Screen Film Radiology

Optical Density
v OD = -log 10 (T) = log 10 (1/T) = log 10 (I 0 /I), inverse relationship is T
= 10 -OD
v As OD increases, transmittance decreases
v The OD of superimposed films is additive
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 158.
The Hurter and Driffield (H&D) Curve
v H&D (characteristic) curve
describes how film responds to
x-ray exposure
v Non-linear, sigmoidal shape
v log 10 -log 10 plot (OD vs. log
relative exposure)
v Film base → OD = 0.11 – 0.15
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2 nd ed., p. 159.
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