Manufacturers and Filters – Is Manufacturers and Filters – Is it all still a bit fuzzy?it all still a bit fuzzy?
Bill Thomson, James Cullis, Joe O’BrienBill Thomson, James Cullis, Joe O’Brien
Physics & Nuclear MedicinePhysics & Nuclear Medicine
City Hospital. BirminghamCity Hospital. Birmingham
BackgroundBackground
MPI ImagesMPI ImagesPhilips OdysseyButterworth 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
What’s Going on ?What’s Going on ?
Variation in filter definition ?Variation in filter definition ? Companies often make subtle changes to things Companies often make subtle changes to things
Butterworth FilterButterworth Filter
Fix cut-off = 0.3 pixel-1
Alter the ORDER
Filter shape set by
order and cutoff
Butterworth cut-off 0.3 pixel-1
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6
Frequency pixel-1
order 3 order 6 order 9 order 12
Butterworth FilterButterworth Filter
Fix order = 6
Alter the CUT-OFF
Butterworth order 6
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6Frequency - pixel-1
0.25 0.3 0.35 0.4
0.12 pixel-1 0.18 pixel-1 0.24 pixel-1
0.3 pixel-1 0.36 pixel-1
Effect of Varying Cut-off
How Does All This Help?How Does All This Help?
More higher frequency terms, more noiseMore higher frequency terms, more noise
Need the ‘lower-mid’ frequencies for structureNeed the ‘lower-mid’ frequencies for structure
Butterworth Filter ComparisonButterworth Filter Comparison Philips: Order 6 , Cut-off 0.4 cm-1
GE: Power 6 , Critical Freq. 0.4cm-1
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
Cycles/cm
Am
plitu
de
Filter DefinitionsFilter Definitions PhilipsPhilips GEGE
n
cff
2
1
1
n
cff
2
1
1
( )
1/2
Order Number n Power = 2n Order Number n Power = 2n
Cut-off at 0.5 Cut-off at 0.5 critical freq. = 0.707 critical freq. = 0.707
Butterworth Filter ComparisonButterworth Filter Comparison Philips: Order 6 , Cut-off 0.4 cm-1
GE: Power 12 , Critical Freq. 0.4cm-1
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
Cycles/cm
Am
plitu
de
Butterworth Filter ComparisonButterworth Filter Comparison Philips: Order 6 , Cut-off 0.4 cm-1
GE: Power 12 , Critical Freq. 0.37cm-1
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
Cycles/cm
Am
plitu
de
Short Axis ComparisonShort Axis Comparison
Order 6.0 Cut-off 0.4 cycles/cm
Power 12.0 Critical Freq. 0.37 cycles/cm
PhilipsPhilips GEGE
Manufacturers ComparisonManufacturers Comparison
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 )
Ordernumber
Cut-offDefinition
Cut-offunits
Published recommendationsPublished recommendationsQGS
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
Published recommendations (2)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 0.707
* “Technical Aspects of Myocardial Imaging”JNM, 42: no 10 1499-1507 , 2001
SNM
Useless
SummarySummary
• 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