Angela Marie Klohs Foudray

Performance Characterization of a Miniature, High Sensitivity Gamma Ray Camera

A compact, hand-held gamma camera with excellent intrinsic and extrinsic performance has been developed for the rapid identification and localization of sentinel lymph nodes during the surgical staging of cancer. The camera comprises a 5×5 cm2 field of view NaI (Tl) pixellated crystal array, a high sensitivity lead (2.0 cm thick) hexagonal parallel-hole collimator, a position sensitive photomultiplier tube (PSPMT), and a novel highly multiplexed electrical readout. The intrinsic energy resolution (12.1±2.0%) at 140 keV including edge crystals, extrinsic sensitivity (5 cps/μCi from 1-5 cm with a 24% energy window) and extrinsic spatial resolution (1.81±0.1 mm at 0.6 cm) facilitate rapid identification of a radioactive node. Using a node phantom we performed an ideal observer study to estimate the detectability of small spheres in the range of 3 mm to 8 mm with different exposure times. With a 5 seconds exposure the camera can detect a 3 mm diameter sphere at a depth of 3.6 cm containing 1 μCi of Tc-99m within a cold background, and a 4 mm diameter sphere containing 2 μCi at 2.6 cm depth within a warm background (>10:1 contrast) with a 5 second exposure duration.

Finite element model based spatial linearity correction for scintillation detectors that use position sensitive avalanche photodiodes

We have developed a method using polynomial correction derived from a finite element model (FEM) to correct spatial linearity of scintillation detectors that use position sensitive avalanche photodiodes (PSAPD). A PSAPD is a planar avalanche photodiode with a resistive coating that allows continuous, but non-linear, positioning of scintillation light pulses over the entire active area. The spatial response of a scintillation crystal array coupled to a PSAPD shows a characteristic pincushion distortion when using a linear center of mass positioning calculation. A finite element simulation was used to derive the spatial response function for the PSAPD detector, and then used to calculate a polynomial linearity correction. We performed spatial linearity correction of several flood histograms from a 22Na point source on discrete, scored and continuous crystal arrays of lutetium orthosilicate (LSO). For discrete arrays, spatial linearity correction improves crystal segmentation by correcting the pincushion distorted peaks in the 2-D crystal positioning histogram. For continuous sheets, spatial linearity correction allows for linear positioning over the central part of the crystal array without the use of time consuming spatial calibration measurements

Prototype parallel readout system for position sensitive PMT based gamma ray imaging systems

A fully parallel prototype readout system that allows digitization and acquisition of 64 or more anode signals from a position sensitive photomultiplier tube (PSPMT) was developed for miniature hand-held gamma camera. This acquisition system was developed to study the benefits of a fully digital readout system compared to charge multiplexed techniques such as resistive division. The system is based on the CAMAC instrumentation standard and is controlled via a Macintosh Computer. Four 16-channel charge-to-digital conversion (QDC) CAMAC modules were used to digitize individual anode signals from the PSPMT. To maximize the data transfer rate, a list processor module is also added in the system. Kmax software from SPARROW is used for acquisition and processing of the digitized data. The system provided greater than 0.5% improvement in linearity and /spl sim/ 3% improvement in energy resolution when a gain correction is applied to list-mode data. More than 6% improvement in linearity, better image quality with 16:1 peak to valley ratio, and higher edge sensitivity is obtained compared to resistive division readout method.

Characterization of Two Thin Postion-Sensitive Avalanche Photodiodes on a Single Flex Circuit for Use in 3-D Positioning PET Detectors

We are developing 1 mm resolution, 3-D positioning, high sensitivity positron emission tomography imaging systems for small animal and breast-dedicated imaging applications. The system uses detector modules that comprises two adjacent position-sensitive avalanche photodiodes (PSAPDs), each with a segmented lutetium oxyorthosilicate (LSO) scintillation crystal array, mounted on a single flex cable readout circuit. The module is oriented two PSAPDs deep in the depth of interaction direction, which facilitates high photon sensitivity (~2 cm of LSO). In this study, we are testing two PSAPD chips in a single flex-cable module simultaneously to investigate their performance and access the degree of inter-device cross-talk. The inter-device cross-talk was measured by looking at the signal in an un-irradiated PSAPD chip while the adjacent PSAPD chip was irradiated, both chips under bias. The signal was three orders of magnitude lower in the unirradiated PSAPD versus the irradiated PSAPD. Individual crystal energy resolution for an array coupled to a PSAPD was measured to be 10.03 plusmn 4.45%for the 1 mm times 1 mm times 3 mm array. All crystals were resolved in a flood histogram, with an average peak-to valley ratio of ~11, with the lowest PVR being 1.8. The average coincidence time resolution was 3.75 ns FWHM across the devices. These measurements indicate that degradation due to the close proximity of two PSAPDs on the same readout circuit is small.

Monte Carlo simulation study of a dual-plate PET camera dedicated to breast cancer imaging

We studied the performance of a dual-plate positron emission tomography (PET) camera dedicated to breast cancer imaging using Monte Carlo simulation based on GATE open code software. The PET camera under development has two 10times15 cm2 plates that are constructed from arrays of 1times1times3 mm3 LSO crystals coupled to novel silicon-based ultra-thin (<300 mum) position-sensitive avalanche photodiodes (PSAPD). With the photodetector configured edge-on, incoming photons see effectively 2-cm-thick of LSO with directly measured 3-mm photon depth-of-interaction. Simulations predict that this camera will have >10% sensitivity, and detector measurements show ~1 mm3 intrinsic spatial resolution, <12% energy resolution, and ~2 ns coincidence time resolution. With a breast phantom including breast, heart and torso activity (concentration ratio of 1:10:1, respectively), count performance was studied under varying time and energy windows. We also studied visualization of hot spheres within the breast for 1times1times3 mm3, 2times2times10 mm3, 3times3times30 mm3 and 4times4times20 mm3 crystal resolutions at different plate separations. Images were reconstructed by focal plane tomography and 3D OS-EM with attenuation and solid angle corrections applied. With an activity concentration ratio of tumor:breast:heart:torso of 10:1:10:1, only the dual-plate PET camera comprising 1times1times3 mm3 crystals can resolve 2.5-mm tumor spheres with an average peak-to-valley ratio of 1.3 in only 30 seconds of acquisition time

Positioning Annihilation Photon Interactions in a Thin LSO Crystal Sheet With a Position-Sensitive Avalanche Photodiode

Using scintillation crystal sheets instead of discrete crystal arrays in high-resolution PET has the advantage of reduced complexity. In order to evaluate the positioning capability of a position sensitive avalanche photodiode (PSAPD) using a sheet lutetium oxyorthosilicate (LSO) crystal scintillator, we need to understand the functional dependence of a detected event position on the known source position. We studied positioning with both collimated 57Co 122keV and coincidence-triggered 22Na 511keV sources, which were stepped across the face of an 8 mmtimes8 mm LSO sheet crystal coupled to an 8 mmtimes8mm PSAPD at 160 mum intervals using a voltage-driven mechanical stage with a LabVIEW controlled acquisition system. We analyze the energy resolution, sensitivity, photopeak position and energy gated full width at half maximum (FWHM) spread of the detected position for a particular known source position. We have observed a 10% variation in average energy from the center of the crystal to the edge with 57Co and <1% for 22Na and an average point spread function FWHM of 2.86 mm and 1.12 mm for 57Co and 22Na respectively. We investigated methods to create a 1-1 map between (1) the four positioning signals from the PSAPD and the recorded energy and (2) the true position of the annihilation photon interaction. We found the average energy change over the 1.2 mm near the edge of the continuous LSO crystal to be ~5% - insufficient to resolve with the prototype PSAPD (energy resolution 12%) with an Anger-type logic positioning algorithm. Simulation using the annihilation photon interactions from GATE and scintillation photon transport from DETECT2000 have confirmed the effects observed in experiment

Incident Photon Direction Calculation Using Bayesian Estimation for High Energy Photon Detector Systems with 3D Positioning Capability

We are developing high resolution PET systems comprising detectors that can position photon interactions in three dimensions and distinguish between individual interactions for single photons entering the system. Having these interaction locations and energies, algorithms can be employed to gain more information about the photon before interaction, including the incident direction and energy. We have developed a maximum likelihood algorithm using Bayesian methods to estimate a single photons incident angle and interaction location, provided that it produced more than one detectable interaction. Both the phi (Phi) and theta (thetas) angle are calculated in this algorithm. The forward model was simulated using Geant4 Application for Tomographic Emission (GATE). The probabilities were calculated for a system comprised of 1 mm times 1 mm times 3 mm LSO arrays coupled to position-sensitive avalanche photodiodes with 12% energy and 2 ns coincidence time resolution. A training set of ~20,000 single photon events were used per angle to calculate the likelihood and prior probabilities for all angles in the range detected by the system (nearly 180 degrees in theta and phi). A test set of singles for particular angles produced an incoming photon angular estimation with a ~16 degrees root mean squared (RMS) deviation in both thetas and Phi.

Bayesian Estimator for Angle Recovery: Event Classification and Reconstruction in Positron Emission Tomography

PET at the highest level is an inverse problem: reconstruct the location of the emission (which localize biological function) from detected photons. Ideally, one would like to directly measure an annihilation photons incident direction on the detector. In the developed algorithm, Bayesian Estimation for Angle Recovery (BEAR), we utilized the increased information gathered from localizing photon interactions in the detector and developed a Bayesian estimator for a photons incident direction. Probability distribution functions (PDFs) were filled using an interaction energy weighted mean or center of mass (COM) reference space, which had the following computational advantages: (1) a significant reduction in the size of the data in measurement space, making further manipulation and searches faster (2) the construction of COM space does not depend on measurement location, it takes advantage of measurement symmetries, and data can be added to the training set without knowledge and recalculation of prior train...