Characteristics and quality assurance of a dedicated open 0.23 T MRI for radiation therapy simulation Dennis Mah a) Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, Pennsylvania19111 Michael Steckner Philips Medical Systems, Cleveland, Ohio 44143 Elizabeth Palacio, Raj Mitra, Theresa Richardson, and Gerald E. Hanks Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, Pennsylvania19111 Received 27 December 2001; accepted for publication 8 August 2002; published 23 October 2002 A commercially available open MRI unit is under routine use for radiation therapy simulation. The effects of a gradient distortion correction GDC program used to post process the images were assessed by comparison with the known geometry of a phantom. The GDC reduced the magnitude of the distortions at the periphery of the axial images from 12 mm to 2 mm horizontally along the central axis and distortions exceeding 20 mm were reduced to as little as 2 mm at the image periphery. Coronal and sagittal scans produced similar results. Coalescing these data into distortion as a function of radial distance, we found that for radial distances of 10 cm, the distortion after GDC was 2 mm and for radial distances up to 20 cm, the distortion was 5 mm. The dosimetric errors resulting from homogeneous dose calculations with this level of distortion of the external contour is 2%. A set of triangulation lasers has been added to establish a virtual isocenter for convenient setup and marking of patients and phantoms. Repeated measurements of geometric phantoms over several months showed variations in position between the virtual isocenter and the magnetic isocenter were constrained to 2 mm. Additionally, the interscan variations of 12 ran- domly selected points in space defined by a rectangular grid phantom was found to be within the intraobserver error of 1 mm in the coronal, sagittal, and transverse planes. Thus, the open MRI has sufficient geometric accuracy for most radiation therapy planning and is temporally stable. © 2002 American Association of Physicists in Medicine. DOI: 10.1118/1.1513991 Key words: quality assurance, open MRI, treatment planning, MRI simulation, distortions INTRODUCTION For radiation treatments to be effective, the target volume must receive the prescribed dose while keeping the dose to surrounding tissues below their tolerance. Treatment plan- ning and delivery consists of multiple steps 1 each contribut- ing to the overall uncertainty. These uncertainties include target definition, organ motion, setup error, accelerator out- put, dose calculation, and biological response. Because of its greater soft tissue contrast, MRI has been shown to provide more consistent target delineation than CT for a variety of sites 2–4 and MRI based planning has been shown to reduce interphysician variation and produce a more accurate plan. 5 MRI based planning, usually via fusion of the MRI to CT is used for treatment planning of cranial sites 2,6–9 and recently has also been investigated 5 and routinely implemented for the prostate. 10 However, the fusion process itself requires additional time and effort and may add uncertainty from the imperfect registration of the two image sets. 11 CT and MRI provide largely redundant anatomical information, but due to concerns about MRI 12 image distortion, both scans are nor- mally acquired for treatment planning. We and other investi- gators are attempting to determine if just an MRI scan is sufficient for treatment planning, i.e., if an MRI simulator can be developed. 12,13 For MRI to replace CT there are a variety of challenges that must be overcome. Some of these were initially de- scribed by Fraass et al., 12 and include production of digitally reconstructed radiographs DRRs, development of appropri- ate MRI treatment planning software with inhomogeneity corrections, and correction or at least quantification of dis- tortions due to chemical shift, susceptibility, and field gradi- ents. Many of these challenges are the subject of active re- search. For instance, Ramsey et al. 14 have considered the possibility of producing DRRs using a T1 weighted pulse sequence. Another approach is to contour the bones on the cross sectional slices to produce a DRR. 15 Recently, Frans- son et al. 16 have suggested using bulk inhomogeneities e.g., setting lung to a density of 0.3 g/cc for dose calculation in MRI data sets. Distortions in MRI arise from both the MRI and from patient induced effects. Patient induced effects include sus- ceptibility and chemical shift distortions, which for low field i.e., 0.2 T magnets are limited to 1 pixel, 17 implying that the distortions of internal anatomy for these MRIs are clinically acceptable. Distortions arising from the magnet it- self fall into two categories: field inhomogeneity and nonlin- ear gradients. Since MRI images derive spatial information from the spatially varying frequencies of the signals, any distortion in the main field ( B 0 ) maps into spatial uncer- 2541 2541 Med. Phys. 29 „11…, November 2002 0094-2405Õ2002Õ29„11…Õ2541Õ7Õ$19.00 © 2002 Am. Assoc. Phys. Med.