CTUG, 14 Oct '04

Assessment of automatic exposure control systems on CT scanners using a custom made phantom

Nicholas Keat, David Platten, Maria Lewis, Sue EdyveanImPACT GroupSt George’s HospitalLondon, UKnick@impactscan.org

CTUG, 14 Oct '04

AEC systems

• Problem in defining CT protocols: Attenuation of x-rays is different– From patient to patient– Along patient length – z-axis– In AP and lateral directions – rotational

• All manufacturers have introduced or updated their 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

CTUG, 14 Oct '04

AEC systems

• mA adjusted to compensate for attenuation differences– dose applied to patient only where needed, avoiding dose

where it isn’t needed

mA

position

CTUG, 14 Oct '04

Advantages of AEC

• 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

CTUG, 14 Oct '04

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

*GE LightSpeed Pro16 only**Siemens Sensation 10/16 upwards only

Current systems

Z-axis AEC

SUREExposure / Real EC

CAREDose 4D**

DoseRight ACS

Auto mA

Patient size AEC

DoseRight DOMPhilips

CareDoseSiemens

Toshiba

SmartmA*GE

mA modulation

CTUG, 14 Oct '04

Methods to set AEC exposure level

• Different methods exist to define the exposure level using the AEC system

System attempts to give same level of image noise as a ‘Reference Image’ acquired earlier

Philips

‘Equivalent mA’ set for standard sized patientSiemens

Set required standard deviation (noise)Toshiba

‘Noise Index’ sets required image noise levelGE

Method for setting exposure levelManufacturer

CTUG, 14 Oct '04

ImPACT phantom

• Based on ‘Apollo’ phantom developed by Muramatsu, National Cancer Centre, Tokyo– Has ellipse, not circle cross section– Ellipse diameters in ratio 3:2

End view Side view

CT scanner couch

Catphan carrying case

CTUG, 14 Oct '04

ImPACT phantom

• Images along length of phantom (AEC off)

CTUG, 14 Oct '04

Coronal view Sagittal view

ImPACT phantom

z-axisAEC off

z-axis AEC on

Noise increases

Constant noise

CTUG, 14 Oct '04

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

• Look at effect of:– Exposure level – change desired standard deviation or

reference mA– kV– Axial and helical scanning– Helical pitch and direction of couch movement

• Store DICOM images on CD

CTUG, 14 Oct '04

Image analysis

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

at centre and edge of imageusing automatic analysis tool

• Region of Interest (ROI) size2000 mm2

• Results analysed using Excel

CTUG, 14 Oct '04

Results from testing

• Testing takes 1-2 hours.– Delays for tube cooling can be long

• 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 so far, much more data has been gathered

CTUG, 14 Oct '04

Results: GE

0

5

10

15

20

25

30

35

50 100 150 200 250 300

AP phantom diameter (mm)

Mea

sure

d S

D

Auto mA OFF

NI = 5

NI = 10

NI = 15

NI = 20

27.310-28020

18.010-50015

11.010-78310

10-783

200

mA

4.45

-AEC off

Mean SD

Noise Index

CTUG, 14 Oct '04

Results: GE

• Noise Index 12, different helical pitch, table movement in and out of gantry

0

2

4

6

8

10

12

14

16

50 70 90 110 130 150 170 190 210 230

AP phantom diameter (mm)

Mea

sure

d S

D

0.563, in

0.938, in

1.375, in

1.75, in

1.375 out

1.75 out

CTUG, 14 Oct '04

Results: GE

Axial

0

100

200

300

400

500

600

700

800

900

50 100 150 200 250 300

AP phantom diameter (mm)

Tu

be

curr

ent

(mA

)

Auto mA OFF

NI = 5

NI = 10

NI = 15

NI = 20

Axial

10

100

1000

50 100 150 200 250 300

AP phantom diameter (mm)

Tu

be

curr

ent

(mA

)

Auto mA OFF

NI = 5

NI = 10

NI = 15

NI = 20

Helical 10

100

1000

50 100 150 200 250

AP phantom diameter (mm)

Tu

be

curr

ent

(mA

)

0.563, in

0.938, in

1.375, in

1.75, in

1.375 out

1.75 out

CTUG, 14 Oct '04

Results: GE

• Different kVs, Noise Index 10

73

105

179

451

mA (Smart mA off)

10.9100

10.5120

10.7140

10.680

Measured SDkV

CTUG, 14 Oct '04

Results: Philips

• Mx8000 IDT has patient size AEC, and mA modulation

3 scans planned, at different z-axis positions,

patient AEC off

0

2

4

6

8

10

12

14

16

18

20

50 100 150 200 250

AP phantom diameter (mm)

Mea

sure

d S

D

Series 1 - 200 mA

Series 2 - 200 mA

Series 3 - 200 mA

3 scans, patient AEC on

0

2

4

6

8

10

12

14

50 100 150 200 250

AP phantom diameter (mm)

Mea

sure

d S

D

Reference Image

Series 1 - ACS ON

Series 2 - ACS ON

Series 3 - ACS ON

CTUG, 14 Oct '04

Results: Siemens

• CAREDose – only mA modulation tested• With CAREDose on, effective mA, and therefore dose

decreases (noise increases)

0

2

4

6

8

10

12

14

16

18

50 100 150 200 250AP phantom diameter (mm)

Mea

sure

d S

D

CAREDose on

CAREDose off

CTUG, 14 Oct '04

Results: Siemens

• Set mA = 200• mA modulation is greatest at wide end

of phantom – lower mean mA at wide end• Improvements of up to 9% in noise and

dose (9% lower noise for same dose).• Greater improvements would be seen

in a shoulder shaped phantom

0.91

0.93

Relative SD

12.2

SD -CAREDose

134

mean mA / mA

50.0

13.0

SD constant mA

47.2120

CTUG, 14 Oct '04

Results: Toshiba

• Data from RealEC on Aquilion 16

0

5

10

15

20

25

30

50 100 150 200 250 300

AP phantom diameter (mm)

Mea

sure

d S

DFixed mA

SD 5

SD 10

SD 12

SD 17

CTUG, 14 Oct '04

Future work plans

• Need to incorporate CAREDose 4D data:– Scanned last week, not yet fully analysed

• Could look at mA modulation systems in greater depth– Would require a less symmetrical phantom: our abdomen

phantom diameter is in ratio 3:2, shoulders are closer to 5:2

Abdomen Shoulder

CTUG, 14 Oct '04

Challenges for manufacturers and users

• Optimisation of scan protocols– AEC systems provide a method to define protocols in terms of

image quality, rather than dose level– Work required to ensure that radiologists are getting good

image quality, and patient doses are under control

• Standardisation of method to set exposure level– Currently, the user can choose noise index, reference mA,

reference image, required SD– A single method would aid comparison of scan protocols from

many scanners or scanning centres

• Education of users– AEC systems do not automatically lead to dose reduction –

correct definition of required IQ is important

CTUG, 14 Oct '04

Conclusions

• AEC systems offer potential benefits for everyone!– Radiologists: image quality consistent from patient to patient– Radiographers: setting image quality for different sized

patients is now much easier– 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 opportunity to improve them further– A common method for defining exposure would be useful– Potential to vary scan times and kV automatically