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Copenhagen University Hospital at Herlev
Herlev DK-2730,
Denmark
We have with great interest read the article by Magee et al. [1], in which they compare high-field-strength versus low-field-strength MRI. Forty patients (of 153 invited patients) had complete shoulder imaging on an open 0.2-T system and limited imaging on a 1.5-T system. The authors conclude that spatial and contrast resolution of the high-field-strength unit may result in more accurate interpretation of full-thickness supraspinatus tendon tears and labral tears in some patients than would be possible with low-field-strength units. It is of great importance that comparative studies like this are undertaken. As the authors state, the study is limited by the fact that all the musculoskeletal radiologists were from the same institution and performed consensus interpretations on retrospective review.
The current 1.5-T MRI units available from different vendors do not differ much with regard to image quality; the competition is too high to allow any outliers, although details may differ. The same is not the case with the current open mid- and low-field-strength MRI units. That is an area with differences. The knowledge acquired on closed horizontal high- and mid-field-strength systems cannot be copied to open vertical low- and mid-field-strength systems. The difference is not only a question of field strength. Other important factors include the construction of the magnet, the consequences of field orientation, the design of the coils, and the development of sequences. A special benefit of an open unit for MRI of the shoulder is that the shoulder may be positioned in the magnet isocenter. Finally, the experience of the technicians may influence the result.
For the last 2 years, we have worked with closed horizontal high-field-strength units and open vertical mid- and low-field-strength units. Our experience is different from that of Magee et al. [1]. The diagnostic quality of shoulder MR images has been clearly superior for the open units (Figs. 1A, 1B and 2A, 2B). They have shown better spatial and contrast resolution. Lower-field-strength machines also add fewer artifacts and smaller chemical shift artifacts, which are of importance in musculoskeletal MRI. Today, we perform shoulder examinations only in our open units and have stopped off-center imaging of shoulders in the closed high-field-strength unit simply because of image quality considerations. Prospective studies [2, 3] have shown that open low-field-strength units can achieve diagnostic accuracy comparable to that achieved with a closed high-field-strength scanner.
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In our opinion, the conclusion of Magee et al. [1] only applies to the open 0.2-T unit they used, not to all other open MRI units as they stated. General Electric Medical Systems withdrew the open 0.2-T system from the Danish market several years ago for a reason (Petersen N, letter addressed to the Central Buyers of Copenhagen County, November 1999). Conclusions based on one open MRI unit may not apply to all open units.
References
Neuroimaging Institute
Melbourne, FL 32901
We appreciate the comments made by Dr. Thomsen and colleagues pertaining to our article "Comparison of high-field-strength versus low-field-strength MRI of the shoulder" [1]. We take exception to the following issues:
They state that current 1.5-T MRI units available from different vendors do not differ very much with regard to image quality. We have worked with 1.5-T MRI units from three vendors and found considerable differences in image quality between them.
They state that there are considerable differences in image quality for different mid- and low-field-strength MRI systems. This is probably true. We simply compared two scanners from the same vendor (GE Health-care)—a high-field-strength and a low-field-strength MR scanner—on the same patients. We found that some small supraspinatus tendon tears and labral tears were seen on the high-field-strength unit that were not seen on the low-field-strength unit. We had the same experience on a 0.2-T Siemens unit as we had on our 0.2-T GE unit.
Thomsen et al. did not compare high-versus low-field-strength units on the same patients the way we did. They simply state "the diagnostic quality of shoulder MR images has been clearly superior for the open units." We do not know what the open units are being compared with in this case. They may be missing small supraspinatus tendon tears and labral tears on the low-field-strength unit without knowing it.
Thomsen et al. provide images of large supraspinatus tendon tears (i.e., 4-5 cm of retraction) to support their position. Tears of this size were all clearly visible on our 0.2-T unit. The tears not seen on our 0.2-T unit were all small ones.
Thomsen et al. state that lower field strength adds the advantage of fewer artifacts and smaller chemical shift artifacts. The only time we have experienced artifacts on high-field-strength units is when metal is present.
We retain the same conclusion as in our original article: The superior spatial and contrast resolution of the high-field-strength units may result in more accurate interpretation of full-thickness supraspinatus tendon tears and labral tears in some patients than would be possible with low-field-strength units. Changes in reviewer interpretation may have an effect on clinical treatment.
References
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