Dose Modulation Technique in CT

(1)

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

Dose Modulation Technique in CT

Short Overview

For internal use only / Copyright © Siemens AG 2009. All rights reserved. Stone Chen

Senior Application specialist Siemens Limited, Taiwan

Page 2 Apr-10

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

Multi Multi Multi

Multi----phase CCA / Single heart beatphase CCA / Single heart beatphase CCA / Single heart beatphase CCA / Single heart beat Power injector Volume Scan ? Multi-Tube design ? MTCT ? MTCT ? MTCT ? MTCT

Multi-phase CCA / ? Single heart beat Power injector Volume Scan ? Flat Plan CT ? FPCT ? FPCT ? FPCT ? FPCT

Multi-phase CTA / multi-organ dual head injector

Volume Scan Matrix formating design SS detector MDCT MDCT MDCT MDCT Multi Multi Multi Multi----phase / Multiphase / Multiphase / Multiphase / Multi----organorganorganorgan dual head injector

Volume Scan Matrix formating design SS detector MSCT

MSCT MSCT MSCT

Multi-phase / single organ Power injector

Helical Scan Low Voltage Slip Ring

Spiral CT Spiral CT Spiral CT Spiral CT Multi Multi Multi Multi----phase / single organphase / single organphase / single organphase / single organ Power injector

Heart Scan Electron gun & multi target design EBCT EBCT EBCT EBCT Dynamic scan Dynamic scan Dynamic scan Dynamic scan Hand-push Dynamical Scan Continuous X-ray Whole body CT Whole body CT Whole body CT Whole body CT Contrast enhancement Contrast enhancement Contrast enhancement Contrast enhancement air-drip infusion

Topo & Fast Scan Pulse X-ray Whole body CT Whole body CT Whole body CT Whole body CT Contrast enhancement infusion Transaxial Scan Brain CT Brain CT Brain CT Brain 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

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)

(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):

280 47 Lumbar Spine 770 25 Upper Abdomen 750 28 Pelvis 1500 24 Abdomen 650 22 Thorax 360 35

Cranium (Face) / Paranasal Sinuses

1050 60 Brain 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

60mAs

160mAs

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

(5)

Page 25 Apr-10

Advantage and Risk of Diagnostic with X-rays

Body Part Spatial Resoluction Low contrast Noise kVp mA Sec Algorithm AEC

E&T basically radiation _{induced cancer*}

information about pathology

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

Advantage and Risk of Diagnostic with X-rays

Diagnostic Information Radiation Risk basically radiation induced cancer* *) "stochastic damage".

information about pathology

Quality

Page 29 Apr-10

Advantage and Risk of Diagnostic with X-rays

Lesion Location Size

characteristic Radiation _Risk

basically radiation induced cancer*

information about pathology

Quality

Page 30 Apr-10

Define diagnostic image quality by adding noise

20mAs

160mAs

40mAs

60mAs

160mAs

103mAs

56mAs

28mAs

(6)

Page 31 Apr-10

- Dose Saving Features

Page 32 Apr-10

An Innovation Leader in

Low Dose Computed Tomography

CARE Dose 4D 1999 2002 Hand CARE 2005 1994 1997 Pediatric 80 kV Protocols DSCT 2008 Ultra Fast Ceramic (UFC) 2008 2008 Flash Spiral 2008 2007 4D Noise Reduction Adaptive Dose Shield Selective Photon Shield 2009 IRIS X-CARE

Up to 68%

Up to 30%

Up to 70%

Up to 50%

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 body axis a.p. lateral a.p. lateral a.p. lateral a.p. lateral atte nua_tion

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-axis

and to patient size

Example:

Thorax Protocol

Page 38 Apr-10

Define diagnostic image quality by adding noise

20mAs

160mAs

40mAs

60mAs

160mAs

103mAs

56mAs

28mAs

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

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

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

mAs-Adaptation along the patients z-axis

and 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

body axis a.p. lateral a.p. late ral a.p. lateral a.p. lateral atte_nua tion

(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

0 100 200 300 400 500 600 table position tu b e c u rr e n t in m A a.p. late ral mea n max Page 45 Apr-10

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

189mAs 199mAs

(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. Example: Shoulder Scan Saved Dose lateral a.p. , p.a. Example: Shoulder Scan Page 51 Apr-10

0

500

1000

1500

2000

2500

3000

3500

4000

0

100

200

300

400

500

600

table position in mm

a

tt

e

n

u

a

ti

o

n

I

_

0

/

I

0

50

100

150

200

250

300

350

400

tu

b

e

c

u

rr

e

n

t

Attenuation

tube current

Optimal mA for AP and lateral Views:

On-line mA modulation

Page 52 Apr-10

Headline

55mAs

130mAs

110mAs

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 pos ition

a tt e n u a ti o n I _ 0 / I 0 50 100 150 200 250 300 350 400 tu b e c u rr e n t Att mA 0 500 1000 1500 2000 -250 -200 -150 -100 -50 table position a tt e n u a ti o n I _ 0 / I a.p. lateral -250 -200 -150 -100 -50 table position tu b e c u rr e n t in m A a .p.

late ral max

Page 54 Apr-10

CARE Dose 4D – First Results

Dose reduction in %, compared to SOMATOM Sensation16 standard

protocols (140 patient studies)

Pelvis

Abdomen

Thorax

Average

34%

40%

67%

39%

Abdomen

/ Pelvis

33%

37%

64%

26%

37%

Thorax /

Abdomen

Shoulder

-13%

Shoulder

51%

Head

Pelvis

Abdomen

Thorax

Average

4%

38%

13%

Thorax

Abdomen

Thorax

Average

17%

66%

38%

Neck

Shoulder

Neck

Average

Organ specific Reduction in Scan Range

Dose

Reduction

Protocols

(10)

Page 55 Apr-10

CARE Dose 4D

Automatic

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

Isocenter positioning of patient is essential!

X-ray tube

Detector

Patient

(not centered)

Distorted Topogram does

influence the mAs Calculation!

CARE Dose 4D:

Topogram

Page 60 Apr-10

Isocenter positioning of patient is essential!

X-ray tube

Detector

Patient

(not centered)

Distorted Topogram does

influence the mAs Calculation!

CARE Dose 4D

Topogram

(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

displays

displaysthetheaverageaverageeffeff. . mAsmAsthatthat will

will bebeappliedappliedbybyCARE Dose 4D CARE Dose 4D forfor the

thecurrentcurrentscanscanrangerange(will (will bebe updated

updatedto to thethereal real appliedappliedeff. eff. mAsmAs after

afterthethescanscan--migthmigthdifferdiffer somewhat somewhat)) 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

Switch SwitchforforCARE 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 tu b e c u rr e n t mean tube current per rotation angle modulated tube current

Impact of system limits

Page 68 Apr-10

For internal use only / Copyright © Siemens AG 2010. All rights reserved. Medical CS SLT Stone Chen scan range z tu b e c u rr e n t mean tube current per rotation angle modulated tube current

upper system Limit (max. tube current)

Impact of system limits

Page 69 Apr-10

For internal use only / Copyright © Siemens AG 2010. All rights reserved. Medical CS SLT Stone Chen scan range z tu b e c u rr e n t mean tube current per rotation angle modulated tube current

upper system Limit (max. tube current)

Impact of system limits

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 0 100 200 300 400 500 600 table position in mm a tt e nu at io n I_ 0 / I 0 50 100 150 200 250 300 350 400 tu b e c u rr en t Attenuation tube current

1.) Reference-Level-based Dose-Adaptation to patient size 2.) Dose-Modulation along z-axis

3.) 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-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 of

26 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 (

γ

) * kVp * mA * sec

CXR = Lung

↑

* (

γ

)

↓

* 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 80 100 120 140 0 2 4 6 8 10 12 14 x 104 keV q ua n ta p e r m m 2 a nd m A s i n 1 m d is ta nc e 80 kV 100 kV 120 kV 140 kV

(16)

Page 91 Apr-10

Dose correlation - Overview

1.) CTDIvol~ mAs

80 kV 100kV 120kV140kV ≈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

n

collimatio

detector

Total

rotation

per

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…

SureView

a unique Feature of all SOMATOM Scanners

Page 96 Apr-10

SureView

Effective mAs Concept

r

PitchFacto

RotTime

mA

⋅

=

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

Overview

Scan & Recon Parameters

improved

worse

(motion)

same

(SureView)

Rot Time

worse

(more)

same

more

Kernel

improved

less

less (z)

less

Eff. Slice

improved

same

less

Eff. mAs

improved

same

more

same

less

kV

improved

worse

less (z)

less

Coll. Slice

slightly

worse

(without

z-sharp)

same

(SureView)

Pitch

Obese

Patients

Artifacts

Contrast

Sharpness

Noise

Parameter

In

c

re

a

s

in

g

…

Will affect…

S

c

a

n

P

a

ra

m

e

te

rs

R

e

c

o

n

P

a

ra

m

s

.

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 => CTDIvoland 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