Planar Imaging

A single crystal camera produces a two dimensional or planar image. Multicrystal cameras also produce planar images but are also able to perform fast, dynamic imaging used in first pass and gated studies. Planar images are unable to reflect the depth of an image.⁶ Planar imaging may be the only option available for obese patients who do not “fit” the single-photon emission computed tomography (SPECT) camera or for those who cannot remain immobile.¹¹

Single-Photon Emission Computed Tomography Imaging

The most common nuclear study of myocardial perfusion is SPECT imaging.¹² Planar images or tomographs are collected by rotating the scintillation detectors (single-, dual-, or triple-head scintillation cameras) around the patient, taking images at 3-degree increments.¹⁰ SPECT images allow for quantification of radioactivity.⁶ However, attenuation artifacts (absorption of the radioactive energy by other body tissues, preventing the camera from detecting it) are greater than with planar images, particularly those images involving the inferior wall of the heart in men and the anterior wall in women, and may produce false-positive scans.¹³ Quality of the images is dependent on the use of stringent quality control measures and experience of the staff. The most commonly used radionuclides are thallium-201, technetium-99m-sestamibi, and technetium-99m-tetrofosmin.¹⁴ Image quality is also patient-dependent. Patients must be able to lie perfectly still for 15 to 20 minutes, with their hands over their heads, while data are collected. Newer systems allow for a 50% reduction in duration of the scan as well as reduced processing time.¹⁰ Currently available systems also allow for seated positioning.¹⁰

Therapists should identify patients who would have difficulty with arm movements or with remaining still. For these patients, other imaging modalities such as echocardiography may be appropriate.

Positron Emission Tomography

Positron emission tomography (PET) uses positron-emitting radionuclides. The images obtained by PET can provide information regarding cellular processes in living tissue. Both cardiac and lung tissue can be quantitatively examined to obtain information on metabolism, blood perfusion, tissue viability, autonomic regulation, and other processes.15,16 PET is valuable in delineating myocardial areas with reversible and irreversible injury, thus assessing the feasibility of revascularization, in patients with CAD or left ventricular dysfunction.¹⁷ PET studies use short scanning times (5-35 minutes) and because of the short half-life of PET tracers, multiple scans can be repeated in a relatively short period of time.¹⁸ Although PET scans offer superior imaging, in part due to greater correction for attenuation artifact,¹⁸ PET’s technological complexity, short-lived radio tracers, and specialized equipment have resulted in high costs and more limited clinical application.¹⁵ However, because of increased PET use in oncology imaging, opportunities for imaging of the cardiorespiratory system are now increasing.^15,19 Current FDA-approved PET nuclides are rubidium-82 (82-Rb), 13N-ammonia (13-NH3), and fluorine-18-fluorodeoxyglucose (18F-FDG).¹⁵

Exercise Stress Studies

Many of the tests performed using radionuclides are coupled with an exercise test. A treadmill exercise test is performed, and at the peak of exercise, a tracer is injected. Shortly afterward, depending on the tracer, images are acquired. Exercise imaging enhances the identification of ischemic areas. These tests are then often compared with rest studies. Rest/stress or stress/rest protocols can be used.¹⁴

Pharmacological Stress Studies

Some patients are unable to exercise to an intensity sufficient to produce stress on the cardiovascular system. When patients are unable to exercise, pharmacological agents can be administered instead. For example, dipyridamole (Persantine) can be used to dilate the patient’s coronary arteries; or to increase the metabolic demand of the heart, beta-agonists such as adenosine and dobutamine are most commonly used. The newest approved agent is regadenoson, a selective A2A adenosine receptor agonist. Pharmacological stress tests can be coupled with SPECT and PET nuclear imaging techniques or with echocardiography to determine regions of ischemic myocardium suggesting functionally significant blockage in the coronary artery supplying that area.

Nuclear-Derived Measurements

Data derived from radionuclide images provide information about perfusion, function, and viability. Radionuclides taken up by the myocardium provide a picture of the heart that includes wall thickness and an outline of the chamber. In radionuclide angiography, which includes first-pass and gated studies, the blood is highlighted as it passes through the chambers. Qualitative and quantitative measurements can be taken.

The most important of these measurements, the ejection fraction (right and left ventricle) is a measure of myocardial function. It is derived from quantitative counts of the ventricular area during diastole and systole:²⁰

< ?xml:namespace prefix = "mml" />([end-diastolic counts−end-systolic counts]×100)/end-diastolic counts

Left ventricular ejection fraction (LVEF) has been shown to be predictive of mortality. LVEF normally ranges from 50% to 85%, whereas LVEF below 40% indicates moderate-severe congestive heart failure.21,22 LVEF is linearly related to mortality up to 45% EF²³ and inversely related to moderate- to large-sized infarcts.²⁴ Right ventricular ejection fraction (RVEF) has been found to have a wide range of normal values (35% to 75%)²⁵ but is not as predictive of function without data about wall motion abnormalities. The strongest radionuclide predictor of outcome is the exercise LVEF. In terms of predicting outcome, multivariate analysis of clinical data, catheterization data, and radionuclide measurements have shown that the radionuclide results (exercise LVEF, resting end-diastolic volume, and change in heart rate [HR]) have the same prognostic power as the catheterization data.²⁶

Another important measurement is wall motion. Quantitative and qualitative evaluation of the movement of the myocardium can be made. Wall thickness and wall movement are compared during systole and diastole. Assessments are made about akinesia (absence of wall motion), global or regional hypokinesia (reduction of wall motion), and dyskinesia (outward bulging of the wall during systole).²⁷ Global left ventricular function is a strong predictor of survival. It can help clinicians differentiate a weak heart from one that is stiff, thus allowing the appropriate treatment. Regional wall function, when correlated with knowledge of coronary artery anatomy, allows for the identification of potential blockages or areas of infarct. Assessing wall thickness can provide information about hypertrophy or aneurysm but is done more reliably by echocardiography.

Infarct size is an important measurement because it predicts short-term mortality. It is also important as a clinical endpoint in determining the effectiveness of reperfusion strategies and pharmacological interventions, which should result in a decrease of infarct size.²⁸ The size of a myocardial infarct can be estimated by using the measurements previously described (i.e., ejection fraction, end-systolic volume, and global or regional wall motion).²⁸ Perfusion defects of the myocardium can be measured acutely, after intervention, or during recovery.

Anatomical measurements of chamber size can be made. From these data, chamber volume can be calculated, as well as stroke volume and cardiac output (Table 14-2).

Myocardial Perfusion Imaging

Perfusion of the myocardium is a vital factor in the viability and function of the heart. Information about myocardial perfusion is used for diagnostic decisions, treatment decisions, and prognosis. Perfusion of the myocardium under rest and exercise conditions is important in the diagnosis of CAD. For those patients with left ventricular dysfunction, identifying tissue that is viable but still at risk is critical for improving long-term outcomes. The efficacy of reperfusion strategies, such as angioplasty and thrombolytic therapy, must be evaluated. Information gained from combined perfusion and metabolic studies has helped to increase the understanding of ischemic myocardium. Two types of contractile dysfunction have been delineated. Hibernating myocardium is the result of prolonged ischemia. In this case, the contractility of the muscle fiber is affected so that there may appear to be regional wall motion abnormalities. The tissue is alive but not contracting. It is theorized that this is a measure to reduce energy expenditure and ensure myocyte survival. The second condition, myocardial stunning, occurs under conditions of acute ischemia. In this case, contractile dysfunction that initially occurs during the acute ischemic episode persists for some time after perfusion has returned to normal. Both conditions are reversible. Patients demonstrating hibernating myocardium may benefit from revascularization procedures. Patients with stunned myocardium may only require supportive care until contractile function returns.²⁹ The type of imaging performed and the specific radiopharmaceutical used provides similar but not necessarily interchangeable imaging results and measurements.¹⁴

Thallium-201

Thallium-201 (201-TL) is the oldest radioactive isotope used in myocardial perfusion studies.¹⁴ It is a cyclotron-produced isotope that emits low-energy radiation (68 to 80 kiloelectron volts [keV]). Thallium concentrations in the myocardium depend on 201-TL properties, uptake, and redistribution. Administered intravenously, its distribution throughout the body depends on blood flow, and within 5 minutes, myocardial uptake represents myocardial blood flow.³⁰ The first-pass myocardial uptake coefficient is 85%.¹⁴ Extraction from the blood and transport across the cell membrane depend on active and passive transport mechanisms. Thallium is transported across the cell membrane through the sodium-potassium ATPase pump and facilitative diffusion.³⁰ Normally perfused and functioning myocardium will take up the thallium tracer, while damaged cells will demonstrate weaker uptake. Within 15 minutes thallium begins washout (also called redistribution) in normal myocardium. Images taken 4-24 hours later show clearance of the 201-Tl, again as a result of normal blood flow, while abnormal cells have more tracer as it has not been washed out.³⁰

Thallium-201 images can be qualitatively and quantitatively evaluated. Normally perfused myocardium demonstrates uniform uptake of the tracer. Uniform uptake can occur as long as blockages are less than 50% of the artery. Ischemic but viable myocardium initially appears as areas of decreased uptake; these areas fill in over time, a function of redistribution. Because blood flow is decreased, clearance of thallium-201 from the defect region is slower.³⁰ In infarcted areas, these defects remain unchanged over time. Thus perfusion and tissue viability can be assessed with 201-TL. Qualitative evaluation requires visual inspection of the images. Quantitative evaluation is performed by specialized software individual to the imaging system used. The computer analyzes the amount of radioactivity taken up in a particular region of interest (ROI). It then quantifies this count so that regions can be compared with each other and with normalized data. In this way, unperfused or hypoperfused areas can be identified (Figure 14-2).

Thallium-201 is used to acquire planar and SPECT images for rest studies, exercise stress studies, and pharmacological stress studies. The traditional thallium-201 stress study calls for injection of the tracer at the peak of exercise with collection of the stress data within 10 to 15 minutes. The redistribution study is completed at a minimum of 4 hours later. Alternative study protocols include reinjection of thallium before the 4-hour redistribution study and use in dual-isotope studies.³⁰ Reinjection of thallium and repeat imaging 24 hours after initial study can help identify severely ischemic (greater than 90% blockage) areas, which may take much longer to demonstrate redistribution.

Positron Emission Tomography Tracers

PET tracers are also used to evaluate perfusion. The most commonly used tracers are 13N-ammonia, rubidium-82, and 15-O-labeled water.33,34 The benefit of using PET tracers is that it is possible to quantify myocardial blood flow in mL/g/min.

Fluorine-18-fluorodeoxyglucose (18F-FDG) is a glucose analogue that crosses the capillary and sarcolemmal membrane at a rate proportional to that of glucose. 18F-FDG becomes trapped in the myocardium, because it is not a useful substrate in metabolic pathways. The distribution of 18F-FDG is relative to uptake of unlabeled glucose.³⁵ Glucose utilization is therefore used as a marker of tissue viability.

Identification of areas with normal metabolism, altered metabolism, or no metabolism is based on the use of glucose as an energy substrate. Normally perfused myocardial cells use a combination of fatty acids and carbohydrates for adenosine triphosphate (ATP) production when in a fasting state and use glucose as the primary energy source in the postprandial state. Ischemic myocardium uses glucose in the fasting state and the postprandial state. Infarcted or scarred areas show no metabolic activity. Therefore increased uptake of 18F-FDG in the postprandial state occurs in both hibernating and normal myocardium, but in the fasting state, it occurs only in the hibernating myocardium. Clinical studies in patients with infarct showed that areas with persistent thallium-201 perfusion defects have evidence of remaining metabolic activity in 47% of regions when studied with a PET scan, indicating overestimation of irreversible injury. Use of reinjection and late redistribution imaging has significantly lowered this number.³⁶ The implication derived from this finding is that in areas with perfusion defects, if glucose activity remains, the region is viable (mismatch pattern); conversely, if such activity is absent, the area is likely to be infarcted or necrotic (match pattern).³⁷ Viewed in functional terms, mismatch simply implies the potential for improvement; match implies no potential for improvement in contractility after successful revascularization.³⁸

Positron Emission Tomography

PET scans using 18F-FDG isotopes are being used to identify metabolically active tissue (glucose metabolism) in the lungs. It has found application in the differential diagnosis of pulmonary nodules, inflammation, and potential malignant lesions.⁴⁶ PET with 18F-FDG is used as a diagnostic tool for pulmonary nodules and mass lesions.⁴⁶ Solitary pulmonary nodules (SPN) are often found during screening tests. They are of significant concern because of the risk for malignancy. Gould and colleagues⁴⁶ performed a meta-analysis and concluded that sensitivity is 96.8% and specificity is 77.8% for detecting malignancy in SPNs. PET 18F-FDG is not recommended for nodules less than 1 cm in size.⁴⁷ In addition to difficulty accessing PET scanners, cost may be a limiting factor—based on Medicare reimbursement figures, PET scans using 18F-FDG can cost six times more than conventional CT scans of the chest.⁴⁷

Gallium Scintigraphy

PET or SPECT imaging has largely replaced gallium scanning for lung disease.⁴⁸ However, this isotope is still used in some centers and in specific clinical situations. Gallium-67-citrate binds with transferrin. This complex binds with transferrin-receptors in neoplastic cells while in areas of inflammation and infection gallium binds to lactoferrin and siderophores.⁴⁸ The radioisotope is injected intravenously, but it takes 48 to 72 hours before imaging can be performed; this delay is one reason it is out of favor.

Gallium is used in the evaluation of patients with inflammation and infection. Non-infectious inflammatory processes include: sarcoidosis, amiodarone toxicity, following cytotoxic therapy with bleomycin, and interstitial pulmonary fibrosis, as well as many other entities.⁴⁸ Gallium scanning is used to monitor response to treatment in lymphoma and where PET scans are not available can be useful in the diagnosis of lung cancer.⁴⁸ Gallium continues to be useful in infections (e.g., TB) as it helps discriminate active infection from fibrosis. Similarly, discriminating active disease from scarring is useful in evaluating AIDS patients with Pneumocystis carinii pneumonia (PCP).⁴⁸ The sensitivity of gallium scanning for PCP in patients with AIDS approaches 94% to 96%.⁴³

Cardiac Imaging

The gold standard with which many of the cardiac noninvasive tests are compared is that of cardiac catheterization. It is an invasive test, with a small but serious risk to the patient. Complications include death, stroke, MI, bleeding, arterial trauma or thrombus, renal dysfunction, and arrhythmias.⁴⁹ The test is indicated in patients presenting with ST elevated MI (STEMI) who are candidates for reperfusion as well as those presenting with chest pain and non-STEMI who are at risk for further cardiac events.⁵¹ There is some debate as to whether the NSTEMI population should undergo a coronary angiogram with potential for revascularization within 24 hours or should wait. Current guidelines recommend the early procedure.⁵² Patients who have CAD with angina that is unresponsive to medical therapy may also need catheterization.

The procedure for cardiac catheterization of the left side of the heart requires the threading of a catheter, guided by fluoroscopy, through the femoral artery or brachial artery to the aorta. Direct measurements of chamber pressures, blood flow, and oxygen saturation can be obtained. When selective coronary arteriography is performed, radiopaque contrast material is injected from the catheter into the left main or right coronary artery. Cine recordings are made after the injection, and injections are repeated until the entire coronary tree is visualized. The location of an arterial obstruction can be identified, as well as the percentage of blockage (stenosis) (Figure 14-4).

More recently, additional physiological measures can be obtained that provide flow and pressure data at the site of the lesion within the coronary artery.⁵³ These measures have become particularly important for monitoring outcomes after the administration of reperfusion therapy (e.g. thrombolytics, angioplasty, stenting).^54,55 Coronary artery flow grades—for example, thrombolysis in myocardial infarction (TIMI) grades 0-3—are based on the visual analysis of the rate of opacification of the artery of interest, typically 90 minutes after intervention.⁵⁴ TIMI myocardial perfusion grades (TMPG 0-3), or myocardial blush, grade the reperfusion of the myocardium by the contrast medium.⁵⁵ Higher TIMI and TMPG grades have been associated with reduced mortality over 2 years.⁵⁵

Cardiac ventriculography is a procedure in which dye is injected into the ventricle and the entire chamber becomes outlined. Several cardiac cycles are recorded on cine. This provides valuable information about global and segmental wall motion, valve motion, and the presence of abnormal anatomy.

Pulmonary Imaging

Pulmonary angiography is used primarily for the diagnosis of PE. It is the gold standard with which ventilation/perfusion scans are compared.⁴¹ Contrast dye is injected into the pulmonary circulation through the pulmonary artery. Additional techniques such as balloon occlusion, segmental injections, or increased magnification improve the sensitivity of the test.⁴⁰ Interobserver reliability studies performed on the interpretation of pulmonary angiograms are 83% to 86%.⁴¹

Physics of Echocardiography

Imaging systems that use ultrasound require a source for generation of the sound wave, a transducer, and a receiver to pick up the reflected sound waves. Sound is absorbed or reflected differentially by specific tissues. Therefore the sound wave reflected from muscle differs from that reflected from vascular space, or valvular structures. Returning signals are converted into electrical signals that generate an image. This is the basis for M-mode and 2-D echo. In M-mode echocardiography, the transducer transmits a single sound wave through the chest wall (transthoracic echocardiography, or TTE). A thin slice of the heart, an “ice pick” view, directly under the sound wave, reflects the wave back to the transducer. M-mode, though useful when a high-resolution image is required, is rarely used.⁵⁹ The primary diagnostic mode is 2-D. In 2-D echocardiography, the transducer is able to pick up a wedge section of the heart. In 2-D mode, the transducer acts as the transmitter and receiver. A “real time” two-dimensional picture of the heart is recorded on videotape. The heart can be viewed in motion, and wall motion can be evaluated (Figure 14-5).

Doppler Echocardiography

Doppler echocardiography relies on the Doppler effect to determine velocity. When a sound wave is aimed at moving objects, such as red blood cells, the known transmitted frequency from the soundhead and the reflected frequency differ, a phenomenon known as a frequency shift. The greater the frequency shift, the greater the speed of the targeted object.⁵⁷ Velocity of flow is the term used to express Doppler frequency shifts. Both continuous-wave and pulsed-wave Doppler modes are used.

Color flow Doppler echocardiography was introduced in the mid-1980s. It is usually superimposed on a two-dimensional image in real time but can be used with M-mode. It is a method that displays anatomical and spatial blood flow velocity estimates with indication of flow direction in real time.⁶⁰ Color is matched based on the direction of flow, and shading is indicative of intensity based on the mean velocity flow estimates.⁶⁰ It is primarily used to assess the extent of regurgitant jets across “leaky valves,” as well as to indicate areas of abnormal shunts such as atrial septal defects. Turbulence created by narrowed valves creates wide color shading.

Measurements Derived from Echocardiography

Echocardiographic results can be viewed qualitatively and quantitatively. An experienced viewer can analyze the anatomical relationships and structure of the heart, wall motion, valve movement and configuration, and chamber size.

Quantitative measures are calculated from the Doppler frequency shifts. Velocity measures can be used in other equations to estimate pressure and valve area. The modified Bernoulli equation is used to calculate pressure changes around the valve:

ΔP (mm Hg)=4×Vmax(m/sec)

where P is the pressure gradient across the valve and V is the velocity of blood flow through the valve.

These measurements allow calculation of stenotic valve gradients and right-sided filling pressures, valve regurgitation, and shunt volumes.⁵⁷ Valve regurgitation and shunt calculations are more complicated than the Bernoulli equation alone.

Another important quantitative measure is the estimated area of the valve. The continuity equation is used to estimate valve area. It is based on the principle of conservation of mass; that is, in a normal heart, the volume of blood that enters the right atrium should be conserved as it passes through the other chambers (stroke volume₁ = stroke volume₂). Since the volume before the valve must be equal to the volume after the valve, it is possible to measure flow velocity before and after the valve and then solve for the area of the valve:

A1×V1=A2×V2,therefore A2=(A1×V1)/V2

where V₁ is the velocity of flow at one side of the valve, V₂ is the flow velocity at the other side of the valve, and A is the cross-sectional area.⁵⁷

Transesophageal Echocardiography

Because of consistent technical difficulties with TTE, the technique of transesophageal echocardiography (TEE) was developed to improve spatial resolution. TEE can obtain M-mode, 2-D, 3-D, and color flow Doppler images by using a small probe mounted on the tip of a modified gastroscope. TEE is considered minimally invasive. The probe is advanced into the esophagus, similar to placement of a gastric tube by nasal or oral route, and positioned so that multiple views of the heart are available.⁶² For cardiac output measures, it is advanced to the midthoracic level.⁶³ The technique can be performed on anesthetized patients during surgery and monitored postoperatively; on mildly sedated patients (e.g., those on mechanical ventilation or trauma patients in the intensive care unit); and on those suspected of having aortic dissection or aortic injury.⁵⁸ TEE is frequently used for diagnosis of congenital heart disease, evaluation of valve function, and determination of thromboembolic risk. It has become an important monitoring tool during cardiac catheterization procedures and surgery.⁶⁴

TEE is used to assess cardiac valve function as well as to monitor treatment. Image quality of TEE is better than that of TTE. This is especially true when visualizing the mitral valve, because of the proximity to the transducer (valves positioned farther from the transducer afford lower-quality images).⁶⁵ It is used to monitor prosthetic valve function, as well as to diagnose and monitor infective endocarditis, particularly the status of valve vegetation. TEE provides superior visualization (compared with TTE) of the left atrium and left atrial appendage, which are often the source of thrombi causing transient ischemic attack (TIA) or stroke. TEE Doppler studies can examine left atrial flow velocities, which also indicate increased stroke risk. TEE Doppler has been successfully used to decrease morbidity and length of stay when hemodynamic management of the patient is critical.⁶³ TEE has now progressed to 3-D real-time imaging, which allows for improved imaging and quantification of LV function, including preload, afterload, and contractility.⁶⁵

Myocardial Contrast Echocardiography

The ability of echocardiography to evaluate myocardial perfusion was historically based on observation of wall motion abnormalities. The development of a contrast agent that can be used with echocardiography has ushered in noninvasive techniques that do not rely on radioactive agents (see Figure 14-3). In myocardial contrast echocardiography (MCE), microbubbles of air encapsulated in a thin shell are introduced by IV infusion or bolus injection into the venous system as the contrast agent.⁶⁶ These small bubbles (<6-10 µm) oscillate when exposed to ultrasound energy and produce harmonic signals (a multiple of the ultrasound frequency) that are then used to produce the images.⁶⁷ Although numerous studies have shown the potential benefit of MCE for bedside and stress perfusion imaging, the FDA has not approved it for this use.⁶⁸

Physics of Computed Tomography

A computed tomographic scan (CT scan) acquires images of a fixed thickness (a cross-sectional slice), which allows examination of a specific part of the body. CT scanners use an x-ray source, collimators, x-ray detectors, and computers to reconstruct the cross-sectional images from the acquired data.⁶⁹ The x-ray source rotates around the patient, the speed of rotation dictating how long it takes to acquire a single slice (about 1 second). Slice thickness and the space between slices determine the quality of the spatial resolution of the images. The spatial resolution required depends on the type of tissue being examined and the suspected diagnosis. Imaging can be done with a single-detector or newer multiple-detector row scanners (4, 8, or 16 rows).⁷² Multidetector CT (MDCT) scanners allow for faster acquisition of images. Spiral CT scanners simultaneously rotate the source and move the table and patient for continuous data acquisition and improved resolution.⁷³ The most rapid scanning times are achieved by electron beam computed tomography (EBCT), also called ultrafast CT. EBCT uses an electron gun to generate x-rays to a fixed tungsten “target.”⁷¹ The electron beam is electronically moved, rather than mechanically rotated, resulting in images that can be made in milliseconds. The cost and the limited number of available EBCT scanners are factors that limit their routine use.⁷²

Cardiac Imaging

Electron-beam tomography is currently used to identify calcium in atherosclerotic plaques within the coronary arteries.⁷¹ A scoring system developed to quantify the amount of calcium in the coronary arteries is used to identify future risk of cardiac events.⁷¹ Scores are based on Housenfeld units (HU), which are based on the tissue attenuation factor. Calcified areas are identified in the coronary tree and given scores based on HU > 130. Scores are summed across all identified lesions (Agatston method) to provide a total coronary calcium score.⁷⁴ In general, the greater the amount of calcium detected, the greater the amount of atherosclerotic plaque in the coronary arteries. A limitation of EBCT is that it cannot localize the plaque anatomically. Sensitivity and specificity for EBCT for CAD is 91% and 49%, respectively.⁷¹ Research is currently being done to determine whether EBCT tests adds valuable data to cardiac risk assessment.⁷⁵ In a sample of nearly 9000 asymptomatic low- to intermediate-risk middle-aged adults, the relative risk for a cardiac event significantly increased at higher quartiles of calcium scores.⁷⁵ However, it is not recommended for screening in asymptomatic patients or for patients at high pretest probability for CAD.^76,77 Cardiac CT has been deemed appropriate for symptomatic individuals with low to intermediate pretest probability of significant coronary stenosis or pre-operative evaluation in low-risk patients.⁷⁷ A particular strength of CT is its ability to image cardiac structure and ventricular function.⁷⁷

Pulmonary Imaging

Compared with standard radiography, CT is not commonly used in the initial diagnosis of pulmonary disease. However, the CT scan has been found to be a useful first-line diagnostic tool in the evaluation of mediastinal disease, including mediastinal masses, staging of mediastinal cancers, and identification of cysts.⁴⁰

High-resolution computed tomography (HRCT) has improved the documentation of morphological changes associated with chronic airflow obstruction. Webb’s 1994 review⁷⁸ of the application of this new technology identified the utility of this test in a variety of obstructive diseases. HRCT is sensitive in the diagnosis of early emphysema, although it is rarely used clinically. In cystic disease, such as histiocytosis X and lymphangiomyomatosis, HRCT is able to demonstrate cysts less than 10 mm in diameter and can also visualize wall thickness.

In the diagnosis of bronchiectasis, HRCT demonstrates a high specificity (Figure 14-6) and is the procedure of choice after standard chest radiography. It can discriminate among the three different types of abnormal bronchi seen in bronchiectasis (cylindrical, varicose, and cystic); however, there is little clinical significance in this distinction.⁷⁸ HRCT can be used to differentiate lymphangitic spread of cancer versus heart failure when that distinction is clinically unclear. It also can be used to evaluate clearing of persistent pneumonias. Overall, HRCT has helped clarify disease morphology, but its clinical significance for diagnosis and for patient pulmonary function is less clear.

Spiral CT, with and without a contrast agent, has become the test of choice in the diagnosis of acute pulmonary embolism.⁷⁹ Multidetector scans (>4 detectors) provide sensitivity and specificity >83% and 89% respectively.⁷⁹ The combination of CT and V/Q scans provide examination of lung function (V/Q) and structure (CT) which may surpass CT alone for diagnosis of PE.⁸⁰ Hybrid cameras that can perform both V/Q SPECT and CT will increase utilization of these scans. Future applications may be seen in planning radiation therapy of the lung and lung volume reduction surgery.⁸¹

Physics of Magnetic Resonance Imaging

MRI relies on the creation of a stable, powerful magnetic field that causes hydrogen (also carbon-13, fluorine-19, and sodium-23) nuclei to align with the field. A radiofrequency pulse specific to the hydrogen (or other) nuclei causes the nuclei to resonate.82,83 Additional magnets that apply magnetic fields of different gradients are used to locate the image. When the radiofrequency is turned off, the nuclei release the stored energy. This energy is analyzed via Fourier transformation, which results in the image. The images are highly dependent on the timing and magnitude of the radiofrequency.⁸⁴ Data acquisition can take 20 to 90 minutes, during which the patient must remain immobile. Application of MRI to the cardiac and pulmonary system was initially limited by the continuous movement of the heart and lungs, but the development of ultrafast imaging has overcome some of these problems.⁸⁵ Contrast materials such as gadolinium diethylenetriamine pentaacetate (Gd-DTPA) can be used to enhance signal intensity.

Cardiac Imaging

MRI is a useful technique in evaluating cardiac structures. It can identify cardiac masses and is the modality of choice for evaluating the pericardium. MRI is still secondary to CT in evaluating the mediastinum and detecting CAD.84,86 Coronary artery imaging is still problematic due to cardiac motion.⁸⁷ Cardiac images from MRI do allow the measurement of right and left ventricular volumes, ejection fraction, and cardiac output.^{82, 87} MRI is able to detect tissue composition such as myocardial fat infiltration and identification of myocardial edema caused by inflammation or ischemia.⁸⁷ Data collection of the heart requires the use of different testing sequences and specialized equipment to capture the beating heart.⁸⁷

Pulmonary Imaging

Evaluation of the lung parenchyma presents unique problems because of respiratory movement, low proton density, and the air-tissue interface.⁸⁵ The development of ultrafast imaging pulse sequences, called dynamic MRI, has improved the utility of this noninvasive test for evaluation of the pulmonary system. Contrast agents such as Gd-DTPA are used to perform MR angiography to diagnose pulmonary embolism.⁸⁸ The use of MRI to provide noninvasive evaluation of PE has application in special cases, where typical tests are inconclusive.⁸⁸ Without contrast, dynamic MRI is being used to study biomechanical function of the diaphragm and chest wall in patients with and without pulmonary disease. These studies are important for patients undergoing lung volume reduction surgery.⁸⁹

The use of gaseous contrast agents such as hyperpolarized helium-3 (HHe-3), with dynamic MRI and lung motion correction, is a promising new development that allows examination of gas distribution in the lungs.85, 90 These studies have shown that dynamic MRI with contrast HHe-3 contrast can discriminate healthy lung grafts from native diseased lung in transplant patients⁸⁵ and identify ventilation defects that correlate with decreased FEV₁ in patients with asthma.⁹⁰