Induced hepatic fibrosis in rats: hepatic steatosis, macromolecule content, perfusion parameters, and their correlations--preliminary MR imaging in rats.

PURPOSE: To prospectively evaluate magnetic resonance (MR) imaging for the characterization of liver fibrosis by estimating fat and extracellular matrix content and hepatic perfusion parameters in CCl(4)-treated rats. MATERIALS AND METHODS: The animal research protocol was approved by the Institutional Animal Care and Use Committee. Fifty-two rats (38 treated, 14 control) were included. A CCl(4) mixture was injected three times per week for 2-16 weeks. Fat-to-water ratios (FWRs) were calculated. Images were obtained with 12 saturation offset frequencies; magnetization transfer ratios (MTRs) were calculated. Distribution volume (DV), mean transit time (MTT), and portal fraction (PF) of blood inflow were calculated. For pairwise group comparisons, an unequal two-tailed Student t test was used. For pairwise correlations between variables, Pearson correlation coefficients were calculated. For multiple pairwise comparisons, Bonferroni correction was performed by adjusting the significance level (alpha). RESULTS: FWR and DV were correlated with CCl(4) treatment duration from 0 through 8 weeks (r = 0.658, P < .001 and r = -0.664, P < .001, respectively; alpha = .010). PF and MTT were correlated with CCl(4) treatment duration from 0 through 16 weeks (r = -0.483, P = .002 and r = 0.414, P = .008, respectively; alpha = .010). DV was inversely correlated with FWR over the same period (r = -0.581, P < .001; alpha = .007). Fibrotic rats without cirrhosis had a higher FWR and lower DV and PF (P < .001, P < .001, and P = .004, respectively; alpha = .017) than control rats, and lower MTR, DV, and MTT (P = .014, .001, and .010, respectively; alpha = .017) than cirrhotic rats. Cirrhotic rats had a higher FWR and a lower PF (P < .001, alpha = .017) than control rats. CONCLUSION: Magnetization transfer contrast is not a specific indicator of increased fibrosis in diseased liver; steatosis may influence some perfusion parameters.

Origin of the radio frequency pulse artifact in simultaneous EEG-fMRI recording: rectification at the carbon-metal interface.

Simultaneous electroencephalograph-functional magnetic resonance imaging (EEG-fMRI) recording has become an important tool for investigating spatiotemporal properties of brain events, such as epilepsy, evoked brain responses, and changes in brain rhythms. Reduction of noise in EEG signals during fMRI recording is crucial for acquiring high-quality EEG-fMRI data. The main source of the noise includes the gradient artifact, the radio frequency (RF) pulse artifact, and the cardiac pulse artifact. Since the RF pulse artifact is relatively small in amplitude, little attention has been paid to this artifact, and its origin is not well understood. However, the amplitude of the RF pulse artifact fluctuates randomly even if a very high EEG sampling rate is used, making it more salient than the gradient artifact after postprocessing for noise removal. In this paper, we investigate the cause of the RF pulse artifact in EEG systems that use carbon wires.

On the application of chemical shift-based multipoint water-fat separation methods in balanced SSFP imaging.

Chemical shift-based multipoint water-fat separation methods have been applied in balanced steady-state free precession (bSSFP) sequences because of the high signal-to-noise-ratio (SNR) attainable. In this approach the echo formation is approximated to occur concurrently for both water and fat at an echo time (TE) equal to half the repetition time (TR/2 approximation). However, the degree to which the imaging conditions underlying the TR/2 approximation are satisfied can significantly vary in vivo depending upon the imaging region of interest (ROI) and the pixels across a field of view (FOV). The consequence of the TR/2 approximation on chemical shift-based multipoint water-fat separation was investigated. The influence of a mismatch between the pass-band profiles of water and fat (pass-band mismatch) on fat quantification was also examined. Theoretical and experimental results demonstrate that the TR/2 approximation can result in spatially dependent noise performance of multipoint water-fat separation methods, and the pass-band mismatch can render the precision of fat quantification spatially dependent. Given that local tissue characteristics in affected liver can be substantially variable, this study is of particular importance in liver imaging.

BOLD fMRI of the visual cortex: quantitative responses measured with a graded stimulus at 1.5 Tesla.

PURPOSE: To measure and quantitatively characterize an activity generated by the neurons of the visual cortex (VC) in response to graded luminous intensity contrast stimuli using a 1.5 Tesla scanner. MATERIALS AND METHODS: Functional magnetic resonance imaging (fMRI) of the vc with the intrinsic blood oxygenation level dependent (BOLD) mechanism was performed by using a paradigm with a 5 x 5 flashing checkerboard pattern flickering eight times per second at eight luminance contrasts presented in a randomized order. The changes of the luminance contrast were obtained by varying the luminance intensity of the white checkerboard squares. Each of eight trials, corresponding to eight luminance contrasts, consisted of six "rest" and six "activation" epochs, repeated five times, amounting to 60 measurement periods per trial. During each epoch, 10 contiguous oblique axial-to-coronal slices covering the calcarine fissure region and parallel to a line through the anterior-posterior commissure (AC-PC) markers were acquired using a gradient-recalled echo planar imaging (GRE-EPI) sequence. RESULTS: The measurements showed changes in the activation extent in the VC following the stimulus' rising luminance intensity contrast. In addition, the fMRI signal in those activated areas present throughout all eight trials, referred to as "common" voxels in this report, showed an increasing trend as a function of the rising luminance intensity contrast. CONCLUSION: These results suggest that the processes of the neuronal recruitment that affects the extent and number of activated neurons, and the neuronal enhancement that defines the magnitude of the neuronal activation are dependent on the luminance intensity contrast. These changes can be visualized and quantified using BOLD fMRI at 1.5 Tesla.

Quantitative functional MR imaging of the visual cortex at 1.5 T as a function of luminance contrast in healthy volunteers and patients with multiple sclerosis.

BACKGROUND AND PURPOSE: In patients with multiple sclerosis (MS), a few preliminary functional MR (fMR) imaging studies of the visual cortex reveal information about magnitude differences between healthy individuals and patients with MS at only a single luminance level. We therefore investigated whether varying luminance contrast levels can help uncover subtle changes in fMR imaging characteristics of the visual cortex in healthy volunteers and patients with MS. METHODS: Blood oxygenation level-dependent fMR imaging signal changes in the primary visual cortex were examined as a function of luminance contrast at 1.5 T in 10 healthy volunteers and nine patients with MS. Ten axial sections through the calcarine fissure were obtained with an echo-planar T2*-weighted imaging sequence (4000/54/1 [TR/TE/excitation]; field of view, 220 mm; voxel size, 1.72 x 1.72 x 5 mm). The imaging series consisted of an alternating 20-second rest epoch (black screen) with a 20-second activation epoch (flickering checkerboard) repeated six times. Each imaging series used a graded increase of eight luminance contrast levels. A paired t test between rest and activation images was used to analyze significant (P <.001) contiguous voxels in the region of interest (primary visual cortex). RESULTS: A progressive increase in fMR imaging activation across all luminance contrast levels in healthy controls and patients with MS was shown. The patients with MS had a significantly lower magnitude in the number of fMR imaging activated voxels at all luminance contrast levels (P <.001). A statistically significant increase in fMR imaging activation (activation threshold) was seen at the second luminance contrast level in controls and at the seventh level in patients with MS. CONCLUSION: Quantifiable changes in blood oxygenation level-dependent signal and a progressive increase in activated voxels within the primary visual cortex with increasing luminance contrast were demonstrated at 1.5 T in controls. The patients with MS showed a significant decrease in the number of activated voxels and an increase in activation threshold compared with healthy controls.