Medical Radiography-Chapter 4 PDF

Chapter 4 Radiography The x-ray film: 1) The base: it is flexible & transparent, either cellulose acetate or a polyester resin. Single emulsion film is less sensitive to radiation and is used primarily when exceptionally fine detail is required in the imag...

Chapter 4 Radiography The x-ray film: 1) The base: it is flexible & transparent, either cellulose acetate or a polyester resin. Single emulsion film is less sensitive to radiation and is used primarily when exceptionally fine detail is required in the image (e.g., laser printing and mammography). 2) The emulsion: It is composed of silver halide granules, 3) Usually silver bromide that are suspended in a gelatin matrix. The emulsion is sensitive to visible and ultraviolet light and to ionizing radiation. 4) The super coat: a protective coating. 5) The adhesive: binds the emulsion to the base. The photographic process: 1) When an x-ray film is exposed to ionizing radiation, the energy 36  absorbed by the silver bromide granules in the emulsion causes the release of electrons from bromide (Br-) ions. 2) These electrons are trapped at "sensitivity centers" in the crystal lattice of the silver bromide granules. 3) The trapped electrons attract and neutralize mobile silver ions (Ag +) in the lattice. Hence small quantities of metallic silver are deposited in the emulsion. 4) Although these changes in the granules are not visible, the distribution of deposited of metallic silver across a film exposed to an x-ray beam is a reflection of the information transmitted to the film by x-rays. This information is captured and stored as a latent image in the photographic emulsion. 5) When the film is placed in a developing solution, additional silver is deposited at the sensitivity centers. 6) Since no silver is deposited along granules that are unaffected during exposure of the film to radiation, these granules are removed by the sodium or ammonium thiosulfate present in the fixing solution. 7) The degree of blackening of a region of the processed film depends on the amount of free silver deposited in the region and, consequently, on the number of x- rays absorbed in the region. 37 Optical density and film gamma: - The amount of light transmitted by a region of processed film is described by the transmittance T, where: - The degree of blackening of a region of film is described as the optical density (OD) of the region: OD = log ( ) - The optical density across most radiographs varies from 0.3 to 2. The characteristic curve of a film: - It is the optical density of a particular film or combination of film and 38  intensifying screens plotted as a function of the exposure or (log (exposure)) to the film. - The radiation exposure to an x-ray film should be sufficient to replace the range of optical densities exhibited by the processed film along the essentially straight-line portion of the characteristics curve. - The average slope of this portion of the curve is referred to as the average gradient of the film: Average gradient = Films with higher average gradients produce images with higher contrast (more blacks and whites and fewer shades of gray). - The gamma (Y) of the film: it is the maximum value of the slope of the characteristic curve. - Film speed (film sensitivity): is is a measure of the position of the characteristic curve at the specific value of the exposure axis. Film speed = - Speed is gained by requiring fewer x-ray or light photons to form an 39  image. Hence, a fast film furnishes a "noisier" image in which fine detail in the object may be less visible. The intensifying screens: - The problem: for an x-ray beam of diagnostic quality, only about 2% to 6% of the total energy of the beam is absorbed in the emulsion of an x- ray film exposed directly to the beam. - Consequently, direct exposure of film to x- rays film very inefficient utilization of energy available the x-ray beam. This procedure is used only when images with very fine detail are required. The solution: for most radiographic examinations, the x-ray film is sandwiched between intensifying screens. - The intensifying screens furnish a light image that reflects the variation in exposure across the x-ray beam. - This light image is recorded by film that is sensitive to the wavelengths of light emitted by the screen. Composition and properties of intensifying screens: - Protective coating that is transparent to the light produced in the active 40  layer. - Small granules of fluorescent material embedded in a plastic matrix. - X- rays transmitted through the patient will be absorbed in the active layer producing fluorescent light which passes through the coating towards the x-ray film. - Light produced in the direction opposite to the film will be reflected towards the film by the reflecting layer. Desirable properties of an x-ray intensifying screen: 1. A high attenuation coefficient for x-rays of diagnostic quality. 2. A high efficiency for the conversion of energy in the x-ray beam to light. 3. A high transparency to light released by the fluorescent granules. 4. A low refractive index so that light from the granules will be released from the screen and will not be reflected internally. 5. An insensitivity to handling. 41 6. An emission spectrum for the radiation- induced fluorescence that matches the spectral sensitivity of the film used. 7. A reasonably short time for fluorescence decay. 8. A minimum loss of light by lateral diffusion through the fluorescent layer. To reduce this loss of light, the fluorescent layer is composed of granules and is not constructed as single sheet of fluorescent material. 9. Low cost. Types of x-ray intensifying screens: A) Before early 1970s: (Screens were made of crystalline calcium tungstate) The efficiency for the conversion of energy in the x-ray beam to light is about 20% to 50%. One reason is because the K- absorption edge of tungsten, (69.5keV) is above the energy of most photons in a diagnostic x-ray beam. B) After early 1970s: (Rare- earth screens): -They are gadolinium, lanthanum, and yttrium (K- absorption edge 35 to 50keV) complexed in oxysulfide or oxybromide crystals and embedded in a plastic matrix. - They exhibit not only an increased absorption of diagnostic x-rays but also an increased efficiency in the conversion of absorbed x-ray energy to light. Hence rare- earth screens are faster than their calcium tungstate counterparts. Advantages of rare-earth intensifying screens: 1. Reduced exposure time and decreased motion un sharpness in the image. 42 2. Reduced tube current allowing more frequent use of small focal spots. 3. Reduced tube voltage and improved contrast in the image. 4. Reduced production of heat in the x-ray tube. 5. Reduced patient exposure. Efficiency & resolution of intensifying screens: The efficiency of energy conversion is greater for a screen with a thick active layer (i.e. a fast screen) than for a slow screen (a detail screen). However, the resolution of the radiographic image decreases as the thickness of the active layer increases. The intensification factor of intensifying screens: The intensification factor = - For a particular screen – film combination this factor varies greatly with the energy of the x-rays used for the radiation exposure, but a range of 50 to 100 is common. - Therefore patient exposure is reduced greatly through the use of screens. 43 Examples: - Compare the number of visible light photons emitted by a calcium tungstate screen with the number emitted by a gadolinium oxysulfide screen. Assume that 100 x-ray photons, each having an energy of 30keV, strike the screen. Assume that all visible photons are emitted by the screen at the spectral peak energy. The following data are given: Absorption Conversion Spectral emission (%) (%) peak (nm) Calcium 40 5 420 tungstate Gadolinium 60 2 550 Oxysulfide Answer: A) The energy per visible photon emitted: E(kev) = = 3 x 10-3 keV = 3eV for calcium tungstate = = 2.25 x 10-3 keV = 2 eV for gadolinium oxysulfide B) The energy converted to visible light photons emitted from each screen is: Energy emitted = (100) (30 x 103 eV) (0.40) (0.05) = 6 x 104 eV for calcium tungstate = (100) (30 x 103 eV) (0.60) (0.20) = 36 x 104 eV for gadolinium oxysulfide 44 C) The number of visible photons emitted from each screen is: = 2 x 104 photons for calcium tungstate. = 18 x 104 photons for gadolinium oxysulfide Thus the lower energy per visible photon and the higher absorption and conversion efficiencies all act to produce more visible photons per incident photon for the gadolinium oxysulfide screen than for the calcium tungstate screen. The radiographic grids: The problem: - Information is transmitted to an x- ray film by un attenuated primary radiation emerging from a patient. - Radiation scattered within the patient and compinging on the film tends to conceal this information by producing a general photographic log on the film. - The amount of radiation scattered to a film increases with the volume of tissue exposed to the x-ray beam. The solution: - Much of the scattered radiation that would reduce the quality of the radiographic image may be removed by a radiographic grid between the patient and the x-ray film or screen – film combination. - A radiographic grid is composed of strips of a use, high- Z material separated by a material is relatively transparent to x rays. 45 Construction of Grids: 1. The thickness of the strips is reduced so that images of the strips are not visible in the radiographic image. 2. The strips should be completely opaque to scattered radiation and should not release characteristic x- rays (grid fluorescence) as scattered x-ray photons are absorbed. 3. Thus lead foil about 0.05mm thick is the material used for the strips while the interspace material between the grid strips may be aluminum, fibre, or plastic. Magnification radiography: The magnification of a radiographic image is given by: 46  Magnification = The magnification is achieved either by: - Object– shift enlargement: the ratio of image to object size is increased by moving the object toward the x – ray tube at constant target to film distance. - Image- shift enlargement: the ratio of image to object size is increased by moving the film farther from the x-ray tube at constant target to object distance. Digital Radiography - The objective of a digital detector in radiography is: to convert the energy of x-ray photons into electrical signals that signify the magnitude to the exposure in different pixels. - This can be achieved either by acquiring radiographic images digitally or by converting images acquired in an analog fashion into digital format. Why do we need digital detectors? - 1. The need for increased dynamic range (of allowed exposures), 2. Multaneous viewing at multiple modifications, and 3. The use of digital image processing and enhancement. A) Discrete digital detectors: They are a number of detectors that produce electronic signals that are easily digitized, including fluorescent crystals (e.g. sodium iodide & bismuth germinate), photodiodes, and various semiconductor devices. 47 B) Photostimulable (storage) phosphor or computed radiography: -The image is acquired on a plate containing crystals of a photostimulable phosphor. - A material such as barium fluorobromide (BaFBr) is capable of storing the energy from an x- ray exposure by the excitation of electrons from the valence band to trapping centres close to the conduction band. Electron vacancies left in the valence band that have an overall positive charge are called holes. The holes are also trapped. - When exposed to a strong light source of the appropriate wavelength, the trapped electrons and holes gain enough energy to jump out of the traps and recombine. Consequently, energy in the form of visible light is released and can be detected by a photomultiplier tube. - Thus the storage phosphor plate records a latent image that may be read out some time after x-ray exposure. -The readout may be accomplished with a well collimated intense light source, such as a helium- neon laser beam, so that the size of the stimulated region of the plate remains small to yield good resolution. - The electrical signals from the photomultiplier or other light – sensing 48  device are digitized with an analog to digital converter. - Once the image is stored in digital form, it may be image processed, viewed on a high- resolution monitor or printed out on film. Advantages of a stimulable phosphor plate system over conventional film: 1. Improvement in dynamic range: radiographic film operates over exposure differences of a factor of approximately 100. The dynamic range of a storage phosphor is on the order of 10.000. thus the storage phosphor has greater range of accepted exposures. 2. Reduction of patient exposure: Advantages of a stimulable phosphor plate system over other digital radiographics techniques: - The Storage phosphor plate simply replaces the screen/film cassette and does not represent a significant change in patient positioning or other imaging procedures. C) Large – area digital image receptors for radiography (flat panel detectors): - Advances in the technology of fabrication and manufacture allowed the production of large area flat panel detectors that equal, or even surpass the performance of film- screen and computed radiography systems. - They have (1) higher quantum efficiency for detection and conversion of x-rays into digital signals with (2) acceptably low noise over a (3) large exposure range. 49 Direct conversion flat panel x-ray detection systems: - The energy of the x-rays is converted to an electrical signal in a single layer of a material called a photoconductor (e.g. amorphous selenium, cadmium zinc telluride, and lead iodide). - When x- rays strike a photoconductor conversion plate, electron-hole pairs are created. - Electrical fields are applied between the front and back surfaces of the photoconductor to force separation of the electron- hole pairs and their transfer to one of the charged surfaces. - Each surface is a thin- film transistor (TFT) array that can be electronically read out to determine the amount of electrical charge present above each transistor. -The TFT array corresponds to an array of pixels in the final image, with the number of transistors equaling the number of pixels. 50 A direct conversion flat panel x-ray detection systems: - A scintillation material (e.g. amorphous silicon and magnesium iodide) converts the energy of the x-ray photons to visible light. - The visible light produced by the scintillation conversion plate is converted into electrical signal by a photoconductor. 51