Speaker
Dr
Eric Wulf
(Naval Research Laboratory)
Description
We have demonstrated two PET imaging system one based on double-sided germanium strip
detectors and another using silicon strip detectors. The germanium system uses two
planar detectors with three-dimensional position resolution, and demonstrates an
intrinsic resolution of 1.35 mm FWHM for both a Ge-68 line source and Na-22 disk
source. The silicon system consists of two planar detectors and has an intrinsic
resolution of 0.75 mm FWHM for a 1 mm diameter Na-22 point source. Both types of
semi-conducting detectors are capable of three-dimensional position resolution
limited by the strip pitch of the detector and have excellent energy resolution: 1.5
keV FWHM for germanium and 2 keV FWHM at room temperature for silicon at 60 keV.
The germanium system consisted of two planar germanium detectors that are 1 cm thick
and separated by ~9 cm. Each detector has orthogonal strips on the two faces at 2 mm
pitch and an active area of 25 cm^2. The depth of interaction within the crystal was
determined from timing the arrival of the signal on the two faces of the detector.
The timing resolution was ~10 ns which corresponds to a depth resolution of ~0.5 mm.
The germanium detectors have much better stopping power and efficiency than silicon
because of their higher atomic number and 1 cm thickness, but need to be cooled to
liquid nitrogen temperatures. The system can trigger at energies as low as 15 keV
without any image degradation. By selecting events within the 2 keV-wide photopeak,
events that scattered within the sampling volume were totally eliminated. Reducing
the strip pitch will generate an instrument whose position resolution is limited by
the range of the positron.
The thick silicon instrument consisted of two layers of 2x2 arrays of detectors with
an active area of 130 cm^2 per layer. The detectors are double-sided cross strip 2
mm thick silicon detectors with 64 strips per side, a strip pitch of ~0.9 mm, and an
active area of 32.5 cm^2. The detectors are daisy-chained together in the array so
that only 256 channels of electronics are needed to read-out each layer of the
instrument. The silicon detectors were operated at room temperature, but are only 2
mm thick. Research is on-going to fabricate thicker silicon detectors that are
operational at room temperature. Four Na-22 sources were placed between the two
silicon layers which are separated by 6.1 cm. Two of the sources had an activity of
2.94 uCi and two others had an activity of 0.47 uCi. One of the weak sources is a
cylinder 1 mm in diameter and 1 mm deep and it is located on the right hand side.
The FWHM of the image differs slightly in the x and y direction due to the source
being off-axis in the x dimension. Averaging the two dimensions yields a FWHM of
1.25 mm and subtracting the source diameter in quadrature yields an intrinsic
resolution of 0.75 mm. The other weak source is a 5 mm diameter cylinder that is 2
mm deep. The Na-22 salt used to fill the cylinder is clearly imaged along the inner
edge of the source capsule.
Authors
Dr
Bernard Phlips
(Naval Research Laboratory)
Dr
Eric Wulf
(Naval Research Laboratory)
Co-author
Dr
Elena Novikova
(Naval Research Laboratory)
