Gary X. Shen

High-resolution echo-planar fMRI of human visual cortex at 3.0 tesla

Known specialized properties of the human visual cortex have been used to investigate the role of spatial resolution on fMRI using blood oxygenation level dependent (BOLD) echo‐planar MRI at 3.0 tesla. The magnitude of BOLD signal changes has been examined at low (3.1×3.1×3.0 mm3) and high (0.8×1.6×3.0 mm3) resolution using both gradient‐echo and spin‐echo EPI. Paradigms were designed to activate primary visual cortex (V1/V2) and more specialized areas associated with detection of color (V4) and motion (V5). Sensitivity of activation maps increased at higher resolution despite the decreased total signal intensity at the smaller voxel size, presumably from reduced partial volume averaging. The greater microvascular selectivity of high‐resolution spin‐echo imaging enabled distinct activation patterns sensitive to motion to be detected in V1/V2 that were not apparent with gradient‐echo imaging. The spatial resolution at 3.0 tesla was constrained by the size of physiological head motion relative to the voxel dimensions rather than SNR or the hemodynamic response of BOLD contrast. The higher spatial resolution at 3.0 tesla with more selective spin‐echo EPI can further refine functional mapping within the cerebral cortex.

Three-dimensional triple-quantum-filtered 23Na imaging of in vivo human brain

A scheme for the generation of three‐dimensional, triple‐quantum–filtered (TQ) sodium images from normal human brain is presented. In this approach, a three‐pulse, six‐step, coherence transfer filter was used in conjunction with a fast twisted projection imaging sequence to generate spatial maps of the TQ signal across the entire brain. It is demonstrated, theoretically as well as experimentally, that the use of the three‐pulse coherence filter leads to TQ sodium images in which the dependence of the image intensity on the spatial variation of the flip angle is less pronounced than it is in the “standard,” four‐pulse, TQ filter. Correction for the variation of the TQ signal intensity across the field of view because of radio‐frequency (RF) inhomogeneity is straightforward with this approach. This imaging scheme allows the generation of RF inhomogeneity–corrected, TQ, sodium images from human brain at moderate field strength (3.0 T) in times acceptable for routine clinical examinations (20 minutes). Magn Reson Med 42:1146–1154, 1999.

Experimentally verified, theoretical design of dual-tuned, low-pass birdcage radiofrequency resonators for magnetic resonance imaging and magnetic resonance spectroscopy of human brain at 3.0 Tesla

A new theoretical method is presented for designing frequency responses of double‐tuned, low‐pass birdcage coils. This method is based on Kirchhoffs equations through a nonsymmetric matrix algorithm and extended through a modification of the corresponding eigenvalue system from a single‐tuned mode. Designs from this method are verified for sodium/proton, dual‐tuned, double‐quadrature, low‐pass birdcage coils at 1.5 Telsa and 3.0 Tesla and then are used to design dual‐tuned, double‐quadrature, lithium/proton and phosphorus/proton birdcage coils for 3.0 Tesla. All frequencies show experimental deviations of less than 3% from theory under unloaded conditions. The frequency shifts caused by loading and radiofrequency shielding are less than 1 MHz and can be compensated readily by adjustment of variable capacitors. Applications to human neuroimaging and spectroscopy are demonstrated.Magn Reson Med 41:268–275, 1999.

Functional, physiological, and metabolic toolbox for clinical magnetic resonance imaging: Integration of acquisition and analysis strategies

Selected neuroimaging strategies have been integrated into a clinical brain imaging protocol to provide quantitative high‐resolution functional, physiological, and metabolic maps to complement exquisitely detailed anatomic images without excessively prolonging the conventional clinical examination or analysis time. The physiological maps of blood pool parameters (relative cerebral blood volume, tissue transit time, and arrival time), apparent diffusion coefficient, tissue water content, and functional neuronal activation maps are derived from series of images acquired with echo‐planar imaging. The metabolic map reflecting tissue sodium homeostasis (tissue sodium concentration) is acquired using twisted projection imaging and a customized dual‐tuned, dual‐quadrature 23Na/1H brain radiofrequency coil that ensures coregistration of data and avoids moving the patient. The different types of acquired images are transferred to a common file format with customized file management software and the corresponding maps are derived by applying appropriate fitting algorithms. Customized software allows rapid interrogation and manipulation of all resultant images and maps for detailed but rapid interpretation, printing, and archiving immediately following completion of acquisition. As all acquisitions and processing are performed by the magnetic resonance technologist, the neuroradiologist is able to focus on the interpretation of this immensely rich data set.

DATA ACQUISITION AND POSTPROCESSING STRATEGIES FOR FAST QUANTITATIVE SODIUM IMAGING

An efficient scheme for quantitatively mapping the three‐dimensional distribution of the sodium ion in vivo using magnetic resonance imaging is described. To make the methodology totally quantitative, the data acquisition scheme is performed with very short echo times and negligible T1 saturation. Removal of signal variation due to imperfect radiofrequency (RF) response is accomplished using RF inhomogeneity maps acquired during each study. The high efficiency of the k‐space trajectories allows the entire data collection process to be performed in under 10 min. The theory underlying the data collection and processing scheme is described along with representative examples acquired at 1.5 and 3.0 T.