In vitro evaluation of a mechanical injector for infusion of magnetic resonance contrast media.

The authors evaluated the performance of a prototype mechanical injector developed for infusion of magnetic resonance (MR) contrast media. The injector was installed in a 0.6-T clinical MR scanner, and evaluation was made using saline (viscosity: 1.0 cP) and gadopentetate dimeglumine (viscosity: 4.9 cP) with 16-, 21-, and 25-gauge needles, and 244- and 366-cm connecting tubing. At a fixed flow rate of 1 mL/second, volume infused was within 10% of the desired volume except for infusions less than 1 mL. Reproduction of flow rates was less reliable. With saline, maximal flow rates were 5.5 mL/second, 4.2 mL/second, and 1.1 mL/second for the 16-, 21-, and 25-gauge needles, respectively, and 3.7 mL/second, 2.4 mL/second, and 0.9 mL/second, respectively, for gadopentetate dimeglumine. Needle size was identified as the major factor limiting flow rate. The magnet was reshimmed after the injector was installed, and no deleterious effects were identified on gradient echo (TR: 250/TE:18/flip angle 60 degrees) MR images.

Contrast agents and cerebral hemodynamics.

Contrast-enhanced magnetic resonance imaging of regional cerebral hemodynamics is discussed. Techniques for measuring cerebral blood volume (CBV) have been validated in animal models and have recently been applied to human studies. Factors affecting CBV measurement in pathologic tissue are addressed. Extension of these techniques to the measurement of cerebral blood flow is presented.

MR contrast due to microscopically heterogeneous magnetic susceptibility: numerical simulations and applications to cerebral physiology.

We calculate the effects of subvoxel variations in magnetic susceptibility on MR image intensity for spin-echo (SE) and gradient-echo (GE) experiments for a range of microscopic physical parameters. The model used neglects the overlap of gradients from one magnetic inclusion to the next, and so is valid for low volume fractions and weak perturbations of the magnetic field. Transverse relaxation is predicted to deviate significantly from linear exponential decay in both SE and GE at a particle radius of 2.5 microns. Calculated changes in transverse relaxation rates for SE and GE increase linearly with volume fraction of high-susceptibility regions of 5 microns diameter, but increase with about the 3/2 power of volume fraction of regions with 15 micron spacing between centers. This sensitivity to the actual size and spacing of magnetized regions may allow them to be measured on the basis of contrast. without being resolved in images. GE and SE decay rates are approximately twice as sensitive to long cylinders of 5 microns diameter than to spheres of the same size, for diffusion constants of 2.5 micron 2/ms. Calculated changes in transverse decay rates increase with approximately the square of field and susceptibility variation for 5-microns spheres and a diffusion constant of 2.5 microns 2/ms. This exponent is smaller for cylindrical magnetized regions of the same size, and also depends on the diffusion constant. We discuss possible applications of our theoretical results to the analysis of the effects of high-susceptibility contrast agents in brain. Experimental data from the literature are compared with calculated signal changes according to the model. The monotonic dependence of decay rates on the volume of distribution of the contrast agent suggests that cerebral blood volume and flow could be measured using MR contrast.

Susceptibility induced MR line broadening: applications to brain iron mapping.

Magnetic susceptibility variations due to the presence of iron in neural tissue can result in a shift of local resonance frequency, decreased T2 resulting from water diffusion through local field gradients, and line broadening due to field inhomogeneity within a voxel. In this study, modified spin echo phase contrast pulse sequences were used to map proton resonance line widths in phantoms and in vivo. In agar gels containing varying concentrations of Fe3O4 (magnetite), line broadening mechanisms permitted accurate spatial localization of iron deposits from measurement of local resonance line widths. Furthermore, line widths estimated from the data were strongly correlated with iron concentrations, indicating the potential quantitative applications of the method. The technique was applied clinically to map iron in subacute brain hemorrhages. These data suggest that resonance line widths may be a useful measure of brain iron content.