Paul Royston Harvey

Preliminary report on in vivo coronary MRA at 3 Tesla in humans

Current limitations of coronary magnetic resonance angiography (MRA) include a suboptimal signal‐to‐noise ratio (SNR), which limits spatial resolution and the ability to visualize distal and branch vessel coronary segments. Improved SNR is expected at higher field strengths, which may provide improved spatial resolution. However, a number of potential adverse effects on image quality have been reported at higher field strengths. The limited availability of high‐field systems equipped with cardiac‐specific hardware and software has previously precluded successful in vivo human high‐field coronary MRA data acquisition. In the present study we investigated the feasibility of human coronary MRA at 3.0T in vivo. The first results obtained in nine healthy adult subjects are presented. Magn Reson Med 48:425–429, 2002.

Design of a radiative surface coil array element at 7 T:the single-side adapted dipole antenna

Ultra high field MR imaging (≥7 T) of deeply located targets in the body is facing some radiofrequency‐field related challenges: interference patterns, reduced penetration depth, and higher Specific Absorbtion Ratio (SAR) levels. These can be alleviated by redesigning the elements of the transmit or transceive array. This is because at these high excitation field (B1) frequencies, conventional array element designs may have become suboptimal. In this work, an alternative design approach is presented, regarding coil array elements as antennas. Following this approach, the Poynting vector of the element should be oriented towards the imaging target region. The single‐side adapted dipole antenna is a novel design that fulfills this requirement. The performance of this design as a transmit coil array element has been characterized by comparison with three other, more conventional designs using finite difference time domain (FDTD) simulations and B  +1 measurements on a phantom. Results show that the B  +1 level at the deeper regions is higher while maintaining relatively low SAR levels. Also, the B  +1 field distribution is more symmetrical and more uniform, promising better image homogeneity. Eight radiative antennas have been combined into a belt‐like surface array for prostate imaging. T1‐weighted (T1W) and T2‐weighted (T2W) volunteer images are presented along with B  +1 measurements to demonstrate the improved efficiency. Magn Reson Med, 2011.

A specific absorption rate prediction concept for parallel transmission MR

The specific absorption rate (SAR) is a limiting factor in high‐field MR. SAR estimation is typically performed by numerical simulations using generic human body models. However, SAR concepts for single‐channel radiofrequency transmission cannot be directly applied to multichannel systems. In this study, a novel and comprehensive SAR prediction concept for parallel radiofrequency transmission MRI is presented, based on precalculated magnetic and electric fields obtained from electromagnetic simulations of numerical body models. The application of so‐called Q‐matrices and further computational optimizations allow for a real‐time estimation of the SAR prior to scanning. This SAR estimation method was fully integrated into an eight‐channel whole body MRI system, and it facilitated the selection of different body models and body positions. Experimental validation of the global SAR in phantoms demonstrated a good qualitative and quantitative agreement with the predictions. An initial in vivo validation showed good qualitative agreement between simulated and measured amplitude of (excitation) radiofrequency field. The feasibility and practicability of this SAR prediction concept was shown paving the way for safe parallel radiofrequency transmission in high‐field MR. Magn Reson Med, 2012.

Modular gradient coil: A new concept in high-performance whole-body gradient coil design.

A new concept in high‐performance MR gradient coil design is presented which we have called the Modular Gradient Coil (MGC). This novel design approach results in an actively shielded whole‐body gradient coil containing multiple and independent elements, integrated onto a single former, for generating gradient fields along each of the three axes (x, y, and z). These elements can be energized in a number of configurations, using a single gradient power supply unit (PSU), to generate a whole range of gradient performance levels. The design criteria for the MGC also include a requirement to prohibit peripheral nerve stimulation in all of its modes of operation. This requirement is achieved, while simultaneously providing high performance, by specifying different volumes of gradient linearity for each of the operating modes. Magn Reson Med 42:561–570, 1999.

Rapid iterative reconstruction for echo planar imaging

A rapid automated method for reconstructing echo planar imaging (EPI) data has been developed and is shown to improve image quality by suppressing the troublesome ghost artifact. The algorithm can be applied without prior knowledge obtained from either reference scans or operator intervention. It first estimates, then improves iteratively, the parameters for a linear phase correction applied directly to the complex image data derived from odd and even echoes. The theory used to derive the criteria employed in the iteration provides insight into mechanisms that allow the process to work. Magn Reson Med 42:541–547, 1999.

Correction of proton resonance frequency shift temperature maps for magnetic field disturbances using fat signal

To improve the immunity of the proton resonance frequency shift (PRFS) method of MRI temperature mapping against magnetic field disturbances. Since PRFS is a phase‐sensitive method, it misinterprets magnetic field disturbances as artifact temperature changes. If not corrected, the resulting temperature artifacts can completely obscure the true temperature estimation, especially if the temperature elevations are small.

Navigator motion correction of diffusion weighted 3D SSFP imaging

Diffusion weighted (DW) 3D steady state MR (SSFP) head imaging technique using navigator echo’s motion correction is presented. This new scheme enables acquisition of DW images even at regions where severe susceptibility is present. Another advantage is the moderate gradient performance requirements. DW imaging methods are sensitive to any kind of motion, thus, most of these methods might suffer from bulk motion artifacts. The common solution to avoid motion artifacts in a 2D DW SSFP acquisition is multi averaging. To avoid the time consuming multi averaging, the new scheme, described here, utilizes navigator echo’s motion correction to remove respiratory bulk motion artifacts. At some brain regions, where the motion is governed by blood or CSF pulsation, the navigator motion correction fails. At these regions the correction is an interpolation of corrections from regions where the motion is particularly of the respiratory type. The combination of a 3D sequence with a navigator echo motion correction, enables acquisition of 10 DW slices within a time of 0:50-2:30 min.

Comprehensive RF safety concept for parallel transmission MR.

The goal of this study is to increase patient safety in parallel transmission (pTx) MRI systems. A major concern in these systems is radiofrequency‐induced tissue heating, which can be avoided by specific absorption rate (SAR) prediction and SAR monitoring before and during the scan.

SAR in Parallel Transmission

Parallel transmission bears the potential of compensating B1 fleld inhomogeneities induced by wave propagation efiects in (ultra) high fleld whole body MR imaging. However, with increasing fleld strength, the RF power deposition and the associated local speciflc absorption rate (SAR) represent an important attention point with respect to patient safety. This paper presents simulations of a 3T whole body eight-channel transmit/receive body coil loaded with a human bio-mesh model. Phantom SAR simulations were carried out and validated by temperature measurements. A good correlation between SAR simulations and measured temperature was obtained, so that the FDTD method can be considered to be a valuable tool in determining (local) SAR for patient safety in multi-channel transmission MRI systems.

The modular gradient coil: an holistic approach to power efficient and high performance whole-body MRI without peripheral nerve stimulation.

The performance levels of whole-body MRI gradient systems have increased dramatically over recent years. Two areas of most concern in such gradient systems are (i) the power requirements for the gradient coil and (ii) the physiological side effects that can be produced. Considerable progress has been made in the design of gradient power supply units (PSU). A resonant or semi-resonant PSU operated in conjunction with a tuned gradient coil has the capability to provide almost unlimited gradient performance. As a less expensive alternative, a modern high current switch mode gradient PSU, coupled to a low inductance gradient coil, is also capable of providing a very respectable gradient performance. By stark contrast, research into the physiological side effects of high performance gradient systems, such as peripheral nerve stimulation (PNS), is still very much lacking. Few investigative works have succeeded in presenting a clear picture of the issue and fewer still have attempted to suggest a solution to the problem that is fully acceptable in the clinical environment. We have reported previously ([1-3]) a new approach to high performance whole-body gradient coil design which we have called the modular gradient coil (MGC). Central to the design strategy of the MGC is the requirement for high performance, with relatively low power requirements, and the complete avoidance of PNS. Our novel design approach results in an actively