effect of skull thickness on quantitative brain imaging

Effect of skull thickness on quantitative brain imaging

Dr Nigel Williams

Nuclear Medicine Department

Walsgrave Hospital, Coventry

DaTSCAN: Quantification

Software to aid quantification has been developed

Accuracy depends on attenuation, scatter and septal penetration

AimImpact of skull thickness on attenuation

Attenuation

• 140keV

• Soft Tissue µ 0.11cm-1 HVL 4.5cm

• Bone µ 0.24cm-1 HVL 2.9cm

• Skull Thickness Variation 0.3 – 0.9cm

A Suitable Phantom

GE Healthcare Biosciences

Walsgrave Phantom Mk I

Perspex Cylinder Diameter 19cm

‘Skull’ Aluminium 1.5mm (x3)

15MBq Tc99m (10ml vial)

266MBq Tc99m (3l cylinder)

Acquisitions

(All Beacon)

No Attenuation

Water only

1.5mm Al

3.0mm Al

4.5mm Al

Reconstruction

• Same filter LP Order 6 / Cut off 0.6 cycles per pixel

• TransaxialsNo correctionChang uniform attenuation correctionBeacon NUAC

Results Phantom Mk INo

AttenuationWater Only 3mm 6mm 9mm

No Correction 179164 66524 63330 58894 54846

Chang 166283 156613 149433 145175

Beacon 278164 276124 265456 238434

No Correction 100% 37 35 33 31

Chang 93 87 83 81

Beacon 100 99 95 86

Walsgrave Phantom Mk II

Construction

Perspex cylinder

Cylinder of dense foam with skull shape removed

Grout layer for bone tissue

Two conical chambers to simulate striata

CT Numbers

979 ± 281050 ± 36

Acquisitions

Phantom with simulated skull (10mm)

Phantom without simulated skull

Reconstruction

• Same filter LP Order 6 / Cut off 0.6 cycles per pixel

• TransaxialsNo correctionChang uniform attenuation correctionBeacon NUAC

Chang Uniform Attenuation Correction

Right Left

No Corr Chang Beacon No Corr Chang Beacon

No Skull 419160 776836 1061549 346811 616050 852320

Skull 351924 633336 1040277 281961 489305 866383

% Difference -16.0 -18.5 -2.0 -18.7 -20.6 1.6

Results Phantom Mk II

Conclusions

For absolute quantification

Variation in skull thickness will have to be considered

The accuracy of uniform attenuation correction is influenced by region delineation

Beacon type attenuation correction appears to offer a solution although system specific phantom acquisitions will be required.