Whole-body hybrid imaging concept for the integration of PET/MR into radiation therapy treatment planning

Abstract

Modern radiation therapy (RT) treatment planning is based on multimodality imaging. With the recent availability of whole-body PET/MR hybrid imaging new opportunities arise to improve target volume delineation in RT treatment planning. This, however, requires dedicated RT equipment for reproducible patient positioning on the PET/MR system, which has to be compatible with MR and PET imaging. A prototype flat RT table overlay, radiofrequency (RF) coil holders for head imaging, and RF body bridges for body imaging were developed and tested towards PET/MR system integration. Attenuation correction (AC) of all individual RT components was performed by generating 3D CT-based template models. A custom-built program for μ-map generation assembles all AC templates depending on the presence and position of each RT component. All RT devices were evaluated in phantom experiments with regards to MR and PET imaging compatibility, attenuation correction, PET quantification, and position accuracy. The entire RT setup was then evaluated in a first PET/MR patient study on five patients at different body regions. All tested devices are PET/MR compatible and do not produce visible artifacts or disturb image quality. The RT components showed a repositioning accuracy of better than 2 mm. Photon attenuation of -11.8% in the top part of the phantom was observable, which was reduced to -1.7% with AC using the μ-map generator. Active lesions of 3 subjects were evaluated in terms of SUVmean and an underestimation of -10.0% and -2.4% was calculated without and with AC of the RF body bridges, respectively. The new dedicated RT equipment for hybrid PET/MR imaging enables acquisitions in all body regions. It is compatible with PET/MR imaging and all hardware components can be corrected in hardware AC by using the suggested μ-map generator. These developments provide the technical and methodological basis for integration of PET/MR hybrid imaging into RT planning.

A dual‐tuned13C/1H head coil forPET/MRhybrid neuroimaging: Development, attenuation correction, and first evaluation

“…The attenuation profile across the phantom volume as displayed on the resulting difference maps shows a rather homogeneous attenuation, reflecting the open birdcage design of the RF coil. Homogeneous and low‐level attenuation of an RF coil can be seen as advantageous features and a precondition for subsequent attenuation correction . The overall attenuation caused by the RF head coil of 8.8% as measured in the phantom experiments can be considered as comparably low, when compared to the results of other studies.…”

“…Following attenuation correction by applying the CT‐based AC template of the RF head coil, the phantom measurements revealed a measured difference across the phantom volume of 0.96% ± 0.29%, which is the measured difference between phantom measurement without RF coil and a second phantom measurement with RF coil and its attenuation correction. This rather low remaining quantification bias indicates successful attenuation correction of the RF head coil and a successful implementation of the RF coil on the PET/MR system and in the PET data reconstruction process …”

“…They have found an attenuation bias in the range of 13%–19% . The article by Paulus and Quick provides a comprehensive overview about numerous studies investigating the attenuation of several hardware components for use in PET/MR. No studies are available today specifically investigating dual‐tuned RF head coils for PET/MR use.…”

“…For attenuation and scatter correction of hardware components and RF coils in PET/MR, the method of using different CT‐based attenuation templates has been established . During the phantom experiments in this study, the MR‐based Dixon‐VIBE images of the phantom only serve as the basis for position detection of the phantom during the experiments.…”

“…The hardware component μmaps are based on CT transmission scans of the RF coils and provide the spatial distribution of attenuation factors in a 3D model of the respective hardware component . Due to the patient table position, the position of rigid RF coils (e.g., RF head coil, RF spine array coil) during a PET/MR examination is known and the template‐based AC using the CT‐derived μmap can be performed during the PET data reconstruction process …”