J.W. LeBlanc

C-SPRINT: a prototype Compton camera system for low energy gamma ray imaging

An electronically-collimated imaging system is being built using pixellated, low-noise, position-sensitive silicon as the first detector, and a sodium iodide scintillation detector ring as the second detector. The system consists of a single 3/spl times/3/spl times/0.1 cm/sup 3/ silicon pad detector module with 1 keV FWHM (noise-limited) energy resolution centered at the front face of a 50 cm diameter, 10 cm long NaI detector annulus. Custom acquisition and timing electronics have been manufactured to minimize system dead time. Monte Carlo modeling is used to predict system sensitivity and position resolution. Simulations using the existing setup show angular uncertainties of 4.1/spl deg/ and 2.1/spl deg/ FWHM for /sup 99m/Tc and /sup 131/I point sources, respectively (7.2 mm and 3.7 mm at 10 cm). Sensitivity can be improved by more than a factor of a hundred over the existing setup by stacking five 1 mm thick 9/spl times/9 cm/sup 2/ silicon arrays and redesigning the second detector geometry to accept a wider range of scattering angles. Lower bound calculations show that our electronically-collimated camera system challenges current mechanically-collimated systems for both /sup 99m/Tc and /sup 131/I despite the deleterious effects of Doppler broadening. Preliminary measurements show a timing resolution of 41 ns FWHM between the silicon detector and a single SPRINT module.

Improved modeling of system response in list mode EM reconstruction of Compton scatter camera images

An improved List Mode EM method for reconstructing Compton scattering camera images has been developed. First, an approximate method for computation of the spatial variation in the detector sensitivity has been derived and validated by Monte Carlo computation. A technique for estimating the relative weight of system matrix coefficients for each gamma in the list has also been employed, as has a method for determining the relative probabilities of emission having some from pixels tallied in each list-mode back-projection. Finally, a technique has been developed for modeling the effects of Doppler broadening and finite detector energy resolution on the relative weights for pixels neighbor to those intersected by the back-projection, based on values for the FWHM of the spread in the cone angle computed by Monte Carlo. Memory issues typically associated with list mode reconstruction are circumvented by storing only a list of the pixels intersected by the back-projections, and computing the weights of the neighboring pixels at each iteration step. Simulated projection data has been generated for a representative Compton camera system (CSPRINT) for several source distributions and reconstructions performed. Reconstructions have also been performed for experimental data for distributed sources.

Experimental results from the C-SPRINT prototype Compton camera

A Compton camera is being tested for nuclear medicine applications. Our design uses a single 3 cm by 3 cm silicon pad detector as the first detector system, and SPRINT, an array of position-sensitive sodium iodide modules, as the second detector. Experimental results with a /sup 99m/Tc point source show coincidence energy spectra agreeing with theoretical predictions. The coincidence energy spectra for both silicon and SPRINT detectors correspond to the geometry-determined scattering angle range. Recorded energy falls outside of strict geometric limits because of Doppler broadening and detector energy resolution effects. The summed energy peak in the initial data run for a /sup 99m/Tc source has a FWHM energy resolution of 33 keV, primarily due to energy uncertainty in the SPRINT modules. A second data run showed an improvement to 25 keV in summed energy resolution due to careful calibration of, and correction for, significant first and second detector gain non-uniformities. Images generated from the second acquired data set result in a backprojection image resolution of 1.5 cm at a source distance of 10 cm. Analytical and Monte Carlo calculations show a very close agreement of 1.6 cm. Using a list-mode maximum likelihood EM reconstruction algorithm, the image resolution is improved to 7 mm, although the resolution recovery is at the expense of increased noise in the image.

A Compton camera for nuclear medicine applications using 113mIn1

Abstract Theoretical studies show our prototype Compton camera, C-SPRINT, matches the 99m Tc performance of clinically available mechanically collimated systems if an advantage in sensitivity of ∼45 can be achieved. Imaging at higher energies substantially reduces the required sensitivity advantage. At ∼400xa0keV, our Compton camera system needs only five times the raw count rate of a mechanically collimated system imaging at 99m Tc energy to reach the performance “break even” point. We analyze our C-SPRINT system performance for the isotope 113m In (391.7xa0keV), and compare it to a collimated system imaging 99m Tc. 113m In has been used in nuclear medicine applications in the past, and can potentially be used to label many of the same radiopharmaceuticals as 99m Tc. In order to fully compare the two systems, their relative sensitivities are combined with the relative amount of useful gamma rays that escape the object being imaged (the patient) for the same patient radiation dose. Results for uniformly distributed sources show that for equal lifetime radiation dose, the ratio of useful 99m Tc to 113m In gamma rays is 1.59. For a point source of activity centered inside the ellipsoid, the useful ratio decreases to 1.33. These fractions scale up the required raw sensitivity advantage to yield a required sensitivity advantage of 5 – 8. Monte Carlo simulations have shown that a raw sensitivity advantage of 25 can be achieved by improving C-SPRINT geometry and using a larger volume of silicon detectors. We conclude that gains of 3–5 in noise equivalent sensitivity are achievable when imaging 113m In with our Compton camera relative to a collimated system imaging 99m Tc.

Quantitative evaluation of information loss for Compton cameras

Compton cameras decouple the inverse relationship between spatial resolution and detection sensitivity which compromises the performance of conventional collimated cameras. However, this improvement is usually achieved at the expense of the amount of information conveyed by each detected photon. In this paper, we propose a simple approach to calculate the information loss for a ring Compton camera. We describe this information loss as decoding penalty, defined as the ratio of the variance of reconstructed intensity for a pixel of interest for a ring Compton camera to that for a mechanically collimated camera normalized on a per-detected-photon basis. The uniform Crame/spl acute/r-Rao bound, our mathematical tool, provides a lower bound on the variance that is dependent on the system and data statistics alone, rather than the estimator. The results suggest that ring Compton cameras perform comparably to conventional collimated cameras at an incident photon energy of 140 keV and substantially outperform their counterparts at 364 keV.

Theoretical performance comparison of a Compton-scatter aperture and parallel-hole collimator

Using the Uniform Cramer-Rao bound on variance, the performance of a conventional parallel-hole collimator has been compared with that of a ring-geometry Compton-scatter aperture. Objects used for the evaluation were uniformly emitting disk sources with diameters ranging from about 4-30 cm. Results for a 7.5 cm diameter disk and 141 keV incident energy show that a Compton-scatter aperture, constructed with Si detectors having 230 eV FWHM energy resolution, can be 15 times more efficient than a standard parallel-hole collimator at the same operating point.

DETERMINING DETECTOR REQUIREMENTS FOR MEDICAL IMAGING APPLICATIONS

Abstract The Cramer–Rao lower bound on estimator variance is used as a tool for assessing radiation detector requirements for nuclear medicine imaging. The classical bound for unbiased estimators is reviewed and its relationship to the well-known propagation-of-error formula is demonstrated. The uniform Cramer–Rao bound is described for situations where unbiased estimation is impractical such as in tomographic imaging. The bounds are used to evaluate the necessary detector requirements for a CsI(Tl)/Si photodiode scintillation camera, and to compare the performance of a Compton-scatter camera to that of a parallel-hole collimator/Anger camera system.

/sup 99m/Tc imaging performance of the C-SPRINT Compton camera

C-SPRINT is an electronically-collimated imaging system that has been built using pixelated, low-noise, position-sensitive Si as the first detector, and a NaI scintillation detector ring as the second detector. The system consists of a single 4.5/spl times/1.5/spl times/0.03 cm/sup 3/ Si pad detector module with /spl sim/2 keV energy resolution centered at the front face of a 50 cm diameter, 12 cm long sodium iodide detector annulus. System sensitivity measurements using /sup 99m/Tc show an absolute efficiency of 1.8/spl times/10/sup -7/. This value is an order of magnitude lower than predicted due to the combination of worse than expected silicon detector triggering performance, timing resolution issues, and system dead time effects. Spatial resolution measurements are within 10% of analytical predictions. Measured resolution for the /sup 99m/Tc point source is 15 mm FWHM. Images obtained were compared with measurements using a clinically-available mechanically collimated Anger camera. A resolution-variance study shows that the C-SPRINT camera performance on a per-detected photon basis is worse than the Anger camera for /sup 99m/Tc, but is similar for /sup 131/I, as predicted by theory. Gains in raw system sensitivity of a Compton camera similar in design to C-SPRINT can potentially lead to substantial improvements in noise-equivalent performance of electronically-collimated cameras over mechanical systems.

Decoding penalty calculation for a ring Compton camera using uniform Cramer-Rao bound

Compton cameras completely decouple the inverse relationship between spatial resolution and detection sensitivity which compromises the performance of conventional collimated cameras. However, this improvement is usually achieved at the expense of the amount of information conveyed by each detected photon. In this paper, we propose a simple approach to calculate the information loss for a ring Compton camera. We describe this information loss as decoding penalty, defined as the ratio of variance of reconstructed intensity for a pixel of interest for a ring Compton camera to a mechanically collimated camera normalized to a per-detected-photon basis. The uniform Cramer-Rao bound, our mathematical tool, provides a lower bound on the variance dependent on the system and data statistics alone rather than the estimator. Results of a decoding penalty calculation suggest that ring Compton cameras outperform conventional cameras at incident photon energies of 364 keV and higher.