Samuel G. Armato III, PhD

Chest radiography continues to rank as the most commonly performed imaging examination in the United States. The evolution from film to electronic image formats allows the augmentation of this core imaging modality with new “enhanced radiography” techniques for improved visualization. 1,2 One such enhanced radiography technique is dual-energy imaging, which exploits the different physical properties of soft-tissue and bony structures that affect the attenuation of x-ray photons at different x-ray energies. 3,4 Dual-energy imaging uses dedicated hardware, which is now commercially available, to capture a “low-energy image” and a “high-energy image” of the patient during a single examination. The practical result is the construction of a pair of “energy subtraction” images.

One configuration through which these low- and high-energy images may be acquired involves a single x-ray exposure and two stacked detectors separated by a copper filter; these detectors (eg, storage phosphor plates) differentially record the fluence of x-ray photons transmitted through the patient (ie, those photons that are not attenuated within the patient). To understand what happens next, we must recall two principles of physics. First, x-ray tubes generate x-ray beams with a continuous spectrum of energies, the maximum of which is specified by the kVp. Second, x-ray photons with lower energies are more readily attenuated relative to x-ray photons with higher energies, and materials with higher atomic numbers (eg, the calcium in bone) attenuate x-rays to a greater degree than materials with lower atomic numbers (eg, water, the dominant component of soft tissue). When the portion of the x-ray beam that emerges unattenuated from the patient strikes the first detector, some fraction of these photons is absorbed to record an image, while the portion of the remaining photons that emerges from the filter arrives at the second detector to record a second image. Due to the differential attenuation of x-ray photons in the first detector and the filter, the energy spectrum of x-ray photons that arrives at the second detector is shifted toward higher energies relative to the spectrum of x-ray photons that arrives at the first detector. Accordingly, the second detector records information regarding the attenuation of higher-energy x-ray photons in the patient, while the first detector records information regarding the attenuation of lower-energy x-ray photons in the patient.

The second configuration through which the low- and high-energy images may be acquired involves the dual x-ray exposure of a single detector (eg, an amorphous-silicon flat panel detector). In this scenario, rather than the spectrum of a single x-ray beam being altered through penetration of an initial detector and a copper filter, the x-ray tube directly generates x-ray beams with two distinct spectra, temporally separated by an interval of approximately 200 milliseconds. This time interval is sufficient for the electronics to record the image captured from the first exposure and to prepare the detector for acquisition of the second exposure. Although some motion artifacts may result from this time interval, especially in the vicinity of vascular structures, image preprocessing techniques are applied to minimize their effect.

Under either physical configuration, the two captured images are mathematically combined through a “weighted subtraction” to create a “soft tissue image” that demonstrates structures with attenuation close to that of water and a “bone image” that demonstrates structures with attenuation close to that of calcium. These two images provide a diagnostically powerful combination, 5,6 especially since the energy subtraction images are interpreted along with the standard radiographic image, which also is generated. While the energy subtraction process may be performed for the lateral view, energy subtraction images typically are produced only for the posteroanterior (PA) view to limit the dose delivered to the patient.

(Click the image for a larger version.)
(Click the image for a larger version.)
(Click the image for a larger version.)

Figure 1. Dual-energy chest radiography may be used to produce (a) a standard radiographic image, (b) a soft-tissue image, and (c) a bone image. The nodule demonstrated in the standard radiographic image (arrow) is identified as a calcified granuloma due to its appearance in the bone image (arrow) but not the soft-tissue image (courtesy of H. MacMahon, MD, The University of Chicago; reprinted with permission from [1])

Energy subtraction images provide important advantages over standard radiographic images. First, subtle intrapulmonary lesions and bone may appear superimposed when projected in two dimensions; the soft-tissue image, with bone effectively removed, has the potential to improve radiologists’ ability to detect these lesions. Moreover, the margins of such lesions may appear more distinct in the soft-tissue image, thus facilitating lesion characterization. Second, calcified nodules may be differentiated from noncalcified nodules, since only calcified nodules will appear on the bone image (Figure 1). The same is true for calcifications in hilar lymph nodes. Certain rib abnormalities, such as sclerotic metastases or bone islands, and calcified pleural plaques can mimic soft-tissue abnormalities in standard radiographic images; such lesions, however, usually may be accurately characterized on the bone image. Accordingly, energy subtraction images may obviate the need for follow-up CT scans in some situations.

Samuel G. Armato III, PhD, is medical physicist, Department of Radiology, The University of Chicago. The author would like to thank Heber MacMahon, MD, for his assistance. Armato holds warrants to shares in a Sunnyvale, Calif-based vendor of computer-aided detection software.


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