Faculty Bibliography

A variable-density spiral method is presented for reducing three-dimensional tailored radiofrequency pulse duration. Pulse length reductions of 21-32% are possible, with only a small error in the desired excitation profile. The method is demonstrated using simulations, phantom experiments, and T(2)*-weighted images of brain regions with susceptibility-induced intravoxel dephasing. Four 19.7-ms shots were needed to excite a 5-mm-thick slice with reduced susceptibility artifacts in the sinus region at 3T.

We have developed a model of reversible cerebral ischemia in a high-level nonhuman primate. By using endovascular techniques, the posterior cerebral artery is permanently occluded with coils, and the ipsilateral middle cerebral artery is temporarily occluded with a balloon. The balloon can be deflated and/or removed to reestablish flow at precise time intervals. Functional imaging of the brain can be performed during occlusion and reperfusion, since the balloon can be deflated or removed in a scanner

This paper addresses the problem of enhancing spatiotemporal resolution of ultra-small superparamagnetic iron oxide (USPIO)-enhanced dynamic MRI of rat kidneys. To alleviate the limited resolution problem of conventional full-scan Fourier imaging methods, we use a generalized series-based imaging scheme to reduce coverage of kappa-space. Experimental results demonstrate that the generalized series imaging method with basis functions constructed using two references (pre- and post-contrast) can reduce the number of phase encodings measured during the dynamic contrast wash-in process by a factor of 4 with a negligible or minimal loss of image quality. The method is expected to make 3D studies possible using USPIO-enhanced dynamic imaging of rat kidneys, and prove valuable for early detection of renal rejection after kidney transplantation.

A multishot 3D slice-select tailored RF pulse method is presented for the excitation of slice profiles with arbitrary resolution. This method is derived from the linearity of the small tip angle approximation, allowing for the decomposition of small tip angle tailored RF pulses into separate excitations. The final image is created by complex summation of the images acquired from the individual excitations. This technique overcomes the limitation of requiring a long pulse to excite thin slices with adequate resolution. This has implications in applications including T*(2)-weighted functional MRI in brain regions corrupted by intravoxel dephasing artifacts due to susceptibility variations. Simulations, phantom experiments, and human brain images are presented. It is demonstrated that at most four shots of 40 ms pulse length are needed to excite a 5 mm-thick slice in the brain with reduced susceptibility artifacts at 3T.

BACKGROUND: Cerebrovascular disease is thought to play a role in the pathogenesis of geriatric major depression. One finding supporting such a "vascular depression" is the increased neuropathology in the form of white matter hyperintensities (WMH) found in patients diagnosed with a late-onset depression. However, at present there is little evidence that a longitudinal increase in WMH burden within an individual is associated with the onset of a late-life depression. METHODS: This study examined three-year longitudinal change in WMH volume and in cognition in: (a) an older man who developed his first episode of major depression during the study period, and (b) a comparison group of twelve older individuals who remained depression free. All subjects received at baseline and three years later a structural magnetic resonance imaging (MRI) using fast-FLAIR technology. The images were analyzed with semi-automated computerized software to obtain WMH volumes. Subjects also received at both time points the Mini Mental State Exam (MMSE) as well a series of cognitive tasks assessing executive abilities (verbal fluency, Trail Making Test and Stroop test) since executive dysfunction is thought to be characteristic of a vascular depression. RESULTS: The individual who became depressed during the followup showed an increase in WMH volume that exceeded the 95% Confidence Intervals (CI) for change in the comparison group. This individual also showed a similar decline on the measures of executive function but not on the MMSE. CONCLUSIONS: These results are consistent with cerebrovascular disease being a factor in the pathogenesis of late-onset depression (i.e. "vascular depression").

A new method for selectively detecting sodium ions in anisotropic environments is presented. A spin-lock (SL) sequence, followed by a coherence transfer pulse, generates rank-two zero-quantum coherences, and converts them into observable transverse magnetization. The quadrupolar polarization is only generated when there are residual quadrupolar couplings in the sample, and provided the SL field strength is comparable to these couplings. This filter has proved to be more efficient than a double-quantum magic-angle (DQ-MA) filter in generating observable signal from ions in anisotropic media in both a nasal bovine cartilage sample and a liquid crystalline DNA sample. Finally, the SL filtering technique does not rely on a flip angle effect for the selection of the desired signal component, as does a DQ-MA filter, and may therefore prove desirable in an imaging experiment, due to its better tolerance to phase and flip angle imperfections.

A new interface combining phased arrays and echo-planar imaging (EPI) technologies was developed for two channel breast MR EPI applications. A detailed design for a dual-channel, EPI-compatible, phased array breast coil is described. EPI digital data multiplexing, signal controlling and sampling schemes are also presented. Results from breast phantoms and patients demonstrate a 55% improvement in signal-to-noise ratio when compared to a conventional two-loop, single channel coil configuration. This method can be easily expanded to a four or more channel, EPI-compatible, phased array system to improve field-of-view coverage and signal-to-noise ratio.