8/12/2011
Call to Action!
Calculating Patient-specific Skin Dose
William Pavlicek PhD
Dept of Radiology
Mayo Clinic Arizona
pavlicek.william@mayo.edu
©2011 MFMER | slide-1
Fluoroscopy time – ‘poor’ measure of PSD
©2011 MFMER | slide-2
Outline
1. Peak Skin Dose Defined
2. Conditions Likely to Result in High PSD
3. Calculating PSD with (some) DICOM info
4. Informatics – Tools for PSD (Wednesday)
5. Take away points
300 patients
©2011 MFMER | slide-3
©2011 MFMER | slide-4
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8/12/2011
1. DEFINE ‘Peak’ Skin Dose
Fluoroscopic/Acquisition Exposure
• Air Kerma
• Kerma accumulated in air at a specific point
• Ka,i – incident air kerma *
• Units: gray (Gy)
• Without backscatter
• Ka,ref commonly computed from PKA
* ICRU Report 74, “Patient Dosimetry for X Rays Used in
Medical Imaging”, 2005.
• Dskin,max –the most highly-irradiated skin region
Compute Dskin from Ka,i:
• Validated Vendor Ka,r
• Backscatter
• Mass-energy absorption coefficient ratio
tissue:air
• Table and pad attenuation
• Equipment/patient specific information
• Account for locations of the x-ray beam/patient
• Actual source-skin entrance distances
©2011 MFMER | slide-5
C-arm Reference Point Location
©2011 MFMER | slide-6
Validate vendors Ka,r
• Ka,r – total air kerma at
the reference point
• 15 cm from isocenter
toward x-ray tube
• Other reference points
exist
• Computed by vendor
from PKA
Isocenter
• Reference point ~
patient’s entrance
surface
• Defined by IEC (2000
and 2008) and FDA
(2009)
©2011 MFMER | slide-7
©2011 MFMER | slide-8
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8/12/2011
Correction factor for Acquisition exposure
(fluoro correction may differ!)
Field size
PMMA
kV
Air kerma (mGy)
Machine
Air kerma (mGy)
Chamber
Correction Factor
22
4 inches
69.98
148
125.328
0.85 = 125.328/148
22
4 inches
70
148
125.41
0.85
22
8 inches
80.66
608
511.049
0.84
22
12 inches
110.34
819
680.623
0.83
32
6 inches
67.15
108.98
87.976
0.81
32
8 inches
80.36
275.78
237.515
0.86
32
8 inches
82.74
644.67
556.161
0.86
32
10 inches
86.13
710.48
608.873
0.86
32
12 inches
94.67
718
601.748
0.84
42
8 inches
73.4
140.2
114.251
0.81
42
10 inches
77.36
581.42
505.795
0.87
42
12 inches
77.92
664.68
573.753
0.86
42
12 inches
88.69
669
573.687
0.86
Measure Table and Pad Attenuation*
kV
M2o
(mGy)
M2i
(mGy)
96
115.00
83.01
81
105.1
73.32
71
90.10
61.20
Table/Pad
Transmission
Factor
Correction
Factor to
be applied
to DSR
0.721826 = 83.01/115
0.721826
0.697621
0.697621
0.679245
0.679245
*fixed technique – service
mode
©2011 MFMER | slide-9
Backscatter Correction (ICRU)
Tube
voltage/kV
Example – Simple Geometry
Field 10cm x
10cm
Field 20cm x
20cm
Field 25cm x
25cm
Filte
r
HVL/mm
AI
ICRU tissue
ICRU tissue
ICRU tissue
50
2.5
1.74
1.25
1.27
1.28
60
2.5
2.08
1.28
1.32
1.32
70
2.5
2.41
1.31
1.36
1.36
70
3.0
2.64
1.32
1.37
1.38
3.96
1.39
1.47
1.47
2.78
1.33
1.39
1.39
3.04
1.34
1.40
1.41
4.55
1.40
1.50
1.51
3.17
1.34
1.41
1.42
3.45
1.36
1.43
1.44
5.12
1.41
1.51
1.53
3.24
1.34
1.41
1.42
70
3.0
80
2.5
80
3.0
80
3.0
90
2.5
90
3.0
90
3.0
100
2.5
+0.1 mm
Cu
+0.1 mm
Cu
+0.1 mm
Cu
©2011 MFMER | slide-10
Ka,r = 5 Gy
Machine correction = 0.85
BSF = 1.4
Tissue/Air = 1.06
Table/Pad = 0.7
Ref Pt/Skin = ‘0’
PSD = 4.4 Gy
Both Fluoro AND
Acquistions are
localized to one area!
Mass Energy Absorption Coefficient: Tissue/Air 1.06
McParland BJR, 71(1998)
©2011 MFMER | slide-11
©2011 MFMER | slide-12
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PSD versus Ka,r
Physics Notes
• Under table fluoroscopy is most common gantry
PSD vs. Air kerma
orientation
4.5
y=x
4
• Backscatter correction ~ negates table/pad
3.5
correction
PSD (Gy)
3
y = 0.592x + 0.013
2.5
• Vendor Ka,r commonly higher than your
2
measurements*
1.5
1
• Localization takes some effort
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Air kerma (Gy)
* nominal & x-ray field size agree
13
©2011 MFMER | slide-13
Nominal/X-ray
Field Agreement
MU
WA
HI
KA
Source to Detector (mm)
1199
1010
1012
1100
Image Area Dose Product
(dGy*cm 2)
7
6
1
166
320
Intensifier Size (mm)
480
420
480
Imager Pixel Spacing (mm)
0
0
0
0
Collimator Left Vertical Edge
6
7
229
59
Collimator Right Vertical Edge
954
955
731
661
Collimator Upper Horizontal
Edge
78
7
8
1
Collimator Lower Horizontal
Edge
1162
955
1232
720
Distance Source to Isocenter
635
635
635
635
Number of Vertical Pixels
948
948
502
602
Number of Horizontal Pixels
1084
948
1224
719
Vertical Distance (mm)
292
292
155
Horizontal Distance (mm)
334
292
377
221
Image Area (mm 2)
97485
85255
58289
41061
Image Area at Reference Point
(mm 2)
27343
33699
22949
13683
0
0
0
0
©2011 MFMER | slide-14
Validate Ka,r
• Tableside/Console shows Ka,r (sums Fluoro and
Acq, continuously updated)
Exposed Area at Reference
Point (m 2)
Exposed Area at Reference
Point (mm 2)
26071
32081
21455
% Error
4.88%
5.05%
6.97%
185
• Console shows Exam Protocol Ka,r for acquisitions as
acquired.
• At completion, the Exam Protocol shows separate
summed Fluoro and Acq Ka,r
• PACs ‘may’ show Exam Protocol (screen capture)
• DICOM RD SR shows Fluoro and Acq Ka,r, but need a
report reader
*Acquisitions: Digital Spots, Cine, DSA (Rot Angio only in Exam
Protocol)
12699
7.75%
©2011 MFMER | slide-15
©2011 MFMER | slide-16
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Complicated Procedures > 3 Gy
2. Conditions for High(er) PSD
• Long procedure – due to complicated episode
• Lateral and steep angle projection procedures
• Large patient (~125+ Kg)
• Extended/high radiation use due lack of training
• High dose Protocols set as ‘default’
Procedures
Percent
Renal Angioplasty (156)
8
Liver Chemoembolization (126)
10
TIPS (135)
21
GI Bleed Dx/Rx (94)
30
Uterine Fibroid Embolization (90)
30
Head Embolization (382)
61
Spine Embolization (24)
92
from Rad-IR Study Table 5, JVIR v14, June 2003
©2011 MFMER | slide-17
©2011 MFMER | slide-18
Skin Dose ‘angled views and patient size’
Steep angles used with cardiac cath
(470 patients) Ken Fetterly, RSNA 2010
©2011 MFMER | slide-19
©2011 MFMER | slide-20
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8/12/2011
Approx 5% Cardiac Cath pts >125 kg
Magnification - use (only) when needed!
Dose Response with 20cm Acrylic
800
Female
6000
Skin Dose Rate (mR/min)
Male
600
400
200
5000
Low
Normal
4000
High
3000
2000
1000
0
19
17
13
10.5
8
7
6
FOV Diagonal SIze (in)
0
16
18
20
22
24
26
28
30
32
34
36
38
Water-equivalent Thickness (cm)
40
42
©2011 MFMER | slide-21
©2011 MFMER | slide-22
Poor Protocol: High DSA and Fluoro Rate
Collimation Is Key!
< 1 Gy
1 -2 Gy
2 - 3 Gy
3 - 4 Gy
4 - 5 Gy
5 - 6 Gy
6- 7 Gy
7-8 Gy
8-9 Gy
9-10 Gy
23
©2011 MFMER | slide-23
©2011 MFMER | slide-24
6
8/12/2011
Vas Aorta
Arch / Carotids
Upper Ext
Chemo Embo
Vas Arch / Carotids
Vas Upper Ext
Protocol Names:
Variability
of quality!
Mesenteric
Pulmonary – enemy
Pulmonary
Unit 1
VasIRMesenteric
Unit 2
VasIRPulmonary
Unit 3
VasIRPulmonary
Upper Extremities
Mesenteric
Tube Check
Vas Upper Extremities
Vas Mesenteric
Aorta
Pelvis / Peri
Upper Extremity
Vas Aorta
Vas Pelvis
Vas Upper Extremity
Mesenteric
Renal
Lower Extremity
Vas Mesenteric
Vas Renal
Vas Lower Extremity
Chemo Embo
IVC
Renal
Renal
Vas IVC
Vas Renal
Vas Renal
CO2 / Gad
DSA
Pulmonary
TIPS
Vas IVC
Vas Pulmonary
Fistulogram
CO2 / Gado
Neuro
Vas Fistulogram
Pelvis
Vertebroplasty
Tube Check
Vas Pelvis
• DICOM Dose Structured Reports
• Contains Fluoro, Acquisitions, Rot Angio, etc.
• Reports are separate ‘Objects’ – may or may
IVC
not be sent to PACs
Perc Neph Dyna
IVC
Perivision
Endoleaks Dyna
Vas IVC
Vas Perivision
DynaCT Body
PICC Line
Singel Leg
InSpace 3D
Catheter Placement
Vas Single Leg
DynaCT Head
Tube Placement
General Dyna
Cardiac
Vertebroplasty
4. Informatics – Tools for PSD Calculation
• Multiple vendors offering dose ‘reports’
• More Physics tools needed
Vertebroplasty
Cardiac OLD
GenDYNA
DSA 0
2D/3D Calib
BILATERAL PERI
Physic's Testing
©2011 MFMER | slide-25
DICOM RD SR
Patient having 287 separate ‘events’
DICOM RDSR
Tag(Group,Element)
NAME
0040,A730-0040,A3000040,A30A
Dose Area Product Total (Gym2)
0040,A730-0040,A3000040,A30A
Dose (RP) Total (Gy)
0040,A730-0040,A3000040,A30A
Fluoro Dose Area Product Total
(Gym2)
0040,A730-0040,A3000040,A30A
Fluoro Dose (RP) Total
0040,A730-0040,A3000040,A30A
Total Fluoro Time(s)
0040,A730-0040,A3000040,A30A
Acquisition Dose Area Product Total
(Gym2)
0040,A730-0040,A3000040,A30A
Acquisition Dose (RP) Total
©2011 MFMER | slide-26
Value
0.0124
0.48313
0.00614
0.2267
2504
0.00624
0.2564
0040,A730-0040,A7300040,A124
Irradiation Event UID
1.2.556.8268.402.12
00…
0008,1030
Performing Physician's Name
Physician
©2011 MFMER | slide-27
©2011 MFMER | slide-28
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DICOM & Info
FLUORO
USE
Primary angle
Table
Table
Secondary
Longitudinal Lateral
angle
Position Position
Table
Height
PERP
(mGy)
Fluoroscopy
Event 1
-17
4.5
-21.8
672
130
1.3
Fluoroscopy
Event 2
-0.2
4.6
-28.2
612
130
3.4
0.9
4.4
-23.1
583
130
6.3
Fluoroscopy
Event 4
32.7
4.8
-23.1
576
130
13.3
Acquisition
Event 1
-0.2
4.6
-9.6
640
130
179.7
Acquisition
Event 2
0.9
4.4
-0.6
702
130
78.7
0.9
4.4
-27.9
568
130
312.0
32.7
4.8
-23.1
576
130
358.5
Fluoroscopy
Event 3
Local dose
mGy @ skin
System Architecture
DICOM
HL7
DICOM SR
Exam
Information Database
DICOM, HL7 Gateway
RIMS
Dose
Tracking
System
WAN
DSA
USE
Knowledge Base
Dosimetry Analyzer
Reporting &
Alerting Module
Emails
Acquisition
Event 3
Acquisition
Event 4
Texts
©2011 MFMER | slide-29
Informatics for FGI
Reporting Tools
• Accessible online
• Need statistics, charts and data view
• Auto-formatted, ready to distribute
©2011 MFMER | slide-30
ID
Gender
Procedure
KAP/air kerma
(cm2)
Number of
the scans
Number of
Fluoro
Peak skin
dose
Total area
radiated
(cm2)
Test
f
IR Emobalization
(non-Neuro)
245.7
180
162
4.3Gy
1068
Triggers and Alert Tools
• db monitoring
• Messages sent by email/ text messaging
Table: 0.8; Machine : 0.8; BSF: 1.4 TAR:1.06
©2011 MFMER | slide-31
32
©2011 MFMER | slide-32
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QA Report: PSD Exceeding 3Gy
PATIENT_NAME
PATIENT_ID
M/F
AGE
ACCESSION
DICOM_EXAM_ID
PSD
EXAM_DATE
TESTING^JOE
55505479
M
73
555063793-1
1.2.124.113532…
3.12
2001.01.03
TESTY^SMITH
55500348
M
84
555028234-1
1.2.124.113532…
3.06
2001.01.04
PHANTOM^JANE
55503229
M
76
555075490-1
1.2.124.113532…
3.24
2001.01.05
TESTT^JOE
55502419
M
71
555051025-1
1.2.124.113532…
3.77
2001.01.06
PHANTOMS^SMITH
55509381
M
68
555069339-1
1.2.124.113532…
3.71
2001.01.07
TESTS^TEST
55507826
M
82
555085478-1
1.2.124.113532…
3.16
2001.01.08
TESTIE^JOE
55509396
F
89
555048986-1
1.2.124.113532…
5.63
2001.01.09
©2011 MFMER | slide-33
©2011 MFMER | slide-34
Take Away
• Know/validate Fluoro and Acquisition Ka,r
• Measure Table/Pad Transmission
• Promote software upgrades to DICOM
Radiation Dose SR
• Review ‘default’ Protocol settings for high F/s
(DSA, Spots, or cine) or Fluoro Pulse/s
• Encourage collimation, not magnification
• Embrace Informatics!
©2011 MFMER | slide-35
©2011 MFMER | slide-36
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Basic References
Team Members and I ‘Thank you’!
Yasaman
Khodadadegan
Dr. Steve Langer,
MCR
©2011 MFMER | slide-37
Dr. Brian
Chong, MCA
Dr. Beth Schueler,
MCR
Dr. Eric Huettl,
MCA
Dr. Richard Morin,
MCJ
Gene
Paden, MS., MCA
Dr. Kevin
Nelson, MCJ
Stephen Sabyan,
R.T. (R), MCA
Dr. Teresa Wu,
ASU
Dr. Kenneth
Fetterly, MCR
Dr. Muhong
Zhang, ASU
©2011 MFMER | slide-38
10