Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Update on Fluoroscopy Physics
• Understand dosimetric concepts
relating to interventional fluoroscopy
• Characterize the dosimetric features
and performance of a modern
fluoroscope
• Be able to set up a clinical dose
recording and reporting policy that will
meet clinical and JC requirements.
Stephen Balter, PhD
Columbia University
Draft for
MO-A-210A-1 2009 AAPM
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Educational objectives
© S. Balter 2009
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© S. Balter 2009
Outline
Notation
• ICRU diagnostic dosimetric quantities and
their fluoroscopic extensions
• Construction, dosimetric features, and
performance characteristics of modern
fluoroscopes
• Extended QA protocols for compliance
measurements, system characterization and
clinical dosimetry.
• Dose recording and reporting
• The Joint Commission fluoroscopy sentinel
event.
• This presentation uses the notation
given in ICRU-74 (2006)
• The notation has been extended to
encompass additional quantities
specifically related to fluoroscopy
• Organizations that have accepted or
are in the process of accepting this
notation include the IAEA, IEC, NCRP
and NEMA.
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© S. Balter 2009
Air KERMA (Ka)
© S. Balter 2009
From the lab to the clinic
• Measurements are made under lowscatter conditions
• Metric usually used by calibration
laboratories to report instrument
factors.
• Unless the detector is air, there may be
additional corrections needed to
account for instrument construction
and X-ray spectrum.
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© S. Balter 2009
• Ka,i is the air kerma at the location of the
patient’s skin in the absence of the patient.
• Ka,e is the air kerma at the location of the
patient’s skin with the patient present.
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© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Backscatter factors Ka,i → Ka,e
Reference Point Air Kerma (Ka,r)
• Reference point originally defined in
IEC 60601-2-43 (2000)
• Included in 2005 FDA fluoro
regulations.
• Intent is to approximate the location of
the patient’s skin
• Reported as Ka without scatter
• IEC, FDA ± 35 %
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© S. Balter 2009
Reference Point Air Kerma (Ka,r)
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Reference Point ≠ FDA Point
• Total air kerma accumulated at the reference
point from the beginning of the procedure.
(called “cumulative dose” in the regs)
• Displayed to operators at the working
position and in the control room.
• Defined under low scatter conditions.
• Table top and mattress attenuation?
Image
Receptor
30 cm
© S. Balter 2009
Reference point ≈ skin
Isocenter
SID = Any
15 cm
– Not standardized.
– Propose measurement without attenuation unless
the attenuators are always in the beam.
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© S. Balter 2009
IEC & FDA Ka,r
Reference Point
FDA “Dose”
Compliance Point
Focal
Spot
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© S. Balter 2009
Conversion of Ka,r to Dskin
• Dmax,skin is seldom numerically equal to Ka,r
• For teaching purposes, assume that the beam
does not move during the procedure.
Ka,i = Ka,r * (SRD/SSD)2
where SRD is the source to reference point distance
Ka,e = Ka,i * Backscatter f(kVp, filter, field size)
Dskin = Ka,e * (µskin / µair)
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© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Kerma Area Product (PKA)
PKA is independent of distance!
• Total KAP accumulated from the
beginning of the procedure.
• Displayed to operators at the working
position and in the control room.
• Can be used to estimate Ka,r
• “Where should DAP
be measured?”
• Area = a * d2
• Output = k *d-2
(need to know field size at reference point)
• Defined under low scatter conditions.
• Table top and mattress attenuation?
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© S. Balter 2009
• PKA = a * d2 * k*d-2
=a*k
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© S. Balter 2009
Outline
Geometry (generic system)
• ICRU diagnostic dosimetric quantities and
their fluoroscopic extensions
• Construction, dosimetric features, and
performance characteristics of modern
fluoroscopes
• Extended QA protocols for compliance
measurements, system characterization and
clinical dosimetry.
• Dose recording and reporting
• The Joint Commission fluoroscopy sentinel
event.
•
•
•
•
© S. Balter 2009
Radiation technique range
•
•
•
•
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© S. Balter 2009
Table-Top Outputs FP
Ka,i range <1 – 4,000 mGy/min
kVp 60 – 120
Fixed filtration 2 – 6 mm Al
Added Filter 0.0 – 0.9 mm Cu
10000
Chamber 30 cm from FP
Minimum SID
1000
– Brand A : Added Cu fluoro filter independent of
kVp
– Brands B,C: Added Cu fluoro filter tends to
decrease as kVp increases
– All Brands: Less or no Cu for acquisition modes
• New systems do not provide physicist level
control of basic X-ray factors
25NF
20NF
15NF
25CI
20CI
15CI
mGy/min
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Min SSD = 38 cm (<30 cm)
SID 80 cm – 125 cm
SAD = 70 - 80 cm (focus to isocenter)
Table top height - 30 to + 20 cm
(relative to isocenter)
• 110º LAO – 110º RAO
• 45º CRA – 45º CAU
• FS ≈ 0 – 2000 cm2 (at image receptor)
100
10
38 mm Al
+0.5 mm
Cu
+1.0 mm
Cu
+1.5 mm
Cu
+2.0 mm
Cu
+3 mm Cu +4 mm Cu
Attenuator
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© S. Balter 2009
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6 mm Cu +10 mm Cu
0902D
© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Observed values
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Dose rates are down; BUT!!!
© S. Balter 2009
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© S. Balter 2009
Conventional Wisdom
Source to Image Receptor Tracking
• FDA limits normal-mode table-top
outputs to 87 mGy/min (10 R/min)
• Using more magnification increases
patient skin-dose-rate
• Using more magnification increases
limiting spatial resolution
• FDA regulations limit normal mode
fluoroscopy to less than 87 mGy/min
at 30 cm from the image receptor.
• Source to image receptor distance (SID)
is not mentioned in the regulations.
• Most interventional fluoroscopes limit output
to less than 87 mGy/min at any SID
• What happens at the table top when SID is
varied?
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© S. Balter 2009
Minimum SID Table-Top Ka,i
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© S. Balter 2009
Maximum SID Table-Top Ka,i
Minimum SID
Maximum SID
30 cm
Flu 75 mGy/m
Flu 230 mGy/m
Cin 1700 mGy/m
30 cm
Flu 75 mGy/m
Cin 1700 mGy/m
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© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Low Table/Max SID Table-Top Ka,i
Table Top Maximum Air Kerma Rate
Maximum SID
Relative Table Top Output
2.75
30 cm
Flu 75 mGy/m
Flu 270 mGy/m
Cin 2000 mGy/m
Table Top (chamber)
fixed 55 cm from FS
2.25
Fluoroscopy
Table Top Rate
Inverse Square
1.75
Cine
Table Top Rate
1.25
0.75
85
90
95
100
105
110
115
Source to Image Receptor Distance (cm)
Minimum SSD
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© S. Balter 2009
Effect of zoom on EERII
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120
0907 Alpha
© S. Balter 2009
Field of view considerations
• An image intensifier zooms a variable size
input field onto a constant output field.
Unmodified: Dose per frame ≈ 1/ FOV2
Programmed: Dose per frame ≈ 1/ FOV
• A first generation flat-panel electronically
zooms a variable number of pixels onto a
full size image display.
Dose per frame ≈ constant
Dose per frame ≈ 1/ FOV
Dose per frame ≈ 1/ FOV2
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© S. Balter 2009
6
Base 38 Al
5
Normalized Phantom Input (fluoroscopy)
Program
kVp
Beam filter
Dose Rate
Constraint
© S. Balter 2009
Image intensifier zoom
Measured Phantom EER vs. f(FOV)
•
•
•
•
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4
0.5 Cu
1.0 Cu
2.0 Cu
4.0 Cu
8.0 Cu
1/r
1/r2
3
2
1
48 cm
42 cm
32 cm
22 cm
16 cm
11cm
Field of View - Flat Panel
0805 BETA
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© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Pixel size – image intensifier zoom
DR – Indirect Detector
X-ray
Structured X--ray
phosphor (CsI)
Light
S
G
+
D
Photodiode
Storage capacitor
Charge
X-rays to light to electrons to electronic signal:
a - Si TFT/ CsI phosphor
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© S. Balter 2009
Pixel size: flat panel zoom (1st gen)
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© S. Balter 2009
Pixel size: zoom comparison
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Smaller FOV = Fewer Pixels
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© S. Balter 2009
Rebinning
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© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Display
Outline
• Display Processing
• ICRU diagnostic dosimetric quantities and
their fluoroscopic extensions
• Construction, dosimetric features, and
performance characteristics of modern
fluoroscopes
• Extended QA protocols for compliance
measurements, system characterization and
clinical dosimetry.
• Dose recording and reporting
• The Joint Commission fluoroscopy sentinel
event.
– Zoomed FP images are digitally transformed from the
detected pixel matrix to the full display matrix.
– All images are highly processed to enhance perception of
detail.
• Dose per frame is programmed by the system to
standardize the perception of noise.
• These measures are designed to maximize
perception
– High Contrast Objects (small interventional devices < 1 mm)
– Low Contrast Objects (relatively large tissue blush > 5 mm)
• Different acquisition and display optimization is
needed for different clinical procedures.
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© S. Balter 2009
What instrument?
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© S. Balter 2009
Reference Point Locations
• If you are super-fussy (and want to try to
stay below the ICRU-74 tolerance of 7%)
the functionally closer to a free air chamber the
better.
• For most work, energy compensation of solid-state
detectors is adequate
Image
Receptor
30 cm
SID = Any
Absolute accuracy should improve when appropriate
calibration beams are defined and implemented.
• Consider the influence of any instrument on the
fluoroscope’s ADRC.
• Caution: You, your service engineers, and your
regulators may be using different types of
instrumentation.
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© S. Balter 2009
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IEC & FDA Ka,r
Reference Point
FDA “Dose”
Compliance Point
Focal
Spot
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© S. Balter 2009
Compliance
Equipment placed on table top
Attenuators 15 cm above ion chamber
Minimum SID
Adjust table height for 30 cm between
center of chamber to entrance surface
of image receptor
• Collimate to just inside smallest FOV
• Test ‘common’ modes at all FOVs.
•
•
•
•
50%
30 cm
minimum SID
Compliance Setup
Isocenter
15 cm
© S. Balter 2009
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© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Attenuator protocol
•
•
•
•
•
Acquisition modes
• There is reluctance to test acquisition
modes at full output due to concerns
about damaging the fluoroscope.
19 mm Al (children)
38 mm Al (small adult)
38 mm Al + 0.5 mm Cu (medium adult)
38 mm Al + 2.0 mm Cu (large adult)
38 Al + 2 Cu + 3 mm Pb (maximum) or
38 mm Al + 8-10 mm Cu (maximum)
This is reflected in regulatory requirements
• Modern systems are actually clinically
operated at or near full output a
significant fraction of the time.
Maximum outputs are often more than
double the 38 Al + 2 Cu outputs
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© S. Balter 2009
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Clinical kVp in one laboratory
© S. Balter 2009
Typical compliance data
Compliance
Image Intensifier - 17 cm FOV FDA Geometry
Cine n = 1856
Fluoro n = 6594
1000
mGy/min
110
FL
FN
CL
CN
100
100
kVp
10
fKVP
cKVP
90
Display = Cine actual/2, fC actual
mGy/min
10000
120
1000
100
10
cine S
fluoro
1
0
1
38 Al
+ 0.5 Cu
+ 1 Cu
+ 2 Cu
+ 4 Cu
+ 8 Cu
+ 8Cu+3Pb
Attenuator
2
4
6
8
10
38 mm Al + X mm Cu
80
Flat Panel Detector
Image Intensifier
70
60
1%
5%
10%
20%
25%
30%
40%
50%
60%
70%
75%
80%
90%
95%
Both have variable Cu beam filters
99%
Percentile
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© S. Balter 2009
“Dose” instrumentation
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© S. Balter 2009
Dose displays in the lab
• Locations
– Integrated
– Interfaced
– External
• Measurement technologies
– None – Calculated from settings
– PKA chamber + calculation
– Ka,r chamber + calculation
– PKA & Ka,r dual channel chamber
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The same RPDose will display as 5678
on all three systems
The same KAP will display as
12345 on GE and SIEMENS and 123450 on PHILIPS
© S. Balter 2009
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© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Verification of displayed “dose”
Stability of Ka,r instruments
• Accuracy of display
AVE
– IEC ± 50% (RPDose &KAP)
– FDA ± 35% (RPDose)
– Stability usually better
A
B
C
DA
DB
E
F
GB
GA
• Usually validated at factory
• Seldom validated by installers
• When verified as part of QA
constancy should be ± 5%
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© S. Balter 2009
Ka,i and PKA at isocenter
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0.84
0.85
0.85
0.87
0.88
0.91
0.98
1.17
1.18
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© S. Balter 2009
Field size measurement
© S. Balter 2009
Outline
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© S. Balter 2009
Skin dose map and beam position
• ICRU diagnostic dosimetric quantities and
their fluoroscopic extensions
• Construction, dosimetric features, and
performance characteristics of modern
fluoroscopes
• Extended QA protocols for compliance
measurements, system characterization and
clinical dosimetry.
• Dose recording and reporting
• The Joint Commission fluoroscopy sentinel
event.
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STDEV
Jul-Dec 05Jan-Jun 06Jul-Dec 06Jan-Jun 07Jul-Dec 07
0.03
0.85
0.82
0.80
0.84
0.89
0.04
0.82
0.82
0.84
0.89
0.89
0.04
0.85
0.80
0.85
0.88
0.89
0.03
0.85
0.82
0.86
0.90
0.90
0.04
0.85
0.86
0.86
0.93
0.92
0.01
0.89
0.91
0.91
0.93
0.92
0.06
0.89
0.99
0.96
1.04
1.02
0.05
1.09
1.16
1.21
1.19
1.18
0.04
1.14
1.13
1.22
1.21
1.18
© S. Balter 2009
Illustration courtesy of Siemens
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© S. Balter 2009
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
What can be measured in the lab?
Dose displays in the lab
• Fluoroscopy time
• Number of acquisition
– Runs
– Frames
•
•
•
•
Kerma Area Product PKA
Reference Point Air Kerma Ka,r
Peak Skin Dose
Dose maps
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The same RPDose will display as 5678
on all three systems
The same KAP will display as
12345 on GE and SIEMENS and 123450 on PHILIPS
© S. Balter 2009
In the control room
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© S. Balter 2009
SIR Standard of Practice
• Record all available data for every
interventional procedure
• Order of Precedence
–
–
–
–
Skin dose map (not currently available)
Reference Point Air Kerma Ka,r
Kerma Area Product PKA
Fluoro time and number of images
• Uses for the data
–
–
–
–
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© S. Balter 2009
Patient pre discharge instruction
Dose reconstruction when necessary
Clinical QA
Physics QA against guidance levels
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© S. Balter 2009
A New IHE Profile
Manual log
IHE Radiation Exposure Monitoring Profile
 Integration of systems reporting dose and systems
which receive, store, or process those reports
 Modalities, PACS, RIS, Workstations, Registries
• Technician hand writes data to log
• Data retyped into clinical records or dose
data base
• Time lags and overflow can be problems
• Process is error prone
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© S. Balter 2009
 Facilitate compliance with Euratom 97/43,
ACR Guidelines, etc.
 Directly based on DICOM Dose Reports
 Creation, Collection, Distribution, Processing
IHE 2008 Webinar Series
© S.Balter 2009
Update on Fluoroscopy Physics AAPM MO-A-210A-1
Stephen Balter, Ph.D.
Outline
JC Radiation Sentinel Event
• ICRU diagnostic dosimetric quantities and
their fluoroscopic extensions
• Construction, dosimetric features, and
performance characteristics of modern
fluoroscopes
• Extended QA protocols for compliance
measurements, system characterization and
clinical dosimetry.
• Dose recording and reporting
• The Joint Commission fluoroscopy sentinel
event.
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© S. Balter 2009
MedPhys list – May 2007
• “A facility without a full-time
physicist needs a quick,
easy number that they can
refer to, so we use fluoro
time, as archaic and
inaccurate as it might be.
• If you want to know if your
patient received more than
the threshold for skin
injuries, look at the patient's
skin, not the KAP
meter or fluoro time!”
Etiology not identified for
more than one year
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An unexpected occurrence involving death or serious physical or
psychological injury, or the risk thereof. Serious injury specifically
includes loss of limb or function. The phrase “or the risk thereof”
includes any process variation for which a recurrence would carry a
significant chance of a serious adverse outcome. Such events are
called “sentinel” because they signal the need for immediate
investigation and response.
• Prolonged fluoroscopy with cumulative
dose >1500 rads to a single field
or any delivery of radiotherapy to the wrong body region or >25%
above the planned radiotherapy dose
• Can you demonstrate to the surveyors
that Sentinel Events did not occur?
• How to be accurate enough? (not easy)
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© S. Balter 2009
What was covered
Fluoro 35 minutes
Cine 12 minutes
© S. Balter 2009
• ICRU diagnostic dosimetric quantities and
their fluoroscopic extensions
• Construction, dosimetric features, and
performance characteristics of modern
fluoroscopes
• Extended QA protocols for compliance
measurements, system characterization and
clinical dosimetry.
• Dose recording and reporting
• The Joint Commission fluoroscopy sentinel
event.
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© S. Balter 2009
© S.Balter 2009