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