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Extremity Magnetic Resonance Imaging
To the Editor: In the October 2006 issue of The Journal, an editorial by Dr. J.T. Sharp discussed the issue of magnetic resonance imaging (MRI) using a low-field 0.2 Tesla (T) scanner versus a standard whole-body 1.5 T scanner for imaging rheumatoid arthritis (RA)1. Dr. Sharp acknowledges the significant benefits of a low-field extremity machine over large-bore, whole-body magnets including lower purchase, installation and maintenance costs, the ability to install the machine in a smaller clinical center because such machines do not require extensive shielding, and the ease with which patients can be placed inside the device for imaging without the concern of claustrophobia. We agree with all these benefits of using an extremity MRI machine and have also acknowledged the benefits in a recent publication2. Dr. Sharp also notes the significant weaknesses of low-field MR scanners, the primary one being the compromise in image quality. The low-field scanner that Dr. Sharp refers to in his criticism is a 0.2 T scanner, images of which are shown in Dr. T.S. Chen's article in the same issue3. The "tradeoff" of image quality versus cost and convenience of an extremity scanner is certainly a concern and raises issues such as those addressed by Sharp. However, it is important to recognize that other extremity MRI machines exist in which image quality is not sacrificed. For instance, ONI Medical Systems Inc. (Wilmington, MA, USA) manufactures a higher-field 1.0 T extremity scanner (OrthOne™) that affords the same advantages as those of the 0.2 T machine but without the disadvantage of poor image quality. For instance, this scanner can also be sited in a clinical office, affording easy access to patients and clinicians and offering patients a quiet, comfortable and convenient experience. This scanner is simple to operate and offers robust pulse sequences for contrast-enhanced studies, late-echo imaging and expanded-volume fat-suppressed visualization. The manufacturers of the 1.0 T machine report that the return on investment is typically less than 2 patients per day compared to 6–8 patients per day required to support a whole-body scanner4. The OrthOne™ extremity scanner has been used for research purposes in the investigation of knee osteoarthritis2,5,6. Our group has recently reported that knee cartilage morphometry can be quantified with precision similar to that achieved using a 1.5 T system2. While these studies have focused upon imaging of the knee joint, extremity scanners are also capable of imaging the hand, wrist, elbow, foot, and ankle. Images of a hand affected by RA acquired with our OrthOne™ system are shown in Figure 1: image quality is clearly superior to that obtained using a 0.2 T machine. In other words, the use of an extremity MR scanner such as the 1.0 T system discussed here does not inevitably equate to a loss of image quality. To the contrary, the existence of higher field dedicated extremity scanners affords advantages of economy and accessibility, while providing images of comparable quality to those obtained with larger whole-body clinical scanners operating at high-field strengths.
KAREN BEATTIE, BSc, PhD, Post-Doctoral Fellow, Department of Medicine, McMaster University, 501-25 Charlton Avenue East, Hamilton, Ontario L8N 1Y2, Canada; DEAN INGLIS, PhD, Department of Civil Engineering; JONATHAN D. ADACHI, MD, FRCPC, Department of Medicine; XIAOMING XIE, MSc, Department of Medical Sciences, McMaster University. Address reprint requests to Ms Beattie. E-mail: karen.beattie@camris.ca 2. Inglis D, Pui M, Ioannidis G, et al. Accuracy and test-retest precision of quantitative cartilage morphology on a 1.0 T peripheral magnetic resonance imaging system. Osteoarthritis Cartilage 2007;15:110-5. [MEDLINE] 3. Chen TS, Crues JV III, Ali M, Troum OM. Magnetic resonance imaging is more sensitive than radiographs in detecting change in size of erosions in rheumatoid arthritis. J Rheumatol 2006; 33:1957-67. [MEDLINE] 4. ONI Medical Systems, Inc. Available at: http://www.onicorp.com/radiologists.html (Accessed April 18, 2007). 5. Roemer FW, Guermazi A, Lynch JA, et al. Short tau inversion recovery and proton density-weighted fat suppressed sequences for the evaluation of osteoarthritis of the knee with a 1.0 T dedicated extremity MRI: development of a time-efficient sequence protocol. Eur Radiol 2005;15:978-87. [MEDLINE] 6. Beattie KA, Boulos P, Pui M, et al. Abnormalities identified in the knees of asymptomatic volunteers using peripheral magnetic resonance imaging. Osteoarthritis Cartilage 2005;13:181-6. [MEDLINE]
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