This trial compares the accuracy provided by plain film as well as ultra-low-dose-computed-tomography (ULDCT) of the chest. It also aims to analyze differences in diagnostic confidence and therapeutic consequence offered by these two imaging modalities.

For nearly a century, chest X-ray (plain film, projectional radiography) has been the established primary imaging modality for patients with acute chest pain, suspected pneumonia, and / or acute dyspnoea in the diagnostic pathway of emergency departments, although the sensitivity and specificity of this X-ray technique are moderate.

The widespread availability and use of chest X-ray is due to the low acquisition and operating costs for projectional radiography equipment, the short examination time, and the very low radiation exposure. In addition, projectional radiography of the chest often serves as a guide for further, more sensitive, diagnostical procedures. However, these advantages are partially offset by the disadvantages inherent to projectional methods: anatomical structures may superpose or mask pathological structures. As a result, some areas of the lung may be obscured, and assessment may therefore be limited.

Whereas computed tomography was reserved for certain clinical questions over the last few decades, and, in most cases, served as a second imaging approach after plain film radiography, it has increasingly evolved as a primary imaging modality for several indications (e.g., suspicion of pulmonary embolism, highly suspected aortic dissection). This rise of computed tomography was due not only to its significant advantage of no superposition, but also partly driven by the marked reduction in radiation dose needed without sacrificing image quality. This was driven by the development of new detectors, modulation of tube current and voltage, as well as iterative reconstruction techniques. As a result, recent computed tomography scanners currently offer not only a more precise visualization of differences in tissue-attenuation and the significant advantage of the absence of artefacts due to superposition, but also allow for imaging with a considerably reduced radiation dosage compared to older scanners. Therefore, computed tomography may now be utilized as screening method in specific indications that carry an increased risk of certain pathologies (e.g., in long-time smokers). The introduction of the latest generation of computed tomography devices about three years ago allowed for an even further reduction in dose by filtering out low-energy photons using a tin filter, which offers the possibility of a reduction in radiation dose by another 50 % or more for established CT indications. For specific indications (e.g., lung nodules in follow-up), the radiation dose may even be reduced to a tenth or one-hundredth of a standard-dose CT (SDCT).

The current reference dose-length-product (DLP) in Germany for thoracic standard-dose CT (SDCT) is ~350 mGycm (effective dose ~6 mSv) and, for thoracic low-dose-CT (LDCT / HR-CT), ~100 mGycm (effective dose ~1.7 mSv). However, the latest devices (third-generation dual-energy CT) provide the opportunity to considerably reduce the reference dose of thoracic low-dose CT. In the current literature, these scans are referred to as Ultra-Low-Dose-CT (ULDCT) and are usually associated with a radiation dosage of 0.14 to 0.5 mSv. For this dose range, no standardized reference values have been published as yet.

The limiting factors of ULDCT are quantum noise, loss of spatial resolution, and other image artefacts. Therefore, careful selection of appropriate CT protocols and dosage is mandatory in order to achieve sufficient image quality to answer the respective diagnostic question.

Several papers have been published on the subject of ULDCT, which are dedicated to the comparison of ULDCT with LDCT and/or SDCT. These papers conclude that this technology may be used with sufficient sensitivity and specificity for indications such as dyspnea, emphysema, or lung nodules. Due to the potential to reduce the radiation dose to less than 1/30 of a standard-dose CT while still providing acceptable image quality with the latest generation of devices, ULDCT of the chest is emerging as an interesting alternative to conventional chest X-ray.

To the best of the authors' knowledge, there are currently no studies comparing and evaluating ultra-low-dose-CT as a primary imaging alternative to chest X-ray in emergency department patients.

This study aims to compare ULDCT and plain film of the chest with regard to their accuracy in an unfiltered patient cohort of an emergency department. For this purpose, the investigator's ULDCT protocol will use the lowest possible dose at which image quality is diagnostically sufficient (approximately 0.2 mSv effective dose). This corresponds to less than 1/30 of the radiation dose of a standard-dose CT of the chest and to only about 2.5 times the dose of a chest X-ray in two views. This dose is equal to less than a month of natural background radiation in Austria and less than the radiation exposure on an intercontinental flight.

In addition to the accuracy of ULDCT of the chest compared to plain film of the chest, this trial also aims to analyze the clinical relevance of both methods by assessing the respective impact on final diagnosis, as well as possible changes in therapy.

Inclusion Criteria:

• all patients who are assigned to a clinically indicated chest X-ray by the emergency department of Vienna General Hospital
• ability to provide informed consent
• informed consent after detailed patient briefing

Exclusion Criteria:

• the emergency departments assessment of a critical clinical condition oppose an examination with both modalities (ULDCT of the chest, chest X-ray)
• assigned to chest X-ray as follow-up
• women with positive ß-HCG-test