FIGURE 4. Comparison of SPECT without (NC) and with attenuation correction (AC) with smoothed PET ¹³N-ammonia uptake. Top row, rest studies. Bottom row, stress studies. (A) Mean differences between NC SPECT and ¹³N-ammonia uptake from smoothed images. (B) Mean differences between AC SPECT and ¹³N-ammonia uptake from smoothed images. 0 = no significant difference between regional SPECT uptake and regional ¹³N-ammonia uptake (smoothed images).