Proceeding of: 2008 IEEE Nuclear Science Symposium Conference Record (NSS '08), Dresden, Germany, 19-25 Oct. 2008A fully 3D statistical image reconstruction
algorithm has been developed for a high-resolution coplanar PETtCT scanner based on rotating planar PET detectors. The
system matrix has been modeled with custom Monte Carlo techniques optimized for the specific scanner architecture. The system model includes positron range, non-colinearity of gamma rays and crystal interaction modelling with attenuation and
Compton scattering effects. Only 0.21 % of the system matrix columns are modeled in detail, obtaining the rest of the values
with axial and transaxial voxel-driven symmetries. The iterative algorithm is a fully 3D approach, regularized with the anatomical registered image using a novel version of the minimum cross entropy (MXE) scheme, and accelerated employing ordered
subsets. The proposed method has been shown to produce images with superior quality than 3D hybrid (FORE+2D-OSEM) algorithms applied on synthetic GATE data, as well as on real
small animal acquisitionsThis work has been partly funded by the CDTEAM project and CENIT programme (Spanish Ministry of Industry), EMIL (ED Network of Excellence), CIBER CB07/09/0031 and
RETIC-RECAVA (Spanish Ministry of Health) and TEC2007-64731/TCM(Spanish Ministry of Education and Science

Desco Menéndez, Manuel

Guerra, Pedro

Kontaxakis, George

Ortuño, Juan E.

Santos, Andrés

Vaquero López, Juan José

Proceeding of: 2008 IEEE Nuclear Science Symposium Conference Record (NSS '08), Dresden, Germany, 19-25 Oct. 2008A fully 3D statistical image reconstruction
algorithm has been developed for a high-resolution coplanar PETtCT scanner based on rotating planar PET detectors. The
system matrix has been modeled with custom Monte Carlo techniques optimized for the specific scanner architecture. The system model includes positron range, non-colinearity of gamma rays and crystal interaction modelling with attenuation and
Compton scattering effects. Only 0.21 % of the system matrix columns are modeled in detail, obtaining the rest of the values
with axial and transaxial voxel-driven symmetries. The iterative algorithm is a fully 3D approach, regularized with the anatomical registered image using a novel version of the minimum cross entropy (MXE) scheme, and accelerated employing ordered
subsets. The proposed method has been shown to produce images with superior quality than 3D hybrid (FORE+2D-OSEM) algorithms applied on synthetic GATE data, as well as on real
small animal acquisitionsThis work has been partly funded by the CDTEAM project and CENIT programme (Spanish Ministry of Industry), EMIL (ED Network of Excellence), CIBER CB07/09/0031 and
RETIC-RECAVA (Spanish Ministry of Health) and TEC2007-64731/TCM(Spanish Ministry of Education and Science

Desco, Manuel

Vaquero, Juan José

Name not available

Efficient methodology for 3D statistical reconstruction of high resolution coplanar PET/CT scanner

Crossref

Universidad Carlos III de Madrid e-Archivo

Efficient Methodology for 3D StatisticalReconstruction ofHigh Resolution Coplanar PET/CTScannerJuan E. Ortuno, Jose L. Rubio, Student Member, IEEE, Pedro Guerra, Member, IEEE, George Kontaxakis, SeniorMember, IEEE, Manuel Desco, Juan J. Vaquero, Senior Member, IEEE, and Andres Santos, Senior Member, IEEE,.... , ," I\ I"\,'I"I," I/ I;I' III~------)~III. MATERIALS AND METHODSThe OSEM (ordered subsets-expectation maximization)algorithm [4] with pre-calculated system matrix modeled withMonte Carlo methods and stored in sparse format, has beendemonstrated to be a powerful and computationally efficientPET detector blocksThe VrPET/CT scanner has higher sensitivity for PET in thecenter of the FOV than the previous rPET design, since fourpairs of detectors are now enabled in coincidence mode,instead of two of the former camera. Furthermore, the FOVdiameter within complete angular projections reaches 86 mmin the transaxial plane.We have developed an efficient methodology for 3Dstatistical reconstruction for this system. The proposediterative algorithms improve the computational efficiency andthe image quality of methods previously applied to rPET [3]but adapted to the new geometry, also being able to use CTco-registered anatomical information.Fig. 1. VrPET/CT transaxial view. A X-ray flat panel detector and fourgamma ray detector blocks are mounted on a common gantry with 370°rotation span. Coincidences are allowed between four pairs of detector blocks.in-plane and on the same rotational gantry. Opposite detectorsare separated by 140 mm. Each detector is composed of a30x30 array of 1.5x 1.5x 12 mm LYSO crystals, coupled to aflat panel-type large area PS-PMTs.Abstract- A fully 3D statistical image reconstructionalgorithm has been developed for a high-resolution coplanarPETtCT scanner based on rotating planar PET detectors. Thesystem matrix has been modeled with custom Monte Carlotechniques optimized for the specific scanner architecture. Thesystem model includes positron range, non-colinearity of gammarays and crystal interaction modelling with attenuation andCompton scattering effects. Only 0.21 % of the system matrixcolumns are modeled in detail, obtaining the rest of the valueswith axial and transaxial voxel-driven symmetries. The iterativealgorith m is a fully 3D approach, regularized with the anatomicalregistered image using a novel version of the minimum crossentropy (MXE) scheme, and accelerated employing orderedsubsets. The proposed method has been shown to produce imageswith superior quality than 3D hybrid (FORE+2D-OSEM)algorithms applied on synthetic GATE data, as well as on realsmall animal acquisitions.Manuscript received November 14, 2008. This work has been partlyfunded by the CDTEAM project and CENIT programme (Spanish Ministry ofIndustry), EMIL (ED Network of Excellence), CIBER CB07/09/0031 andRETIC-RECAVA (Spanish Ministry of Health) and TEC2007-64731/TCM(Spanish Ministry ofEducation and Science).1. E. Ortufio, 1. R Rubio, G. Kontaxakis and A. Santos are with theBiomedical Image Technology Group, Dpto. Ingenieria Electronica,Universidad Politecnica de Madrid, Spain, and with the Biomedical ResearchCenter on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN),Madrid, Spain E-28040 (e-mail: juanen.jlrubio.gkont.andres@die.upm.es).P. Guerra is with the Biomedical Research Center of Bioengineering,Biomaterials and Nanomedicine (CIBER-BBN), Madrid, Spain E-28040 (e-mail: pguerra@ciber-bbn.es).M. Desco and 1. 1. Vaquero are with the Unidad de Medicina y CirugiaExperimental. Hospital General Universitario Gregorio Marafion, Madrid,Spain E-28007 (e-mail: desco.juanjo@mce.hggm.es).I. INTRODUCTIONPRECLINICAL PET scanners are designed to achieve highspatial resolution to detect small animal functional targetvolumes. One affordable solution consists of opposed planardetectors mounted on a rotating gantry. This configuration canachieve state-of-the-art spatial resolution at the cost of lowerdetection sensitivity than full ring alternatives.The rotational PET (rPET) [1] (SUINSA Medical Systems,Spain) pre-clinical camera consists of two pairs of planardetector blocks, composed ofpixelated scintillator crystals andPS-PMTs. The gantry has 3700 rotation span. This design hasbeen recently upgraded to incorporate multimodalitycapabilities by integrating an X-ray tube and a semiconductorX-ray detector [2]. The PET block detectors are disposed in"V" arrangements as seen in Fig. I to allow enough space toaccommodate the X-ray CT system with cone beam geometry12Fig. 4. Surface rendering of gray level images represented in Fig. 3. (a) 3DOS-MXE reconstruction with pixel size of 0.5xO.5><0.375 mm. (b) 3D OS-MXE reconstruction with pixel size of (b) 0.5xO.5xO.75 mm. (c) FORE & 2DOSEM reconstruction.rods. The voxel size in this case is 0.75 mm axially. 5iterations with 19 subsets per iteration were employed in allcases.Fig. 4 shows a surface rendering of the partial views shownin Fig. 3, for better appreciating the better contrast of 30-MXE reconstruction using the finest grid.Gamma penetration and Compton scattering in scintillationcrystals is carefully simulated with attenuation cross-sectionstables and the Klein-Nishina formula. To avoid poorsparseness of the SM with negligible values, an auxiliary LUTis employed, which contains the relative probabilities ofdetection in neighboring crystals, according to the angle andpoint of intersection of the gamma rays. The pre-calculatedSM values are then stored in less than 3GB for theconfigurations described aboveIII. RESULTSA. Derenzo-Type phantomsThe proposed image reconstruction scheme has beenevaluated with data generated using the GATE platform [9]. ADerenzo-type phantom was simulated with water-filled rods of{4.8, 3.6, 2.4, 1.8, 1.2} mm diameter and 16 mm length.Activity density was selected to be 5 IlCi/cm3 (F-18radiopharmaceutical). A total amount of 10.8 millioncoincidences was collected with an energy window of 400-700keV and 12 ns coincidence window.The phantom was situated with rods perpendicular to thescanner axial axis, in order to better evaluate the spatialresolution in the axial direction. No anatomical informationwas used in this experiment.a) (b) (c)Fig. 3. GATE acquisition of Derenzo-type phantom with rods aligned withthe transaxial plane: Partial coronal views. (a) 3D OS-MXE reconstructionwith pixel size of 0.5xO.5><0.375 mm. (b) 3D OS-MXE reconstruction withpixel size of (b) 0.5xO.5xO.75 mm. (c) FORE & 2DOSEM reconstnlction.,..-.....O 1.6 lTIlTI • :... •••• •r+••••••••:.•.::.-.:....o 1.2 mm ~:..:::.........'-.....-:.:::;:." I -:.•••t ' -~oronal viewx~y•••••••••••••••••••-..............__ 111.-_..........••(a) (b)HIli IIIzL;-y-.-T-ran-s-a-xl-'al-Vl-'e-w----ll(c)B. Phantom with registered anatomical informationA quality phantom has been modeled with GATE in orderto evaluate the noise characteristics and edge contrast inpresence of registered anatomical data.This phantom consists of several rods filled with watercontaining F-18, embedded in a main chamber cylinder (orbackground) with 1/5th of activity density. 5.2x 106coincidences were collected with the same energy andcoincidence window of the Derenzo-type experiment. Ascheme of the activity distribution is shown in Fig. 5 (up) witheight hot rods of {8, 7, 6, 5, 4, 3, 2, I} mm diameter and 25mm length in the upper part (as seen in Fig. 5), and 2 hotcylinders of 16 mm diameter and 9 mm length in the lowerpart of the phantom. There are anatomical boundaries withoutactivity gradient for evaluating the robustness of thereconstruction algorithm against anatomical information notcorrelated with functional PET regions (see the anatomicalmap of Fig. 5)In Fig. 6, 3D OS-MXE reconstructions of the phantomstudy are shown at three transaxial and one coronal view. Thefinest grid was used (i.e, 0.5xO.5x0.375 mm) with 4 iterationsand 19 subsets per iteration (i.e, 76 sub-iterations). The edgesaligned with anatomical interfaces are enhanced, while thereare no significant artifacts related to non-eorrelatedanatomical (i.e, simulated CT) and PET data.Fig. 3 shows a partial view of the Derenzo-type phantomacquisition. Rods have better contrast in 3D mode with voxelsize of 0.375 mm axially. 2D-OSEM with Fourier rebinning(FORE) shows poorer results in terms of contrast between3Activity map Anatomical data PET images.',..f.'' :,(d)(a)~(b) • e'. .,.(c)Fig. 6. OS-MXE reconstructions of the synthetic phantom with registeredanatomical images (blue palette pictures). (a) Transaxial view of region withsmall rods with aligned anatomical information. (b) Transaxial view where thesmall diameter rods were not modeled with anatomical gradients, showingpoorer edge resolution. (3) Transaxial view of upper part of the phantom,showing enhanced edge resolution in the cylinder with aligned anatomicaldata. (d) Coronal view of the phantom.3D-OSEM0.471.001.181.150.20'.,Coronal (lower rods)Coronal (lower rods)3D-MXE0.440.720.960.930.14Coronal (upper cyl.)Coronal (upper cyl.)Anatomical (CT) mapTransaxialTransaxialRegion of intererest05 mmrod06 mmrod07 mmrod08 mm rodbackgroundTABLE 2COEFFICIENT OF VARIATION (CoV) OF REGIONS OF INTEREST WITH OSEM ANDMXE RECONSTRUCTION METHODSFig. 5. Schematical representation of the of quality phantom simulatedwith GATE. Hot rods filled with water labeled with F-I8 are aligned withanatomical boundariesTable 2 shows the coefficient of variation (CoV) in regionsof interest containing active rods of different radii (see Fig. 5)as well as the background, compared with results obtainedwithout anatomical data using 30-0SEM reconstructionregularized with MRP scheme.C. Rat studyExperimental acquisition with rat injected with F-I8-FOGrevealed superior spatial resolution for the 3D method in theaxial direction. Fig. 7 shows that contrast between functionalstructures located in the vertebral column of a rat study issuperior in the 3D reconstructed image.4REFERENCESJ. 1. Vaquero, E. Lage, L. Rican, M. Abella, E. Vicente, and M. Desco,"rPET detectors design and data processing", at Nuclear ScienceSymposium Conference Record, IEEE, vol 5, pp. 2885-2889, PuertoRico (USA) 2005.1. 1. Vaquero, et aI., "Co-planar PETICT for small animal imaging", atNuclear Science Symposium Conference Record, IEEE, vol 3, pp. 1748-1751, Puerto Rico (USA) 2005.1. E. Ortufto, P. Guerra, 1. L. Rubio, G. Kontaxakis, and A. Santos, "3D-OSEM iterative image reconstruction for high-resolution PET usingprecalculated system matrix", Nuclear Instruments & Method,' inPhysics Research Section a-Accelerators Spectrometers Detectors andAssociated r;quipment, vol. 569, no. 2, pp. 440-444,2006.[4] H. M. Hudson and R. S. Larkin, "Accelerated image-reconstnlctionusing ordered subsets of projection data", IEEE Transactions on MedicalImaging, vol. 13, no. 4, pp. 601-609, 1994.[5] B. A. Ardekani, M. Braun, B. F. Hutton, I. Kanno, and H. Iida,"Minimum cross-entropy reconstruction of PET images using prioranatomical information", Physics in Medicine and Biology, vol. 41, no.11, pp. 2497-2517,1996.[6] B. F. Hutton, A. Olsson, S. Som, K. Erlandsson, and M. Braun,"Reducing the influence of spatial resolution to improve quantitativeaccuracy in emission tomography: A comparison of potential strategies",Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment, vol.569, no. 2, pp. 462-466, 2006.[7] S. Som, B. F. Hutton, and M. Braun, "Properties of minimum cross-entropy reconstnlction of emission tomography with anatomically basedprior",IEt.;/;; Transactions on Nuclear Science, vol. 45, no. 6, pp. 3014-3021,1998.[8] C. A. Johnson, Y. C. Van, R. E. Carson, R. L. Martino, and M. E.Daube-Witherspoon, "A system for the 3D reconstnlction of retracted-septa PET data using the EM algorithm", IEEE Transactions on NuclearScience, vol. 42, no. 4, pp. 1223-1227,1995.[9] S. Jan, et al.,"GATE: a simulation toolkit for PET and SPECT", Physicsin Medicine and Biology, vol. 49, no. 4, pp. 4543-4561, 2004.[1],III [2]IIII,... [3](b) (c)IV. CONCLUSSIONSWe have proposed a novel fully 3D algorithm for theiterative reconstruction of PET data~ taking into considerationanatomical correlated data~ based on the maximum entropyapproach and accelerated using the ordered subsets technique'.The algorithm has been validated on the VrPET/CT rotationalcamera. The anatomical correlates correspond to the CTimages obtained with this system.For simulation data using the GATE platform the proposed3D OS-MXE method demonstrates superior image qualityover the FORE-2D-OSEM alternative in terms of spatialresolution in the axial axis. The edges aligned with anatomicalinterfaces are enhanced while there are no significant artifactsrelated to non-eorrelated anatomical data.In real data acquisitions with small animals, even withoutthe anatomically correlated images~ a substantial improvementis obtained in the axial spatial resolution over FORE-2D-OSEM method. Future work will include CT registered data toevaluate edge improvements with rat studies.(a)Fig. 7. (a) Coronal view of rat injected with FDG and reconstructed with3D OS-MXE algorithm; (b) Detail showing vertebrae; (c) FORE & 2D-OSEMreconstruction.5

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Efficient methodology for 3D statistical reconstruction of high resolution coplanar PET/CT scanner

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