Measured count-rate performance of the Discovery STE PET/CT scanner in 2D, 3D and partial collimation acquisition modes

Abstract

We measured count rates and scatter fraction on the Discovery STE PET/CT scanner in conventional 2D and 3D acquisition modes, and in a partial collimation mode between 2D and 3D. As part of the evaluation of using partial collimation, we estimated global count rates using a scanner model that combined computer simulations with an empirical live-time function. Our measurements followed the NEMA NU2 count rate and scatter-fraction protocol to obtain true, scattered and random coincidence events, from which noise equivalent count (NEC) rates were calculated. The effect of patient size was considered by using 27 cm and 35 cm diameter phantoms, in addition to the standard 20 cm diameter cylindrical count-rate phantom. Using the scanner model, we evaluated two partial collimation cases: removing half of the septa (2.5D) and removing two-thirds of the septa (2.7D). Based on predictions of the model, a 2.7D collimator was constructed. Count rates and scatter fractions were then measured in 2D, 2.7D and 3D. The scanner model predicted relative NEC variation with activity, as confirmed by measurements. The measured 2.7D NEC was equal or greater than 3D NEC for all activity levels in the 27 cm and 35 cm phantoms. In the 20 cm phantom, 3D NEC was somewhat higher (~15%) than 2.7D NEC at 100 MBq. For all higher activity concentrations, 2.7D NEC was greater and peaked 26% above the 3D peak NEC. The peak NEC in 2.7D mode occurred at ~425 MBq, and was 26-50% greater than the peak 3D NEC, depending on object size. NEC in 2D was considerably lower, except at relatively high activity concentrations. Partial collimation shows promise for improved noise equivalent count rates in clinical imaging without altering other detector parameters.

PeneloPET simulations of the Biograph ToF clinical PET scanner

“…The larger ring difference of the mCT also leads to a 10% increase of the peak NEC, compared to the B-TPTV (Table II). The simulation of MacDonald et al [31] with SimSET also agrees with the observed increase of sensitivity and NEC with increasing axial FOV and maximum ring difference predicted by PeneloPET.…”

Effective count rates for PET scanners with reduced and extended axial field of view

“…We also simulated the scanner with one septum between every two crystals. This collimation mode is referred to as 2.5D partial collimation, and was shown in a previous study to have advantages over 2D and 3D mode for NEC on the Advance and DSTE scanners [10–11]. Our simulations of reduced and extended AFOV modeled these three collimation modes.…”

“…Adjustments for sensitivity losses due to inter-crystal and inter-block material in the block detector were made with a multiplicative packing fraction [11]. Random coincidence counts were estimated from the rate of single photons absorbed in the detector following Stearns et al [15].…”

“…We used objects longer than the standard cylinder to account for scattered and random coincidence events originating farther outside the field of view, as would be present in clinical scanning. We included the use of full and sparse axial septa in the simulations following previous work that showed potential advantages of sparse septa [10–11]. We then estimated total scan times for whole-body exams that acquire integrated NEC density consistent with current oncology protocols.…”

Determination of the optimal acquisition protocol of breath-hold PET/CT for the diagnosis of thoracic lesions

“…Recently developed high-sensitivity PET scanners and 3D acquisition modes have enabled clinicians to obtain images within a short acquisition time [18,19]. Breath-hold (BH)-PET/CT has been recommended for the diagnosis of thoracic lesions.…”