S.G. Kohlmyer

The effect of camera geometry on singles flux, scatter fraction and trues and randoms sensitivity for cylindrical 3D PET-a simulation study

Preliminary results of an assessment of the effects of changing the axial field of view (AFOV) and detector ring diameter (DRD) of a cylindrical PET tomograph on count rate performance are presented. The assessment was made using Monte Carlo simulations of an anthropomorphic phantom based on the Zubal phantom. This phantom was modified to include cylinders approximating arms and legs, and was sequentially stepped through the AFOV to simulate a whole-body scan covering an axial region of interest of 1 m. DRD was varied from /spl sim/60 cm to /spl sim/108 cm, and AFOV was varied from 10 cm to 60 cm. A simple activity distribution and dead time model was assumed to allow the calculation of noise-equivalent count (NEC) rates for a situation similar to that of a typical 18F-FDG study. Both the scatter fraction and singles flux were found to be strongly dependent on DRD, but only weakly dependent on AFOV when the latter was greater than /spl sim/25 cm. Trues and randoms sensitivity were strongly dependent on AFOV, and randoms sensitivity was also strongly dependent on DRD. Scatter and singles flux do not appear to be limiting factors for extended AFOV configurations, and randoms rates, while high, appear to be manageable with existing detector technology. This initial assessment suggests that for whole-body applications, substantial gains in NEC may be possible by extending the AFOV.

Investigation of bias-free positioning estimators for the scintillation cameras

Bias-free positioning estimators for scintillation cameras are investigated. A linear correlation coefficient (LCC) and Chi square error (CSE) method are evaluated with respect to linearity and spatial resolution performance. The LCC method uses correlation information between the true function (i.e., light response function) and measured data in mapping an event characterization vector to the associated position. The CSE method estimates the position where the Chi square error between two functions becomes minimized. In order to determine true statistics as a function of position, the light response function (LRF) was estimated based on sample measurements by using a cubic spline interpolation technique which provides smooth first-order and continuous second-order derivative of the LRF. Both methods have superior linearity properties compared to the weighted centroid. Each method call be considered as a bias-free positioning estimator within the effective field of view of the detector. The spatial resolution performance of the CSE method is /spl sim/7% and /spl sim/16% better than the weighted centroid method for a 16- and 25-mm-thick crystal, respectively. The spatial resolution performance of the LCC method is comparable to that of the weighted centroid method.

Improving the performance of the SP-3000 PET detector modules

A major concern in the use of BaF/sub 2/ detectors in a positron emission tomograph (PET) is the collection of the UV light from the fast decay component of the scintillator. A series of experiments with the standard SP-3000 commercial detector module confirmed that the coupling compound formulation was reasonable, but that the reflector material used to wrap the crystals was not optimal. Rebuilding the detector modules with PTFE Teflon increased the light output from the detector modules by factors of three. Initial tests with phantoms using two rings of rebuilt detectors in the University of Washington BaF/sub 2/ PET system confirmed improvements in resolution and contrast in comparison to the original SP-3000 modules.<<ETX>>

An XYE acquisition interface for General Electric Starcam Anger cameras

To support more detailed experimental studies into various scatter correction techniques for single photon imaging studies, an interface was designed and built for acquiring position and energy data from the General Electric Starcam series of scintillation cameras. The interface monitors the main camera data pipeline without disturbing normal camera operations. Data events are latched and presented to a 16-b parallel output port. The 32-b data word is shifted out as two 16-b data words using a simple handshake protocol. If data are not transferred within a preset time, the interface resets and latches a data set. In addition to spatial and energy information, the data word includes information about detector motion and a run time-clock.<<ETX>>

Performance characteristics of a second generation micro crystal element (MiCE2) detector

This work reports on performance characteristics of a second generation micro crystal element (MiCE2) detector for a dedicated PET system to image mice. Our MiCE2 detector consists of a 22/spl times/22, array of 0.8/spl times/0.8/spl times/6 mm mixed lutetium silicate (MLS) crystals. Five sides of the crystals are polished with one 0.80.8 mm face left unpolished. The crystals are placed within a grid made of a highly reflective polymer film material. The grid optically isolates the crystals and also functions as a reflective wrap. The detector unit is directly coupled to a 6+6 cross-anode position sensitive PMT. The crystal of interaction is determined using simple Anger style logic. Crystal maps have been created for a 22/spl times/22 crystal array. All 484 crystals are visualized in the full detector modules crystal map. The average peak to valley ratio between neighboring crystals was 6.4. There was greater than a factor of two difference between the photopeak energy channel for high versus low light collection efficiency crystals. The energy resolution for individual crystals varied between 14% and 23%. Partial detector arrays (e.g., 22/spl times/4) using 0.8/spl times/0.8/spl times/10 mm crystals with an etched surface finish have also been built and decoded. A miniature line source phantom consisting of five 1 mm diameter lines with 2 mm center-to-center spacing has been imaged using two partial MiCE2 detectors. All lines are distinguished and. the average peak to valley ratio between lines is 2.7.

Evaluation of low energy threshold settings for PVI PET systems

We studied the effects of adjusting the low level energy discriminator (LLD) for positron volume imaging (PVI) mode acquisitions using a GE Advance PET system. NEMA scatter fraction and count loss measurements were performed for a 20 cm right circular cylinder at LLD between 300 keV and 450 keV. From these data, noise equivalent count rate LLD (NECR) curves were calculated to estimate effects of LLD on scans of head sized objects. To evaluate the effect of LLD on whole-body image quality, contrast-to-noise ratio (CNR) values were obtained from images processed without attenuation or scatter correction. CNR data were acquired for 1.6 cm and 1.0 cm spherical lesions within the liver region of a Data Spectrum torso phantom scanned with arms, and activity outside the field of view. The results indicate that the NECR for a head-sized object is fairly insensitive to LLD differences between 300 keV and 425 keV for activity concentrations up to 0.4 /spl mu/Ci/cc. The NECR improved slightly for higher concentrations with 375 keV<LLD<425 keV. Torso phantom images processed without scatter or attenuation correction show some CNR improvement with increasing LLD up to 425 keV for clinically relevant activity concentrations.

Characterization of single and multiple scatter from matter and activity distributions outside the FOV in 3-D PET

Operation of PET in 3-D mode, without internal septa results in increased sensitivity but at the cost of increased scatter fraction. Significant amounts of scattered events in 3-D PET either originate from activity outside the FOV (OFOV) or scatter from matter outside the FOV, complicating scatter correction methods. We performed 3-D PET simulations with scanner geometry based on the GE Advance with several phantom geometries designed to characterize all scatter with particular attention to that from outside the FOV. Scatter from OFOV matter was deduced by performing simulations with matter and activity within the FOV only (short phantom). Studies were repeated with phantoms with identical characteristics inside the FOV but with additional OFOV material (long phantom) but no activity outside the FOV. The incremental effect of OFOV activity was then determined by repeating the long phantom studies with additional activity. Scatter was also classified as single or multiple. In summary, the data allow extraction of yields from unscattered and all types of scattered events. The effect of source position, both within and external to the FOV, on scatter was determined by performing a series of simulations with point sources and techniques similar to those described above. The effect of energy discriminator setting (with BGO detectors) was also studied and data sets were generated for five settings from 300 to 425 keV. Using a Zubal phantom with the lungs and heart in the FOV and the abdomen outside the FOV and with a 300 keV discriminator, we find a total scatter fraction of 57%. Of the total scatter, 7.4% comes from OFOV matter, and 24% from OFOV activity. If the discriminator is increased to 425 keV the scatter fraction drops to 36%. OFOV scatter, particularly from OFOV activity, becomes relatively less important with the OFOV matter contribution dropping to 5.3% and OFOV activity contribution to 17%. Additionally, the spatial distributions of scatter from external matter and external activity were found to be different.

Slat collimator design issues for dual head coincidence imaging systems

This paper investigates optimum slat collimator design parameters for dual-head coincidence imaging (DHCI) systems. The noise equivalent count (NEC) rate was examined with respect to the activity concentration under various system conditions. All results are derived from Monte Carlo simulations with a digital anthropomorphic (Zubal) phantom. The DHCI system was modeled after the Millennium VG gamma camera (GEMS, Waukesha, WI). The dead-time characteristics of the camera were experimentally determined. Our results suggests that substantial NEC gains can be achieved by varying the slat-to-slat separation, such that the peak of the NEC curve is located at clinically relevant levels (i.e., 0.07 /spl sim/ 0.10 /spl mu/Ci/cc). The NEC was also found to increase with the use of longer slats with appropriately selected slat-to-slat separation. Furthermore, the NEC performance also depends on the count-rate performance (i.e., dead-time losses) of the system. Therefore, as improvements are made to the count-rate capabilities of DHCI systems, the slat geometry should be modified. Further study is required to determine the effect that slat collimator design has on image quality and lesion detection for clinically realistic imaging situations.

Comparison of NEC and subjective image quality measures in 2D and 3D whole-body PET imaging

Twelve clinical 3D whole-body PET studies have been used to compute noise-effective counts (NEC) for comparison with observer study results. Clinical data was collected using data acquired on a GE Advance/spl trade/ PET imaging system. Data acquisition for the studies was performed by acquisition of multi-frame 2D & 3D emission data along with a transmission scan. Patient weights ranged from 112 lb to 320 lb (average=178 lb); injected activity ranged from 7.7-12.9mCi (average=10.1); 2D and 3D post-injection frame acquisition start times averaged 74 and 125 minutes respectively. NEC was calculated using a k factor based upon the fraction of randoms within the patient boundary, found by segmentation of the transmission sinograms, and assuming real-time randoms subtraction.

Design And Development Of A Collimator And Robot For Use In Detector/collimator Studies

As part of a project aimed at understanding and accounting for collimator effects in SPECT (single photon emission computed tomography) and PET (positron emission tomography) images, a robot and collimator system is being constructed to accurately position a narrow beam of radiation. The beam from the collimator will have a diameter of no more than 5 mm FWTM (full width tenth maximum) at a distance of 30 cm and will improve, due to penumbra effects, as the distance decreases. Positioning will be done by a wrist (two degrees of angular motion) on a Cartesian arm (three degrees of linear motion) controlled by a CAMAC (Computer Automated Measurement and Control) system and LabView software on the Macintosh. Beam positioning error should be less than 2.5 mm at a distance of 30 cm. While the majority of the use will be in a bench environment, the design will also function within the tunnel of a whole body PET scanner or with a SPECT camera. With this in mind, the stepping motor drivers will toggle a signal, compatible with standard cardiac gating signals, to signify if the arm is stationary or in motion. This will allow sorting of many positions taken during the same acquisition on most PET and SPECT scanners. As a further convenience, the collimator assembly will have an internal laser that will aid in calibration of the robot and accurate positioning of the beam.