Three-dimensional MR microscopy of a transgenic mouse model of dilated cardiomyopathy

4D cardiac MRI in the mouse

With the introduction of mouse models for the study of cardiac morphogenesis, there arises a need for new imaging protocols that can capture both morphological and functional information. High-resolution 2D cardiac cine MRI has often been used to quantify left and right ventricular function. In this study we propose a 3D isotropic cardiac cine MRI protocol with a voxel size of 200 microm(3) as a means of studying cardiac multi-chamber morphology and function. A black blood sequence was used to enhance blood myocardium contrast. Manual segmentation of the ventricles was used to measure ventricular volumes at end diastole and end systole. This method is demonstrated on an Irx4-deficient mouse model. We have been able to identify the volumes of both ventricles dynamically and to show differences in ejection fraction in the mutant. We have also identified an abnormality of the papillary muscle in the mutant that had been missed in previous phenotyping with ultrasound and histology.

Diffusivity- and T2 imaging at 3 Tesla for the detection of degenerative changes in human-excised tissue with high resolution: atherosclerotic arteries

RATIONALE AND OBJECTIVES

We investigated whether it is possible to investigate degenerative changes in human tissue on a sub-100-microm resolution scale not only on special high-field small-bore MR-microscopy systems but also on a 3T whole-body MR-scanner.

METHODS

Spin-spin relaxation, proton density, and diffusion microimaging were investigated in studying human atherosclerotic arteries. Strong diffusion weighting and high spatial resolution was achieved by means of a strong dedicated gradient system and a small birdcage radiofrequency resonator.

RESULTS

Quantitative parameter maps were obtained at voxel sizes down to 73 x 73 x 600 microm3. The morphologic structure and pathology connected to lipid deposits, plaques, small thrombi, and bifurcations were well visualized.

CONCLUSION

High-resolution parameter-weighted and parameter-imaging at sub-100-microm pixel resolution can be achieved for excised tissue on a 3.0 T whole body MR system. Perspectives for the characterization of atherosclerotic plaques imply not only cost advantages but also equivalence of contrast, especially as to T(2), for in vivo and high-resolution ex vivo investigations on the same MR scanner.

Improved preparation of chick embryonic samples for magnetic resonance microscopy

Previous work demonstrated the power of three-dimensional (3D) magnetic resonance microscopy (MRM) to follow complicated morphologic development in the embryonic cardiovascular system. In this study we describe a new dual-contrast method for specimen preparation that combines perfusion fixation and immersion in fixative with macro- and small molecular gadolinium agents to provide enhanced definition of both the heart wall and chamber. MRM was performed at 9.4 T with image resolutions of 25, 31, and 50 microm isotropic voxels for three stages of chick embryos (day 4, day 5.5, and day 9), and compared to histological sections of the same embryos. The results show considerable improvement of image quality over previous efforts, with better signal-to-noise ratio (SNR) and contrast between the cardiac chamber and myocardial wall. Excellent correlation was shown between the MRM images and histological sections. Thus, 3D high-resolution MRM in combination with the dual-contrast technique is useful for acquiring quantitative 3D morphologic data regarding heart development.

MR imaging at high magnetic fields

Recently, more investigators have been applying higher magnetic field strengths (3-4 Tesla) in research and clinical settings. Higher magnetic field strength is expected to afford higher spatial resolution and/or a decrease in the length of total scan time due to its higher signal intensity. Besides MR signal intensity, however, there are several factors which are magnetic field dependent, thus the same set of imaging parameters at lower magnetic field strengths would provide differences in signal or contrast to noise ratios at 3 T or higher. Therefore, an outcome of the combined effect of all these factors should be considered to estimate the change in usefulness at different magnetic fields. The objective of this article is to illustrate the practical scientific applications, focusing on MR imaging, of higher magnetic field strength. First, we will discuss previous literature and our experiments to demonstrate several changes that lead to a number of practical applications in MR imaging, e.g. in relaxation times, effects of contrast agent, design of RF coils, maintaining a safety profile and in switching magnetic field strength. Second, we discuss what will be required to gain the maximum benefit of high magnetic field when the current magnetic field (< or = 1.5 T) is switched to 3 or 4 T. In addition, we discuss MR microscopy, which is one of the anticipated applications of high magnetic field strength to understand the quantitative estimation of the gain benefit and other considerations to help establish a practically available imaging protocol.