CBCT scatter

1
Scatter measurements foroptical cone beam
computed tomography Kevin Jordan Jerry
Battista DOSGEL 2008 Hersonissos, Crete, Greece
Schulich School of Medicine Dentistry
The University of Western Ontario
2
Optical cone beam CT geometry
75 cm
85 cm
Scatter ( CT artifacts cupping and
streaking) Beam hardening ( CT artifacts cupping
and streaking) relative magnitudes ?
3
Image correction steps
Rinkel et al, 2006 SPIE 6142
4
Radiological Physics Centre (RPC)

• An independent QA watch dog based out of the US

Anterior
Anterior
point dose measurement (1D)
Superior
Lt
Rt
Rt
Ant
film measurement (2D)
Lt
Rt
Jordan
5
Lower contrast in Vista CBCT 3D
reconstruction Scatter effect?
Babic 2008
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Lower contrast in Vista CBCT 3D
reconstruction Scatter effect?
Babic 2008
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Scatter corrections common for x-ray imaging
8
Experimental optical cone beam CT scanner with
LED light
9
Camera view
Scatter measurement geometries
Shadow disk
Beamlet array
Shadow dot array
Hole array
85 cm
75 cm
10
Scatter geometry shadow disk
75 cm
85 cm
OD25 mm
Iopen
Iscatter
11
Open field reference images
H2O teflon cylinder
H2O
12
Shadow disk
water
LCV gel
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Low light level background image
light blocked at diffuser background image
14
Shadow dot array
array at diffuser
array at aquarium
overlay 2 printed sheets to increase dot optical
density sheets introduce additional scatter!!
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Shadow dot array choosing optimum shadow size
leuco crystal violet gel, patent blue violet 8
propylene glycol H2O
1 mm diameter, 3 mm grid
2 mm diameter, 6 mm grid
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Beamlet array
primary in air
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Beamlet array
H2O
LCV gel
18
Primary beamlet array geometry aperture size (F)
75 cm
85 cm
Verify that aperture does not backscatter to
diffuser Divergent collimator (pair of aperture
planes), 9 cm separation, OD12.5mm
Iaperture
Iprimary (Iaperture-Ibkgd)/(Iopen-Ibkgd)
Ibackground
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Shadow dot array scatter vs light separation
L cm
85 cm
print array on transparent plastic sheet (Corel
Draw, laser printer) black dots 2 mm diameter, 6
mm grid size
Iprimary Iopen-Iscatter
Iopen
Iscatter
Note plastic sheet also scatters
20
Hole array array geometry
75 cm
85 cm
hole array 19x19, machined on CNC mill aluminum
plate, flat black paint, hole diameter 3mm, 10 mm
grid spacing
Iprimary Iopen-Iscatter
Iscatter
Iopen
21
Scatter light with hole array, F11
0.7
2 1
water
LCV 4 gelatin, teflon, 8 propylene glycol
Note reflections (not all stray light due to
scatter
22
Primary light with hole array, F11
89 91 95
Iprimary Iopen-Ihole
LCV 4 gelatin, teflon, 8 propylene glycol
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central primary summary
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Scatter correction constant
Note Vista jar with carbon black micelle gel
fingers
25
Summary scatter components 3 due to teflon
walls 3 due to refractive index matching
liquid (8 propylene glycol, filtered to 5 micron
to remove algae and molds) 5 due to 4 gelatin
Triton X100 micelle LCV, F 8 scatter decreases
with larger light separation (acceptance
angle) Conclusions Ibkg constant scatter
magnitude 10 low angle scatter, local
correction gt ratio appropriate Future once
scatter is quantified, magnitude of beam
hardening effects can be determined?
26
Image correction steps
ring artefacts beam hardening
ratio(post/pre)
Rinkel et al, 2006 SPIE 6142
27
Primary light vs beamlet diameter, F11
0 2
0 100
LCV 4 gelatin, teflon, 8 propylene glycol