In this work, 7 body background VOIs were generated from thresholded 24 hr SPECT images in the patient study. There is a tradeoff between the number of VOIs and the estimability of VOI activity. More VOIs are preferred to model the nonuniform activity concentration, but, on the other hand, too many VOIs could make the estimation problem ill posed. In our experience, 5 to 7 VOIs was a reasonable and sufficient number. We used 7 body background VOIs for patient data in this work. However, for other agents a different number may be appropriate.
In the CPlanar method, the mean counts in a small adjacent background region times the organ thickness factor was subtracted to perform the background correction. The background correction is thus highly dependent on the location and the size of this small region. As a result, the performance of the background correction is very sensitive to the experience of the operator, and high variability can be expected. The QPlanar method also depends on the accuracy of organ VOI definition. However, in our experience, drawing VOIs in 3D, especially when using fused anatomic and SPECT images, is less subjective than drawing them on planar projections. In previous work we have also shown that the QPlanar method is somewhat less sensitive to VOI misdefinition and misregistration than SPECT. As a result, it is expected that the operator variations, which were not explicitly evaluated in this work, would be smaller for the EQPlanar method than for CPlanar methods.
We observed in both the simulation and patient studies that the accuracy for the left kidney was better than for the right kidney. One explanation for this is that the right kidney is located adjacent to the liver, which usually has high uptake for 111In-Zevalin. Both residual misregistration and partial volume effects from the liver could thus increase errors in activity estimates for the right kidney. This suggests that it may, in general, be difficult to estimate low-activity features that are partially shadowed by high activity organs.
The data in this work indicated that EQPlanar provides a major improvement over the CPlanar method. Previous work has shown that the accuracy of the closely-related QPlanar method approaches the accuracy and precision of QSPECT (He and Frey, 2006
). Thus, one conclusion that one might draw is that these methods obviate the need for SPECT imaging in dosimetry applications. However, upon closer examination this is not a valid conclusion. First, although the EQPlanar method estimates organ activities from the wholebody planar projections, it uses SPECT images to generate the multiple background VOIs required to model accurately the nonuniform activity distribution in background regions. Second, in the context of residence time estimation, the acquisition of a SPECT image at one time point enables the use of hybrid SPECT/Planar residence time estimation methods (Koral et al., 1994
; He et al., 2009a
). Hybrid residence time estimation methods use the planar images to estimate the time activity curve and renormalize the curve so that it passes through the activity estimate from the QSPECT image. These methods have been shown, in both simulation (He et al., 2009a
) and patient studies (Assie et al., 2008
) to provide improved residence time estimates compared to purely planar residence time estimation. Combined with EQPlanar planar activity quantification, hybrid methods should produce organ residence time methods with reliability approaching that obtained with methods requiring a SPECT image at every time point.
Even if hybrid EQPlanar/QSPECT organ residence time estimates have reliability approaching those from purely QSPECT based approaches, there are situations where acquiring SPECT at every time point may be desirable. First, it is likely that QSPECT is superior for quantification of small objects, e.g., tumors. Second, having a QSPECT image at every time point is important for allowing implementation of voxel-based dosimetry. Advanced voxel-based dosimetry methods allow taking into account the radiobiological effects of the nonuniformity of the dose distribution in tumors and organs as well as dose-rate effects. In situations where these effects are important, for example in radio-peptide therapy where the kidney is dose limiting and the dose-distribution is highly nonuniform, performing SPECT at every time point is likely the method of choice.
In this work, the accuracy of organ activity estimates from EQPlanar method was validated using data at 24 hr time point because of the availability of 24 hr SPECT images, which was considered a golden standard. For 1, 5, 72 and 144 hr time points, the goodness of fit between estimated and measured projections was investigated. It should be noted that although the improvement of goodness of fit cannot serve as direct evidence of improved accuracy for organ activity estimates, it does indicate the improved reliability of organ activity estimates since improved goodness of fit and improved organ activity estimates were observed in the simulation studies.
Note that in the EQPlanar method we used background VOIs obtained from the 24 hr SPECT images for other time points. This would not work in cases where the tracer redistribution was such that the shapes of the background VOIs regions changed at the other time points. However, since the activity in the background VOIs was not fixed, but was fit to the data, and since the 3D background regions did not overlap the organs, the use of the multiple backgrounds should not, in general, degrade the performance of the method and would also be a problem for conventional planar processing. In cases where the shape of background regions changes at every time point the use of SPECT at every time point would be preferred.