MAP.18 Gamma camera and Positron Emission Tomography (PET)
Page 1: Introduction
Topic: Gamma Camera & Positron Emission Tomography (PET)
Presenter: Dr. Andy Ma
Page 2: Learning Outcomes
Requirement for radioactive measurement techniques
Principles of film badges, Geiger counters, photomultiplier tubes
Structure of a gamma camera and its application in nuclear imaging
Properties of an ideal radiopharmaceutical for radio-imaging
Thyroid 24-hour uptake test description
Principles of positron emission tomography (PET)
Definition of coincident gamma photons and lines, and their role in PET imaging
Overview of basic PET instrumentation
Page 3: Detecting Radiation
Human senses cannot directly detect radioactive decay; indirect measurement is required.
Key workers at risk of radiation exposure: Nuclear radiation workers, hospital staff.
Page 4: Film Badge Detector
Basics: A film badge indicates cumulative radiation exposure through film blackening.
Page 5: Film Badge Details
Contains filters of various thicknesses/materials to estimate radiation energy/wavelength.
National agencies monitor workers' radiation doses annually.
TLDs (thermoluminescence detectors): modern, reusable detectors more common in hospitals.
Page 6: Advanced Radiation Detectors
More complex detectors measure ionization from radiation rather than radiation itself.
Common detectors: Geiger-Muller (GM) tube, Scintillation counters.
Page 7: Geiger-Muller Tube Basics
Filled with gas (Helium, Neon, Argon).
High voltage electrode compared to outer casing.
Radiation enters through mica window, causing ionization.
Page 8: Geiger-Muller Tube Image
Diagrammatic representation of GM tube.
Page 9: Ionization Process in GM Tube
α/β/γ radiation causes gas ionization.
Electrons accelerate towards high voltage electrode and ionize other gas molecules.
Positive ions move towards tube walls, contributing to further ionization.
Results in a short, intense current pulse indicating an event.
Page 10: Quenching Process
After detection, discharge must be 'quenched' to allow further detection.
'Dead Time': ~100 to 500ms where no new events can be detected.
Quenching is essential as a single discharge prevents detection of subsequent events.
Page 11: Limitations of GM Tube
Does not differentiate between radiation types or provide specific radiation data.
Page 12: Scintillation Detection
Scintillation: small flashes of light from scintillators absorbing high energy radiation.
Scintillator examples: Sodium iodide (NaI) crystal fluoresces upon ionizing radiation impact.
Page 13: Photoelectron Production
Visible photons absorbed by PMT photocathode produce photoelectrons (photoelectric effect).
Page 14: Electron Cascade in PMT
Photoelectrons accelerate towards dynodes which emit more electrons upon impact.
Cascade effect leading to increased electron production.
Page 15: PMT Output
Accumulation of electrons at the anode results in a pulse indicating photon detection.
Commercial PMTs may have ~15 dynodes operating at high voltages (1000-2000V).
Page 16: Scintillation Counters in Imaging
Commonly used in Gamma Cameras for medical imaging.
Page 17: Structure of Gamma Camera
Contains large sodium iodide crystal (~45 cm dia., 1 cm thick) with PMT tubes arranged hexagonally.
Collimator made of lead with drilled holes for spatial discrimination of detected photons.
Page 18: Functional Map of Gamma Camera
Collimator determines exact location of γ-rays and generates activity maps.
Page 19: Function of PMTs in Gamma Camera
PMTs convert scintillation light into electronic signals and magnify them.
Position logic circuits process impulses to map scintillation events.
Page 20: Gamma Camera Imaging Process
A radiopharmaceutical is injected, absorbed by the target organ, and Gamma Camera detects its intensity.
Generates a 2-D image plotting radioactive intensity relative to position.
Page 21: Applications in Organ Function Studies
Enables monitoring of organ function over time, useful in kidney diagnostics (e.g. blockage detection).
Page 22: Full Body Scans with Gamma Cameras
Utilizes 99mTc for bone disease assessment. Increased uptake indicates abnormalities.
Page 23: Dynamic Organ Function Studies
Gamma Cameras measure organ functions such as kidneys, lungs, heart.
Ideal radionuclide properties include short life, low patient dosage, and non-toxicity.
Page 24: Characteristics of Technetium 99m
Half-life: 6 hours; emits low energy γ-ray (140 keV).
Flexible tracing capabilities due to incorporation into diverse molecules.
Page 25: Medically Useful Radioactive Isotopes
Summary of isotopes, their uses in diagnostics, and specific examples provided.
Page 26: Thyroid Gland Function
Utilizes iodine for hormone production; uptake varies with thyroid activity level.
Page 27: 24-hour Thyroid Uptake Test Process
8mCi of 123I administered orally; post-24 hours, counts measured and compared to a standard.
Ratio gives % 24-hour uptake.
Page 28: Interpretation of Thyroid Uptake Test
Determines thyroid activity; imaging example shows 'cold' nodule.
Page 29: PET Imaging Technology
Constructs images using co-aligned γ-rays from positron-electron annihilation.
Objective: 3-D visualization of radio-labeled drug distribution post-injection.
Page 30: Process of Positron Annihilation
Positrons produced through various means, annihilate with electrons to produce γ-rays.
Two identical 511 keV γ-rays emitted 180 degrees apart.
Page 31: PET Imaging Process
Illustration of positron annihilation producing γ-rays crucial for imaging.
Page 32: Coincidence Detection in PET
Detecting coincident γ-rays (simultaneous detection) for localization of the source.
New systems utilize time-of-flight data for improved localization accuracy.
Page 33: Image Reconstruction in PET
Software determines angular/linear coincidence events to reconstruct images based on annihilation activity.
Page 34: PET Scan Procedure
Short-lived radioactive tracer injected into subject; concentration in tissues involves a waiting period.
Page 35: Typical PET Radionuclides
Used in PET for metabolic processes: Carbon-11, Nitrogen-13, Oxygen-15, Fluorine-18 with short half-lives.
Page 36: PET in Metabolic Imaging
PET scanners excellent for metabolic information; often integrated with CT for comprehensive imaging.
Page 37: Co-registration of PET and CT Scans
Allows for anatomical correlation with metabolism data, enhancing diagnostic precision.
Page 38: Example of Co-registered PET-CT Images
Visual representation of combined data from PET and CT imaging modalities.
Page 39: Effectiveness of PET in Treatment Response
Comparison of pre and post-treatment imaging to assess effectiveness of chemotherapy.
Page 40: Comparison of Imaging Techniques
Key differences among PET, CT, and MRI in radiation type, information type, and scan duration.
Page 41: Summary of Imaging Technique Differences
Features of PET versus CT and MRI highlighted.
Page 42: Recap of Learning Outcomes
Review of expectations and learning objectives from the presentation.
Page 43: Conclusion
Thank you note from Dr. Andy Ma with contact information for further inquiries.