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Threshold segmentation for PET target volume delineation in radiation treatment planning: The role of target‐to‐background ratio and target size
Author(s) -
Brambilla M.,
Matheoud R.,
Secco C.,
Loi G.,
Krengli M.,
Inglese E.
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2870215
Subject(s) - imaging phantom , scanner , nuclear medicine , medical imaging , dosimetry , physics , radiation treatment planning , partial volume , biomedical engineering , materials science , optics , medicine , computer science , radiation therapy , artificial intelligence , radiology
A multivariable approach was adopted to study the dependence of the percentage threshold[ TH ( % ) ]used to define the boundaries ofF18 ‐ FDG positive tissue on emission scan duration (ESD) and activity at the start of acquisition( A acq)for different target sizes and target‐to‐background( T / B )ratios. An anthropomorphic model, at least for counting rate characteristics, was used to study this dependence in conditions resembling the ones that can be encountered in the clinical studies. An annular ring of water bags of 3 cm thickness was fitted over an International Electrotechnical Commission (IEC) phantom in order to obtain counting rates similar to those found in average patients. The scatter fraction of the modified IEC phantom was similar to the mean scatter fraction measured on patients, with a similar scanner. A supplemental set of microhollow spheres was positioned inside the phantom. The NEMA NU 2‐2001 scatter phantom was positioned at the end of the IEC phantom to approximate the clinical situation of having activity that extends beyond the scanner field of view. The phantoms were filled with a solution of water andF18(12 kBq/mL) and the spheres with various T / B ratios of 22.5, 10.3, and 3.6. Sequential imaging was performed to acquire PET images with varying background activity concentrations of about 12, 9, 6.4, 5.3, and 3.1 kBq/mL. The ESD was set to 60, 120, 180, and 240 s/bed. Data were fitted using two distinct multiple linear regression models for sphere ID ≤ 10 mm and sphere ID > 10 mm . The fittings of both models were good with anR 2of 0.86 in both cases. Neither ESD norA acqresulted as significant predictors of the TH ( % ) . For sphere ID ≤ 10 mm the target size was the most significant predictor of the TH ( % ) , followed by the T / B ratio, while for sphere ID > 10 mm the explanatory power of the target size and T / B ratio were reversed, the T / B ratio being now the most important predictor of the TH ( % ) . Both the target size and T / B ratio play a major role in explaining the variance of the TH ( % ) , throughout the whole range of target sizes and T / B ratios examined. Thus, algorithms aimed at automatic threshold segmentation should incorporate both variables with a relative weight which critically depends on target size.