technical development of a high resolution ccd-based scanner for 3-d gel dosimetry: (i) scanner...
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Technical development of a high resolution CCD-based scanner for 3-D gel dosimetry:
(I) Scanner construction
S J Doran, K K Koerkamp*, M A Bero, P Jenneson, E J Morton and W B Gilboy
S Department of Physics,University of Surrey,Guildford, GU2 7XH, UK
Department of PhysicsUniversity of Surrey
Department of Applied PhysicsUniversity of Twente, NL
*
S J Doran, K K KoerkampM A Bero, P Jenneson,E J Morton, W B Gilboy
Dept of Applied Physics,University of Twente,Enschede, NL
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Structure of talk
• Historical perspective and context of the research
• Original scanner
• Components of the new scanner
Light source
Collimation
Tank
Stepper motors
Detection
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Historical perspective
• Colour-change gels introduced in 1991(Appleby and Leghrouz, Med. Phys. 18, 309-312, 1991)
• 2-D imaging of radiation dose with CCD(Tarte et al. Med. Phys. 24(9), 1521-1525, 1997)
• Pencil-beam, laser-based systems Typically one plane in ~15 mins. (Kelly et al. Med. Phys. 25(9), 1741-1750, 1998)
• Imaging of stacked gels (Gambarini et al. DOSGEL ’99)
• First CCD tomography scanners(Wolodzko et al., Bero et al., DOSGEL ’99)
Typically 512 planes in ~30 mins.
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X-ray tomography lab at UniS
• The University of Surrey Physics Department already has a large investment in experimental X-ray computed tomography (Dr E Morton).
• Optical tomography fits perfectly into this scheme.
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Original scanner: DOSGEL ’99
• First image obtained in 1999 with the simple setup below demonstrates principle, but images not adequate.
Scanning tank Reconstructed OT image of optical
density
Optical density profile
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Premise for further development
• Up until now optical dosimetry has been seen as a cheap alternative to MRI.
• In designing the original CCD scanner, we originally tried to prove the principle on an ultra-low budget (< £5000).
• To obtain a credible medical instrument, one must purchase components with a higher specification.
• The key question is “How expensive does it need to be?”
• Strategy is to upgrade components gradually and evaluate which ones make the greatest difference.
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New scanner schematic
Hglamp
Cylindrical lens, pinhole and filter pseudo
point-source
Lens parallel beamScanning tank with matching medium
Exposed gel
Unexposed gel Diffuser screen on which real shadow image forms
CCDdetector
Standard 50mmcamera lens
PC with frame-grabber card
Turntable controlled by acquisition computer via stepper motors
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New scanner
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Light source: mercury vapour lamp
• The mercury emission spectrum contains a number of strong emission lines in the visible.
• In particular, there are lines on either side of the isobestic point.
Wavelength / nm
(o
pti
cal a
bso
rban
ce)
/ cm
-1
FXG spectraldose-response
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Collimation of light source
Short dimension of discharge tubeCircularconverging lens
CollimatingapertureLight source
(elongated discharge tube)
Cylindrical lens,focussing dimension
Cylindrical lens,non-focussing dimension
Long dimension of discharge tube
Filter
Cylindricallens
Pinhole
Filter
7 cm
• Aim is to produce large diameter, parallel light beam.
• Hence, need good point source.
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Scanning tank
• Purpose-built perspex tank, 30 cm3
• Turntable attached to “rotation table” below(Time and Precision, Basingstoke,
UK, model TR48) via watertight seal
• Projection screen stuck to back face of tank
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Stepper motors
• Motor controllers (Parker Hannifin Corporation, Rohnert Park, CA, USA, model 6K4)
precision 0.05° in rotation table positioning
• Ethernet interface to host computer
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Signal detection
• 50 mm camera lens
• Off-the-shelf CCD detector (RS ~£120)
• CCIR framegrabber (Matrox pulsar, ~£1000)
• Acquisition PC, Visual Basic
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Conclusions
• The new tomography scanner operates on the same principles as the previous prototype, but each of the components now has a higher specification.
• As will be shown subsequently, further improvement is needed in the CCD detector and projection screen.