<p class="first" id="d3024527e132">The amount of coronary artery calcification (CAC)
is a strong and independent predictor
of cardiovascular events. CAC is clinically quantified in cardiac calcium scoring
CT (CSCT), but it has been shown that cardiac CT angiography (CCTA) may also be used
for this purpose. We present a method for automatic CAC quantification in CCTA. This
method uses supervised learning to directly identify and quantify CAC without a need
for coronary artery extraction commonly used in existing methods. The study included
cardiac CT exams of 250 patients for whom both a CCTA and a CSCT scan were available.
To restrict the volume-of-interest for analysis, a bounding box around the heart is
automatically determined. The bounding box detection algorithm employs a combination
of three ConvNets, where each detects the heart in a different orthogonal plane (axial,
sagittal, coronal). These ConvNets were trained using 50 cardiac CT exams. In the
remaining 200 exams, a reference standard for CAC was defined in CSCT and CCTA. Out
of these, 100 CCTA scans were used for training, and the remaining 100 for evaluation
of a voxel classification method for CAC identification. The method uses ConvPairs,
pairs of convolutional neural networks (ConvNets). The first ConvNet in a pair identifies
voxels likely to be CAC, thereby discarding the majority of non-CAC-like voxels such
as lung and fatty tissue. The identified CAC-like voxels are further classified by
the second ConvNet in the pair, which distinguishes between CAC and CAC-like negatives.
Given the different task of each ConvNet, they share their architecture, but not their
weights. Input patches are either 2.5D or 3D. The ConvNets are purely convolutional,
i.e. no pooling layers are present and fully connected layers are implemented as convolutions,
thereby allowing efficient voxel classification. The performance of individual 2.5D
and 3D ConvPairs with input sizes of 15 and 25 voxels, as well as the performance
of ensembles of these ConvPairs, were evaluated by a comparison with reference annotations
in CCTA and CSCT. In all cases, ensembles of ConvPairs outperformed their individual
members. The best performing individual ConvPair detected 72% of lesions in the test
set, with on average 0.85 false positive (FP) errors per scan. The best performing
ensemble combined all ConvPairs and obtained a sensitivity of 71% at 0.48 FP errors
per scan. For this ensemble, agreement with the reference mass score in CSCT was excellent
(ICC 0.944 [0.918-0.962]). Aditionally, based on the Agatston score in CCTA, this
ensemble assigned 83% of patients to the same cardiovascular risk category as reference
CSCT. In conclusion, CAC can be accurately automatically identified and quantified
in CCTA using the proposed pattern recognition method. This might obviate the need
to acquire a dedicated CSCT scan for CAC scoring, which is regularly acquired prior
to a CCTA, and thus reduce the CT radiation dose received by patients.
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