Patent Number: 060382856
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a crystal monochromator, such as a bent Laue crystal monochromator, which diffracts a large area of divergent monochromatic beams or rays, such as X-rays. 2. Description of Prior Art Conventional angiography systems use polychromatic X-rays and intra-arterial injection of contrast agents. Dual-energy subtraction imaging with intravenous injection of the contrast agent can produce usefuil images with much reduced risk. Early attempts at intravenous angiography with non-synchrotron X-rays included the use of filtered or kVp-modulated polychromatic X-rays and dual-energy subtraction methods. The broad spectra of the X-rays used by conventional methods requires three energies in order to minimize bone artefacts. Prior synchrotron-based patient studies using the dual-energy digital subtraction intravenous coronary angiography technique with monochromatic X-rays have obtained research quality images of the coronary artery anatomy. However, the cost of a synchrotron prevents its general use for clinical diagnostic imaging. Development of a compact source and a corresponding X-ray optics system is necessary for the technology to be widely utilized. One of the recent developments of compact sources which would have sufficient intensity for digital subtraction coronary angiography is an X-ray generator with a rotating anode coated with barium and cerium. In addition to the desired characteristic X-rays, the bremsstrahlung radiation from the source is also present. This continuum in the emitted X-ray spectrum increases the dose to a patient, creates subtraction artifacts due to beam hardening effects, reduces contrast and adds noise to the subtracted image. Medical imaging with monochromatic beams produced with synchrotron X-rays and crystal monochromators show significantly improved image quality compared to conventional methods in several fields, including transvenous coronary angiography, mammography and computed tomography. However, the use of synchrotron radiation for clinical applications may not become widespread due to the synchrotron size, cost and complexity of operation. The development of compact sources of narrow energy-band X-rays for radiography has been the subject of several studies in recent years. One such proposal is for the use of a rotating anode X-ray source for digital subtraction coronary angiography. The source utilizes a high-energy (up to 1 MeV) electron beam in conjunction with selected rare-earth anodes. Anode materials can be chosen so that their characteristic emission lines bracket the iodine K absorption edge. The source provides adequate beam intensity for digital subtraction imaging of the coronary arteries with an iodine contrast agent delivered intravenously. In that particular system design, however, the resulting energy bandwidth is not narrow because the beam, along with the characteristic X-rays, includes a substantial amount of bremsstrahlung radiation. The bremsstrahlung continuum increases noise to the subtracted image. SUMMARY OF THE INVENTION It is one object of this invention to provide a method and apparatus that transmits X-ray beams through a bent crystal, such as a bent Laue crystal, which produces a diffracted highly monochromatic X-ray beam. It is another object of this invention to provide a method and apparatus that uses a compact divergent source, such as a rotating anode X-ray tube, to transmit through a bent crystal and emit X-rays having a solid angle of at least about 5.degree. by at least about 5.degree.. It is still another object of this invention to provide a method and apparatus for a transmitting X-ray beams through a non-cylindrical shaped or logarithmic spiral shaped crystal and to rock a position of the bent crystal until a large area of divergent monochromatic beams are emitted from the bent crystal. A bent Laue crystal monochromator according to this invention can diffract an area beam of characteristic X-rays from a rotating anode X-ray tube, for example, thereby eliminating the bremsstrahlung problem associated with conventional systems. An area beam is known as a beam having an area large enough (e.g., about 5 cm.times.about 5 cm) for radiography. A monochromator according to this invention was initially tested at the X12A beam line at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, Upton, N.Y., using molybdenum, silver and barium fluorescence targets excited by a synchrotron white beam. The Laue crystal monochromator of this invention produces a two-dimensional, uniform, monochromatic beam, which can be used for radiography purposes, using standard X-ray generators. The energy bandwidth of the monochromatic beam is about 2% (.DELTA.E/E) which makes possible the selection of a single emission line from a target or a tube. The Laue crystal monochromator, according to one preferred embodiment of this invention, is able to vary the Bragg angle and bending parameters to accept different energies produced by various targets. At the same time, the monochromatic area beam with a solid angle of greater than at least about 5.degree..times.at least about 5.degree. can be separated from the direct beam and bremsstrahlung radiation at distances of less than about one meter. The properties of the Laue crystal monochromator of this invention make it nearly ideal for monochromatic beam diagnostic radiography. The monochromatic beam can be tuned in energy to bracket the K-edge of radiographic contrast elements, such as iodine. Dual-energy subtraction techniques, such as digital subtraction, can then be used to enhance image contrast in diagnostic radiography programs, such as coronary angiography and computed tomography. In addition, a bent crystal monochromator of this invention can be easily incorporated into an existing X-ray source as an add-on device.