Presentation on theme: "Quantitative Quality Assurance in Hull & East Yorkshire Dr. Craig Moore & Dr. Tim Wood Radiation Physics Department."— Presentation transcript:
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
Measurement of CR MTF Acquire ‘MTF image’ using a tungsten metal edge tool, 50 x 50 mm and 1 mm thick 50mm
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 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)
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!!!
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
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
MicroDose Noise We also measure detector response with dose And quantum and structure variance
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?
RT 2D Imaging - Variance Genuine due to structure noise?
RT 3D Imaging Use Catphan phantom Measure the MTF with the bead tool Measure CT numbers etc
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
MRI Quantitative QA Use Leeds Test Object MagIQ phantom Measure: –SNR –Linearity –Uniformity –MTF
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
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