Systematic Evaluation of Phantom Fluids for Simultaneous PET/MR Hybrid Imaging

Abstract

With the recent advent of integrated PET/MR hybrid systems, the need for simultaneous PET and MR phantom measurements arises. Phantom fluids that are used in stand-alone MR systems, especially in larger phantoms and at a high magnetic field strength, are not necessarily applicable in PET imaging and vice versa. In this study, different approaches to fluid selection were considered and systematically evaluated with respect to their usability for simultaneous PET/MR phantom imaging. Methods: Demineralized water, water with increased electrical conductivity, a water-oil emulsion, and monoethylene and triethylene glycol were investigated in MR and PET measurements using the most common PET tracer 18 F-FDG. As an alternative to 18 F-FDG, a modified PET tracer ( 18 F-fluoride Kryptofix 222 complex) was investigated toward its ability to dissolve in pure oil, which provides good signal homogeneity in MR imaging. Measurements were performed on a 3.0 T integrated PET/ MR whole-body system using a National Electrical Manufacturers Association quality-standard phantom. Results: All tested fluids dissolved the radiotracer 18 F-FDG homogeneously. Regarding their suitability for MR at 3.0 T, all fluids significantly improved the homogeneity compared to pure water (increase of excitation flip angle within the tested phantom by a factor of 2.0). When the use of 18 F-FDG was preferred, triethylene glycol provided the best compromise (flip angle increase by a factor of 1.13). The potential alternative tracer, 18 F-fluoride Kryptofix 222 complex, dissolved in pure oil; however, it is not optimal in its tested composition because it accumulates at the bottom of the phantom during the time of measurement. Conclusion: This study provides a systematic approach toward phantom fluid selection for imaging a given quality-standard body phantom-and phantoms of comparable size-at 3.0 T. For simultaneous PET/MR scans using the standard tracer 18 F-FDG, an alternative fluid to water and oil is proposed that serves as a viable option for both imaging modalities. Nevertheless, when water is preferred, ways to improve MR image homogeneity are presented. The tested alternative PET tracer enables the use of pure oil in combined scans, but the tracer composition needs to be optimized for phantom measurement applications.

Simultaneous PET/MR imaging: MR-based attenuation correction of local radiofrequency surface coils

“…29 This might hamper the reconstruction of MRbased attenuation maps. For this study, consequently a phantom consisting of two water filled phantom bottles in a stable and reproducible configuration was constructed in order to provide a reasonable sized geometry for body RF surface coil evaluation and that provides homogenous images in PET, MR, and CT imaging.…”

Attenuation Correction for Magnetic Resonance Coils in Combined PET/MR Imaging

“…An important aspect in using phantom measurements successfully is to insure that the attenuation map for the phantom contains the correct attenuation coefficients and that it is well aligned with the emission. This issue is relevant for PET/MRIs where the system generated attenuation maps are not designed to work for phantoms 42 . Data visualization in the phantom scans is often performed either by displaying a difference image or by plotting the mean activity within a ROI over all planes in the phantom, which provides a volumetric evaluation of the attenuation profile rather than just one plane.…”

Effect of Attenuation Correction on Regional Quantification Between PET/MR and PET/CT: A Multicenter Study Using a 3-Dimensional Brain Phantom

“…The Ingenuity TF m-map had the best quality, despite the fact that the polymer scalp remained invisible. Phantom materials have been considered challenging for PET/MR (13,14). Therefore, comparison of MR-based m-maps is not likely to be clinically relevant.…”

“…Measured Hounsfield units (HUs) are converted to m-values by simple bilinear scaling (10-12). Therefore, a multicenter evaluation between PET/MR and PET/CT systems with CTAC and an anthropomorphic phantom would be highly desirable for determining image quantification in a controlled manner.Previous PET/MR investigations have been conducted with National Electrical Manufacturers Association whole-body or Hoffman phantoms, which do not model attenuation realistically and have not specifically addressed brain PET/MR (5,13,14). However, an anatomic brain phantom that has recently been developed models gray matter uptake and has a realistic head contour, including air spaces and the attenuation effect of bone (15,16).…”