In-line Hard X-ray Holography for Biomedical Imaging

Abstract

X-ray imaging for biomedical applications is based on detecting interactions of electromagnetic (EM) radiation with energy in the 15–200 keV range (corresponding to wavelength in the 0.8– 0.06 A range) with biological tissues. Such ultra short EM waves called x-rays can be generated through many different mechanisms (e.g. Bremsstrahlung in solid targets, characteristic x-ray emission, synchrotron radiation or in free-electron lasers) and may interact with biological objects being imaged via the coherent Rayleigh scattering, photoelectric absorption or Compton scattering. During x-ray imaging x-ray beam is directed on the object of interest and specially designed detectors detect the transmitted or scattered x-rays. Some x-ray detectors can count photons that interacted with the detector element while other simply record the average x-ray fluence over exposure time in a given detector element. Consequently, even though x-rays are EM radiation conventional detectors can only measure its amplitude but not phase. For biological tissues the complex refractive index is used to describe their interaction with x-rays. It is very close to unity and usually it is written as: n = 1 – δ – iβ, where the δ >0 (called the decrement of the real part of the refractive index) and β >0 describe the phase and amplitude change of x-ray wave traversing tissue, respectively (Als-Nielsen & McMorrow, 2001; Lewis, 2004; Zhou & Brahme, 2008). In conventional x-ray imaging only information on attenuation (i.e. on the imaginary part β) is obtained. For water and soft tissue the ratio δ/β increases as a square of the x-ray energy up to E~40 keV and δ/β is in the 100–1200 range. The ratio δ/β is higher for lower effective Z compounds. Therefore, it is expected that effect of phase shift will be more pronounced in soft tissue imaging, as compared to bone. It follows that two x-ray waves traversing different biological tissues may exit the object with large phase difference even though the absorption they suffered was similar. Consequently, the ability to detect the xray phase shift during x-ray imaging could provide additional important information on the structure of the imaged object in addition to information on electron density that could be elucidated from conventional x-ray absorption imaging. The optimum energy for such phase-contrast imaging is higher than in absorption imaging possibly resulting in the radiation dose savings. We note that absorption–based x-ray imaging suffers from limited low-contrast resolution, i.e. the ability to differentiate two adjacent regions with similar electron density is rather poor. This can be remedied by use of imaging probes– contrast agents that provide higher or lower electron density around or in the structures of interest.