Evaluation of PET attenuation correction accuracy with ultra-low-dose CT on the Vereos digital PET/CT scanner

Objectives Determine ultra-low dose CT scan parameters for PET attenuation correction on the Vereos digital PET/CT scanner when used with novel CT algorithms such as iDose (noise reduction) and O-MAR (metal artifact reduction).

Methods A CT system phantom with teflon, acrylic, polyethylene and water inserts was used to evaluate attenuation correction accuracy. For whole body (WB) studies, the CT phantom was placed inside a large water-filled elliptical phantom (simulating the 95th percentile chest circumference of North American patients) while it was scanned alone for head studies. Different combinations of tube current, iDose levels and O-MAR were tested for 4 voltage settings (80, 100, 120 and 140 kVp). The 511keV linear attenuation coefficients (LACs) were evaluated for the 4 inserts to determine the minimal operating conditions where attenuation accuracy was still maintained. A NEMA image quality (IQ) study was also acquired with 80kVp/15mAs, 120kVp/50mAs and 140kVp/15mAs CT scans and iDose levels 4 and 6. The variation in contrast and background variability as a function of different low-dose CT scans was evaluated.

Results For WB studies, LACs at 120kVp/15mAs and 140 kVp/15mAs were within 5% of the values obtained with the default setting of 120kVp/50mAs (and within 10% of the true values), with a corresponding dose reduction of 70 and 55%, respectively. For head studies, 15 mAs at 80 or 140 kVp provided LACs within 5% of the default (120kVp/50mAs) setting. The above results were also observed with iDose or O-MAR indicating that they did not adversely affect LAC measurements. NEMA IQ results indicated that changes in contrast and variability measurements were within 2.5% over the range of CT settings tested.

Conclusions Based on phantom studies, the minimum recommended operating conditions for low-dose CT scans were determined and found to maintain attenuation accuracy. CT techniques such as iDose and O-MAR, introduced for improving CT image quality, maintained LAC accuracy at these ultra-low-dose settings.