Heterogeneity Compared to Conventional SUV is a More Sensitive Method for Distinguishing Thoracic Aortic Pathologies

Objectives 18F-fluorodeoxyglucose (18F-FDG) uptake by positron emission tomography (PET) has been utilized to assess increased metabolic activity in the aneurysmal abdominal aortic wall and the dissected thoracic aortic wall. The ability to distinguish pathologies of the thoracic aortic wall has not been well demonstrated utilizing 18F-FDG PET. Variability of 18F-FDG uptake over a volume of tissue may be quantified by calculating heterogeneity values. A non-invasive test that identifies markers of accelerated growth and/or impending rupture associated with thoracic dissection and thoracic aortic aneurysms is needed. This study compared 18F-FDG uptake in the thoracic aorta of non-aneurysmal patients with carotid atherosclerosis to that of thoracic aortic aneurysms patients, as well as Type A & B thoracic aortic dissections. Maximum standard uptake values (SUVmax) were compared with heterogeneity values.

Methods Patients that had undergone 18F-FDG PET scans with documented carotid disease by duplex and no aneurysmal degeneration (control, N=6), thoracic aortic aneurysms (TAA, N=7), and Type A & B thoracic aortic dissections (TAD, N=10) were identified. Regions of interest were drawn around the outer wall of the aorta in each case, from distal to the left subclavian artery to the celiac axis. SUVmax and heterogeneity values were determined for each segment. The global Moran I(d) analysis is performed to characterize the heterogeneity on PET images. Other than texture analysis that widely used in heterogeneity prediction, I(d) statistic is a measure of spatial correlation among 3D neighboring voxels normalized by autocorrelation. Student t-tests were performed to compare control, TAA, and TAD patients.

Results Patient demographics were well matched between the groups. Mean aortic diameters for the control, TAA, and TAD groups were 2.5±0.1 cm, 5.0±0.4 cm, and 4.7±0.2 cm, respectively. SUVmax values for controls, TAAs, and TADs were 2.4±0.3, 3.0±0.2, and 3.3±0.3, respectively. 18F-FDG uptake was significantly higher in TAA(p=0.001) and TAD(p=0.01) patients when compared to controls, but no difference was noted between the two aortic pathologies. Heterogeneity ratios for controls, TAAs, and TADs were 0.42±0.12, 0.55±0.04, and 0.62±0.02, respectively. Heterogeneity was significantly higher in TAAs(p=0.02) and TADs(p=0.001) compared to controls, and a difference in heterogeneity between TAA and TAD patients(p=0.02) was appreciated. See Figure. Greater TAD false lumen 18F-FDG uptake was associated with greater heterogeneity(p=0.04).

Conclusions This study describes one institution’s experience assessing inflammation in the thoracic aortic wall by 18F-FDG PET. The ability to detect greater inflammation in TAA and TAD walls compared with controls was demonstrated with use of conventional SUVmax measures; however, this approach was not sensitive enough to detect differences in uptake when comparing TAAs and TADs. Greater heterogeneity in the aortic walls of TADs compared to TAAs, as well as increased heterogeneity associated with greater false lumen wall uptake of TADs would suggest that heterogeneity is a more sensitive measure of thoracic aortic 18F-FDG uptake and inflammation. Further investigation of 18F-FDG uptake by TAAs and TADs is necessary to thoroughly evaluate the predictive value of PET heterogeneity as a tool that may be used to survey TAA and TAD patients in the future and determine TAA and TAD growth and rupture risks.

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