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Biomedical subjects

T T Lewis

Publications and source records attributed to T T Lewis.

17 recordsLinked to original sources
PubMed1997

Changes in tissue oxyhaemoglobin concentration measured using multichannel near infrared spectroscopy during internal carotid angiography.

OBJECTIVE: To develop an in vivo model for testing spatially resolved spectroscopy and quantified near infrared spectroscopy (NIRS) cerebral blood flow measurements. METHOD: Multiple detector NIRS has been used to study changes in tissue oxyhaemoglobin (O2Hb) concentration during selective internal carotid angiography. A significant reduction in O2Hb occurred in tissue interrogated by detectors situated between 0.7 and 4.1 cm from the NIRS light source. RESULTS: The time course of O2Hb concentration change was consistent with displacement of oxygenated blood by the radiocontrast medium from vascular beds of differing flow and NIR light attenuation. Increasing changes in O2Hb concentration per unit photon path length--predicted to occur at greater emitter-detector separations if those changes had occurred predominantly in cerebral tissue--were found in the first four seconds after injection of radiocontrast medium. However, later changes (6-10 s) were larger and were not proportional to emitter-detector separation. CONCLUSION: The findings indicate that simple assumptions regarding the distribution of the internal carotid artery blood supply to cerebral and extracerebral tissues, the photon path length through those tissues, and their relative contributions to attenuation of NIR light may not be justified.

Brain↗
PubMed1996

Blunt carotid arterial trauma.

Blunt injury to the carotid arteries in the neck, either by direct or indirect forces, is rare but may produce a devastating outcome with long term morbidity. Injury results in either dissection of the vessel wall or pseudoaneurysm formation. Secondary thromboembolic disease results in cerebral infarction. In a seven year period, from April 1987 to April 1994, 2024 patients with blunt injury to the head and neck were admitted to our institution. In this group eight patients (0.4%) were identified with injury to the carotid arteries, seven with dissection and one with pseudoaneurysm formation. Injury was caused by either hyperextension or lateral hyperflexion of the neck. External signs of trauma to the neck were absent in three patients. In all patients there was an associated head injury. Three suffered transient loss of consciousness and a fourth had a transient ischaemic attack 36 h after the injury. CT brain scans performed at the time of initial injury in these patients were all normal. The remaining four patients had no neurological deficit at initial presentation. In no case was injury to the carotid artery suspected. Focal neurological signs and symptoms developed in all cases following a variable latent period ranging from 4 h to 75 days. CT or MRI at this time revealed areas of cerebral infarction. Diagnosis of carotid injury was made at angiography in all cases. These lesions are treatable if diagnosed prior to the development of extensive infarction. Patients developing focal neurological signs secondary to cerebral infarction after a latent period following head and neck trauma should therefore be considered as having a vascular lesion in the neck and referred for urgent cerebral angiography.

Adult↗
PubMed1992

Paraplegia following a rivet gun injury: the use of magnetic resonance imaging in the management.

We describe the case of a 39-year-old man who was accidentally shot in the back by a rivet gun. This resulted in paraplegia and partial sensory loss. Magnetic resonance imaging (MRI) was helpful in the accurate localization of the foreign body and in the assessment of the spinal cord damage. MRI enabled accurate pre-operative localization of the rivet and provided information on the degree of damage to the spinal cord. The artefact produced on computed tomography plus the limited anatomical detail of the spinal cord in the absence of intrathecal contrast makes this technique unhelpful.

Accidents, Occupational↗
PubMed1991

Radiological assessment of prolapsed disc.

Low back pain is an extremely common disorder, usually self-limiting and not requiring investigation or treatment. The correct approach to investigation results in the lowest radiation dose to the patient and the right diagnosis. There are many imaging techniques available and some traditional techniques are now of questionable value. The best approach to imaging is discussed, along with some common appearances.

Humans↗
PubMed1987

Regional cerebral blood flow of patients with focal epilepsy studied using xenon enhanced CT brain scanning.

Stable xenon-enhanced X-ray computed tomography (XeCT) was used to measure the regional cerebral blood flow (rCBF) of 12 patients with drug resistant partial epilepsy and a marked unilateral focus on electroencephalography (EEG). Interictal mean rCBF of fixed regions of interest (ROIs) was reduced by 25% in the cortex of the epileptogenic cerebral lobe compared with the same regions on the contralateral side (p less than 0.02). Six control scans showed a mean side to side cortical difference in rCBF of 14%, whereas the epileptogenic focus was associated with a reduction in the cortical rCBF of greater than 30% in six out of the 12 patients. In an additional patient with partial epilepsy XeCT demonstrated significant focal hypoperfusion when interictal EEGs and conventional CT scans showed no abnormalities.

Adult↗
PubMed1983

Fracture toughness of acrylic denture base.

This paper describes the application of two fracture toughness tests, the compact tension (CT) and the double torsion (DT) to homogenous poly(methyl methacrylate) and two-phase acrylics. Materials were tested in air as processed and after saturation for one month in water. Most materials gave stable crack propagation with these tests, allowing accurate measurement of crack velocity. Modulus and un-notched fracture strength (sigma f) were determined at an identical strain rate in bending to that used in the fracture toughness tests, allowing accurate calculation of the flaw size (a) for a material and an assessment of how applicable Linear Elastic Fracture Mechanics are to these materials.

Denture Bases↗
PubMed1982

Measurement of strain rate behaviour in complete mandibular dentures.

A major cause of fracture in complete lower dentures is due to the denture falling onto a hard floor. Impact failure has never been investigated in complete dentures and in a material which is strain rate dependent, a knowledge of the maximum strain and strain rate at failure is essential if laboratory tests are to evaluate materials in a realistic manner. The work described includes brittle lacquer studies to establish the strain pattern which occurs when a denture is dropped from a height of 1 metre. This was used to orientate strain gauges and experimental dentures were then hit by a specially constructed ram driven at known velocities by a spring, whilst the strain was recorded on a storage oscilloscope. Photographs of the stored wave form were then used to establish the results at ram velocities equal to that of a denture falling 1 metre. Maximum strains were achieved of 11 x 10(3) microstrain and strain rates of 1.04 x 10(3) microstrain/ms. Laboratory impact machines for testing denture materials should therefore be designed to achieve these figures.

Dental Materials↗
PubMed1980

Fatigue test machine for denture base polymers.

A description of a single station fatigue test machine suitable for denture base polymers is given. This machine has been especially designed for dental purposes. A group of representative denture base polymers has been tested, which includes dental poly(methylmethacrylate) of the standard heat polymerized type, autopolymerizing pour type and a heat polymerized rubber graft polymer. These materials were compared and some preliminary results are given. These show that the rubber demonstrated the most satisfactory fatigue life and the pour types were least satisfactory.

Dental Stress Analysis↗
PubMed1975

The development of an abrasion testing machine for dental materials.

Wear testing should be an important part of the investigation into the physical and mechanical properties of some dental materials. It has, however, largely been ignored because of conflicting and unreproducible results. It was decided, therefore, to review the work done by other researchers and to examine the human masticatory cycle, and then present new parameters to design and construct a new dental abrasion testing machine. This new machine is described in detail and its capabilities briefly illustrated.

Dental Materials↗