3 Projection Radiography I Flashcards
Analog radiography uses film to do what?
Capture, display, and store radiographic images.
Film consists of what?
- ∼10-μm-thick emulsion supported by a 150- to 200-μm-thick polyester (Mylar) base.
- Emulsion layer on both sides of the base.
- Protective coating, antistatic, or anti-crossover layer.
What are the machines uses single emulsion films?
Laser and mammography.
What is the contains of the emulsion?
Silver halide (iodobromide) grains, which can be sensitized by radiation or light to hold a latent image.
What is the contains of Silver halide?
1 μm in diameter and contain between 10^6 and
10^7 silver atoms.
How many grains?
10^9 grains per cubic centimetre.
How the grain will be sensitized?
- Several light photons must be absorbed to sensitize each grain.
- A grain may also be sensitized by absorbing a single x-ray photon.
- Can also be sensitized by thermal and chemical processes without photons (i.e., fogging).
Fading and fogging can be aggravated by what?
Environmental heat and humidity.
What are the steps of film processing?
Exposure, Developer, Fixer, Wash, and Dry.
What will happen after exposure?
After exposure, grains have a few neutral silver atoms in the speck along with millions of Ag+ ions.
What is the film development process?
Converts the invisible latent image to a permanent visible image.
How sensitized grains are reduced in the alkaline developer solution?
By the addition of electrons, which converts the positive silver ions to silver atoms.
What is the result of a developed step?
- A developed grain results in a speck of silver that appears black on the film.
- Unexposed grains with no latent image are developed at a much slower rate.
Film speed, contrast, and fog levels are all affected by what?
Developer chemistry and temperature.
What is the effect of increasing the developer temperature or developer time?
It can increase film contrast and density also increases the level of fogging on the processed film.
What is developer temperatures typically range?
From 31◦C to 35◦C.
What is (replenishment)?
The processor must supply fresh developer as more films are run.
What is the fixing solution contains? and what will do to the image?
- The fixing solution contains acetic acid to inhibit further development and remove unexposed silver halide grains.
- Fixing makes the image stable.
What is the result of poor or Inadequate fixation?
It can result in a milky appearance to the film.
Why washing the film again?
- After fixing, the film is washed again to eliminate all chemicals and is then dried by heaters or infrared lamps.
- Incomplete removal of the fixer causes the film to turn brown.
How long is the total processing time?
Typically 90 seconds (e.g., 25 seconds developer time, 21 seconds fixer time, 44 seconds washing and drying time).
What is the effect of dirty, uneven, or maladjusted rollers?
Can leave lines or other artifacts (e.g., π lines) on
the film.
Static electricity also causes severe film artifacts.
Processor QC involves what?
Measuring developer temperature and monitoring the density and contrast of film exposed to a light source in a sensitometer.
Film blackening is measured using what?
Optical density (OD) = log10(I_0/I_t), where I_0 is the light intensity incident on the film, and I_t is the light transmitted through the film. OD can be measured using a densitometer.
What is the transmittance of the film?
Transmittance is the fraction of incident light passing through the film, where the transmittance = I_t/I_0. As OD increases, transmittance decreases.
What is the useful range of film ODs?
Is from∼0.3 (50% transmittance)to∼2 (1% transmittance). Densities greater than about 2 require the use of a hot (bright) light.
What are characteristic curves are also known as H and D curves?
- Represents the relation between radiation intensity (air kerma) and resultant film optical density.
- The toe is the low-exposure region, and the shoulder is the high-exposure region of the curve.
- Fog is the level of blackening due to a few grains being developed in the absence of any radiation exposure.
- Base refers to the density of the film base alone, which will absorb a small fraction of any incident light.
- Base plus fog levels are ∼0.2 OD units.
- An unexposed film that is processed will thus have a film density of ∼0.2.
What is the relationship between film time and radiation?
Fast films require less radiation to achieve a given film density. Slow films require more radiation.
What are the Intensifying screens?
- Contain phosphor crystals that absorb about 50 times more of the incident x-rays than a radiographic film.
- For each x-ray absorbed in a screen, hundreds of visible light photons are produced that expose the film.
What is the job of the Intensifying screen?
The screen converts the x-ray pattern to a light pattern, which is subsequently recorded on radiographic film.
What are the advantages of the Intensifying screen?
The use of intensifying screens decreases the exposure time required for a given film density.
- Shorter exposures result in a lower patient dose.
- Shorter exposure times decrease x-ray tube loading.
- Shorter exposures also decrease blur caused by patient motion.
What are the contains of the Intensifying screen?
High atomic number materials to maximize the absorption of x-rays.
What are the disadvantages of using calcium tungsten in the Intensifying screen?
Tungsten has a high K-shell binding energy (70 keV), which is higher than the mean photon energy levels normally used in diagnostic radiology.
*The high K-edge energy of tungsten (W) means that x-ray absorption is less than optimal.
What is the common screen thickness?
A common screen thickness is ∼200 μm.
What are the emitted colors of Lanthanum oxybromide and calcium tungstate?
*Lanthanum oxybromide (LaOBr) and calcium tungstate (CaWO4) emit mainly blue light.
What are the emitted color of Gadolinium oxysulfide?
*Gadolinium oxysulfide (Gd2O2S) emits mainly green light.
Define spectral matching?
- The light color from a screen and the light sensitivity of the film must be matched.
- Matching the light emitted by the screen with film sensitivity is known as spectral matching.
Conventional film is sensitive to what?
Conventional film is sensitive to ultraviolet and blue light.
Orthochromatic film is also sensitive to what?
Orthochromatic film is also sensitive to green light.
What is absorption efficiency?
The percentage of x-ray photons absorbed in the screen.
What is the intensification factor?
The ratio of exposures, without and with intensifying screens, required to obtain a given film density.
The intensification factor depends on what?
The absorption and conversion efficiency of the screen.
What are the typical intensification factors?
Typical intensification factors are 30 to 50.
What is the name of the computer system?
Computers use the binary system (base two).
Defined the bit?
A bit (binary digit) is the fundamental information element used by computers and can be assigned one of two discrete values.
What are the types of detectors?
- Gas detectors.
- Solid state detectors (Semiconductor Radiation Detector).
- Photostimulable phosphors.
- Scintillators.
- Photoconductors.
Define Gas detectors?
*A high voltage across an air or gas chamber measures the electrons liberated by the
incident x-rays.
*Used to accurately measure
x-ray beam intensities and are calibrated in air kerma (Gy).
For imaging, what is the type of gas detector?
Gas detectors must have a high x-ray absorption efficiency and would not use air. Therefore, high atomic number gases and/or high pressures.
Example of imaging gas detector?
*Xenon is a high atomic number gas (Z = 54; K-edge energy 34.6 keV) and is an efficient x-ray detector at high pressure.
Define solid-state detectors?
In solid-state crystals (e.g., NaCl), atoms are arranged in a regular three-dimensional structure.
In solid-state crystals, electrons occupy energy bands.
Define Photostimulable phosphors detectors?
The release of stored energy within a phosphor by stimulation with visible light
Computed radiography (CR) uses photostimulable phosphor plates made of barium
fluorohalides.
X-ray photons interact with the electrons in the phosphor, creating a latent image.
Define Scintillators detectors?
Scintillators are materials that emit light when exposed to radiation. Scintillators are also known as phosphors.
Define photoconductor detectors?
A solid state device that detects x-rays directly.
Selenium (Z = 34; K edge energy 12.7 keV) is the most common photoconductor in use in digital radiography.
Define Image window width?
Refers to the range of gray scale values displayed.
Define Image contrast?
Within the window range is increased more as the window is narrowed.
Define Windows level?
The center value of the window width and therefore overall image brightness.
Define Histogram?
Equalization eliminates white and black pixels that contribute little diagnostic information, and expands the remaining data to the full display range.
Define Low-pass spatial filtering?
A method of noise reduction in which a portion of the
averaged value of the surrounding pixels is added to each pixel.
Define Unsharp masking?
Involves subtraction of a smoothed version from the original, which is then added to a replicate original.
Visibility of tubes, lines, and catheters is improved, but noise increases and artifacts may be introduced.
Define Background subtraction?
It can digitally reduce the effect of x-ray scatter to increase image contrast.
Define Energy subtraction?
Techniques are based on subtracting projection radiographs obtained at two x-ray generator settings (e.g., 60 and 110 kV).
AEC is also known as phototiming
–Measures the actual amount of radiation incident on the screen–film and terminates the exposure when the correct amount has been received.
–To ensure the correct film density,
Screen film speed
–As the speed increases, the air kerma required decreases.
Screen speed increases with
–Increasing screen thickness, absorption efficiency, as well as conversion efficiency.
