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.     

Multislice 1H MRSI of the human brain at 7 T using dynamic B0 and B1 shimming

Proton MR spectroscopic imaging of the human brain at ultra-high field (≥7 T) is challenging due to increased radio frequency power deposition, increased magnetic field B(0) inhomogeneity, and increased radio frequency magnetic field inhomogeneity. In addition, especially for multislice sequences, these effects directly inhibit the potential gains of higher magnetic field and can even cause a reduction in data quality. However, recent developments in dynamic B(0) magnetic field shimming and dynamic multitransmit radio frequency control allow for new acquisition strategies. Therefore, in this work, slice-by-slice B(0) and B(1) shimming was developed to optimize both B(0) magnetic field homogeneity and nutation angle over a large portion of the brain. Together with a low-power water and lipid suppression sequence and pulse-acquire spectroscopic imaging, a multislice MR spectroscopic imaging sequence is shown to be feasible at 7 T. This now allows for multislice metabolic imaging of the human brain with high sensitivity and high chemical shift resolution at ultra-high field.     

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.     

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.     

Initial results of cardiac imaging at 7 tesla

This work reports preliminary results from the first human cardiac imaging at 7 Tesla (T). Images were acquired using an eight‐channel transmission line (TEM) array together with local B₁ shimming. The TEM array consisted of anterior and posterior plates closely positioned to the subjects' thorax. The currents in the independent elements of these arrays were phased to promote constructive interference of the complex, short wavelength radio frequency field over the entire heart. Anatomic and functional images were acquired within a single breath hold to reduce respiratory motion artifacts while a vector cardiogram (VCG) was used to mitigate cardiac motion artifacts and gating. SAR exposure was modeled, monitored, and was limited to FDA guidelines for the human torso in subject studies. Preliminary results including short‐axis and four‐chamber VCG‐retrogated FLASH cines, as well as, short‐axis TSE images demonstrate the feasibility of safe and accurate human cardiac imaging at 7T. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Dynamically applied B1+ shimming solutions for non‐contrast enhanced renal angiography at 7.0 tesla

The purpose of this study was to detail a strategy for performing non‐contrast enhanced renal magnetic resonance angiography studies at 7.0 T. It is demonstrated that with proper B management, these studies can be successfully performed at ultrahigh field within local specific absorption rate constraints. An inversion prepared gradient echo acquisition, standard for non‐contrast renal magnetic resonance angiography studies, required radiofrequency pulse specific B shimming solutions to be dynamically applied to address the field dependent increases in both B₀ and B inhomogeneity as well as to accommodate limitation in available power. By using more efficient B shimming solutions for the inversion preparation and more homogeneous solutions for the excitation, high quality images of the renal arteries were obtained without venous and background signal artifacts while working within hardware and safety constraints. Finite difference time domain simulations confirmed in vivo measurements with respect to B distributions and homogeneity for the range of shimming strategies used and allowed the calculation of peak local specific absorption rate values normalized by input power and B. Increasing B homogeneity was accompanied by decreasing local specific absorption rate per Watt and increasing maximum local specific absorption rate per [B]², which must be considered, along with body size and respiratory rate, when finalizing acquisition parameters for a given individual. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Short echo spectroscopic imaging of the human brain at 7T using transceiver arrays

Recent advances in magnet technology have enabled the construction of ultrahigh‐field magnets (7T and higher) that can accommodate the human head and body. Despite the intrinsic advantages of performing spectroscopic imaging at 7T, increased signal‐to‐noise ratio (SNR), and spectral resolution, few studies have been reported to date. This limitation is largely due to increased power deposition and B₁ inhomogeneity. To overcome these limitations, we used an 8‐channel transceiver array with a short TE (15 ms) spectroscopic imaging sequence. Utilizing phase and amplitude mapping and optimization schemes, the 8‐element transceiver array provided both improved efficiency (17% less power for equivalent peak B₁) and homogeneity (SD(B₁) = ±10% versus ±22%) in comparison to a transverse electromagnetic (TEM) volume coil. To minimize the echo time to measure J‐modulating compounds such as glutamate, we developed a short TE sequence utilizing a single‐slice selective excitation pulse followed by a broadband semiselective refocusing pulse. Extracerebral lipid resonances were suppressed with an inversion recovery pulse and delay. The short TE sequence enabled visualization of a variety of resonances, including glutamate, in both a control subject and a patient with a Grade II oligodendroglioma. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Fully automated shim mapping method for spectroscopic imaging of the mouse brain at 9.4 T.

For spectroscopic imaging studies of the mouse brain, it is critical to obtain optimal B(0) homogeneity over a large region of interest (ROI). In this paper, a fully automated shimming method for mouse brain at 9.4 T, based on B(0) mapping, is described. B(0) maps were obtained using a multislice gradient echo sequence with multiple phase evolution time delays with a novel unwrapping scheme. The unwrapping method allows phase maps with large bandwidths (+/-1 kHz) but with high resolution (0.3 Hz/ degrees ) to be acquired in a single acquisition, thereby minimizing the number of iterations required. The SD of the B(0) over the ROI (8 x 5 x 1 mm) was less than 10 Hz after shimming. Application of this method to the in vivo mouse brain allowed reproducible, high-quality spectroscopic data to be collected with 1-microl voxels.     

Maximum efficiency radiofrequency shimming: Theory and initial application for hip imaging at 7 tesla

Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high‐frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power‐demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal‐to‐noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

SENSE shimming (SSH): A fast approach for determining B0 field inhomogeneities using sensitivity coding

The pursuit of ever higher field strengths and faster data acquisitions has led to the construction of coil arrays with high numbers of elements. With the sensitivity encoding (SENSE) technique, it has been shown that the sensitivity of those elements can be used for spatial image encoding. Here, a proof‐of‐principle is presented of a method that can be considered an extreme case of the SENSE approach, completely abstaining from using encoding gradients. The resulting sensitivity encoded free‐induction decay (FID) data are then not used for imaging, but for determining B₀ field inhomogeneity distribution. The method has therefore been termed “SENSE shimming” (SSH). In phantom experiments the method's ability to detect inhomogeneities of up to the second order is demonstrated. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements

High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B(1) (B(1) (+)) shimming to minimize B(1) (+) losses resulting from destructive interferences. B(1) (+) shimming was performed by altering the input phase of individual RF channels based on relative B(1) (+) phase maps rapidly obtained in vivo for each channel of an eight-element stripline coil. The relative transmit phases needed to maximize B(1) (+) coherence within a limited region around the prostate greatly differed from those dictated by coil geometry and were highly subject-dependent. A set of transmit phases determined by B(1) (+) shimming provided a gain in transmit efficiency of 4.2 +/- 2.7 in the prostate when compared to the standard transmit phases determined by coil geometry. This increased efficiency resulted in large reductions in required RF power for a given flip angle in the prostate which, when accounted for in modeling studies, resulted in significant reductions of local specific absorption rates. Additionally, B(1) (+) shimming decreased B(1) (+) nonuniformity within the prostate from (24 +/- 9%) to (5 +/- 4%). This study demonstrates the tremendous impact of fast local B(1) (+) phase shimming on ultrahigh magnetic field body imaging.     

Proton MR spectroscopic imaging of rhesus macaque brain in vivo at 7T

Due to the overall similarity of their brains' structure and physiology to its human counterpart, nonhuman primates provide excellent model systems for the pathogenesis of neurological diseases and their response to treatments. Its much smaller size, 80 versus 1250 cm(3), however, requires proportionally higher spatial resolution to study, nondestructively, as many analogous regions as efficiently as possible in anesthetized animals. The confluence of these requirements underscores the need for the highest sensitivity, spatial coverage, resolution, and exam speed. Accordingly, we demonstrate the feasibility of 3D multi-voxel, proton ((1)H) MRSI at (0.375 cm)(3)=0.05 cm(3) isotropic spatial resolution over 21 cm(3) (approximately 25%) of the anesthetized rhesus macaques brain at 7T in 25 min. These voxels are x10(2)-10(1) times smaller than the 8-1 cm(3) common to (1)H-MRS in humans, retaining similar proportions between the macaque and human brain. The spectra showed a signal-to-noise-ratio (SNR) approximately 9-10 for the major metabolites and the interanimal SNR spatial distribution reproducibility was in the +/-10% range for the standard error of their means (SEMs). Their metabolites' linewidths, 9+/-2 Hz, yield excellent spectral resolution as well. These results indicate that 3D (1)H-MRSI can be integrated into comprehensive MR studies in primates at such high fields.     

Direct B0 field monitoring and real-time B0 field updating in the human breast at 7 Tesla

Large dynamic fluctuations of the static magnetic field (B(0)) are observed in the human body during MR scanning, compromising image quality and detection sensitivity in several MR imaging and spectroscopy sequences. Partially, these dynamic B(0) fluctuations are due to physiological motion such as breathing, but other sources of temporal B(0) field fluctuations are also present in the MR system (e.g., eddy currents). Especially at ultrahigh field (≥7 T), the increased susceptibility effects lead to large B(0) field variations over time. Direct measurement and correction of these temporal field variations of up to 70 Hz will lead to a significant reduction of artifacts and improved measurement stability/reproducibility. For direct measurement of the temporally changing B(0) field, a simple field probe was developed, that was placed in proximity to the tissue of interest. In this work, it is shown how such a field probe system can be used to monitor temporal B(0) field variations in the human body during MRI and magnetic resonance spectroscopy. Furthermore, it is shown how the acquired temporal B(0) field information can drive a dynamic shim module to directly correct the B(0) magnetic field in real time.