AJR 2004; 182:1601-1602
© American Roentgen Ray Society
Open Low-Field-Strength MRI of the Shoulder Is Not So Bad
Henrik S. Thomsen,
Lone Larsen,
Elizaveta Chabanova and
Jakob M. Moller
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
(General Electric Medical Systems) and limited imaging on a 1.5-T system
(General Electric Medical Systems). 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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Fig. 1A. —63-year-old man with supraspinatus rupture. Images were
obtained on low-field-strength 0.23-T open MRI scanner. Examination was
performed with shoulder in isocenter. Turbo spin-echo comparable image (TR/TE,
3,000/85) (A) and inversion recovery fast-field echo image (2,000/22;
inversion time, 80 msec) (B) show supraspinatus rupture
(arrowheads).
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Fig. 1B. —63-year-old man with supraspinatus rupture. Images were
obtained on low-field-strength 0.23-T open MRI scanner. Examination was
performed with shoulder in isocenter. Turbo spin-echo comparable image (TR/TE,
3,000/85) (A) and inversion recovery fast-field echo image (2,000/22;
inversion time, 80 msec) (B) show supraspinatus rupture
(arrowheads).
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Fig. 2A. —71-year-old man with supraspinatus rupture. Images were
obtained on mid-field-strength 0.6-T open MRI scanner. Examination was
performed with shoulder in isocenter. Fast-field echo image (TR/TE, 800/28)
(A) and inversion recovery fast-field echo image (2,455/16) (B)
show supraspinatis rupture (arrowheads).
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Fig. 2B. —71-year-old man with supraspinatus rupture. Images were
obtained on mid-field-strength 0.6-T open MRI scanner. Examination was
performed with shoulder in isocenter. Fast-field echo image (TR/TE, 800/28)
(A) and inversion recovery fast-field echo image (2,455/16) (B)
show supraspinatis rupture (arrowheads).
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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
- Magee T, Shapiro M, Williams D. Comparison of high-field-strength
versus low-field-strength MRI of the shoulder. AJR2003; 181:1211
–1215[Abstract/Free Full Text]
- Merl T, Scholz M, Gerhard P, et al. Results of prospective
multicenter study for evaluation of the diagnostic quality of an open
whole-body low-field MRI unit: a comparison with high-field MRI measured by
the applicable gold standard. Eur J Radiol1999; 30:434
–454
- Loew R, Kreitner K-F, Runkel M, Zoellner J, Thelen M. MR
arthrography of the shoulder: comparison of low-field (0.2 T) vs high-field
(1.5 T) imaging. Eur Radiol2000; 10:989
–996[Medline]
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