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State of the art and future development
for standardized estimation of organ
doses in CT
March 2015
William J. O’Connel, Dr. Ph, Senior Medical Physicist
Imagination at work.
Agenda
•
Introduction
•
Duke
•
Florida
•
UCLA / MD Anderson
•
RPI
•
AAPM TG 246
•
Conclusion
2
Introduction
3
Introduction
What is the radiation dose?
Multi-faceted calculation requiring
knowledge of energy deposited in
defined mass (organ / tissue)
Is the procedure safe? (risk)
Growing interest in role of organ
dose as descriptor of risk
4
National Radiological Protection Board
• Monte Carlo simulations of
calculated x-ray spectra in an
adult, hermaphrodite,
mathematical model (MIRD)
• 75 scanners (out of ≈ 200)
operating in the UK at the time.
• 23 data sets produced for
surveyed scanner models
• Original data from contiguous
axial scans
5
National Radiological Protection Board
6
ImPACT
Estimating patient dose on current CT scanners: Results of
the ImPACT* CT dose survey
M.A. Lewis, S. Edyvean, S.A. Sassi, H. Kiremidjian, N. Keat and A.J. Britten.
ImPACT, Medical Physics, St. George's Hospital, London
7
Introduction
• Estimated effective doses are not patient specific
• DLP / k-factor method (k is not scanner specific)
• Effective Dose in obese patients is problematic
8
Overview
9
Introduction
Limitations to existing patient dose metrics
CTDIVOL is a useful benchmarking tool but is not ideal
indicator of organ dose and radiation risk
• Broader beam widths
• Bow-tie filters
• Variable Pitch
• Non-contiguous slices
10
Introduction
Researchers are looking at many aspects of organ dose
estimation
• Validated Monte Carlo modeling
• Computational Phantoms
• Tube Current Modulation
• Obese Patients
Roadmap to Organ Dose in computed tomography
11
Florida
12
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
13
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
Stylized (Mathematical) Phantom
ORNL stylized adult phantom
Flexible
Anatomically unrealistic
Errors
14
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
Voxel (Tomographic) Phantom
Constructed from patient
acquisitions
Anatomically realistic
Not flexible
15
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
Hybrid Phantom
Nurbs (non-uniform rational
B-spline)
mathematical model used for
generating / representing
curves and surfaces.
Realistic
Flexible
16
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
Hierarchy of phantom morphometric categories
Patient-Specific
patient match: individual patient morphology
Patient-Sculpted
patient match: height, weight, body contour
Patient-Dependent
patient match: nearest height and weight
Reference
50th percentile individual, patient matching by age only
17
Florida - Long et al, Med. Phys. 40 (1), January 2013
18
Florida - Long et al, Med. Phys. 40 (1), January 2013
Benchmark Monte Carlo
simulations against
anthropomorphic
phantoms
SOMATOM Sensation 16
multidetector CT
scanner
Multiple axial and
helical acquisitions
UF computational adult male
reference hybrid phantom
19
Florida - Long et al, Med. Phys. 40 (1), January 2013
Monte Carlo radiation
transport code, MCNPX version
2.6.
SPEC78 spectrum generation
program
Bow-tie filter and over-ranging
UF Series-B 9-month-old
voxel phantom
fiber-optic coupled
plastic scintillator dosimetry
(PSD) system
20
Florida - Long et al, Med. Phys. 40 (1), January 2013
21
Florida - Long et al, Med. Phys. 40 (1), January 2013
22
Florida - Long et al, Med. Phys. 40 (1), January 2013
On average, organ doses from the Monte Carlo simulations
agreed with physically measured doses within 8%-9% for
axial and helical imaging of the reference adult phantom
Agreement is within 6%-7% for the 9-month old child
Individual organ doses were found to be within 15% of
measurements of organ dose for both phantoms
23
Duke
24
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
25
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
• How are dose results affected by choice of computational
anthropomorphic phantom?
• What uncertainties exist in the estimation of dose with
different types of phantoms?
• Organ doses, effective doses, risk indices, and conversion
coefficients to effective dose and risk index were estimated
for ten body and three neurological examination
categories
26
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
1. Male and Female Extended Cardiac-Torso (XCAT)
2. ICRP No. 110 reference male and female phantoms
3. Impact Group phantoms
4. CT-Expo
27
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
28
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
29
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
XCAT Hybrid Phantoms
Visible Human anatomical
data – National Library of
Medicine
NURBS based phantoms
modified to match ICRP 89
reference values
Brains modeled separately
on MRI models
30
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
ICRP 110 Voxelized Phantoms
Tomographic data of
individuals whose body height
matched reference values in
ICRP Publication 89
Radiosensitive organs were
directly segmented from
tomographic data
31
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
ImPACT Phantoms
Stylized mathematical phantom
208 contiguous 5 cm slabs
extending from upper legs to
head
NRPB-R186
32
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
CT-Expo Phantoms
Stylized mathematical phantom
Design characteristics of MIRD5 phantom
ADAM and EVA – GSF – ICRP
Publication 23
33
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
34
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
35
Duke - Tian et al, Radiology 270 (2), February 2013
36
Duke - Tian et al, Radiology 270 (2), February 2013
Validated Monte Carlo
simulations performed on 42
pediatric patient models
(normals)
Organ dose estimates for
routine chest and
abdominopelvic examinations
Feasible to estimate patientspecific organ dose with
knowledge of patient size and
CTDIVOL
37
Duke - Tian et al, Radiology 270 (2), February 2013
38
Duke - Tian et al, Radiology 270 (2), February 2013
Multi scanner study – Lightspeed VCT and SOMATOM
Definition Flash
CTDIVOL is used as an index of scanner radiation output
Calculate CTDIVOL conversion factor (hO, S, P ) – specific to each
organ, scanner and patient model
CTDIVOL determined with 100 mm chamber and 16-cm
phantom
CTDIVOL conversion factor showed exponential relationship
with average patient diameter
39
Duke - Tian et al, Radiology 270 (2), February 2013
40
Duke - Tian et al, Radiology 270 (2), February 2013
41
Duke - Tian et al, Radiology 270 (2), February 2013
42
Duke - Tian et al, Radiology 270 (2), February 2013
43
Duke - Tian et al, Radiology 270 (2), February 2013
44
Duke - Tian et al, Radiology 270 (2), February 2013
45
UCLA / MD Anderson
46
UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
47
UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
•
Most methods estimating patient dose from computed
tomography are based on fixed tube current scans
•
A growing number of CT scans are performed with tube
current modulation (TCM)
•
Detailed TCM data is difficult to obtain
•
What is accuracy of organ dose estimates obtained using
methods that approximate detailed TCM function?
48
UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
MCNPX (Monte Carlo NParticle eXtended v2.6.0)
Two MDCT scanners:
Sensation 64 and LightSpeed
16
Twenty adult female chest
voxelized models
Twenty pediatric female
models (whole body)
49
UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
For each patient model,
detailed TCM function was
extracted from the raw
projection data
Over-ranging region can be
determined from start and
end locations of the image
data and locations of x-ray
beam on and x-ray beam
off
50
UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
51
UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
Longitudinal approximated TCM function obtained from the
image data is reasonable surrogate to detailed TCM function
for use in Monte Carlo dose simulations.
Longitudinal approximated TCM function only represents the
z-axis modulation of the TCM algorithm and it does not
capture the over-ranging information that the detailed TCM
function
Results suggest angular modulation has a stronger effect on
smaller peripheral organs (breasts) compared to larger and
more central organs (lungs).
52
RPI
53
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
54
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
Visceral adipose tissue (VAT)
Study the effect of obesity on
the calculated radiation dose
to organs and tissues
Developed BMI-adjustable
phantoms ( Range 23.5 – 46.4)
Subcutaneous adipose tissue (SAT)
55
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
SAT layer is added to
phantom in the space
between the body surface
and internal organs
Thickness of adipose
tissue is only considered
for attenuation properties
Dose to adipose tissue is
not estimated
56
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
No data to estimate the
effect of VAT on internal
organ placement /
deformity
Internal organ size and
VAT volume held
constant for all BMI
settings
VAT density is corrected
for obesity with waist
circumference (WC)
57
RPI – 3D - BMI Adjustable Phantoms
58
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
Data validated against
anthropomorphic data
from literature (Ogden
2004)
AP and LAT
measurements
performed at mid-chest
and mid-abdomen
59
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
Good correlation between
AP and LAT measurements
and Ogden data
BMI-adjustable phantoms
are realistic representation
of overweight and obese
patients for the purpose of
estimating CT imaging
doses
60
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
61
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
62
AAPM Task Group 246
63
AAPM TG 246
William Pavlicek – Chair
Daniel Bednarek
Wesley Bolch
Dianna Cody
Frank Dong
Sue Edyvan
Aaron Jones
Cynthia McCollough
Ed McDonagh
Michael McNitt-Gray
Donald Miller
Donald Peck
Madan Rehani
Ehsan Samei
Mark Supanich
64
AAPM TG 246
True Patient
Dose
Phantom
Organ Dose
Enhanced
SSDE
SSDE
Dose
Page
CTDIVOL
DLP
65
AAPM TG 246
•
MC calculation tools have been validated numerous times
with physical measurements and are considered capable
of accuracy equal to measured values
•
Possible to assemble tables of dose coefficients to convert
individual episodes of patient exposures to dose
•
Approach would reduce need for long duration MC
computations for each patient
•
MC with a near matched scanning device and patient
matched phantom computation
66
Conclusion
67
Conclusion
•
Dose to radiosensitive organs is a useful basis for
estimating metrics related to risk
•
Organ dose is more informative than CTDIVOL, DLP or
Effective Dose
•
Better accounts for scanner differences
•
Better accounts for variability in patient size
•
Better accounts for changes in target region
•
Better accounts for tube current modulation
68
Conclusion
•
Not quite ready for implementation
•
How many computational phantoms are required?
•
What level of accuracy is required? ± 20%, ± 35%
THANK YOU !
69