Image distortion in MRI-based polymer gel dosimetry of Gamma Knife stereotactic radiosurgery systems

• Published in: Medical physics (Lancaster) 2002-05, Vol.29 (5), p.797-802
• Main Authors: Watanabe, Yoichi, Perera, Gerard M., Mooij, Robertus B.
• Format: Article
• Language: English
• Subjects: absorbed dose; BANG‐3 polymer gel; biomedical MRI; Biophysical Phenomena; Biophysics; Clinical applications; Computed radiography; Computed tomography; computerised tomography; dosimetry; Dosimetry/exposure assessment; Encoding; Gamma Knife; Gamma ray effects; Gels; Gels and sols; Humans; image coding; Magnetic Resonance Imaging - statistics & numerical data; magnetic susceptibility; Medical image distortion; medical image processing; Medical imaging; Medical magnetic resonance imaging; MRI; Phantoms, Imaging; Polymers; Quality assurance equipment; radiation therapy; Radiometry - methods; Radiometry - statistics & numerical data; Radiosurgery; Radiosurgery - statistics & numerical data; Radiotherapy Planning, Computer-Assisted - statistics & numerical data; surgery
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.1470204

We have studied effects of MR (magnetic resonance) image distortion on polymer gel dosimetry of Gamma Knife stereotactic radiosurgery systems. MR images of BANG polymer gel phantoms were acquired by using a Hahn spin-echo sequence and a fast 3D imaging GRASS sequence. Image artifacts were studied by varying the directions of frequency encoding and the receiver bandwidth. The phantoms were also CT (computed tomography) scanned. The studies showed that the measured dose distributions are shifted by 1.8±0.5  mm (2 pixels) in the frequency encoding direction. The magnitude of the shift is inversely proportional to the receiver bandwidth in agreement with theory. Comparison of MRI with CT showed the same image shift. We concluded that the discrepancy is caused by MR image distortion due to a difference in susceptibility effects between the phantom and the fiducial markers of the Leksell localization box.