Echo-planar imaging of diffusion and perfusion

Gradient coil design: a review of methods

Methods of coil design for MRI gradient coils and other purposes are discussed. The conflicting requirements for satisfactory gradient coil performance are described and analyzed. Design methods are classified into discrete wire techniques and current density techniques. The advantages and disadvantages of each group of methods are described, and optimization methods are summarized. These include the target field approach, and constrained inductance and power minimization. Design for low fringe fields is described, including active shielding.

Evaluation of experimental early acute cerebral ischemia before the development of edema: use of dynamic, contrast-enhanced and diffusion-weighted MR scanning

The ability of dynamic, contrast-enhanced, magnetic susceptibility-weighted scanning to delineate early experimental acute cerebral infarction was compared with that of heavily T2-weighted and diffusion-weighted spin echo scanning. Spontaneously hypertensive rats, which had undergone right middle cerebral artery occlusion, were studied from 15 min to 3 h post ligation on a 1.5-T clinical whole-body imager. In contrast to the diffusion- and T2-weighted spin echo scans, the dynamic, contrast-enhanced technique clearly and consistently delineated the nonperfused regions as early as 15 min post ligation.

Ultrafast MR imaging of water mobility: animal models of altered cerebral perfusion

"Single shot" magnetic resonance (MR) diffusion imaging was used to study the details of signal decay curves in experimental perturbations of cerebral perfusion induced by hypercapnia or death. Despite large perfusion increases observed with dynamic susceptibility-contrast MR imaging, no correlation with these changes was seen in either the diffusion coefficient or any other intravoxel incoherent motion (IVIM) model parameters in dog gray matter as arterial carbon dioxide pressure increased. Non-monoexponential signal decay in cat gray matter was seen both before and after death. In addition, cat gray matter demonstrated a steady decrease in the diffusion coefficient after death. These data are strong evidence that the fast component of the non-monoexponential diffusion-related signal decay is not due solely to perfusion. The authors believe that a second compartment of nonexchanging spins, most likely cerebrospinal fluid, accounts for the non-monoexponential decay.

Diffusion imaging of the human brain in vivo using high-speed STEAM MRI

This paper describes a new method for diffusion imaging of the human brain in vivo that is based on a combination of diffusion-encoding gradients with high-speed STEAM MR imaging. The single-shot sequence 90 degrees-TE/2-90 degrees-TM-(alpha-TE/2-STE)n generates n = 32-64 differently phase-encoded stimulated echoes STE yielding image acquisition times of 576 ms for a 48 x 128 data matrix. Diffusion encoding is performed during the first TE/2-interval as well as during each readout period. Phantom studies reveal a quantitative agreement of calculated diffusion coefficients with literature values. EKG triggering completely eliminates motion artifacts from diffusion-weighted single-shot STEAM images of human brain in vivo. While signal attenuation of the cerebrospinal fluid (CSF) is predominantly due to flow, that observed for gray and white matter results from diffusion. Evaluated diffusion coefficients yield (1.0 +/- 0.1) x 10(-5) cm2 s-1 for gray matter, (0.5 +/- 0.1) x 10(-5) cm2 s-1 for white matter with the diffusion encoding parallel to the main orientation of the myelin sheath of the neurofibrils, and (0.3 +/- 0.1) x 10(-5) cm2 s-1 for white matter and a perpendicular orientation. All studies were performed at 2.0 T using a conventional 10 mT m-1 gradient system.