The new techniques of digital tomosynthesis, dualenergy subtraction radiography and temporal subtraction radiography have potential clinical application, chiefly for nodule detection to varying degrees, but at present are confined mainly to research studies.
Digital tomosynthesis of the chest involves the capture of a series of radiographic acquisitions with a digital detector system and computer-controlled apparatus, from which an unlimited number of section images at arbitrary depths and focus can be reconstructed. In this way, an object such as a pulmonary nodule may be rendered more conspicuous, with an exposure equivalent to that of a single film-screen lateral chest radiograph, and therefore with a dose substantially lower than that of CT. Preliminary data suggest improved sensitivity as compared with standard chest radiography for the detection of pulmonary nodules.
Dual-energy subtraction radiography is essentially a bone suppression technique that takes advantage of the differential attenuation of X-ray photons of high atomic number materials (such as calcium and iodine) at different photon energies. Such differential attenuation causes the contrast from calcium and bone in a high kVp image to be reduced. As such, subtraction of the lowenergy from the high-energy image allows subtraction of obscuring bony structures, potentially increasing pulmonary nodule conspicuity (Fig. 1). Low- and high-kVp images can be generated in two ways. First, a single shot exposure, using a two-panel storage phosphor detection system with a copper filter, can be used to generate low- (usually 60) and high- (usually 120) kVp exposures, which can then be subtracted from each other. Alternatively, digital detectors are used to capture a sequential exposure, first at low and then high tube voltages, in a short
interval.
Temporal subtraction radiography involves the subtraction of a current and a previous radiograph to generate a difference image, so that any temporal change (such as a new lesion) is made more conspicuous. This technique relies heavily on co-registration of the two images using computer-generated techniques.
FIGURE 1 ■ Series of dual-energy subtraction chest radiographs in a healthy man. A right apical opacity is seen on a conventional posteroanterior radiograph (A), but a soft-tissue nodule in the left apex only becomes conspicuous on a bone-subtracted image (B). Additionally, a soft-tissue subtracted image (C) reveals that the right apical opacity is actually calcification of the first costochondral junction. (With permission from McAdams HP, Samei E, Dobbins J III, et al 2006 Recent advances in chest radiography.
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