CT Dose Metrics and Current Dose
Methods
Sue Edyvean: Medical (Radiation) Dosimetry Group, CRCE, PHE, Chilton, UK
sue.edyvean@phe.gov.uk
IPEM Dosimetry in Diagnostic Radiology
IPEM Dose in DR_Oct2018_SE
4 Oct 2018
Patient Dosimetry in CT
•
•
•
•
•
Introduction
Technology and dose distributions
‘Physics Metrics’
Understanding and use of these metrics
Monte Carlo - Calculation of organ doses and
Effective Dose
IPEM Dose in DR_Oct2018_SE
Scanner Radiation Output to Patient Specific Dosimetry
Air
Air
kerma
Regular shape,
homogenous
Anthropomorphic
--------------------------------Numerical/ Stylised/Voxel
Anthropomorphic phantom –
estimate of ‘dose’ taking
‘closer’ to ‘human body’
into account quality of
beam
Stylised, numerical, voxel phantoms
~‘Surrogate organ
dose’
Acceptance, QC,
optimisation
patient
Patient specific
dosimetry
Organ doses , Effective dose
IPEM Dose in DR_Oct2018_SE
Patient
Scanner Radiation Output to Patient Specific Dosimetry
Air
Air
kerma
Regular shape,
homogenous
Anthropomorphic
--------------------------------Numerical/ Stylised/Voxel
estimate of ‘dose’ taking
into account quality of
beamMeasurements (ion chambers, TLD, solid state diodes)
Stylised, numerical, voxel phantoms
~‘Surrogate organ
Calculations (Monte Carlo)
dose’
Acceptance, QC,
optimisation
patient
Patient specific
dosimetry
Organ doses , Effective dose
IPEM Dose in DR_Oct2018_SE
Patient
Patient Dosimetry in CT
•
•
•
•
•
Introduction
Technology and dose distributions
‘Physics Metrics’
Understanding and use of these metrics
Monte Carlo - Calculation of organ doses and
Effective Dose
IPEM Dose in DR_Oct2018_SE
The CT scanner – multi-slice
fan beam -> X-ray cone beam
Y
Y
X
Image
Slice
width
Y
X
X
Z
Z
Z
Typical detector length ~ 40 mm
(range 20-160 mm)
IPEM Dose in DR_Oct2018_SE
Picture courtesy of K. Gelijns, Leiden
MSCT dose distribution in Scan Plane
higher
lower
Periphery to centre ratio:
Body ~ 2:1
IPEM Dose in DR_Oct2018_SE
Head ~ 1:1
MSCT dose distribution in Scan Plane
0
lower
0
lower
IPEM Dose in DR_Oct2018_SE
Kalendar
0.4
higher
0.6
higher
Courtesy Mika Kortesienmi
IPEM Dose in DR_Oct2018_SE
MSCT dose distribution in Z-Axis
Z-Axis
higher
lower
xy plane
IPEM Dose in DR_Oct2018_SE
Complex dose distribution
MSCT dose
distribution in Z-Axis
MC simulated dose map for a
helical scan
Courtesy Mika Kortesienmi
IPEM Dose in DR_Oct2018_SE
Absorbed dose from CT Exam
• Calculating the conventional way…CTDI
• …pencil chamber is used to measure dose from single
rotation including some of the scatter tails
IPEM Dose in DR_Oct2018_SE
Courtesy M.A. Lewis
CTDI100
10 cm
Collect
10 mm beam
of dose in 100 mm
(lose ~ 40%)
Dose collection
distance
Dose profile (log scale) at
centre of body phantom
Courtesy John Boone
IPEM Dose in DR_Oct2018_SE
Dose distribution in CT
Dose profile from one slice
IPEM Dose in DR_Oct2018_SE
CTDI - general
A descriptor telling about the
type of CTDI
(integration length, or medium measured in)
The dose profile
+L / 2
1
D (z) dz
CTDI L =
∫
( N × T ) -L/ 2
The nominal
beam width
IPEM Dose in DR_Oct2018_SE
Integral limits – how
much dose we collect
from the dose profile
CTDI100
A descriptor telling about the
type of CTDI
(integration length, or medium measured in)
The dose profile
+50
+L
/2
1
D (z) dz
=
CTDI 100
L
∫
( N × T ) --50
L/ 2
The nominal
beam width
IPEM Dose in DR_Oct2018_SE
Integral limits – how
much dose we collect
from the dose profile
CTDI100
• 100 mm long ion chamber used
• One ‘slice’ scanned (n.T = beam width)
• The dose from the profile is collected over 100 mm
• That value is divided by the nominal beam width
+50
1
D (z) dz
CTDI 100 =
∫
( N × T ) - 50
CTDI100 = integral dose 100 mm
nominal beam width
measured dose
IPEM Dose in DR_Oct2018_SE
100 mm
CTDIair
• Very little scatter
• Used for QC
• Used to scale normalised organ dose
datasets (MC generated) in a dose
calculator (NRPBSR250, ImPACT calculator)
dose
profile
Chamber stand
z - axis
electrometer
Scanner couch
IPEM Dose in DR_Oct2018_SE
Ion chamber (Lc)
‘gap’
Weighted CTDI (CTDIw)
• Weighted Computed Tomography Dose Index (CTDIw)
– CTDI100 measured in a Perspex phantom (32 cm or 16 cm diam.)
– (quoted as dose to air)
• Weighted average = 1/3 centre + 2/3 periphery
x- ray tube
x- ray beam
CTDI phantom
electrometer
ion chamber
scanner
z-axis
Scanner couch
detectors
Tolerances 10 – 40%
IPEM Dose in DR_Oct2018_SE
Volume CTDI (CTDIvol)
• Apply CTDIw to a protocol
• Effect of pitch taken into account
CTDIvol = CTDIw / pitch
• CTDIvol ~ represents average local absorbed dose
CTDIvol B = ½ CTDIvol A
B
A
P1 = 1
IPEM Dose in DR_Oct2018_SE
P2 = 2
• Everything’s simple …isn’t it ?
IPEM Dose in DR_Oct2018_SE
CTDIair - Wide beam MSCT ?
• Wide beam
dose
profile
Chamber stand
z - axis
electrometer
Scanner couch
IPEM Dose in DR_Oct2018_SE
Ion chamber (Lc)
‘gap’
IEC : Wide beam CTDIvol
2011
Ed 3 Amm.1
2010
BIR 2012
IPEM Dose in DR_Oct2018_SE
Wide Beam CTDIfree-in-air
beam width
of 160 mm
100 mm
ion
chamber
step
increments
equal to the ion
chamber
length
The 200 mm integration length is sufficient according to the minimum requirement
IAEA Wide Beam
of
IEC,
the 300 mm integration length can also be used
IPEM
Dosehowever
in DR_Oct2018_SE
Report / SE
Weighted CTDI (CTDIw) - Wide beam MSCT ?
x- ray tube
x- ray beam
CTDI phantom
electrometer
ion chamber
scanner
Scanner couch
detectors
IPEM Dose in DR_Oct2018_SE
z-axis
IEC : Wide beam CTDIvol
• CTDIvol :nominal beam widths (NxT) < 40 mm .. no change
• CTDIvol :nominal beam widths (NxT) greater than 40 mm
– Measure for an ~ 20 mm beam, correct with CTDIair ratios
 CTDI free − air , N ×T 


CTDI
CTDI
=
×
vol , :~ 20 mm
CTDI vol vol
: , N ×T CTDI vol
CTDI air : beam (N
xT) mm


CTDI
free − air , ~ 20 mm 
beam width = beam width x 
(NxT) mm
~ 20 mm
CTDI air : beam ~ 20 mm
IEC 60601-2-44 Ed 3.1
IPEM Dose in DR_Oct2018_SE
CTDIvol is a Dose Index
• Good for optimisation – as an indicator of relative dose
between protocols, scanners, and standard size patients
CTDI is ‘what it says on the can’ – ‘A Dose Index’
IPEM Dose in DR_Oct2018_SE
CTDIvol is not patient dose
• All scatter is not considered
• CTDI under or over estimates the ‘dose’
for scanned lengths > or < 100 mm
IPEM Dose in DR_Oct2018_SE
Equivalence of approaches
• CTDI (single slice integration/T)
• MSAD (or cumulative dose D0 (L))
CTDIvol = MSAD100
CTDI100 (vol) MSAD100
Cumulative dose D0(100)
Absorbed dose
from multiple rotations
IPEM Dose in DR_Oct2018_SE
29
Courtesy M.A. Lewis
Equivalence of approaches
• CTDI (single slice integration/T)
• MSAD (or cumulative dose D0 (L))
MSAD200
CTDI100(vol)
Cumulative dose D0(200)
CTDIvol < MSAD200
Absorbed dose
from multiple rotations
IPEM Dose in DR_Oct2018_SE
30
Courtesy M.A. Lewis
Absorbed dose from CT Exam
• Measuring the obvious way…
helical
Also wide beam / cone beam
IPEM Dose in DR_Oct2018_SE
31
Courtesy M.A. Lewis
AAPM TG111 and ICRU 87
ICRU Report 87
Radiation Dose and Image-Quality
in Computed Tomography
Also coming out ..AAPM TG 200 - Practical
implementation of AAPM Report 111
IPEM Dose in DR_Oct2018_SE
Practical
implementation
of AAPM Report
111
AAPM Report
from TG-220
pending
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
Measuring dose the obvious way…
Normalised D0(L)
Dose from different scanned lengths: D0 (L) (or MSADL)
Integration chamber
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
Measuring dose the obvious way…
Dose from different scanned lengths: D0 (L) (or MSADL)
Normalised D0(L)
Z= 0
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
Measuring dose the obvious way…
Dose from different scanned lengths: D0 (L) (or MSADL)
Normalised D0(L)
Z= 0
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
Measuring dose the obvious way…
Dose from different scanned volumes: D0 (L) (or MSADL)
Normalised D0(L)
Z= 0
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
Measuring the single slice integration way…
(the ‘CTDI’ approach)
Conventional CT chamber
One scan - real time probe
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
Measuring the single slice integration way…
(the ‘CTDI’ approach)
Time (representing distance)
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
Measuring the single slice integration way…
Total integrated dose (→ CTDIL = integral /nT)
Full profile
Different integration lengths
Time (representing distance)
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
CTDIvol is not patient dose
• The patient is not
–
–
–
–
made of Perspex (PMMA)
circular in cross-section
16 cm (head) in diameter
or 32 cm (body) in diameter
IPEM Dose in DR_Oct2018_SE
Patients come in different sizes
• Patients come in different sizes
• But we quote CTDIvol to the same size phantom
For same scan parameters – quoted CTDI = 7 mGy in each case
Actual dose >> 7 mGy
IPEM Dose in DR_Oct2018_SE
<< 7 mGy
7 mGy
Dose and Patient Size
•
•
•
•
Same mAs
CTDIvol same
DLP same
Absorbed dose lower
IPEM Dose in DR_Oct2018_SE
Size Specific Dose Estimate (SSDE)
AAPM Reports 204, 220
IPEM Dose in DR_Oct2018_SE
December 2012 Radiology, 265, 666-668.
Size Specific Dose Estimate (SSDE)
for CTDI
• Conversion factors established to convert CTDI values to a
dose to water equivalent diameter of patient
IPEM Dose in DR_Oct2018_SE
IPEM Dose in DR_Oct2018_SE
Size Specific Dose Estimate (SSDE)
• Convert the patient diameter (AP, lateral or both), taking
into account attenuation, to a water equivalent diameter
– Using patient images or SPR
• Tables supplied of conversion factors
IPEM Dose in DR_Oct2018_SE
Courtesy John Boone
Variation of CTDI & DLP with patient size
• For same kV and mAs settings
CTDIvol
DLP
SSDE*
20 mGy
600 mGy.cm
31 mGy
* From AAPM 204
IPEM Dose in DR_Oct2018_SE
20 mGy
29 mGy
20 mGy
600 mGy.cm
25 mGy
Adapted from E Castellano
SSDE – A surrogate for organ dose
• “when the organ is fully contained in the scan volume, SSDE
can be used as an estimate of organ dose” (AAPM 204)
─ E.g. Pediatric
─ Moore BM, Brady SL, Mirro AE, Kaufman RA. Size-specific dose estimate (SSDE)
provides a simple method to calculate organ dose for pediatric CT
examinations. Med Phys. 2014;41(7):071917.
─ E.g. Abdomen and Pelvis
─ Wang J, Duan X, Christner JA, Leng S, Yu L, McCollough CH. Attenuation-based
estimation of patient size for the purpose of size specific dose estimation in CT.
Part I. Development and validation of methods using the CT image. Med Phys.
2012;39(11):6764-6771.
IPEM Dose in DR_Oct2018_SE
Dose length product (DLP)
• Dose descriptor used to indicate overall exposure for CT
• DLP = CTDIvol x scan length
IPEM Dose in DR_Oct2018_SE
DLP to E Conversion Factors
– Dose Length Product for typical examination (survey data)
– Effective Dose (ED) for examination from MC calculations
– DLP to ED conversion factors for exam
T
DLP = CTDIvol x L
IPEM Dose in DR_Oct2018_SE
DLP to E Conversion Factors
– Dose Length Product for typical examination (survey data)
– Effective Dose (ED) for examination from MC calculations
– DLP to ED conversion factors for exam
Pay attention to source data, CTDI phantom size,
MC phantom, E103 or E60
- values vary
T
DLP = CTDIvol x L
IPEM Dose in DR_Oct2018_SE
DLP to E60 Conversion Factors
• Conversion factors: E60 = EDLP.DLP (mSv)
Body
Region
Norm. effective dose,EDLP
(mSv mGy-1 cm-1) E / DLP
Head
0.0023
Neck
0.0054
Chest
0.017 (0.014^)
Abdomen
0.015
Pelvis
0.019
• European Guidelines on Quality Criteria for Computed Tomography, EUR
16262, May 1999
• ^UK 2005 Dose Survey, Shrimpton et al - modified (chest 0.014)
• ICRP 60
IPEM Dose in DR_Oct2018_SE
AAPM (2007) The Measurement, Reporting and Management of Radiation Dose in
CT: Report of the AAPM Task Group 23. Report No.96, American Association of
Physicists in Medicine, New York.
Note phantom size
IPEM Dose in DR_Oct2018_SE
BJR 2016 (89), Jan 2016
• Survey DLP values – 3 sets
– 1996, 2003, and 2011
• MC calculations :
• 2 types of Phantom
– NRPB SR250 stylised/numerical
– ICRP AM/AF voxel
• 2 Organ dose weighting factors
– ICRP 60 vs ICRP 103
• Using older and newer scanner
models (average)
IPEM Dose in DR_Oct2018_SE
E/DLP (E60, E103)
(MIRD/CRISTY phantom)
2011 survey data
IPEM Dose in DR_Oct2018_SE
examinations
•
•
•
IPEM Dose in DR_Oct2018_SE
ICRP 60
MIRD phantoms
2003 Survey
•
•
•
ICRP 103
ICRP voxel phantoms
2011 Survey
Abdo E/DLP
From 0.015
to 0.024
examinations
•
•
•
ICRP 60
MIRD phantoms
2003 Survey
•
•
•
ICRP 103
ICRP voxel phantoms
2011 Survey
Abdo E/DLP
From 0.015
to 0.024
Pay attention to source data, CTDI phantom size,
MC phantom, E103 or E60
- values vary
IPEM Dose in DR_Oct2018_SE
DLP to E Conversion Factors - Cardiac CT
• E to DLP factor
– Higher for a cardiac scan than for chest
– Gosling et al suggest 0.028 (E103)
• ~0.019 for E60
• Study uses GE HD750, GE VCT, local protocols and
ImPACT calculator
O. Gosling et al. Clinical Radiology 65 (2010) 1013e1017
IPEM Dose in DR_Oct2018_SE
Organ doses and Effective Doses
• Obtain average organ doses^ (DT)
• Apply tissue radio-sensitivity factors (wT)
• Effective dose (Sieverts) is the sum of these
• Relates to overall radiation risk
─
─
For a population, not an individual (ICRP 103)
Draft new ICRP report on ED modifies that approach
^ Apply quality factor: for X-rays = 1
IPEM Dose in DR_Oct2018_SE
Patient Dosimetry – Effective Dose
• ED calculated from all sources of phantoms
– high variability in ‘ED’
ICRP-110 AF, AM
IPEM Dose in DR_Oct2018_SE
Phantoms Used for MC Calculations in CT
• Numerical / stylised phantoms
– e.g. Cristy-Eckerman, Adam and Eva, NRPB 18+
• Voxel phantoms – based on real scans
– E.g. Laura, Gollum, MAX and FAX, ICRP (AF, AM)
The FAX06
phantom
comprises
around
143 millio
n 1.2 mm
cubic
voxels.
IPEM Dose in DR_Oct2018_SE
Male and female adult reference
computational phantoms
ICRP: Effective Dose should be
calculated with ICRP 110
https://www.helmholtz-muenchen.de/amsd/research/groups/radiation-physicsin-medical-diagnostics/numerical-dosimetry-and-voxel-models/referencecomputational-phantoms/index.html
IPEM Dose in DR_Oct2018_SE
Review paper – computational phantoms
An exponential growth of computational phantom research in radiation
protection, imaging, and radiotherapy: a review of the fifty-year history
X George Xu 2014 Phys. Med. Biol. 59 R233 doi:10.1088/0031-9155/59/18/R233
IPEM Dose in DR_Oct2018_SE
IPEM Dose in DR_Oct2018_SE
https://sites.google.com/site/nacpctcoursehelsinkimaterials/
NACP_Helsinki_2014_Deak_
Monte Carlo Calculations in CT
• Model the x-ray interactions
– Computer simulated irradiation, and statistical calculations of
photon interactions (MCNP, EGS4 …)
• Model the scanner
– X-ray spectrum, filtration (including beam shaping), tube voltage
– Many tens to hundreds of models potentially needed
• Model the patient
– Mathematical anthropomorphic or voxel phantom
• Model physical phantoms
– for verification with actual measurements
IPEM Dose in DR_Oct2018_SE
Doses for every day use?
• Use a front end package to utilise Monte Carlo
generated organ dose datasets
– organ doses per voxel or slab of tissue.
• All require some input (e.g. CTDIair, CTDIphantom, mAs)
to adjust the normalised datasets
IPEM Dose in DR_Oct2018_SE
mGy per slab, per mAs, per CTDI air
NRPB – SR250
Normalised Organ dose
co-efficients
IPEM Dose in DR_Oct2018_SE
Jones, D G and Shrimpton, P C (1991). Survey of CT practice in the UK. Part 3: Normalised
organ doses calculated using Monte Carlo techniques. NRPB-R250, Chilton.
Doses for every day use?
• Examples
–
–
–
–
–
ImPACT CTDosimetry calculator (plus NRPBSR250 datsets)
CTExpo
NCICT
CT-Dose
ImpactDose (unrelated to the one above)
IPEM Dose in DR_Oct2018_SE
Dose Calculators for every day use, e.g.
Virtual DoseTM CT
7
Adapted from
http://www.cirms.org/pdf/2012_conference_pdf/Medical/cirms2012%20Ding%20Med.pdf
ESMP_Prague_Software for dose calculations_SE
KITEC 2015: CT in PET-CT and SPECT-CT SE/MAL
Dose Management Systems
• A number of DMS have organ and ED options
• 2 examples shown here
Qalum
Radimetrics
IPEM Dose in DR_Oct2018_SE
First created ~ 2002
Utilises NRPB –
SR250 Organ dose
co-efficients
IPEM Dose in DR_Oct2018_SE
Paediatrics (rough guide)
Khursheed A, Hillier MC, Shrimpton PC and Wall BF. Influence of patient age on normalized effective
doses calculated for CT examinations. Br J Radiol 2002; 75:819-830
IPEM Dose in DR_Oct2018_SE
Patient Dosimetry in CT
•
•
•
•
•
Introduction
Technology and dose distributions
‘Physics Metrics’
Understanding and use of these metrics
Monte Carlo - Calculation of organ doses and
Effective Dose
IPEM Dose in DR_Oct2018_SE
CT Dose Metrics and Current Dose
Methods
Sue Edyvean: Medical (Radiation) Dosimetry Group, CRCE, PHE, Chilton, UK
sue.edyvean@phe.gov.uk
IPEM Dosimetry in Diagnostic Radiology
IPEM Dose in DR_Oct2018_SE
4 Oct 2018