The Detection and Intensification of X-Ray Images
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THE DETECTION AND INTENSIFICATION OF X-RAY IMAGES CYRIL WILLIAM SMITH ABSTRACT "THE DETECTION AND INTENSIFICATION OF X—RAY IMAGES". An historical introduction is followed by a survey of the physics of X—rays, and the production of X—rays for radiology. The possibility of detecting X—ray images is related to measurable quantities, and existing techniques are shown to be close to the quantum limit. Improvements in the diagnostic value of the X—ray image, and the reduction in the patient dose require: the reduction of scattered radiation; increased X—ray absorption in the primary detector; the development of contrast media; the control of image contrast; the presentation of the final image at a high brightness level. The methods of X—ray image intensification are surveyed. Work towards these ends is described in respect of an X—ray sensitive photo— conductive camera tube for converting an X—ray image into a television signal. Its time lag was reduced to 1/25th second by reducing the sulphur content of the amorphous selenium used for the photoconductor, but the tube was shown to be of little use for dose rates of less than 1 r/Min at the tube. This was on account of the small photocurrents which were due to the difficulty of extracting charge from a thick layer by an applied electric field, 2. without also producing a background of white spots. The intensification of the weak light images from X—ray fluorescent screens becomes practicable with reduction in the background glow in intensifier tubes. A tube design in which this had been reduced by excluding caesium from the high voltage part of the tube was improved by using a spiral accelerating electrode to give a more linear electric field, and the remaining background glow was shown to depend on the presence of the solenoidal magnetic focusing field in the region of the tube cathode. 3. CONTENTS Abstract. Page 1. Section 1: Introduction. Page 4.- Section 2: X—ray Absorption. Page 14. Section 3: X—ray Images. Page 26. Section 4: An X—ray Sensitive Photoconductive Camera Tube for X—ray Image Intensification. Page 63. Section 5: The Intensification of the Faint Light Images from an X—ray Fluorescent Screen. Page 130. 1G5 Conclusions. Page i-&t. 16 Acknowledgements. Page -1-frd-. 170 References. Page 4-69.. 4. The Dection and Intensification of X-ray Images. Section 1: Introductj.on. Historical Survey. While repeating Hertz"and Lenard's experiments with electrons, RUntgen made an observation on November 8th, 1895, which resulted in the discovery of X-rays. In his "Preliminary Communication", Rtntgen (1896) described the fluorescence which the rays produced in various substances and their power of penetrating solid bodies. Of the X-ray image of his hand he wrote: "... if the hand be held between the discharge tube and the screen, the darker shadow of the bones is seen within the slightly dark shadow image of the hand itself ...". He published photographs from X-ray images and observed that the rays were subject to rectilinear propagation with the intensity decreasing as the square of the distance from their origin at the bright spot on the wall of the discharge tube where the cathode-rays impinged. Discharge tubes and high voltage induction coils were a part of the equipment of most physics laboratories at the time and Rbntgen's experiments were immediately confirmed and extended. There were at least 1044 publications concerned with X-rays during 1896. (Glasser, 1933). The subsequent development 5. of radiology is described in a series of articles entitled "Sixty Years of Radiology" (Stead et al., 1956). Rlintgen's work was followed by a development of the physics of X-rays. Stokes and J.J. Thomson (1898) put forward the theory that X-rays were irregular electro- magnetic pulses produced when charged particles were decelerated on striking a target. Rdntgenis failure to obtain refraction of X-rays with a prism was explained by Malt.ezos (1896) on the Helmholtz dispersion formula which gives a refractive index tending to unity for very short wavelengths. Haga and Wind (1899) and Walter and Pohl (1909) confirmed that the wavelength of X-rays was of the order of 10-8 cm, by diffraction experiments. Imbert and Bertin-Sans (1896) found that X-rays underwent scattering rather than specular reflection and J.J. Thompson applied the classical pulse theory of X-rays to this scattering. Barkia (1906) went far towards confirming the predicted angular distribution and polarization of the scattered X-rays and deduced the number of electrons in the carbon atom. He recognised series of line spectra in the scattered X-rays which were characteristic of the scattering substance, and used these X-rays (Barkia and Sadler, 1909) as a source of homogeneous radiation to show that the general law for the absorption of radiation could be applied to X-rays. 6. On the basis of the electromagnetic theory, Laue predicted that X—rays would be diffracted by a crystal lattice acting as a three—dimensional grating, this was confirmed (Friedrich, Knipping and Laue, 1913). W.I. Bragg (1912) introduced the concept of reflection from a set of equidistant planes in the crystal and deduced the spatial position of the diffraction pattern for a given wavelength of X—rays. W.H. Bragg and W.L. Bragg (1913) developed the X—ray spectrometer and observed the characteristic X—ray emission spectra for the target material superimposed on the continuous X—ray spectrum. Moseley (1913, 1914) made a systematic study of the L-ray emission spectra of various elements from which he derived a relation between the frequency of the X—ray spectral lines and the atomic number of the radiating element. Bohr (1913) used the Rutherford (1911) model of the atom to calculate the energies of the electrons in their orbits and the frequencies of the radiation they would emit in making transitions between different energy levels. These were found to correspond to the observed absorption limits. Besides the discontinuous energy states which give rise to the characteristic spectra, there are states forming a continuous range which correspond to the hyper— bolic orbits of Bohr's theory. Transitions involving 7. these give rise to the continuous X-ray spectrum (Born, 1952). The continuous spectrum has a well defined lower wavelength (maximum energy) limit corresponding to the kinetic energy of the incident electrons and is given by Einstein's (1905) equation for the photoelectric effect. With very thin targets the maximum of the X-ray energy spectrum occurs at this lower-wavelength-limit. For thicker targets the energy spectrum is modified by the loss of energy by some incident electrons and by the absorption of the emitted X-rays in the target material. The wavelength of maximum energy occurs at about 12 times the wavelength of the lower-wavelength-limit. KUlenkampff (1926) suggested that the energy distribution in the con- tinuous X-ray spectrum can be given by a system of parallel straight lines for values of the tube voltage V and the frequency of the radiation ;v. Corresponding to each value of V there is a maximum frequency vo given by Einstein's (1905) photoelectric equation: Ve hvo where e is the electronic charge and h is Plank's constant. Then the equation is expressed in terms of the minimum wavelength X0 and the values of the constants are sub- stituted it becomes x _ 12,354 0 - V 8. where X o is in Angstrom units if V is expressed in volts. The energy E, of X—radiation of frequency v emitted per second is proportional to (/ 0 v) and to the tube current is Fiv = Ki(v•0 — v• ) where IC is the constant of proportionality. The energy emitted per second in the frequency interval v 2 —v 1 is given by 2 E d.v = Ki( vo v )cv . v-1 Expressing the energy emitted per second in terms of the wavelength: E =Li ,1) ^ with a new constant L. This expression must be further adjusted to take account of X—ray absorption in the target, the tube window and any filtration used (de Waard, 1947). X—ray Tubes. The early discharge tubes were improved by the use of a concave cathode and adjustable electrodes to enable the electron beam to be focused on the target. A potash—bulb enabled oxygen to be generated as required to adjust the gas pressure and hence the break—down potential of the tube which determined the effective wavelength of the 9. X-radiation. Coolidge (1913) introduced a high-vacuum tube with a thermionic cathode in which the tube current was independent of the applied voltage. Greatorex (1960b) points out that no fundamental change in the operation of the tube has taken place since that time. There have been considerable refinements in the tube technology and electron optics and the high-voltage generators and control units now offer precise and fully automatic selection of the appropriate parameters by means of a single push-button. The X-radiation is generated by a beam of high energy electrons which are decelerated by striking a target. The electron source is a cathode of tungsten wire heated by an electric current and mounted in a metal focusing cup. The anode is usually a tungsten block about 0.25 cm thick around which is vacuum cast a copper block to conduct the heat away from the tungsten. The efficiency of X-ray generation n is proportional to the atomic number z of the target and to the tube voltage V volts (usually 40 to 120 kVp).