Ioannis A. Tsalafoutasa, Shady AlKhazzama, Huda AlNaemia,b, and Mohammed Hassan Kharitaa
aMedical Physics Section, OHS Department, Hamad Medical Corporation, Doha, Qatar
bWeill Cornell Medicine-Qatar, Doha, Qatar
EI for QC of clinical images
The brightness and contrast of digital radiography images are automatically adjusted, and therefore the problems with over- or under-exposed images, which were very common in screen/film radiography, no longer occur. However, the disconnection of image brightness from the image receptor’s exposure can be a disadvantage, since higher exposures can be allowed without detriment on the image quality. Indeed, a trend towards increased patient dose was observed during the first years of application of digital imaging, which was reported as the ‘exposure creep’.
To deal with this problem, all major manufacturers of computed radiography (CR) and digital radiography (DR) systems devised an indicator, referred to as exposure index or exposure indicator (EI), to inform the users about the incidence air- kerma (IAK) on the image receptor, and alert them in cases of over- or under- exposure. Initially, various definitions of EI were used, some of which were complicated and not easy to use in the everyday clinical practice. For this reason, a new IEC standard was issued to universalize EI. According to the current definition, EI is 100 times the (IAK in μGy) on the image receptor, when an X-ray beam with certain characteristics is used (RQA-5: 70 kV, HVL=6.8 mm Al). In this IEC document, the definitions of target EI (EIT) and the deviation index (DI), were also introduced.
In brief, EIT is the desired EI that is achieved, when the IAK to the image receptor is equal to that intended. For example, for examinations where the Automatic Exposure Control (AEC) system is activated, the intended EI is the IAK at which the AEC system has been adjusted (usually around 2.5 μGy). Since the anatomy radiographed in different examinations varies, the image pixels which constitute what is considered as ‘clinical content’, correspond to detector elements of the image receptor irradiated with various IAK levels. Thus, the calculated EI values will vary, and consequently different EIT are required. The DI informs the user how much the EI of an actual radiograph deviates from the EIT, without the user to need to remember what the EIT is. For example, a DI value of +1/-1 denotes a deviation of +25%/−20% from the EIT whereas a DI value of +3/-3 denotes a deviation of +100%/−50%.
Α study (Eur J Radiol Open, 2022 Nov 9;9:100454) was performedin order to assess the level of implementation regarding the latest recommendations on EI, EIT and DI in the digital radiological equipment installed in Hamad Medical Corporation hospitals. Flat field (QC) and clinical images acquired using anthropomorphic phantom body parts and different examination protocols, tube potential settings and radiation field sizes, and two X-ray units of different manufacturers. Furthermore, a survey on EIT and DI data from clinical images acquired in 58 radiographic systems connected to the Radiation Dose Monitoring system of HMC was performed.
Though automatic exposure control (AEC) systems have been adjusted for an IAK of about 2.5 μGy, for most anthropomorphic phantom images the EIs were far from 250, depending on the manufacturer, the anatomy imaged, and the examination protocol. Thus, customization of EIT is necessary and the methodology of selecting appropriate EIT must be refined. Regarding the survey results, DI calculation was feasible in only 38% of the systems, since for the rest EIT values have not been set. However, the rationale based on which EIT values have been selected is unclear. Some systems use only one while others many different EIT values.
Though this study presents preliminary results of work-in-progress, we expect that the medical physicists who use the EI, and may face similar problems regarding implementation, will be interested in the findings of the study regarding the factors that may affect the calculation of EI in both QC and clinical images, and how the EIT should be selected so that the calculated DI values are a meaningful image quality metric for radiographic images of patients.
Note that the American Association of Physicists in Medicine has recently formed a task group to work on this issue (AAPM TG368: Methodology for Establishing Exam-Specific Target Exposure Indices in General Radiography). Dr Ioannis Tsalafoutas and Dr Shady AlKhazzam are members of AAPM TG368.
- Tsalafoutas IA, AlKhazzam S, AlNaemi H, Kharita MH. From the use of exposure index in quality control testing to the use of exposure index for quality control of clinical images. Eur J Radiol Open. 2022 Nov 9;9:100454. doi: 10.1016/j.ejro.2022.100454. PMID: 36386764; PMCID: PMC9647429.
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