This paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo time TE is less than a known characteristic time tc for a given system, then the signal decays exponentially with an argument which depends quadratically on TE. This is equivalent to an R2* relaxation rate which is a linear function of TE. In the opposite case, when TE is greater than tc, the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction sigma occupied by the field-creating objects, b) magnetic field Bo or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference delta chi between the objects and the medium.

No abstract

Modeling the effects of RF penetration in magnetic resonance (MR) imaging requires a knowledge of the local values of conductivity and permittivity. The inverse problem of determining the electlic properties of the materid under investigation using the M R images themselves has not previously been addressed.W e review such an approach for the heterogeneous layer model and examine the parsmeter sensitivity l o geometry and signal-to-noise ratio. For a few-layer system, it is within the realm of present day M a systems to extract the electric properties 10 within 10% or better. Knowledge of the electrical propertis will then allow a better prediction of the R F penetration and power deposition at high fields.

The authors discuss the appropriate FISP (fast imaging with steady-state precession) sequence structure to maintain constant phase at the radio-frequency pulse in the presence of motion. They present preliminary results of its application to head and spine imaging in an effort to maintain contrast between the cerebrospinal fluid (CSF) and the soft tissue. In the usual application of these FISP-like sequences, the gradient structure is modified to avoid unwanted signal (and contrast) variations due to field inhomogeneities. This change makes the signal sensitive to motion with a resulting decrease in signal intensity for moving tissue. The expected high contrast at large flip angles for tissues with low T1/T2 ratios such as CSF is not obtained. The technique discussed here overcomes the effects of field inhomogeneities and compensates for moving spins so that the transverse steady-state equilibrium and hence high contrast are obtained simultaneously.

No abstract

No abstract

Numerous techniquesexist for suppressing ghosting artifacts due to respiratory motion on MR images. Although such methods can remove coherent ghosting artifacts, motion during gradient pulses also leads to poor image quality. This is due to phase variations at the echo caused by changes in velocity from one phase-encoding view to the next. The effect becomes severe for long samplIng times and long TE values and can lead to low estimates of T2. We discuss general, robust modifications of the standard gradient or spin-echo sequences by using rephasing gradients that force the phase of constant-velocity moving spins to be zero at the echo. These sequences lead to a significant reduction in motion artifacts and hence improvement in image quality. They can be applied to muftislice, multiecho, water/fat, and gating schemes as well. Since motion problems are universal, ft would appear that these modified sequences should come into common usage for MR imaging.

The authors assessed the clinical utility of a magnetic resonance angiography technique in the evaluation of intracranial circulation. Eighteen patients with a low likelihood of cerebrovascular disease (control group) and 40 patients with suspected cerebrovascular disease were imaged with a FISP (fast imaging with steady precession) sequence (repetition time of 50 msec, echo time of 15 msec, velocity compensation in the read and section-select directions with acceleration compensation in the read direction, 15 degrees anisotropic volume, and a 1.25-mm partition thickness). Ninety-four percent of images in the control group and 72% of images in the group with cerebrovascular disease were considered useful for diagnosis. This technique can provide accurate images of intracranial circulation and can be performed in conjunction with two-dimensional spin-echo or gradient-echo imaging. It was most useful in the evaluation of patent intracranial aneurysms, vessel displacement, and large-vessel occlusive disease. Disadvantages included limited field of view, persistent signal voids, limited spatial resolution, and inadequate depiction of lesions with slow flow.