Chapter 9: Principles of Radiographic Image Production PDF

PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production FACULTY OF HEALTH SCIENCE BACHELOR OF MEDICAL IMAGING PHY 6123 – INSTRUMENTATION & IMAGE PROCESSING I By: UMYSARAH BT ZULKIPLI ([email protected]) PHY 6123 : INSTRUMENTATION...

PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production FACULTY OF HEALTH SCIENCE BACHELOR OF MEDICAL IMAGING PHY 6123 – INSTRUMENTATION & IMAGE PROCESSING I By: UMYSARAH BT ZULKIPLI ([email protected]) PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production PRINCIPLES OF RADIOGRAPHIC IMAGE PRODUCTION  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production INTRODUCTION Médical diagnostics are essential to the extraction of anatomical and physiological information from a subject(the patient) and the interpretation of this information in such a way that corrective treatment may be prescribed. There are 3 distinct stages in the flow of information from the patient to the observer: i) The formation of an invisible or ‘aerial’ x-ray image ii) The conversion of the invisible x-ray image into a visible light image. iii) The viewing, perception, and interpretation of the visible image.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production X-ray beam of uniform intensity passes from x- ray tube into the patient INVISIBLE X- X-ray beam penetrates RAY IMAGE through the body tissues X-ray beam is modulated (depends on the degree of attenuation) Beam emerges from patient body (no longer uniform) Pattern of intensities →Invisible x-ray image PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production VISIBLE LIGHT IMAGE Photographic effect of X-rays on a sheet of sensitive film ~ Direct Exposure X-ray Film. Fluorescent effect of X-rays on a sensitive screen. When a fluorescent screen is exposed to x-ray, it emits visible light whose brightness depends on the level of exposures it receives.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production PHOTOGRAPHIC EFFECT Some chemical compounds undergo subtle structural changes when they are exposed to electromagnetic radiations such as visible light, ultraviolet radiation, or x-rays. The changes in most of the cases are not immediately visible, but they are associated with an alteration in the chemical behaviors of the exposed materials.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production In certain reactions, the exposed materials will responds differently from similar materials which have not been exposed. By careful chemical processing, it is possible to differentiate between exposed and unexposed materials. The effect of electromagnetic radiation on the chemical nature of the material is known as the photographic effect of radiation. An invisible image formed on the materials as a result of exposure to electromagnetic radiation is called a latent image. The chemical processing that is necessary to make the invisible changes visible is known as photographic development.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Photosensitive materials In photography photosensitive materials used are Silver halides. Radiography is a specialized application of the photographic process Therefore in radiography too silver halides are used as photosensitive material to record the images  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Silver halides Are a group of chemical compounds consisting of atoms of element silver combined with atoms of halogen elements Silver bromide Silver iodide Silver chloride ▪They are sensitive to light and x-rays ▪Silver bromide is commonly used  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Physical properties of silver halides White or pale-yellow crystalline salts similar in appearance to common salt The links between silver and halogen atoms are ‘ionic bonds’. The electrical forces between positively charged silver ions and negatively charged halide ions fix the positions of the ions in a regular three-dimensional crystal structure or lattice  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Crystal /lattice of AgX  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Chemical properties of Silver halide Pure silver halide crystals are relatively stable and do not suffer a chemical breakdown. But under certain conditions it is possible to convert silver ions to atoms of metallic silver by donating electrons. The agents that supply electrons for this process are called reducing agents. The action is the chemical reduction of silver halide to silver. Chemical reduction takes place more rapidly on silver halide crystals that have been exposed to radiation than those are not. This is the key feature of photographic processing.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Effects of exposure on silver halide X-ray X-ray photon photon e Sensitivity spec Silver Bromine ion Some x-ray photons Some photons give its ion passes through energy to a bromine ion without action and emit an electron Silver halide lattice  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Effect of Exposure on Silver Halide Photon is absorbed by the crystal Interact with bromide ion, release an electron Electron free to move inside the crystal for short time Electron becomes lodged in a low-energy electron trap at the surface of crystal called sensitivity speck Sensitivity speck acts as a development center in the exposed crystal, enabling them to be reduced completely to silver during the development process.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Some X-ray photons pass through without any interaction Some photons interact with bromine ions and release an electron The electron moves around the crystal In a short time it loges in a low energy electron trap near the surface of the crystal This low-energy electron trap is called a ‘sensitivity spec’  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Sensitivity specs are formed by the deliberate introduction of ‘impurities’ into the crystal during its manufacture As it collects more and more electrons sensitivity specs acquire a negative charge When this negative charge becomes strong enough some positively charge silver ions are drawn towards it. As they come the silver ions each gain an electron to become a neutral silver atom (metallic silver)  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production The latent image The existence of a film of the number of silver halide crystals possessing development centers is said to constitute a latent image. A latent image is an invisible image formed on a film as a result of exposure to radiation and which may be made visible by photographic development (This explanation of latent image formation is based on the Gerney-Mott theory)  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Significance of the formation of metallic silver In normal circumstances only a few hundred silver atoms are formed. It is inadequate to produce a visible change in the crystal. But the tiny collection of silver atoms renders the crystal much more vulnerable than unexposed crystals to attack by the reducing agent in the photographic developer. The sensitivity spec now becomes a development center in exposed crystals enabling them to be reduced completely to metallic silver during the development process  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Differences between silver halide & metallic silver Silver halide Metallic Silver Can be converted to Unaffected by fixing soluble compounds by agents fixing agents Not sensitive to light Sensitive to light Image containing only Image containing traces metallic silver is of silver halide undergo permanent changes when exposed Opaque to light and is to light therefore, Image visible as darkened area containing silver halide is against a light not permanent. background  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Radiation interacts releasing e- Silver atoms buildup at the sensitivity center, building the latent image center The group of silver atoms is called the latent image center PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production LIGHT VS DARK AREAS ON FILM Dark Spots-Silver Halide Crystals that have been exposed to photons turn to black metallic silver after processing. Light Areas-Unexposed silver halide crystals are washed away with processing.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production SPECTRAL SENSITIVITY For the maximum effect of the emulsion, the screen-type x-ray films must absorb the light photons being received from the intensifying screen. Light can be of many different colors, representing different wavelengths of electromagnetic radiation. The spectrum is the display of the different colors arranged in order of wavelength (Figure 1). PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Figure 1  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Red light has a wavelength of about 700nm; violet light has a wavelength of about 400nm. Ultraviolet and infrared rays with wavelengths outside of these limits are not visible to the human eye, but photographic emulsions can be made to respond to some of these invisible radiations. It is found that light of different colors is absorbed differently in the emulsion. The behavior of emulsion in this respect is known as the spectral response.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production During manufacture, the color sensitizers are incorporated into the emulsion to control its spectral response. Without sensitizers, a silver bromide emulsion is far more sensitive to blue, violet, and ultraviolet light than the rest of the spectrum monochromatic emulsion. It may require prolonged or intense exposure to red light before any photographic effect is produced. This property of limited spectral response forms the basis of the safelight employed in a photographic darkroom.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Sensitizer If sensitizers are incorporates, the spectral response can be extended into green (up to 570nm ~orthochromatic emulsion) or even as far as the red (up to 700nm~panchromatic emulsion). The spectral response of the screen-type x-ray films must be correspond to the spectral emission of the intensifying screen phosphors. Poor matched leads to loss of sensitivity and increase in the required x-ray exposure.  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production Spectral Sensitivity Curve The graph is a plot of the sensitivity (speed) of the film against the wavelength of the light to which it is exposed. Application of spectral sensitivity curve: 1)Matching the film spectral response to intensifying screen spectral emission. 2)Ensuring the choice of safelight illumination is appropriate to the spectral response of the film being handled in the darkroom. PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production  PHY 6123 : INSTRUMENTATION & IMAGE PROCESSING I LESSON 9 : Principles of Radiographic image production REFERENCES Chesneys’ Radiographic Imaging. John Ball & Tony Price, 6th edition. 1995.Blackwell Science Ltd. Thank you