PROOF COPY 014204MPH
PROOF COPY 014204MPH
Efficient simulation of SPECT down-scatter including photon interactions
with crystal and lead
Hugo W. A. M. de Jong
Image Sciences Institute, University Medical Center, Utrecht, The Netherlands
Wen-Tung Wang and Eric C. Frey
Department of Biomedical Engineering and Department of Radiology, The University of North Carolina
at Chapel Hill, Chapel Hill, North Carolina
Max A. Viergever and Freek J. Beekman
Image Sciences Institute, University Medical Center, Utrecht, The Netherlands
Received 28 June 2001; accepted for publication 29 January 2002
A major image degrading factor in simultaneous Dual Isotope DI SPECT or simultaneous
Emission–Transmission ECT–TCT imaging, is the detection of photons emitted by the higher
energy isotope in the energy window used for imaging the lower energy isotope. In Tc-99m/Tl-201
DI-SPECT typically tens of percents of the total detected down-scatter is caused by lead x rays. In
Tc-99m/Gd-153 ECT–TCT, a comparable fraction of the down-scatter originates from Tc-99m
photons which only partly deposit their energy in the detector crystal i.e., due to crystal interac-
tions. Efficient simulation methods which model down-scatter can be used to optimize DI-SPECT
or ECT–TCT imaging acquisition or reconstruction protocols. In this paper we adapt a previously
proposed efficient down-scatter simulation method, to include the interactions of photons with the
detector crystal and collimator lead. To this end, point spread function tables including crystal and
lead interactions are precalculated. Subsequently, photons are traced through the patient body until
their last scatter position, and the precalculated responses are used to project the photons onto the
detector plane, while photon attenuation in the patient is taken into account. The approach is
evaluated by comparing simulated Tc-99m down-scatter projections with measured projections.
Incorporation of photon interaction with crystal and lead leads to significantly improved accuracy
of the shape of down-scatter responses, while differences in total counts between simulated and
measured projections typically decrease from tens of percents to a couple of percents. Calculating
60 down-scatter projections of an extended distribution on a 646464 grid takes about three
minutes on a PC with two 1.2 GHz processors. We conclude that accurate and efficient simulation
of down-scatter is now possible including the major effects of the nonuniform mass density of the
patient as well as photon interactions with the crystal and collimator lead. © 2002 American
Association of Physicists in Medicine. DOI: 10.1118/1.1462638
Key words: dual-isotope SPECT, Monte Carlo simulation, down-scatter, lead x-rays, scintigraphy.
I. INTRODUCTION
Simultaneous Dual-Isotope DI SPECT allows for the
studying different biological properties of organs or tissues
with a single acquisition. This is possible by acquiring events
in separate energy windows corresponding to the energies of
the isotopes in question. The projection sets obtained in such
a way are reconstructed to give radionuclide distributions of
the isotopes with different information captured by each. The
advantages of DI-SPECT over sequential imaging are the
avoidance of image registration problems and the reduction
of total scan time. The latter results in increased patient
throughout and patient comfort. Two examples of DI-SPECT
are found in myocardial perfusion imaging
1
where Tc-99m
sestamibi 140 keV can be used in conjunction with Tl-201
majority of emissions have an energy between 69 keV and
80 keV for simultaneous rest and stress imaging or with
Gd-153 around 100 keV in combined transmission–
emission CT TCT–ECT.
2,3
Other examples of DI-SPECT
include Tc-99m/FDG
4
and Tc-99m/I-123 SPECT.
5
In a typical Tc-99m/Tl-201 DI-SPECT cardiac study the
number of Tc-99m photons detected on the Tl-201 photopeak
window is significant e.g., typically 50% of total counts.
Only a small amount less than 2–3% of the Tc-99m
counts
6
of higher energy Tl-201 photons 135 keV and 167
keV is detected in the Tc-99m photopeak window. This
down-scatter causes an overestimation of activity and im-
proper fill-in of defects in the reconstructed Tl-201
distribution.
7–9
In a similar way, in TCT–ECT down-scatter
of Tc-99m photons in the Gd-153 transmission window cen-
tered around 100 keV hampers the reconstruction of the
transmission map.
3,10–12
The number of detected down-scattered photons depends
on several factors, i.e., the SPECT system used, the energy
window settings and the activity ratio of the two isotopes.
Furthermore, the amount of down-scatter is patient depen-
1 1 Med. Phys. 29 „4…, April 2002 0094-2405Õ2002Õ29„4…Õ1Õ11Õ$19.00 © 2002 Am. Assoc. Phys. Med.
PROOF COPY 014204MPH