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Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
A novel scatter detector based on the Controlled-Drift
mechanism suitable for Compton prostate imaging
A. Castoldia and C. Guazzonib
a Politecnico di Milano, Dip. Ingegneria Nucleare Ce.S.N.E.F. and INFN Sez. Milano, Italy
b Politecnico di Milano, Dip. Elettronica e Informazione and INFN Sez. Milano, Italy
Abstract
A very promising technique for prostate imaging is given by Compton imaging. The use of the Compton imaging
principle allows gaining both in position resolution and in sensitivity with respect to state-of-the art systems based
on the Anger Camera approach since Compton imaging provides electronic collimation of the incoming photon
without limiting the detection solid angle. In this paper we propose a novel silicon detector, called Controlled-Drift
Detector that can match the demanding requirements of the scatter detector in a high-resolution Compton camera for
prostate imaging. The results of the Compton electron tracking within one detector layer are presented and
discussed. This feature reduces the event circle to a short arc of uncertainty centered on the source direction.
KEYWORDS: Compton imaging, scatter detector, prostate imaging, Controlled-Drift Detector.
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
1. Introduction
Prostate cancer is one of the most frequently occurring cancers in men. In the last decade several
imaging techniques (both based on MR and on CT/radiotracers detection) have been applied to
prostate cancer diagnosis and staging. [1-3]. The main requirements of the radiotracers imaging
system are in terms of spatial resolution and count rate that in state-of-the-art systems based on
mechanical collimation of the incident -rays are limited and prevent good quality images. A very
promising technique is a Compton imaging system with an intra-rectal probe housing the scatter
detector [4, 5]. The use of the Compton imaging principle allows gaining both in position resolution
and in sensitivity. In fact a Compton Camera provides electronic collimation of the incoming photon
without limiting the detection solid angle as it is done with mechanical apertures, since Compton
scattering preserves information about the direction and energy of incident  -rays. Moreover the
position resolution achievable in Compton imaging improves for energies of the incoming  -ray
above 140keV where the Anger Camera approach is less suitable. This fact is of particular interest in
the case of prostate imaging, since the antibody (Prostascint®) which targets an antigen produced by
prostate cells is combined with
111
In, a radioisotope emitting two -rays at 171 and 245 keV. In this
paper we propose a novel silicon detector, called Controlled-Drift Detector (CDD) that can fulfill all
the demanding requirements of the scatter detector in a high-resolution Compton camera for prostate
imaging. Section 2 reviews the theoretical background of Compton imaging while Section 3
describes the features of the proposed detector relevant to Compton imaging. Section 4 discusses the
results of Compton electrons tracking. Section 5 ends with conclusions and future plans.
2. Theoretical background
The performance of a Compton imager is mainly dominated by the parameters of the scatter detector
that must track with high accuracy the primary recoil electron and any secondary interactions of the
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
scattered photons. Compton kinematics allows then the reconstruction of the direction of incidence of
the -ray that lies on a cone whose axis and aperture  can be calculated from the measured quantities
[6]. The overall angular uncertainty is defined by the following equation:
2
2
 2   Doppler
  E2   Geom
(1)
where  Doppler is due to Doppler broadening in the scatter detector material [7] and defines the
ultimate performance of the camera,  Geom takes into account geometrical effects (finite pixel size
and detectors thickness, source-detector and inter-detector distance) and  E is due to the finite
energy resolution of the scatter detector. The last contribution can be expressed by:
 E 
m0 c 2

sin  E  E e
2
E
(2)
where m0c2 is the electron rest energy, E is the energy of the incident photon and Ee is the recoil
electron energy, E is the energy resolution of the scatter detector. Fig. 1 shows the total angular
uncertainty  as a function of the scatter angle for the two -ray lines of
111
In with the energy
resolution of the silicon scatter detector as parameter. The geometric contribution has been neglected
and the absorption detector is assumed ideal. Since the energies are not so high the detector energy
resolution impacts on the achievable resolution and sets a stringent requirement on the scatter
detector performances. The possibility of near-field operation offered by the Compton probe
increases the subtended solid-angle accounting for good efficiency even with small-size instruments
Moreover, the effects of the angular uncertainty translates in a small spatial uncertainty.
3. Material and methods
The combination of position-sensing capability, high energy-resolution and very fast event timing are
peculiar features of novel multi-linear silicon detectors based on the Controlled-Drift mechanism.
The additional advantage of the relatively small Doppler broadening of silicon make these detectors a
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
very promising choice for the scatter detector in Compton telescopes for prostate imaging making
possible the reconstruction of the original location of the gamma-ray even with sub-millimeter
position resolution [8]. The scheme of principle of the novel detector that we propose is based on the
Controlled Drift Detector architecture [9, 10] and is reported in Fig. 2. The active volume is a
detector-grade high-resistivity silicon substrate 300 or 450 m thick. In this detector the signal
electrons generated by the interaction are transported at constant velocity within parallel drifting
columns towards point-like anodes by the electrostatic field. The drift time of the electron packet
gives one interaction coordinate while the second coordinate is obtained from the 1-D array of
readout anodes. The multi-linear transport mechanism based on electrons’ drift has a three-fold
advantage: i) it reduces dramatically the number of channels required for true 2D position sensing
(i.e. no. of channels equal to the square root of the pixels), ii) it naturally allows the connection of the
front-end electronics aside the detector chip thus minimizing the interconnection issue and iii) it
leads to readout times of only few microseconds. Moreover the deposited energy can be measured
with spectroscopic resolution at (or very close to) room temperature thanks to the very low anode
capacitance (integrated front-end capacitance plus strays <100 fF). We have successfully tested
detector prototypes based on the Controlled Drift principle, with active areas up to 6x6 mm 2,
qualifying their performances in terms of energy resolution (300eV FWHM @ 6keV corresponding
to an Equivalent Noise Charge of about 30 electrons r.m.s.), position resolution (120-180µm pixel
size) and maximum readout frequency (full-frame readout frequency up to 100 kHz) [9-13].
The p+ back electrodes are instrumented to pick-up the fast induction pulse of the signal electrons
and holes for event timing. During the initial charge separation a current pulse, lasting few
nanoseconds or more (depending on substrate resistivity and on the ionization profile), is induced on
the p+ back electrode that collects the holes. A time resolution of 3.5 ns FWHM has been measured
in the case of injection of 30,000 electrons with an infrared laser pulse [14]. Coincidence
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
measurements carried out using a fast scintillator-PMT as the time reference and the annihilation
photons of a 22Na source revealed an overall time resolution (including the statistics of the generation
process) of 6 ns FWHM [15]. This signal provides the fast trigger needed for event coincidence.
4. Results and discussion
The possibility of electron tracking of the first Compton scatter can significantly increase the
sensitivity of Compton telescopes because the estimation of the recoil electron direction can reduce
the event cone to a short arc with uncertainty  centred on the source direction. Unfortunately the
determination of the initial electron direction from two adjacent interaction layers of silicon does not
apply to low gamma energies (<0.5 MeV) due to the short electron range [16]. Therefore, instead of
the conventional approach of tracking Compton electrons in successive layers of silicon detectors
which requires only the measurement of the total deposited energy per layer and the average
interaction position, we have tested the possibility to resolve the Compton electron track within one
silicon layer. This is made possible by the excellent energy and position resolution of the proposed
detector. Fig. 3a shows a simplified sketch of one detector layer in which a Compton interaction
takes place. The detector images the 2D projection of the electron track and samples the deposited
charge with spatial resolution of the order 100 µm. The (projected) initial direction of the recoil
electron can therefore be estimated for wide range of Compton electron energies. Additional
information can be obtained by estimating the specific energy loss per pixel. The comparison of the
data with the known theoretical curve (dE/dx versus E) allows us to derive a least-square estimate of
the recoil electron energy (Te) and, if not fully absorbed, of the escape energy.
A small-area detector prototype having 13 120µm-wide drift channels was operated at room
temperature and irradiated with a
22
Na source to image Compton electron tracks. The induction
signal picked up at the uniform back contact provided the interaction time (and the start of the
electron drift) with a measured time jitter of about 6 ns FWHM. Figs. 3b, c, d show selected
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
examples of recorded tracks. In each figure the left inset shows the pixelated (120m×120m pixel
size) image of the electron ionization track. Pixel colors correspond to the deposited energy (in keV)
according to the color map aside. The black arrows indicate the direction of the recoil electron
motion. The right inset shows the estimated specific energy loss after least square fit to the
theoretical curve (solid line) that gives the initial Compton electron energy. Fig. 3b shows the case in
which a 415 keV electron is fully absorbed in the silicon detector. The marked increase of the
deposited energy near the end of the track is clearly visible. Fig. 3c shows the case in which the
ionization track appears inside the detector volume and stops at the edge of the active area or beyond
it with partial charge loss. Fig. 3d shows a different case in which an energetic electron (915 keV)
crosses the detector thickness and escapes through the surface with a residual energy of about 760
keV.
5. Conclusions and future plans
In this paper we presented a novel silicon detector, based on the Controlled-Drift Detector concept
that can fulfil all the demanding requirements of the scatter detector in a high-resolution Compton
camera for prostate imaging. Although more refined data analysis can be performed the shown
results demonstrate how the proposed detector can add direct ‘true imaging’ capability to the
Compton telescope. This possibility is an attractive way to significantly reduce background and
increase sensitivity in prostate imaging. The lack of the Depth-Of-Interaction (DOI) information
affects the estimation of dE/dx and limits the accuracy of this analysis. However, it must be pointed
out that the shape of the induced signals on segmented back electrodes could also be used to provide
DOI information at the expense of a more sophisticated signal processing. Larger detectors (up to
3cm2) have been designed, produced and are presently under test.
Acknowledgements
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
This work has been fully supported by INFN. We acknowledge the staff of the Max Planck Institut Halbleiterlabor
in Munich together with H. Soltau, P. Holl, R. Hartmann and the staff of PNSensor GmbH in Munich for detector
production. We want to acknowledge S.Masci for careful and professional detector bonding. We also thank D.
Arceri for the help in the development of the DAQ system and C. Mantovani for the help in the experimental
measurements.
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[5] Lacasta C et al., Results from a First Prototype of a Compton Prostate Probe, IEEE 2005 Nuclear Science Symposium
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Parameters on a Compton Camera, IEEE Trans.Nucl.Sci. 2002: 49; 817-821.
[7] Zoglauer A, Kanbach G, “Doppler broadening as a lower limit to the angular resolution of next-generation Compton
telescopes, Proc. SPIE 2003: 4851; 1302-1309.
[8] Walenta AH, Brill AB, Castoldi A, Conka Nurdan T, Guazzoni C, Hartmann K, Longoni A, Nurdan K, Strüder LWJ,
Vertex Detection in a Stack of Silicon Drift Detectors for High Resolution Gamma-ray Imaging, IEEE Trans. Nucl. Sci.
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[9] Castoldi A, Guazzoni C, A New Position Sensing X-Ray Detector: Working Principle And Experimental Results,
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[10] Castoldi A, Guazzoni C, Rehak P, Strüder L, Spectroscopic-grade X-ray imaging up to 100 kHz frame rate with
Controlled-Drift Detectors, IEEE Trans. Nucl. Sci. 2001: 48; 982-986.
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
[10] Castoldi A, Cattaneo G, Galimberti A, Guazzoni C, Rehak P, Strüder L, Room-temperature 2-D X-ray imaging with
the Controlled-Drift Detector, IEEE Trans. Nucl. Sci. 2002: 49; 989-994.
[11] Castoldi A, Galimberti A, Guazzoni C, Rehak P, Strüder L, Menk RH, Energy-resolved X-ray radiography with
controlled-drift detectors at Sincrotrone Trieste, Nucl. Instrum. Meth. 2003: A510; 57-62.
[12] Castoldi A, Galimberti A, Guazzoni C, Rehak P, Strüder L, X-ray imaging and spectroscopy with Controlled-Drift
Detectors: experimental results and perspectives, Nucl. Instrum. Meth. 2003: A512; 250-256.
[13] Castoldi A, Gatti E, Guazzoni C, Fast triggering in silicon drift detectors by means of holes’induction, Nucl. Instr.
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Silicon Drift Detectors, IEEE Trans. Nucl Science 2006 in press.
[15] O'Neill TJ, Ait-Ouamer F, Schwartz I, Tumer OT, White RS, Zych AD, Compton recoil electron tracking with silicon
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angular uncertainty FWHM [deg]
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
10
perfect resolution
9
0.3 keV FWHM
8
0.5 keV FWHM
7
1 keV FWHM
6
5
4
171.3 keV
3
245.4 keV
2
1
0
20
40
60
80
100
120
140
 scatter angle [deg]
Fig. 1. Angular uncertainty due to the scatter detector energy resolution and to Doppler broadening (in silicon) as a function of
the scatter angle for the two -ray lines of 111In. The scatter detector energy resolution varies from the ideal case to 1 keV
FWHM. The geometric contribution has been neglected.
Fig. 2. 3D schematic view of the proposed detector. The high energy n-implant close to the top detector surface defines the
potential minimum for the electrons drift. The p+ strips on the back side are instrumented to measure the signals induced by the
generated charge. The combination of channel-stops (deep p-implants) and deep n-implants provides charge confinement in the
direction transversal to the drift direction.
Advanced Molecular Imaging Techniques in the Detection, Diagnosis,
Therapy and Follow-up of Prostate Cancer
Roma (Italy), December 6-7, 2005
(a)
(b)
(c)
(d)
Fig. 3 a) Simplified sketch of one silicon detector layer in which a Compton interaction takes place. The detector images
the 2D projection of the electron track and samples the deposited charge with spatial resolution of the order 100 µm. (b,
c, d) Selected examples of recorded Compton electron tracks originated by -rays coming from a 22Na source The black
arrows indicate the direction of the recoil electron motion. b) A 415 keV electron is fully absorbed in the silicon
detector. c) The ionisation track appears inside the detector volume and stops at the edge of the active area or beyond it
with partial charge loss. d) An energetic electron (915 keV) crosses the detector thickness and escapes through the
surface with a residual energy of about 760 keV.