CT Organ Dose - Part 1 Estimating organ dose for patients Michael F. McNitt-Gray, Ph.D., DABR David...
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Transcript of CT Organ Dose - Part 1 Estimating organ dose for patients Michael F. McNitt-Gray, Ph.D., DABR David...
CT Organ Dose - Part 1
Estimating organ dose for patients
Michael F. McNitt-Gray, Ph.D., DABRDavid Geffen School of Medicine at UCLA
CTDIvol and DLP
• CTDIvol currently reported on the scanner – (though not required in US)
• Is Dose to one of two phantoms– (16 or 32 cm diameter)
• Is NOT dose to the patient• Does not tell you whether scan was done
“correctly” or “Alara” without other information (such as body region or patient size)
• MAY be used as an index to patient dose with some additional information
Scenario 1: No adjustment in technical factors for patient size
32 cm phantom 32 cm phantom
CTDIvol = 20 mGy CTDIvol = 20 mGy
The CTDIvol (dose to phantom) for these two would be the same
100 mAs 100 mAs
Scenario 2: Adjustment in technical factors for patient size
32 cm phantom 32 cm phantom
CTDIvol = 10 mGy CTDIvol = 20 mGy
The CTDIvol (dose to phantom) indicates larger patient received 2X dose
50 mAs 100 mAs
Did CTDI Really Tell Us Patient Dose?
For same exposure parameters, the smaller patient absorbs more dose (think “Image Gently” here)– Scenario 1: CTDI is same but dose to smaller Patient is higher– Scenario 2: CTDI is smaller for smaller Patient, but dose is closer to
equal for both.
– CTDI DOES describe system output (Reduced for small patient?)– CTDI DOES NOT describe patient dose
• At least not without other info. such as patient size, body region, etc.)
Radiation Dose Basics: Organ Dose
• BEIR VII report (2005)– Risk based on radiation dose to organ, age,
gender, etc.
• ICRP 103 (2007) – Calculates “effective dose” based on weighted
sum of organ dose
• All use dose to radiosensitive organs as a basis for estimating metrics that relate to risk
Effective Dose (mSv)• Tissue ICRP 60 weighting factor (wT) ICRP 103wT• Gonads 0.20 .08• Red Bone Marrow 0.12 .12• Colon 0.12 .12• Lung 0.12 .12• Stomach 0.12 .12• Bladder 0.05 .04• Breast 0.05 .12• Liver 0.05 .04• Esophagus 0.05 .04• Thyroid 0.05 .04• Skin 0.01 .01• Bone Surface 0.01 .01• Brain (Remainder) .01• Salivary Glands (Remainder) .01• Remainder (Adrenals, etc.) 0.05 .12
Can We Get There?
• W/o doing detailed analysis on EACH patient?
Monte Carlo Simulation Methods for Estimating Radiation Dose
• Monte Carlo methods– Used in CT for some time
• NRPB report 250 (1990)• GSF (Zankl)
Background
• These early reports used:– Detailed Models of Single Detector, Axial Scanners– Idealized (Nominal) collimation– Standard Man Phantom
Background
• These form the basis for:– CT Dose computer program– CT Expo– ImPACT dose calculator– k factor approach (Effective dose = k* DLP),
which was derived from NRPB simulated data
Current Approaches
• Model Scanner (e.g MDCT) in detail• Model Patient (Geometric, Voxelized)• Simulate Scan• Tally Organ Dose
Modeling the CT scanner• Spectra• Geometry• Beam Collimation• Filtration• Tube Current Modulation Scheme
– x-y only, z-only, x-y-z, etc.
Photon Fluence Spectra
0.000E+00
5.000E+10
1.000E+11
1.500E+11
2.000E+11
2.500E+11
3.000E+11
0 50 100 150 200
Energy in keV
Ph
oto
n F
luen
ce
80 kVp Spectra
125 kVp Spectra
150 kVp
Normalized Dose
0.000
0.250
0.500
0.750
1.000
1.250
40 60 80 100 120
Distance (mm)
No
rmal
ized
Do
se
128 mm in air at iso
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0.900
1.000
1.100
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
distance in mm
rela
tive
do
se
128 mm in air at iso
0
100
200
300
400
500
600
0 50 100 150 200 250 300
Table Position (mm)
Tu
be
Cu
rre
nt
(mA
)
90 degrees (AP)
Shoulder Region
Lung Region Abdomen
180 degrees (LAT)
Breast Tissue
Long Axis Modulation
Modeling the Patient• Geometric• Voxelized Models
Modeling (Parts of) the Patient• Embryo/Fetus• Breast, Lung, Kidney, Liver, Spleen
Contoured Image Voxelized Model
Ea
rly
Ge
sta
tio
nL
ate
Ge
sta
tio
n
Original Image
Original Image
Threshold Image
Contoured Image
Voxelized Model
Simulating the Scan• Select Technical Parameters• Select Anatomic Region• Translate this to:
– Start/stop location -> Source Path
Organ Dose Independent of Scanner
Organ dose (in mGy/mAs) and effective dose (in mSv/mAs) for GSF model Irene resulting from a whole body scan with similar parameters for each scanner
Organ dose and effective dose normalized by measured CTDIvol for GSF model Irene resulting from a whole body scan.
Normalized Organ Dose as function of Pt. Size(Abdomen Scans for each Patient)
25 50 75 100 125 1500.0
0.5
1.0
1.5
2.0
2.5
3.0
f(x) = 3.77967656053616 exp( − 0.0112933588640899 x )R² = 0.969917543699663
Stomach
Exponential (S-tomach)
Liver
Adrenals
Gall Bladder
Kidney
Pancreas
Spleen
Patient Perimeter (cm)
Mea
n or
gan
dose
/CT
DIv
ol a
cros
s sc
anne
rs
Baby
Irene
Child
GolemDonna
Visible Human
Helga
Frank
Turner et al RSNA 2009
Tube Current Modulation
0 50 100 150 200 250 300 350 4000
100
200
300
400
500
600
700
800
Tube Current Modulation
Table Position in mm
Tub
e C
urr
ent
in m
A
a) detailed TCM function from raw data
b) discrete TCM function from image data
c) single tube current value (average mA across the scan)
Approx TCM(DICOM header)
Actual TCM
Average mAs
Future of Dosimetry? Patient Size info
CTDIvol
(or TG 111)
Size Coefficients
Patient Organ Dose• Accounting for scanner and
acquisition parameters• Accounting for anatomic
region• Accounting for patient size
Summary Estimating Organ Doses Part 1
• Organ Doses are meaningful indicators of Dose• More informative than CTDI
Summary Estimating Organ Doses Part 1
• Demonstrating concept and feasibility of NOT doing detailed analysis on each Patient
• Not quite ready for implementation• A path to estimate organ doses that takes into
account:– Scanner– Acquisition parameters (including TCM)– Anatomic Region– Patient Size