Custom Phantoms for MRI Geometric Accuracy Quantification Using CT-Based Image Matching In Brain Stereotactic Radiosurgery
Abstract
Purpose
Stereotactic radiosurgery (SRS) requires submillimeter geometric accuracy and commonly relies on combined computed tomography (CT) and magnetic resonance imaging (MRI). However, MRI is susceptible to geometric distortions that increase with distance from the scanner isocenter, potentially compromising target localization and treatment accuracy. This study assesses the MR and CT properties of contrast agents for three potential in-house developed phantoms designed to quantify MRI geometric distortions relevant to brain SRS.
Methods
Three cylindrical phantom prototypes were developed: a LEGO-based phantom (19.2 cm height, 9.54 cm radius), an in-house CNC-machined acrylic phantom (20 cm height, 10 cm radius), and a 3D-printed, filament-based (PETG) lattice phantom (22 cm height, 22 cm radius). MR contrast agent materials were selected for their moldability into LEGO wells, low CT density, and high MRI signal intensity. The materials included mineral oil-ethylene/propylene/styrene copolymer (MO-gelatin), paraffin-mineral oil (PMO) mixtures (1:1 and 1:4), paraffin, and silicone rubber. Materials were assessed for long-term stability, physical consistency, and CT imaging performance using a GE Healthcare Optima CT scanner.
Results
MO-gelatin, paraffin, and silicone demonstrated stable physical properties over extended periods, whereas both PMO mixtures exhibited surface sweating and instability following changes in ambient conditions. Mean CT Hounsfield units (HU) values were -197.75 ± 1.29, -166.57 ± 0.88, -167.88 ± 0.93, -132.36 ± 1.21, and 456.67 ± 17.91 for MO-gelatin, PMO (1:1), PMO (1:4), paraffin, and silicone, respectively. Imaging within LEGO structures resulted in reduced HU values for MO-gelatin, PMO (1:4), and silicone, suggesting container-dependent density effects. In addition, the 3D-printed phantom design was finalized for fabrication and construction of the acrylic phantom is currently underway.
Conclusion
These results demonstrate the feasibility of low-cost, in-house built phantoms for MRI geometric distortion assessment in brain SRS, supporting future MRI-based distortion measurements and offering a customizable alternative to commercial solutions.