W. Leslie Rogers

A Hybrid Maximum Likelihood Position Computer for Scintillation Cameras

Maximum likelihood (ML) estimators offer advantages of improved spatial resolution and linearity over traditional position estimates in position sensitive detectors. We have constructed a two board, multibus based hybrid position computer capable of performing the ML estimate at SPECT countrates. In addition, the board can implement any estimate linear in the photomultiplier tube outputs.

Potential of a Compton camera for high performance scintimammography

In this paper, we present a novel approach to scintimammography that is based on the Compton camera principle. We analyse the performance of our scheme using Monte Carlo simulations. In particular, we evaluate the detection efficiency, spatial resolution and lesion visibility of the system at several gamma photon energies. The simulation results show that the proposed technique achieves an absolute detection efficiency of 0.03 and a full width at half maximum resolution of 3.8 mm at 141 keV photon energy for point sources 2.5 cm deep in a 5 cm thick breast phantom using 5 mm thick silicon detectors. Furthermore, our approach shows good performance in lesion detection, especially at high gamma photon energies, where mechanically collimated systems perform poorly due to severe septal penetration. With total collected counts of 1.35 million, equivalent to a 30 s acquisition time for an activity concentration level of 3.7 kBq ml(-1) (100 nCi cm(-3)) in normal breast tissue, and a tumour-to-background ratio of 8:1, our system can clearly reveal an 8 mm diameter tumour that is located 2.5 cm deep in a 500 ml breast phantom. We also present a simulation-based quantitative performance comparison between the proposed scintimammographic system and the compact collimated scintimammographic system in the task of lesion detection under a clinical imaging situation using a non-prewhitening matched filter observer model. Our comparison demonstrates that for the same imaging time, the two systems have a comparable performance in detecting an 8 mm tumour at 141 keV, with the proposed system performing marginally better. However, the proposed scintimammographic system clearly outperforms the compact collimated counterpart in the detection of a 5 mm tumour. We also investigate the contribution of scatter and direct radiation from adjacent organs. We find that the background contribution of liver to the right breast is 30% at 141 keV, which can be reduced to 4.8% with shielding.

Design of a very high-resolution small animal PET scanner using a silicon scatter detector insert

A small animal positron emission tomography (PET) instrument using a high-resolution solid-state detector insert in a conventional PET system was investigated for its potential to achieve sub-millimeter spatial resolution for mouse imaging. Monte Carlo simulations were used to estimate the effect of detector configurations (thickness, length and radius) on sensitivity. From this initial study, a PET system having an inner cylindrical silicon detector (4 cm ID, 4 cm length and 1.6 cm thickness composed of 16 layers of 300 microm x 300 microm x 1 mm pads), for scattering, surrounded by an outer cylindrical BGO scintillation detector (17.6 cm ID, 16 cm length and 2 cm thickness segmented into 3 mm x 3 mm x 20 mm crystals), for capture was evaluated in detail. In order to evaluate spatial resolution, sensitivity and image quality of the PET system, 2D images of multiple point and cylinder sources were reconstructed with the simulation data including blurring from positron range and annihilation photon acollinearity using filtered backprojection (FBP). Simulation results for (18)F demonstrate 340 microm FWHM at the center of the field of view with 1.0% sensitivity from the coincidence of single scattering events in both silicon detectors and 1.0 mm FWHM with 9.0% sensitivity from the coincidence of single scattering in the silicon and full energy absorption of the second photon in the BGO detector.

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.

Application of Monte-Carlo Methods to the Design of Spect Detector Systems

Monte-Carlo methods have been applied to the design of a detector suitable for use in a SPECT cylindrically shaped scintillation camera. Included in the study are the calculated detection characteristics of two scintillator materials and the optical performance of several geometric configurations. Results include maps of the light distribution for several rectangular crystal-light guide combinations, a comparison of various approaches to specifying the optical properties of detector surfaces, and estimates of relative light output for various geometries.

ω-123I-Hexadecanoic acid metabolic probe of cardiomypathy

The utility of ω-123I-hexadecanoic acid myocardial scintigraphy as a metabolic probe of cardiomyopathies was investigated. Sixteen patients with a variety of cardiomyopathies and myopathies that involve cardiac muscle and ten volunteers were imaged in the postabsorptive state in a 40° LAO projection after a standard dose of ω-123I-hexadecanoic acid. An elimination T1/2 was calculated from the left ventricular myocardial time-activity curve. An uptake index, corrected for chest wall attenuation, was also computed in 7 of 10 volunteers and 8 of 16 patients.Of the 16 patients, only 2 had distinctly abnormal ω-123I-hexadecanoic acid myocardial tracer kinetics. The first patient had a metabolic disorder of which cartine deficiency was one component. The second patient had endocardial fibroelastosis, a process which has been linked to disorders which deprive the myocardium of oxygen and energy. Therefore, the cardiomyopathy may have been caused by some abnormality of cardiac metabolism other than carnitine deficiency. Although of limited utility in the overall cardiomyopathic population, ω-123I-hexadecanoic acid myocardial scintigraphy should be further investigated as a screening test for carnitine deficiency and related metabolic abnormalities in patients at risk.

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.

Computer analysis of cardiac radionuclide data

HE impact of computers on Nuclear Cardiology and the progress which has been made through computer applications is perhaps best illustrated by several historical observations. First, it is of note that all clinically important types of nuclear cardiologic procedures were initially performed without the aid of a computer. For example, the first gated equilibrium, blood pool ventriculograms performed by Zaret et al.‘** were accomplished by analog imaging. A physiologic trigger or gate was used to turn the gamma camera image recording system on and off to define “end systole” and “end diastole” which were the only two images obtained. The gating “window” used for end diastole was the 60-msec interval immediately prior to the Rwave. For end systole, data were recorded during the T-wave. Quantitative analysis was then accomplished manually. The analog images were projected to life size and the ventricular borders traced by hand. Ventricular volumes and ejection fractions were calculated using area length formulae adapted from contrast angiography. The data obtained in this fashion correlated well with results from contrast angiography and provided a strong motivation for further technical development. Likewise, in early applications of the first pass technique, computers were not used. Rather, data were recorded on video tape for subsequent replay and analysis.3 These analog imaging techniques were extremely time consuming, particularly for purposes of quantitative analysis, took advantage of only a small fraction of the potentially available data, and provided limited visual appreciation of cardiac dynamics. The first logical adaptation of the computer to radionuclide ventriculography was simply to acquire the gated end diastolic and end systolic frames into computer memory, instead of onto film, one frame at a time. Initially the data analysis was again performed by hand by tracing ventricular contours on the computer screen. As before, this analytic approach was quite tedious and partly for this reason very little clinical use was made of radionuclide ventriculography, initially. However, several breakthroughs in computer techniques occurred in the early 1970s which established radionuclide ventriculography as a feasible clinical procedure. First, Seeker-Walker and Parker4,’ recognized that after equilibration in the blood pool the net ventricular count rate resulting from an intravascular tracer (e.g., Tc-99m albumin or Tc-99m RBC) at any time in the cardiac cycle is proportional to ventricular blood volume. This germinal observation permits calculation of the ventricular ejection fraction and other quantitative parameters through analysis of net counts (i.e., background corrected) in the ventricles (a task for which the computer is uniquely suited) rather than by the more laborious geometric technique. The formula for the count based ejection fraction is:

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.

A fast least-squares arrival time estimator for scintillation pulses

The true weighted least-squares (WLS) arrival time estimator for scintillation pulse detection was previously found to out-perform conventional arrival time estimators such as leading-edge and constant-fraction timers, but has limited applications because of its complexity. A new diagonalized version of the weighted least-squares (DWLS) estimator has been developed which, like the true WLS, incorporates the statistical properties of the scintillation detector. The new DWLS reduces estimator complexity at the expense of fundamental timing resolution. The advantage of the DWLS implementation is that only scalar multiplications and additions are needed instead of the matrix operations used in the true WLS. It also preserves the true WLSs ability to effectively separate piled-up pulses. The DWLS estimator has been applied to pulses which approximate the response of BGO and NaI(Tl) scintillation detectors. The timing resolution obtained with the DWLS estimator is then compared to conventional analog timers along with the Cramer-Rao lower bound on achievable timing error. The DWLS out-performs the conventional arrival time estimators but does not provide optimal performance compared to the lower bound; however, it is more robust than the true WLS estimator. >