Sam S. Huh

Performance evaluation of a very high resolution small animal PET imager using silicon scatter detectors

A very high resolution positron emission tomography (PET) scanner for small animal imaging based on the idea of inserting a ring of high-granularity solid-state detectors into a conventional PET scanner is under investigation. A particularly interesting configuration of this concept, which takes the form of a degenerate Compton camera, is shown capable of providing sub-millimeter resolution with good sensitivity. We present a Compton PET system and estimate its performance using a proof-of-concept prototype. A prototype single-slice imaging instrument was constructed with two silicon detectors 1 mm thick, each having 512 1.4 mm x 1.4 mm pads arranged in a 32 x 16 array. The silicon detectors were located edgewise on opposite sides and flanked by two non-position sensitive BGO detectors. The scanner performance was measured for its sensitivity, energy, timing, spatial resolution and resolution uniformity. Using the experimental scanner, energy resolution for the silicon detectors is 1%. However, system energy resolution is dominated by the 23% FWHM BGO resolution. Timing resolution for silicon is 82.1 ns FWHM due to time-walk in trigger devices. Using the scattered photons, time resolution between the BGO detectors is 19.4 ns FWHM. Image resolution of 980 microm FWHM at the center of the field-of-view (FOV) is obtained from a 1D profile of a 0.254 mm diameter (18)F line source image reconstructed using the conventional 2D filtered back-projection (FBP). The 0.4 mm gap between two line sources is resolved in the image reconstructed with both FBP and the maximum likelihood expectation maximization (ML-EM) algorithm. The experimental instrument demonstrates sub-millimeter resolution. A prototype having sensitivity high enough for initial small animal images can be used for in vivo studies of small animal models of metabolism, molecular mechanism and the development of new radiotracers.

Statistical performance evaluation and comparison of a Compton medical imaging system and a collimated Anger camera for higher energy photon imaging

In radionuclide treatment, tumor cells are primarily destroyed by charged particles emitted by the compound while associated higher energy photons are used to image the tumor in order to determine radiation dose and monitor shrinkage. However, the higher energy photons are difficult to image with conventional collimated Anger cameras, since a tradeoff exists between resolution and sensitivity, and the collimator septal penetration and scattering is increased due to the high energy photons. This research compares imaging performance of the conventional Anger camera to a Compton imaging system that can have improved spatial resolution and sensitivity for high energy photons because this tradeoff is decoupled, and the effect of Doppler broadening at higher gamma energies is decreased. System performance is analyzed by the modified uniform Cramer-Rao bound (M-UCRB) algorithms based on the developed system modeling. The bound shows that the effect of Doppler broadening is the limiting factor for Compton camera performance for imaging 364.4 keV photons emitted from 131I. According to the bound, the Compton camera outperforms the collimated system for an equal number of detected events when the desired spatial resolution for a 26 cm diameter uniform disk object is better than 12 mm FWHM. For a 3D cylindrical phantom, the lower bound on variance for the collimated camera is greater than for the Compton imaginer over the resolution range from 0.5 to 2 cm FWHM. Furthermore, the detection sensitivity of the proposed Compton imaging system is about 15-20 times higher than that of the collimated Anger camera.

Real time image reconstruction using GPUs for a surgical PET imaging probe system

We present an on-line list-mode image reconstruction system using GPUs for a surgical PET imaging probe system. We used the nVidia GeForce 9800GTX+ and CUDA to reconstruct images. The proposed system can generate a three-dimensional image from simulated data in 70 msec. We also compared the processing time with respect to the number of LORs per subset. We are working on optimizing the CUDA code to reduce the processing time. The data for image reconstruction were obtained from Monte Carlo simulations. The PET imaging probe system provides intermediately reconstructed 3-dimensional images online in order to detect small tumors or remnants of surgically removed tumors either during surgery or preoperatively. The on line image reconstruction requires high speed computation. Even multi-processor computers may not satisfy the requirement. Fortunately, currently available GPUs provide high speed computation. We are exploring a variant of one-pass list-mode OSEM in conjunction with GPU-based coding to provide real time image reconstruction. We present intermediately reconstructed images by consecutively applying a non-overlapping data window or an overlapping data window on data. The PET probe system itself consists of a segment of a conventional PET scanner and a small, high-resolution PET detector equipped with a position-tracker. Coincidence lines-of-response (LORs) are collected between the two detectors. The imaging probes proximity to the lesions and the small size crystals in the probe contribute to high spatial resolution. The continuously updated 3D image is reprojected in real-time onto a plane whose orientation is driven by the tracking system. Based on the intermediate images and the preoperative PET scans, surgeons can survey suspicious regions in detail. If online reconstruction and display are achievable, the PET imaging probe can be viewed as a handheld, clinician-guided camera capable of seeing the distribution of the radiotracer. Note that the probe suffers from an inherent limited angle tomography problem. However, because we are interested in the FOV that is close to the PET imaging probe, the effect of limited angle tomography may not be so severe. The GPU based image reconstruction system will be integrated with a prototype PET imaging probe. While computation speed is not yet sufficient, it can be improved by using high-end GPUs and optimized CUDA code.

On-line sliding-window list-mode PET image reconstruction for a surgical pet imaging probe

We present an on-line image reconstruction method for a surgical PET imaging probe. We have been investigating the PET imaging probe in order to detect small tumors or remnants of surgically removed tumors either during surgery or preoperatively. The probe system consists of a segment of a conventional PET scanner and a small, high-resolution PET detector equipped with a position-tracker and collects coincidence events between them. The imaging probe’s proximity to the lesions and the small size crystals in the probe contribute to high spatial resolution. It is anticipated that in the clinical system, the acquired data will be presented to the clinicians or surgeons on line. In order to display the images on line, we use a variant of the list mode OSEM. By consecutively applying a nonoverlapping data window or an overlapping data window on data, we can present intermediately reconstructed images on line. The continuously updated 3D image is re-projected in real-time onto a plane whose orientation is driven by the tracking system. We are interested in implementing the real time PET imaging system using graphic process units (GPUs) and parallel processing. We tested out the algorithm using simulated data.

Timing in silicon pad detectors for Compton cameras and high resolution PET

Silicon pad detectors and self-triggering readout electronics have been developed for use in Compton cameras and very high resolution PET applications. Events of interest in both techniques are registered when one or more detector interactions occur within a narrow time coincidence window. And in both methods poor time resolution leads directly to poor discrimination between true and random coincidences. Initial coincidence timing measurements between two silicon detectors each having 512 1.4 mmtimes1.4 mm pads in a 32times16 array have demonstrated a cusp-shaped spectrum of 80 ns FWHM with long tails. Ideally, for PET and Compton camera applications, time resolution would be less than 10 ns FWHM. This investigation demonstrates that the shape of the spectrum and to a large extent its width result from time-walk due to (1) the wide range of energies deposited in the silicon detectors from Compton interactions, (2) the leading-edge threshold trigger employed in the readout ASIC, and (3) the ~200 ns peaking time in the fast-channel of the readout ASIC. Although walk is a major component of the timing spectrum, it is not the only reason for broadening as determined by both simulations and measurements correlating pulse-height with threshold-crossing time

A study of the effects of strong magnetic fields on the image resolution of PET scanners

Very high resolution images can be achieved in small animal PET systems utilizing solid state silicon pad detectors. In such systems with sub-millimeter intrinsic resolutions, the range of the positron is becoming the dominant contribution to image blur. The size of the positron range effect depends on the initial positron energy and hence the radioactive tracer used. For higher energy positron emitters, such as 68Ga and 94mTc which are gaining importance in small animal studies, the width of the annihilation point distribution dominates the spatial resolution. This positron range effect can be reduced by embedding the field of view of the PET scanner in a strong magnetic field. In order to confirm this effect experimentally we have developed a high resolution PET instrument based on silicon pad detectors that can operate in a 7 T magnetic field. In this paper we present the preliminary results of a study of the effects of magnetic fields up to 7 T on PET image resolution for 22Na and 68Ga point sources.

TOPEM: A multimodality probe (PET TOF, MRI, and MRS) for diagnosis and follow up of prostate cancer

Multimodality imaging plays a significant role on specific diagnosis of prostate cancer. An endorectal PET-TOF MRI probe, designed here, allows for improved SNR and NECR with respect to standard imagers, providing better functional diagnosis of prostate diseases.

An FPGA based DAQ system for the readout of Madeira PET probe

Madeira project aims to significantly improve three-dimensional (3D) nuclear medicine imaging technologies via a compact photon-sensitive probe interfaced to an external conventional PET ring and placed close to the region of interest. The probe consists of several modules densely packed. Each module is made of two high-resistivity silicon detectors of 1 mm thickness and 1040 square pixels of 1 mm2. The detectors are placed back-to-back at a distance of 0.8 mm. The pads are read out with the VATAGP7 chip, a Gamma Medica — Ideas designed application specific integrated circuit (ASIC). A FPGA based DAQ system has been designed and developed at IFIC-Valencia to read out the probe. The system consists of several DAQ boards working in parallel which control the acquisition process. The DAQ board has trigger and coincidence capabilities to be used in coincidence with a conventional PET scanner. This work describes the characteristics of the system and its architecture, and also the future activities.

An Investigation of an Intra-Operative PET imaging Probe

We present a Monte Carlo simulation of an intraoperative PET imaging probe system with high spatial resolution capability. We have been investigating imaging systems in which a detector having high spatial resolution operates in conjunction with a conventional imaging system. Based on this concept, the intra-operative PET imaging probe system consists of a segment of a conventional PET scanner and a small PET imaging probe. The coincidence events between the segment of the conventional PET scanner and the small imaging probe are collected. The high spatial resolution is obtained due mainly to the imaging probes proximity to the lesions and the small size crystals in the PET imaging probe. The PET imaging probe is equipped with a position tracker. By employing the position tracker, surgeons or clinicians can survey suspicious regions in detail by moving the probe during the imaging process. We cautiously expect that the image degradation due to limited-angle tomography will not be severe for small lesions in the near-field when using a simple list- mode ordered-subsets EM algorithm for the image reconstruction. We plan to continue characterizing the proposed imaging system in terms of adaptive imaging described by Barrett and limited angle tomography.

Very High Resolution Small Animal PET in Strong Magnetic Fields

Very high resolution images can be achieved in small animal PET systems utilizing solid state silicon detectors (Compton PET). In such systems with sub-millimeter intrinsic resolutions, the range of the positron is the largest contribution to image blur. The size of the positron range effect depends on the initial positron energy and hence the radioactive tracer used. For higher energy positron emitters, such as 124I and 94mTc which are gaining importance in small animal studies, the variation of the annihilation point dominates the spatial resolution. It has been suggested that this positron range effect can be reduced by embedding the PET field of view in a strong magnetic field. Conventional PET systems using scintillators and photomultiplier tubes require extensive modifications to operate in magnet fields; however, our silicon detector based system can operate in magnetic fields with minimal modifications. In this paper we present a progress report of embedding our small animal PET test-bench in magnetic fields up to 7 Tesla.