Purpose: In simultaneous positron emission tomography/magnetic resonance (PET/MR) imaging, local receiver surface radiofrequency (RF) coils are positioned in the field-of-view (FOV) of the PET detector during PET/MR data acquisition and potentially attenuate the PET signal. For flexible body RF surface coils placed on top of the patient's body, MR-based attenuation correction (AC) is an unsolved problem since the RF coils are not inherently visible in MR images and their individual position in the FOV is patient specific and not known a priori. The aim of this work was to quantify the effect of local body RF coils used in the Biograph mMR hybrid PET/MR system on PET emission data and to present techniques for MR-based position determination of these specific local RF coils.
Methods: Acquisitions of a homogeneous phantom were performed on a whole-body PET/MRI scanner. Two different PET emission scans were performed, with and without the local body matrix RF coil placed on the top of the phantom. For position determination of the coil, two methods were applied. First, cod liver oil capsules were attached to the surface of the coil and second, an ultrashort echo time (UTE) sequence was used. PET images were reconstructed in five different ways: (1) PET reference scan without the coil, (2) PET scan with the coil, but omitting the coil in AC (PET/MR scanning conditions), (3) AC of the coil using a CT scan of the same phantom setup and registration via capsules, (4) same setup as 3, but registration was done using UTE images, neglecting the capsules, and (5) registration using the capsules, but the CT was performed with the coil placed flat on the CT table and the outer regions of the coil were cropped. The activity concentrations were then compared to the reference scan. For clinical evaluation of the concept, the presented methods were also evaluated on a patient.
Results: The oil capsules were visible in the MR and CT images and image registration was straightforward. The UTE images show only parts of the coil's plastic housing and image registration was more difficult. The overall loss of true counts due to the presence of the surface coil is 4.7%. However, a spatially dependent analysis shows larger deviation (10%-15% attenuation) of the activity concentration in the top part of the phantom close to the coil. When accounting for the RF coil for PET AC, attenuation due to the RF coil could mostly be corrected. These results of the phantom studies were confirmed by the patient measurements.
Conclusions: Disregarding local coils in PET AC can lead to a bias of the AC PET images that is regional dependent. The closer the analyzed region is located to the coil, the higher the bias. Cod liver oil capsules or the UTE sequence can be used for RF coil position determination. The middle part of the examined RF coil hosting the preamplifiers and electronic components provides the highest attenuating part. Consequently, emphasis should be put on correcting for this portion of the RF coils with the suggested methods.