Abstract
Most patients suffering from community acquired pneumonia do not appear at a radiology department since diagnosis is made on a clinical basis. In severe or unclear situations, a chest X-ray is done and analysis is frequently done by interns. Radiologists frequently see those patients that suffer from recurrent, nosocomial pneumonia, or an additional predisposing disease. The appropriate investigational technique, frequently targeted differential diagnosis, and the special needs of these patients need to be understood by radiologists. Early detection of a focus of infection is the major goal in immunocompromised patients. As pneumonia is the most common focus, chest imaging is to be done at the beginning. The sensitivity of chest X-rays, especially in the supine position, is known to be low. Therefore the very sensitive high-resolution CT (HRCT) became the gold standard in neutropenic hosts and is widely replaced by thin-section multi-detector-row-CT (MDCT). Underlying diseases such as pulmonary embolism or bronchial carcinoma might also be depicted. Furthermore, the costs of CT are low in comparison to antibiotics. The infiltrate needs to be localised, so that a physician can utilise this information as a guidance for invasive procedures for further microbiological work-up. The radiological characterisation of infiltrates gives a first and rapid hint to differentiate between different sorts of infectious (typical bacterial, atypical bacterial, fungal) and non-infectious aetiologies. Follow-up investigations need careful interpretation according to disease and concomitant treatment. Temporary exclusion of infectious involvement of the lung with high accuracy is, besides of pneumonia management, a hot topic for clinicians.