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Published byDerrick Clarkin Modified over 2 years ago

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

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Background Background

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MPI Images Philips Odyssey Butterworth filter: Order = 6.0 Cut-off = 0.24 cycles / pixel 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 6 mm pixel, = 0.4 cycles / cm

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Whats Going on ? Variation in filter definition ? Companies often make subtle changes to things

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

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Butterworth Filter Fix order = 6 Alter the CUT-OFF

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0.12 pixel pixel pixel pixel pixel -1 Effect of Varying Cut-off

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

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

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

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

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

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Short Axis Comparison Order 6.0 Cut-off 0.4 cycles/cm Power 12.0 Critical Freq cycles/cm Philips GE

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

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Published recommendations QGS order = 5, cut-off 0.41 cycles per cm -1 EANM guidelines Says correct cut-off is at 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

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Published recommendations (2) G Germano, Cedars-Sinai* Equates cut-off and critical frequency States is at 0.5, but graphs show both 0.5 and * Technical Aspects of Myocardial ImagingJNM, 42: no , 2001 SNM Useless

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Summary Take great care! Check filter definition and units Dont always trust the manufacturer to be right Convert units and double-check! Beware! Confusion in other areas eg DaTSCAN

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