Many disease processes increase the tissue stiffness by edema, inflammation, or fibrosis. Liver diseases including viral hepatitis and alcoholi/non-alcoholic steatohepatitis increase stiffness of the liver by fibrosis. MR Elastography (MRE) is an MRI-based technique for quantifying tissue stiffness. MRE is based on the physical principle that the propagation characteristics of mechanical waves within various materials are determined by their mechanical properties.
Through a research collaboration with Mayo Clinic, we have been using MR Elastography for long time in all patients who undergo liver MRI in our institution. We revealed that liver stiffness value by MRE can be used for many applications, which include fibrosis staging, prediction of HCC development, and prediction of cirrhosis progression.
MRE is not only for the liver, but for other organ. Recently, brain MRE is getting attention from the world. Brain MRE is performed with a pillow-like driver to generate vibration in the brain appropriately. We are excited to see what we can do with this new equipment.
Visualization and Quantification of Blood Flow in the Body
As you may or may not know, MRI can measure the velocity of blood stream. Non-invasive assessment of the cardiac and aortic flow, pulmonary arterial flow, hepatic flow of both artery and portal is essential to evaluate the function of these organs. Current approaches such as Doppler ultrasound and conventional 2D phase contrast MRI (2D PC-MRI) is challenging due to the limited anatomical coverage and complex and variable anatomy. The use of 4D magnetic resonance imaging methods for assessing blood flow, allows the co-registration of anatomical and hemodynamic information in the vessels of interest. A time resolved 3D PC-MRI method is one of the research theme at our department. The purpose of this project is to comprehensively investigate the vasculature and flow dynamics in patients with congenital heart disease, aortic diseases, and portal hypertension. Through a research collaboration with University of Wisconsin, we use a time-resolved 3D (“4D”) PC-MRI method featuring radially undersampling (PC-VIPR), which enables great flexibility for post-processing, wide anatomical coverage, and excellent signal-to-noise ratio even in a limited acquisition time.
Clinical Application of new Hepatobiliary Contrast Agents
Gadoxetic acid (Primovist/Eovist, Bayer Pharmaceuticals) is a new gadolinium based contrast agent, which was launched in 2008. Gadoexetic acid has important pharmacokinetic properties, namely the uptake of approximately 50% of the agent into functioning hepatocytes, with subsequent excretion into bile. This behavior offers new opportunities for the detection and characterization of focal liver lesions such as metastatic disease, focal nodular hyperplasia, hepatic adenoma, and hepatocellular carcinoma, as well as providing new means of functional biliary imaging for direct T1 weighted MR cholangiography.
Especially, the hepatobiliary phase makes difference in the diagnosis of nodules in the cirrhotic livers. Nodules showing non-hypervascular and low signal intensity in hepatobiliary phase (NLH nodule) would be an important biomarker to indicate that the patient is at high risk of HCC development near future.
Diffusion-weighted imaging of the body
Diffusion-weighted (DW) MRI is a powerful technique for assessment of tissue microstructure in healthy and diseased tissue. It’s been widely used for detecting small liver metastases in the clinic. However, the image quality of body DW-MRI is not yet excellent in all cases due to: motion effects, low SNR, presence of fat, perfusion effects, as well as a number of imaging artifacts. In this project, we aim to identify and address these issues using various technique in image acquisition, reconstruction, and post-processing, in order to improve the robustness, reproducibility and clinical utility of body DW-MRI.