International Journal of Radiation Oncology*Biology*Physics
Physics ContributionAccurate Automatic Delineation of Heterogeneous Functional Volumes in Positron Emission Tomography for Oncology Applications
Introduction
Although most clinical applications of positron emission tomography (PET) rely on manual and visual analysis, accurate functional volume delineation in PET is crucial for numerous oncology applications. These include the use of tumor volume and associated determination of semiquantitative indices of activity concentration for diagnosis and therapy response evaluation (1) or the definition of target volumes in intensity-modulated radiation therapy (IMRT) (2). Subjective (1) and tedious manual delineation cannot perform accurate and reproducible segmentation, particularly when considering complex shapes and nonhomogeneous uptake. This results from the low quality of PET images due to statistical noise and partial volume effects (PVE) (3), arising from the scanner's limited spatial resolution.
Most of the previously proposed methods for PET volume definition are semiautomatic and threshold-based, using either fixed (30%–75% of the maximum activity) 2, 4, 5 or adaptive approaches incorporating the background activity 6, 7, 8, 9, 10. Unfortunately, these approaches often require additional a priori information and are user- and system-dependent. They require manual background regions of interest (ROIs), and their performance depends on parameters requiring optimization using phantom acquisitions for each scanner and reconstruction. Finally, all of these approaches are strictly binary and were not validated considering heterogeneous volumes.
Numerous works have addressed PET lesion segmentation using more advanced image segmentation methodologies 11, 12, 13, 14, 15, 16, 17, 18, 19. However, the majority of these approaches often depend on pre- or postprocessing steps such as deconvolution or denoising, are often binary only, and are validated on phantom acquisitions or clinical data without rigorous ground truth.
We previously developed an algorithm for PET volume definition by combining a fuzzy measure with a locally adaptive Bayesian-based classification (FLAB) that has been shown to perform better with respect to fixed thresholding, fuzzy C-means (FCM), or fuzzy hidden Markov chains (FHMC) for PET volume definition, as far as homogeneous spheres or slightly heterogeneous and nonspherical tumors are concerned (20). Preliminary results show that FLAB is also robust with respect to variability of the acquisition and reconstruction parameters (24).
Clinical tumors may be characterized by heterogeneous uptake, thus demanding a nonbinary approach for an accurate segmentation that may have a significant impact in defining biological target volumes for dose painting (21). The goals of this work were to (1) improve the FLAB model by incorporating the use of three hard classes and three fuzzy transitions and (2) evaluate its accuracy on real (with known diameter measured in histology) and simulated (with known ground truth) data sets containing inhomogeneous tumors.
Section snippets
Three-class fuzzy Bayesian segmentation (3-FLAB)
The 3-FLAB algorithm is an extension of our previous work considering only a binary segmentation (20). FLAB automatically estimates parameters of interest from the image, maximizing the probability of each voxel to belong to one of the considered classes. This probability is estimated for each voxel as a function of its value and the values of its neighbors relative to the voxels' statistical distributions in the image, which corresponds to an estimation of the noise within each class. Hence,
Results
Figure 5 contains one axial slice of the segmentations obtained on three simulated tumors of Data Set 1 and one tumor of Data Set 2. Figure 6a contains the mean classification errors and standard deviation obtained by all the methods on the 20 tumors of Data Set 1. FLAB (binary only) performed well on homogeneous tumors but failed as expected on strongly heterogeneous lesions, leading to overall errors of 15% ± 11%. 3-FLAB, in contrast, produced segmentation maps closer to the ground truth,
Discussion
Functional volume delineation represents an area of interest for multiple clinical applications (routine and research) of PET. Such areas include response to therapy studies and the use of biological tumor volumes in radiotherapy treatment planning. Although several fully automatic algorithms have recently been proposed 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, segmentation methodologies currently used in clinical practice are based on the use of fixed and adaptive thresholding 4, 5, 6, 7, 8, 9,
Conclusion
A modified version of the FLAB algorithm has been developed to include the estimation of three hard classes and three fuzzy transitions. This automatic approach combines statistical and fuzzy modeling to address specific issues associated with 3D-PET images, such as noise and PVE. Its accuracy has been assessed on both simulated and clinical images of complex shapes containing inhomogeneous activities and small regions. The results demonstrate the ability of 3-FLAB to delineate such lesions,
Acknowledgments
This work was supported by the Brittany Region grant program (Grant No. 1202-2004), the French National Research Agency (Grant Nos. ANR-06-CIS6-004-03 and ANR-08-ETEC-005-01), and Cancéropôle Grand Ouest (R05014NG).
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Conflict of interest: none.