P H D COURSE ON N UCLEAR M ICROELECTRONICS Luca Miari 785753 Alessandro Ruggeri 802434 T OWARDS M...

PHD COURSE ONNUCLEAR

MICROELECTRONICSLuca Miari785753

Alessandro Ruggeri802434

TOWARDS MILLIMETER RESOLUTION IN

TIME-OF-FLIGHT PET

A. Ruggeri, L. Miari2013, July 10th2

OUTLINE

1. Positron Emission Tomography

2. Image Reconstruction

3. SNR Enhancement in TOF PET

4. Scintillators & Detectors Evolution

5. Discrete vs Integrated Acquisition Chain

6. State-of-the-art timing TAC


POSITRON EMISSION TOMOGRAPHY

Patients are injected with radioactive drug

Radionuclide decays emitting a positron

Positron annihilates with an electron of the tissue

Back-to-back gamma ray@ 511 keV is emitted

Two detectors register a hit

Positron lies on line defined by detector pair.


IMAGE RECONSTRUCTION

It contains information about the positron emission rate along the line of response

The projection of a 2D image on a 1D line is called Radon Transform

By rotating the projection line of an angle θ I get the sinogram of the 2D image

Sinogram is 2π periodic



The back-projection algorithm allows the reconstruction of the image original image (G)

Star artifacts arises when a limited number of projection angles are used (E,F)

Blurring (G) arises from the the finite sample- width

By HP-filtering the sinogram the resulting image becomes sharper



The back-projection algorithm allows the reconstruction of the image original image (G)

Star artifacts arises when a limited number of projection angles are used (E,F)

Blurring (G) arises from the the finite sample- width

By HP-filtering the sinogram the resulting image becomes sharper



Arc correction resampling is necessary before back-projecting because of the circular distribution of the detectors

False coincidence events decrease SNR

A. Ruggeri, L. Miari2013, July 10th

SIGNAL-TO-NOISE RATIO

8

W. W. Moses, “Time of Flight in PET Revisited”, IEEE Transactions on Nuclear Science, 2003

𝑆𝑁 𝑅𝑐𝑜𝑛𝑣∝1

√𝑛⋅

𝑇√𝑇 +𝑆+𝑅

T is the True coincidence event rate ( indep.)

S is the Scattered event rate ()

R is the Random event rate ()

n is the number of volume elements influencing the noise (

M. E. Casey, “Improving PET With HD • PET + Time of Flight”, Siemens AG, 2008


SIGNAL-TO-NOISE RATIO

9

When n become smaller:

TOF SNR Gain:

Δ𝑥=Δ𝑡 ⋅𝑐2

M. Conti, “State of the art and challenges of time-of-flight PET”, Physica Medica, 2009

M. E. Casey, “Improving PET With HD • PET + Time of Flight”, Siemens AG, 2008


TOF: BENEFITS & REQUIREMENTS

10

BENEFITS Temporal resolution of 50 ps allows sub-

centimeter spatial resolution Current resolution of 500 ps () do not

improve spatial resolution but reduces statistical noise:

Usually : reduces computational time and numbers of iteration

REQUIREMENTS FOR MILLIMETER RESOLUTION Fast scintillators (with ns decay

constant) Low Jitter detectors Timing ElectronicsM. Conti, “Focus on time-of-flight PET: the benefits of improved time resolution”, Eur J Nucl

Med Mol Imaging, 2011W. W. Moses, “Time of Flight in PET Revisited”, IEEE Transactions on Nuclear Science, 2003


SCINTILLATORS: 1980

M. Ter-Pogossian, D. Ficke, M. Yamamoto, J. Hood, “Super PETT 1: A Positron Emission Tomography Utilizing Photon Time-of-Flight Information“, IEEE Transactions on Medical Imaging, 1982

First TOF faced scintillators limitations: High Z scintillator had high

output yield, high energy resolution, could be made smaller but had slow decay constant

Scintillator with fast decay constant (like CsF) had opposite performance and so TOF studies were abandoned


SCINTILLATORS: 2000

S. Surti, J.S. Karp, G. Muehllehner, “Investigation of Lanthanum Scintillators for 3-D PET“, IEEE Transactions on Nuclear Science, 2003

1980: CsF and BaF2

Very Fast Decay ConstantVery Low Light YieldLow Energy Resolution

2000: LaCl3 and LaBr3

Fast Decay Constant Very High Light Yield High Energy Resolution

New scintillators renewed scientific interest in TOF PET

Measurements @ 664 keV shows Energy Resolution of 4.6 %

FWHM Timing Resolution of 350 ps

FWHM


SCINTILLATORS: 2010

Recent studies broke the 100 ps barrier

3x3x5mm LaBr3:Ce(5%) scintillator

Hamamatsu SiPM Digital Signal Processing

D.R. Schaart et al., “LaBr3:Ce and SiPMs for time-of-flight PET: achieving 100 ps coincidence resolving time“, Phys Med Biol, 2010

Promising new scintillators

LaBr3:Ce(30%) provides

better timing resolution

New materials are under

investigation:

CeBr3

LuI3:Ce

Necessity of optimization

for commercial system


DETECTORS

PMT

SiPM

SiPM has slower 10-90% rise time than PMT (9 vs 3ns)

SiPM has higher PDE than PMT (45% vs 16% @380nm)

Higher PDE brings more primary charges which reduce statistical fluctuations on timing resolution even with a slower dv/dt

SiPM can be integrated in CMOS technology

“Fine spatial resolution of today’s PET scanners requires up

to 104 detectors”

D.R. Schaart et al., “LaBr3:Ce and SiPMs for time-of-flight PET: achieving 100 ps coincidence resolving time“, Phys Med Biol, 2010


ADVANTAGES OF CMOS TECHNOLOGY

PET analysis requires a large number of

channels that must be processed in parallel

Integration of the front-end in standard CMOS tecnhnology Reduces size Reduces costs Improves system reliability Increases system

performance Reduces power dissipationB.K. Swann et al., “A 100ps Time-Resolution CMOS Time-to-Digital Converter for PET Imaging Application“, IEEE Journal of

Solid State Circuits, 2004


Timing section

Energy section

ACQUISITION CHAIN

Pulse discriminator

Scin

tilla

tor Detector

TDC

Charge preamp SA ADC

Storage PC

Common sync

Peak stretcher

BLH

-


PULSE DISCRIMINATORS

J.F. Genat et al, “Signal processing for picosecond resolution timing measurements”, Nucl Inst and Met in Physics Research Section A, 2009

Leading-edge discriminator: Very simple to implement

(comparator) Suffers from variation of pulse

amplitude/baseline level Constant-fraction discriminator:

No pulse amplitude dependance Complex circuit involving delays


DISCRETE CFD: IMPLEMENTATION

C1: reject input pulses below a certain

threshold amplitude (noise level)

C2: implement CFD difference amplifier

Down to picosecond resolution

W. Becker, “Advanced Time-Correlated Single Photon Counting Techniques”, 1st ed. Springer, 2005


FULLY INTEGRATED CFD

D.M. Binkley et al , “A 10-Mc/s, 0.5-μm CMOS constant-fraction discriminator having built-in pulse tail cancellation”, IEEE Transactions on Nuclear Science, 2002


FULLY INTEGRATED CFD: EXAMPLE

Param Value

Jitter (FWHM)

40 ps @ 85 mV

80 ps @ 23 mV

Time walk 10 ps

Count rate 10 Mcps



FULLY INTEGRATED CFD: EXAMPLE

Baseline restorer removes pulse tail → reduced pileup

Current mode and differential circuits minimize harmonic distortion and noise coupling



FULLY INTEGRATED ACQUISITION CHAIN

H. Matsuda et al., “Development of ultra-fast ASIC for future PET scanners using TOF-capable MPPC detectors“, Nucl Inst&Met in Physics Research, 2013

CHAIN1 32 parallel channels 2 ICON based current

conveyor 2nd order shaping filter Energy discrimination for

position information Timing discrimination by

both leading edge and zero-crossing method

CHAIN2 Analog summing circuit for

energy information TAC for timing information Internal baseline restorer


FULLY INTEGRATED ACQUISITION CHAIN

ASUM provides the energy spectrum (to be digitally converted)

LEDGE comparator acquires fast initial rising edges

Zero-crossing discriminator (FAST) provides a digital time-walk free signal (DSUM)

Performance 10.5%

(FWHM) @ 662 keV

9.8% (FWHM) @ 511 keV

491ps (FWHM) TOF info

H. Matsuda et al., “Development of ultra-fast ASIC for future PET scanners using TOF-capable MPPC detectors“, Nucl Inst&Met in Physics Research, 2013


TIME-TO-AMPLITUDE CONVERTER

𝑉 𝑂𝑈𝑇=𝑉 𝑠𝑡𝑎𝑟𝑡+𝐼 𝑐𝑜𝑛𝑣𝐶𝑐𝑜𝑛𝑣

(𝑇 𝑠𝑡𝑜𝑝−𝑇 𝑠𝑡𝑎𝑟𝑡 )

M. Crotti et al., “Four Channel, 40 ps Resolution, Fully Integrated Time-to-Amplitude Converter for Time-Resolved Photon Counting“, IEEE JSSC, 2012

An extremelysimple operating

principlewith

several requirementslead to a

complex circuitwith

high performance

Time resolution down to 15.9 ps

Less than 2.5% LSB peak-to-peak DNL

200 ns maximumdead-time


TIME-TO-AMPLITUDE CONVERTER

Idle Phase

TG1 ON (OA in buffer configuration)

Differential stage switches on M2

Conversion Phase (Start Event)

TG1 OFF Vout increases

Wait Phase (Stop Event)

Differential stage switches on M1

Conversion capacitor holds Vout

Reset Phase

TG1 ON discharges the capacitor

M2 ON resets to Idle Phase

M. Crotti et al., “Four Channel, 40 ps Resolution, Fully Integrated Time-to-Amplitude Converter for Time-Resolved Photon Counting“, IEEE JSSC, 2012


CONCLUSIONS

Improve image SNR

TOF PET

ScintillatorsDetectors

CFDTAC

Millimeter Resolution

Fast DecayHigh Light Yield

Ruggedness

Low JitterCMOS Integration

Fast AnalysisImproved Resolution

Higher EfficiencyCompactness

High ResolutionLow Dead-Time



FULL DIGITAL TDC

B. Markovic et al., “A High-Linearity, 17 ps Precision Time-to-Digital Converter Based on a Single-Stage Vernier Delay Loop Fine Interpolation”, IEEE Transaction on Circuits and Systems I: Regular Papers, 2013

𝑇𝑚𝑒𝑎𝑠=𝑇 𝑐𝑜𝑢𝑛𝑡𝑒𝑟+𝑇11−𝑇 21+𝑇 12−𝑇22


FULL DIGITAL TDC

B. Markovic et al., “A High-Linearity, 17 ps Precision Time-to-Digital Converter Based on a Single-Stage Vernier Delay Loop Fine Interpolation”, IEEE Transaction on Circuits and Systems I: Regular Papers, 2013

Performances 10 ps bin < 20 psrms

precision 160 ns range 0.9%RMS LSB DNL

4%MAX LSB DNL 0.3mm2 core area

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