CT AEC Part 1 - automatic mA modulation

Module Learning Objectives

• Critically reflect on the relationship between the physical principles of computed Tomography (CT) and the design and applications of CT technology.

• Critically appraise a broad range of CT applications and protocols.

• Evaluate complex issues in a systematic and creative manner.

Session Objectives

• Patient size and shape.

• Tube Current Modulation

  • AEC (Automatic Exposure Control)

  • How it works

  • System differences

  • Tips, tricks, and pitfalls

Patient Size Metrics:

• Weight

• Height

  • BMI (Body Mass Index)

• Diameter

  • Measured from scout view

• Circumference

  • Measured pre-scan

Patient Size In Real Terms

• BMI data from CT Lung Screening Trial in Leeds (55-80 age range)

Dose Issues with Specific Patient Types

• Paediatrics

• Thin patients

• Elderly

• Obese

• Scans through both the thorax and abdomen

• Scans for patients with regular follow-up

• Major trauma

Size vs. Attenuation

• Which is more important in imaging?

• Why?

Attenuation Variation Along Patient

Attenuation Data from Scout View

Helical Scan Considerations

• Fixed tube current:

  • What will we get if we use a fixed tube current for this scan?

  • Why or when might we do this?

• Alternative: Automatic Tube Current Modulation (ATCM)

The Problem: Patient Size Variation

• No patient is a 'standard' size.

  • What is the supposed "standard" adult male patient size? (70kg)

  • Is that the "average" patient size?

Patient Size/Shape Problem

• Patients are different shapes & sizes.

• Difficult to achieve constant Image Quality (IQ).

• Actually obtain:

  • Varying noise levels throughout a scan

  • Varying noise levels from patient to patient

• Potential for missing detail.

• Potential for incorrect dosage.

AP & Lateral Attenuation Changes

• X-ray tube and detector positions influence attenuation.

Automatic Exposure Control (AEC)

What is AEC?

• A system that automatically adjusts exposure parameters according to patient size/attenuation.

• Types:

  • Automatic mA modulation

  • Organ dose modulation

  • Automatic kV selection

Automatic mA Modulation

• Aims to achieve broadly consistent image quality throughout a scan.

  • Accounts for longitudinal and rotational changes in attenuation.

• Aims to achieve broadly consistent image quality for all patients.

  • Accounts for changes in patient size & attenuation.

• Adjusts tube current (mA) based on scout scan to achieve image quality level requested in protocol.

How It Works (Automatic mA Modulation)

• Initial exposure parameters determined using attenuation profiles obtained from scout.

• mA modulation may be programmed to take account of angular variations in patient shape, e.g., shoulders.

• Exposure may be modified during acquisition using feedback from previous detector readings at the same angle.

  • Feedback during scan adjusts the mA in real-time based on changes in detector readings as patient size varies throughout the scan.

Benefits of Varying mA

• Vary mA according to patient size.

• Vary mA along patient length.

• Vary mA during rotation.

• Benefits:

  • ‘Consistent’ noise in all images.

  • Artifact reduction.

  • Significant dose reduction (potentially).

  • Tube heat capacity conserved.

Automatic mA Control

• Tube current is adjusted during scanning to compensate for attenuation differences.

• Dose applied to patient only where needed, avoiding dose where it isn’t.

mAs Adjustment Theory

• Theory for x-ray CT:

  • Half Value Thickness in tissue $\sim$ 4cm

  • To maintain constant noise, double mAs for every extra 4 cm of tissue.

  • $mAs<em>{new} = mAs</em>{original} * 2^{(\frac{\Delta Diameter}{4})}$

AEC System Differences

• Two types of AEC system:

  • Constant noise:

    • Strong adjustment of mA with patient size.

    • Yields same image quality (noise) for all patients.

    • Large variation in doses across range of patient sizes.

    • Used by Canon, GE & FujiFilm

  • Adequate noise:

    • Asserts that better image quality (lower noise) is needed for small patients due to lack of internal fat – higher noise can be accepted for large patients.

    • More gentle adjustment of mA with patient size.

    • Smaller range of dose across range of patient sizes.

    • Used by Siemens & Philips & GE (!)

Control Methods for CT Automatic mA Modulation

1) Standard Deviation (SD) of CT Numbers

• Acceptable image quality is based on acceptable noise level (SD) in a region of patient-equivalent water phantom.

• Higher standard deviation = higher noise, therefore lower dose (allowing for limits of scanner).

• mA modulated based on the difference between the phantom and the patient to achieve the pre-specified image quality.

• Constant Noise AEC system.

Canon SUREExposure 3D

• Scannogram(s) used to convert patient into series of water equivalent cylinders.

• Scanner works out mA needed to generate a reconstructed reference image of each water-equivalent diameter cylinder that has a noise level equal to your specified SD value.

• Reference image is reconstructed using the specified SureIQ settings, including the reference reconstruction filter.

• Changing the reference recon filter strongly influences how much mA is needed to achieve the specified SD level, therefore affects dose!

• For example, changing it from FC11 to FC18 will double the dose to every patient.

• Operator can set maximum mA value for each examination.

• Dose information prediction available during scan planning.

• Rotational (x-y) modulation can be switched on or off separately from z-axis modulation.

FujiFilm IntelliEC

• Scannogram(s) used to convert patient into series of water equivalent cylinders.

• Scanner works out mA needed to generate a reconstructed reference image of each water-equivalent diameter cylinder that has a noise level equal to your specified SD value.

• Reference image is reconstructed using the specified reconstruction settings, including the level of iterative reconstruction.

• Changing the iterative recon level strongly influences how much mA is needed to achieve the specified SD level, therefore affects dose!

• Operator can set min and max mA value for each examination.

• Dose information prediction available during scan planning.

2) Noise Index (NI)

• Acceptable image quality is based on a Noise Index (acceptable noise) set from a patient equivalent elliptical water phantom.

• NI relates to sd in phantom rather than in patient.

• NI must be matched in every image through the scan series.

• Constant Noise AEC system.

• GE

  • Auto mA

    • Z-axis modulation only

  • Smart mA

    • Combined modulation

  • Minimum and maximum mA values specified by user.

  • Data gathered from scout – most recent scout view used.

  • Modulates 4 times per rotation.

  • Some small ramp time in adjusting mA between the 4 sectors.

Size-adjusted noise index (SANI)

• Available as part of Auto Prescription software

  • Automatically adjusts prescribed NI according to patient size

    • "Allows users to better tailor noise and dose levels to their preference"

  • Higher NI for large patients, lower NI for smaller patients

• Adequate noise AEC system!

3) Reference Case / Image

• An adequate ‘ideal reference image’ is selected from an existing clinical case

• mA is then modulated to match the image quality in this image

• Set up so that 90% of examinations will have noise levels less than or equal to the reference image

  • 10%, therefore, have a higher noise level

• Adequate Noise AEC system

Philips

• DoseRight 3D

  • ACS – Automatic Current Selection (patient size)

  • D-Dom (angular)

  • Z-Dom (longitudinal)

  • Can’t use all three together

• iPatient

  • Range of reference patient sizes

  • Patient-specific optimisation of image quality and dose

  • Dependent on patient type (age, size, fast exam needed?) and diagnostic task

• Attenuation data from SPR

  • Matches patient to a water equivalent diameter

4) Image Quality Reference mAs

• A reference effective mAs ($mA \times gantry \ rotation \ time / pitch$) is selected for an average sized patient (75kg) for a particular examination

• Provides adequate image quality based on the reference mAs

• Allows higher image noise for larger patients and less for smaller ones

• Adequate Noise AEC system

Siemens

• CAREDose 4D

  • assesses patient attenuation from topogram and plans mA adjustment

  • x-y modulation is controlled in near real-time, from information from the previous 180º of rotation

  • Very weak to very strong compensation settings can be selected to adjust for patient attenuation

  • Functionality is body region dependent

• The determination of quality reference mAs according to patient size is not a linear function.

System Summary

*some compensation at extremes of patient size

Summary

• Many systems specifying a level of image quality, some specify a dose level for a standard patient/phantom size

• Difficult to set up comparable protocols if you have systems from different manufacturers

• Very important to assess image quality when setting up AEC / auto mA systems

  • Dose alone does not tell the whole story

  • How might you objectively measure image quality?

Image Quality Measurement

• ROI placed in liver

• Record CT# (HU) and noise (s.d.)

• Measure AP & Lat size

• Plot noise against size

Effect on Patient Dose

• Red is constant noise system

• Blue, green, and orange are adequate noise systems with different implementations

Scanning Off End of Scout

• mA modulation may do funny things:

  1. apply minimum mA specified in protocol

  2. apply mA used for previous rotation

  3. apply maximum mA in protocol (or max possible)

  4. apply default manual mA

• ALWAYS perform scout longer than intended helical/axial scan

  • System then has sufficient information to calculate how to modulate tube current

Tips for Optimising AEC Use - Scout

• Always scout longer than length you need to scan

• Additional dose from extra few cm of scout may be saved by AEC knowing what to do

• Better to re-scout than scan off the end of a short scout

Tips for Optimising AEC Use - Centering

• Ensure patient is properly centered

• AEC chooses mA values based on patient size in topogram

• Up to 80% increase in dose for 7cm table offset!!!

• Accurate positioning is essential when AEC is used

Dose Variation with Couch Height

Auto Positioning Systems

AEC Use - Arm Positioning

• Scan chests with arms up wherever possible

• 83% increase in mA

Which Scout View?

• Delivered dose can depend on which scout view is used for mA planning

• Measured attenuation (projection area) different between to views, hence different mA calculations

• Will result in some image quality differences

• GE: AP vs. Lat scout view

  • 16% lower dose when using lateral scout view

Effect of Scan Direction

• Negligible difference in overall CTDI

• Potentially marked differences in organ dose

  • e.g. breast dose in this example

• Effect depends on pitch

When Not to Use Automatic mA Modulation?

• Extremities

• Low dose follow ups (e.g. hydrocephalus)

• Low dose lung nodule assessments

References

• Overview, practical tips and potential pitfalls of using AEC in CT: Siemens CARE Dose 4D, Marcus Soderberg, 2016

• Setting up CT automatic tube current modulation systems, Martin & Sookpeng, 2016

• Automatic exposure control in computed tomography an evaluation of systems from different manufacturers, Soderberg and Gunnarsson, 2010