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S Crozier

Publications and source records attributed to S Crozier.

At least 19 recordsLinked to original sources
PubMed2006

A new decoupling method for quadrature coils in magnetic resonance imaging.

A powerful decoupling method is introduced to obtain decoupled signal voltages from quadrature coils in magnetic resonance imaging (MRI). The new method uses the knowledge of the position of the signal source in MRI, the active slice, to define a new mutual impedance which accurately quantifies the coupling voltages and enables them to be removed almost completely. Results show that by using the new decoupling method, the percentage errors in the decoupled voltages are of the order of 10(-7) % and isolations between two coils are more than 170 dB.

Computer-Aided Design↗
PubMed2006

Electrodynamic heart model construction and ECG simulation.

OBJECTIVES: In this paper, we present a unified electrodynamic heart model that permits simulations of the body surface potentials generated by the heart in motion. The inclusion of motion in the heart model significantly improves the accuracy of the simulated body surface potentials and therefore also the 12-lead ECG. METHODS: The key step is to construct an electromechanical heart model. The cardiac excitation propagation is simulated by an electrical heart model, and the resulting cardiac active forces are used to calculate the ventricular wall motion based on a mechanical model. The source-field point relative position changes during heart systole and diastole. These can be obtained, and then used to calculate body surface ECG based on the electrical heart-torso model. RESULTS: An electromechanical biventricular heart model is constructed and a standard 12-lead ECG is simulated. Compared with a simulated ECG based on the static electrical heart model, the simulated ECG based on the dynamic heart model is more accordant with a clinically recorded ECG, especially for the ST segment and T wave of a V1-V6 lead ECG. For slight-degree myocardial ischemia ECG simulation, the ST segment and T wave changes can be observed from the simulated ECG based on a dynamic heart model, while the ST segment and T wave of simulated ECG based on a static heart model is almost unchanged when compared with a normal ECG. CONCLUSIONS: This study confirms the importance of the mechanical factor in the ECG simulation. The dynamic heart model could provide more accurate ECG simulation, especially for myocardial ischemia or infarction simulation, since the main ECG changes occur at the ST segment and T wave, which correspond with cardiac systole and diastole phases.

Electrocardiography↗
PubMed2005

An experimental and finite element poroelastic creep response analysis of an intervertebral hydrogel disc model in axial compression.

A hydrogel intervertebral disc (IVD) model consisting of an inner nucleus core and an outer anulus ring was manufactured from 30 and 35% by weight Poly(vinyl alcohol) hydrogel (PVA-H) concentrations and subjected to axial compression in between saturated porous endplates at 200 N for 11 h, 30 min. Repeat experiments (n=4) on different samples (N=2) show good reproducibility of fluid loss and axial deformation. An axisymmetric nonlinear poroelastic finite element model with variable permeability was developed using commercial finite element software to compare axial deformation and predicted fluid loss with experimental data. The FE predictions indicate differential fluid loss similar to that of biological IVDs, with the nucleus losing more water than the anulus, and there is overall good agreement between experimental and finite element predicted fluid loss. The stress distribution pattern indicates important similarities with the biological IVD that includes stress transference from the nucleus to the anulus upon sustained loading and renders it suitable as a model that can be used in future studies to better understand the role of fluid and stress in biological IVDs.

Compressive Strength↗
PubMed2005

An object-oriented designed finite-difference time-domain simulator for electromagnetic analysis and design in MRI--applications to high field analyses.

This paper presents a finite-difference time-domain (FDTD) simulator for electromagnetic analysis and design applications in MRI. It is intended to be a complete FDTD model of an MRI system including all RF and low-frequency field generating units and electrical models of the patient. The program has been constructed in an object-oriented framework. The design procedure is detailed and the numerical solver has been verified against analytical solutions for simple cases and also applied to various field calculation problems. In particular, the simulator is demonstrated for inverse RF coil design, optimized source profile generation, and parallel imaging in high-frequency situations. The examples show new developments enabled by the simulator and demonstrate that the proposed FDTD framework can be used to analyze large-scale computational electromagnetic problems in modern MRI engineering.

Algorithms↗
PubMed2004

Portal and umbilical venous blood supply to the liver in the human fetus near term.

OBJECTIVES: To determine the contribution of the umbilical (UV) and portal (PV) veins to blood supply to the human fetal liver in a low-risk population near term, and to assess the distribution between the left and right lobes. METHODS: In 91 low-risk pregnancies with normally grown fetuses at 36 weeks of gestation we measured the distribution of blood in the UV and PV to the right and left hepatic lobes using ultrasound imaging and Doppler techniques. RESULTS: The median (10(th), 90(th) centile) total UV return was 205 (127, 294) mL/min, of which 25% (13, 47%) was shunted through the ductus venosus, 55% (35, 66%) was distributed to the left hepatic lobe, and 20% (11, 30%) was distributed to the right hepatic lobe. While the left lobe was supplied exclusively by the UV, the right lobe received 50% (20, 70%; 37 (16, 65) mL/min) of its venous blood supply from the PV. The total venous blood supply to the liver parenchyma was 185 (114, 277) mL/min; 21% (8, 34%) came from the PV supply to the right lobe and the remainder came from the UV supply to both lobes. CONCLUSIONS: The venous supply to the left lobe is from nutrient-rich UV blood; for the right lobe, half is from UV blood and half from nutrient-poor PV blood. This watershed between the portal and umbilical venous flows to the fetal liver suggests a corresponding functional dichotomy; this may be modified by hemodynamic influences, with long-term consequences.

Adult↗
PubMed2004

Doppler blood flow velocity waveforms in the fetal renal arteries: variability at proximal and distal sites in the right and left arteries.

OBJECTIVE: To investigate variability in pulsatility/resistance indices and absolute velocities of blood flow in fetal renal arteries dependent on side and sampling site within each vessel. METHODS: Doppler blood flow measurements of pulsatility index (PI), resistance index (RI) and peak systolic velocity (PSV) and end-diastolic velocity (EDV) were performed in the renal arteries of 27 fetuses between 236 and 247 days of gestation. Velocity waveforms were sampled in both the right and left renal arteries at two different sites: proximally, close to the aorta, and distally, before any major visible bifurcation of the vessels. The intraobserver variation was measured in 15 additional fetuses. RESULTS: Mean values of PI and RI were similar in the right and left renal arteries and at the proximal and distal sampling sites of these arteries. Mean PSV and EDV were higher at the proximal compared to the distal site on both sides. For all parameters the 95% limits of agreement between measurements made in the right and left arteries and at proximal and distal sites were wide but tended to be narrower for the left renal artery and at the distal site. Intraclass correlation coefficients for intraobserver variability were 0.88, 0.89, 0.83 and 0.81 for PI, RI, PSV and EDV, respectively. CONCLUSIONS: It is important to standardize the precise site at which Doppler blood flow velocity waveform measurements are made in the renal arteries of the fetus. On the basis of our observations, we suggest that the left renal artery is the preferred vessel for sampling and that measurements should be made in the renal arterial trunk away from the aorta and before any visible branches.

Blood Flow Velocity↗
PubMed2003

Acute basilar artery occlusion treated by thromboaspiration in a cocaine and ecstasy abuser.

Thromboaspiration was performed in a young adult in a coma because of acute basilar artery occlusion associated with cocaine and ecstasy abuse 30 hours after symptom onset. There was complete recanalization of the basilar artery and favorable recovery. Because cocaine and ecstasy abuse has been reported to be a risk factor for ischemic stroke and fatal brain hemorrhage, thromboaspiration may be an alternative therapy to thrombolysis.

Abciximab↗
PubMed2003

Actively shielded multi-layer gradient coil designs with improved cooling properties.

In standard cylindrical gradient coils consisting of a single layer of wires, a limiting factor in achieving very large magnetic field gradients is the rapid increase in coil resistance with efficiency. This is a particular problem in small-bore scanners, such as those used for MR microscopy. By adopting a multi-layer design in which the coil wires are allowed to spread out into multiple layers wound at increasing radii, a more favourable scaling of resistance with efficiency is achieved, thus allowing the design of more powerful gradient coils with acceptable resistance values. Previously this approach has been applied to the design of unshielded, longitudinal, and transverse gradient coils. Here, the multi-layer approach has been extended to allow the design of actively shielded multi-layer gradient coils, and also to produce coils exhibiting enhanced cooling characteristics. An iterative approach to modelling the steady-state temperature distribution within the coil has also been developed. Results indicate that a good level of screening can be achieved in multi-layer coils, that small versions of such coils can yield higher efficiencies at fixed resistance than conventional two-layer (primary and screen) coils, and that performance improves as the number of layers of increases. Simulations show that by optimising multi-layer coils for cooling it is possible to achieve significantly higher gradient strengths at a fixed maximum operating temperature. A four-layer coil of 8 mm inner diameter has been constructed and used to test the steady-state temperature model.

Journal Article↗
PubMed2003

Facilitation of renal autoregulation by angiotensin II is mediated through modulation of nitric oxide.

AIMS: This study was designed to investigate the influence of angiotensin II (Ang II) and nitric oxide (NO) on autoregulation of renal perfusion. METHODS: Autoregulation was investigated in isolated perfused kidneys (IPRK) from Sprague-Dawley rats during stepped increases in perfusion pressure. RESULTS: Ang II (75-200 pM) produced dose-dependent enhancement of autoregulation whereas phenylephrine produced no enhancement and impaired autoregulation of GFR. Enhancement by Ang II was inhibited by the AT1 antagonist, Losartan, and the superoxide scavenger, Tempol. Under control conditions nitric oxide synthase (NOS) inhibition by 10 microm N-omega-nitro-L-arginine methyl ester (L-NAME) facilitated autoregulation in the presence of non-specific cyclooxygenase (COX) inhibition by 10 microm indomethacin. Both COX and combined NOS/COX inhibition reduced the autoregulatory threshold concentration of Ang II. Facilitation by 100 pm Ang II was inhibited by 100 microm frusemide. Methacholine (50 nm) antagonised Ang II-facilitated autoregulation in the presence and absence of NOS/COX inhibition. Infusion of the NO donor, 1 microm sodium nitroprusside, inhibited L-NAME enhancement of autoregulation under control conditions and during Ang II infusion. CONCLUSIONS: The results suggest than an excess of NO impairs autoregulation under control conditions in the IPRK and that endogenous and exogenous NO, vasodilatory prostaglandins and endothelium-derived hyperpolarizing factor (EDHF) activity antagonise Ang II-facilitated autoregulation. Ang II also produced a counterregulatory vasodilatory response that included prostaglandin and NO release. We suggest that Ang II facilitates autoregulation by a tubuloglomerular feedback-dependent mechanism through AT1 receptor-mediated depletion of nitric oxide, probably by stimulating generation of superoxide.

Angiotensin II↗
PubMed2002

Progress in high field MRI at the University of Florida.

In this article we report on progress in high magnetic field MRI at the University of Florida in support of our new 750MHz wide bore and 11.7T/40cm MR instruments. The primary emphasis is on the associated rf technology required, particularly high frequency volume and phased array coils. Preliminary imaging results at 750MHz are presented. Our results imply that the pursuit of even higher fields seems warranted.

Animals↗
PubMed2002

[Endoluminal dilatations and stenosis of symptomatic vertebral arteries].

Surgical treatment of symptomatic atherosclerotic stenosis of vertebral arteries has been proposed for many years but this technique remains quite confidential due to technical difficulties and relatively high risks. Transluminal angioplasty has been proposed and we developed a simplified technique using coronary stent placement. The aim of this study was to evaluate the feasibility and efficacy of transluminal angiography with primary stenting for proximal stenosis for vertebral arteries. Eleven patients with symptomatic atherosclerotic stenosis of vertebral arteries were treated by trans luminal angioplasty with primary stent placement. Two patients, one with stenosis of the proximal vertebral artery and one with distal stenosis of the vertebral artery where only treated by transluminal angioplasty. In all cases transluminal angioplasty and stenting were feasible with restitution ad integrum of the diameter of the artery in 98 cases and with residual moderate stenose (<20%) in 5 cases. All patients were followed for more than one year, only one patient had recurrence of symptoms, but he stopped spontaneously the anti platteless drugs. Vertebro basilar symptoms disappeared completely in 12/13 cases and were improved in 1/13 cases. No restenose of the artery was observed on control (echodoppler) excepted in one case, where a tight stenose of pre vertebral sub clavian artery developed. Transluminal angioplasty for symptomatic stenosis of vertebral artery appears as a very successful technique with a low complication rate. It should be proposed in many cases of vertebrobasilar insufficiency related with tight vertebral artery stenosis.

Aged↗
PubMed2002

DWI prediction of symptomatic hemorrhagic transformation in acute MCA infarct.

PURPOSE: Symptomatic hemorrhagic transformation is a severe complication of acute ischemic stroke which occurs at a higher frequency after thrombolysis. The present study was designed to analyze whether early DWI can be used for predicting the risk of hemorrhagic transformation with clinical worsening in MCA stroke patients. MATERIALS AND METHODS: Of 28 patients with a middle cerebral artery (MCA) infarct and proven MCA or carotid T occlusion on DWI and MR angiography performed within 14 hours after onset (mean 6.5 +/- 3.5 hours, median 5.2 hours), 4 developed hemorrhagic transformation with clinical worsening, while 24 did not. For the 2 groups, we compared admission NIHSS score, site of arterial occlusion, volume of DWI abnormalities, and several apparent diffusion coefficient (ADC) measurements: ADC(infarct) (mean ADC value of the whole infarct), ADC(core) (peak ADC decrease as calculated in a 57 mm(2) circular ROI, manually centered on the ischemic area with the lowest ADC value on the ADC maps), ADC(superficial) and ADC(deep). Discriminant function analysis was used to determine the most accurate predictors of symptomatic hemorrhagic transformation. RESULTS: The best predictor was the ADC(core) (F=5.34, p=2.9%, cut-off value=300 x 10(-6) mm(2)/s). This monovariate model allowed to correctly classify all 4 patients (ADC(core) 300 x 10(-6) mm(2)/s) with subsequent symptomatic hemorrhage, and 17 of the 24 patients without symptomatic hemmorrhage (ADC(core)>300 x 10(-6) mm(2)/s) (100% sensitivity, 71% specificity). CONCLUSION: Although preliminary, these results suggest that a simple measurement of minimum ADC values within an acute MCA stroke could be useful in targeting those patients with a high risk of severe hemorrhagic transformation.

Acute Disease↗
PubMed2001

Compact clinical MRI magnet design using a multi-layer current density approach.

In this work, a new method of optimization is successfully applied to the theoretical design of compact, actively shielded, clinical MRI magnets. The problem is formulated as a two-step process in which the desired current densities on multiple, co-axial surface layers are first calculated by solving Fredholm equations of the first kind. Non-linear optimization methods with inequality constraints are then invoked to fit practical magnet coils to the desired current densities. The current density approach allows rapid prototyping of unusual magnet designs. The emphasis of this work is on the optimal design of short, actively-shielded MRI magnets for whole-body imaging. Details of the hybrid numerical model are presented, and the model is used to investigate compact, symmetric, and asymmetric MRI magnets. Magnet designs are presented for actively-shielded, symmetric magnets of coil length 1.0 m, which is considerably shorter than currently available designs of comparable dsv size. Novel, actively-shielded, asymmetric magnet designs are also presented in which the beginning of a 50-cm dsv is positioned just 11 cm from the end of the coil structure, allowing much improved access to the patient and reduced patient claustrophobia. Magn Reson Med 45:331-340, 2001.

Magnetic Resonance Imaging↗
PubMed2001

On a possible mechanism for peripheral nerve stimulation during magnetic resonance imaging scans.

When patients undergo a magnetic resonance imaging scan, they are subject to both strong static and temporal magnetic fields. The temporal fields are designed to vary at each point in the region being imaged. This is achieved by the use of gradient coils. However, when the gradient coils are switched very rapidly, the strongly time-varying magnetic fields produced can be responsible for stimulating nerves in the peripheral regions of the body. This paper gives a somewhat novel explanation for this phenomenon. The physical mechanism suggested is supported by an illustrative theoretical calculation.

Biophysical Phenomena↗
PubMed2001

The role of dorsolateral prefrontal cortex in the preparation of forthcoming actions: an fMRI study.

The dorsolateral prefrontal cortex (DLPFC) plays a key role in working memory (WM). Yet its precise contribution (the storage, manipulation and/or utilization of information for the forthcoming response) remains to be determined. To test the hypothesis that the DLPFC is more involved in the preparation of actions than in the maintenance of information in short-term memory (STM), we undertook a functional magnetic resonance imaging investigation in normal subjects performing two delayed response tasks (matching and reproduction tasks) in a visuospatial task sequence (presentation, delay, response). In the two tasks, the presentation and delay phases were similar, but the expected response was different: in the matching task, subjects had to indicate whether a visuospatial sequence matched the sequence presented before the delay period; in the reproduction task, subjects had to reproduce the sequence and, therefore, to mentally organize their response during the delay. Using a fMRI paradigm focusing on the delay period, we observed a significant DLPFC activation when subjects were required to mentally prepare a sequential action based on the information stored in STM. When subjects had only to maintain a visuospatial stimulus in STM, no DLPFC activation was found. These results suggest that a parietal-premotor network is sufficient to store visuospatial information in STM whereas the DLPFC is involved when it is necessary to mentally prepare a forthcoming sequential action based on the information stored in STM.

Adult↗
PubMed2001

Asymmetric zonal shim coils for magnetic resonance applications.

A method is presented for the systematic design of asymmetric zonal shim coils for magnetic resonance applications. Fourier-series methods are used to represent the magnetic field inside and outside a circular cylinder of length 2L and radius a. The current density on the cylinder is also represented using Fourier series. Any desired field can be specified in advance on the cylinder's radius, over some nonsymmetric portion pL<z<qL of the coil's length (-1<p<q<1). Periodic extension of the field is used, in a way that guarantees the continuity of the field, and therefore gives good convergence of the Fourier series. All that is required is to calculate the Fourier coefficients associated with the specified desired field, and from these, the current density on the coil and the magnetic field components then follow automatically. The method is illustrated by applying it to three sample zonal coil designs, namely, linear, quadratic, and cubic fields located asymmetrically in the coil. Current densities and corresponding coil winding patterns are shown for these three illustrative cases. Field calculations directly from the coil patterns and spherical harmonic deconvolutions of these fields indicate that the example designs match the theory well. Asymmetric shim coils can be used in conventional symmetric MRI magnets, particularly those architected for "head-only" studies. One of their major applications is expected to be in the newly developed asymmetric magnet systems.

Algorithms↗
PubMed2001

Recovered memories:ethical guidelines to support professionals.

There is a need for guidelines on recovered memories to support the work of professionals and provide protection for clients. This paper highlights the value added by the Canadian Psychological Association's Guidelines for Psychologists Addressing Recovered Memories (Crozier and Pettifor, 1996) in assisting professionals to deal with ethical issues in this area. The Guidelines in combination with a rational and intuitive ethical decision-making process are applied to three vignettes involving sexual abuse and possible recovered memories. This paper offers professionals an opportunity to review their practice according to approved Guidelines for dealing with the difficult issue of recovered memories.

Attitude of Health Personnel↗