Session Item

To correct for the PET signal attenuation of RT patient positioning hardware and MRI coils by implementing user-derived CT-based attenuation maps into the PET reconstruction path.

The RT positioning hardware consisted of a flat RT table overlay, coil holders for abdominal scans, coil holders for head and neck scans (all Medibord) and an MRI compatible ProStep system (Innovative Technologie Völp) for hip and leg immobilization. Each hardware element was scanned at a CT (Somatom Confidence, Siemens Healthineers) using 120kV/255mA for low attenuating hardware (RT table overlay, coil holders, ProStep) and 140kV/467mA for high attenuating hardware (coil holders with coils). From the CT images, attenuation maps were derived using the bilinear (low attenuating hardware) [1] and the adapted bilinear (high attenuating hardware) [2] approach. Attenuation maps were implemented into the PET reconstruction path. For experimental validation of attenuation correction accuracy, PET images with each positioning device mounted independently (RT scans) and one reference scan (without any RT positioning hardware present) were acquired at a PET/MRI scanner (Biograph mMR, Siemens Healthineers) using an active 68Ge phantom (32 MBq, 10 min scan time). For each positioning device, PET reconstructions of the RT scans were performed twice: once without hardware attenuation correction (uncorrected RT scan) and once with hardware attenuation correction implemented (corrected RT scan). The average percentage difference in PET signal intensities of the uncorrected and corrected reconstructions to the reference reconstruction was calculated.

Table 1 compares the average percentage difference of PET signal intensities for the corrected and uncorrected RT scans with the reference scan for each positioning device. The PET signal attenuation ranges from 7.9±0.8% (head and neck coil holder with coils) to 1.6±0.4% (abdominal coil holder). With attenuation correction, these values can be reduced to 2.5±2.0% and 0.3±0.7%, respectively.

Integrating hybrid PET/MRI into radiation treatment planning has great potential to improve tumor delineation and dose prescription. Since these scans must be acquired under treatment conditions, additional RT positioning hardware is necessary. The additional patient positioning hardware necessary causes a PET signal attenuation in the order of up to 8% which can be corrected to 2.5% or less by implementing CT-based attenuation maps into the PET reconstruction path.

References:
[1] Carney et al. DOI: 10.1118/1.2175132
[2] Paulus et al. DOI: 10.1088/0031-9155/58/22/8021