Comparison between eight- and sixteen-channel TEM transceive arrays for body imaging at 7 T.

Eight- and sixteen-channel transceive stripline/TEM body arrays were compared at 7 T (297 MHz) both in simulation and experiment. Despite previous demonstrations of similar arrays for use in body applications, a quantitative comparison of the two configurations has not been undertaken to date. Results were obtained on a male pelvis for assessing transmit, signal to noise ratio, and parallel imaging performance and to evaluate local power deposition versus transmit B(1) (B(1) (+) ). All measurements and simulations were conducted after performing local B(1) (+) phase shimming in the region of the prostate. Despite the additional challenges of decoupling immediately adjacent coils, the sixteen-channel array demonstrated improved or nearly equivalent performance to the eight-channel array based on the evaluation criteria. Experimentally, transmit performance and signal to noise ratio were 22% higher for the sixteen-channel array while significantly increased reduction factors were achievable in the left-right direction for parallel imaging. Finite difference time domain simulations demonstrated similar results with respect to transmit and parallel imaging performance, however, a higher transmit efficiency advantage of 33% was predicted. Simulations at both 3 and 7 T verified the expected parallel imaging improvements with increasing field strength and showed that, for a specific B(1) (+) shimming strategy used, the sixteen-channel array exhibited lower local and global specific absorption rate for a given B(1) (+) .

7 Tesla (T) human cardiovascular magnetic resonance imaging using FLASH and SSFP to assess cardiac function: validation against 1.5 T and 3 T.

We report the first comparison of cardiovascular magnetic resonance imaging (CMR) at 1.5 T, 3 T and 7 T field strengths using steady state free precession (SSFP) and fast low angle shot (FLASH) cine sequences. Cardiac volumes and mass measurements were assessed for feasibility, reproducibility and validity at each given field strength using FLASH and SSFP sequences. Ten healthy volunteers underwent retrospectively electrocardiogram (ECG) gated CMR at 1.5 T, 3 T and 7 T using FLASH and SSFP sequences. B1 and B0 shimming and frequency scouts were used to optimise image quality. Cardiac volume and mass measurements were not significantly affected by field strength when using the same imaging sequence (P > 0.05 for all parameters at 1.5 T, 3 T and 7 T). SSFP imaging returned larger end diastolic and end systolic volumes and smaller left ventricular masses than FLASH imaging at 7 T, and at the lower field strengths (P < 0.05 for each parameter). However, univariate general linear model analysis with fixed effects for sequence and field strengths found an interaction between imaging sequence and field strength (P = 0.03), with a smaller difference in volumes and mass measurements between SSFP and FLASH imaging at 7 T than 1.5 T and 3 T. SSFP and FLASH cine imaging at 7 T is technically feasible and provides valid assessment of cardiac volumes and mass compared with CMR imaging at 1.5 T and 3 T field strengths.

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 B(1) 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.

Lactate turnover in rat glioma measured by in vivo nuclear magnetic resonance spectroscopy.

Elevated tissue lactate concentrations typically found in tumors can be measured by in vivo nuclear magnetic resonance (NMR) spectroscopy. In this study, lactate turnover in rat C6 glioma was determined from in vivo 1H NMR measurements of [3-13C]lactate buildup during steady-state hyperglycemia with [1-13C]glucose. With this tumor model, a narrow range of values was observed for the first-order rate constant that describes lactate efflux, k2 = 0.043 +/- 0.007 (n = 12) SD min-1. For individual animals, the standard error in k2 was small (< 18%), which indicated that the NMR data fit the kinetic model well. Lactate measurements before and after infusing [1-13C]glucose showed that the majority of the tumor lactate pool was metabolically active. Signals from 13C-labeled glutamate in tumors were at least 10-fold smaller than the [3-13C]lactate signal, whereas spectra of the contralateral hemispheres revealed the expected labeling of [4-13C]glutamate, as well as [2-13C] and [3-13C]glutamate, which indicates that label cycled through the tricarboxylic acid cycle in the brain tissue. Lack of significant 13C labeling of glutamate was consistent with low respiratory metabolism in this glioma. It is concluded that lactate in rat C6 glioma is actively turning over and that the kinetics of lactate efflux can be quantified noninvasively by 1H NMR detection of 13C label. This noninvasive NMR approach may offer a valuable tool to help evaluate tumor growth and metabolic responsiveness to therapies.

The return of the frequency sweep: designing adiabatic pulses for contemporary NMR.

Frequency-modulated (FM) pulses that function according to adiabatic principles are becoming increasingly popular in many areas of NMR. Often adiabatic pulses can extend experimental capabilities and minimize annoying experimental imperfections. Here, adiabatic principles and some of the current methods used to create these pulses are considered. The classical adiabatic rapid passage, which is a fundamental element upon which all adiabatic pulses and sequences are based, is analyzed using vector models in different rotating frames of reference. Two methods to optimize adiabaticity are described, and ways to tailor modulation functions to best satisfy specific experimental needs are demonstrated. Finally, adiabatic plane rotation pulses and frequency-selective multiple spin-echo sequences are considered.

Effects of continuous localized infusion of granulocyte-macrophage colony-stimulating factor and inoculations of irradiated glioma cells on tumor regression.

OBJECT: Glioblastoma multiforme (GBM) is a malignant tumor of the central nervous system that directly suppresses immunological defenses in vitro and in vivo. The authors used the peripheral delivery of continuously infused granulocyte-macrophage colony-stimulating factor (GM-CSF) in the presence of irradiated tumor antigens as a tumor-specific stimulant to dendritic cells to initiate an immune response to GBM in rats. METHODS: The 9L gliosarcoma tumors were established in the flanks of syngeneic Fischer 344 rats. Osmotic minipumps implanted in the animals' contralateral flanks continuously delivered recombinant GM-CSF (0, 0.1, 1, or 10 ng/day) for 28 days. Irradiated gliosarcoma cells were intermittently injected at the site of the GM-CSF infusion. Animals in the saline control group (0 ng/day GM-CSF) died on Day 59 with average tumor volumes greater than 30,000 mm3. This control group was significantly different from the GM-CSF-treated animals, which all survived with average tumor volumes that peaked on Day 23 and later regressed completely. Tumor growth as well as peak tumor volumes (5833+/-2284 mm3, 3294+/-1632 mm3, and 1979+/-1142 mm3 for 0.1, 1, and 10 ng/day GM-CSF, respectively) in the different treatment groups reflected a significant dose-response relationship with the GM-CSF concentrations. All animals treated with GM-CSF and irradiated cells were resistant to additional challenges of peripheral and intracerebral gliosarcoma, even when they were inoculated 8 months after initial immunotherapy. The colocalization of GM-CSF and inactivated tumor antigens was required to stimulate immunoprotection. To test the efficacy of a peripherally administered immunological therapy on intracerebral brain tumors the authors transplanted 10(6) gliosarcoma cells into the striatum of treated and control animals. Subcutaneous pumps that released GM-CSF (10 ng/day) and irradiated gliosarcoma cells were placed in the treated animals. The control animals all died within 31 days after intracerebral tumor implantation. In contrast, 40% of the animals receiving GM-CSF-irradiated cell vaccinations survived beyond 300 days. These long-term survivors showed no evidence of gliosarcoma at the injection site on evaluation by magnetic resonance imaging. CONCLUSIONS: These results suggest that the continuous localized delivery of subcutaneous GM-CSF in conjunction with inactivated tumor antigens can initiate a systemic response that leads to the regression of distant peripheral and intracerebral tumors. The success of this treatment illustrates the feasibility of tumor-specific peripheral immunological stimulation after tumor resection to prevent the recurrence of malignant brain tumors.

BISTRO: an outer-volume suppression method that tolerates RF field inhomogeneity.

A technique is described for performing frequency-selective signal suppression with a high degree of tolerance to RF field inhomogeneity. The method is called B1-insensitive train to obliterate signal (BISTRO). BISTRO consists of multiple amplitude- and frequency-modulated (FM) pulses interleaved with spoiler gradients. BISTRO was developed for the purpose of accomplishing band-selective signal removal, as in water suppression and outer-volume suppression (OVS), in applications requiring the use of an inhomogeneous RF transmitter, such as a surface coil. In the present work, Bloch simulations were used to illustrate the principles and theoretical performance of BISTRO. Its performance for OVS was evaluated experimentally using MRI and spectroscopic imaging of phantoms and in vivo animal and human brain. By using FM pulses featuring offset-independent adiabaticity, BISTRO permitted high-quality, broadband suppression with one (or two) discrete borders demarcating the edge(s) of the suppression band. Simulations and experiments demonstrated the ability to operate BISTRO with reasonably attainable peak RF power levels and with average RF energy deposition similar to other multipulse OVS techniques.

Detunable transverse electromagnetic (TEM) volume coil for high-field NMR.

Most high-field MRI systems do not have the actively detuned body coils that are integral to clinical systems operating at 1.5T and lower field strengths. Therefore, many clinical applications requiring homogeneous volume excitation in combination with local surface coil reception are not easily implemented at high fields. To solve this problem for neuroimaging applications, actively detunable transverse electromagnetic (TEM) head coils were developed to be used with receive-only surface coils for signal-to-noise ratio (SNR) gains and improved spatial coverage from homogeneously excited regions. These SNR and field of view (FOV) gains were achieved by application of a detunable TEM volume coil to human brain imaging at 4T.

Efficient high-frequency body coil for high-field MRI.

The use of body coils is favored for homogeneous excitation, and such coils are often paired with surface coils or arrays for sensitive reception in many MRI applications. While the body coil's physical size and resultant electrical length make this circuit difficult to design for any field strength, recent efforts to build efficient body coils for applications at 3T and above have been especially challenging. To meet this challenge, we developed an efficient new transverse electromagnetic (TEM) body coil and demonstrated its use in human studies at field strengths up to 4 T. Head, body, and breast images were acquired within peak power constraints of <8 kW. Bench studies indicate that these body coils are feasible to 8 T. RF shimming was used to remove a high-field-related cardiac imaging artifact in these preliminary studies. P41RR13230

7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images.

Signal-to-noise ratio (SNR), RF field (B(1)), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B(1) magnitude was nearly twofold higher at 7T than at 4T ( approximately 42% compared to approximately 23%). The power required for a 90 degrees pulse in the center of the head at 7T was approximately twice that at 4T. The SNR averaged over the brain was at least 1.6 times higher at 7T compared to 4T. These experimental results were consistent with calculations performed using a human head model and Maxwell's equations. Magn Reson Med 46:24-30, 2001.