JPNM Physics
Convolution
A very narrow x-ray beam will broaden when it encounters an x-ray screen:
Now we know what the imaging system response is to a single, infinitely narrow input - a beam of x-rays centered on (x',y'). In real x-ray imaging, we have a continuous 2-dimensional distribution of x-rays emerging from the patient and impinging on the screen:
In summary, the response of the screen has been included as a result of SHIFTING it to all (x,y) locations, SCALING it according to the x-ray intensity, and ADDING it to the others. SHIFT, SCALE, and ADD is the order of the operations performed.
The result is an image which has had the response function applied to all incident photons - the film image has been BLURRED by the response function - it is not as sharp as the incident x-ray distribution.
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Douglas J. Wagenaar, Ph.D., wagenaar@nucmed.bih.harvard.edu