Standard OSEM vs. regularized PET image reconstruction: qualitative and quantitative comparison using phantom data and various clinical radiopharmaceuticals

Abstract

We investigated the block sequential regularized expectation maximization (BSREM) algorithm. ACR phantom measurements with different count statistics and 60 PET/CT research scans from the GE Discovery 600 and 690 scanners were reconstructed using BSREM and the standard-of-care OSEM algorithm. Hot concentration recovery and cold contrast recovery were measured from the phantom data. Two experienced nuclear medicine physicians reviewed the clinical images blindly. Liver SNR liver and SUV_max of the smallest lesion detected in each patient were also measured. The relationship between the maximum and mean hot concentration recovery remained monotonic below 1.5 maximum concentration recovery. The mean cold contrast recovery remained stable even for decreasing statistics with a highest absolute difference of 4% in air and 2% in bone for each reconstruction method. The D600 images resulted in an average 30% higher SNR than the D690 data for BSREM; there was no difference in SNR results between the two scanners with OSEM. The small lesion SUV_max values on the BSREM images with β of 250, 350 and 450, respectively were on average 80%, 60% and 43% (D690) and 42%, 29%, and 21% (D600) higher than in the case of OSEM. In conclusion, BSREM can outperform OSEM in terms of contrast recovery and organ uniformity over a range of PET tracers, but a task dependent regularization strength parameter (beta) selection may be necessary. To avoid image noise and artifacts, our results suggest that using higher beta values (at least 350) may be appropriate, especially if the data has low count statistics.

Keywords: BSREM; OSEM; PET; image; reconstruction.

Figure 1

ACR phantom from left to right with 100%, 50%, 20% and 5% of the total counts, respectively; A: D690 top row OSEM, bottom row BSREM β=350; B: D600 top row OSEM, bottom row BSREM β=350.

Figure 2

Mean vs. maximum hot concentration recovery for the hot cylinders of the ACR phantom (top: D600, bottom: D690). The different marker shapes depict the 4 reconstruction methods; the marker color represents the SNR value measured in the uniform region of the phantom.

Figure 3

Small lesion SUV_max difference between BSREM and OSEM as a function of the OSEM SUV_max value in the 60 patient studies using various radiopharmaceuticals. The different marker shapes depict the 3 BSREM β values; the marker color represents the SNR value measured in the liver.

Figure 5

Arrow indicates focal ¹⁸F FPPRGD₂ uptake visualized on the 3 BSREM reconstructions that is not seen on the OSEM image in a 34-year-old woman with recurrent GBM. Brain MRI follow-up done 1 and 2 months later confirm a growing contrast enhancing lesion in this location.

Figure 4

Relative number of cases when a given method for a given radiopharmaceutical received the highest score out of the 4 images presented to the readers.