In this talk will review the current standing of diffusion imaging and modeling in the heart. With appropriate data quality and processing diffusion weighted imaging and modeling allows to describe both the microstructural properties of heart tissue as well as describe and quantify its macroscopic anatomy.
Highlights
DWI of the can be challenging due to its low T2 relaxation time of only 30 to 50 ms. DWI imaging is typically done using a spin echo - echo planar imaging sequence (SE-EPI) . To allow for ample diffusion weighting echo times are generally between 80 and 110 ms, which results in very low SNR. Luckily, the diffusion of water in the heart is fast (MD ≈ 1.6 to 1.8 mm2/s) so a b-value between 400 and 500 s/mm2 is optimal for DTI analysis. The diffusion in the heart fast, this also means that the diffusion anisotropy is much lower (FA ≈ 0.20 to 0.25) and much more sensitive to noise (2, 3). For accurate estimation of diffusion tensor parameters an SNR of 25 of higher is needed (40 or higher for tractography). Investigating microstructural changes in the heart using DWI is mostly done using the DTI model. However, other models are being explored, e.g. IVIM and DKI.
DWI of the beating heart is challenging due to that the heart is always moving. Measuring the microscopic motion of water in the presence of macroscopic motion needs a different diffusion sequence design. To overcome this two different approaches are possible:
1) using a stimulated echo acquisition (STEAM) which applies the diffusion (4) weighting over two consecutive heartbeats and
2) Designing the diffusion weighting wave form to become insensitive for macroscopic motion (5).
With STEAM acquisition it is essential that both diffusion gradients are applied in the identical location of two subsequent cardiac cycles. Typically this is done in the so called “sweet spots” to prevent strain encoding of the diffusion signal (6, 7). Due to improvements in gradient hardware single shot SE sequence with motion compensated gradient wave forms is feasible (8, 9). Due to the long mixing time in STEAM the FA is higher than in SE acquisition. Furthermore the MD is lower in STEAM than in SE acquisition (8). Both methods provide similar data quality however SE has higher scan time efficiency.
Diffusion tensor imaging in the heart is of great interest since it allows to, next to diffusion parameters, obtain detailed information about the cardiac myocardial architecture. The heart has a unique structure with myocytes aligned in a left-handed helix in the epicardium, to circumferential in the mid wall, to a right handed-helix in the endocardium. The heart muscle fibers are arranged in a laminar structure, called sheetlets. Both these structures can be described by quantitative values derived from the diffusion tensor Eigen-vectors. Changes in the myocardial helix angels over the cardiac cycle (10) during development (11) and in pathology (12) have been studied and can potentially give new insights in cardiac function and disease and monitory of cardiac therapy (13).
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