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Construction of a Phantom for Image Quality Evaluation in PET/MRI System
Yasuyuki Takahashi1, Ayaka Nemoto2, Shota Hosokawa1
1Department of Radiation Science, Hirosaki University Graduate School of Health Sciences, Hirosaki, Japan.
Abstract:
Background: There is no phantom for image quality test in magnetic resonance imaging combined with positron emission tomography systems (PET/MRI systems). In MRI, radioactive water phantom containing 2-deoxy-2-[F-18] fluoro-D-glucose (18F-FDG) cannot be used due to the dielectric effect. Even for phantoms filled with MR-available solutions, the source current of the RF coil is strongly disturbed as the diameter of the phantom increases. Stable MR images require proper phantom size and solution selection. Previous reports have not provided these details. Other than that, few existing phantoms evaluate negative signals such as N-13 ammonia (13N-NH3). We created a phantom for PET/MRI system for image quality test. Methods: The phantom for the PET/MRI system was assembled in two portions. One portion is a signal part containing 18F-FDG radioactive water. The other portion is filled with polyvinyl alcohol glue to construct MRI image to generate µ-map. The glue part is allowed to rewrite the table position overlaps with the first layer, and attenuation correction is performed. Signals are set as positive (4 times and twice higher than background radioactivity) and negative (no radioactivity) columns with different sizes (15 mm φ and 7 mm φ). The PET images with X-ray computed tomography-based attenuation correction (CT-AC) and MRI-AC were evaluated by %-contrasts, variation and uniformity. Results: The %-contrasts of the positive shallow signals with PET/magnetic resonance (MR) and PET/CT were 41.8% and 45.4%, respectively. And it of the positive deep signals with PET/MR and PET/CT were 40.7% and 44.9%. On the other hand, the %-contrasts of the negative shallow signals with PET/MR and PET/CT were 62.3% and 65.6%, respectively. And it of the negative deep signals with PET/MR and PET/CT were 60.7% and 63.7%. Moreover, the % Nj index of uniformity was 2.0% on PET/MRI images and 0.34% on PET/CT images. For negative signals that assume a decrease in myocardial blood flow, The image quality of MR-AC was almost the same as that of CT-AC. Consistency between the images after CT-AC and MR-AC correction were confirmed, and in particular, a stable MR-AC µ-map was obtained in the phantom study. Conclusion: The suggested prototype phantom for generating µ-map is reasonable and useful for evaluating PET/MRI image quality, based on the present standard.
Insights
A new phantom was developed for positron emission tomography/magnetic resonance imaging (PET/MRI) systems to test image quality. This phantom enables accurate evaluation of both positive and negative signals, crucial for advanced medical imaging diagnostics.
Area of Science:
- Medical Imaging Physics
- Radiopharmaceutical Imaging
- Magnetic Resonance Imaging
Background:
- Existing phantoms for PET/MRI systems are insufficient for comprehensive image quality testing.
- Challenges include dielectric effects with radioactive water (e.g., 2-deoxy-2-[F-18] fluoro-D-glucose) and RF coil disturbances with larger phantom sizes.
- Limited evaluation of negative signals (e.g., N-13 ammonia) in current PET/MRI phantoms.
Purpose of the Study:
- To develop and validate a novel phantom for PET/MRI image quality assessment.
- To evaluate the phantom's capability in generating accurate µ-maps for attenuation correction.
- To assess the performance of the phantom with both positive and negative signal contrasts.
Main Methods:
- A two-part phantom was constructed: one with 2-deoxy-2-[F-18] fluoro-D-glucose (18F-FDG) and another with polyvinyl alcohol glue for MRI µ-map generation.
- Positive (2x, 4x background) and negative (0x background) signals were incorporated in varying sizes (15 mm and 7 mm diameter).
- PET images were acquired using X-ray computed tomography-based attenuation correction (CT-AC) and MRI-based attenuation correction (MR-AC) for comparison.
Main Results:
- The phantom demonstrated effective evaluation of both positive and negative signals, with comparable contrast percentages between PET/MR and PET/CT.
- Negative signal contrast percentages ranged from 60.7% to 65.6%, indicating good performance for simulating reduced radiotracer uptake.
- High uniformity (% Nj index of 2.0% for PET/MRI) and stable MR-AC µ-maps were achieved, showing consistency with CT-AC.
Conclusions:
- The developed prototype phantom is a reasonable and effective tool for evaluating PET/MRI image quality.
- It facilitates accurate µ-map generation and assessment of both positive and negative signals, meeting current imaging standards.
- The phantom's design supports reliable quality control for advanced PET/MRI applications.
