Projectional X-ray of the chest

Site: ISRRT e-Learning
Course: International Covid-19 support for Radiographers and Radiological Technologists
Book: Projectional X-ray of the chest
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Date: Friday, 14 May 2021, 9:36 AM

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.  

Table 4. Procedural requirements for mobile chest radiography.
Projection PA erect AP erect
Patient position Patient upright
Chin raised outside of FOV
Shoulders relaxed
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.

Above a Youtube video from Barts Health NHS Trust (Coronavirus: Proning)
Video demonstrating the positioning a patient from supine to prone. Video courtesy of Deborah Harrison (A1 Risk Solutions) and the University of Salford.
Video demonstrating how to position the image receptor for a patient lying prone. Video courtesy of Deborah Harrison (A1 Risk Solutions) and the University of Salford.
Video demonstrating the removal of the image receptor for a patient lying prone. Video courtesy of Deborah Harrison (A1 Risk Solutions) and the University of Salford.

2. Radiation Protection

Appropriate radiation protection for mobile radiography should always be employed.  When examining COVID-19 patients PPE for radiation protection (i.e lead rubber apron) will also need to be combined with PPE for infection control.  Lead rubber aprons should be worn under COVID-19 PPE (Figure 7) and should be correctly cleaned and stored following use.  Radiation protection PPE and practices should comply with relevant legislation and the local rules. Radiographer / RTs and relevant personnel should understand that the ‘Controlled Area’ around a mobile unit exists when the unit is exposing and will extend in the direction of the primary beam until the beam is sufficient attenuated and for 2 metres from the patient and X-ray tube in all other directions.    Access to this Controlled Area should be restricted by the radiographer / RT who will give a verbal indication of the intention to conduct a radiation exposure and ensure that all non-essential persons leave the area.  In some instances, it may be necessary for a non-radiographic staff member or relative to physically support the patient during the examination.  If a staff member then this person should not be used on a regular basis.  Females, of childbearing age, should be asked if they are pregnancy and if they answer anything other than ‘no’ they should not be permitted to support the patient.  Any person supporting a patient during a mobile X-ray examination must wear a protective apron and must be instructed to avoid the primary X-ray beam.  Additional lead rubber aprons should be available on mobile X-ray equipment.  The radiographer / RT should take special care to minimise the exposure of patients in adjoining beds.  Any operating key should be removed when the mobile X-ray unit is not in use or if password protected the system must be adequately ‘logged off’.      

Mobile X-ray unit and radiographer

Figure 7.  Radiographer preparing to undertake a mobile chest X-ray on a suspected COVID-19 patient. Note that the radiographer is wearing a lead rubber apron under a disposable apron.  Image courtesy of the Countess of Chester Hospital NHS Foundation Trust.    

3. Ensure environment is safe and patient equipment (e.g ventilators) are not affected

Patient and staff safety is paramount during mobile radiography.  Mobile units are very sensitive to the forward/backward force asserted by the operator at the driving control. Sudden acceleration can lead to accidents resulting in unintended consequences.  Radiographer / RTs should ensure that the mobile X-ray equipment, does not strike equipment at a patient’s bedside when being manoeuvred into position.  Radiographer / RTs, for mobile equipment requiring mains electricity, should ensure that there are no trip hazards.  Radiographer / RTs should ensure that patients and staff members are aware of the close proximity of X-ray equipment and that they do not suffer any physical injuries from accidental contact (for example, head or foot injuries).  It is a primary responsibility of the radiographer / RT to ensure that the environment for mobile radiography is as safe as possible for both the patient and staff members.