WO2015028610A1

WO2015028610A1 - A method and a system for determining parameters of a position of a gamma quantum reaction within a scintillator detector of a pet scanner

Info

Publication number: WO2015028610A1
Application number: PCT/EP2014/068382
Authority: WO
Inventors: Pawel Moskal; Lukasz KAPLON
Original assignee: Uniwersytet Jagiellonski
Priority date: 2013-08-30
Filing date: 2014-08-29
Publication date: 2015-03-05
Also published as: PL227661B1; PL405188A1

Abstract

A method for determining parameters of a position of a gamma quantum reaction within a scintillator detector of a PET scanner wherein the signal measured in the scintillator is transformed using at least two converters into electric measurement signals, the method comprising the steps of: accessing a database (130) comprising calibration curves p(S) that define the parameters of the reaction position (p) depending on a value of a signal ratio (S); calculating, by means of a processor (120), the parameters of the reaction position (p) based on the value of the signal ratio (S) and the calibration curve p(S); wherein the signal ratio (S) is determined as a ratio of the electric measurement signals measured using at least two converters (111A, 111B; 112A, 112B) having different quantum efficiency spectra.

Description

A METHOD AND A SYSTEM FOR DETERMINING PARAMETERS OF A POSITION OF A GAMMA QUANTUM REACTION WITHIN A SCINTILLATOR

DETECTOR OF A PET SCANNER DESCRIPTION

TECHNICAL FIELD

The present disclosure relates to a method and a system for determining position of ionization within scintillator detectors of PET scanners.

BACKGROUND

Images of the interiors of bodies may be acquired using various types of tomographic techniques, which involve recording and measuring radiation from tissues and processing acquired data into images.

One of these tomographic techniques is positron emission tomography (PET), which involves determining spatial distribution of a selected substance throughout the body and facilitates detection of changes in the concentration of that substance over time, thus allowing to determine the metabolic rates in tissue cells.

The selected substance is a radiopharmaceutical administered to the examined object (e.g. a patient) before the PET scan. The radiopharmaceutical, also referred to as an isotopic tracer, is a chemical substance having at least one atom replaced by a radioactive isotope, e.g. ¹¹C, ¹⁵O, ¹³N, ¹⁸F, selected so that it undergoes radioactive decay including the emission of a positron (antielectron). The positron is emitted from the atom nucleus and penetrates into the object's tissue, where it is annihilated in reaction with an electron present within the object's body.

The phenomenon of positron and electron annihilation, constituting the principle of PET imaging, consists in converting the masses of both particles into energy emitted as annihilation photons, each having the energy of 51 1 keV. A single annihilation event usually leads to formation of two photons that diverge in opposite directions at the angle of 180° in accordance with the law of conservation of the momentum within the electron-positron pair's rest frame, with the straight line of photon emission being referred to as the line of response (LOR). The stream of photons generated in the above process is referred to as gamma radiation and each photon is referred to as gamma quantum to highlight the nuclear origin of this radiation. The gamma quanta are capable of penetrating matter, including tissues of living organisms, facilitating their detection at certain distance from object's body. The process of annihilation of the positron-electron pair usually occurs at a distance of several millimeters from the position of the radioactive decay of the isotopic tracer. This distance constitutes a natural limitation of the spatial resolution of PET images to a few millimeters.

A PET scanner comprises detection devices used to detect gamma radiation as well as electronic hardware and software allowing to determine the position of the positron-electron pair annihilation event on the basis of the position and time of detection of a particular pair of the gamma quanta. The radiation detectors are usually arranged in layers forming a ring around object's body and are mainly made of an inorganic scintillation material. A gamma quantum enters the scintillator, which absorbs its energy to re-emit it in the form of light (a stream of photons). The mechanism of gamma quantum energy absorption within the scintillator may be of dual nature, occurring either by means of the Compton's effect or by means of the photoelectric phenomenon, with only the photoelectric phenomenon being taken into account in calculations carried out by current PET scanners. Thus, it is assumed that the number of photons generated in the scintillator material is proportional to the energy of gamma quanta deposited within the scintillator.

When two annihilation gamma quanta are detected by a pair of detectors at a time interval not larger than several nanoseconds, i.e. in coincidence, the position of annihilation point along the line of response may be determined, i.e. along the line connecting the detector centers or the points within the scintillator strips where the energy of the gamma quanta was deposited. The coordinates of annihilation position are obtained from the difference in times of arrival of two gamma quanta to the detectors located at both ends of the LOR. In the prior art literature, this technique is referred to as the time of flight (TOF) technique and the PET scanners utilizing time measurements are referred to as TOF-PET scanners. This technique requires that the scintillator has a time resolution of a few hundred picoseconds.

Currently, the state of the art methods of determining the positions of interactions of the gamma quanta in positron emission tomography are based on the measurements of charges of signals generated in vacuum tube photomultipliers, silicon photomultipliers, or avalanche diodes optically connected to inorganic crystals notched into smaller elements. Position of the gamma quantum reaction is determined with the accuracy of the smaller crystal element size on the basis of the differences in charges of the signals from different converters optically connected to the same crystal. In the state of the art PET scanners, reconstruction of the set of LOR and TOF data is based on the relationships between charges and times of signals recorded for a particular event without reference to external signals.

Currently, the state of the art methods of recording the gamma quanta in positron emission tomography involve the use of inorganic crystals optically connected to photomultipliers that facilitate determination of the position of reaction of the gamma quantum with the accuracy of crystal size on the basis of the differences in the charges of the signals from individual photomultipliers. Usually, a solution is applied that involves four photomultipliers connected to one side of the crystal, allowing to determine the reaction position on the basis of "Anger logic". In most recent solutions, standard vacuum tube photomultipliers are replaced by silicon photomultiplier or avalanche diode matrices. Solutions of this type were described in the article titled "Whole-Body MR/PET Hybrid Imaging" (Quick H. et al., MAGNETOM Flash 1/201 1 p. 88-100) as well as in a US patent US7626389. Some solutions allow for determination of the depth of interactions of the gamma quanta by means of simultaneous use of two or three interconnected layers of different crystals with photomultipliers connected to one end. This was described for instance in the article titled "A Modular VME Or IBM PC Based Data Acquisition System For Multi-Modality PET/CT Scanners Of Different Sizes And Detector Types " (D.B. Crosetto, The Internet Journal of Medical Technology. 2003 Vol. 1 No. 1 .). Simultaneous detection of light from two sides with one avalanche diode matrix on one side and an Anger's system of photomultipliers on the other side is also possible.

A PCT application WO 201 1/0081 19 discloses a strip device and a method to determine the gamma quanta reaction positions and times as well as the use of said device in positron emission tomography. In this invention, photomultipliers are not arranged around the diagnostic chamber and thus the solution allows for the use of multiple scintillation layers so as to facilitate determination of the depth of interaction with the accuracy of the thickness of scintillator strips.

A PCT application WO 201 1/0081 18 discloses an invention related to a matrix device and a method to determine the gamma quanta reaction positions and times as well as the use of said device in positron emission tomography. The matrix tomography scanner disclosed in this application allows for determination of the depth of interactions on the basis of distribution of signal amplitudes in a matrix of photomultipliers surrounding a scintillator plate.

It would be expedient to develop a method for determination of the position of interaction of the gamma quantum that might be used either independently of or in combination with the aforementioned methods, allowing to achieve a higher imaging precision in positron emission tomography and other imaging techniques that involve the recording of ionizing radiation. SUMMARY

There is presented a method for determining parameters of a position of a gamma quantum reaction within a scintillator detector of a PET scanner wherein the signal measured in the scintillator is transformed using at least two converters into electric measurement signals, the method comprising the steps of: accessing a database comprising calibration curves p(S) that define the parameters of the reaction position (p) depending on a value of a signal ratio (S); calculating, by means of a processor, the parameters of the reaction position (p) based on the value of the signal ratio (S) and the calibration curve p(S); wherein the signal ratio (S) is determined as a ratio of the electric measurement signals measured using at least two converters having different quantum efficiency spectra.

Preferably, the parameters of the reaction position (p) comprise at least one (x, y) coordinate.

Preferably, the parameters of the reaction position (p) comprise a depth of interaction.

Preferably, the signal ratio (S) is determined as a ratio of the amplitudes of the electric measurement signals.

There is also presented a system for determining parameters of a position of a gamma quantum reaction within a scintillator detector of a PET scanner wherein the signal measured in the scintillator is transformed using at least two converters into electric measurement signals, the system comprising: a database comprising calibration curves p(S) that define the parameters of the reaction position (p) depending on a value of a signal ratio (S); a processor configured to calculate the parameters of the reaction position (p) from values of the signal ratio (S) on the basis of calibration curve p(S); wherein the signal ratio is the ratio of the electric measurement signals measured using at least two converters having different quantum efficiency spectra.

Preferably, the scintillator detector comprises scintillator strips to which pairs of photomultipliers are connected at opposite ends, the photomultipliers having different quantum efficiency spectra within each pair.

Preferably, the scintillator detector comprises a photomultiplier matrix assembly with neighboring photomultipliers (E, F) having different quantum efficiency spectra.

The presented method and system make use of the changes of light attenuation as the function of wavelength and facilitates determination of the distance between the light pulse generation position and the converters. The invention facilitates determination of the depth of interactions of the gamma quanta in positron emission tomography. BRIEF DESCRIPTION OF FIGURES

Example embodiments are presented on a drawing wherein:

Fig. 1 presents the dependence of the absorption on the wavelength and the emission spectrum of scintillator UPS-923A for different distances between the interaction position and the detector;

Fig. 2 presents a graph comparing the emission spectra of selected scintillators with quantum efficiencies of several selected converters;

Fig. 3 presents a general outline of a system to determine the ionization position according to the invention using the strip detector as an example.

Fig. 4 and 5 present example embodiments of the invention.

DETAILED DESCRIPTION

The solution according to the invention relates to determination of the position of interaction of the gamma quantum in a scintillator detector using at least two photomultipliers or other converters that are able to transfer light pulses into electric pulses and are characterized by different dependencies of quantum efficiencies and the recorded photon wavelengths.

Absorption of photons in scintillators was observed to be strongly dependent on the wavelength. The shorter the wavelength, the higher the photon suppression. For example, photons with higher energies ("violet" photons) are absorbed more than photons with lower energies ("red" photons). Therefore, the energy spectrum (referred to as "color") of the light pulse being propagated within the scintillator is changed, and it shifts towards red color as presented in Fig. 1 published by V. Senchyshyn et al. in the article titled "Accounting for self-absorption in calculation of light collection in plastic scintillators", Nuclear Instruments and Methods in Physics Research A 566 (2006) 286. Therefore, the further away the photomultiplier is located from the reaction position, the higher the ratio of photons in the red part of the spectrum to photons in the violet part of the spectrum in the light pulse incident on the photomultiplier. If the distance between the position of reaction of the gamma quantum from the converter is large, than the pulse approaching the converter is reduced and shifted from violet through blue and green towards yellow, as presented in Fig. 1 . If one of the converters is characterized by a higher efficiency of recording photons within the green range as compared to the violet range (for example, a silicon avalanche photodiode: SI_APD) and the other converter is inversely characterized by a higher efficiency within the violet range as compared to the green range (for example, a R5320 photomultiplier), then the likelihood of the photon being recorded by the first converter (SI APD) will increase and the likelihood of the photon being recorded by the other converter (R5320) will decrease along with the distance from the gamma quantum reaction position, as shown in Fig. 2. In this case, the ratio of the charge of the signal generated by the SI_APD converter to the charge of the signal generated by the R5320 converter will increase along with the distance between the gamma quantum reaction position and the converters. Thus, the ratio of signals generated by converters characterized by different quantum efficiency spectra is a measure of the distance between the ionization position and the converters.

An advantage of determination of the position of ionization within the scintillator on the basis of the ratio of signals from converters characterized by different quantum efficiency is that the method is independent of the value of energy deposited in the scintillator as the result of ionization.

The method is characterized in that the position of the reaction within the scintillator is determined from the ratio of charges of signals generated by at least two converters optically connected to the scintillator, whereas the quantum efficiency spectra of said converters must be different and the relationship between the ratio of charges from different converters and the distance is determined from previous calibration by means of the measurements of ratios for known detector irradiation positions.

Information on the position and time of annihilation of the positron- electron pair within object's body is carried by two gamma quanta recorded in scintillating detectors of PET scanners. The result of reactions of the gamma quanta within the scintillating detectors 101 are light pulses that, after reaching the scintillator edges, are transformed into electric pulses by means of photomultipliers 1 1 1 A, 1 1 1 B, 1 12A, 1 12B. The measurement signals from photomultipliers are transferred into electronic readout systems (readers) 1 10 that facilitate the measurement of the signal charges and times of crossing preset voltage thresholds.

In the presented example, a strip detector is used as disclosed in the PCT application WO 201 1/0081 19.

Two photomultipliers are attached to each side of the scintillator strip 101 having the length of L: photomultipliers 1 1 1 A and 1 1 1 B on the left and 1 12A and 1 12B on the right. An important fact is that photomultipliers 1 1 1A and 1 12A are characterized by different quantum efficiency than photomultipliers 1 1 1 B and 1 12B. Determination of position x of radiation's interaction within the scintillator along the axis parallel to L, said position x being a reaction position parameter p, may be determined from the ratio of signal amplitudes or charges S1 = Si e / S111A and independently from S2 = S 2B / S 2A, where SI HA, SI H B, S112A, S112B are amplitudes or charges of measurement signals as measured by the respective photomultipliers. Buffer 1 15 may store the information on charges SI HA, SI HB, SH2A, SH2B or information on the ratios of amplitudes or charges S1 , S2.

Processor 120 is used to calculate the reaction position parameters p from signal ratio S values (in this case characterized, among others, by the values SI HA, SI HB, SH2A, SH2B) on the basis of calibration curve p(S). Calibration curves p(S) for particular converter pairs may be stored in the database. The processor receives the signal ratio S via the signal buffer 1 15. The signal buffer 1 15 may transmit data into processor 120 either in real time or after a delay - for instance, immediately after completion of the measurement or after a certain time from the measurement (in such case, buffer 1 15 is the memory for storing the signal ratio S values).

Reference calibration curves p(S) recorded in database 130 are generated at the stage of tomography scanner calibration. Curve generation involves scanning the strip 101 using a collimated beam of annihilation gamma quanta with a profile smaller than the positional resolution to be achieved. For instance, a beam of the width of 1 mm is generated and moved along the strip while recording measurements and adding tags indicating the irradiation position to each pulse data. Scanning is performed using the source 102 of annihilation radiation located within a collimator that may rotate around the scanner axis and move along this axis so as to permit irradiation of every point within the detector with a beam of an appropriately chosen profile.

Fig. 4 presents an example use of "Anger logic" as known from most state of the art PET scanners. Identification of the crystal in which the gamma quantum reaction took place is achieved by means of an assembly of four photomultipliers as presented schematically in Figure 4. The x coordinates are determined from the differences in amplitudes of signals from photomultipliers A, C and B, D while the y coordinates are determined accordingly from the differences in amplitudes of signals from photomultipliers A, B and C, D:

x = ((B+D) - (A+C)) / (A+B+C+D); y = ((A+B) - (C+D)) / (A+B+C+D) (1 )

Application of the method according to the invention consists in the use of two types of photomultipliers, for example in the use of a system where photomultipliers A and D are characterized by a quantum efficiency spectrum different from that of photomultipliers C and B. Thus, the reaction position parameters - in this case, the reaction position coordinates x and y may be determined from formula (1 ) and the other reaction position parameter, i.e. DOI, may be determined for instance from the ratio S = (B+C) / (A+D) following previous calibration of the DOI vs. S relationship for every position x, y.

Fig. 5 presents an example of the use of the solution of the invention in a matrix PET scanner disclosed in the PCT application WO 201 1/0081 18. Individual types of photomultipliers are marked by lighter (E) and darker (F) colors. In this case, the depth of interaction as a reaction position parameter can be calculated for example from the ratio S = sigma_over_i(E_i) / sigma_over_j(FJ), where indices "i" and "j" are used to number the photomultipliers attached to the outer surface of the plate and E_i and FJ correspond to the charges or amplitudes of signals generated in the i_th and j_th photomultiplier, respectively.

In addition, the method of the invention facilitates determination of other reaction position parameters, particularly the position along the plate on the basis of the charges or amplitudes of signals generated by photomultipliers attached to the sides of the plate in a manner analogous to that in Fig. 3. In this case, ratios may be determined as S1 = sigma_over_i(E1_i) / sigma_over_j(F1 J), where E1_i and F1 J are amplitudes of charges of signals measured on the right side of the plate and, analogously, S2 = sigma_over_i(E2_i) / sigma_over_j(F2J), where E2_i and F2J are amplitudes of charges of signals measured on the left side of the plate. The S3 and S4 ratios for signals at the front and at the back of the plate may be determined in a similar manner. Next, following previous calibration, the method described in this invention may be used to determine the reaction position parameters that determine the (x, y, z) coordinates of the reaction of the gamma quantum in the scintillator on the basis of the S, S1 , S2, S3, and S4 values, where S allows determination of the depth of interaction, S1 and independently S2 allow determination of one coordinate along the plate, while S3 and independently S4 allow determination of the second coordinate.

While the technical solutions presented herein have been depicted, described, and defined with reference to particular preferred embodiment(s), such references and examples of implementation in the foregoing specification do not imply any limitation on the invention. Various modifications and changes may be made thereto without departing from the scope of the technical solutions presented. The presented embodiments are given as example only, and are not exhaustive of the scope of the technical solutions presented herein. Accordingly, the scope of protection is not limited to the preferred embodiments described in the specification, but is only limited by the claims that follow.

Claims

1 . A method for determining parameters of a position of a gamma quantum reaction within a scintillator detector of a PET scanner wherein the signal measured in the scintillator is transformed using at least two converters into electric measurement signals, the method comprising the steps of:

- accessing a database (130) comprising calibration curves p(S) that define the parameters of the reaction position (p) depending on a value of a signal ratio (S);

- calculating, by means of a processor (120), the parameters of the reaction position (p) based on the value of the signal ratio (S) and the calibration curve p(S);

wherein the signal ratio (S) is determined as a ratio of the electric measurement signals measured using at least two converters (1 1 1 A, 1 1 1 B; 1 12A, 1 12B) having different quantum efficiency spectra.

2. The method according to claim 1 wherein the parameters of the reaction position (p) comprise at least one (x, y) coordinate.

3. The method according to claim 1 wherein the parameters of the reaction position (p) comprise a depth of interaction (DOI).

4. The method according to claim 1 wherein the signal ratio (S) is determined as a ratio of the amplitudes of the electric measurement signals.

5. A system for determining parameters of a position of a gamma quantum reaction within a scintillator detector of a PET scanner wherein the signal measured in the scintillator is transformed using at least two converters into electric measurement signals, the system comprising:

- a database (130) comprising calibration curves p(S) that define the parameters of the reaction position (p) depending on a value of a signal ratio (S); - a processor (120) configured to calculate the parameters of the reaction position (p) from values of the signal ratio (S) on the basis of calibration curve p(S);

- wherein the signal ratio is the ratio of the electric measurement signals measured using at least two converters (1 1 1 A, 1 1 1 B; 1 12A, 1 12B) having different quantum efficiency spectra.

6. The system according to claim 5 wherein the scintillator detector comprises scintillator strips to which pairs of photomultipliers (1 1 1 A, 1 1 1 B; 1 12A, 1 12B) are connected at opposite ends, the photomultipliers having different quantum efficiency spectra within each pair.

7. The system according to claim 5 wherein the scintillator detector comprises a photomultiplier matrix assembly with neighboring photomultipliers (E, F) having different quantum efficiency spectra.