Screen Film Radiography
1-Screen Film Radiography
The imaging principle:
· Patient’s body is composed of different materials with different attenuation properties.
· If an X-ray incident on the patient, the high-density material (Bone) will absorb a high amount of radiation, while the low-density material (Air) will absorb less amount of radiation.The transmitted X-ray photons will be received by imaging receptors to represent the attenuation properties of different materials.
The role of screen in the imaging process:
· The imaging receptor differs from the past up to today, starting from screen film combination up to digital receptor.
· If The transmitted X-Ray photons interact with the film directly, you have to expose the patient too much to receive the required number of photons for optimum image quality.
· The role of the screen is to receive the X-Ray photon and convert it to hundreds of light photons
· The screen is in front of the film to absorb the incident x-ray photon, however, there is another screen on the back side of the film to detect any X-ray photons passing from the front screen.
· The light photons are reflected back towards the film from the back screen.
· The screen is composed of phosphor material (fluorescent material) which has a high Z number, High density, and favorable K-edge.
· The fluorescent material converts the X-ray photon to visible light photons immediately (used in screen film radiography), while phosphorescent material requires energy to release the light photons (used in Computed radiography).
· The conversion of X-Ray photons to light photons is called intensification, so the screen is called intensifying screen.
· There are different materials that can be used as an intensifying screen. although tungsten has a higher Z = 74, rare earth materials (gadolinium Gd Z=64 and lanthanum La Z=56) are commonly used. The reason is that Gd and La have K-absorption edges at 50 Kev and 39 Kev respectively, while Tungsten at 70 Kev. The mean energy of the X-ray beam used in radiography is close to the K-absorption edge of Gd and La which increases the absorption efficiency of X-ray photons.
· The conversion efficacy of rare earth material is 20 %, and 5% for tungstate.
· The higher sensitivity of rare earth material causes lower usage of KV and mAs, Lower patient exposure, and lower tube loading.
· The intensifying screen is maximum when the X-Ray energy is just above the K-Edge energy.
· If the Kv increase, the mean energy increase which causes less attenuation and less screen sensitivity.
· Calcium tungstate screen releases blue light after x-ray exposure, while rare earth material emits green light.
Screen thickness affecting image quality and patient dose.
· If the screen thickness increase, the higher absorption of X-ray photons, as the probability of passing X-Ray photons without interaction will be less.
· Not many X-ray photons are required to get the optimum image quality in the case of thicker screens, so the patient radiation exposure will be less.
· In the thicker screen, the light photons diffuse over a long distance before reaching the film
· The film will receive the light photons from a location different from the site of light photon production inside the screen.
· This mechanism happens with a thicker screen causing a loss in resolution of the X-ray image.
· The thinner screen enables the light photons to diffuse over a short distance which has less effect on image resolution, however patient radiation exposure increases to compensate for the loss of X-ray photon absorption.
· The selection of screen size must be compromised between image resolution and patient dose.
· Thin screen is preferable in high-detail imaging procedures (breast imaging, wrist, and ankle), while a thicker screen can be used for abdomen, chest, and pelvis imaging.
The film and light photons interaction mechanism:
· The film is in contact with the screen to receive the light photons by the emulsion.
· The emulsion is the sensitive part composed of grains that contain silver bromide crystals.
· The bromide is excited by light photons to release an electron.
· As silver bromide is in a crystal structure, so sensitive spot is present which traps the released electron.
· The sensitive spot will attract the silver positive ions to form an exposed sensitive spot.
· Many sensitive spots are formed in the emulsion to form a latent image which has to be converted to the real image.
· More silver ions will be attracted to the sensitive spots when higher radiation exposure happens.
· The more radiation incident on the screen, the more the light photons are received by the film emulsion, the more bromide ions excitation, the more electrons are released, and the more silver ions are attracted.
· The latent image is formed by silver ions accumulation in the sensitive centers.
· To convert the latent image to the real image, we have to blacken the silver ions and remove the unexposed crystals (grains).
· The degree of blackening depends on the number of silver ions accumulated in the sensitive spots.
Film Developing
· Developing process occurs by adding a source of electrons (phenidone solution) that converts the silver ion into a black metallic silver.
· These electrons invade all exposed and exposed grains.
· The electrons invade the unexposed grains to form the background fog of the film.
Fixing
· The film is immersed in fixer solution so that the unexposed and unreduced silver halide is removed to render the image stable in white light.
· The fixer solution contains:
Neutralizer: acetic acid to stop ongoing development after removal from developer solution
Clearer: ammonium or sodium thiosulphate is used to clear the undeveloped silver halide grains from the film
Preserver: sodium sulphite
Hardener: aluminum chloride shrinks and hardens the emulsion to produce the final film.
Automatic processors
· An automatic processor:
1. Unloads the film from the cassette
2. Transports the film through the processing chemicals in the right order
3. Produces a developed, hardened, and dried film
The role of Film screen contact
· Film should be in direct contact with the screen
· Inadequate film-screen contact usually produces blurring in only a portion of the image area.
The Film composition:
· Supercoat protection layer
· Base polyester layer that gives the film the physical stability
· Adhesive adhere layers together
· Emulsion radiation and light sensitive layer
Screen Film combination:
· No screen films
· Only film is used to detect the X ray photons directly.
· High radiation exposure (air KERMA) is required to get a proper image quality
· High resolution is produced as no light diffusion
· Single Screen Film
· The film is attached to on screen in the front side
· Each Xray photon is absorbed and converted to hundreds of light photons
Double-screen film
· the film is sandwiched between a double screen.
· Double emulsion layer is used
· the back screen is thicker than the front one to absorb the X-ray photons that passed without absorption.
· One-third of X-ray photons are absorbed by the front screen.
· Half of the light photons produced, are absorbed by front screen
· The back screen is used to absorb the X ray passed and reflect the produced light towards the emulsion
· Double screen film is a highly sensitive combination but affects image sharpness.
· Reduced the property of motion un-sharpness, as less exposure time is required.
· Reduce the tube loading and less power and acquisition parameters can be used for imaging.
Film Sensitivity
· Film sensitivity (Speed) express how much radiation exposure is required to get an image.
· High Sensitivity (speed) require less radiation exposure than low speed.
· High sensitivity (speed) films are chosen when the reduction of patient exposure and heat loading of the x-ray equipment are important considerations.
· Low sensitivity (speed) films are used to reduce image noise. The relationship of film sensitivity to image noise is considered in the section titled, “Image Noise.”