Search

The MEGA experiment, which searched for the muon- and electron-number violating decay ${\ensuremath{\mu}}^{+}\ensuremath{\rightarrow}{e}^{+}\ensuremath{\gamma},$ is described. The spectrometer system, the calibrations, the data taking procedures, the data analysis, and the sensitivity of the experiment are discussed. The most stringent upper limit on the branching ratio, $\mathcal{B}({\ensuremath{\mu}}^{+}\ensuremath{\rightarrow}{e}^{+}\ensuremath{\gamma})l1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ with $90%$ confidence, is derived from a likelihood analysis.

The MEGA experiment is designed to search for the rare decay μ→eγ with a branching ratio sensitivity of ∼5×10−13. Production data have been taken during 1992 and 1993, and the detector is working as expected. Following a complete analysis, the present data set should represent an improvement of 12–15 in sensitivity over the previous limit of μ→eγ.

A new measurement of the Michel parameter ρ in normal muon decay has been performed using the MEGA positron spectrometer. Over 500 million triggers were recorded and the data are currently being analyzed. The previous result has a precision on the value of ρ±0.0026. The present experiment expects to improve the precision to ±0.0008 or better. The improved result will be a precise test of the standard model of electroweak interactions for a purely leptonic process. It also will provide a better constraint on the WR−WL mixing angle in the left‐right symmetric models.

This study is related to an evaluation of synchrotron radiation for the non-invasive assessment of coronary artery disease in humans. In vivo canine studies are reported for an imaging system developed for use with synchrotron X-ray beams. The probable configuration of an expanded system for pilot human subject studies is described.

Imaging of coronary arteries using a venous instead of an arterial injection of contrast agent could provide a much safer method to diagnose heart disease. The tunability, intensity, and collimation of synchrotron radiation x‐ray beams makes possible imaging systems with greatly improved imaging sensitivity. A pair of fan x‐ray beams, a movable patient chair, and a multielement x‐ray detector are used to acquire a pair of x‐ray images above and below the iodine K edge. The logarithmic subtraction of these two images produces an image with excellent sensitivity to contrast agent and minimal sensitivity to bone and tissue. High‐quality images from a dog and preliminary images from five humans have been obtained. Improvements are being made to the system to increase the effective radiation flux and to measure the position of both x‐ray beams.

X-ray computed tomography (CT) is a widely used method of obtaining cross sectional views of objects. The high intensity, natural collimation, monochromaticity and energy tunability of synchrotron X-ray sources could potentially be used to provide CT images of improved quality. The advantages of these systems would be that images could be produced more rapidly with better spatial resolution and reduced beam artifacts. In addition images, in some cases, could be acquired with elemental sensitivity. As a demonstration of the capability of such a system, CT images were obtained of four slices of an excised pig heart, in which the arteries and cardiac chambers were filled with an iodinated medium. Images were taken with incident X-rays tuned successively to energies just above and below the iodine K edge. Iodine specific images were obtained by logarithmically subtracting the low energy image data from the high energy data and then reconstructing the image. CT imaging using synchrotron radiation may become a convenient and non-destructive method of imaging samples difficult to study by other methods.

The risks and costs of the present method of visualizing the coronary arteries have limited the use of coronary angiography in long-term serial studies needed to establish the natural history of coronary atherosclerosis and its response to interventions. A less invasive method, in which the contrast agent is administered intravenously, has been developed using synchrotron radiation as the illuminating source. The present report describes the initial results in human subjects. The findings indicate that transvenous coronary angiograms can be acquired in this manner. Further refinements in the x-ray imaging system are expected to result in increased x-ray fluence and improved image quality.

X-ray-based angiography continues to be the standard method of assessing the severity and the extent of coronary atherosclerosis. The relative insensitivity of conventional X-ray imaging systems to iodine-containing contrast agents necessitates that the images be acquired with essentially undiluted contrast agent in the lumen of the vessels being studied. This, in turn, requires arterial catheterization and the insertion of the catheter tip in or near the ostia of the vessels and the direct injection of the contrast agent. The health risks and monetary costs of the procedure have limited its use to circumstances in which there is a high probability of the presence of severe disease. Despite these problems, coronary angiography has been employed in serial studies to evaluate the effects of drug therapy and diet on coronary atherosclerosis (1).

A safe and sensitive method of visualizing the coronary arteries is needed. The presently available method of visualization is highly invasive and carries too high a risk and cost. The intensity and tunability of synchrotron X-ray beams provides a new source of X-rays highly suited to the in vivo detection of iodinated contrast agents. The method offers the prospect of non-invasive visualization of the arteries by peripheral venous injection of contrast agent. The studies already conducted and those in preparation for human subjects are reviewed.

The intensity and tunability of synchrotron X-ray beams are highly suited to the non-invasive detection of iodine by K-edge digital subtraction angiography. An imnaging system developed for in vivo canine studies is described. Considerations relating to an expanded system for human subject studies are discussed.

A rotating slit system has been designed and tested which can effectively translate a fan-shaped X-ray beam at the same average speed and in the same direction of travel as a patient moving continuously during a line-scan dual-energy medical imaging procedure. The slit has been shown to eliminate the artifacts due to bone in logarithmically subtracted images obtained with monochromatic X-ray beams derived from synchrotron radiation, where one beam is just above and the other just below the K-edge of iodine. Images of iodine-containing phantoms and in vivo canine images after intravenous iodine injection show vessel detail without artifacts due to bone. Corresponding images taken without the rotating slit have severe artifacts.

An engineering run with a partial detector is scheduled for Fall of 1987. The run will test the response of Phillips TDCs and nonsparse latches, the performance of SEs and SIs, and the data path from Fastbus through the ACP system to the MicroVAX II. Debugging of the GPM and FBBC is currently in progress. Software development for the GPM, ACP nodes, and MicroVAX II host is in progress and will be ready for use by the Fall run. Production data taking will start in late 1988.

The application of coronary angiography is limited because it requires arterial invasion and the direct injection of contrast agent into the coronary arteries. A prototype system has been developed which achieves sufficient sensitivity to the iodinated contrast agent to allow the visualization of coronary arteries in dogs after its intravenous injection. The system uses two fan beams of x-rays from an electron storage ring and a 300 element linear silicon detector. Two interlaced images, spaced at 150 eV above and below the K absorption edge of iodine (33.2 keV), are acquired and the logarithmic subtraction of these two images produces an image which has maximal sensitivity to iodine and minimal sensitivity to soft tissue and bone. This approach appears suitable for studies on human subjects.

Synchrotron radiation provides a new source of X-rays highly-suited to iodine K-edge Digital Subtraction Angiography (DSA). The use of such beams provides maximum sensitivity to intra-arterial iodine and virtually eliminates image contrast due to non-vascular body structures. The intensity of the beams permits short exposure times and allows images to be recorded, in line-scan fashion, in sharp focus despite arterial motions. The sensitivity of this method offers the prospect of visualizing arteries, and in particular the coronary arteries, by peripheral venous injection. The principles of DSA have been demonstrated using phantoms and excised animal hearts, and in vivo studies in animals have begun. The instrumentation developed for this purpose and the results obtained to date are summarized.

Two 30 cm diameter steel drums with four facets rotating at 1800 RPM have produced high-quality digital-subtraction radiographs by alternately blocking and transmitting dual-energy synchrotron X-ray beams. An eight times faster system with 16.5 cm diameter tenfaceted tin-plated aluminum drums rotating at 5760 RPM is presently under construction.

Because coronary disease represents the principal health problem in the Western, industrialized world, and because of the risks and costs associated with conventional methods of visualizing the coronary arteries, an effort has been underway at the Stanford Synchrotron Radiation Laboratory to develop a less invasive coronary imaging procedure based on iodine K-edge dichromography. A pair of line images, recorded within a few milliseconds of each other, is taken with two monochromatic X-ray beams whose energy closely brackets the K-edge of iodine, 33.17 keV. The logarithmic subtraction of the images produced by these beams results in an image which greatly enhances signals arising from attenuation by iodine and almost totally suppresses signals arising from attenuation by soft tissue and bone. The high sensitivity to iodine allows the visualization of arterial structures after an intravenous injection of contrast agent and its subsequent 20-30 fold dilution. The experiments began in 1979, with initial studies done on phantoms and excised pig hearts. The first images of anesthetized dogs were taken in 1982. The results of experiments on dogs will be reviewed, showing the stepwise evolution of the imaging system, leading to the use of the system on human subjects in 1986. The images recorded on human subjects will be described and the remaining problems discussed.

A program is underway at the Stanford Synchrotron Radiation Laboratory to evaluate at minimally invasive method of visualizing the coronary arteries in man using synchrotron radiation. The design of an X-ray imaging system to accomodate a newly-available X-ray beam 12.3 cm in width is described. The system performance as revealed with test images of phantoms and excised hearts is indicated, and the system components to be tested before clinical studies can begin are identified.

Measurements of K L O and neutron total cross sections are reported for C, Al, Cu and Pb at average momenta of 9.0 GeV/ c for K L O and 3.8 GeV/ c for neutrons. Comparison is made with the predictions of an optical model calculation.

In the past two years we have reported on the properties of large NaI(Tl) crystals as total absorption spectrometers foy electrons and γ-rays in the range 0.1 to 14 GeV1,2. We have recently made similar measurements on a NaI(Tl) spectrometer which is both larger than any we have used before and assembled in a different way. In the following paragraphs we indicate the results of our initial tests.

Multichannel lithium-drifted silicon (Si(Li)) semiconductor detectors are well suited to line-scan medical imaging systems. They have excellent linearity, high efficiency, large dynamic range and good stability. A major limitation in many medical imaging systems using image intensifiers is the veiling-glare associated with the large differences in intensity between different parts of the image field. Images acquired with linear Si(Li) detectors should suffer less from veiling-glare, primarily because they are line-scan images, but also because the Si(Li) detector responds to X-rays directly rather than to the light produced in a scintillator. The effective veiling-glare has been measured in a linear, 64-channel Si(Li) detector of thickness 5 mm and center-to-center spacing between adjacent sensitive areas of 0.5 mm. Each elemental contact was 6 mm high and 0.4 mm wide. The measurement was made in two different ways, both employing a 33 keV beam of synchrotron X-rays at the Stanford Synchrotron Radiation Laboratory. In one method, a lead phantom in the shape of an isosceles triangle was illuminated across its width with a fan beam of 33 keV X-rays 0.5 mm high by 20 mm wide. The X-ray flux passing by the phantom was measured in each channel of the Si(Li) detector. In the second method, a highly collimated beam of 33 keV X-rays (0.5 mm high by 0.025 mm wide) was scanned across 15 channels of the detector in 0.01 mm steps. The measured intensity in each channel was recorded at each position of the beam and the photon flux fell to less than 0.1% at a distance of 3 mm from the edge of the beam. The results of both measurements were in mutual agreement. Both were compared also to calculated results based on the known total X-ray cross sections for Compton scattering and photoelectric absorption. To within the experimental error, the observed and calculated results were found to be generally in good agreement.

Experimental results on hadron-nucleus scattering at high energy can be used to investigate nuclear structure or, conversely, if the nucleus is sufficiently well understood, to determine hadron-nucleon amplitudes. In the analysis of such experiments the nucleus is described by a complex optical potential and the collision treated according to Glauber scattering theory. The data available at high energy, involving the stable or relatively long-lived hadrons, are not plentiful. It was with the aim of increasing the variety of the available data, against which the basic theoretical approach can be tested, that the experiments to be described were carried out.

The effective veiling glare has been measured in a linear, 64 element Si(Li) detector of 5 mm thickness and 0.5 mm center-to-center spacing between adjacent sensitive areas. Each elemental contact was 6 mm high and 0.4 mm wide. The measurement was made in two different ways, both employing a 33 keV beam of synchrotron x-rays at the Stanford Synchrotron Radiation Laboratory. In one method, a lead phantom in the shape of an isosceles triangle was illuminated across its width with a fan beam of 33 keV x-rays 0.5 mm high by 20 mm wide. The x-ray flux passing by the phantom was measured in each channel of the Si(Li) detector. In the second method, a highly collimated beam of 33 keV x-rays (0.5 mm high by 0.025 mm wide) was scanned across 15 channels of the detector in 0.01 mm steps. The measured intensity in each channel of the detector was recorded at each position of the beam. The photon flux fell to less than 0.1% at a distance of 3 mm from the edge of the beam. The results of both measurements were in agreement. They were also compared to calculated results based on the known total x-ray cross sections for Compton scattering and photoelectric absorption. To within the experimental error, the observed and calculated results were found to be in good agreement.