Design and initial evaluation of a low-cost 3-Tesla research system for combined optical and functional MR imaging with interventional capability

Effects of combining field strengths on auditory functional MRI group analysis: 1.5T and 3T

PURPOSE

To evaluate effects of combining functional magnetic resonance imaging (fMRI) data acquired from different field strengths on group analysis as a function of the number of subjects at each field strength.

MATERIALS AND METHODS

In all, 28 subjects (18 at 3T) participated in an auditory task of passively listening to a 0.75s segment of jazz music in an event-related design. Results of single-subject analysis were combined to create all possible subject combinations for a group size of eight subjects from each of the 3T and 1.5T pools, comprising subject mixtures of (3T/1.5T) 0/8, 2/6, 4/4, 6/2, and 8/0. Group analysis performance of each subject permutation was measured by receiver operating characteristic (ROC) curves and activation overlap maps.

RESULTS

While area under ROC curves, extent of activation in the gold standard region, and reliability of activation increased with the number of 3T subjects, marginal gain decreased. ROC performance overlap across mixtures was observed, indicating that some combinations of subjects markedly outperformed others. For detection of activation, 4/4 was arguably the minimum mixture level that was comparable to 3T-only group results.

CONCLUSION

Inclusion of 1.5T data does not necessarily reduce the validity of group analysis. Lower field strength data was found only to limit detection power, but did not affect specificity. Within the limits of realignment error, these results should also extend to group longitudinal analyses of subject mixtures from different field strengths.

Method for rapid MRI needle tracking

A new method for MRI needle tracking within a given two-dimensional (2D) image slice is presented. The method is based on k-space investigation of the difference image between the current dynamic frame and a reference frame. Using only a few central k-lines of the difference image and a nonlinear optimization procedure, one can resolve the parameters that define the 2D sinc function that best characterizes the needle in k-space. The spatial location and orientation of the needle are determined from these parameters. Rapid needle tracking is obtained by repeated acquisitions of the same set of several central k-lines (as in a "keyhole" protocol) and repeated computation of these parameters. The calculated needle tip is depicted on the reference image by means of a graphic overlay. The procedure was tested in computer simulations and in actual MRI scans (the computations were done offline). It was demonstrated that six k-lines out of 128 usually suffice to locate the needle. The refresh rate of the needle location depends on the time required to sample the subset of k-lines, calculate the current needle location, and refresh the reference image.

3T MR for Clinical Use: Update

Clinical experience with 3-tesla MRI for body applications has only a short history; to date, it has no proven practical advantage over 1.5-tesla or less powerful systems. However, the theoretical advantage of higher field strength-which includes a higher signal-to-noise ratio with identical scan parameters, higher special resolution available within clinically acceptable scan time, and better spectral resolution-can contribute to the clinical outcome. This paper seeks to demonstrate some of the clinical advantages of the 3-tesla system through preliminary experiences as well as the potential advantages of a 3-tesla system when combined with much of the hardware and software that is clinically accepted in the 1.5-tesla world.