X-ray phase-contrast imaging of the breast--advances towards clinical implementation

Abstract

Breast cancer constitutes about one-quarter of all cancers and is the leading cause of cancer death in women. To reduce breast cancer mortality, mammographic screening programmes have been implemented in many Western countries. However, these programmes remain controversial because of the associated radiation exposure and the need for improvement in terms of diagnostic accuracy. Phase-contrast imaging is a new X-ray-based technology that has been shown to provide enhanced soft-tissue contrast and improved visualization of cancerous structures. Furthermore, there is some indication that these improvements of image quality can be maintained at reduced radiation doses. Thus, X-ray phase-contrast mammography may significantly contribute to advancements in early breast cancer diagnosis. Feasibility studies of X-ray phase-contrast breast CT have provided images that allow resolution of the fine structure of tissue that can otherwise only be obtained by histology. This implies that X-ray phase-contrast imaging may also lead to the development of entirely new (micro-) radiological applications. This review provides a brief overview of the physical characteristics of this new technology and describes recent developments towards clinical implementation of X-ray phase-contrast imaging of the breast.

Figure 1.

X-rays encounter changes in intensity and phase when traversing an object. Compared with an X-ray wave bypassing the object (bottom), an X-ray wave travelling through an object (top) experiences changes in amplitude (attenuation) and phase. The degree of attenuation roughly depends on material density; the strength of the phase shift on the electron density. The complex index of refraction n for X-ray waves can be expressed as , where δ is proportional to the phase shift and iβ is proportional to absorption. For biological tissues and the X-ray energies typically used in breast imaging, δ is significantly larger than iβ.

Figure 2.

Examples of phase detection techniques. (a) Crystal interferometry. A monochromatic X-ray beam is split by a silicon crystal. One-half of the beam traverses the object before both beams are reunited in front of the detector. (b) Propagation-based imaging. Partially coherent (polychromatic) X-rays pass through the object before meeting the detector. If the distance between object and detector is small (d₁), only absorption information is recorded in the resulting image. Increasing the distance between object and detector (d₂) yields a composite image that contains both absorption and phase information. (c) Analyser-based imaging. Phase information is extracted by a silicon analyser positioned in the X-ray beam. (d) Grating-based interferometry set-up with a conventional X-ray tube source. X-rays originating from the tube source are masked by a source grating before traversing the object. Subsequent phase and analyser gratings allow detection of pure phase, dark-field and absorption information, which can be calculated from the intensities recorded by the detector.

Figure 3.

Comparison of clinical mammograms with propagation-based imaging mammograms recorded at a synchrotron source. (a) Mediolateral oblique digital mammography image and (b) corresponding digital zoom image, showing a suspicious mass (arrow). (c) Findings on the synchrotron radiation mammographic image (recorded with propagation-based imaging technology) and (d) corresponding digital zoom image confirm and better depict the spiculated mass. Reproduced with permission from the Radiological Society of North America.

Figure 4.

Absorption (a), phase-contrast (b) and dark-field (c) grating-based mammography images (craniocaudal) of an ablated breast recorded with a conventional X-ray tube source. The images are of a 52-year-old patient and show the complete remission of an angiosarcoma after neoadjuvant chemotherapy. The differential phase image enhances the edges of tissue structures and thus depicts collagen strands more clearly. The dark-field image is particularly sensitive for calcifications.

Figure 5.

Direct comparison of absorption and phase-contrast breast CT. Experimental absorption-contrast (a) and phase-contrast (b) tomographic images recorded at the European synchrotron radiation facility. The grey-scale bars represent Hounsfield units (absorption contrast) and phase-contrast Hounsfield units (phase contrast), respectively. The sample presented here contains invasive ductal carcinoma, as well as ductal carcinoma in situ. Phase-contrast images reveal contrast differences within the tumour that are not resolved by absorption-based imaging. In particular, circular structures of high phase contrast within ductal carcinoma in situ are better resolved in phase-contrast images and coincide with ductal walls and the basement membrane. Conversely, calcifications are better seen in absorption-based images, underlining the complementary nature of the methods. For more details, see Sztrokay et al.