Method for calibration of a spectrozonal X-ray diagnostic system

Abstract

Spectrozonal X ray detectors allow X ray image to be obtained at different energies and information value of projection X ray to be increased without increasing dose load on the patient [1,3].Subtraction algorithms of spectrozonal X ray imag ing can restore separate images of bone and soft tissues [5]. The resulting images are equivalent to images obtained using linear tomography of skeleton hidden pathology. Multienergy methods of radiography would provide an opportunity of not only density imaging, but also imaging in grades of atomic number or electron den sity [4,6], thereby providing noninvasive analysis of the nature of a lesion [7].Let us consider a method for calibration of an X ray diagnostic system equipped with a spectrozonal X ray detector.Let the detector have spectrometric properties and be able to distinguish X ray photons of 3 energy groups: low (l), medium (m), and high (h).A tissue identification criterion was suggested in [2]:where µ ob,h , µ ob,m , µ ob,l are X ray mass extinction coeffi cients of inclusion tissues at high, medium, and low ener gies, respectively. Practical implementation of this method includes recasting of Eq. (1) as:where I f и I ob are radiation intensities at background and inclusion regions, respectively.In case of calibration, Eq.(2) allows the effective atomic number of tissue to be determined using feedback filtration of background component of the image itself.Given the fact that X ray radiation extinction coeffi cient is a function of quantum energy:where summand τ(E)Z 4 determines photon absorption; σ(E)Z is Compton scattering; and χ(E)Z 2 is pair forma tion effect for energies used in medical radiography:Effective atomic number can be calculated from angle ϕ. Dependence of τ(E) and σ(E) is universal for all substances: Φ = tanϕ = µ ob,m /µ ob,l ; the expression for ϕ is:It follows from Eq. (2) that Φ = ln(I f /I ob ) m /ln(I f /I ob ) l .If coefficient Φ is experimentally found, effective atomic number of inclusion tissue can be derived from Eq. (4) as:Coefficients τ(E m ), τ(E l ), and σ(E m ), σ(E l ) are determined by calibration using reference substances with standard parameters, by radiation intensity ratio in direct and attenuated beams at different energies. A basis of two materials A and B (with standard parameters ρ A , t A , Z A and unknown parameters ρ B , t B , Z B meeting the condition Z A ≠ Z B , respectively) is a necessary con dition of calibration. Values Z A and Z B should fall