Biomedical subjects

David B Clayton

Publications and source records attributed to David B Clayton.

5 recordsLinked to original sources

PubMed2007

Identifying systematic errors in quantitative dynamic-susceptibility contrast perfusion imaging by high-resolution multi-echo parallel EPI.

Several obstacles usually confound a straightforward perfusion analysis using dynamic-susceptibility contrast-based magnetic resonance imaging (DSC-MRI). In this work, it became possible to eliminate some of these sources of error by combining a multiple gradient-echo technique with parallel imaging (PI): first, the large dynamic range of tracer concentrations could be covered satisfactorily with multiple echo times (TE) which would otherwise result in overestimation of image magnitude in the presence of noise. Second, any bias from T(1) relaxation could be avoided by fitting to the signal magnitude of multiple TEs. Finally, with PI, a good tradeoff can be achieved between number of echoes, brain coverage, temporal resolution and spatial resolution. The latter reduces partial voluming, which could distort calculation of the arterial input function. Having ruled out these sources of error, a 4-fold overestimation of cerebral blood volume and flow remained, which was most likely due to the completely different relaxation mechanisms that are effective in arterial voxels compared with tissue. Hence, the uniform tissue-independent linear dependency of relaxation rate upon tracer concentration, which is usually assumed, must be questioned. Therefore, DSC-MRI requires knowledge of the exact dependency of transverse relaxation rate upon tracer concentration in order to calculate truly quantitative perfusion maps.

Brain↗

PubMed2005

Foundations of advanced magnetic resonance imaging.

During the past decade, major breakthroughs in magnetic resonance imaging (MRI) quality were made by means of quantum leaps in scanner hardware and pulse sequences. Some advanced MRI techniques have truly revolutionized the detection of disease states and MRI can now-within a few minutes-acquire important quantitative information noninvasively from an individual in any plane or volume at comparatively high resolution. This article provides an overview of the most common advanced MRI methods including diffusion MRI, perfusion MRI, functional MRI, and the strengths and weaknesses of MRI at high magnetic field strengths.

Animals↗

PubMed2003

MR imaging of sodium in the human brain with a fast three-dimensional gradient-recalled-echo sequence at 4 T.

RATIONALE AND OBJECTIVES: Sodium ions play a vital role in cellular homeostasis and electrochemical activity throughout the human body. However, the in vivo detection of sodium (23Na) with magnetic resonance (MR) techniques is hindered by the fast transverse relaxation, low tissue equivalent concentration, and small gyromagnetic ratio of sodium ions compared with protons (1H). The goals of this study were to acquire MR images of sodium in the whole human brain by using a fast three-dimensional gradient-recalled-echo sequence and to investigate the effect that restrictions on specific absorption ratio have on MR imaging of sodium at 4 T. MATERIALS AND METHODS: A three-dimensional gradient-recalled-echo sequence with short echo time was developed for MR imaging of sodium. Slab encoding was removed and a hard excitation pulse was used. Five healthy human volunteers were examined in a whole-body MR imager with the use of a custom transmit-and-receive birdcage coil. Fields of view were selected to cover the entire brain: 38 x 38 cm in the axial plane, with 24 sections of 5.8 mm each or 12 sections of 1.1 cm each. The in-plane acquisition matrix was 64 x 128, and voxel size was 0.2 cm(3). RESULTS: Sodium in white matter was depicted with an acceptable signal-to-noise ratio of 20-25. The echo time, and hence the signal-to-noise ratio, was limited by the MR imager's maximum allowable gradient strength. To keep the specific absorption ratio below 3 W/kg (the limit established by the Food and Drug Administration), it was necessary to prolong the repetition time to 30 msec. CONCLUSION: The MR imaging protocol used in this study provided acceptable visualization of sodium in the whole brain in a tolerable total acquisition time of 15 minutes.

Brain↗

PubMed2002

Localized proton spectroscopy without water suppression: removal of gradient induced frequency modulations by modulus signal selection.

Most Magnetic Resonance Spectroscopy (MRS) localization methods can generate gradient vibrations at acoustic frequencies and/or magnetic field oscillation, which can cause a time-varying magnetic field superimposed onto the static one. This effect can produce frequency modulations of the spectral resonances. When localized MRS data are acquired without water suppression, the associated frequency modulations are manifested as a manifold of spurious peaks, called sidebands, which occur symmetrically around the water resonance. These sidebands can be larger than the small metabolite resonances and can present a problem for the quantitation of the spectra, especially at short echo times. Furthermore, the resonance lineshapes may be distorted if any low frequency modulations are present. A simple solution is presented which consists of selecting the modulus of the acquired Free Induction Decay (FID) signal. Since the frequency modulations affect only the phase of the FID signal, the obtained real spectrum of the modulus is free from the spurious peaks where quantitative results may be directly obtained. Using this method, the distortions caused by the sidebands are removed. This is demonstrated by processing proton MRS spectra acquired without water suppression collected from a phantom containing metabolites at concentrations comparable to those in human brain and from a human subject using two different localization methods (PRESS and Chemical Shift Imaging PRESS-(CSI)). The results obtained illustrate the ability of this approach to remove the spurious peaks. The corrected spectra can then be fit accurately. This is confirmed by the results obtained from both the relative and the absolute metabolites concentrations in phantoms and in vivo.

Brain Chemistry↗

PubMed2002

In vivo sodium MR imaging of the intervertebral disk at 4 T.

RATIONALE AND OBJECTIVES: The authors performed this study to evaluate whether a semiquantitative method of in vivo sodium imaging of the human intervertebral disk could provide diagnostic quality images in a reasonable time. MATERIALS AND METHODS: In vivo sodium imaging of the human spine was performed with a 4-T whole-body magnetic resonance (MR) unit by using custom-built hardware and software. Short-echo-time images were obtained with a modified three-dimensional gradient-echo sequence, a custom-built surface coil, and receiver modifications to allow for nonproton data acquisition. Corrections for surface coil image intensity were performed with phantom image data. An estimation of the fixed charge density within the intervertebral disk was made with the surface coil-corrected images by using cerebrospinal fluid as an internal reference standard. RESULTS: In vivo sodium images of the spine with high signal-to-noise ratio and resolution were obtained in approximately 10 minutes. Correction of in vivo images for surface coil effects demonstrated that the sodium content of the cerebrospinal fluid in the spinal canal may be used as an internal standard to estimate the proteoglycan content of the intervertebral disk. CONCLUSION: Diagnostic quality in vivo sodium images of the intervertebral disk are technically feasible and may be useful for assessing the proteoglycan content of the intervertebral disk. This would provide a method to detect early degenerative changes in the disk and evaluate the effects of any subsequent therapy.

Adult↗