RAD 254 Chapter 27 Digital Fluoroscopy Terms to remember DSA digital subtraction angiography Registration Interrogation time Hybrid subtraction CCD ROI PACS

Advantages of DF Speed of image acquisition Post processing “tweaking” Spatial resolution is determined by the matrix size (usually 1024 X 1024) and the size of the image intensifier DF operates at “conventional mAs” (hundreds of mA rather than less than 5 mA as conventional fluoro) BUT DF operates in pulsed progressive fluoro

Pulsing terms Interrogation time = time to switched on and reach the mA and kVp level Extinction time = time for the tube to be switched off (usually times less than 1 ms)

Receptor The receptor is usually a “charge coupled device” CCD’s are very sensitive to light and have a much lower level of noise than TV camera This results in much higher SNR than conventional TV cameras/systems Thy also have NO lag time or “blooming” and require NO maintenance CCD’s can be “docked” directly to the II’s output phosphor

Advantages of CCD’s High spatial resolution High SNR High Detective Quantum Efficiency (DQE) No warm up required No lag/blooming No spatial distortion/maintenance Unlimited life Unaffected by magnetic fields Lower dose

DR Subtraction Two primary types Temporal subtraction Single kVp setting Normal filtration Good contrast resolution (1% @ 1mm) Simple arithmetic image subtraction used Motion artifacts are a problem (misrepresentation) Total subtraction is able to be achieved Subtraction limited by number of images

IF BOTH ARE COMBINED = HYBRID Energy Subtraction Rapid voltage switching is used Filter switching is preferred Higher x-ray energy used for contrast res. Complex image subtraction required Motion artifacts (misrepresentation) are reduced Some residual bone is survived (shows) More types of subtraction are possible IF BOTH ARE COMBINED = HYBRID

DF/D subtraction dose DF & D Subtraction usually result in much higher patient dose and PULSED imaging is required to lower it! Storage and image distribution as already discussed are utilized

Chapt 28 The Digital Image Spatial resolution Contrast resolution Contrast-detail curve Pt. Dose considerations

Spatial resolution -the ability of an imaging system to RESOLVE and render on the image a HIGH CONTRAST object (shades of gray differences in close objects to one another)

Spatial FREQUENCY Refers to LINE PAIR! Line pair per millimeter )lp/mm) The higher the spatial frequency, the HIGHER the spatial resolution! In digital imaging, spatial resolution is limited by PIXEL size.

Modulation Transfer Function (MTF) The ability of an imaging system to render objects of DIFFERENT sizes onto an image (ratio of image to object due to spatial resolution)

Contrast Resolution The HIGHEST possible contrast is TWO shades of gray – Black and White!

Dynamic Range -is really the OD shown on the H & D curve = ranges from base + fog (.2 or lower) to the solarization point The number of shades of GRAY that an imaging system can produce (remember the human eye can only see about 30 different shades of gray!)

NOISE Image noise limits contrast resolution (SNR) Post processing of the image allows for manipulation and visualization of shades of gray (window and leveling)

Patient dose Digital imaging should reduce pt. dose (in reality, it had done just the reverse!) – the “in in doubt, burn it out” syndrome and post image manipulation – “dose creep!”

Misc. information Focal spot size determines spatial resolution in film/screen systems Post image manipulation = contrast resolution in digital regardless of pt. dose Pt. dose in digital should be LOW because of DQE (probability of a photon interacting with an image receptor due to atomic number, etc) DQE = measure of x-ray absorption efficiency