Quantitative Quality Assurance in Hull & East Yorkshire Dr. Craig Moore & Dr. Tim Wood Radiation Physics Department.

Quantitative Quality Assurance in Hull & East Yorkshire

Dr. Craig Moore & Dr. Tim WoodRadiation Physics Department

What imaging modalities do we have in Hull & East Yorks?

• Lots of Computed Radiography (CR)• Some Direct Radiography (DR)• CT • MRI• Full Field Digital Mammo (FFDM)• Cone Beam CT

– Dental– Radiotherapy imaging

• Digital Fluoro labs• In this talk I’ll try to give you a flavour of the quantitative

QA measurements we do on some of the above (with some results!!)

What do we measure for CR and DR?

• We (try to!!) conform to the requirements of IPEM Report 32 part 7– MTF– NNPS– SNR– Variance as a function

of dose

Computed Radiography

• Have been measuring MTF and NNPS of our CR systems for approx 5 years

• Only have Agfa CR in Hull

• Originally wrote own software in Matlab

• But have since moved to IQWorks

Measurement of CR MTF• First need to derive the

system transfer properties (STP) of the system, i.e. flat field images at doses of:– 1 uGy– 4 uGy– 12 uGy– 50 uGy

• Always with 1 mm Cu filtration and 70 kVp

y = 4265.1Ln(x) + 13221

R2 = 0.9981

0.00E+00

5.00E+03

1.00E+04

1.50E+04

2.00E+04

2.50E+04

3.00E+04

3.50E+04

0.00 10.00 20.00 30.00 40.00 50.00 60.00

Dose (uGy)

mea

n PV

Measurement of CR MTF

• Acquire ‘MTF image’ using a tungsten metal edge tool, 50 x 50 mm and 1 mm thick

50mm

50mm

Measurement of CR MTFtube

CR cassette

1.5m

MTF edge tool

Typical results…

0

0.1

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0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Frequency (cycles mm -1)

MT

F

Current

Baseline

Measurement of CR NNPS

• Use the 4 uGy STP image

• Use IQWorks (or Matlab) to do the number crunching

Typical Results…

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

3.5E-05

4.0E-05

0 1 2 3 4 5 6

Frequency (cycles mm - 1 )

Current Scan

Current Sub-scan

Baseline Scan

Baseline Sub-scan

Measurement of SNR and Variance

• Use STP images– Linearise– Chop out central 5 cm

square portion of image

– Calc mean, standard deviation and variance

– SNR = mean/SD– Plot STP corrected

variance against dose

STP Corrected Variance

y = 0.0019x1.3266

R2 = 0.9924

0.00E+00

1.00E-02

2.00E-02

3.00E-02

4.00E-02

5.00E-02

6.00E-02

-1.00 1.00 3.00 5.00 7.00 9.00 11.00 13.00 15.00

Dose (uGy)

ST

P C

orr

ecte

d V

aria

nce

Find relationship and quote the exponent

If only quantum noise the exponent should be unity

CR results over time…

• Over the last 5 years, we have seen no significant changes in– MTF– NNPS – SNR– Variance– STP– DDI calibration (Agfa’s ‘SAL log’ number)

• Do these quantitative QA tests take any extra time?– No! – only one extra exposure required (MTF edge) as STP images

would be taken anyway for detector response

Getting the images off the system……

• For Agfa CR we set up a DICOM node on each system and send the image to our laptops– Must have DICOM server

running on laptop– We use K-PACS– Images must always be

acquired as:• System diagnosis

• Flat field

• Speed class 200

General DR measurements

• We only have one general Philips DR system

• Measure MTF, NNPS etc in the same manner as for CR

• Grid removed

MTF edge on detector housing

Typical DR results…

• STP is logarithmic (usually linear for a DR system)

0

0.2

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0.6

0.8

1

1.2

0 1 2 3 4 5 6

Spatial Frequency lp/mm

MT

F

Current

Baseline

Typical CR

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

0 0.5 1 1.5 2 2.5 3 3.5 4

Spatial Frequency lp/mm

NN

PS

mm

2

NNPS current

NNPS Baseline

Typical CR NNPS

y = -320.39Ln(x) + 2286.7

R2 = 0.9999

0

500

1000

1500

2000

2500

0.00 10.00 20.00 30.00 40.00 50.00 60.00

Dose (uGy)

Mea

n P

ixel

Val

ue

DR Variance

• Variance as function of dose

y = 0.0012x1.2578

R2 = 0.9967

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.00 10.00 20.00 30.00 40.00 50.00 60.00

Dose (uGy)

ST

P C

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ec

ted

Va

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nc

e

Hot off the press…

• Last week we found significant difference in MTF and NNPS from baseline of our DR system

• Service engineer attending this Friday– Will be interesting to see if we find any

genuine issues!!!

0

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1.2

0 1 2 3 4 5 6 7

Spatial frequency (lp mm-1)

MT

F(f

)

Measured

Baseline

1.00E-06

1.00E-05

1.00E-04

0 0.5 1 1.5 2 2.5 3 3.5 4

Spatial frequency (lp mm-1)

NN

PS

Measured X

Baseline X

DR image transfer

• Not easy to transfer images– Same method as

Agfa CR but Philips were very reluctant to help us with this

FFDM Philips MicroDose• Photon counting full field digital

mammo system• STP measured with 2 mm Al

filter at the end of the collimator

• MTF measured in a similar way to CR/DR

• Edge tool placed on breast platform– No extra filtration– 32 kV/12 mAs– Approx 4cm from chest wall

edge• Easy to take images off the

system directly to an external hard drive

Philips MicroDose MTFModulation Transfer Function

0 1 2 3 4 5 6 7

Spatial Frequency (lp per mm)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

MT

F(f

)

We did have an issue with one of our MicroDose systems!!!

• Strange shape to the MTF

• Corresponding dip in LSF

• No issues with basic TOR MAX line pair test

• We went back to test the system but it passed

Line Spread Function

-6 -4 -2 0 2 4 6

Distance (mm)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Va

lue

Modulation Transfer Function

0 1 2 3 4 5 6 7 8

Spatial Frequency (lp per mm)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

MT

F(f

)

MicroDose Noise

• We also measure detector response with dose• And quantum and structure variance

0

20

40

60

80

100

1 10 100 1000

Detector dose (uGy)

% o

f to

tal v

ari

an

ce

Quantum variance Stucture variance AKref Baseline quantum variance Baseline structure variance

y = 24.93x - 245.87

R2 = 1.00

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

0 200 400 600 800

Detector Air Kerma (uGy)

Me

an

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e

Radiotherapy 2D on-board imaging

• We work very closely with our RT colleagues wrt QA and testing of kV imaging systems– 2D and 3D

• STP images– 1 mm Cu at tube port– 70 kVp– 1 to 20 uGy

• MTF edge tool placed on imager– Approx 70 uGy

• Same analysis and reporting as for diagnostic CR/DR

• Get images off the system with the help of RT physicists

kV tube

imager

RT 2D Imaging

• Huttner image for this detector scored 12 groups (1.8 lp/mm)

• This is much worse than this MTF suggests

• Possible image processing on the acquisition monitor not applied to image sent for MTF analysis– Does this demonstrate the usefulness of

doing quantitative QA?

Modulation Transfer Function

0 1 2 3 4 5 6 7

Spatial Frequency (lp per mm)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

MT

F(f

)

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0 0.5 1.0 1.5 2.0 2.5

Frequency lp/mm

NN

PS

RT 2D Imaging - Variance

y = 0.0176x2.0081

R2 = 1

0.00E+00

2.00E-02

4.00E-02

6.00E-02

8.00E-02

1.00E-01

1.20E-01

1.40E-01

0.00 1.00 2.00 3.00 4.00 5.00

Dose (uGy)

ST

P C

orr

ecte

d V

aria

nce

Genuine due to structure noise?

RT 3D Imaging• Use Catphan phantom• Measure the MTF with the bead tool• Measure CT numbers etc

0

0.1

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Spatial frequency (lp / mm)

MTF

Measured Baseline

Cone Beam Dental Imaging

• Tests conform to HPA-CRCE-010– Guidance on safe use

of dental cone beam CT equipment

• We use a specially designed phantom shaped like a jaw to hold Sedentex inserts

CBCT Dental MTF

Modulation Transfer Function

0 1 2 3 4 5 6 7

Spatial Frequency (lp per mm)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

MT

F(f

)

Dental CBCT

MRI Quantitative QA

• Use Leeds Test Object MagIQ phantom

• Measure:– SNR– Linearity– Uniformity– MTF 0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Spatial frequency (mm-1)

MT

F

256 x 256 horizontal 256 x 256 vertical 384 x 384 horizontal

384 x 384 vertical 512 x 512 horizontal 512 x 512 vertical

Future Work

• Want to start quantitative QA on our digital fluoro labs– Just recently found out (we think!!) how to take

images off our Siemens and Philips systems • Thorough CT analysis via automated Catphan

measurements• Non-CBCT digital dental• Do we want to start measuring effective MTF,

NNPS, DQE etc????– Similar to clinical set up with an appropriate phantom

so not detector centric– But would be much more time consuming

Conclusions• We have implemented routine quantitative testing (e,g. MTF, NNPS,

SNR, Variance) into our QA programme, specifically for the following modalities:– CR– DR – FFDM– CBCT (RT and Dental)– MRI

• No significant time penalty in doing these quantitative tests if software is already set up

• We haven’t seen any significant changes in results over the years – but is this to be expected???– We rarely see gross changes in Leeds Test Objects either

• Vital we get cooperation from manufacturers to ensure we can easily acquire images in correct format and easily get them off the system