DISCUSS THE RADIOLOGICAL
REQUIREMENTS FOR DSA /MASKED SUBTRACTION?
Require:
·
stable/repeatable x-ray generator and x-ray tube
·
Excellent image intensifier
·
CCD camera and read out electronics
X-ray generator
·
A small difference in the tube current or kV
will result in improper mask subtraction
Xray tube
·
X-ray tube loading to handle rapid exposures
·
Small FS size for magnified images – reduce
geometric blurring
·
Small focal spot limitation – increase in heat
loading required by rapid exposures – may have to INCREASE FS size to increase
target heat capacity
Image intensifier
·
A high quality image intensifier is needed for
digital application
o
Very high contrast ratio is needed; but high
spacial resolution is not needed. So thick input phosphor can be used for high
x-ray detection efficiency
o
Power
supply to II must be stable – small changes in accelerating potential
for the electrons can produce changes in brightness gain of II
o
Fluctuating power - incorrect DSA image
CCD
·
Quality of light proportional to x-ray photons
that exit patient
·
No lag, low electronic noise, flicker free
DEFINE MASK SUBTRACTION?
·
Pre contrast image is taken
·
Contrast is injected IV or intra-arterial with
high Z – iodine
·
Contrast image is taken and post contrast image
is taken
·
Subtracted image is displayed on pixel matrix
·
Series of images
·
Angiographic film pair – precontrast image and
post contrast image are subtracted from one another
