Assessment of left ventricular perfusion by intraperitoneal administration of 99mTc-MiBIc
using high-resolution, high efficiency single photon detector
Abstract–SPECT systems using pinhole apertures
permit radiolabeled molecular distributions to be
imaged in vivo in small animals. Nevertheless
studying cardiovascular diseases by means of
small animal models is very challenging.
Specifically, submillimeter spatial resolution,
good energy resolution and high sensitivity are
required. We designed what we consider the
“optimal” radionuclide detector system for this
task. It should allow studying both detection of
unstable atherosclerotic plaques and monitoring
the effect of therapies. Using mice is particularly
challenging in situations that require several
intravenous injections of radiotracers, possibly for
week or even months, in chronically ill animals.
Thus, alternative routes of delivering the
radiotracer in tail vein should be investigated. In
this study we have performed preliminary
measurements of detection of atherosclerotic
plaques in genetic modified mice with highresolution prototype detector. We have also
evaluated the feasibility of assessing left
ventricular perfusion by intraperitoneal delivering
of MIBI-Tc in healthy mice.
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.
On the other hand a novel treatment strategy, the
delivery of different kind of stem cells and
different modalities for research with animals and
even for clinical trials. For this reason 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 of therapy [1-3]. Studies with
animals, namely mice are very important for the
similarities with human coronary artery diseases.
Mice models both genetically modified and
artificially induced are available. Molecular
imaging by radionuclides is the most reliable non
invasive technique for myocardial perfusion
studies. SPECT better than PET is the technique
of choice. It has been shown [14,17] that after
careful calibration, using standard nuclear
medicine software, perfusion ECG gated 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 99mTcMIBI uptake predicts the response of myocardium
with
abnormal
function
to
subsequent
revascularization in 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 because
submillimeter spatial resolution, good energy
resolution and high sensitivity are required. The
goal of the experiment dictates the spatial
resolution and sensitivity required for
the
imaging system. Many devices have been
proposed or developed, each with different
performance characteristic [20]. Most of them are
based on Anger cameras with pinholes and
multipinholes [20]. Due to the high efficiency and
to the magnification factor that can be used good
performances have been showed especially using
multipinhole techniques. Nevertheless the Anger
camera based systems have limitation [21] for
many reasons. The ideal system should be an
“open”
and flexible, to be integrated in
multimodality with other detectors (MRI, optical).
This is not possible with the Anger camera based
systems.
Our group started research studies on detection of
vulnerable atherosclerotic plaques and of stem cell
therapy on mice.
Many clinical trials have been performed recently
on this subject but the results are contradictory.
More basic studies with small animals have to be
performed [2].
Injecting radiotracers in mice modelled for
infarction many times, possibly for week or even
months, as needed, is impossible using the usual
route of delivery (tail vein). Alternative routes
have to be used.
This paper describes the research started by our
collaboration in outlining the best detectors suited
for these studies, the preliminary measurements,
with an high resolution detector prototype, for the
detection of atherosclerotic plaques on genetic
modified mice and perfusion measurements
1
comparing the uptake of Tc-MIBI with the two
modalities: tail vein and peritoneum.
is obtained coupling such a scintillator to a
PSPMT Hamamatsu H9500 (3 x 3 mm2 anode
pixel).
2. Materials and Methods
We designed what we consider the “optimal”
radionuclide detector system for this task, flexible
enough to be integrated in a multimodality
system: 8 detectors to optimize 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 m, sensitivity
of 0.3 cps/kBq, and active area 100 x 100 mm2,
using tungsten pinhole collimator(s) and a high
granularity scintillator (CsI(Na) with 0.8 mm
pitch (the smallest so far achieved for SPECT
detectors)) or continuous LaBr3.
The performances of such a detector system
compared to what can be obtained with Anger
camera based systems are shown in Fig.1
Fig. 2 Flood field (Co-57) and pixel identification
of CsI(Na) 0.8 mm pitch coupled to Hamamatsu
H9500 (see text
Nevertheless this layout could be not optimized in
the dead areas between the PSPMT’s [9,12].
Moreover good energy resolution would be
needed to be able to use the multilabeling
technique. For this reason careful comparison has
to be done between CsI(Tl) 0,.8 mm pitch and
LaBr3(Ce) that, provided thin sheets (4 mm) (so
small pixels) of it can be built. This solution
would allow to get rid of the dead area problem
and would provide very good energy resolution
2.1 Prototypes available
Fig. 1 Spatial resolution and efficiency for High
resolution based system and Anger camera based
system
2.1 Detector layout
The reasons for the choice of the layout comes
from the fact that in the SPECT with
multipinholes, with 3D reconstruction, a sufficient
number of “resolution elements” has to be used.
This translates in the need of 120 pixels in 100
mm, that means an intrinsic spatial resolution of
ri=0.8 mm. Scintillator arrays of very small pixels
have to be used and identifying so small pixels is
challenging. It will require to fully exploit the
characteristics of the electronics we have designed
and built, capable of reading out up to ~ 4096
channels individually at 20 KHz [9, 16].
Preliminary test measurements with small samples
(seeFig.2) show that very good pixel identification
Two prototypes with peformances close to the
needs in terms of spatial resolution were available,
a NaI(Tl) 1.5 mm pitch 100 x 100 mm2, coupled
to PSPMT Hamamatsu H8500 (6 x 6 mm2 anode
pixel) and CsI(Tl) 1 mm pitch coupled to PSPMT
Hamamatsu H9500 (3 x 3 mm2 anode pixel).
These prototypes allowed performing the
measurements described in this paper. We used
pinhole collimator that according to the
magnification allows obtaining a FOV of ~ 30 x
30 mm2 (M=3) sufficient for imaging the mouse
body relevant for studying the stem cell
traffickling and a FOV of 25 x 25 mm2, sufficient
for heart perfusion imaging. The spatial resolution
and sensitivity depend strongly on the hole
dimension. The hole dimension was 0.5 mm.
This allowed us to study the basic problems of the
detection system and animal handling.
2.2 Micro SPECT System prototype.
The prototype SPECT system is a detector
equipped with a 2.5-cm-diameter acrylic
cylindrical bed-holder (3 mm thick) that keeps the
mouse horizontal (see Fig. 3).
2
(MIBI-99mTc) 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. The single pinhole projection
data were acquired in 60 angular intervals over
360 degrees. Thoracic bone scan was performed to
Fig. 3 The SPECT system prototype
Two detectors are mounted on a motorized gantry
that can rotate (only one detector was used for the
measurement described here). The bed holder stay
fixed. The system could be manually adjusted to
optimize the distance between the pinhole and the
axis of rotation, giving the possibility to resize the
camera parameters depending on measurements
requirements. The detector characteristic and
performance parameters are listed in Tab1.
Tab.1
Pinhole Diameter (mm)
NaI (Tl) Scintillator:
- pitch (mm)
- Thickness (mm)
0.5
1.5
6
- Dimension (mm)
100 × 100
Photomultiplier Array
(2 × 2) H8500
Resolution (mm)
Efficiency (cps/MBq)
Magnification Factor
Field of View (mm)
evaluate system image quality (mouse was injected
with 2 mCi of 99mTc-MDP). Tomographic acquisitions
started 2 hour after tracer administration. Projection
data were acquired at 2 min/projection. Myocardial
perfusion scan was performed. Live mouse was
injected with 6.7 mCi of 99mTc-MIBI; acquisitions
started 1 hour after tracer administration to ensure a
better contrast of heart to soft tissues. Projection data
were acquired at 60 sec/projection. The same procedure
was used for the second mouse but was injected with
6.7 mCi of 99mTc-MIBI intraperitoneally. To assure
high-resolution and artefacts free SPECT image
reconstruction, mechanical calibration of detector was
needed. For this reason, tomographic acquisition 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, was also performed.
< 0.8
35
3
33
2.3 PHANTOM STUDIES
To test the tomographic spatial resolution
capabilities of our detector, a miniature acrylic
resolution phantom was manufactured, as shown
in Fig. 4. It consists of 6 sectors, each containing
equally sized sets of capillaries (0.8, 0.9, 1.0, 1.1,
1.2, 1.3 mm). The overall phantom diameter was
2.5 mm. The total activity in all filled capillary
was ~ 4.5 mCi of 99mTc. The single pinhole
projection data were acquired in 60 angular
intervals over 360 degrees at 2 min/projection.
2.4 Animal procedures, Anaesthesia, and
Tracer administration
Two adult mice three-month-old VFB/N male
mice, weighted 30 g, were intraperitoneally
anesthetized. For one of the mice, the radiotracer
2.5 Image reconstruction technique
Upper head projection data were reconstructed
using a 3D pinhole OS-EM algorithm.Which
takes into account system geometric misalignment
parameters, including the centre-of-rotation error,
the tilt angles between the axis-of-rotation and the
detector plane in the 3D space. Reconstruction
matrix size was 90°×°90°×°90 with a voxel size of
0.25 mm. A 3D Butterworth filter was used for
the post-reconstruction
2.6 Myocardial Perfusion Analysis
There is no true standard for quantification of
SPECT [15]. We used the Standardized uptake
value, SUV, also referred to the dose uptake ratio,
DUR, calculated as a ratio of tissue radioactivity
concentration (in units kBq/ml) at time T,
CPET(T) and injected dose (in units MBq) at the
time of injection divided by body weight (in units
kg).
SUV=CPET(T)/(Injected dose / Patient's weight).
If the is radiotracer uniformly distributed, taking
into account the delay time, we calculated it for
the region of interest (heart) (Regional Uptake
3
value (RUV) RUV=[(counts)eλt)inROI/(Volume
ROI/CPET).
2.7 Detection of atherosclerotic plaques
Another detector prototype using CsI(Tl)
scintilator array, 1.0 mm pitch and the same setup
and the same procedure has been used for
detecting atherosclerotic plaques.
and horizontal long-axis slice (right) obtained
from reconstructed projection of myocardial
perfusion images are shown. Left and right
ventricular cavities and corresponding walls can
be easily identified
5 mm
5 mm
LV
LV
RV
3. RESULTS
3.1 Phantom
Sensitivity
studies,
Spatial
Resolution,
For the resolution phantom as well as for
myocardial perfusion we used a FOV of 33
mm. The spatial resolution of the system is
~0.8 mm. The intrinsic spatial resolution of
the detector was 1.5 mm.
R
V
Fig. 5. Mutual perpenddicular cross section 3-dimensional
myocardial perfusion image volume of living mouse. Shortaxis slice (left) shows myocardial perfusion in right
ventricular (RV) and left ventricular (LV) wall. Horizontal
long-axis slice
The need of a different route of delivery brought
us to new scan with comparison of two different
modalities, injection through the tail vein and
injection through the peritoneum. The procedure
adopted was the same. In Fig. 5 we show images
of the mouse-injected trough the tail vein;
transverse, sagittal and coronal views are shown.
1.3 mm
0.8 mm
1.2 mm
0.9 mm
1.1 mm
1.0 mm
Fig. 4 Miniature acrylic resolution phantom (left),
and reconstructed image (right), sum of 21 transaxial slices. 0.8 mm capillaries are clearly
separated on image
Fig. 6 Transverse, sagittal and coronal heart views. Tail
vein injection
The other mouse had the radiotracer injected
peritoneally. The procedures were the same. Fig 5
shows the perfusion images.
The sensitivity of the system was ~ 35 cps/MBq.
It was measured by using a source 370 kBq of Co57 in the centre of the FOV at a distance of 10
mm. The energy resolution was 14%.
Fig. 7 The same as Fig. 3 for the mouse injected
peritoneal.
3.2 Perfusion images
Three-month-old VFB/N male mice, weighted 30
g, were intraperitoneally anesthetized. Care was
taken to minimize the volume of injected tracers
into the tail vein (0.02-0.05 ml) to avoid
significant changes in the whole blood volume of
the mice. Fig. 5 shows a perfusion image obtained
with the detector prototype described (NaI(Tl) 1.5
mm pitch). Mid-ventricular short-axis slice (left)
3.3 Uptake
Tab.2 shows the results of the calculated uptake
for the two delivery modalities.
4
4. CONCLUSIONS AND OUTLOOK
Tab.2
Peritoneum
Tail vein
Dose (MBq)
170
130
Weight (g)
37
37
Age (week)
12
12
Duration/view
(min)
1
1
Transverse
0.62
1.09
Coronal
0.51
1.22
Sagittal
0.53
1.31
A reduction of uptake occurred but the ventricular
cavities are identified in both cases.
3.4 Detecting atherosclerotic plaques
Mice of different ages were scanned. Preliminary
results are shown in Fig. 8. Young mouse didn’t
show, in the limit of the sensitivity of the detector,
plaques. Uptake of Annexin V by liver is shown.
Fog. 8 b show the result for the control mouse, 25
weeks old. It doesn’t show any suspicious spot.
Fig. 8 c shows the result for the APOE (+/-)
mouse 25 weeks old. Suspicious spots seem to be
shown in the image. No definitive conclusions can
be extracted form this preliminary analysis.
Hystological findings will confirm eventually the
detection of atherosclerotic plaques.
A single head high-resolution detector prototype
has been built for studying optimized SPECT
system for researches on the detection of
atherosclerotic vulnerable plaques and on the
diffusion of stem cells, their fate and the effect of
the therapy.
The scope was to determine the detector
characteristics and to study animal handling
issues.
The spatial resolution of the prototype showed to
be sufficient for perfusion studies. The energy
resolution allows using dual tracers techniques.
The sensitivity would have been significantly
higher using a larger dimension of the hole (up to
1.5 mm [14]).
We demonstrated that injecting the radiotracer
through the peritoneum instead of the tail vein
allows obtaining a good perfusion.
The price to be payed is a reduction of the uptake
by the heart. The sensitivity of the system has to
be increased to compensate the reduction. This
can be obtained by fine-tuning the parameters (the
hole dimension and the magnification), using the
multipinhole technique and adding as many
detectors as possible to the system.
It has to be underscored that to fully accomplish
the objectives of the study will probably require
the integration of other modalities [3], essentially
optical and MRI with significant modifications of
the layout, and of the materials and components
starting with substitution of PSPMT’s with Silicon
Photomultipliers (SiPm) insensitive to the
magnetic fields.
Research in this sense is
ongoing.
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Fig. 6 a. Apoe mouse 6 weeks old
Fig.6b 25 weeks
Control
Fig.6c APOE mouse
25 weeks old
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