OID - Chapter 23 Flashcards
What is OID?
The distance between the object being radiographed and the image receptor
What is SID?
The distance between the xray sources and the image receptor
What is SOD?
The distance between the xray source and the object being radiographed
Why is OID unique?
Because it impacts every aspect of the image (Exposure, Subject Contrast, Noise, Sharpness & Magnification) except shape distortion
What impact does increased OID have on subject contrast?
As OID increases subject contrast increases (improves)
What is subject contrast?
The difference in radiation intensity between different portions of the remnant beam
Why does increased OID improve subject contrast?
When OID is increased the intensity of scattered photons decreases, which reduces the scatter to primary ratio to improve contrast
What is an increased OID technique called?
Air gap technique
Where does scatter in an image come from?
Within the patient as a result of compton interaction
What is a compton interaction?
When xray photons undergo interactions with matter and change directions, which contribute to image noise
Which method is better for reducing scatter grids or OID?
Grids because it prevents magnification
What effect does OID have on the primary beam?
No effect, since OID starts after the patient and the primary beam starts before it even hits the patient
What effect does increased OID have on exposure?
Similar to SID, increasing OID reduces exposure following the same rules as the inverse square law
What effect does increased OID have on sharpness?
As OID increases, the image sharpness (spatial resolution) decreases
Why does sharpness decrease with increased OID?
The penumbral shadow spreads out which reduces the amount of recorded detail/spatial resolution
What is penumbra?
The blurred, unsharp margin of a projected imaged caused by the finite size of the xray source
What is sharpness?
The clarity and distinctness of the edges and fine structures in the image
What effect does increased OID have on magnification?
Increasing the OID also increases the magnification of an image (exponentially proportional)
Why does magnification occur with increased OID?
Because both the umbra and penumbra increase, which alter the image size but not shape
What are some reasons OID increases are useful in imaging?
To visualize vessels such as small arteries or veins in the head
What is the increase OID technique used in angiographic procedures?
Magnification radiography or macroradiograph
What is a fractional focal spot?
A smaller focal spot that is used in the tube to improve sharpness, when magnification is used
What is the controlling factor for magnification in an image?
The SID/SOD ratio
What is the formula for finding SOD if not provided?
SOD=SID-OID
What is the controlling factor for sharpness in an image?
The SOD/OID ratio
If both SID and OID are doubled what effect will if have on sharpness/magnification?
There will be no change in sharpness since the ratio is increasing by the same amount
If both SID and OID are increased what is the effect?
There is an exaggerated loss of exposure to the image
