The definition of blurring or un-sharpness on mammography images is: the amount of lateral border spread along the edges of our image. A distance of border spread that is visually acute is unacceptable.
What causes our images to look blurry? Why do the x-rays sometimes look indistinct? What can we do, where can we seek an answer?
There are three distinct causes of mammography blurring:
- Motion
- Geometric
- Receptor
Geometric:
Geometric blur is caused by the physical geometry or characteristics of the mammography unit. The indistinct quality of an image produced by the x-ray unit is due to the amount of penumbra or shadowing formed on either side of the image’s edge.
The units that cause the smallest penumbra have certain characteristics:
- They utilize the smallest focal spot allowable and still achieve adequate exposure (.3)
- They utilize the longest SID (source to image distance) possible and still maintain reasonable doses
- They allow the shortest OFD (object film distance) possible
All these characteristics can be easily illustrated by simple geometric diagrams. It is
as simple as high school geometry; it’s not rocket science.
Small Focal Spot Versus Large Focal Spot:
1. .3 focal spot
2. Object
3. Receptor
4. Tiny penumbra surrounding image
1. .6 focal spot
2. Object
3. Receptor
4. Large penumbra surrounding image
Long Source Image Distance Versus Short Source Image Distance:
1. 66cm SID
2. Object
3. Receptor
4. Tiny penumbra surrounding image
1. 50cm SID
2. Object
3. Receptor
4. Large penumbra surrounding image
Short Object Film Distance Versus Long Object Film Distance:
1. Short Object to Film Distance
2. Object
3. Receptor
4. Tiny penumbra surrounding image
1. Long Object film Distance
2. Object
3. Receptor
4. Large penumbra surrounding image
Summary
Blurring is a major cause of error in interpretation of mammography images. It is imperative we understand and address all the causes of this difficulty. Motion & Geometric are the first two in the series on blurred images. Stay tuned for Receptor problems.
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