A novel high resolution and high efficiency dual head detector for Molecular Breast Imaging
F. Garibaldi et al.
ISS Dipartimento TESA
1. INTRODUCTION
Several designs of dedicated gamma cameras for
Molecular Breast Imaging (MBI) have been
implemented, tested and shown to increase the
detection sensitivity for sub-centimeter size lesions
[1,3]. Nevertheless the technique can be further
improved. Key parameters for detecting small lesions
are: spatial resolution, Signal to Noise Ratio (SNR) and
contrast. Energy resolution plays only a secondary
additional role in imaging breast under compression.
The intrinsic properties of the gamma detector have
been optimized and clinical trials have been
successfully performed with a single head detector. In
order to improve the performances of the MBI system
we have implemented a new layout, a dual detector
setup that allows spot compression for detecting very
small tumors.
further improved by using a pinhole collimator. The
system has another advantage, the possibility of
rotating the small detector and allowing focusing on
the suspicious lesion in case of lesion proximity to the
chest wall.
a
b
c
Fig. 1. Schematic layout of the detector. Standard dual head
system (a) was replaced with an asymmetric system with one
detector head with a spot compression and pinhole collimator (b).
Rotating the small detector allows better “focusing “on the lesions
close to the chest wall (c).
I. MATERIALS AND METHODS
We designed a dual head high-resolution high
sensitivity detector for MBI. The layout is shown in
Fig. 1. Fig. 2 shows the system prototype. The larger
detector has the same dimension as the mammographic
screen (150 x 200 mm2); the smaller is 50 x 50 mm2.
The
first
detector
consists
of
a
high
efficiency/multipurpose parallel hole collimator, a
Saint Gobain Crystals array of pixellated NaI(Tl) with
1.5 mm pixel step (1.3 mm pixel size + 0.2mm thick
separation septa) coupled to an array of Hamamatsu
H8500 flat panel PMT’s. More optimal design has
been also considered with NaI(Tl) 1.2 mm pitch array
coupled to an array of Hamamatsu H9500 flat panel
PMTs. Indeed smaller pixel size has been shown to
produce greater SNR [7,8,9]. Pixel size (and
correspondingly the number of pixels in the detector) is
to our best knowledge, the smallest for this kind of
detectors (high resolution MBI). The second detector
consists of a pinhole collimator with a 2 mm hole,
magnification factor M=2 and FOV ~ 25 x 25 mm2,
coupled to a continuous LaBr3(Ce) scintillator (4 mm
thick) and Hamamatsu H9500 PMT. A commercial
electronic system1 capable of reading out all channels
separately has been used (12 x 64 = 768 channels in
this case). A new readout. described elsewhere, has
been successively implemented to overcome some
limitation of this system [10,11].
The layout allows performing spot compression and
therefore bringing the detector closer to the lesion with
significant increase in the efficiency and consequently
the SNR. The sensitivity and spatial resolution are
1
IDEAS
Fig.2. The clinical prototype system.
2. Simulations and calculations
Both spatial resolution and sensitivity are greatly
improved by using the pinhole collimator. Fig. 3 shows
the comparison of spatial resolution of detectors with
parallel hole and pinhole collimators as function of the
source distance.
Fig. 3: Comparison of spatial resolution of 2mm pinhole and parallel
hole collimators.
Fig. 4 shows the efficiency and FOV for parallel hole
and pinhole collimators as function of source distance
for different values of the “focusing”. A system able to
tune the focus in order to avoid too small FOV is
needed. It has been implemented by using simple
plastic spacers allowing to tune the FOV and optimize
the tradeoff spatial resolution/efficiency/FOV. In fact
Fig. 5 shows the same as Fig. 4 but with different
focusing distances. It is possible to choose the right
compromise FOV/efficiency.
Fig. 4. Top: Efficiency of parallel hole and pinhole collimators.
Bottom: effective FOV for a pinhole collimator system versus
“focusing”.
3. Phantom measurements
to be done
Fig. 6. SNR for single and dual head detctors. The single head
parallel hole points are phantom data. The other points are
extrapolated from the calculated efficiencies.
4. CLINICAL TRIALS
The single head detector has been used on 10
patients in clinical trials at the University of Tor
Vergata (Rome) where a commercial high resolution
detector was available, the LUMAGEM by Gamma
Medica. Fig. 7 shows the image of the breast with a
suspicious lesion from mammography. It showed to be
negative to the MBI both for our detector and for
LUMAGEM. Biopsy confirmed this finding.
Fig. 5. Efficiency of parallel hole and pinhole collimators for a
different focusing geometry.
A dual detector setup improves the performances
because of the possibility of combining the counts of
the two detectors [12]. Fig. 5 shows the comparison of
the efficiencies obtained combining counts coming
from two parallel-parallel collimators with parallelpinhole collimators. The advantage of our system is
evident. Finally Fig. 6 shows the advantage in terms of
SNR of the pinhole with respect to parallel hole for
single head setup and parallel-parallel to parallelpinhole dual head setup.
Fig. 7. Scintimamography performed with LumaGem (left) and
our detector (right). There was a suspicious lesion from
mammography, ultrasound and MRI.
There were other such negative cases and few
(relatively) big tumors (>10mm). All trials showed
comparable results with respect to the LUMAGEM
detector.
Other trials will be performed soon with the novel dual
detector both in Rome and in Naples hospitals. Results
will be shown during the workshop.
II. SUMMARY
A novel MBI system has been built and tested with
phantoms and in clinical trials. 10 patient trials have
been performed with the high resolution single head
detector. The images have been compared with a
commercial
high resolution gamma
camera
(LUMAGEM2) showing similar results. A novel setup
has been implemented by adding a second, smaller
detector, to allow spot compression. Calculations and
preliminary phantom measurements show that the
performances of the system are significantly increased
allowing to detect small tumors. The new system is
going to be used in clinic in patient trials. Results will
be presented during the workshop.
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