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