Cine CT for attenuation correction in cardiac PET/CT

Abstract

In dual-modality PET/CT systems, the CT scan provides the attenuation map for PET attenuation correction. The current clinical practice of obtaining a single helical CT scan provides only a snapshot of the respiratory cycle, whereas PET occurs over multiple respiratory cycles. Misalignment of the attenuation map and emission image because of respiratory motion causes errors in the attenuation correction factors and artifacts in the attenuation-corrected PET image. To rectify this problem, we evaluated the use of cine CT, which acquires multiple low-dose CT images during a respiratory cycle. We evaluated the average and the intensity-maximum image of cine CT for cardiac PET attenuation correction.

Methods: Cine CT data and cardiac PET data were acquired from a cardiac phantom and from multiple patient studies. The conventional helical CT, cine CT, and PET data of an axially translating phantom were evaluated with and without respiratory motion. For the patient studies, we acquired 2 cine CT studies for each PET acquisition in a rest-stress (13)N-ammonia protocol. Three readers visually evaluated the alignment of 74 attenuation image sets versus the corresponding emission image and determined whether the alignment provided acceptable or unacceptable attenuation-corrected PET images.

Results: In the phantom study, the attenuation correction from helical CT caused a major artifactual defect in the lateral wall on the PET image. The attenuation correction from the average and from the intensity-maximum cine CT images reduced the defect by 20% and 60%, respectively. In the patient studies, 77% of the cases using the average of the cine CT images had acceptable alignment and 88% of the cases using the intensity maximum of the cine CT images had acceptable alignment.

Conclusion: Cine CT offers an alternative to helical CT for compensating for respiratory motion in the attenuation correction of cardiac PET studies. Phantom studies suggest that the average and the intensity maximum of the cine CT images can reduce potential respiration-induced misalignment errors in attenuation correction. Patient studies reveal that cine CT provides acceptable alignment in most cases and suggest that the intensity-maximum cine image offers a more robust alternative to the average cine image.

FIGURE 1

Cardiac PET/CT example of attenuation and emission scan mismatch, which often occurs along lateral free wall or right diaphragm. Axial and coronal images of CT scan and of attenuation-corrected NH₃ PET scan show, as indicated with same solid line drawn on all slices, 24.7-mm mismatch at lateral wall.

FIGURE 2

Transaxial slice through CT scans of oscillating box and water-filled sphere. (A) Image acquired in conventional helical mode (0.8-s rotation, 2.5-mm slice, pitch of 1). (B and C) Images formed from cine CT images: average image of 5 time frames at same slice (B) and intensity-maximum image of 5 time frames (C). Because PET is performed during multiple complete respirations, attenuation correction derived from cine CT images may be more accurate than that derived from helical images.

FIGURE 3

Comparison of attenuation correction techniques for cardiac PET, with CT images for attenuation correction at top and attenuation-corrected PET images at bottom. (A) PET/CT data acquired from stationary phantom processed to induce motion and generate the true case. (B–D) PET data acquired from moving phantom corrected with helical CT, as performed in current clinical practice (B); with average cine CT (C); and with intensity-maximum cine CT (D). The proposed cine methods reduce bias and variance in myocardium.

FIGURE 4

Coronal slice through patient example of aligned CT and PET images. At top are average (A) and intensity maximum (B) of 1 set of cine CT data. Top row was used for attenuation correction of PET images in bottom row.

FIGURE 5

Patient example of unacceptable alignment from average of cine CT images (column A) and acceptable alignment from intensity maximum of cine CT images (column B). Top row contains coronal slice of CT used for attenuation correction of PET image seen in second row from top. Third row from top contains transaxial slice of same CT image used for attenuation correction of PET image seen in bottom row. Identical myocardial outline is drawn on all coronal views, and separate identical outline is drawn on all transaxial views.

FIGURE 6

Average quantification in myocardium along circumferential profiles through short-axis view of basal segments (A and C) and mid segments (B and D) for identical emission data attenuation-corrected with different methods: plots of average kBq/mL in segments (A and B) and plots of same segments with units normalized (norm) such that images from all methods have same maximum value (C and D). This patient scan was classified as normal, with unacceptable alignment for average of first cine and with acceptable alignment for all other methods.

FIGURE 7

Coronal slices through average of cine CT data (A), intensity-maximum image (B), and weighted combination of average and intensity-maximum image (C). Weighted-combination image reduces some of the stair-step axial-field-of-view artifacts present on average image and shows a potentially more accurate boundary extent than does intensity-maximum image.