Case 8

Published on 02/03/2015 by admin

Filed under Internal Medicine

Last modified 22/04/2025

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Case 8

A 51-year-old man with hyperlipidemia was referred for exercise SPECT MPI for evaluation of nonexertional localized chest pain.

He exercised for 8:54 minutes on a Bruce protocol, achieving an estimated workload of 10 METs. No exercise-induced symptoms were reported, and the blood pressure and ECG responses were normal.

SPECT Images

The SPECT images demonstrate a mild, nonreversible reduction in tracer activity in the lateral, anterior, inferior, and apical regions (relative to the septum). This perfusion pattern (highest myocardial tracer activity in the septum) is abnormal. In men, the highest myocardial tracer activity is typically found in the anterolateral region, since this region is least susceptible to photon attenuation. The observed perfusion pattern in this patient could represent multivessel coronary artery disease (CAD) with extensive subendocardial infarction, severe soft-tissue attenuation artifact, or increased septal wall thickness. Because of the limited spatial resolution of the SPECT camera, the apparent tracer activity in a myocardial region is directly related to the thickness of the myocardial wall. This is referred to as the “partial volume effect.”

Defect Severity Polar Maps

When the tracer distribution is compared to a male-specific normal database, there is a fixed reduction in tracer activity in the lateral, anterior, and apical regions. The mild inferior defect does not exceed normal limits compared to a male-specific normal database.

Gated SPECT Images

The relationship between regional wall thickness and apparent tracer activity (the “partial volume effect”) is used to measure regional myocardial systolic wall thickening on gated SPECT images. The gated images in this patient demonstrate normal regional systolic thickening in the regions with reduced tracer activity. This finding argues against extensive subendocardial infarction as the cause for the observed fixed perfusion defects.

The septal wall demonstrates mildly reduced regional systolic thickening (see color-coded thickening polar map at bottom). An apparent reduction in regional systolic thickening can occur due to increased diastolic wall thickness. The relationship between regional wall thickness and apparent tracer activity (the “partial volume effect”) does not apply when the wall thickness approaches the spatial resolution of the detector. Therefore, regional systolic wall thickening will be underestimated in myocardial regions with markedly increased wall thickness.

Echocardiography

Hypertrophic cardiomyopathy (HCM) with severe asymmetric septal hypertrophy (23 mm).

Echocardiography demonstrated asymmetric septal hypertrophy (23 mm; normal = 7 to 12 mm) consistent with hypertrophic cardiomyopathy.

Echocardiogram-SPECT Correlation

Side-by-side comparison of the apical four-chamber view on echocardiography (left) and the horizontal long-axis SPECT image (right) is shown. The area of increased septal tracer activity on SPECT correlates with the region of increased septal wall thickness on echocardiography. The apparent increase in tracer activity is therefore consistent with the “partial volume effect.”

Teaching Points: MPI in Hypertrophic Cardiomyopathy

1. Quantitative perfusion polar maps compare regional tracer activity to a “normal” database, and patients with HCM will appear to have perfusion defects in myocardial regions with less hypertrophy (relative to regions with more severe hypertrophy).

2. Myocardial regions with severe hypertrophy will demonstrate an apparent reduction of regional systolic wall thickening due to the decreasing impact of the “partial volume effect” (relationship between wall thickness and apparent tracer activity).

3. Ischemia can be seen in HCM even in the absence of epicardial CAD, presumably due to myocardial fibrosis and “microvascular” disease. Therefore, specificity for diagnosis of CAD is reduced in patients with HCM.

4. Sensitivity of SPECT MPI is preserved in HCM. Therefore, the absence of reversible perfusion defects is useful for excluding ischemia. Normal SPECT MPI studies are associated with a more favorable prognosis in patients with HCM.

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