Biomedical subjects

Ashley D Harris

Publications and source records attributed to Ashley D Harris.

4 recordsLinked to original sources

PubMed2005

Control of end-tidal PCO2 reduces middle cerebral artery blood velocity variability: implications for physiological neuroimaging.

Breath-by-breath variability of the end-tidal partial pressure of CO2 (Pet(CO2)) has been shown to be associated with cerebral blood flow (CBF) fluctuations. These fluctuations can impact neuroimaging techniques that depend on cerebrovascular blood flow. We hypothesized that controlling Pet(CO2) would reduce CBF variability. Dynamic end-tidal forcing was used to control Pet(CO2) at 1.5 mm Hg above the resting level and to hold the end-tidal partial pressure of oxygen (Pet(O2)) at the resting level. Peak blood velocity in the middle cerebral artery (MCA) was measured by transcranial Doppler ultrasound (TCD) as an index of CBF. Blood velocity parameters and timing features were determined on each waveform and the variance of these parameters was compared between Normal (air breathing) and Forcing (end-tidal gas control) sessions. The variability of all velocity parameters was significantly reduced in the Forcing session. In particular, the variability of the average velocity over the cardiac cycle was decreased by 18.2% (P < 0.001). For the most part, the variability of the timing parameters was unchanged. Thus, we conclude that controlling Pet(CO2) is effective in reducing CBF variability, which would have important implications for physiologic neuroimaging.

Adult↗

PubMed2004

A comparison of images generated from diffusion-weighted and diffusion-tensor imaging data in hyper-acute stroke.

PURPOSE: To compare isotropic (combined diffusion-weighted image [CMB], apparent diffusion coefficient [ADC], TRACE, exponential ADC [eADC], and isotropically-weighted diffusion image [isoDWI]) and anisotropic (relative anisotropy [RA], fractional anisotropy [FA], and volume ratio [VR]) diffusion images collected with fast magnetic resonance (MR) diffusion-weighted (DWI) and diffusion-tensor (DTI) acquisition strategies (each less than one minute) in hyper-acute stroke. MATERIALS AND METHODS: Twenty-one patients suffering from ischemic stroke-imaged within six hours of symptom onset using both DWI and DTI-were analyzed. Regions of interest were placed in the ischemic lesion and in normal contralateral tissue and the percent difference in image intensity was calculated for all nine generated images. RESULTS: The average absolute percent changes for the isotropic strategies were all > 38%, with isoDWI found to have a difference of 50.7% +/- 7.9% (mean +/- standard error, P < 0.001). The ADC maps had the most significant difference (-42.4% +/- 2.0%, P < 0.001, coefficient of variation = 0.22). No anisotropic images had significant differences. CONCLUSION: Anisotropic maps do not consistently show changes in the first six hours of ischemic stroke; therefore, isotropic maps, such as those obtained using DWI, are more appropriate for detecting hyper-acute stroke. Anisotropic images, however, may be useful to differentiate hyper-acute stroke from acute and sub-acute stroke.

Aged↗

PubMed2002

Effect of b value on contrast during diffusion-weighted magnetic resonance imaging assessment of acute ischemic stroke.

PURPOSE: To examine the effect of varying the diffusion encoding strength (b value) on the contrast (signal difference, Delta S) between damaged and normal tissue during diffusion-weighted magnetic resonance imaging (DWI) assessment of acute ischemic stroke. MATERIALS AND METHODS: Phantoms with diffusion values approximating those expected in acutely infarcted and normal tissue were constructed from a mixture of agar and formaldehyde and imaged at varying b values (0-3000 mm(-2) second). Ten patients were imaged with multiple b values (500-2500 mm(-2) second) within 12 hours of stroke onset. RESULTS: Theoretical calculations showed that for any combination of diffusion coefficients there existed an optimal b value that was higher than the standard setting of 1000 mm(-2) second, and this was confirmed by the phantom studies. In the patients, increasing b from 1000 to 1500 mm(-2) second increased Delta S (average, 22.4%; P = 0.001), but no consistent benefit was seen at b = 2000 mm(-2) second (P = 0.408). This compared favorably with the average optimal b value of 1662 mm(- 2) second calculated from the patients. CONCLUSION: These results suggest that increasing the b value from 1000 to 1500 mm(-2) second would increase contrast between infarcted and normal tissue in the setting of acute ischemic stroke.

Acute Disease↗

PubMedDate not supplied

Fluid-attenuated inversion recovery preparation: not an improvement over conventional diffusion-weighted imaging at 3T in acute ischemic stroke.

BACKGROUND AND PURPOSE: Change in signal intensity due to acute ischemic stroke can be detected on diffusion-weighted (DW) images soon after symptom onset. Fluid-attenuated inversion recovery (FLAIR) DW imaging suppresses signal intensity from water and has been suggested to be better than conventional DW imaging as a diagnostic imaging technique in acute stroke. We compared the signal intensity-to-noise ratio (SNR) and contrast-to-noise-ratio (CNR) between ischemic and normal tissues by using these two sequences. METHODS: Twenty stroke patients underwent imaging less than 6 hours after stroke onset by using both acquisition methods. The SNR of six regions of interest in normal brain and one region in ischemic brain were compared on both DW imaging and FLAIR DW imaging. We also compared CNR in normal and ischemic tissues. The calculated apparent diffusion coefficient (ADC) maps from each acquisition technique were similarly assessed. RESULTS: The SNR was significantly lower for FLAIR DW imaging than for DW imaging (P < .05). The CNR between normal and ischemic tissue was also lower on FLAIR DW imaging (P < .05). SNR and CNR of the ADC maps were significantly different (P < .05) for all tissues except the putamen and white matter (for SNR and CNR) and globus pallidus (for CNR only). CONCLUSION: Ischemic tissue on FLAIR DW imaging was significantly less conspicuous than on DW imaging and potentially limits the clinical utility of this sequence.

Adult↗