PET attenuation correction for flexible MRI surface coils in hybrid PET/MRI using a 3D depth camera

Abstract

PET attenuation correction for flexible MRI radio frequency surface coils in hybrid PET/MRI is still a challenging task, as position and shape of these coils conform to large inter-patient variabilities. The purpose of this feasibility study is to develop a novel method for the incorporation of attenuation information about flexible surface coils in PET reconstruction using the Microsoft Kinect V2 depth camera. The depth information is used to determine a dense point cloud of the coil's surface representing the shape of the coil. From a CT template-acquired once in advance-surface information of the coil is extracted likewise and converted into a point cloud. The two point clouds are then registered using a combination of an iterative-closest-point (ICP) method and a partially rigid registration step. Using the transformation derived through the point clouds, the CT template is warped and thereby adapted to the PET/MRI scan setup. The transformed CT template is converted into an attenuation map from Hounsfield units into linear attenuation coefficients. The resulting fitted attenuation map is then integrated into the MRI-based patient-specific DIXON-based attenuation map of the actual PET/MRI scan. A reconstruction of phantom PET data acquired with the coil present in the field-of-view (FoV), but without the corresponding coil attenuation map, shows large artifacts in regions close to the coil. The overall count loss is determined to be around 13% compared to a PET scan without the coil present in the FoV. A reconstruction using the new μ-map resulted in strongly reduced artifacts as well as increased overall PET intensities with a remaining relative difference of about 1% to a PET scan without the coil in the FoV.

Applications of PET‐MRI in musculoskeletal disease

“…Large flexible arrays are used for body or pelvis exams as well as for efficient whole‐body scanning while small flexible extremity coils are optimal for MRI and PET‐MRI of the joints. New methods have shown the feasibility of using MR visible markers, 3D cameras, or uTE sequences to detect the position of flexible MRI surface coils, allowing a template CT‐based attenuation map of the coil to be localized and registered in the MRAC map. Furthermore, new coil designs specifically for PET‐MRI systems aim to reduce PET attenuation from MR hardware while simultaneously improving MR sensitivity …”

Assessment of a novel 32-channel phased array for cardiovascular hybrid PET/MRI imaging: MRI performance

“…Meanwhile, developing a dedicated PET/MRI phased array with a higher number of channels is technically challenging, leading researchers to focus on the approach of correcting the attenuation of the currently available MRI arrays (Eldib et al 2015;Ferguson et al 2014;Frohwein et al 2018;Kartmann et al 2013). The common approach to correct for attenuation of an RF phased array is to generate a hardware AC map, also known as a μ-map, produced from a CT scan at a specific tube voltage (Carney et al 2006;Patrick et al 2017).…”

Evaluation of a Dedicated Radiofrequency Carotid PET/MRI Coil

“…A phantom study was set up to determine whether the PET/MRI compatible prototype reduced PET attenuation. A cylindrical phantom, 14 cm in diameter and 20 cm in length, was filled with an aqueous solution of 18 F-fluorodeoxyglucose ( 18 F-FDG; 117.0 MBq). Three measurements were performed: (1) no surface coil (i.e., built-in body coil), ( 2) with the PET/MRI compatible prototype, and (3) with the original MRI-only coil.…”