Method for determining dead time in a gamma camera and system for achieving the same

Abstract

Method for estimating count loss in a gamma camera, wherein the method comprises the following: Injecting a synthetic pulse at a given rate into a data stream originating from a photodetector, wherein both analog and digital synthetic pulses are injected simultaneously or sequentially into the data stream, wherein analog synthetic pulses are injected prior to digitization of the data stream, and wherein digital synthetic pulses are injected after digitization of the data stream; Integrating the synthetic pulse into the data stream to form an integrated data stream; Determining a number of synthetic pulses in the integrated data stream that are passed on to a final image; and Determining the counting loss from equation (2) P r o z e n t u a l e r Z a ¨ h l v e r l u s t = ( Z a h l e i n g e f u ¨ h r t e r I m p u l s e − Z a h l e r k a n n t e r _ I m p u l s e ) × 100 _ Z a h l e i n g e f u ¨ h r t e r I m p u l s e

Description

GENERAL STATE OF THE ART

[0001] This disclosure relates to a method for determining the dead time in a gamma camera and a system for achieving it.

[0002] US 4,058,728 A describes the monitoring and correction of counting losses in a gamma camera to increase the camera's accuracy. Bolotin HH et al.: Simple Technique For Precise Determinations Of Counting Losses In Nuclear Pulse Processing Systems (In: Nuclear Instruments and Methods, 83, 1970, 1-12) describes a method for determining counting losses in processing systems based on nuclear pulses. US 4,369,495 A concerns a method and means for compensating for dead times in a gamma camera.

[0003] The dead time (also called count loss) of a nuclear medicine imaging system is the time during which the system processes one or more events (i.e., the interaction of a particle or stimulus from a radiation field with the system) and is therefore unavailable for processing subsequent events. It arises from the multitude of electronic circuits in a nuclear medicine imaging system, each with its own dead time, and the complex interaction between such circuits. Furthermore, the count rate losses of a system are a function of the total number of particles generated by the radiation field under investigation, including those outside the energy window of the system's single-channel analyzer, because the interactions of such particles occupy the system's circuitry while a decision is made regarding further processing.Thus, the dead time of a nuclear medicine imaging system depends on the nature of the system and the type of field interacting with it.

[0004] Due to the dead-time phenomenon in nuclear medicine imaging systems, the rate at which events are processed by the system is a non-linear function of the incoming event rate. In a gamma camera, the curve relating processed events to incoming events eventually reaches a maximum, and this maximum defines a turning point. At this turning point, the camera processes only about 50% of the incoming events, while at higher count rates, the camera's efficiency drops below 50%. Thus, if a radiation field generates particles that interact with the camera at rates higher than those at the turning point, less than half of these events will be processed by the camera and appear on a map of the radiation field.

[0005] One approach to compensating for the dead time of a gamma camera (to account for the dependence of the camera's efficiency on the rate of incoming events) is to empirically measure the true rate as a function of the apparent rate for a system that approximates a clinical system and store this information in a look-up table (LUT). The initial count rate for future patients is then estimated using the look-up table (LUT). Because this method is time-variant or case-variant, it reduces the accuracy of the count loss correction.

[0006] As previously stated, the dead time is extremely complex and depends not only on the inherent limitations of the camera itself, but also on the nuclear spectra with which the camera interacts. Therefore, using an empirical function to compensate for the dead time is prone to errors.

[0007] It is therefore desirable to accurately estimate the dead time of the gamma camera. It is also desirable to be able to correct for the efficiency, which decreases with increasing count rate due to the detector's dead time. This allows the generation of functional images that are quantitative, in which the numbers in the images represent, for example, the absolute amount of a radioactive tracer taken up by the tissue, e.g., in megabecquerels per cubic centimeter (MBq / cc). For this to work, the system must be precisely calibrated for its sensitivity to activity.

[0008] It is therefore desirable to provide a new and improved method and means for compensating the dead time of a gamma camera, which is less complex than previously used techniques and is more likely to produce accurate results. BRIEF SUMMARY OF THE INVENTION

[0009] Disclosed herein is a method for estimating a count loss in a gamma camera, comprising injecting a synthetic pulse at a given rate into a data stream originating from a photodetector; integrating the synthetic pulse into the data stream to form an integrated data stream; determining a number of synthetic pulses from the data stream to be passed on to a final image; and determining the count loss from the following equation (2). Percentage count loss = (Number of introduced pulses − Number of detected pulses) × 100 / (Number of introduced pulses)

[0010] Disclosed herein is a system comprising a scintillation crystal; a photodetector capable of generating a data stream based on photons received by the scintillation crystal; an amplifier capable of amplifying voltages based on the data stream received by the photodetector; an analog-to-digital converter capable of converting the data stream into a digital data stream; an integrator; and a pulse generator; wherein the pulse generator is capable of introducing a synthetic pulse into the data stream generated by the photodetector; and wherein the integrator is capable of integrating the data stream generated by the detector and the synthetic pulses generated by the pulse generator; and wherein the system is for injecting the synthetic pulses into the data stream emanating from the photodetector at a given rate.for integrating the synthetic pulses into the data stream to form an integrated data stream; for determining a number of synthetic pulses in the integrated data stream which are passed on to the final image; and for determining the count loss from the following equation (2). Percentage count loss = (Number of introduced pulses − Number of detected pulses) × 100 / (Number of introduced pulses) BRIEF DESCRIPTION OF THE FIGURES Fig. Figure 1 is an exemplary schematic illustration of a system for injecting an analog pulsed event into a data stream received from a crystal; Fig. Figure 2 is an exemplary schematic illustration of an arrangement of photomultiplier tubes, showing the position of the pulsed event relative to the photomultiplier tubes; and Fig. Figure 3 is an exemplary schematic illustration of a system for injecting a digital pulsed event into a data stream received from a crystal. DETAILED DESCRIPTION

[0011] Disclosed herein is a system and a method that estimates the dead time (also known as count loss) of a nuclear medicine imaging system by injecting pulsed events into a data stream. These events, in terms of amplitude and shape, resemble pulses generated by the system in response to interaction with stimuli from a radiation field. The injected pulses can be introduced in an analog format via an analog pulse generator prior to analog-to-digital conversion of the pulse, or alternatively, they can be introduced digitally via a digital pulse generator electrically connected to the system downstream of an analog-to-digital converter. The injected pulses are counted using one or more windows (space windows and / or energy windows) to determine the system count loss. These injected pulses are synthetic pulses and are introduced via a pulse generator.They are sometimes also referred to as pseudo-pulses. In one embodiment, both analog and digital pulses can be injected simultaneously or sequentially into the data stream emanating from the photomultiplier tube (PMT) to determine a counting loss.

[0012] The method involves introducing (referred to hereafter as "injecting") pulsed events at a predetermined rate into a data stream originating from a detector (e.g., a photomultiplier) and counting these events after digitizing the data stream to determine the amount of count loss or dead time. The injected pulses have a shape identical or substantially identical to a normal gamma event pulse. The injected pulses are injected at a controlled rate (and have a selected amplitude) to place them in a region of low counts in the spectrum of an isotope of interest. The use of a selected amplitude simplifies detection using an energy window. The difference between the number of injected pulses and the number of pulses measured in the final image provides an estimate of the dead time or count loss.One or more windows can be used to identify and determine the number of injected pulses. For example, if 100,000 pulses are injected and only 90,000 are measured after digitizing the data stream, the count loss or dead time is estimated at 10%. This information can be used to simplify the generation of functional images, which are quantitative in nature, where the number of counts in the image represents, for example, the absolute amount of a radioactive tracer taken up by the tissue of a patient under investigation.

[0013] This system of injecting pulsed events into the data stream is advantageous because it measures counting losses throughout the entire imaging chain, which includes the pre-processing electronics, pulse processing, position calculation, event stream generation, and transmission to an external computer. Because the pulses are "typical" and identical in form, or substantially identical, to normal gamma pulses, they encounter the same sources of loss throughout the acquisition chain. The use of triple windowing (windows containing a spatial position (x and y position) and an energy intensity (E)) ensures a clean pulse generator peak in the final image (though an actual image does not necessarily have to be formed). The ratio of collected events in the pulse generator windows to the number of generated pulses is a direct measure of the fractional live time, or alternatively, the dead time.This method for estimating dead time does not depend on the camera's state, such as the number of defined windows or the ratio of the windowed rate to the total pre-detector rate. It also avoids the time-consuming calibration of multiple dead-time lookup tables (LUTs), each of which may depend on one or more isotope energy windows and an object. Therefore, this method is inherently more accurate than the LUT-based approach.

[0014] Fig. Figure 1 illustrates a schematic representation of an exemplary system 100 for injecting an analog pulsed event into a data stream received from an object 101 (e.g. a patient) via one or more photomultiplier tubes 104. The system 100 comprises a gamma camera head 200, which includes a scintillation crystal 102, the photodetectors 104 (hereinafter referred to as the photomultiplier tube(s) 104), the amplifiers 108, an analog pulse generator 106, which is functional for injecting a pulsed event into a data stream generated by the detector crystal 102, the analog-to-digital converters 110 (ADCs), the integrators 112 (which integrate the area under the pulse to determine its energy, which is equivalent to summing the number of scintillator photons generated by the gamma-ray interaction), and a computer 114 for performing the desired calculations.The photomultiplier tubes 104 are arranged in a predetermined arrangement with respect to a scintillation crystal (not shown), with which stimuli from a radiation field interact, thereby generating light photons in the crystal and causing the photomultiplier tubes to generate signals whose amplitudes are directly related to the relative distance of a gamma event from the photomultiplier tubes 104.

[0015] While the photodetectors described in detail herein are referred to as photomultiplier tubes, other photodetectors, such as silicon photomultipliers or silicon drift diodes, can also be used instead. The scintillation crystal 102 generally comprises sodium iodide. The amplifier 108 can be a preamplifier, preferably a transimpedance preamplifier. The analog-to-digital converter can be a flash analog-to-digital converter (FADC).

[0016] In Fig. 1. A gamma ray emanating from object 101 strikes the scintillation crystal 102, which interacts with it and releases a large number of scintillation photons. The scintillation radiation is collected in the several photomultiplier tubes 104. Light photons generated in the scintillation crystal decay with a characteristic lifetime, resulting in electronic signals with a distinctive shape characterized by a rapidly rising leading edge and a trailing edge with a time constant corresponding to the scintillator decay time. This is due to the Fig. Curve A, shown in Figure 1, illustrates this process. During the data acquisition process from object 101, the analog pulse generator 106 introduces analog pulses at a predetermined rate and amplitude (energy) into the data stream emanating from the photomultiplier tubes. The analog pulses can be introduced at any point before digitization by the ADC.

[0017] The analog-to-digital converter 110 digitizes the data arriving from the crystal and introduced by the analog pulse generator 106. The integrator 112 integrates the pulse segments received from the scintillation crystal 102 and the analog pulse generator 106. It integrates the subtractive portion of the pulse to determine its energy. This is equivalent to summing the number of scintillator photons generated by the gamma-ray interaction. The computer 114 counts the number of digitized pulses. Since the pulses are introduced at a given rate, the dead time is estimated by the computer 114 by counting the number of detected pulses in a final image, subtracting these pulses from the total number of pulses introduced in the final image, and calculating the percentage of counting loss or dead time.Some additional details about the circuitry associated with the Gamma Camera Head 200 are provided below.

[0018] The gamma camera head 200 includes electronic circuits (not shown) that receive the signals generated by the photomultiplier tubes 104, such signals being referred to below as input signals related to interactions of stimuli with the scintillation crystal 102. For each gamma (γ) event in the crystal 102, a distribution of scintillation light signals is provided to the PMTs 104. The spatial distribution of the scintillation light signals is a measure of the position in the crystal of the gamma event that causes such signals in the crystal.

[0019] The electronic circuits integrate the signals generated by the photomultiplier 104 and, for each group of input signals, produce a group of output signals from which the coordinates of a gamma event can be calculated. These coordinates can be spatial coordinates (xy coordinates) or energy coordinates (E coordinates). The electronic circuits thus process groups of input signals and generate groups of output signals that represent an interaction of a stimulus with the camera head 200.

[0020] System 100 also includes coordinate calculation circuits (not shown), which can be analog or digital in operation, for processing the output signals generated by electronic circuits (not shown). These circuits calculate the coordinates of the interaction represented by the group of output signals applied to the calculation circuits. The operation of the calculation circuits generates the x-coordinate, y-coordinate, and z-coordinate (i.e., the energy (E) coordinate) of each event. The latter represents the total energy of a gamma event in the crystal and is used to associate the spatial coordinate signals, x and y, with a gamma event corresponding to a specific energy.In other words, if the energy signal lies within a predetermined energy window of a single-channel analyzer, which is part of the coordinate calculation circuitry, it is concluded that the event that triggered the generation of the coordinates originated from a primary radiation stimulus with the scintillation crystal and not from secondary radiation. In such a case, the z-coordinate signal directs the x, y coordinates to an image memory stored in computer 114, indexing a register at an address in the image memory corresponding to the calculated coordinates. In this way, a record of the event is stored in the image memory. If the z-signal lies outside the energy window of a single-channel analyzer, no record is made in the image memory corresponding to that energy window.In one embodiment, it may be desirable to define 2 adjacent windows in order to record not only the primary gamma ray (photo-peak energy window) but also a lower energy scattering / secondary gamma ray to correct the image for the scattering radiation.

[0021] Once again with reference to Fig. 1 To estimate the counting loss, a series of analog pulses are injected into a data stream by an analog pulse generator 106 at the preamplifier 108. Fig. Figure 1, however, illustrates a single impulse, and for the purpose of discussing the analysis described in detail herein, reference should be made to a single impulse. While Fig. 1 shows the analog pulse that is introduced into the preamplifier; it can actually be introduced at any point before the digitization of the data stream into System 100. For example, with regard to Fig. 1, the analog pulse can also be introduced upstream of the flash analog / digital converter 110 and downstream of the preamplifier 108, if desired.

[0022] The concept employed here involves identifying the injected pulses in the final image. This can be achieved by selecting a spectrum where the total energy of the injected pulse lies within an energy window with few competing data events. Since this is occasionally insufficient for high accuracy, a further step of isolating the injected pulse can be performed. This next level of isolation involves selecting the pulse characteristics (which depend on the type of detector) such that the injected events appear in a spatial window located in a part of the image with very few competing data events. One possibility is a corner of the image where it is unlikely the patient would be located.In the example (gamma camera), there is a technique based on the properties of energy-weighted center-of-mass calculation according to the Anger principle to place the injected events beyond the field of view in which real events might potentially be located. The imaging field of view is defined as the region of image space occupied by the gamma rays interacting within the crystal.

[0023] In the case of an exemplary (Anger) camera, the spatial position of the analog pulse can be determined as described in detail below. After an analog pulse has been injected, it generates a steep peak just outside the imaging field of view (see Fig. 2) This occurs because the position weights of the photomultiplier tubes for a typical gamma camera (e.g., an Anger camera) become progressively larger than their true geometric value as the photomultiplier tube approaches the edge. Because all injected energy signals, e.g., i Since z is equal to the position calculation, the spatial position ([x, y] position) of the pulsed event can be calculated as an energy-weighted center of mass, as shown in equations (1) below: y^=∑i=1NZi⋅Wyi∑i=1NZi Zi≡Z Wyi=Wy→ N⋅Z⋅WyN⋅Z=Wyx^=∑i=1NZi⋅Wxi∑iNZi Zi≡Z→ Z⋅∑i=1NWxiN⋅Z=〈Wx〉 where Z i the energy signal (center of gravity) of the i-th photomultiplier tube is and W x and W yThe effective photomultiplier tube positions are given. The effective positions for an edge photomultiplier tube are significantly larger than their geometric values. N is the number of photomultiplier tubes with injected events (for example, 4 in ). Fig. 2, which is discussed in detail below).

[0024] The y-position of pulsed events should therefore be equal to the y-weight value of the topmost photomultiplier tube row, placing it outside the imaged field of view of normal events. The x-position is equal to the average of the x-weight values. This is in Fig. 2 illustrated below.

[0025] Fig. Figure 2 is an exemplary schematic illustration of an arrangement of photomultiplier tubes, with the position of the pulse generator tip shown in relation to an arrangement of photomultiplier tubes into whose corresponding data streams identical analog pulses were injected, as in Fig. 1 shown. As in Fig. As shown in Figure 2, the exemplary gamma camera has several photomultiplier tubes placed in a two-dimensional arrangement, with the signals from the different photomultiplier tubes being combined to provide an indication of the positions and energies of detected gamma rays. Fig. 2. In an exemplary embodiment, 4 identical pulses are injected into the data streams of the 4 uppermost photomultiplier tubes. The identical pulses are then processed to generate a pulse generator peak. As in Fig. As illustrated in Figure 2, the pulse generator tip, with the aforementioned x and y coordinates, is located outside the imaging field of view of normal events. The total effective energy of the pulse generator tip can be dissipated by injecting additional identical analog pulses into the data streams of additional photomultiplier tubes, which are located in the photomultiplier tube array of Fig. The 2 shown will be increased.

[0026] During the normal acquisition of data from an object 101, such as a patient, the analog pulse generator 106 is used (see Fig. 1) Analog pulses are injected into the photomultiplier data stream at a predetermined rate, as detailed above. The injected pulses are introduced at a rate that does not interfere with the normal acquired data. The rate of injected pulses is low enough that the dead time does not increase significantly, but high enough that the statistical Poisson distribution of the number of injected events actually counted in the image is small (approximately 1 to 2%). In short, the analog pulsed events are introduced at a position and with an energy that makes them isolable from normal data acquisition crystal events. The analog pulse generator tip generally has the same shape as the crystal pulse, so pulse separation can function normally. It should be noted that if the detector can perform the separation, the pulse generator tip has exactly the same shape as the crystal pulse.For detectors that do not perform the separation, this limitation may be less of an issue.

[0027] In short, it is desirable that peaks in the pulsed and normal crystal events be spatially and energetically distinguishable. In one embodiment, it is desirable that the pulse generator peak is located in a low-count region in both the image and the energy spectrum, and that the pulse generator is capable of injecting pulses whose energy can be varied depending on the isotope used in a given scan.

[0028] Following the data acquisition process, the analog-to-digital converter 110 digitizes the data, and the integrator 112 integrates the pulse segments received by the analog-to-digital converter 110. The computer 114 then separates those pulses whose pulse shapes overlap in time. The spatial (x, y) and energy (E) data are passed to the acquisition computer, and an image is formed. It calculates the number of injected pulses passing through the image using one or more windows, preferably two or more windows and preferably at least three windows. The windows can be spatial windows—an x-window for spatial identification of the injected pulse along the x-coordinate, a y-window for spatial identification, and an energy (E) window for energetic identification of the injected pulse.Knowing the number of analog pulses initially introduced and the number of pulses counted by combining the information received from the windows, the computer can calculate the amount of dead time or counting loss. The counting loss is calculated as shown in equation (2) below. Percentage count loss = (Number of introduced pulses − Number of detected pulses) × 100 / (Number of introduced pulses)

[0029] The counting loss or dead time of the gamma camera head 200 can then be corrected based on the value obtained from equation (2).

[0030] As stated above, the impulses can be introduced in an analog format, as in Fig. As shown in Figure 1, the data can be injected digitally after the data acquired from the scintillation crystal has been digitized. Digital injection is advantageous because it is not adversely affected by temperature fluctuations that might affect the instruments used in data acquisition (e.g., the preamplifier or the analog-to-digital converter). With digital injection, a digital pulse is introduced during the summation phase of the process; that is, it is generally introduced after the data obtained from the crystal has been digitized. In one embodiment, the digital pulse is generally introduced either at the integrator or further downstream of the integrator, after the digitization of the original data obtained from the crystal.

[0031] Fig. Figure 3 is a schematic diagram showing the digital injection of a pulse into the data stream at the integrator 112. The system 100 of Fig. 3 is similar to system 100 of Fig. 1, except that it has a digital pulse generator 107 instead of the analog pulse generator 106. While the system 100 of Fig. 3 illustrates the digital pulse generator 107 and the system 100 of Fig. As illustrated by the analog pulse generator, it is possible for a system to have both a digital pulse generator and an analog pulse generator, and to use both either simultaneously or sequentially.

[0032] As in Fig.As shown in Figure 3, a digital pulse is injected directly into the integrator 112, located downstream of the analog-to-digital converter 110, via a digital pulse generator 107. The injected digital pulse has the same properties as the injected analog pulse described in detail above—that is, the injected pulse has a shape identical to a normal gamma event pulse and is introduced at a controllable rate (and has a selected amplitude) to place it in a low-count region of the spectrum of an isotope of interest, etc. All these properties and the detection of the peak using space and / or energy windows are not repeated here for the sake of brevity.

[0033] In one embodiment, the digitally injected pulses can be scaled to lie within the energy window of the imaged isotope(s), or alternatively, they can be scaled to lie outside the energy window of the imaged isotope(s). These digitally injected pulses can lie either within or outside the imaging field of view. All possible combinations of the energy window and the imaging field of view are considered here. In other words, the scaling of the digital gamma event injection pulses can be performed such that the injected pulse is placed within the energy window of the imaged isotope, but outside the imaging field of view.Alternatively, the scaling of the digital gamma event injection pulses can be performed such that the digital pulse is placed outside the energy window(s) of the imaged isotopes and outside the imaging field of view. In other embodiments, the scaling of the digital gamma event injection pulse can be performed such that the digital pulse is placed within the energy window(s) of the imaged isotopes and outside the imaging field of view of the photomultiplier tubes. In yet another embodiment, the scaling of the digital gamma event injection pulse can be performed such that the digital pulse is placed outside the energy window(s) of the imaged isotopes and within the imaging field of view.

[0034] The use of analog and / or digital injected pulses is advantageous because count loss can be estimated without the need for lookup tables. Temperature fluctuations of equipment components do not affect the image position of the injected events, even if they distribute the position of the injected pulse generator energy peak. The disclosed system measures count losses throughout the entire acquisition chain. Because the pulses are "typical" and identical in form to normal gamma pulses, they encounter the same loss sources throughout the entire acquisition chain.

[0035] It will be understood that, although the terms "first," "second," "third," etc., may be used herein to describe different elements, components, regions, layers, and / or sections, these elements, components, regions, layers, and / or sections are not meant to be limited by these terms. These terms are used only to distinguish one element, component, region, layer, or section from another. Thus, "a first element," "component," "region," "layer," or "section" discussed below could also be referred to as a second element, component, region, layer, or section without departing from the teachings herein.

[0036] The terminology used herein serves only to describe certain embodiments and is not intended to be restrictive. As used herein, singular forms such as "a" or "an" and "the" are to include the plural forms unless the context clearly indicates otherwise. Furthermore, the terms "comprises" and / or "comprehensive" or "includes" and / or "containing," when used in this specification, shall specify the presence of the specified features, regions, integers, steps, operations, elements, and / or components, but shall not exclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and / or groups thereof.

[0037] Furthermore, relative terms, such as "lower" or "under" and "upper" or "highest," may be used herein to describe the relationship of one element to other elements, as illustrated in the figures. It will be understood that relative terms are intended to include different orientations of the device in addition to the orientation illustrated in the figures. For example, if the device is inverted in one of the figures, elements described as being on the "lower" side of other elements would then be oriented on the "upper" sides of those other elements. The exemplary term "lower" can therefore include both a "lower" and a "higher" orientation, depending on the specific orientation of the figure.Similarly, if the device is turned upside down in one of the figures, elements described as being "under" or "below" other elements would then be aligned "above" the other elements. The exemplary terms "under" or "below" can therefore include both an alignment above and below.

[0038] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as generally understood by an average person of knowledge in the field to which this revelation belongs. Furthermore, it shall be understood that terms such as those defined in commonly used dictionaries are to be interpreted as having a meaning consistent with their meaning in the context of the relevant technology and the present revelation, and not in an idealized or overly formal sense, unless expressly defined herein.

[0039] Exemplary embodiments are described herein with reference to cross-sectional illustrations, which are schematic illustrations of idealized embodiments. As such, deviations from the shapes of the illustrations are to be expected due to, for example, manufacturing techniques and / or tolerances. Thus, the embodiments described herein should not be interpreted as being limited to the respective shapes of regions as illustrated herein, but should include deviations in shapes that are, for example, caused by the manufacturing process. For example, a region illustrated or described as flat may typically exhibit rough and / or nonlinear features. Furthermore, illustrated acute angles may be rounded.Therefore, the regions illustrated in the figures are schematic in nature and their shapes are not intended to represent the precise shape of a region and are not intended to limit the scope of the claims presented.

[0040] The features of the system and method have been disclosed, and further variations will be obvious to a person skilled in the art. All such variations are said to be within the scope of the appended claims. For the true scope of the disclosed method, reference should be made to the appended claims rather than to the foregoing specification.

[0041] The functions and process steps disclosed herein can be performed automatically or wholly or partially in response to a user command. An activity (including a step) that is performed automatically occurs in response to an executable instruction or device operation without direct initiation of the activity by a user.

[0042] The disclosed systems and processes are not exclusive. Other systems and processes can be derived in accordance with the principles of the invention to achieve the same objectives. Although this invention has been described with reference to certain embodiments, it should be understood that the embodiments and variations shown and described herein serve only as illustrations. Modifications to the current design can be implemented by a person skilled in the art without departing from the scope of the invention. In alternative embodiments, the processes and applications can be located on one or more (e.g., distributed) processing devices that access a network connecting the elements of the disclosed system.Furthermore, any functions and steps provided in this disclosure may be implemented in hardware, software, or a combination of both, and may be located on one or more processing devices situated at any position in a network connecting the elements of the disclosed system or any other connected network, including the Internet.

[0043] The term "and / or" is intended to mean both "and" and "or" in this context. For example, "A and / or B" should mean A, B, or A and B.

[0044] The transitional term "comprehensive" includes the transitional terms "essentially consisting of" and "consisting of", and these can be replaced by "comprehensive".

[0045] While this disclosure describes exemplary embodiments, the person skilled in the art will understand that various modifications can be made and equivalents can be used for elements thereof without departing from the scope of the disclosed embodiments. Furthermore, many modifications can be made to adapt a particular situation or material to the teachings of this disclosure without departing from its essential scope. Therefore, it is intended that this disclosure is not limited to the specific embodiment disclosed as the best considered way of carrying out this disclosure.

Claims

[1] Method for estimating count loss in a gamma camera, the method comprising: Injecting a synthetic pulse at a given rate into a data stream originating from a photodetector, wherein both analog and digital synthetic pulses are injected simultaneously or sequentially into the data stream, wherein analog synthetic pulses are injected prior to digitization of the data stream, and wherein digital synthetic pulses are injected after digitization of the data stream; Integrating the synthetic pulse into the data stream to form an integrated data stream; Determining a number of synthetic pulses in the integrated data stream that are passed on to a final image; and Determining the counting loss from equation (2) Percentage count loss = (Number of introduced pulses − Number of detected pulses) × 100 / (Number of introduced pulses) [2] Method according to claim 1, wherein the analog pulse is generated by an analog pulse generator located upstream of an amplifier or upstream of an analog-to-digital converter. [3] Method according to claim 1, wherein the digital pulse is introduced via a digital pulse generator located upstream of an integrator and downstream of an analog-to-digital converter. [4] Method according to claim 1, wherein the number of detected pulses is counted by one or more windows that detect pulses spatially or by means of an energy range. [5] Method according to claim 4, wherein at least two windows spatially detect impulses and at least one window detects impulses by means of a defined energy range. [6] Method according to claim 1, wherein the synthetically injected pulse has a substantially identical shape to a normal gamma pulse event. [7] Method according to claim 1, wherein the synthetically injected pulse is injected in a region of low counts in a spectrum of an isotope of interest. [8] Method according to claim 1, wherein the photodetector is a photomultiplier tube. [9] Method according to claim 1, wherein the synthetically injected pulse is injected outside an imaging field of view. [10] Method according to claim 1, wherein the synthetically injected pulse is injected within an imaging field of view. [11] Method according to claim 5, wherein a spatial position of the synthetic impulse can be calculated using equation (1) as follows: y^=∑i=1NZi⋅Wyi∑i=1NZi Zi≡Z Wyi=Wy→ N⋅Z⋅WyN⋅Z=Wyx^=∑i=1NZi⋅Wxi∑iNZi Zi≡Z→ Z⋅∑i=1NWxiN⋅Z=〈Wx〉 where Z i the energy signal of the i-th photodetector is and W x and W ythe effective photodetector positions are and where N is the number of photodetectors with injected events. [12] System which includes the following: a scintillation crystal; a photodetector capable of generating a data stream based on photons received by the scintillation crystal; an amplifier capable of amplifying voltages based on the data stream received from the photodetector; an analog-to-digital converter capable of converting the data stream into a digital data stream; an integrator; and a pulse generator; wherein the pulse generator is capable of introducing a synthetic pulse into the data stream generated by the photodetector; and wherein the integrator is capable of integrating the data stream generated by the photodetector and the synthetic pulse generated by the pulse generator; wherein the system is capable of performing the following: Injecting synthetic pulses at a given rate into the data stream emanating from the photodetector, wherein both analog and digital synthetic pulses are injected simultaneously or sequentially into the data stream, wherein analog synthetic pulses are injected by an analog pulse generator prior to digitization of the data stream by the analog-to-digital converter, and wherein digital synthetic pulses are injected by a digital pulse generator after digitization of the data stream by the analog-to-digital converter; Integrating the synthetic pulse into the data stream to form an integrated data stream; Determining a number of synthetic pulses in the integrated data stream, which are passed on to form a final image; and Determining the counting loss from equation (2) Percentage count loss = (Number of introduced pulses − Number of detected pulses) × 100 / (Number of introduced pulses) [13] System according to claim 12, wherein the photodetector is a photomultiplier tube, a silicon photomultiplier or a silicon drift diode. [14] System according to claim 12, wherein the scintillator crystal comprises NaI. [15] System according to claim 12, wherein the amplifier is a transimpedance preamplifier.

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