X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2001-03-13
2003-04-29
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S098200
Reexamination Certificate
active
06556650
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and apparatus for obtaining radiography images. Moreover it relates to a detector for detecting incident radiation.
BACKGROUND OF THE INVENTION
X-rays have been used in radiographic imaging for a long time, and have been subject to great developments. In its simplest form, imaging is conducted by providing a source of X-ray radiation, an object to be imaged, through which the radiation is transmitted, and a detector for the detection and recording of the transmitted radiation. The X-ray detector used today, at hospitals, is normally a screen-film combination. In a phosphor screen (e.g. Gd
2
O
2
S), X-ray photons are converted and thereby produce secondary light, which is registered on a photographic film. The use of a film limits the dynamic range of the image. The increased efficiency achieved by using a phosphor screen is provided at the expense of the resolution, since the secondary light is emitted isotropically.
To visualise an object within an image, it is necessary that the signal to noise ratio exceeds a certain threshold. The ideal system would have the image noise determined only by photon statistics. This is typically not the case for systems operating with a screen-film combination. To obtain a useful diagnostic image one must increase the patient dose of X-ray radiation.
X-ray photon flux is, by nature, digital. However, one has to distinguish between two different methods in producing digital images:
Integrating technique is an intrinsically analogue method. The response in each pixel is proportional to the total X-ray energy flux. The image is then built up digitally by means of the pixels. Examples of the integrating approach to imaging are CCD (charge-coupled device), storage phosphors, selenium plates, etc. The dynamic range of many of these “digital” detectors is similar to that of film. As in the film technique, the photon flux energy (not the number of photons) is integrated, and thus add noise, since X-ray tubes produce a wide energy spectrum. The most significant noise sources are the “dark current” and the fluctuations in photon energy.
Photon counting is an intrinsically digital method, in which each photon is detected, and detection signals are counted.
A two-dimensional photon counting detector requires many readout elements, and a huge number of interconnections. This leads to typical manufacturing and reliability problems, which has been experienced in such systems. It is difficult to make a large two-dimensional detector with high resolution and high probability for interaction of a major fraction of the X-ray photons.
One way to overcome size and cost limitations, in connection with two-dimensional detector readout systems, is to create an image receptor that is essentially one-dimensional and acquires the second dimension for the image by scanning the X-ray beam and detector across the object to be imaged. Scanning can be done by employing a single line detector and a highly collimated planar X-ray beam. In addition, this approach eliminates the scattered radiation noise but imposes a large heat load on the X-ray tube. To ease the tube loading and simplify the mechanics (by reducing the scanning distance), a multi line set of low cost one-dimensional detectors is beneficial.
One advantage with a line detector is a significant reduction of image noise, which is caused by radiation scattering in the object to be imaged. An X-ray photon that is Compton-scattered in the object will not be detected in a line detector.
Several attempts have been made to develop a photon counting X-ray imaging system based on the scanning technique. This requires detectors that produce fast signals with a rise time of a few nanoseconds. Only a few detection media can produce signals that fast, e.g. a gas or a semiconductor (for example silicon). Semiconductor detectors are expensive and are thus not practical in a multi line configuration. In a gas medium, an X-ray photon interacts with a gas atom which emits a primary ionisation electron, which in [its] turn produces electron-ion pairs that are further multiplied in a gas avalanche. The advantage of a gas detector is low cost, a high noiseless signal amplification in the gas (up to 10
6
), and a uniformity of the detection media.
Several imaging systems described in published articles utilise a multi wire proportional chamber as detector. In its basic configuration, the multi wire proportional chamber consists of a set of thin anode wires stretched between, and parallel with, two cathode planes. Application of a voltage between the anode wires and the cathode planes creates an electric field in the chamber. Electrons emitted in the gas by ionisation of gas atoms, caused by incident X-ray photons, drift towards the anode wires, and when approaching the thin wires they experience ionising interactions, with gas molecules, in the strong electric field. The ensuing avalanche multiplication provides a noiseless amplification of the charge signal, by a factor as large as 10
5
or more.
An example of a digital imaging system based on photon counting is described in the article, “Multi wire proportional chamber for a digital radiographic installation”, by S. E. Baru et. al., in Nuclear Instruments and Methods in Physics Research A, vol. 283 (Nov. 10 1989), pages 431-435. This detector is a combination of a drift chamber and a multi wire proportional chamber with non-parallel anode wires aiming at the focal point of the X-ray source. The radial wires enable the use of a thick interaction volume without parallax error. The uniformity of gain along the anode wires is guaranteed by an increasing gap between the anode wires and the cathode planes.
The described device has, however, the following drawbacks.
The need for providing sufficient space for wire mounting and high voltage isolation results in losses of X-ray detection efficiency.
The use of radial wires to solve the parallax problem results in a position resolution limited by the smallest practical anode wire pitch of about 1 mm. The problem can be overcome by using cathode strip readout that provides the ultimate multi wire proportional chamber resolution. One possibility of a practically feasible fast cathode strip readout is described in the article, “The OD-3 fast one-co-ordinate X-ray detector”, by V. M. Aulchenco et. al., in Nuclear Instruments and Methods in Physics Research A, vol. 367 (Dec. 11, 1995), pages 79-82. In this solution, an increasing anode-cathode gap is combined with a decreasing high voltage applied to different anode wire groups.
A known problem with using multi wire proportional chambers for medical imaging is the space charge effect that degrades the detector performance at high X-ray fluxes above 10 kHz/mm
2
. To decrease the space charge effect, the anode plane has been modified by adding alternating cathode wires in a prior art device, disclosed in U.S. Pat. No. 5,521,956 (G. Charpak).
The use of thin wires (typically less than 100 &mgr;m in diameter) in multiwire proportional chambers makes them difficult to construct, and reduces reliability, since one broken wire disables operation of the whole detector.
A gas avalanche detector that is very simple in construction and does not use anode wires is the gaseous parallel plate avalanche chamber. This detector is basically a gas-filled capacitor, comprising two essentially parallel conducting plates, an anode and a cathode, subjected to a high voltage. The high voltage is chosen such that electrons released by ionisation in the gas produce avalanches in a strong electric field between the plates. Typically, the distance between the plates is on the order of one millimetre, and the field strength is in the order of kilovolts per millimetre, depending on the type of gas used. A wide variety of gases can be used depending on the application. In such a detector X-ray photons are incident on a plane parallel to the detector plane, or on the cathode, which is made of a material that emits electrons, so called photoelectrons, when X-ray
Porta David P.
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