Tomography by emission of positrons (PET) system

1. A Positron Emission Tomography (PET) system dedicated to close examination, at a few millimeters from the skin, of human body parts including the breast, axilla, head, neck, liver, heart, lungs, prostate region and other body extremities or, to the detection and follow-up of different types of cancers in of the human body, that integrates in two movable, light-weight and compact PET detector heads the large number of individual detection channels, more than 12000 channels, based on LYSO (Cerium Doped Lutetium Yttrium Orthosilicate) crystals and avalanche photo-diodes (APD) arrays, with a small number of interconnections to a trigger and data acquisition system, necessary to allow high-sensitivity and image resolution of 1 mm in the full field-of-view comprised between two detection plates of LYSO crystals, and that is characterized by:

a. two detector heads housing more than 6000 LYSO crystals each with dimensions of the order of 2×2×20 mm 3 , two avalanche photodiodes per crystal pixel, electronic front-end readout system for each APD detection channel, and ancillary systems, the detector heads having a density larger than 0.5 detection channels per cm 3 ;

b. means for measuring of the coordinates of the photon interaction point in the detector with a precision of the order of 1 mm in the three space directions, by using fine-grained crystal granularity and a means for measuring a depth of interaction based on the sharing of scintillating light at the two ends of the crystal pixels;

c. a means for detecting and measuring individual hits of Compton events in the detector and in consequence to use in image reconstruction the events where at least one of the two PET photons has Compton diffusion in the detector, without significant degradation of the image resolution;

d. motorized mechanical means to allow the movement of the detector heads under manual or computer control, including the rotation around two independent axis and the translation along three perpendicular axis, plus the relative positioning of the two detector heads, making it possible to place the detector heads in plural orientations, as more appropriate for the organ under examination, and in contact with the patient skin for maximum sensitivity, and to collect data in several orientations as needed for tomographic image reconstruction with sensitivity as high as 0.10 cps/Bq;

e. a data-driven and synchronous architecture of the electronic front-end readout system that provides to each individual APD detection channel, low-noise amplification, pulse sampling at clock frequency, analog pipeline storage, event detection, and that provides input channel selection and multiplexing to the circuit output of the two-highest energy inputs, operating in pipeline mode with fixed latency at frequencies up to 100 MHz, implemented in an application-specific integrated electronics circuit handling 192 input channels;

f. a means for on-line measurement by the trigger system of the photons detection time, without introducing dead time in the data acquisition process, based on the analog-to-digital conversion of detector pulse samples and on a digital algorithm to compute the photon event time, as required by a two-photon coincidence trigger with time resolution of the order of one nanosecond; and

g. an architecture of the trigger and data acquisition system, based on the combination of a pipelined synchronous section followed by a dual-bus asynchronous readout system, capable to operate at a rate of one million coincidence events per second, for a background photon interaction rate of 10 million photons per second as expected in partial-body PET system operation under the large background of radiation from the whole-body and without shielding in the detector heads.

2. The Positron Emission Tomography (PET) system according to claim 1 , characterized in that each of the detector heads is composed of a plurality of detector modules, which consist of units providing a plurality of high density scintillating photon detector crystals LYSO (Cerium doped Lutetium Yttrium Orthosilicate) with a density of 7.4 g.cm −3 , a light yield of about 27 photons/keV, an emission spectrum peaking at 430 nm, a signal with a time constant of 40 ns and with dimensions 20×2×2 mm, optically isolated in a 4×8 arrangement coupled to APD (Avalanche Photo-Diode) arrays at both ends, the crystals and the APD arrays being housed and sealed in a plastic mechanical assembly and the transverse dimensions of the crystals being determined by the desired spatial resolution, whereas the longitudinal dimension is dictated by the required detector sensitivity.

3. The Positron Emission Tomography (PET) system according to claim 2 , characterized in that the APD arrays of 4×8 diodes convert the crystal light into an electrical signal, the pixel effective size being compatible with the planned crystal transverse size, the APD pixel effective size is 1.6×1.6 mm 2 and the quantum efficiency at the LYSO emission peak is about 75%, the APD gain is around 50, the dark current is of the order of 2-4 nA, the capacitance is 9 pF and the said gain has a temperature gradient of −2.4%/° C., which implies that the system has to operate under stabilized thermal conditions.

4. The Positron Emission Tomography (PET) system according to claim 2 , characterized in that the optical parameters of the crystal light collection into the APD pixels are designed and experimentally validated to achieve the gradient of light sharing between the top and bottom APDs necessary for a measurement of Depth-of-Interaction with a resolution of the order of two millimeters, the said parameters being that the LYSO crystal lateral surfaces are slightly polished with the roughness parameter Ra comprised between 1500 and 2000 Angstroms, the top and bottom surfaces are polished with Ra less than 100 Angstroms, the crystal pixel wrapping is made of BaSO 4 (barium sulphate) with 250 μm thickness, which wrapping provides the crystal support and enclosure, the crystal top and bottom surfaces are optically coupled to the APD array epoxy with a layer of thickness 20 μm of optical grease with refraction index 1.6, matching the epoxy refraction index.

5. The Positron Emission Tomography (PET) system according to claim 1 , characterized in that the motorized mechanical means permit:

a. vertical movement, allowing the adjustment of the plates height;

b. horizontal movements in two directions, allowing the adjustment of the plates position;

c. relative positioning of the two detector heads, allowing to adjust the plate separation;

d. rotation of the PET plates around the PET axis, allowing optimal positioning of the detector heads and optimizing tomographic image reconstruction; and

e. rotation of the PET system around a second axis, which combined with first rotation axis allowing examinations with the detector heads oriented in plural directions.

6. The Positron Emission Tomography (PET) system according to claim 1 , characterized in that the electronic front-end readout system, based in an application-specific integrated circuit (ASIC) handling 192 input channels, has a specific data-driven synchronous architecture operating in pipeline mode at frequencies up to 100 MHz as required for the operation of the described PET system, the said architecture providing for each input APD pulse the corresponding output data samples synchronously with the system clock, the output data being generated only in case the data corresponds to a photon with energy above a programmable threshold (data-driven auto-trigger), and with a fixed time interval (latency) between the input signal and the output data as required by the coincidence trigger system.

7. The Positron Emission Tomography (PET) system according to claim 6 , characterized in that the electronic front-end readout system has for each APD detection channel a low-noise amplifier characterized by a maximum input charge of 30 femto-Coulomb, an input equivalent noise charge (ENC) below 1000 electrons, a power dissipation below 3 milli-Watt per channel, a radiation tolerance up to 5 kilo rad total dose, an output signal shaping with peaking time of 30 nanoseconds and a gain of 30 milliVolt/femtoCoulomb.

8. The Positron Emission Tomography (PET) system according to claim 6 , characterized in that the electronic front-end readout system provides signal amplitude sampling at the output of the amplifiers and at every clock period, stores the sampled charges in capacitor arrays which work as a circular pipeline analog memories, and in parallel processes the analog signals by comparators followed by digital logic to determine the channels above threshold.

9. The Positron Emission Tomography (PET) system according to claim 6 , characterized in that the electronic front-end readout system, for the input signals above threshold, reads from the analog memories ten data samples and multiplexes the data to one of two available output channels at system clock frequency, together with the corresponding input channel identification coded as a ten bit data stream synchronous to the analog data samples, thus providing a data compression factor of 192 to 2, as required by the compact and movable PET detector heads.

10. The Positron Emission Tomography (PET) system according to claim 9 , characterized in that the electronic front-end readout system is composed of a certain number of front-end boards (FEB) per detector head, providing a direct interface to the top and bottom sides of detector modules, and a service board for clock distribution, power supply distribution, bias voltage regulation and temperature monitoring, each FEB corresponding to a certain number of crystal modules, and each FEB including:

a certain number of front-end integrated circuits (ASIC) with 192 channels;

a free running analog-to-digital (ADC) sampling converter at frequency up to 100 MHz for each of the ASIC output channels, followed by a parallel-to-serial converter for each group of two ASICs;

plural fast data links, each working at frequencies up to 680 MHz, using electrical flat cables with 10 differential pairs each;

plural flat cable connectors for control and clock lines, for reading the temperature sensors and for distribution of low voltage;

plural flat cable connectors for APD bias voltage distribution, one independent voltage per 16 APD pixels.

11. The Positron Emission Tomography (PET) system according to claim 1 , characterized in that the trigger and data acquisition system receives from the front-end system digitized data streams synchronously to the electron-positron annihilation events, estimates in a time shorter than 100 ns the energy and time of the detected photons in the two opposite detector heads, selects those photons with energy compatible with 511 keV or finds groups of hits that correspond to photons with Compton diffusion in the detector heads, compares the times of plural combinations, and reads the data of the relevant channels into the computer memory and disk when a coincidence is found.

12. The Positron Emission Tomography (PET) system according to claim 11 , characterized in that trigger and data acquisition system comprises pipeline processing structures used to compute the pulse amplitude and time, combined with pipeline memories for data storage during the trigger processing time, which combination allows for on-line coincidence triggering without introducing data acquisition dead-time.

13. The Positron Emission Tomography (PET) system according to claim 11 , characterized in that the trigger and data acquisition system is composed of three logic modules, the Data Acquisition and Filter (DAQ) modules, one per front-end data link, DAQ modules which process in parallel the front-end data, the Trigger module and the Data Concentrator (DCC) module, all modules interconnected through two data buses, a trigger bus and a data acquisition bus, and the DCC module is connected to the PC by a fast data link.

14. The Positron Emission Tomography (PET) system according to claim 13 , characterized in that the DAQ module is responsible for computing the photon energy and the interaction time for every input channel, performing the following operations in pipeline mode:

a. Peak search: search in the input data stream for a data sample higher than the previous and the following data samples in each channel;

b. Channel sorting and matching: sorting of the detector channel identifiers received from the front-end digital lines and matching between detector channels identifiers in the two sides of the detector plate;

c. Baseline estimation: computation of signal baseline as the average of two pre-samples (samples before the pulse rising-edge) and subtraction from the data samples;

d. Normalization: multiplication of the baseline corrected samples by a programmable normalization coefficient;

e. Photon Energy sum: summation of the pulse peak amplitude of the four output channels of two matching front-end ASICs; and

f. Photon Interaction time: computation of the pulse time, given by the clock number at pulse reception (coarse grain time) associated to the phase between the pulse peak and the clock (fine grain time), the fine grain time being computed has the ratio of the peak sample amplitude to the previous sample amplitude, multiplied by a normalization coefficient, of the highest pulse in the four channels involved in the energy computation.

15. The Positron Emission Tomography (PET) system according to claim 14 , characterized in that the four-channel energy sum is compared to two programmable thresholds, the event threshold and the Compton threshold, the Compton threshold being used to identify the occurrence of a Compton diffusion in the detector and the event threshold being used to identify the absorption of a 511 keV photon, and when the energy is above one of the threshold levels the channel information is transmitted, via the Trigger bus, to the Trigger module which receives information from all the DAQ modules that have detected the occurrence of either a photoelectric absorption or a Compton diffusion.

16. The Positron Emission Tomography (PET) system according to claim 13 , characterized in that a Trigger module detects a coincidence trigger when energy deposits above the event threshold in the two crystal planes are detected within a programmable time window, in which case the Trigger module sends a signal the corresponding DAQ modules that the event is valid, and the DAQ modules send the associated data, via the data acquisition bus, to the DCC module, which receives data from all the corresponding DAQs and organizes it into a single package to be sent to the data acquisition PC through a separated data link.

17. The Positron Emission Tomography (PET) system according to claim 13 , characterized in that the trigger and data acquisition system is implemented in five boards of two different types, four data acquisition boards implementing the DAQ modules, each data acquisition board corresponding to 8 DAQ modules, and one central trigger and data concentrator board, implementing the Trigger and DCC modules, using Field Programmable Gate Arrays (FPGAs) with four million gates to implement the trigger and data acquisition logic and memories, which boards are housed in a crate of 6 U format with two backplane buses with Peripheral Computer Interface (PCI) connectors implementing the trigger and the data acquisition transmission buses.

18. A Positron Emission Tomography (PET) system dedicated to close examination of human body parts, comprising:

(a) two detector heads, each comprising (1) a plurality detection modules, each module having a plurality of LYSO crystals and a plurality of avalanche photodiodes, each LYSO crystal being combined at opposite ends with an avalanche photodiode (APD) to define a crystal pixel coupled to an APD detection channel, (2) an electronic front-end readout system for each APD detection channel, and (3) ancillary systems,

wherein each detector head has a density larger than 0.5 detection channels per cm 3 ,

wherein the measurement of coordinates of a photon interaction point in the detector has a precision of the order of at least 1 mm in three spacial directions, by using fine-grained crystal granularity and by measuring a depth of interaction based on the sharing of scintillating light at the two ends of the crystal pixels,

wherein the detector head is operative to detect and measure individual hits of Compton events and in consequence to use in image reconstruction events where at least one of two PET photons has Compton diffusion in a detector module, without significant degradation of image resolution; and

wherein each electronic front-end readout system has a data-driven and synchronous architecture that provides to each individual APD detection channel, low-noise amplification, pulse sampling at clock frequency, analog pipeline storage and event detection, and that provides input channel selection and multiplexing to the circuit output of the two-highest energy inputs, operating in pipeline mode with fixed latency at frequencies up to 100 MHz, implemented in an application-specific integrated electronics circuit;

(b) motorized mechanical means to provide movement of said detector heads under manual or computer control, by rotation around two independent axis and translation along three perpendicular axis, plus the relative positioning of the detector heads,

wherein said detector heads may be placed in plural orientations, as appropriate for a target under examination and in contact with a patient's skin for maximum sensitivity, and for collecting data in several orientations as needed for tomographic image reconstruction with sensitivity as high as 0.10 cps/Bq; and

(c) a trigger and data acquisition system, having an architecture comprising a combination of a pipelined synchronous section followed by a dual-bus asynchronous readout system, operate at a rate of at least one million coincidence events per second, for a background photon interaction rate of at least 10 million photons per second as expected in partial-body PET system operation under the large background of radiation from the whole-body and without shielding in the detector heads,

wherein the trigger and data acquisition system performs on-line measurement of the photons detection time, without introducing dead time in the data acquisition process, based on the analog-to-digital conversion of detector pulse samples and on a digital algorithm to compute the photon event time, as required by a two-photon coincidence trigger with time resolution of the order of at least one nanosecond.