Angela M. K. Foudray

Investigation of position sensitive avalanche photodiodes for a new high-resolution PET detector design

We are developing a high-resolution PET detector design with a goal of nearly complete scintillation light collection in /spl les/1 mm width, /spl ges/20 mm effective thickness LSO crystals. The design uses position sensitive avalanche photodiodes in novel layered configurations that significantly improve the light collection aspect ratio. To reduce design complexity and dead area we are investigating the use of 1 mm thick sheets of LSO in addition to discrete crystal rods, and the use of PSAPDs which require only four readout channels per device. The raw spatial response of a 1 mm thick crystal sheet coupled to a PSAPD exhibits a compressed dynamic range compared to that observed with discrete crystals. Measurements with the proposed configurations using /sup 22/Na irradiation achieved 10%-13% FWHM energy resolution at 511 keV and 2 ns coincidence time resolution. 1 mm width crystals with a saw cut surface finish an no inter-crystal reflector were well resolved in flood images.

Impact of high energy resolution detectors on the performance of a PET system dedicated to breast cancer imaging.

We are developing a high resolution, high sensitivity PET camera dedicated to breast cancer imaging. We are studying two novel detector technologies for this imaging system: a scintillation detector comprising layers of small lutetium oxyorthosilicate (LSO) crystals coupled to new position sensitive avalanche photodiodes (PSAPDs), and a pure semiconductor detector comprising cadmium zinc telluride (CZT) crystal slabs with thin anode and cathode strips deposited in orthogonal directions on either side of each slab. Both detectors achieve 1 mm spatial resolution with 3-5 mm directly measured photon interaction depth resolution, which promotes uniform reconstructed spatial resolution throughout a compact, breast-size field of view. Both detector types also achieve outstanding energy resolution (<3% and <12%, respectively for LSO-PSAPD and CZT at 511 keV). This paper studies the effects that this excellent energy resolution has on the expected system performance. Results indicate the importance that high energy resolution and narrow energy window settings have in reducing background random as well as scatter coincidences without compromising statistical quality of the dedicated breast PET data. Simulations predict that using either detector type the excellent performance and novel arrangement of these detectors proposed for the system facilitate approximately 20% instrument sensitivity at the system center and a peak noise-equivalent count rate of >4 kcps for 200 microCi in a simulated breast phantom.

A Method to Include Single Photon Events in Image Reconstruction for a 1 mm Resolution PET System Built with Advanced 3-D Positioning Detectors

We are developing cadmium zinc telluride detectors with three-dimensional photon positioning capabilities for high-resolution PET imaging. These detectors exhibit high spatial resolution (1 mm), energy resolution (2.5% full width at half maximum for 511 keV photons), and the ability to resolve individual Compton interactions within the detector. Using these measurements, non-coincident single photons can be reconstructed by estimating the incoming direction of the photon using the kinematics of Compton scatter within the detector. In this paper, we investigated image reconstruction methods for combining two different types of measurements: conventional coincidence photon events and non-coincident single photon events. We introduce a new image reconstruction method that uses a Bayesian projector function. Using Monte Carlo simulated data generated by GATE (Geant4), we showed that this new approach has the potential to improve contrast and resolution with comparable signal-to-noise ratio.

Performance characterization of a novel thin position-sensitive avalanche photodiode-based detector for high resolution PET

We are developing advanced dedicated breast and small animal positron emission tomography (PET) systems with detectors comprising arrays of 1times1times3 mm3 LSO crystals coupled to novel, extremely thin position-sensitive avalanche photodiodes (PSAPD). This thin PSAPD is essential for achieving high crystal packing fraction for a PET system design which has 2 cm effective LSO crystal thickness, 1 mm transaxial spatial resolution and 3 mm directly measured photon DOI resolution. These properties facilitate high detection efficiency and three dimensional positioning capability in the detector. With this very thin PSAPD, preliminary results indicated that we can expect 1 mm3 intrinsic spatial resolution. We have observed an average of 1.1 mm transaxial and 3 mm DOI spatial resolution, ~11% energy resolution and ~2 ns temporal resolution. We have also implemented a position algorithm for reducing the effect of pincushioning on events detected over the face of the PSAPD

Evaluation of free-running ADCs for high resolution PET data acquisition

Advances in field-programmable gate arrays (FPGA) allow a larger part of analog circuits to be replaced by digital logic. We are prototyping a high-resolution position- sensitive APD (PSAPD) based PET system, using a fully digital data acquisition system with free-running analog-to-digital converters (ADC) and FPGA logic. Numerical algorithms for digital shaping and digital timing pickup were tested and the results were compared to those obtained by using analog NIM modules for a LSO/PSAPD detector module, in terms of energy resolution, time resolution and positioning ability. A significant improvement in the coincidence time resolution was found using a digital CFD (4.71+/-0.07 ns FWHM) over the analog counterpart (3.15+/- 0.07 ns). A linear time interpolation model provides the best time resolution (1.72+/-0.07 ns).

Accurately Positioning and Incorporating Tissue-Scattered Photons into PET Image Reconstruction

We are developing cadmium zinc telluride detectors with three-dimensional positioning capabilities for high-resolution PET imaging. These detectors exhibit high spatial resolution (1 mm), energy resolution (2.5% full width at half maximum for 511 keV photons), and the ability to identify the 3-D coordinates of individual Compton and photoelectric interactions within the detector. These detectors can operate in conventional PET mode measuring photons in coincidence and as a Compton camera for single photon events. In this work, we show how the capabilities of this detector can be used to reconstruct tissue-scatter coincidence events. We present a scatter projector function for positioning tissue-scatter coincidence events in the field of view. Using Monte Carlo simulated data generated by GATE (Geant4), we showed that this new approach might be used to increase the number of usable counts in PET.

Component based normalization for PET systems with depth of interaction measurement capability

The number of lines of response (LOR) in a system with small crystals and depth of interaction (DOI) measurement capability can require prohibitive amounts of memory and computational resources. The number of independent LORs in the small animal imaging system evaluated in this study is /spl sim/2 /spl times/ 10/sup 9/. We propose a method to adapt the component-based normalization model to include (1) DOI and (2) large field of view (FOV) to system inner bore volume percentage (near 100%) or percentage of view (POV) considerations without an order of magnitude increase in computational resources. This method can also compensate for bin location errors made during the LOR calculation. Included in the study were radial, axial and interference geometric efficiency and crystal absorption factors. We have observed a large effect on sensitivity from axial geometric factors, distance between positioned coincident photon interactions, and amount of crystal surrounding any particular crystal in our system. Assessing direct plane sinograms of direct normalization data (infinitely thin cylindrical annulus), we have observed that there are LORs that received zero counts due to inter-module gaps. Component based normalization pre-correction weights significantly improved the coefficient of variation for the reconstructed 1.5 mm and 1.75 mm spheres in the row nearest the center line in the axial image slices by 17%, 32%, and 51% in the z = 0 cm, z = 1.5 cm, and z = 3.0 cm axial planes, respectively.

Comparing geometries for a PET system with 3-D photon positioning capability

We compared two PET system geometries, a box and a cylinder, for a novel PET 3-D detector design with axial, transaxial, and depth of interaction resolution of 1 mm, 1 mm, and 3 mm, respectively. Monte Carlo simulations have shown that arranging this detector into an 8/spl times/8/spl times/8 cm/sup 3/ FOV box has twice the sensitivity of a conventional cylinder of comparable dimensions (8 cm diameter, 8 cm axial length) due to a reduction in inter-module gaps. We compared image reconstruction performance for the box and the cylinder by computing the mean square error (MSE) for a weighted least squares estimator and contrast recovery coefficient (CRC) for rod phantoms with rods located at the radial center and radial offset of 2 cm. The contrast was comparable (near 100% CRC) for both box and cylinder. However, the cylinder showed slightly better contrast uniformity (CRC varied 2% for cylinder, 8% for box) as the radial position of the lesion was varied. The box had a MSE that was 7-14 times better than the cylinder system. We performed an ROC study by computing the detection SNR for computer observer models. A non-prewhitening and a prewhitening test statistic were computed to provide an upper and lower bound for the performance of human observers. The prewhitening filter detection SNR for the cylinder design was superior (by <15%) to the box at the center position. For all other source locations, the box showed an improvement (by >20%) in detection SNR. Finally, we reconstructed a resolution phantom with 3-D data for both systems and showed that both systems were qualitatively comparable.

Performance characterization of a new high resolution PET scintillation detector

We present measurements of a new high resolution PET scintillation detector. The detector is capable of recording the 3-D coordinates, energy, and arrival time of the individual photon interactions in the scintillation crystal. The incident radiation interacts edge-on with the crystal array. This new detection concept is very different than the standard block design. Measured energy, time and spatial resolution of the detector modules are reported.

Investigation of scintillation light multiplexing for PET detectors based on position sensitive avalanche photodiodes

Dedicated high resolution compact PET systems are being developed for breast and small animal imaging that use lutetium oxyorthosilicate (LSO) crystals (1/spl times/1/spl times/3 mm/sup 3/) coupled to highly compact position sensitive avalanche photodiodes (PSAPDs) with 8/spl times/8 mm active area and 200 /spl mu/m thickness. The full system will consist of thousands of PSAPDs. In this paper, we investigated methods to multiplex scintillation light from the LSO crystals in order to significantly reduce the number of PSAPDs required without compromising the system performance. Initial experiments were performed using dual-layer slotted LSO crystals forming crystal pixels that are offset with respect to either layer by half crystal pixel width in either 1D or 2D configuration. For the 1D crystal configuration, the results show that all crystal pixels (1/spl times/1/spl times/3 mm/sup 3/ ) except the edge crystal pixels can be resolved with an average of 3:1 peak-to-valley ratio when a specular reflector is used between the crystal slots. An average energy resolution of (/spl sim/13%) is also obtained. Compared to the 1D offset crystal configuration, the 2D offset crystal configuration showed slightly better performance with an average energy resolution of (/spl sim/12%) and peak-to-valley ratio of (/spl sim/4.5:1). This result is comparable to the previous published data using discrete single layer crystal arrays. A relatively uniform photo peak energy distribution (standard deviation /spl sigma/ < 0.3 volts) is also observed. The result showed that light multiplexing using dual layer LSO crystals with offset crystal slot configuration has great potential. In addition, the slot crystal offset mechanism will simplify the mechanical assembly of the crystals.