Hello, everyone. I'm Vasvi Singh. I'm a multimodality advanced division cardiologist at HCA Midwest Health in Kansas City. Today, we're going to talk about CT for anatomic localization and attenuation correction in FDG PET and PYP SPECT. These are my disclosures. None are relevant to this talk. The objectives of this talk are the following. To understand what is attenuation, what are the factors determining attenuation, how do we perform attenuation correction with CT, associated artifacts, and we're going to go over some case examples focusing on PYP SPECT CT and FDG PET CT. So attenuation is any interaction of photons within the body tissue. Using conventional SPECT techniques, even though the source distribution is uniform, the reconstructed images shows an apparent decrease in activity that reaches a minimum at the center of the image. This effect is due to attenuation of photons within the source, that is the human body, before exiting the source and being detected by the camera system. The primary mechanism for attenuation in tissue throughout the diagnostic energy range in conventional nuclear medicine is Compton scattering, which results in change in photon directions of travel with loss of energy. The effects of attenuation are more intense at lower energies, but are still significant at the highest energy value, as shown in the top figure. In addition, the magnitude of the attenuation effect depends on the tissue type, as shown in the bottom figure. The figure shows the percentage transmission of 140 KeV photons versus depth in lung, soft tissue, and bone. In order to accurately represent the activity distribution measured with SPECT, it is necessary to accurately correct for the effects of attenuation. The deficiencies in SPECT identified previously can be addressed by incorporating the techniques of CT in image acquisition and reconstruction process. CT provides high quality and high spatial resolution images of cross-sectional anatomy. CT images are acquired as transmission maps with a high photon flux and are actually high quality representations of tissue attenuation, and thus can provide the basis for attenuation correction. Therefore, combining SPECT and CT modalities into an integrated system is a significant advance because the two modalities are complementary in that the weaknesses of one are often the strengths of the other in specific imaging situations. Therefore, integrated SPECT-CT technology makes it possible to acquire physiologic and anatomic images in a registered format and fuse them so that the precise anatomic localizations of radiopharmaceutical distributions can readily be visualized. An additional benefit of this technologic advance is that the anatomic images can be used to perform high quality attenuation corrections of the radiopharmaceutical distributions. Similar advantages are noted with integrated SPECT-CT technology. The figure on the left is demonstrating SPECT-CT scanner anatomy, while the figure on the right is demonstration of data flow for data processing. So let's review the role of SPECT-CT integrated technology in amyloid imaging. Radionuclide imaging plays a critical role in the diagnosis of transthyretin cardiac amyloidosis. Historically used in myocardial infarct imaging, technetium-99m bone avid radiotracers like technetium pyrophosphate or PYP is used to specifically image ATTR, obviating the need for an endomyocardial biopsy to be used in for an endomyocardial biopsy in most cases when following the recommended diagnostic algorithm that excludes for the presence of light chain amyloidosis. Shown here is the visual grading scheme on PYP SPECT-CT where grade 0 is when there is no myocardial uptake. Grade 1 is when myocardial uptake is less than that of rib uptake. Grade 2 is when myocardial uptake is equal to rib uptake and grade 3 is when myocardial uptake is greater than rib uptake. A score of 2 or more is a positive PYP scan. Another metric that can be calculated on anteroposterior planar images is the heart to contralateral lung ratio or the HCL ratio by drawing two identical region of interest or RYs, one on the heart and the other on the contralateral lung. A ratio of greater than 1.5 at 1 hour and greater than 1.3 at 3 hours is considered to be positive. However, there can be several causes of false positive scans if only planar images are used for scan interpretation such as hot blood pool activity and focal infarct that can give a falsely elevated HCL ratio. But the diagnosis can easily be established with SPECT technology as shown in the panels below with SPECT-CT images. Another example that can also cause falsely elevated HCL ratio is a large right-sided pleural effusion. Shown here is an example of technetium-99m pyrophosphate planar and SPECT-CT study planar and SPECT-CT study showing blood pool and therefore a negative study. And shown here is an example of technetium-99m PYP planar and SPECT-CT study showing diffuse LV myocardial uptake and therefore a positive study. In addition, CT co-registration may improve diagnostic confidence for PYP scan interpretation, especially in cases where there may be focal septal uptake as has been described in early disease in cardiac amyloidosis. Panel A shows AP planar image with low visual score and hard to contralateral lung ratio. Panel B shows cardiac SPECT with no discernible cardiac tracer activity and high uptake in the sternum is noted. However, panel C shows SPECT-CT depicting focal tracer activity in the septum, confirming LV myocardial activity. And when correct techniques are employed, all of these scans were SPECT-CT scans. We showed that there was excellent inter-observer reproducibility and intra-observer repeatability of technetium-99m PYP visual scan interpretation and HCL ratio for the diagnosis of transthyretin cardiac amyloidosis. So what are the protocol considerations for attenuation correction? As listed, low CT current for attenuation correction is preferred. The breathing protocol is not standardized, but usually shallow free breathing is preferred with a slow rotation speed that helps blur cardiac motion. ECG gating is typically not recommended. Tube potential is based on manufacturer specification. Slice collimation and reconstruction should approximate slice thickness of PET. Following techniques are employed for optimizing CT for attenuation correction. Axial coverage should include all of the myocardium and sufficient coverage should be used to avoid image truncation. Perform SPECT prior to CT. Ensure appropriate patient positioning with patient in the center of the CT field of view. Both arms above head and similar position between rest and stress image acquisition when employing for perfusion imaging. When reading through a hybrid imaging scan such as PET-CT shown in the figure, the first step is to ensure registration of emission and transmission scans in axial, sagittal, and coronal imaging planes. Shown is an example of misregistration artifact of emission and transmission images on stress and rest M13 ammonia PET-CT scan. Stress images are shown in the top row and rest images in the bottom row. The short axis images are shown from apex to base, horizontal long axis images from inferior wall to anterior wall, and the vertical long axis images from the septum to the lateral wall. This figure demonstrates a large size and severe perfusion defect in the mid to basal anterolateral wall and the apical lateral wall on stress images that is absent on rest images. Therefore, may be misread as completely reversible or ischemia if quality control is not exercised. An overlay of the stress emission and transmission images as shown in the right panel demonstrates significant misregistration with the emission images overlying the CT lung field, most notable on the short axis and horizontal long axis views, explaining the cause of this artifactual reversible perfusion defect. This figure shows new reconstructed stress images after appropriate alignment of the transmission and emission images. The perfusion defect on stress images has now resolved after corrected registration of stress and rest images as seen on the right side panel and a new image reconstruction using this alignment. Shown here are emission only FDG PET images on the left panel and FDG PET-CT hybrid images overlay in the right panel. Both attenuation correction and non-attenuation correction images demonstrate avid FDG uptake in the region of the prosthetic aortic valve, consistent with aortic valve endocarditis in the appropriate clinical context. This is a negative case of prosthetic valve endocarditis where there is false low-grade FDG uptake in the region of metallic prosthesis on CT attenuation corrected FDG PET images that completely disappears when looking at the non-attenuation correction FDG PET images. So this case highlights the importance of reviewing both CT attenuation corrected and non-attenuation corrected FDG PET-CT images in cases of metallic prosthesis infection. Thank you all for your time.

cardiology

attenuation correction

SPECT-CT

hybrid imaging

CT co-registration