X-ray detectors for use in medical computerized tomography are well known in the art. Medical x-ray detectors, such as described and claimed in commonly assigned U.S. Pat. No. 4,031,396 to Whetten et al., are subject to the resolution-limiting effects of k-band x-ray fluorescence. This phenomenon relates to the emission of x-ray photons from the incident x-ray energy-excited gas molecules of the detecting medium and is fully explained in the above-cited patent. The atoms of a gaseous detecting medium, upon interacting with the incident x-ray energy, may themselves emit low energy x-ray photons. When the detecting medium allows a long absorption length for a fluorescent x-ray photon, the photon thus generated by secondary emission may be detected within the detector at a location remote from the point where the incident x-ray entered the detector. Such event degrades spatial resolution and decreases detector efficiency since a portion of the incident x-ray energy is thus not usefully detected. To address this problem, the Whetten et al. patent discloses a detector embodiment which employs voltage and collector plates comprising metals of high atomic number, such as tungsten. Such plates are substantially opaque to x-ray energy and effective to absorb the fluorescent x-rays, thereby minimizing their adverse effect on resolution.
The use of tungsten plates, however, causes a decrease in operating efficiency of an x-ray detector. The detector operating efficiency may be expressed as the percentage of incident x-ray energy that is sensed by the detector. It will be appreciated that absorption of fluorescent x-rays by tungsten plates aborts the liberation of charged particles in the detecting medium which can be detected at the collector plates. Hence, that portion of incident x-ray energy responsible for the emission of fluorescent x-rays goes undetected.
Another drawback to using tungsten plates is that they can absorb incident x-ray energy just as well as fluorescent x-rays. In the above-cited Whetten et al. patent, the tungsten plates are oriented parallel to the incident x-ray beam. While these plates are quite thin (0.05 mm), x-ray energy striking a plate edgewise will be absorbed. Therefore, in the aggregate, the sum of the thicknesses of the plurality of voltage and collector plates used in the Whetten et al. detector represents a significant incident x-ray absorption area and materially contributes to loss of detector efficiency.
Typical prior art medical x-ray detectors, such as disclosed in the Whetten et al. patent, employ a relatively low pressure gaseous detecting medium having a correspondingly low density. As a result, the absorption lengths of both the incident x-rays and fluorescent x-rays are relatively long. Such long absorption lengths necessitate the use of tungsten plates which extend a considerable distance into the gaseous detecting medium, e.g. approximatey one inch. The resulting large area presenting by each plate may cause a number of problems. First, microphonics are produced by any motion of the voltage plates with respect to adjacent collector plates. This motion may occur as a result of detector rotation in computerized axial tomography scanning, or even as a result of vibrations transmitted through the building structure supporting the equipment. Such voltage plate movements cause changes in the relative capacitances between the plates, which induce signals in the collector plates. The induced signals are superimposed on the true signals associated with the x-ray image and thus tend to distort spatial resolution. Moreover, the larger plate size must be accommodated, structurally, in order to maintain it rigid with respect to its adjacent collector plates.
Second, the large plates in the tungsten plate detector embodiment disclosed in the Whetten et al. patent are arranged in radial fashion aimed at the focal spot of the incident x-ray source. As a result, the position of the detector in relation to the x-ray source must be maintained precisely; otherwise, much of the incident beam will impinge on and be absorbed by the tungsten plates and hence not be detected.
Third, use of larger voltage plates can result in degradation of kilovolt peak (KVP) linearity. KVP linearity relates in general to a detector's sensitivity to different energy spectra of incident x-rays. The individual collector elements of an x-ray detector are usually calibrated in accordance with a particular x-ray energy spectrum. A patient undergoing examination with a medical computerized tomography x-ray detector may be exposed to an incident x-ray energy spectrum comprising a range of energies. Absorption of x-rays by the patient's body results in a shifting of the average energy of the spectrum toward a higher value. In order to maintain acceptable tomographic image resolution, it is necessary that the individual collector elements respond uniformly to this shifting of the average energy. To this end, it is necessary that the volume of the detecting medium allocated to each collector element be exactly the same. When the respective portions of the detecting medium volume associated with the individual collector elements become unequal, KVP linearity is degraded. Uniformly increasing these volume portions improves KVP linearity. However, in prior art medical detectors, the large tungsten plates require extra care in manufacture to maintain, with precision, equality between the adjacent volume portions respectively associated with the collector elements.
Industrial x-ray detectors known in the art are typically employed in such applications as inspection of jet engine turbine buckets, nuclear fuel rods, weldments, and other industrial inspection functions. Typically, these applications require higher resolution and higher energy x-rays than do medical applications. For example, industrial detectors may be used at an average x-ray energy of 300 to 400 KEV while the average x-ray energy in a medical detector application is only on the other of 50 to 125 KEV.
In order to achieve the higher resolution required in industrial applications, such industrial detectors have utilized structures in which the individual detector elements are extremely narrow and closely spaced, e.g. individual detector widths of only 2 to 3 mils and spacing of only 1 to 2 mils. Such detector element structure provides a degree of spatial resolution that is neither necessary nor desirable in medical computerized tomography. If such a high resolution detector structure were to be applied in a medical x-ray detector, the lower signal-to-noise ratio that would be experienced would require an intolerable increase in the x-ray dosage to the patient under study in order to achieve acceptable results.
Industrial x-ray detectors typically employ a high density gaseous detecting medium in order to minimize the absorption length of the higher energy x-rays being detected. For example, the detector disclosed in U.S. Pat. No. 4,394,578 to Houston et al. employs a detecting medium gas pressure of up to 200 atmospheres.
The extremely high (1) detecting medium gas pressures, (2) image resolution structures and (3) x-ray energies are important industrial x-ray detector design considerations which are not applicable to medical x-ray detectors.