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
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…
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
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)
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
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)
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
pix
el v
alu
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
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)
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