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Biomedical subjects

T K Lewellen

Publications and source records attributed to T K Lewellen.

33 records · Page 2Linked to original sources
PubMed1988

Blood flow changes following 137Cs irradiation in a rat glioma model.

Blood flow changes in response to 20 Gy 137Cs whole brain irradiation were measured with quantitative autoradiography of [14C]iodoantipyrine (IAP) in intracerebral grafts of the 36B-10 rat glioma, the brain around tumor (BAT), the contralateral corpus callosum, and the contralateral cerebral cortex. Irradiations were delivered on Day 14 post-transplantation, and measurements of flow (F) were performed with IAP on Day 15 or Day 16. Mean values of F were determined in individual tumors and in treatment groups. In 15- and 16-day-old unirradiated control tumors, the group mean F was 0.31 ml.g-1.min-1. In both 15- and 16-day-old tumor groups irradiated on Day 14 (Day 1 and 2 postirradiation tumors) the mean F for each day's group was 0.52 ml.g-1.min-1, 68% higher than the control (P less than 0.01). Flow in the BAT and the contralateral corpus callosum similarly was increased at these times (P less than 0.01). Flow in the contralateral cerebral cortex was 1.1, 1.5, and 1.3 ml.g-1.min-1 in the control, 1 day postirradiated, and 2 day postirradiated groups, respectively, but these increases were not significantly different from the control. These data indicate that flow increases in the intracerebral gliomas as well as in normal brain regions during the 2 days following 20 Gy irradiation. Changes such as these following radiotherapy may have important effects on the bioavailability of chemotherapeutic drugs.

Animals↗
PubMed1988

Considerations in the purchase of a nuclear medicine computer system.

Selection of a nuclear medicine computer system is a process that should be approached with care and forethought. The general scheme should be to define your needs and constraints, determine what is available, investigate the leading candidates, make a site visit, and, finally, submit an order. Through a series of discussions between members of the Computer Council of the Society of Nuclear Medicine and representatives from the manufacturers of computer systems, a set of important considerations emerged, which are reported in this paper. This paper is not intended to be a step-by-step guideline to the purchase of a computer system. Rather, it is a set of concepts and considerations with which the prospective purchaser should be familiar before undertaking such a purchase.

Computer Systems↗
PubMed1987

Regional blood-to-tissue transport in an irradiated rat glioma model.

To assess vascular permeability in intracerebral grafts of the 36B-10, F-344 rat glioma following 20 Gy 137Cs whole brain irradiation, the blood-to-tissue transport constant, K, of [14C]-alpha-aminoisobutyric acid (AIB) was measured with quantitative autoradiography. Mean, 90th percentile, and 95th percentile values of K were determined in individual tumors and in treatment groups. In 15-day-old unirradiated control tumors, mean, 90th percentile, and 95th percentile values of K were, respectively, 11.3, 18.4, and 20.8 ml kg-1 min-1. In 15-day-old tumors irradiated on Day 14 (Day 1 postirradiation tumors) the K values were 5.9, 9.4, and 10.4, all of which were significantly less than the respective control values (P less than 0.01). In 16-day-old tumors irradiated on Day 14 (Day 2 postirradiation tumors), the K values were 10.8, 15.0, and 16.0, respectively, none of which was significantly different from control tumors. Mean K values for Day 2 vs Day 1 postirradiation tumors (10.8 vs 5.9) yielded P less than 0.05, but the 90th percentile and 95th percentile values for Day 2 vs Day 1 yielded 0.05 less than P less than 0.10. Separate experiments measured AIB and 86RbCl uptake in 36B-10 cells in vitro 1 and 2 days following 20 Gy irradiation to assess whether this radiation dose reduced the capacity of tumor cells to trap AIB or Rb+. Irradiation did not reduce the accumulation of either tracer, but rather was associated with an increased accumulation of AIB. Therefore, the AIB transport data suggest that vascular permeability and/or surface area decreases significantly in the day following 20 Gy irradiation and that this decrease reverses by the second day following irradiation.

Aminoisobutyric Acids↗
PubMed1986

Quantitative autoradiographic evidence for insulin receptors in the choroid plexus of the rat brain.

Techniques of in vitro receptor autoradiography were used to visualize binding of 125I-insulin on slices of frozen rat brain. Slide-mounted sections of frozen rat brain were incubated in 0.05 nM porcine 125I-monoiodoinsulin, alone or mixed with 1 microM unlabeled porcine insulin, ribonuclease, or glucagon, for 2 h at 22 degrees C. The labeled brain slices were apposed to LKB Ultrofilm to generate autoradiograms. The method permitted equal access of labeled insulin to both sides of the blood-brain barrier and localization of insulin binding sites in small anatomic regions. Quantitative estimates of specific iodoinsulin binding were made by computer digital image densitometry of the autoradiographic film images. High concentrations of specific binding sites for iodoinsulin were present in the choroid plexus of the lateral (26.9 +/- 2.0 X 10(-3) fmol/mm2), fourth (18.3 +/- 3.0 X 10(-3) fmol/mm2), and third (13.2 +/- 1.5 X 10(-3) fmol/mm2) ventricles (insulin binding is expressed per unit area of autoradiographic image). Binding to the third ventricular choroid plexus was similar to the concentrations observed for liver slices and the external plexiform layer of the olfactory bulb. Specific binding of iodoinsulin in the cingulate cortex and other surrounding regions was less than in choroid plexus. Ribonuclease or glucagon had no measurable effect on binding when mixed with labeled insulin. The results support the hypothesis that the choroid plexus has a high density of receptors for insulin, and suggests that the choroid plexus may be a target of CSF insulin action and/or a site of insulin transport into the CSF.

Animals↗
PubMed1986

Quantitative autoradiography using a personal computer.

A video digitizing system based on an Apple II personal computer has been developed for use in quantitative autoradiography. The selection process of an acceptable video camera, an illumination system, and the functions provided in the software are discussed. This system is in routine use and has been duplicated by several other laboratories.

Autoradiography↗
PubMed1983

Total body calcium by neutron activation analysis in normals and osteoporotic populations: a discriminator of significant bone mass loss.

This paper reports total body calcium by neutron activation (TBC) measurements in 94 normal individuals and 86 osteoporotic patients. The ability of TBC to discriminate normal from osteoporotic females was evaluated with decision analysis. Bone mineral content (BMC) by single-photon absorptiometry was also measured. TBC was higher in males (range 826 to 1363 gm vs 537 to 1054 in females) and correlated with height in all normals (r = 0.55 in males, 0.58 in females; p less than 0.001). In females over age 55 there was a negative correlation with age (r = -0.51, p less than 0.01). Thus, for normals an algorithm was derived to allow comparison between measured TBC and that predicted by sex, age, and height (TBCp). In the 28 normal females over age 55, the TBC was 764 +/- 115 gm vs. 616 +/- 90 in the osteoporotics (p less than 0.001). In 63 of the osteoporotic females an estimated height, from tibial length, was used to predict TBC. In normals the TBC/TBCp ratio was 1.00 +/- 0.12, whereas in osteoporotic females it was 0.80 +/- 0.12 (p less than 0.001). A receiver operating characteristic curve showed better discrimination of osteoporosis with TBC/TBCp than with wrist BMC. By using Bayes' theorem, with a 25% prevalence of osteoporosis (estimate for postmenopausal women), the posttest probability of disease was 90% when the TBC/TBCp ratio was less than 0.84. We conclude that a low TBC/TBCp ratio is very helpful in determining osteoporosis.

Activation Analysis↗
PubMed1981

A low-contrast phantom for daily quality control.

A simple phantom is proposed to provide a low-contrast test object for daily quality assurance. The phantom consists of four quarters and five dimes taped to a Plexiglas plate. For daily quality control, the phantom is used with 5 cm of Plexiglas as scattering material and a flood source. Examples of images are presented for several gamma cameras, illustrating some of the information that can be obtained. In particular, we present examples of cameras providing improper imaging performance with the coin phantom but with "normal" floods and bar-phantom images. The major conclusion is that daily quality-control images should include significant of scattering material and low-contrast objects.

Models, Structural↗
PubMed1981

A comparison of count rate parameters in gamma cameras.

Pulse pile-up is a fundamental problem that limits the ability of a gamma camera to produce high quality images at high count rates. Pulse pile-up results in loss of events and spatial distortions in the image. The question asked in this study is how well do some of the test procedures for measuring count rate performance in gamma cameras compare. In four gamma cameras we compared measurements of spectral fraction, count-rate curve, pulse-pair resolution, deadtime, maximum count rate, full width at half maximum of the line spread function, and misplaced event count rate. The results indicated that no one technique provides a complete description of the count-rate effects in gamma cameras. The misplaced-event measurements provided the most information.

Evaluation Studies as Topic↗
PubMed1979

A system for measuring total body calcium in man using the 40Ca(n, alpha)37 Ar reaction.

A technique to measure total body calcium using the 40Ca(n, alpha)37 Ar reaction has been developed. The technique is based on collecting 37Ar exhaled in the breath following a 10 mrad uniform total body irradiation by 14 MeV neutrons. The 37Ar in the exhaled breath is extracted by selective absorption and its radioactivity is measured inside a low-background proportional detector. The facilities developed include an activation facility providing a +/-2.7% activation uniformity, a closed circuit rebreathing and gas collection system, and a gas purification and counting system. The technique provides a precision of +/-2.4% as determined by repetitive measurements of human volunteers and has an accuracy for determination of total body calcium in grams of +/-5%.

Argon↗
PubMed1978

A field procedure for the quantitative assessment of nuclear imaging cameras.

Techniques developed for the quantitative assessment of gamma cameras are described. The performance parameters discussed include energy resolution, intransic line spread function, uniformity, and extrinsic line spread function. In addition to describing the equipment used and indicating methods used in analyzing data, some of the potential problems in interfacing multichannel analyzers to gamma cameras are discussed.

Electronics, Medical↗
PubMedDate not supplied

The use of importance sampling techniques to improve the efficiency of photon tracking in emission tomography simulations.

Monte Carlo simulations are widely used to study the transmission and scattering of gamma rays. Use of this method for simulations of emission tomographs suffers from geometric inefficiency resulting from the low solid angle of acceptance of most tomograph designs. We have applied several importance sampling techniques--stratification, forced detection, and weight control through Russian roulette and splitting--to increase the computational efficiency of the Monte Carlo method 10- to 300-fold. A description of these techniques, their validation, and sample performance results are given. Application of importance sampling methods makes it practical to study photon scattering in heterogeneous attenuators on workstations and minicomputers.

Electrons↗