4. Optimization of the procedure ALARP / ALARA
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.