evaluation of an instrument to improve pet timing alignment

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Nuclear Instruments and Methods in Physics Research A 620 (2010) 343–350

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

(J. Han1 N

Univers2 Pr

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

Evaluation of an instrument to improve PET timing alignment

J. Hancock a,n,1, C.J. Thompson b,2

a Department of Medical Physics, McGill University, Montreal General Hospital, 1650 Avenue Cedar, Montreal, Quebec, Canada H3G 1A4b Montreal Neurological Institute, McGill University, Montreal, Canada

a r t i c l e i n f o

Article history:

Received 14 October 2009

Received in revised form

9 March 2010

Accepted 18 March 2010Available online 8 April 2010

Keywords:

Time alignment

PET

Calibration

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.03.131

esponding author.

ail addresses: [email protected], ja

cock).

ow with Department of Medical Physics, Cr

ity Avenue, Edmonton, Alberta, Canada T6G I

esent address: 11870 Lavigne, Montreal, Can

a b s t r a c t

Purpose: In order to increase the simplicity and accuracy of performing the time alignment on a

positron emission tomography (PET) scanner, a new generation timing alignment probe has been

developed.

Methods: A timing alignment probe containing a plastic scintillator with an embedded sodium-22

source which is optically coupled to a fast photomultiplier tube (PMT) is described and tested. When a

positron is ejected from the radioactive atom’s nucleus, its kinetic energy is absorbed in and can be

detected as a light flash from the scintillator. This is used as the reference time for each atom’s positron

decay. It is only after the positron slows that it can combine with an electron, forming positronium after

which the 511 keV annihilation photons will be created, possibly traveling to the PET detectors. In

practice, the probe is placed in the center of the scanner’s field of view and connected to the coincidence

circuit. Since the delay between an annihilation photon’s detection and positron detection is almost

identical (the lifetime of positronium in a solid is extremely short, and the gamma rays’ path lengths are

equal with the probe in the center of the scanner), the probe’s signal provides a fixed reference time to

which the response of individual crystals in the PET detectors can be compared. We present an

evaluation of the performance of this probe. We first investigated the intrinsic performance of the time-

alignment probe comparing its timing resolution with two barium fluoride crystals in coincidence. We

then investigated the timing performance of the probe in coincidence with various individual

scintillation crystals and with detectors from two commercial PET scanners.

Results: The best full-width at half-maximum (FWHM) timing resolution of the probe was found when

in coincidence with BaF2 at 400 ps. The common commercial scintillator lutetium oxy-orthosilicate

(LSO) was tested and its FWHM was 510 ps. When testing the crystal arrays used in two commercial

block detectors it was found that there are significant, systematic timing delays among the crystals. In

the Siemens HiRezs detector the average time difference between the positron detection and

annihilation photon detection is 500 ps with a standard deviation of 115 ps. In the Siemens Focuss

detector the average was 1000 ps and a standard deviation of 400 ps. The reasons for the variation in

apparent arrival times among the crystals appear to be due to the electronic readout rather than light

transport from the crystals to the PMT.

Conclusion: Many PET scanners use a global time offset for each detector, and these variations in

apparent time delay from individual crystals in the detector would have a significant impact if these

detectors were employed in a time-of-flight (ToF) PET scanner.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

A PET timing alignment probe, based on a patent by Thompsonand Camborde [1] was previously described by our group [2]. Theoriginal version used a nominal 10 mCi (0.37 MBq) source and a

ll rights reserved.

[email protected]

oss Cancer Institute, 11560

Z2.

ada H4J 1X8.

9 mm photomultiplier. Further testing by Moses and Thompson[3] and McElroy et al. [4] showed that it could be useful inproviding more precise timing alignment than conventionaltechniques in prototype scanners. However, other tests atSiemens Molecular Imaging on commercial scanners (personalcommunication) showed that the activity in the central sourceproduced fewer counts that the intrinsic activity in the detectorsdue to the presence of 176Lu in scanners Lutetium oxy-orthosi-licate (LSO) crystals. This made it difficult to correlate signals fromthe probe and detectors in order to estimate the time delays.Based on these results a new version with 10 times more activityand a faster PMT was developed. This new version has since been

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J. Hancock, C.J. Thompson / Nuclear Instruments and Methods in Physics Research A 620 (2010) 343–350344

certified as a radiation emitting device by the Canadian NuclearSafety Commission thus allowing it to be used without a licensefor the 3.7 MBq of 22Na it contains.

The concept would facilitate an improved and more conve-nient method of time alignment for PET scanners [1]. Consisting ofa positron source embedded in a plastic scintillator, which isattached to a photomultiplier tube, the probe serves as a commonreference clock, with respect to which the time of response ofeach crystal in each PET detector can be measured.

Time alignment is a critical procedure needed for a properlyfunctioning and efficient PET scanner. With the recent developmentand ongoing research into faster and more efficient scintillators, thedevelopment of more advanced scanners has followed. In thesescanners, primarily ToF scanners, proper time alignment and event-time recording becomes integral to the operation of the scanner. Inits simplest terms timing alignment is the ability of two detectors toreport two temporally aligned detections as happening at the sametime. If each detector has a photon interacting with it at the sametime, then, through the path of the PMT and the electronics, theevents should record an identical time stamp for the time of theirinteractions. Small deviations in the time stamp are inevitable as aconsequence of imprecise timing information from the scintillator,PMTs and electronics, but systematic differences can be estimatedand compensated for. Time alignment is the process used tominimize these systematic errors.

One can think of timing alignment process as shown in Fig. 1,which shows the coincidence counting response of two detectorsto a source placed between them represented as a Gaussian curve.The vertical lines represent the scanner’s coincidence window. Ifone of the detectors reports the detector of a gamma ray in amanner which is systematically late or early (triangle curve inFig. 1), this detector pair will record fewer true coincident counts.By performing repeated experiments with different timing offsetsfor each detector one can maximize the coincident count rate, andthus find the best time offset for each detector. The offset is anamount of time that is added to or subtracted from a signal toensure the same time report from each detector. Doing this foreach detector pair would be very time consuming, and is notpractical for a large number of detectors in a PET scanner. Theother problem is the adjustments to the time offset are doneblindly since there is, initially, no indication whether there needsto be more or less offset or how much offset may be needed.

Several other methods have been developed to perform the timingalignment of PET scanners to ensure that all detectors report thearrival time of gamma rays to the coincidence circuit and thus

Fig. 1. Schematic of the changing number of counts caused by time alignment. The

timing window is shown as a pair of vertical lines, and the recorded coincident

count rate, is the area under the curve bounded by these two lines.

compensate for the various instrumental delays in the processing ofindividual detection channels. The most common method takesadvantage of the radioactivity in the orbiting transmission source [5].For example, this method is used to optimize the timing resolution ofall the detectors of the CPS HRRT PET scanner to 2.82 ns, enabling theuse of a 6 ns timing window [6]. Another method takes advantage ofthe intrinsic radioactivity embedded in LSO scintillators that are usedin new high-speed PET scanners [7]. This is hampered by the lowcount rate of LSO, so it is not commonly used.

Researchers at General Electric Medical Systems have reportedprocedures which are able to perform timing alignment in a closedloop fashion [8], allowing the timing alignment process to convergemore rapidly than previous methods. This technique is the subject ofUS patent issued in 2005 [9]. This technique allows the measure-ment of block-to-block timing differences but also estimates theindividual time differences within the blocks of crystals. The authorspoint out that they measure the time differences between recordedevents using this technique not the absolute time taken from acommon reference source. This requires an iterative process which issaid to converge rapidly requiring only three iterations. Anothergroup [10], recognizing the benefits of a central source for timingalignment, places a centrally positioned source in a small steelcylinder which causes some of the 511 keV gamma rays to scatterallowing those which scatter though small angles to remain in thescanner’s photo-peak energy window, but spread enough so that asmall array containing several diagonally opposite crystals to appearin coincidence, even though the pairs of crystals are not preciselycollinear with the source. They have successfully used this toperform the timing alignment of a ToF PET scanner. This method stillrequires iterations to determine whether the crystals are early orlate, since there is no common time reference.

In conventional PET scanners the quality of the time alignmenthas the effect of reducing noise and improving image quality in thePET image. The reason for the improvement is the reduction in theratio of random counts to the true counts in the image. Randomcounts are the result of a recorded annihilation being the result ofthe detection of one photon from two separate annihilations. Thenumber of random counts is proportional to the width of thecoincidence timing window: the time gap allowed betweenthe detection of two events in order for them to be considered tooriginate from one annihilation. The coincidence timing windowcan be reduced without a great reduction in the number of countswith good time alignment; however if the timing is misaligned (asin Fig. 1), there will be fewer true coincident counts, but therandom count rate will not change. The count rate for randomsis [11]

_Rij ¼ 2t _C i_C j

where Ci and Cj are the individual single count rates for twodetectors in coincidence and t is the width of the timingcoincidence window.

2. Materials and methods

The time alignment probe that our group developed consists ofa plastic scintillator optically attached to a fast PMT. Thescintillator is machined into a cylinder with a small cavity inthe center. The cavity has been filled with 3.7 MBq of activity frompositron emitting 22Na (Fig. 2). The scintillator and PMT arecapped with a black plastic cover in order to provide a light seal.When each sodium atom decays a positron is emitted. Thispositron’s kinetic energy is absorbed in the plastic scintillator andis converted into a light flash, which is detected by the PMT andconverted to an electrical signal. The positron annihilates in theplastic creating two photons, each with an energy of 511 keV.

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Fig. 2. The PVT scintillator with embedded source.

Fig. 3. The Hamamatsu H6610 photomultiplier tube used in the probe.

Table 1Time resolution results from various detectors and crystals in coincidence with the

timing probe. BaF2/BaF2 is not in coincidence with the timing probe, this was used

to set a minimum resolution.

Crystal/detector Dimensions Time resolution

FWHM (ns)

LSO Square: 3.9�3.9�20 mm3 0.51

PVT Cylinder: 10.0 mm diameter 0.46

18.8 mm height

BaF2 Cylinder: 12.5 mm diameter 0.40

12.7 mm height

BaF2/BaF2 Cylinder: 12.5 mm diameter 0.23

12.7 mm height

LaBr3 Cylinder: 16.3 mm diameter 0.44

19.9 mm height

LYSO (Lu:95%, Y:5%) Cylinder: 13.0 mm diameter 0.49

12.9 mm height

HiRezs (13�13 array

of LSO)

Square: 4.0�4.0�20 mm3 0.62

Focuss (12�12 array

of LSO)

Square:

1.51�1.51�10 mm3

0.96

J. Hancock, C.J. Thompson / Nuclear Instruments and Methods in Physics Research A 620 (2010) 343–350 345

Either or both of these photons may be detected by the scanner’sdetectors. The time stamp from each detector in the PET scannermay now be aligned with the time of the detection of the positronfrom the probe and its single detector and thus a unique referencetime is obtained, something that is lacking in other alignmentmethods. The plastic scintillator is polyvinyl toluene [12] and thePMT is a Hamamatsu H6610 (lmin¼160 nm, lmax¼650 nm,lpeak¼420 nm, 160 ps transit time spread, Fig. 3) [13]. This PMTwas selected for its low transit time spread (TTS) and because ithas a synthetic silica window, enabling the use of BaF2 crystals,which emit in the UV spectrum. A lower cost PMT, whichpossesses the same time characteristics, but lacks the silicawindow, is also available.

A series of experiments was performed to evaluate the timingprobe and the timing resolution of a variety of individualscintillation crystals, and two commercial PET detectors. Pairs ofsample crystals: BaF2, LYSO and LaBr3 were obtained from SaintGobain Crystals. The LSO crystal was provided by SiemensMolecular Imaging and the plastic scintillator from Alpha Spectra.Most were right circular cylinders whose dimensions are given inTable 1. In addition, two commercial PET detectors blocks: aHiRezs detector and Focuss Micro-PET both provided by SiemensMolecular Imaging detector were tested. The HiRez detector has a13�13 matrix of LSO crystals coupled to four cylindrical PMTs.The Focuss detector has a 12�12 matrix of LSO crystals coupledto a Hamamatsu position-sensitive PMT.

When measuring the timing characteristics of individualcrystals they were attached to the same Hamamatsu H6610PMT. Individual crystals were tested by optically coupling them tothe PMT and light sealing them; they were then placed incoincidence with the timing probe (with one exception). The

output from each PMT was connected to one of the inputs of aCanberra 454 constant fraction discriminator (CFD) [14]. Theoutputs of the CFD were connected to the ‘‘start’’ and ‘‘stop’’inputs of a Canberra 2145 time to amplitude converter (TAC) [15].The ‘‘stop’’ signal is routed through a Canberra 2058 switchabledelay box before being sent to the TAC. This is done to ensure apositive TAC signal in all cases, and the addition of a small delay inmeasurements provides twin peaks in order to calibrate the timespectrum in channels per nanosecond.

When a commercial PET detector was tested, the PMTs fromthat PET detector were used. The setup for the testing of acommercial PET block detector is more complicated, but funda-mentally similar. A non-commercial nuclear instrument module(NIM) sums the four signals from the PET detector to produce asingle fast timing signal for input to a CFD and individualintegrated signals ready for sampling by a Jorway Aurora 14 sixchannel ADC. Another channel of the ADC receives the time delayoutput from the TAC. The ADC is readout by a Compac Alphaworkstation to which the ADC is interfaced via its SCSI bus.

The acquisition software can save a ‘‘list-mode’’ file for lateranalysis with different energy windows or time calibration, andcan also build images during acquisition using preset time andenergy windows. Individual energy gains can be applied to eachcrystal as is normally required in conventional PET detectors. Thesoftware thus allows an energy window to be applied so that onlyevents within 511 keV will be included.

The display and analysis software uses an algorithm appro-priate to the detector under evaluation to encode the position thedetection of each gamma ray in a 256�256 matrix. This allowseach crystal in the detector to be visualized as in a conventionalPET detector. Sixty-four of these matrices are assigned to timedelays according to an adjustable calibration factor. The indivi-dual crystal identification matrices are displayed in raster formatas shown in Fig. 4. The top left image contains the sum of allothers and so appears like the conventional PET detector crystalidentification matrix. The image just to the right of the sum imageshows those detections when there was no timing signal from theTAC due to the TAC’s intrinsic dead time, or the timing signal wassmaller than set up for this experiment. This will also occur eitherwhen the positron is not detected due to its energy being belowthe probe’s threshold, but a gamma ray from the probe isdetected, or when the intrinsic activity from the 176Lu causesthe detector to trigger. The image in the bottom right cornershows those gamma rays which were detected beyond the timerange selected in that study. These may be due to random counts,

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Fig. 4. Example where the lower right PMT signals arrive early.


or result from a very narrow time range. These ‘‘too early’’ and‘‘too late’’ images are very useful in setting up the timing study.From the time analysis it can be seen if there is a crystal or set ofcrystals that arrives chronically early or late. By way of example,the data shown in Fig. 4 were acquired with a shorter cableconnecting the lower right PMT of the detector so gamma raysdetected by crystals closest to the lower right PMT appear toarrive earlier than those from the rest of the block.

The number of counts required to generate a timing spectrumdepends on the timing accuracy required, the efficiency of theprobe in detecting positrons, the solid angle subtended by thecrystal and its detector efficiency for 511 keV gamma rays. From areview of the recent PET scanner performance literature, the tablepresented in Fig. 5 shows the expected count rate for each crystalin a variety of well-known PET scanners based on the timingprobe’s measured efficiency of 50% positron detection. The timingspectrum for the Focuss detector was collected for ten individual1 min scans in order to estimate the accuracy of this timingmethod. The time offset for the crystals was then averaged andthe standard deviation calculated.

3. Results

In order to provide an ideal time resolution, we attached BaF2

to two PMTs and placed a source between them. Since BaF2 is thefastest scintillator presently available, this would give a value towhich the timing resolution of the other crystals can becompared. Since the plastic scintillator is slower than BaF2

(2.5 ns vs. 0.6–0.8 ns), none of the results with the timing probeis expected to be better than the minimum resolution set in theBaF2–BaF2 experiments (Table 1). In current practice, the mostcommercially relevant scintillators tested were the individual LSO

crystal and the two LSO crystal blocks (HiRezs and Focuss

detectors). Their importance is due to being the most commonscintillators in modern commercial PET scanners and the crystalof choice in time-of-flight PET scanners.

The setup of the tests for the block detectors was designed tomimic what is done in a complete PET scanner. The goal was tosee the block-wide differences in time resolution that can be seenamongst the crystal matrix and the PMT(s). Both of the blockdetectors had a set of LSO crystals as the scintillator, butemployed different PMT technologies. The HiRezs detector hasa set of four PMTs attached to the crystal block and used Angerlogic to position the event in the crystal. The Focuss detector useda single position-sensitive PMT (PSPMT) with a resistor chainreadout to place the event.

Fig. 6 shows the raw display of the arrival time distribution inthe HiRez detector. The frame-to-frame difference is 250 ps. Theanalysis software allows individual crystals in the matrix to beidentified and the time dispersion can be fitted to a sum of threeGaussian functions. The FWHM and FWTM along with thecentroid (representing the average arrival time) can bemeasured. The arrival times are plotted as a bar-graph andshown in Fig. 7 and the FWHM time resolution in Fig. 8.

The analysis of the time resolution of the individual crystals inthe block reveals the advantage that would be had if thealignment were performed on a crystal by crystal basis (Fig. 5).In the HiRezs detector the signals from two of the corners arriveto the electronics much earlier (the short bars, Fig. 7) then thoseof the other two corners (the long bars, Fig. 7). The fastest crystalhad the events detected 252 ps after the probe detected thepositrons; the slowest was 781 ps after the probe. The average is495 ps with a standard deviation of 114 ps.

In the Focuss detector there is a very different pattern apparentin the arrival time crystal map (Fig. 9). The events in the center of

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Fig. 5. Important specifications of several PET scanners and the count rates per crystal per second expected when the timing probe is placed in the center of the scanner.

Fig. 6. Arrival time with the HiRezs detector of signals as a difference from positron detection, 250 ps per frame.


the crystal block are reported at a much latter time than those fromthe edge crystals (Fig. 10). The maximum and minimum differencesare 1.648 and 0.281 ns, with an average difference of 1.02 ns and astandard deviation of 0.39 ns. If these detectors were used in atime-of-flight PET scanner, which uses the time of arrival differencebetween the annihilation photons, a difference of 1.4 ns wouldresult in an event misplacement of 20.5 cm, which is completelyunacceptable. The FWHM time resolution of the individual crystalsis seen in Fig. 11. When ten 1 min scans were performed, the

standard deviation in the delay between the trigger pulse and asingle crystal’s response was 71 ps.

4. Discussion

Our evaluation of the Scanwell Systems timing alignmentprobe with various scintillation crystals demonstrates that theplastic scintillator is not as fast as the BaF2 crystal. Comparing the

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Fig. 7. Time difference between positron detection and reported photon detection

with the HiRezs detector, each bar is one crystal.

Fig. 8. Time resolution of the individual crystals in the HiRez block detector. The

average value is 1.26 ns.


timing resolutions presented in Table 1 shows that it iscomparable with LaBr3, and better than LSO. The principalapplication of this device is in the alignment of all the crystalsin a complete PET scanner. This was not possible in this studysince none of the scanners available had an input to which theprobe could be connected. The main attraction of this conceptcompared with other techniques for time alignment is that noneof the other techniques can use a common reference. In order tohave a valid coincidence between detectors in a normal PETscanner, the source must be on the line joining the two detectors.If the source were in the center, only diametrically opposeddetectors can be aligned to each other. If the source is movedaway from the center, ToF delays are introduced. If the sourceused for transmission scanning is employed, these ToF delays arequite significant. (�4 ns). The probe used here detects positron

decay rather than one of the collinear gamma rays resulting frompositron annihilation. Thus it no longer needs to be on the linejoining two detectors. Placing the source in the center of thescanner eliminates ToF delays. Furthermore, having one uniquesignal with which to align all the crystals allows the collection of acomplete set of time delay spectra from each crystal (with respectto the probe’s signals) simultaneously. This enables the possibilityof a single pass time alignment.

The count rates in each crystal for various PET scanners show awide variation according to the crystal size and ring radius. If oneconsiders those scanners which can operate in the time-of-flightmode, the Siemens ‘‘HiRez’’ and the Philips ‘‘Gemini TF’’ singlecrystal count rates of about 30 cps are possible. In a study toestimate the error expected in the timing offset as determinedfrom ten 1 min measurements with the Focuss detector thestandard deviation was found to be 70 ps. A detector from theGemini scanner was not available to test.

In a traditional block detector, an array of scintillating crystalsis attached to four PMTs. The ratio of the amount of light reachingeach PMT is used to calculate the position of each photon’sdetection in the scintillator. In order to enable this function, eachPMT in the block must output the same signal amplitude for thesame input light. This is done by adjusting the gain of each PMT inthe block. Adjusting the gain essentially changes the voltageapplied across the dynodes in a single PMT. Reducing the voltagereduces the energy of the electrons released from each dynode, sothat when they strike the next dynode, fewer secondary electronsare liberated. However, reducing the electrons’ energy to changethe gain implies that they travel more slowly. For example, in aseries of four PMTs, with 1200 V applied to the block, theindividual voltages could be 1150, 1200, 1225 and 1250 V sothat each PMT has a matched output.

While adjusting the gain works well making the amplitude ofthe PMT outputs same, the time taken by the PMT to convert lightto an electrical signal will no longer be consistent. This property ofPMTs is known as the transit time. The transit time given by aPMT manufacturer is usually given at or beyond the maximumvoltage rating. For example, the Photonis XP1452 PMT used insome PET scanners has a transit time of about 34 ns. Using theparameters of these PMTs, Fig. 12 shows the effect of relativelysmall voltage changes on their transit time. From the number ofdynodes nd, the distance between each dynode, Dd and the appliedvoltage, V, one can estimate the total transit time in a PMT as afunction of applied voltage.

tcathode�anode ¼ 2Ddðndþ1Þ=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2eV=mðndþ1Þ

p

A plot of this function is given in Fig. 11.A voltage adjustment of 50 V can make a difference of about

half a nanosecond. This difference will cause a relatively largeinconsistency in the arrival time in the entire block detector. Ifeach crystal were aligned to a common reference this differencecould be compensated for and the output matching would also besuccessful.

The construction of the Siemens Focuss detector is different.Other than a different number of crystals, the primary differenceis that it employs a position-sensitive PMT. These PMTs have twoorthogonal sets of anode wires the relative signal amplitudes onwhich provide positional information about the signal. In order tosimplify the readout of these anodes, they are connected by aresistor network. However, each anode has a stray capacitanceassociated with its surrounding structure. We believe that theeffect shown in Figs. 9 and 10 can be explained by considering thepropagation of the signals from the anode which receives the peaksignal to the ends of the inter-anode resistor network. We havewritten a simulation to show the output of each anode (anddifferent position on the PMT face) for a given input signal. In

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Fig. 9. Arrival time with the Focuss detector of signals as a difference from positron detection, 250 ps per frame.

Fig. 10. Time difference between positron detection and reported photon

detection with the Focuss detector, each bar is one crystal.

Fig. 11. Time resolution of the individual crystals in the Focuss block detector.

The average value is 1.21 ns.


Fig. 13, the simulated output, shown normalized to the maximum,from each anode is shown. In this case, the signal is actuallygreatest on anode #1. The top line shows a fast rising signal with arealistic decay time of 40 ns. The other signals appear later andwould have lower amplitudes. When these weighted signals aresummed an early signal is seen from the end of the inter-anoderesistor chain at anode 1, and a smaller delayed signal at the otherend of the resistor chain. However, if the gamma ray is detectednear the middle of the PMT the signals from both ends of the

anode resistor chain are of equal amplitude, but would be delayedby the signal shown for anode 3.

Since the temporal position of the peak is a very importantfactor in the time reporting in time pickoff electronics, it is clearfrom Fig. 13 that which anode the signal is on becomes animportant factor. Once again, this problem could be solved byemploying a crystal by crystal time offset to account for this.

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Fig. 12. Transit time of an XP1452 PMT when using various voltages.

Fig. 13. Signal output from different anodes of a PSPMT with the top line being the

input signal.


5. Conclusions

In testing the block detectors we were able to show that thereare systematic time delays across each crystal matrix. For thisreason there needs to be a system wide, crystal by crystal timealignment. Just as there is a need for an adapted energy windowapplied to each crystal, so should a separate time offset be appliedrather than just one for each block. Advanced techniques such astime-of-flight require very accurate time stamps from the systemin order to operate properly. On a conventional scanner, this isalso very useful to decrease the system wide time resolution,which will decrease random counts and increase image quality.The time alignment probe we have developed and tested providesa superior method to perform time alignment due to itssufficiently high count rate and most importantly, its constantclock/frame of reference for each event in the PET scanner. As seenin the work by McElroy, the previous generation of the probe wasused to improve the time resolution of their scanner from 16.5 to10.2 ns [4].

System wide time-stamp values seen in the studies wouldresult in time-of-flight errors as large as 20.5 cm, which isunacceptable. In a conventional scanner, the value to beconsidered in reducing random counts is also very advantageousas the number of random counts is linearly related to the width ofthe timing window. If properly aligned, which the timing probecan do accurately and less labour intensively, the timing windowcan be reduced leading to more efficient use of the detector dataand higher-quality images. Our repeat study results suggest that asingle 1 min scan would provide sufficient data to time-alignevery crystal in the scanner to an accuracy of 70 ps, which isprobably a sufficient accuracy for even a ToF scanner. With thisrelatively fast procedure in place may be even desirable toincorporate such a scan into the daily quality control protocolused to optimize the scanner’s ToF performance for each day’sscanning.

Acknowledgements

This work was sponsored by Grant # OPG-003672 from theNatural Science and Engineering Council of Canada to C.J.Thompson. The PET block detectors were provided by SiemensMolecular Imaging. Lissa Tegleman of Eckert and Ziegler preparedthe sources used in these timing probes.

References

[1] C.J. Thompson, M. Camborde, Instrument and method to facilitate and improvethe timing alignment of a PET scanner, US Patent 7247844, July 24, 2007.

[2] C.J. Thompson, M.L. Camborde, M.E. CaseyIEEE Trans. Nucl. Sci. NS-52 (2005)1300.

[3] W. Moses, C. ThompsonIEEE Trans. Nucl. Sci. NS-53 (2006) 2660.[4] D.P. McElroy, C.J. Thompson, V. Spanoudaki, S.I. Ziegler, Nucl. Sci. Symp. Conf.

Rec. 3 (2005) 4 October.[5] J.J. Williams, Automated Coincidence Timing Calibration for a Pet Scanner, US

Patent 5272344, December 21, 1993.[6] M.W. Lenox, Z. Burbar, J. Young, T. Gremillion, C. Knoess, C.P.S. Innovations,

T.N. Knoxville, Coincidence time alignment of high resolution planardetectors, 2002 IEEE Nucl. Sci. Symp. Conf. Rec., 2002.

[7] J.J. Williams, C.W. Stearns, D.L. McDaniel, A. Ganin, Method and Apparatus forTiming Calibration in a PET Scanner, US Patent 7129495, October 31, 2006.

[8] D. Luo, J.J. Williams, M.K. Limkeman, M.J. Cook, E.L. Oswalt, M.P. Feilen, D.L.McDaniel, G.E. Syst, W.I. Waukesha, Crystal-based coincidence timingcalibration for PET scanner, 2002 IEEE Nucl. Sci. Symp. Conf. Rec., 2002.

[9] J.J. Williams, D. Luo, M.K. Limkeman, M.J. Cook, D.L. McDaniel, E.L. Oswalt,M.P. Feilen, Crystal-Based Coincidence Timing Calibration Method, US Patent6,852,978.

[10] A.E. Perkins, M. Werner, A. Kuhn, S. Surti, G. Muehllehner, J.S. Karp, P. Res,Time of flight coincidence timing calibration techniques using radioactivesources, 2005 IEEE Nucl. Sci. Symp. Conf. Rec., 2005.

[11] G.B. Saha, in: Basics of Pet Imaging: Physics, Chemistry, and Regulations,Springer, 2005.

[12] L. Torrisi, Radiat. Phys. Chem. 63 (2002) 89.[13] Hamamatsu Photonics Corp.: Photomultiplier Tube Assemblies H6533/

H6610.[14] Canberra Industries Inc.: Model 454 200 MHz Quad Constant Fraction

Discriminator.[15] Canberra Industries Inc.: Model 2145 Time-to-Amplitude Converter/Single

Channel Analyzer.

evaluation of an instrument to improve pet timing alignment

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