LEARNING ZONE
 
NONINVASIVE CARDIAC IMAGING
 

Echocardiography and Stress Echocardiography (Table: 1)
2D echocardiography provides excellent images of the heart and great vessels, as well as the assessment of regional and global left and right ventricular function. Of all the noninvasive techniques, it is the most versatile and provides additional information at the lower cost Stress echocardiography with exercise or dobutamine can assess for the presence of left ventricular systolic or diastolic dysfunction, valvular heart disease, and the extent of infarction and stress-induced ischemia. Exercise echocardiography may be performed using treadmill, supine bicycle, or upright bicycle. In patients who cannot exercise, dobutamine is the most commonly used pharmacologic stress agent; vasodilator stress echocardiography (with dipyridamole or adenosine) has been reported to have a lower diagnostic sensitivity for single vessel disease. Transient regional wall motion abnormality, if visualized in the setting of exercise or dobutamine stress, is a marker of CAD.

In patients with chronic CAD, left ventricular ejection fraction (LVEF) measured at rest has an important impact on prognosis, with increasing mortality associated with declining LVEF. For stress echocardiography, the presence or absence of inducible myocardial ischemia is useful for risk stratification in patients with known or suspected CAD. The number of segments with stress-induced wall motion abnormalities has a strong correlation with cardiac events, death, and MI. Negative stress echocardiography generally denotes a low rate of cardiac events. Stress echocardiography with exercise or dobutamine is an effective and highly accurate noninvasive means of detecting CAD and risk-stratifying symptomatic patients with an intermediate to high pretest likelihood of CAD.

 
Table 1: Clinical uses of echocardiography
A - 2D Echocardiography:
  • Cardiac chambers:  size; left ventricular hypertrophy; regional wall motion abnormalities
  • Valve abnormalities: thickness, mobility, calcification
  • Pericardium: effusion, tamponade
  • Intracardiac masses
  • Aortic disease
B - Stress echocardiography:
2D: confirm myocardial ischemia and estimate its severity, i.e. viable myocardium
 
C - Doppler : for valvular disease
Radionuclide myocardial perfusion imaging.
Stress myocardial perfusion imaging (MPI) with single-photon emission computed tomography (SPECT) uses radioactive tracers to provide information about regional blood flow, coronary artery perfusion, and ventricular function. Over the past decade, significant advances in MPI have resulted in substantial improvements in its diagnostic accuracy in the evaluation of patients with known and suspected ischemic heart disease. The lower-energy isotope (thallium-201) is now largely replaced by the use of technetium-based imaging agents with characteristics leading to improvement in specificity, particularly with ECG-gated SPECT imaging in which resting and post stress wall motion and ejection fraction can be evaluated.
According to the ACC/AHA guidelines, MPI is recommended for patients with suspected or known IHD and is combined with exercise treadmill testing or pharmacologic stress. A radiopharmaceutic agent, typically technetium 99m, is infused IV. Areas of the myocardium that are perfusing well, poorly, or not at all at rest and during exercise can be readily differentiated. Some areas may be well perfused at rest but demonstrate decreased perfusion during exercise or pharmacologic stress and are thus ischemic. Areas that are poorly perfused both at rest and during stress (termed glucose/blood flow mismatch) are highly suggestive of scarred myocardium. SPECT is also useful in the non-invasive evaluation of patients, particularly women, with equivocal or suboptimal exercise test results caused by breast attenuation.

Positron emission tomography MPI (PET MPI) is an advance over conventional MPI. It utilizes rubidium-81 and other radioactive tracers including fluorodeoxyglucose (FDG) for the assessment of metabolic activity and viable myocardium and radioactive ammonia for perfusion. PET has a higher sensitivity and specificity than SPECT for the detection of ischemia as well as the evaluation of viable myocardium. PET has become a useful investigation in the detection of viable myocardium in patients who are suitable for revascularization, such as angioplasty or bypass surgery. However, PET is not as widely available as SPECT. PET MPI is particularly useful in patients with excessive soft tissue.

Coronary Calcium scoring
Calcium scoring is performed via state-of-the-art CT techniques, using either electron beam tomography or multidetector CT, in an ambulatory setting. A patient's calcium score correlates strongly with the degree of plaque buildup, but it provides a less precise estimate of coronary occlusion. The test's negative predictive value is excellent, meaning that a low calcium score (101 to 400) is reassuring and reflects a low-risk plaque situation. A high plaque score (over 400) requires follow-up with another imaging study, such as stress echocardiography. However, although CT calcium scoring has a high sensitivity for the detection of CAD, it has low specificity, and should not be used for the diagnosis of obstructive coronary disease. The test exposes the patient to less radiation than a CT coronary angiogram (CTCA), and no contrast agents are used.

CT Angiography (CTA)
With the high temporal (i.e. the amount of time taken to acquire necessary scan data to reconstruct an image), and spatial resolution( i.e. the ability of the system to effectively depict and reproduce fine details within an image) of multislice spiral CT, accurate assessment of luminal narrowing in the major branches of the coronary arteries is possible in selected patients. Some experts believe that CTA has the potential to one day replace invasive coronary angiography as the diagnostic gold standard of cardiac imaging. Others see a stronger role for this modality in patients with suspected false-positive stress test results or equivocal stress test results. The modality has high sensitivity (>85%), and specificity (>90%) as compared to cardiac catheterization. The highest accuracy has been noted in the left main and the proximal portions of the left anterior descending and left circumflex arteries, probably due to the rapid motion of the right and circumflex arteries. Complete visualization of all epicardial vessels is limited by cardiac motion, small size of distal segments, and tortuous course through imaging plane.

The reputation of the so-called triple-rule-out finding with CTA is increasing among emergency department physicians. Pain suggestive of MI may also signal an aortic aneurysm or pulmonary embolism, and CTA can exclude all 3 of these conditions in 1 examination. This application of CTA remains to be rigorously tested in clinical settings, however.

Limitations of CT :
Exposures to radiation and contrast agents are concerns with CT angiography (in contrast to MRI). It is clear that the higher the radiation exposure, the better the image. A CT angiogram exposes the patient to approximately twice the exposure associated with a conventional diagnostic catheterization. The radiation exposure is typically confined to a small area with few tissues that are highly radiation sensitive. The radiation exposure associated with CT angiography is similar to the exposure that patients experience during a nuclear study, which is often performed more than once Radiation exposures, especially to the breast in women, are a greater concern in younger patients than they are in older ones. Careful consideration of the benefits and risks of CT angiography is necessary in any patient, especially in patients with renal insufficiency or contrast allergy.

CT Artifacts :
Artifacts such as physics-based (e.g. beam hardening artifact, blooming artifact); patient-based (e.g. respiratory and patient motion, arrhythmias, metallic sources); and scanner based (e.g. ring artifact, cone beam artifacts) can significantly affect image quality and sometimes make images impossible to interpret.

Bottom line :
“The concept of ‘noninvasive coronary angiography’ has generated great interest in the widespread utility of CTA. However, at present, the major well-accepted indication for coronary CTA is in the evaluation of suspected coronary anomalies. CTA may be of use in patients with chest pain syndromes who have an intermediate pretest probability of CAD who are unable to exercise or have an uninterpretable or equivocal stress test”.