Microstructural Impact of Ischemia and Bone Marrow–Derived Cell Therapy Revealed With Diffusion Tensor Magnetic Resonance Imaging Tractography of the Heart In Vivo

Abstract

Background The arrangement of myofibers in the heart is highly complex and must be replicated by injected cells to produce functional myocardium. A novel approach to characterize the microstructural response of the myocardium to ischemia and cell therapy, using serial diffusion tensor MRI (DTI) tractography of the heart in vivo, is presented. Methods and Results Validation of the approach was performed in normal (n=6) and infarcted mice (n=6) as well as healthy human volunteers. Mice (n=12) were then injected with bone marrow mononuclear cells (BMMCs) 3 weeks after coronary ligation. In half the mice the donor and recipient strains were identical and in half the strains were different. A positive response to cell injection was defined by a decrease in mean diffusivity, an increase in fractional anisotropy, and the appearance of new myofiber tracts with the correct orientation. A positive response to BMMC injection was seen in one mouse. The response of the majority of mice to BMMC injection was neutral (9/12) or negative (2/12). The in vivo tractography findings were confirmed with histology. Conclusions DTI-tractography was able to directly resolve the ability of injected cells to generate new myofiber tracts and provided a fundamental readout of their regenerative capacity. A highly novel and translatable approach to assess the efficacy of cell therapy in the heart is thus presented.

Clinical imaging in regenerative medicine

“…In humans receiving genetically unmodified cells, it should be possible to identify new myocardium by MRI if it is large enough (i.e., >2% of the left-ventricle mass) and reasonably localized. Structural alignment of grafted cells with host myofibers can be visualized in humans with diffusion tensor MRI 77 . However, demonstrating functional integration currently requires more-invasive techniques.…”

“…Quantitative imaging, such as diffusion-tensor imaging (DTI), can be used to assess the fiber architecture in remodeled myocardium in animals and humans 77, 87, 88 . Myocardial fibers have a complicated helical structure that is critical for efficient contractile and conductive function of the heart.…”

Myocardial Scar Delineation Using Diffusion Tensor Magnetic Resonance Tractography

“…Noncontrast magnetic resonance imaging approaches based on the measurement of myocardial relaxation rates,34, 35 magnetization transfer,36 and apparent diffusivity21 have been reported; however, with the exception of T1 mapping–based techniques, these approaches remain experimental. In addition, traditional indexes of diffusion such as MD and FA may not show large variations in fully healed chronic infarcts 20, 37…”

“…Normal mice (C57BL6, n=6) were imaged on a 9.4‐T horizontal‐bore scanner (Biospec; Bruker) equipped with a 1500 mT/m gradient 20. DTI of the heart was performed with cardiorespiratory gating using a fat‐suppressed single‐shot spin echo sequence with an echo planar imaging readout.…”

“…DTI of the heart was performed with cardiorespiratory gating using a fat‐suppressed single‐shot spin echo sequence with an echo planar imaging readout. First‐order motion‐compensated, diffusion‐encoding gradients were placed on both sides of the 180° refocusing pulse 20. Acquisition parameters included field of view, 20×20×8 mm 3 ; matrix, 70×70×28; spatial resolution, 156 μm (isotropic); echo time, 14 ms; b values, 0 and 500 to 650 s/mm 2 ; 24 diffusion‐encoding directions; and 2 averages.…”