SOLID STATE BETA-SENSITIVE SURGICAL PROBE

Abstract: An intraoperative probe system (1) for detecting beta radiation over gamma radiation emitted from a radiopharmaceutical. The probe system has an ion-implanted silicon charged-particle detector (2) for generating an electrical signal in response to received beta particles. A preamplifier (3) located in close proximity to a processing means (4) for amplifying and filtering the electrical signal. A wire (9) couples the probe to a processing means (4) for amplifying and filtering the electrical signal. A counter (6) analyzes the resulting electrical signal to determine the number of beta particles being received by the detector (2). In an alternative embodiment, the wire (9) can be replaced with an infrared transmitter and receiver or a radio transmitter and receiver for "wireless" operation of the probe (1).

Description: SOLID STATE BETA-SENSITIVE SURGICAL PROBE FIELD OF THE INVENTION The present invention relates generally to the field of radiation detection probes, and more specifically to a beta-sensitive radiation probe used to detect tissue labelled with beta- emitting radiopharmaceuticals BACKGROUND OF THE INVENTION The surgical excision of diseased tissue within the body, such as a tumor or abcess, is often complicated by the inability of the surgeon to visually differentiate the diseased tissue from the normal tissue This problem is particularly acute in the field ol surgical oncology, where small numbers of tumor cells can infiltrate areas of normal tissue both adjacent to and remote from the main tumor mass Importantly, the failure to remove all of the diseased tissue during the procedure often results in a continuation or recurrence of the original problem One potential solution to this problem involves the detection of radiolabelled monoclonal antibodies and other radiopharmaceuticals, which are preferentially accumulated in diseased tissues such as cancer ceils Although intraoperative probes have been developed for use with several types of radioactive materials, the historical emphasis has been on the detection of gamma radiation in particular (gamma rays or photons) See Harris et al , Nucleonics 14 102-8 (1956), Morris et al , Phys Med Biol 16 397-404 (1971), Wooifenden et al , Chest 85 84-88 (1984) Unfortunately, the prior art devices designed for use with gamma-emitting radiopharmaceuticals suffer from two significant problems 1 ) the tumor-to- background ratios are non-optimal for the reliable differentiation of tumors, and 2) the detection of distant sources of gamma rays further reduces the already low tumor-to- background contrast The longer path length of gamma radiation in body tissues creates significant background contamination from distant accumulations of the radiopharmaceutical, making the detection of nearby tagged tissues difficult or impossible As a result, there has been a renewed interest in developing intraoperative probes which focus primarily on the detection of beta emissions (positrons and/or electrons), particularly in light of the recent discovery of positron emitters with high affinity for cancers, such as l8 F-labeled-Fluoro-2-Deoxy-D-Glucose (FOG) See Wahl et al , Cancer 67 1550-54 (1991) However, recent attempts to design an accurate beta-sensitive intraoperative probe have been complicated by the fact that positron-emitting radiopharmaceuticals such as l 8 F- FDG create two 51 1 keV annihilation photons when the positron subsequently collides with an electron. The detection of these highly penetrating gamma rays greatly reduces the observed tumor-to-background contrast gained by the use of these radiopharmaceuticals. In response, several attempts have been made to design detectors to maximize the detected positron-to-photon ratio. One possible approach to limit the effect of the annihilation photon emissions relies upon energy discrimination to reduce the photon contribution to the signal. See Raylman et al., J. Nucl. Med. 36: 1869-74 (1995). The dominant mode of interaction for the 51 1 keV photons produced during annihilation of the positron and electron is a Compton scattering of electrons, generally below 340 keV. In contrast, the positron interacts with the detector by producing a spectrum of energies, some of which are above the Compton edge of approximately 340 keV for annihilation photons. Accordingly, by selectively counting only those events with energies above the Compton edge, the probe becomes selectively sensitized to the electrons and positrons emitted by radiopharmaceuticals such as l8 F, which create annihilation photons. Unfortunately, most of the detectors proposed thus far for use as positron probes have utilized plastic scintillators. See Lerch et al., Am. J. Physiol. 242:H62-H67 (1982); Raylman et al, J. Nucl. Med. 35:909-13 ( 1994); Daghighian et al., Med. Phys. 36: 1869-74 (1995). The application of the energy discrimination technique with plastic scintillators is problematic due to the poor energy resolution of this material, which is a measure of how well the energy of a specific type of radiation (such as gamma rays) can be defined. Moreover, inefficiencies in the collection of the scintillation light produced by the plastic scintillators also reduce the energy resolution of these detection devices. An alternative method and device proposed and patented by Daghighian et al. involves the use of two separate plastic scintillation detectors, whereby the signals from the shielded outer detector are used to correct for photon contamination of the signal from the inner detector. See Daghighian et al, Med. Phys. 21 :153-7 ( 1994); U.S. Patent No. 5,008.546 to Mazziotta et al. Correction of signal contamination is accomplished by a weighted subtraction of the outer detector count rate from the inner detector count rate. The weighting factor is the ratio of the gamma counting efficiencies of the two detectors, which is calculated during a relatively simple calibration procedure. While the use of a second detector to measure the background contamination is somewhat effective, this addition unfortunately results in a probe tip which is always physically larger than a single detector. Therefore, the practical application of this type of probe is problematic where space is a premium, such as with intraluminal probes and other situations where the surgical field is small. Moreover, the reduction of the surgical field continues to increase as minimally invasive surgical procedures are developed, and therefore a useful alternative to the two-detector method is needed. Furthermore, it is not clear that the background subtraction/weighting function remains constant when the probe is presented with gamma rays entering the detector volume other than through the front window. This problem is very often present in many common surgical applications. Accordingly, there is still a substantial need in the art for an intraoperative probe which can differentiate diseased tissue based on beta emissions from a radiopharmaceutical. The probe must also have a minimal size for less intrusive operation during surgery, while at the same time provide increased sensitivity and selectivity. SUMMARY OF THE INVENTION The present invention contemplates a method for detecting radiopharmaceuticals within diseased tissue, comprising the steps of a) providing: i) a patient having a region of diseased tissue, ii) a radiopharmaceutical capable of emitting beta particles and gamma radiation, and iii) an ion-implanted silicon detector; b)administering said radiopharmaceutical to said patient; and c) using said ion-implanted silicon detector to preferentially detect beta particles emitted from said radiopharmaceutical within said region over gamma radiation. In one preferred embodiment, the step of discriminating a component of an electrical signal produced by said detector when struck by said beta particles and said gamma radiation. In another preferred embodiment, said component of said electrical signal is produced by said detector when struck by said gamma radiation. In yet another preferred embodiment, said radiopharmaceutical is ,8 F-labeled-Fluoro-2- Deoxy-D-Glucose. The present invention also contemplates a probe system comprising a probe having an ion-implanted silicon detector, whereby beta particles emitted from a radiopharmaceutical within a diseased tissue are preferentially detected over gamma radiation. In another preferred embodiment, said probe further comprises a means for discriminating a component of an electrical signal produced by said detector when struck by said beta particles and said gamma radiation, said means for discriminating coupled to said detector. In another preferred embodiment, said component of said electrical signal is produced by said detector when struck by said gamma radiation. In another preferred embodiment, said probe further comprises a) a preamplifier for amplifying said electrical signal, said preamplifier coupled to said detector; and b) an amplifier for further amplifying said electrical signal, said amplifier coupled between said preamplifier and said means for discriminating. In another preferred embodiment, the probe system further comprises a counter for counting the number of received beta particles, said counter coupled to said means for discriminating. In another preferred embodiment, the probe system further comprises a) a transmitter for transmitting said electrical signal as a transmitted signal, said transmitter coupled to said means for discriminating; and b) a receiver for receiving said transmitted signal, said receiver coupled to said counter. In another embodiment, said probe further comprises battery. In another preferred embodiment, said transmitter is an optical transmitter, said receiver is an optical receiver and said transmitted signal is an optical signal. In an alternative embodiment, said optical transmitter is an infrared transmitter, said optical receiver is an infrared receiver and said optical signal is an infrared signal. In an alternative embodiment, said transmitter is a radio transmitter, said receiver is a radio receiver and said transmitted signal is a radio signal. The present invention also contemplates a probe system for detecting radiation emitted from a radiopharmaceutical in a diseased tissue, comprising a) a probe having a radiation detector which generates an electrical signal in response to the passage of radiation into said detector from the radiopharmaceutical in the diseased tissue; and b) an optical transmitter coupled to said probe for transmitting said electrical signal as an optical signal to a remote location. The present invention also contemplates a probe comprising an ion-implanted silicon detector, whereby beta particles emitted from a radiopharmaceutical within a diseased tissue are preferentially detected over gamma radiation. In another preferred embodiment, the probe further comprises a means for discriminating a component of an electrical signal produced by said detector when struck by said beta particles and said gamma radiation, said means for discriminating coupled to said detector. In another preferred embodiment, said component of said electrical signal is produced by said detector when struck by said gamma radiation. In another preferred embodiment, the probe further comprises a) a preamplifier for amplifying said electrical signal, said preamplifier coupled to said detector; and b) an amplifier for further amplifying said electrical signal, said amplifier coupled between said preamplifier and said means for discriminating. In another preferred embodiment, the probe further comprises a counter for counting the number of received beta particles, said counter coupled between said amplifier and said means for discriminating. BRIEF DESCRIPTION OF THE FIGURES Figure 1 illustrates a schematic diagram of the probe system of the present invention. Figure 2 illustrates a schematic diagram of the wireless probe system of the present invention incorporating infrared transmitters and receivers. Figure 3 A illustrates a graph of the energy spectra measured using an USD. Figure 3B illustrates the energy spectra measured using a plastic scintillator detector. Figure 4 illustrates a graph showing resolution of an USD versus a threshold energy setting. Figure 5A illustrates a graph showing the detection sensitivity for positrons versus threshold settings. This displays data acquired with both an USD and a plastic scintillator. Figure 5B illustrates the detection sensitivity for 51 1 keV photons versus threshold energy settings. This displays data acquired with both an USD and a plastic scintillator. Figure 6 illustrates the positron/photon detection ratio versus threshold energy. Figure 7A illustrates the Z-score versus disk diameter. Measurements were made with an USD and a threshold energy setting of 14 keV. The hollow circle indicates the surrounding paper while the dark circle indicates the small disk. Figure 7B illustrates the Z-score versus disk diameter. Measurements were made with a plastic scintillator probe and a threshold energy setting of 170 keV. The hollow circle indicates the surrounding paper while the dark circle indicates the small disk. GENERAL DESCRIPTION OF THE INVENTION The present invention solves the problems presented in the prior art by providing a solid state beta-sensitive radiopharmaceutical probe incorporating an ion-implanted silicon detector (USD), which is highly sensitive to beta particles and much less sensitive to gamma ray contamination. The use of an USD in the probe of the present invention therefore provides an effective means for preferentially detecting beta particles over gamma radiation, while at the same time substantially minimizing the requisite size of the probe instrument itself. Semiconductor-Based Detection Devices Semiconductor-based detectors typically provide "doped" regions in materials such as silicon (Si) or cadmium telluride (CdTe), for the detection of radiation. The "doping" process involves the introduction of ions into defined areas, creating an interface between positively- and negatively-charged areas in the detection material. The thickness of the interface between the positively- and negatively-charged regions is defined as the "depletion depth", which is the active region of the detector responding to the passage of radiation particles or gamma rays. In semiconductor-based detectors, a biasing voltage is typically applied to increase the size of the active region. However, conventional radiopharmaceutical probes employing semiconductor technology, such as the CdTe-based Neoprobe- 1000 ™ (Neoprobe Corporation. Columbus, Ohio), are designed primarily for use as gamma detectors. In these devices the depletion depth can be as large as 5 mm, and is often located further within the detector material itself, which makes these devices highly susceptible not only to the desirable gamma rays entering through the front detector window, but also to contaminating radiation entering through the side wall of the device. The usefulness of these gamma probes for the detection of beta-emitting radiopharmaceuticals is therefore severely limited, since several inches of additional lead shielding would have to be incoφorated in order to sufficiently minimize the effects of background gamma rays and 51 1 keV annihilation photons. The addition of such a large amount of shielding would obviously produce an awkward and unwieldy surgical instrument, which would clearly be unsuitable for use in small surgical fields. Conventional semiconductor-based gamma detectors utilizing the lithium drift process for doping the silicon (Si(Li)) suffer from the same insurmountable problem. One potential solution to this problem would be to incorporate another class of semiconductor detectors known as surface barrier detectors, in which the active region or depletion depth of the detector is both smaller and closer to the surface. A thinner depletion depth located closer to the surface greatly decreases the detection of unwanted gamma radiation entering from the sides of the probe, thereby substantially reducing the amount of shielding necessary to create an effective beta-sensitive radiopharmaceutical probe. The optimal depletion depth is dependent upon the particular radionuclide sought to be detected. For example, with respect to l8 F, a preferred range for the depletion depth would be 100-2000 microns. In a more preferred embodiment the range is 300-1000 microns, and in a most preferred embodiment, the range is 400-600 microns. However, while surface barrier detectors are one possible candidate for use in beta- sensitive radiopharmaceutical probes, USDs are a superior choice. The ion-implantation process utilized in the doping of these semiconductor detectors provides a more uniform depth and distribution of the doped material, which results in better energy resolution. Moreover, the placement of the electrical contacts in ion-implanted detectors also results in less statistical noise from leakage current. For example, it has been reported that USDs produce approximately 40% less noise than surface barrier detectors at body temperature, and their operating parameters are more stable over a wider range of ambient conditions. See EG&G Ortec, Modular Pulse-processing Electronics and Semiconductor Radiation Detectors, p. 400 (EG&G Ortec. Oak Ridge, 1995). This significantly improves the energy discrimination capability of the probe, allowing further reduction of the photon contribution to the signal as described above. Moreover. USD detectors are much less prone to permanent damage than surface detectors if the detector material itself is accidentally touched or contacted by fluids, due to a tear in the detector window cover. The exposed front face of the surface barrier detectors is more susceptible to damage because the electrical contacts are placed on the surface of the detectors, in contrast with ion-implanted detectors, where the electrical contacts are deposited below the surface. Accordingly, the use of an USD in the probe of the present invention substantially reduces the detection of background gamma rays from distant accumulations of a radiopharmaceutical, and therefore allows a user to detect the presence of positron-emitting radiopharmaceuticals, such as l8 F-labeled-Fluoro-2-Deoxy-D-Glucose (FDG), while substantially minimizing the contaminating effect of the 51 1 keV photons created by annihilation of the positron. It is further contemplated that similar particles such as Auger or conversion electrons could also be detected by the probe of the present invention. The present invention contemplates the preferential detection of beta particles through the incorporation of a highly-selective USD detector into the tip of a probe. When struck by a beta particle or equivalent, a small electrical current or pulse is produced in the detector. This pulse is shaped and amplified by a preamplifier, which in a preferred embodiment is placed as close to the detector as possible to reduce electrical noise. The signal then travels to an amplifier. Impedance matching with the amplifier is also performed by the preamplifier. In a preferred embodiment, the signal then travels to a discriminating means, which selectively filters the gamma ray component of the amplified signal, and reduces the effect of noise, such as that caused by the leakage current, on the amplified signal. Finally, the resulting pulses are counted to give an indication to the user as to whether a significant number of beta particles are being detected. The probe of the present invention is therefore useful for detecting and localizing the preferential uptake of beta-emitting radiopharmaceuticals by a lesion at the time of surgery. This probe can also be advantageously utilized to detect and localize other types of lesions, such as suspected infections or inflammatory processes, or any other disease process which can be detected by the accumulation of beta-emitting radiopharmaceuticals. Thus, a surgeon could utilize this probe as an aid in differentiating diseased from normal tissue, and thus tailor his or her choice of tissue for resection. The probe would also assist in detecting additional diseased tissue remaining after the main tissue section has been removed, as well as searching for other regions of diseased tissue within the operative field. Moreover, the incorporation of an ion-implanted silicon detector into the present invention in lieu of the dual probe design suggested by Daghighian et al. provides a substantial size advantage. As a result, the intraoperative probe of the present invention can be made very small, facilitating its application to endoscopy, bronchoscopy, colposcopy, colonoscopy, cytoscopy, laparoscopy and thorascopy, as well as other forms of minimally- invasive or non-invasive surgical biopsy. The present invention also contemplates a remote embodiment of the probe which can easily be used in almost any size surgical field. Furthermore, the probe of the present invention is very simple to operate, and does not require any special calibration procedures or signal processing. Although the foregoing description has described the use of the positron-emitting radionuclide ( l8 F), the USD-based probe of the present invention can also used with more common radiopharmaceuticals labeled with UI I. 32 P, and '"In, and other useful radionuclides such as ,24 L "Cu, 90 Y, 86 Y and "C, as well as other beta-emitters of appropriate energy. DEFINITIONS In considering the present medical probe invention, some definitions are helpful. For example, an "amplifier" is an electrical circuit that amplifies an electrical signal. "Background gamma radiation" refers to unwanted gamma radiation from any source. A "counter" is any means that is able to count the number of electrical pulses it receives. A "means for discriminating" or "discriminating means" prevents a portion of an input signal from passing through the device, such as filter. For example, it is contemplated that one embodiment of the discriminating means of the present invention can selectively prevent pulses below a specific energy level from passing through. In this manner, only those signals having a pulse above a desired energy level are allowed to pass through. A "pre-amplifier" is an electrical circuit having a high gain for amplifying a weak or small signal. "Infrared transmitter" refers to any transmitter capable of converting an electrical signal into an infrared signal and transmitting the infrared signal. "Infrared receiver" refers to any receiver capable of receiving an infrared signal containing data and converting the data into an electrical signal. "Optical transmitter" refers to any transmitter capable of converting an electrical signal into an optical signal and transmitting the optical signal. "Optical receiver" refers to any receiver capable of receiving an optical signal and converting the signal to an electrical signal. "Radio transmitter" refers to any transmitter capable of converting an electrical signal into a radio signal and transmitting the radio signal. "Radiopharmaceutical" refers to a pharmaceutical compound with a radionuclide. It can also be referred to as a radiolabelled compound. "Radio receiver" refers to any receiver capable of receiving a radio signal and converting the signal to an electrical signal. "Wire means" refers to any means capable of transmitting data (i.e. wires, cables, and fiber optics.) An "ion-implanted silicon detector" is a silicon substrate doped with ions from a low energy accelerator for converting received beta particles into electrical signals. A "surface barrier detector" is a semiconductor detector formed by doping the silicon through a drifting process wherein the silicon is exposed to a gas of the doping material thereby introducing the doping material to the silicon. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description serves to illustrate a preferred embodiment and aspects of the present invention and is not to be construed as limiting the scope thereof. Figure 1 illustrates a schematic diagram of one embodiment of the radiopharmaceutical probe of the present invention. An USD 2 is located in the tip of the probe 1 to provide an electrical signal in response to received beta particles from a radiopharmaceutical accumulated within a region of diseased tissue. As detailed above, USDs are superior for this particular application because of their high positron to photon ratio. In other words, they have a high sensitivity to beta particles in comparison to gamma radiation. In one embodiment, a preamplifier 3 is coupled to the detector 2 for amplifying and shaping the electrical signal into an appropriate voltage pulse. Ideally, the preamplifier 3 is located as close to the detector as possible in order to reduce electrical noise. An amplifier 4 is then coupled to the preamplifier for further amplifying the electrical signal. In a preferred embodiment, a means for discriminating a component of the electrical signal 5 is coupled to the amplifier 4 for further filtering of noise and to reduce the gamma ray component of the electrical signal. A counter 6 is coupled to the discriminator 5 for counting the received beta particles. Finally, a power supply 7 is coupled to the preamplifier for supplying power to the probe. It should be noted that while the different circuit elements described above have been listed as separate elements and illustrated in the figures as separate hardware elements, it may be possible for them to be combined together. Furthermore, it may be possible to provide software running on a processor which performs the same functions. Therefore, the present invention is not limited to strictly hardware elements, but may comprise a combination of hardware and software. In operation, a patient having a body part with a suspected diseased tissue is administered a radiopharmaceutical. After the radiopharmaceutical accumulates in the diseased tissue, the probe is placed in close proximity to the diseased tissue. Beta particles emitted from the radiopharmaceutical accumulated in the diseased tissue are detected by the detector 2, which generates corresponding electrical signal or pulse. Gamma radiation produced by the radiopharmaceutical either directly or by annihilation may also be detected by the detector 2. The electrical signal therefore comprises a component corresponding to the detected beta particles and a component corresponding to the detected gamma radiation. The electrical signal is then amplified first by the preamplifier 3, and then by amplifier 4. In a preferred embodiment, the amplified signal then passes through a discriminating means 5 acts as a filter to prevent that portion of the electrical signal (or those electrical pulses) below a certain energy level from passing through. Ideally, the discriminating means provides a circuit for adjusting the detection threshold energy of the USD. in order to balance the sensitivity of the detector with the elimination of unwanted background radiation and noise. Finally, the signal is passed through a counter which counts the pulses present in the electrical signal. These pulses generally correspond to the number of beta particles detected by the detector 2. although it is possible that some of the pulses resulting from beta particles striking the detector may be filtered by the discriminating means 5, and that some stray gamma pulses may also pass through. However, the majority of the remaining pulses should correspond to the detected beta particles. Based on the received beta particle count, a user can localize radiopharmaceuticals accumulated within a region of diseased tissue, such as a tumorous lesion. This localization can be performed by determining a threshold level of beta particles which indicates the presence of a lesion. In one embodiment, localization is performed by calculating a "Z-score" which indicates the beta particle count. The Z-score is calculated as: Z-score = [(measured particle count) - reference]/STD where the reference is determined by placing the probe at a location remote from the region suspected of having the diseased tissue. This reference indicates the background radiation emitted by the patient where no diseased tissue is present. The measured particle count is the count of the received particles by the probe at the region suspected of having diseased tissue. The difference of these two values is then divided by the standard deviation (STD) reference count, which is the standard deviation for a series of counts over the remote reference area. Regions having a Z-score of at least two to three indicate the presence of a radiopharmaceutical-avid diseased tissue. When the probe 1 is in close proximity to a tissue emitting a sufficient number of beta particles, an audio or visual signal can be activated to alert the user. In an alternative embodiment (not shown), the remote elements (4-7) can be housed completely within the probe 1. In this embodiment, the power supply could be a battery housed within the probe housing. In the alternative preferred embodiment illustrated in Figure 2. the wire means 8 and 9 can be substituted with an optical transmitter 11 and receiver 10 (i.e. infrared) thereby providing greater ease of use. Optical transmitters and receivers incorporating, for example, infrared light, are well known in the art and readily available. For example, suitable components could be purchased from NEC Optoelectronics and easily incoφorated into the probe of the present invention. Therefore, the optical components will not be discussed with any specificity. The electrical signal output from the discriminator 5 is converted by the optical transmitter 1 1 to an optical signal for transmission to a remote receiver 10, which converts the optical signal back into an electrical signal within the means for processing 13. Here, the electrical signal is processed by counter 6. It should be apparent to one skilled in the art that the electrical signal generated by the detector 2 can be transmitted from the probe 1 at any point. In other words, the optical transmitter 1 1 can also be positioned immediately after the preamplifier 3, so that the amplifier 4. discriminating means 5 and counter 6 are all located within the means for processing 13. Alternatively, the infrared transmitter 1 1 can be located after the amplifier 4 so that the discriminating means 5 and counter 6 are both located remotely from the probe 1. It should be noted that because the probe will be "wireless" it will require an internal power supply such as battery 12 illustrated in Figure 2. In an alternative preferred embodiment (not illustrated), the optical transmitter and receiver can be substituted with a radio transmitter and receiver. Again, the radio transmitter can be located at different points similarly to the optical transmitter. EXPERIMENTAL The following examples serve to illustrate certain preferred embodiments and aspects of the present invention and are not to be construed as limiting the scope thereof. In the experimental disclosure which follows, the following abbreviations apply: eq (equivalents); M (Molar); μM (micromolar); N (Normal); mol (moles); mmol (millimoles); μmol (micromoles); nmol (nanomoles); gm (grams); mg (milligrams); kg (kilograms); μg (micrograms); L (liters); ml (milliliters); μl (microliters); cm (centimeters); mm (millimeters); μm (micrometers); nm (nanometers); °C (degrees Centigrade); sec (seconds); msec (milliseconds); kBq (kiloBecquerels); mBq (milliBecquerels); MBq (megaBecquerels); STD (standard deviation); SUV (standard uptake values); keV (kiloelectronvolt); cps (counts per second). EXAMPLE 1 Comparison of USD Probe with Scintillation Probe The ion-implanted-silicon detector used in this example was manufactured by EG&G Ortec (Ultra™: Model Number U-013-050-500; Oak Ridge, TN). The detector chosen possesses a nominal 500 μm thick depletion layer and operates at 100 volts. This amount of silicon is sufficient to stop most of the positrons emitted by l8 F. The factory-specified beta energy resolution of this device is 4.2 keV. The circular silicon wafer which makes up the sensitive area of the device has a diameter of 8 mm. In the embodiment of the probe used for the present experiment, the detector unit was mounted in a light-tight aluminum cylinder. The entrance window consisted of a thin ( 1.27 x 10 '3 cm thick) sheet of aluminum foil; the cap on the other end of the cylinder contained the electrical feedthru necessary for providing the biasing voltage to the device. Current pulses were conditioned with an EG&G Ortec Model 142 charged-coupled preamplifier. The length of cable between the detector and preamplifier was minimized to help reduce noise and maintain good energy resolution. Amplification of the pre-amplified pulses was performed with an EG&G Ortec Model 572 shaping amplifier (shaping time= 0.5 μs). The resulting voltage signals were routed to an EG&G Ortec Model 583 pulse-height discriminator and the pulses created by the discriminator were counted by an EG&G Ortec Model 772 sealer module. A high voltage EG&G Ortec Model 556 power supply was also incoφorated to feed power to the probe. Many of the operating characteristics of the USD were then compared to those possessed by a plastic scintillation probe tested by the present inventors in preclinical studies of FDG-guided breast cancer surgery. See Raylman et al. J. Nucl. Med. 36: 1869-1874 ( 1995). Briefly, this scintillation probe consisted of a cylinder of BC-408 (Bicron Corp; Newbury.OH) plastic scintillator (diameter = 8.4 mm; length = 4 mm) fiber optically coupled to a photomultiplier tube (XP-1911; RCA Electronics). The plastic scintillator was enclosed in a stainless steel tube with a slanted front end piece to facilitate use in surgical procedures. The device further employed energy discrimination in an attempt to reduce the effect of background photon detection. Energy Spectra The combined positron and annihilation photon energy spectrum measured by each detector was acquired by centering the detectors 0.4 mm above a circular (8 mm diameter) piece of filter paper containing 37 kBq of l 8 F. Output from the amplifiers were "pulse-height analyzed" for 20 seconds by a Canberra Series 30 Multichannel Analyzer (Canberra Nuclear; Meriden, CT). A pure positron energy spectrum was obtained by covering the filter paper with a 0.2 mm thick piece of stainless steel, and the Multichannel Analyzer was then set to subtract mode and a 20 second acquisition performed. The stainless steel prevented positrons from reaching the detector, and thus the annihilation photon signal was subtracted from the previously measured combined spectrum, resulting in a pure positron energy spectrum. Detected photon energy spectra were obtained by acquiring a spectrum for 20 seconds with the stainless steel in place. Figures 3A & 3B illustrate the l8 F positron and 51 1 keV photon energy spectra obtained with the USD and plastic scintillator detectors. Figure 3A also shows the theoretical shape of the positron energy spectrum calculated from the Fermi theory of beta decay. See R.D. Evans. The Atomic Nucleus 3rd ed., pp. 278-279 (McGraw-Hill, New York, 1976). The two positron energy spectra obtained with the USD presented in Figure 3A illustrate that there is very little annihilation photon contamination of the combined spectrum. This is demonstrated by the very minor differences between the combined and positron spectra. Also note that the minor differences in the spectra vanish at energies above the position of the Compton edge (0.7 keV). In addition, the excellent energy resolution of the USD is evident from the good agreement between the measured and calculated positron energy spectra. Figure 3B shows the energy spectra from the plastic scintillator detector in addition, for the sake of comparison, to the positron spectrum obtained with the USD. Clearly, the positron energy spectra acquired with the plastic scintillator are distorted compared to the USD spectrum. These spectra appear to have an overabundance of lower energy events. Since energy discrimination of the signals from the detector is used to reduce photon contamination, the positron and photon energy spectra measured by the detector are important. The spectra in Figure 3A demonstrate the very good response of the ion-implanted silicon detectors to positrons and annihilation photons. It is important to note that measured positron energy spectrum agrees very well with the positron energy spectrum calculated for ,8 F. Thus, it seems that very little energy is lost in the window of the detector. In addition, the position of the Compton edge measured by the USD correlates reasonably well with the calculated value of 340.7 keV for 511 keV photons. While Figure 3A demonstrates the very good performance characteristics of the USD, the energy spectra in Figure 3B illustrates that the problems with the plastic scintillator detector. The positron spectrum appears distorted and skewed towards the low energy end of the scale, compared to the USD spectrum, thus indicating that not all of the scintillation light from each event is being collected. There are several additional possible sources of light loss in the system. First, some of the light pulses undergo numerous reflections from the walls of the scintillator, and, although reflective coatings were applied, many reflections might produce some attenuation of signal. Also, there is a slight mismatch between the indices of refraction of the optical coupling compound and the scintillator. The index of refraction of the optical coupling compound is 1.46 and that of the scintillator is 1.58. The critical angle for total internal reflection, therefore, is 67.5°. Hence, some of the light signals are not coupled to the fiber optic cable. The photon spectrum seems also to be affected by inefficient light collection in a fashion similar to that of the positron spectrum. Spatial Resolution The spatial resolution of the USD was measured by translating a 2.54x10 "2 mm thick piece of thread soaked in l8 F (15.5 kBq) across the face of the detector. The distance between the detector and thread was 0.4 mm. At each point a series of five, five second acquisition was obtained; a mean count rate and standard deviation for each position was calculated from this data. The resulting curve of count rate versus position was fit to a Normal distribution and the full width at half maximum (FWHM) extracted as a measure of resolution. Measurements of resolution were acquired for a number of different threshold energies ranging from 6.8 keV to 495 keV. To determine the effect of background radiation on the resolution of the USD, a large circular petri dish containing 1.55 MBq of l8 F (5.2 kBq/ml) (this concentration simulates an SUV of 1 for an assumed injection of 370 MBq to a 70 kg patient one hour prior to the surgical procedure) was placed behind the thread. The same method for acquiring and processing data to calculate resolution was performed. Figure 4 illustrates the change in USD detector resolution as a function of energy threshold. Both curves, with and without the presence of background annihilation photon flux, are relatively flat; indicating a minimal dependence of resolution on threshold energy. In addition, the fact that the two curves are so similar supports data displayed in Figure 3A that very little photon contamination of the positron signal is present. The effect of background photon contamination on USD resolution appears to be minimal. The data presented in Figure 4 indicates that the change in spatial resolution as a function of energy threshold remains virtually unchanged; even when an annihilation photon background flux representative of that anticipated in a normal human upper chest is added. This is an improvement over other plastic scintillation detectors which use energy discrimination to remove background contamination. See R. Raylman et al. , J. Nucl. Med. 35:909-913 (1994). Sensitivity and Selectivity To measure detection sensitivity, a disk of filter paper (8 mm diameter) containing 55.5 kBq of l 8 F was positioned 0.4 mm below the front face of the USD. A series of five, five second acquisitions were obtained from which mean and standard deviation counts per second were calculated. A second series of measurements, measuring only photon interactions, were obtained following the placement of a 0.2 mm thick piece of stainless steel in front of the detector face. By dividing the results from the first set of measurements by the amount of activity present on the disk, the combined (positron and photon) sensitivity was calculated. The positron sensitivity was calculated by first subtracting the pure photon count rate from the combined count rate and dividing by the amount of activity on the disk. Finally, the photon sensitivity was calculated by dividing the pure photon count rate by the amount of activity on the disk. The ability of the detector to distinguish between positron events and photon events, or its "selectivity." was determined by dividing the pure positron sensitivity by the photon sensitivity. All parameters were determined at energy thresholds ranging from 6.8 keV to 635 keV. Both sensitivity and selectivity were determined for the USD and plastic scintillation probe. The plot in Figure 5A compares the combined and positron sensitivities measured with the USD as a function of threshold energy to the sensitivities measured with the plastic scintillator. Note that at most threshold energies the USD has a higher positron detection sensitivity than the plastic scintillator. The graph in Figure 5B demonstrates that the plastic scintillator has a higher photon detection sensitivity than the USD at most threshold energies. The combination of a high positron detection sensitivity and low photon sensitivity results in a very high detected positron-to-photon ratio (selectivity) for the USD, as demonstrated by the plot in Figure 6. Detection sensitivity is at least as important of an operating characteristic as is resolution. The data shown in Figure 5A reveals the excellent sensitivity of the USD detector. Indeed, at an energy threshold of 14 keV, combined detection sensitivity is 108.7 cps/kBq, which is virtually identical to that reported by Daghighian et al. (108.0 cps/kBq) for the dual plastic scintillation probe. At this threshold energy, the pure positron sensitivity is 101.3 cps/kBq. As expected, detection sensitivity decreases rapidly as the energy threshold is increased. But, because of the peaked shape of the plastic scintillator energy spectrum the reduction in sensitivity as a function of energy threshold is steeper than for the USD. This effect is most prominent at low threshold energy values. Combined signal sensitivity is also greater than pure positron sensitivity due to detection of signals from annihilation photons. The difference between combined and positron sensitivity is most pronounced in the data from the plastic scintillator. The large disparity is due mostly to the higher photon detection cross section of the plastic scintillation detector compared to the USD, as demonstrated by the data presented in Figure 5B. Sensitivity to photons can be reduced by using a thinner piece of plastic scintillator. For total absorption of all of the positron emitted by l8 F the piece of scintillator must be at least 2 mm thick. However, a disadvantage of using a thin piece of scintillator is the increased probability that positrons not directed normal to the front surface can escape through the side of the detector before depositing all of their energy; further reducing the amount of light collected per event. A thinner detector will also reduce the detection cross section of photons originating from areas located to the sides of the detector. The presence of sources of radiation which are not parallel to the face of the probe is common in situations where the device is used to survey a tumor bed. These regions are usually concave and therefore the probe will be presented with a flux of positrons and photons from the side. While proper shielding can eliminate the effect of positron flux, the photon flux is very difficult to shield so the geometry of the detector itself must be optimized to reduce detection. Since the USD has a much thinner active area (500 μm in this case) than most scintillation detectors, the detection of "off-axis" source of photons will be reduced significantly; making USDs superior for use in surgical applications. Perhaps the most important operating characteristic of a positron sensitive detector for use in FDG-guided procedures is the ratio of detected positron-to-photon events. Figure 6 shows the ratio between positron and photon sensitivity as a function of threshold energy. The differences in the data for the USD and plastic scintillator at the lower and intermediate threshold energies are due to the higher photon detection cross section and lower positron sensitivity of the plastic scintillator detector at these energy threshold settings. At higher energies, the positron/photon ratio for the plastic detector doesn't increase at the same energy as the USD because even though the amount of photons detected has been reduced almost to zero; this is because the lower positron sensitivity at higher energies forces the ratio to stay small until absolutely no photons are detected and then the selectivity rises rapidly. The data presented in Figure 6, in conjunction with the data displayed in Figure 5, can be used to determine the optimal threshold energy setting for the USD. Clearly, a trade-off in operating characteristics must be made. From Figure 5 we see that the greatest sensitivity is achieved at very low threshold settings. This phenomenon is due to the detection of annihilation photons and noise present in the detector. Therefore, the selectivity should be poor, this is confirmed by the data shown in Figure 6. Also we see from Figure 6 that, because of the complete exclusion photon events, selectivity is greatest at threshold energies above -350 keV. However, because of the large overlap of the photon and positron energy spectra, illustrated by the plots in Figure 3A, sensitivity at these settings is poor. Therefore a compromise must be made. Since the positron/photon ratio is relatively constant from a threshold energy of 14 keV to 21 1 keV, the threshold setting which results in the greatest sensitivity was chosen, 14 keV. EXAMPLE 2 Simulation of Surgical Field Following Excision To test the effectiveness of the USD in a search for the presence of residual diseased tissue left in the surgical field following attempts at excision, a surgical field following attempted lesion excision was simulated. Differing amounts of residual tumor were modeled by soaking various sized disks (2 mm, 5 mm, 10 mm and 15 mm diameter) in a solution of l8 F (concentration= 22.2 kBq/ml; simulating an SUV of 4.2 assuming a 370 MBq infusion and a 70 kg patient). To model exposed normal tissue present in the tumor bed, a piece of filter paper (7 cm x 3 cm) was soaked in a solution of l8 F (concentration= 5.2 kBq/ml; simulating an SUV of 1 , assuming a 370 MBq infusion and a 70 kg patient) and mounted on similar sized piece of tissue equivalent plastic (Gammex/RMI; Madison, WI). In order to simulate the background photon signal emanating from distant areas of normal tissue, a large petri dish (volume= 300 ml, diameter= 14 cm) was filled with a solution of ,8 F (concentration= 5.2 kBq/ml; simulating an SUV of 1 assuming a 370 MBq infusion and a 70 kg patient). The large piece of filter paper and the piece of tissue equivalent plastic it was mounted on were placed on top of the petri dish. The disks simulating the tumor residue were then placed (one at a time) in the center of the large sheet of filter paper. A series of five, 5-second measurements were acquired while the probe was positioned 0.4 mm above the petri dish, the large piece filter paper and the small disk of filter paper. Readings were acquired at a threshold energy of 14 keV. Mean and standard deviations calculated from this data were used to determine the Z-scores of the readings acquired above the large piece of filter paper (normal tissue) and the small disks (residual tumor), relative to the petri dish (sources of distant background) measurements. This experiment was repeated using the plastic scintillator probe. The plot in Figure 7A shows the results of this experiment to determine if it is possible to locate simulated residual FDG-avid disease in a tumor bed. Graphs of Z-scores versus disk size for the cases where the USD detector was placed above the simulated tumor bed and above the simulated residual tumor are displayed. The plot in Figure 7B shows the results for the plastic scintillator, with the energy threshold at 170 keV. At energy threshold below 170 keV signal from the disks of activity were completely indistinguishable from background. Having decided upon the optimal energy threshold value for the USD. the use of this detector in a simulated search for tumor remnants was performed. The data shown in Figure 7A demonstrates the potential effectiveness of this device in this application. Even when the amount of l8 F and the disk were smallest (2 mm and 0.52 kBq [14 nCi], respectively), the Z- score was greater than simulated surrounding tissues. As the disk and total amounts of activity grew, the Z-score relative to the simulated background activity grew, until the disk size exceeded the diameter of the detector (8 mm). At the point where the diameter of the disk was greater than the detector, those positrons and photons not emitted from the areas viewed by the USD were not detected; hence the static Z-score for the 15 mm diameter disk. While the USD was able to detect the disks of activity in the presence of realistic sources of background, the plastic-scintillator probe had a great deal of difficulty in accomplishing this task. Indeed, in order to approach the performance of the USD. the energy threshold had to be set to 170 keV. At this level the sensitivity to positrons was measured to be 29.4 cps/kBq, in contrast the USD's positron sensitivity at 14 keV (the level utilized in the experiment) is 101.3 cps/kBq. From the above it is clear that the present invention provides for a method and apparatus which can accurately detect beta-emitting radiopharmaceuticals accumulated within lesions in the body, while substantially reducing the effect of background gamma ray contamination. While a preferred embodiment has been described in some detail, it should be apparent from the above discussion that many modifications and variations are possible without deviating from the invention. For example, while the examples provided herein described the preferential detection of beta emissions from l 8 F-FDG, similar results have been obtained by the present inventors for both m I and '"In. Similarly, the substantial reduction in the size of the probe enabled by the present invention facilitates its application to endoscopy, bronchoscopy, colposcopy, colonoscopy, cytoscopy, laparoscopy and thorascopy, as well as other forms of minimally-invasive or non-invasive surgical biopsy, It is also contemplated that an array of detectors as described herein could be incorporated into a radiopharmaceutical probe and arranged to produce an image of the signal distribution. Furthermore, other improvements and modifications which become apparent to persons of ordinary skill in the art only after reading this disclosure, the drawings and the following claims are deemed within the spirit and scope of the present invention.

Claims: CLAIMS We claim: 1. A method for detecting radiopharmaceuticals within diseased tissue, comprising the steps of a) providing: i) a patient having a region of diseased tissue, ii) a radiopharmaceutical capable of emitting beta particles and gamma radiation, and iii) an ion-implanted silicon detector; b) administering said radiopharmaceutical to said patient; and c) using said ion-implanted silicon detector to preferentially detect beta particles emitted from said radiopharmaceutical within said region over gamma radiation. 2. The method of Claim 1. further comprising the step of discriminating a component of an electrical signal produced by said detector when struck by said beta particles and said gamma radiation. 3. The method of Claim 2, wherein said component of said electrical signal is produced by said detector when struck by said gamma radiation. 4. The method of Claim 1 wherein said radiopharmaceutical is l 8 F-labeled-Fluoro-2- Deoxy-D-Glucose. 5. A probe system comprising a probe having an ion-implanted silicon detector, whereby beta particles emitted from a radiopharmaceutical within a diseased tissue are preferentially detected over gamma radiation. 6. The probe system of Claim 5. wherein said probe further comprises a means for discriminating a component of an electrical signal produced by said detector when struck by said beta particles and said gamma radiation, said means for discriminating coupled to said detector. 7. The probe system of Claim 6. wherein said component of said electrical signal is produced by said detector when struck by said gamma radiation. 8. The probe system of Claim 7, wherein said probe further comprises: 5 a) a preamplifier for amplifying said electrical signal, said preamplifier coupled to said detector; and b) an amplifier for further amplifying said electrical signal, said amplifier coupled between said preamplifier and said means for discriminating. 0 9. The probe system of Claim 8, further comprising a counter for counting the number of received beta particles, said counter coupled to said means for discriminating. 10. The probe system of Claim 9, further comprising: a) a transmitter for transmitting said electrical signal as a transmitted signal, said 5 transmitter coupled to said means for discriminating; and b) a receiver for receiving said transmitted signal, said receiver coupled to said counter. 1 1. The probe system of Claim 10 wherein said probe further comprises battery. 0 12. The probe system of Claim 10 wherein said transmitter is an optical transmitter, said receiver is an optical receiver and said transmitted signal is an optical signal. 13. The probe system of Claim 12 wherein said optical transmitter is an infrared 5 transmitter, said optical receiver is an infrared receiver and said optical signal is an infrared signal. 14. The probe system of Claim 10 wherein said transmitter is a radio transmitter, said receiver is a radio receiver and said transmitted signal is a radio signal. 15. A probe system for detecting radiation emitted from a radiopharmaceutical in a diseased tissue, comprising: a) a probe having a radiation detector which generates an electrical signal in response to the passage of radiation into said detector from the radiopharmaceutical in the diseased tissue; and b) an optical transmitter coupled to said probe for transmitting said electrical signal as an optical signal to a remote location. 16. A probe comprising an ion-implanted silicon detector, whereby beta particles emitted from a radiopharmaceutical within a diseased tissue are preferentially detected over gamma radiation. 17. The probe of Claim 16, further comprising a means for discriminating a component of an electrical signal produced by said detector when struck by said beta particles and said gamma radiation, said means for discriminating coupled to said detector. 18. The probe system of Claim 17, wherein said component of said electrical signal is produced by said detector when struck by said gamma radiation. 19. The probe of Claim 18, further comprising: a) a preamplifier for amplifying said electrical signal, said preamplifier coupled to said detector; and b) an amplifier for further amplifying said electrical signal, said amplifier coupled between said preamplifier and said means for discriminating. 20. The probe of Claim 19, further comprising a counter for counting the number of received beta particles, said counter coupled between said amplifier and said means for discriminating.

Inventors: RAYLMAN RAYMOND R, WAHL RICHARD L, RAYLMAN, RAYMOND, R., WAHL, RICHARD, L.
Applicants: RAYLMAN RAYMOND R, UNIV MICHIGAN, WAHL RICHARD L, RAYLMAN, RAYMOND, R., THE REGENTS OF THE UNIVERSITY OF MICHIGAN, WAHL, RICHARD, L.
Application year: 1997
Main classification: G01T1
IPC codes: G01T1