Introduction to PET/CT in Oncology: Practical Aspects
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Transcript of Introduction to PET/CT in Oncology: Practical Aspects
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Introduction to PET/CT in Oncology:Practical Aspects
Jeffrey T. Yap, PhD
Department of ImagingDana-Farber Cancer Institute
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Outline
• Image registration and fusion• Visualization of PET and CT images• Acquisition considerations• Potential artifacts• Considerations for Quantitative Imaging• Standardized Uptake Value (SUV)
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Whole-body PET/CT protocol
Clinical Review
CT TopogramSpiral CT WB PET Fused PET/CT
Patient prep18FDG injectionUptake period
ROI Definitio
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SUV Analysis
CT PET
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Definitions
• Image registration: Process of matching the spatial coordinates between two or more images
• Image fusion: Process of combining multiple images of a scene to obtain a single composite image
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Image registration: the problem
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Image registration: zoom
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Image registration: rotate
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Image registration: translate
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Image registration: complete
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2D display methods
• 2 dimensional reconstructed planes: transaxial, sagittal, coronal, oblique
• Gray scale versus color tables• Windowing (contrast enhancement)– PET: linear scale specified by min & max– CT: linear scale specified by center and width
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PET windowing:Bottom = 0, Top = 10
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PET windowing:Bottom = 2, Top = 10
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PET windowing:Bottom = 2, Top = 8
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CT windowing notation:Center = 5, Width = 10
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CT Windowing
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PET/CT Fusion Display
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3D display methods• Maximum intensity projection: Displays the
maximum intensity along each projection• Shaded surface: Displays lighted surface of
segmented voxels • Volume rendering: Displays multiple rendered
objects with various transparencies and color tables
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Maximum Intensity Projection (MIP)
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3D Display: Shaded Surface (SSD)
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3D Display: Volume Rendering Technique
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Acquisition Considerations
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Options with CT Acquisition
• Breathing protocols• Arms up for thorax/abdomen• Arms down for head/neck• Dose options: attenuation correction only,
anatomical localization, full diagnostic• I.V. and/or oral contrast• Gated (cardiac or respiratory)• Dynamic (e.g. perfusion imaging)
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Options with PET acquisition• Static• Whole body (multiple bed positions)– Step-and-shoot– Continuous bed movement
• List-mode• Dynamic• Gated (cardiac and/or respiratory)
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Potential Artifacts
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CT-Based attenuation artifacts: respiratory motion
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Arms downArms down Arms upArms upArms downArms down Arms upArms up
PET image qualityPET image quality
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Intravenous contrast mediaIntravenous contrast media
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Chemotherapy port artifact
CT PET PET/CT
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Movement artifact
CT PET Uncorrected PET
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Considerations for Quantitative Imaging
• Calibration of all instrumentation is required at commissioning and regular intervals (PET/CT scanners, dose calibrators, scales, clocks)
• Consistent patient preparation is critical (e.g. fasting)
• Technical acquisition should be standardized and critical parameters should be recorded
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Hardware/Software Requirements for Accurate SUV Quantification
• Dose calibrator accuracy – traceable standard• Scanner normalization (detector efficiency)• Scanner calibration• PET corrections: attenuation, scatter, randoms,
decay (images and doses)• Partial volume correction for small objects• Appropriate reconstruction algorithm• Daily/weekly/monthly scanner QC
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Requirements for Reproducible SUV Quantification
• PET technique: 18FDG dose, 18FDG uptake period, emission scan length, scanning range, scanning direction (e.g. head to toe)
• Patient preparation: fasting, resting, medication• Reconstruction parameters: slice thickness,
filters• Region-of-interest definition methods (mostly
manual or semi-automated)• Consistency is the most important factor!
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Mandatory measurements• Acquisition parameters• Patient height, weight• Injected activity, residual, and time• Circulating glucose• Infiltrated doses• Patient compliance (e.g. fasting state, movement)• Protocol deviations– Injection time/scan delays – Injected activity
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Standardized Uptake Value
• Under certain circumstances, 18FDG SUV correlates with metabolic rate of glucose and/or the number of viable tumor cells
• Simplified semi-quantitative measure that can be routinely performed in clinical PET studies
• Adjusts for differences in patient size and injected activity
SUV (time) = Radioactive Concentration x Weight Injected Activity
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SUV Units
• Assuming the following:– water-equivalent tissue– a body weight correction in grams– decay-correction to the time of injection– Concentration in consistent units of mCi/ml or MBq/cc
• The SUV is a unit-less quantity• The SUV has a value of 1 if the radiotracer is
uniformly distributed
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SUV Example
• Consider 0.8 ml volume containing 12 mCi of 18FDG is “injected” into a 1.5 liter volume of water
• SUV = Radioactive Conc. x Weight Injected Activity
• SUV = (12 mCi / 1500 ml) * 1500 g (1 ml/g) 12 mCi
SUV = 1
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Variations in SUV
• Use of body surface area or lean body mass instead of weight
• Sometimes denoted with subscripts (SUVBW, SUVBSA, SUVLBM)
• Linear correction for circulating glucose• Designated uptake period – delayed scanning
may reduce background physiologic uptake• Various statistics: mean, max, peak
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Histogram of percentage residual FDG activity(N = 10,680 FDG injections)
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Stdev = 2.29Min = 0.00%Max = 41.4%
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Histogram of Coefficient of Variation in weight for patients with repeat visits (N = 2275 patients)
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The Good News
• Most differences in scanner hardware and reconstruction software cancel out in longitudinal studies of the same patient
• Although there is no universal cutoff, the SUV can help differentiate malignancy from normal tissue
• Changes in SUV after therapy have been shown to correlate with clinical outcome (e.g. survival)