4. Optimization of the procedure ALARP / ALARA
3. Dose Optimization
Patient dose can be optimised in several ways. Firstly, consistent high standards of radiographic technique are essential to reduce the need for repeat imaging and thus further exposure (Do it right – first time!). Most publications cite positioning errors (50%) and anatomy cut off (20%) (Atkinson et al, 2020) as the leading causes of image rejection, so radiographers should take specific care in these areas, which are often particularly difficult in the mobile radiography setting.
Collimation to the anatomy is very important to minimize scatter radiation and patient dose but must include the required anatomy (lungs and diaphragms). Increasing kVp may be necessary to penetrate larger patients on CR and DR x-ray units and will help to reduce patient dose when accompanied by a reduction in mAs. The radiographer / radiologic technologist may have to increase kVp on a patient with known pneumonia to allow penetration of the pathology. With CR and DR systems, assessment of correct exposure cannot usually be assessed visually. The exposure index must be utilized to understand if the exposure is optimal, over-exposed, or under-exposed. Since exposure index has yet to be standardized across manufacturers, it is not the purview of this article to address specific numbers for each x-ray unit / manufacturer , however, radiographers should be aware of recommended ranges for their particular equipment.
Shielding – Patient shielding has been a subject of much debate since late 2019 when the American Association of Physicists in Medicine (AAPM, 2019) published a position statement on the use of patient gonadal and fetal shielding. The AAPM’s position states that patient shielding should be discontinued as a routine practice. The Society and College of Radiographers in the UK (2020) issued similar advice. The rationale for this policy considers the “advances in technology and current evidence of radiation exposure risks.” (p.123). Given this statement and the extremely high infection rate of the COVID-19 virus, and the optimal disinfection of the patients gonadal shielding, the use of patient shielding during mobile CXR should be reconsidered.
Staff dose: The radiographer / radiologic technologist should use the three cardinal rules of radiation protection - time, distance and shielding - to reduce occupational exposure to secondary radiation. Distance is the best protection, and the radiographer should stand a minimum of 2 meters (6 feet) from the patient, since the patient is the primary source of scatter radiation. If the radiographer must be in the room and cannot position themselves behind a secondary barrier, the radiographer should stand behind the mobile x-ray unit and try to achieve a maximum distance from the patient. The radiographer / radiologic technologist must communicate with the other medical staff that may be in the approximate area so as to protect them from any secondary radiation exposure. It is the radiographer’s responsibility to enforce radiation protection guidelines for any medical staff that may be involved in the x-ray exam. (Bushong, 2016, p. 188)Another radiation protection option that has been purported is the use of a ‘through glass’ technique. (Harborview Medical Center, 2020) This approach involves positioning the mobile x-ray unit outside the patient room and expose the image receptor through the glass window of the room (Figure below). This requires there to be a nurse or additional radiographer / radiologic technologist to don personal protective equipment (PPE) and position the image receptor plate behind the patient. The in-room personnel then directs the primary radiographer / radiologic technologist by speakerphone or hand gestures to the optimal positioning of the tube for the exposure (Mossa-Basha et al, 2020) and maybe a useful addition to infection control measures by limiting direct contact with patients.
Figure: Through glass mobile CXR technique. Courtesy of Mossa-Basha et al (Radiology, 2020)
Lead aprons and thyroid shields are recommended if the appropriate distance of 2 meters (6 feet) from the patient cannot be achieved and no reliable secondary barriers are available. Lead aprons and thyroid shields should have a minimum of 0.25 mm Pb equivalent, although 0.5 Pb is recommended.