MR-IMAGING OF FAT-CONTAINING TISSUES - VALUATION OF 2 QUANTITATIVE IMAGING TECHNIQUES IN COMPARISON WITH LOCALIZED PROTON SPECTROSCOPY

Since lipid protons, consisting mainly of triacylglycerols (TAG), are rather mobile, magnetic resonance imaging (MRI) is ideally suited for the examination of fat-containing tissues such as bone marrow. In cont rast to water protons, however, lipid protons are chemically distinct and give rise to at least eight resonance peaks with different T1 and T2 relaxation times in the H-1 spectrum. This is why the characterizat ion of fat-containing tissues by quantitative MRI is much more difficu lt than that of most other tissues. In our study we wanted to examine the accuracy and the potential of a H-1 chemical shift imaging (CSI) t echnique and a multiple spin-echo imaging (MSEI) technique. A stimulat ed-echo (STEAM) sequence for spatially localized proton spectroscopy w as used as the reference method. In the first part of this paper, we d escribe quantitative imaging experiments which were performed to asses s the accuracy of the fat-water separation according to the Dixon meth od and the bi-exponential decomposition of the MSEI data. For that pur pose, we used a two-compartment phantom filled with either an aqueous Gd-DTPA solution and vegetable oil or with two different aqueous Gd-DT PA solutions, respectively. The analysis of the H-1 CSI data revealed that the presence of non-methylen protons in neutral fats leads to a s light under-estimation (of about 15%) of the relative fat signal fract ion. The error is described theoretically and verified quantitatively by STEAM measurements. The bi-exponential analysis of the transverse r elaxation data, on the other hand, yields reliable T2 values if the re lative proton density of both components is higher than 15%. In the se cond part of our investigation, the same techniques were applied to ac quire data from the subcutaneous fatty tissue, the femoral head, and t he lumbar vertebrae of three healthy volunteers. In the bone marrow sp ectra, only two broad resonances could be resolved; they were superpos itions of diverse molecular groups with different T1 and T2 relaxation times. In these cases, localized proton spectroscopy does not provide additional information with respect to H-1 CSI. The MSEI data of the three examined fat containing tissue regions were adequately fitted by a bi-exponential function despite the fact that there were much more chemically distinct protons present in fatty tissues.