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(Radiology. 2001;221:274.)
© RSNA, 2001

Letters to the Editor

Routine Opposed-Phase FLASH MR Imaging Does Not Quantify Fat Content

Lawrence Yao, MD

Med-Tel International, 1430 Spring Hill Road, Suite 500, McLean, VA 22102, e-mail: yaol@medtel.com

Editor:

In an article by Dr Namimoto and colleagues, which appeared in the March 2001 issue of Radiology (1), the authors profess to derive "fat fractions" from a signal intensity (SI) index obtained from in- and out-of-phase gradient-echo magnetic resonance imaging. The desire to objectify this technique, which is now commonly used to characterize adrenal masses, is laudable.

While the phantom data in this study nicely illustrate the dependence of the SI index on the T1 relaxation of the nonfatty tissue component, Dr Namimoto and colleagues have not illustrated how a fat fraction is derived from the standard in- and out-of-phase fast low-angle shot (FLASH) images.

Dr Namimoto and colleagues assume that T2* contributions to the SI index can be ignored in assessing adrenal lesions, which is unproven. They reference an article in this regard on benign fatty infiltration of the liver, a condition hardly parallel to the situation of neoplastic tissue infiltration, where a variable prolongation of T2* is expected, referable to normal tissue.

Even accepting this point and ignoring the T2* contribution to the SI index, one remains puzzled about the derivation of fat-fraction values in this study. I surmise Dr Namimoto and colleagues use the phantom data as a nomogram, selecting an SI index versus fat-fraction curve (Figure 2 in their article) corresponding to a published value for the "normal" T1 of adrenal tissue. In this way, they translate the SI index to fat fraction by using a fixed nomogram. This exercise hardly requires a phantom study and assumes that benign and malignant adrenal tissues share the same T1.

I may be missing something. However, I sense there is no shortcut to derive a true fat fraction from routine in- and out-of-phase imaging. A relative fat fraction might be derived in the case of healthy tissues characterized by a relatively stable and homogeneous bulk T1. Any indiscriminate assumptions used to derive a secondary parameter such as fat fraction only seem to further complicate the clinical discrimination of benign from malignant lesions.

REFERENCES

  1. Namimoto T, Yamashita Y, Mitsuzaki K, et al. Adrenal masses: quantification of fat content with double-echo chemical shift in-phase and opposed-phase FLASH MR images for differentiation of adrenal adenomas. Radiology 2001; 218:642-646.[Abstract/Free Full Text]
  2. Levenson H, Greensite F, Hoefs J, et al. Fatty infiltration of the liver: quantification with phase-contrast MR imaging at 1.5 T vs biopsy. AJR Am J Roentgenol 1991; 156:307-312.[Abstract/Free Full Text]

Drs Namimoto and colleagues respond:

Tomohiro Namimoto, MD, Yasuyuki Yamashita, MD, Katsuhiko Mitsuzaki, MD, Yoshiharu Nakayama, MD, Osamu Makita, MD, Masataka Katoda, MD and Mutsumasa Takahashi, MD

Department of Radiology, Kumamoto University School of Medicine, 1-1-1 Honjo, Kumamoto 860-8556, Japan, e-mail: tomohiro@kaiju.medic.kumamoto-u.ac.jp

We appreciate Dr Yao’s interest in our recent article (1). Dr Yao raises a few limitations to the study that we did not fully discuss in our article. Dr Yao’s concern is that the T2* and T1 were not considered for our fat-fraction calculation. As pointed out, theoretically, both T2* and T1 should be considered for accurate measurement of fat fraction.

According to the articles by Baker et al (2) and Glazer et al (3), most T2 values of adrenal tumors range from 30 to 130 msec. These values are small enough in comparison with T2 elongated structures (eg, cerebrospinal fluid, 200 msec). Accordingly, T2* of variable adrenal adenomas can be negligible, less than 1% of fat fraction, when we use a gradient-echo sequence with a very short echo time (4).

The T1 value of the adrenal adenomas ranges from 300 to 800 msec, and that of normal adrenal tissue is similar to the average value of 504 msec (3). In this range, however, the contribution of the T1 value to the fat fraction is relatively small, less than 10%, with using the same phantoms and methods as those used previously (1) (Table). Although it would be desirable to obtain a value for each tumor, the contribution to the difference in T1 value to fat fraction can be clinically negligible. We should have discussed this potential limitation in the article.


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  		Estimated Fat Fraction Versus T1 Value

 
As pointed out, measurements of T1 and T2/T2* values are required to obtain an accurate fat fraction of tissue, but this may be extremely time-consuming. Although our measurement may contain potential limitations, the errors may be negligible in clinical circumstances. Actually, our data are similar to the results by Leroy-Willig et al (5), who used MR spectroscopy. In that sense, we believe our technique is valuable in clinical applications.

REFERENCES

  1. Namimoto T, Yamashita Y, Mitsuzaki K, et al. Adrenal masses: quantification of fat content with double-echo chemical shift in-phase and opposed-phase FLASH MR images for differentiation of adrenal adenomas. Radiology 2001; 218:642-646.
  2. Baker ME, Blinder R, Spritzer C, et al. MR evaluation of adrenal masses at 1.5 T. AJR Am J Roentgenol 1989; 153:307-312.[Abstract/Free Full Text]
  3. Glazer GM, Woolsey EJ, Borrello J, et al. Adrenal tissue characterization using MR imaging. Radiology 1986; 158:73-79.[Abstract/Free Full Text]
  4. Levenson H, Greensite F, Hoefs J, et al. Fatty infiltration of the liver: quantification with phase-contrast MR imaging at 1.5 T vs biopsy. AJR Am J Roentgenol 1991; 156:307-312.
  5. Leroy-Willig A, Bittoun J, Luton JP, et al. In vivo MR spectroscopic imaging of the adrenal glands: distinction between adenomas and carcinomas larger than 15 mm based on lipid content. AJR Am J Roentgenol 1989; 153:771-773.[Abstract/Free Full Text]



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H. K. Hussain, T. L. Chenevert, F. J. Londy, V. Gulani, S. D. Swanson, B. J. McKenna, H. D. Appelman, S. Adusumilli, J. K. Greenson, and H. S. Conjeevaram
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[Abstract] [Full Text] [PDF]

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