Download Development of a high-resolution and high efficiency single photon

Development of a high-resolution and high efficiency single photon detector for
cardiovascular diseases study in mice. SPECT assessment of left ventricular
perfusion using different routes of delivery of 99mTc-MIBI
F. Garibaldi1, E. Cisbani1, F. Cusanno1, S. Colilli1, R. Fratoni1, F, Giuliani1, M. Gricia1, R. Fratoni1,
M. Lucentini1, M. L. Magliozzi1, F. Santanvenere1, S. Torrioli1 , G. Marano2, M. Musumeci2, M.
Baiocchi3, L. Vitelli3, P. Musico4, A. Argentieri5, G. De Vincentis6, S. Majewski7, Y. Wang8, B. M.
W. Tsui8
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1Dipartimento TESA, Istituto Superiore di Sanita’ and INFN - gr. Sanita’ – Rome, Italy
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2Dipartimento del Farmaco, Istituto Superiore di Sanita’ – Rome, Italy
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3Istituto Superiore di Sanita’ Dipartimento di Oncologia – Rome, Italy
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4INFN Genova – Genova, Italy
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5INFN Bari- Bari, Italy
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6 Dipartimento di Scienze Radiologiche Universita’ degli Studi La Sapienza, Rome, Italy
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7University of West Virginia – Morgantown, USA
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8Johns Hopkins University, Baltimore, MD, USA
Corresponding author: Franco Garibaldi
[email protected]
Fax number +39 0649902317
Tel number +390649902243
Abstract – Introduction: We describe an open and flexible detector system for studying cardiovascular
diseases on mice, namely the detection of atherosclerotic plaques and stem cell therapy of heart infarction.
Tests with a prototype on phantom, perfusion SPECT imaging on mice, with different routes of the
radiotracer and detection of atherosclerotic plaques on mice have been performed.
Methods: A Geant4 code has been used to design a detector system optimized for studying cardiovascular
disease on mice, open and flexible enough to be able to host detectors with different modalities (MRI). In
order to perform test measurements on detection of atherosclerotic plaques and on perfusion SPECT
imaging of mouse heart two prototype modules of the detector made of CsI (Tl) and NaI (Tl) pixellated
scintillators coupled to pinhole collimators and PSPMTs. A readout electronics capable of reading out
4096 channels individually at 10-20 KHz was specifically designed and built.
Results: Tests performed on phantom show a spatial resolution of 0.8 mm. It can be improved down to 0.3
mm as tradeoff with needed sensitivity. Measurement on transgenic mice after injection of 99Tc-AnnexinV showed suspicious focal activity indicating possible plaques. Myocardial perfusion SPECT study with
two different routes of delivery (tail vein and peritoneum) showed that also injecting the radiotracer trough
the peritoneum gives good images making possible the use of this technique to monitor the effects of stem
cell therapy of infarction on mice. To fully accomplish the objectives of the study will probably require the
integration of other modalities (MRI) with significant modifications of the layout, and of the materials and
components.
Key words: small animal imaging, high-resolution single photon detector, 99Tc-MIBI, 99Tc-Annexin-V,
intraperitoneal injection.
1.
Introduction
Cardiovascular disease (CVD) is the leading cause of disability and mortality in the developed
countries. Atherosclerosis is a systemic disease that develops slowly and often asymptomatically, so
that for many patients its first manifestation is sudden cardiac death, stroke, or myocardial infarction.
The clinical challenge is not just in identifying the patient with atheroma but in recognizing specific
lesions likely to cause clinical events, that means “vulnerable” plaque. Also monitoring novel treatment
strategies, for example delivery of different variants of stem cells. For these reasons, the assessment of
myocardial perfusion plays an important role in the diagnostic work-up of patients as well as in the
assessment of prognosis and guiding the therapy [1-6]. Studies with mice are very important due to the
similarities of the disease onset and progression with human coronary artery diseases. Both genetically
modified and artificially induced mouse model are available for research purposes. From the imaging
side, molecular imaging by radionuclides is the most reliable non-invasive technique for myocardial
perfusion studies. SPECT is the technique of choice here over PET. In fact SPECT techniques have a
special role in small animal imaging research [7]. Although SPECT have limited sensitivity due to the
use of traditional collimation, PET has intrinsic limitations such as spatial resolution [8]. Also, a large
spectrum of SPECT radiotracers is accessible, and, provided the detector has good energy resolution,
multi-isotope imaging allows the study of different molecular probes simultaneously. For example,
dual-isotope small animal SPECT would allow simultaneous imaging of 99mTc-labeled MIBI to assess
myocardial perfusion and of 111In labelled stem cells to delineate stem cells engraftment [6,9]. It has
been shown [10,11] that after careful calibration, using standard nuclear medicine software, ECG gated
myocardial perfusion SPECT in mice permits quantification of LV volumes and motion. This would
allow evaluating the effects of therapy in the limit of the sensitivity attained by the system. In fact the
magnitude of 99mTc-MIBI uptake predicts the response of myocardium with abnormal function to
subsequent revascularization in the chronic coronary artery disease, and the recovery of myocardium
after reperfusion therapy for acute MI.Studying cardiovascular diseases by means of small animal
models is very challenging due to the need of sub millimeter spatial resolution, high energy resolution
and high sensitivity. The goal of an experiment dictates the spatial resolution and the sensitivity
required for the imaging system. Many devices have been proposed or developed, each with different
performance characteristics [12]. Most of them are based on the standard Anger camera-based detector
with single pinhole and multipinhole collimation [12]. Taking advantage of high geometric efficiency
and high magnification factor that can be employed from pinhole collimator when imaging small
animal, good imaging performance SPECT have been showed, especially using multipinhole
techniques. Nevertheless, the Anger camera-based systems have limitations [13] for several reasons.
The ideal system should have an “open” and flexible design, to be integrated in a multimodality system
with other detectors (MRI, CT, optical). This is difficult with the standard Anger camera-based
systems.
Our group started research studies on detection of atherosclerotic plaques and of stem cell therapy on
mice using pinhole SPECT techniques. From the review of prior art, many clinical trials have been
performed recently on this subject but the results are contradictory. Therefore, indeed more basic
studies with small animals have to be performed [5]. Also, we found that repeated injections of
radiotracers in mice modeled for infarction, possibly for weeks or even months, when needed, is not
possible using the usual route of delivery (tail vein). Alternative delivery routes have to be developed.
This paper describes the research started by our collaboration in outlining the best detectors suited for
these studies, the preliminary measurements, with a high resolution detector prototype, in detection of
atherosclerotic plaques on genetically modified mice, and perfusion images comparing the uptake of
99mTc-MIBI with the two injection options: via tail vein and peritoneum injection techniques.
2. Materials and Methods
2.1 Detector layout
We designed an optimized radionuclide detector system for this task, flexible enough to be integrated
in a multimodality system: 8 detectors to maximize the trade-off between spatial resolution and
sensitivity. One of these special modules is a detector with spatial resolution in the range of 300-500
μm, sensitivity of 0.3 cps/kBq, and active area 100 x 100 mm2, using tungsten pinhole collimator(s)
and a high granularity pixilated scintillator (0.8 mm pitch and sufficient light yield) or a continuous.
Details on the basic studies on detector prototypes can be found in [14-19] . The (calculated)
performances of such a detector system compared to what can be obtained with Anger camera-based
systems are shown in Fig.1
The arguments for the selection of the particular components come from the fact that in multipinhole
SPECT with 3D reconstruction, a sufficient number of “resolution elements” has to be used [20]. This
translates in the requirement of approximately 120 pixels in a 100 mm dimension of the detector, that
means an intrinsic spatial resolution of Ri = 0.8 mm. Scintillator arrays composed of very small pixels
have to be used and identifying these small pixels is challenging. It would require the use of
multichannel readout to fully exploit the detector characteristics.
2.2 Detector prototypes
A scintillator array with 0.8 mm pitch with the needed light yield pixel was not available on the market.
In order to be able to study the basic performances of the detector and issues of radiotracer delivery in
monitoring the possible repair of infarcted mouse heart by stem cell therapy, prototype detector was
designed and built: a pinhole collimator, a pixilated NaI (Tl) scintillator 100 x 100 mm2 (1.5 mm
pitch) coupled to a Position Sensitive PhotoMuliplier (PSPMT) Hamamatsu H8500 (6 x 6 mm 2 anode
pixel). The pinhole collimators provide a imaging geometry that allows obtaining a FOV of ~ 30 x 30
mm2 (M=3) sufficient for imaging the fraction of the mouse body relevant for studying stem cell
trafficking, or a FOV of 25 x 25 mm2 (M=2) for heart perfusion imaging and detection of
atherosclerotic plaque. The spatial resolution and sensitivity depend strongly on the size of the pinhole
aperture. The selected pinhole diameter for both the measurements quoted in this paper was 0.5 mm.
The above prototype detectors allowed us to study phantom test measurements, the basic imaging
properties of detection system, animal handling issues and radiotracer delivery issues.
A compact individual-channel, self-triggering readout electronics, based on MAROC2 chip controlled
with FPGA, was specifically designed, built, and successfully employed [21]. The front-end operates
with both H8500 and H9500 PMTs. The electronics interface to the DAQ via a USB2 high-speed
interface.
The prototype SPECT system was equipped with a 2.5-cm-diameter acrylic cylindrical bed-holder
(with 3 mm wall thickness) that kept the mouse in a horizontal position (see Fig. 3). The detector was
mounted on a motorized gantry that could rotate around the animal bed. The bed holder stayed in a
fixed position. The system could be manually adjusted to optimize the distance between the pinhole
and the axis-of-rotation, giving the possibility to configure the imaging parameters depending on
measurement requirements. The detector design parameters and imaging performance characteristics
are listed in Table 1.
2.3 Animal procedures, Anesthesia, and Tracer administration
Two three-month-old adult FVB/N male mice, weighing 30 g, were intraperitoneally anesthetized. The
single pinhole projection data were acquired in 60 angular intervals over 360 degrees. Thoracic bone
scan was performed to evaluate system’s image quality (a mouse was injected with 2 mCi of 99mTcMDP). Acquisition of projection data started 2 hours post injection of radiotracer at 2 min/projection.
For one of the mice, 247.9 MBq of 99mTc-MIBI was injected into the tail vein. Care was taken to
minimize, as much as possible, the volume of injected tracers around 0.02-0.05 ml to avoid significant
changes in the whole blood volume of the mice. Myocardial perfusion scan was 1 hour after tracer
administration to ensure a better contrast of heart to soft tissues. The same procedure was used for the
second mouse but it was injected with 247.9 MBq of 99mTc-MIBI intraperitoneally. To assure highresolution and artifacts free SPECT image reconstruction, mechanical calibration of the imager was
needed. The calibration procedures required a SPECT acquisition and reconstruction of a set of 2 point
sources (~ 1 mm in size) positioned as far as possible both along the axis-of-rotation and away of it.
Table 1
Pinhole Diameter (mm)
0.5
NaI (Tl) Scintillator:
1.5
- pitch (mm)
- Thickness (mm)
- Dimension (mm)
6
100 × 100
Photomultiplier Array
(2 × 2) H8500
Resolution (mm)
< 0.8
Efficiency (cps/MBq)
35
Magnification Factor
3
Field of View (mm)
33
Pinhole Diameter (mm)
0.
2.4 Image reconstruction technique
The acquired projection data were reconstructed using a 3D pinhole OS-EM image reconstruction
algorithm that takes into account geometric misalignment parameters of the system, including the
centre-of-rotation error, the tilt angles between the axis-of-rotation and the detector plane in 3D space.
Size of the reconstruction matrix was 90°×°90°×°90 with a voxel size of 0.25 mm. A 3D Butterworth
post-filter was used to smooth noise and to improve the final reconstructed image quality.
2.5 Myocardial perfusion analysis
There is no true standard for quantification of SPECT [22]. We used the Standardized Uptake Value
(SUV) also referred to the dose uptake ratio, DUR, defined as a ratio of tissue radioactivity
concentration (in units kBq/ml) at time T, i.e., CPET(T), and injected dose (in units MBq) at the time of
injection divided by body weight (in kg units).
SUV = CPET(T)/(Injected dose/animal's weight). If radiotracer is uniformly distributed, and delay time
is taken into account, we calculated it as Regional Uptake Value (RUV) for the region of interest
(heart).
2.8 Detection of atherosclerotic plaque
Another prototype detector using pixilated CsI (Tl) scintillator array, 1.0 mm pitch (close to what is
needed) coupled to PSPMT Hamamatsu H9500 (3 x 3 mm2 anode pixel). The same collimator, the
same setup and procedure for the SPECT system have been used in an experiment to detect
atherosclerotic plaques in mice. In fact pixilated scintillator arrays with 0.8 mm pixels and sufficient
light yield was not available on the market. We decided also to evaluate a very small CsI (Na) array
with 0.8 mm pixel, a good candidate for our detector.
3. Results
2.4 Phantom studies
In order to test the reconstructed spatial resolution of our prototype SPECT system, a miniature acrylic
resolution phantom was manufactured, as shown in Fig. 4. It consists of 6 sectors, each containing
equally sized sets of small diameter holes (0.8, 0.9, 1.0, 1.1, 1.2, 1.3 mm). The overall phantom
diameter was 25 mm. The total activity in all filled capillary holes was ~ 4.5 mCi of 99mTc. The single
pinhole projection data were acquired in 60 angular intervals over 360 degrees at 2 min/projection. For
imaging the resolution phantom as well as the myocardial perfusion study we used a FOV with a
diameter of 33 mm. The spatial resolution of the system is then ~0.8 mm.
The sensitivity of the system was ~35 cps/MBq. It was measured by using a 370 kBq source of 57Co
placed in the centre of the FOV at a distance of 10 mm. The energy resolution was 14% at 122 keV.
3.2 Perfusion images
Figure 5 shows sample short-axis (left) and horizontal long-axis images (right) from the 99mTc-MIBI
myocardial perfusion SPECT study obtained using the prototype detector with the pixilated NaI (Tl)
crystal with 1.5 mm pitch. The left and right ventricular cavities and corresponding walls can be easily
identified. The need for a different route of delivery brought us to a new scan with comparison of two
different injection methods, through the tail vein and through the peritoneum. The same imaging
acquisition and reconstruction procedures were adopted. In Fig. 6 we show images of the mouse
injected trough the tail vein; transversal, sagittal and coronal views are shown.
The second mouse had the radiotracer injected intra-peritoneal. All other procedures were the same.
Fig. 7 shows the obtained 99mTc-MIBI myocardial perfusion images.
3.3 Uptake
Tab. 2 shows the results of the calculated uptake for the two delivery modalities.
A reduction of uptake occurred but the ventricular cavities are identified in both cases.
3.4 Detecting atherosclerotic plaques
A transgenic ApoE-/- mouse was scanned using 99mTc-Annexin-V at different ages. Preliminary
results showing uptake of 99mTc-Annexin-V are demonstrated in Fig. 8. At a young age no focal
activity uptake in plaque (Fig. 8a) is observed. Uptake of Annexin V by liver is seen. It can attribute to
the limitations of resolution and sensitivity of the detector. Figure. 8b shows the results from the
control mouse at 25 weeks old. No suspicious focal uptake is observed. Fig. 8c shows the results from
the ApoE-/- mouse at 25 weeks old. Suspicious focal activity uptakes indicating possible plaques can
be seen in the image. However, no definitive conclusions can be extracted from this preliminary
analysis.
Table 2
SUV
Peritoneum
Tail vein
Transversal
0.47
1.09
Coronal
0.39
1.22
Sagittal
0.41
1.31
4. Test measurements on CsI (Na)
In order to confirm the feasibility of a detector system fulfilling our design study, preliminary test
measurements with a small CsI (Na) array, coupled to a Hamamatsu H9500 (3 x 3 mm 2 anode pixel)
was performed. Fig.8 shows very good pixel identification confirming the feasibility of the detector
with wanted performances in terms of spatial resolution. Nevertheless this layout could be not
optimized in the dead areas between the PSPMT’s [16,19]. Moreover, good energy resolution across
the detector field of view would be necessary to be able to use the multi-isotope imaging technique. For
this reason careful comparison has to be done between pixilated CsI (Tl) detector with 0.8 mm pitch
and continuous LaBr3(Ce) with high intrinsic resolution when robust, reliable thin sheets will be
available.
4. Conclusions
A single-head high-resolution prototype SPECT system with different prototype detectors has been
built for studying pinhole SPECT system for molecular imaging of small animal. Possible applications
of the system include the detection of atherosclerotic plaques and stem cells tracking for their fate and
the effect of the therapy. The goal of our research was to determine the imaging characteristics of the
SPECT system and to study animal handling issues.
The spatial resolution of the prototype SPECT system showed to be sufficient for perfusion studies.
The energy resolution allows the use of dual-isotope image techniques. The sensitivity of the system
can be increased by using a larger dimension of the pinhole (up to 1.5 mm [10] with concurrent
degradation of spatial resolution. We demonstrated that peritoneum injection the radiotracer shown
different SUV values as compared to tail vein injection in a 99mTc-MIBI myocardial perfusion SPECT
study. In particular, a reduction of the uptake by the heart muscle was observed. However, both show
the same regional perfusion defect at the same location. It has to be emphasized that to fully
accomplish the objectives of molecular imaging, an integration of SPECT with other imaging
modalities is necessary [6]. Examples are additional integrated optical and MRI systems. This will
require modifications of system configuration, and materials and components of the radiation detector,
for example, starting with substitution of PSPMT’s with silicon photomultipliers (SiPMs) insensitive to
the magnetic fields. Research in this direction is ongoing.
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Figure Captions
Fig. 1. (Top) Efficiency (EFF) and (Bottom) Spatial resolution (Rt) for high-resolution Anger camera-based
SPECT system
Fig.2a. APOE(-/-) mouse 6 weeks old
Fig.2b 25 weeks: Control
Fig.2c APOE(-/-) 25 weeks old
Fig. 3 The SPECT prototype system.
Fig. 4 Miniature acrylic resolution phantom (left), and reconstructed image (right), sum of 21 trans-axial slices.
0.8 mm capillaries are clearly separated in the image.
Fig. 5. Short-axis (Left) and vertical long-axis (Right) images through the 3-D 99mTc-MIBI myocardial
perfusion SPECT image of a living mouse.
Fig. 6. Transversal, sagittal and coronal heart views. Tail vein injection.
Fig. 7. The same as Fig. 3 except that for the mouse injected peritoneally.
Fig. 8 Flood raw image (57Co) and pixel identification of the CsI(Na) 0.8 mm pitch array coupled to a
Hamamatsu H9500 flat panel PMT (see text).