University of Ljubljana Faculty of mathematics and physics Andrej Studen Compton Camera with PositionSensitive Silicon Detectors Doctoral thesis Supervisor: Professor Marko Mikuž Outline: Motivation Basic Principles New Compton camera prototype Image reconstruction Conclusion
Warning This is a presentation of a doctoral thesis. If your plans for today do not include listening to a presentation of a doctoral thesis, this would be a perfect time to leave.
Motivation In short: Compton camera could substantially improve SPECT imaging. With explanation: Single Photon Emission Computed Tomography (SPECT) is a diagnostic tool in nuclear medicine (brain scan, whole body tumor search,...) Selected substance is labeled by radio-active sources emitting g-rays and injected into a patient. A gamma camera is required to trace the substance within the body of a patient.
Anger camera Traditional cameras: Mechanically collimated or Anger cameras. (sold by Siemens, ELSCINT, GE) Inverse coupling (trade-off) between resolution and efficiency limits SPECT. Many possibilities for alternative detection methods. Hal Anger (1953)
Compton camera Compton camera replaces the mechanical collimator with a special scattering detector where Compton scattering occurs. Collimation is based on kinematics of Compton scattering. ANGER COMPTON
Compton camera Compton camera replaces the mechanical collimator with a special scattering detector where Compton scattering occurs. Collimation is based on kinematics of Compton scattering. ANGER COMPTON
Compton camera (cont'd) Compton imaging is a successful tool in astronomy (COMPTEL, MEGA,...), where photon energies exceed 1 MeV. The photon energy range in SPECT (140.5-511 kev) introduces harsh requirements for Compton camera. COMPTEL 1991-2000 This work continues the efforts in the field, represented by prototype CSPRINT, constructed in 1999 at University of Michigan. C-SPRINT 1999
Basic Principles: Compton scattering Kinematics of Compton scattering relates the scattering angle q of the photon to the energy Ee of the Compton electron. 2 2 sin / 2 =mc E / E e E E e Compton equation Random movement of bound electron prior to scattering broadens the scattering angles Doppler broadening (indicated by the blue arrows).
Division of Compton camera in subdetectors Compton camera consists of two sub-detectors: Scatterer measures the position of Compton scattering and energy Ee of the Compton electron Absorber measures the impact position of the scattered photon. 2 2 sin / 2 =mc E / E e E E e Compton equation
Position reconstruction Possible directions of incoming photons form a cone with axis along the scattered photon track, apex on the interaction point in the scatterer and opening angle equal to the scattering angle.
Position reconstruction Intersection of cones formed for consecutive events reproduces the position of the source.
Requirements: Angular resolution Task: reduce contributions of geometry and energy resolution below Doppler broadening: Geometry => simple, Energy => the harsh requirement of SPECT, goal 1 kev FWHM. astronomy Angular resolution of the reconstruction is due to three contributions: Doppler broadening (inherent to the method). Energy resolution of the scatterer (through Compton equation). Geometrical factor related to spatial resolution of sub-detectors.
Requirements: Efficiency & Background Efficiency important in SPECT: collateral damage by radiation uptake. Management of radiation dose absorbed by the patient. => Efficient setup required. Plenty of background gamma radiation expected: large activity used (0.1 GBq scale), uptake in tissue surrounding the imaged object. Steps taken to ensure signal data is recorded: maximize timing correlation between sub-detectors => timing resolution required, shield the absorber from direct hits => adjust geometry, reject events based on sum of energies in both sub-detectors => measure energy in the absorber.
New Compton camera prototype emphasis Advantages of silicon as scattering material: mature production and processing yielding: excellent spatial resolution, excellent inherent energy resolution at room temperature, low Doppler broadening, high Compton to photoabsorption ratio making efficiency grow with sensor thicknes with no signal loss. Scatterer: based on recent development in silicon technology. resolution of 1 kev FWHM optimize timing. + Absorber (borrowed from existing SPRINT setup) DAQ mechanics
The scatterer Scatterer module consists of: silicon pad sensor : 4.8 x 1.2 cm2, up to 1 mm thick, 256 pads, 32 x 8 array, designed by W. Dulinski & produced by SINTEF associated front-end electronics: pair of VATAGP3, made by Ideas, A.S.A., Norway The scatterer consisted of several identical modules (efficiency grows linearly with total thickness). VATAGP3 STACK
The scatterer: DAQ system Receive trigger, read, digitize & store values. Distribution board: daisy-chain the modules Intermediate board: analog signals for VATAGP3-s, sensor bias VME board: digital signals for VATAGP3-s, ADC, PC communication PC: data storage and processing
The scatterer: Energy resolution Important for angular resolution of prototype. VATAGP3 provides a slow shaped (3 ms) CR-RC output for precise measurement of Compton electron energy. Noise (= error) : electronic + (inherent) silicon, 99mTc (126 e)2 : (120 e)2 + (37 e)2 Pedestal noise of module with 0.5 mm thick sensor Electronic noise dominates: signal must be collected in a single pad, pad size (1.4 mm) exceeds expected Compton electron range (< 0.7 mm). Spatial resolution of the scatterer equals to pad size (1.4 mm FWHM).
The scatterer: Energy resolution Thick sensor desired - larger efficiency: 0.5 and 1 mm sensor tested, exceeding standard thickness (0.3 mm). tests performed with 241Am (Eg= 59.5 kev). performance of both sensor types close to design goal. 0.5 mm 1 mm
The scatterer: Timing resolution Important for proper matching of interactions in scatterer and absorber. VATAGP3 trigger: fast (150 ns) CR-RC shaper and constant level discriminator. Shaping time > signal collection time (1 mm Si) => 150 ns minimum. Measurement (varying threshold at fixed charge injection): jitter: 17 50 ns time-walk: 150 ns (dominant) Possible gain with alternative triggering methods (e.g. CFD) : reduced time-walk (at the expense of jitter). 150 ns time-walk
The absorber Borrowed from SPRINT. 3 layers of associated electronics 20 Hamamatsu R980 PMTs 44 NaI(Tl) bars, each 3 mm wide, 15 cm long and 1.27 cm thick
The absorber: DAQ system Generate trigger from Energy sum; collect and digitize maximum value of shaped outputs of 20 PMT-s. Based on VME, PC stores & manipulates data.
The absorber: Spatial resolution Modules were calibrated on a rectangular (5 x 5 mm2) mesh. Position was determined using the centroid method and distortion was corrected with the known position of the source. variance [mm] Corrected resolution below 4 mm FWHM across the sensitive area.
The absorber: Energy resolution Sum of energies recorded in scatterer and absorber is used for separation of true events from random background. Energy resolution of the sum dominated by energy resolution of NaI(Tl), which is much lower than in silicon.
The absorber: Timing resolution The timing resolution measured using 22Na positron source: one photon absorbed by the plastic scintillator (timing reference), second photon absorbed in SPRINT. Timing resolution of absorber is 24 ns FWHM; small compared to 150 ns resolution of the scatterer.
Mechanics Custom designed gantry: up to 5 scatterer modules; at the center 3 absorber modules form a semi-ring (reduced background) optimized for on-axis source good efficiency for scattering angles around 1 radian where imaging is optimal
Efficiency Composed of combined probabilities : that emitted photon hits the scatterer (solid angle, 1.6 % @ 5 cm) Oskar Klein of Compton scattering (P(q), Klein-Nishina) of geometrical acceptance of scattered photons (a(q)) photon absorber energy [kev] efficiency [%] that scattered photon interacts in absorber (ea) 140.5 250 511 100 60 10 Total efficiency (e): = a 1 0 a P d 1.6 for 99mTc @ 5 cm, 4.5 mm scatterer The data throughput of the prototype was saturated by the speed of the DAQ system electronics at 50 Hz.
Angular resolution Worst case - 99mTc: Doppler broadening: 60 mrad @ 1 rad Energy resolution of the scatterer: 45-60 mrad @ 1 rad Geometry contribution: 50 mrad @ 1 rad Total: 105-125 mrad @ 1 rad Increasing Eg helps! optimal range Eg [kev] 140.5 250 511 Doppler E scat (1.5 kev) [mrad] [mrad] 60 60 40 30 30 10 Geometry [mrad] 50 50 50 Total [mrad] 125 70 60
DAQ system Joint scatterer and absorber DAQ systems. SYSTEM TRIGGER: timing coincidence of any SPRINT AND scatterer. Event combined in VME, stored and manipulated by PC.
Timing resolution Triggers from scatterer and absorber combined. low-energy Compton electrons Timing resolution dominated by the scatterer. Low energy (low angle) Compton electrons cut by the coincidence window.
Image reconstruction To determine the performance of the prototype, simple back-projection was used instead of elaborate techniques (EM/LML). Reduce the problem to 2D by reconstruction on a focal plane; multiple planes give 3D information. For each event: construct the cone of possible directions of initial photon, determine and properly weight pixels hit by the cone
Image reconstruction Image recorded with 2 out of 3 absorber modules: y direction lacks resolution. 99m Tc point source; 1 x 1 mm2 Representative profile along X:
Image reconstruction Angular resolution is determined as spatial resolution divided by the source-scatterer separation. Performance matches the prediction. Best resolution (170 mrad FWHM) for lowest detected energies of Compton electrons.
Conclusion: Prototype evaluation for (99m) Tc resolution [mrad] efficiency [ ] C-SPRINT 160 80** 0.001 * equivalent geometry assumed this prototype 170 100* 50*** 1.6 e v i t a t i l qua ** with EM/LML *** assumed both equiv. geom. and EM/LML better (due to improved scatterer) simple CC prototype, worst conditions (99mTc), stateof-the art Anger camera for this (99m)Tc prototype resolution [mm] 8 11 (@ 5 cm) efficiency 1.6 e [ ] v i t a it l a qu Anger camera 5 15 0.05 0.4 average factor of 50 required for cone reconstruction (Compton) to break even with line reconstruction (Anger) Possible improvements of the prototype: Resolution: optimized geometry elaborate reconstruction techniques (in C-SPRINT factor 2 improvement). Performance comparable Improved efficiency: larger scatterer sensors, more scatterer modules. Shift to larger Eg Compton camera is a possible tool of the future.
Prostate probe A possible future application; prostate probe. Aimed at detection of prostate cancer, second most common cancer in men, which develops in 1/6 of population. finger-thick probe inserted intra-rectally, viewed by a large absorber surrounding the patient.
Prostate probe The probe relies on: near-angle view of the prostate, convenient radio-tracer used (111mIn, 250 and 171 kev). Simulation predicts: Prostate probe conventional SPECT spatial resolution and efficiency far better than in Anger cameras, possible detection of primary prostate tumors (only secondary detectable with Anger camera).
Concluding remarks Compton camera offers an alternative to mechanically collimated imagers, removing the resolution-efficiency trade-off. Harsh requirements for SPECT applications. Developed position-sensitive silicon sensors with associated readout electronics meet those requirements (1 kev FWHM resolution, up to 1 mm thickness). Simple prototype with developed scatterer and conventional absorber, data reconstructed with a simple back-projection, was found comparable to contemporary Anger cameras. Possible improvements and shift to other radio-tracers can make Compton camera the tool of the future. A possible application is a prostate probe which could directly image primary tumors, allowing for early disease diagnostics and treatment.