aapm scientific meeting patient dose in ct: imaging … · california sb 1237 requires reporting on...

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AAPM Scientific Meeting

Imaging Symposium

Patient Dose in CT: Calculating

Patient Specific Doses in CT

(Joint with Education)

Michael McNitt-Gray, PhD, DABR, FAAPM; UCLA

Peter Caracappa, PhD, CHP; RPI

Ehsan Samei, PhD, DABR, FAAPM, Duke

Patient Dose in CT:

Calculating Patient Specific Doses in CT

1. Limitations of current metrics (e.g. CTDIvol) and methods in

estimating patient dose (McNitt-Gray)

2. Methods to more accurately estimate dose that take into

account scanner, exam and patient factors (McNitt-Gray)

3. The role of Computational Phantoms, Monte Carlo

simulation in developing patient-specific dose estimates

(Caracappa)

4. Estimate patient-specific organ doses, effective doses, and

radiation risk for comparison and optimization purposes

(Samei)

Acknowledgements

• Funded by NIBIB grant R01EB004898

• Recipient of Research Grant Support

from Siemens Medical Solutions

Patient Dose in CT:

Calculating Patient Specific Doses in CT

Questions to keep in mind:– Should we calculate patient specific dose?

– How accurately do we need to calculate this?• Does it vary from purpose to purpose (fetal dose,

meet legal requirements, etc.)

– Do we need to calculate for EACH patient? Or just for a “class” of patients (Large adult male)

• Should we do online realtime Monte Carlo for each patient?

• or precalculate doses somehow?

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Background• NCRP 160 and (Mettler et al, Health Physics, Nov 2008)

Estimated US averages 1980 2006

Estimated Average Annual

Radiation Dose

(whole body eff. dose in mSv)

3.6 mSv/yr 6.0 mSv/yr

From Medical Radiation 0.54 mSv/yr

15% of total

3.0 mSv/yr

50% of total

From CT --- 1.5 mSv/yr

25% of total

Background• CT procedures

– Estimate 18.3 million in 1993

– Estimate 62.0 million in 2006

– 10% annual growth• Slightly higher since introduction of MDCT (1994-1998)

– Could be over 100 million by now

Current Dose Metrics

Many Organizations suggesting that CT dose be

tracked (NCI, IAEA, ACR, FDA, etc.)

California SB 1237 requires reporting on CT dose

by July 1st, 2012 , one of the following:

“The computed tomography index volume (CTDIvol) and dose

length product (DLP), as defined by the IEC and recognized by

FDA; The dose unit as recommended by the American

Association of Physicists in Medicine”.


What is available to be recorded now?

CTDIvol and DLP

• What could we do in the future?

• Organ Dose

– Could be tracked across scans and across time

– Accumulated organ dose over time

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What is available to be recorded now?

CTDIvol and DLP

• What could we do in the future?

• Organ Dose

– Could be tracked across scans and across time

– Accumulated organ dose over time

CTDIvol and DLP

• CTDIvol reported on the scanner

• Is Dose to one of two phantoms

• 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

CTDI and Patient Dose :

They Are Not the Same Thing• McCollough et al, Radiology, May 2011 ; 259:311–316

• CTDI DOES REPRESENT:

– A measure of scanner output (with limitations

being addressed by TG 111 and TG 200)

– Well defined and highly reproducible across CTs

• CTDI DOES NOT REPRESENT

– Patient dose – does not take into account patient

size, shape, composition, scan length

– Dose from scans with no table motion (perfusion)

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

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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 Patient Dose Really Increase ?

For same tech. factors, smaller patient absorbs more dose

– Scenario 1: CTDI is same but smaller patient’s dose

is higher

– Scenario 2: CTDI is smaller for smaller patient, but

patient dose is closer to equal for both.

AAPM TG 204

Size Specific Dose Estimates

Based on Both

Simulations and

Measured Data

CTDIvol

• UNDER estimates dose for small patients

(have to multiply by > 1)

• OVER estimates dose for large patients

• (have to multiply by < 1)

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CTDIvol

• Not patient Dose

• By itself can be misleading

• CTDIvol should be recorded with:

– Description of phantom size (clarify 16 or 32 cm diameter)

– Description of patient size (lat. Width, perimeter, height/weight, BMI)

– Description of anatomic region

Tracking/Reporting Dose?

• What should we record/report?

• What do we tell patient?

• What do we tell referring physician?

• CTDIvol? DLP?

• Total CTDIvol? Total DLP?

• Calculate Effective Dose from Total DLP*k?

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

• MIRD V (geometric model)

• Eva, Adam

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Monte Carlo for CT Dose - Details• Monte Carlo Packages

– MCNP (Los Alamos)

– EGS

• Model Transport of Photons from modified (CT) source

• Probabilistic interactions of photons with Tissues

– Photoelectric, Compton Scatter, Coherent Scatter

• Tissues need detailed descriptions

– Density

– Chemical composition (e.g. from NIST web site)

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

– Function of beam energy

• Geometry

– Focal spot to isocenter, fan angle

• Beam Collimation

– Nominal or actual

• Filtration

– Bowtie filter (typically proprietary)

– Other add’l filtration (also proprietary)

• 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

rmali

zed

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 d

ose

128 mm in air at iso

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Modeling the CT scanner

• Source Path - dependent on scan parameters:

• Nominal collimation

• Pitch

• Start and Stop Locations (of the source)

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

Validating the CT Scanner Model

• Benchmark MC Model against physical

measurements

– CTDI Phantoms

• Head and Body

• Simulate a tally in a pencil chamber

• Each kVp and beam collimation combination

• Measured vs. Simulated

– Aim for < 5% difference between Simulated

and Measured

Modeling the Patient• Geometric

– e.g MIRD

– Standard man

– Often androgynous (male/female organs)

– Usually single size

• Size and age variations

– newborn, ages 1, 5, 10, and 15 years

– adult female, and adult male

– Including pregnant patient

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Modeling the Patient

• All radiosensitive organs identified

– Location

– Size

– Composition and density

Modeling the Patient

• Voxelized Models

– Based on actual patient scans

– Identify radiosensitive organs –

usually manually

– Non-geometric

• Different age and gender

• Different sizes

Modeling the Patient• GSF models (Petoussi-Henss N, Zankl M et al,

2002)

– Baby, Child, three adult females (shown), two adult

males, Visible Human

– All radiosensitive organs identified manually (ugh!)

Modeling (Parts of) the Patient

• Embryo/Fetus

• Breast

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Uterus

Gest. Sac

Uterus

Gest. Sac

7 weeks (embryo not visible)Mature Fetus:

36 weeks

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

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Simulating the Scan• Select Technical Parameters

– Type of scan (helical, axial)

– Beam energy

– Collimation

– Pitch

– Tube Current/rotation time (or tube current modulation)

• Select Anatomic Region

– Head/Chest/Abdomen/Pelvis/etc.

• Translate this to:

– Start/stop location -> Source Path

Monte Carlo Methods and Patient Size

Fetal Dose as a Function of Patient Perimeter

y = -0.12x + 23.11

R2 = 0.68

0

2

4

6

8

10

12

14

16

85 90 95 100 105 110 115 120 125

Perimeter of Mother (cm)

No

rmalized

Feta

l D

ose (

mG

y/1

00m

As)

Angel et al Radiology 2008

Angel et al, PMB Feb 2009

Tube current versus x-axis location of the TCM schema for a

patient model with a perimeter of 125cm. Background is a

sagittal view of the patient.

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Breast dose versus patient perimeter for all 30 patient models

in the fixed tube current simulations. Breast dose decreases

linearly with an increase in patient perimeter (R2=0.76).


Breast dose versus patient perimeter for all 30 patient models

in the TCM simulations. Breast dose increases linearly with an

increase in patient perimeter (R2=0.46).


Percent dose reduction for the TCM simulations as compared

to the fixed tube current simulations. Dose reduction decreases

linearly with an increase in patient perimeter (R2=0.81).

Dose Increase

Dose Savings

Organ Dose Independent of Scanner

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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

Turner et al Med Phys 2010

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)

y = 3.780e-0.011x

R² = 0.970

0.0

0.5

1.0

1.5

2.0

2.5

3.0

25 50 75 100 125 150

Mea

n o

rgan

do

se/C

TD

I volac

ross

sca

nner

s

Patient Perimeter (cm)

Stomach

Liver

Adrenals

Gall Bladder

Kidney

Pancreas

Spleen

Expon. (Stomach)

Baby

Irene

Child

GolemDonna

Visible

Human

Helga

Frank


Future of Dosimetry?

Patient

Size info

CTDIvol

(or TG 111)

Size

Coefficients

Patient Organ Dose

•Accounting for patient size

•Accounting for scanner

•Accounting for anatomic region

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Radiation Dose : 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

• Use dose to radiosensitive organs as a basis

for estimating metrics that relate to risk

Summary - Estimating Organ Doses

• Organ Doses are meaningful indicators of Dose

• More informative than CTDI, DLP, E alone

– Take into account differences in scanner

– Take into account differences in patient size

– Take into account differences in body region

– Take into account dose reduction methods (TCM)

• Will be a better indicator as to when we truly reach sub mSv exam

Summary - Estimating Organ Doses

• Demonstrate feasibility of NOT having to do 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

Acknowledgements

• Funded by NIBIB grant R01EB004898

• Technical Support from:

– Siemens Medical Solutions

– GE Healthcare

– Toshiba Medical Systems

– Philips Healthcare

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