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Shine-Through in PET/MR Imaging: Effects of the Magnetic Field on Positron Range and Subsequent Image Artifacts
Armin Kolb1, Alexander W Sauter2, Lars Eriksson3
1Werner Siemens Imaging Center, Department of Preclinical Imaging and Radiopharmacy, Eberhard Karls University, Tübingen, Germany armin.kolb@med.uni-tuebingen.de.
Unlabelled:
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 shine-through 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 NiSO4 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-with-trachea 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 volume-of-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.
Insights
The magnetic field in simultaneous PET/MR imaging can cause significant shine-through artifacts, especially with high-energy radioisotopes near air cavities. Recognizing and compensating for these artifacts is crucial for accurate medical imaging interpretation and staging.
Area of Science:
- Medical Imaging
- Nuclear Medicine
- Biophysics
Background:
- Simultaneous Positron Emission Tomography/Magnetic Resonance (PET/MR) imaging is an advanced hybrid technique.
- The strong magnetic field in MR scanners influences positron trajectories from PET radioisotopes.
Purpose of the Study:
- To investigate the impact of the MR magnetic field on PET imaging, specifically focusing on "shine-through" artifacts.
- To quantify the relevance and characteristics of these artifacts in preclinical and simulated clinical scenarios.
Main Methods:
- Experiments were conducted using a 3-tesla PET/MR scanner.
- Radioactive sources (paper soaked with radioisotopes) and simulated lesions (in swine lung and larynx) were imaged.
- Shine-through artifact magnitudes were quantified and analyzed in relation to radioisotope type, distance, and anatomical orientation.
Main Results:
- Shine-through artifacts were observed, appearing in locations distant from the actual radioactive source.
- Artifact intensity varied significantly with different radioisotopes, with higher energy isotopes (e.g., 68Ga) producing more pronounced artifacts.
- In simulated lesions, artifact activity reached up to 46% of the true lesion activity, with specific patterns related to the magnetic field alignment.
Conclusions:
- The interaction between the MR magnetic field and positron emission can lead to substantial artifacts in PET/MR imaging.
- These artifacts are particularly problematic when imaging near air cavities or using high-energy radioisotopes.
- Accurate interpretation and staging require awareness of these magnetic field-induced artifacts and the development of mitigation strategies.
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