ISSN:
1619-7089
Keywords:
Multiwire proportional chambers
;
Singlephoton imaging
;
Positron emission tomography
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract Detectors used for radioisotope imaging have, historically, been based on scintillating crystal/photomultiplier combinations in various forms. From the rectilinear scanner through to modern gamma cameras and positron cameras, the basic technology has remained much the same. Efforts to overcome the limitations of this form of technology have foundered on the inability to reproduce the required sensitivity, spatial resolution and sensitive area at acceptable cost. Multiwire proportional chambers (MWPCs) have long been used as position-sensitive charged particle detectors in nuclear and high-energy physics. MWPCs are large-area gas-filled ionisation chambers in which large arrays of fine wires are used to measure the position of ionisation produced in the gas by the passage of charged particles. The important properties of MWPCs are high-spatial-resolution, large-area, high-count-rate performance at low cost. For research applications, detectors several metres square have been built and small-area detectors have a charged particle resolution of 0.4 mm at a count rate of several million per second. Modification is required to MWPCs for nuclear medicine imaging. As gamma rays or X-rays cannot be detected directly, they must be converted into photo- or Compton scatter electrons. Photon-electron conversion requires the use of high atomic number materials in the body of the chamber. Pressurised xenon is the most useful form of “gas only” photon-electron convertor and has been used successfully in a gamma camera for the detection of gamma rays at energies below 100 keV This camera has been developed specifically for highcount-rate first-pass cardiac imaging. This high-pressure xenon gas MWPC is the key to a highly competitive system which can outperform scintillator-based systems. The count rate performance is close to a million counts per second and the intrinsic spatial resolution is better than the best scintillator-based camera. The MWPC camera produces quantitative ejection fraction information of the highest quality. The detection of higher energy gamma rays has proved more problematical, needing a solid photon-electron convertor to be incorporated into the chamber. Several groups have been working on this problem with modest success so far. The only clinical detectors have been developed for positron emission tomography, where thin lead or lead-glass can provide an acceptable convertor for 511 keV photons. Two MWPC positron cameras have been evaluated clinically and one is now in routine use in clinical oncology. The problems of detection efficiency have not been solved by these detectors although reliability and large-area PET imaging have been proven. The latest development involves a hybrid system in which crystals of barium fluoride are viewed by an MWPC filled with a photosensitive gas. The high detection efficiency of the scintillator is combined with the large sensitive area and good spatial resolution of the MWPC to overcome the limitations of previous MWPC-based positron cameras. The first large-area, clinical camera using this technology is now under development and is expected to perform at least as well as multicrystal positron cameras but at a fraction of the cost.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00169813
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