Shine-Through in PET/MR Imaging: Effects of the Magnetic Field on Positron Range and Subsequent Image Artifacts

Abstract

Simultaneous PET/MR imaging is an emerging hybrid modality for clinical and preclinical imaging. The static magnetic field of the MR imaging device affects the trajectory of the positrons emitted by the PET radioisotopes. This effect translates into an improvement of the spatial resolution in transaxial images. However, because of the elongation of the positron range distribution along the magnetic field, the axial resolution worsens and shine-through artifacts may appear. These artifacts can lead to misinterpretation and overstaging. The aim of this work was to study the relevance of this effect. Methods: Measurements were performed in a 3-tesla PET/ MR scanner. A 1-cm 2 piece of paper, soaked with a radioisotope and placed in air, was scanned, and the magnitude of the shinethrough was quantified from the PET images for various radioisotopes. Additionally, PET/MR and PET/CT images of the lungs and the larynx with trachea of a deceased swine were obtained after injecting a mixture of NiSO 4 and 68 Ga to simulate hot tumor lesions. Results: For the radioactive paper, shine-through artifacts appeared in the location of the acrylic glass backplane, located 3 cm from the source in the axial direction. The ratio between the activity of the shine-through and the activity reconstructed in the original location ranged from 0.9 ( 18 F) to 5.7 ( 68 Ga). For the larynx-withtrachea images, the magnitude of the artifacts depended on the organ orientation with respect to the magnetic field. The shine-through activity could reach 46% of the reconstructed activity (larynx lesion). The lesion within the trachea produced 2 artifacts, symmetrically aligned with the magnetic field and characterized by artifact-to-lesion volumeof-interest ratios ranging from 21% to 30%. Conclusion: In simultaneous PET/MR imaging, the effect of the magnetic field on positrons may cause severe artifacts in the PET image when the lesions are close to air cavities and high-energy radioisotopes are used. For accurate staging and interpretation, this effect needs to be recognized and adequate compensation techniques should be developed. PET/ MR imaging is a powerful technology that is now expanding worldwide (1); integrated PET/MR imaging systems are already commercially available for clinical applications (2,3). Special attention is thus being paid to the ability of simultaneous PET/MR imaging to provide quantitative information and artifact-free images. A particular phenomenon of this integrated technology is the effect of the static magnetic field on positrons. Because of the Lorentz force, positrons follow helical paths along the magnetic field lines. Consequently, the positron range distribution narrows in the plane perpendicular to the magnetic field, thus improving the transaxial resolution (4-9). On the other hand, little attention has been devoted to the behavior of positrons in the axial direction. The positron range distribution elongates along the magnetic field, and the axial resolution subsequently degrades (9-11). The positron range can rea...

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“…Alternatively, Abdul-Fatah and colleagues [46] identified shine-trough artifacts when imaging thyroid tumors in the trachea with I-124 PET/CT due to its large PR. This effect may be even more important in PET/MR, when the high magnetic field in the axial axis creates an elongation of the PR in that direction [47,48], as recently discussed by Kolb and colleagues [49]. To the best of our knowledge, these issues are still not solved.…”

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“…As reported by Kolb et al [7], the static magnetic field (B0) of the MRI scanner affects the trajectory of positrons by reducing the positron range in the plane perpendicular to B0 (axial plane) and by elongating the positron range along the direction of B0 (craniocaudal direction). In PET/MRI, the shine-through artifact can be seen with low-energy PET radionuclides only if the area of high tracer uptake is in the immediate vicinity of an air cavity, a situation typically observed in the head and neck region.…”

“…A recently described effect observed in simultaneous PET/ MRI systems is the shine-through artifact [7]. As reported by Kolb et al [7], the static magnetic field (B0) of the MRI scanner affects the trajectory of positrons by reducing the positron range in the plane perpendicular to B0 (axial plane) and by elongating the positron range along the direction of B0 (craniocaudal direction).…”

“…It results in an elongated shape of tracer uptake on coronal and sagittal images and in an apparently increased tracer concentration diametrically opposed to the location of the actual lesion. The shine-through artifact can potentially lead to overestimation of tumor involvement in PET/MRI [7]. Adequate compensation techniques for this artifact are not yet available.…”

“…Monte Carlo simulation studies demonstrated that the use of a magnetic field collinear with a PET scanner's axis improves the transaxial spatial resolution without impinging on the axial spatial resolution [5]. For instance, Wirrwar et al [6] reported foreseen improvements in spatial resolution for high-energy positron-emitting tracers ranging between 18.5% (2.73 mm instead of 3.35 mm) for 68 Ga and 26.8% (2.68 mm instead of 3.66 mm) for 82 Rb at a field strength of 7 T. Another effect, which has been characterized only very recently, is the degradation of the axial spatial resolution owing to the elongation of the positron range distribution along the magnetic field B0 or the so-called shine-through artifact [7]. It was reported that this effect might cause severe artifacts in PET images for malignant lesions located close to air cavities, particularly when using high-energy positron-emitting radionuclides (see the section "Pitfalls and Artifacts").…”