Ch 36 Flashcards
Why is fluoroscopy the domain of the radiologist?
It involves diagnosis during an examination
What is the typical basic fluoroscopic imaging chain?
A fluoroscopic imaging chain consists of a specialized x-ray tube with an image receptor, called the fluoroscopic screen that can be viewed during an x-ray exposure
How does a fluoroscopic x-ray differ from a diagnostic x-ray tube?
Fluoroscopic tubs are designed to operate for longer periods of time at much lower mA than x-ray tubes
The x-ray tube operates at 50-1.2 mA, the fluoroscopic mA range is 0.5- 5.0 mA.
Converts x-ray photons to light photons
Built into the image intensifier as its input screen that absorbs the x-ray photons and emits light photons which immediately encounter the photocathode that’s in contact with the input screen to prevent divergence of the light beam
Fluorescent screen
Converts the light photons to electrons
Absorbs the light photons and emits electrons which are accelerated from the cathode toward the anode and the output screen by the potential difference that exists between the cathode and anode
Emits electrons when struck by light emitted by input screen
Made of photoemissive materials which is usually cesium and antimony compounds
Photocathode
Focus the electrons towards the anode focal point
Charged electrodes inside tube
Accelerate and focus electron pattern across tube to anode
Primary source of brightness gain
Electrostatic lenses
Converts the electrons to light photons
Absorbs the electrons and emits light photons, which are then available for viewing or further electronic processing by a video system
Glass fluorescent screen
Very thin (4-8 mm) to produce high resolution (about 70 lp/mm)
Silver-activated zinc-cadmium sulfide phosphor (ZnS-CdS:A); phosphor is extremely small (1-2 mm)
Electrons that strike this are converted into green light photons that exit the tube
Because all phosphors admit light isotropically, an opaque filter is used under the output phosphor layer to prevent most of the light emitted in that direction from returning to the input screen (where it’d degrade the image)
Typical one has a diameter of 1” (2.5 cm)
Output screen
What is the formula used for determining brightness gain?
Total brightness gain = magnification gain x flux gain
Maintain the brightness of the image by automatically adjusting the exposure factors as necessary according to subject density and contrast
Most systems monitor the current flowing between the cathode and anode of the image intensification tube or the intensity of the output screen
In all systems, the primary beam is changed when current and intensity fall below established levels; regulation of the primary beam can be accomplished by varying kVp, mA & pulse time
Most use combinations of these methods in a manner similar to a stepped variable kVp technique
All have a relatively slow response time (which is noticeable during routine fluoroscopy scanning because the image density adjustment lags a moment behind rapid changes in tissue density)
Automatic brightness control (ABC) [most common term]
Automatic dose control (ADC)
Automatic brightness stabilization (ABS)
What is the primary limitation of resolution of the fluoroscopic image?
The 525-line raster pattern of the video monitor
What radiation protection practices should be adhered by the radiographer during fluoroscopy?
Lead apron of at least 0.5 mm Pb/eq must be worn by all persons (other than patient) who are present in the fluoroscopy room during exposure; apron designed to cover front and sides of body is usually sufficient
If the hands must be placed within the primary beam, lead gloves of at least 0.5 mm Pb/q must be worn
Dynamic radiographic exam which is static (still pictures) in character and involves active diagnosis during an examination
Fluoroscopy
Fluoroscopic imaging change consists of a specialized x-ray tube with an IR that can be viewed during an x-ray exposure
Fluoroscopic screen
Makes image brighter, video camera and monitor used to view fluoroscopic images (1948)
Capable of increasing image brightness 500-8000 times
Vacuum tube with cathode and anode
Primary beam exits the patient and strikes the input screen of this
Encased in a lead-lined housing that effectively absorbs the primary beam while permitting the intensified light photon image to be transmitted to the viewer
Image intensification tubes
Visual acuity is controlled by the cones and requires daylight
Photopic
Night vision controlled by rods
Scotopic
4 fluoroscopy uses (functional studies)
Gastrointestinal (GI) tract studies
Angiograms
Cardiac cath (femoral artery to heart)
Head
Permits the IR to be raised and lowered to vary the beam geometry for maximum resolution while the x-ray tube remains in position and permits scanning the length and width of the table
C-arm
2 types of fluoroscopy equipment
C-arm
Carriage
2 types of C-arms described by the location of the x-ray tube
Under-table unit
Over-table unit
Arm that supports the equipment suspended over the table
Typically includes an image intensification tube (under-table unit) or x-ray tube (over-table), controls for power drive to the this, brightness (regulates tube mA), spot image selection tube shutters, spot imaging and/or cine cam, video input tube and other controls
Although it can be disengaged and pushed away from the table to gain access to the patient, exposure can’t commence until this is returned to a full beam intercept position
Carriage
What is the mA range of a fluoroscopy tube?
0.5-5.0 mA
What is the minimum source-to skin distance for mobile and stationary fluoroscopy?
12” (30 cm) for mobile
15” (38 cm) for stationary
Occurs from the acceleration and focusing of the electron beam
The acceleration of the electron beam increases its energy and its ability to emit light at the output screen
The focusing of the electron beam intensifies the image into a smaller area
Primary brightness gain
Ion pump device used to removed ions produced during operation and to maintain the vacuum within the tube
Getter
0.1-0.2 mm layer of sodium activated cesium iodide (Csl)
Either made of glass, titanium, steel or aluminum
Ranges from 6” (15 cm) to 23” (58 cm) in diameter
Concave in surface to help keep distance the same between the input (bigger) and output screen
Converts intercepted x-ray beam to light
Most image intensification tubes have these of 6” (15 cm) or 9” (23 cm) (although 12” (30 cm) and larger tubes have been available for special apps)
Input screen
Greater voltage to electrostatic lenses: increases acceleration of electrons and shifts focal point away from anode
Can be designed to magnify the image electronically by changing the voltage on the electrostatic lenses often called: multi field, dual-field, triple-field or quad-field intensifiers
Capable of 1.5-4 magnification which is usually controlled at the fluoroscopy carriage
Described according to the diameter of the area of the input screen that’ll be imaged
Magnification tubes
What is the dual focus of magnification tubes?
23/15
Has 9” (23 cm) input screen when operating normally
Uses a 6” (15 cm) area when magnifying
What is the magnification factor formula?
Magnification factor = input screen diameter/diameter of input screen used during magnification
Positively charged and normally supplied with about 25 kV
Has a hole in its center that permits the accelerated electrons to pass thru the anode field and onto the output screen
Anode
Measurement of the increase in image intensity achieved by an image intensification tube
Its measure is a conversion factor that’s the ratio of the intensity of the output phosphor to the input exposure rate: conversion factor = intensity of output phosphor/(mR/sec)
Output phosphor intensity is measured in candelas (cd), the unit of luminous intensity
Deteriorates as much as 10% per yeat because (just as with intensifying screen phosphors) the input and output screen phosphors age
Total brightness gain
2 factors that determine total brightness gain
Minification gain
Flux gain
Occurs as a result of the same number of electrons that were produced at the large input screen being compressed into the area of the small output screen (increase in brightness or intensity, not an improvement in the quality or number of photons making up the image)
Calculated as the ratio between the area of the input and output screens
Minification gain
What is the formula for minification gain?
Minification gain = input screen diameter^2/output screen diameter^2
Measurement of the increase in light photons due to the conversion efficiency of the output screen (how well output screen converts photons to light)
Causes a decreases in image quality
Flux gain
Part of the video camera control system that responds very quickly to input information but doesn’t change the x-ray exposure factors
Automatic gain control (AGC)
2 systems used to automatically maintain satisfactory fluoro image density and contrast
Automatic brightness control (ABC) [most common term]/automatic dose control (ADC)/automatic brightness stabilization (ABS)
Automatic gain control (AGC)
4 factors that affect each of the quality elements of the image (because the imaging chain of the fluoroscopy system is so complex there are more than in static radiography)
Contrast
Resolution
Distortion
Quantum mottle
Controlled by increasing the amplitude of the video signal (although it’s affected by other factors)
Digital fluoroscopy system can also use post-processing functions (especially window width and various filtering algorithms) to improve this
Image intensified fluoroscopy this isn’t only affected by scattered ionizing radiation but also by penumbral light scatter in the input and output screens and light scatter within the image intensification tube itself
Raising the lowest density value decreases the total visible this; overall effect = reduced this and decrease in this near the edges of the images
Contrast
Will vary depending on the geometrical factors
CsI image intensifiers are capable of about 2 lp/mm
Digital fluoro systems often achieve 2 lp/mm
Resolution
What is size distortion caused primarily by?
OID
What is shape distortion caused primarily by?
Geometric problems in the shape of the image intensification tube
Although the image intensifier input screen is concave, it doesn’t completely eliminate edge distortion at the output screen
Electrons at the outer edges of image tend to flare outward as they’re electrostatically focused
Part of this problem is caused by the repulsion of electrons from one another due to their like charges
May comprise 8-10% of the image area
Some also caused by the effect of the divergence of the primary beam from the x-ray tube focal spot
Also causes image intensity to be greater at the center of the image and less at the edges; consequently distortion is minimized and contrast is improved at the center of the fluoroscopy image
This problem has been completely overcome with the use of TFT matrices, as they have uniform resolution across the entire surface of the detector array
Vignetting/pincushion distortion
Blotchy/grainy appearance caused by insufficient radiation to create a uniform image
Because the quantity of photons is controlled by the mA and time settings, with static radiography any mA and time combination can be used to accumulate sufficient radiation to create a uniform image
With fluoroscopy the time factor is controlled by the length of time the eye can integrate/accumulate light photons from the fluoro imaging chain
Because this period is 2 seconds fluoroscopy must provide sufficient photons through mA to avoid this
Large part of video noise and is a special problem during fluoroscopy because the units operate with the minimum number of photons possible to activate the fluoroscopy screen
Quantum mottle
6 factors that influence mottle are also those that affect the total number of photons arriving at the retina of the eye (increasing the efficiency of any of these factors can assist in reducing mottle but the most common solution is to increase the fluoroscopy tube mA)
Radiation output
Beam attenuation by subject
Conversion efficiency of the input screen
Minification, flux and total brightness gain
Viewing system
Distance of the eye from the viewing system
What is the most commonly used fluoroscopy viewing system?
Video system includes video camera attached to the image intensification tube output phosphor and a display monitor for viewing
Fluoroscopy video cameras use charge-coupled devices (CCDs)
Semiconducting device capable of storing a charge from light photons striking a photosensitive surface
When light strikes the photoelectric cathode of this, electrons are released proportionally to the intensity of the incident light
Has ability to store the freed electrons in a series of P and N holes thus storing the image in a latent form (as with all semiconductors)
Video signal is emitted in a raster scanning pattern by moving the stored charges along the P and N holes to the edge of this where they are discharged as pulses into a conductor
Charged couple devices (CCDs)
4 advantages of CCDs
Extremely fast discharge time which eliminates image lag; extremely useful in high-speed imaging applications such as cardiac catheterization (primary)
More sensitive than video tubes
Operate at much lower voltages which prolongs their life
Have acceptable resolution
Not as susceptible to damage from rough handling
High resolution output screen that receives the final processed signal from the fluoro processor
Flat screen these require regular calibration and quality assurance testing
Video monitor
Achieved through the use of a high-power generator operating in a pulse progressive fluoroscopy mode
Pulses the x-ray production from the fluoroscopy x-ray tube in sync with the detector signal so that pulses of signals are received by the image processing unit
This tech combined with a detector comprised of a thin film transistor (TFT) in contact with the image intensifier (II) output anode screen (TFT replaces the image intensifier in nondigital fluoroscopy systems)
Most systems capable of 8-bit processing
Capable of very good separation of acquisition and display which produces much higher contrast
Reduces patient exposure considerable with both dynamic and static images recorded at reduced doses; can be on the order of elimination of up to 90% of the patient exposure
Digital fluoroscopy
Length of time required for the generator to come on and achieve the necessary kVp & mAs levels
Interrogation time
Time required to shut the generator down in preperation for the next pulse
Extinction time
Pixelated unit with a photodiode connect to each pixel element
Relatively intensive to x-ray photons
Some manufacturers add a CsI scintillator element to the image receptor to increase sensitivity by adding back the x-ray photon energy
CsI phosphors absorb x-ray photons and emit light that can be recorded by this
Limitations primarily concerned w/the electronic noise limits for flat panel amplification
Thin film transistor (TFT)
How many pixels are in digital fluoroscopy?
200-300 micrometers (1-2 lp/mm) as compared to radiography pixels which are 100-150 micrometers (10-12 lp/mm)
Maintains the last real-time fluoroscopy image until it’s replaced by the unit being activated again (allows physicians to continue work from the most recent image without exposing patient to added radiation)
Last image hold
3 post-exposure image processing functions of digital fluoroscopy
Window level and width Filtering techniques (edge enhancement. temporal filtering, etc.) Digital subtraction technology (which can digitally remove background structures when a previous image is superimposed)
How are fluoroscopy images recorded?
Dynamic recording of fluoroscopy images can be achieved thru the use of any recording media with adequate memory (ex: DVDs), also possible to record static images from fluoroscopy with any digital recording device
4 steps of recording images in digital fluoroscopy
Process the image coming from the charge coupled device by sending an analog signal thru an analog-to-digital converter (ADC) microchip
Once the fluoroscopy image has been converted to a digital signal, it can be manipulated as desired and transferred repeatedly without loss of quality
Display on a monitor permits changes in density (digital window level), contrast (width), magnification, filtration enhancements and other techniques to be applied to the image
Digitization also permits storage on computer disk and transfer via electronic means such as tele-radiology systems or the internet
What is the maximum and average tabletop exposure rate?
Tabletop exposure rate shouldn’t exceed 10 R/min and for most units should range from 1-3 R/min
Why do magnification image intensifiers cause increased patient dose?
The ABC will increase the tube output to compensate for the loss of electrons within the image intensification tube during magnification
