Cause of apical thinning on attenuation corrected myocardial perfusion SPECT

Objectives Decreased apical count in attenuation corrected (AC) myocardial perfusion, namely “apical thinning”, is a well known phenomenon and may induce false-positive perfusion defects. The aim of this study was to compare actual myocardial thickness and AC perfusion count to investigate the cause of apical thinning.

Methods Both Tc-99m MIBI SPECT-CT and 64-slice multi-detector computed tomography (MDCT) scans were independently acquired on 21 patients (mean age 65 ± 20 years, 13 male). Attenuation correction was performed by using the SPECT-CT scan. Myocardial thickness was measured on short-axis and vertical long-axis views using the MDCT images. AC and non-AC perfusion counts were normalized by maximums. Myocardial perfusion count and thickness were measured at the apex, apical and mid 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, 9.9 ± 2.4 mm, respectively, p < 0.003). AC count at the apex was significantly lower than that at the apical and mid regions (0.81 ± 0.05, 0.88 ± 0.04, 0.91 ± 0.04 %, respectively, p < 0.001). Better relationship was observed between myocardial thickness and AC count (y = -10.66 + 0.22x, r = 0.59, p < 0.0001). No relationship was found between thickness and non-AC count (y = 4.88 + 0.03x, r = 0.01, p = 0.97).

Conclusions The low apical count on the AC image was caused by anatomical thinning of the myocardium. AC provided true relationship between myocardial thickness and count due to the partial volume effect