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Biomedical subjects

Timothy J Carroll

Publications and source records attributed to Timothy J Carroll.

9 recordsLinked to original sources
PubMed2003

Automatic calculation of the arterial input function for cerebral perfusion imaging with MR imaging.

An automated method for determination of arterial input function (AIF) for rapid determination of cerebral perfusion with dynamic susceptibility contrast magnetic resonance (MR) imaging was derived. In 100 patients, the automated method was used to create images of relative blood flow, relative cerebral blood volume, and mean transit time. In 20 patients, the voxel chosen with the automated AIF correlated with a large cerebral artery and exhibited less partial-volume averaging when compared with an AIF chosen manually. It is possible to reliably determine the AIF at dynamic susceptibility contrast MR imaging and eliminate the need for operator input and lengthy postprocessing.

Adolescent↗
PubMed2002

The sites of neural adaptation induced by resistance training in humans.

Although it has long been supposed that resistance training causes adaptive changes in the CNS, the sites and nature of these adaptations have not previously been identified. In order to determine whether the neural adaptations to resistance training occur to a greater extent at cortical or subcortical sites in the CNS, we compared the effects of resistance training on the electromyographic (EMG) responses to transcranial magnetic (TMS) and electrical (TES) stimulation. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous muscle of 16 individuals before and after 4 weeks of resistance training for the index finger abductors (n = 8), or training involving finger abduction-adduction without external resistance (n = 8). TMS was delivered at rest at intensities from 5 % below the passive threshold to the maximal output of the stimulator. TMS and TES were also delivered at the active threshold intensity while the participants exerted torques ranging from 5 to 60 % of their maximum voluntary contraction (MVC) torque. The average latency of MEPs elicited by TES was significantly shorter than that of TMS MEPs (TES latency = 21.5 +/- 1.4 ms; TMS latency = 23.4 +/- 1.4 ms; P < 0.05), which indicates that the site of activation differed between the two forms of stimulation. Training resulted in a significant increase in MVC torque for the resistance-training group, but not the control group. There were no statistically significant changes in the corticospinal properties measured at rest for either group. For the active trials involving both TMS and TES, however, the slope of the relationship between MEP size and the torque exerted was significantly lower after training for the resistance-training group (P < 0.05). Thus, for a specific level of muscle activity, the magnitude of the EMG responses to both forms of transcranial stimulation were smaller following resistance training. These results suggest that resistance training changes the functional properties of spinal cord circuitry in humans, but does not substantially affect the organisation of the motor cortex.

Adaptation, Physiological↗
PubMed2002

Rapid generation of preview images for real-time 3D MR angiography.

In 3D real-time MR angiography the reconstruction of images from large raw datasets via Fourier transforms with minimal delays is problematic. In this study strategies for reconstructing time-resolved three-dimensional (3D) datasets at a rate substantially faster than conventional 3D MR image reconstruction were investigated on general-purpose computer hardware. Moderate quality 'preview images' were generated from k-space subsets to reduce image reconstruction times from more than 50 s to 0.3 s per image volume. A blinded review of 3D TRICKS patient examinations showed that these moderate-quality images were sufficient for providing immediate feedback and guiding the subsequent reconstruction of selected time frames (p < 0.05). Fourier projection (reconstruction from a central k-space slice) was the most efficient reconstruction technique. However, the reduction of the reconstructed volume in all three dimensions resulted in higher contrast and better image quality while allowing reconstruction in near-to-real-time (less than 1 s per image volume). The use of such preview images in a real-time system allows for fast feedback from dynamic 3D datasets, enables scanner interaction with minimal latencies and can substantially reduce the postprocessing times.

Angiography↗
PubMed2002

Combined time-resolved and high-spatial-resolution 3D MRA using an extended adaptive acquisition.

PURPOSE: To combine the benefits of time-resolved dynamic imaging and single elliptical centric acquisitions in a reasonable scan time. MATERIALS AND METHODS: A time series of images with moderate spatial resolution was acquired using the 3D Time-Resolved Imaging of Contrast KineticS (3D TRICKS) technique with elliptical centric encoding during contrast arrival. Following venous opacification, a complete large centrically encoded k-space volume was acquired. The high-spatial-frequency portions of this volume were combined with a 3D TRICKS time frame to form a high-resolution image. An additional single image is formed by suppressing background and signal averaging all acquired data, including post-venous low-spatial-frequency data. For this image, 2D temporal correlation analysis is used to suppress low-spatial-frequency vein contributions. Arrival time and spatial correlations are used to suppress background. RESULTS: The 3D TRICKS time frame may be selected to ensure a combined high-resolution image that has optimal central k-space sampling for any vascular region. The single image formed by signal averaging all acquired data has increased contrast-to-noise (CNR) and signal-to-noise (SNR) ratios. CONCLUSION: The advantages of time-resolved and high-spatial-resolution imaging were combined using an extended dual-phase acquisition. Some SNR and CNR gain was achieved by signal averaging. This process is facilitated by background and vein suppression.

Animals↗
PubMed2002

Technical developments in MR angiography.

CE MRA has evolved rapidly since the early studies by Prince et al [3]. Whereas many of the procedures in clinical use today rely heavily on the use of gadolinium contrast agents and standard. Fourier transform acquisition techniques, advances will have a significant impact on MRA by shortening the acquisition time, improving the reproducibility of the image-acquisition techniques, and improving spatial resolution or SNR. From a technical basis, shorter acquisition times associated with fast gradients are likely to improve spatial resolution and allow for acquisition of MR images over large FOVs. In addition, alternative k-space sampling techniques, such as parallel imaging and PR, are expected to further reduce acquisition time, while maintaining or improving spatial resolution. The approval and subsequent use of new contrast agents will also have a beneficial impact on the image quality of contrast-enhanced MRA applications. It is likely that these contrast agents will be coupled with advanced acquisition techniques to improve spatial resolution and technical success rates of MRA examinations.

Biophysical Phenomena↗
PubMed2002

Absolute quantification of cerebral blood flow with magnetic resonance, reproducibility of the method, and comparison with H2(15)O positron emission tomography.

While H2(15)O positron emission tomography (PET) is still the gold standard in the quantitative assessment of cerebral perfusion (rCBF), its technical challenge, limited availability, and radiation exposure are disadvantages of the method. Recent work demonstrated the feasibility of magnetic resonance (MR) for quantitative cerebral perfusion imaging. There remain open questions, however, especially regarding reproducibility. The main purpose of this study was to assess the accuracy and reproducibility of MR-derived flow values to those derived from H2(15)O PET. Positron emission tomography and MR perfusion imaging was performed in 20 healthy male volunteers, who were chronic smokers, on day 1 and day 3 of a 4-day hospitalization. Subjects were randomly assigned to one of two groups, each with 10 subjects. One group was allowed to smoke as usual during the hospitalization, while the other group stopped smoking from day 2. Positron emission tomography and MR images were coregistered and rCBF was determined in two regions of interest, defined over gray matter (gm) and white matter (wm), yielding rCBF(PET)gm, rCBF(MR)gm, rCBF(PET)wm, and rCBF(MR)wm. Bland-Altman analysis was used to investigate reproducibility by assessing the difference rCBFday3 - rCBFday1 in eight continual-smoker volunteers. The analysis showed a good reproducibility for PET, but not for MR. Mean +/- SD of the difference rCBFday3 - rCBFday1 in gray matter was 6.35 +/- 21.06 and 0.49 +/- 5.27 mL x min(-1) x 100 g(-1) for MR and PET, respectively; the corresponding values in white matter were 2.60 +/- 15.64 and -1.14 +/- 4.16 mL x min(-1) x 100 g(-1). The Bland-Altman analysis was also used to assess MRI and PET agreement comparing rCBF measured on day 1. The analysis demonstrated a reasonably good agreement of MR and PET in white matter (rCBF(PET)wm - rCBF(MR)wm; -0.09 +/- 7.23 mL x min(-1) x 100 g(-1)), while in gray matter a reasonable agreement was only achieved after removing vascular artifacts in the MR perfusion maps (rCBF(PET)gm - rCBF(MR)gm; -11.73 +/- 14.52 mL x min(-1) x 100 g(-1)). In line with prior work, these results demonstrate that reproducibility was overall considerably better for PET than for MR. Until reproducibility is improved and vascular artifacts are efficiently removed, MR is not suitable for reliable quantitative perfusion measurements.

Adult↗
PubMed2002

Time-resolved three-dimensional contrast-enhanced MR angiography of the peripheral vessels.

PURPOSE: To compare the diagnostic accuracy of time-resolved three-dimensional contrast material-enhanced magnetic resonance (MR) angiography with that of conventional angiography for imaging the lower extremity vasculature. MATERIALS AND METHODS: Sixty-nine patients who were evaluated for possible surgical intervention underwent conventional angiography (ie, digital subtraction angiography [DSA]) and contrast-enhanced MR angiography (ie, time-resolved imaging of contrast kinetics [TRICKS]). Two independent, blinded readers evaluated vessel stenosis and occlusion at DSA and MR angiographic image readings. Sensitivity, specificity, positive and negative predictive values, and area under the receiver operating characteristic curve were analyzed with repeated-measures analysis of variance. The Cohen kappa test was performed to examine interreader variability. RESULTS: At pooled readings, contrast-enhanced MR angiography had a sensitivity of 78% and a specificity of 98% for detection of occlusion. For detection of significant stenosis (at least one > or = 50% stenosis), sensitivity and specificity were 77% and 91%, respectively. Interreader agreement was high for detection of both occlusion (kappa = 0.76) and significant stenosis (kappa = 0.68). Sensitivity increased as MR angiographic technical parameters were optimized. When improvements resulting from coil type and injection protocol were considered, the sensitivity and specificity of TRICKS MR angiography were 89% and 97%, respectively, for occlusion detection and 87% and 90%, respectively, for significant stenosis detection. CONCLUSION: Contrast-enhanced TRICKS MR angiography is a feasible and minimally invasive means of acquiring angiograms of the peripheral vasculature with high sensitivity and specificity.

Adult↗
PubMedDate not supplied

Confounding effect of large vessels on MR perfusion images analyzed with independent component analysis.

BACKGROUND AND PURPOSE: First pass contrast-enhanced MR imaging using gradient-echo acquisitions is commonly used to assess cerebral blood flow, despite the confounding signal from large blood vessels. We hypothesized that removal of this unwanted intravascular signal using independent component analysis would result in a more accurate depiction of cerebral blood flow. METHODS: Images of 11 patients, acquired with our acute stroke imaging protocol, were post processed to produce images of relative cerebral blood flow (rCBF). The same images were processed with independent component analysis to identify and remove the signal from large blood vessels, with a second set of rCBF images produced. Both sets of rCBF maps were pooled, randomized in order, and read in a blinded fashion by two neuroradiologists to assess the level of large artery artifact and overall image quality. Significance was determined using a Wilcoxon signed rank test. RESULTS: Results from both readers indicated that the level of large artery artifact was significantly reduced in the images processed using independent component analysis component removal (P <.05). In addition, both readers indicated significantly (P <.05) improved image quality of the images processed using independent component analysis. CONCLUSION: The removal of the signal resulting from large blood vessels before calculation of rCBF resulted in images with significantly less artifact and higher image quality.

Adolescent↗