Development of a single-ring OpenPET prototype

Eiji Yoshida n, Hideaki Tashima, Hidekatsu Wakizaka, Fumihiko Nishikido,Yoshiyuki Hirano, Naoko Inadama, Hideo Murayama, Hiroshi Ito, Taiga YamayaNational Institute of Radiological Sciences, 4-9-1, Inage-ku, Chiba 263-8555, Japan

a r t i c l e i n f o

Article history:Received 3 June 2013Received in revised form8 August 2013Accepted 10 August 2013Available online 28 August 2013

Keywords:PET (Positron emission tomography)DOI (Depth-of-interaction) detectorIn-beam pet

a b s t r a c t

One of the challenging applications of PET is implementing it for in-beam PET, which is an in situmonitoring method for charged particle therapy. For this purpose, we have previously proposed an open-type PET scanner, OpenPET. The original OpenPET had a physically opened field-of-view (FOV) betweentwo detector rings through which irradiation beams pass. This dual-ring OpenPET (DROP) had a wideaxial FOV including the gap. This geometry was not necessarily the most efficient for application to in-beam PET in which only a limited FOV around the irradiation field is required. Therefore, we haveproposed a new single-ring OpenPET (SROP) geometry which can provide an accessible and observableopen space with higher sensitivity and a reduced number of detectors than the DROP. The proposedgeometry was a cylinder shape with its ends cut at a slant, in which the shape of each cut end became anellipse. In this work, we developed and evaluated a small prototype of the SROP geometry for proof-of-concept. The SROP prototype was designed with 2 ellipse-shaped detector rings of 16 depth-of-interaction (DOI) detectors each. The DOI detectors consisted of 1024 GSOZ scintillator crystals whichwere arranged in 4 layers of 16�16 arrays, coupled to a 64-channel FP-PMT. Each ellipse-shapeddetector ring had a major axis of 281.6 mm and a minor axis of 207.5 mm. For the slant mode, the ringswere placed at a 45-deg slant from the axial direction and for the non-slant mode (used as a reference)they were at 90 deg from the axial direction with no gap. The system sensitivity measured from a 22Napoint source was 5.0% for the slant mode. The average spatial resolutions of major and minor axisdirections were calculated as 3.8 mm FWHM and 4.9 mm FWHM, respectively for the slant mode. Thisdifference resulted from the ellipsoidal ring geometry and the spatial resolution of the minor axisdirection degraded by the parallax error. Comparison between the slant mode and the non-slant modeshowed no remarkable difference in terms of the sensitivity distribution and spatial resolutionperformance. Therefore, we concluded that the SROP geometry has a good potential as an opengeometry especially suitable for in-beam imaging.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

One of the applications being attempted for PET isits implementation for in-beam PET [1], which is an in situmonitoring method for charged particle therapy. We havepreviously proposed an open-type PET scanner, OpenPET[2,3], for this purpose. Our original OpenPET had a physicallyopened field-of-view (FOV) between two detector ringsthrough which irradiation beams pass as shown in Fig. 1(a).This dual-ring OpenPET (DROP) had a wide axial FOV includingthe gap which was not necessarily the most efficient geometrywhen it is applied to in-beam PET that requires only a limitedFOV around the irradiation field.

On the other hand, in another report [4], we proposed our 2ndgeneration OpenPET, a single-ring OpenPET (SROP) geometry(Fig. 1(b)) which can provide an accessible and observable openspace with higher sensitivity and a reduced number of detectorsthan the DROP. The proposed geometry is a cylinder shape with itsends cut at a slant, in which the shape of each cut end becomes anellipse. Numerical simulation results for the imaging property ofthe SROP geometry have indicated that the depth-of-interaction(DOI) detector [5] can provide uniform resolution even when thedetectors are arranged in an ellipsoidal ring. In this work, wedeveloped and evaluated a small prototype of the SROP geometryfor a proof-of-concept.

We are studying the feasibility of OpenPET especially forin-beam PET experiments. In the in-beam PET experiments, theactivity due to irradiation is generally low, although the activitydistribution is confined to a localized area in the body [6,7]. Also, itis well known that lutetium-based scintillators commonly used inPET imaging include the intrinsic radioactivity of 176Lu [8–10].

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Nuclear Instruments and Methods inPhysics Research A

0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.nima.2013.08.041

n Corresponding author. Tel.: þ81 43 206 3439; fax: þ81 43 206 0819.E-mail address: rush@nirs.go.jp (E. Yoshida).

Nuclear Instruments and Methods in Physics Research A 729 (2013) 800–808

For clinical studies, the effects of the intrinsic radioactivity oflutetium-based scintillators can be ignored with a narrow energywindow. However, the intrinsic radioactivity becomes problematic[11] when used in low count rate situations such as in-beam PETimaging. Therefore, low noise and highly sensitive PET scannersare required for in-beam PET.

2. Materials and methods

2.1. Single ring OpenPET prototype

Fig. 2 shows photographs of the SROP prototype having2 modes, slant and non-slant. The SROP prototype was designed

Fig. 1. Illustrations of (a) The 1st generation OpenPET, dual-ring OpenPET (DROP) and (b) The 2nd generation OpenPET, single-ring OpenPET (SROP).

Fig. 2. Photographs of the SROP prototype showing its 2 modes: (a) slant mode with detector rings placed at an angle of 45 deg; and (b) non-slant mode (which was areference) with detector rings placed at an angle of 90 deg with no gap.

Fig. 3. Detector arrangements for the OpenPET geometry: (a) the slant-mode (45 deg) and (b) the non-slant mode (90 deg). The Z direction is equal to the axial direction forthe non-slant mode. The slice direction for the filtered back projection (FBP) is slanted at 45 deg to the axial direction for the slant-mode and is equal to the axial direction forthe non-slant mode.

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with 2 ellipse-shaped detector rings of 16 detectors each. Eachellipse-shaped detector ring had a major axis of 281.6 mm and aminor axis of 207.5 mm. For the slant mode, the rings were slantedby 45 deg from the axial direction to obtain an open space of74.6 mm width (Fig. 3(a)). We evaluated the non-slant mode as areference; in it, the two detector rings were placed at 90 deg fromthe axial direction with no gap (Fig. 3(b)).

Table 1 shows basic scanner specifications of the DROP proto-type [3,12] and the SROP prototype. The previous DROP prototypeused a lutetium gadolinium oxyorthosilicate (LGSO) scintillator;low activity imaging such as for in-beam use was sometimesdegraded due to the intrinsic radioactivity of 176Lu. The intrinsiccoincidence rate of the DROP prototype has been counted as about50 cps. In the SROP prototype, therefore, we used a scintillatorwithout intrinsic radioactivity. This DOI detector consisted of 1024Zr-doped gadolinium oxyorthosilicate (GSOZ) scintillators whichwere arranged in 4 layers of 16�16 arrays, coupled to a64-channel flat panel position sensitive photomultiplier tube(FP-PMT). Each GSOZ crystal was 2.8�2.8�7.5 mm3 as shown inFig. 4. By removing reflectors between crystals for every two linesand shifting reflector patterns depending on layers, responses ofall crystal elements in the four layers can be mapped to a 2Dposition histogram after applying a conventional Anger-typecalculation [13].

After 64-channel gain correction of the FP-PMT, the outputsignals of two axially stacked DOI detectors were projected ontoone 2D position histogram by the Anger calculation. This signalprocessing can be reduced to the scale of a coincidence processor.Front-end circuits were separated from the detector head by60-cm coaxial cables for protection of electronic circuits from

Table 1Basic scanner specifications of the DROP prototype and the SROP prototype.

DROP SROP

(1st generation) (2nd generation)

Scintillator crystal LGSO GSOZSize of a scintillator crystal (mm) 2.9�2.9�5 2.8�2.8�7.5Number of crystals per detector 14�14�4 16�16�4Block size of the detector (mm) 42�42�30 46�46�30Number of detector rings 2 2Number of detectors per ring 8 16Ring diameter (mm) 86 281.6 (major axis)

207.5 (minor axis)Open space (mm) 27 74.6

Fig. 4. Illustration of the 4-layer DOI detector.

Fig. 5. (a) Photograph of the rotating stage for the slant mode and illustration of the line source behavior of (b) The slant mode and (c) The non-slant mode.

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radiation damage. Each DOI detector was in coincidence with theopposing 13 DOI detectors. Also, the coincidence circuits havedelayed coincidence circuits for random estimation. Coincidencedata were acquired only for the list mode.

Front-end circuits of the SROP prototype had almost the samearchitecture as that of the DROP prototype. But, the data acquisi-tion system of the SROP prototype was extended from 16 detectorsto 32 detectors. The coincidence timing window and energywindow were set as 20 ns and 400–600 keV, respectively. Theseparameter values of the SROP prototype were the same as those ofthe DROP prototype.

The optional rotating stage was used and the data for thenormalization of the detector sensitivity were obtained by rotatingthree 68Ge–68Ga line sources having a 26 cm long activity area and0.34 MBq radioactivity. For the slant mode, each line source wasrotated along the slant circle orbit inside the ellipse-shaped detectorring using a spacer. This spacer, which had a cylinder shape with itsends cut at a slant as shown in Fig. 5, cannot only extend the linesources to the FOV but also control position of line sources as thecenter of the line sources always equal to the center of the axial FOV.On the other hand, for the non-slant mode, each line source wasrotated around the simple circle orbit by replacing another spacer

Fig. 6. Illustration of setup for the (a) small rod phantom and (b) rabbit brain study.

Fig. 7. 2D position histogram from the 511-keV uniform irradiation for the non-slant mode. Two axially stacked DOI detectors were projected onto one 2D positionhistogram by the Anger calculation.

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with a simple cylinder shape. The rotational diameter of these linesources was 150mm for both modes. Normalization data wasacquired for 24 h while rotating line sources.

2.2. Image reconstruction

For the image reconstruction algorithm, we mainly used the 3Dmaximum likelihood expectation maximization (ML-EM) algorithm[14]. Acquired list-mode data were transformed to histogram databefore image reconstruction. The ML-EM algorithm can be applied tonot only the non-slant mode but also the slant mode by definingaccurate detector geometry. The system matrix was calculated incor-porating detector response functions modeled by the sub line-of-response (LOR) model [15]. Random correction was applied bysubtracting delayed coincidence, but attenuation correction and scat-ter correction were not applied. The FOV defined in the imagereconstruction was 150mm in diameter and 150mm in axial lengthand voxel size was 1.5 mm3 for the slant mode and the non-slantmode. The number of iterations was 100 for both modes.

For only spatial resolution measurement, the 2D FBP algorithmwas applied to the data rebinned by the Fourier rebinning (FORE).

The FOV defined in the image reconstruction was 200mm indiameter and 110mm in axial length and voxel size was 0.78�0.78�1.45 mm3 for the slant mode and the non-slant mode. Maximum ringdifference was 30. In this case, due to the limitation of the projectiondata, the slice direction was perpendicular to the plane of detectorrings. For the non-slant mode, the slice direction was equal to theaxial direction in Fig. 3. For the slant mode, however, the slicedirection was slanted by 45 deg of the axial direction.

2.3. Performance evaluation

Performance of the DOI detector was tested for 511-keV uni-form irradiation (68Ge–68Ga line source, 26 cm long with a 1.7 mminner diameter, 0.34 MBq). We evaluated crystal identificationperformance at the peripheral crystals from uniform irradiation,because the scintillator block of the SROP prototype extended form42 mm2 (the DROP prototype) to 46 mm2 as shown in Table 1.Also, energy resolutions were calculated from the energy spec-trum of each DOI detector.

The sensitivity profiles and spatial resolutions were measuredfrom a 22Na point source (0.03 MBq). Measurement times of the22Na point source were 5 min for each position. The reportedvalues characterize the width of the reconstructed image, definingthe width as its full width at half maximum (FWHM).

We used a cylinder phantom (8 cm long with a 4-cm diameter)for the count rate performance test. This phantom was filled with18F solution and was placed in the center of the FOV for the slantmode. All data were acquired in 1-min frames with a delay of10 min between frames.

For an imaging demonstration, we measured a small rod phantomand a rabbit brain. The rod phantom consisted of an outer hollowcylinder and an inner solid cylinder of 36.1 mm diameter and 17.8 mmlength. The rod phantom was filled with 18F solution (about 2 MBq)and was measured in the center of the FOV as shown in Fig. 6(a). Scantime was 50min for the rod phantom. The rabbit was placed so thatits brain was located in the center of the FOV for the slant mode asshown in Fig. 6(b). The rabbit brain was scanned for 30 min afterinjection of 24 MBq 18F-FDG and 72min uptake period.

3. Results

3.1. DOI detector

Fig. 7 shows the 2D position histogram from the 511-keVuniform irradiation. Almost all crystals could be identified exceptfor several peripheral crystals. Fig. 8 shows profiles of the 2D

Fig. 8. Profiles of 2D position histogram along the arrows indicated in Fig. 7.Profiles (A) and (C) show 1st and 3rd layers. Profiles (B) and (D) show 2nd and 4thlayers. For each profile, bold numbers are upper layers. Fig. 9. Energy resolution of each DOI detector.

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position histogram along the arrows indicated in Fig. 7. Fig. 9shows energy resolutions of each DOI detector. The average energyresolution was 1470.4% for the 32 DOI detectors. Therefore, theenergy window was set 400–600 keV. With the energy window of400–600 keV, the background coincidence of the SROP prototypecould be lowered from about 50 cps (the DROP prototype) tounder 1 cps.

3.2. Sensitivity

Fig. 10 shows sensitivity profiles for the slant mode and thenon-slant mode. In the X direction, the sensitivity profile of theslant mode was degraded compared to that of the non-slant mode.The system sensitivities of the slant mode and the non-slant modewere 5.0% and 4.9%, respectively.

3.3. Spatial resolution

Fig. 11 shows reconstruction images of point sources for theslant mode and the non-slant mode using the ML-EM. Fig. 12shows spatial resolutions for the slant mode and the non-slantmode using the ML-EM. The SROP prototype had uniform spatialresolution for both modes. For the slant mode, average spatialresolutions of X and Y directions were 2.470.3 mm FWHM and3.270.6 mm FWHM, respectively. Also, for the non-slant mode,average spatial resolutions of X and Y directions were2.870.3 mm FWHM and 3.670.4 mm FWHM, respectively.Spatial resolutions at Y direction increased 20% larger than theseat X direction on average. Also, in the Z direction, average spatialresolution of the slant mode and the non-slant mode were2.570.3 mm FWHM and 2.670.4 mm FWHM, respectively.

Fig. 10. Sensitivity profiles of the SROP prototype for (a) The slant mode and (b) The non-slant mode. Each direction corresponds to directions of Fig. 3.

Fig. 11. Reconstruction images of point sources for (a) The slant mode and (b) The non-slant mode using the ML-EM. In the coronal image of the slant mode, the missingregion of the FOV was due to the cylinder shape with its ends cut at a slant.

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Fig. 13 shows spatial resolutions at Y direction for the slantmode and the non-slant mode using the 2D-FBP. For the 2D-FBP,the average tangential spatial resolutions of the slant mode andthe non-slant mode were 3.870.2 mm FWHM and 3.770.2 mmFWHM, respectively. Also, the average radial spatial resolutions ofthe slant mode and the non-slant mode were 4.970.4 mm FWHMand 5.370.5 mm FWHM, respectively. Spatial resolutions at theradial direction increased 30% larger than these at the tangentialdirection on average.

3.4. Count rate performance

Fig. 14 shows dead time of the SROP prototype for the slantmode and Fig. 15 shows its count rate performance. The maximumevent rate of 360 kcps (promptþrandom) was seen for activitiesexceeding 30 MBq. The trueþscatter rate was equal to the randomrate at 27 MBq.

Fig. 12. Spatial resolutions of the SROP prototype for (a) The slant mode and (b) The non-slant mode using the ML-EM. Each direction corresponds to directions of Fig. 3.

Fig. 13. Spatial resolutions at the offset in the Y direction for (a) The slant mode and (b) The non-slant mode using the 2D-FBP. The radial direction is equal to Y direction ofFig. 3. The tangential direction is equal to X direction for the non-slant mode and the X0 direction for the slant mode.

Fig. 14. Dead time of the SROP prototype for the slant mode.

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3.5. Imaging demonstration

Fig. 16 shows reconstructed images of the small rod phantomfor the slant mode and non-slant mode using the ML-EM. Thisimage is a transverse slice at the center of the axial FOV. In theboth modes, the rods of 3.0 mm diameter were clearly separated.For the non-slant mode, we found low activity of some rods. Thiswas because that airs were mixed into some 4 mm rods. Fig. 17shows reconstructed images of the rabbit brain for the slant modeusing the ML-EM. In the coronal image, the missing region of theFOV was due to the cylinder shape with its ends cut at a slant.

4. Discussion

In the SROP prototype, we used GSOZ scintillators for the DOIdetectors so that the background coincidence could be ignored.A pair of the DOI detectors was projected onto one 2D positionhistogram. Therefore, the total number of crystals projected ontothe 2D position histogram was 2048. Almost all crystals could beidentified except for several peripheral crystals as shown in Fig. 7.Among the peripheral crystals, some had a larger crystal decodingerror than the center crystals. This was because the output signalsof two DOI detectors were projected onto one 2D positionhistogram. In the future systems, we plan to develop a system asthat each DOI detector projects onto each 2D position histogramindividually.

System sensitivity was 5.0%. The SROP prototype achievedsignificantly high sensitivity. In comparing results for the slantmode and the non-slant mode, we did not observe any remarkabledifferences in the sensitivity distribution between the two modes.The dynamic range of the SROP prototype was limited to below27 MBq based on count rate performance results. However, thisrange limit is sufficient for in-beam PET studies, because thegenerated activity of in-beam PET is very low.

We evaluated the spatial resolution performance of the SROPprototype using ML-EM and 2D-FBP. For the ML-EM, averagespatial resolutions of X and Y directions were calculated as2.4 mm FWHM and 3.2 mm FWHM, respectively. Also, the averagespatial resolution of the Z direction was 2.5 mm FWHM. The rodsof 3.0 mm diameter of the small rod phantom were clearlyseparated. On the other hand, for the 2D-FBP, average spatialresolution of major and minor axis directions was calculated as

Fig. 16. Reconstructed images of the small rod phantom.

Fig. 17. Reconstructed images of the rabbit brain.

Fig. 15. Count rate performance of the SROP prototype for the slant mode.

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3.8 mm FWHM and 4.9 mm FWHM, respectively. The ML-EM hadbetter spatial resolution than that of the 2D-FBP. This was becausethe ML-EM used the accurate system matrix calculated incorpor-ating detector response functions modeled by the sub LOR modeleven for the ellipsoidal ring geometry. As a result of spatialresolution measurements, the SROP prototype had uniform spatialresolution in the whole FOV. However, the spatial resolution of theminor axis direction was worse than that of the major axisdirection. This difference resulted from the ellipsoidal ring geo-metry and the spatial resolution of the minor axis directiondegraded by the parallax error. Also, in X and Y directions of theML-EM, spatial resolutions of the slant mode were slightly betterthan that of the non-slant mode. We think that this was because ofsampling pitch improved by shifting detector rings for the slant-mode although this difference was within statistical error.

The SROP geometry promises imaging performance equivalentto that of the DROP geometry though the SROP geometry has acylinder shape with its ends cut at a slant. Therefore, we con-cluded that the SROP geometry has a good potential as an opengeometry especially suitable for in-beam imaging.

5. Conclusions

We developed and evaluated a small prototype of the SROPgeometry for a proof-of-concept. The DOI detector based on theGSOZ scintillators promises good performance for in-beam PETstudies. We plan to develop the human size OpenPET using thisDOI detector.

Acknowledgment

This work was supported by the Grants-in-Aid for ScientistsResearch (A) of Kakenhi (22240065, 25242052).

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