Speaker
Neal Clinthorne
(University of Michigan)
Description
Several years ago we presented the idea that PET resolution better than the
intrinsic range of the positron at good sensitivity was achievable by placing a high
resolution detector—ideally taking the form of a small diameter ring—within the bore
of a conventional PET detector ring (J. Nucl. Med. Supp. 2000, 41(5):20P, 2001, 42
(5):55P, 102P). While we examined constructing the inner detector from a number of
materials that could potentially support high spatial resolution including cadmium
zinc telluride and various scintillators, particularly intriguing was the fact
excellent performance appeared achievable by using low proton-number (Z) materials
such as silicon for the inner detector. Although nearly all interactions result in
a Compton scatter, the Compton-scatter cross-section does not drop quickly with
increasing energy. Furthermore, energy resolution is still possible by collecting
the scattered photon in the outer ring. Detectors having high propensity for
Compton interactions followed by escape of the scattered photon are capable of
supporting extremely high spatial resolution that is limited by only the range of
the Compton recoil electron. (Of course, photoelectric interactions are usable
also). Although detector materials having higher Z are useful for packing more
detection efficiency into a given volume, on the basis of the same detection
efficiency, higher-Z materials suffer from more multiple interactions, which must be
resolved to determine the correct coincidence line-of-response.
Various instrument configurations ranging from an intrarectal prostate imaging probe
constructed using higher-Z LSO to a device for imaging mice at submillimeter
resolution using a low-Z silicon inner detector are under active investigation. For
the latter instrument simulation studies have shown that image resolution of
~350microns FWHM is achievable with good sensitivity (~1%) while ~1mm FWHM
resolution can be achieved with outstanding sensitivity (9%). Both these figures
include the effects of F-18 positron range and acolinearity. Resolution was
estimated from images reconstructed using filtered backprojection, which has no
intrinsic resolution recovery.
Results from such Monte Carlo investigations are encouraging and are presently being
validated via experiment. To this end a single-slice proof-of-concept PET
instrument was constructed using silicon and BGO detectors. Each 2.2cm x 4.4cm x
1mm silicon detector consisted of a 16 x 32 array of 1.4mm x 1.4mm pads. The
silicon detectors were placed edgewise (for detection efficiency) on opposite sides
of the 4.4 cm field-of-view and the source was collimated to a 1mm thick slice using
thick tungsten plates. Measured efficiency was ~0.7x lower than Monte Carlo
predictions which is fully explained by the detectors being biased slightly lower
than depletion and by the coincidence timing window. Most encouraging was the
spatial resolution which ranged between 700 to 800 microns FWHM across the field-of-
view.
Even though the high spatial resolution predicted in simulation studies is borne out
by experiment, construction of an instrument supporting such resolution at high
efficiency remains challenging but not outside the realm of practicality.
Author
Neal Clinthorne
(University of Michigan)
Co-authors
Andrej Studen
(Dept. Experimental Particle Physics, IJS, Ljubljana, Slovenia)
Don Burdette
(Dept. Physics, Ohio State University, USA)
Enrico Chesi
(CERN, Geneva, Switzerland)
Gabriela Llosa
(IFIC/CSIC University of Valencia, Spain)
Klaus Honscheid
(Dept. Physics, Ohio State University, USA)
Marko Mikuz
(Dept. Experimental Particle Physics, IJS, Ljubljana, Slovenia)
Peter Weilhammer
(CERN / University of Perugia, Italy)
Sang-June Park
(Brookhaven National Laboratory, USA)
W. Leslie Rogers
(Nuclear Medicine, University of Michigan, USA)
carlos lacasta
(IFIC/CSIC University of Valencia, Spain)
harris kagan
(Dept. Physics, Ohio State University, USA)
sam huh
(Dept. Biomedical Eng. University of Michgan, USA)
