Abstract
BackgroundPET/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.MethodsA 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.ResultsThe 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.ConclusionThe 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.
Highlights
positron emission tomography/magnetic resonance imaging (PET/MRI) phantom studies are challenged by the need of phantomspecific attenuation templates to account for attenuation properties of the phantom material
We investigate if attenuation correction (AC) for PET/MRI phantoms can be performed with standard MR-AC approaches when using an MRI visible polymer as phantom material
The phantom material signal was only partly separated from the liquid compartment using the Dixon water-fat separation, as the Dixon water image still contained signal from the phantom housing (Fig. 2)
Summary
PET/MRI phantom studies are challenged by the need of phantomspecific 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. Several technical and methodological challenges had to be overcome [2,3,4] the most prominent being the use of MR images for the purpose of attenuation correction (AC) of the PET emission data [5]. Several MRI-based AC methods are available, which work sufficiently well for assessing the attenuation properties of a human subject in most standard clinical PET/MRI investigations [6,7,8,9], their accuracy in specific organs such as lung are still a matter of debate [10, 11]. MRI-based AC for hardware components and phantom studies is in general not sufficiently addressed
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