Chapter 20. Ischemic Heart Disease

The available imaging arsenal for the detection and management of ischemic heart disease has never been larger. Many new noninvasive imaging techniques have obviated the need for many invasive diagnostic procedures and offer risk stratification of patients with known and newly detected disease. Imaging allows the stratification of patients with preclinical disease, beyond that of clinical risk factors. Finally, in cases of invasive diagnostic workups, new imaging modalities now offer highly accurate quantification and adjunctive information that was previously not possible.

Given the often overwhelming array of imaging modalities and potential findings from each given imaging test, a framework with which to categorize imaging findings is useful. The "ischemic cascade" that follows coronary artery occlusion (Fig. 20–1), as first described by Nesto and Kowalchuk,1 provides such a framework. By establishing the progression from clinically unrecognized parameters to myocardial infarction, the concept of the ischemic cascade allows the physician to recognize and categorize the potential findings from a given imaging study and interpret them along a clinically meaningful spectrum.

The progression along the continuum of detectable abnormalities of the myocardium is preceded by detectable precursors affecting the coronary arteries, namely atherosclerotic changes, although simple coronary arterial disease may not lead to ischemic consequences. In the presence of a discrete coronary artery stenosis or generalized arteriopathy significant enough to cause significantly decreased myocardial blood flow at rest or during stress (exercise or pharmacologically induced), perfusion defects can be initially detected. Diastolic myocardial dysfunction follows and then systolic dysfunction. Eventually, electrocardiogram (ECG) changes and infarction can be detected. Various imaging tests can detect or infer the presence of myocardial ischemia or infarct. These findings are summarized in Table 20–1.

Therefore, in this chapter, we discuss each imaging modality and outline the potential findings offered by each modality along the preclinical and clinical sequence of the ischemic cascade. When selecting the appropriate test for a given patient, the multisociety appropriateness criteria can serve as guidelines, allowing evidence-based expert consensus to guide the clinician in evaluating complex imaging decisions in an efficient manner.2-5 In many cases, the complementary information provided by multiple tests is essential to patient management.6 In certain instances, hybrid imaging allows simultaneous acquisition of this information (ie, positron emission tomography [PET]/computed tomography angiography [CTA] or single photon emission computed tomography [SPECT]/computed tomography [CT]).7

Chest radiography is a universally available, relatively inexpensive test that allows reliable assessment of cardiac size and of the sequelae of ischemic heart disease such as pulmonary venous hypertension, pulmonary edema from cardiogenic origin, and focal edema from ischemic valve disease (such as mitral regurgitation). Although normal radiographs cannot exclude ischemic heart disease, this easily performed test can reliably detect changes over time in patients with known heart disease.8 X-ray fluoroscopy has historically been used to detect coronary calcification and cardiac size but is no longer used due to the availability of more reliable methods.

Cardiac ultrasound, or echocardiography, is a widely available, radiation-free modality that is routinely used in the diagnosis and management of all forms of heart disease. Transthoracic echocardiography is the most commonly performed imaging examination in the United States.9 Although limited by operator-dependent techniques, acoustic windows, and body habitus in obese patients, echocardiography plays a key role, offering robust information without the use of iodinated contrast or ionizing radiation. Transesophageal echocardiography, an invasive test, allows more reliable acoustic windows and imaging planes at the expense of time and invasiveness. Echocardiography allows the assessment of ventricular and atria size, global and regional left ventricular function, diastolic function (tissue relaxation/ventricular filling), myocardial wall thickness, and assessment for valvular disease. Doppler ultrasound also allows measurement of flow and calculation of gradients, such as in cases of valve dysfunction.3 New techniques in echocardiography such as deformation (tissue tracking imaging), application of contrast, and three-dimensional imaging can expand the potential role of echocardiography and aid in selected cases.

Stress echocardiography, performed with dobutamine (pharmacologic stress) and/or low-level exercise, allows the detection of induced wall motion abnormalities, a key finding along the ischemic cascade. This finding alone results in a high diagnostic accuracy for the detection of significant coronary artery disease (CAD).2 Two different meta-analyses have been performed comparing the performance of stress echocardiography to conventional coronary angiography. The first included patients with an intermediate pretest likelihood of CAD (25%-75%), and the angiographic definition of significant coronary heart disease was either ≥50% or ≥70%. Stress echocardiography had a sensitivity and specificity of 76% and 88%, respectively, in six studies of 510 patients.10

The second meta analysis compared exercise echocardiography and exercise SPECT myocardial perfusion imaging with coronary angiography, with extent of stenosis varying between ≥50% and 75%. The two tests had similar sensitivity (85% for echocardiography and 87% for SPECT), but the specificity for stress echocardiography was significantly higher (77% for echocardiography vs 64% for SPECT).11

CT of the heart has been performed for decades, but the challenges of cardiac motion and ECG gating have until recently been daunting. Early success with electron-beam CT allowed reliable quantification of calcium scores, which highly correlate with (preclinical) CAD burden.12 However, this expensive technology has not become widely available.

With the advent of multidetector CT (MDCT), routine assessment of the coronary arteries became widely available. MDCT offers reliable calcium scoring. The use of calcium scoring can be used to lower the likelihood of CAD, but numerous recent studies have shown that a significant proportion of patients have only noncalcified plaque, which will be missed by calcium scoring CT.13 Calcium scoring can help manage asymptomatic patients at intermediate risk for CAD by providing additional information compared with traditional risk factors alone,14 which may help assess for preclinical manifestations of ischemic heart disease.

ECG-gated cardiac CT with intravenous bolus contrast enhancement has allowed the collection of a dynamic, three- or four-dimensional data set. Case 1 (illustrated in Fig. 20–2) demonstrates three-dimensional coregistration of CTA data in the multimodality workup of a patient for ischemic heart disease. Combined with advanced multiplanar reconstruction workstations, MDCT offers reliable coronary angiography by using 64-slice MDCT technology or higher. The direct visualization of coronary arteries and their lumen has been proven to be highly accurate in patients with satisfactory low and regular heart rates (often requiring β-blockade to achieve the necessary heart rate for scanning).15 New scanner technologies such as increasing z-axis detector length, dual-source scanners, and prospective ECG triggering have allowed continual improvements in image quality, temporal resolution, and radiation doses.16

ECG-gated cardiac multidetector CTA can be used to reliably assess for CAD and can reliably exclude significant CAD, particularly in patients with low to intermediate probability of CAD.5 One growing application of coronary CTA is the assessment of low- to intermediate-risk patents presenting to the emergency department with acute chest pain. Several trials demonstrated that a strategy using coronary CTA can decrease time for diagnosis, length of stay, and hospital costs, with zero major adverse cardiac events 30 days after discharge with a negative coronary CTA.17-19

Other established applications are assessment of coronary anomalies; patients presenting with atypical angina and low to intermediate likelihood of CAD; patients presenting with nondiagnostic or inconclusive stress tests; patients with new onset of heart failure and low to intermediate probability of CAD; and patients before valvular surgery to rule out CAD. Three recent multicenter trials confirmed prior observations of single-center studies demonstrating that 64-slice MDCT has a high sensitivity and specificity with a moderate positive predictive value and an excellent negative predictive value.20-22

More recent publications have validated the use of ECG-gated cardiac CT for complications of ischemic disease, such as coronary and ventricular aneurysms, myocardial thinning, and global and regional dysfunction. Case 2 (illustrated in Fig. 20–3) demonstrates CT of a left ventricular aneurysm. Preliminary research has demonstrated potential use for other manifestations of ischemic heart disease, such as myocardial characterization, and rest or stress perfusion defects.23,24 Contrast CT can be contraindicated in cases of severe renal dysfunction, contrast allergy, and severe congestive heart failure.

The use of cardiac magnetic resonance imaging (MRI) for ischemic heart disease is promising and still in its early stages. This modality allows accurate assessment of myocardial function (see Fig. 20–2, Case 1, for an example of ventricular thinning consistent with infarction), as well as first-pass myocardial perfusion (bolus of gadolinium agent during pharmacologic stress) and delayed enhancement (myocardial necrosis and scar). Because of the exquisite contrast resolution and excellent temporal resolution, MRI can identify subendocardial and transmural perfusion defects and characterize the transmyocardial perfusion gradient.25 Use of cardiac MRI to assess the late manifestations of ischemic heart disease is well established.26 The use of MRI to identify delayed enhancement allows reliable assessment of myocardial necrosis and scar and the identification of patterns of scar consistent with ischemic myocardial infarction. Case 2 (see Fig. 20–3) demonstrates findings of scar in a left ventricular aneurysm. In cases of cardiomyopathy of unknown origin, MRI allows excellent differentiation of ischemic cardiomyopathy from other causes of nonischemic cardiomyopathy.27 MRI can also offer unmatched ability for tissue characterization, which can be useful in differentiating true aneurysm from pseudoaneurysm, evaluating for pericardial adhesions, and imaging in cases where echocardiography windows are limited. Phase-contrast MRI can evaluate and quantify flow in cases of shunting or valvular dysfunction. Limitations of MRI include safety issues relating to implantable hardware, such as automatic implantable cardioverter-defibrillators/pacemakers, and issues with contrast-enhanced MRI, such as gadolinium-induced nephrogenic systemic fibrosis (particularly in patients with severe renal dysfunction).5

The use of radionuclide imaging is often a vital part of the imaging of ischemic heart disease. Stress myocardial perfusion imaging can be performed to detect relative perfusion defects, a sign of unbalanced coronary blood flow from significant stenosis. This test is frequently used and can identify ischemic territories, infer the distribution of coronary lesions, demonstrate fixed defects (infarcts), and quantify systolic function, each of which carries independent prognostic value beyond or in addition to the simple presence of ischemic myocardium. In certain situations, the test is limited, such as in cases of balanced ischemic lesions, obesity, and breast attenuation and diaphragmatic artifacts.4 Myocardial perfusion imaging can be performed with SPECT or PET. SPECT and PET can be used with radiotracers targeted at perfusion (eg, technetium-based and rubidium agents, respectively) and myocardial viability (eg, thallium and fluorine 18 fluorodeoxyglucose, respectively). Myocardial perfusion can be especially helpful in cases of known ischemic heart disease, a situation where quantitation of ischemic burden can be essential to guide management. Newer hybrid scanners (PET/CT and SPECT/CT) can offer simultaneous acquisition of nuclear and CT data, offering the benefits of calcium scoring, attenuation correction, and even anatomic CTA in conjunction with myocardial perfusion data.28 Case 1 (see Fig. 20–2) demonstrates rubidium PET and fluorodeoxyglucose PET in the workup for myocardial ischemia and viability, respectively. Advances in nuclear cardiology can include evaluation of ischemic memory with iodine 123–15-(p-iodophenyl)-3-(R,S)-methylpentadecanoic acid (BMIPP) in which an area at risk of acute myocardial infarction can be detected as a defect even a couple of weeks after the myocardial insult.

The multimodality imaging workup of ischemic heart disease often ends in selective coronary angiography (SCA), which offers the ability to both diagnose and treat CAD in a single setting. Imaging options at SCA include contrast angiography and intravascular ultrasound, which can characterize the luminal wall and allow evaluation of stents.29 Invasive coronary angiography also allows simultaneous functional measurements by assessing the functional flow reserve (ie, pressure measurements).30