Projectional X-ray of the chest
1. Radiographic technique
Mobile chest radiography can be undertaken with the patient in multiple positions. Patient position will largely be determined by the physical condition of the patient. An erect postero-anterior (PA) projection is universally considered to be the gold standard. This is often not practically possible when undertaking mobile examinations and the most frequent projection would be an erect antero-posterior (AP) projection. Such a projection can be performed with the patient sat in a chair or in bed. Due to the options available resultant images should be correctly annotated in order to accurately describe the acquired projection. Whichever position is selected it is important that the image receptor is carefully fixed into position and that the relevant image receptor surface is facing towards the X-ray tube. Both of these can be difficult to achieve with an agitated patient and with the receptor placed inside an opaque protective cover. In many instances ward staff can be consulted regarding the most appropriate imaging position for the patient. In some instances, it will not be possible to acquire a fully erect image and the radiographer / RT will need to opt for a semi-erect or supine image. Consideration should also be given to undertaking mobile chest radiography with the patient in the prone position (PA supine). Recent reports have documented the use of prone patient positioning to help support ventilation in the severest of COVID-19 cases12,13. A study in 201314 reported that pronation helped lower mortality rates for patients with severe acute respiratory disease syndrome (SARS), a type of respiratory failure caused by inflammation in the lungs, not unlike some patients with severe COVID-19. Further specific issues for prone positioning will include insuring that the patient is not rotated, no injuries result from positioning the imaging receptor and making a judgement regarding full inspiration. Ideally, prone positioning should only be undertaken when it is not possible to delay the examination until a later time, when the patient is positioned supine. Early experience of imaging severe COVID-19 patients in Italy has lead to the Italian Federation of Scientific Radiographers' Societies (FASTeR) recommending that all radiographers have an understand of mobile prone radiography. It may be beneficial for radiographers to have an understanding of how to safely move a patient from a supine to prone position and vice versa (see Youtube Videos 1 to 4 at the foot of this page).
Space is a consideration when undertaking mobile chest radiography. Where possible the radiographer / RT should aim to achieve a source-to-image distance (SID) of 180 cm. This is likely to be practically unachievable when undertaking supine examinations or when confined to a small side-room or cubicle. In any case the radiographer / RT should attempt to achieve a SID as close to 180 cm as possible. Both the image receptor and X-ray tube should be perpendicular to each other. This is more easily achievable when a patient is in a true erect or supine position and more difficult when the patient is recumbent. The radiographer / RT should align the X-ray tube with the angulation of the image receptor or use a skin landmark such as the angle of the sternum. For some aspects of radiographic practice there is a tendency to provide additional caudal angulation of the X-ray tube. The effect of this is to reduce the radiation exposure to the head and neck regions. This is a viable option for some mobile radiographic examinations and angulation of between 5 to 10 degrees can be applied. Positioning of the central ray is controlled by the light beam diaphragm (LBD) and together with the collimator will ensure an adequate field size. Visualisation of the LBD can be difficult in areas of high intensity lighting, for example close to a window and the radiographer / RT should take steps to ensure adequate visualisation. Centring points and field of views are described for both AP and PA projections in Table 4. Erect techniques are largely transferable to supine / prone patient positions, but will generally require a reduced SID.
|Projection||PA erect||AP erect|
|Patient position||Patient upright
Chin raised outside of FOV
|Anterior surface against IR||Posterior surface against IR|
|Arms by sides, shoulders rotated anterior
to remove scapulae from lung fields
|Arms by sides or if possible, elbows bent,
backs of hands on hips and arms moved forward
|Technical factors||2nd Arrested (full) inspiration|
|- Centring point||7th thoracic vertebra||7 cm below sternal notch|
|- Central ray||5 – 10 degrees caudal angulation|
|- Collimation||Superiorly – 1 cm above upper skin border of shoulder
Inferiorly – inferior to 12th rib
Laterally – lateral skin borders
|- Detector size / orientation||35 cm by 43 cm (portrait or landscape)|
|- Antiscatter grid||Depending on local protocol / patient size.
Consider the option of using a virtual grid (if available)
|- SID||180 cm|
|FOV, field of view; IR, image receptor; SID, source-to-image distance.
NB- collimation and orientation of the image receptor should consider body habitus.