Neuroblastoma imaging using a combined CT scanner-scintillation camera andI-131-MIBG

High-dose administration of I-131-metaiodobenzylguanidine (I-131-MIBG) cont inues to be a promising treatment for neuroblastoma. However, currently use d methods of estimating I-131-MIBG uptake in vivo may be too inaccurate to properly monitor patient radiation exposure doses. To improve localization and uptake measurements over currently practiced techniques, we evaluated d ifferent methodologies that take advantage of the correlated patient data a vailable from a combined CT-scintillation camera imaging system. Methods: S erial CT and radionuclide scans of three patients were obtained on a combin ed imaging system. SPECT images were reconstructed using both filtered back projection and maximum-likelihood expectation maximization (MLEM). Volumes of interest (VOIs) were defined on anatomic images and automatically correl ated to spatial volumes in reconstructed SPECT images. Several radionuclide quantification methods were then compared. First, the mean reconstructed v alues within coregistered SPECT VOIs were estimated from MLEM reconstructed images. Next, we assumed that reconstructed activity in SPECT voxels were linear combinations of activities present in individual objects, weighted b y geometric factors derived from CT images. After calculating the weight fa ctors by modeling the SPECT imaging process with anatomically defined VOIs, least-squares fitting was used to estimate the activities within lesion vo lumes. We also estimated the lesion activities directly from planar radionu clide images of the patients using similar linearity assumptions. Finally, for comparison, lesion activities were estimated using a standard conjugate view method. Results: Activities were quantified from three patients havin g a total of six lesions with volumes ranging from 0.67 to 117 mL. Methods that used CT data to quantify lesion activities gave similar results for pl anar and tomographic radionuclide data. Estimating activity directly from m ean VOI values in MLEM-reconstructed images alone consistently provided est imates lower than CT-aided methods because of the limited spatial resolutio n of SPECT. Values obtained with conjugate views produced differences up to fivefold in comparison with CT-aided methods. Conclusion: These results sh ow that anatomic information available from coregistered CT images may impr ove in vivo localization and measurement of I-131-MIBG uptake in tumors.