^{A 3D Tissue-Printing Approach for Validation of Diffusion Tensor Imaging in Skeletal Muscle}

Abstract

The ability to noninvasively assess skeletal muscle microstructure, which predicts function and disease, would be of significant clinical value. One method that holds this promise is diffusion tensor magnetic resonance imaging (DT-MRI), which is sensitive to the microscopic diffusion of water within tissues and has become ubiquitous in neuroimaging as a way of assessing neuronal structure and damage. However, its application to the assessment of changes in muscle microstructure associated with injury, pathology, or age remains poorly defined, because it is difficult to precisely control muscle microstructural features in vivo. However, recent advances in additive manufacturing technologies allow precision-engineered diffusion phantoms with histology informed skeletal muscle geometry to be manufactured. Therefore, the goal of this study was to develop skeletal muscle phantoms at relevant size scales to relate microstructural features to MRI-based diffusion measurements. A digital light projection based rapid 3D printing method was used to fabricate polyethylene glycol diacrylate based diffusion phantoms with (1) idealized muscle geometry (no geometry; fiber sizes of 30, 50, or 70 mm or fiber size of 50 mm with 40% of walls randomly deleted) or (2) histology-based geometry (normal and after 30-days of denervation) containing 20% or 50% phosphate-buffered saline (PBS). Mean absolute percent error (8%) of the printed phantoms indicated high conformity to templates when ''fibers'' were >50 mm. A multiple spin-echo echo planar imaging diffusion sequence, capable of acquiring diffusion weighted data at several echo times, was used in an attempt to combine relaxometry and diffusion techniques with the goal of separating intracellular and extracellular diffusion signals. When fiber size increased (30-70 mm) in the 20% PBS phantom, fractional anisotropy (FA) decreased (0.32-0.26) and mean diffusivity (MD) increased (0.44 · 10 -3 mm 2 /s-0.70 · 10 -3 mm 2 /s). Similarly, when fiber size increased from 30 to 70 mm in the 50% PBS diffusion phantoms, a small change in FA was observed (0.18-0.22), but MD increased from 0.86 · 10 -3 mm 2 /s to 1.79 · 10 -3 mm 2 /s. This study demonstrates a novel application of tissue engineering to understand complex diffusion signals in skeletal muscle. Through this work, we have also demonstrated the feasibility of 3D printing for skeletal muscle with relevant matrix geometries and physiologically relevant tissue characteristics.

Relationships between tissue microstructure and the diffusion tensor in simulated skeletal muscle

“…For example, Hoojimans et al rejected regions of interest in muscle with SNR < 20, as inaccurate estimation of the diffusion tensor is common under this threshold as determined in . In an experiment using the exact pulse sequence used in this manuscript on a 7T scanner at our institution, we reported an SNR of 33.7 on a 3‐dimensional (3D) printed, hydrogel diffusion phantom with muscle microstructural features . Similar SNR values have been measured in unreported data in skeletal muscle tissue in our lab.…”

Lumbar Muscle Structure Predicts Operational Postures in Active-Duty Marines

“…The quantitative relationship of diffusion variables to specific features of muscle microstructure is the focus of current work, although there is some evidence that they are related to muscle fiber [ research report ] size. 5,8,12,35 Mean diffusivity describes the average restricted diffusion coefficient of λ 1-3 and is normally between 1 × 10 -3 mm 2 /s and 2 × 10 -3 mm 2 /s. 24 Fractional anisotropy is a unitless measurement from 0 to 1 that indicates the shape of the diffusion tensor.…”

“…17,21,22 It is also difficult to measure muscle microstructure in vivo, although there is some evidence that diffusion-based imaging techniques are sensitive to different features of muscle microstructure, in particular fiber area. 5,8,12,35 With injury and age, atrophy of muscle fibers and replacement of muscle tis- in this cross-sectional study, using an upright magnetic resonance imaging scanner while standing without load and standing, sitting, and prone on elbows with body armor. Cobb, horizontal, and sacral angles were measured.…”

“…1 As the sarcolemma is considered to be the primary [ research report ] barrier to diffusion, DTI is believed to be most sensitive to changes in fiber size, because radial diffusion of water across a muscle fiber is more restricted (by the sarcolemma) than longitudinal diffusion within a muscle fiber. 44,45 While it has been shown that FA and fiber area are inversely related, 2,5,8,12,35 it is important to note that the exact relationship has not been validated. However, it is well established that muscle fiber area and isometric force are directly related.…”

Erbium-Based Perfusion Contrast Agent for Small-Animal Microvessel Imaging

“…Confirmation of perfusion of microvessels (i.e., <10 μ m) was achieved by high-resolution micro-CT. Fabricated microvessel constructs or synthetic capillaries have been utilized to evaluate microvessel perfusions in the past [ 34 , 35 ], but fabricating synthetic vessels with diameters on the order of 10 μ m remains technically challenging. Fortunately, the mouse kidney is a well-characterized organ system, with known vessel diameters ranging from the renal artery (~0.3 mm) to capillaries (~10 μ m) [ 36 , 37 ].…”