Jonathan C. Sharp

Pilot study of dermal and subcutaneous fat structures by MRI in individuals who differ in gender, BMI, and cellulite grading

Background/aims: Puckered, dimply skin on the thighs, hips, and buttocks is known as cellulite. The cause of cellulite is not known, although there are a number of different hypotheses. In this study, we use magnetic resonance (MR) micro‐imaging to study cellulite skin. To the best of our knowledge, this is the first reported MR study of cellulite.

In vivo morphological characterisation of skin by MRI micro-imaging methods.

Background/purpose: Quantitative assessments in skin layers using images obtained with standard magnetic resonance imaging (MRI) sequences are limited, since the stratum corneum and dermis, the layers of most clinical interest, have low signal due to their short spin–spin relaxation, T2.

In vivo quantitative analysis of the effect of hydration (immersion and Vaseline treatment) in skin layers using high-resolution MRI and magnetisation transfer contrast*

Background/aims: Many claims are made as to the efficacy of topical preparations in moisturising the skin, yet most of these claims cannot be substantiated by scientific study for the skin layers beneath the stratum corneum, and yield no information on the remainder of the epidermis and dermis. This argues for an in vivo quantitative method for measuring the effect of water loading extended to various layers of the skin.

The engineering of an interventional MRI with a movable 1.5 Tesla magnet.

The engineering of a novel intra‐operative MRI system is described. A movable, 1.5 Tesla MRI magnet was placed in a neurosurgical operating room without affecting established neurosurgical procedure. The system allows fast, high‐quality MR intra‐operative imaging of the brain and spine without the necessity of patient transportation. A neuro‐navigational device capable of displaying and updating spatially referenced MR images in the operating room was integrated with the MRI system. Over 100 procedures have been carried out with this system without limiting surgical access and without compromising traditional neurosurgical, nursing or anesthetic techniques. J. Magn. Reson. Imaging 2001;13:78–86.

MRI using radiofrequency magnetic field phase gradients.

Conventionally, MR images are formed by applying gradients to the main static magnetic field (B0). However, the B0 gradient equipment is expensive, power‐hungry, complex, and noisy and can induce eddy currents in nearby conducting structures, including the patient. Here, we describe a new silent, B0 gradient‐free MRI principle, Transmit Array Spatial Encoding (TRASE), based on phase gradients of the radio‐frequency (RF) field. RF phase gradients offer a new method of k‐space traversal. Echo trains using at least two different RF phase gradients allow spin phase to accumulate, causing k‐space traversal. Two such RF fields provide one‐dimensional imaging and three are sufficient for two‐dimensional imaging. Since TRASE is a k‐space method, analogues of many conventional pulse sequences are possible. Experimental results demonstrate one‐dimensional and two‐dimensional RF MRI and slice selection using a single‐channel, transmit/receive, 0.2 T, permanent magnet, human MR system. The experimentally demonstrated spatial resolution is much higher than that provided by RF receive coil array sensitivity encoding alone but lower than generally achievable with B0 gradients. Potential applications are those in which one or more of the features of simplified equipment, lower costs, silent MRI, or the different physics of the image formation process are particularly advantageous. Magn Reson Med, 2010.

MR spectroscopy using multi-ring surface coils.

A spatially uniform B1‐field is preferred for MR imaging and spectroscopy. Unfortunately, volume coils are sometimes unavailable, or do not provide adequate RF power or SNR for some applications. In quantitative MRS, mean metabolite concentration cannot be evaluated when the coil response is nonuniform, unless an assumption is made concerning the metabolite spatial distribution. It is well known that standard single‐loop surface coils, although offering high SNR characteristics, have poor B1 homogeneity. New multi‐ring surface coils are proposed which produce a locally uniform B1 field, with sensitivity and power requirements comparable to those of standard surface coils. MR spectroscopy using two and three‐ring versions of this “local volume coil” result in spatial localization essentially identical to that obtained with a volume coil but with much improved RF power and SNR characteristics. When compared to standard surface coils, the multi‐ring coil offers much improved water suppression and localization, as well as reduced outer voxel contamination, with only a small loss in SNR and moderate increase in SAR. In summary, the multi‐ring coil operates midway between the volume coil and the standard surface coil, retaining the most advantageous properties of both. Magn Reson Med 42:655–664, 1999. Published 1999 Wiley‐Liss, Inc.

An NMR technique for measurement of magnetic field gradient waveforms

Almost all NMR imaging and localized spectroscopic methods fundamentally rely on the use of magnetic field gradients. It follows that precise information on gradient waveform shape and rise-times is often most useful in experimental MRI. We present a very simple and robust method for measuring the time evolution of a magnetic field gradient. The method is based on the analysis of the NMR signal in the time domain, and requires no specialized field measurement probes for its implementation. The technique makes use of the principal that for small flip angles the excitation profile is a good approximation to the Fourier transform of the radio frequency pulse shape. Creation of the NMR signal can be considered as an inverse Fourier transform and thus variation of the gradient strength during the excitation pulse influences the shape of the NMR signal. Although originally designed for measurement of the rise time only, we have now extended the technique to measure the exact time course of the gradient. The theory is confirmed by experimental results for gradient waveform field measurements in a high-field vertical bore system.

Molecular susceptibility weighted imaging of the glioma rim in a mouse model

BACKGROUND Glioma is the most common and most difficult to treat brain cancer. Despite many efforts treatment, efficacy remains low. As neurosurgical removal is the standard procedure for glioma, a method, allowing for both early detection and exact determination of the location, size and extent of the tumor, could improve a patients positive response to therapy. NEW METHOD We propose application of susceptibility weighted molecular magnetic resonance imaging using, targeted contrast agents, based on superparamagnetic iron oxide nanoparticles, for imaging of the, glioma rim, namely brain-tumor interface. Iron oxide attached to the targeted cells increases, susceptibility differences at the boundary between tumor and normal tissue, providing the opportunity, to utilize susceptibility weighted imaging for improved tumor delineation. We investigated potential, enhancement of the tumor-brain contrast, including tumor core and rim when using susceptibility, weighted MRI for molecular imaging of glioma. RESULTS There were significant differences in contrast-to-noise ratio before, 12 and 120min after contrast, agent injection between standard gradient echo pulse sequence and susceptibility weighted molecular, magnetic resonance imaging for the core-brain, tumor rim-core and tumor rim-brain areas. COMPARISON WITH EXISTING METHODS Currently, the most common MRI contrast agent used for glioma diagnosis is a non-specific, gadolinium-based agent providing T1-weighted enhancement. Susceptibility-weighted magnetic, resonance imaging is much less efficient when no targeted superparamagnetic contrast agents are, used. CONCLUSION The improved determination of glioma extent provided by SWI offers an important new tool for, diagnosis and surgical planning.

B1 transmit phase gradient coil for single-axis TRASE RF encoding.

PURPOSE TRASE (Transmit Array Spatial Encoding) MRI uses RF transmit phase gradients instead of B0 field gradients for k-space traversal and high-resolution MR image formation. Transmit coil performance is a key determinant of TRASE image quality. The purpose of this work is to design an optimized RF transmit phase gradient array for spatial encoding in a transverse direction (x- or y- axis) for a 0.2T vertical B0 field MRI system, using a single transmitter channel. This requires the generation of two transmit B1 RF fields with uniform amplitude and positive and negative linear phase gradients respectively over the imaging volume. MATERIALS AND METHODS A two-element array consisting of a double Maxwell-type coil and a Helmholtz-type coil was designed using 3D field simulations. The phase gradient polarity is set by the relative phase of the RF signals driving the simultaneously energized elements. RESULTS Field mapping and 1D TRASE imaging experiments confirmed that the constructed coil produced the fields and operated as designed. A substantially larger imaging volume relative to that obtainable from a non-optimized Maxwell-Helmholtz design was achieved. CONCLUSION The Maxwell (sine)-Helmholtz (cosine) approach has proven successful for a horizontal phase gradient coil. A similar approach may be useful for other phase-gradient coil designs.

The integration of real and virtual magnetic resonance imaging experiments in a single instrument

We present the design of an integrated system for performing both real and virtual (simulated) magnetic resonance imaging (MRI) experiments. We emphasize the approaches used to maximize the level of integration and also the benefits that tight real-virtual integration brings for a scientific instrument. The system has been implemented for both low field (0.2 T) and high field (9.4 T) imaging systems. The simulations can run for any MRI experiment and we demonstrate the operation of the system for T(1), T(2), T(2) ( *), and diffusion contrasts.