In this work we describe a new modality for bedside chest DTS. In the presented simulations of ICU patients, portable DTS improved the detection of abnormalities in the bedside CXR. Whereas the clinical use of chest DTS for patients in the radiology department can be evaluated against CT, MRI and other modalities, far fewer alternatives exist for bedside chest examination of patients in the ICU, for whom transport to a radiology department is often complicated and expensive. The cost of performing bedside ultrasound as a daily follow-up is relatively high, since this exam takes longer to perform and requires a high level of expertise. Mobile CT scanners exist but are not widespread and they are mainly limited to dedicated applications such as imaging of the head [23,24,25]. Portable chest DTS could be implemented on a mobile X-ray device, which is already widely accepted as a mobile examination device. Portable DTS might also hold potential to improve bedside exams in the emergency room, sterile rooms in haematology wards or transplantation units, burn units, surgical departments, etc. In future work, a more elaborate prospective clinical study of potential applications will be conducted, in combination with an optimisation of the acquisition technique.
The radiation dose for a chest DTS exam using a dedicated wall-mounted flat panel is around 0.12 mSv for a typical acquisition of 60 projections . In our experiment, however, only 15 DTS projections were taken at 90 kVp and 0.1 mAs. The effective dose of 0.029 mSv corresponds to approximately 1.5 times the dose of a portable CXR, obtained at 90 kV and 1 mAs.
Söderman et al.  showed that decreasing the tube travel distance, and thus the angular range, has a positive effect on the reproduction of the trachea and paratracheal tissue, vessels, and aorta. However, a smaller angular range resulted in decreased image quality related to following vessels through the volume. Image quality for findings more specifically related to critically ill patients, such as pneumothorax/pneumomediastinum, and the capability of separating overlapping devices into different slices were not discussed. Also, the aforementioned study investigated different configuration parameters for chest DTS with an effective dose comparable to a standard DTS exam. Our simulations showed portable chest DTS in the ICU with a small angular scanning range (only 6°) and a radiation dose of only 0.029 mSv. The effective slice thickness of the simulations presented in this study was obtained experimentally by adding a thin line throughout the simulated patient from head-left-posterior to foot-right-anterior. The amount of this line that was visible in the reconstructed tomosynthesis slices illustrated an effective slice thickness corresponding to a patient slab of 30 mm. Despite this relatively high effective slice thickness, the diagnostic quality of the bedside CXR was improved.
The presented simulation study has obvious limitations. First, the quality of the bedside DTS reconstruction relies heavily on the accuracy of the measured relative positions and orientation of the X-ray source trajectory and detector, which was assumed to be error-free in our simulation. In a real-life acquisition, our previous work could be applied to correct possible misalignment [28, 29]. Secondly, patient motion during the acquisition of the DTS projections is another well-known cause of degradation of DTS image quality, which was not included in our simulation. Despite a reduced total acquisition time of a few seconds, due to the reduced number of 15 exposures, breathing or other types of patient motion might still occur. Many ICU patients might not be able to hold their breath, although for intubated patients, a short interruption in mechanical ventilation could be considered in some cases. Reconstruction and motion correction methods might need to be incorporated and improved in a portable chest DTS device [30, 31]. Finally, physical phenomena such as scatter and quantum noise were excluded from these preliminary simulations. However, the reconstructions from the experimentally acquired DTS exam of the humanoid phantom strengthen our belief that the conducted experiments are at least indicative of the expected performance of mobile chest DTS.
In conclusion, we have shown with preliminary simulations that portable chest DTS holds potential to improve the diagnostic accuracy of bedside CXR in the ICU. Possible benefits include: improved localisation of parenchymal consolidations (anterior versus posterior), detection of pneumothorax or pneumomediastinum patients in the supine position, verification of the correct position of drains and lines, differentiation between pleural effusions and consolidations, and other applications. It is technically feasible to perform mobile chest DTS with a modified mobile X-ray unit, which is already widely accepted as a mobile examination tool.