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

• 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