John Thomas Vaughan

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

Signal‐to‐noise ratio (SNR), RF field (B1), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B1 magnitude was nearly twofold higher at 7T than at 4T (∼42% compared to ∼23%). The power required for a 90° 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 Maxwells equations. Magn Reson Med 46:24–30, 2001.

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 coils 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 4T. Head, body, and breast images were acquired within peak power constraints of <8 kW. Bench studies indicate that these body coils are feasible to 8T. RF shimming was used to remove a high‐field‐related cardiac imaging artifact in these preliminary studies. Magn Reson Med 52:851–859, 2004.

Whole-body imaging at 7T

The objective of this study was to investigate the feasibility of whole‐body imaging at 7T. To achieve this objective, new technology and methods were developed. Radio frequency (RF) field distribution and specific absorption rate (SAR) were first explored through numerical modeling. A body coil was then designed and built. Multichannel transmit and receive coils were also developed and implemented. With this new technology in hand, an imaging survey of the “landscape” of the human body at 7T was conducted. Cardiac imaging at 7T appeared to be possible. The potential for breast imaging and spectroscopy was demonstrated. Preliminary results of the first human body imaging at 7T suggest both promise and directions for further development. Magn Reson Med 61:244–248, 2009.

Different excitation and reception distributions with a single‐loop transmit‐receive surface coil near a head‐sized spherical phantom at 300 MHz

Calculations and experiments were used to examine the B1 field behavior and signal intensity distribution in a 16‐cm diameter spherical phantom excited by a 10‐cm diameter surface coil at 300 MHz. In this simple system at this high frequency very complex RF field behavior exists, resulting in different excitation and reception distributions. Included in this work is a straightforward demonstration that coil receptivity is proportional to the magnitude of the circularly polarized component of the B1 field that rotates in the direction opposite to that of nuclear precession. It is clearly apparent that even in very simple systems in head‐sized samples at this frequency it is important to consider the separate excitation and reception distributions in order to understand the signal intensity distribution. Magn Reson Med 47:1026–1028, 2002.

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. Magn Reson Med 47:990–1000, 2002.

Quantitative 31P spectroscopic imaging of human brain at 4 Tesla: Assessment of gray and white matter differences of phosphocreatine and ATP

This report describes the implementation and application of a multicompartment analysis of 31P spectroscopic imaging data to determine the tissue‐specific heterogeneities in metabolite content in the human brain and surrounding tissue. Using this information and a multicompartment regression analysis the phosphocreatine and ATP content of “pure” cerebral gray and white matter, the cerebellum, and skeletal muscle was determined in a group of 10 healthy volunteers. The data were converted to mM units using previously reported values for the T1s of phosphocreatine and ATP at 4 T, the water content of human brain, and an external reference for absolute quantification. The phosphocreatine concentration in cerebral gray and white matter, the cerebellum, and skeletal muscle was 3.53 ± 0.33, 3.33 ± 0.37, 3.75 ± 0.66, and 25.8 ± 2.3 mM, respectively. The ATP concentration in cerebral gray and white matter, the cerebellum, and skeletal muscle was 2.19 ± 0.33, 3.41 ± 0.33, 1.75 ± 0.58, and 8.5 ± 1.9 mM, respectively. Magn Reson Med 45:46–52, 2001.

In vivo 1H NMR spectroscopy of the human brain at 9.4 T: initial results.

In vivo proton NMR spectroscopy allows non-invasive detection and quantification of a wide range of biochemical compounds in the brain. Higher field strength is generally considered advantageous for spectroscopy due to increased signal-to-noise and increased spectral dispersion. So far (1)H NMR spectra have been reported in the human brain up to 7 T. In this study we show that excellent quality short echo time STEAM and LASER (1)H NMR spectra can be measured in the human brain at 9.4 T. The information content of the human brain spectra appears very similar to that measured in the past decade in rodent brains at the same field strength, in spite of broader linewidth in human brain. Compared to lower fields, the T(1) relaxation times of metabolites were slightly longer while T(2) relaxation values of metabolites were shorter (<100 ms) at 9.4 T. The linewidth of the total creatine (tCr) resonance at 3.03 ppm increased linearly with magnetic field (1.35 Hz/T from 1.5 T to 9.4 T), with a minimum achievable tCr linewidth of around 12.5 Hz at 9.4 T. At very high field, B(0) microsusceptibility effects are the main contributor to the minimum achievable linewidth.

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 B1 (B1+). All measurements and simulations were conducted after performing local B1+ 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 B1+ shimming strategy used, the sixteen‐channel array exhibited lower local and global specific absorption rate for a given B1+. Magn Reson Med, 2011.

Cardiac imaging at 7 tesla: Single- and two-spoke radiofrequency pulse design with 16-channel parallel excitation

Higher signal to noise ratio (SNR) and improved contrast have been demonstrated at ultra‐high magnetic fields (≥7 Tesla [T]) in multiple targets, often with multi‐channel transmit methods to address the deleterious impact on tissue contrast due to spatial variations in B1+ profiles. When imaging the heart at 7T, however, respiratory and cardiac motion, as well as B0 inhomogeneity, greatly increase the methodological challenge. In this study we compare two‐spoke parallel transmit (pTX) RF pulses with static B1+ shimming in cardiac imaging at 7T.

RF Head Coil Design with Improved RF Magnetic Near-Fields Uniformity for Magnetic Resonance Imaging (MRI) Systems.

Higher magnetic field strength in magnetic resonance imaging (MRI) systems offers higher signal-to-noise ratio, contrast, and spatial resolution in magnetic resonance (MR) images. However, the wavelength in ultrahigh fields (7 T and beyond) becomes shorter than the human body at the Larmor frequency with increasing static magnetic field (B0) of the MRI system. At short wavelengths, an interference effect appears, resulting in nonuniformity of the RF magnetic near-field (B1) over the subject and MR images may have spatially anomalous contrast. The B1 near-field generated by the transverse electromagnetic RF coils microstrip line element has a maximum near the center of its length and falls off towards both ends. In this study, a double trapezoidal-shaped microstrip transmission line element is proposed to obtain uniform B1 field distribution by gradual impedance variation. Two multi-channel RF head coils with uniform and trapezoidal shape elements were built and tested with a phantom at 7-T MRI scanner for comparison. The simulation and experimental results show stronger and more uniform B1+ near-field with the trapezoidal shape.