Chapter 9 - Fluoroscopy Flashcards
The principal feature of the imaging chain that distinguishes fluoroscopy from radiography…
Ability to produce real-time X-ray images with high frame rates and a low-dose per image
Basic product of a fluoroscopic imaging system
Projection X-Ray image
4 principal components of an II(image intensifier):
- Vacuum housing
- Input layer
- Electron optics system
- Output phosphor
(II component) keeps air out and allow unimpeded electron flow
Vacuum housing
(II component) converts the absorbed incident X-rays into light, which in turn releases electrons
Input layer
(II component) accelerates and focuses the electrons emitted by input layer into the output layer
Electron optics system
(II component) converts the accelerated electrons into a visible light image
Output phosphor
In IIs, an antiscatter grid is mounted _____ to the input layer on the housing
Adjacent
Virtually all modern IIs use _____ for the input phosphor
Cesium Iodide (CsI)
The input phosphor/photocathode surface is CURVED to accommodate the physics of electronic focusing…distortion produce in the output image?
-result of projecting the image with a CURVED input phosphor to the flat output phosphor
Pincushion distortion
*remedy: position the desired anatomy in the central area of the FOV
Five-component (“pentode”) electronic lens system of the II
1-3. G1, G2, and G3 electrodes
- Input phosphor substrate (the cathode)
- Anode just proximal to the output phosphor
Made of zinc cadmium sulfide doped with silver (ZnCdS: Ag) , which has a green (530nm) emission spectrum
Output phosphor
A very thin (0.2 um) coating of aluminum on the vacuum side of the output phosphor, which is electrically conductive to carry away the electrons once they deposit their kinetic energy in the phosphor
Anode
The ratio of the area of the input phosphor to that of the output phosphor
Minification gain
Stray light reflecting inside the output window contributes to ______, which can reduce image contrast
Veiling glare
*reduce by using a thick (about 14 mm) clear glass window
Vidicon video systems typically operate at 30 FPS in an ____ scanning mode to reduce ____,the perception of the image flashing on and off.
INTERLACED, FLICKER
The human eye-brain system can detect temporal fluctuations slower than about ______
47 images/s
*therefore at 30 FPS, flicker would be perceptible
With interlaced systems, each frame is composed of two fields called _____
Odd and even fields
- corresponding to every other row in the raster
- ODD FIELD - starts at row 1
- EVEN FIELD - starts at row 2
Solid-state electronic arrays (2) that convert a projection image into a digital image and then into a video signal
- Charge coupled device
2. Complementary metal oxide semiconductor (CMOS) photosensitive cameras
Characteristics unique to image intensifier systems (3):
- Brightness gain
- Pincushion distortion
- S distortion
Product of the electronic and minification gains of the II
Brightness gain
*as the effective diameter (FOV) of the input phosphor DECREASES, the brightness gain DECREASES.
A spatial warping of the image in an S shape thru the image
- usually subtle
- result of stray magnetic fields and the earth’s magnetic field affecting the electron trajectory from the cathode to the anode inside the II
S distortion
Comprised of thin film transistor (TFT) arrays of individual detector elements (dexels) that are packaged in a square or rectangular area
Flat panel detectors
Regulates the SNR of the image constant when possible
- Regulates the X-Ray exposure rate incident on the input phosphor of the II or flat panel detector
Automatic exposure rate control (AERC)
*formerly referred to as Automatic Brightness Control (ABC)
Increasing kV, _____ the dose rate but contrast is _____
AERC
DECREASES; REDUCED
Increasing mA, _____ the dose rate but ______ contrast
AERC
Raises; Preserves
Produces a continuous X-Ray beam typically using 0.5 to 6 mA (depending on patient thickness and system gain)
Continuous fluoroscopy
X-Ray generator produces a series of short X-Ray pulses
Pulsed fluoroscopy
Yields higher spatial resolution for a smaller field of view
Magnification modes
Brightness gain of the II ____ as the magnification increases
Decreases
Can cause noticeable image lag with reduced temporal resolution
Frame averaging
A common approach to frame averaging
- the image just acquired is added together with the last displayed image
Recursive filtering
Continuously displays the last acquired image or set of images on the fluoroscopy monitor
Last-frame-hold
A software- and video-enhanced variant of the last-frame-hold feature and is useful for angiography procedures
Road mapping
Capable of motorized rotation of about 220 degrees around the patient, which allows two-dimensional projection images to be acquired at many different angles around the patient
C-arm
For routine QC testing in fluoroscopy, a ______ is commonly used to assess spatial resolution
Visual assessment of a resolution test object imaged with minimal geometric magnification
Describes the maximum possible resolution in a digitally sampled imaging system
Nyquist Frequency
Contrast resolution of fluoroscopy compared to radiography is ____
LOW
*chiefly due to the low SNR
Contrast resolution is usually measured subjectively by viewing ___ under fluoroscopic imaging conditions
Contrast-detail phantoms
2 advantages of fluoroscopy in comparison to radiography
- Excellent temporal resolution
2. Ability to provide real-time images
Temporal blurring
Image lag
Implies that a fraction of the image data from one frame carries over into the next frame
Lag
Photoreceptors in the human eye produce a lag of about _____
0.2 s
***page 301-303
FLUOROSCOPY SUITES
- Genitourinary
- Remote Fluoroscopy Rooms
- Peripheral angiography suites
- Cardiology catheterization suite
- Cardiology electrophysiology laboratory
- Biplane angiographic systems
- Mobile fluoroscopy - C arms
The maximum permissible exposure rate to the patient
A. for normal fluoroscopy
B. For specially activated fluoroscopy
A. 87.3 mGy/m or (10 R/m)
B. 175 mGy/m (20 R/m)
Dose rate limits are assessed at specified positions:
A. For systems in which the X-ray tube is below the patient
B. For c-arm fluoroscopes
A. One cm above the table
B. 30 cm from the image receptor toward the X-ray source along the central axis of the beam
Typical entrance exposure rates for fluoroscopic imaging are about
A. For thin body parts
B. For the average patient
A. 8.7 to 17 mGy/m(1 to 2 R/min)
B. 26 to 44 mGy/m(3 to 5 R/min)
Dose rates are evaluated in fluoroscopy by using a ___
Tissue-equivalent phantom
*made of a material such as polymethyl methacrylate (PMMA, aka Lucite or Perspex) placed in the field, with an ionization chamber positioned in front of it to measure the entrance skin dose
To determine the maximum exposure rate , a ___ is placed between the PMMA and the detector system to attenuate the X-Ray beam and drive the AERC system to its maximum output
Sheet of lead
Typical entrance Kerma rates for fluoroscopic detectors range from _____
8.7 nGy (1 uR) to 44 nGy (5uR) per frame
Comprised of both leakage and scatter radiation
- has a distinctly non-uniform distribution
- chiefly due to backscattered radiation from the entrant beam surface
Stray radiation
Required by some institutions when the integrated skin dose reaches predefined levels, in order to evaluate the benefits and risk of continuing with the procedure
Procedural pause
Cumulative air Kerma may significantly over or underestimate the actual peak skin dose for three reasons (3)
- It does not account for backscattered radiation
- The X-ray beam may be incident upon a single area on the skin, or may be incident on multiple areas
- The skin may be closer or farther from the X-ray tube than the reference point at which the cumulative air Kerma is specified
It is customary to wear a single dosimeter at the ______, in ____ of the protective apron
Collar level ; front
*second dosimeter may be worn on the body under the apron
Most effective way to reduce patient dose during fluoroscopy
Use less fluoroscopy time
