The value of local normal limits in quantitative Thallium-201 CZT MPI SPECT.

BACKGROUND: Considering the distinctive characteristics of CZT detectors, automatic quantification of ischemia using normal limits included with software package may deliver suboptimal results for CAD detection. The present study aims to evaluate the benefits of creating normal limits specific to a local population and laboratory protocol. METHODS AND RESULTS: Two groups were selected from patients who had undergone a CZT MPI. Normal limits were generated with the QPS application based on the population with low likelihood of CAD. Using the vendor-supplied and the population-specific normal limits i-TPD and vessel-specific SDS results obtained for patients who had subsequently undergone coronary angiography were compared with coronary angiography data. A weak correlation was observed for low i-TPD (stress TPD minus rest TPD) and SDS values. Both databases gave similar values for the area under the ROC curve concerning i-TPD (0.75 to 0.74) and SDS results (0.72 to 0.75 for the LAD, 0.62 to 0.64 for the LCx, and 0.63 to 0.67 for the RCA). Sensitivity (60%), specificity (78%), and predictive positive (84%) and negative (52%) values were also similar with a diagnostic and prognostic threshold value. CONCLUSION: The use of a population-specific created database did not influence the diagnostic value of thallium-201 MPI QPS results using a CZT camera.

Activity standardisation of 45Ca and 204Tl using the new TDCR system at CMI.

A new triple-to-double coincidence ratio (TDCR) system has been established at CMI as an additional technique for primary activity standardisation. Details of the detectors and the electronics are given. Construction of the optical chamber was validated using measurement of a (3)H standard-the efficiency achieved with Ultima Gold was 58%. Several methods of efficiency variation (coloured bands painted on vial, positioning and photomultipliers defocussing) are compared. Activity concentrations of (204)Tl and (45)Ca were determined with the TDCR method and with the efficiency tracer technique. Advantages of the TDCR method compared to the efficiency tracing method and agreement between results are shown.

An approach based on the SIR measurement model for determining the ionization chamber efficiency curves, and a study of 65Zn and 201Tl photon emission intensities.

The measurement model used to determine ionization chamber efficiency curves accounts from the outset for impurity corrections and beta spectrum shapes. The curves are represented by exponentials of polynomials whose coefficients are adjusted using non-linear least-squares minimization. The curves are validated by comparing with SIR key comparison reference values (KCRVs) and other published curves. The associated covariance matrix is also evaluated. Deviations from model predictions for 65Zn and 201Tl using recommended nuclear data are studied.

Primary standardisation of 204Tl using the efficiency tracing method.

The electron-capture and beta(-) -emitting radionuclide 204Tl has been known from previous experience to show discrepancies between different methods of standardisation. Source preparation is also difficult due to the complex chemistry of thallium. The Bureau International des Poids et Mesures (BIPM) intercomparison held in 1997, showed discrepancies of up to 10% between measurements of solid sources and liquid scintillation methods. These problems have been the subject of a BIPM CCRI(II) working group since 1999. This paper presents the results from a primary standardisation of a 204Tl solution using 4pibeta-gamma coincidence counting and liquid scintillation counting. The tracer technique was used for the 4pibeta-gamma coincidence counting, where 204Tl was traced with 134Cs and 60Co. The extrapolation to 100% beta-efficiency was performed by three different approaches: foiling, adding carrier and measuring sources of different initial masses. The results showed that tracing with 60Co and using external foiling gave the same result as obtained by liquid scintillation counting. A comparison of the results achieved by the different methods of measurement, tracers and methods for extrapolation is presented and discussed in this paper.

The absolute standardization of 32P and 204Tl at LNMRI.

32P and 204Tl solutions were standardized within the frame of the international key comparisons organized by bureau international des poids et mesures, in 2002. The activity concentration of 32P was measured by counting solid sources in a 4pibeta proportional gas flow counter and by liquid scintillation counting. The self-absorption in solid sources for 4pibeta counting and the presence of 33P as an impurity were evaluated. The combined standard uncertainty for 32P was 0.59% in the 4pibeta counting and 0.38% in the liquid scintillation counting. Liquid scintillation counting was used to measure the activity concentration of 204Tl with combined standard uncertainty of 0.35%.

Standardization of tritiated water and 204Tl by TDCR liquid scintillation counting.

The triple to double coincidence ratio method was used with a combined uncertainty of 1-2% to standardize tritiated water and a solution of 204Tl. The vial with liquid scintillator, in which the sample to be measured was dissolved, is optically coupled to three photomultipliers. The electronic module MAC-3 assures the selection of double and triple coincidences count rates, D and T, from the three counting channels. It contains the gating circuits, necessary to obtain the livetime value and the extended deadtime circuit. Specific computer programs were used to calculate the free parameter value, the efficiency of D and so, the value of the activity. The optimal value of the Birks ionisation-quenching parameter, kB, was evaluated by changing the detection efficiency with grey filters. Three types of liquid scintillators, namely InstaGel, PPO+POPOP+Triton in toluene and Ultima Gold, were employed. 204Tl was measured in the frame of an international comparison organized by BIPM. For tritiated water a comparison was made with LNHB-Saclay; the relative difference between the obtained values for the massic activity was only 0.2%.

Standardization of 32P/33P and 204Tl by liquid scintillation counting.

Two solutions, one of 32P with a significant contribution of 33P, and another one of 204Tl were standardized by liquid scintillation counting in the frame of two intercomparison exercises organized by the BIPM in 2002. For the 32P solution, the counting efficiencies for 32P and 33P were obtained by the CIEMAT/NIST method. The individual contributions of 32P and 33P were determined by fitting a set of measurements to a function of the half-lives and counting efficiencies of both component nuclides. 204Tl was also standardized by the CIEMAT/NIST method using two counters, three scintillators, HiSafe 3, Insta-Gel Plus and Ultima-Gold, and two different kinds of samples. A total of 480 samples were measured over 45 days with results in full agreement within the uncertainties considered. The efficiency values are also compared to those used in the previous 204Tl intercomparison (1997).