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Biomedical subjects

G Johnson

Publications and source records attributed to G Johnson.

At least 199 records · Page 11Linked to original sources
PubMed1995

GRASE improves spatial resolution in single shot imaging.

In single shot echo train imaging all the data required for a two dimensional image is acquired from a series of echoes generated following a single RF excitation pulse. Spatial resolution is limited because all samples must be acquired before the signal decays. In this paper we show theoretically that more echoes and hence better spatial resolution can be obtained with single shot GRASE imaging than with either echo planar imaging or single shot RARE imaging. This conclusion holds for both conventional imaging hardware and specialized gradient hardware designed for EPI. High quality single shot GRASE images support the theoretical conclusions.

Brain↗
PubMed1995

Increased flexibility in GRASE imaging by k space-banded phase encoding.

GRASE (GRadient and spin Echo) is an echo train imaging technique that combines gradient and RF refocusing. Although overall signal decay is with T2 and field inhomogeneity phase errors do not accumulate, the small residual phase errors are periodic with echo number. The echo order described previously eliminates the phase error periodicity in k space but instead creates periodicity in the T2 modulation function that can also cause artifacts. In addition, with this order, the effective TE must be half the echo train time, and asymmetric Fourier sampling is difficult to implement. A new method is described that greatly reduces artifacts due to T2 decay, permits greater control of T2 contrast, and lends itself to asymmetric Fourier sampling. Different time segments of the echo train are encoded with different bands of spatial frequency in k space (hence "k banding"). Both computer simulations and experimental results demonstrate improvements in GRASE images acquired by this method.

Abdomen↗
PubMed1995

A methodology for co-registering abdominal MR images over multiple breath-holds.

Previous studies have demonstrated that the SNR of abdominal MR images can be increased by averaging images obtained in different breath-hold acquisitions. In this note, the authors present a simple new methodology for ensuring that images acquired in multiple breath-hold periods are accurately co-registered. Within each breath-hold, a quick coronal scout scan is followed by a longer axial scan. The scout is used to position the axial slices in a fixed position relative to the organ under examination. This MR technique can, in principle, be automated so as to add less than 1 s to the imaging time of the axial scan. The method can be used to increase SNR by signal averaging or to co-register images acquired during, for example, uptake of contrast agents. SNR improvement with negligible blurring is demonstrated in liver images acquired by this method from healthy volunteers.

Abdomen↗
PubMed1995

Multiple breath-hold averaging (MBA) method for increased SNR in abdominal MRI.

Breath-holding during MR imaging eliminates respiratory motion artifacts but places a major time constraint on data acquisition. This constraint limits image signal-to-noise ratio and hence spatial resolution. A new method, multiple breath-hold averaging, is presented that overcomes these time limitations. Several images are acquired in sequential breath-hold periods, separated by periods of normal breathing, and averaged. This averaged image shows the expected increase in SNR with surprisingly little blurring due to misregistration. SNR improvements can be traded for increased spatial resolution. The MBA methodology can also be applied to 3D data acquisitions, dynamic contrast acquisitions, and image subtractions.

Abdomen↗
PubMed1995

Technetium-99m sestamibi kinetics in reperfused canine myocardium.

The purpose of the current study was to clarify the myocardial kinetics of technetium-99m sestamibi when the latter is administered during reperfusion. Sestamibi has in the past been given to patients following thrombolytic therapy to document reperfusion and assess salvage. However, the factors which affect sestamibi kinetics during reperfusion are not clearly defined. In this study the left circumflex coronary artery was occluded for 2 h in six dogs (group 1) and for 3 h in six dogs (group 2), followed by reperfusion. Five additional dogs were not reperfused (group 3). Sestamibi was administered during reperfusion in groups 1 and 2, and during ongoing occlusion in group 3. Regional myocardial sestamibi activity was monitored for 3 h using miniature implanted radiation detectors and gamma camera imaging. Group 1 dogs had no infarcts, group 2 had moderate infarcts (mean: 13.9%), and group 3 had large infarcts (mean: 25.2%). Three-hour fractional myocardial clearances were significantly greater for reperfused infarcted (group 2) (0.23 +/- 0.02 SEM) and for nonreperfused infarcted myocardium (group 3) (0.24 +/- 0.02) compared to control (0.10 +/- 0.01) and reperfused noninfarcted myocardium (group 1) (0.07 +/- 0.02; P < 0.01). Quantitative image analysis demonstrated a significant reduction in the left circumflex/left anterior descending count ratios from initial to final scans for group 2 (0.74 +/- 0.03 to 0.65 +/- 0.03, P < 0.05), and a trend towards a reduction in the count ratios from initial to final scans for group 3 (0.38 +/- 0.04 to 0.30 +/- 0.04; P = 0.06).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗
PubMed1995

Mechanical complications after implantation of multiple-lead nonthoracotomy defibrillator systems: implications for management and future system design.

Nonthoracotomy lead system (NTL) implantable cardioverter defibrillators (ICDs) provide excellent protection against sudden death from ventricular tachyarrhythmias. However, these devices have unique mechanical complications and management issues. We reviewed the major complications occurring in 159 patients who underwent attempted implantation of a multilead NTL system. Successful implantation was obtained in 98% of patients. Two-year, all-cause actuarial survival on an intention-to-treat basis was 94%. Major complications occurred in 28 (17.6%) patients over a follow-up period of 21 +/- 10 months. Complications included 11 (6.9%) lead dislodgements, 10 (5.7%) lead fractures in 9 patients, 2 (1.3%) pocket infections, 1 frozen shoulder, 1 right ventricular perforation, 1 pneumothorax, 1 bleed requiring transfusion, 1 thromboembolism, and 1 "twiddle"-induced torsion of leads. Most of the lead dislodgements and fractures were identified by routine x-ray surveillance. Single-lead systems may significantly reduce complication rates in the future and maintain excellent survival rates.

Actuarial Analysis↗
PubMed1995

Adsorption of fibronectin and vitronectin onto Primaria and tissue culture polystyrene and relationship to the mechanism of initial attachment of human vein endothelial cells and BHK-21 fibroblasts.

The two cell culture substrata, tissue culture polystyrene (TCPS) and Primaria, were compared in order to determine whether a nitrogen-containing surface such as Primaria attracts cells by a different mechanism to an oxygen-containing surface (TCPS). The amounts of vitronectin (Vn) and fibronectin (Fn) which adsorb from the fetal bovine serum (FBS) component of the culture medium onto Primaria and TCPS were determined. Primaria adsorbed two- to threefold more Fn than TCPS, but adsorbed similar amounts of Vn from medium containing FBS. The Fn and Vn binding sites on Primaria were distinct, as adsorption was non-competitive between these two proteins. The amounts of Fn and Vn that adsorbed onto the two surfaces were compared to the concentration dependence of the cell attachment activities of Fn and Vn. Whereas the amounts of Fn which adsorbed onto TCPS were suboptimal for cell attachment, Primaria adsorbed an Fn surface density that was supraoptimal for attachment of human vein endothelial cells and BHK-21 fibroblasts. We conclude that Primaria differs from TCPS in that both Fn and Vn mediate initial cell attachment to Primaria when the culture medium contains FBS, whereas cell attachment to TCPS is dependent upon Vn.

Adsorption↗
PubMed1995

Clinical predictors of the defibrillation threshold with the unipolar implantable defibrillation system.

OBJECTIVES: The purpose of this study was to determine the relation between clinical variables and the defibrillation threshold by using a standardized testing protocol and a uniform implantable defibrillator system. BACKGROUND: Past studied have not revealed useful correlations between clinical variables and the energy required to terminate ventricular fibrillation. Most of these studies did not use a uniform implantable defibrillator system or a standardized protocol to measure the defibrillation threshold and, thus, did not control for the influence of these technical influences. We postulated that defibrillator and defibrillation threshold measurement-based sources of variability overshadowed important clinical predictors. METHODS: The defibrillation threshold was measured by using a standardized protocol in 101 consecutive patients. We used a transvenous unipolar pectoral defibrillation system that employed a single endocardial right ventricular defibrillation coil as the anode and the shell of an 80-cm3 pulse generator as the cathode to deliver a 65% tilt biphasic pulse. RESULTS: Several clinical variables were found to be significantly associated with the defibrillation threshold: patient gender, height, weight, body surface area, heart rate at rest, QRS and corrected QT (QTc) intervals, left ventricular mass and several measures of heart and chest size by chest roentgenogram. None of these variables had a correlation coefficient > 0.45 with the defibrillation threshold. On multivariate analysis, left ventricular mass and heart rate at rest were the only independent predictors of the defibrillation threshold and explained only 25% of the observed variability. CONCLUSIONS: Despite the use of a uniform transvenous defibrillation system and a standardized protocol to measure the defibrillation threshold, no clinically relevant correlation was found between clinical variables and the defibrillation threshold. The defibrillation threshold is probably a function of a complex interaction of anatomic, physiologic and cellular variables that are not adequately represented by easily obtainable clinical information. It is probably not possible to predict defibrillation outcome from standard clinical variables.

Amiodarone↗
PubMed1995

Predicting cardiothoracic voltages during high energy shocks: methodology and comparison of experimental to finite element model data.

Finite element modeling has been used as a method to investigate the voltage distribution within the thorax during high energy shocks. However, there have been few quantitative methods developed to assess how well the calculations derived from the models correspond to measured voltages. In this paper, we present a methodology for recording thoracic voltages and the results of comparisons of these voltages to those predicted by finite element models. We constructed detailed 3-D subject-specific thorax models of six pigs based on their individual CT images. The models were correlated with the results of experiments conducted on the animals to measure the voltage distribution in the thorax at 52 locations during synchronized high energy shocks. One transthoracic and two transvenous electrode configurations were used in the study. The measured voltage values were compared to the model predictions resulting in a correlation coefficient of 0.927 +/- 0.036 (average +/- standard deviation) and a relative rms error of 22.13 +/- 5.99%. The model predictions of voltage gradient within the myocardium were also examined revealing differences in the percent of the myocardium above a threshold value for various electrode configurations and variability between individual animals. This variability reinforces the potential benefit of patient-specific modeling.

Animals↗
PubMed1995

Prospective randomized comparison of biphasic waveform tilt using a unipolar defibrillation system.

BACKGROUND: A unipolar defibrillation system using a single right ventricular (RV) electrode and the active shell or container of an implantable cardioverter defibrillator situated in a left infraclavicular pocket has been shown to be as efficient in defibrillation as an epicardial lead system. Additional improvements in this system would have favorable practice implications and could derive from alterations in pulse waveform shape. The specific purpose of this study is to determine whether defibrillation efficacy can be improved further in humans by lowering biphasic waveform tilt. METHODS: We prospectively and randomly compared the defibrillation efficacy of a 50% and a 65% tilt asymmetric biphasic waveform using the unipolar defibrillation system in 15 consecutive cardiac arrest survivors prior to implantation of a presently available standard transvenous defibrillation system. The RV defibrillation electrode has a 5-cm coil located on a 10.5 French lead and was used as the anode. The system cathode was the active 108 cm2 surface area shell (or "CAN") of a prototype titanium alloy pulse generator placed in the left infraclavicular pocket. The defibrillation pulse derived from a 120-microF capacitor and was delivered from RV-->CAN, with RV positive with respect to the CAN during the initial portion of the cycle. Defibrillation threshold (DFT) stored energy, delivered energy, leading edge voltage and current, pulse resistance, and pulse width were measured for both tilts examined. RESULTS: The unipolar single lead system, RV-->CAN, using a 65% tilt biphasic pulse resulted in a stored energy DFT of 8.7 +/- 5.7 J and a delivered energy DFT of 7.6 +/- 5.0 J. In all 15 patients, stored and delivered energy DFTs were < 20 J. The 50% tilt biphasic pulse resulted in a stored energy DFT of 8.2 +/- 5.4 J and a delivered energy DFT of 6.1 +/- 4.0 J; P = 0.69 and 0.17, respectively. As with the 65% tilt pulse, all 15 patients had stored and delivered energy DFTs < 20 J. CONCLUSION: The unipolar single lead transvenous defibrillation system provides defibrillation at energy levels comparable to that reported with epicardial lead systems. This system is not improved by use of a 50% tilt biphasic waveform instead of a standard 65% tilt biphasic pulse.

Adolescent↗
PubMed1995

A prospective randomized comparison in humans of 90-mu F and 120-mu F biphasic pulse defibrillation using a unipolar defibrillation system.

INTRODUCTION: Capacitance is known to influence defibrillation. Optimal biphasic waveform capacitance for transvenous unipolar defibrillation systems in man is currently being defined. In an effort to improve defibrillation efficacy, we examined the relative defibrillation efficacy of a 65% tilt biphasic pulse from a 90-mu F capacitor compared to a 65% tilt biphasic pulse from a 120-mu F capacitor in a prospective, randomized fashion in 16 consecutive cardiac arrest survivors undergoing defibrillator surgery. METHODS AND RESULTS: The transvenous unipolar pectoral defibrillation system uses a single endocardial RV anodal defibrillation coil and the shell of an 80-cc volume (88 cm2 surface area) pulse generator (Medtronic Model 7219C PCD "active CAN") as the cathode for the first phase of the biphasic shock: RV+ --> CAN-. Defibrillation thresholds for each capacitance were determined prospectively in a randomized fashion. The defibrillation threshold results for the 90-mu F capacitance were: leading edge voltage 383 +/- 132 V; stored energy 7.4 +/- 5.0 J; and resistance 57 +/- 10 omega. The results for the 120-mu F capacitance were: leading edge voltage 315 +/- 93 V (P = 0.002); stored energy 6.5 +/- 3.7 J (P = 0.21); and resistance 57.0 +/- 11 omega (P = 0.87). CONCLUSIONS: We conclude that 90-mu F, 65% tilt biphasic pulses used with unipolar pectoral defibrillation systems have equivalent stored energy defibrillation efficacy compared to 120-mu F, 65% tilt pulses. Use of lower capacitance is possible in present implantable defibrillators without compromising defibrillation.

Adult↗
PubMed1995

Serial defibrillation threshold measures in man: a prospective controlled study.

UNLABELLED: Serial DFT Measures in Man. INTRODUCTION: The defibrillation threshold (DFT) may change throughout the first year following implantation of a cardioverter defibrillator, but it remains uncertain if changes are a consequence of changes in clinical condition or are related to fundamental alterations at the electrode-tissue interface. The purpose of this study was to evaluate the extent and time course of DFT changes over the first year following implantable cardioverter defibrillator (ICD) surgery when extraneous clinical and device variables potentially affecting the DFT were excluded. METHODS AND RESULTS: We prospectively enrolled 61 patients undergoing epicardial or nonthoracotomy/transvenous ICD therapy into a series of follow-up studies where the DFT was measured at implant and at 1, 6, 12, and 52 weeks following implantation in a uniform manner. Stored energy DFT was measured and recorded for all patients. Patient exclusion criteria were: (1) inability to complete all five measures of the DFT; (2) institution of Class I or Class III antiarrhythmic drugs at any time during the study; (3) lead system changes (relocation or new leads) or programming changes in pulse width or current pathway; or (4) development of a significant change in their clinical status, such as decompensated congestive heart failure or acute ischemia. Only 20 of the 61 patients satisfied the criteria required to complete the study. Two of the excluded patients developed high DFTs, which required reprogramming of the current pathway. Eight patients had an epicardial lead system, and 12 had a nonthoracotomy lead system.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

Screening instruments for depression and anxiety following stroke: experience in the Perth community stroke study.

Evaluation of the relative efficacy of three screening instruments for depression and anxiety in a group of stroke patients was undertaken as part of the Perth community stroke study. Data are presented on the sensitivity and specificity of the Hospital Anxiety and Depression Scale (HAPS), the Geriatric Depression Scale and the General Health Questionnaire (GHQ) (28-item version) in screening patients 4 months after stroke for depressive and anxiety disorders diagnosed according to DSM-III criteria. The GHQ-28 and GDS but not the HADS depression, were shown to be satisfactory screening instruments for depression, with the GHQ-28 having an overall superiority. The performance of all 3 scales for screening post-stroke anxiety disorders was less satisfactory. The HADS anxiety had the best level of sensitivity, but the specificity and positive predictive values were low and the misclassification rate high.

Adult↗
PubMed1995

Causes of suppurative keratitis in Ghana.

AIMS: Suppurative keratitis is a serious problem in all tropical countries, but very little information is available about the causative organisms in Africa. The objectives were to identify the causative organisms and the proportion of cases caused by fungi in southern Ghana, and to determine whether correct decisions about treatment could be made on the basis of Gram stain in the eye clinic. METHODS: Scrapings were taken from corneal ulcers of consecutive new patients presenting at Korle Bu Hospital, Accra, and inoculated on 'chocolate' and Sabouraud's agars. Further scrapings were taken for Gram staining and interpretation in the eye clinic. Duplicate slides were assessed by an experienced microbiologist in the UK. RESULTS: One or more organisms were cultured from 114 of 199 patients (57.3%), the most common being Fusarium species, Pseudomonas aeruginosa, and Staphylococcus epidermidis. Fungi, alone or in combination, were isolated from 56% of the patients who had positive cultures. In total, 122 patients (61.3%) had their treatment either determined or altered based on the results of the microbiological diagnosis; in 87 of these solely on the basis of direct microscopic examination. CONCLUSIONS: Infection by filamentous fungi accounted for more than half of the ulcers from which cultures were obtained. Both training in technique and experience in interpretation are necessary for microscopy based diagnosis by staff in the clinic to be of greatest value. Direct microscopy was particularly useful for detecting fungi.

Adolescent↗
PubMed1995

Suppression of apoptosis by v-ABL protein tyrosine kinase is associated with nuclear translocation and activation of protein kinase C in an interleukin-3-dependent haemopoietic cell line.

We previously demonstrated that activation of v-ABL protein tyrosine kinase resulted in suppression of apoptosis following interleukin-3 removal using an interleukin-3-dependent haemopoietic cell line transfected with a temperature-sensitive mutant of the v-abl oncoprotein (IC.DP). Cellular signalling events associated with the activation of v-ABL included increased levels of sn-1,2-diacylglycerol, an activator of protein kinase C. Calphostin C, a PKC inhibitor, restored apoptosis to interleukin-3-deprived IC.DP cells expressing active v-ABL. However, chronic exposure to the phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate to downregulate protein kinase C did not attenuate the survival of IC.DP cells expressing active v-ABL. Translocation of a classical protein kinase C isozyme(s) to the nuclear fraction was observed 6 hours after activation of v-ABL, when nuclear protein kinase C activity was increased approximately 2-fold. The protein kinase C isozyme responsible, which was only partially downregulated by 12-O-tetradecanoyl phorbol 13-acetate, was identified as protein kinase C beta II. This translocation of protein kinase C beta II to the nucleus was inhibited by calphostin C. Taken together, these results suggest that nuclear translocation and activation of PKC beta II may play a role in v-ABL-mediated suppression of apoptosis.

Animals↗
PubMed1995

Training to administer electroconvulsive therapy: a survey of attitudes and experiences.

Recent Royal Australian and New Zealand College of Psychiatrists guidelines regarding Electroconvulsive Therapy (ECT) call for "specific training in both the practical and theoretical aspects of ECT", involving provision of an "educational programme" by centres where ECT is administered, and "supervised administration of ECT prior to administering this treatment alone". This survey was undertaken to elicit the attitudes and experiences of current trainees in relation to training to administer ECT. It was found that ECT is given entirely by the registrars, that consultants are rarely or never present, and in most centres, training typically consists of registrars being supervised once or twice by another registrar, and thereafter administering ECT alone. Twenty percent of those who had given ECT, however, reported not being supervised the first time they administered it. Most trainees indicated limited theoretical teaching in this area, and almost none were aware of a formal training scheme at their respective hospitals. The College guidelines, as stated, address these issues, and priority should be given to their implementation.

Attitude of Health Personnel↗