DR [28]. Fully DR became available more than a decade ago as stationary equipment intended for general applications in hospitals and clinics. Equipment costs range from US$43 000 to US $140 000, with CE- or US Food and Drug Administration-approved products generally costing US$100 000. The equipment requires a stable power supply for the X-ray generator and cannot easily be transported to multiple sites, making it unsuitable for many remote settings. By contrast, a CR digitiser that can be used with an analogue generator costs ∼US$10 000 [28]. More recently, developments in detector technology have allowed the production of so-called portable and ultra-portable X-ray systems, although the nomenclature used to describe them sometimes varies and overlaps. According to technical specifications produced by the WHO and the International Atomic Energy Agency, these systems “are designed to be used mainly, but not exclusively, when the planned diagnostic and/or screening activities are located far from health structures or in any case when multiple outreach interventions are socially or economically convenient and considered an advantage” [29]. Despite their increased portability, these systems must still follow the “as low as reasonably achievable” principle of managing radiation exposure for healthcare workers [30]. Portable systems are lighter and easier to assemble and disassemble than stationary systems. They can be loaded into cars for transportation to healthcare facilities or temporary clinics, including those with intermittent power supplies, or installed inside large vans for community-based screening [28]. Ultra-portable systems (figure 1) are the newest hardware development. They are small enough to fit in a backpack, weigh 20 kg and are fully battery operated, making them ideal for use in hard-to-reach areas lacking a power supply or as part of event-based screening [28]. Portable and ultra-portable systems have similar, or slightly lower, upfront costs compared with stationary equipment, but each increasing level of portability is associated with a reduction in throughput: while stationary equipment can process more than 300 CXRs per day, ultra-portable systems can manage fewer than 100 per day [28]. FIGURE 1 An ultra-portable digital X-ray system. https://doi.org/10.1183/2312508X.10024322 81 IMAGING AND DIAGNOSIS |J. BIGIO ET AL.