The characteristics of transverse magnetisation decay of 120 longstanding lesions and 40 regions of normal-appearing white matter have been analysed in 40 patients with multiple sclerosis (MS) and 10 normal controls. Fifty lesions showed a biexponential decay in which two water compartments-one probably intracellular, the other extracellular-could be defined. There was a higher frequency of biexponential lesions in patients with a primary progressive course but no significant difference between benign and secondary progressive groups. Seventy lesions showed a monoexponential decay, of which 31 showed a T2 of greater than 200 ms, implying that these lesions were predominantly composed of extracellular rather than intracellular water. The results imply that an expanded extracellular space within chronic MS brain lesions is a common finding at all levels of disability and disease course. In so far as an expanded extracellular space implies axonal loss, the results suggest that the latter occurs commonly in longstanding MS lesions. The lack of correlation with disability suggests a limited role for the technique in therapeutic monitoring.

A new OCCA phantom using Oblique Cylinders and Contrast Adjustment for measuring the accuracy of brain lesion volume estimation schemes is described. It uses obliquely oriented cylinders made from acrylic rod, mounted in a water bath, to give realistic partial volume errors. Image intensities are inverted, scaled, and shifted, and noise is added, to form images that have realistic values of lesion-white matter contrast (5-30%) and contrast-to-noise ratio (3-20%). Artificial gray matter, CSF (cerebrospinal fluid), and scalp lipid are added because these bright areas may determine how the gray level display window is set. The performance of manual and contouring methods for estimating lesion volume was measured for three observers and nine lesions with individual volumes from 0.3 to 6.2 ml. There was a large variation, depending on the choice of method, the observer, and the lesion contrast. Volumes were usually overestimated, with the error increasing at high contrasts. The average error in estimating total lesion volume was 17% (range -16% to +30%). The OCCA phantom may have a role in training observers to improve their accuracy (and hence inter- and intraobserver reproducibility).

The selection of a suitable low dielectric constant liquid for radiofrequency coil nonuniformity measurements is described. Measurements of dielectric constant (relative permittivity) were made on a range of candidate liquids. After excluding liquids that were too inflammable or too viscous, Esso Bayol 82 oil (dielectric constant epsilon' = 2.37) was chosen. At 1.5 T, a 27 cm diameter cylindrical phantom (test object) filled with Bayol 82 has a maximum nonuniformity of 1.9% arising from radiofrequency standing waves. The maximum diameter cylinder that can be used without the nonuniformity exceeding 2% is given for a range of liquids and field strengths. The construction of customized containers that fit closely inside a radiofrequency head coil from glass fiber reinforced resin ("fiberglass") is described. Thermal expansion of the liquid takes place without a rise in the internal pressure in the container.

Three major models (from Tofts, Larsson, and Brix) for collecting and analyzing dynamic MRI gadolinium-diethylene-triamine penta-acetic acid (Gd-DTPA) data are examined. All models use compartments representing the blood plasma and the abnormal extravascular extracellular space (EES), and they are intercompatible. All measure combinations of three parameters; (1) kPSp is the influx volume transfer constant (min-1), or permeability surface area product per unit volume of tissue, between plasma and EES; (2) ve is the volume of EES space per unit volume of tissue (0 < ve < 1); and (3) K(ep), the efflux rate constant (min-1), is the ratio of the first two parameters (k(ep) = kPSp/ve). The ratio K(ep) is the simplest to measure, requiring only signal linearity with Gd tracer concentration or, alternatively, a measurement of T1 before injection of Gd (T10). To measure the physiologic parameters kPSp and ve separately requires knowledge of T10 and of the tissue relaxivity R1 (approximately in vitro value).