Cause of apical thinning on attenuation-corrected myocardial perfusion SPECT.

OBJECTIVES Decreases in apical and apex activities - namely, 'apical thinning' - are a well-known phenomenon in attenuation-corrected (AC) myocardial perfusion. The aim of this study was to compare actual myocardial thickness derived from a multidetector-row computed tomography with AC myocardial perfusion count from a hybrid single-photon emission computed tomography/computed tomography to investigate the cause of apical thinning. METHODS We enrolled 21 participants with a low likelihood of coronary artery disease (mean age 65 ± 21 years, 13 men) from 185 consecutive patients and 11 healthy volunteers, who independently underwent ⁹⁹mTc-sestamibi single-photon emission computed tomography/computed tomography and 64-slice multidetector-row computed tomography scans. AC and non-AC myocardial perfusion counts and thickness were measured on the basis of a 17-segment model and averaged at the apex, apical, mid, and basal walls. RESULTS Myocardial thickness at the apex was significantly thinner than that at the apical and mid walls (5.1 ± 1.3, 7.3 ± 1.3, and 9.9 ± 2.4 mm, respectively; P<0.005). AC count at the apex was significantly lower than that at the apical and mid regions (76.0 ± 5.5, 82.8 ± 4.7, and 85.6 ± 3.8, respectively; P<0.002). Moderate relationship was observed between myocardial thickness and AC count (y=-10.5 + 0.22x, r=0.54, P<0.0001. No relationship was found between thickness and non-AC count (r=0.16, P=0.263). CONCLUSION The low apex and apical counts were caused by anatomical thinning of the myocardium in AC myocardial perfusion imaging. Attenuation correction provided an accurate relationship between myocardial count and thickness because of the partial volume effect.