Decreased Perfusion in the Lateral Wall of the Left Ventricle in PET/CT Studies with 13N-Ammonia: Evaluation in Healthy Adults

Abstract

This study evaluated the causes of the frequently observed decreased myocardial perfusion in the lateral wall of the left ventricle in PET/CT studies with 13 N-ammonia in healthy adults. Methods: Thirty-two adults who were undergoing PET/CT of the heart with 13 N-ammonia as part of a study of the effects of posttraumatic stress disorder in American Indians were included. Participants who had evidence of heart disease by a physician-administered questionnaire or by pharmacologic stress during the myocardial perfusion study were excluded. The ratio of counts in the lateral wall relative to the septum at stress and rest were compared with the degree of misalignment between the PET and CT images at the left border of the heart and at the right hemidiaphragm. Results: No participant had evidence of heart disease. For the left heart border, 41% of participants showed misalignment at rest and 62% showed misalignment at stress. For the right hemidiaphragm, 28% showed misalignment at rest and 53% showed misalignment at stress. Among all participants, the decrease in lateral wall counts relative to the septum correlated with the degree of misalignment of the PET and CT images at the left heart border at rest using both average (P 5 0.01) and maximum (P , 0.01) counts per pixel and at stress using maximum counts per pixel (P 5 0.02) but not average counts per pixel (P 5 0.14). There was also a correlation between the degree of misalignment of the PET and CT images at the left heart border and at the right hemidiaphragm at stress (P 5 0.01) but not at rest (P 5 0.09). The decreased counts in the lateral wall relative to the septum and the misalignment of the PET and CT images at the left heart border and at the right hemidiaphragm were all greater at stress than at rest (all P , 0.05). In those participants who had no misalignment of the PET and CT images at the left heart border, there was still a residual decrease in relative counts in the lateral wall at both stress and rest, using both average and maximum counts (all P , 0.05). Conclusion: Relative decreased counts in the lateral wall of the heart in PET/CT studies with 13 N-ammonia are commonly seen in healthy adults. The decreased counts in the lateral wall appear to be caused by an attenuation artifact from misalignment at the left border of the heart between the PET and CT images, possibly related to differences in respiratory motion during acquisition of the PET and CT images. In addition, there was a small, but significant, decrease in relative counts in the lateral wall even in participants without misalignment at the left heart border or at the right hemidiaphragm. The development of combined PET/CT scanners has changed the method of generating density maps for attenuation correction of PET images. The previous method of acquiring a density map with an external rotating radioactive source had the disadvantages of requiring considerable time and being of low spatial resolution. On the other hand, the method had the advantage that the blurring effect of respirator...

The Effect of Misregistration Between CT-Attenuation and PET-Emission Images in 13N-Ammonia Myocardial PET/CT

“…Lautamäki et al also performed a 2D PET/CT study and found that misregistrations of 10 and 14 mm resulted in up to a 19% and 225% change in relative uptake in the septum and the lateral wall, respectively, whereas a 5-mm misregistration had no noticeable effect on uptake (8). Our results are in line with the results of those 2D PET/CT studies (4)(5)(6)(7)(8), indicating that the influence of misregistration on altered uptake is similar between 3D PET/CT and 2D PET/CT.…”

“…Previous 2D PET/CT studies demonstrated that artifactual defects occurred predominantly in the LV anterior and lateral walls (4,5,7), which have the largest surface area in common with the lung. When there is a substantial overlap between lung and heart, the marked difference in attenuation factors between lung and heart is considered a main reason for the observed changes in measured uptake (6).…”

“…Four LV segments (anterior wall, inferior wall, lateral wall, and septum) were visually assessed in short-axis basal, mid, and apical PET/CT slices (12 segments total) (7,19) in random order by 2 board-certified nuclear medicine specialists, who worked in consensus and did not know the degree of misregistration applied to each dataset. The erroneous decrease in uptake was graded using a 4-point scale (0 5 none, 1 5 mild, 2 5 moderate, and 3 5 severe).…”

“…PET image was moved over CT image in transaxial (A), coronal (B), and sagittal (C) views with special attention to preventing LV myocardial uptake in PET from overlapping lung parenchyma in CT. A color version of this figure is available as a supplemental file at http://tech.snmjournals.org. (6,7,19). The smallest uptake ratio among the 3 slices for each of the 3 walls was recorded.…”

“…However, misregistration between PET-emission and CT-transmission data may occur up to 40% of the time (4). Studies using 2-dimensional (2D) PET/CT scanners have found that a misregistration of more than 5 mm causes a statistically significant artifactual reduction in myocardial uptake and inaccurate quantification of myocardial blood flow, predominantly in the anterior and lateral walls, due to errors in attenuation correction (4)(5)(6)(7)(8).Recently, the use of 3-dimensional (3D) PET in clinical PET/CT scanners has been facilitated by improvements in detectors and scatter correction techniques (9). 3D PET has higher sensitivity than 2D PET, substantially improving the signal-to-noise ratio in PET images (9).…”