Blanching defects at the pressure points: a potential pitfall in dynamic Total-Body PET/CT studies

186 Purpose: Total body PET/CT scanners enable capturing dynamic tracer changes from the entire body simultaneously, however, this task may be challenging from multiple perspectives. We aimed to describe blanching uptake defects seen opposite the bony prominences at the pressure points, which may represent a potential pitfall in dynamic studies performed using high-sensitivity high-resolution total-body scanners.\n Methods: This is a retrospective review of two prospectively acquired IRB-approved studies. Group 1 (FDG) included 30 adult healthy volunteers (16 females, 14 males; mean age: 47.3±13). Images were acquired for 60 min, starting immediately after IV injection of a median 18F-FDG dose of 374.3 MBq (range: 337.3-393.8; high dose subgroup; n=15) or 18.9 MBq (range: 17.2-23.5; low dose subgroup; n=15). The last 20-min of acquisition (40-60 min after injection) were reconstructed for reviewing. All participants were re-imaged twice, each for 20-minutes, starting at 90- and 180-min post-injection. Group 2 (Fluciclovine) included 23 men (mean age 69±7.5) with biochemical recurrence of prostate cancer. Images were acquired for a total of 25-minutes, starting immediately after IV injection of a median 18F-Fluciclovine dose of 312.0 MBq (range: 287.2 - 389.6). The data were reconstructed into different time frames; frames from 4-14 minutes were used for reviewing. Both attenuation-corrected (AC) and non-AC (NAC) images were reviewed side-by-side by two nuclear medicine physicians for any areas of skin/subcutaneous tissue, opposite 4 pressure points (skull, scapulae, sacrum, and heels), that demonstrate absent or markedly decreased tracer uptake without CT correlate. The association of these defects with different clinical and scan characteristics was evaluated using chi-square (for categorical variables) or Mann-Whitney U test (for continuous non-normally distributed data).\n Results: Blanching defects were seen in 44/53 participants (83%) at one or more pressure points. The total number of defects was 103: of them 54 were seen around the heels, 30 opposite the scapulae, 12 in the scalp and 7 against the sacra. The presence or absence of these blanching defects did not vary according to age, sex, height, weight, BMI, injection dose, fasting hours, blood glucose level or the use of back and/or knee pillows. Also, the presence of metallic prosthesis or mis-registration between the PET and CT was not associated with these defects. However, their distribution varied according to the radiopharmaceutical. Defects opposite the skull and scapulae were encountered more frequently with Group 2 (Fluciclovine), who were all males, and older, taller and heavier on average. In Group 1 participants, 24/30 showed blanching defects on initial dynamic imaging. All of them demonstrated no clear photopenia at the same pressure points on subsequent 90- and 180-min imaging. This suggests that the defects are associated with pressure-related ischemia that resolves after moving around between the scans.\n Conclusions: Blanching defects are commonly encountered at the pressure points on dynamic total body PET/CT imaging. Careful review of this finding is crucial; not to be misinterpreted as a pathology. These findings from the high sensitivity total body scans may be exploited towards studying different mechano-pathological conditions like pressure ulcers.