Diffusion coefficients in abdominal organs and hepatic lesions: evaluation with intravoxel incoherent motion echo-planar MR imaging

Technologic advances in abdominal MR imaging

Magnetic resonance (MR) imaging is finding an ever-growing role in the evaluation of a wide range of conditions in the abdomen. No longer confined to problem solving regarding abnormalities in solid organs, such as the liver and kidneys, MR imaging is increasingly being applied to the evaluation of the pancreatic and biliary ductal systems and even the bowel. Recent technical advances in hardware and software have allowed the acquisition of MR images that are largely free of artifact secondary to bowel peristalsis or respiratory motion; images providing excellent anatomic detail can now be obtained routinely. Faster sequences have reduced image acquisition time, thereby improving patient acceptance and allowing more efficient utilization of machine time. New three-dimensional sequences allow rapid image acquisition, reducing section misregistration and motion artifact while improving multiplanar reformations. The potential of MR imaging to provide functional and anatomic information is intriguing, and new techniques, including diffusion and perfusion imaging, are being evaluated. This review considers the advances in imaging hardware and pulse sequence design that underlie the increasing role of MR imaging in evaluation of the abdomen and discusses evolving clinical applications.

Diffusion tensor MRI of the human kidney

This study characterizes the diffusion anisotropy of the human kidney using a diffusion-weighted, single-shot echo planar imaging (EPI) sequence in order to access the full apparent diffusion tensor (ADT) within one breathhold. The fractional anisotropy (FA) of the cortex and the medulla were found to be 0.22 +/- 0.12 and 0.39 +/- 0.11, respectively (N = 10), which emphasizes the need for rotationally invariant diffusion measurements for clinical applications. Additional limitations for clinical diffusion imaging on the kidney are the strong susceptibility variations within the abdomen that restrict the use of imaging techniques employing long echo trains, and the severe motion sensitivity that limits the available imaging time to one breath-hold. To overcome these problems an isotropic, diffusion-weighted, segmented EPI protocol that facilitates the acquisition of high-resolution diffusion-weighted images within a single breath-hold was implemented. Using this method, the apparent diffusion coefficient (ADC) of the cortex and medulla were found to be 2.89 +/- 0.28. 10(-9) m2/s and 2.18 +/- 0.36. 10(-9) m2/s (N = 10).

Diffusion tensor imaging: concepts and applications

The success of diffusion magnetic resonance imaging (MRI) is deeply rooted in the powerful concept that during their random, diffusion-driven displacements molecules probe tissue structure at a microscopic scale well beyond the usual image resolution. As diffusion is truly a three-dimensional process, molecular mobility in tissues may be anisotropic, as in brain white matter. With diffusion tensor imaging (DTI), diffusion anisotropy effects can be fully extracted, characterized, and exploited, providing even more exquisite details on tissue microstructure. The most advanced application is certainly that of fiber tracking in the brain, which, in combination with functional MRI, might open a window on the important issue of connectivity. DTI has also been used to demonstrate subtle abnormalities in a variety of diseases (including stroke, multiple sclerosis, dyslexia, and schizophrenia) and is currently becoming part of many routine clinical protocols. The aim of this article is to review the concepts behind DTI and to present potential applications.

In vivo diffusion measurements as an indication of fetal lung maturation using echo planar imaging at 0.5T

The aim of this study was twofold: First, to establish the normal range of fetal lung diffusion values measured during healthy pregnancy; and second, to determine whether fetal lung diffusion could be used as an indication of fetal lung maturity. The apparent diffusion coefficient (ADC), averaged over all 26 subjects with an average gestational age of 29 +/- 6 weeks (mean +/- sd), was found to be 2.0 +/- 0.6 x 10(-9) m(2)/sec (mean +/- sd), but a trend was found indicating that ADC increased with gestational age at the rate of 0.07 x 10(-9) m(2)/sec per week (P = 4 x 10(-5)). To determine the usefulness of this data in predicting lung maturity, a simple three-compartment model was proposed which was comprised of intra-lung amniotic fluid, intra-tissue water, and vascular blood. The relative proportions of each compartment were taken from the literature, and exchange between the compartments was assumed to be minimal. This model predicted the in vivo data reasonably well, and indicated that MR measurements of fetal lung diffusion are a marker for the degree of vascularization of the terminal tubules. Magn Reson Med 45:247-253, 2001.

Hemodynamic effect of iodinated high-viscosity contrast medium in the rat kidney: a diffusion-weighted MRI feasibility study

RATIONALE AND OBJECTIVES

To assess the abilities of dynamic diffusion-weighted MRI to demonstrate the effects in vivo of a high-viscosity iodinated contrast agent on medullary and cortical blood flow in the rat kidney.

METHODS

Dynamic diffusion-weighted, echoplanar MR images obtained from five b-value single-shot acquisitions and their isotropic apparent diffusion coefficient maps were obtained from nine rats anesthetized by pentobarbital sedation, before and after intravenous injection of a high-viscosity, dimeric iso-osmolar iodinated contrast medium (iodixanol), and compared with those obtained from four control rats that received saline.

RESULTS

The mean baseline apparent diffusion coefficient values were 1.64 +/- 0.05 x 10(-3) mm2/s for the cortex and 1.75 +/- 0.06 x 10(-3) mm2/s for the medulla. In the iodixanol group, a significant decrease in renal diffusion was observed at 12 minutes and lasted at least until 24 minutes. The decrease in diffusion occurred earlier for the cortex and lasted less than for the medulla. There was no significant modification in diffusion over time in the control group.

CONCLUSIONS

This preliminary experience in rats shows that dynamic diffusion-weighted MRI can be used to study noninvasively the in vivo renal hemodynamic response after injection of iodinated contrast.

Apparent diffusion coefficient in thoracolumbar intervertebral discs of healthy young volunteers

Apparent diffusion coefficient values (ADC) of healthy intervertebral discs of young volunteers in the thoracolumbar spine were determined using a single-shot EPI sequence. ADC(z) was in the lumbar spine slightly higher than ADC(x) or ADC(y). In vivo diffusion measurements of intervertebral discs may offer a novel diagnostic tool to evaluate disc diseases in early phases.

Multi-component apparent diffusion coefficients in human brain: relationship to spin-lattice relaxation

In vivo measurements of the human brain tissue water signal decay with b-factor over an extended b-factor range up to 6,000 s/mm(2) reveal a nonmonoexponential decay behavior for both gray and white matter. Biexponential parametrization of the decay curves from cortical gray (CG) and white matter voxels from the internal capsule (IC) of healthy adult volunteers describes the decay process and serves to differentiate between these two tissues. Inversion recovery experiments performed in conjunction with the extended b-factor signal decay measurements are used to make separate measurements of the spin-lattice relaxation times of the fast and slow apparent diffusion coefficient (ADC) components. Differences between the spin-lattice relaxation times of the fast and slow ADC components were not statistically significant in either the CG or IC voxels. It is possible that the two ADC components observed from the extended b-factor measurements arise from two distinct water compartments with different intrinsic diffusion coefficients. If so, then the relaxation results are consistent with two possibilities. Either the spin-lattice relaxation times within the compartments are similar or the rate of water exchange between compartments is "fast" enough to ensure volume averaged T(1) relaxation yet "slow" enough to allow for the observation of biexponential ADC decay curves over an extended b-factor range. Magn Reson Med 44:292-300, 2000.