Faculty Bibliography

PURPOSE: To test the hypotheses that (i) the regional heterogeneity of brain sodium concentration ([Na(+)](br)) provides a parameter for ischemic progression not available from apparent diffusion coefficient (ADC) data, and (ii) [Na(+)](br) increases more in ischemic cortex than in the caudate putamen (CP) with its lesser collateral circulation after middle cerebral artery occlusion in the rat. MATERIALS AND METHODS: (23)Na twisted projection MRI was performed at 3 Tesla. [Na(+)](br) was independently determined by flame photometry. The ischemic core was localized by ADC, by microtubule-associated protein-2 immunohistochemistry, and by changes in surface reflectivity. RESULTS: Within the ischemic core, the ADC ratio relative to the contralateral tissue was homogeneous (0.63 +/- 0.07), whereas the rate of [Na(+)](br) increase (slope) was heterogeneous (P < 0.005): 22 +/- 4%/h in the sites of maximum slope versus 14 +/- 1%/h elsewhere (here 100% is [Na(+)](br) in the contralateral brain). Maximum slopes in the cortex were higher than in CP (P < 0.05). In the ischemic regions, there was no slope/ADC correlation between animals and within the same brain (P > 0.1). Maximum slope was located at the periphery of ischemic core in 8/10 animals. CONCLUSION: Unlike ADC, (23)Na MRI detected within-core ischemic lesion heterogeneity.

PURPOSE: To optimize the Rosette trajectories for fast, high sensitivity spectroscopic imaging experiments and to compare this acquisition technique with other chemical shift imaging (CSI) methods. MATERIALS AND METHODS: A framework for comparing the sensitivity of the Rosette Spectroscopic Imaging (RSI) acquisition to other spectroscopic imaging experiments is outlined. Accounting for hardware constraints, trajectory parameters that provide for optimal sampling and minimal artifact production are found. Along with an analytical expression for the number of excitations to be used in an RSI experiment that is provided, the theoretical precompensation weights used for optimal image reconstruction are derived. RESULTS: The spectral response function for RSI is shown to be approximately the same as the point spread function of standard Fourier reconstructions. While the signal-to-noise ratio (SNR) for an RSI experiment is reduced by the inherent nonuniform sampling of these trajectories, their circular k-space support and speed of spatial encoding leads to greater SNR efficiency and improvements in the total data acquisition time relative to the gold standard CSI approach with square k-space support and to similar efficiency to spiral CSI acquisitions. Numerical simulations and in vivo experimental data are presented to demonstrate the properties of this data acquisition technique. CONCLUSION: This work demonstrates the use of Rosette trajectories and how to achieve improved efficiency for these trajectories in a two-dimensional spectroscopic imaging experiment.

Three-dimensional (3D) twisted projection imaging (TPI) trajectory has a unique advantage in sodium ((23)Na) imaging on clinical MRI scanners at 1.5 or 3 T, generating a high signal-to-noise ratio (SNR) with a short acquisition time (approximately 10 min). Parallel imaging with an array of coil elements transits SNR benefits from small coil elements to acquisition efficiency by sampling partial k-space. This study investigates the feasibility of parallel sodium imaging with emphases on SNR and acceleration benefits provided by the 3D TPI trajectory. Computer simulations were used to find available acceleration factors and noise amplification. Human head studies were performed on clinical 1.5/3-T scanners with four-element coil arrays to verify simulation outcomes. In in vivo studies, proton ((1)H) data, however, were acquired for concept-proof purpose. The sensitivity encoding (SENSE) method with the conjugate gradient algorithm was used to reconstruct images from accelerated TPI-SENSE data sets. Self-calibration was employed to estimate coil sensitivities. Noise amplification in TPI-SENSE was evaluated using multiple noise trials. It was found that the acceleration factor was as high as 5.53 (corresponding to acceleration number 2 x 3, ring-by-rotation), with a small image error of 6.9% when TPI projections were reduced in both polar (ring) and azimuthal (rotation) directions. The average noise amplification was as low as 98.7%, or 27% lower than Cartesian SENSE at that acceleration factor. The 3D nature of both TPI trajectory and coil sensitivities might be responsible for the high acceleration and low noise amplification. Consequently, TPI-SENSE may have potential advantages for parallel sodium imaging.

Spiral MRI has several advantages over Cartesian MRI such as faster acquisitions and reduced demand in gradient. In parallel imaging, spiral trajectories are especially of great interest due to their inherent self-calibration capabilities, which is especially useful for dynamic imaging applications such as fMRI and cardiac imaging. The existing self-calibration techniques use the central spiral data that are sampled densely in the accelerated acquisition for coil sensitivity estimation. However, the resulting sensitivities are not sufficiently accurate for SENSE reconstruction due to the data truncation. In this paper, JSENSE which has been successfully used in Cartesian trajectories is extended to spiral trajectory such that the coil sensitivities and the desired image are reconstructed jointly to improve accuracy through alternating optimization. The improved sensitivities lead to a more accurate SENSE reconstruction. The results from both phantom and in vivo data are shown to demonstrate the effectiveness of JSENSE for spiral trajectory.

PURPOSE: To validate (23)Na twisted projection magnetic resonance imaging (MRI) as a quantitative technique to assess local brain sodium concentration ([Na(+)](br)) during rat focal ischemia every 5.3 minutes. MATERIALS AND METHODS: The MRI protocol included an ultrashort echo-time (0.4 msec), a correction of radiofrequency (RF) inhomogeneities by B(1) mapping, and the use of 0-154 mM NaCl calibration standards. To compare MRI [Na(+)](br) values with those obtained by emission flame photometry in precision-punched brain samples of about 0.5 mm(3) size, MR images were aligned with a histological three-dimensional reconstruction of the punched brain and regions of interest (ROIs) were placed precisely over the punch voids. RESULTS: The Bland-Altman analysis of [Na(+)](br) in normal and ischemic cortex and caudate putamen of seven rats quantitated by (23)Na MRI and flame photometry yielded a mean bias and limits of agreement (at +/-1.96 SD) of 2% and 43% of average, respectively. A linear increase in [Na(+)](br) was observed between 1 and 6 hours after middle cerebral artery occlusion. CONCLUSION: (23)Na MRI provides accurate and reliable results within the whole range of [Na(+)](br) in ischemia with a temporal resolution of 5.3 minutes and precisely targeted submicroliter ROIs in selected brain structures.

The signal loss susceptibility artifact is a major limitation in gradient-echo MRI applications. Various methods, including z-shim techniques and multidimensional tailored radio frequency (RF) pulses, have been proposed to mitigate the through-plane signal loss artifact, which is dominant in axial slices above the sinus region. Unfortunately, z-shim techniques require multiple steps and multidimensional RF methods are complex, with long pulse lengths. Parallel transmission methods were recently shown to be promising for improving B1 inhomogeneity and reducing the specific absorption rate. In this work, a novel method using time-shifted slice-select RF pulses is presented for reducing the through-plane signal loss artifact in parallel transmission applications. A simultaneous z-shim is obtained by concurrently applying unique time-shifted pulses on each transmitter. The method is shown to reduce the signal loss susceptibility artifact in gradient-echo images using a four-channel parallel transmission system at 3T.

Ultra-short echo time (UTE) MRI requires both short excitation ( approximately 0.5 ms) and short acquisition delay (<0.2 ms) to minimize T(2)-induced signal decay. These requirements currently lead to low acquisition efficiency when high resolution (<1 mm) is pursued. A novel pulse sequence, acquisition-weighted stack of spirals (AWSOS), is proposed here to acquire high-resolution three-dimensional (3D) UTE images with short scan time ( approximately 72 s). The AWSOS sequence uses variable-duration slice encoding to minimize T(2) decay, separates slice thickness from in-plane resolution to reduce the number of slice encodings, and uses spiral trajectories to accelerate in-plane data collections. T(2)- and off-resonance induced slice widening and image blurring were calculated from 1.5 to 7 Tesla (T) through point spread function. Computer simulations were performed to optimize spiral interleaves and readout times. Phantom scans and in vivo experiments on human heads were implemented on a clinical 1.5T scanner (G(max) = 40 mT/m, S(max) = 150 T/m/s). Accounting for the limits on B(1) maximum, specific absorption rate (SAR), and the lowered amplitude of slab-select gradient, a sinc radiofrequency (RF) pulse of 0.8ms duration and 1.5 cycles was found to produce a flat slab profile. High in-plane resolution (0.86 mm) images were obtained for the human head using echo time (TE) = 0.608 ms and total shots = 720 (30 slice-encodings x 24 spirals). Compared with long-TE (10 ms) images, the ultrashort-TE AWSOS images provided clear visualization of short-T(2) tissues such as the nose cartilage, the eye optic nerve, and the brain meninges and parenchyma.

Parallel transmitter techniques are a promising approach for reducing transmitter B1 inhomogeneity due to the potential for adjusting the spatial excitation profile with independent RF pulses. These techniques may be further improved with transmit sensitivity encoding (SENSE) methods because the sensitivity information in pulse design provides an excitation that is inherently compensated for transmitter B1 inhomogeneity. This paper presents a proof of this concept using transmit SENSE 3D tailored RF pulses designed for small flip angles. An eight-channel receiver coil was used to mimic parallel transmission for brain imaging at 3T. The transmit SENSE pulses were based on the fast-k(z) design and produced 5-mm-thick slices at a flip angle of 30 degrees with only a 4.3-ms pulse length. It was found that the transmit SENSE pulses produced more homogeneous images than those obtained from the complex sum of images from all receivers excited with a standard RF pulse.