Introduction

The imaging of patients who have, or who are suspected of, having a lung infection should be based on two important and widely differing concepts. The first is the analysis of the chest radiograph, a basic investigation of unparalleled use to any clinician. This radiograph should be used as the start of any investigative chain leading to other more complicated diagnostic tests and occasionally to radiological therapeutic intervention. The second concept is based on the understanding of disease as related to the individual patient under investigation. To make full use of any imaging modality, the following fundamental points have to be considered.

1. (a) Does the patient have a pre-existing lung and/or a systemic condition? and,

2. (b) Does the patient have an altered immunity state for whatever reason?

It is also important to ascertain the ethnic origin of the patient, whether the patient comes from the tropics or has recently travelled to a known site of infection. The radiographical and clinical picture may also be atypical if the patient has either been given inappropriate therapy particularly antibiotics or has only taken a partial course of the correct drugs. A chest radiograph or indeed any imaging investigation, can rarely be specific in identifying the causal organism unless the answers to the above are known. Classical infections such as Mycobacterium tuberculosis and Pneumocystis carinii may give pathognomonic changes on a chest radiograph providing the patient was ‘normal’ beforehand, but both conditions can present very different radiological signs in other patients, for example, those with HIV disease.