1. X-ray radiography Seminar 22.10. 2008.

2. The "geometrical" unsharpness of X-ray images is limited by finite dimension of the source, i.e. the window on X-ray tube. Therefore the optical shadow is always surrounded by half- shadow. By adjustment of distance between the tube, object and film we try to minimalize the half-shadow. Secondary photons, emitted by relaxation of atoms after Compton scattering, leave the body and uniformly increase the exposure of the film reducing the contrast. The quality of radiographs can be improved by the use of special filters, X-ray film coated with fluorescence layer and image intensifier. Difficulties and problems of classical radiography

3. Contrast of the image The X-ray beams will be absorbed in different amount in particulate tissues. The transmitted beams will have different intensities - the basis for the contrast in the image. The contrast is satisfactory if the intensity difference is at least 10%. Using the contrast agents we can improve the contrast. I0I0 I 1 (bones) I 2 (soft tissues)

4. Soft tissues are absorbing X- radiation by the mechanism of Compton effect. The probability for absorption depends on electron density which is almost the same for the atoms of soft tissue and the contrast is rather low. It can be enhanced by introduction of substance with high density and high Z number (contrast agent) in organ under investigation. These agents are absorbing X-radiation by photoelectric effect. Using the lower voltage, only the soft tissues with contrast agent will have strong absorption. intestines with solution of barium salt

5. Influence of half-shadow on contrast D half-shadow is the consequence of finite dimension of the source window apertureobject L d x half-shadow film x = D d/L shadow half-shadow is thinner if the distance from object to film is small in comparison with distance from object to source

6. Use of intensifying screen on X-ray film Photographic emulsion is not enough sensitive for X- photons, only 1% of incident photons are absorbed in film - it would require the long irradiation of patient to get the good image Intensifying screen (phosphor in low Z matrix) enables the transformation of X-photons into light photons (1 X - 1000 VIS) which can be completely absorbed in film The intensity of image can be enhanced about 30 times

7. Image intensifier - in fluorescent screen X- photons are transformed in photons of visible light - these photons eject electrons from photocathode - in evacuated tube, electrons are accelerated toward anode; their motion is controlled by electron optics - electrons hit the other fluorescent screen and create the image of high intensity which can be monitored in the dark convenient for surgery halls, teaching, distance learning

8. Image intensifier in surgery

9. Xerography Xerography uses photoconductivity - photoelectric effect on a semiconductor the images have sharp boundary lines - the borders between different tissues The beam transmitted through the body contacts with positively charged selenium coated aluminum plate; at the collision site, the photon energy causes selenium to become conductive and the charge is neutralized; the distribution of positive charge density is inversely proportional to the number of incident photons (electrostatic image) by applying negatively charged dye powder we can get visible image edges are sharp due to changes of electric field as a step function X-beam objects with different density