In recent years, there has been growing interest in the use of chest x-rays (CXR) as a screening tool for TB within active and enhanced case finding programs . This growing interest is facilitated by innovations in digital radiology. Recent TB prevalence surveys have shown that CXR has higher sensitivity than verbal screening for identifying pulmonary TB . Previously, costs, limited access to x-ray facilities, maintenance of equipment, availability of trained personnel, poor specificity and inter-observer variation meant that the role of CXR within diagnostic algorithms was limited .
Digital radiography presents numerous advantages over analog film-based systems, such as lower running costs, high image quality, lower radiation dose and automated reading (CAD) eliminates inter-observer variation. CXR/CAD can be used as a low cost and rapid triage test to select individuals for sputum testing, it allows for high throughput systematic screening of large groups for active TB. Chest radiography is also essential for the diagnosis of many other lung diseases. dCXR/CAD has a high negative predictive value for pulmonary TB , is rapid (1 minute) and costs about 25% of one Xpert® test, depending on settings.
Check for TB using digital X-ray will give patients access to fast, accurate and convenient TB screening, reducing their travel cost, waiting time and speeding up access to diagnosis and treatment. Intensified TB case finding using digital radiology will support the combat against TB in people living with HIV who often present with smear negative sputum. New technology X-ray allows for low dose exposure for patients, making it an appropriate tool also for TB prevalence surveys and systematic and regular screening of high risk groups. In combination with molecular tests, the time to diagnose active TB can be shortened from often 10-20 days to about 2.5 hours. Digital radiology can also be used to spot pre-clinical TB and can support the health system in general, through diagnostic imaging of also other body parts.
High digital image quality
Digital X-rays are instantly available on the diagnostic monitor with constant high quality. Software tools to zoom in or to add contrast to the digital image are available and enhance ease and accuracy of TB screening. Issues of poor readability and deterioration or loss of films are eliminated.
Low image cost
Films and developer are no longer needed, storage of images is done electronically and superior readability allow for a very low cost per image. Cost savings relative to analog systems using films in high throughput settings can exceed Euro 50.000 per year per system. Even at a low throughput of 30 images per day, digital images are more economic.
Instant image retrieval
Digital radiology systems using an integrated Picture Archiving and Communications System (PACS) can efficiently archive and retrieve large numbers of digital X-rays. This eliminates the high archiving cost of analog film based X-ray images. Using the universal Digital Imaging and Communications in Medicine (DICOM) format, the PACS attaches patient information to the right X-ray , eliminating the risk of “storing a film in the wrong envelope” and, enhancing patient safety.
Digital X-ray is clean technology as chemical waste from film developing, that can cause serious environmental damage, is completely eliminated.
Remote diagnosis - tele-radiology
If the X-ray image cannot be interpreted locally because no specific knowledge is available or the image is particularly difficult to interpret, it can be sent over the internet or even over a low bandwidth (mobile phone) connection to a centre where accurate reading can be done. The diagnosis can be sent back immediately and continues to be attached to the right digital Chest X-ray.
Computer Aided Detection
CAD allows for the automated recognition and quantification of abnormalities consistent with TB in a digital Chest X-rays. CAD4TB already outperforms the trained human reader in terms of accuracy as demonstrated in studies in Philippines and Tanzania. In order to support TB prevalence surveys and early case detection in risk groups, this diagnostic software is available for use in the field as a “second opinion” or to automate triaging before Xpert MTB/RIF. Regular CAD software updates now become available as the diagnostic accuracy continues to improve and will also support the detection of other lung diseases such as silicosis, pneumonia, emphysema and lung cancer. In case of prevalence surveys the CAD sensitivity can be pre-selected to reach highest levels, allowing radiologists to focus more time on reading abnormal images and decimating the risk of missing cases. Depending on the budget NTP's can select an abnormality threshold for Xpert eligibility which allows for opting for higher specificity at reduced sensitivity (higher CAD score threshold) and vice versa (lower CAD score theshold).
Zambart in Zambia put this threshold at > 60% CAD abnormality score resulting in a high predictive value (23%) for a positive Xpert MTB/RIF test; a CAD abnormality score of > 80% resulted in 44% positive Xpert MTB/RIF tests (please refer to the table below) .
At recent UNION World Conferences the high sensitivity of Chest X-ray was illustrated by findings in the TB prevalence surveys in Viet Nam, Ethiopia, Kenya, Malawi, Rwanda, Ghana and Cambodia. The fact that screening on symptoms suggestive of TB only can result in missing more than 50% of TB cases, now makes Chest X-ray with high sensitivity levels a mandatory part of any TB prevalence survey, as mentioned in the WHO guidelines for Prevalence Surveys.