Evaluation of a method for activity estimation in Sm‐153 EDTMP imaging
Authors: Vanzi Eleonora , Genovesi Dario , Di Martino Fabio 
Published: March 2009

Absolute activity evaluation is fundamental for internal radionuclide dosimetry when patient‐specific therapy optimization is wanted. Often, quantification is attempted with 3D SPECT image based (IB) methods, but the true concentration values can be underestimated due to the partial volume effect (PVE). This is especially true when small diffuse lesions are present. In this paper, we describe a 3D region of interest (ROI) based quantification method (LS‐ROI), which estimates the ROI concentration values directly from the projection data acquired in the tomographic scan once ROIs have been segmented on a CT and/or a SPECT image. The method, which has inherent PVE correction capabilities, was applied both on simulated and on real phantom data. Simulations reflected the case of a patient with bone metastases treated with ‐EDTMP: Both the activity in the metastases and the total retention in the skeleton were evaluated. Thirty noisy data sets were produced in order to evaluate the accuracy and precision of the method. The effect of region segmentation errors on estimated concentrations was thoroughly investigated. Real data were acquired on a NEMA phantom, where a cylindrical central region simulated the bone and two spheres (10.3 and ) simulated the metastases. The results obtained with the LS‐ROI method were compared with those of a conventional 3D IB method and those of a quantitative conjugate view approach derived from LS‐ROI and applied to the anterior and posterior views acquired in the tomographic scan (LS‐ROI anterior‐posterior: LS‐ROI‐AP). Simulations showed that when the geometry of regions is known, the LS‐ROI method recovered the simulated concentration values within 20%, while the IB method underestimated the concentration in high activity small lesions by as much as 49%. Segmentation errors, up to 44% of the true region volume, produced a higher variation in LS‐ROI estimates than in IB ones; however, the overall bias of the LS‐ROI estimates remained lower than that of IB estimates. In the case of the evaluation of the total retention in the skeleton, the LS‐ROI method recovered the simulated value within 2%, while IB underestimated it up to 13%. In all the cases, the LS‐ROI‐AP method showed an accuracy comparable with that of the LS‐ROI one, and a worse precision just because of the lower number of counts used in the analysis. However, a worsening of LS‐ROI‐AP performances was demonstrated in the case of strong overlap of regions: In this case, a bias of up to 40% was observed. The results obtained on real phantom data confirmed the simulation results: The IB method underestimated activity up to 47% in the smallest sphere, while the bias was reduced to 13% with LS‐ROI and LS‐ROI‐AP estimates. The good quantification capabilities of the LS‐ROI method can be useful for absolute activity quantification in the case of small active diffused lesions and constitute the basis for the development of an accurate patient‐specific planning strategy in internal radionuclide treatments, provided there is a reliable segmentation of lesions.