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Automated dose control in multi-slice CT Nicholas Keat Formerly - - PowerPoint PPT Presentation

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Automated dose control in multi-slice CT Nicholas Keat Formerly ImPACT, St George's Hospital, London UKRC 2006 Introduction to presentation CT contributes ~50+ % of all medical radiation dose Ideally all patients would receive just

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UKRC 2006

Automated dose control in multi-slice CT

Nicholas Keat Formerly ImPACT, St George's Hospital, London

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UKRC 2006

Introduction to presentation

  • CT contributes ~50+ % of all medical radiation dose
  • Ideally all patients would receive ‘just enough’ radiation to

produce a diagnostic image

– Extra radiation provides no clinical benefit, but extra dose

  • Controlling exposure usually achieved with ‘standard’

protocols

– These usually err on the side of over-exposure

  • Automatic exposure controls (AECs) introduced on CT

scanners to address these issues

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UKRC 2006

X-ray exposure

  • X-ray film needs correct exposure to get the best image
  • Phototimers used since ~1940 to set x-ray exposure time
  • verexposed

underexposed

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AEC systems in CT

  • CT uses digital detectors, not easily under or over-exposed
  • Over-exposure leads to better image quality!

– Under-exposure gives noisy or streaky images

  • Manufacturers have introduced CT AEC systems in last

three years

  • CT has caught up with general x-ray, 60 years after

introduction of the phototimer

– In CT, tube current, not exposure time is being controlled

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CT scanner exposure pattern

  • CT scanner exposure is highly localised

– Good opportunity for AEC optimisation

Power Data

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Variable patient attenuation

  • Attenuation of x-rays varies according to patient density and

thickness

– Each patient is a different size – Cross sectional diameters change along patient length – Bones highly attenuating, lungs low attenuation

  • Signal to detectors varies inversely to attenuation

Pelvis Shoulder

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CT AEC principles

  • mA adjusted to compensate for attenuation differences

– dose applied to patient only where needed – image quality less variable

mA position

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Patient attenuation

  • Assessed from SPR (plan) view, or from feedback from

previous rotations

  • Tube current adjusted accordingly

z-axis position attenuation

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Advantages of AEC

  • More constant level of x-ray signal to detectors

– Avoids under- and over-exposing detectors

  • Image quality is kept at a constant level

– From patient to patient, and during single study

  • Tube heat capacity is conserved

– Avoids tube cooling delays

  • Reduction in ‘photon starvation’ streak artefact

– Caused by under exposure of detectors

  • Dose optimisation becomes easier

– CT scan setup is based on image quality, not tube current

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Dose and image quality

  • Dose and image quality are opposite sides of the same coin

– Good image quality ‘costs’ x-ray exposure

  • AEC systems operate by varying tube current (mA)

– Patient dose proportional to mA – Image noise proportional to 1/√mA

  • AECs are generally operated by specifying image noise

characteristics

  • Specifying patient protocols using image noise levels has

implications for patient dose

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Present AEC systems

  • AEC systems available on multi-slice systems are applied at
  • ne or more levels:

*GE LightSpeed Pro scanners only ** Work in progress

Patient size AEC Z-axis AEC Auto mA DoseRight ACS DoseRight ZDOM CAREDose 4D

SUREExposure

mA modulation GE SmartmA* Philips DoseRight DOM Siemens Toshiba **

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Methods to set AEC exposure level

  • Different methods exist to define the exposure level using

AEC systems

* new method based on reference mAs forthcoming Manufacturer Method for setting exposure level GE ‘Noise Index’ sets required image noise level Philips A ‘Reference Image’ is used, which has the desired level of image noise.* Siemens ‘Equivalent mA’ set for standard sized patient Toshiba Set required standard deviation (noise)

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ImPACT cone phantom

  • Conical Perspex phantom with elliptical cross section
  • Based on ‘Apollo’ phantom developed by Muramatsu,

National Cancer Centre, Tokyo Side view

CT scanner couch Catphan carrying case

End view

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Cone phantom

  • Images along length of phantom (AEC off)
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Cone phantom

Coronal view Sagittal view z-axis AEC off z-axis AEC on

Noise increases Constant noise

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Scan protocol

  • Standard conditions:

– 120 kV, approx 200 mA, 1 s or less rotation time, – wide collimation e.g. 20 mm, 5 mm slice, 45 cm reconstruction field of view

  • Scan along phantom with AEC off and on

– If possible select different features of AEC separately

  • Change exposure level – increase desired standard

deviation or reference mA

  • Look at effect of different kVs
  • Change helical pitch and direction of tube movement
  • Store DICOM images on CD
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Image analysis

  • mA information retrieved from DICOM files
  • Standard deviation (SD) and average CT number calculated

at centre and edge of image using automatic analysis tool

  • Region of Interest (ROI) size

2000 mm2

  • Results analysed using Excel
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Results from testing

  • Aims of each AEC system are slightly different, so it is

difficult to compare results

  • In general, all systems successfully achieved their aims
  • Following slides show a selection of the results, much more

data has been gathered

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UKRC 2006

Results: GE - axial

5 10 15 20 25 30 35 50 100 150 200 250 300 AP phantom diameter (mm) Measured SD

Auto mA OFF NI = 5 NI = 10 NI = 15 NI = 20

Noise Index mA Mean SD AEC off 200 10-783 10 10-783 11.0 15 10-500 18.0 20 10-280 27.3

  • 5

4.4

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Results: GE - axial

100 200 300 400 500 600 700 800 900 50 100 150 200 250 300 AP phantom diameter (mm) Tube current (mA) Auto mA OFF NI = 5 NI = 10 NI = 15 NI = 20

10 100 1000 50 100 150 200 250 300 AP phantom diameter (mm) Tube current (mA)

Auto mA OFF NI = 5 NI = 10 NI = 15 NI = 20

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Results: GE - helical

  • Noise index 12, different helical pitch, table movement in

and out of gantry

2 4 6 8 10 12 14 16 50 100 150 200 250 AP phantom diameter (mm) Measured SD 0.563, in 0.938, in 1.375, in 1.75, in 1.375 out 1.75 out

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Results: Toshiba

  • Data from RealEC on Aquilion 16

5 10 15 20 25 30 50 100 150 200 250 300 AP phantom diameter (mm) Measured SD

Fixed mA SD 5 SD 10 SD 12 SD 17

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Results: Philips

  • Mx8000 IDT has patient size AEC, and mA modulation

3 scans planned, at different z-axis positions, patient AEC off

2 4 6 8 10 12 14 16 18 20 50 100 150 200 250

AP phantom diameter (mm) Measured SD Series 1 - 200 mA Series 2 - 200 mA Series 3 - 200 mA

3 scans, patient AEC on

2 4 6 8 10 12 14 50 100 150 200 250

AP phantom diameter (mm) Measured SD Reference Image Series 1 - ACS ON Series 2 - ACS ON Series 3 - ACS ON

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Results: Siemens

  • System does not aim to keep noise constant

– Smaller patients may need better quality images

  • Three ‘strengths’ of AEC

2 4 6 8 10 12 14 16 50 100 150 200 250 AP phantom diameter (mm) Measured SD Average Weak 10 100 1000 50 100 150 200 250 300 AP phantom diameter (mm) Tube current (mA) Strong Average Weak Strong Constant Noise

AEC OFF AEC OFF

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Know your AEC!

  • Each AEC responds differently to changes in scan and

recon parameters

– Important to know how your system will react!

Manufacturer Tube voltage Rotation time Helical pitch Image thickness Recon kernel GE

  • Philips
  • Siemens

Toshiba

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What is the is optimum AEC setting?

  • Depends on the application

– One body part may require different IQ levels depending upon clinical requirements

  • How do we find this out?

– Critical evaluation of image quality, feedback – Simulation studies

  • Responsibility for manufacturer to develop good default

protocol settings

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What IQ or dose is needed?

  • What image quality is required?

Simulated dose: 0.9 Simulated dose: 0.8 Simulated dose: 0.7 Simulated dose: 0.6 Simulated dose: 0.5 Simulated dose: 0.4 Simulated dose: 0.3 Simulated dose: 0.2 Simulated dose: 0.15 Simulated dose: 0.1 Simulated dose: 0.075 Scanned dose: 1

Images courtesy Y. Muramatsu, NCC Tokyo

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What do AECs give us?

  • Lower patient doses than before?

– Possibly, but this is by no means a foregone conclusion – It is possible to use AEC and give higher dose than previously – Keep monitoring CTDIvol and DLP – expect larger variations

  • More consistent image quality?

– Yes…

  • The optimum image quality?

– If they are used well

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Conclusions

  • AEC systems offer potential benefits for everyone

– Radiologists: image quality consistent from patient to patient – Radiographers: consistent IQ for different sizes is now simple – Patients: potential for dose reduction, repeat exams less likely – Physicists: protocol optimisation is easier

  • Users need to understand the systems

– How does mA vary when changing slice thickness or kernel?

  • The current systems work as intended, but there is
  • pportunity for manufacturers to improve them further

– Optimisation of scan protocols with AEC – A common method for defining image quality would be useful – Potential for AEC to control scan times and kV too

  • ImPACT AEC report: www.impactscan.org/bluecover.htm
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Challenges for manufacturers and users

  • Optimisation of scan protocols

– Work required to ensure that radiologists are getting good image quality, and patient doses are under control

  • Standardisation of method to set exposure/IQ

– A single method would aid comparison of scan protocols from many scanners or scanning centres

  • Education of users

– AEC users need to know the details of their system, how it differs from others