DSA /MASKED SUBTRACTION

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

·         Reduced diagnostic value of image –contrast resolution fluctuations ?

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