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E G Walsh

Publications and source records attributed to E G Walsh.

At least 19 recordsLinked to original sources
PubMed2001

A TiO(2) dielectric filled toroidal radio frequency cavity resonator for high-field NMR.

R(F) performance in high-field MRI applications is improved by filling the resonator with material of relative dielectric constant approximating that of human soft tissue. We demonstrate this by filling a toroidal cavity resonator operating in TEM(00) (cyclotron) mode with titanium dioxide (TiO(2)) in powdered rutile form, and acquiring phantom, human lower leg and human breast images of good quality at 4.1 T. Images made with this resonator had unusually high SNR, while the level of R(f) power required to produce a 90 degrees flip angle pulse was about a quartes as high for the filled resonator as for the same resonator before filling. Phantom images obtained with the filled resonator had an SNR of nearly 300 at a resolution of 256 x 256 voxels, nearly three times that of images of the same phantom obtained using a standard volume R(f) coil in frequent use at this laboratory. Breast images made at 256 x 256 voxels resolution had an SNR of 174, also unusually high for a volume coil. High-resolution (512 x 512 voxels) were also obtained, with SNR = 60. Preliminary phantom and in vivo human images are presented in this article. Acquiring the phantom and leg images required significantly less R(f) power than did comparable imaging using a conventional coil. In addition, the field lines were focused as they penetrated into the sample, and this resulted in a more homogeneous B(1)-field. We believe that these improvements occurred because the dielectric presence minimizes the large dielectric mismatch between air and sample.

Breast↗
PubMed2001

Validation of rapid velocity encoded cine imaging of a dynamically complex flow field using turbo block regional interpolation scheme for k space.

Block regional interpolation scheme for k space (BRISK) is a sparse sampling approach to allow rapid magnetic resonance imaging of dynamic events. Rapid velocity encoded cine (VEC) imaging with Turbo BRISK is potentially an important clinical diagnostic technique for cardiovascular diseases. Previously we applied BRISK and Turbo BRISK to imaging pulsatile flow in a straight tube. To evaluate the capabilities of Turbo BRISK imaging in more complex dynamic flow fields such as might exist in the human vasculature, an in vitro curved tube model, similar in geometry to the aortic arch, was fabricated and imaged under pulsatile flow conditions. Velocity maps were obtained using conventional VEC and Turbo BRISK (turbo factors 1 through 5). Comparison of the flow fields obtained with each higher order turbo factor showed excellent agreement with conventional VEC with minimal loss of information. Similarly, flow maps showed good agreement with the profiles from a laser Doppler velocimetry model. Turbo-5 BRISK, for example, allowed a 94% savings in imaging time, reducing the conventional imaging time from over 8 min to a near breath-hold imaging period of 31 s. Turbo BRISK shows excellent promise toward the development of a clinical tool to evaluate complex dynamic intravascular flow fields.

Biomedical Engineering↗
PubMed2000

Hemodynamic evaluation with TURBO BRISK--a rapid phase contrast angiography technique.

Hemodynamic imaging by phase contrast angiography was significantly accelerated by selective interpolation and segmentation in k-space using TURBO BRISK. The method was tested in vitro on three independent flowfields, representative of human blood rheology: a straight tube simulating the descending aorta, a curved tube simulating the aortic arch and a two-chamber orifice flow model simulating valvular regurgitation. The results were compared to data obtained by Laser Doppler Velocimetry (LDV) and showed good agreement. For the straight tube, the flow velocity obtained by five TURBO BRISK methods with increasing segmentation factors and corresponding time savings showed good agreement with LDV. For the curved tube, the velocity showed good general agreement with some differences in the decelerating part of the cycle, and in the low-velocity secondary flow structures. The orifice flow evaluation, the most time consuming case, was performed by the control volume method. It showed good agreement with actual flows through the orifice. Data acquisitions for TURBO-4 BRISK could be performed in 20s for each velocity component. The method shows promise for breath-hold acquisitions in clinical applications, including calculation of blood flow volumes through diseased arteries, measurement of blood backflow volumes through dysfunctional heart valves to time valve replacement operations, and evaluation of arterial wall shear stress, an important factor in the genesis of atherosclerosis.

Aorta, Thoracic↗
PubMed1999

Coronary flow velocity response to adenosine characterizes coronary microvascular function in women with chest pain and no obstructive coronary disease. Results from the pilot phase of the Women's Ischemia Syndrome Evaluation (WISE) study.

OBJECTIVES: We sought to develop and validate a definition of coronary microvascular dysfunction in women with chest pain and no significant epicardial obstruction based on adenosine-induced changes in coronary flow velocity (i.e., coronary velocity reserve). BACKGROUND: Chest pain is frequently not caused by fixed obstructive coronary artery disease (CAD) of large vessels in women. Coronary microvascular dysfunction is an alternative mechanism of chest pain that is more prevalent in women and is associated with attenuated coronary volumetric flow augmentation in response to hyperemic stimuli (i.e., abnormal coronary flow reserve). However, traditional assessment of coronary volumetric flow reserve is time-consuming and not uniformly available. METHODS: As part of the Women's Ischemia Syndrome Evaluation (WISE) study, 48 women with chest pain and normal coronary arteries or minimal coronary luminal irregularities (mean stenosis = 7%) underwent assessment of coronary blood flow reserve and coronary flow velocity reserve. Blood flow responses to intracoronary adenosine were measured using intracoronary Doppler ultrasonography and quantitative angiography. RESULTS: Coronary volumetric flow reserve correlated with coronary velocity reserve (Pearson correlation = 0.87, p < 0.001). In 29 (60%) women with abnormal coronary microcirculation (mean coronary flow reserve = 1.84), adenosine increased coronary velocity by 89% (p < 0.001) but did not change coronary cross-sectional area. In 19 (40%) women with normal microcirculation (mean flow reserve = 3.24), adenosine increased coronary velocity and area by 179% (p < 0.001) and 17% (p < 0.001), respectively. A coronary velocity reserve threshold of 2.24 provided the best balance between sensitivity and specificity (90% and 89%, respectively) for the diagnosis of microvascular dysfunction. In addition, failure of the epicardial coronary to dilate at least 9% was found to be a sensitive (79%) and specific (79%) surrogate marker of microvascular dysfunction. CONCLUSIONS: Coronary flow velocity response to intracoronary adenosine characterizes coronary microvascular function in women with chest pain in the absence of obstructive CAD. Attenuated epicardial coronary dilation response to adenosine may be a surrogate marker of microvascular dysfunction in women with chest pain and no obstructive CAD.

Adenosine↗
PubMed1999

Continuum of reflex excitability in hemiplegia: influence of muscle length and muscular transformation after heel-cord lengthening and immobilization on the pathophysiology of spasticity and clonus.

The electromyographic (EMG) responses to tendon taps at the ankle and ensuing muscular twitch forces and temporal parameters were studied at varying angles across the joint range in 18 children, aged 3 to 14.9 years, with congenital hemiparetic cerebral palsy and 22 healthy (control) children, aged 3 to 13.6 years. Those subjects with hemiparesis were community ambulators without assistance. In all subjects, passive muscle stretch caused a waxing of the reflex EMG and twitch force near neutral (with the sole of the foot at right angles to the tibia) and a diminution of these with further dorsiflexion. Twitch times increased with each dorsiflexing increment, being slowest at maximum dorsiflexion and fastest at the resting plantarflexion angle. Heterogeneity of the hemiparetic-limb data is evident when compared with data of non-paretic and unaffected limbs, with clear differences in the clonic (fast twitch) as opposed to non-clonic (slow twitch) muscles. In four cases with clinical clonus, clonus frequency was reduced by passive dorsiflexion. Plaster immobilization for 1 month produced clonus which was previously absent in one subject, and caused a fast-twitch phenotype to emerge in two subjects. Follow-up after heel-cord lengthening in one subject showed that clonus frequency diminished from 9 to 3 Hz with slowing and strengthening of muscle-twitch phenotype. Short- and long-term peripheral manipulations appear to regulate neuromuscular excitability according to whether muscles are loaded or unloaded. Although damage to the nervous system provides the setting for reflex excitability, the data suggest that the muscle length (which specifies the joint angle) and the muscle-twitch phenotype of any given limb for any given case appear to dictate the actual speed and strength of reflex muscle-twitch and clonus profiles. This study illustrates how peripheral manipulations of muscles and tendons may alter the expression of what have hitherto been considered as exclusively central phenomena.

Adolescent↗
PubMed1999

Rapid velocity-encoded cine imaging with turbo-BRISK.

Velocity-encoded cine (VEC) imaging is potentially an important clinical diagnostic technique for cardiovascular diseases. Advances in gradient technology combined with segmentation approaches have made possible breathhold VEC imaging, allowing data to be obtained free of respiratory artifacts. However, when using conventional segmentation approaches, spatial and temporal resolutions are typically compromised to accommodate short breathhold times. Here we apply a sparse sampling technique, turbo-BRISK (i.e., segmented block regional interpolation scheme for k-space) to VEC imaging, allowing increased spatial and temporal resolution to be obtained in a short breathhold period. BRISK is a sparse sampling technique with interpolation used to generate unsampled data. BRISK was implemented to reduce the scan time by 70% compared with a conventional scan. Further, turbo-BRISK scans, using segmentation factors up to 5, reduce the scan time by up to 94%. Phantom and in vivo results are presented that demonstrate the accuracy of turbo-BRISK VEC imaging. In vitro validation is performed using conventional magnetic resonance VEC. Pulsatile centerline flow velocity measurements obtained with turbo-BRISK acquisitions were correlated with conventional magnetic resonance imaging measurements and achieved r values of 0.99 +/- 0.004 (mean +/- SD) with stroke volumes agreeing to within 4%. A potential limitation of BRISK is reduced accuracy for rapidly varying velocity profiles. We present low- and high-resolution data sets to illustrate the resolution dependence of this phenomenon and demonstrate that at conventional resolutions, turbo-BRISK can accurately represent rapid velocity changes. In vivo results indicate that centerline velocity waveforms in the descending aorta correlate well with conventional measurements with an average r value of 0.98 +/- 0.01.

Blood Flow Velocity↗
PubMed1997

Rapid cardiac imaging with turbo BRISK.

A variety of variable and constant rate, sparse sampling strategies have previously been proposed to rapidly image dynamically changing objects. The majority of these strategies compile a k-space data set for any given time point by substituting k-space data from the most recently sampled time positions (extracted from the sparsely sampled set). The BRISK technique, is a variable rate, sparse sampling technique which additionally incorporates an interpolation scheme to more accurately represent k-space data at positions which were not directly sampled. Here, strategies are introduced that allow tubo concepts to be incorporated with BRISK. Simulations are conducted to compare the efficacy of the turbo BRISK acquisition and processing strategy against a constant rate, sparse sampling strategy with direct substitution of the most recently acquired k-space lines. It is shown that turbo BRISK generates images of similar quality in approximately half the time as the uniform sampling rate, sparse sampling strategy. Data from turbo BRISK acquisitions of multicardiac phase image sets, obtained on a normal volunteer and cardiac patients are presented.

Heart↗
PubMed1997

Common k-space acquisition: a method to improve myocardial grid-tag contrast.

A current limitation of many myocardial tag acquisitions employing SPAMM encoding is the relatively rapid loss of tag contrast over the cardiac cycle. Acquisition schemes that apply line tags produce prolonged tag contrast compared with directly excited grid tags. However, a grid-tag series can be generated by combining two orthogonal line-tag series. To be time efficient, each line-tag series can be acquired with the read gradient oriented perpendicular to the line-tag direction. There are several disadvantages associated with this approach, including the requirement to avoid signal fold-over and that fat shift artifacts appear in different directions in each line-tag series. Presented here is a method of applying separate line tags that does not require interchanging the read and phase encoding gradients and does not extend the scan time compared with a conventional grid-tag acquisition. Additionally, the means of generating grid tags results in a doubling of the tag contrast to noise ratio compared a line-tag set. Computer simulations are presented along with phantom and volunteer scans.

Heart↗
PubMed1997

Botulinum toxin A in the hemiplegic upper limb: a double-blind trial.

In a randomised, double-blind study, the effects of intramuscular injection of botulinum toxin type A (BtA) into the upper limb were compared with those of normal saline solution in 14 patients with cerebral palsy; their mean age was 9 years. Range of movement and function were assessed before injection and at 2 and 12 weeks after injection. BtA injection significantly increased maximum active elbow and thumb extension and significantly reduced tone at wrist and elbow. The hand grasp-and-release score improved, representing a modest functional change, but fine motor function, assessed by the ability to pick up coins, did not improve and in some cases deteriorated temporarily. The most notable subjective change was the cosmetic benefit of reduced involuntary elbow flexion. The tone-reducing effect of BtA was clinically detectable in comparison with the placebo and patients and parents perceived the change as beneficial. The median of changes in the treatment group was small but the range was large, suggesting that BtA can be useful in selected patients.

Adolescent↗
PubMed1997

Soleus muscle length, stretch reflex excitability, and the contractile properties of muscle in children and adults: a study of the functional joint angle.

The influence of the joint angle on stretch reflex excitability of the soleus muscle at the ankle has been studied in 22 children aged 3.9 to 13.6 years and 9 adults aged 19 to 70 years. For all subjects, reflex EMG and mechanical twitch torque gain were trivial at resting plantar flexion. The reflex EMG gain reached a maximum between -15 degrees and -10 degrees of plantar flexion beyond the neutral angle, 0 degrees, defined as the foot at right angle to the tibia, diminishing steeply with further dorsiflexion. The reflex mechanical gain rose to a peak between 0 degrees and +10 degrees of dorsiflexion beyond neutral, declining steeply thereafter. By contrast, axonally stimulated muscle twitch torque increased serially up to +30 degrees dorsiflexion beyond neutral. For the soleus muscle, the optimal reflex neuromechanical angle lies approximately midway between the angle for optimal reflex EMG gain (in mild plantar flexion, at which the largest and strongest motor units can be activated) and the optimal muscle mechanical angle (at the extreme of soleus muscle dorsiflexion). These studies confirm that the excitability of the spinal alpha motor neuron pool in vivo is strongly influenced by muscle length and explain the variability in reflex excitability within and between subjects, if the joint angle is not controlled. They also indicate how posture influences movement, agreeing with the known function of the soleus muscle in the stance phase of gait and the modulation of motor unit recruitment during voluntary alternating movements at the ankle. Soleus muscle twitch characteristics show a fivefold to eightfold increase in peak force associated with a tenfold reduction in compliance in the first two decades of life and an apparent speeding up of twitch time in the first decade.

Adolescent↗
PubMed1997

Synchronization of human finger movements: delays and sex differences with isotonic 'antiphase' motion.

The interval between the operation of two Morse keys, or the moments at which fingers touched or broke contact with electrical conductors, has been measured, the subjects making the movements as synchronously as possible. The data obtained deal with timings when two fingers flexed or extended together ('in phase') and when one finger extended whilst the other flexed ('antiphase'). For antiphase movements of the index fingers with the conductors, flexion usually occurred before extension so there was 'overlap'. Statistically, this period was greater in women and unusually high values were found in one subject with mild cerebral palsy. Overlap was not found using the keys. Using conductors, the timing of antiphase movements for the index and middle fingers of the right and left hand showed overlap again. No significant relationships were found with handedness. When the conductors were placed above the fingers, and the contact was thus with the dorsum, extension took place before flexion: both fingers were extended for a while, with overlap occurring again. The significance of these findings for the understanding of motor control is discussed.

Adolescent↗
PubMed1996

The maturation of motor dexterity: or why Johnny can't go any faster.

The speed of alternating movements at the ankle, metacarpophalangeal and wrist joints in 11 healthy children and 13 adults doubled between age 3 and 11 years, despite a 32-fold increase in limb-segment inertia produced by the doubling in limb length over the same period. The data for the children showed little or no practice effect. The speeds for the adults, though faster than those for the children, were more widely dispersed, indicating the possibility that training might increase the speed of the slowest adult. The findings are consistent with a previous report demonstrating a parallel increase in the speed of calf muscles over the first 10 years of life and it is inferred that the increase in dexterity at the wrist and metacarpophalangeal joints also depends on an increase in muscle speed with age. Muscle maturation may impose a rate-limiting envelope for all motor tasks which is particularly evident in rapidly alternating movements. These findings have implications for training in sport and music and for the understanding of motor delay, clumsiness and speech difficulties.

Adult↗
PubMed1995

Block regional interpolation scheme for k-space (BRISK): a rapid cardiac imaging technique.

We introduce an acquisition method, "block regional interpolation scheme for k-space" (BRISK), to reduce the acquisition time for cardiac imaging. The method exploits the high degree of correlation that exists between time-resolved cardiac images. For representative k-space data sets, Fourier analysis was applied along the cardiac phase dimension to reveal that different regions of k-space can be effectively sampled at different rates. A reduced sampling strategy was implemented, and unsampled points were generated by Fourier interpolation. Time savings of up to 75% are quite feasible and 25% BRISK scans compare well with 100% scans. Simulations and acquisitions using a normal volunteer and patients are presented.

Aortic Valve Insufficiency↗
PubMed1995

Multislice first-pass myocardial perfusion imaging on a conventional clinical scanner.

A technique is demonstrated for the acquisition and processing of multislice, first-pass contrast-enhanced perfusion images in the myocardium. The acquisition is a modification of "keyhole" imaging in which time series images are acquired by sampling a limited segment of k-space, corresponding to the low spatial frequencies. In the modification demonstrated here, keyhole samples are divided into two groups that are sampled on alternate cardiac cycles. The alternate "missing" k-space portions are synthesized by Fourier interpolation. Visualization of contrast agent accumulation by image subtraction is demonstrated. A motion artifact reduction process using time domain Fourier filtering is used to reduce artifacts from respiration. Studies were performed on 46 patients at 1.5 T using gadoteridol (0.05-0.1 mmol/kg) injected into the right antecubital vein in conjunction with radionuclide imaging. Fully concordant studies were noted in 27 of these patients. Remaining studies were either partially or completely discordant for reasons relating to the differing natures of radionuclide versus MR contrast agent characteristics.

Artifacts↗
PubMed1994

Physiological maturation of muscles in childhood.

Little is known about the motor muscle physiology of growing children and it has been assumed that increasing motor dexterity and speed correspond solely to brain maturation, although there is evidence that muscles mature from studies of other mammalian species. Such a finding in children would throw light on the possible mechanics of motor delays in speech, hand function, athletic, and musical skills as well as helping towards a better understanding of pathological motor disorders such as the cerebral palsies. Our non-invasive observations of soleus-muscle relaxation times in healthy children suggest that a child's muscles are initially slow to relax but relaxation doubles in speed up to adult rates by early adolescence. Half-relaxation times halved from about 90 ms at age 3 to about 40 ms at age 10. On the basis of these studies, we suggest that muscle maturation rate-limits motor tasks; which should be borne in mind when considering motor development in muscles and when attempting to determine the effects of early brain or spinal cord damage. Such methods of measuring the dynamics of muscle open up new areas of research and offer the possibility of developing operationally-defined in-vivo measurements for use in motor assessment and for following up the effects on muscle or treatments such as physiotherapy, orthotics, surgery, and drugs.

Adolescent↗
PubMed1994

Radioactive microsphere validation of a volume localized continuous saturation perfusion measurement.

Detre et al. (Magn. Reson. Med. 23, 37-45 (1992)) and Zhang et al., (Magn. Reson. Med. 25, 362 (1992)) have recently demonstrated a technique for the measurement of regional cerebral blood flow (rCBF) based on the continuous saturation (or inversion) of the arterial blood supply to the brain in rats at 4.7 T. In the work reported here, we combined this technique with volume localized (PRESS) readouts to benefit from recording "perfusion" signals averaged over a larger volume, resulting in rapid acquisition of data with sufficient signal-to-noise ratio for application at 2.0 T. In 10 baseline flow measurements, the mean error between the NMR technique and the microsphere flow measurement was -1.5% with a standard deviation of 15.2%. For five measurements obtained with occlusion of the middle cerebral artery, the mean error was -32.4 +/- 20.2%. Perfusion measurements from a single animal under hypercapnic conditions indicated that the NMR technique could underestimate rCBF at high flow rates. An error analysis of the NMR perfusion model is also presented, along with results for typical parameters encountered at 2.0 T.

Animals↗