ct: dose estimation and measurementdiscuss challenges related to dose estimation 3. discuss factors...

AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation

CT: Dose Estimation and Measurement

Tessa S. Cook, MD PhD, DABRDepartment of Radiology

Hospital of the University of PennsylvaniaPhiladelphia, PA

[email protected]


Disclosures

• No financial disclosures


Acknowledgments

• Dianna Cody, PhD• Cynthia McCollough, PhD• Michael McNitt-Gray, PhD


References

• McCollough, CH et al. “How Effective is Effective Dose as a Predictor of Radiation Risk?” AJR 194(4): 890-96, 2010.

• McNitt-Gray, M. “AAPM/RSNA Physics Tutorial for Residents: Topics in CT”. RadioGraphics22(6): 1541-1553, 2002.

• McCollough, CH et al. “CT Dose Index and Patient Dose: They Are Not the Same Thing”. Radiology 259: 311–316, 2011.


Learning Objectives

1. Describe CT-specific dose parameters2. Discuss challenges related to dose estimation3. Discuss factors that affect CT dose4. Discuss new alternatives to existing CT dose

metrics


General Radiation Dose Parameters

• Exposure (C/kg, R)– The ability of x-rays to ionize air– Measured with ionization chamber– No information about energy absorbed by tissues

• Absorbed dose (Gy, rad)– Energy absorbed per unit mass at specific point– No information about radiosensitivity

• Effective dose (Sv, rem)– Accounts for radiosensitivity of tissues that have

absorbed radiation


CT-Specific Radiation Dose Parameters

• CTDI (Computed Tomography Dose Index)– CTDI100

– CTDIw

– CTDIvol

• DLP (Dose-Length Product)


CTDI – CT Dose Index

• Dose normalized to beam width, measured from 14 contiguous sections (not easily achievable)

N – number of sections/data channels per scanT – width of interval, i.e. section thicknessDsingle(z) – dose at point z along line parallel

to z-axis

CTDI 1

NTDsingle z dz

7T

7T


CTDI100

• Integrated measurement along the entire length of a 100 mm pencil ionization chamber

• Dependent on position within scan plane

N – number of sections/data channels per scanT – width of interval, i.e. section thicknessDsingle(z) – dose at point z along line parallel

to z-axis

CTDI100 1

NTDsingle z dz

5

5


Another Definition of CTDI100

f *– conversion factor from exposure to dose in air (0.87 rad/R)

C – calibration factorE* – integrated exposure to 100 mm chamber (R)L – length of pencil ionization chamberNT – beam width

* if air kerma (Gy) instead of exposure (R), f =1

CTDI100 f C E L / NT


NT – Beam Profile

• N – number of data channels (≤ # detector rows)• T – thickness of a data channel• NT – total nominal width of collimated x-ray beam

– Independent of and not necessarily the same as the reconstructed image thickness!


CTDIw – Weighted CTDI

• Weighted average of central and peripheral contributions to dose within a scan plane

• Position independent dose index• Periphery = 1 cm from phantom surface• Phantom size matters!

CTDIw 13

CTDI100,center 23

CTDI100, periphery


CT Phantoms

Polymethyl methacrylate (PMMA) cylindrical CT dosimetry phantoms


How CTDI100 Varies with Phantom Size

McNitt-Gray, RadioGraphics 22(6): 1541-53, 2002

“Head” phantom – 16 cm “Body” phantom – 32 cm

Same kVp, mAs, collimation used

The same thing happens with real patients, not just with phantoms!


CTDIvol – Volume CTDI

• Accounts for helical pitch or axial scan spacing• For axial scans:

NT – total collimated beam widthI – spacing between acquisitions

• For helical scans:

pitch – table distance traveled in 1 rotation divided by NT

CTDIvol CTDIw NT I

CTDIvol CTDIw pitch


Scenario 1: No adjustment 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

Courtesy of Michael McNitt-Gray, PhD


Scenario 2: Adjustment 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 the same technical factors, the smaller patient absorbs more dose

• Scenario 1: – CTDIvol is same but smaller patient’s dose is higher

• Scenario 2: – CTDIvol is smaller for smaller patient, but patient dose is

closer to equal for both



DLP – Dose-Length Product

• Incorporates scan length, including “over-ranging” from helical scanning

• Can be used to calculate a value of effective dose• Often reported on a dose sheet with CTDIvol for

each imaging series– Vendors account for “over-ranging” automatically

DLP CTDIvol scan length


Dose Sheets – Many Flavors


Estimating Effective Dose in CT

• From Monte Carlo simulations with mathematical phantoms– Derived under certain assumptions re. patient model and

scanner geometry– Computationally intensive

• From DLP– k factors (specific to an anatomic region)

• Derived from Monte Carlo simulations with MIRD phantom

• Assumes “reference man”: 70 kg, 170 cm tall

E DLP k


Mathematical Patient Model used by NRPB

McCollough et al., AJR 194(4): 890-96, 2010

NRPB = National Radiological Protection Board, UKTheir CT organ dose calculations are the basis for the derived k factors.


Adult Conversion (“k”) Factors for DLP to E

Christner et al., AJR 194(4): 881-89, 2010


Effective Dose Estimate from DLP

Phantom size

CTDIvol and DLPHead: 1243 x 0.0021 = 2.6 mSv (16 cm)

C-Spine: 597 x 0.0059 = 3.5 mSv (32 cm)

Thorax: 552 x 0.014 = 7.7 mSv (32 cm)

AP: 1166 x 0.015 = 17.5 mSv (32 cm)


Dose Monitoring: Challenges

• Actual patient dose depends on– Scanner parameters

• Geometry• Settings• Number of imaging phases

– Patient parameters• Gender• Age• Body habitus• Anatomy imaged

dose sheet? In the

How many of these factors are represented on a

dose sheet? In the RDSR?


So How to Monitor and Report Dose?

• CTDIvol…is not patient dose• DLP…is not patient dose• Effective dose…is not patient dose

– It’s not a dose at all; it’s a parameter that describes risk– It is certainly not patient specific

• Currently there is no estimate of actual dose to the patient provided by a CT scanner


What to Do When Law Says Report Dose

SSDE (mean dose in center of scan volume)


CTDIvol is NOT Patient Dose!

• CTDI quantifies scanner radiation output• Patient size must be considered to estimate patient dose

McCollough, et al, Radiology, May 2011


Effective diameter

Rel

ativ

e do

se

CTDIvol

SAME scanner output

DIFFERENT patient doses for different size patients


2011


Size Specific Dose Estimate = CTDIvol × size conversion factor

Conversion factor is exponentially related to patient size (attenuation)


How SSDE Came to Be

• Four different research groups– Tom Toth & Keith Strauss– Cynthia McCollough– Mike McNitt-Gray– John Boone

• Relationship between dose and patient size• Goal: know scanner output & size of object in

scanner, calculate how much energy per unit mass (i.e., dose) will be absorbed


Standard 10- ,16-, 32-cm PMMA phantomsTom Toth & Keith Strauss

16- and 64-slice scanners from the four major vendors; water-equivalent diameter for x-ray attenuation


8 anthropomorphic tissue-equivalent phantomsCynthia McCollough, Mayo Clinic

Newborn (9 cm lat) to large adult (39 cm lat) plus 3 custom “round” patients; 4 CT models from 2 vendors.


7 anthropomorphic Monte Carlo phantomsMike McNitt-Gray, UCLA

GSF phantoms, newborn to large adult, M & F, simulating abdominal CT across four major vendors


Cylindrical Monte Carlo phantoms (1 – 50 cm)John M. Boone, UC Davis

. 22 increments of cylinders from 1-50 cm in length


patient size

dose

CTDIvol

32 cm PMMA

normalization point


corr

ectio

n fa

ctor

CTDIvol

32 cm

after normalization

1.0

patient size


Con

vers

ion

Fact

or32 cm 120 kVp

Normalized to CTDIvol


How to Determine SSDE

• Patient size metrics– Anteroposterior thickness (AP)– Lateral width (LAT)– AP+LAT– Effective diameter

• Tabulated* conversion factors, fsize

* AAPM TG Report 204, 2011

SSDE = fsize x CTDIvol

AP

LAT


AAPM Report 204, Figure 2

effective diameter

circle of equal area

AP

lateral

Effective Diameter


Practical Implementation


CTDIvol is on most scanners…..

Pay attention to phantom size!


Table 2: For reference when the 16 cm diameter PMMA phantom was used to compute CTDIvol

16 cm CTDIvol Conversion Factors


Table 1: For reference when the 32 cm diameter PMMA phantom was used to compute CTDIvol

32 cm CTDIvol Conversion Factors


Estimating Patient Size

CT radiograph

Can obtain before the scan,requires proper centering

CT image

Requires waiting till after scan,and full FOV reconstruction


CT RadiographSSDE = 14.0 mGy × 1.13

SSDE = 15.8 mGy

CTDIvol (16 cm phantom) = 14 mGy


CT Image SSDE = 5.4 mGy × 2.5

SSDE = 13.5 mGy

CTDIvol (32 cm phantom) = 5.4 mGy


Suggested Reporting Language


y = 0.36x - 3.24R2 = 0.42

5

15

25

35

40 50 60 70 80 90AP + LAT (cm)

CTD

Ivol

(mG

y)

CTDIvol depended on patient sizePatient size explained 42% of the

variation in CTDIvol

y = 0.04x + 19.98R2 = 0.005

5

15

25

35

40 50 60 70 80 90AP + LAT (cm)

SSD

E (m

Gy)

“Dose” vs. Patient Size

SSDE independent of patient sizePatient size explained <1% of

variation in SSDE


Download from www.aapm.org!


Summary: Dose Estimation

• CTDIvol…is not patient dose• DLP…is not patient dose• Effective dose…is not patient dose• SSDE is a good estimate of actual patient dose in

the scan volume

ct: dose estimation and measurementdiscuss challenges related to dose estimation 3. discuss factors...

Documents