of Physics Radiography
Part 2: image characteristics Laura Ziegler, DVM
What makes a good radiograph?
Should be able to see everything you are supposed to see given the study Intent of the study may change what constitutes a good radiograph
(e.g. a good cervical spinal radiograph is generally poor for assessing the pharynx)
We will leave off talking about positioning, centering, etc.
Excellent Reference
Radiograph Quality Evaluation for Exposure VariablesA Review
Robert M. Kirberger, Vet Radiol Ultrasound Vol 40, NO. 3, 1999, pp 220-226
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What makes a good radiograph?
Appropriate density (referring to optical density) of the ENTIRE film
High density film=relatively BLACK Low density film=relatively WHITE
Density is related to the number of photons interacting with the silver in the film
Film Density
Directly related to mAs Directly, but not linearly, related to kVp Affected by film/screen type (remember characteristic curves?) Also affected by processing Affected by grid (if not compensated) MUST have a good technique chart (see lab!)
Film Densitytoo white
Periphery of film (where the patient isnt) should be BLACK
Quick test-you should not be able to see a finger through it
If not, either have:
Inadequate mAs Inadequate developing
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Film Densitytoo white
Film Densitytoo white
Inadequate mAs
If pure technique issue, either double mAs or change to a more efficient (faster) film/screen Make sure you have compensated for
Using a grid Using a slower film/screen
Make sure no obstruction to beam path (two films in cassette, paper in new cassette)
Film Densitytoo white
Inadequate mAs
Make sure the incoming voltage is constant (can be a problem with portables) May need to call someone to service the tube
Inadequate developing
Developer exhausted, diluted, incorrectly mixed Developing time too short Developer too cold
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Film Densitytoo white
If the periphery is black, but the overall density is too white, check gross image detail
If you cant see anything, kVp is way too low, and no amount of mAs will make up for it If you can see gross detail, can increase kVp *or* mAs
Film Densitytoo white
Corrected image
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Film Densitytoo white
Roughly speaking, halving or doubling mAs is equivalent to increasing or decreasing kVp by 10-15% Must change kVp by at least 5% to detect a visible difference kVp change of 15% will approximately double the density of the film
Film Densitytoo black
Evaluate under a hot light for small, thin bones If they are visible, the kVp is ok If they are not visible, the kVp is too high
Film Densitytoo black
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Film Densitytoo black
Film Densitytoo black
If kVp is ok, there is either
Too many photons hitting the film Over development in processing
Film Densitytoo black
Too many photons
mAs set too high
Make sure to compensate for grids (or absence of), film/screen, SID etc.
Line surge Double exposure Fog (light leak, radiation scatter, etc.)
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Film Densitytoo black
Over development
Developer incorrectly mixed Developer time too long Developer temperature too high
Film Densitytoo black
If kVp is too high (small detail not visible), must decrease kVp However, the factors mentioned above (usually too high mAs) are also in play If kVp is too high but mAs is fine, overall film density is ok, but IMAGE CONTRAST suffers
Film Densitycorrected image
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Film Density
When in doubt, err on (slightly) the side of OVEREXPOSURE (higher kVp or mAs), as information can frequently be retrieved with a hot light
Contrast
Contrast is defined as the the difference between an object and surrounding background Degree of contrast in an image depends on:
the composition of the object and background (calcified nodule in lung vs. soft tissue nodule in fat) Film characteristics kVp
Contrast
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Contrast
As kVp (remember p is for Peak) increases, a few photons can squeak through the higher density materials
Gas
Fat
Soft tissue/ Fluid
High Z mineral
Metal
Contrast
As kVp (remember p is for Peak) increases, a few photons can squeak through the higher density materials
Gas
Fat
Soft tissue/ Fluid
High Z mineral
Metal
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Contrast
Contrast
When radiographing the thorax, we like a long scale of contrast (many shades of gray)
The subject contrast is quite high (soft tissue vs. air for the most part) so it doesnt need a lot of help Many of the things that we are looking for can be quite subtle shadings of gray that may get burned out with a high contrast technique
Contrast-thorax
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Contrast
Relatively HIGH kVp for thorax radiographs
Contrast
When radiographing the abdomen, we like higher contrast (shorter scale)
This is mostly because the subject contrast is quite low (soft tissue vs. fat) In order to tell the difference between the two, we need to compensate with our technique
Contrast-abdomen
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Contrast
Relatively LOW kVp for abdomen radiographs
Contrast
For orthopedic studies, we generally like quite high contrast techniques (very short scale)
Subject contrast often even lower than abdomen (more dense bone vs. less dense bone) Dont care too much about peripheral soft tissues (although soft tissues should NOT be ignored or burned out completely)
Contrastorthopedic
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Contrast
Relatively LOW kVp for orthopedic studies
Detail
Detail (resolution) is the ability to discern two discrete objects as being separate from one another Objectively measured in line pairs per millimeter
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