IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 3, JUNE 2004 789
Properties of LYSO and Recent LSO Scintillators for
Phoswich PET Detectors
Catherine Michelle Pepin, Student Member, IEEE, Philippe Bérard, Student Member, IEEE,
Anne-Laure Perrot, Member, IEEE, Claude Pépin, Daniel Houde, Roger Lecomte, Member, IEEE,
Charles L. Melcher, Member, IEEE, and Henri Dautet, Member, IEEE
properties of the new cerium-doped rare-earth scintillator
lutetium-yttrium oxyorthosilicate (Lu? ?Y? ?Si? ?:Ce, LYSO)
were investigated and compared to those of both recent and older
LSO crystals. UV-excited luminescent spectra outline important
similarities between LYSO and LSO scintillators. The two distinct
Ce1 and Ce2 luminescence mechanisms previously identified in
LSO are also present in LYSO scintillators. The energy and timing
resolutions were measured using avalanche photodiode (APD) and
photomultiplier tube (PMT) readouts. The dependence of energy
resolution on gamma-ray energy was also assessed to unveil the
crystal intrinsic resolution parameters. In spite of significant
progress in light output and luminescence properties, the energy
resolution of these scintillators appears to still suffer from an
excess variance in the number of scintillation photons. Pulse-shape
discrimination between LYSO and LSO scintillators has been
successfully achieved in phoswich assemblies, confirming LYSO,
with a sufficient amount of yttrium to modify the decay time, to
be a potential candidate for depth-of-interaction determination in
multicrystal PET detectors.
Index Terms—Intrinsic resolution, LSO, LYSO, scintillation de-
tector, scintillation processes.
for applications in medical imaging. The availability of a va-
riety of crystals with a range of different scintillation properties
has triggered a renewed interest for phoswich detectors, which
allow the depth of interaction (DOI) information within mul-
ticrystal assemblies tobe determined simply bypulse shape dis-
crimination (PSD). However, the number of scintillators with
suitable characteristics for PSD identification and coincidence
detection in PET remains fairly limited. The first choice is evi-
BGO, but is seven times faster with a light output three (APD)
HE last decade has seen the introduction of several new
high luminosity scintillators that are promising candidates
Manuscript received January 28, 2003; revised April 5, 2004. This work
was supported in part by the Natural Sciences and Engineering Council of
Canada under Collaborative R&D Grant CRD 217968-98, in collaboration
with PerkinElmer Optoelectronics and CTI, Inc.
C. M. Pepin, P. Bérard, C. Pépin, D. Houde, and R. Lecomte are with the
Department of Nuclear Medicine and Radiobiology, Université de Sherbrooke,
CH-12111 Geneva 23, Switzerland (e-mail: Anne-Laure.Perrot@cern.ch).
C. L. Melcher is with CTI, Inc., Knoxville, TN 37932 USA (e-mail:
H. Dautet is with PerkinElmer Optoelectronics, Vaudreuil, QC J7V 8P7,
Canada (e-mail: Henri.Dautet@perkinelmer.com).
Digital Object Identifier 10.1109/TNS.2004.829781
to five(PMT)times higher. LSO is now a mature crystal that can
be produced in quantity with stable scintillation properties ,
A variant of LSO in which some of the lutetium is replaced
by yttrium atoms has recently been developed at CTI, Inc.
(Knoxville, TN). Cerium-doped lutetium-yttrium oxyortho-sil-
Y SiO :Ce, LYSO) has a comparable light yield
to LSO with a slightly longer decay time of 53 ns, making
it an attractive candidate for PSD identification in phoswich
detectors. In this work, the scintillation performance of the
new LYSO scintillator was investigated with PMT and APD
readouts, and compared to the most recent LSO production.
Older LSO crystals were also measured concurrently as a
The scintillation properties of Ce-activated rare-earth oxy-
orthosilicates are controlled by two distinct luminescence cen-
ters, known as Ce1 and Ce2 , , which are attributed to Ce
substituted lattice sites and interstitial Ce centers, respectively
lieved to be detrimental to the energy resolution of Ce-activated
Theenergyresolutionof scintillatorscoupled toAPDscanbe
, and scintillator resolution
. The scintillator resolution
further broken down into a statistical term
which is dependent on the photon yield
. The excess noise factor
variance in excess of Poisson statistics, and
parameters to describe scintillator intrinsic characteristics.
, and the crystal in-
, describing the
can be used as
III. MATERIALS AND METHODS
A. Description of Crystals
The properties of the scintillators LYSO, LSO, GSO and
BGO are summarized in Table I. For nuclear spectroscopy
measurements, crystals were wrapped in several layers
Teflon tape and optically coupled with optical grade silicone.
All crystals were cleaned successively with ethanol, acetone
and methanol prior to wrapping. As a reference, three older
LSO crystals previously used in scintillation studies with APDs
(, , , LSOA and LSOB ) were also tested. The di-
mensions and surface treatment of the crystals are summarized
in Table II.
0018-9499/04$20.00 © 2004 IEEE
790 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 3, JUNE 2004
Fig. 1.UV-excitation and emission spectra for LSO scintillator.
DIMENSIONS AND SURFACE TREATMENT OF SCINTILLATORS
A standard procedure described elsewhere – was
used to chemically etch the crystals. Measurements of light
output and energy resolution were performed with the crystals
optically coupled to a PMT before and after polishing to assess
the efficiency and reproducibility of the etching process.
B. UV- and -Excited Luminescence Measurements
UV-excited luminescence spectra were obtained using a
Hitachi F2000 spectrofluorometer with the 2
LSO and LYSO scintillators. Using the same crystals, -excited
2 10 mm
luminescence spectra were obtained by irradiation with a
Cs source (662 keV). Light emitted by the crystals was
concentrated onto the aperture of an ISA monochromator (HR
320) with a BK7 lens of 100 mm focal length. The
cited spectra were then collected by an intensified photodiode
array equipped with an S-20 photocathode (IPDA 1024 from
Princeton Instruments). All measurements were performed at
room temperature and corrected for the spectral efficiencies of
the spectral-dependent monochromator and photodetector.
C. Nuclear Spectroscopy Measurements
The second set of measurements involved standard nuclear
spectroscopy techniques to determine the relative light output,
energy resolution at several gamma-ray energies, coincidence
timing resolution, as well as thepulse-shape discrimination per-
formance of LYSO and LSO crystals. All measurements were
performed using PerkinElmer APDs connected to a low-noise
tomultiplier tube (PMT). An ORTEC 452 spectroscopy ampli-
fier was employed for all energy resolution measurements with
a shaping time of 0.25
s. Scintillation decay time measure-
ments were carried out for the LSO#9g and LYSO#9g scintil-
lators using standard time correlated single photon technique
originally developed by Bollinger and Thomas . The crys-
tals were excited by a
Na source. The data were fitted with
a single exponential term to extract the decay time. Coinci-
dence timing measurements as well as PSD measurements of
LYSO/LSO phoswich were performed using a zero-cross time
circuit based on two CF discriminators with 10 ns (Ortec 579)
and 100 ns (Ortec 474) shaping times .
A. UV-Excited Luminescence Spectra
Typical LSO and LYSO UV spectra are shown in Figs. 1 and
2. At first glance, the luminescence spectra of the new genera-
tion LSO and LYSO crystals are quite similar. However, when
compared to the emission spectra of the older LSOref scintil-
lator , , one observes that the excitation bands at
and nm, associated with Ce2 emission, are significantly
PEPIN et al.: PROPERTIES OF LYSO AND RECENT LSO SCINTILLATORS FOR PHOSWICH PET DETECTORS 791
Fig. 2.UV-excitation and emission spectra for LYSO scintillator.
?-excited luminescence spectra for LSOref and new generation LSO and LYSO.
RELATIVE LIGHT OUTPUT AND ENERGY RESOLUTION OF SCINTILLATORS AT 662 keV (
Cs) BEFORE AND AFTER ETCHING MEASURED WITH A PMT READOUT
enhanced. ThisCe2 luminescence mode is revealed by themax-
imum of the emission spectra, which is clearly shifted toward
440–445 nm for both LSO and LYSO when the crystals are ex-
-Excited Luminescence Spectra
The -excited luminescence spectra for LSOref and the new
UV-excited spectra. For the newer LSO and LYSO, the max-
imum emission is shifted toward
shoulder at longer wavelengths (see arrow in Fig. 3), which can
be correlated with the more efficient Ce2 luminescence.
nm with a prominent
C. Effect of Surface Treatment
The light output and energy resolution of unpolished and
chemically etched LSO and LYSO scintillators have been
measured. The results are summarized in Table III. The etching
792 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 3, JUNE 2004
with APD readout.
Typical energy spectrum from Cs obtained for an LYSO scintillator
with APD readout.
Typical energy spectrum fromCs obtained for an LSO scintillator
LIGHT OUTPUT AND ENERGY RESOLUTION OF 4?4?10 MM
SCINTILLATORS AT 662 keV ( Cs)
process slightly reduced the weight of the crystals (
% for LYSO), the smaller crystals being more
affected because of their larger surface to weight ratio. The
LYSO was also more sensitive to etching.
Before etching, the light yield of the recent LSO and LYSO
scintillators with a 4
4 mm cross section was already supe-
rior to that of LSOref. For 2
yield could be dramatically lower, by up to a factor of two in
LYSO. While the gain in light output is only marginal
with the cubic 44 4 mm crystals after etching, it is signif-
% with 44 10 mm crystals and striking with
2 10mm crystals(20%–40%).Inallcases,thegainin
2 10 mm crystals, the light
function of APD bias at 662 keV (
Energy resolution of 4?4?4 mm LSO and LYSO scintillators as a
Cs, ? ? ??? ns).
Fig. 7. Components of energy resolution in LSO#9g.
light output was more important in LYSO than LSO. Similarly,
the improvement in energy resolution after etching is impres-
sive with the 2
2 10 mm , dropping from
662 keV. For the larger crystals, the gain in energy resolution is
still significant and the resulting resolution is comparable to the
D. Nuclear Spectroscopy
The averaged light output and energy resolution of
4 10 mm LYSO and recent LSO crystals irradiated
Cs-ray source (662 keV) are reported in Table IV
and compared to data obtained for GSO, BGO and older LSO
scintillators. Typical LSO and LYSO spectra are shown in
Figs. 4 and 5. In spite of higher light outputs (from 11 to 36%)
than the older LSOref, the newer crystals show little gain in
Fig. 6 shows the energy resolution of scintillators LSO#9g
and LYSO#9g as a function of the APD bias at 662 keV. The
behavior of these two crystals is very similar, both achieving
an energy resolution of about 10%. Further investigation of the
tron yield and energy resolution as a function of -ray energy.
Figs. 7 and 8 display the components of energy resolution in
typical LSO and LYSO scintillators, respectively, as a function
PEPIN et al.: PROPERTIES OF LYSO AND RECENT LSO SCINTILLATORS FOR PHOSWICH PET DETECTORS 793
Fig. 8.Components of energy resolution in LYSO#9g.
Fig. 9. Components of energy resolution in LSOref.
COMPARISON OF MEASURED SCINTILLATION CHARACTERISTICS
of primary electrons generated in the APD. These results can be
The scintillation parameters extracted from similar measure-
ments performed on several recent LSO and LYSO samples are
reported and compared to previously reported data in Table V.
% higher than the electron yield of the older
LSOref. However, there is no progress in energy resolution, as
this is reflected in the values obtained for the excess variance
in the number of scintillation photons
. The fitted value
LSO is similar to the one of 6.5 reported in a previous work 
and to the one of 5.2 measured in the present study for LSOref.
However, the intrinsic energy resolution of newer LSO is even
worse than the one of LSOref (
LYSO crystals achieve a better intrinsic resolution than LSOref
and the intrinsic en-
% versus 3.5%). The
Nonproportionality of LYSO light output as a function of incident
butadeteriorated of .Significant
nonproportionalities, as shown in Figs. 10 and 11, were noticed
in the scintillation response of the recent LSO and LYSO crys-
tals, in agreement with previously reported data –.
E. Coincidence Timing and Pulse Shape Discrimination
Figs. 12 and 13 present the scintillation decay time measure-
ment for LSO#9g and LYSO#9g scintillators. The decay time
constants are 41.9 ns for LSO#9g and 50.8 ns for LYSO#9g.
to be successfully achieved with APD readout in an LYSO/LSO
phoswich assembly, provided that photoelectric energy gating
was implemented(Fig. 14). PSD timeresolutions of 1.94 ns and
2.34 ns were obtained for LSO and LYSO, respectively, with a
in reference to a fast plastic/PMT detector as a function of the
APD bias are presented in Figs. 15 and 16 for LSO#9g and
LYSO#9g, respectively. These measurements yielded timing
resolutions of 1.3 ns FWHM for LSO#9g and 1.5 ns FWHM
for LYSO#9g when read out by the APD operated at a gain of
57 (bias of 560V).
794 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 3, JUNE 2004
Fig. 12.Scintillation decay time measurement for LSO#9g.
Fig. 13. Scintillation decay time measurement for LYSO#9g.
optically coupled to an APD.
Energy-gated PSD time spectrum of a LYSO/LSO phoswich detector
A. Effect of Surface Treatment
The etching process used to polish the crystals had a
beneficial effect on light yield, but variable outcome for
energy resolution. The small crystals with a large aspect ratio
2 10 mm ) clearly benefited the most from the
Fig. 15.Coincidencetiming resolutionas afunctionof APDbiasfor LSO#9g.
Coincidence timing resolution as a function of APD bias for
surface treatment. Chemical etching would therefore be the
preferred procedure with these small crystals. The gain in
performance was observed to be slightly superior for LYSO,
probably as a result of the more aggressive etching process
with this lower density material.
B. Scintillator Performance Characteristics
The light yield of the most recent LSO scintillators has
improved significantly in comparison to older LSO. The new
scintillator LYSO also exhibits a high light output, though
slightly lower than LSO, in spite of the presence of yttrium
atoms, which would be expected to enhance scintillation effi-
ciency. Unfortunately, the higher light output does not translate
into any significant improvement of the energy resolution.
Even with highly optimized detector setups enhancing light
collection and avoiding systematic position dependence, the
energy resolution can not be improved.As this has been pointed
out previously , –, some fundamental process other
than photon statistics obviously must be responsible for the
limited energy resolution of these scintillators.
and the excess variance in the number of scintillation photons
contribute to degrade the overall energy resolution. Since
the material is clear and exempt of bubbles or impurities on
PEPIN et al.: PROPERTIES OF LYSO AND RECENT LSO SCINTILLATORS FOR PHOSWICH PET DETECTORS 795 Download full-text
visual inspection, the nonzero intrinsic resolution in the small
4 4 mm crystals used in this study is likely due to
Further investigation could be planned to verifyif such spatially
dependent luminous efficiency is present on a macroscopic or
microscopic scale by irradiating selected regions of the crystals
with a highly collimated source.
The excess variance
cannot be excluded as another sig-
nificant source of resolution degradation in LSO and LYSO.
The excess variance is commonly attributed to the important
nonproportionality of the photon yield observed in rare-earth
oxyorthosilicate crystals –. The origin of this nonlinear
response is not well understood. However, the energy resolu-
tion was related to the existence of two competing scintilla-
tion processes in cerium-activated crystals . The higher light
emission, while preserving high Ce1 efficiency. As previously
postulated, the coexistence of the two efficient luminescence
processes, which appears to be necessary to achieve a high light
yield in rare-earth Ce-activated oxyorthosilicate crystals, may
C. Pulse Shape Discrimination
The discrimination of LSO and LYSO in a phoswich
assembly was shown to be possible using the PSD technique,
proving LYSO to be a potential candidate for use in conjunc-
tion with LSO in multicrystal PET detectors. However, PSD
is achieved at the expense of a significant loss of detection
efficiency because photoelectric energy gating must be ap-
plied. Progress in light output, APD performance (quantum
efficiency, gain) and pulse shape discrimination techniques will
be required for this feature to be easily implemented in PET
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