H h j i i

Figure 14. 70 year old male with stable angina. The MPI study was performed for risk assessment. There is reduced activity in the anterior wall (narrow arrowhead), apex (short arrow) and septum (long arrow) at rest with improvement at rest. There is reduced activity inferiorly at stress with improvement of the distal inferior wall at rest (broad arrowhead). There is evidence of both LAD and RCA disease.

perfusion imaging than with quantitative coronary angiography. An additional limitation of coronary angiography relates to the considerable variability in the extent of jeopardized myocardium in patients with stenosis of similar anatomic severity and location (Fig. 15).

Specificity

The specificity of perfusion imaging is between 70 and 80%. One potential explanation for the apparent low specificity is referral bias i.e., the selective referral of patients with an abnormal perfusion study for coronary angiography. It has been suggested that the "normalcy rate" (the percentage of patients with a low pre-test probability of coronary disease with normal images) should be used instead of specificity. The normalcy rate for SPECT imaging is approximately 90%.

False positive studies may be due to technical factors (e.g., patient motion), soft tissue attenuation and left bundle branch block (septal hypoperfusion at rapid heart rates). Patient motion during acquisition of a study may cause apparent myocardial defects on the reconstructed images. To ensure that there was no significant patient

Figure 15. 71 year old male with post MI angina. An angiogram showed moderately severe stenosis of the proximal RCA and severe stenosis of the circumflex artery. The MPI study was performed to identify the "culprit" artery prior to revascularization. At stress, severe reduction of activity is seen in the lateral wall on the short axis and HLA images (arrows) with improvement at rest. Based on the finding of a reversible lateral wall abnormality, the patient underwent a PTCA of the circumflex artery with excellent results.

Figure 15. 71 year old male with post MI angina. An angiogram showed moderately severe stenosis of the proximal RCA and severe stenosis of the circumflex artery. The MPI study was performed to identify the "culprit" artery prior to revascularization. At stress, severe reduction of activity is seen in the lateral wall on the short axis and HLA images (arrows) with improvement at rest. Based on the finding of a reversible lateral wall abnormality, the patient underwent a PTCA of the circumflex artery with excellent results.

motion the projection images are examined. This is typically done by reviewing the images in cine mode looking for evidence of vertical or horizontal movement of the heart. Another cause of an apparent myocardial defect is attenuation from overlying soft tissue structures. In women breast tissue frequently overlaps the anterior myocardial wall on many of the SPECT projections. In these projections fewer anterior wall counts will be detected and consequently, upon reconstruction of the data, the anterior wall will show apparently diminished uptake (Fig. 7). Attenuation of the inferior wall is frequently seen in men with a protuberant abdomen and an elevated left hemidiaphragm.

Clinical Role in Prognosis and Risk Stratification

MPI provides important prognostic information in a wide spectrum of patients with CAD. The ability to identify patients at high and low risk has been validated for both planar and tomographic techniques using 201Tl or 99mTc-sestamibi with exercise or pharmacologic stress.

Patients with Known or Suspected Coronary Disease

A normal myocardial perfusion study in patients with angiographically documented coronary artery stenosis as well as in patients with suspected coronary disease identifies a low risk group with a rate of cardiac death or non-fatal myocardial infarction less than 1% per year which is similar to the general population. On the other hand (a) reversible or fixed perfusion abnormalities representing jeopardized viable or infarcted myocardium particularly in multiple vascular territories (b) increased lung thallium uptake or (c) transient ischemic dilatation identify patients at increased risk of future cardiac events. There is a significant relation between the extent and severity of the perfusion abnormality and cardiac death or non-fatal infarction. Importantly, MPI provides independent and incremental prognostic information to that already available from clinical and exercise variables.

A cost-effective strategy (Fig. 16) is to use perfusion imaging in patients who have an intermediate probability of coronary disease after a GXT. Patients at low risk after clinical assessment and stress testing have a low event rate and may not require additional investigation. Patients with a high risk GXT (based on a low workload, angina, exercise induced hypotension, ST segment depression that is severe, occurs at a low workload and persists into the recovery period) warrant coronary angiography while those considered at intermediate risk after GXT should be evaluated with a myocardial perfusion study. While significant increases in cardiac death and/or myocardial infarction (MI) occur as a function of worsening scan results, patients with mildly abnormal perfusion studies (SSS = 4-8) are at intermediate risk for MI but low risk for cardiac death. Since myocardial revascularization (CABG or PTCA) has not been shown to reduce the rate of subsequent MI, patients with a mildly abnormal perfusion study can be managed with medical therapy unless they have disabling symptoms. Similarly patients with an intermediate risk treadmill test but a normal exercise perfusion study are at low risk for subsequent cardiac death and can be managed medically without coronary angiography until they develop disabling symptoms requiring revascularization.

Following Myocardial Infarction

MPI is ideally suited to identify the presence and extent of jeopardized viable myocardium after acute myocardial infarction thereby identifying patients at risk for future cardiac events (Fig. 17). Compared with exercise stress testing, MPI has an increased sensitivity for detecting multi-vessel coronary disease and identifying patients at risk for subsequent cardiac events. MPI can also localize ischemia, identify viable jeopardized myocardium and determine global left ventricular systolic function with gated acquisition.

Pre-discharge post-infarction myocardial perfusion imaging with either 201Tl or 99mTc-sestamibi in conjunction with submaximal exercise or vasodilator stress (dipyridamole or adenosine) stratifies patients into low, intermediate and high risk. The extent and severity of the perfusion defect and the degree of reversibility are predictors for in-hospital and late cardiac events whether or not patients receive thrombolytic therapy.

Early risk stratification is important, since most early (< 1 year) post-MI cardiac events (e.g., unstable angina, recurrent infarction, cardiac death) occur within 4-6

Kent County Health Flow Chart

Figure 16. Flow chart outlining a cost-effective strategy for the evaluation of coronary artery disease (MPI-myocardial perfusion imaging; GXT-graded exercise stress test).

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