Slice-by-slice B(1) (+) shimming at 7 T

Parallel transmission has been used to reduce the inevitable inhomogeneous radiofrequency fields produced in human high‐field MRI greater than 3 T. Further improvements in the transmit homogeneity and efficiency are possible by leveraging the additional degree of freedom permitted by multislice acquisitions. Compared to simple scaling of the flip angle to compensate for B₁⁺ falloff along the radiofrequency coil, calculation of B₁⁺ shim solutions on a slice‐by‐slice basis can markedly improve homogeneity and/or reduce transmitted power and global SAR. Performance measures were acquired at 7 T with a 15‐channel head‐only transceive array featuring elements distributed over all three logical axes, facilitating B₁⁺ shimming over arbitrary orientations. Compared to a circularly polarized volume mode of the same coil, shimming to maximize excitation efficiency on a slice‐by‐slice basis yielded improvements in mean B₁⁺ by 12.8 ± 2.4% and a reduction in standard deviation of B₁⁺ of 16.3 ± 6.8%, while reducing relative SAR by 6.2 ± 3.1%. When shimming for greater uniformity, the mean and standard deviation of B₁⁺ were further improved by 15.9 ± 2.6% and 26.2 ± 10.4%, respectively, at the expense of a 135 ± 8% increase in global SAR. Robust multislice‐shim solutions are demonstrated that can be quickly calculated, applied in real time, and reliably improve on volume coil modes. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Role of very high order and degree B(0) shimming for spectroscopic imaging of the human brain at 7 tesla

With the advent of ultrahigh field systems (7T), significant improvements in spectroscopic imaging (SI) studies of the human brain have been anticipated. These gains are dependent upon the achievable B₀ homogeneity, both globally (σB, over the entire regions of interest or slice) and locally (σB, influencing the linewidth of individual SI voxels within the regions of interest). Typically the B₀ homogeneity is adjusted using shim coils with spatial distributions modeled on spherical harmonics which can be characterized by a degree (radial dependence) and order (azimuthal symmetry). However, the role of very high order and degree shimming (e.g., 3rd and 4th degree) in MRSI studies has been controversial. Measurements of σB and σB were determined from B₀ field maps of 64 × 64 resolution. In a 10 mm thick slice taken through the region of the subcortical nuclei, we find that in comparison to 1st–2nd degree shims, use of 1st–3rd and 1st–4th degree shims reduces σB by 29% and 55%, respectively. Using a SI voxel size of ～1cc with an estimate of σB from 3 × 3 × 3 B₀ map pixels in this subcortical region, the number of pixels with σB of less than 5 Hz increased from 24 to 59% with 1st–3rd and 1st–4th over 1st–2nd degree shims, respectively. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Robust fully automated shimming of the human brain for high-field 1H spectroscopic imaging.

Although a variety of methods have been proposed to provide automated adjustment of shim homogeneity, these methods typically fail or require large numbers of iterations in vivo when applied to regions with poor homogeneity, such as the temporal lobe. These limitations are largely due to 1) the limited accuracy of single evolution time measurements when full B0 mapping studies are used, and 2) inaccuracies arising from projection-based methods when the projections pass through regions where the inhomogeneity exceeds the order of the fitted parameters. To overcome these limitations we developed a novel B0 mapping method using multiple evolution times with a novel unwrapping scheme in combination with a user-defined ROI selection tool. We used these methods at 4T on 10 control subjects to obtain high-resolution spectroscopic images of glutamate from the bilateral hippocampi.     

RF shimming for spectroscopic localization in the human brain at 7 T

Spectroscopic imaging of the human head at short echo times (≤15 ms) typically requires suppression of signals from extracerebral tissues. However, at 7 T, decreasing efficiency in B generation (hertz/watt) and increasing spectral bandwidth result in dramatic increases in power deposition and increased chemical shift registration artifacts for conventional gradient‐based in‐plane localization. In this work, we describe a novel method using radiofrequency shimming and an eight‐element transceiver array to generate a B field distribution that excites a ring about the periphery of the head and leaves central brain regions largely unaffected. We have used this novel B distribution to provide in‐plane outer volume suppression (>98% suppression of extracerebral lipids) without the use of gradients. This novel B distribution is used in conjunction with a double inversion recovery method to provide suppression of extracerebral resonances with T₁s greater than 400 ms, while having negligible effect on metabolite ratios of cerebral resonances with T₁s > 1000 ms. Despite the use of two adiabatic pulses, the high efficiency of the ring distribution allows radiofrequency power deposition to be limited to 3‐4 W for a pulse repetition time of 1.5 sec. The short echo time enabled the acquisition of images of the human brain, displaying glutamate, glutamine, macromolecules, and other major cerebral metabolites. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

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

Proton magnetic resonance spectroscopic imaging ((1)H 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. (1)H 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 B(1) and B(0) inhomogeneity as well as chemical shift localization (CSL) error, degrade the performance of conventional (1)H 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% B(1) 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 (1)H 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.     

B1 and B0 inhomogeneity mitigation in the human brain at 7 T with selective pulses by using average Hamiltonian theory

A novel method based on average Hamiltonian theory to design selective pulses is reported. With this tool, it is first shown how to shape the radiofrequency and gradient pulses to generate a desired rotation matrix, which is independent of the position through the slice of interest. After theoretical examination of the concept, it is applied to the strongly modulating pulses' recipe developed by the same authors and initially designed to be nonselective, to mitigate the amplitude of (excitation) radiofrequency field and amplitude of static (polarizing) field inhomogeneity problems at high field. Two in vivo human brain imaging experiments at 7 T are reported to prove the validity of the technique. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

1H NMR spectroscopic imaging of the mouse brain at 9.4 T

Purpose

To assess the feasibility of ¹H spectroscopic imaging (SI) in the mouse brain at 9.4 T, and investigate regional variations in brain metabolites.

Materials and Methods

A total of 21 SI studies were performed in CD‐1 mice to evaluate the basal ganglia (N = 5), hippocampus and thalamus (N = 11), and cerebellum (N = 5). We adjusted the B₀ homogeneity for each slice using a fully automated shim calculation method based on the B₀ map, which we measured using a multislice gradient‐echo sequence with multiple phase evolution delays. The SI employed a modified localization by adiabatic selective refocusing (LASER) sequence with TE/TR of 50/2000 msec, 24 × 24 encodes over a field of view (FOV) of 24 mm × 24 mm, 1 μL voxel resolution, and two averages, for a total acquisition time of 38 minutes.

Results

Sufficient shimming was achieved and high‐quality spectra were consistently obtained in each slice. N‐acetyl aspartate (NAA)/creatine (Cr) ratios in the basal ganglia and thalamus (0.86 ± 0.07, and 0.87 ± 0.07, respectively) were significantly higher than those in the hippocampus and cerebellum (0.76 ± 0.03 and 0.67 ± 0.07), which were also significantly different from each other.

Conclusion

¹H SI of the mouse brain is highly reproducible and allows differences in regional metabolite ratios to be easily visualized. J. Magn. Reson. Imaging 2006. © 2006 Wiley‐Liss, Inc.