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US-12625285-B2 - 3D semiconductor detector system

US12625285B2US 12625285 B2US12625285 B2US 12625285B2US-12625285-B2

Abstract

A detector system for molecular imaging of a radionuclide comprises a 3D semiconductor detector comprising a plurality of sensor stacks of sensors made of a semiconductor material having an average atomic number Z below 40. A read-out circuitry connected to the pixels is configured to output, for each interaction induced by an incident gamma ray in the detector, a signal representative of a time, a position and an energy of the interaction in the detector. The interactions in the detector belonging to a same event induced by the incident gamma ray are predicted based on the output signals and used to estimate a direction of the incident gamma ray and reconstruct an image based on the estimated directions of incident gamma rays.

Inventors

  • Mats Danielsson
  • Elias RIEGER

Assignees

  • SiSnap AB

Dates

Publication Date
20260512
Application Date
20240514

Claims (20)

  1. 1 . A detector system for molecular imaging of a radionuclide, comprising: a three-dimensional (3D) semiconductor detector comprising a plurality of sensor stacks, wherein each sensor stack of the plurality of sensor stacks comprises a plurality of semiconductor sensors each comprising a plurality of pixels, wherein the plurality of semiconductor sensors is made of a semiconductor material having an average atomic number Z below 40; a read-out circuitry connected to the pixels in the 3D semiconductor detector and configured to output, for each pixel along an electron track in the 3D semiconductor detector, a pixel value representative of an energy deposited at the pixel by a Compton recoil electron along the electron track in the 3D semiconductor detector, wherein the Compton recoil electron is created by a Compton scatter interaction induced by an incident gamma ray in the 3D semiconductor detector; at least one processor; and at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to, if an estimated energy of the Compton recoil electron is below a first threshold value, predict a position of a start of the electron track based on a center of a charge cloud in the 3D semiconductor detector by a Gaussian fit to the pixel values output by the read-out circuitry by: summing pixel values over a first dimension in the 3D semiconductor detector to obtain a first one-dimensional projection, summing pixel values over a second dimension in the 3D semiconductor detector to obtain a second one-dimensional projection, fitting a first Gaussian function to the first one-dimensional projection, fitting a second Gaussian function to the second one-dimensional projection, determining a first coordinate in the first dimension in the 3D semiconductor detector based on a mean of the first Gaussian function, determining a second coordinate in the second dimension in the 3D semiconductor detector based on a mean of the second Gaussian function, and predicting the position of a start of the electron track based on the first coordinate and the second coordinate.
  2. 2 . The detector system according to claim 1 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to, if the estimated energy of the Compton recoil electron is below the first threshold value: estimate a third coordinate in a third dimension in the 3D semiconductor detector based on a width of the first Gaussian function and a width of the second Gaussian function; and predict the position of the start of the electron track based on the first coordinate, the second coordinate and the third coordinate.
  3. 3 . The detector system according to claim 2 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to, if the estimated energy of the Compton recoil electron is below the first threshold value: compare the value of the third coordinate with a threshold value representing a physical constraint of the plurality of semiconductor sensors in the third dimension in the 3D semiconductor detector; and determine an updated value of the third coordinate based on the mean of the first Gaussian function and the mean of the second Gaussian function if the value of the third coordinate is below 0 or above the threshold value.
  4. 4 . The detector system according to claim 1 , wherein the first threshold value is 100 keV.
  5. 5 . The detector system according to claim 1 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to predict the position of the start of the electron track by identifying a pixel position associated with least amount of energy deposition along the electron track in the 3D semiconductor detector if the estimated energy of the Compton recoil electron is above a second threshold value.
  6. 6 . The detector system according to claim 5 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to, if the estimated energy of the Compton recoil electron is above the second threshold value: perform linear regression to fit a line to the pixel values; compare a pixel value at a start of the line with a pixel value at an end of the line; and predict the position of the start of the electron track as the one of the start of the line and the end of the line having a pixel value representing a lowest amount of energy deposited at the pixel.
  7. 7 . The detector system according to claim 5 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to, if the estimated energy of the Compton recoil electron is above the second threshold value: sum, for each pixel having a pixel value above a minimum threshold value, pixel values within a pixel area of N×M pixels centered at the pixel; and predict the position of the start of the electron track based on a position of the pixel having the smallest sum.
  8. 8 . The detector system according to claim 7 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to, if the estimated energy of the Compton recoil electron is above the second threshold value: set each pixel value below the minimum threshold value to zero; and sum, for each pixel having a non-zero pixel value, pixel values within the pixel area of N×M pixels centered at the pixel.
  9. 9 . The detector system according to claim 8 , wherein N=2×k+1, M=2×h+1 and k, h are each a positive integer equal to or larger than one.
  10. 10 . The detector system according to claim 9 , wherein k=h.
  11. 11 . The detector system according to claim 5 , wherein the second threshold value is 100 keV.
  12. 12 . The detector system according to claim 1 , wherein each semiconductor sensor of the plurality of semiconductor sensors comprises: a plurality of electrodes; at least one counter electrode; and an electric field circuitry connected to the plurality of electrodes and the least one counter electrode and configured to apply a bias voltage between each electrode of the plurality of electrodes and a counter electrode of the at least one counter electrode.
  13. 13 . The detector system according to claim 1 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to: estimate a momentum of a Compton recoil electron based on the pixel values output by the read-out circuitry; and calculate a kinematic constraint for the Compton scatter interaction based on the estimated momentum of the Compton recoil electron.
  14. 14 . The detector system according to claim 13 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to estimate the momentum of the Compton recoil electron by a linear fit to a first part of the electron track in the 3D semiconductor detector.
  15. 15 . The detector system according to claim 13 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to calculate an opening angle of a constrained cone based on the estimated momentum of the Compton recoil electron; and the constrained cone restricts a volume in the 3D semiconductor detector, within which a next interaction belonging to the same event induced by the incident gamma ray is allowed to take place.
  16. 16 . The detector system according to claim 1 , wherein the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to estimate a direction of the incident gamma ray by a maximum likelihood estimation based on the pixel values output by the read-out circuitry.
  17. 17 . The detector system according to claim 1 , wherein: the read-out circuitry is configured to output, for each interaction induced by an incident gamma ray in the 3D semiconductor detector, a signal representative of a time, a position and an energy of the interaction in the 3D semiconductor detector; and the at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to: predict, based on the pixel output by the read-out circuitry, the interactions in the 3D semiconductor detector belonging to a same event induced by the incident gamma ray; estimate, based on the predicted interactions in the 3D semiconductor detector belonging to the same event, a direction of the incident gamma ray inducing the same event; and reconstruct an image based on the estimated directions of incident gamma rays.
  18. 18 . The detector system according to claim 1 , wherein the plurality of semiconductor sensors comprises complementary metal oxide semiconductor (CMOS) electronics comprising an application specific integrated circuit (ASIC) comprising analogue to digital converts (ADCs) and the read-out circuitry.
  19. 19 . The detector system according to claim 18 , wherein each semiconductor sensor of the plurality of semiconductor sensors is a monolithic semiconductor sensor integrating the CMOS electronics and the plurality of pixels on the monolithic semiconductor sensor.
  20. 20 . The detector system according to claim 18 , wherein each semiconductor sensor of the plurality of semiconductor sensors is a hybrid semiconductor sensor comprising the CMOS electronics flip chipped at a side of the plurality of pixels in the semiconductor sensor.

Description

TECHNICAL FIELD The present invention generally relates to a three-dimensional (3D) semiconductor detector system for molecular imaging of radioactive nuclides, and in particular such a 3D semiconductor detector system with outstanding efficiency and high spatial resolution. BACKGROUND In molecular imaging, radiolabeled biologically relevant probes are used as tracers, also referred to as radiotracers, to map biological function and processes in the body. To measure the location of the radioactive nuclides, also referred to as radionuclides, radioisotopes or radioactive isotopes in the art, the incident direction of the detected photons needs to be measured. For this, in positron emission tomography (PET), two 511 keV photons from the positron annihilation are detected in time coincidence and the radionuclide is assumed to be on the line between the two points where the photons were detected. In single photon emission computed tomography (SPECT), the individual photons emitted from the radionuclide must pass through a collimator with holes that define the allowed incident directions. The detector positioned behind the collimator determines the detected position of the small fraction of the photons that make it through the collimator. Both PET and SPECT are widely used for clinical imaging and for research, each having its own advantages and challenges. For instance, PET is fundamentally limited in resolution due to the positron range (0.5-6 mm) and requires expensive cyclotrons in the vicinity to produce the radionuclides. The problem with SPECT systems is the mechanical collimator, which is very inefficient, rejecting most of the photons that carry information about the object, with only about 1 out of 106 photons passed to the detector. The collimators used in SPECT systems have an unfortunate built-in trade-off between efficiency and spatial resolution, limiting the latter to around 10 mm. There is therefore a need for a detector system that can be used for molecular imaging of radioactive nuclides and that is not marred by the shortcomings of existing PET and SPECT systems. SUMMARY It is a general objective a detector system having high efficiency and spatial resolution. This and other objectives are met by embodiments disclosed herein. An aspect of the invention relates to a detector system for molecular imaging of a radionuclide. The detector system comprises a three-dimensional (3D) semiconductor detector comprising a plurality of sensor stacks. Each sensor stack of the plurality of sensor stacks comprises a plurality of semiconductor sensors each comprising a plurality of pixels. The plurality of semiconductor sensors is made of a semiconductor material having an average atomic number Z below 40. The detector system also comprises a read-out circuitry connected to the pixels in the 3D semiconductor detector and configured to output, for each pixel along an electron track in the 3D semiconductor detector, a pixel value representative of an energy deposited at the pixel by a Compton recoil electron along the electron track in the 3D semiconductor detector. The Compton recoil electron is created by a Compton scatter interaction induced by an incident gamma ray in the 3D semiconductor detector. The detector system further comprises at least one processor and at least one memory comprising instructions, which when executed by the at least one processor, cause the at least one processor to predict a position of a start of the electron track based on a distribution of the energies deposited at each pixel along the electron track in the 3D semiconductor detector. Gamma rays incident into the 3D semiconductor detector generate Compton recoil electrons by Compton scatter interaction. The detector system of the invention is able to detect energy deposition caused by a Compton recoil electron along an electron track and to predict the start position of the electron track in the 3D semiconductor detector. Such as detection of Compton recoil electrons and prediction of the start positions thereof enables the detector system to sort Compton interactions belonging to the same event and prediction of a direction of the incident gamma ray inducing the same event of Compton interactions. This means, together with a high probability for interaction in the semiconductor material, that the detector system will have a higher efficiency and spatial resolution as compared to prior art detector systems for molecular imaging of radionuclides. BRIEF DESCRIPTION OF THE DRAWINGS The embodiments, together with further objects and advantages thereof, may best be understood by referring to the following description taken together with the accompanying drawings, in which: FIG. 1 schematically illustrates an embodiment of assembling a 3D semiconductor detector. FIG. 2 is an illustration of a monolithic semiconductor sensor according to an embodiment. FIG. 3 schematically illustrates the outline of assembly of the 3D semiconductor det