Biomedical subjects

P N Goodwin

Publications and source records attributed to P N Goodwin.

16 recordsLinked to original sources

PubMed1987

Methodologies for the measurement of bone density and their precision and accuracy.

Radiographic methods of determining bone density have been available for many years, but recently most of the efforts in this field have focused on the development of instruments which would accurately and automatically measure bone density by absorption, or by the use of x-ray computed tomography (CT). Single energy absorptiometers using I-125 have been available for some years, and have been used primarily for measurements on the radius, although recently equipment for measuring the os calcis has become available. Accuracy of single energy measurements is about 3% to 5%; precision, which has been poor because of the difficulty of exact repositioning, has recently been improved by automatic methods so that it now approaches 1% or better. Dual energy sources offer the advantages of greater accuracy and the ability to measure the spine and other large bones. A number of dual energy scanners are now on the market, mostly using gadolinium-153 as a source. Dual energy scanning is capable of an accuracy of a few percent, but the precision when scanning patients can vary widely, due to the difficulty of comparing exactly the same areas; 2 to 4% would appear to be typical. Quantitative computed tomography (QCT) can be used to directly measure the trabecular bone within the vertebral body. The accuracy of single-energy QCT is affected by the amount of marrow fat present, which can lead to underestimations of 10% or more. An increase in marrow fat would cause an apparent decrease in bone mineral. However, the precision can be quite good, 1% or 2% on phantoms, and nearly as good on patients when four vertebrae are averaged. Dual energy scanning can correct for the presence of fat, but is less precise, and not available on all CT units. QCT of the femoral neck has recently been attempted, but presents difficulties that make it unlikely to become widely accepted. Recently there has been much interest in using gamma cameras for dual energy bone density measurements. Although this can present some difficulties, several groups reportedly have overcome them, and this technique may have more widespread application in the future.

Absorptiometry, Photon↗

PubMed1987

SPECT instrumentation: performance, lesion detection, and recent innovations.

The use of a gamma camera for single photon emission computed tomography (SPECT) imaging places greater demand on camera performance than does planar imaging, since camera specifications that are adequate for planar imaging may produce serious image artifacts in SPECT. Manufacturers have responded to the challenge with improvements in both hardware and software, but the user must pay careful attention to quality control procedures. Field nonuniformity is caused mainly by spatial distortion, which may vary during rotation; some cameras incorporate a means of automatically correcting for angular changes. The purpose of performing SPECT is to improve lesion detection, ie, to improve contrast, which is influenced by many factors. Attenuation corrections may be applied either before or after reconstruction. For Tc-99m an attenuation coefficient less than that for water, ie, either 0.11 or 0.12, would appear to give better results; the visual appearance is improved but the effect on contrast is minimal. Scattered photons are the major cause of loss of contrast in SPECT images. Recent developments in on-line energy corrections allow the use of narrower photopeak windows as well as asymmetric energy windows. Offpeak energy windows have demonstrated significant improvements in contrast; however, the increase in nonuniformity can cause artifacts which may limit their use. Simultaneous collection of separate images, one in the photopeak region and the other in the scatter region (92 to 125 keV) may enable a transaxial scatter image to be subtracted from the transaxial photopeak image. This may permit quantification of the true radioactivity distribution. New collimator designs introduced to improve SPECT resolution include cast collimators which can be made with more uniform hole construction than the lead-foil type; for brain imaging, long bore parallel hole and converging fan beam collimators as well as astigmatic collimators which converge in both planes with different lines of focus. Some of these improve both sensitivity and resolution compared to parallel hole collimators. Noncircular orbits have also been introduced in order to improve resolution, but their use presents many problems which require careful monitoring. Transaxial multicrystal systems have been developed over many years, but have not been widely used, primarily because of their inherent complexity, high costs, and limited applicability. Some of these drawbacks may be overcome by a new type of SPECT camera currently under development by several different groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Humans↗

PubMed1982

Thyroid uptake measurements with I-123: problems and pitfalls: concise communication.

The measurement of radioiodine uptake is generally considered to be straight-forward and accurate. However, during the past two decades, discrepancies in "normal" thyroid uptake values have been noticed between Montefiore Hospital and Medical Center and the Hospital of Albert Einstein College of Medicine. These differences were attributed to differences in patient population. Further investigation revealed that the persisting uptake discrepancies arose from neck-phantom differences and variations in procedure. Differences in presumably standardized neck-phantom attenuation characteristics have been shown to cause large variations in count rates from I-123 and I-131 standard capsules. The effect of high-voltage fluctuation on phantom count rates is more pronounced with I-123 than with I-131. In constant levels of high-energy contaminants in I-123 also affect the uptake measurements. large errors in the measurements of thyroid uptake values may result from seemingly unimportant variations in technique. A stable high-voltage power supply, precise high-voltage adjustment, careful selection of energy window, and the use of a standardized neck phantom with generally accepted attenuation characteristics are absolutely essential if RAIU values are to be compared and appropriate therapeutic doses are to be administered based on these measurements.

Humans↗

PubMed1982

A dual collimator design for beat-to-beat measurement of cardiac performance with an Anger camera.

A dual collimator was designed for an Anger camera to permit measurement of cardiac performance on a beat-to-beat basis. Special all-purpose (SAP) and special high-sensitivity (SHS) collimator sections can be interchanged without movement of the patient. Thus, left-ventricular regions of interest delineated on SAP multigated images can be transferred to SHS dynamic images to generate beat-to-beat volume curves. Preliminary balloon studies demonstrated an excellent correlation between ejection fractions calculated with the two collimators: r greater than 0.99, n = 17, p less than 0.001. Varying the volume of an adjacent "right ventricle" balloon failed to alter significantly the count rate from the "left ventricle" balloon's region of interest. Preliminary results on 12 patients, comparing standard-camera ejection fractions with average beat-to-beat ejection fractions, showed that is is possible to measure cardiac function on a beat-to-beat basis with a single-crystal gamma camera. There was minimal difference between the ejection fractions calculated by the dual-collimator method and a standard gated technique (r = 0.98, n = 12, p less than 0.001).

Erythrocytes↗

PubMed1980

Recent developments in instrumentation for emission computed tomography.

Equipment for emission tomography is currently undergoing a rapidly changing stage of development, both for single-gamma detection and for tomography using positron emitters. For single-gamma longitudinal tomography, the 7-pinhole collimator has won wide acceptance because of it simplicity and rapid reconstruction times. However, rotating slant-hole collimators overcome some of the disadvantages of the 7-pinhole method and may eventually be used more widely. For transverse single-gamma imaging, rotating gamma cameras are currently attracting the most interest and offer the best prospects for wise-spread application, since such instruments can be used also for routine studies. In the field of positron tomography, development of new positron cameras has moved from the research center to the commerical area, with at least four manufacturers now marketing tomographic units, all of the multiple-ring design. Small cyclotrons suitable for in-hospital use also are being offered by these companies. Most of the new positron tomographic units employ BGO crystals, which offer substantial advantages over Nal for this purpose. However, the recent introduction of cesium fluoride (CsF) as a detector for tomographic cameras offers the exciting possibility of using time-of-flight techniques for positron detection. This should substantially improve the attainable resolution, which presently is slightly less than 1 cm FWHM. The number of institutions involved in research using positron tomography has suddenly increased, in part because of the recent award of substantial research grants from NIH. Thus, a field which has grown very slowly over the past decade has taken a sudden spurt, and we can anticipate further growth during the coming decade as clinical utility improves.

Elementary Particles↗

PubMed1977

Methods for comparing the performance of different gamma cameras.

Meaningful comparisons of different gamma cameras require the acquisition of numerical data characterizing each of the characteristics of interest. These include resolution, contrast, sensitivity, uniformity, dead time (and the resulting percent loss), and the maximum counting rate. Resolution is best determined by using a line source to measure the linespread function, from which the modulation transfer function can be calculated. Bar patterns are useful in showing the effect on resolution of different collimators and different source distances. Sensitivity comparisons depend on collimator choice, and require counting the same object under identical conditions. Accurate comparisons of dead times and maximum counting rates require extreme care that measurements are taken under identical conditions of scatter, source volume, window setting, etc. Comparisons of the intrinsic dead times should be made with the collimator off, while actual counting losses should be determined using the collimators and appropriate phantoms. With today's high resolution and high-speed gamma cameras, the conditions under which a camera is used would appear to play a larger role in determining its performance than would the small differences in inherent capabilities between the cameras of various manufacturers.

Methods↗

PubMed1977

Comparative study of superficial whole-body radiotherapeutic techniques using a 4-MeV nonangulated electron beam.

Various whole-body, multiple-field, electron-beam therapy techniques are analyzed. Accelerator characteristics, various dosimetric techniques, and experimental setup are described. A flattening filter was constructed which produced an uncollimated electron-beam profile (4 MeV) with a uniformity of +/- 8% across a 200-cm diameter area at 7 m from the source. Irradiation techniques include 2-, 4-, 6-, and 8-field therapy and rotation. Conclusive evidence is presented to support adoption of either the 8-field technique or rotation as the methods of choice for superficial whole-body treatments.

Dose-Response Relationship, Radiation↗

PubMed1974