Phantom fluids: Finding a solution for PET/MR

Steady advancements in simultaneous PET/MR technology make the need for new phantom designs more salient, but the challenge is tricky due to the physics involved in MR imaging. While PET research usually involves watery models, glycols in combination with precise adjustments in tracer composition may be the ideal solution, according to a study published June 21 in the Journal of Nuclear Medicine.

Susanne Ziegler, from the Institute of Medical Physics at the University of Erlangen-Nuremberg in Erlangen, Germany, and colleagues tested various fluids including demineralized water, water with increased electrical conductivity, a water-oil emulsion and monoethylene and triethylene glycol. MR and PET measurements were taken with F-13 FDG. To improve MR signal homogeneity, researchers also proposed an alternative tracer, a modified F-18 Fluoride Kryptofix 222 complex, and evaluated it for its dissolution properties in pure oil.

“In both imaging modalities, phantoms usually consist of plastic containers filled with signal-providing liquid,” the researchers wrote. “In PET imaging, pure water is used in phantom inserts and in the surrounding background fluid because it can dissolve the most commonly used radioactive tracer, F-18 FDG, well. However, water may induce strong artifacts in MR imaging when scanning large phantoms at a high magnetic field strength (above 1.5 T).”

Inhomogeneous radiofrequency excitation is the main culprit when it comes to these MR-related artifacts, which can flood imaging of water and other liquids of high relative permittivity.

All fluids investigated were found to dissolve evenly in F-18 FDG and significantly better than in pure water, considered an increase in excitation flip angle in the phantom by a factor of 2.0. In situations where F-18 FDG is the ideal tracer, the fluid of choice for PET/MR was triethylene glycol, for an increased flip angle by a factor of 1.13. The modified F-18 Fluoride Kryptofix 222 complex was successfully dissolved in pure oil, but it was found to be lacking due to its tendency to settle to the bottom of the phantom during measurements.

“Compared to water, both triethylene glycol and monoethylene glycol improve MR homogeneity notably and therefore enable phantom imaging, even with RF intensive MR sequences in the given phantom and in phantoms of comparable size,” wrote the authors. “The emulsion was superior in terms of homogeneity of RF excitation in MR imaging; however, its preparation is time consuming and cumbersome. Additionally, air bubbles may appear within the emulsion and impair potential quantitative phantom measurements. A different emulsifier composition might exist that yields a more suitable emulsion with better stability and homogeneity.”

One complication is the need for additional attenuation correction. PET/MR only differentiates between fat and water attenuation values. Operators will have to adjust attenuation values manually for glycols and the plastic and glass materials of phantom models to achieve an optimal PET scan.