Biomedical subjects

Mark R Symms

Publications and source records attributed to Mark R Symms.

21 records · Page 2Linked to original sources

PubMed2002

ADC mapping of the human optic nerve: increased resolution, coverage, and reliability with CSF-suppressed ZOOM-EPI.

The mean apparent diffusion coefficient (ADC) of the human optic nerve (ON) has been quantified in vivo, and mean ADC maps are shown along the complete length of the nerve from the globe to the optic chiasm. The mean ADC, over the whole nerve, is shown to be 1058 x 10(-6) mm(2) s(-1) (standard deviation (SD), over nine 3-mm slices, 101x10(-6) mm(2) s(-1); range (833-1178)x10(-6) mm(2) s(-1)). The robustness of the method relies on acquisition of high-resolution coronal images of the ON using the ZOOM-EPI technique, which makes use of a shortened echo train length for increased resolution with decreased susceptibility-induced distortions. Suppression of the cerebrospinal fluid (CSF) and fat signals from tissues that surround the ON also helps successful identification and delineation of the nerve. Averaging of magnitude images is used to compensate for the inherently low signal-to-noise ratio (SNR) of the acquired images; the effects of the Rayleigh distributed noise in such images are allowed for during ADC calculations.

Cerebrospinal Fluid↗

PubMed2002

Investigating cervical spinal cord structure using axial diffusion tensor imaging.

This study describes a new technique for Diffusion Tensor Imaging (DTI) that acquires axial (transverse) images of the cervical spinal cord. The DTI images depict axonal fiber orientation, enable quantification of diffusion characteristics along the spinal cord, and have the potential to demonstrate the connectivity of cord white matter tracts. Because of the high sensitivity to motion of diffusion-weighted magnetic resonance imaging and the small size of the spinal cord, a fast imaging method with high in-plane resolution was developed. Images were acquired with a single-shot EPI technique, named ZOOM-EPI (zonally magnified oblique multislice echo planar imaging), which selects localized areas and reduces artefacts caused by susceptibility changes between soft tissue and the adjacent vertebrae. Cardiac gating was used to reduce pulsatile flow artefacts from the surrounding cerebrospinal fluid. Voxel resolution was 1.25 x 1.25 mm(2) in-plane with 5-mm slice thickness. Both the mean diffusivity (MD) and the fractional anisotropy (FA) indices of the cervical spinal cord were measured. The FA index demonstrated high anisotropy of the spinal cord with an average value of 0.61 +/- 0.05 (highest value of 0.66 +/- 0.03 at C3), comparable to white matter tracts in the brain. The diffusivity components parallel and orthogonal to the longitudinal axes of the cord were lambda( parallel) = (1648 +/- 123) x 10(-6) mm(2)s(-1) and lambda( perpendicular) = (570 +/- 47) x 10(-6) mm(2) s(-1), respectively. The high axial resolution allowed preliminary evaluation of fiber connectivity using the fast-marching tractography algorithm, which generated traces of fiber paths consistent with the well-known cord anatomy.

Algorithms↗

PubMed2002

Functional magnetic resonance neuroimaging of drug dependence: naloxone-precipitated morphine withdrawal.

This study investigated the potential utility of fMRI as a neuroimaging technique to examine drug dependence using a robust animal model of drug withdrawal. Two groups of rats chronically pretreated with incremental doses of morphine sulfate (2, 7, 15, 30, 40, 50, 50, and 50 mg/kg--subcutaneous injection) were subjected to opioid precipitated withdrawal (using the opioid antagonist, naloxone) and subsequently behaviorally assessed or gradient-echo imaged under urethane anesthesia. Whole brain, group statistical parametric maps revealed statistically significant changes in signal intensity following administration of 1 mg/kg naloxone (corrected for multiple comparisons: P < 0.05, T > 5.03). Control groups within the fully crossed designs did not exhibit any statistically significant changes in behavior or signal intensity changes. Regional patterns of modulated activity include the retrosplenial, piriform, insular, entorhinal, cingulate, visual and auditory cortices, posterior fields of the hippocampus, and in particular the dentate gyrus. Such areas are consistent with biochemical correlates of morphine withdrawal and time profiles derived from our behavioral observations (P < 0.02). A notable lack of signal intensity changes in a number of subcortical areas suggests a possible confound associated with fMRI under anesthesia. This paper reports the first whole brain fMRI examination of an animal model of drug withdrawal, we believe there is considerable scope for extrapolation of our methods to a multitude of pharmacological applications-most notably in conjunction with other techniques in the development of potential therapeutic agents for drug dependence.

Animals↗