Radiography Flashcards
This is the production of a two-dimensional image from a three-dimensonal object, the patient.
Radiography
The procedure projects the x-ray shadows of the patient’s anatomy onto the detector and is often called what?
Projection radiograph
For most radiographic examinations, the source to imager distance (SID) is fixed at how many cm?
100 cm
And there are usually detents in the radiographic equipment that help the technologists to set this distance.
An exemption to the 100-cm (40 inches) SID is for upright chest radiography, where the SID is typically set to how many cm?
183 (72 inches)
To reduce the differential magnification in lung parenchyma
This occurs due to beam divergence
Magnification
The collimated xray beam becomes larger in area and less intense with increasing distance from the source
This is determined by the ratio of the length of the image to the length of the object.
Magnification
Largest when the object is close to the focal spot
Decreases with distance to the focal spot
Approaches a value of 1 as the object approaches image plane
How can focal spot blur can be minimized?
By keeping the object close to the image plane
In _______ radiography, a sheet of film with a light-sensitive emulsion on both sides is sandwiched between two intensifying screens.
Screen-film radiography
A light tight cassette encapsulates the screens and film
This is composed of scintillator (Gd 2 O2 S crystals) held together by a binder material.
Intensifying screen
What is the purpose of a scintillator?
It converts incident xray photons to visible light, which then exposes the silver halid emulsion on film
Why is close physical contact between screen and film important?
Gaps promote excessive lateral light propagation which blur the image
Why are intensifying screens composed of high atomic compounds? (Gd2 O2 S, LaOBr, YTaO4)
They have high absorption efficiency for x-rays
Thicker intensifying screens absorbs more x-ray photons than thinner screens, but the scintillation light that is emitted in a thicker screen diffuses greater distances to reach and expose film emulsion.
What is the result?
This results in more light spread, which reduces the spatial resolution of the screen.
This is composed of a thin plastic base coated on one or both sides with a layer of light sensitive emulsion consisting of silver halide crystals held in water soluble gelatin.
Film
Why are the silver halide crystal engineered in a tabular “T” grain shape?
To increase surface
For improving light capture efficiency and reducing silver content to save cost
Silver halide grains have small of amount of AgS, that does what?
That introduces defects in the ionic crystal structures, where negative charge builds up on the surface and a net positive charge is more central in the crystals
This is a protrusion of the positive charge that reaches the surface of the crystal.
A sensitivity speck
It is induced by the lattice defects caused by AgS
When silver halide grain is exposed to visible light, what will happen?
A small number of Ag ions are reduced and converted to metallic Ag
If there is greater than or equal to 5 Ag ions are reduced , what will be formed?
A stable latent image center
An exposed film not yet chemically develop contains what?
Contains a latent image consisting of a pattern of invisible silver halide grains with latent image centers
There is also a slight intensity rate effect, giving rise to slight differences in film density depending on the exposure rate, called what?
Reciprocity law failure
The cassette is usually made of what?
Usually made of carbon fiber or a material of low atomic number
The inside is light tight to keep ambient light from exposing the film
Designed to maximize transmission of xrays
Why is the gelatin layer permeable to aqueous solution?
It is necessary for the aqueous developing chemicals to come into contact with the silver crystals.
When the film is developed in an aqueous chemical bath containing a reducing agent, called the developer, the metallic Ag atoms at the latent image centers acts as a catalyst causing what?
Causing the remaining silver ions in the grain to be reduced.
A grain reduced metallic silver atoms appears as a black speck on the film.
After the film has passed through the developer, it passes through a bath of an aqueous oxidizing solution called the fixer, that does what?
That dissolves the remaining (inactivated) silver halide from the emulsion layer areas that were not exposed (or were underexposed) to light.
The film is then rinsed with water to remove residual developer and fixer and fixer, and is dried.
It is worth noting that the catalytic reaction that is promoted by the sensitivity speck in the presence of a strong reducing agent results in what?
In an approximately 10(9) gain in optical densityon the film with very little added noise - this is a chemical amplification process that few electronic detector systems can match.
This illustrates the optical density of film changes nonlinearly with the xray exposure level.
Hurter and Driffield Curve
Higher contrast in linear region
If unexposed film is developeed, its OD will be the base + fog level
Systems cans be more sensitive (and have hight speed), which means what?
Less x-ray radiation is required to produce a give OD
Slower screen-film systems required more radiation to produce a given level of film darkening, which results in what?
Results in more radiation dose, but because more x-ray photons are used to make the image, the statistical integrity of the image is better (lower quantum noise)
A change in screen-film speed is seen as what?
It is seen as a lateral shift of the H and D curve
How is contrast in screen film being adjusted?
By altering the grain size and other parameters in the film emulsion.
Is high contrast suitable for every application?
No.
For example, a large difference in x-ray fluence to the detector occurs in chest radiography, because of the highly attenuating mediastinum and the hightly transmissive lung fields - a low contrast, wide-latitude film is required for this setting, otherwise the heart silhoutte would be too white and the lung fields be to dark on the processed film.
In mammography, what is more applicable, a high or low contrast setting?
The compressed breast is almost uniformly thick and this the x-ray fluence striking the detector is relatively uniform , and thus a very high contrast (narrow latitude) film can be used.
This refers to a photostimulable phosphor detector (PSP) systems, which are historically housed in a cassette similar to a screen-film cassette.
Computed radiography
When x-rays are absorbed by photostimulable phosphors, what will happen?
Some light is promptly emitted, but a fraction of the absorbed x-ray energy is trapped in the PSP screen and can be red out later using laser light.
This are composed of mixture of BaFBr and other halide-based phosphors, often referred to as barium fluorohalid.
CR imaging plates
It is a flexible screen that is enclosed in a light-tight cassette.
The CR cassette is exposed to x-rays during the radiographic examination and is subsequently placed in a CR reader.
What are steps once the plate is placed on the CR reader?
- Cassette is moved into reader unit and the imaging plate is mechanically removed
- Imaging plate is translated vertically and scanned horizontally by a laser beam of approximately 700 nm wavelength.
- Red laser light stimulated emission of trapped energy. Blue-green visible light is emitted from storage phosphor
- Emitted light is collected by a fiber optic light guide and strikes a multiplier tube - produces an electronic signal
- Plate is exposed to bright white light to erase residual trapped energy
- Imaging plate is returned to cassette.