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Manufacturers and Filters – Is it all still a bit fuzzy?

Manufacturers and Filters – Is it all still a bit fuzzy?. Bill Thomson, James Cullis, Joe O’Brien Physics & Nuclear Medicine City Hospital. Birmingham. Background. MPI Images. Philips Odyssey Butterworth filter: Order = 6.0 Cut-off = 0.24 cycles / pixel.

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Manufacturers and Filters – Is it all still a bit fuzzy?

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  1. Manufacturers and Filters – Is it all still a bit fuzzy? Bill Thomson, James Cullis, Joe O’Brien Physics & Nuclear Medicine City Hospital. Birmingham

  2. Background

  3. MPI Images Philips Odyssey Butterworth filter: Order = 6.0 Cut-off = 0.24 cycles / pixel 6 mm pixel , = 0.4 cycles / cm G* X*l*r*s Manufacturer recommendations Butterworth filter: Power = 5 Critical Freq. = 0.5 cycles/cm G* X*l*r*s Butterworth filter: Power = 6 Critical Freq. = 0.4 cycles/cm

  4. What’s Going on ? Variation in filter definition ? Companies often make subtle changes to things

  5. Butterworth Filter Filter shape set by order and cutoff Fix cut-off = 0.3 pixel-1 Alter the ORDER

  6. Butterworth Filter Fix order = 6 Alter the CUT-OFF

  7. Effect of Varying Cut-off 0.12 pixel-1 0.18 pixel-1 0.24 pixel-1 0.3 pixel-1 0.36 pixel-1

  8. How Does All This Help? • More higher frequency terms, more noise • Need the ‘lower-mid’ frequencies for structure

  9. Butterworth Filter Comparison Philips: Order 6 , Cut-off 0.4 cm-1 GE: Power 6 , Critical Freq. 0.4cm-1

  10. Filter Definitions PhilipsGE 1/2 ( ) Order Number n Power = 2n Cut-off at 0.5 critical freq. = 0.707

  11. Butterworth Filter Comparison Philips: Order 6 , Cut-off 0.4 cm-1 GE: Power 12 , Critical Freq. 0.4cm-1

  12. Butterworth Filter Comparison Philips: Order 6 , Cut-off 0.4 cm-1 GE: Power 12 , Critical Freq. 0.37cm-1

  13. PhilipsGE Short Axis Comparison Order 6.0 Cut-off 0.4 cycles/cm Power 12.0Critical Freq.0.37 cycles/cm

  14. Manufacturers Comparison Cut-off Definition Cut-off units Order number Philips n 0.5 per pixel Philips n 0.5 per 2 pixels GE power = 2n 0.707 per cm-1 Hermes n 0.5 per cm-1 Siemens n 0.5 per Nyquist* * ( Nyquist is 0.5 pixel -1 )

  15. Published recommendations QGS order = 5 , cut-off 0.41 cycles per cm-1 EANM guidelines Says ‘ correct cut-off is at 0.707 ‘ Cut-off = 0.3 – 0.4 Nyquist , pixel size = 6mm Different cut-off values depending on MBq. Definition of Nyquist and pixel size relationship is wrong. BNMS Order 5 or 10 , cut-off 0.5 cycles per cm-1 (? Pixel size ) but cut-off = 0.75 for Hanning ? SNM Useless

  16. Published recommendations (2) SNM Useless G Germano, Cedars-Sinai* Equates cut-off and critical frequency States is at 0.5, but graphs show both 0.5 and 0.707 * “Technical Aspects of Myocardial Imaging”JNM, 42: no 10 1499-1507 , 2001

  17. Summary • Take great care! • Check filter definition and units • Don’t always trust the manufacturer to be right • Convert units and double-check! • Beware! Confusion in other areas eg DaTSCAN

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