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
–
–
–
–
1 uGy
4 uGy
12 uGy
50 uGy
• Always with 1 mm Cu
filtration and 70 kVp
3.50E+04
3.00E+04
mean PV
• First need to derive the
system transfer
properties (STP) of the
system, i.e. flat field
images at doses of:
2.50E+04
y = 4265.1Ln(x) + 13221
2.00E+04
R = 0.9981
2
1.50E+04
1.00E+04
5.00E+03
0.00E+00
0.00
10.00
20.00
30.00
Dose (uGy)
40.00
50.00
60.00
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
1.5m
CR cassette
MTF edge tool
Typical results…
Current
1
Baseline
0.9
0.8
0.7
MTF
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.5
1
1.5
2
2.5
3
Frequency (cycles m m -1)
3.5
4
4.5
5
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
6.00E-02
STP Corrected Variance
5.00E-02
y = 0.0019x 1.3266
R2 = 0.9924
4.00E-02
3.00E-02
2.00E-02
1.00E-02
0.00E+00
-1.00
1.00
3.00
5.00
7.00
9.00
11.00
13.00
Dose (uGy)
Find relationship and quote
the exponent
If only quantum noise
the exponent should
be unity
15.00
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…
1.2
7.00E-05
• STP is logarithmic (usually linear for a DR
system)
1
6.00E-05
2500
0.8
5.00E-05
Current
4.00E-05
3.00E-05
0.4
2.00E-05
Mean Pixel Value
0.6
NNPS mm2
MTF
2000
Baseline
NNPS current
Typical CR
NNPS Baseline
1500
Typical CR NNPS
1000
y = -320.39Ln(x) + 2286.7
R2 = 0.9999
0.2
500
1.00E-05
0
0 0.00E+00
0
0
1
0.00
0.5
10.00
1
2
3
20.00
30.00
1.5
2
lp/mm2.5
Spatial Frequency
Dose (uGy)
Spatial Frequency lp/mm
4
40.00
3
50.00
3.5
5
60.00
4
6
DR Variance
• Variance as function of dose
0.2
0.18
1.2578
y = 0.0012x
2
R = 0.9967
STP Corrected Variance
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0.00
10.00
20.00
30.00
Dose (uGy)
40.00
50.00
60.00
Hot off the press…
0
0.5
1
1.5
2
2.5
3
3.5
4
• Last week we found significant difference
in MTF and NNPS from baseline of our DR
system
• Service engineer attending this Friday
1.00E-04
1.2
Measured
Baseline
1
MTF(f)
NNPS
0.8
0.6
1.00E-05
–
Will be interesting to see if we find any
0.4
genuine issues!!!
Measured X
0.2
Baseline X
0
0
1.00E-06
1
2
3
4
-1
SpatialSpatial
frequency
(lp (lp
mm
frequency
mm -1) )
5
6
7
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
1.2
1.0
MTF(f)
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
Spatial Frequency (lp per mm)
5
6
7
We did have an issue with one of
our MicroDose systems!!!
1.0
MTF(f)
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
5
6
7
8
Spatial Frequency (lp per mm)
Line Spread Function
1.2
1.0
0.8
Value
• 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
Modulation Transfer Function
1.2
0.6
0.4
0.2
0.0
-0.2
-6
-4
-2
0
Distance (mm)
2
4
6
MicroDose Noise
• We also measure detector response with dose
• And quantum and structure variance
18000 Stucture variance
Quantum variance
AKref
16000
Baseline structure variance
y = 24.93x - 245.87
2
R = 1.00
100
14000
Mean pixel value
80
% of total variance
Baseline quantum variance
60
40
12000
10000
8000
6000
4000
20
2000
0
1
0
10
0
100
200
400
Detector
dose (uGy)
600
Detector Air Kerma (uGy)
1000
800
Radiotherapy 2D on-board imaging
• We work very closely with our
RT colleagues wrt QA and
testing of kV imaging systems
kV tube
– 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
imager
RT 2D Imaging
0.0006
Modulation Transfer Function
• 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
1.2
0.0005
1.0
0.0004
NNPS
MTF(f)
0.8
0.6
0.0003
0.4
0.0002
0.2
0.0001
0.0
6
5
4
3
2
1
– 0Does this
demonstrate
the
usefulness
of
Spatial Frequency (lp per mm)
doing quantitative QA?
7
0
0.0
0.5
1.0
1.5
Frequency lp/mm
2.0
2.5
RT 2D Imaging - Variance
1.40E-01
STP Corrected Variance
1.20E-01
1.00E-01
y = 0.0176x 2.0081
R2 = 1
8.00E-02
6.00E-02
4.00E-02
2.00E-02
0.00E+00
0.00
1.00
2.00
3.00
Dose (uGy)
Genuine due to
structure noise?
4.00
5.00
RT 3D Imaging
• Use Catphan phantom
• Measure the MTF with the bead tool
• Measure CT numbers etc
Measured
Baseline
1
0.9
0.8
0.7
MTF
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
Spatial frequency (lp / mm)
1.2
1.4
1.6
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
1.2
1.0
MTF(f)
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
Spatial Frequency (lp per mm)
5
6
7
Dental CBCT
MRI Quantitative QA
• Use Leeds Test
Object MagIQ
phantom
• Measure:
SNR
Linearity
Uniformity
MTF
256 x 256 vertical
384 x 384 horizontal
384 x 384 vertical
512 x 512 horizontal
512 x 512 vertical
1
0.8
MTF
–
–
–
–
256 x 256 horizontal
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
-1
Spatial frequency (mm )
1.4
1.6
1.8
2
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