Conventional diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) are assumed to reflect Gaussian distribution models. These images are very useful and have become essential in clinical practice. They can express simple diffusion phenomena, but not complex diffusion phenomena, in a living body more accurately.
The method of diffusion weighting and analysis with regard to the non-normality of diffusion distribution is called “advanced diffusion magnetic resonance imaging” (MRI). Various types of advanced diffusion MRI that are used in clinical practice—q-space diffusion imaging (QSI), diffusion spectrum imaging (DSI), q-ball imaging (QBI), diffusion kurtosis imaging (DKI), and neurite orientation and dispersion density imaging (NODDI)—are described in this paper. In addition, the MR g-ratio, which is expected to be applied in the future, is also reviewed.
Advanced diffusion MRI also has disadvantages: More imaging conditions are necessary, and the scanning time is longer than that with the conventional DWI. However, it is possible to acquire complicated diffusion information in a living body that could not be obtained by DWI or DTI, and this method is provides much information about various diseases. It cannot be undertaken easily and quickly, but further development and use in clinical settings are expected in the future.
Hepatobiliary specific contrast medium Gd-EOB-DTPA-enhanced MRI (EOB-MRI) is excellent for the detection and characterization of nodular lesions and plays an important role in the diagnosis of hepatocellular carcinoma (HCC). Gd-EOB-DTPA is received by normal hepatocytes, and then, excreted into bile ducts, under mediation by hepatocyte membrane transporters. The expression of uptake transporter organic anion transporting polypeptide 1B3 (OATP1B3) correlates with the enhancement ratio in the hepatobiliary phase of HCC. Consequently, OATP1B3 is the main uptake transporter of Gd-EOB-DTPA in HCC. The hepatobiliary phase of EOB-MRI can sensitively detect pathologically early HCC as a hypointense nodule, because the OATP1B3 expression decreases at an early stage of multistep hepatocarcinogenesis. Hypervascular HCC commonly presents hypointensity in the hepatobiliary phase with the decrease in the OATP1B3 expression ; however, approximately 10% of HCC atypically demonstrates hyperintensity, owing to OATP1B3 overexpression. HCC presenting hyperintensity in the hepatobiliary phase is a unique genetic subtype of HCC with a biologically less aggressive nature and mature hepatocyte-like molecular/genetic features. The interaction between β-catenin signaling and hepatocyte nuclear factor (HNF) 4α may plays an important role in the OATP1B3 expression and less aggressive biological nature of the hyperintense HCC in the hepatobiliary phase. Thus, EOB-MRI is crucial for the detection and characterization of HCC as well as for personalized medicine, such as an imaging biomarker.
Magnetic resonance elastography (MRE) is a relatively new technology for quantitatively assessing the mechanical properties of tissues. It can be considered an imaging-based virtual palpation. The purpose of this review is to introduce this technology to clinicians/technologists and summarize its basic mechanism. First, a brief overview of MRE is provided. Second, the method of MRE is explained. Finally, an example of psoas major MRE is introduced.
Diffusion-weighted whole body imaging with background body signal suppression uses the short time inversion (TI) recovery (STIR) method, which is effective against the inhomogeneity of a magnetic field, for fat suppression. Oil causes chemical shifts with different transfer amounts, owing to the change in its chemical structure, and the null points of CH2 and olefins present different values. Consequently, fat cannot be completely suppressed by the STIR method, and it is necessary to combine it with other fat suppression methods. In this study, using phantom imaging proposed by the authors with SSRF, both 1.5T and 3.0T fat suppression effects were improved. The null point was prolonged using CHESS with STIR. In this case, the fat suppression effect decreased with the same TI as that of the STIR.