Second, the latent image received by the receptor is processed into a radiographic image [24]. In conventional analogue radiography, the receptor is a piece of X-ray film. Analogue radiography has several limitations: film processing is labour intensive, often requiring a dark room and involving chemicals that can be hazardous to those using them [25, 26], image quality is variable [27], and without an additional digitalisation step, the images cannot be digitally stored or used for automated image reading. Despite the 20th-century history of mobile, mass CXR screening, it is also difficult to implement as a portable technology unattached to health facilities, given the complex image processing requirements [28]. Two newer technologies, computed radiography (CR) and digital radiography (DR), provide improvements over analogue radiography. In CR, the receptor is a reusable phosphor plate, which is scanned by a digitiser and processed into a digital image [25, 28]. This allows substantially faster throughput than analogue radiography but is still slower than DR, in which the receptor is a digital flat-panel detector, removing the need for the intermediate processing stage [28]. Both CR and DR require computing infrastructure, as well as picture archiving and communication systems [28]. As well as offering speed improvements, DR offers the best image quality and is safer for patients because it requires lower doses of radiation than the other technologies. Nevertheless, the preferred system in low–middle-income countries remains CR, primarily due to the capital investment required to implement fully digital stationary radiography equipment, as described later. Moreover, DR requires an X-ray generator calibrated to the specific detector for optimal use, and these generators are fragile, making their transportation to remote settings challenging. Although analogue generators can be retrofitted with digital detector panels, providing better performance than analogue radiography or CR, their performance is still slightly inferior to fully TABLE 1 Current WHO recommendations for the use of CXR in TB WHO consolidated TB guideline module #Recommendations around the use of CXR Study [ref.] Prevention (2020) CXR may be offered to PLHIV on ART, and TPT given to those with no abnormal radiographic findings The absence of any symptoms of TB and the absence of abnormal CXR findings may be used to rule out active TB disease among HIV-negative household contacts aged ⩾5 years and other risk groups before TPT WHO [23] Screening (2021) Among individuals aged ⩾15 years in populations in which TB screening is recommended, systematic screening for TB disease may be conducted using a symptom screen, CXR or molecular WHO-recommended rapid diagnostic tests, alone or in combination Among individuals aged ⩾15 years in populations in which TB screening is recommended, computer-aided detection software programmes may be used in place of human readers for interpreting digital CXR for screening for TB disease Among adults and adolescents living with HIV, CXR may be used to screen for TB disease WHO [9] Children and adolescents (2022) In children with presumptive PTB attending healthcare facilities, integrated treatment decision algorithms (flowcharts allocating evidence-based scores to microbiological, clinical and radiological features) may be used to diagnose PTB WHO [16] #:module refers to the five modules currently comprising the WHO consolidated guidelines on TB. 80 https://doi.org/10.1183/2312508X.10024322 ERS MONOGRAPH |THE CHALLENGE OF TB IN THE 21ST CENTURY