Unit 4: Fluoroscopy Flashcards
What is responsible in fluoroscopy for the converting of x-rays into visible light?
A phosphor screen
What is a spot film?
Digital images taken without the interruption of the dynamic examination that takes a still picture
What are the 2 main areas of angiography?
- Neuroradiology
- Vascular radiology
True or false?
Not all fluoroscopy treatments aloow digital images (spot films) to be obtianed.
False
What is the x-ray tube in fluoroscopy generally operated at in terms of mA?
Less than 5 mA
Does the 15% rule of thumb apply to fluoroscopy?
Yes
True or false?
ABC is always being used in fluoroscopy.
True
How does the fluoroscopy machine maintain ABC?
By varying kVp, mA or both as the x ray tube moves over body parts of differing thickness and attenuation
Why is an image intensifier necessary for the visualization of images in fluoroscopy?
W/o an image intensifier, we would only be using the rods in our eyes to visualize the images produced. An I.I. increases CR and SR.
What is the function of an I.I.?
Converts remnant x ray beams into a high intensity visible light image
What is responsible for converting the remnant x rays and its energy to light in fluoroscopy?
The input phosphor made of cesium iodide (CsI)
Why do we used CsI as the input phosphor in fluoroscopy?
- Has a high intrinic efficiancy which reduces patient dose
- It absorbs 2/3 of the incident beam
- Increase of SR
True or false?
CsI has a low atomic number.
False; high-it results in higher x ray absorption
Why does CsI increase SR?
1. Thinness: Reduces the amount of diverging photons since they dont have to go through as much material
2. Shape of crystals: They help direct light with little lateral dispersion and less blurring
What are the advantages of increasing the image phosphor thickness?
- There is a higher x ray absorption efficiency
- Patient dose is reduced
What are the disadvangaes of increasing the image phosphor thickness?
- Decreased spatial resolution
What is photoemission?
Occurs when the photocathode emits e- when light from the IP strikes it
What is the photocathode made of in fluoroscopy? What is it bonded to?
-Made of cesium and antimony compound
-Bonded directly to the input phosphor
True or false?
The number of e- emitted by the photocathode is indirectly proportional to the intensity of light that reaches it.
False; directly proportional
Why is the image phosphor curved?
To ensure that the e- emitted at the peripheral regions of the photocathode travel the same distance as those emitted from the central region to maintain resolution
How long is the image intensifier?
50 cm long
What is the anode plate in fluoroscopy?
A plate with a hole in the middle which allows e- to pass from the photocathode to the output phosphor attached to it
What is the potential difference in the accelerating anode across the tube between the PC and the anode?
25000-35000 V
What is the term that describes the engineering aspects required to maintain proper e- travel across the accelerating anode?
Electron optics
What is the funciton of electrostatic focusing lenses in fluoroscopy?
Uses negatively charged focusing lenses placed on the inside of the glass envelop to focus all electrons to the output phosphor
What is the point of inversion?
Electron focusing inverts and reverses the image at the focal point (just prior to reaching the O.P.)
True or false?
The output phosphor is smatter than the IP?
True
What is the function of the output phosphor?
Converts electrons to visible light
What do electrons that reach the O.P contain?
The electrons that arrive at the O.P with high KE contain the image of the IP in the minified form.
What is located on the vaccum side of the O.P surface and why?
-A thin Aluminum film coating to allow e- pass through, but prevent light photons from being scattered back to the photocathode. Also serves as a reflector to increase the output luminesence.
What gains occur at the ouput phosphor?
- Flux gain
- Minification gain
- Brighness gain
How many light photons has a photoelectron produced by the time that it arrives at the OP?
It has produced 50-75 times as many light photons as were nescessary to create it.
What is flux gain?
The ratio of the number of light photons at the OP to the number of x ray photons at the IP
What is flux gain caused by?
The acceleration of photoelectrons to a higher kinetic energy
What causes an increase in image brightness in fluoroscopy?
The concentration of photons per unit area from a large input screen onto a small output screen
The amount of photoelectrons hitting the output phosphor is the same as those leaving the IP
True or false.
Minification gain changes contrast by making the image brighter.
False; the minification gain does not change contrast, but it will make the image brighter.
What is the minification gain?
The ratio of the square of the diameter of the input phosphor to the square of the diameter of the output phosphor.
What is the range of sizes of the image phosorphor in fluoroscopy?
Varies from 10-40cm
What is the usual OP diameter?
Ussually 2.5 or 5cm
What is the brighness gain for most image intensifiers?
5000-30000
True or false?
As brightness gain decreases, patient dose increases.
True
Why does brightness gain decrease?
Due to tube age and use
What is the brightness gain?
The increased ilumination of the image due to the multiplication of the light photons at the OP, and the image minifaction from the IP to the OP
How is fluoroscopic magnification achieved?
- Geometrically OR
- Electronically
Will magnification increase or decrease as a patient moves closer to the x ray source?
Increase
True or false?
As SEE increases, so does magnification.
True
Skin entrance exposure
What is the advantage of modern day multi-feild image intensification II?
It procides greater flexibility in fluoroscopic exams since a part of the image or the whole image can be viewed due to the ability to change the diameter of the IP
How is multifield image intenisifcation achieved?
By the ability to manipulate the voltage on the electrostatic lenses
What does an increaase in voltage on the electrostatic lense cause?
- Increased acceleration of electrons
- Shifts the focal spot away from the anode (magnified image)
True or false?
MII can be used to penetrate though larger parts due to resulting increased radiation dose, or enlarge smaller structures.
True
True or false?
The use of the larger dimension of the milti-field II always results in a magnified image.
False; The use of the SMALLER dimension of the milti-field II always results in a magnified image.
What is the magnification factor?
The ratio of the 2 diameters in a multifield image intensifier
True or false?
In the mag mode, minification gain is reduced, and fewer electrons are insident on the OP.
True
In mag mode, why are fewer electrons incident on the OP? What does this result in?
Because of the reduced field of view at the IP, resulting in a dimmer image
How do you find the increase in dose when in mag mode?
The increase in dose is equal to the square of the ratio of the 2 IP diameters
Why is SR increased in mag mode?
Since olny the central region of the IP is used while in mag mode
T or F; Mag mode increases SR, CR, but increases patient dose.
True
What are 2 advantages of television monitoring?
- Brightness level and contrast can be controlled electronically
- Several observes can view the floro image at the same time
What is fiber optic coupling in fluoroscopy?
Bundles that are only a few mm thick and contain thoughstands of glass fibers per square mm at the end of the II that is coupled to the TV monitor
What are the advantages of fiber optic coupling in fluoroscopy?
- Compact assembly-makes it easy to move the II
- Rugged-can withstand rough handling
What is better for patient dose, conventional fluoroscopy (I.I.), or digital fluoroscopy (flat panel detectors)
Digital fluoroscopy
What are the limitations of I.I. systems?
- Size
- Internal stricutres in the I.I. must be under high vaccum
- Defocusing effect
- S distortion
- Geometric limitations
- Spatial resolution vs. radiation dose
- Veiling glare
What are the 2 types of geometric limitations?
1.Pin cushion distortion
2. Vignetting
Why is size a limitation for image intensifiers?
Its large size makes it difficult to position; especially in the OR
Why is it a disadvantage to have high pressure in vaccums with image intensifiers?
Air leakage could occur, and would interfere with the transit of the electrons between the PC and the OP, and would degrate image quality
When does the defocusing effect occur? What does this cause?
When the V of the electrostatic lenses are not adjusted correctly, and causes the image to be blurry since the electrons are not able to pass through the appropriate focal point.
Why does S distortion occur with image intensifiers?
S distortion within the image occurs due to close proximity to an MRI or stray electromagnetic fields
Why do image intensifiers cause geometric distortion from its geometric limitations?
Since the OP is flat, the distances from the focal point to the OP are not the same; distances to the periphery of the output surface are longer than they are to the center.
What is pin cushion distortion caused by?
Caused due to the curved IP and the flat OP
How can you decrease pin cushion distortion in image intensifiers? Why does this work?
-Use mag mode, since e- from the center of the FOV are more in focus
What is vignetting?
Since the longer diverging paths reduce the concentration of e- that hit the outside of the OP, this causes the center of the OP to brighter than the periphery.
At what rate does radiation dose increase at in mag mode?
1/FOV^2
What is veiling glare caused by?
Internal scatter radiation in the form of x rays, light, and electrons causing a reduction in contrast at the OP.
What type of fluoroscopy is predominently used in angiography suits and cardiac catheterization labs?
Flat panel detector fluoroscopy
T or F; Flat panel detector floro allows for more flexible movement.
True
What type of aquisition is mostly used in FPD fluoroscopy machines?
Indirect aquisition
What element is the photodiode FPD in fluoroscopy generally made of?
Solid state detectors made of amorphus silicon dopped with thallium
Explain the process of how a FPD produces an image from the Phoshor screen to the photodiode:
- Phosphor screen scintillates when exposed to x rays, emitting light
- Strikes TFT
- Crystal CSI layer forms light channels to confine the dispersion of light
- Light directed toward DELS in the AMA
- Hits A-Si, which allows the diode to conduct electricity
In the absence of light on the surface of A-si in FPD, what is the function of the photodiode?
It acts like an insulator, preventing the flow of electrons
What happens when the electronic switch in the photodiode is closed?
The power supply charges the capacitor
What process occur from the photodiode to the display?
- An electronic swtich is closed and the power supply charges capacitor
- Electronic swtich is opened and charge remains on the capacitor
- The amount of light causes photodiode to conduct to different degrees. As more light produced, the more charge is drained from capacitor
- Capacitor left with remnent charge
- Another electronic switch is closed and remnent charge is withdrawn from storage capacitor and sent to display system
- Eletronic image that is incident on the FPD can be formed by reading each DEL in the FPD
True or false?
The loss in charge (E) in the photodiode is related to the amount of x-ray radiation and light incident on the DEL.
True
What are the advanages of the FPD system in floro?
- Images dont exhibit geometric distortions because individual DELS in the FPD are manufactured in straight rows and columns
- No vignetting
- No deforcusing because there are no electrostatic plates to change focal spot. Each DEL is in a constant position
What is binning?
Size of data rates can be reduced by grouping the data from several DELS together for larger fields of views (instead of reading each individual pixel, it reads a group as one)
What are the disadvantages of binning?
Less spatial resolution becuase the effective area of each image pixel is larger.
What are the advantages of binning?
Loer data rates and less image mottle than ungrouped DELS
What are the limitations of FPDs?
5
- Often contain defective/degraded DELs
- Software interpolates values for defective elements that may cause artifacts
- They are temperature sensitive
- SR is limited by size of DEL
- Large FPDs have high data rates which are difficult for electronic systems to handle
Why can having a smaller DEL be a bad thing?
Although it increases SR, the images have more ottle due to less SNR, so we would have to increase patient dose
T/F?
For smaller FOVs in FPD imaging, binning is not required, and the image becomes more magnified.
True
What is the disadvantage of using a smaller FOV in FPD systems to increase magnification?
Makes the image noise more apparent; so an increase in technique would have to by applyed to reduce noise.
What has a high increase in dose with magnification; FPDs or II?
II
T/F?
Ghosting can occur in FPD systems.
True
Why does ghosting occur in FPD systems? How do we get rid of this?
-It occurs because the phosphorescent light on the scintillaton surfaces undergoes a period of decay, causing light emissions from previous images to persist
-Bright internal light source flashes cause the FPD to reset
T/F
II is a type of digital fluoroscopy.
True
What technique in digital fluoroscopy allows us to use a higher mAs?
Pulse progressive floro
What would happen if the tube were energized continuously in floro?
It would fail because of thermal overloading and patient dose would be extremely high
T/F
Floro images always appear with an inverted grayscale.
True
T/F
Tube current is measured in hundreds of mA as opposed to less than 5 mA in conventional floro
True
What is one way to reduce dose with pulse progressive floro?
-Reducing frame rates
-Using last image hold
If you were to cut the frame rate by half, how much would this reduce your dose?
22-28%
What is one downside to pulse progressive floro?
-Increases image noise
-Effects temporal resolution
What is the interrogation time in pulse progressive floro?
The time that is required fo rthe x-ray tube to be switched on and reach the selected kVp and mA
What is the extiniction time in pulse progressive floro?
The time required for the x-ray tube to be switched off
What is the duty cycle?
The fraction of time that the x ray tube is energized
How long do we wnat the extinction and interrogation time (duty cycle) in high f generators with DR systems to be?
Less than 1 ms
What is electronic collimation in digital floro?
Electronic collimation overlays a collimator blade on the last image hold so that one can adjust feild dimensions without exposing the patient
What are 2 advantages to conventional colimation?
- Better SC bc the extra light isnt there
- Reduction in dose
What is better; conventional collimation or spot collimation? Why?
Spot collimation
-Enables the operator to collimate a rectangular, square or asymmetrical collimation anywhere within the FOV in the LIH
-As soon as collimation is confirmed, the collimator blades are positioned accordingly and the LIH is superimposed of the colllimated part
What does ROI stand for?
Region of interest
T/F
Spot collimation has ABC.
True
How does spot collimation and LIH comprimise with ABC?
A collimation adaptive ABC-ROI adjustment algorithm prevents interpretation that could result in a dose increase
Label what the functions of these buttons is;
- Rotates image
- Saves image
- Changes monitor
- Increases quality
T/F
Conventional floro is no longer used.
True
What are the 2 methods of temporal subtraction in DSA?
- Mask mode
- Time intervale difference mode
What is temporal subtraction?
A computer assited technique whereby an image obrained at one time is subtracted from an image obtained later
Explain how mask mode DSA works:
- Power injector injects contrast after initial image without contrast has been obtained
- When the exposure starts, the mA increases 20 to 100 times and activates pulse image aquisition
- The initial image is displayed on monitor and stored in primary memory. This is our mask image
- As subsequent images are taken, the mask image is subtracted from each image and the new image is displayed on monitor B
What is the disadvantages of mask mode and what are 2 ways we can fix it?
Misregistration artifacts: Occurs when patient moves between exposures.
-Fixed by using pixel shifting (moves the mask, so that the anatomy is superimposed)
-Fixed by remasking (using another image as the mask
Explain how Time Interval Difference (TID) mode works:
- A total of 60 images are obtained in a study for a total of 4 seconds and each image is identified by their frame numbers (1-60) (15 frames per second)
- A TID (ex:4) is selected
- In this case, the first image to appear will be frame 5; using frame 1 ask its mask
- This pattern continues for all frames past frame 5 and is always subtacting from a different frame
What is dual energy subtraction?
-Uses 2 different x ray beam energy to provide a subtraction image that results from differences in photoelectric interaction
-This allows you to see structers more clearly
(it subtracts the differences from the 2 acquired images)
What are the 2 differnent methods of aquiring an anternating x ray beam for dual energy subtraction?
- Alternately pulsing the x-ray beam at 70 kVp and 90 kVp
- Alternately introducing metal filteres into the x-ray beam
T/F?
The SC when using dual energy subtraction is not a good as temporal subtraction.
True
What is one way to increase SC when using dual energy subtraction?
High quality images can be obtained by combining energy and temporal subtraction into hybrid subraction (using both dual energy subtraction and temporal subtraction)
What are the differences between the kVp settings in temporal subtraction vs. Dual energy?
Temporal: Single kVp setting
Energy: Rapid kVp switching
What are the differences between the beam filtration settings in temporal subtraction vs. Dual energy?
Temporal: Normal filtration
Energy: X ray beam filter switching is used
What are the differences between the image subtraction in the computer in temporal subtraction vs. Dual energy?
Temporal: Usies simple arithmetic image subtraction
Energy: Uses complex image subtraction
What are the differences between the motion artifiacts in temporal subtraction vs. Dual energy?
Temporal: Motion artifacts area problem
Energy: Motion artifacts greatly reduced
What are the differences between total subtraction of the image in temporal subtraction vs. Dual energy?
Temporal: Total subtraction is often achieved
Energy: Some residual bone may still be in the subtracted image
What are the differences between the types of subtraction avalible in temporal subtraction vs. Dual energy?
Temporal: Not many other types of subtaction avalible because they are limited by the number of images
Energy: Many more types of subtraction are avalible
What is road mapping?
-An application of DSA
1. Image subtraction occurs for the the vessel of interest after recieving contrast. This becomes the road map/mask.
2. The road map is subtracted from the live non contrast images.
3. This produces real time subtracted floro images of an inserted cathetier overlaied on a static image and no distracting underlying tissue
