Gamma and SPECT Imaging

These flashcards cover key concepts, definitions, and comparisons related to gamma cameras, SPECT, and PET imaging techniques.

Gamma Camera

A device used to map the distribution of radionuclides in the body, showing organ function (physiology) rather than structure.

Gamma Rays

High-energy electromagnetic radiation emitted from radioactive tracers inside the patient.

Collimator

A device used in gamma cameras to determine photon direction, block scattered photons, and improve image resolution.

Scintillation Crystal

Converts gamma photons into visible light in a gamma camera.

Photomultiplier Tubes (PMTs)

Devices that convert light into electrical pulses and amplify them through dynodes.

Z Pulse

Represents the energy of the detected photon in a gamma camera.

Performance Factors of Collimators

Determined by hole size (d), length (L), and septal thickness (t).

SPECT

Single Photon Emission Computed Tomography, which constructs 3D images from multiple planar images.

Coincidence Detection

Counting only photon pairs detected simultaneously on opposite detectors in PET.

Attenuation

Absorption of photons in tissue, reducing the detected signal.

Energy Window

A range around the photopeak used to accept valid events and reject scattered photons.

Spatial Resolution

Ability to distinguish two close points in an image.

Sensitivity

Fraction of emitted photons detected; higher with thicker crystals and closer detectors.

How does Positron Emission Tomography (PET) work?

It detects two 511 keV photons emitted \sim180^\circ apart after positron annihilation.

SPECT vs PET

SPECT uses single photon detection while PET relies on coincidence detection of photon pairs, leading to different sensitivity and resolution.

Uniformity

The detector’s ability to produce a consistent response across its surface.

Non-colinearity

When the two annihilation photons aren’t emitted at exactly 180^\circ, causing slight blur in PET images.

Image Quality Metrics

Characterizes various attributes of images such as spatial resolution, energy resolution, and uniformity.

Scattering

Effects that cause photons to be mis-positioned, leading to reduced contrast in images.

What information do detectors need to know about radiation?

Energy, position, and amount (intensity).

What are the main parts of a gamma camera?

Collimator \to Scintillation Crystal \to Light Guide \to PMTs \to Electronics.

What material is most common for the scintillation crystal?

NaI(Tl) — Sodium Iodide doped with Thallium.

What determines where the gamma ray came from?

Relative signal strengths from multiple PMTs.

Why are collimators needed?

Gamma rays can’t be focused; collimators absorb photons not traveling in the correct direction.

What trade-off exists in collimator design?

Better resolution = lower sensitivity.

How does hole size affect image quality?

Smaller holes improve resolution but reduce sensitivity.

How does length affect image quality?

Longer holes improve resolution but reduce sensitivity.

What material are collimators made of?

Lead.

What happens if septa are too thin?

Scattered photons penetrate adjacent holes, lowering contrast.

Match collimator type with image property:

• Parallel-hole: True size, not inverted
• Converging: Magnified image
• Diverging: Reduced image
• Pin-hole: Magnified & inverted image

What does SPECT stand for?

Single Photon Emission Computed Tomography.

How does SPECT work?

Gamma camera rotates around patient to collect multiple planar images \to computer reconstructs 3D image.

What types of collimators are used in SPECT?

Fan-beam and cone-beam for small field-of-view (brain, heart).

Why is 180^\circ acquisition used in cardiac SPECT?

Because of the heart’s position in the chest; reduces attenuation artifacts.

What is the benefit of SPECT/CT?

Combines functional (SPECT) and anatomical (CT) info, allowing attenuation correction and precise localization.

What does PET stand for?

Positron Emission Tomography.

Why doesn’t PET need a collimator?

Direction is determined electronically from coincident detections.

What affects PET spatial resolution?

Detector size, positron range, non-colinearity, and depth of interaction.

Why must PET crystals have a high attenuation coefficient?

To efficiently stop 511 keV photons.

Common PET scintillators and features:

• BGO: Light Yield 7k, Decay 300 ns, Notes: High efficiency, slow
• LSO/LYSO: Light Yield 25k, Decay 42 ns, Notes: Fast, good resolution
• LaBr_3: Light Yield 60k, Decay 27 ns, Notes: Excellent timing
• LuI_3: Light Yield 100k, Decay 30 ns, Notes: Highest light yield

Why is LSO often used in modern PET?

Fast decay and high stopping power \to better timing & spatial resolution.

Compare SPECT vs PET based on key properties:

• Photon energy: SPECT 100-300 keV, PET 511 keV
• Detection: SPECT Single photon, PET Coincidence pair
• Collimator: SPECT Physical lead, PET Electronic
• Sensitivity: SPECT Low, PET High
• Resolution: SPECT \sim8-10 mm, PET \sim4-6 mm
• Quantification: SPECT Relative, PET Absolute
• Main Isotopes: SPECT Tc-99m, I-123, PET F-18, C-11, O-15

Compare Scintillation vs Semiconductor Detectors:

• Conversion: Scintillator (NaI:Tl) \gamma \to light \to electrons, Semiconductor (CZT, CdTe) \gamma \to electrons directly
• Energy Resolution: Scintillator \sim10\%, Semiconductor <5\%
• Temperature Sensitivity: Scintillator High, Semiconductor Low
• Fragility: Scintillator Fragile, Semiconductor Rugged
• Advantage: Scintillator High efficiency, Semiconductor Better resolution
• Example: Scintillator Anger Camera, Semiconductor D-SPECT CZT camera

What defines energy resolution?

Ability to distinguish photons of different energies.

What improves spatial resolution?

Smaller detector elements, thinner crystals, better collimation.

What improves energy resolution?

Semiconductor detectors and precise electronics.

What percent of photons can be scattered even with a narrow energy window?

Up to 40\%

What is an energy window?

A range around the photopeak used to accept valid events (e.g., 140 keV \pm 10\%).

Why are energy windows used?

To reject scattered photons and improve contrast.

What factors affect SPECT spatial resolution?

Collimator geometry, detector size, photon energy, and distance from source.

What factors affect PET spatial resolution? (Section 11)

Positron range, non-colinearity, detector thickness, and geometry.

Explain a gamma camera in simple terms.

Detects gamma rays, converts to light in a crystal, light converted to electronic signal, mapped to create image.

What is the main advantage of PET over SPECT?

Higher sensitivity and resolution due to coincidence detection.

What is the main advantage of SPECT/CT?

Combines anatomical precision with functional detail.

What happens when scattered photons enter the detector?

They reduce image contrast and spatial accuracy.