Acknowledgements. PET Fundamentals: Ideal Case. Why Are We Excited About PET/CT? PET Fundamentals: Real Case

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1 PET/CT and Fusion Issues Jon A. Anderson Department of Radiology The University of Texas Southwestern Medical Center at Dallas American Associate of Physicists in Medicine 2003 Annual Meeting Acknowledgements Tay Pritchett, RT, CNMT (UTSW) Michael Viguet, CNMT, (UTSW) Dana Mathews, MD (UTSW) Thomas Lane, PhD (UTSW) Jonathan Frey (Siemens) Jim McCann (Siemens) James Bland (CPS) Ken Halliday (CTI) Alex Ganin, PhD (GE) Jeff Simer (GE) Johann Fernando, PhD (Philips) and many others Why Are We Excited About PET/CT? Use of sequential CT and PET in the same scanner to obtain detailed anatomical and functional images yields automatic and reliable co-registration of the two images for improved diagnostic performance, reduced acquisition time for PET study to enable better patient compliance and higher patient throughput, lower noise in the attenuation correction step of the PET data processing knock your socks off images that referring physicians can readily appreciate and use. PET Fundamentals: Ideal Case Positron-emitting Nucleus Detector Ring *Same Time = within 6-2 ns (typ) γ e+ Annihilation Event Two Events ( ) in Detector at the Same Time* Define Line of Response (LOR) Used in Reconstruction PET Fundamentals: Real Case γ TRUES with correct LOR γ3 RANDOMS with false LOR suppress with small coincidence time and collimation scatter γ γ SCATTERS with misplaced LOR suppress with energy resolution and collimation Sensitivity 2D and PET (Distinctly similar to single-row vs multi-detector CT) collimators Plane 2D 2D Collimators, restricted span between rings, lower sensitivity, reduced scatter No collimators, larger span between rings, high sensitivity, higher scatter

Corrections for Quantitative Studies (All PET is Quantitative) Raw Sinogram Data (T +S+ R) Remove Randoms Normalize Detector Responses Correct for Deadtime Correct for Scatter Correct for

entire LOR -- exactly the data from transmission CT! γ s 2 s s P = e µ ds s 2 P2 = e Pcoincidence = P P2 µ ds s + s2 = e ACF is inverse of this probability.

Attenuation Correction Factor in Soft Tissue 40 20 00 80 60 40 20 0 0 0 20 30 40 60 Path Length [] d = 23 ACF 9 d = 35 ACF 28 In A Sense, We Have Had PET/CT For A Long Time, but.

2 Corrections for Quantitative Studies (All PET is Quantitative) Raw Sinogram Data (T +S+ R) Remove Randoms Normalize Detector Responses Correct for Deadtime Correct for Scatter Correct for Attenuation T Sinogram Ready for Reconstruction Attenuation Correction Factors (ACFs) µ ds Probability of both photons escaping depends on the integral of the absorption coefficient along the entire LOR -- exactly the data from transmission CT! γ s 2 s s P = e µ ds s 2 P2 = e Pcoincidence = P P2 µ ds s + s2 = e ACF is inverse of this probability. Attenuation Correction Factor How Big is the Attenuation Correction Factor (ACF)? Attenuation Correction Factor in Soft Tissue Path Length [] d = 23 ACF 9 d = 35 ACF 28 In A Sense, We Have Had PET/CT For A Long Time, but. Corrections to PET scans for attenuation and possibly scatter require an attenuation map (µ-map, AKA CT image) at 5 kev obtained by measuring transmission of 5 or 662 kev photons from isotopic sources. - obtained sequentially to emission scan - acquired on same time scale (requires about /3 of total imaging time, say 3 minutes out of 8 minute scan) PET µ-map images are CT scans that - are made at 5 kev - have low spatial resolution - visualize bone, tissue, and contrast differently than x-ray CT Basic Arrangement of a PET/CT Siemens (#6) HR+ PET (BGO) Somatom Emotion CT (single slice) Current Implementations (August 2003) CPS (marketed by CTI and Siemens) Reveal RT/Reveal XVI LSO/ Sensation 6 GE PET CT apologies to any vendor that was inadvertently omitted! Philips GEMINI

3 Available Technology Crystal Technology BGO LSO GSO Manufacturer CPS (CTI and Siemens) GE Philips Product Reveal RT LSO Reveal XVI Sensation 6 GEMINI CT Siemens Somatom Emotion (2 Slice) Siemens Somatom Sensation (6 Slice) GE Lightspeed (4, 8, 6 slice) GE Lightspeed (4, 8 slice)* Philips MX8000 (2 slice) PET Reveal ECAT ACCEL Advance NXI Allegro Detectors LSO BGO GSO *6 expected by end of 2003 Material Decay Time Attenuation Length Relative Light Out Energy Resolution (single crystal) ns % % bismuth germanate % 0.2 lutetium oxyorthosilicate:ce % 0 gadolinium oxyorthosilicate:ce % 8.5 DS Bailey et al in Valk et al Positron Emission Tomography (Springer, 2003) PET Configurations Product Reveal RT LSO Reveal XVI Sensation 6 GEMINI Crystal Size and (#) 6.45 x 6.45 x 25 (926) 4 x 8 x 30 (2096) 6.3 x 6.3 x30 (0080) 4 x 6 x 20 (7864) Axial Field of View Transverse Field of View # Image Planes Axial Sampling Ring and (Port) Diameter 82.7 (70) 92.7 (70/59)* 88.6 (70) 2 90 (70/63)* * CT port/ PET port PET Performance Product Reveal RT LSO Reveal XVI Sensation 6 GEMINI Sensitivity (Trues) kcps/kbq/ml 27** 5.4** 3** 8.** 35** 25** (t+s), 40 LLD Mode 2D 2D Axial Resolution Axis/0 5.8/7.* 4.8/5.4** 6.0/6.3** 6.2/6.8** 6.2/6.8** 5.0/6.* Transverse Resolution Axis/0 6.3/7.4* 4.8/5.4** 4.8/5.4** 6.2/6.8** 6.2/6.8** Energy Resolution % 25 (recently announced 5 w new electronics) 20 7 PET Xmis n No Yes Ge-68 No 4.9/5.5* 6 Yes Cs-37 * NEMA 200 ** NEMA 994 CT Configuration Product Reveal RT LSO Reveal XVI Sensation 6 GEMINI Generator kw Anode Heat Rating MHU Minimum Slice Thickness Field of View Noise Equivalent Count Rate: NECR Noise equivalent count rate (NECR) is used to compare the effective count rate performance of systems and to determine the optimal operating regime of particular combinations of hardware and data collection strategy It represents the count rate in a perfect system (no randoms or scatters) that would have the same SNR as the measured count rate in the system Under certain 2 assumptions, can T NEC = be calculated as: ( T S nfr) + + n = 2 for direct subtraction of randoms and if variation reduction scheme is applied to randoms; f = fraction of field of view filled by phantom

4 Specific Issues for PET/CT Bone Differs From Other Tissues When Changing µ 70keV µ 5keV Derivation of proper attenuation corrections from CT image Recognition of PET artifacts due to Attenuation correction model failures Patient motion CT artifacts QA and test procedures Design features of PET/CT facilities µ/ρ [ 2 /g] Photoelectric Component of Attenuation Coefficient Mass Attenuation Coefficients for Principal Tissue Types CT Energy 70 kev Soft Tissue Total Soft Tissue PE Adipose Tissue Total Adipose Tissue PE Bone Total Bone PE PET Energy 5 kev Compton Component Accounts for Almost All of Attenuation Coefficient Energy [kev] Conclusions Regarding the Relation of µ 70keV to µ 5keV Initial Scheme for Converting µ 70keV µ 5keV ) The attenuation coefficient at 5 kev can be obtained from that at 70 kev by simple multiplication by a constant, independent of tissue type, for all tissues having attenuation dominated by Compton scatter at CT energies ( 70 kev). 2) For materials having substantial contribution from photoelectric absorption, the scaling factor will be dependent on the nature of the material. ) Adjust resolution in CT to agree with resolution of PET 2) Convert CT numbers to µ 70 kev, µ = ((CT/000)+)*µ H2O,70 kev 3) Scale all µ values corresponding to CT values below by a factor of µ H2O,5 kev /µ H2O,70 kev 4) Scale all µ values corresponding to CT values above by a factor of µ bone,5 kev /µ bone,70 kev Used in Siemens/CTI Scanners hybrid segmentation CT Histogram of CT image, showing separation of tissue types [Kinahan et al.] Kinahan et al.,med Phys 25, (998) A Second Algorithm for Converting µ 70keV µ 5keV CT# 0 HU: µ PET = [((CT/000)+)*µ H 2O,80 kev]* µ H2O,5 kev µ H 2O,80 kev CT# > 0 HU: Equivalent to formula in Kinahan for CT# 0 HU µ PET = µ H2O,5 kev + (CT/000)*µ H 2O,80 kev * µ bone,5 kev - µ H 2O,5 kev µ bone,80 kev - µ H 2O,80 kev Tissue is treated as a mixture of air and water below 0 HU and a mixture of water and bone above 0 HU. GE Approach Burger et al. EJNM (2002) Comparison of Attenuation Correction Methods Attenuation Coefficient at 5 kev Hybrid Segmentation Approach Mixture Approach CT Number (HU) Exact forms depend on choice of parameters; for this plot Hybrid Segmentation threshold=200 HU PET/CT<200 = 0.5 PET/CT>200 = 0.4 Mixture threshold = 0 HU µ H2O,80 kev = µ bone,80 kev = µ H2O,5 = µ bone,5 kev =

Principal Artifacts Specific to PET/CT Artifacts due to fundamental failure of the attenuation

to patient extending outside the CT field of view Failure of the Attenuation Correction Model There is

don t know what we ve got or how much, only what the total attenuation is at CT energies Materials not

values give incorrect activities, too high or too low The Origin of Contrast and Metal Artifacts in

Materials 0.00 CT Energy 70 kev Soft Tissue Bone Barium Sulfate Sodium Iodide µ/ρ [ 2 /g].00 0.

tissues (including bone) due to the large photoelectric component; the scaling to 5 kev is different

PET Energy 5 kev 0 00 000 Energy [kev] Bolus of contrast in bowel generates apparent area of high

surrounding tissues Non- AC Artifacts from Hardware: Mediports Artifacts from Metal: Orthopedic

5 Principal Artifacts Specific to PET/CT Artifacts due to fundamental failure of the attenuation correction model Artifacts due to patient motion between the CT and PET scans Truncation artifacts due to patient extending outside the CT field of view Failure of the Attenuation Correction Model There is no way to intrinsically deconvolve the measured attenuation coefficients µ into µ µ = ρi i ρ i so we don t know what we ve got or how much, only what the total attenuation is at CT energies Materials not in the model will scale differently and give the wrong attenuation correction Incorrect attenuation values give incorrect activities, too high or too low The Origin of Contrast and Metal Artifacts in PET/CT Artifacts from Oral Contrast CT AC Mass Attenuation Coefficients for Tissue and Contrast Materials 0.00 CT Energy 70 kev Soft Tissue Bone Barium Sulfate Sodium Iodide µ/ρ [ 2 /g] The mass attenuation coefficients for contrast materials are significantly larger than those for tissues (including bone) due to the large photoelectric component; the scaling to 5 kev is different and the attenuation correction will be overestimated! PET Energy 5 kev Energy [kev] Bolus of contrast in bowel generates apparent area of high uptake on attenuationcorrected image Non-attenuationcorrected image reveals little contrast wrt surrounding tissues Non- AC Artifacts from Hardware: Mediports Artifacts from Metal: Orthopedic Hardware Intense activity shown on PET/CT (SUV = 6) is associated with metallic hardware having CT# > 3000HU Hot spot on PET Correlates to Mediport placement No anomalous uptake in nonattenuation corrected image

6 Artifacts from Metal: Orthopedic Hardware, continued Artifacts from Patient Motion Artifacts arise because the CT and PET scans are taken - at different times (i.e. sequentially) - on different time scales (seconds for CT, minutes for PET) Attenuation Corrected PET Non-attenuation Corrected PET Voluntary movements: Patient shifts position between CT scan and PET scan; principally movements of head and arms Involuntary movements: CT catches snapshot of patient position, but PET averages over minutes - CT is not normally done with breath hold (contrary to normal CT practice), but with shallow breathing - PET artifact not seen on conventional PET because both emission and transmission averaged over similar periods Breathing Artifact Breathing during helical CT acquisition can lead to floating liver artifact (variously called banana or mushroom artifact) which can then be reflected in PET scan. Quality Assurance Program: QA for PET -- Daily check per mfg with line or volume source to assure that system performance is not drifting Period Physics check per standard PET practice QA for CT -- Daily air cals and image quality check per mfg Period Physics check per standard CT practice Registration check -- check for proper registration of PET and CT images on periodic basis Examples: QA Procedures CPS (Siemens-CTI): Obtained with volume phantom (.5 mci 68Ge); suary report presented and logged; sinogram viewer for details. Gantry Offset Correction/Check: Phantom Geometry GE: Obtained with rotating line source in gantry. Report generated with fan-sum views and comparisons to previous trends.

Registration Test CT FUSION PET In our case, Siemens/CT Gantry Offset Procedure could be used to numerically evaluate x (horizontal) y(vertical) and z(axial) registration without modifying machine

5-3 Registration Stability 2/23 3/5 3/5 3/25 4/4 4/4 4/24 Date Z-900 X Y The reproducibility of the acquisition process was better than σ = 0.

7 Registration Test CT FUSION PET In our case, Siemens/CT Gantry Offset Procedure could be used to numerically evaluate x (horizontal) y(vertical) and z(axial) registration without modifying machine calibration. Results can also be evaluated visually or with other programs Stability of Registration: An Example Correction [] Registration Stability 2/23 3/5 3/5 3/25 4/4 4/4 4/24 Date Z-900 X Y The reproducibility of the acquisition process was better than σ = 0. (registration repeated without removing the dual line phantom) The reproducibility of the registration procedure (multiple replacement of the phantom) was better than σ = 0.4. The stability of the system over the first 45 days was characterized by σ = 0.5 or better for all 3 axes. Examples of PET Testing Tools Provided for PET/CT Machines CPS (Siemens, CTI) Uniformity and Sensitivity test software shipped with machine, based on volume 68 Ge source used for daily QC GE Will provide physicist with testing software (NEMA NU-2 200) Philips Will ship NEMA NU software with machine What Tests are Important? (You may get different answers from different folks!) NEMA NU Spatial resolution (xverse,axial) Scatter fraction Sensitivity Deadtime,count-rate losses Uniformity Scatter correction accuracy Count-rate correction accuracy Attenuation correction accuracy NEMA NU Spatial resolution Scatter fraction, count losses, randoms Sensitivity {now part of scatter fraction} {now part of image quality test} Count rate correction accuracy {now part of image quality test} Image quality (scatter, AC accuracy) 94 Scatter fraction test 94 Count rate tests Many of these tests were developed to compare classes of machines rather than to provide practical acceptance test procedures. NU2-94 may be more appropriate for brain scanners and NU2-0 for WB oncology scans. Workflow at the PET Center (FDG Whole Body Scans) Arrival of patient Pt instruction and prep min 0 min 5-30 min Release Pt Injection of Pt Uptake of pharmaceutical Have Pt empty bladder Transport Pt to scanner Position Pt Scan QA Check of Scan Read study Receive doses Assay of dose Print for file and referring; distribute to PACS * * * * steps with highest technologist exposure Workload Estimation Patient Number PET Facility Throughput Example: Hour Uptake, 30 Minute Scan 8:00 9:00 0:00 :00 2:00 3:00 4:00 5:00 6:00 7:00 #pts/day = (T work -T uptake )/T scan_rm Time of Day Phase Uptake Scanning # uptake rooms = T uptake /T scan_rm

8 Effective Dose Comparison: Conventional PET vs PET/CT Whole Body Scan, HR+ in 2D mode 2 mci 8 F FDG.3 rem transmission scan (average exposure of 56 mr on axis, free air, 6 bed scan, 8 min/bed,) Whole Body Scan, 0 mci 8 F FDG. rem WB CT scan (30 kvp, 20 mas, 5, p =.5) Total.3 rem 2.4 rem The End

CT-PET calibration : physical principles and operating procedures F.Bonutti. Faustino Bonutti Ph.D. Medical Physics, Udine University Hospital.

CT-PET calibration : physical principles and operating procedures Faustino Bonutti Ph.D. Medical Physics, Udine University Hospital Topics Introduction to PET physics F-18 production β + decay and annichilation