MRI under hyperbaric air and oxygen: effects on local magnetic field and relaxation times.

PURPOSE: Hyperbaric oxygen therapy has shown efficacies in the treatment of a number of diseases. The goal of this study was to develop a rodent hyperbaric chamber for MRI studies and to investigate the effects of hyperbaric air and hyperbaric oxygen on local magnetic field (B0 ) and MRI relaxation parameters in the rat brain. METHODS: A hyperbaric chamber, constructed to fit inside an animal MRI scanner, was pressurized with air to four atmospheres, while oxygen was delivered locally via nose cone. B0 , T2 , T2 *, and T1 maps in the rat brain were evaluated under normobaric air, hyperbaric air, and hyperbaric oxygen at 7T. RESULTS: Under hyperbaric oxygen, images exhibited artifacts and temporal instability, attributable to fluctuating oxygen concentration from air and oxygen mixing near the imaging region. Physically shielding the imaging region from fluctuating oxygen concentration resolved the problems. With increasing oxygen at hyperbaric pressure, B0 was shifted downfield with increased inhomogeneity near the ear canals and nose. Brain T2 and T2 * were lengthened, and T1 was shortened. CONCLUSION: This study establishes the means to perform MRI on rodents under hyperbaric conditions. Hyperbaric air and hyperbaric oxygen have significant effects on B0 and tissue relaxation parameters compared with normobaric air.

A low cost color visual stimulator for fMRI.

This low cost visual stimulator was developed for use in small animal imaging. The stimulator uses a single tri-color LED for each eye and can output red, green, or blue light or any combination of the three. When all three LED colors are illuminated at the same time achromatic light is the output. The stimulator is almost entirely implemented in software with only minimal electronics. The LEDs are controlled via the parallel port of a desktop computer. Flicker frequency, wavelength, intensity and waveform shape are under software control. The LEDs are coupled to fiber optic cables which run into the MRI scanner room leaving the LEDs and the power source in the control room. Calibration with a radiometer shows the light output to be very linear from zero to full intensity. The stimulator was used in fMRI visual stimulation studies performed on Sprague Dawley rats with an 11.7Tesla magnet. As the stimulator is software driven, modifications to accommodate other protocols and extensions for new functionality can be readily incorporated. With this in mind, the visual stimulator circuit diagram and software including source code are available upon request.

Routine testing of magnetic field homogeneity on clinical MRI systems.

Poor main magnetic field (B0) homogeneity (H(B)) leads to artifacts and signal losses in magnetic resonance imaging (MRI). The American College of Radiology's MRI quality control manual mandates annual checks of H(B), suggesting tests using spectral linewidth and phase-difference (delta phi) maps. A new method, the bandwidth-difference (deltaBW) method, which compares the distortion for small and large BW acquisitions to determine the HB, is proposed. The deltaBW method has the advantage that it can be used to measure multiple diameters of spherical volumes (DSV) in a single phantom. A phantom has been developed to exploit this method and results obtained with it are compared to those using three standard methods. Small receiver BW in the presence of poor H(B) leads to geometric distortions because gradients are reduced to the level of the B0 inhomogeneities. Data were acquired using seven MRI systems from different manufacturers, ranging in field strength from 0.2 to 3.0 T. Fast gradient echo pulse sequences were scanned twice using small and large BWs. H(B) was measured from the shift of landmarks between the two BW acquisitions. Results were compared with data from the full width at half maximum (FWHM) method, the delta phi method and one manufacturer's resonant frequency mapping data. The FWHM method was available on two systems and the detla phi method was available on one. The deltaBW method could be performed in all scanners investigated. The H(B) measured ranged 0.11-0.32 ppm to 6.7-12.9 ppm for DSV of 13-22.6 cm. Direct comparisons of the data obtained using the deltaBW method showed good agreement with data obtained using the FWHM method. Data obtained using the deltaBW method compared favorably with the manufacturer's resonant frequency map. The deltaBW method produces measurements of H(B) at various DSV values that can be obtained from a single set of phantom images. The accuracy of deltaBW B0 homogeneity measurements are comparable to the other methods tested.

Evaluation of an image-guided, robotically positioned transcranial magnetic stimulation system.

The emergence of transcranial magnetic stimulation (TMS) as a tool for investigating the brain has been remarkable over the past decade. While many centers are now using TMS, little has been done to automate the delivery of planned TMS stimulation for research and/or clinical use. We report on an image-guided robotically positioned TMS system (irTMS) developed for this purpose. Stimulation sites are selected from functional images overlaid onto anatomical MR images, and the system calculates a treatment plan and robotically positions the TMS coil following that plan. A new theory, stating that cortical response to TMS is highest when the induced E-field is oriented parallel to cortical columns, is used by the irTMS system for planning the position and orientation of the TMS coil. This automated approach to TMS planning and delivery provides a consistent and optimized method for TMS stimulation of cortical regions of the brain. We evaluated the positional accuracy and utility of the irTMS system with a B-shaped TMS coil. Treatment plans were evaluated for sites widely distributed about a head phantom with well-defined landmarks. The overall accuracy in positioning the planned site of the TMS coil was approximately 2 mm, similar to that reported for the robot alone. The estimated maximum range of error in planned vs. delivered E-field strength was +4%, suggesting a high degree of accuracy and reproducibility in the planned use of the irTMS system.