Nuclear Instruments and Methods in Physics Research A 648 (2011) S72–S74

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Development of a multimodality sensor for spectral photon counting CT,standard CT and PET

Andreas Persson �, Bo Cederwall

The Royal Institute of Technology, SE-10691, Stockholm, Sweden

a r t i c l e i n f o

Available online 29 December 2010

Keywords:

SiPM

CT

PET

02/$ - see front matter & 2011 Elsevier B.V. A

016/j.nima.2010.12.148

esponding author.

ail address: aper@kth.se (A. Persson).

a b s t r a c t

A prototype sensor module for multimodality medical imaging applications requiring a wide intensity

range has been developed. It consists of a silicon photomultiplier (SiPM)-scintillator sensor connected to a

100 kHz bandwidth current amplifier integrated with a four stage energy discriminator and a charge

sensitive preamplifier. The electronics design allows for simultaneous read out of current level and

discriminatory information of single photon energy or, optionally, high-resolution energy information via

the charge preamplifier. This single-channel device is a proof-of-principle system designed primarily for

combined spectral photon counting computed tomography (CT)/standard CT or combined with positron

emission tomography (PET).

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Multimodality medical imaging systems, e.g. utilizing anato-mical structures from CT and functional information from PET areessential diagnostic tools in modern medicine [1]. Today, differentdetector and readout systems are used for CT and PET due to thedifference in photon energies and radiation intensities betweenthese two imaging techniques. A sensor technology that could spanthe energy and intensity ranges necessary for these applicationswould enable enhanced diagnostic performance due to higherimage accuracy, as well as cost benefits. A similar sensor technol-ogy is needed to realize a CT detector system that combines spectralphoton counting with standard high-rate current mode operation.The SiPM [2] light sensor is a matrix microstructure of Geiger-modeavalanche photodiode pixels which are connected in parallel. It is adevice that is identified as highly promising for the next generationPET scanners (see e.g. Refs. [3,4]), that could also realize the dreamof a ‘‘universal’’ wide range radiation sensor [5] with propertiessuitable to either characterize single primary X-ray or g-rayphotons or to measure the incident radiation flux with highprecision. It utilizes Geiger discharge to create numerous (exceed-ing 106) charge carriers per photoelectron in each pixel. At incidentphoton fluxes which are small with respect to the single pixelrecovery time of typically a few tens of ns and the pixel density ofup to a few times 103 per mm2 the sensor has good linearity in pulsemode. The high intrinsic gain eliminates the need for other thanrelatively simple amplifiers later in the electronics chain; the rawpulse amplitude is typically of the order of tens of mV depending on

ll rights reserved.

the number of pixels firing. The short recovery times of theindividual pixels enable spectral photon counting CT detectorsthat may operate in photon fluxes up to 107 photons/(mm2 s) (withincreasing losses in energy resolution). This limit in photon flux is,however, significant compared to standard CT operation whereorders of magnitude higher X-ray flux is used routinely.

We have developed a detector concept based on dual-modereadout of SiPM-based radiation sensors that enables imagingsystems where conventional CT imaging is combined with PET, aswell as in CT systems where both conventional current mode (high-flux) and spectral photon counting (low-dose) operation can beselected. Such dual-mode SiPMs coupled to scintillators can beused to detect and characterize single primary X-ray or g-rayphotons up to fluxes of millions of photons per mm2/s in pulsemode. Current mode operation allows the dynamic intensity rangeto be extended to the much higher rates found in standard CTimages. Detector systems based on this idea can generate a numberof advantages, e.g. higher image quality, lower patient dose withphoton counting CT, easier calibration procedures and higherpatient throughput. The SiPM is insensitive to magnetic fieldswhich yields a possibility to merge the detector system with amagnetic resonance imaging (MRI) system.

2. Principles of operation

The SiPM is a versatile device, which allows for either high-gainamplification of photo-electrons in Geiger mode or with low gain,similar to the avalanche photodiode, in sub-Geiger operation. In thelow-gain regime, the SiPM pixels do not suffer from recovery timelosses which is ideal for applications with high photon fluxes. Both

Fig. 1. Photograph of the prototype PCB. Signal sensitive areas that are normally

shielded are open in the figure.

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Energy resolution with LSO scintillator on3x3 mm2 SiPM with Cs137

Energy [keV]

Cou

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Cs137 spectrumFWHM at 662 keV = 12.1 %

Fig. 2. Measurement of 137Cs with the charge output enabled together with a SiPM

connected to a LSO scintillator.

A. Persson, B. Cederwall / Nuclear Instruments and Methods in Physics Research A 648 (2011) S72–S74 S73

of these properties are embodied in the detector concept that weare developing. In applications with photon fluxes exceeding thelimit imposed by the pixel dead times, the device is operated belowthe Geiger breakdown voltage and the current flowing through thesensor is measured with a bandwidth of around 100 kHz. On theother hand, when the conditions are met for pulse mode operation,the sensor is operated at a bias voltage above the breakdown level,where the energy and timing information from individual incidentX-ray or g-ray photons can be collected with excellent resolution.These two levels of operation can be switched seamlessly with noloss of information for either charge or current mode. A front-endelectronics board for a single-channel proof-of-principle systemhas been produced, see Fig. 1. In the next stage the sensor conceptwill be adapted to clinical or preclinical settings by using compactASICS readout of high-granularity SiPM-scintillator matrices.

2.1. Amplifier specifications

Two types of amplifiers and an energy discriminator circuit havebeen integrated onto the common PCB. The charge sensitivepreamplifier used in high-resolution pulse mode is based on acharge integrating circuit from Cremat Inc [6]. The current ampli-fier is designed with adjustable time constant width a bandwidthranging from a few kHz to well above 100 kHz, controlled by ananalog frequency control (FC) input on the amplifier board. Viajumpers it is possible to choose tens, hundreds or thousands of kHzin frequency range. Simultaneous operation is possible for two ofthe three modes; either charge and current amplifier or discrimi-nator and current amplifier. The discriminator circuit consists of

four comparators which can be individually adjusted to fit thespecific application. It operates directly on the SiPM signal whichmakes it faster than the high-resolution charge preamplifier. Thediscriminator outputs are of TTL type with a width of 10 ns. TheSiPM sensor bias voltage is controlled via another analog input onthe amplifier system. A voltage control (VC) input ranging from�10 to +10 V, is transformed to 60–74.5 V bias voltage, suitable forthe SiPM working range. This input is controlled by an analogoutput of a multi-task data acquisition module from NationalInstruments, USB-6212 [7]. Also bandwidth of the current readoutelectronics is controlled by an analog output from the same device.A LabVIEW [8] based software monitors and registers the analoginput signal from the current integrating output of the amplifierwith a sample rate ranging up to 400 kHz [7]. The same softwarehas live control over the bias voltage and current samplingfrequency, enabling possibility to have feedback control.

3. Preliminary test results

3.1. Energy resolution in charge mode

With the charge integrating output enabled, we have tested a3�3 mm2 Hamamatsu SiPM sensor coupled to a 2�2 mm2 LSOscintillator. The charge preamplifier output is fed to a shapingamplifier before sampled by a multichannel analyzer. In themeasurement a 137Cs source was used. A FWHM of 12.1% couldbe measured at 662 keV, see Fig. 2.

3.2. Integrated current

Fig. 3 shows the current flowing through the sensor, sampled bythe current amplifier circuit when illuminated by an LED, driven bya pulse generator operating at 10 kHz with 20 ms wide squarepulses. The variations in pulse height are mainly due to the non-optimal operating conditions for the LED in the measurement.

Another NI device, PCI-6602 [9], a 80 MHz counter registers thefour discriminator outputs simultaneously. Both NI modules arecontrolled and monitored via the LabVIEW software package.

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Fig. 3. Sampled current output from a pulsed LED.

A. Persson, B. Cederwall / Nuclear Instruments and Methods in Physics Research A 648 (2011) S72–S74S74

4. Summary

A prototype SiPM-based scintillator module for multimodalitymedical imaging applications requiring a wide intensity range has

been developed. The sensor design allows for simultaneous readoutin pulse mode with excellent energy resolution and current mode,enabling an extension of the intensity range up the levels used instandard CT applications and beyond.

Acknowledgements

This work was supported by the Swedish Governmental Agencyfor Innovation Systems (Vinnova) and the Goran Gustafssonfoundation. The authors would like to thank Lars Eriksson, SiemensMedical Solutions for providing the scintillators. A.P. gratefullyacknowledges support from the Knut och Alice Wallenbergfoundation.

References

[1] D.W. Townsend, et al., Br. J. Rad. 75 (2002) S24.[2] V. Saveliev, V. Golovin, Nucl. Instr. and Meth. A 442 (2000) 223.[3] A. Del Guerra, et al., this issue.[4] A.N. Otte, et al., Nucl. Instr. and Meth. A 545 (2005) 705.[5] IEEE Trans. Nucl. Sci., M11-4 Conference Record of the NSS/MIC 2009.[6] /http://www.cremat.comS.[7] /http://sine.ni.com/nips/cds/view/p/lang/en/nid/207097S.[8] /http://www.ni.com/labviewS.[9] /http://sine.ni.com/nips/cds/view/p/lang/en/nid/1123S.