An optoelectronic/digital flash x-ray imaging system has been invented for use in quantitative physiological studies and general radiology. The system could be implemented in a relatively compact, lightweight apparatus suitable for use aboard a spacecraft as well as on Earth. Applications for which this system is especially well suited include monitoring changes in bone density; monitoring redistribution of fat, muscle, and body fluids; mammography; and three-dimensional tomography.
The system is based on use of imaging devices in the form of the most recently developed large-format planar arrays of integrated semiconductor x-ray detectors. Three especially notable features of the system are the following:
- The system hardware and software are designed so that in digital processing of image data, the contribution of scattering of x-rays is subtracted out, leaving an essentially scatter-free image. This is a significant advantage because scattering tends to reduce contrast, reduce the signal-to-noise ratio, and cause blurring.
- he system employs a dual-x-ray-energy imaging method that facilitates discrimination between soft tissue and bone.
- A compact flash x-ray source makes it possible to obtain stop-motion images.
The design and principle of operation of the system are amenable to several variations. The figure depicts the x-ray imaging geometry of the system to assist in understanding the principle of operation of a basic version of the system. X rays from a compact flash source (regarded as a point source) pass through the human body or other object under examination to an imaging apparatus that comprises an x-ray collimator sandwiched between front and rear x-ray detector assemblies. In this version, the source is capable of generating photons with energies that are concentrated toward either the lower or the higher end of a range from about 10 to about 500 keV, and is operated in such a manner that it alternates between the low and high photon energies on successive pulses.
The x rays that impinge on the front detector assembly include the primary x rays, which are the desired x rays that pass directly from the source to the detectors along radial lines and form the projection image of the body under examination. Scattered x rays (which deviate from the radial lines) also impinge on the front detector assembly. Because all the front detectors are illuminated by both primary and scattered x rays, the front detector assembly yields a high-resolution image.
The collimator is made from a plate of an x-ray-absorbing material. Holes that are radial to the x-ray source are drilled or otherwise formed in the plate. These holes are wide enough to pass a detectable fraction of the primary (radially propagating) x rays and not so wide as to pass a substantial fraction of scattered (non-radially-propagating) x rays. Of course, the x rays are blocked from those rear detectors that are covered by the solid portions of the collimator; hence, the image detected by the rear detector assembly is a low-resolution image formed in primary x rays.
To obtain a complete snapshot of the object under examination, one acquires two sets of images: one in low-energy and one in high-energy x rays. The detector outputs are digitized, then processed in a computer to correct for scattering and to determine the amounts of bone and soft tissue along the radial line to each pixel. The processing involves taking account of the differences between the behaviors of the low- and high-energy x rays, the different resolutions of the front- and rear-detector images, and the known radial-line geometric relationships between coordinates in images on the front and rear detector planes. The steps of the image-processing algorithm (somewhat oversimplified to fit the space available for this article) are summarized as follows:
- The low- and high-energy front- and rear-detector outputs are normalized and corrected for "dark" signals according to established x-ray-image-processing procedures.
- The quantities of soft tissue and bone along the projection line from the source to each pixel in the low-resolution image are computed from the low- and high-energy rear-detector outputs, by use of a numerical inversion of the equations for attenuation of the low- and high-energy x rays in soft tissue and bone.
- For each image and each front-detector position connected with a rear-detector position along the projection line of a radial collimator hole, the scatter component of the image is obtained by subtracting front-detector (scatter + primary) reading from the rear-detector (primary only) reading. This operation effectively yields the scatter component of the image.
- The low-resolution scatter image is interpolated to all pixels between the projection-line points to synthesize a high-resolution scatter image. The interpolation is justified by theoretical calculations and empirical data that show that scatter has a smooth distribution on an image plane and, consequently, the interpolation error can be assumed to be small.
- The synthetic high-resolution scatter image is subtracted from the high-resolution front-detector (primary + scatter) image to obtain the desired high-resolution image in primary x rays.
This work was done by Yong-Sheng Chao of Advanced Optical Technologies, Inc., for Johnson Space Center.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Advanced Optical Technologies, Inc.
111 Founders Plaza
East Hartford, CT 06108
Refer to MSC-23071, volume and number of this NASA Tech Briefs issue, and the page number.