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.