Tube current modulation and dose reduction : How TCM works · 2017-11-16 · 09/11/2017 1 Tube current modulation and dose reduction : How TCM works Dean Pekarovič UMC Ljubljana,

09/11/2017

1

Tube current modulation and dose reduction :

How TCM works

Dean PekarovičUMC Ljubljana,

Institute of Radiology

Quality and Safety office

IAEA RER/9/135COURSE ON OPTIMIZATION IN COMPUTED TOMOGRAPHY

Sofia, Bulgaria, 2017

Pekarovic, Sofia, 2017

Plain Radiography

one kV valueone mAs value

What should be on Image ?Pulmo, Fat, Air, Bones …

Do we have same attenuation in all parts of the Image ?

09/11/2017

2


kV increase will

Decrease the mAs required to achieve a constant detector exposure (AEC on).

Decrease ESD and effective dose. Decrease image contrast. Increase scattered radiation.

Ref: ICRP 121, Radiological protection in paediatric diagnostic and interventional radiology

79 Kv0,71 mAsDAP 0,128 dGy cm2

EI: 250,9DI: -0,4524,98 µGy

67 Kv0,71mAsDAP 0,085 dGy cm2

EI 191,5

DI -0,7134,57 µGy

Radiologist to decide – contrast ribs/pulmo


mAs increase willIncrease detector exposure (without AEC) Increase radiation dose to the patient.

Improve image quality – increased CNR and SNR ?.


DI 3,23NO AEC


DI -0,9NO AEC

noise / higher mAs

09/11/2017

3


Lookin for proper ImQ DX

3 weeks, girl, 2760 g

65 kV DAP: 0,021 dGycm2

0,71 mAs DI : 0,35


0,9 mAs DI : 1,1558 kV DAP: 0,053 dGycm2

1 mAs DI : 2,26


1,21 mAs DI : 3,04, + Cu Filter70 kV DAP: 0,021 dGycm2

0,56 mAs DI : - 0,42, + Cu Filter

newborn girl 7 days2750 g


What we need is how to control theImQ and Dose

kV mAs EI DI DAP µGy mGy/s

90 1 231,53 -0,32 0,861

90 1,1 256,4 0,12 0,958 71,9 16,6

90 1,2 284,8 0,57 1,045

80 2 268,5 0,32 1,483

80 1,8 1246,5 -0,03 1,343 101 15,15

80 1,6 217,9 -0,6 1,181

72 3,6 238,5 -0,2 2,215 166 16,6

kV mAs EI DI DAP

90 1 233 -0,28 0,872

92 1 261 0,26 0,41691 1 245,7 -0,07 0,883

DI %3 1002 581 260 0-1 -21-2 -37-3 -50

How to improve DI1mAs cca 0,4 DI1 Kv cca 0,3 DI

CsJ Nastavljeni protokoli

Size range kv mAs focal spot target EI

OTROK PC 1 KG SMALL 68 0,45 1 226,22MEDIUM 70 0,56 1 226,22

LARGE 70 0,63 1 226,22range kv mAs focal spot target EI

OTROK PC 2 KG SMALL 70 0,45 1 226,22

MEDIUM 72 0,56 1 226,22LARGE 72 0,63 1 226,22

range kv mAs focal spot target EI

OTROK PC 3 KG SMALL 71 0,5 1 226,22MEDIUM 73 0,56 1 226,22


OTROK PC 4 KG SMALL 72 0,56 1 226,22

MEDIUM 74 0,63 1 226,22


OTROK PC 5-10 KG SMALL 75 0,63 1 226,22

MEDIUM 75 0,71 1 226,22


OTROK PC 10-15 KG SMALL 75 0,8 1 226,22

MEDIUM 77 0,8 1 226,22


OTROK PC 15-20 KG SMALL 80 0,71 1 226,22

MEDIUM 80 0,8 1 226,22


OTROK PC 20-30 KG SMALL 80 0,71 1 226,22

MEDIUM 85 0,8 1 226,22


OTROK PC 30-50 KG SMALL 85 0,9 1 226,22MEDIUM 90 0,9 1 226,22

LARGE 95 1,1 1 226,22

09/11/2017

4


From plain to CT …


CT -problem

09/11/2017

5


Tube Current Modulation

Purpose is to ensure the same picture quality regardless of the characteristics of the patient.

= same CNR in all images


Tube Current

• Determines the number of electrons accelerated across the x-ray tube per unit time

• Units: milliAmperes (mA)

• CTDIvol is directly proportional to Tube Current

CTDIvol Tube Current

REF : AAPM Computed Tomography Radiation Dose Education Slides

09/11/2017

6


TCM – along z axis mAs changes automatically

55mAs

130mAs110mAs

140mAs


Tube Current Modulation (TCM) / Automatic Exposure Control (AEC)

• Automatically adapts the Tube Current or Tube Potential according to patient

attenuation to achieve a specified image quality

– Automatic adjustment of Tube Current may not occur when Tube

Potential is changed

– Centering the patient in the gantry is VITAL for most AEC systems

• AEC aims to deliver a specified image quality across a range of patient sizes.

• It tends to increase CTDIvol for large patients and decrease it for small patients

relative to a reference patient size.

The use of Automatic Exposure Control may decrease or increase CTDIvol depending on the patient size and body area imaged and

image quality requested

REF : AAPM Computed Tomography Radiation Dose Education Slides

09/11/2017

7


TCM - Principe

• Reference data is collected from attenuation of anatomical structures during scanned projection radiograph -SPR

(one or two)

• Data collected from previous rotation.


TCM -principle

• TCM could be defined as a CT technique that performs automatic modulation of tube current in the x, y plane (angular modulation), or along the scanning direction, z-axis, (longitudinal modulation), or both (combined modulation).

Is really total „Automatic“ ? • The modification is done according to each patient’s size,

shape and attenuation of body parts being scanned.

• The operator must select a required image quality level and then the system can adjust the tube current to obtain the predetermined image quality with improved radiation efficiency.

Ref :Kalra MK et all, Computed tomography radiation dose optimization: scanning protocols and clinical applications of automatic exposure control.Curr Probl Diagn Radiol 2005; 34:171-181

09/11/2017

8


Start info..

• TCM needs input WHAT is needed Image quality = radiologist input, but team should be

involved; + radiographer + medical physicist

• Different vendor solutions :– Noise = SD CT-numbers on the Image

• (GE, Toshiba)

– Reference mA or mAs for a standard patient for specified quality

• (Philips, Siemens)


TCM idea

AP diameter : 36 cm

LAT diameter : 28 cm

09/11/2017

9


Scan where Tube Current Modulation was used

Blue Curve Represents actual instantaneous mARed Curve Represents avg mA for each imageYellow Curve Represents avg mA over entire scanOverall avg is used for CTDIvol reported in Dose Report


Dose Modulation -AEC

• Many CT scanners automatically adjust the technique parameters (and as a result the CTDIvol) to achieve adesired level of image quality and/or to reduce dose

• Dose Modulation and Reduction techniques vary by scanner manufacturer, model and software version.

Tip : read the manual and then test the AEC.

09/11/2017

10


Variations in mA modulation

Spatial mA modulation

– Longitudinal -Z axis modulation

– Angular / x-y axis / transverse modulation

– Combined / x-y-z modulation

Temporal mA modulation

– mA adapted at different time points using ECG gating (Cardiac CTA);not included in this PPT


TCM Modulations

Longitudinal (Z direction )

• Is an TCM feature that adjusts the Tube Current as patient size and attenuation changes of the anatomic region in the longitudinal direction.

• The CT Localizer Radiograph is used to estimate patient attenuation.

REF : AAPM Computed Tomography Radiation Dose Education SlideS

09/11/2017

11


TCM Modulations

Angular (X,Y direction)

• Is an TCM feature that adjusts the Tube Current as the x-ray tube rotates around the patient according to the size, shape, and attenuation of body region being scanned to compensate for attenuation changes with view angle.

• Angular Tube Current Modulation is used to adjust the Tube Current to attempt to deliver similar dose to the detector at all view angles.


Ref: M.K.Karla, Automatic Exposure Control in Multidetector-row CT

180°-mirror


TCM Modulations

Angular + Longitudinal (x, y, z direction)

• Is an AEC feature that incorporates the properties of both Angular and Longitudinal Tube Current Modulation to Adjust the Tube Current based on the patient’s overall attenuation.

• Modulate the Tube Current in the angular (X-Y) and longitudinal (Z) dimensions to adapt to the patient’s shape.

Ref: M.K.Karla, Automatic Exposure Control in Multidetector-row CT


09/11/2017

12


All in one

Ref : Marcus Söderberg; Automatic exposure control in CT


Principles are similar, but ..?

How it works on my CT modality

09/11/2017

13


Siemens : Care Dose 4D

• Implies need for image quality equal to that obtained with the use of specified reference mAs in a standard adult (70 kg -80 kg) or child (20 kg).

• Head Protocols –only x,y modulation

• Quality Reference mAs is NOT the max or min.

AEC System Operator-Controlled Z-Axis Angular X,Y

Vendor Name Parameter Parameter Explanation Principles Technique Modulation Modulation

Angular modulation of

tube current in the x,

Image quality y, and z axes on the

Siemens CAREDose4D reference mAs mAs that would be used for basis of patient size x-y-z/combined Yes Yes

an average-sized patient relative to the mAs

specified by the user

for a standard-sized

reference patient


Siemens : Care Dose 4D

• The degree to which the tube current is adjusted for patient size can be selected, using ‘very weak’ to‘very strong in 5 steps (high degree of mA adjustment)’, compensation settings.

Ref : Siemens and Sodberg, Automatic exposure control in CT

09/11/2017

14




Modulation of tube

current on the basis of Z-DOM D-DOM

Reference image Image quality expressed in patient size to achieve with ACS

Phillips Dose Right Ref mAs/slice terms of noise level of an the same image noise Yes Yes

ACS existing optimal clinical level as in a previously

image defined reference

image

Philips : Dose Right

• “Baseline mAs” is used as a reference to obtain constant image noise along the z axis.

• DoseRight ACS (automatic current selector) provides patients based AEC(object size), by use of a reference image

• DoseRight DOM (Dose Modulation)– Z-Dom: Z-axis AEC– D-Dom: Tube current is set so that 90% of images will have

equal or lower noise than the reference image, with remaining 10% of images in a series having equal or higher noise than the reference image.

• No ACS for Head CT protocols.

*miss new technique with x, y and z axis modulation


Philips : Dose Right*

DoseRight 3D-DOM (three dimensional dose modulation) combines angular and longitudinal patient information to modulate dose in three dimensions (x-y-z-axis). It incorporates modulation of tube current time product (mAs) according to changes in individual patient’s size and shape in the transverse (x-y-axis; angular) direction during helical scans, in addition to changes in the

craniocaudal or caudocranial (z-axis; longitudinal) direction, as the tube rotates.

Additional Parameter: Reference Noise Index

09/11/2017

15


GE : AutomA 3D



Measure of image quality/ Modulation of tube

noise level defined current only in the

Auto mA Noise index relative to uniform water longitudinal direction z axis/longitudinal Yes No

phantom to maintain a constant NI

Measure of image quality/ Modulation of tube

GE noise level defined current in the x, y, and

Smart mA Noise index relative to uniform water z axes to maintain a x-y axis/angular Yes Yes

phantom constant noise index

Auto mA 3D* Noise index x-y-z/combined Yes Yes

• GE : defined with NI (Noise Index)• NI : Std.Dev. of HU number in water phantom with standard algorithm used • NI CTDI vol

Auto mA use last scout if more then one were performed.

Smart mA MUST works with Auto mA


GE : AutomA 3D

Noise Index is Image Quality Parameter which sets the image noise in the image. Scout is used to determine patient attenuation characteristics and size and along with Noise Index the mA per rotation for the acquisition is determined. Noise Index will vary based on the slice thickness selected due to the difference in image noise relative to slice thickness. The same NI should never be used across all slice thicknesses.

Minimum and maximum mA - Range of allowed mA to achieve desired noise index

09/11/2017

16


Toshiba – SureExposure 3D

• Sure Exposure 3D is Longitudinal Modulation• XY – Modulation is Angular Modulation

• 2 possibilities for radiographer :• Standard Deviation –SD• Image Quality level –High, Standard..

SD of pixel values measured in a patient-equivalent water phantom.



User prespecifies image

quality on the basis of

Standard deviation of pixel a patient-equivalent

Sure Exposure Target Image quality values in an image water phantom, and x-y-z/combined Yes Yes

3D level Higher SD= higher noise mAs is modulated on Always ON

the basis of patient

Toshiba size to maintain image

quality

Sure Exposure z axis/longitudinal


LL (50cm), L (40cm), M (32cm), S (24cm), SS (18cm)

AEC setup allows tube current to be limited by max. and min. values.

Be careful with reconstructed slice thickness and reconstruction kernel.

Toshiba – SureExposure 3D

09/11/2017

17


Siemens -ref. mAs

Image Quality Parameters -TCM

Ref :M.P.Supanich,Tube Current Modulation,3rd CT Dose Summit

GE NI (Noise Index) Philips – mAs/slice


Scan and Reconstruction parameters

Which parameter effect TCM ?

Ref :

CARE Dose 4D * Yes Yes Yes

Dose Right Yes Yes Yes Yes Yes

AutomA 3D Yes Yes Yes Yes Yes

SureExposure 3d Yes Yes Yes Yes Yes Yes

*New Versions of CARE Dose 4D, a change in the tube volatage will result in a change in tube current by the AEC

** add to original Table

Localizer**AEC systemTube

Voltage

Rotation

timePitch Slice Thickness

Reconstruction

kernel

09/11/2017

18


Localizer -SPR

• Start and end of scan range.• Used for appropriate tube current modulation

(AEC in use).• Incorrect position in iso center leads to

inadequate tube current modulation (higher or lower).

• Incorrect position lead to incorrect geometry and size of scanned object.


6 cm from Center

TH 0 kV 120mAs 190AEC ON

LocalizerBoth has same exposure parameters

• kV 120• mAs 35• Length 256 mm• CTDI vol (32cm) 0,13mGy• DLP 3 mGycm• Table Height Δ 6cm

Positioned above isocenter (+6 cm)

TH +6 cm kV 120mAs 230AEC ON

1. Spiral mode• kV 120• mAs/ref mAs 204/180• Length 117 mm• Slice thickness 5 mm• CTDI vol (32cm) 13.6 mGy• DLP 145 mGycm

2. Spiral mode• kV 120• mAs/ref mAs 219/180• Length 117 mm• Slice thickness 5 mm• CTDI vol (32cm) 16.7 mGy• DLP 178 mGycm

22 % difference in dose from 6 cm table height difference.

09/11/2017

19


Isocenter – noise and CR

noise increase

isocenter

8

10

12

14

16

18

20

0 50 100 150

No

ise

(1

SD H

U)

y-coordinate (mm)centered 60 mm below

Ref : Eurosafe, M. Kortesniemi, D. Pekarovic, D.Sheppard

• Cylindrical 16 cm CTDI-head phantom scanned in center position and while lowered by 60 mm.

• Noise (1SD HU) increases vertically across the phantom in lower position as the beam shape is non-optimally targeted in the scan.

60

70

80

90

100

110

0 50 100 150

Co

ntr

ast

(HU

)

y-coordinate (mm)centered 60 mm below

isocenter

slight contrast change


Spr DIRECTION

09/11/2017

20


Scanning Outside The Localizer

• Localizer is basic source for TCM

• If scan region extends outside of localizer „we have a problem“.

Four possible outcomes in scan region not covered by localizer :

• Tube Current goes to maximum

• Tube Current goes to minimum

• Tube Current stays what it was at edge of localizer

• Tube Current goes to manual settingRef : TCM, M.Supanich,3rd CT Dose Summit


One or two and direction of thelocalizer

09/11/2017

21


TCM performance on QA phantom

Ref :Iball: A QA test for CT AEC systems , Journal Of Applied Clinical Medical Physics


0

25

50

75

100

125

150

175

200

225

250

122 43 64 85 10

612

714

816

919

021

123

225

327

4

mA

mA

120 kV53 mAs

120 kV146 mAs

mA mA mA mA mA mA

215 56,5 57 119,5 54,5 62

211 56 57,5 112,5 54,5 62

208,5 56 57,5 109 54,5 62

206 55,5 58 106,5 54,5 62

200 55,5 58,5 101,5 54,5 62

197 55,5 59 99,5 54,5 62

194 55,5 59 96 54 62

188 55,5 59,5 91 54 62

185 55,5 59,5 90 54 62

181,5 55,5 59,5 89 54 62

175,5 56 59,5 87 54 62

174 56 59,5 85 54 62

172,5 56 59,5 83,5 53,5 62

169 56 59,5 80,5 53,5 62

167 56,5 60 79,5 53,5 62

164 56 60 79 53,5 62

157 55,5 60 76,5 53,5 62

153,5 55,5 60,5 76 53,5 62

150 55 60,5 75 53,5 62

142,5 55 60,5 74 53,5 62

138,5 55 61 74 53,5 62,5

134,5 54,5 61,5 73 53,5 62,5

127 54,5 61,5 70 53,5 62,5

123 54,5 62 69 53,5 62,5

Example – how to start

09/11/2017

22


0

20

40

60

80

63

3

64

6.3

65

9.6

67

2.9

68

6.2

69

9.5

71

2.8

72

6.1

73

9.4

75

2.7

76

6

77

9.3

mA

26 Months

Thorax wo CM

Topogram Cr/Ca100kV 35 mAs

Helical100 Kv30 mAs AEC off

g/rot : 0,5 sCTDIvol: 1.23 mGyDLP : 23 mGy cm

Example

kV 100mAs 30


19 Months

Thorax +Adomen wo CM

Topogram Cr/Ca100kV 100 mAs

Helical100 Kv100 mA AEC off

g/rot 0,5 sCTDIvol : 4.50 mGyDLP : 182 mGy cm (+Abd)

kV 100mAs 100

0

100

200

1 27 53 79 105

131

mA

mA

Example

kV 100mAs 100

09/11/2017

23


18 Months

Thorax wo CM

Topogram Cr/Ca100kV

Helical100 Kv30 ref/ 27 mAs AEC on

g/rot 0,5 s

CTDIvol 4.50 mGyDLP 182 mGy cm

kV 100mAs 18

0

20

40

60

80

1 16

31

46

61

76

91

10

6

12

1

13

6

mA

mA

30 % mAs deviation in Z plane

Example


26 Months

Thorax wo CMTopogram Cr/Ca

120kV Helical

100 Kv20 ref/ 15 mAs AEC on

g/rot 0,5 sCTDIvol : 0.80 mGyDLP : 14 mGy cm

kV 100mAs 11

Example

09/11/2017

24


When should it be avoided?

A great tool, but sometimes ..

• Head exams? – Brain

– Orbits

– Sinuses

• CT Perfusion – Little or no table motion

– No chance to change cross section shape

– low manual technique


Team

• Therefore, it is paramount that radiologists know how to use the AEC systems in their own scanners. AEC systems do not reduce radiation dose per se; rather, they control radiation exposure relative to the requiredimage quality.

Only Radiologist ?

09/11/2017

25


EMAN - WP 8 :Training & Education

1.The medical practitioners requesting a CT examination. This group requires knowledge about indications for CT, its alternatives and the associated risks and benefits.

2. The core CT team that defines and optimizes the set of standard scan protocols on a specific scanner (radiographer, medical physicist and radiologist).This team will usually start with a standard set of protocols provided by the manufacturer and adapt it to the local needs. This team requires in-depth knowledge of scan parameters and how to optimize them.

3. The professionals (radiologists, radiographers) define the CT protocols. This group has to have knowledge when not to use CT, but another image technique, according to patient clinical indication. They are ultimately responsible for the individual choice of the correct protocol associated with each of the set of available standard protocols at a specific scanner / institution.

4. The radiographers that actually perform the examination. This group requires knowledge about individual routine adaptations required for each patient, such as centring of patients, adapting scan range, adapting protocol to patient size, optimizing modality performance in order to obtain the best diagnostic image at the lowest possible dose.


How to evaluate

• Understanding CTDIvol

• Dose Reference Levels ( will be disuseed..)

09/11/2017

26


CTDIvol – DLP – Effective Dose

CT “Dose Numbers”


Why CTDIvol ?

• CTDIvol provides information about the amount of radiation used to perform the study.

• CTDIvol is a useful index to track across patients and protocols for quality assurance purposes .

• CTDIvol can be used as a metric to compare protocols across different practices and scanners when related variables, such as resultant image quality, are also taken in account.

09/11/2017

27


DLP

• The Dose Length Product (DLP) is also calculated by the scanner.

• DLP is the product of the length of the irradiated scan volume and the average CTDIvol over that distance.

• DLP has units of mGy*cm.


CTDI

Absorbed dose in the slice.

Absorbed dose including scattercontributions from outside the slice

(CTDI).

The CTDI is calculated as the integral of the absorbed dose along the z axis, divided by the nominal slice thickness S.

Ref : Easy Guide to low dose,Siemens

09/11/2017

28


CTDIw

There are different ways to calculate the CTDI. One of them is to consider the differences between the absorbed dose in the periphery and in the centre of the patient’s body by a weighted sum of the central and peripheral CTDI values.



CTDIvol -DLP

The Dose Length Product (DLP) is the product of CTDIvol and the examination range.


The Dose Length Product (DLP) is also calculated by the scanner.

DLP is the product of the length of the irradiated scan volume and the average CTDIvol over that distanceDLP has units of mGy*cm

09/11/2017

29


Effective dose

Normalised values of effective dose per dose-length product (DLP) over various body regions.

Region of body

Normalised effective dose, EDLP

(mSv mGy-1 cm-1)

Head 0.0023

Neck 0.0054

Chest 0.017

Abdomen 0.015

Pelvis 0.019

E = EDLP · DLP

Effective dose E -measured in Sievert (Sv) units, includes the sensitivity to radiation of the different organs. It is the

sum of the equivalent doses in all irradiated organs multiplied by the respective tissue weighting factors wi.


ICRP 60 – ICRP 103

The Recommendations of the International Commission on Radiological Protection of 2007 (ICRP 103) has different coefficients than that of 1990 (ICRP 60).

In particular, gonads are less radiosensitive and the breast is more radiosensitive than previously assumed.

The effective dose E is an approximate measure that was introduced to compare the stochastic risk of a nonuniform exposure of ionizing radiation with the risk caused by a

uniform exposure of the whole body.

09/11/2017

30


Conclusions

Users must possess a good understanding of the concepts of noise index, standard deviation, reference images, and reference mA(mAs) as they relate to each AEC system.

Use User manual and ask vendor for additional papers.

Evaluate your own AEC system, for each software and scanner type.

Don`t accept one solution. Understanding will improve reports and CT Dose will become controllable.


Thank you

Documents

Tube current modulation and dose reduction : How TCM works · 2017-11-16 · 09/11/2017 1 Tube current modulation and dose reduction : How TCM works Dean Pekarovič UMC Ljubljana,