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This release introduces two new features and implements four bug fixes. There are also some warnings/work-arounds about other bugs still present in the software.
A personnel note: Steven Vannoy has left the SimSET team to pursue a Ph.D. Robert Harrison is assuming Steven’s duties. If you have recently sent email to simset@u.washington.edu, please forgive our tardy or non-existent response—we have been focusing on the changeover of duties and the new release.
New Features
This release introduces two new features intended to improve simulation efficiency: tomograph files and finer stratification tables. It also reintroduces the new features from version 2.6, which had been temporarily withdrawn until the completion of bug fixes 1 and 2 below.Tomograph files allow the user to simultaneously simulate multiple tomographs. The finer stratification tables improve the performance of stratification for simulations with large acceptance angles.
- Tomograph files: The tomograph file feature enables SimSET to send the output from the photon history generator (the part of SimSET that models the object) to several tomographs at once. For instance, with this modification the user can test several different slat collimation geometries using only one SimSET run. Each tomograph file specifies a collimator, detector and binning parameter file. Formerly these files were specified in the run file (and the user can still specify them there—this modification is backward compatible). However, to simulate multiple tomographs the user deletes the references to these parameter files and adds a reference to a tomograph file for each tomograph to be simulated. This feature is described in greater detail on the tomograph files web page.
- Improved stratification: Formerly, stratification tables did not allow for fine enough sampling when the acceptance angle was set to large values. The acceptance angle was split into 12 stratification cells. As the acceptance angle often needs to be set to 90 degrees for simulations where scatter in the collimator is modeled, this was very coarse stratification. We have changed the software to use 48 stratification cells within the acceptance angle. Stratification is still used as it was in the past, however old productivity tables will not work with the new software. They need to be regenerated.
Bug fixes
This release fixes four bugs in the SimSET software. The first caused corruption of the weight and weight squared data in many PET simulations using Monte Carlo or slat collimation. The second caused the program to terminate unsuccessfully when certain features of the cylindrical PET detector were used. The third bug caused the number of accepted counts to be misreported in the screen output when 3DRP or a user binning function (PhgUsrBinPETPhotons2 or PhgUsrBinSPECTPhotons2) was used. The last bug resulted in some less-than-meaningful screen output from the collimator module.
- Bug 1: SimSET gave inaccurate results for any simulation that included (1) scatter in the collimator and (2) photon weight changes (i.e. stratification, forced detection, forced non-absorption, and forced interaction in the detector). The size of the effect depended on the collimator and detector modules used, and the proportion of detected coincidences that do not undergo scatter in the object, but do undergo scatter in the collimator.
As the current SPECT collimator is geometric (i.e., does not allow for scatter or penetration), this bug has no effect on SPECT simulations.
- Bug 2: When using cylindrical detectors with multiple axial rings, SimSET would terminate with an error message in detCylCompFreePathsToExit. This bug had no effect on output as none was produced. (I.e., as there was a fatal error, the program did not produce any erroneous output.)
- Bug 3: Any photons rejected by the distance limits in 3DRP or the user binning functions PhgUsrBinPETPhotons2 or PhgUsrBinSPECTPhotons2 were nevertheless included in the total accepted coincidences in the screen output. The number of counts in the output histograms were, however, correct, and could be summed to get the correct figure.
- Bug 4: The weight statistics reported to the screen by the collimator module did not include the decay weight. This had no effect on the weight statistics in the output histograms.
- Multiple binning files: We have had reports that the multiple binning files feature is misfunctioning intermittently. We have not yet determined under what circumstances this occurs. We suggest using the new tomograph file feature instead until we have tracked down the problem.
- MSRB: The number of counts is ambiguous for MSRB binning in PET. It is not clear whether, for the screen output of total coincidences, to count an event just once for each coincidence (the method we use) or once for each sinogram to which it contributes. In the former case, the total coincidences reported will not be equal to the sum of the count histogram (as some coincidences are credited to multiple slices); in the latter case, more coincidences are reported than were actually detected. In either case, it is not clear how to compute quality factors for subsets of bins when using MSRB.
- PhgUsrBinPETPhotons2 and MSRB: When using PhgUsrBinPETPhotons2 and MSRB together, the screen output of total coincidences cannot accurately reflect the number of coincidences as PhgUsrBinPETPhotons2 may reject a given event for one slice, but not for others.
- Mac OS10 timing info: The screen output of CPU times is not working for Macintosh OS10.
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