MR spectroscopy using multi-ring surface coils

Enhancing surface coil sensitive volume with hybridized electric dipoles at 17.2 T

Preclinical MR applications at 17.2 T can require field of views on the order of a few square centimeters. This is a challenging task as the proton Larmor frequency reaches 730 MHz. Most of the protocols at such frequencies are performed with surface transceiver coils for which the sensitive volume and the signal to noise ratio (SNR) is given by their size. Here we propose an approach based on metamaterials in order to enhance the sensitive volume of a commercial surface coil for small animal imaging at 17.2 T. We designed a passive resonator composed of four hybridized electric dipoles placed onto the floor of the MRI bed. Combining numerical and experimental results on a phantom and in vivo, we demonstrate a 20% increase of the sensitive volume in depth and 25% along the rostro-caudal axis while maintaining more than 85% of the local SNR right beneath the surface coil plane. Moreover, our solution gives the ability to double the average SNR in the region between 1.2 and 2 cm away from the loop using a single layer of 1 mm thick metallic wires easy to design and manufacture.

Applications of RF current sources for transmit phased array

Current practice in MRI trends towards higher static magnetic field (B/sub 0/) because of the advantage of signal-to-noise ratio (SNR). However at high magnetic fields, the interaction between a coil and a load becomes significant, thus making current distribution on RF coils disturbed and causing transverse magnetic (B/sub 1/) field inhomogeneity. A novel approach to optimize B/sub 1/, field homogeneity is to use a transmit phased array and adjust the amplitudes and phases of the currents on each element independently. However, independent control is difficult to achieve in conventional arrays due to coupling between elements. In addition, the currents are generally load dependent. In this paper we show that a RF current source is an effective method for controlling the current on each element of the transmit phased array, and that the RF current source is much less sensitive to loading and a interelement coupling than a conventional 50Omega matched coil.

Proton spectroscopy provides accurate pathology on biopsy and in vivo

In the last 25 years, MR spectroscopy (MRS) has moved from being a basic research tool into routine clinical use. The spectroscopy method reports on those chemicals that are mobile on the MR time scale. Many of these chemicals reflect specific pathological processes but are complicated by the fact that many chemicals change at one time. There are currently two clinical applications for spectroscopy. The first is in the pathology laboratory, where it can be an adjunct to, and in some cases replacement, for difficult pathologies like Barrett's esophagus and follicular adenoma of the thyroid. The spectroscopy method on a breast biopsy can also report on prognostic indicators, including the potential for spread, from information present in the primary tumor alone. The second application for spectroscopy is in vivo to provide a preoperative diagnosis and this is now achievable for several organs including the prostate. The development of spectroscopy for clinical purposes has relied heavily on the serially-sectioned histopathology to confirm the high accuracy of the method. The combination of in vivo MRI, in vivo MRS, and ex vivo MRS on biopsy samples offers a modality of very high accuracy for preoperative diagnosis and provision of prognostic information for human cancers.

Fluorine electron double resonance imaging for 19F MRI in low magnetic fields

This work demonstrates the feasibility of generating fluorine NMR images at a very low magnetic field of 0.015 T by making use of the Overhauser enhancement of (19)F NMR signal brought about by a stable, water-soluble, narrow-line paramagnetic contrast agent. The enhancement in the (19)F NMR images depends on the concentration of the single electron contrast agent, the pO(2), and the electron paramagnetic resonance (EPR) irradiation power. The applicability of this technique for (19)F NMR imaging is demonstrated with phantom samples, where a time resolution of 4-10 min is achieved. Proton electron double resonance imaging (PEDRI) and fluorine electron double resonance imaging (FEDRI) images were also obtained from rat kidneys ex vivo, perfused with 10 mM Oxo63 and 10 M trifluoroacetic acid. The spatial and temporal resolutions of these images are comparable to those obtained at magnetic fields 2-3 orders of magnitude larger. Constant NMR frequency (628 kHz) operation permits both FEDRI and PEDRI of identical slices without removing the object under investigation. This feasibility of coregistration of proton-based anatomical PEDRI image with physiological FEDRI image offers good potential for studying fluorine-containing tracers.

Overhauser enhanced magnetic resonance imaging for tumor oximetry: coregistration of tumor anatomy and tissue oxygen concentration

An efficient noninvasive method for in vivo imaging of tumor oxygenation by using a low-field magnetic resonance scanner and a paramagnetic contrast agent is described. The methodology is based on Overhauser enhanced magnetic resonance imaging (OMRI), a functional imaging technique. OMRI experiments were performed on tumor-bearing mice (squamous cell carcinoma) by i.v. administration of the contrast agent Oxo63 (a highly derivatized triarylmethyl radical) at nontoxic doses in the range of 2-7 mmol/kg either as a bolus or as a continuous infusion. Spatially resolved pO(2) (oxygen concentration) images from OMRI experiments of tumor-bearing mice exhibited heterogeneous oxygenation profiles and revealed regions of hypoxia in tumors (<10 mmHg; 1 mmHg = 133 Pa). Oxygenation of tumors was enhanced on carbogen (95% O(2)/5% CO(2)) inhalation. The pO(2) measurements from OMRI were found to be in agreement with those obtained by independent polarographic measurements using a pO(2) Eppendorf electrode. This work illustrates that anatomically coregistered pO(2) maps of tumors can be readily obtained by combining the good anatomical resolution of water proton-based MRI, and the superior pO(2) sensitivity of EPR. OMRI affords the opportunity to perform noninvasive and repeated pO(2) measurements of the same animal with useful spatial (approximately 1 mm) and temporal (2 min) resolution, making this method a powerful imaging modality for small animal research to understand tumor physiology and potentially for human applications.

Ultrafast pulse sequence techniques for cardiac magnetic resonance imaging

Cardiac magnetic resonance imaging is a rapidly emerging field that has seen tremendous advances in the past decade. Central to the development of effective imaging strategies has been the advent of high-performance gradient hardware and the exploitation of their speed characteristics through specialized pulse sequences well suited for cardiac imaging. These advances have facilitated unprecedented acquisition times that now approach echocardiographic frame rates, while maintaining excellent image quality. This article provides a detailed overview of advanced pulse sequence technology and approaches currently taken to maximize speed performance and image quality. In particular, segmented K-space techniques that include single-echo and multiecho spoiled gradient-echo imaging as well as steady-state free precession imaging are discussed. Finally, spiral and fast spin-echo techniques are explored. Examples of common applications of these pulse sequences are presented.