Abstract:
A novel detector element structure and method for its use is provided. In a preferred embodiment, one or more inorganic scintillating crystals are coupled through wavelength shifting optical fibers (WLSFs) to position sensitive photomultipliers (PS-PMTs). The superior detector configuration in accordance with this invention is designed for an array of applications in high spatial resolution gamma ray sensing with particular application to SPECT, PET and PVI imaging systems. The design provides better position resolution than prior art devices at a lower total cost. By employing wavelength shifting fibers (WLSFs), the sensor configuration of this invention can operate with a significant reduction in the number of photomultipliers and electronics channels, while potentially improving the resolution of the system by allowing three dimensional reconstruction of energy deposition positions.

Description:
This invention was made with Government support under Contract No. DE-FG02-91ER40676 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to a device and technique for detection and imaging of ionizing radiation and more specifically to a gamma ray detector employing inorganic scintillating crystals coupled to wavelength shifting optical fibers. 
     BACKGROUND OF THE INVENTION 
     Current nuclear medicine provides various techniques for non-invasive diagnosis of internal physical structures and biochemical processes occurring within a patient. Computerized axial tomography (CAT), magnetic resonance imaging (MRI) scans and conventional X-ray methods are examples of such structure-imaging systems. These techniques, which identify and localize only physical structures, suffer from the drawback that by the time an abnormality appears, and is detected, the pathological condition creating such an abnormality is often well advanced. 
     In contrast, positron-emission tomography (PET) systems are used to image functioning metabolic systems in the brain as well as the rest of the body. By imaging function rather than structure, these systems provide a unique complement to X-ray, CAT and MRI systems. PET is accomplished through the coincident detection of pairs of gamma rays. These gamma rays are produced when positrons emitted from the source (which is typically placed within the patient&#39;s body) annihilate with electrons in the tissues surrounding the source location. When the gamma rays are emitted at annihilation, the physical properties of these subatomic particles dictate that the two gamma rays are emitted at a single energy and with other known properties; for example, it is known that the gamma rays will be emitted such that they travel in directions very nearly opposite one another. 
     The PET imaging process includes a number of steps. Proton rich radioisotopes are first placed within the patient&#39;s body by, for example, injection or ingestion. In most cases these isotopes will be localized at or near the area where diagnosis is desired. Once incorporated into the body, the isotope continues to emit positrons as part of a naturally occurring decay process. The positron is an antielectron that, after traveling a short distance, will combine with an electron from the surrounding tissue and annihilate. On annihilation, the masses of both the electron and positron will be converted into electromagnetic radiation. In order to conserve energy and linear momentum, the electromagnetic radiation is in the form of two gamma rays which are of equal energy and which are emitted approximately 180 degrees to each other. It is this annihilation radiation that is detected externally in a PET device in order to measure both the quantity and the location of the positron emitter as it moves through the body. 
     The concentration of the radioisotope as it moves through and is processed by the patient&#39;s body can be measured and displayed as a cross section gray scale image. In this image, the intensity of each pixel (picture element) is proportional to the concentration of the radioisotope at that position within the body. This type of so called &#34;kinematic&#34; technique has been and will likely continue to be one of the most powerful methods for diagnosing and analyzing dynamic processes such as blood flow, substrate transport and biochemical reactions within the human body. 
     PET systems currently existing can record and process a large number of tomographic images of a human brain or torso simultaneously. Moreover, sensors can be placed either in a planar ring structure capable of forming a two dimensional image or in a volumetric layout to achieve a three dimensional image. The latter layout is termed positron volume imaging (PVI), although some authors will use the term &#34;PET&#34; when referring to PVI as well. PVI can be set up to process data initially as PET data, later combining planar images to form a volumetric image. Alternatively, PVI can be achieved by permitting inter-plane coincidences at the sensors and processing these coincident detections accordingly. The obtainable resolution with either of these systems has been recently narrowed to under one centimeter, and the distribution of radioactivity within the subject can be assessed to within a few percent. 
     In a typical PET implantation, a ring of gamma ray sensors are positioned to surround the patient in a position local to the radioisotope source. The detection process takes advantage of both the fact that gamma ray emission occurs at 180 degrees to each other and the fact that gamma rays are created simultaneously. Simultaneous or coincidence detection of the gamma ray by sensors on opposite sides of the patient places the site of the annihilation on or near a line connecting the centers of the two sensors. If only one detection takes place, then the annihilation has typically occurred outside of the volume or plane between the two detectors. In this case no event is recorded, since the source would be located outside of the diagnosed area. 
     An operational PET system typically includes the above described data acquisition subsystem including the radiation sensors and their associated circuitry, a fast computer with the necessary imaging software, and large amounts of memory for storing and processing sensor and other input data. A display system for immediate viewing of the image is also typically provided. Finally, a means for interactive processing and system control by the user is generally included. 
     As early as 1986, dozens of regional cyclotron-PET centers were in operation or under development worldwide, and that number continues to grow. A cyclotron-PET center typically consists of an accelerator (usually a small medical cyclotron) for generating radioisotopes, a positron emission tomograph (PET) and a chemistry laboratory for the synthesis of short-lived biological radiotracers. In the U.S., many such centers may be found at university-based medical research centers. The cost of a modern high resolution PET detector is more than $1 million, including approximately $100,000 for crystals and approximately $250,000 for photosensors (typically photomultipliers). 
     Since PET was first implemented in the 1970 &#39;s, it has undergone successive refinements. Unfortunately, the newest high-resolution PET systems operate at or near the intrinsic limitations imposed by the physics of this technology. Statistical limits from limited patient exposure to positron-emitting radioisotopes and source position-smearing from positron range and residual momentum at annihilation serve to limit image resolution and accuracy. In addition, systems operating near this intrinsic limit are quite complex and very expensive. This is due, in part, to the requirement for a very large number of sensor elements to achieve the desired resolution. 
     Current PET detection techniques also suffer from various inaccuracies that result from decreases in crystal width as higher resolutions are sought. These inaccuracies are generally termed &#34;imaging artifacts.&#34; One such artifact is that of radial blurring, which results from crystal penetration from sources away from the axis of the system. In other words, if the line of coincidence is located at some distance from the diameter of the detector ring, the gamma ray may pass through one or several crystals before being absorbed by the detecting crystal. This, in turn, causes a broadening of the coincidence aperture function towards the edges of the field of view. This problem is additionally complicated as the attenuation length of the crystal material increases. In order to achieve equivalent efficiency when using a crystal material having a longer attenuation length (where such crystal may have otherwise desirable properties such as high brightness, high speed, or low cost) the corresponding crystal depth must be increased. But once the crystal is deepened, radial blurring is increased. Even with crystals with the shortest attenuation lengths in current use, radial blurring limits system resolution for objects a few centimeters from the central axis of the detector. There have been various proposals made in an attempt to solve this problem, with the primary solution being the use of a depth of interaction measurement for the photons interacting within the detector. 
     Modern imaging systems have attempted to minimize imaging artifacts by using dense scintillation crystals such as bismuth germinate (BGO), by employing very narrow crystals, and by using specialized sensors to determine the particular location of interaction of the gamma ray in larger crystals. A variety of methods have been proposed to accomplish depth of interaction measurements in very high resolution PET detectors, but such measurements all have required either many additional photosensors (such as photodiodes) with their associated electronics, or complex coding schemes. 
     Another class of imaging instruments used in clinical nuclear medicine applications is that of single photon emission computed tomography (SPECT) systems. A detector used for SPECT can have many attributes in common with a PET detector, although position resolution requirements are typically much less demanding. In the SPECT imaging process, a radioactive tracer is first placed within the patient&#39;s body by injection or ingestion. This radioisotope decays by continually emitting low energy gamma rays (photons) as it travels throughout the patient&#39;s body. It is this photon radiation that is detected externally by the SPECT device. The photon radiation energy detected by SPECT devices is typically between the range of 55-400 keV, which is lower than the annihilation gamma&#39;s (511 keV) in PET systems. The most widely used radionuclide is an isomer of technetium,  m   99  Tc, which has a half-life of 6 hours (the time required for exactly half of the radionuclide initially present to decay). The radionuclide decays by continually emitting gamma rays, in the case of  m   99  Tc, the gamma energy being 140 keV. Some of the commonly used isotopes are listed here with the decay photon energy and corresponding bodily imaging function: 
       201  T1, 80 keV; used for heart and tumor imaging. 
       176  Ta, 55-65 keV, used for imaging the heart. 
       133  Xe, 80 keV, used for lung and ventilation studies. 
     Conventional SPECT implemention calls for the use of a collimator, usually consisting of a thick lead sheet perforated with thousands of small holes, placed directly in front of a gamma camera (crystal detector). Generally, the collimator holes are perpendicular to the crystal so as to block the passage of obliquely incident photons to the crystal detector and to thereby select the direction of the incident photon. By rotating the gamma ray camera and/or the collimator around the patient, a series of two-dimensional projections can be formed from different directions. By applying various reconstruction techniques, the internal distribution of radioactive tracers can be recovered simultaneously for parallel two-dimensional transverse sections. This SPECT technique can then be used for three-dimensional imaging of radioactive tracer distributions located in the lungs, heart and brain. Conventional gamma cameras for use in SPECT generally employ from 36 to 90 photomultipliers as photosensors, with their associated readout electronics. In part due to this internal complexity, commercial SPECT systems range in cost from approximately $200,000 to $500,000. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a simplified gamma ray detector structure. 
     It is a further object of this invention to provide a detector structure with high image resolution. 
     It is a yet further object of this invention to decrease the number of sensor components and the resulting cost of each sensor element. 
     It is a still further object of this invention to increase the resolution of the imaging system while at the same time decreasing the number of sensor elements required. 
     It is a yet further object of this invention to provide depth of interaction measurements in order to alleviate imaging artifacts. 
     The above objects are accomplished by a novel detector element structure and method for its use which is described as follows. In a preferred embodiment, one or more inorganic scintillating crystals are coupled through wavelength shifting optical fibers (WLSFs) to position sensitive photomultipliers (PS-PMTs). The superior detector configuration in accordance with this invention is designed for an array of applications in high spatial resolution gamma ray sensing with particular application to SPECT, PET and PVI imaging systems. The design provides better position resolution than prior art devices at a lower total cost. By employing wavelength shifting fibers (WLSFs), the sensor configuration of this invention can operate with a significant reduction in the number of photomultipliers and electronics channels, while potentially improving the resolution of the system by allowing three dimensional reconstruction of energy deposition positions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of the physical process by/which the PET technique functions; 
     FIG. 2 is a diagrammatic illustration of a PET device into which the present invention may be embodied; 
     FIG. 3 is a diagrammatic illustration of the cones of direct light in an inorganic scintillation crystal; 
     FIGS. 4(a) and 4(b) illustrate a scintillation crystal coupled to a WLSF readout in accordance with a first embodiment and a second embodiment, respectively, of the present invention; 
     FIG. 5(a) is an end view of a single WLSF; 
     FIG. 5(b) is a side view of the same single WLSF; 
     FIG. 6(a) illustrates a crystal array coupled to WLSF ribbons according to a preferred embodiment of this invention; 
     FIG. 6(b) is a detailed view of one crystal coupled to WLSF according to the embodiment of FIG. 6(a); 
     FIG. 6(c) illustrates a multianode PS-PMT coupled to a WLSF readout according to a preferred embodiment of the invention herein; and 
     FIG. 7 illustrates a preferred embodiment of the invention as employed in a PET application. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. I illustrates an example of tracer kinetic modeling and the associated physics in forming a PET image. First, a chemical compound having the desired biological activity is labeled with a radioactive isotope and is introduced into the subject. As an example, in FIG. 1, a radioactive isotope of carbon monoxide 60 is used. Such a molecule could act as a tracer for hemoglobin by forming  11  C-carboxyhemoglobin. For purposes of this discussion, it will be assumed that the subject is a human being composed of numerous materials, one of which is tissue 10. It should, however, be understood that the present invention can be employed to image any type of subject having active biological processes occurring internally therein. 
     As the radioactive isotope 60 is processed within the subject it is continually decaying by emitting positrons or positive electrons 20. In the example of FIG. 1, the  11  C-carbon monoxide is disassociated to form  11  B-boron 40 and oxygen 50. Once emitted, the positrons almost immediately combine with electrons 30 in the immediately surrounding tissue 10 of the subject. Each of the positrons 20 combines with one electron 30 such that the both particles are annihilated. 
     As a result of this annihilation, two gamma rays (each 511 keV) 70 and 80 are produced. These two gamma rays are emitted simultaneously in very nearly opposite directions. They pass through the tissue of the subject and are recorded externally by a large number of sensors which are the subject of this invention. 
     FIG. 2 illustrates a typical implementation of a PET device which may be used to image a radioisotope located within an object to be diagnosed. In this embodiment, gamma ray sensors 110 are formed in a ring structure so as to surround the object 130 being imaged. Radioactive isotope 140 is contained within object 130. While the figure shows a relatively small number of sensors 110, an operational system, in order to achieve a reasonably effective resolution, will have many more. It can be further seen that in this apparatus coincident sensors 150 and 160, respectively, concurrently detect gamma rays 170 and 180. 
     As mentioned above, the primary limitation in PET is spatial resolution. Ultimately, resolution is limited by the physics of the annihilation process. This includes positron range broadening of from 0.2 mm to 2.6 mm depending upon the particular radioisotope selected as the labeling material. Further, resolution is adversely affected by the gamma pair opening angle variation of 180° +/-0.3° which amounts to 1.4 mm to 2.8 mm at a detector separation of, for example, 50 cm and 100 cm respectively. Thus, it can be seen that PET resolution is theoretically limited to approximately 2 or 3 mm despite limitless numbers of sensors and/or expense of materials. 
     Many structures that are of interest for PET imaging are not much larger than 2 mm or 3 mm or are, in fact, somewhat smaller. This is especially true in many non-human (i.e., laboratory animal) applications. A requirement for fine spatial resolution is that the sensor be segmented at a scale comparable to the size of the objects being imaged. Therefore, a device limited by the physics of the PET process rather than the detector technology should have sensor cells with an effective size of 3 mm or less transverse to the gamma incident direction. 
     The succession of events in the operation of a preferred embodiment of this novel sensor is as follows: 
     A positron is emitted by a radioisotope within the object (patient) being imaged, which then annihilates within the surrounding tissue to produce back-to-back 511 keV annihilation gamma rays, or a radionuclide within the object being imaged decays to produce low energy gammas (50-400 keV). 
     Each gamma ray then interacts within an inorganic scintillating crystal, either through Compton scattering or photoabsorption. Energy deposition by the electron produced in the interaction results in the production of light in the crystal. The wavelength of the light produced in the crystal is an intrinsic property of the crystal. For example, the peak emission of sodium iodide (NaI) is 415 nm, while for cesium iodide (CsI) it is 310 nm. 
     The light produced in the crystal next propagates to the surface of the crystal. If it strikes the surface at glancing incidence, it is internally reflected and emerges on another crystal face. If it is travelling nearly perpendicular to the surface, it emerges from the crystal and enters a wavelength shifting optical fiber (WLSF). Light emerging from other crystal surfaces may be sensed with other techniques (such as direct optical coupling to a photosensor), or with other fibers. 
     The light entering a WLSF is absorbed, resulting in the emission of light with a different wavelength; the range of light wavelengths which are absorbed or emitted depends on the type of WLSF used. A typical WLSF absorbs blue light and emits green light although fibers with other optical behavior can also be used. For example, a fiber absorbing in the UV and emitting in the blue is also possible. An array (ribbon) of WLSFs encodes the position where the light emerged from the crystal and thereby encodes the location of the energy deposition. The WLSF ribbon then transmits a fraction of the wavelength-shifted light to a photosensor, with this photosensor typically less than 1 meter distant from the crystal. 
     At the photosensor, the energy deposition magnitude as encoded by the amount of WLSF light, and the position of energy deposition as encoded by the position of the wavelength shifted light within the ribbon, are sensed. If a position-sensitive photosensor is used, then the position where the light from the fiber ribbon entered the photosensor is encoded into electrical pulses. The time and amplitudes of these pulses also indicate the gamma arrival time and the amount of energy deposited into the crystal. Alternatively, individual photosensors may be used to sense light from single fibers or sets of fibers; by multiplexing fibers from separate ribbons on individual photosensors, the total number of photosensors required may be minimized. 
     
                                           TABLE 1__________________________________________________________________________Properties of various inorganic scintillating crystals.      Radiation         Rel. Light                               Decay                                    PeakDensity    Length           Index of     Output Time Emmission(g/cm.sup.3)      (cm) Refraction                 Hydroscopic?                        (NaI = 1.0)                               (ns) (nm)__________________________________________________________________________Nal (Tl) 3.67 2.59 1.85  Yes    1.00   230  415CsI (pure) 4.53 1.86 1.80  Slight 0.10   10,36                                    310                        0.02   &gt;1000                                    &gt;400CsI (Tl) 4.53 1.86 1.80  Slight 0.40   &gt;1000                                    550BGO   7.13 1.11 2.15  No     0.20   300  480BaF.sub.2 4.90 2.10 1.56  Slight 0.05   0.6  220                        0.20   620  310YAlO.sub.2 Ce 5.35 2.63 1.94  No     0.49   31   380LSO   7.40 1.14 1.82  No     0.75   40   420__________________________________________________________________________ 
    
     The novel detector configuration disclosed herein includes an inorganic scintillating crystal. Table 1 provides a partial list of common and newly developed inorganic scintillating crystals, along with their intrinsic characteristics. It will be understood that any of the crystal materials shown in Table 1, as well as others, may be employed to practice the invention described herein. A preferred embodiment for PET applications may incorporate LSO, while a preferred embodiment for SPECT applications may be NaI(T1). The choice of an optimal crystal for a given application of this technique involves considerations of light yield/energy resolution requirements, speed requirements, crystal cost, and simplicity of detector construction. 
     The sensor structures further employ wavelength shifting optical fibers (WLSFs) to couple to and read out the scintillating crystal. The choice of WLSF is determined by matching the light emission of the crystal to the light absorption of the fiber. Detailed optimization must be done in order to meet the requirements of the performance as dictated by the use of a particular device, e.g., mobile gamma ray camera, SPECT detector, or PET detector. In general, however, the novel detector configuration functions similarly independent of the particular choice of crystal and of WLSF. 
     As noted earlier, this technique may be used either to identify which of a set of very small crystals contained a gamma ray interaction, or to locate where within a larger crystal a gamma ray interaction occurred. In a preferred embodiment of this invention, scintillation light which is produced within the scintillating crystal by an ionizing electron (which is in turn produced by an incident annihilation gamma ray or a low energy gamma ray through either Compton scattering or photocapture) is emitted isotopically within the crystal. The fraction of the light which strikes the polished crystal face at an angle greater than the critical angle (determined by the index of refraction of the crystal) with respect to the surface normal is totally internally reflected within the crystal. The distribution of the remaining, non-reflected light on the surface of the crystal is shown in FIG. 3. This light 230 provides information on the position of the energy deposition 220 within the crystal 210, as discussed below. The direct light 230 which exits the crystal 210 provides useful information regarding the timing and amount of energy deposition 220 occurring within the crystal 210. 
     FIGS. 4(a) and 4(b) show alternative embodiments of the placement of the WLSFs with respect to the crystal. Totally internally reflected light may be reflected from other crystal faces, and may eventually exit from another crystal surface. More than one surface of the crystal 210 may be equipped with fiber-ribbon readout; by arranging two ribbons 310 and 320 at right angles on opposite sides of a given crystal (as shown in FIG. 4(a)), the energy deposition location 220 may be determined in two dimensions. Alternatively, partially translucent WLSFs may be used (as shown in FIG. 4(b)), so that both orthogonal fiber ribbons 410 and 420 may be placed on single side of crystal 210. In addition to sensing that fraction of the light which is transmitted down the WLSF, light which is not trapped within the fiber optic (as discussed below) may be sensed by one or more photosensors 330 coupled to the WLSF ribbons 310 and 320 or 410 and 420. By using one photosensor 330 directly coupled to the crystal 210 or to the WLSF ribbon at the location where the WLSF contacts the crystal 210, and a second photosensor at the end of the WLSF light pipe (not shown), it is possible to separate the functions of energy depositions amplitude measurement (direct coupling) from energy deposition localization (piped measurement). In a preferred embodiment for gamma cameras or SPECT, perpendicular fiber ribbons are read out by a position-sensitive photomultiplier, with one such fiber ribbon sandwiched between the crystal and a second photomultiplier. 
     Referring to FIGS. 5(a) and 5(b), an end view and a side view, respectively, of an individual WLSF as used herein, is illustrated. WLSFs combine an optical waveguide with a dopant chemical. The dopant chemical is selected such that it absorbs short wavelength light and re-emits longer wavelength light in a manner such that each absorbed photon provides a corresponding emitted photon. A fraction of the re-emitted light is captured within the total internal reflection aperture of the optical sensor such as the position-sensitive photomultiplier tube used in a preferred embodiment herein. 
     As shown in FIGS. 5(a) and 5(b), an incident photon 520 is absorbed after passing through fiber cladding 530 of fiber 510. Once absorbed, a portion of the light is totally internally reflected and a re-emitted photon 570 is transmitted through fiber 510 within fiber core 590. 
     Plastic WLSFs are manufactured by several vendors worldwide and are frequently assembled into ribbons one fiber thick and many fibers wide. One such vendor is the Bicron Corporation in Newbury, Ohio. A suitable WLSF for the use herein is the BCF-92 fibers (in ribbon form) manufactured by Bicron although others may be substituted without departing from this invention. The efficiency for the absorption and re-emission of light incident on a 1.0 mm thick fiber may approach 90%, with an attenuation length of 2 mm for the light which is optically captured along the fiber axis. It will be understood by one of ordinary skill in the art that of the light exiting the crystal, on 8% will be optically transmitted toward the fiber ends for use in readout. For the Bicron BCF-92 fibers, the peak absorption wavelength is 405 nm and the peak of the emission spectrum is at 494 nm. The refractive index of the core is 1.60, while that of the cladding is 1.49. Light yield may further be increased by utilizing doubly clad fibers which have a second low index cladding outside the first. This typically results in capture fractions which are approximately 50% higher than an equivalent singly clad fiber. 
     FIGS. 6(a), 6(b) and 6(c) collectively illustrate a preferred embodiment of a detector design for a gamma ray camera. This embodiment consists of a 3×3 array of optically isolated (meaning the crystal contact surfaces have either specular or diffuse reflectors between them) inorganic scintillating crystals 710 measuring 12 cm square by 1 cm deep. Thus, the crystal array in a preferred embodiment, measures 36 cm×36 cm by 1 cm deep. The crystal array is placed between two perpendicular wavelength shifting fiber ribbons 730 and 740. In this manner, information on the location (in both the x and y directions), time and magnitude of energy deposition may be transferred from the crystals 710, wavelength shifted and captured within the WLSFs 730 and 740. The light is then propagated through the WLSFs 730 and 740 to a photosensor 830. Light yield can be increased either by reading out both ends of the WLSFs 730 and 740 with the same photosensor 830 or by mirroring one end of the WLSF 730 and 740. FIG. 6(a) illustrates an embodiment having mirrored fiber ends. 
     FIG. 6(b) shows, in detail, a close-up of the 12 cm square×1 cm deep crystal indicated by a circle in FIG. 6(a). The WLSF ribbon used in a preferred embodiment of this invention is a pair of 1 mm×60 mm ribbons 730 on one surface of a 12 cm×12 cm×1 cm crystal 710, and another pair of identical WLSF ribbons 740 on the opposite crystal face, with the second pair oriented at 90 degrees with respect to the first. Alternatively, a single WLSF ribbon wide enough to cover the crystal face may be used. For obtaining a direct coupling readout of the energy deposition in the crystal 710, a photomultiplier 760 is placed directly on top of one set of the orthogonal WLSF ribbons 710, as shown in FIG. 6(b). Although not shown in FIGS. 6(a) and 6(b), additional WLSF ribbons may be placed on the remaining small area faces of the crystals 710 in order to gather the above-mentioned totally internally reflected light. While relatively short sections of WLSFs are shown in the figures, it will be understood that the actual ribbons used may be as long as 100 cm. 
     A position sensitive photomultiplier (PS-PMT) 830 is shown in FIG. 6(c). In a preferred embodiment of this invention, an X-Y PS-PMT readout is used; this results in reduced channel count, finer position resolution and greater efficiency than in prior art devices in which scintillators are coupled directly to PS-PMTs. Alternatively, a pixillated type of PS-PMT could be used in high event-rate applications. In a specific implementation of an embodiment using the X-Y PS-PMT, the Hamamatsu 4 135-01 PS-PMT may be employed. This device is currently available from the Hamamatsu Corporation which is located in Hamamatsu, Japan. In the PS-PMT device, up to 3 ribbons of 1.0 mm×60 mm WLSF may be coupled to each of the 8 input regions 840 on this PS-PMT 830. While the operation of the PS-PMT, in general, is known in the art, a brief description is provided herein as follows. 
     When the output light carried from the WLSF 730 reaches a photocathode, the photocathode is caused to emit photoelectrons. The photoelectrons are applied to a group of dynodes where they are subjected to photomultiplication while maintaining their spatial spread. Next, they are applied to a resistive division type anode having a plurality of output terminals. The incident position and the quantity of the photoelectrons are output as data though the output terminals. U.S. Pat. No. 4,929,835 to Emaciate describes additional details on this process. 
     By measuring the ratio of the charge in the two pulses associated with each input region, the coordinate across the ribbon 730 (along the 60 mm width) may be reconstructed to within 2 mm. Each of the 8 input regions 840 within the PS-PMT 830 is capable of encoding information from 3 ribbons. One 3×3 array of crystals contains a total of 12 WLSF ribbons, with each ribbon measuring 1 mm thick and 60 mm across. The photomultiplier 830 further includes two signal lines 890 (change division readout) that provide the data to additional processing elements. 
     It should be noted that although a preferred embodiment of the invention herein has been described in the context of a PS-PMT (either X-Y or pixillated), it is possible to substitute other photosensors for the PS-PMT. In particular, photodiodes, avalanche photodiodes, or hybrid photodiodes (photocathode/photodiode combinations) are all possible configurations for achieving the objects of this invention. The concentration of light within WLSF makes possible a device with smaller photocathode readout area than with the prior art methods, thereby providing the ability to employ smaller photosensors having finer spatial resolution. 
     In addition to providing fine traverse and axial spatial resolution, the inorganic scintillating crystal WLSF/PS-PMT detector can readily measure the depth of interaction in the crystals. For high resolution detector applications, depth of interaction ambiguities lead to imaging artifacts (radial elongation) for positron source positions at some distance from the axis of the ring. By orienting readout fiber ribbons with their 60 mm widths along the depth-of-interaction axis, we may use the technique described above to measure the depth of interaction in a crystal (or in any of several crystals which are read out in parallel by the fiber ribbon). A 5 mm FWHM (Full Width Half Max.) depth of interaction resolution is sufficient to completely eliminate radial elongation in PET applications, while a 3 mm depth of interaction resolution is readily obtainable with WLSF ribbon readout. 
     A preferred embodiment of the inorganic crystal WLSF/PS-PMT detector for PET applications is illustrated in FIG. 7. In this embodiment, direct readout photosensors 910 are formed in a ring structure so as to surround the object 930 being imaged. Radioactive isotope 940 is contained within object 930. While the figure shows a relatively small number of direct readout photosensors 910, an operational system, in order to achieve a reasonably effective resolution, will have many more. It can be seen that in this apparatus coincident sensors 950 and 960, respectively, concurrently detect gamma rays 970 and 980. 
     The functions of energy and timing measurement (direct readout) and localization (fiber readout) are separated. Several PET rings (separate units duplicated along the central axis of the ring) may be read out with a single set of axial fibers 945. Circumferential fibers 935 may perform depth-of-interaction measurements for each PET ring separately. For ultra-high precision PET, a single axial fiber can be used to read out each optically isolated crystal, which may be 1 mm in width or less. Such a PET detector has superior resolution to current art at lower system cost. 
     Application of the inorganic scintillating crystal/WLSF/PS-PMT detector configuration is not limited to medical imaging alone. Applications to astrophysics, experimental nuclear physics, commercial nuclear reactors, and biomedical imaging of a radioisotope-labelled materials are possible. In the astrophysics and nuclear experiment examples, a large-area gamma camera could be implemented at low cost using inorganic crystals and wavelength-shifting fibers. For nuclear reactors and other installations involving radioactive materials this technique may be incorporated in imaging devices used to locate sources of radiation, again improving system sensitivity while lowering costs. Radioisotopes are commonly used in biomedical research to label research biochemicals; imaging radiation detectors used for this purpose may again be upgraded in sensitivity and decreased in cost by using the proposed simplified readout technique. In all such applications, the decreased number (and area) of photosensors and their associated electronics results in either costs savings or increased detector size and sensitivity. In addition, the use of wavelength shifting fibers in combination with inorganic crystal scintillators is not limited to the imaging of gamma radiation alone; other forms of ionizing radiation such as alpha and beta particles, neutrons, protons, and nuclear fragments can all be imaged at decreased system cost and increased sensitivity. 
     While the invention has been particularly described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various other changes in detail may be made therein without departing from the spirit, scope or teachings of this invention.