Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
47 Cards in this Set
- Front
- Back
|
What is sensitometry |
Measurement of the responses of a film to exposure and processing. The film is exposed and then processed and the resulting densities are measured and evaluated. NB: This procedure is the responsibility of the radiographer |
|
|
What are the 3 pieces of equipment used in sensitometry |
1. Penetrometer 2. Sensitometer 3. Densitometer |
|
|
What is a Penetrometer |
A solid step wedge that is used, in conjunction with the x-ray unit, to produce a uniform range of densities on the test film. |
|
|
What is the step wedge itself made from |
Aluminium or tissue-equivalent plastic |
|
|
What is a Penetrometer good for? |
Monitoring x-ray equipment and the response of different film/screen combinations. |
|
|
What is the purpose of a Sensitometer |
Another method of producing a uniform range of densities on the test film though this time the exposure to the film is from a light-box, not the x-ray unit. |
|
|
How does a sensitometer work? |
The unit uses a controlled intensity light source, and a standardised optical step wedge image. The optical step wedge is a calibrated strip that has been exposed and developed to produce different patches of grey. These patches control how much light will get through to the test film during exposure. |
|
|
How are voltage fluctuations and other factors that could change the intensity controlled in the sensitometer? |
By circuits (so that the same power is used each time the unit is triggered) |
|
|
What can happen if you touch the optical step wedge on the sensitometer? |
Oil from your fingers can interfere with the light intensity |
|
|
What are the 2 types of step wedge? |
11 step: increase in density by 100% (factor of 2) per step 21 step: increase in density by 41% (factor of 1.41) per step |
|
|
What is a densitometer |
Measures how dark the test film is after it has been processed. It compares the amount of light emitted by the light source with the amount of light transmitted through the film. |
|
|
What is important to do each time the densitometer is used? |
Calibrate it |
|
|
How is the densitometer calibrated each time it is used? |
Acheived by measuring the intensity of the light from the unit without the film. |
|
|
What does the readout display show on a densitometer? |
Numbers known as 'optical density numbers' and is written as 'OD number'. |
|
|
On a densitometer what do radiographic film densities range from? |
OD 0.0 to OD 4.0 |
|
|
What is a D log E curve? |
The curve shows the results of sensitometry and are plots of film density against log10 of the exposure to the film. |
|
|
What are D Log E Curves also referred to as? |
Sensitometric curves, characteristic curves or Hurter and Driffield (H&D) curves. |
|
|
What are the areas of the D Log E curve? |
A: Base plus fog B: Toe C: Gamma portion D: Shoulder E: Dmax |
|
|
What is Base plus Fog? |
The inherent density of the film (i.e density of the film without any exposure to light) This darkening of the film is due to any tints or dyes in the film base, and any fogging that has occured before exposure. |
|
|
What are Base plus Fog levels usually between? |
OD 0.05 and OD 0.10, and processing can add an extra OD 0.05 to OD 0.10. So, the total base plus fog level of a film is seldom below OD 0.10, but should never be above OD 0.22 |
|
|
What is the gamma portion |
The straight-line region after the toe. It is the region of primary sensitivity and the range of diagnostic densities. |
|
|
Why should our image density be within this region of the film response? |
It is here we obtain the best contrast from the film. |
|
|
What do diagnostic densities range from (Gamma Portion) |
Start between OD 0.25 and OD 0.50 and end between OD 2.0 and OD 3.0 |
|
|
What is the shoulder? |
Where the film contrast starts to tail off as the film reaches its maximum optical density. |
|
|
What is the shoulder part controlled by? |
The developing agent in the automatic processor. |
|
|
What is the Dmax? |
The maximum density that the film is capable of recording. Exposure beyond this point results in "reversal" or "solarisation" |
|
|
What is solarisation? |
Exposure beyond that which causes the maximum density to be recorded in the film will re-ionsise some of the silver atoms, reverse their charge, and cause them to be repelled. The net effect would be to reduce the optical density produced in the film (known as "reversal" or "solarisation". |
|
|
What are the 4 primary characteristics of the film? |
1. Resolution 2. Speed 3. Contrast 4. Latitude |
|
|
What is resolution? |
Ability of a system to image an object accurately. |
|
|
What are the 4 things that resolution is also referred to as |
Detail Sharpness Definition Resolving power |
|
|
What is resolution measured by? |
The ability to see an object consisting of "line pairs", where a line pair is an opaque line of a certain width followed by a transparent line of the same width. |
|
|
What is resolution of the film determined by |
The size of the silver halide crystals within the emulsion layer of the film: the bigger the crystals the poorer the film resolution. |
|
|
What are the silver halide crystals sometimes referred to as? |
Grains (hence "graininess" means poor resolution) |
|
|
What is resolution of a film-screen combination usually determined by (not the silver halide crystals) |
Size of the crystals in the intensifying screen. |
|
|
What is 'speed' a meaure of |
The exposure needed to produce the required film density.
It is determined by the sensitivity of the film to the exposure and is due primarily to the size of the to the size of the silver halide crystals although it does also depend on the thickness of the emulsion. |
|
|
Are large or small silver halide crystals in the film capable of receiving more light photons and therefore darken larger areas than the opposite using the same exposure? |
Large |
|
|
What do larger crystals mean in terms of film speed |
Faster film speed |
|
|
The thicker the emulsion the ____ crystals there are in a given area, so more photons are likely to interact with the emulsion. Therefore, the _________ the emulsion the faster the film. |
more , thicker |
|
|
What is the speed of the film determined by? |
Measuring the exposure required to reach the "speed point" of the film. |
|
|
What is the speed point? |
The point where a film density of OD 1.0 above the "base plus fog level" is acheived i.e OD 1.0 + (b+f) |
|
|
With knowing the speed points, what can we calculate? |
Difference in exposure that will produce a quality image on a new film when the proper fators are known for a previous one. |
|
|
What is contrast? |
The difference in the optical densities of neighbouring areas on the film. |
|
|
What is contrast represented by? |
The slope (or gradient) of the straight-line portion of the D log E curve i.e. where film contrast is at its maximum. |
|
|
If we remain within the "straight-line portion" will changing the exposure have an affect on image contrast? |
No. (hence this is where we want to operate when taking an image) |
|
|
A change in exposure (mAs) will only affect the contrast of an image if we are exposing in the ______ or ______ regions of the curve. |
Shoulder or toe |
|
|
What is a measure of the contrast of a film and is the gradient of the slope at the speed point. Typical values range from about 2.5 to 3.5. So the steeper the graph, the more the film "amplifies" the exposure by producing a greater amount of density per exposure. Toe and shoulder gradients are much smaller and actually decrease the contrast. |
Gamma value |
|
|
What is latitude |
The range of exposures that will produce densities within the diagnostic range (i.e. the straight-line portion of the D log E curve) Hence a wide latitude film allows variation in exposure while still showing densities in the diagnostic range, in other words it is a "forgiving" film. |
