Medical Biophysics 3503G Lecture Notes - Lecture 4: Medical Physics, Photon, Photodiode

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Medical biophysics
Contrast-detail anaylsis
object contrast = (physical property of feature) – (physical property of
backgound)/physical property of background
Contrast-detail procedure
build test objects (phantoms) containing spherical features of diff sizes and
differences in the measured object property
acquire test iamges of all phantoms as well as some phantoms without spherical
features
blinded observer determines whether a feature is detected in each image
(oblivious to what they are looking for)
each feature corresponds to a point in the contrast-detail plane
the dots on the graph represent a feature
flag each point ot indicate whether that feature was detected or missed by the
blinded observer
draw smooth curve that seperates detected from missed features
repeat steps 3-6 with many observers to obtain average curve
Detection limited by resolution
idealized scenario:
imaging systems A and B both have SNR = infinity
system b has finer spatial resolution than system A
finer spatial resolution higher resolution (better)
In graph, everything to the left (of B) is undetected
Everything to right of A is detected
High SNR is ideal (infinity)
Detection limited by local SNR
idealized scenario:
imaging systems A and B both have resolution  0
system B has higher local SNR than A
both lines in form logC = -logd + constant
an increase in SNR, results in a curve with the same slope but closer to the origin
(ie. Curve B)
Realistic contrast-detail curve
vertical line for SNR = infinity system and diagonal line for perfect resolution
system form asymptotes of realistic curve.
Diagonal line for ideal resolution
Vertical line for ideal SNR
Resolution vs. SNR trade-off in digital mammorgraphy
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