Accuracy of noninvasive quantification of brain NAA concentrations using PRESS sequence: verification in a swine model with external standard.

The metabolite ratios had been employed in the field of MR spectroscopy (MRS) for a long period. The main drawback of metabolite ratio is that ratio results are not comparable with absolute metabolite concentration in vivo. The purpose of this study was to examine the accuracy of noninvasive quantification of brain N-acetylaspartate (NAA) concentrations using previously reported MR external standard method. Eight swine were scanned on a GE 1.5 T scanner with a standard head coil. The external standard method was utilized with a sphere filled with NAA, GABA, glutamine, glutamate, creatine, choline chloride, and myo-inositol. The position resolved spectroscopy (PRESS) sequence was used with TE=135 msec, TR=1500 msec, and 128 scan averages. The analysis of MRS was done with SAGE/IDL program. In vivo NAA concentration was obtained using the equation S=N * e(-TE/T2) * [1-e(-TR/T1). In vitro NAA concentration was measured by high performance liquid chromatography (HPLC). In the MRS group, the mean concentration of NAA was 10.03 plusmn 0.74 mmol/kg. In the HPLC group, the mean concentration of NAA was 9.22 plusmn 0.55 mmol/kg. There was no significant difference between the two groups (p = 0.46). However, slightly higher value was observed in the MRS group (7/8 swine), compared with HPLC group. The range of differences was between 0.02~2.05 mmol/kg. MRS external reference method could be more accurate than internal reference method. 1H MRS does not distinguish between N-acetyl resonance frequencies and other N-acetylated amino acids.

Improving spatial signal homogeneity in MR 2D chemical shift imaging using outer volume saturation bands.

Many endeavors of improving chemical shift imaging (CSI) techniques have been made during last two decades. Good examples of two-dimensional CSI and three-dimensional CSI can be found in the literature. However, clinical CSI using available sequences is still not satisfactory. The purpose of this study was to assess the effect of outer volume saturation bands on signal homogeneity in MR 2D chemical shift imaging. The 2D CSI scans were acquired using a point resolved spectroscopy (PRESS) CSI sequence on a phantom filled with brain metabolites. A single PRESS volume of interest was prescribed graphically. The acquisition matrix was 18x18 phase encodings over a 24-cm FOV. Identical acquisitions were obtained with and without outer-volume saturation bands. After initial acquisition was obtained, four more acquisitions were repeated for both studies with and without saturation bands. Identical five groups of voxels were compared for both studies. Standard deviations of metabolite ratios were calculated in each group for both studies. Spectra obtained without outer-volume saturation bands showed signal to noise gradient with higher concentration of signal within voxels at the center of the volume of interest. Outer volume saturation bands reduced this gradient. In general, standard deviations of metabolite ratios with saturation bands were smaller than those without saturation bands. Improved spatial homogeneity of spectra in voxels of CSI with saturation bands was obtained. Outer-volume saturation bands improve spatial signal homogeneity of chemical shift imaging.

Pathological studies of CT findings in supratentorial astrocytoma. Correlation between low-density lesions and changes in fine structure.

CT findings and pathological features of 102 supratentorial astrocytomas were evaluated, and the surgically removed specimens of 10 cases studied by electron microscopy. Five pathological indices-anaplasia, vascularity, interstitial water content, necrosis and tumor fibers were graded from 1 to 4, and studies by the double-blind method. CT scans were divided into 4 groups according to attenuation coefficients and contrast enhancement. The data were statistically analysed. The results showed that the interstitial water content and tumor fibers were statistically different in low-density lesions and isodensity or hyperdensity lesions. The peritumoral low-density specimens correlated well with infiltrated tumor cells, which had swollen mitochondria. No definite correlation was found between the histological subtypes of astrocytomas and CT images. These findings suggest that high water content and lower physical density are responsible for the low-density of astrocytomas. Abnormalities of capillary ultrastructure result in the accumulation of contrast medium. The peritumoral area of low-density seen on CT scans is the real extent of the edematous tumor. But it is impossible to distinguish the histological subtypes of astrocytomas by CT findings.