Transcript RHB Rad Prot & Fluoro Syllabus

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Fluoro Equipment
Review
RHB Fluoro Syllabus
&
Rad Prot
Review #1 255
June 2008 ccrt55 revised
History of fluoroscopy
• Thomas Edison invented the fluoroscope
in 1896
• He was the first one to try to image the
brain
• Fluoroscopy was performed in total
darkness so the eyes had to be adjusted
for 30 minutes by wearing red goggles
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General Fluoroscopic Concepts
1. Primary factors associated with fluoroscopy
2. High-level fluoroscopic procedures
3. Pulsed fluoroscopic procedures
4. Scatter, magnitude, directionality & leakage
radiation
5. Dose trade offs versus quantum mottle
6. Potential image distortions
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Basic “Imaging Chain”
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Basic Componets of “old”
Fluoroscopy “Imaging Chain”
Primary
Fluoro
TUBE
EXIT
Radiation
PATIENT
Radiation
Cassette
105
Photospot
Image
Intensifier
ABC
Fiber Optics
OR
Image
Recording
Devices
CINE
VIDICON
CONTROL
Camera Tube
UNIT
TV
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Basic Componets of “NEW
DIGITAL” Fluoro“Imaging Chain”
Fluoro
TUBE
Primary
Radiation
EXIT
Radiation
PATIENT
Analog to
Image
Intensifier
ABC
CCD
Digital
TV
Converter
ADC
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Modern fluoroscopic system
components
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The image intensifier (I.I.)
I.I. Input Screen
Electrode E1
Electrode E2
Electrode E3
I.I.Output Screen
Photocathode
+
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Image Intensifier
• VACUUM TUBE
• ENCASED IN A
LEAD HOUSING
• = 2MM PB
• (PRIMARY
BARRIER)
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Cesium Iodide (CsI) Phosphor
on Input Phosphor
CsI crystals grown linear
and packed closely
together
The column shaped
“pipes” helps to direct
the Light with less
blurring
SIDE VIEW
Converts x-ray photons to
visible light
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Input phosphor
• X-rays hit this concave surface first.
• It is made of cesium iodide crystals shaped like
tiny needles packed tightly together.
• The more crystals, the better the spatial
resolution.
• The crystals convert the x-ray photons to light
energy so the more crystals, the more energy
converted to light the less radiation needed so
patient dose goes down.
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photocathode
• The photocathode is close to the input phosphor
but can’t touch it otherwise there would be a
chemical reaction that would destroy the
phosphor.
• The photocathode is made of antimony and
cesium compounds. When these compounds
come into contact with light they emit electrons
• The photocathode has the job of turning the light
into photoelectrons.
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Veiling glare
•
Scatter in the form of
x-rays, light &
electrons can
• reduce contrast
• of an image intensifier
tube.
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Functioning of Image
Intensifier
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Intensifier Format and Modes
Note focal point
moves farther
from output in
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mag mode
BG = MG X FG
• FLUX GAIN – increase of light brightness
due to the conversion efficiency of the
output screen
• 1 electron = 50 light photons is 50FG
• Can decrease as II ages
• Output phosphor almost always ____ inch
• Made of _____________phosphor
• Flux gain is almost always 50
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Raster pattern
scan lines
Video Field
Interlacing
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Horizontal resolution
• The number of dots on the horizontal scan
line.
• How close together are they?
• It is the product of scan lines, frame rate
and frequency rate
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Vertical resolution
• How far apart are the horizontal scan
lines? Since we can’t have more than 525
scan lines, we can have more dots that
are smaller
• The Kell factor is a component of vertical
resolution. Memorize the number 0.7 and
know that it is the Kell factor & part of
vertical resolution
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Viewing Fluoroscopic Images
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Image Quality
•
•
•
•
Contrast
Resolution
Distortion
Quantum mottle
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Contrast
• Controlled by amplitude of video signal
• Affected by:
– Scattered ionizing radiation
– Penumbral light scatter
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Resolution
• Video viewing
– Limited by 525 line raster pattern of monitor
– Newer digital monitors 1024 - better
resolution
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Size Distortion
• Affected by same parameters as static
radiography
– Primarily OID
– Can be combated by bringing image
intensifier as close to patient as possible
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Shape Distortion
• Geometric problems in shape of input
screen
– Concave shape helps reduce shape
distortion, but does not remove it all
– Vignetting or pin cushion effect
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Image distortion
PINCUSHION
EFFECT
PINCUSHION
?? WHAT IS VIGNETTING??
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Image Quality
• Terms that are necessary to know:
– Vignetting is the loss of brightness at the periphery of
the II due to the concave surface
– Pincushion effect is the drop off at the edges of the II
due to the curved surface
– Quantum mottle is the grainy appearance on the
image due to statistical fluctuations
– The center of the II will always have the best
resolution.
– Lag is the blurry image from moving the II too fast
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Regulations about the operation
• Fluoroscopic tubes operate at currents
that range from0.5 to 5 mA with 3 - 4 the
most common
• AEC rate controls: equipment built after
1974 with AEC shall not expose in excess
of 10 R/min; equipment after 1974 without
AEC shall not expose in excess of 5 R/min
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Other regulations
• Must have a dead man switch
• Must have audible 5 min. exposure timer
• Must have an interlock to prevent exposure
without II in place
• Tube potential must be tested weekly
• Brightness/contrast must be tested annually
• Beam alignment and resolution must be tested
monthly
• Leakage cannot exceed 100mR/hr/meter
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more regulations
• Must have a device to prevent operation at a
SOD of less than 12”
• A bucky slot cover must be provided
• Aprons must be at least 0.25 mm Pb equivalent
• 2.5 mm Al equivalent filtration is required
• Must provide at least 12” and preferably 15”
between source and table top
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Fluoroscopy exposure rate
• For radiation protection purposes the
fluroscopic table top exposure rate must
not exceed 10 mR/min.
• The table top intensity should not exceed
2.2 R/min for each mA of current at 80 kVp
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Patient Protection
1. A 2 minute UGI results in an exposure of
approximately 5 R!!
2. After 5 minutes of fluoro time the exposure is
10-30 R
3. Use of pulsed fluoro is best (means no matter
how long you are on pedal there is only a short
burst of radiation)
4. ESE must not be more than 5 rads/min
5. W/0 AEC 10 WITH & 20 BOOST
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• Always keep the II as close to the patient
as possible to decrease dose
• Highest patient exposure happens from
the photoelectric effect (absorption)
• Boost control increases tube current and
tube potential above normal limits
– Must have continuous audible warning
– Must have continuous manual activation
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ESE FOR FLUORO
• TLD PLACED AT SKIN ENTRACE
• POINT for fluoro
• 1 – 5 R/MINUTE AVE IS
________R/MIN
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ESE FOR FLUORO
•
•
•
•
TLD PLACED AT SKIN ENTRACE POINT
1 – 5 R/MINUTE AVE IS 4 R/MIN
(now closer to 1 – 3 R/min)
When not known – assume 4 R
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Framing and patient dose
syll = Pg 31 which is BEST
• The use of the available film area to
control the image as seen from the output
phosphor.
– Underframing
– Exact Framing, (58 % lost film surface)
– Overframing,(part of image is lost)
– Total overframing
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EXPOSURE RATES FLUORO
• MA IS 0.5 MA TO 5 MA PER MIN
• AVE DOSE IS 4 R / MIN
• IF MACHINE OUTPUT IS 2 R/MA/MIN =
WHAT IS PT DOSE AT 1.5 MA FOR 5
MIN STUDY?
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DOSE REGULATIONS
• BEFORE 1974 - AT TABLETOP
• 5R/MIN (WITHOUT AEC)
• 5R/MIN (WITHOUT AEC) – BOOST
MODE
• After 1974 with AEC
• 10 R/MIN
20R/MIN BOOST
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ISOEXPOSURE CURVES
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Where should a dosimeter
badge be worn?
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RHB “RULES” RHB RP PG61
• LICENTIATES OF THE HEALING ARTS
(MD, DO, DC, DPM)
• MUST HAVE A
• RADIOLOGY SUPERVISOR &
OPERATORS PERMIT & CERTIFICATE
• TO OPERATE OR SUPERVISE THE USE
OF X-RAYS ON HUMANS
• SUPEVISORS MUST POST THEIR
LICENSES
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RHB “RULES” RHB RP PG62
• ALL XRAYS MUST BE ORDERED BY A
PHYSICIAN
• VERBAL OR WRITTEN PRESCRIPTION
• See Section C – “Technologist
Restrictions”
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Declared Pregnant Worker
• Must declare pregnancy – 2 badges provided
• 1 worn at collar (Mother’s exposure)
• 1 worn inside apron at waist level
Under 5 rad – negligible risk
Risk increases above __________rad
Recommend abortion (spontaneous) 25 rad
• (“Baby exposure” approx 1/1000 of ESE
• 1/50 th dose of mother***)
•
www.ntc.gov/NRC/RG/08/08-013.html
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CARDINAL RULES
• TIME
• DISTANCE
• SHEILDING
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RHB – Rad Prot – CH. IX p 51
• ALARA (no minimum threshold)
• STOCHASTIC EFFECTS –
NON TRESHOLD (CA + GENETIC)
• NON STHOCAHSTIC (DETERMINISTIC)
SEVERITY OF EFFECTS VARIES WITH
RADIATION DOSE (THRESHOLD)
(CATARACTS, SKIN, BONE MARROW,
STERILITY
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#7
– Dose Response Relationships
LINEAR NON THRESHOLD
• ASSUMES ANY AMOUNT OF
RADIATION IS CAPABLE OF CAUSING
A BIOLOGIC RESPONSE
• THE RELATIONSHIP BETWEEN THE
RADIATION DOSE AND BIOLOGIC
RESPONSE IS CONSIDERED TO BE
DIRECTLY PROPORTIONAL
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