Quantitative analysis of the effect of the magnetic field generated by a PET/MR scanner on positron range

Image quality in positron emission tomography (PET) is influenced by positron range. In this work, the effect of the magnetic field of a PET/MR Siemens Biograph mMR 3T on the quality of PET images was studied.

Experimental measurements were conducted using¹⁸F and⁶⁸Ga-filled phantoms to quantify image uniformity, recovery coefficients (RCs), spill-over ratios and percent contrast for spherical lesions. Tissue-equivalent phantoms (lung inhale and exhale, adipose, water, trabecular and cortical bone) were used together with a line source to quantify the impact of the magnetic field on the reconstructed PET images. A comparative analysis was made with images obtained with a PET/CT Biograph Vision 600, using the same radionuclides and phantoms.

Higher RCs values were obtained when the image quality phantom was filled with⁶⁸Ga and scanned with the PET/MR system compared to those obtained with the PET/CT scanner. Hot spheres in the lesion detectability phantom, appear contracted in the transverse direction in the PET/MR system, an effect more evident for⁶⁸Ga compared to¹⁸F, but no elongation in the direction parallel to the magnetic field was observed. In the PET/CT scanner, radial profiles taken from axial slices of line sources, show longer distribution tails extending beyond 20 mm when filled with⁶⁸Ga and placed inside lung-inhale tissue. In the PET/MR scanner the radial profiles of all materials collapsed into a single distribution with tails extending no more than 10 mm in the direction perpendicular to the magnetic field.

Positron range depends on positron energy and material density in which they traverse. The results show an evident improvement in image quality in the transaxial direction only, particularly in phantoms filled with⁶⁸Ga when using a PET/MR system as opposed to images acquired in the PET/CT system due to the presence of the magnetic field.