Quantitative MRI for assessment of myocardial fat infiltration at ultra-high field strength
PIs: Tobias Schäffter, Jeanette Schulz-Menger, Thoralf Niendorf
Application area: Cardiovascular
Background: The concept of fatty myocardium has received attention because of its role in cardiomyopathy and obesity. Furthermore, a relationship between severity of atrial fibrillation and epicardial fat and myocardial fat infiltration has recently been reported. The healthy heart obtains approximately 70% of its energy from oxidation of long-chain fatty acids (FA). Under normal conditions, FAs are sequestered as triglycerides (TGs) and stored in adipocytes as lipid droplets, with a small amount also stored in non-adipose tissues, such as the myocardium. There is increasing evidence suggesting altered myocardial substrate utilization and subsequent excessive TG accumulation (steatosis) in myocardial disease. The concept of fatty myocardium has received attention because of its role in cardiomyopathy.
Aim: We suggest that ultra-high-field MRI allows quantification of fat infiltration in correlation with functional changes in cardiomyopathy.
Methods: During this project, a novel high-resolution 3D chemical shift imaging (CSI) sequence will be developed that enables acquisition of water and fat images of at least two cardiac phases as well as TG quantification within the myocardium at 7 Tesla. Advanced, multi-slice B0 and B1+field mapping methods will be modified to deal with respiratory and cardiac motion-induced field changes. Parallel transmission (pTX) methods will be included to counteract heterogeneous flip angle distributions at 7 Tesla. Furthermore, advanced reconstruction algorithms will be developed for multi-frequency fat spectrum modelling.
A series of N images taken with small echo times (TE) increments ΔTE is the basis for CS artifact removal. The spectral bandwidth b of this acquisition is given by b = 1/ΔTE, while the spectral resolution is determined by Δf = 1/(N ΔTE). For a known and fixed MR spectrum, the spectral resolution and bandwidth can be adjusted such that each resonance line coincides with one of the N spectral windows. If the chemical shift δ of a component exceeds the bandwidth, it is aliasing back into the encoded spectral region. In an ideal case, the number of necessary echo-time increments equals the number of resonance lines
With the acquired data, we will perform a voxel-wise FT in the TE direction. All CS components will be separated and due to the known high resolution 7T spectrum and the pixel bandwidth, the CS can be compensated by sub-pixel translations.
With the proposed approach, we will perform efficient imaging of TG. To validate the result we will built a dedicated phantom containing multiple oil-filled vials of different sizes. The phantom will allow for fat quantification of tiny structures that mimic tiny fat infiltration in tissue. Finally, a small clinical feasibility study is planned.