Fat quantification and T2 measurement

Abstract

Sir, We have read with interest the paper by Kim et al. [1] published in Pediatric Radiology in the November 2013 issue about fat measurement in healthy children using T2 maps and nuclear magnetic resonance (NMR) spectroscopy. As the author stated, it is important to quantify structural muscle changes such as fatty infiltration. NMR spectroscopy is a possible way to quantify fat, with the limitation of providing localized measures and this was shown in the paper. Besides, many other alternatives are available and they exploit the frequency shift between fat and water components [2]. However, Kim et al. [1] did not consider these techniques and presented a quantification approach based on T2 mapping and fat T2, which was defined as the difference between the T2 maps with and without fat suppression. We found confounding and misleading the choice by the authors to coin an inappropriate term «T2 fat value» and to show that «T2 fat value» is proportional to the muscle fat content. At best, one should speak of an “apparent delta T2 related to fat content.”As a result of this unfortunate decision, we read that mean fat T2 value in the gluteus is 5 ms! To the best of our knowledge, fat T2 is in the 80 to 150 ms range depending on the method used [3–6] and it is independent of fat concentration. The idea of operating a subtraction on T2s is physically incorrect, at best, 1/T2s can. The authors also used proton spectroscopy for fat quantification as it is the case of several studies [7, 8]. But, they acquired only one spectrum at TE=31 ms where the amplitudes of water and fat peaks are T2-weighted. In other words, even for equal quantities of water and fat in the voxel, the signal of fat will be twice the signal of water for TE=31 ms. For an accurate fat quantification using NMR, several spectrums at different TEsmust be acquired to account for the T2 relaxation of each component. Regarding fat saturation, it is well known that it is sensitive to B0 inhomogeneities and it may fail in cases with large volumes. To identify the exams with unsuccessful fat suppression, Kim et al. [1] hypothesized that if fat suppression was correctly achieved, the T2 in the subcutaneous fat region should be equal to zero. This claim is questionable because in case of perfect fat suppression, only Rician noise is measured in the subcutaneous fat and mono-exponential fit will give a long T2 not a zero value. At the end of the discussion, the authors mention the existence of the Dixon method; in reality, a large group of water-fat imaging techniques are now available from all vendors. There is no indication that the indirect method introduced in this paper adds to the well-established Dixon methods.