Visualization of myocardial microstructure using high‐resolution T imaging at high magnetic field

Abstract

The analysis of myocardial microstructure in vivo is important for the determination of myocardial contractility and function. The purpose of the present study was to demonstrate that high-resolution T* 2 imaging has the potential to visualize the microstructure of beating, isolated rat hearts. To perform T* 2 imaging, a multiple gradient-echo sequence was implemented on an 11.75 Tesla microscopy system. An in-plane resolution of 78 m and a slice thickness of 250 m were achieved in 24 min. In comparison to histological sections, the T* 2 maps showed an excellent spatial correspondence to the myocardial fiber structure. To demonstrate the utility of this technique, morphologic alterations in myocardial microstructure were investigated in hearts with chronic myocardial infarction. Scar tissue and the extent of the infarcted region were clearly visualized and quantified using high-resolution T* 2 imaging. The myofiber structure of the heart plays a critical role in force production and contraction of the heart. Fiber orientation is also an important determinant of myocardial stress and strain, and, consequently, of cardiac perfusion and oxygen consumption (1). Furthermore, the structural fiber arrangement is known to be modified during the time course of different cardiac diseases (2,3).Because molecular movements are strongly influenced by the organization and geometry of the microscopic environment, diffusion-weighted MRI has become a useful tool for investigation of the microstructure of materials and biological tissues. Diffusion tensor imaging (DTI), for example, has so far been used to characterize the fiber architectures of the brain white matter (4), spinal cord (5,6), cartilage (7), myocardium (1,8 -10), and other musculature (11).Because the tissue microstructure of the myocardium may also be characterized by its distribution of field inhomogeneities, this structure should be observable in T* 2 maps. The relative scale of these inhomogeneities compared with an imaging voxel can be divided roughly into two relevant categories (12-14): macroscopic and mesoscopic field inhomogeneities. Macroscopic field inhomogeneities arise from magnet imperfections, imperfect shimming, air-tissue interfaces inside animals, and similar effects. These types of inhomogeneities contain no structural or physiological information. Mesoscopic field inhomogeneities do provide structural information about biological tissue, and have proved to be a useful tool for investigating biological tissue structure and function. The goal of the present study was to demonstrate that the microstructure of biological tissue can be investigated with high-resolution T* 2 imaging. In particular, we show that the fiber orientation in the isolated rat heart can be characterized with T* 2 imaging. Furthermore, scar tissue can be visualized in the chronically infarcted isolated rat heart by T* 2 imaging. METHODS Isolated Heart ModelHearts of male Wistar rats were excised and perfused in the Langendorff mode (15) with oxygenated (95% O 2 and 5% CO 2 ) K...

High Spatial Resolution and Temporally Resolved T2* Mapping of Normal Human Myocardium at 7.0 Tesla: An Ultrahigh Field Magnetic Resonance Feasibility Study

“…This is in line with T 2 *  = (15.8±0.2) ms recently observed for hind limb skeletal muscle in rats at 7 Tesla [52] Admittedly, the absolute spatial resolution demonstrated for T 2 * mapping of the human heart at 7.0 T is still by an order of magnitude below that previously reported for ex vivo MR microscopy based T 2 * mapping of the isolated rat heart [7], which demonstrated that T 2 * mapping provides an insight into the complex architecture of the heart musculature. However, the effective anatomical spatial resolution – voxel size per anatomy – is getting close to what has been demonstrated for animal models.…”

“…Methodological developments in T 2 * sensitized imaging [1]–[4] and simulations of myocardial vasculature [5], [6] have been indispensable. Applications include investigation of the microstructure of the isolated rat heart [7], detection of myocardial ischemia [8]–[14], probing of vasodilator or dipyridamole-induced changes in myocardial perfusion [15]–[18], visualization of scarred myocardium [19], imaging of capillary recruitment [20] and assessment of tissue oxygenation related to endothelium-dependent blood flow changes [21]. T 2 * mapping has also been shown to be of substantial clinical value for the ascertainment of myocardial iron levels [22]–[29].…”

Investigation of the microstructure of the isolated rat heart: A comparison between T*2‐ and diffusion‐weighted MRI

“…In validation studies performed in muscular tissues, it has been shown that the principal direction of the diffusion tensor corresponding to the largest diffusivity is associated with the direction of the myofiber (9–12). Because the tissue microstructure of the myocardium can also be characterized by the distribution of field inhomogeneities within the tissue, the myocardial fiber orientation is also observable in T * 2 maps, as we demonstrated in a previous study (13). In particular, we showed that fiber orientation in the intact isolated rat heart can be characterized by T * 2 imaging, as can scar tissue in the chronically infarcted isolated rat heart.…”

Improved correlation of histological data with DCE MRI parameter maps by 3D reconstruction, reslicing and parameterization of the histological images

“…There are, however, modern non-invasive techniques to image a tissue probe from any direction and also offer the opportunity for a three-dimensional reconstruction. One of these is magnetic resonance (MR) microscopy [7], which provides an in plane resolution of less than 20 μm using three-dimensional spin-echo acquisitions [8]. This resolution is only marginally below that of a histological slice.…”