There is a growing interest in the use of chest x-rays (CXR) as a screening tool for TB within enhanced case finding programs . TB prevalence surveys show 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 and inter-observer variation meant that the role of CXR within diagnostic algorithms was limited .
CXR is widely used for detection of TB for a long time in especially North America and Europe, but many factors until recently complicated its use in low and medium resource countries. Digital radiology allows for low running costs, high image quality, lower radiation dose and automated reading (CAD) eliminates inter-observer variation. dCXR can be used as a low cost and rapid triage test to select individuals for further sputum examination and it allows for high throughput of large groups of people. dCXR has a high negative predictive value for pulmonary TB , is rapid (1 minute) and costs less than 25% of one Xpert® test, depending on settings.
Digital X-ray will give patients access to fast, accurate and convenient TB screening, which can contribute to 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 for screening of key populations. In combination with molecular tests, the time to diagnose active TB can be shortened from often 10-20 days to several 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 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 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 € 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 the specific skills are not 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 expert 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 (please see CAD description). Depending on the budget NTPs can select an abnormality threshold for Xpert® eligibility which allows for opting for higher sensitivity at reduced specificity (lower CAD score threshold) and vice versa (higher CAD score threshold). 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 .
At recent UNION World Conferences the high sensitivity of Chest X-ray was illustrated by findings in the TB prevalence surveys in Namibia, Viet Nam, Ethiopia, Kenya, Malawi, Rwanda, Ghana and Cambodia. IRD presented at the 2018 UNION in The Hague the Zero TB initiative (cost) effectiveness using digital radiology as a rapid triage before Xpert®, compared to screening on symptoms only (please see below). 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 a mandatory part of any TB prevalence survey.
IRD Zero TB initiative presentation at the 2018 UNION Conference
Key differences analog vs digital radiography
In the next table three X-ray technologies – analog (film based), Computed Radiology (CR) and Direct Digital Radiology (DR) - are compared on key aspects.
Digital Radiology technology fit for low and medium resource areas is to be designed as a: “single phase (220V), solar powered system and easy to use in the field; a system that allows to make, store, and diagnose CXRs from anywhere and can cope with high ambient temperatures and humidity"
Latest technology can ensure low dose for the patient and instant high quality images for the medical doctor or radiologist. A DR system includes a workstation for diagnosis and an archive server. Integration with archive and viewing software makes the system suitable for fast and efficient medical use. Image reading can be done immediately on the diagnostic monitor, or at any location in the world by sending a compressed image to a centre with qualified staff to read radiographs. A daily throughput of more than 300 images can be achieved per system.
Maintainability in the field is generally high if the X-ray unit comprises components that can be swapped easily. In addition components have to be engineered for shock resistance and the system is to be supported by rubber shock absorbers. Remote system testing using internet and built in test equipment can indicate any component to be replaced or serviced.
Basic digital radiology system technical characteristics
Commonly used for mobile and stationary use
- Single arm Design of Tube and Detector with motorised height adjustment and the ability to rotate the arm to accommodate exposures on standing, seated and recumbent patients
- Fixed focus-detector distance between 140 cm minimum and 155 cm maximum
- Direct digital system with throughput capacity: of at least 60 images per hour
- Capacitor x ray generator, recharge of the condenser in maximum 3 sec. for high throughput
- Single phase 220/240 VAC mains input to allow for connection to national grid and in case of back-up power for the use of solar panels recharged batteries
- Mobile internet connectivity
- CE certified system, delivered with set of manuals
- Input power 220 VAC 8/16 A (single phase connectable to national grid and to batteries/inverter)
- Output power 50 KW (125 KV @ 630 mA)
- Integration with flat panel detector, one touch panel for both X-ray setup and review
- kV range 40-125
- mA range 10-630
- mAs range 0.1-630
- AEC integration mAs 01 to 500 mAs shortest exposure time min 1 ms
- X-Ray tube should be a IEC Classification Class I Type B or better
- Temperature range 0 °C ÷ 40 °C
- Rel humidity range 10% ÷ 90%, non-condensing
X-RAY FLATPANEL DETECTOR
- Suitable for chest X-ray and general radiographic examinations (such as abdomen & extremities)
- Min detector size 43 x 42 cm
- Scintillator & amorphous silicon (a-Si)
- 5.9 megapixels min Gray scale: 12 bits minimum 4000 levels
- Calibration of the detector less or equal - once per year
- Max weight 5 kg
- Pixel size min 160x160 μm
- A/D 14 bit
- Non moving grid, grid elimination software
- Preview time 3-6 seconds after X-ray exposure
- Non moving grid with GRID removing software
- DICOM 3.0 compatible
- Integration with X-ray generator, one touch panel for best quality X-ray at lowest possible dose
X-RAY OPERATORS STATION
- Touch screen operation, Size min 15”
- Windows 10
- Min processor capacity I3, 8GB RAM, 1TB HDD (RAID 1)
First installations in-country are recommended to be done by factory trained engineers.
3 steps to detect a patient with presumptive TB
The imaging process: Software when designed with the input of users and radiology specialists warrants an intuitive user interface. Most user interfaces are Windows™ based and appear similar to well-known applications.
Making an image: First, patient demographic information is entered by the keyboard, scanner or DICOM Modality Worklist. Second, the image is taken. The Automatic Exposure Control will give high quality Chest X-rays with the lowest possible dose for the client.
Diagnosing everywhere: Immediately after the image appears on the screen it can be relayed to any central database anywhere in the world and is typically available just three seconds after the exposure has been made. When using the mobile phone GSM network sending a compressed 350KB image to a diagnostic centre will take about 35 seconds provided image compressing and communication software is used.