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Fluoroscopy
Chapter 9
89
Medical
Post-Graduate
07/05/2016

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Term
The principal feature that distinguishes fluoroscopy from radiography.
Definition
Real-time x-ray images with high frame rates and low-dose per image.
Term
Standard fluoroscopy requires how much exposure on the detector?  How much does a radiographic CR system require?
Definition

Fluoro - 1 to 2 μR, CR system - 0.5 to 1 mR

Fluoro requires about a 1000x less than CR

Term
Four primary components of an II.
Definition
1) vacuum housing, 2) Input layer - converts x-rays into light into electrons, 3) Electron optics that accelerate and focus the electrons, 4) output phosphor that converts accelerated electrons into visible light
Term
What are the layers of the input screen of the II and what do they do?
Definition
1) vacuum window - keeps air out 2) support layer - supports the remaining two layers 3) input phosphor - converts x-rays to visible light -  4) photocathode - emits electrons when struck by light
Term
What is the II vacuum window and support layer made of?
Definition
Vacuum window - 1 mm Al, Support layer - 0.5 mm Al
Term
What is the input phosphor made of?
Definition
Cesium Iodide (CsI)
Term
What is the input photocathode made off?
Definition
Antimony and alkali metals (Sb2S3)
Term
The electronic lens system in the II consists of
Definition
The cathode (input), anode (output), and 3 focusing electrodes (G1, G2, G3)
Term
Electronic gain
Definition
The number of photons generatred at the output phosphor for every photon generated at the input phosphor.  Also known as the flux gain.
Term
Pincushion distortion is the result of.
Definition
The input surface of the II being curved and the output surface being flat.
Term
Why is the input surface of the II curved.
Definition
To allow for electronic focusing.
Term
The image at the output phosphor of the II is _______.
Definition
minified
Term
Minification gain of the II
Definition
Ratio of the area of the input phosphor to the area of the output phosphor
Term
Output phosphor of an II is made of
Definition

zinc cadmium sulfate doped with silver

ZnCdS: Ag

Term
Each electron that impacts the output phosphor produces about how many light photons?
Definition
1000
Term
What is the size of the output phosphor?
Definition
2.5 cm (1 inch) diameter circle
Term
What is veiling glare and how is it reduced?
Definition
It is caused by light reflecting inside the output window.  The sides of the window are coated in black to absorb the scattered light.
Term
What is coupled to the output screen of the II.
Definition
A light sensitive camera such as an analog vidicon or a solid-state CCD or CMOS.
Term
The amount of light from the output phosphor that gets to the camera is controlled by
Definition
the aperature.
Term
The amount of light reaching the camera is determined by the
Definition
area of the aperature.
Term
f-number
Definition
f = focal length / aperture diameter
Term
Increasing the f-number by one f-stop reduces the amount of light passing through by a factor of
Definition
2
Term
Increasing the gain of the system ______ the exposure rate and the dose, but also ______ the image quality.
Definition
reduces, reduces
Term
Brightness gain
Definition
Product of the electronic gain and the minification gain
Term
Typical electronic gain of an II
Definition
50 - 100
Term
As the electronic magnification increases (decreasing the effective FOV), the minification gain _______.
Definition
decreases
Term
S Distortion
Definition
Spatial warping of an image into an S shape due to stray magnetic fields or the earth's magnetic field.
Term
II's can be replaced by
Definition
flat panel image receptors
Term
Flat panel detectors are made of
Definition
thin film transistors (TFT)
Term
In a flat panel image receptor, each detector element accumulates and stores a charge that is proportional to the _________.
Definition
x-ray flux that is incident on that detector.
Term
Why does a flat panel detector have a higher quantum detection efficiency than an II.
Definition
The II has an aluminum housing and aluminum support layer that absorbs a large fraction of the x-rays.  The flat panel detector only has a carbon fiber cover.
Term
What does AERC stand for?  What did it used to be called?  What is it's purpose?
Definition
Automatic Exposure Rate Control, used to be called ABC - automatic brightness control.  It's purpose is to keep the SNR constant when possible.
Term
What parameters does the AERC control?
Definition
kVp and mA
Term
If the AERC increases kVp it will  ________ subject contrast, but keeps the dose ________.
Definition
reduce subject contrast, but keeps the dose lower.
Term
If the AERC increases mA, is will ________ subject contrast, but also ________ dose.
Definition
increase subject contrast and also increase dose
Term
What trade-off does the AERC make when adjusting for a thicker patient.
Definition
Having more subject contrast but with a higher dose (increased mA) or having less subject contrast but with a lower dose (increased kVp)
Term
Continuous fluoroscopy typically uses what range of tube current?
Definition
0.5 to 6 mA
Term
Cameras display images at what frame rate?
Definition
30 fps
Term
At 30 fps, each image is displayed for how long?
Definition
33 ms
Term
In pulsed fluoroscopy, exposure times range from
Definition
3-10 ms
Term
The shorter exposure times of pulsed fluoroscopy result in
Definition
Reduced blurring from patient motion.
Term
To get the same image quality (same dose) as continous fluroscopy, pulsed fluoroscopy uses
Definition
increased mA
Term
Typical frame rates for pulsed fluroscopy
Definition
30, 15, 7.5, and 3.75 fps
Term
Lowering the frame rate in pulsed fluorscopy does what?
Definition
It reduces the temporal resolution which isn't always needed and it reduces the dose.
Term
What determines the largest field of view?
Definition
The physical size of the II.
Term
Magnification modes yield _______ spatial resolution for a ________ field of view.
Definition
higher, smaller
Term
Magnification is produced by
Definition
focusing a smaller area of the II input screen onto the entire output phosphor
Term
The brightness gain _______ as magnification increases.
Definition
decreases
Term
In mag mode the AERC compensates for reduced brightness gain by increasing the x-ray exposure rate.  The increase in exposure rate is given by
Definition
the ratio of the FOV areas.
Term
To reduce patient dose, one should use the _______ magnification and the ________ collimated area.
Definition
least magnification and the smallest collimated area
Term
Advantage and disadvantage of frame averaging.
Definition

advantage - lower quantum noise

disadvantage - image lag

Term
DSA - what does it stand for and how does it work
Definition
Digital subtraction angiography - a non-iodinated image is subtracted from an iodinated image leaving just the vascular anatomy
Term
In DSA, the anatomy only image is called
Definition
the mask image
Term
What is the benefit of DSA?
Definition
It reduces anatomical noise.
Term
What is a typical range for the intrinsic limiting resolution of a fluoroscopy system?
Definition
3-5 lp/mm
Term
The limiting resolution of a fluoroscopy system is typically limited by
Definition
the video device
Term
The limiting resolution of the fluoroscopy system can be determined by
Definition

FOV (mm) / video display matrix (# of pixels) = Δ mm/pixel,

resolution = 1/(2Δ) cycles/mm

 

Term
What does higher magnification modes do to spatial resolution?
Definition
increases it
Term
Flat panel detectors in fluoroscopy have ________ spatial resolution than II systems?
Definition
similar or better
Term
Contrast resolution in fluoroscopy is ______ compared to radiography due to the _______.
Definition
low, low SNR
Term
What is the compromise for improved contrast resolution?
Definition
increased patient dose
Term
What are the range of Bucky factors and grid ratios for grids used in fluoroscopy?
Definition

Bucky factor: 2-4

Grid ratio: 6:1 to 12:1

Term
Maximum permissible entrance exposure rate for normal and high level fluoroscopy?
Definition

normal - 10 R/min (87.3 mGy/min)

high level - 20 R/min (175 mGy/min)

Term
Where are output exposure rates measured in fluoroscopy?
Definition
1 cm above the table for systems with the tube under the table, 30 cm from the II for C-arms
Term
What is the FDA dose limit for image recording with fluoroscopy?
Definition
There is no limit.
Term
An operator standing 1 m from the patient receives about _________ of the entrance exposure.
Definition
1/1000
Term
The scatter radiation on the entrance side of the patient is about _____ times more than the scatter radiation on the exit side.
Definition
10
Term
Dose monitoring can be done with a ________ meter.  The units are?
Definition
KAP (Kerma-area-product), mGy*cm2
Term
How is skin dose calculated from KAP?
Definition
KAP / exposed area
Term
Fluoroscopy systems manufactured after June 10, 2006 are required to display
Definition
cumulative air kerma (mGy)
Term
For C-arms, the reference point for air kerma rate is
Definition
on the beam axis, 15 cm from the isocenter towards the x-ray tube.
Term
Three reasons why cumulative air kerma may not acurately represent peak skin dose.
Definition
1) CAK doesn't account for backscatter, 2) the beam may be incident on multiple areas of skin, 3) the skin may be closer or farther away from the x-ray tube than the reference point.
Term
8 ways to reduce dose in fluoroscopy besides changing technique
Definition
1) collimate, 2) maximize source to skin distance, 3) remove anti-scatter grids for pediatrics and extremities, 4) use the lowest mag mode possible, 5) vary the location of the entrance field (dose spreading), 6) use low frame rate pulsed fluoroscopy, 7) use last-frame-hold, 8) limit the number of radiographic images
Term
Patient follow-up should occur when cumulative air kerma exceeds
Definition
3-5 Gy
Term

K-edge of Iodine?

Optimal range of kVp for using Iodine contrast?

Definition

33.2 keV

60-80 kVp

Term
Other names for Equalization Filters.  What are they used for?
Definition
Soft filters or soft cones.  They are wedge shaped filters used to taper the radiation profile.
Term
What is a virtual collimator?
Definition
White lines that appear on the last hold image.  The lines indicate where the collimation will occur next time flouro is used.  The results in a dose savings to the patient.
Term

Grid ratios used in interventional systems?

 

Definition
8:1 to 16:1
Term
Instead of an ABC, digital image receptor systems use an
Definition
ADRC - Automatic Dose Rate Control
Term
If the noise of a mask image is σand the noise of a contrast image is σ2 what is the noise of the subtracted image?
Definition

= √(σ12 + σ22)

The subtracted image will be noisier.

Term
TJC defines a sentinel event in flouroscopy as
Definition
a cumulative dose of greater than 15 Gray to a single field
Term
Above what patient thickness should a grid be used.
Definition
10 cm.  The grid should be removed for anatomy that is less than 10 cm thick.
Term
Fifty percent of the patient dose is delivered in the first ___ cm of tissue.
Definition
3-5 cm
Term
Vignetting
Definition
When the image appears brighter in the center than at the edges.  This occurs because the path through the II is farther at the edge of the output phosphor.
Term
Conversion Gain
Definition

A measure of the efficiency of an II in converting incident X-ray radiation into light at the output surface.

Units are Cd/m2 per mR/sec

Term
How is contrast ratio measured for an II?  What is the range of values?
Definition
A lead disk that covers 10-15% of the II is placed on the input surface.  A region adjacent to the disk is compared to the signal behind the disk.  It should be 10:1 to 20:1.
Term
What causes the electronic or flux gain?
Definition
The increase in the kinetic energy of the electrons as they are accelerated from the photocathode to the anode.
Term
What is SRDL?  What document defines it?
Definition

SRDL - Substantial Radiation Dose Level

Defined in NCRP Report No. 168

Term
What are the values for the SRDL's?
Definition

1. 3 Gy Peak skin dose

2. 5 Gy Cummulative Air Kerma

3. 500 Gy·cm2 KAP (assumes 100 cm2 field)

4. 1 hour of fluoro time

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