Evaluating the image quality of combined positron emission tomography-magnetic resonance images acquired in the pelvic radiotherapy position

Abstract

Positron emission tomography-magnetic resonance (PET-MR) scanners could improve radiotherapy planning through combining PET and MR functional imaging. This depends on acquiring high quality and quantitatively accurate images in the radiotherapy position. This study evaluated PET-MR image quality using a flat couch and coil bridge for pelvic radiotherapy. MR and PET image quality phantoms were imaged in three setups: phantom on the PET-MR couch with anterior coil on top (diagnostic), phantom on a flat couch with coil on top (couch), and phantom on the flat couch with coil on a coil bridge (radiotherapy). PET images were also acquired in each setup without the anterior coil. PET attenuation correction of the flat couch and coil bridge were generated using kilovoltage computed tomography (CT) images and of the anterior coil using megavoltage CT images. MR image quality was substantially affected, with MR signal to noise ratio (SNR) relative to the diagnostic setup of 89% ± 2% (mean ± standard error of the mean, couch) and 54% ± 1% (radiotherapy), likely due to the increased distance between the patient and receive coils. The reduction impacted the low-contrast detectability score: 23 ± 1 (diagnostic), 19.7 ± 0.3 (couch) and 15 ± 1 (radiotherapy). All other MR metrics agreed within one standard error. PET quantitative accuracy was also affected, with measured activity with anterior coil being different to diagnostic without anterior coil by −16.7% ± 0.2% (couch) and −17.7 ± 0.1% (radiotherapy), without attenuation correction modification. Including the couch and coil bridge attenuation correction reduced this difference to −7.5% ± 0.1%, and including the anterior coil reduced this to −2.7% ± 0.1%. This was better than the diagnostic setup with anterior coil (difference −8.3% ± 0.2%). This translated into greater PET SNR performance for the fully corrected radiotherapy setup compared to diagnostic with coil. However contrast recovery was unchanged by the modified attenuation correction, with the diagnostic setup remaining ∼2% better. Quantitative PET in the radiotherapy setup is possible if appropriate attenuation correction is used. Pelvic radiotherapy PET-MR imaging protocols will need to consider the impact on PET-MR image quality.

Impact of attenuation correction of radiotherapy hardware for positron emission tomography‐magnetic resonance in ano‐rectal radiotherapy patients

“…In addition, it would have added significant imaging time for patients. Therefore it was decided to compare AC maps with and without the radiotherapy hardware included to assess the impact of changing the AC map and rely on the phantom measurements alone to confirm the accuracy of the AC map with radiotherapy hardware included.The phantom measured difference in activity loss between using the AC map with radiotherapy hardware included was 15.0%, [15] which is close to the whole image difference in SUV in this study of 13.8%, suggesting the phantom measurements do translate to patients. Winter et al did compare PET-MR images in the radiotherapy position with appropriate AC and diagnostic position for head and neck patients.…”

Developing quality assurance tests for simultaneous Positron Emission Tomography – Magnetic Resonance imaging for radiotherapy planning

“…A potential limitation of this study was that the MR image quality and PET SUV accuracy tests were not acquired with the radiotherapy flat couch and coil bridge. This was done because the radiotherapy hardware causes a significant drop in MR SNR of and PET activity of [21] . This would have reduced the sensitivity of the tests to detect changes in MR/PET performance, which was their primary aim.…”

“…Radiotherapy adapted PET-MR systems have been evalauted using QA phantoms. PET and MR image quality has been evaluated in a head and neck radiotherapy setup using uniform PET and MR phantoms [20] and in a pelvic radiotherapy setup using image quality phantoms [21] . Methods to correct the PET attenuation from the radiotherapy setup [22] , [21] have also been evaluated.…”

“…PET and MR image quality has been evaluated in a head and neck radiotherapy setup using uniform PET and MR phantoms [20] and in a pelvic radiotherapy setup using image quality phantoms [21] . Methods to correct the PET attenuation from the radiotherapy setup [22] , [21] have also been evaluated. However, to the best of the author’s knowledge, a comprehensive set of tests for a routine QA programme have not been evaluated in the literature previously.…”

Evaluation of Magnetic Resonance Imaging for Microsurgical Efficacy and Relapse of Rolandic Meningioma

“…At present, the main method of imaging assessment of meningioma invasion of brain tissue is also MRI. Magnetic resonance examination is characterized by multidirectional and external parametric imaging, which can clearly show the lesions of tumor soft tissue and microvessels [ 9 ]. Magnetic resonance examination includes magnetic resonance imaging (MRI), MRI enhancement imaging, MRI functional imaging, MR diffusion imaging, MR perfusion imaging, and proton magnetic resonance spectroscopy ( 1 H-MRS) [ 10 ].…”