E7tools

Because mMR surface coils include some metal components, we decided to acquire the µ-map of the surface coils using a transmission scan to avoid CT metal artifacts. A transmission scan of an anthropomorphic phantom (Data Spectrum Corporation, Hillsborough, NC) with a Siemens mMR surface coil array attached [See Figure 1 (A)] was first acquired on a Siemens ECAT HR+ scanner. The same phantom was filled with F-18 in both the liver and soft-tissue compartments (liver/background concentration ratio: 2.4). Additionally, three 1-cm “tumors” were placed around the liver (tumor/liver concentration ratio: 4). The phantom was placed in a Siemens Biograph PET-CT scanner. First, a CT with 120 kVp was acquired. Second, an mMR surface coil array was placed on the top of the phantom in the same way as we did the transmission scan. Third, a PET acquisition which included total 279 million coincidence events was performed. The CT image was first transformed into a µ-map, denoted phantom-µ-map, using Siemens e7 tools. The coil image obtained from the transmission scan was digitally added to the phantom-µ-map to create another µ-map, denoted phantom-coil-µ-map. The PET data were reconstructed twice: one with phantom-µ-map and one with phantom-coil-µ-map. A bias image was then calculated using the image reconstructed with phantom-coil-µ-map as the reference.

From the mMR PET listmode data, only the first 10 minutes for each patient was used – this reduced the chance of artifacts due to patient motion. All PET reconstructions (OP-OSEM, 3 iterations and 21 subsets) were performed offline using JSRecon and e7tools provided by Siemens, using a 344×344×127 matrix with pixel size of 2.09 mm² and slice thickness of 2.03 mm. Next to the different human μ-maps, the corresponding hardware μ-maps were used to correct for attenuation and scatter due to the head coil and patient table. A post-reconstruction smoothing with a Gaussian filter and kernel width of 2mm full width at half maximum was applied.

CTs and pCTs were converted to PET LAC values by piecewise linear scaling (1 (link)). The vendor-provided e7tools program (Siemens Medical Solutions, Knoxville, TN) was employed to reconstruct PET list mode data acquired from 50–70 minutes post-tracer injection using an ordinary Poisson ordered subset expectations maximization (OP-OSEM) algorithm with 3 iterations, 21 subsets, and a 5 mm Gaussian filter. In the test-retest repeatability analysis, AC maps computed using acquired CTs and DL-TESLA pCTs from visit 1 and visit 2 were used to reconstruct visit 1 PET data. A comparison between PET data from visit 1 and visit 2 was avoided due to potential PET signal changes resulting from pathophysiological progression over 3 years.