Chapter 28 - Density shiz
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Whats the difference between Film and Digital Radiography?
Film is defined by the degree of overall blackening from black metallic silver deposited in the film emulsion.
Digital Radiography has a stimulible phosphor receptor that absorbs x-ray photons and then produces light.
How does DR work? ( What is the first step? )
Take an x-ray and the xray photons are abosrbed by the phosphor screens and hold on to the light producing a latent image.
What happens in Step 2 for DR?
IR is put through a processing unit that shoots laser beams on the screen which causes light to be produced resulting in an image on the surface of the phosphor screen in the form of light.
What happens in Step 3 of DR?
A light detector in the processor measures the light and sends the data in the computer to produce a digitized image.
What happens in 4th step of DR?
When the IR is scanned, the data that is being read is converted into digital values for pixel and stored in the computer memory as a digital image.
What is the difference between brightness and density?
Brightness is a monitor control function that cna change the lighteness and darkness of an image on a display monitor. This is not related to Image Receptor exposure at all unlike density.
Brightness is for digital imaging basically.
What is window level?
Window level describes post-processing that produces changes in brightness.
Does density affect recorded detail? What does it mostly affect?
No it doesn't affect recorded detail at all. But it affects visibility of the image .
If ever the time comes when the radiograph is too light, what would be the wiser decision to produce a darker image?
Double the mAs
If the image is too dark, what would be the sensible thing to do?
Half the mAs.
How much do you increase density in order to see a visible change?
25-35% change both ways ( higher or lower )
What happens to patient exposure as mAs increases?
When mAs goes up, patient exposure increases.
What is the MAIN controlling factor of radiographic density?
What is the recioprocity law?
Any combiniation of mA and time will equal the same mAs.
Regardless of exposure, what does the digital system do?
Digital systems will always attempt to rescale the historgam to provide a display that appears acceptable to the eye.
What is the main INFLUENCING factor in radiographic density?
What does higher kvp do to the amount of photons going through the tube?
Increases the amount of photons going through the tube.
What does kVP do to the penetration ability of photons?
It increases the penetration ability because higher kvp = stronger photons.
How does kVP affect film density?
kVp alters the amount of scatter radiation created which affects film density
How much change in kvp do you need to see a visible change?
5% increase or decrease will show a change.
To double density, how much do you increase kvp by?
You increase kvp by 15% if you need to double the density.
Does focal spot size affect density?
No it does not. Focal spot size affects radiographic detail but has nothing to do with density.
When is the anode heel affect most noticable?
In large casette sizes and short SID's.
How much more photons does the cathode side have?
20% more than the central ray.
How much less photons does the anode side of the tube have in comparsion the the central ray?
25% less photons
How does SID affect density?
- Larger SID = lighter, less dense image.
- Closer SID = Darker, more dense image.
What is the rule of thumb for increasing distance? And why?
40 SID, 56 SID and 72 SID is best used because youll know that when going from 40 -> 56 SID, you just need to double the mAs, and from 56 to 72 SID you also just need to double mAs.
What does OFD ( OID ) really do? How does it affect radiographic density?
It produces less scatter ( kinda works like a grid ) because it angles the scatter away. An increase in OFD will decrease radiographic density though wont be much of a visible change.
How does filtration affect density?
The more filtration, the less dense the image will be
How does collimation affect density?
- The more you collimate, the less dense the image will be.
- The less you collimate, the more dense the image will be.
How does the patient thickness affect density?
As tissue thickens, averaege atomic number of the tissue/tissue density increases, hence radiographic density decreases.
What happens to exposure when tube is angled?
You will need more penentration because you are going through a thicker part of the body. Because of this, you may have to increase exposure by 30%.
What do grids do? How does it affect density?
Grids help abosrb scatter which adds desnity to the film. Hence the higher grid ratio, the less dense the image will be. ( Decreases radiographic density )
How does Film/Screen speed affect how you maintain density?
As speed increases, the amount of exposure required to maintain the same density decreases. So more speed, less exposure.
What do you need to do to kVp when body part is thicker?
You need to increase kVp to increase penetrability.
When people have destructive conditions ( pathology ) what should you do to exposure?
You should decrease overall exposure, hence decrease mAs.
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