A1258583255xnKEv - PowerPoint PPT Presentation

Title: A1258583255xnKEv


1
S
Technical development of a high-resolution
CCD-based scanner 1. Scanner construction
Introduction Three-dimensional gel dosimetry has
been demonstrated over recent years to be an
important tool for verifying experimentally
complex 3-D dose distributions created in
radiotherapy. During much of the 1990s,
researchers focussed on MRI as the data readout
technique. However, this method suffers a number
of limitations, chief among which is the need for
expensive equipment, not always available on a
routine basis to radiotherapy treatment planning
departments. In the search for a less expensive
readout technique, Gore et al. 1 developed a
novel technique in which radiation dose
measurements are made using optical computed
tomography (OCT). However, the laser scanning
technique developed by Gore et al. is relatively
slow (approximately 15 minutes per slice). We
have developed a new type of OCT scanner, faster
by two orders of magnitude, the schematic diagram
for which is shown in Fig. 1. It is based on a
CCD detector which obtains a 2-D projection
images at every rotation step, acquiring the data
for a full 3-D dose map in under 40 minutes. The
total cost of the scanner illustrated was less
than 5000, including the PC used for data
acquisition.

• Key components of the scanner system
• An earlier prototype of this scanner was
  presented at DOSGEL 99 2. Since that time, a
  number of key components have been investigated
  in detail
• light source and collimation
• scanning tank and turntable
• detection system.
• The CCD-tomography approach adopted requires the
  creation of a parallel beam of light. Creating a
  beam that is both highly parallel and has a large
  cross-sectional area is difficult. We require (a)
  a high intensity point source with its output
  concentrated in the desired spectral lines and
  (b) a large diameter converging lens.
• Various options were examined to create the
  point source, including a beam-expanded laser and
  the output of an optical fibre, but the solution
  eventually chosen was to use a mercury vapour
  discharge tube as the light source. Its elongated
  shape requires the insertion of a cylindrical
  lens in front of the pinhole see Fig. 2. A
  photograph of the actual device is shown in Fig.
  3.

Figure 1 Schematic diagram of the new optical
computed tomography scanner
Figure 2 Purpose-built optics to turn the
extended mercury vapour discharge tube into a
pseudo point source
Conclusions We have described the construction of
a high-resolution scanner for 3-D gel dosimetry.
The 3-D images may be obtained on timescales that
are similar to 2-D acquisitions using the laser
based optical method. Our acquisition data matrix
size of up to (768 ? 536 pixels) spans a
field-of-view whose minimum size is limited only
by the minimum focal length of the camera lens
and we thus have the potential to acquire
ultra-high resolution images.
Previous CCD-based OCT scanners have had
relatively primitive turntable control. The proof
of principle by Wolodzko et al. 3 used a
turntable positioned by hand, whilst our own
previous prototype used a stepper motor attached
to the turntable by a belt drive. The current
system is more sophisticated, with a rotation
table that can be positioned, under computer
control, with a precision of 0.05. This is
attached to the turntable through the bottom of
the tank with a specialised watertight
seal. The detection system consists of a
projection screen (recently changed from
engineering tracing film to opal white perspex to
reduce its granularity and hence reduce
artefact level in the images) and a specialist
CCD detector (recently upgraded to Pulnix model
62-EX) connected to a standard 50 mm camera lens.
The signal is digitised using a 10-bit
framegrabber card (Matrox Pulsar) for high
dynamic range measurements.
Acknowledgements KKK was supported by the
Socrates exchange programme of the European
Union. The authors thank Mr C Bunton for
technical help in construction of the scanner.
References 1 JC Gore et al., Phys. Med. Biol.
41, 2695, 1996 2 M A Bero et al. Proc. DOSGEL
99 p. 136 3 J G Wolodzko et al. Med. Phys. 26,
2508, 1999