4. Optimization of the procedure ALARP / ALARA
|Course:||International Covid-19 support for Radiographers and Radiological Technologists|
|Book:||4. Optimization of the procedure ALARP / ALARA|
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|Date:||Monday, 15 August 2022, 12:37 PM|
1. Justification: Is the exam needed now?
The justification for immediate mobile CXR on COVID-19 patients lies in how the imaging outcomes will impact patient care. Because COVID-19 is highly infectious and can progress into respiratory distress and secondary pneumonia, it may be critical to assess the patient’s condition to optimize patient care or to confirm the position of tubes and lines. The question that must be asked is whether the imaging will alter the patient’s management. According to a recent article in Radiology (Mossa-Basha et al, 2020), mobile CXR should not be considered as a screening tool, but should only be considered if the patient has a specific clinical need. The article also states that “emergent imaging may be necessary for evaluation of other urgent conditions, including stroke, trauma, infection, and other disease conditions” (p. 123). There are several reports of other disease processes that appear as a result of infection with COVID-19.
2. Exposure Factors
In COVID-19 patients, mobile chest x-rays (CXR) are performed via a single projection (anterior-posterior (AP) or postero-anterior (PA)) only. Although the lateral projection could be performed it is not normally performed in these circumstances. Technical considerations for the AP/PA projection include, source to image distance (SID), kVp, mAs, body habitus, suspected disease process, grid use (& ratio), and anode heel effect. The radiographer / radiologic technologist must be able to use the technique chart that is attached to the unit, if possible, to achieve an optimized technique.
Worldwide there are a range of image receptor technologies in use. Although most hospitals have converted to computed radiography (CR) or digital radiography (DR), there may be some locations that are utilizing screen/film radiography.SID: For CR and DR systems, the standard SID for mobile CXR AP projections is 102 centimeters (40 inches) (Bushong, 2016, pp.187). Although this is the standard, it may not be practical in all situations. There may be equipment in the way or physical limitations within the patient room. A recent protocol established by Harborview Medical Center in Seattle, WA. (2020) states that the COVID-19 patient “will be at distances varying from 6 feet [ 2 meters] (if standing) to as far as 18 feet [ 5.5 meters] (when upright in a stretcher) from the x-ray tube”.(COVID-19 Patient Portable).
The radiographer must compensate for any variance in SID by utilizing the inverse square law and evaluating the necessity / choice of grid. According to Merrill’s Atlas of Radiographic Positioning and Procedures, the average technique for mobile AP CXR for a patient thickness of 21 cm is 120 kVp @ 3.2 mAs with a grid ratio of 6:1 to 8:1 depending on the actual SID.
Grid use: Most grids used for mobile CXR are focused grids and the radiographer must be aware of the SID so as to use the correct grid. Newer digital (DR) mobile x-ray units can utilize a 12:1 focused grid with a high kVp technique of 115-125 kVp @ 2 to 4 mAs. Grids are usually necessary for mobile CXR if the kVp is greater than 90 kVp. There are some radiographers/ radiologic technologists who may not use a grid and some newer DR units do not require a grid, instead of having ‘virtual grid’ software which digitally removes scatter signal.
Figure 1. Virtual grid software comparison - courtesy of Fuji Film Corporation (FDR Go Plus)
According to the revised protocol from Harborview Medical Center, with the use of a DR system utilizing a ‘virtual’ grid, and an SID of 17 feet (5.2 meters), the optimal technique was 125 kVp @ 8 mAs. It must also be noted that with a large SID and high kVp, the mA is limited, so the exposure time must be extended, resulting in increased susceptibility to motion artifacts.
If using a grid, care must be taken to avoid misalignment and grid cut off which will degrade the resulting image quality. This can be visualised as reduced exposure to one side of the image and is important not to be confused with pathology especially in the chest region
Figure 2. Grid cut off. Courtesy of Fuji Film Corporation (FDR Go Plus)
Figure 3. Grid cut off on portable chest radiography. Courtesy of Dr Henry Knipe Radiopaedia.org (Case ID: 2779). Note decrease in exposure to the patients left side
Anode-heel effect: The radiographer must also
be cognizant of the anode-heel effect. This effect causes the cathode side of
the x-ray tube to have increased density on the image receptor and
consequently, a decrease in density on the anode side of the x-ray tube. Correct
placement utilizing the anode heel effect would be to place the cathode side of
the x-ray tube towards the diaphragm.
Pediatric patients must be imaged with vastly reduced radiation dose. These patients may be better served by bringing them to a dedicated machine in the radiology department that has been designated for suspected COVID-19 pediatric patients and utilize automatic exposure control (AEC) for optimal images at a reduced dose. If a mobile CXR is indicated, a technique of 70-80 kVp @ 1-2 mAs would be an approximate technique. The mobile x-ray machines technique chart should be referred to for an optimal technique that would take into consideration the patient’s size.
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.