Biomedical subjects

Mark A Elliott

Publications and source records attributed to Mark A Elliott.

22 records · Page 2Linked to original sources

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↗

PubMed2002

Metabolic costs of isometric force generation and maintenance of human skeletal muscle.

During isometric contractions, no true work is performed, so the force-time integral (FTI) is often used to approximate isometric work. However, the relationship between FTI and metabolic cost is not as linear. We tested the hypothesis that this nonlinearity was due to the cost of attaining a given force being greater than that of maintaining it. The ATP consumed per contraction in the human medial gastrocnemius muscle (n = 6) was determined by use of (31)P-NMR spectroscopy during eight different electrical stimulation protocols. Each protocol consisted of 8 trains of a single frequency (20 or 80 Hz) and duration (300, 600, 1,200, or 1,800 ms) performed under ischemic conditions. The cost of force generation was determined from the ATP turnover during the short-duration trains that did not attain a steady force level. Estimates of the cost of force maintenance at each frequency were determined by subtracting the ATP turnover during the shorter-duration trains from the turnover during the long-duration trains. The force generation phase of an isometric contraction was indeed more metabolically costly than the force maintenance phase during both 20- and 80-Hz stimulation. Thus the mean rate of ATP hydrolysis appeared to decline as contraction duration increased. Interestingly, the metabolic costs of maintaining force during 20-Hz and 80-Hz stimulation were comparable, although different levels of force were produced.

Adenosine Triphosphate↗

PubMedDate not supplied

Effect of errors in baseline optical properties on accuracy of transabdominal near-infrared spectroscopy in fetal sheep brain during hypoxic stress.

A continuous-wave (cw) near-infrared spectroscopy (NIRS) instrument has been developed to noninvasively quantify fetal cerebral blood oxygen saturation (StO2). A linear Green's function formulism was used to analytically solve the photon diffusion equation and extract the time-varying fetal tissue oxy- and deoxy-hemoglobin concentrations from the NIR measurements. Here we explored the accuracy with which this instrument can be expected to perform over a range of fetal hypoxic states. We investigated the dependence of this accuracy on the accuracy of the reference optical properties chosen based on the literature. The fetal oxygenation of a pregnant ewe model was altered via maternal aortic occlusion. The NIR cw instrument was placed on the maternal abdomen directly above the fetal head, continuously acquiring diffuse optical measurements. Blood was sampled periodically from the fetus to obtain fetal arterial saturation (SaO2) measurements from blood gas analysis. The NIR StO2 values were compared with the fetal SaO2 measurements. Variations in the NIR results due to uncertainty in the reference optical properties were relatively small within the fetal SaO2 range of 30 to 80%. Under hypoxic conditions, however, the variability of the NIR StO2 calculations with changes in the assumed reference properties became more significant.

Abdomen↗

PubMedDate not supplied

Intraobserver variability in the MR determination of tumor volume in squamous cell carcinoma of the pharynx.

BACKGROUND AND PURPOSE: If tumor volumes are to be used for evaluating responses to treatment and long-term outcomes of patients with primary pharyngeal carcinomas, the reproducibility of these measurements must be established. We determined the intraobserver variability of MR imaging-based volume measurements of these cancers and their regional metastases. METHODS: We used an interactive computer program (IDL) that enables the extraction of tumor volumes from 3D MR data to obtain 202 volume measurements in 17 patients with pharyngeal carcinoma (two to five time points each). The primary cancer and largest nodal mass were manually outlined on every T2-weighted image of each MR study. The same neuroradiologist reanalyzed this MR dataset 2-41 weeks later. Measurement error and percentage measurement error (intraobserver variability) were determined. Differences in intraobserver variability between primary lesions and nodes, as well as between stages of treatment were tested with a Wilcoxon rank sum test. RESULTS: The mean and median percentage measurement errors, respectively, were 13% and 12% (range, 0-53%; 95% CI: 10%, 16%) for primary tumors and 9% and 7% (range, 0-37%; 95% CI: 7%, 12%) for nodal metastases. The difference in the percentage measurement error between primary lesions and cervical nodes approached statistical significance (P =.07). Differences in the variation of volume measurements based on the stage of therapy were significant (P =.01). CONCLUSION: Our results suggest that MR imaging-based tumor volumes are reliably reproducible. Such measurements may be important in predicting patient outcome, determining appropriate therapy, and conducting patient follow-up.

Adult↗