Biomedical Engineering Reference
In-Depth Information
planning (e.g., determining tunnel placement for anterior cruciate ligament (ACL)
reconstruction) to avoid stunting the patient's growth or compromising the stability of
the knee. Currently, growth potential ismeasured by a physician using anX-ray scan of
multiple bones; patients receive repeated doses of radiation exposure.
The authors of [ 1 ] describe a semiautomatic segmentation algorithm as one that
uses human input for initialization of an otherwise automatic process and an
interactive segmentation algorithm as an algorithm that actively involves the user
during the segmentation by iteratively requiring human input. Our work presents an
interactive approach to segmentation by formulating it as a human supervisory
control (HSC) problem. Thus, we reduce the time required for segmentation by
leveraging the strength (high level knowledge of anatomy) of the user with the
strength (performing an otherwise tedious task of outlining exact boundary
locations) of an automatic algorithm.
In Sect. 2.1 , we analyze problem of using global descriptors for segmentation of
medical image volumes. In Sects. 2.2 and 2.3 , we present an HSC formulation for
segmentation, novel in this field. Results are shown in Sect. 3 including a software
application developed as part of this work and currently used in medical research.
Finally, in Sect. 4 , we conclude.
2 Methods
2.1 Problem Analysis
While the safety advantages of using magnetic resonance imaging (MRI) for
evaluation of bone structure are substantial, the physical properties of this imaging
modality makes analysis of bony tissue quite challenging. Values in CT images are
in Hounsfield units and their correspondence to various tissues in the body is known.
Hence, segmentation can be accomplished simply knowing these correspondences.
In MRI, intensities are relative and do not correspond to the same tissue for different
patients [ 2 ]. Additionally, there is large variance in intensities that correspond to the
same organ. In Figs. 1 and 2 , we show the degree of intensity inhomogeneity within
and between slices, respectively, of the femur. From this graphic, we see that it is
impossible to find a single narrow range of intensities that would separate the bone
from other organs; also, the intensities that describe bone are spatially varying.
2.2 User Control Framework
2.2.1 Definition of a Control System
A control system is a mechanism (manual or automatic) that regulates the behavior
of a plant; a plant is a combination of a process and an actuator. For instance, the
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