Dose Modulation Technique in CT - aec.gov.tw · PDF fileCT Dose Modulation Technique RadioGraphics 2008; 28:1451–1459, Chang Hyun Lee, MD etc Page 35 Apr-10 For internal use only

1

For internal use only / Copyright © Siemens AG 2010. All rights reserved.

Dose Modulation Technique in CT

Short Overview


Stone Chen

Senior Application specialist

Siemens Limited, Taiwan

Page 2 Apr-10


Medical CS SLT Stone Chen

Content

- Overview

- Dose Saving Features

- Take Home Point

- Conclusions

Page 3 Apr-10



- Overview

Page 4 Apr-10



Scanner Generations

Page 5 Apr-10



Scanner Generations

Page 6 Apr-10



Development of CT

MultiMultiMultiMulti----phase CCA / Single heart beatphase CCA / Single heart beatphase CCA / Single heart beatphase CCA / Single heart beatPower injectorVolume Scan? Multi-Tube design? MTCT? MTCT? MTCT? MTCT Multi-phase CCA / ? Single heart beatPower injectorVolume Scan? Flat Plan CT? FPCT? FPCT? FPCT? FPCT Multi-phase CTA / multi-organdual head injectorVolume ScanMatrix formating design SS detectorMDCTMDCTMDCTMDCT MultiMultiMultiMulti----phase / Multiphase / Multiphase / Multiphase / Multi----organorganorganorgandual head injectorVolume ScanMatrix formating design SS detectorMSCTMSCTMSCTMSCT Multi-phase / single organPower injectorHelical ScanLow Voltage Slip RingSpiral CT Spiral CT Spiral CT Spiral CT MultiMultiMultiMulti----phase / single organphase / single organphase / single organphase / single organPower injectorHeart ScanElectron gun & multi target designEBCTEBCTEBCTEBCT Dynamic scanDynamic scanDynamic scanDynamic scanHand-pushDynamical ScanContinuous X-rayWhole body CTWhole body CTWhole body CTWhole body CT Contrast enhancementContrast enhancementContrast enhancementContrast enhancementair-drip infusionTopo & Fast ScanPulse X-rayWhole body CTWhole body CTWhole body CTWhole body CT Contrast enhancementinfusionTransaxial ScanBrain CTBrain CTBrain CTBrain CT

2

Page 7 Apr-10



Risk Vs Benefit

Page 8 Apr-10



Radiation Risk and Effective Dose

imaginable correlations for low dose

Eff. Dose (mSv)

Radiation Risk

•••• ••••

••••••••

Data from radiation victims(Hiroshima, Nagasaki)

Typ. range of CT

Schematic (!) Graph:

Assumption for radiation protection (ICRP): Linear extrapolation without threshold

For children: Higher Risk !

Page 9 Apr-10



Cataract in eye of interventionist

after repeated use of over table x-ray tube

Page 10 Apr-10



Example of chronic skin injury due to cumulative skin dose of

~20,000 mGy (20 Gy) from coronary angiography and x2 angioplasties

21 months after

first procedure,

base of ulcer

exposes spinous

process

Page 11 Apr-10


Medical CS SLT Stone Chen Page 12 Apr-10



News 1

On Friday, 08 Oct. 2009,

the FDA sent out a notice concerning "Safety Investigation of CT Brain Perfusion Scans: Initial Notification."

According to the FDA notification, 206 patients were exposed over a time period of 18 months with inappropriately high levels of radiation dose

during CT Perfusion (CTP) examinations.

As a consequence of the exposure, some patients suffered from

erythema and partial hair loss.

Take from Managing Patient Dose in Multi-Detector Computed Tomography (MDCT)Madan Rehani C 3 etc.

3

Page 13 Apr-10



News 2

CHICAGO (Reuters) - Radiation from CT scans done in 2007 will cause 29,000 cancers and kill nearly 15,000 Americans, researchers said on Monday CHICAGO Mon Dec 14, 2009 4:30pm EST

About 70 million CT scans were done on Americans in 2007, up from 3 million in 1980.

Amy Berrington de Gonzalez of the National Cancer Institute and colleagues developed a

computer model to estimate the impact of so many scans.

They estimated the scans done in 2007 will cause 29,000 cancers. A third of the projected

cancers will occur in people who were ages 35 to 54 when they got their CT, two-thirds will

occur in women and 15 percent will arise from scans done in children or teens.

The researchers estimated there will be an extra 2,000 excess breast cancers just from CT

scans done in 2007.

Page 14 Apr-10



Page 15 Apr-10





Distribution of S or EUS for the categories of

exposure

early 1981s 2006

Data from NCRP Report 160

Background

83%

medical

15%

consumer

2%

occupationa

/ industrial

0%

medical

48%

Background

50%

consumer

2%

occupationa /

industrial

0%

Page 17 Apr-10



Over view

rest

94%

CT

6%

rest

53%

CT

47%

Frequency of radiological examination Proportions of collective effective dose

The figures shown refer to Germany for the year 2003 [BfS, 2003]

Page 18 Apr-10



4

Page 19 Apr-10





Page 21 Apr-10



Reference Dose Levels

Various countries or organisations have defined Reference Levels for CT dose

e.g. American College of Radiology (ACR):

e.g. Germany (Federal Office of Radiation Protection):

28047Lumbar Spine

77025Upper Abdomen

75028Pelvis

150024Abdomen

65022Thorax

36035Cranium (Face) / Paranasal Sinuses

105060Brain

DLP(mGy ×××× cm)CTDIw (mGy)Examination

Diagnostic Reference Values for CT-Examination for adults

Page 22 Apr-10



Page 23 Apr-10



Define diagnostic image quality by adding noise

20mAs

160mAs

40mAs

60mAs160mAs 103mAs

56mAs 28mAs Page 24 Apr-10



Advantage and Risk of Diagnostic with X-rays

Radiation Risk

basically radiation induced cancer*

*) "stochastic damage".The "deterministic damage" caused by very high doses (e.g. radiation burns) are not taken into account here.

significant information about pathology

A careful consideration of advantage against risk is demanded for every individual patient !

Image Quality

5

Page 25 Apr-10




Body PartSpatial

ResoluctionLow

contrastNoise

kVpmASec

AlgorithmAECE&T




How to give a definition about Good Image Quality ?higher Image Contrast & less noise

or less image contrast & acceptable noise

Page 26 Apr-10



Page 27 Apr-10






Diagnostic Information

Radiation Risk





Image Quality

Page 29 Apr-10




Lesion Location

Sizecharacteristic

Radiation Risk





Image Quality

Page 30 Apr-10




20mAs

160mAs

40mAs

60mAs160mAs 103mAs

56mAs 28mAs

6

Page 31 Apr-10



- Dose Saving Features

Page 32 Apr-10



An Innovation Leader in

Low Dose Computed Tomography

CAREDose 4D

1999 2002

Hand CARE

20051994 1997

Pediatric80 kV Protocols

DSCT

2008

Ultra Fast Ceramic (UFC)

2008 2008

Flash Spiral

20082007

4D Noise Reduction

Adaptive Dose Shield

Selective Photon Shield

2009

IRISX-CARE

Up to 68% Up to 70% Up to 50% Up to 50%Up to 30%

Up to 25% < 1 mSv No penalty Up to 50% Up to 40% Up to 60%

Siemens Exclusive

Siemens Exclusive

Siemens Exclusive

Siemens Exclusive

Siemens Exclusive

Siemens Exclusive

2007

Adaptive ECG-Pulsing/Sequence

1-3 mSv

X-ray low


CARE Dose 4D

Automatic Exposure Control

Page 34 Apr-10



CT Dose Modulation Technique

RadioGraphics 2008; 28:1451–1459, Chang Hyun Lee, MD etc

Page 35 Apr-10



x

X-ray tube

Detector

bodyaxis

a.p.

lateral

a.p.

lateral

a.p.

lateral

a.p.

lateral

attenuation

The human body is not a homogeneous cylinder

⇒⇒⇒⇒ X-ray attenuation varies along the spiral path of a CT scan

Page 36 Apr-10



Optimal diagnostic image quality in every slice at lowest dose levels

⇒Adaptation of tube current to attenuation

Automatic Exposure Control

Accordingly, the noise in projection data varies (for constant tube

output)

"On each CT SCANNER, AUTOMATIC EXPOSURE CONTROL (AEC) shall be provided as a MODE(S) OF OPERATION alternative to the manual selection of CT CONDITIONS OF OPERATION."

7


75kg-Patient: Neck Thorax Abdomen Shoulder

Obese Patient (Example): Neck Thorax Abdomen Shoulder

Constant image noise

mAs-Adaptation along the patients z-axisand to patient size

Example:

Thorax Protocol

Page 38 Apr-10




20mAs

160mAs

40mAs

60mAs160mAs 103mAs

56mAs 28mAs


Results of Clinical Image Quality Assessment

Image Noise should not be constant

resp. mAs should not be proportional to object attenuation

� Noise in pediatric patients would be too high

� mAs for obese patients would deliver excessive dose levels and exceed the

power limits of current scanners

→→→→mAs should be adapted by an empirical function,

according to diagnostic information requirements


mAs-Adaptation along the patients z-axisand to patient size

75kg-Patient: Neck Thorax Abdomen Shoulder

Obese Patient (Example): Neck Thorax Abdomen Shoulder

Constant image noise

Clinical reasonable mAs

Example:

Thorax Protocol

CARE Dose 4D:mA is set proportional to (A/Aref)

b

Default Settings for b:Slim patients / low attenuation: b= 0.5

Obese patients / high attenuation b= 0.33

Page 41 Apr-10



How does CARE Dose 4D work?

1. Evaluation of Topogram for attenuation in lateral and AP direction

2. Calculation of appropriate axial tube current profiles in lateral and AP direction

3. Axial tube current variation during scan

4. Angular online tube current modulation during scan

Page 42 Apr-10



x

Care Dose

bodyaxis

a.p.

lateral

a.p.

late

ral

a.p.

lateral

a.p.

lateral

attenuation

8

Page 43 Apr-10



CareDose

mA = X/Y mA

CARE Dose 4D:

mA is set roportional to (A/Aref) b

Page 44 Apr-10



Measuring the attenuation in z-axis

0100200300400500600

table position

tub

e c

urr

en

t in

mA

a.p.lateralmean max

Page 45 Apr-10



Dataacquisition

system

GeneratorX-ray tube

Detector

Currentmodulation

unit

Dose Modulation Technical Aspects

Page 46 Apr-10



AP

Why is a angular modulation needed?

Detector

lateral

Elliptical/ irregular

objects

Without angular dose modulation:

The image noise is mainly determined by those angles with the highest X-

ray attenuation!

Page 47 Apr-10



Lower Image Noise due to better mA distribution

Using angle modulation

AP

Detector

lateral

Page 48 Apr-10



Dose Modulation – Phantom scan

Scan with constant mA Scan with automatic mA adaption

Effective Dose Reduction:49% measured / 50% calculated

Shoulder phantom, 14cm x 40cm

189mAs199mAs

s = 12.9HU s = 9.4HU

9

Page 49 Apr-10



Dose reduced by 51%

Scan with dose modulation

327mAs 171mAs

Improved image quality at lower dose

Scan without dose modulation

Page 50 Apr-10



Advantage of Online Modulation

lateral

a.p. , p.a.

Example: Shoulder Scan

lateral

a.p. , p.a.


Saved Doselateral

a.p. , p.a.


Page 51 Apr-10



0

500

1000

1500

2000

2500

3000

3500

4000

0100200300400500600

table position in mm

att

en

ua

tio

n I

_0

/ I

0

50

100

150

200

250

300

350

400

tub

e c

urr

en

t

Attenuation

tube current

Optimal mA for AP and lateral Views:

On-line mA modulation

Page 52 Apr-10



Headline

55mAs

130mAs110mAs

140mAs

Optimal image for all organs (adult)

Courtesy of Erlangen University, Germany

CARE Dose 4D: Whole body scan

Page 53 Apr-10



Care Dose 4D Spiral scan of Carotid Arteries

0

500

1000

1500

2000

2500

3000

3500

4000

-250-200-150-100-50

table position

att

en

uati

on

I_0 / I

0

50

100

150

200

250

300

350

400

tub

e c

urr

en

t

Att

mA

0

500

1000

1500

2000

-250-200-150-100-50

table position

att

en

ua

tio

n I

_0

/ I

a.p.

lateral

-250-200-150-100-50

table position

tub

e c

urr

en

t in

mA

a .p.

lateralmax

Page 54 Apr-10



CARE Dose 4D – First Results

Dose reduction in %, compared to SOMATOM Sensation16 standard

protocols (140 patient studies)

PelvisAbdomenThorax Average

34%40%67%39%

Abdomen

/ Pelvis

33%37%64%26%37%

Thorax /

Abdomen

Shoulder

-13%

Shoulder

51%

Head

PelvisAbdomenThorax Average

4%38%13%Thorax

AbdomenThoraxAverage

17%66%38%Neck

ShoulderNeckAverage

Organ specific Reduction in Scan RangeDose

Reduction

Protocols

[Gress et al, RSNA 2003]

10

Page 55 Apr-10



CARE Dose 4DAutomatic Exposure Control

for Siemens CT-Scanners (since SW VB10n)

Features:

� mAs setting automatically adapted to patient size

� full mAs variation over the patient’s long axis

� real-time modulation during tube rotation (current CARE Dose)

Benefits:

� same scan protocols for slim/obese, (adult/pediatric patients)

� optimal diagnostic image quality in every slice

� achieved at lowest dose levels

Page 56 Apr-10



How to work with CARE Dose 4D

Since SW version VB10/VB19, Siemens’ default scan protocols use CARE Dose

4D, resulting in ability to scan immediately without any adjusting of mAs after

performing a Topogram (*resulting in default image quality).

Adjustment of image quality to individual preference is possible.

Page 57 Apr-10



� A Topogram is needed to utilize CARE Dose 4D

� If the scan range exceeds the Topogram range:

�Outside the Topogram range the last measured/calculated mA-value of

the Topogram range will be used (warning pops up).

� If more than one Topogram of the current examination exists:

�Information of all valid Topograms will be used to calculate mA values

�(if overlapping in the same direction, e.g. lateral, the newest information

will be used)

CARE Dose 4D:Topogram Data

Page 58 Apr-10



Isocenter positioning of patient is essential!

X-ray tube

Detector

Patient

(centered)

CARE Dose 4D:Topogram

Page 59 Apr-10




X-ray tube

Detector

Patient

(not centered)

Distorted Topogram does

influence the mAs Calculation!

CARE Dose 4D:Topogram

Page 60 Apr-10




X-ray tube

Detector

Patient

(not centered)

Distorted Topogram does

influence the mAs Calculation!

CARE Dose 4DTopogram

11

Page 61 Apr-10



indicates CARE Dose 4D is

switched on for the current

protocol

displays the average eff. mAs

that will be applied by CARE

Dose 4D for the current scan

range

Page 62 Apr-10



displaysdisplays thethe averageaverage effeff. . mAsmAs thatthat

will will bebe appliedapplied byby CARE Dose 4D CARE Dose 4D forfor

thethe currentcurrent scanscan rangerange (will (will bebe

updatedupdated to to thethe real real appliedapplied effeff. . mAsmAs

afterafter thethe scanscan -- migthmigth differdiffer

somewhatsomewhat))

The Quality Reference mAs value

defines the overall image quality

(noise) of the current protocol and

may be adapted for each protocol to

the user's individual preference of

image quality

SwitchSwitch forfor CARE Dose 4DCARE Dose 4D

Page 63 Apr-10





Page 65 Apr-10





12

Page 67 Apr-10



upper system Limit(max. tube current)

scan range z

tub

ecu

rren

t

mean

tube

currentper

rotation

angle

modulatedtube

current

Impact of system limits

Page 68 Apr-10



scan range z

tub

ecu

rren

t

mean

tube

currentper

rotation

angle

modulatedtube

current

upper system Limit(max. tube current)


Page 69 Apr-10



scan range z

tub

ecu

rren

t

mean

tubecurrent

per

rotation

angle modulated

tubecurrent

upper system Limit

(max. tube current)


Page 70 Apr-10



Why is this CARE Dose a ‘4D’??

CD4D adjusts the tube current over the patient’s long axis

� The ‘1D’ approach, like our competitors on some

scanners

CD4D adjusts the tube current in x and y (tube angle): 3D

� The 3D approach based on Topogram evaluation

� Data needed to plan the scan: dose information, tube load

During the scan:

Real-time measurement of attenuation and mA modulation

� Real 4D modulation in space and time

Page 71 Apr-10



- Automatic Exposure Control (CARE Dose 4D) => see e-learning!

0

500

1000

1500

2000

2500

3000

3500

4000

0100200300400500600

table position in mm

att

en

uat

ion

I_0

/ I

0

50

100

150

200

250

300

350

400

tub

e c

urr

en

t

Attenuation

tube current

1.) Reference-Level-based Dose-Adaptation to patient size

2.) Dose-Modulation along z-axis3.) Angular Online-Dose-Modulation

Note: Without CARE Dose 4D

- the dose has manually adapted to patient size, which will frequently result in too high patient dose or reduced image quality.- the dose is constant over the whole scan range.Excessive dose in thin regions and excessive image noise in thick regions will result! - in inhomogeneous regions (shoulder, pelvis) the dose will be too high, without improvement of image quality.

- Image Noise Reduction (Adaptive Filter, Image filtration)

- Adaptive Dose Shield

- ECG-Pulsing

Dose Saving Features (Extract)

- ...

Page 72 Apr-10



13

Page 73 Apr-10



Principle of Radiation Protection

� Time

� Distance

� Shielding

Page 74 Apr-10



Principle of Radiation Protection for Patient (ASAP)

� Time (exp time) : As soon as possible

� Distance (Coverage) : As short as possible

� Shielding (H/W improvement) : As shielding as possible

Page 75 Apr-10





Page 77 Apr-10





14

Page 79 Apr-10



Adaptive cardiac Sequence scan

Page 80 Apr-10



Page 81 Apr-10



Flash scan

Page 82 Apr-10



1 heart beat scan

Page 83 Apr-10





Cardiac Dose Approaches

Overview

2

4

6

8

10

12

14

16

18

20

SOMATOM Definition Flash

Step-and-ShootStep-and-Shoot

Spiral CTA

Bhatti, SCCT 2007 abstract 19

Dewey, Rofo, Jun 2007

Mori, EurJRad, Jul 2007

Hausleiter, JAMA, Feb 2009

Rybicki, J Card Im, Mar 2008

Earls, SCCT 2007, abstract 16

Cole, SCCT 2007 abstract 15

Sablayrolles, RSNA 2007, abstract

Stolzmann, Eur Radiol 2007

McCollough, Radiology 2007

Hausleiter, SCCT 2007 abstract 17

Scheffel, Heart 2008

Stolzmann, Radiology 2008

Dose

(in mSv)

Run and Shoot

Spiral CTA

15

Page 85 Apr-10



- Take Home Point

Page 86 Apr-10



How dose technologist can do more better?

Page 87 Apr-10



Dose Saving Features (Extract)

- Proper filtration of X-ray beam(avoid quanta energies that produce only dose and don't contribute to the image)

- Shaped Filters

- Predefined Siemens Scan Protocols (in particular for children)

=> Proper setting of kV, eff. mAs, collimation, etc.

Focal Spot

Shaped Filter

Patient

Page 88 Apr-10



Dual Energy - Field of View

Patient Positioning Important Hints

Some general but Important Hints

Dual Energy information is only available in a

FOV of26 cm.

Page 89 Apr-10



Lower Dose Technique

For Conventional Radiography

� Image contrast of Film = Subject Contrast * Film Contrast (γ) * Dose

� Image contrast = Subject Contrast * Film Contrast (γγγγ) * kVp * mA * sec

� CXR = Lung↑ * (γγγγ) ↓* kVp↑* mA * sec↓ (↓↑)

For CT:

� Image contrast of CT = Subject Contrast * kVp *mA*sec

� Lower Dose CT Technique for Lung Screening

Page 90 Apr-10



Tube Voltage (kV)

The tube voltage controls the used x-ray spectrum.

Increasing the kV (at constant mAs) will� increase Pt dose

� decrease image noise

� decrease image contrast � (especially iodine contrast)

� improve image quality for big patients

� Change CT No.

0 20 40 60 80 100 120 1400

2

4

6

8

10

12

14

x 104

keV

quanta

per

mm

2 a

nd m

As in 1

m d

ista

nce

80 kV

100 kV

120 kV

140 kV

16

Page 91 Apr-10



Dose correlation - Overview

1.) CTDIvol ~ mAs

80 kV 100kV 120kV 140kV≈30% ≈60% 100% ≈150%

≈ 6 - 8 mGy/100mAs for typical Body Parameters* at 120kV

(referred to Phantom ∅ 32 cm)

≈ 12 - 18 mGy/100mAs for typical Body Parameters* at 120kV(referred to Phantom ∅ 16 cm)

*) Note: Special parameter settings (e.g. UHR) may lead to significantly higher values

2.) CTDIvol ↔ kV

Page 92 Apr-10



Tube current (mA)

The tube current controls the no, of photon.

Increasing the mA (at constant kVp) will� increase Pt dose

� decrease image noise

� improve image quality for big patients

Page 93 Apr-10



The user controls the SSP by choosing the effective slice width.

Increasing the slice width will have the effect of

� reducing image noise,

� blurring structures in the image with strong z dependence,� reducing contrast of small structures.

Note: The effective slice width can only be wider than the collimation!

Slice Sensitivity Profile (SSP)

Page 94 Apr-10



In spiral CT, the pitch is defined as

Increasing the pitch has the following impact:

� The tube mA has to be increased to achieve the same noise or dose

(this will be done automatically by the �SureView concept)

� The maximum achievable dose is reduced.� The scan time is shortened.

� In scans without z-sharp, stronger windmill artifacts may appear.

Pitch

ncollimatiodetector Total

rotationper feed Table

Page 95 Apr-10



� Due to the patented SOMATOM reconstruction algorithms, Siemens

offers image quality independent of the pitch.

� At a given mA (or mAs) value, the noise will vary with the pitch.

� To achieve the same noise and dose with varying pitch, it makes

sense to introduce a new mA-related quantity…

SureViewa unique Feature of all SOMATOM Scanners

Page 96 Apr-10



SureViewEffective mAs Concept

rPitchFacto

RotTimemA ⋅=mAs effective

� Holding the eff. mAs constant, the tube current increases with pitch.

Image noise depends only on eff. mAs.

Patient dose depends only on eff. mAs

System mAs limitation, effective mA = (mA * sec).

Only eff. mAs appear on UI (without CARE Dose 4D).

17

Page 97 Apr-10



OverviewScan & Recon Parameters

improvedworse

(motion)samesame

same (SureView)

Rot Time

worse(more)samemoremoreKernel

improvedlessless (z)less (z)lessEff. Slice

improvedsamesamesamelessEff. mAs

improvedsamemoresamelesskV

improvedworseless (z)less (z)lessColl. Slice

slightly

worse

worse

(without z-sharp)

samesamesame

(SureView)Pitch

Obese

PatientsArtifactsContrastSharpnessNoiseParameter

Incre

asin

g…

Will affect…

Sca

n P

ara

me

ters

Re

co

n P

ara

ms.

Page 98 Apr-10



-Conclusions

Page 99 Apr-10



Conclusion

=> Patient Dose has to be weighted against the diagnostic use to reduce the radiation risk

=> CTDIvol and DLP are well established to describe the physical dose of a CT-examination

=> Dose saving features help to reduce dose, if applied

=> Careful adjustment of scan parameters to patient type and examination type has to be done,

=> Displayed dose values have to be observed

Thank you for your attention!

Page 100 Apr-10



Siemens Computed Tomography.


Documents

Dose Modulation Technique in CT - aec.gov.tw · PDF fileCT Dose Modulation Technique RadioGraphics 2008; 28:1451–1459, Chang Hyun Lee, MD etc Page 35 Apr-10 For internal use only