Standard MRI-based attenuation correction for PET/MRI phantoms: a novel concept using MRI-visible polymer

Abstract

Background PET/MRI phantom studies are challenged by the need of phantom-specific attenuation templates to account for attenuation properties of the phantom material. We present a PET/MRI phantom built from MRI-visible material for which attenuation correction (AC) can be performed using the standard MRI-based AC. Methods A water-fillable phantom was 3D-printed with a commercially available MRI-visible polymer. The phantom had a cylindrical shape and the fillable compartment consisted of a homogeneous region and a region containing solid rods of different diameters. The phantom was filled with a solution of water and [18F]FDG. A 30 min PET/MRI acquisition including the standard Dixon-based MR-AC method was performed. In addition, a CT scan of the phantom was acquired on a PET/CT system. From the Dixon in-phase, opposed-phase and fat images, a phantom-specific AC map (Phantom MR-AC) was produced by separating the phantom material from the water compartment using a thresholding-based method and assigning fixed attenuation coefficients to the individual compartments. The PET data was reconstructed using the Phantom MR-AC, the original Dixon MR-AC, and an MR-AC just containing the water compartment (NoWall-AC) to estimate the error of ignoring the phantom walls. CT-based AC was employed as the reference standard. Average %-differences in measured activity between the CT corrected PET and the PET corrected with the other AC methods were calculated. Results The phantom housing and the liquid compartment were both visible and distinguishable from each other in the Dixon images and allowed the segmentation of a phantom-specific MR-based AC. Compared to the CT-AC PET, average differences in measured activity in the whole water compartment in the phantom of −0.3%, 9.4%, and −24.1% were found for Dixon phantom MR-AC, MR-AC, and NoWall-AC based PET, respectively. Average differences near the phantom wall in the homogeneous region were −0.3%, 6.6%, and −34.3%, respectively. Around the rods, activity differed from the CT-AC PET by 0.7%, 8.9%, and −45.5%, respectively. Conclusion The presented phantom material is visible using standard MR sequences, and thus, supports the use of standard, phantom-independent MR measurements for MR-AC in PET/MRI phantom studies.

Influences on PET Quantification and Interpretation

“…Systematic underestimation of SUV in bone tissue or lesions close to the bone of >10% compared to PET/CT has been described [ 60 , 62 ], whereas SUV differences were <5% in other normal tissues [ 60 ]. It should also be noted that cross-calibration of PET/MRI scanners is generally hampered by water-filled phantoms, which are commonly used for SUV calibration in PET/CT and can produce substantial artifacts in the MR-AC sequence with corresponding SUV errors [ 63 , 64 ]. In such phantoms, inter-vendor differences in MR-AC maps [ 61 ] can lead to SUV differences in phantom spheres of 10–20% [ 58 ].…”

Multimodal phantoms for clinical PET/MRI

“…In their systematic review to identify trends in the use of 3D printing in the development of medical imaging phantoms, Fillipou and Tsoumpas [ 32 ] found that radiological properties are not commonly tested by manufacturers for 3D printing. However, Rausch et al [ 33 ] have recently created a phantom using 3D printed polymer RGD252 (Stratasys), previously demonstrated as MRI visible [ 34 ], that is visible in both modalities for simultaneous PET/MRI acquisitions.…”

“…Rausch et al [ 33 ] demonstrated the first PET/MRI phantom in-house-made with MRI visible housing. The cylindrical phantom had rod features on the bottom lid, whilst keeping the top section of the phantom homogenous.…”

“…The design and verification of a phantom require a significant time commitment, and so distribution of this information is valuable to the imaging community to accelerate phantom development. Most recently, an MRI visible polymer has been demonstrated as suitable for PET/MRI use [ 33 ], providing a potential solution to MRI-based attenuation correction issues in phantom studies.…”

A Path to Qualification of PET/MRI Scanners for Multicenter Brain Imaging Studies: Evaluation of MRI-Based Attenuation Correction Methods Using a Patient Phantom

“…While substantial progress has been made in manufacturing phantoms capable of mimicking both electron density and MRI contrast characteristics of human tissues (7)(8)(9), no such phantom that could be used to assess the performance of multiple MR-based AC techniques is yet widely available. Additionally, water-filled phantoms, a mainstay in the accreditation of PET scanners (10), produce resonance artifacts in MR images (11).…”