Scanning for the scanner: FMRI of audition by read-out omissions from echo-planar imaging

Combined acquisition technique (CAT) for neuroimaging of multiple sclerosis at low specific absorption rates (SAR)

PURPOSE

To compare a novel combined acquisition technique (CAT) of turbo-spin-echo (TSE) and echo-planar-imaging (EPI) with conventional TSE. CAT reduces the electromagnetic energy load transmitted for spin excitation. This radiofrequency (RF) burden is limited by the specific absorption rate (SAR) for patient safety. SAR limits restrict high-field MRI applications, in particular.

MATERIAL AND METHODS

The study was approved by the local Medical Ethics Committee. Written informed consent was obtained from all participants. T2- and PD-weighted brain images of n = 40 Multiple Sclerosis (MS) patients were acquired by CAT and TSE at 3 Tesla. Lesions were recorded by two blinded, board-certificated neuroradiologists. Diagnostic equivalence of CAT and TSE to detect MS lesions was evaluated along with their SAR, sound pressure level (SPL) and sensations of acoustic noise, heating, vibration and peripheral nerve stimulation.

RESULTS

Every MS lesion revealed on TSE was detected by CAT according to both raters (Cohen's kappa of within-rater/across-CAT/TSE lesion detection κCAT = 1.00, at an inter-rater lesion detection agreement of κLES = 0.82). CAT reduced the SAR burden significantly compared to TSE (p<0.001). Mean SAR differences between TSE and CAT were 29.0 (± 5.7) % for the T2-contrast and 32.7 (± 21.9) % for the PD-contrast (expressed as percentages of the effective SAR limit of 3.2 W/kg for head examinations). Average SPL of CAT was no louder than during TSE. Sensations of CAT- vs. TSE-induced heating, noise and scanning vibrations did not differ.

CONCLUSION

T2-/PD-CAT is diagnostically equivalent to TSE for MS lesion detection yet substantially reduces the RF exposure. Such SAR reduction facilitates high-field MRI applications at 3 Tesla or above and corresponding protocol standardizations but CAT can also be used to scan faster, at higher resolution or with more slices. According to our data, CAT is no more uncomfortable than TSE scanning.

Influence of selecting EPI readout-encoding bandwidths on arterial spin labeling perfusion MRI

OBJECT

The objective of this study was to investigate effects of varying readout bandwidths on the arterial spin labeling (ASL)-perfusion MRI measurements at a high magnetic field MRI system.

MATERIALS AND METHODS

Brain perfusion studies were performed on nine volunteers (four males, five females) using flow sensitive alternating inversion recovery (FAIR) ASL single-shot echo-planar imaging (EPI)-MRI. To investigate EPI bandwidth effects on the time-series perfusion-weighted imaging (PWI) data, two regions-of-interest (ROI) were placed outside the brain to determine the level of noise and another ROI inside the brain to determine the level of signal. Coefficients of variations (CoV) were calculated for the time-series PWI data. One-way analysis of variance (ANOVA) was used to investigate voxel-wise differences in the time-series PWI data between two different bandwidth values.

RESULTS

At the level of ROI, there was no significant effect of changing EPI bandwidths on the time-series PWI data in any of the volunteers (P > 0.031). In contrast, CoV values over the dynamic PWI data varied with depending on selecting EPI bandwidths and voxel-based tests showed that N2 ghosting, modulated by EPI bandwidth, can appear in some brain regions, especially in areas that overlap with the spatial distribution of N2 ghosting artifacts.

CONCLUSIONS

Although N2 ghosting can be reduced by adjusting the bandwidth of EPI on the time-series of PWI data, the effects cannot be entirely eliminated. In particular, N2 ghosting can bias CBF quantification if EPI control scans to determine the equilibrium-state signal are confounded by N2 ghosting. Therefore, careful tuning of the bandwidth of EPI is necessary to avoid artifacts in the ASL signal from N2-ghosting.