The development of computer-aided diagnosis (CAD) methods for the processing of CT lung scans continues to become
increasingly popular due to the potential of these algorithms to reduce image reading time, errors caused by user fatigue,
and user subjectivity when screening for the presence of malignant lesions. This study seeks to address the critical need
for a realistic simulated lung nodule CT image dataset based on real tumor morphologies that can be used for the
quantitative evaluation and comparison of these CAD algorithms. The manual contouring of 17 different lung
metastases was performed and reconstruction of the full 3-D surface of each tumor was achieved through the utilization
of an analytical equation comprised of a spherical harmonics series. 2-D nodule slice representations were then
computed based on these analytical equations to produce realistic simulated nodules that can be inserted into CT datasets
with well-circumscribed, vascularized, or juxtapleural borders and also be scaled to represent nodule growth. The 3-D
shape and intensity profile of each simulated nodule created from the spherical harmonics reconstruction was compared
to the real patient CT lung metastasis from which its contour points were derived through the calculation of a 3-D
correlation coefficient, producing an average value of 0.8897 (±0.0609). This database of realistic simulated nodules can
fulfill the need for a reproducible and reliable gold standard for CAD algorithms with regards to nodule detection and
sizing, especially given its virtually unlimited capacity for expansion to other nodule shape variants, organ systems, and
imaging modalities.
The ability of a clinician to properly detect changes in the size of lung nodules over time is a vital element to both the
diagnosis of malignant growths and the monitoring of the response of cancerous lesions to therapy. We have developed
a novel metastasis sizing algorithm based on 3-D template matching with spherical tumor appearance models that were
created to match the expected geometry of the tumors of interest while accounting for potential spatial offsets of nodules
in the slice thickness direction. The spherical template that best-fits the overall volume of each lung metastasis was
determined through the optimization of the 3-D normalized cross-correlation coefficients (NCCC) calculated between
the templates and the nodules. A total of 17 different lung metastases were extracted manually from real patient CT
datasets and reconstructed in 3-D using spherical harmonics equations to generate simulated nodules for testing our
algorithm. Each metastasis 3-D shape was then subjected to 10%, 25%, 50%, 75% and 90% scaling of its volume to allow for 5 possible volume change combinations relative to the original size per each reconstructed nodule and inserted back into CT datasets with appropriate blurring and noise addition. When plotted against the true volume change, the nodule volume changes calculated by our algorithm for these 85 data points exhibited a high degree of accuracy (slope = 0.9817, R2 = 0.9957). Our results demonstrate that the 3-D template matching method can be an effective, fast, and accurate tool for automated sizing of metastatic tumors.
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