Priti Balchandani

Self-refocused spatial-spectral pulse for positive contrast imaging of cells labeled with SPIO nanoparticles

MRI has been used extensively to noninvasively track the location of cells labeled with superparamagnetic iron‐oxide nanoparticles (SPIOs) in vivo. Typically, SPIOs are employed as a negative contrast agent which makes it difficult to differentiate labeled cells from extraneous sources of inhomogeneity and actual voids in the image. As a result, several novel approaches have been put forth to obtain positive contrast from SPIOs. One technique proposed by Cunningham et al. utilizes spectrally selective pulses to excite and refocus spins in the vicinity of the SPIOs. Although the frequency selectivity of this technique provides effective positive contrast, the lack of slice selectivity results in interfering signal from sources of off‐resonance outside the slice of interest. We have developed a self‐refocused spatial‐spectral (SR‐SPSP) pulse to achieve slice‐selective spin‐echo imaging of off‐resonant spins. Using a self‐refocused pulse affords flexibility in echo‐time selection since the spin echo may be placed at any time after the end of the pulse. The spatial selectivity achieved by the SR‐SPSP RF pulse eliminates background signal from out‐of‐slice regions and reduces the on‐resonant water suppression requirements. Phantom and in vivo data demonstrate that positive contrast and slice‐selectivity are achieved using this novel RF pulse. Magn Reson Med, 2009.

Fat suppression for 1H MRSI at 7T using spectrally selective adiabatic inversion recovery.

Proton magnetic resonance spectroscopic imaging (1H MRSI) at 7T offers many advantages, including increased SNR and spectral resolution. However, technical difficulties associated with operating at high fields, such as increased B1 and B0 inhomogeneity, severe chemical shift localization error, and converging T1 values, make the suppression of the broad lipid peaks which can obscure targeted metabolite signals, particularly challenging. Conventional short tau inversion recovery can successfully suppress fat without restricting the selected volume, but only with significant metabolite signal loss. In this work, we have designed two new pulses for frequency‐selective inversion recovery that achieve B1‐insensitive fat suppression without degrading the signal from the major metabolites of interest. The first is a spectrally selective adiabatic pulse to be used in a volumetric 1H MRSI sequence and the second is a spatial‐spectral adiabatic pulse geared toward multi‐slice 1H MRSI. Partial interior volume selection may be used in addition to the pulses, to exclude areas with severe B0 inhomogeneity. Some differences in the spectral profile as well as degree of suppression make each pulse valuable for different applications. 7T phantom and in vivo data show that both pulses significantly suppress fat, while leaving most of the metabolite signal intact. Magn Reson Med 59:980–988, 2008.

Interleaved narrow-band PRESS sequence with adiabatic spatial-spectral refocusing pulses for 1H MRSI at 7T

Proton magnetic resonance spectroscopic imaging (1H MRSI) is a useful technique for measuring metabolite levels in vivo, with Choline (Cho), Creatine (Cre), and N‐Acetyl‐Aspartate (NAA) being the most prominent MRS‐detectable brain biochemicals. 1H MRSI at very high fields, such as 7T, offers the advantages of higher SNR and improved spectral resolution. However, major technical challenges associated with high‐field systems, such as increased B1 and B0 inhomogeneity as well as chemical shift localization (CSL) error, degrade the performance of conventional 1H MRSI sequences. To address these problems, we have developed a Position Resolved Spectroscopy (PRESS) sequence with adiabatic spatial‐spectral (SPSP) refocusing pulses, to acquire multiple narrow spectral bands in an interleaved fashion. The adiabatic SPSP pulses provide magnetization profiles that are largely invariant over the 40% B1 variation measured across the brain at 7T. Additionally, there is negligible CSL error since the transmit frequency is separately adjusted for each spectral band. in vivo 1H MRSI data were obtained from the brain of a normal volunteer using a standard PRESS sequence and the interleaved narrow‐band PRESS sequence with adiabatic refocusing pulses. In comparison with conventional PRESS, this new approach generated high‐quality spectra from an appreciably larger region of interest and achieved higher overall SNR. Magn Reson Med 59:973–979, 2008.

Designing adiabatic radio frequency pulses using the Shinnar–Le Roux algorithm

Adiabatic pulses are a special class of radio frequency (RF) pulses that may be used to achieve uniform flip angles in the presence of a nonuniform B1 field. In this work, we present a new, systematic method for designing high‐bandwidth (BW), low‐peak‐amplitude adiabatic RF pulses that utilizes the Shinnar–Le Roux (SLR) algorithm for pulse design. Currently, the SLR algorithm is extensively employed to design nonadiabatic pulses for use in magnetic resonance imaging and spectroscopy. We have adapted the SLR algorithm to create RF pulses that also satisfy the adiabatic condition. By overlaying sufficient quadratic phase across the spectral profile before the inverse SLR transform, we generate RF pulses that exhibit the required spectral characteristics and adiabatic behavior. Application of quadratic phase also distributes the RF energy more uniformly, making it possible to obtain the same spectral BW with lower RF peak amplitude. The method enables the pulse designer to specify spectral profile parameters and the degree of quadratic phase before pulse generation. Simulations and phantom experiments demonstrate that RF pulses designed using this new method behave adiabatically. Magn Reson Med, 2010.

SLICE-SELECTIVE TUNABLE-FLIP ADIABATIC LOW PEAK POWER EXCITATION

Adiabatic pulses are useful in achieving uniform excitation profiles in the presence of B1‐inhomogeneity. At higher fields, this inhomogeneity becomes more severe, further amplifying the need for B1‐insensitive excitation. Although gradient modulation techniques for slice‐selective adiabatic excitation have been introduced, a pulse that falls within the gradient and RF amplifier limits for most commercial human scanners is currently unavailable. In this work, we present an alternative gradient modulated approach for pulse design that achieves adiabatic slice selection with significantly lower RF peak power requirements. The resulting Slice‐selective Tunable‐flip AdiaBatic Low peak‐power Excitation (STABLE) pulse consists of an oscillating gradient in conjunction with a BIR‐4‐like RF envelope that is sampled by many short spatial subpulses to achieve spatial selectivity. Simulations show that the expected spatial profile as well as the off‐resonance behavior of the pulse remain invariant for a range of B1 values. Phantom and in vivo results demonstrate the adiabaticity and slice selectivity of the STABLE pulse. Magn Reson Med 59:1072–1078, 2008.

Self-refocused adiabatic pulse for spin echo imaging at 7 T

Spin echo pulse sequences are used to produce clinically important T2 contrast. However, conventional 180° radiofrequency pulses required to generate a spin echo are highly susceptible to the B1 inhomogeneity at high magnetic fields such as 7 Tesla (7 T), resulting in varying signal and contrast over the region of interest. Adiabatic 180° pulses may be used to replace conventional 180° pulses in spin echo sequences to provide greater immunity to the inhomogeneous B1 field at 7 T. However, because the spectral profile of an adiabatic 180° pulse has nonlinear phase, pairs of these pulses are needed for proper refocusing, resulting in increased radiofrequency power deposition and long minimum echo times. We used the adiabatic Shinnar Le‐Roux method to generate a matched‐phase adiabatic 90°–180° pulse pair to obviate the need for a second adiabatic 180° pulse for phase refocusing. The pulse pair was then reformulated into a single self‐refocused pulse to minimize the echo time, and phantom and in vivo experiments were performed to validate pulse performance. The self‐refocused adiabatic pulse produced transmit profiles that were substantially more uniform than those achieved using a conventional spin echo sequence. Magn Reson Med, 2011.

Improved Slice-Selective Adiabatic Excitation

The purpose of this work is to design an improved Slice‐selective Tunable‐flip AdiaBatic Low peak‐power Excitation (STABLE) pulse with shorter duration and increased off‐resonance immunity to make it suitable for use in a greater range of applications and at higher field strengths. An additional aim is to design a variant of this pulse to achieve B1‐insensitive, fat‐suppressed excitation.

Semi-adiabatic Shinnar–Le Roux pulses and their application to diffusion tensor imaging of humans at 7T

The adiabatic Shinnar-Le Roux (SLR) algorithm for radiofrequency (RF) pulse design enables systematic control of pulse parameters such as bandwidth, RF energy distribution and duration. Some applications, such as diffusion-weighted imaging (DWI) at high magnetic fields, would benefit from RF pulses that can provide greater B1 insensitivity while adhering to echo time and specific absorption rate (SAR) limits. In this study, the adiabatic SLR algorithm was employed to generate 6-ms and 4-ms 180° semi-adiabatic RF pulses which were used to replace the refocusing pulses in a twice-refocused spin echo (TRSE) diffusion-weighted echo planar imaging (DW-EPI) sequence to create two versions of a twice-refocused adiabatic spin echo (TRASE) sequence. The two versions were designed for different trade-offs between adiabaticity and echo time. Since a pair of identical refocusing pulses is applied, the quadratic phase imposed by the first is unwound by the second, preserving the linear phase created by the excitation pulse. In vivo images of the human brain obtained at 7Testa (7T) demonstrate that both versions of the TRASE sequence developed in this study achieve more homogeneous signal in the diffusion-weighted images than the conventional TRSE sequence. Semi-adiabatic SLR pulses offer a more B1-insensitive solution for diffusion preparation at 7T, while operating within SAR constraints. This method may be coupled with any EPI readout trajectory and parallel imaging scheme to provide more uniform coverage for diffusion tensor imaging at 7T and 3T.

Thin-film persistent current switch

We have developed a fast, low power heat switch for switching a niobium thin film between the normal and superconducting state. The sputtered niobium film (400 nm thick, 100 /spl mu/m wide) has a critical current density of 5/spl times/10/sup 10/ Am/sup -2/. Switching is produced by joule heating a small section of the niobium film with a titanium thin-film resistor. With the heat switch in vacuum, the minimum heater power needed to switch to the normal state was 4.5/spl times/10/sup -5/ W. A simple three-dimensional thermal model shows that the minimum power is primarily determined by the thermal conductivity of the substrate. We have achieved response times less than 10/sup -6/ s.