⁸² However, in clinical practice, the exact role of β blockade in dual-source imaging is yet to be determined, especially at high heart rates.

Radiation and Dose

There has been rapid evolution in MDCT technology with increasing temporal and spatial resolution, allowing rapid imaging of a greater population of patients than in previous years. With this evolution, the scientific and public awareness of the radiation risks associated with this technology has been increasing. Therefore, it is essential for operators to understand the effects of the radiation exposure delivered and to implement techniques that reduce this exposure.

Potential biologic effects from ionizing radiation depend on the radiation dose and the biologic sensitivity of the tissue or organ system irradiated. Effective dose (E) is the descriptor that reflects this difference in biologic sensitivity. The units of E are sieverts (Sv), often expressed as millisieverts (mSv). Not all tissues are equally sensitive to the effects of ionizing radiation. Therefore, tissue-weighting factors (Wt) assign a particular organ or tissue the proportion of the risk of stochastic effects (e.g., cancer and genetic effects) resulting from irradiation of that tissue compared with uniform whole-body irradiation. The x-ray radiation that everyone is exposed to each year from natural sources amounts to 2 to 5 mSv.

CT contributes to 75% of the total collective dose from ionizing radiation to the public from medical imaging.¹⁷ With the increasing widespread use of CT and individual patients having multiple scans during their lifetime, low-dose techniques are increasingly desired. The goal is to keep doses “as low as reasonably achievable” (ALARA). This represents a practice mandate adhering to the principle of keeping radiation doses to patients and personnel as low as possible. Among the most widely known protocols, such as calcium scoring studies, the effective dose is relatively small, 1 to 3 mSv.⁸³ The effective radiation dose with retrospectively gated coronary angiography by 64-slice MDCT is estimated to be approximately 11 to 22 mSv.⁸⁰

Clinically, studies performed with higher milliamperage have less noise and higher signal-to-noise ratio and contrast-to-noise ratio, which is visually more appealing and useful in heavily calcified or stented vessels. However, doubling of the tube current doubles the radiation dose. Lowering of the tube voltage allows significant reduction in effective dose as increasing the kilovoltage by 15% has the same effect as doubling of the milliampere-seconds. The tradeoff for lower tube voltage is increased noise; however, use of 100 kV for patients with low body mass index results in increased intensity of iodinated contrast media as 100 kV is closer to the k-edge of iodine.

Adaptive scanning reduces the radiation exposure of dose-sensitive anatomic regions, such as the female breast. This is done by switching the x-ray tube assemblies off during the rotation phase in which the anatomic regions concerned are most directly exposed to radiation. In this way, it is possible to reduce the radiation exposure of individual anatomic regions, such as the breasts, by up to 40%. Furthermore, an adaptive dose shield can block irrelevant pre-spiral and post-spiral radiation with dynamic diaphragms, thus ensuring that only a minimum and clinically essential radiation exposure occurs. This enables an additional 25% reduction of the dose required for routine examinations.

Fraioli and colleagues⁸⁴ performed peripheral CTA at 50 mAs, 100 mAs, and 130 mAs in three groups of patients with peripheral arterial disease and compared the findings with the gold standard, DSA.⁸⁴ No difference in qualitative analysis of arterial segments was determined between the three CTA groups; similar sensitivity and specificity for diagnosis of peripheral arterial disease were achieved with CTA performed at 50 mAs, 100 mAs, and 300 mAs. Thus, optimal image quality with diagnostic accuracy for peripheral arterial disease was achieved with the low-dose technique, allowing a 74% reduction in effective dose to the patient.⁸⁴ Similarly, for renal arterial CTA in potential living renal donors, 100 kVp achieved diagnostically acceptable CT images with a significant radiation dose reduction compared with CTA at 120 or 140 kVp.⁸⁵ In our institution, Farrelly and colleagues have demonstrated a significant reduction in radiation dose by performing low peak tube voltage, prospectively ECG-gated CTA of thoracic aortas without loss of image quality. The mean radiation dose of the retrospectively gated–120 kVp was 26 mSv compared with 2.94 mSv for the prospectively gated–100 kVp studies. The latest coronary CTA protocols using prospective gating reduce the radiation doses to less than 3 mSv.²⁴

Strategies for reduction of radiation dose in cardiac MDCT include the following⁸⁶:

The implementation of these techniques and other dose reduction strategies evolving industry-wide will greatly minimize the radiation risks associated with CTA.

Postprocessing

Thinner slice acquisition with CTA has resulted in a significant increase in the number of images acquired. Hence, “data overload” has been a direct side effect of isotropic and near-isotropic scanning. CTA of the abdominal aorta and its branches on the newest scanners generates more than 1000 images.⁸⁷ As the volume of data significantly increases, the processing power of many of the current image processing workstations is pushed to its limits. Because of the large volume of data obtained with CTA, accurate, time-efficient, and reproducible image postprocessing to enable disease interpretation is now a requirement.^88–91 The axial image plane, formerly the plane with highest image quality in a single-detector CT scanner acquisition, is no longer the only plane available for image interpretation⁹²; however, it remains mandatory to review these axial planes to ensure detection of extra-arterial disease.⁸⁹ For the majority of patients with arterial disease, axial image viewing is time-consuming, inefficient, and often less accurate than viewing of reformatted images.⁹³ With MDCTA acquisition, near-isotropic data sets can be obtained and thus manipulated in all imaging planes and projections without significant loss of image quality to enable eloquent display of the arterial anatomy and pathology.⁹⁴ Therefore, a dedicated three-dimensional workstation to enable a real-time interactive approach to image manipulation and interpretation has now become a necessity. Many authors advocate the use of three-dimensional image display, both for diagnosis by the interpreting physician and for procedure planning by the interventionalist.^91,95–97 Available image postprocessing techniques on the clinical three-dimensional workstations include multiplanar reconstruction (MPR), maximum intensity projection (MIP), volume rendering technique (VRT), and shaded surface display (SSD). Postprocessing of the source data not only improves visualization of the vascular structures and their relationship to the adjacent organs but also decreases the number of slices needed for the review of the data set.^98,99 Images can be processed on the scanner or a freestanding image processing workstation. Thicker slices may be produced for review of the axial images on PACS.¹⁰⁰ When the attenuation of vessels is usually more than or nearly 300 HU, the image quality of MPR, MIP, and VRT is usually satisfactory. Most institutions, including that of the authors, routinely use a combination of the MIP and volume rendered display of data for CTA because these two are complementary. Whereas volume rendering is useful for the display of soft tissues and three-dimensional relationships, MIP provides a more detailed view of the vessels within the slab of data and is less operator dependent.¹⁰¹

Postprocessing Limitations

Technical limitations to postprocessing may exist, with the main ones being secondary to heavy arterial wall calcification or the presence of an arterial stent, as discussed before. Review of the axial source data can be a simple measure to overcome these problems. Postprocessing artifacts can also occur secondary to inadvertent vessel subtraction, particularly with MIP images, and production of pseudostenosis or occlusions in MPRs (especially curved MPRs) as a result of inaccurate centerline definition. Volume rendered images may not demonstrate aneurysmal dilation of a vessel because of partial luminal thrombosis. Again, the axial source data can act to solve these problems. To date, there are no fully automated algorithms to detect vessel centerlines, to segment out bony structures, or to detect or to subtract vessel wall calcification. However, with constant software updates, these tools are likely to become available in the near future.⁸⁰ A new investigational tool is dual-energy CTA that allows automatic subtraction of bone and potentially calcified atherosclerotic plaque, based on differential attenuation of bone, calcium, and iodine on a dual-energy scan.