technical development of a high resolution ccd-based scanner for 3-d gel dosimetry: (i) scanner...
TRANSCRIPT
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
Structure of talk
• Historical perspective and context of the research
• Original scanner
• Components of the new scanner
Light source
Collimation
Tank
Stepper motors
Detection
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.
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.
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
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.
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
New scanner
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
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.
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
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
Signal detection
• 50 mm camera lens
• Off-the-shelf CCD detector (RS ~£120)
• CCIR framegrabber (Matrox pulsar, ~£1000)
• Acquisition PC, Visual Basic
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.