Fluoroscopy - Image Intensifiers (11/26/2024 & 12/3/2024)) (copy)

Fluoroscopy is also referred to as ___

dynamic imaging

Early fluoro screens were composed of ___

zinc cadmium sulfide

(emits a yellow-green light)

What is “dark adapt”?

radiologist had to wear dark goggles for 20-30 minutes prior to start of exam (to adjust their eyes)

What are the parts of the eye and what purpose do they serve for vision?

iris: diaphragm

cones: daylight and central vision

rods: night and peripheral vision

What were some early fluoro modifications after biological hazards were recognized?

• primary beam enclosed in lead

• image transmitted to an optical viewer

• no more “dark adapting”

When was the image intensifier introduced?

1948

What did the image intensifier do for early fluoroscopy?

• converts x-rays to visible light with higher energy

• brightened image

• eliminated need for dark adaptation

• use cone vision

• improved visualization of detail and contrast

What is the most common tube/ii configuration?

under-table x-ray tube

• tube and collimator are below tabletop

• ii mounted above table

• protective curtain and bucky slot (lead)

What is the typical mA range for fluoro?

0.5-5 mA (low mA, longer time)

What is the minimum SOD for stationary and mobile fluoro units?

stationary: 15”

mobile: 12”

What are the 5 basic parts of the vacuum tube?

• input phosphor/screen

• photocathode

• electrostatic lenses

• accelerating anode

• output phosphor/screen

Explain the input screen (as one of the 5 basic parts of the ii tube)

• converts x-ray photons to visible light (yellow-green spectrum)

• 6-23” diameter convex screen

  • typically 6”, 9”, 12”, or 16”

  • glass, titanium, steel, or Al

• screen coated with cesium iodide (CsI) crystals

  • 0.1-0.2 mm layer

  • needle shaped (columnar crystals)

  • tightly packed

Explain the photocathode (as one of the 5 basic parts of the ii tube)

• absorbs light photons and emits electrons (process of photoemission)

• photoemissive materials - cesium antimony compounds

• thin protective coating between input screen and photocathode (prevents chemical reaction between layers)

Explain the electrostatic focusing lens (as one of the 5 basic parts of the ii tube)

• series of metal bands or rings with increasing positive charge

• focuses electrons toward anode ring

  • become narrower to focus electrons

• electrons concentrated onto output screen (smaller)

Explain the accelerating anode (as one of the 5 basic parts of the ii tube)

• located in front of output screen

• positive charge attracts electrons and accelerates them toward output screen

  • potential difference = at least 25 kV

  • increased energy from anode to output screen = flux gain

• electrons pass through hole in center

  • strike output screen

Explain the output screen (as one of the 5 basic parts of the ii tube)

• converts electrons to light photons (visible light)

• made of zinc cadmium sulfide or cesium iodide

• electrons strike screen and emit green light

  • diameter = 1”

  • light then passes to CCD or camera tube

• thin aluminum coating prevents light backflow

• newer units have fiber-optic disc in place of screen

What are multi-field ii?

tubes with more than one size input screen

• dual field: 9/6 inch

• tri field: 10/7/5 inch

• have mag mode

• number refers to diameter of input screen of ii

What is the focal point?

• the point where electrons cross

• (moves based on voltage applied to focusing lens)

• image is magnified at output screen

The ___ the voltage applied, the closer the focal point moves to input screen

higher

Explain the effect of magnification mode

• optical system can only see the central part of image

• central, straighter rays strike output (reduces FOV, improves resolution)

• image is less minified

• brightness is decreased

• mA is automatically increased to compensate for decreased brightness (due to minification screen)

mag mode:

___ FOV

___ resolution

___ brightness

___ mA

reduces FOV

improves resolution

decreases brightness

increases mA

Magnification intensifiers can mag ___

between 1.5-4x

Magnified image ___ patient dose

increases

(more x-rays required to form image; ABC boosts technical factors to create more signal)

smaller field = ___ resolution = ___ dose

smaller field = improved resolution = higher dose

magnification equation

magnification = (input screen diameter)/(input screen diameter in mag mode)

What are some common brightness control systems?

• ABC

• ADC

• ABS

What does ABC stand for?

Automatic Brightness Control

What does ADC stand for?

Automatic Dose Control

What does ABS stand for?

Automatic Brightness Stabilization

What does AGC stand for?

Automatic Gain Control

What does AGC do?

• adjusts current flowing to display monitor

• DOES NOT adjust kVp and mA

What does ABC do?

• automatically maintains fluoro image density and contrast (mA/kV)

  • rad selects brightness level desired

  • maintained during exam

Explain image lag in ii systems

ii systems have slow response time to adjusting to changes in tissue density

Brightness gain

the ability of an ii to increase image illumination

Total brightness gain

measurement of the increase in image intensity achieved by the ii

Total brightness gain is determined by ___

minification gain and flux gain

Minification gain

• increase in image brightness or intensity

• occurs as electrons are compressed into smaller area

• same number of light photons are concentrated on a smaller screen, creating a bright image

Minification gain equation

minification gain = (d_i/d_o)²

d_i = diameter of input phosphor

d_o = diameter of output phosphor

Flux gain

• anode accelerates electron toward output screen

• acceleration increases kinetic energy

  • increased light intensity at output phosphor

• electrons arrive at output screen with increased energy

  • 50-75 times more

Flux gain equation

flux gain = (number of output light photons)/(number of input x-ray photons)

Brightness gain equation

brightness gain = minification gain x flux gain

Brightness gain may deteriorate as much as ___

10% per year (due to aging of screens)

Conversion factor

• ICRU rates brightness gain by conversion factor

• ratio light intensity at the output phosphor / radiation intensity at the input phosphor

• measures how efficiently ii converts x-ray energy to light energy

What are 3 types of output screen attachments?

TV/video tubes, CCDs, CMOs

Explain TV/video tubes (old output screen technology)

• coupling device sends signal from output screen to viewing monitor

• beam splitter used to send image data to spot film and cine camera

• image must be converted to electrons to be sent to viewing devices

• vidicon or plumbicon tube converts light from output phosphor to electrical signal

• electrical signal sent to television monitor for viewing

Explain charge coupled devices (CCDs) as a type of output screen attachment

• coupled to output phosphor by optic cables

• small, flat plate

• light strikes CCD

• CCD releases electrons proportional to incident light

• stores image in latent form

• emits signal in raster pattern

• IR is divided into squares (DELs)

• signal is collected, stored, and released

What are some benefits of CCDs?

• faster discharge time (no image lag)

• operates at much lower voltages (extends tube life)

• high contrast images

• high resolution

• high SNR

• high DQE (high sensitivity, decreased technique, decreased exposure)

Explain complimentary metal oxide semiconductor (CMO) as a type of output screen attachment

• coupled to output phosphor by optic cables

• small, flat plate

• converts light form output screen to electronic signal

• lower image quality

• each DEL contains readout components

Explain the difference in DEL readout in CCD and CMO

CCD DELs read out in groups/lines

CMO DELs read out individually

CCD has ___, but CMO has ___

CCD has higher image quality, but CMO has economic advantages

What is the most restrictive component of any fluoro system?

viewing monitors

List 4 components of image quality

1. contrast

2. resolution

3. distortion

4. quantum mottle

Explain contrast as 1 of the 4 components of image quality

• degradation between input and output screen

  • affected by penumbral light scatter

• background fog

  • incident photons striking output screen

• deteriorates approximately 10% per year the fluoro system is in use

Explain resolution as 1 of the 4 components of image quality

• greatest limitation comes from video monitor

• detail determined by geometric factors

• OID

• phosphor size and thickness

  • want thin layers and small crystals

• input and output screen diameter

  • input is much bigger, gets squished down to output size

• screen resolution

Explain distortion as 1 of the 4 components of image quality (know the 3 types)

• pincushion - distortion of lines

  • caused by curvature of input screen and flatness of output screen

• vignetting - brightness varies from center of image to edge

  • potentially a consequence of pincushion

• veiling glare - light scatter from output screen degrades contrast

Explain quantum mottle as 1 of the 4 components of image quality

• blotchy or grainy image caused by insufficient radiation

• factors contributing to quantum mottle:

  • video noise

  • radiation output

  • beam attenuation by subject

  • conversion efficiency of ii

  • minification gain

  • flux gain

  • brightness gain

• solution = increase mA

Explain flat panel digital fluoro

• flat panel replaces ii - TFTs

  • added CsI scintillation

• increased SNR

  • pixel binning - combines up to 4 pixels at once

  • results in lower noise

• resolution - not as high as radiography

  • clinical value outweighed by patient exposure concerns

• less patient dose

• improved contrast

What are some features of dynamic flat panel fluoro detectors?

• zoom feature (can zoom without MAG = no dose increase)

• LED “refreshes” detector between frames

  • prevents ghosting

• last image hold

  • maintains last image real time

• less distortion

• post processing algorithms

  • window width and level

  • edge enhancement

  • digital subtraction

What are some advantages of digital fluoro?

• reduces patient dose by as much as 50%

  • operates at 2-20 mA

  • pulsed fluoro

  • frame grabbing

  • last image hold

  • filtering

• high DQE and SNR

• patient dose is displayed

• durable (doesn’t degrade with age)

• reduced artifacts

  • less veiling glare and pincushion distortion

• less geometric distortion

Explain over-table x-ray tube

• configuration results in increased exposure to personnel

• may be equipped for remote control operation

Explain fixed c-arms

• may be mounted from ceiling or floor

• applications include:

  • cardiac

  • peripheral and neuro angiography

  • interventional procedures

Explain mobile c-arms

• c-shaped arm with x-ray tube mounted on one end an ii on the other

• small, 10-15cm diameter ii

• connected to viewing monitor

• used in ED, ICU, GI, pain, OR, etc.

Explain mini c-arm

• compact system with a small image intensifier

• designed for imaging extremities

Explain Bi-plane equipment

• consists of:

  • 1 floor mounted c-arm

  • 1 ceiling mounted u-arm

• each is capable of individual or simultaneous motion and operation

Explain radiation protection

• tech is gatekeeper

• time

  • timer: 5 minute alarm

• distance

  • SSD not more than 15” with fixed unit, 12” for mobile

• shielding

  • aprons and gloves 0.5 mm Pb

  • 0.25 mm lead drape and lead shield over bucky slot

  • collimate to anatomy when in mag mode

  • stand behind radiologist

• exposure at tabletop not to exceed 10R/min