Abund Ottokar Wist

Stable xenon versus radiolabeled microsphere cerebral blood flow measurements in baboons.

Regional cerebral blood flow was simultaneously determined using the stable xenon computed tomographic and the radioactive microsphere techniques over a wide range of blood flow rates (less than 10-greater than 300 ml/100 g/min) in 12 baboons under conditions of normocapnia, hypocapnia, and hypercapnia. A total of 31 pairs of determinations were made. After anesthetic and surgical preparation of the baboons, cerebral blood flow was repeatedly determined using the stable xenon technique during saturation with 50% xenon in oxygen. Concurrently, cerebral blood flow was determined before and during xenon administration using 15-microns microspheres. In Group 1 (n = 7), xenon and microsphere determinations were made repeatedly during normocapnia. In Group 2 (n = 5), cerebral blood flow was determined using both techniques in each baboon during hypocapnia (PaCO2 = 20 mm Hg), normocapnia (PaCO2 = 40 mm Hg), and hypercapnia (PaCO2 = 60 mm Hg). Xenon and microsphere values in Group 1 were significantly correlated (r = 0.69, p less than 0.01). In Group 2, values from both techniques also correlated closely across all levels of PaCO2 (r = 0.92, p less than 0.001). No significant differences existed between the slopes or y intercepts of the regression lines for either group and the line of identity. Our data indicate that the stable xenon technique yields cerebral blood flow values that correlate well with values determined using radioactive microspheres across a wide range of cerebral blood flow rates.

Systematic errors in bone-mineral measurements by quantitative computed tomography

Bone-mineral measurements using quantitative computed tomography (QCT) are commonly based on comparisons with solutions in water of known concentrations of K2HPO4. In this paper are described theoretical and experimental studies that have led to the conclusion that large systematic errors can arise in these measurements, depending on the soft-tissue and fat concentrations in the vertebral spongiosa. In the case of single energy scanning, such large errors have been identified to be due to the varying water content (displacement effect) in the calibration samples and the varying fat content in the region of interest (ROI) within the patient. In the case of dual energy scanning, the error arises because when normalized to that of water, the mass attenuation coefficient of fat increases with photon energy while the reverse is true for K2HPO4. Our studies have also revealed that total trabecular bone density (which includes the mineral, soft tissue, and fat) can be much more accurately determined by the dual energy QCT method than bone mineral alone. This finding is especially interesting since there have been several reports in the literature suggesting that bone density rather than bone-mineral content is a better predictor of the risk of osteoporosis-related fractures.

Regional cerebral blood flow measurements using stable xenon enhanced computed tomography: a theoretical and experimental evaluation

Several theoretical and practical aspects of regional cerebral blood flow measurements using stable xenon gas and CT are discussed. It is shown that by comparing the enhancement at any time T1 with that at saturation or any other time T2, the need to use arbitrary means to bring the arterial concentration data and the CT enhancement data to the same system of measurement units can be eliminated. If CT is performed continuously during the washin phase, say at intervals of 1 min, least squares analysis of the enhancement data can be used to obtain the best possible estimates for the flow rate constant k and the saturation enhancement. However, if only a limited number of scans can be performed, as may be the case in human studies, it is also possible to get a good estimate of k from a knowledge of the ratio of the enhancement at any time T1 with that at any other time T2. Combinations of T1 = 2.0 min and T2 = 4.0 min, T1 = 1.0 min and T2 = 6.0 min, or T1 = 2.0 min and T2 = 5.0 min were found to be the most convenient. It is also shown that the end-tidal xenon concentration in the exhaled air can be accurately assessed indirectly by measuring the oxygen, CO2, and water vapor concentrations, thereby eliminating the need for more expensive methods involving the use of a mass spectrometer or a thermal conductivity gas analyzer.

Transmission spectra of teeth in the visible to near infrared

Investigations of new transillumination techniques have created a need for detailed transmission studies of teeth and tissues. Recent results have shown that light imaging of extracted teeth can identify small (1 mm2 cross-section) caries in teeth by raster scanning using helium-neon laser light and a post collimated in-line detector. Preliminary frequency (wavelength) dependent transmission studies have been carried out from the visible to the near infrared spectral regions on the dentin of a slice of an extracted tooth 1.85 mm thick. These preliminary results show that in the region from 5,000 to 20,000 cm-1 the dentin of an extracted, healthy, human tooth has a transmission of between 2.5 and 5.5 percent, indicating that the total attenuation coefficient is between 16 and 19.5 cm-1.

Increased spatial resolution for light images of tissues--especially for teeth

A new high resolution transillumination technique has been developed to detect small lesions in teeth and in tissues. It uses collimated, narrow light beams synchronized with a highly sensitive receiver to scan the sample. Extracted teeth and tissues have been examined with this technique. The teeth included a variety of conditions, some with caries, some previously restored, and others with metal pins inserted into them. The results show that a small carious lesion as small as 1 mm2 can be detected from either side of the tooth. Cysts and cancerous lesions in tissues were also detected. The new light imaging technique can find carious lesions better than dental x rays, however, the solution of mental pins inside a tooth is less than that of x rays. The technique is totally harmless to teeth and tissues and can be used as often as necessary even during any clinical procedure. The investigation shows that the resolution for carious lesions could be increased further to about 0.1 mm by incorporating presently available new semiconductor devices and that such a device could be designed for clinical oral use.

Investigation to expand the collimated transillumination method for improved identification and determination of the location of small details in very thick tissues

A collimated transillumination system has been developed for the detection of small lesions in thick tissues. The test object is raster scanned by a collimated light beam and a synchronous in-line post collimated receiver. The images are recorded either in analog form to a film using a light emitting diode, or stored digitally in a microcomputer and displayed by a 256 gray shade monitor. Tests show that the system can achieve high resolution in detecting small details in images of whole teeth, live mice, breast sections, and bones in the regular operating mode. The resolution can be increased by using a newly developed high resolution method and/or by digitizing the image. In addition, different types of details can be separated from each other by using different wavelengths of light.

Investigation to improve the resolution and range of a light imaging system for very thick tissues

A high resolution light imaging system has been developed utilizing an HeNe (628 nm, 32 mW) and a receiver with post collimation mounted on an x, y table to scan the object. The image can be either recorded on a film or stored in a computer for display on a terminal. Tests show that the system in the regular mode is capable of detecting the spine and soft tissues in anesthetized mice, and of transilluminating fully an adult skull bone with a resolution for details better than one third mm. In teeth, all regular carious lesions, including incipient lesions larger than one third of a mm, can be seen in front or in the back of the tooth, none of which could be detected by dental x-ray systems. Applying a new high resolution mode, the resolution can be increased in teeth to less than 0.1 mm. Some difficulty still exists in detecting small lesions on occlusal or approximal surfaces.

Investigation of a light imaging system to detect small lesions in thick tissues

A light imaging system is being developed which can at this time detect and characterize small lesions on the surface and deep inside a tooth or tissue. It depends on the existence of light photons which can fully traverse a tooth in a straight path and therefore can make -- similar to x rays -- a clear shadow of a pathology on the receiver. To observe the very weak, transmitted, unscattered light rays a special patented system has been developed using collimated, narrow light beams and a highly sensitive, post collimated receiver which are rasterscanned over the entire sample. The image can be transferred in analog form to a film or observed after digitization on a 256 grayshade monitor. The light image can be observed in a regular or slower high sensitivity mode.

Study to increase the resolution and range of a light imaging system

Collimated transillumination is a technique based on the observation that some of photons from a laser beam can traverse an entire tooth, and other tissues in an apparent straight light path and therefore make a shadow of any pathology on the receiver similar to x-rays. To observe the very weak, transmitted, unscattered light rays a special patented system has been developed using collimated, narrow light beams and a highly sensitive, post collimated receiver which rasterscans the entire sample. The image is then transferred to a film. To increase the sensitivity a high resolution mode (HR) has been added.

Investigation of a new light imaging technique to detect incipient caries in teeth

Dental x-ray systems are at this time the best method to locate carious lesions but it is difficult to detect them when they are small. Light imaging systems in the past have shown to be more sensitive than x-ray system to carious lesions but one has difficulty in determining the characteristics of these lesions especially when they are small. We developed a new light imaging technique which makes it much easier to determine the size and depth of lesions on most areas of the teeth even though still modifications on the present setup will be necessary to detect them as easily also on occlusal surfaces. This technique is based on a raster scan of the teeth with narrow collimated light beams. The investigation shows that even the area of small incipient lesions (< 0.1 mm2) can be measured and their depth estimated.