Hybrid Collimation to Limit the Field of View for Gamma Detection Probes at High and Low Energies

Abstract

A hybrid collimated probe incorporates two detectors consisting of a scintillating crystal or semiconductor material, such as Cadmium-Zinc-Telluride (CZT). The count rate measured on the rear detector is corrected for the shielding effect of the front detector before the count rate ratio is calculated. This is done by multiplying the rear detector count rate by a factor pre-determined from the thickness and density of the front detector for a specific radionuclide energy. The count rate ratio also must be corrected for the presence of background radiation at the target site. This is done by taking a 3 second average of the count rate over tissue that does not contain a radiotracer sequestered at the site of pathology, but in adjacent tissue that is uniformly perfused by a lower level concentration of the radiotracer circulating in the blood pool background.

Claims

1. A hand-held probe for detecting a source of radiation, which comprises: (a) an elongate annular housing having a forward end and a rear end, and being devoid of side shielding; (b) a pair of co-axial radiation detecting elements, a forward co-axial radiation detecting element and a rear co-axial radiation detecting element, the co-axial radiation detecting elements separated by a material of low gamma absorption, the forward co-axial radiation detecting element located at the annular housing forward end and not being recessed; (c) one or more preampliers located adjacent and rearward of the pair of co-axial radiation detecting elements and in electrical connection therewith; (d) a console in communication with the pair of co-axial radiation detecting elements and housing a software algorithm to determine the distance, d, to a radiation source, according to the following equation: $d=\frac{x}{\left(\sqrt{\frac{N_F}{N_R}}-1\right)}$ where, N.sub.F is the number of gamma counts received by the forward co-axial radiation detecting element; N.sub.R is the number of gamma counts received by the rear co-axial radiation detecting element; d is the distance from the gamma emission source to the detector.

2. The hand-held probe of claim 1, wherein the software algorithm corrects count rates for both co-axial radiation detecting elements by subtracting background radiation count, N.sub.B, from both NE and N.sub.R before determining the distance d.

3. The hand-held probe of claim 2, wherein the software algorithm compensates for shielding by multiplying rear radiation detecting element count rate by the K.sub.SHIELDING factor resulting in the software algorithm for a corrected distance, as follows: $\mathrm{depth}=\frac{x}{\sqrt{\frac{\left(N_F-N_B\right)}{\left(\left(N_R-N_B\right)*K_{\mathrm{SHIELDING}}\right)}-1}}$ $\mathrm{where},K_{\mathrm{SHIELDING}}=\frac{1}{\left(1-{e^{-{\mu }_l}}^T\right)};$ μ.sub.l is the linear attenuation coefficient for the material of the pair of co-axial radiation detecting elements and the energy of the gamma emission; and T is the thickness of the forward co-axial radiation detecting element.

4. The hand-held probe of claim 3, wherein lower end of the energy range for the software algorithm is limited to the value to prevent K.sub.SHIELDING from exceeding 2.00.

5. The hand-held probe of claim 4, wherein the software algorithm calculates the corrected distance to a radiation source, according to the following equations: $\mathrm{depth}=\frac{x}{\sqrt{\frac{\left(N_F-N_B\right)}{\left(\left(N_R-N_B\right)*K_{\mathrm{SHIELDING}}\right)}-1}}$ $\mathrm{where},K_{\mathrm{SHIELDING}}=\frac{1}{\left(1-{e^{-{\mu }_l}}^T\right)};$ μ.sub.l is the linear attenuation coefficient for the detector material and the energy of the gamma emission; and T is the thickness of the forward co-axial radiation detecting element.

6. The hand-held probe of claim 5, wherein the software algorithm further limits the field of view by inhibiting counting whenever the radiation source is outside of the volume specified by a threshold value for count rate ratio, R.sub.THRESHOLD according to the following equation: $\mathrm{FOV}\mathrm{Limit}=\frac{x*\left(\cos \phi +\sqrt{{\cos }^2\phi +R_{\mathrm{THRESHOLD}}-1}\right)}{\left(R_{\mathrm{THRESHOLD}}-1\right)}$ where: x is the fixed distance between the pair of co-axial radiation detecting elements; ϕ is the off-axis angle in the direction of the source; and R.sub.THRESHOLD is the value that the count rate ratio must exceed to enable counting.

7. The hand-held probe of claim 6, which is calibrated for each specific source of radiation at or above about 511 KeV to provide a correction factor for the shielding effect of the forward co-axial radiation detecting element on the rear co-axial radiation detecting element.

8. The hand-held probe of claim 1, wherein the pair of co-axial radiation detecting elements comprise one or more of a semiconductor, a diode, or a scintillation element.

9. The hand-held probe of claim 1, wherein the diameter of the elongate annular housing is less than about 12 millimeters.

10. The hand-held probe of claim 1, wherein the communication of the console with the pair of co-axial radiation detecting elements is electrical communication.

11. The hand-held probe of claim 1, wherein the communication of the console with the pair of co-axial radiation detecting elements is wireless communication.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0013] For a fuller understanding of the nature and advantages of the present method and process, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

[0014] FIG. 1 illustrates a dual detector probe capable of electronic collimation and collimation using heavy metal for low energy radionuclides.

[0015] FIG. 1A is an end view looking into the forward end of the probe.

[0016] FIG. 2 illustrates the various contours for the electronically collimated field of view at different values of R.sub.THRESHOLD. The CZT is recessed within the tungsten shielding to provide hybrid collimation at low energies.

[0017] FIG. 3 illustrates an end-viewing dual detector probe capable of electronic collimation can be placed on an da Vinci® Endowrist® system to provide angulation of the field of view within the surgical cavitation.

[0018] FIG. 4 illustrates a side-viewing probe intended for laparoscopic use. Three detectors are required. Heavy metal shielding is used to increase or decrease the count rate ratio for electronic collimation.

[0019] FIG. 4A is an end view of the laparoscopic probe.

[0020] FIG. 4B is a sectional view through the middle detector of the new laparoscopic probe.

[0021] FIG. 4C is a sectional view through the flanking detectors.

[0022] FIG. 5 illustrates a gamma detection control unit and hand-held probe.

[0023] These drawings will be described in more detail below.

DETAILED DESCRIPTION OF THE INVENTION

[0024] In the end viewing configuration as illustrated in FIG. 1, a hybrid collimated probe, 10, incorporates two detectors, 12 and 14, consisting of a scintillating crystal or semiconductor material, such as Cadmium-Zinc-Telluride (CZT). Semiconductor crystals are energized with a bias voltage (60-240 Volts) on alternating anodes and grounded on alternating cathodes, with the cathode located on the distal aspect of each crystal to form a charge collecting detector. Charge integration and pulse shaping are provided by a pre-amplifier circuit incorporated in the handle of the probe. A Teflon® or other low gamma absorbing spacer, 16, in used to recess the front detector in tungsten shielding, 19, for collimation at energies less than 234 KeV (for CZT). See also FIG. 1A. All probe materials are enclosed within medical grade 316 stainless steel or aluminum annular housing, 18. Also shown is a pre-amplifier bracket, 20, and pre-amplifier(s), 22. Detectors 12 and 14 are separated by an insulated (low gamma absorbing) spacer, 17, such as Teflon®.

[0025] The signal from the dual detector probe consists of two channels of charge pulses. The pre-amplifier is gain trimmed to provide 6 mV/KeV amplitude pulses. Forward crystal 14 acts as the primary count rate detector. Rear crystal 12 is used to measure the count rate ratio of the two detectors separated by a fixed distance. The count rate ratio can be used to calculate the distance to the radiation source, and provide electronic collimation based on the Inverse Squared Law.

[0026] It is essential that the count rate measured on the rear detector is corrected for the shielding effect of the front detector before the count rate ratio is calculated. This is done by multiplying the rear detector count rate by a factor pre-determined from the thickness and density of the front detector for a specific radionuclide energy. The radionuclide is selected on the gamma detection system console and the correction factor is loaded from a database incorporated in the console.

[0027] The count rate ratio also must be corrected for the presence of background radiation at the target site. This is done by taking a 3 second average of the count rate over tissue that does not contain a radiotracer sequestered at the site of pathology, but in adjacent tissue that is uniformly perfused by a lower level concentration of the radiotracer circulating in the blood pool background. The measured background count is subtracted from both the front and rear detector counts as it is assumed to be uniform in the direction of the probe and, unlike the target emission, can be assumed to be a parallel flux field that is constant over small differences in distance.

[0028] Once the rear count rate is corrected for the shielding effect of the front detector and both detectors are corrected for the measured background count, the ratio of the front count rate divided by the rear count rate is compared to a threshold value that defines the extent of the field of view mathematically in the probe control unit. The extent of the field is calculated as:

[00001]$\mathrm{FOV}\mathrm{Limit}=\frac{x*\left(\cos \phi +\sqrt{{\cos }^2\phi +R_{\mathrm{THRESHOLD}}-1}\right)}{\left(R_{\mathrm{THRESHOLD}}-1\right)}$

where,
x is the fixed distance between the two detectors;
ϕ is the off axis angle in the direction of the source; and
R.sub.THRESHOLD is the value that the count rate ratio must exceed to enable counting.

[0029] FIG. 2 illustrates the electronically collimated field of view for various values of R.sub.THRESHOLD.

[0030] The distance to the radioactive source is estimated as:

[00002]$\mathrm{depth}=\frac{x}{\sqrt{\frac{\left(N_F-N_B\right)}{\left(\left(N_R-N_B\right)*K_{\mathrm{SHIELDING}}\right)}-1}}$$\mathrm{where},K_{\mathrm{SHIELDING}}=\frac{1}{\left(1-{e^{-{\mu }_l}}^T\right)};$

μ.sub.l is the linear attenuation coefficient for the detector material and the energy of the gamma emission; and
T is the thickness of the front detector material.

[0031] The lower end of the energy range for the algorithm is limited to the value to prevent K.sub.SHIELDING from exceeding 2.00.

[0032] FIG. 2 illustrates the various contours for the electronically collimated field of view at different values of R.sub.THRESHOLD. The CZT is recessed within the tungsten shielding to provide hybrid collimation at low energies.

[0033] Other configurations using three or more detectors and a combination of metallic and electronic collimation also are possible. Since the outside diameter of the end-viewing probe is 12 mm, it can be introduced into the surgical field through a standard Trocar™ for laparoscopic and robotic approaches. In these applications, the dual detection element and associated electronics can be mounted at the distal end of an articulated probe for robotic surgery as an alternative to a side-viewing probe (FIG. 3). In particular, probe 10 is seen mounted to the end of a robotic arm, 24, such as a da Vinci® Endowrist® system.

[0034] A side viewing probe for laparoscopy can be implemented using three detectors and hybrid collimation as well, as disclosed in U.S. Ser. No. ______ (Attorney Docket ACT 2-009P (see FIGS. 4, 4A, 4B, and 4C). In particular, a laparoscopic probe, 26, uses 3 detector crystals, 28, 30, and 32, configured for lateral view for counting pulses from a gamma radiation source, where measured counts, respectively, are Np, NM, and ND. With tungsten shielding, 34, for central crystal 30 configured for a 60° field of view and flanking crystals 28 and 32 having tungsten shielding, 36. Since the count rate of the middle detector is attenuated outside of the angular field of view, the count rate ratios of the following equation are similarly reduced by 32%, forcing the count rate ratios to a value less than unity. Outside of the 60° arc, the count rate ratios are less than 1, and counting is inhibited by the probe control unit.

[00003]$\frac{N_M}{N_D}\mathrm{or}\frac{N_M}{N_P}=0.68\frac{N_M}{N_{ei\mathrm{ther}}}\mathrm{outside}\mathrm{the}\mathrm{FOV}$

[0035] Probes can interface to a Gamma Detection System console, 38, shown in FIG. 5 using a multiconductor cable and connectors, 40, or wireless standard modules such as Bluetooth™, can be incorporated in the design to obviate the need for a cable with a sufficient number of conductors for two or more channels of gamma radiation pulses (FIG. 5).

[0036] While the apparatus, system, and method have been described with reference to various embodiments, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope and essence of the disclosure. In addition, many modifications may be made to adapt a particular situation or material in accordance with the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiments disclosed, but that the disclosure will include all embodiments falling within the scope of the appended claims. In this application all units are in the metric system and all amounts and percentages are by weight, unless otherwise expressly indicated. Also, all citations referred herein are expressly incorporated herein by reference.