Quantitative Quality Assurance in

Hull & East Yorkshire

Dr. Craig Moore & Dr. Tim Wood

Radiation 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)

me

an

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 MTF

tube

CR cassette

1.5m

MTF edge tool

Typical results…

0

0.1

0.2

0.3

0.4

0.5

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…

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

ari

an

ce

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

0.4

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)

Mean

Pix

el

Valu

e

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

orr

ec

ted

Va

ria

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

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7

Spatial frequency (lp mm-1)

MTF

(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 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

)

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 va

ria

nce

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

pix

el va

lue

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

ari

an

ce

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

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Spatial frequency (lp / mm)

MT

F

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