| Abstract: |
Purpose: In computed tomography (CT) cardiovascular imaging, the numerous
contrast injection protocols used to enhance structures make it difficult to
gather training datasets for deep learning applications supporting diverse protocols.
Moreover, creating annotations on noncontrast scans is extremely tedious.
Recently, spectral CT’s virtual-noncontrast images (VNC) have been used as
data augmentation to train segmentation networks performing on enhanced and
true-noncontrast (TNC) scans alike, while improving results on protocols absent
of their training dataset.However, spectral data are not widely available, making
it difficult to gather specific datasets for each task. As a solution, we present a
data augmentation workflow based on a trained image translation network, to
bring spectral-like augmentation to any conventional CT dataset.
Method: The conventional CT-to-spectral image translation network (HUSpect-
Net) was first trained to generate VNC from conventional housnfied units images
(HU),using an unannotated spectral dataset of 1830 patients. It was then tested
on a second dataset of 300 spectral CT scans by comparing VNC generated
through deep learning (VNCDL) to their true counterparts.To illustrate and compare
our workflow’s efficiency with true spectral augmentation,HUSpectNet was
applied to a third dataset of 112 spectral scans to generate VNCDL along HU
and VNC images. Three different three-dimensional (3D) networks (U-Net, XNet,
and U-Net++) were trained for multilabel heart segmentation, following
four augmentation strategies. As baselines, trainings were performed on contrasted
images without (HUonly) and with conventional gray-values augmentation
(HUaug). Then, the same networks were trained using a proportion of contrasted
and VNC/VNCDL images (TrueSpec/GenSpec). Each training strategy
applied to each architecture was evaluated using Dice coefficients on a fourth
multicentric multivendor single-energy CT dataset of 121 patients, including different
contrast injection protocols and unenhanced scans.The U-Net++ results
were further explored with distance metrics on every label.
Results: Tested on 300 full scans, our HUSpectNet translation network shows
a mean absolute error of 6.70 ± 2.83 HU between VNCDL and VNC, while peak
signal-to-noise ratio reaches 43.89 dB.GenSpec and TrueSpec show very close
results regardless of the protocol and used architecture:mean Dice coefficients
(DSCmean) are equal with a margin of 0.006, ranging from 0.879 to 0.938.
Their performances significantly increase on TNC scans (p-values < 0.017
for all architectures) compared to HUonly and HUaug, with DSCmean of0.448/0.770/0.879/0.885 for HUonly/HUaug/TrueSpec/GenSpec using the UNet++
architecture.Significant improvements are also noted for all architectures
on chest–abdominal–pelvic scans (p-values < 0.007) compared to HUonly and
for pulmonary embolism scans (p-values < 0.039) compared to HUaug. Using
U-Net++,DSCmean reaches 0.892/0.901/0.903 for HUonly/TrueSpec/GenSpec
on pulmonary embolism scans and 0.872/0.896/0.896 for HUonly/TrueSpec/
GenSpec on chest–abdominal–pelvic scans.
Conclusion: Using the proposed workflow,we trained versatile heart segmentation
networks on a dataset of conventional enhanced CT scans,providing robust
predictions on both enhanced scans with different contrast injection protocols
and TNC scans. The performances obtained were not significantly inferior to
training the model on a genuine spectral CT dataset, regardless of the architecture
implemented. Using a general-purpose conventional-to-spectral CT translation
network as data augmentation could therefore contribute to reducing data
collection and annotation requirements for machine learning-based CT studies,
while extending their range of application.
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