Dose reduction

Radiation dose modulation and dose reduction techniques

Introduction to presentationCT Dose Index CTDI100

CTDIvol

DLP Effective dose CT dose reduction techniques AEC Filter Reconstruction algorithm

CT Dose Index CT dose index (CTDI) is the

standardized measure of the radiation output of a CT system,

Currently used measures are the CTDI100 and CTDIw, modified to CTDIvol for modern helical scanners. 

CTDI phantoms(PMMA)

Measurement of CTDI• Ionisation pencil chamber

– Air-filled chamber – X-rays cause ionisations in

chamber, causing current to flow, proportional to exposure

– Instant readings, good accuracy, easy to use

100 mm

Measurement of CTDI• Single slice measurement

(z-axis)

Dose

TNominal beam width

dzzDT1 CTDI

-

dzzDT1 CTDI

50

50-100

-50 +50

50

50

D(z)dzn.T1CTDI100

n = no. slices imaged simultaneously T = nominal imaged width

Davge

T100DavgeCTDI100

CTDI100 • CTDI100 is a measure of radiation dose on a 100mm

long pencil ionization chamber.• Centre • Periphery - 1 cm depth (mean of 4 positions)

C

P1

P2

P3

P4

320 mm

‘Body’

‘Head’

160 mm

140 mm

Measurement of MSAD(Multiple Scan Average Dose)

CTDI ‘Series’ doseMulti-scan

‘Series’ dose

CTDI

I

TN MSAD

T

T is the nominal scan width (mm)

I is the distance between scans (mm)

I

N is the number of scans

N × T is the total nominal scan width

CTDI100 • Typical CTDI100 values (mGy)

40

Head

Body

40

40 40

40

20

20 20

20

10

Periphery : centre 1:1 Periphery : centre 2:1

Derivatives of CTDI100 • CTDIW

• Weighted average CTDI: represents the average dose for contiguous irradiation

• CTDIw = 1/3 CTDI100,C + 2/3 CTDI100,P

CTDIC CTDIP

Derivatives of CTDI100

• CTDI100 & CTDIw refer to a pitch of 1

• Average absorbed dose dependent on pitch

Contiguous Extended Overlapping

CTDIvol

• Volume CTDI: Considers contiguous scanning

• CTDIvol = CTDIw Pitch

Pitch = 1 CTDIvol = CTDIw

Pitch = 2 CTDIvol = CTDIw

2

Pitch = 0.5 CTDIvol = 2 x CTDIw

Radiation risk: DLP• CTDIvol is a measure of absorbed dose, energy

absorbed per unit mass • To measure radiation risk from stochastic effects

the total energy absorbed must be considered • In CT this can be estimated using the Dose Length

Product, DLP

DLP = CTDIvol . L (mGy.cm)

DLP1 DLP2

where L = scan length

Dose length product

L1

T

Pitch 1 8 rotations

Pitch 2 8 rotations

L2

T

as CTDIvol2 = CTDIvol1

2

Estimates of effective dose (E)• Can be obtained from DLP • Effective dose = DLP. CF (mSv) • • • • • • Conversion factors not scanner specific or location

specific

Region of body Conversion factor,EDLP (mSv mGy-1 cm-1)

Head & neck 0.0031 Head 0.0021 Neck 0.0059 Chest 0.014 Abdomen& pelvis 0.015 Trunk 0.015

CT dose reduction techniquesAEC systems have a number of

potential advantages, including better control of patient radiation dose, avoidance of photon starvation artifacts, reduced load on the x-ray tube, and the maintenance of image quality in spite of different attenuation values on CT scans

Types of AECPatient-Size AEC

Z-axis AEC

Rotational or Angular AEC

Operation of AEC Systems on Different Multidetector CT

Scanners Exposure 3D CARE Dose 4D

Patient-Size AEC Patient size AEC: the tube current is adjusted based on the overall size of the patient to reduce the variation in image quality between small patients and large patients. For a given patient size, the appropriate milliamperage is selected and is used for the entire examination or scan series

Hi-mA

Lo-mA

Z-axis AEC: tube current is modulated according to patient attenuation along the z-axis.The goal is to reduce the variation in image quality of images from the same series.

atte

nuat

ion

Patient-Size AEC Rotational AEC: The tube current is decreased and increased rapidly (modulated) during the course of each rotation to compensate for differences in attenuation between lateral (left-right) and A-P (anterior-posterior) projections .

high

atte

nuat

ion

low attenuation

Combination of AEC functions • Tube current is adjusted during scanning to compensate for

attenuation differences – dose applied to patient only where needed, avoiding dose where it isn’t

mA

position

On line’ modulation–uses attenuation data from previous rotation–adapts tube current to patient attenuation ‘on the fly’

Scan projection radiographs (SPRs, known as scout, scanogram or topogram views) are the main way that AEC systems assess the attenuation of the patient in order to set the tube current

To obtain correct attenuation data from SPR always centre the patient carefully

Patient is positioned in the isocenter – optimal dose and image quality

Patient is positioned too high - increased mAsPatient is positioned too low - reduced mAs and increased noise

Toshiba:defining image quality requirements

Specify s.d. level (or ‘image quality level)–patient mAcalculated to achieve this noise level at any scan parameter settings

Set min & max mA

Benefits of CT scanner AEC Consistent image quality

Potential for dose reduction through

exposure optimisation

Reduced tube loading

Extended scan runs(OLP)

Reduction in photon starvation artifact

30

• Images along length of phantom (no AEC)

• • • • • • • •

Constant mA •

Testing the AEC

31

Testing the AEC

• Measure noise with AEC off and on • Monitor mA, CTDIvol

0 4 8

12 16 20 24 28

-150 -100 -50 0 50 100 150 Z-position (mm)

Noi

se (%

)

automA off

Noise Index 12

Increased mA

Decreased mA

Constant mA

Same mA

Increased mA

Decreased mA

32

Coronal view Sagittal view

z-axisAEC off

z-axis AEC on

Noise increases

Constant noise

Testing the AEC – Viewing with MPR

photon starvation artifact

without angular mA modulation with angular mA modulation

Bowtie FilterLonger bowtie path lines up with shorter

patient path

Reduce x-ray scatter (noise and artifact)

Maintain uniform x-ray at detectorReduce surface dose by 50%

• Retrospectively gated CCTA(dose modulation )

~15 mSv 100%

20% ~30% reduction

Image obtained in middiastole (75% of R-R interval)

reconstructed using 1.5-mm slice thickness at high tube current

shows low-noise

Image obtained in midsystole (30% of R-R interval)

reconstructed using 1.5-mm slice thickness shows higher-noise

image obtained at low radiation dose

• Prospectively gated CCTA

~ 1 mSv

Prospective Gating of CTA(snapshot plus)

40mm

5mm overlap

Scan range

Conventional technology without Dose Shield

SOMATOM Definition AS+ with Adaptive Dose Shield

Scan range

Dynamic Collimation• In helical scanning extra rotations are needed at end of imaged

volume – Significant extra dose: wide beam widths and short scans

• Dynamic collimation - collimator blades open and close asymetrically at start and end of scan

(a) conventional and (b) adaptive section collimation CT scanning protocols. Foradaptive section collimation, shape of x-ray cone beam at beginning and end of spiral acquisition is controlled by two collimators made of absorbent material.

Image reconstruction(FBP)

attenuationdetector position

90°2 projections4 projections8 projections16 projectionsat

tenu

atio

ndetector position

0°

0(3)

0(9)

0(7)

0(5)

6 6 6 6

10 14

1212

12 12

5 7 5 7

3 9 7 5

3 5

9 7

816

12

12

+1-1

-2+2

Iterative reconstruction

Iterative reconstruction

120 kVp and 300 mA FBP

120 kVp and 300 mA ASiR 100%

120 kVp and 300 mA ASiR 50%