Accurate Prostate Volume Estimation Using Multifeature Active Shape Models on T2-weighted MRI

Department of Biomedical Engineering, Rutgers, The State University of New Jersey, Piscataway, 08854, USA.
Academic radiology (Impact Factor: 1.75). 06/2011; 18(6):745-54. DOI: 10.1016/j.acra.2011.01.016
Source: PubMed


Accurate prostate volume estimation is useful for calculating prostate-specific antigen density and in evaluating posttreatment response. In the clinic, prostate volume estimation involves modeling the prostate as an ellipsoid or a spheroid from transrectal ultrasound, or T2-weighted magnetic resonance imaging (MRI). However, this requires some degree of manual intervention, and may not always yield accurate estimates. In this article, we present a multifeature active shape model (MFA) based segmentation scheme for estimating prostate volume from in vivo T2-weighted MRI.
We aim to automatically determine the location of the prostate boundary on in vivo T2-weighted MRI, and subsequently determine the area of the prostate on each slice. The resulting planimetric areas are aggregated to yield the volume of the prostate for a given patient. Using a set of training images, the MFA learns the most discriminating statistical texture descriptors of the prostate boundary via a forward feature selection algorithm. After identification of the optimal image features, the MFA is deformed to accurately fit the prostate border. An expert radiologist segmented the prostate boundary on each slice and the planimetric aggregation of the enclosed areas yielded the ground truth prostate volume estimate. The volume estimation obtained via the MFA was then compared against volume estimations obtained via the ellipsoidal, Myschetzky, and prolated spheroids models.
We evaluated our MFA volume estimation method on a total 45 T2-weighted in vivo MRI studies, corresponding to both 1.5 Tesla and 3.0 Tesla field strengths. The results revealed that the ellipsoidal, Myschetzky, and prolate spheroid models overestimated prostate volumes, with volume fractions of 1.14, 1.53, and 1.96, respectively. By comparison, the MFA yielded a mean volume fraction of 1.05, evaluated using a fivefold cross-validation scheme. A correlation with the ground truth volume estimations showed that the MFA had an r(2) value of 0.82, whereas the clinical volume estimation schemes had a maximum value of 0.70.
Our MFA scheme involves minimal user intervention, is computationally efficient and results in volume estimations more accurate than state of the art clinical models.

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Available from: Robert Toth
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    • "Other authors used the mean absolute distance as evaluation strategy, Allen et al , achieved a value of 2.8 ± 0.82 mm (Allen et al 2006) and Toth et al , reported a value of 5 mm (Toth et al 2011b). Toth et al , using volumetric ratio for evaluation, obtained a value 1.05 ± 0.21 (Toth et al 2011a). Dowling et al , used median Dice coefficient for evaluation and obtained a value of 0.86 (Dowling et al 2011). "
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    • "Shape priors and local image statistics were incorporated in an energy function that was minimized to achieve prostate segmentation in a levelset framework. More recently, Toth et al. (2011a) used a series of 50 Gaussian kernels of variable size to extract prostate texture features. ASM constructed from manually delineated contours of training images was automatically initialized depending on the most probable location of the prostate boundary to achieve the segmentation. "
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    • "The MFLAAM was applied to the difficult task of prostate MRI segmentation. Such segmentation is useful in a computer aided cancer diagnosis system [2], and treatment evaluation via planimetry-based volume estimation [3]. Several segmentation schemes for MR imagery of the prostate have been recently presented, including Klein et al. [22], Martin et al. [23], Pasquier et al. [24], and Makni et al. [25]. "
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    ABSTRACT: Active shape models (ASMs) and active appearance models (AAMs) are popular approaches for medical image segmentation that use shape information to drive the segmentation process. Both approaches rely on image derived landmarks (specified either manually or automatically) to define the object's shape, which require accurate triangulation and alignment. An alternative approach to modeling shape is the levelset representation, defined as a set of signed distances to the object's surface. In addition, using multiple image derived attributes (IDAs) such as gradient information has previously shown to offer improved segmentation results when applied to ASMs, yet little work has been done exploring IDAs in the context of AAMs. In this work, we present a novel AAM methodology that utilizes the levelset implementation to overcome the issues relating to specifying landmarks, and locates the object of interest in a new image using a registration based scheme. Additionally, the framework allows for incorporation of multiple IDAs. Our multifeature landmark-free AAM (MFLAAM) utilizes an efficient, intuitive, and accurate algorithm for identifying those IDAs that will offer the most accurate segmentations. In this paper, we evaluate our MFLAAM scheme for the problem of prostate segmentation from T2-w MRI volumes. On a cohort of 108 studies, the levelset MFLAAM yielded a mean Dice accuracy of 88% ± 5%, and a mean surface error of 1.5 mm ±.8 mm with a segmentation time of 150/s per volume. In comparison, a state of the art AAM yielded mean Dice and surface error values of 86% ± 9% and 1.6 mm ± 1.0 mm, respectively. The differences with respect to our levelset-based MFLAAM model are statistically significant . In addition, our results were in most cases superior to several recent state of the art prostate MRI segmentation methods.
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