Comparison of 3-D OSEM versus 1-D SAGE for focal total-activity quantification in I-131 SPECT with HE collimation

Abstract

A three-dimensional (3-D) ordered-subset expectation maximization (OSEM) reconstruction algorithm used fits to the average behavior of the I-131 point-source-response function at different depths to model the detector-collimator response of a SPECT camera. The fitting function was a Gaussian, plus a single exponential to account for septal-penetration of the high-energy (HE) collimation. Results from that unregularized 3-D OSEM were compared to those with regularized one-dimensional (1-D) space-alternating generalized expectation maximization (SAGE). The latter used a simple strip integral. Each reconstruction algorithm employed attenuation correction and scatter compensation. The SPECT camera was calibrated by imaging a 200 cm 3 sphere containing a known I-131 activity. This sphere was located inside an elliptical cylinder. The water in the cylinder contained one of four I-131 activity concentrations. The ratio of background-over-target activity concentration b was used to characterize the different situations. The known activity and the reconstructed counts within the sphere at each b value yielded a counts-to-activity conversion factor, CF. After reconstruction with 3-D OSEM, CF varied much less with b than after reconstruction with 1-D SAGE. A test of activity quantification was carried out with 20 realizations of an anthropomorphic phantom with zero background activity. This phantom simulated a 100 cm 3 spherical tumor centrally located inferior to the lungs. The mean bias in the estimate of "tumor" activity was 7 3% with 3-D-OSEM-based quantification and 4 3% with 1-D-SAGE-based quantification.Recovery-coefficient-based correction was applied with both algorithms. With UHE collimation, the previously-published mean bias in the estimate of the "tumor" activity was 5 2% after adding a recovery-coefficient-based correction. For that collimation, the previously-published mean bias with 1-D-SAGE-based quantification was 10 7%. We conclude that 1) two-component fitting allowed 3-D OSEM to do a good job of compensating for the septal penetration of the HE collimation, 2) the procedure for activity quantification with 3-D OSEM was simpler than that with 1-D SAGE, and 3) activity accuracy for a test phantom had a similar quality for four situations, using 3-D OSEM-based activity quantification and 1-D-SAGE-based activity quantification and combining each with HE and UHE collimation.

Optimizing the number of equivalent iterations of 3D OSEM in SPECT reconstruction of I-131 focal activities

“…The curve shape was that of a typical convergence, a fast increase followed by a slower approach toward an asymptote. Such convergence had been observed earlier for a 200 cm 3 sphere [6].…”

131I-Tositumomab Radioimmunotherapy: Initial Tumor Dose–Response Results Using 3-Dimensional Dosimetry Including Radiobiologic Modeling

“…Posttherapy SPECT projection data were corrected for dead time using a paralyzable model (13). Projection data were reconstructed with 35 iterations (6 subsets) of 3D ordered-subsets expectation maximization including triple-window–based scatter correction, CT-based attenuation correction, and compensation for depth-dependent detector response (14,15). …”

MIRD Pamphlet No. 23: Quantitative SPECT for Patient-Specific 3-Dimensional Dosimetry in Internal Radionuclide Therapy

“…7). When full CDR compensation is desired, the total (i.e., including intrinsic, geometric, penetration, and scatter components) distance-dependent CDR can be determined experimentally by measuring the in-air point-source response at various distances from the collimator face (50,51) or by Monte Carlo simulations of this geometry (52). The total CDR should be determined for the radionuclide of interest, as both septal penetration and scatter are energy-dependent (in contrast to the geometric component).…”

“…The source can then be placed on a low-scatter medium (such as a Styrofoam [The Dow Chemical Co.] cup) positioned at the required distance from the collimator. The planar acquisitions to determine CDR should be performed using a fine image matrix (512 × 512) and for at least 5 source–collimator distances in a clinically relevant range (2–30 cm) (50,51). For medium- and high-energy photon emitters, a gaussian function in combination with decreasing exponential terms can be used to fit the measured CDR (50,51).…”

“…The planar acquisitions to determine CDR should be performed using a fine image matrix (512 × 512) and for at least 5 source–collimator distances in a clinically relevant range (2–30 cm) (50,51). For medium- and high-energy photon emitters, a gaussian function in combination with decreasing exponential terms can be used to fit the measured CDR (50,51). The exponential terms are used to account for the wider tails of the response function due to collimator scatter and penetration.…”