PHYSICS: WEEK 7

Scintillation Camera Performance Characteristics

What is the purpose of a gamma camera in nuclear medicine?

• Images the distribution of radiopharmaceuticals in the body

• Provides mainly functional, rather than anatomical, information

• Ideal gamma-ray energy range = 100–200 keV

• 99mTc is the most commonly used radionuclide

• Camera head contains a NaI(Tl) crystal + photomultiplier tubes (PMTs)

What are the major gamma-camera components and their functions?

• Collimator: only allows gamma rays travelling in appropriate directions to reach detector

• NaI(Tl) crystal: gamma ray → light

• PMTs: light → electrical signal + amplification

• X-Y positioning circuit: determines where interaction occurred in crystal

• Pulse Height Analyzer (PHA): accepts/rejects events based on their energy

What is the function of the collimator?

• Lead collimator sits between patient and crystal

• Holes allow appropriately directed photons to reach the crystal

• Septa (lead walls) absorb unwanted photons/scatter

• Essential for good spatial information and SNR

• Spatial resolution worsens as object-to-collimator distance increases

• Main types:

  • Parallel-hole

  • Pinhole

  • Converging

  • Diverging

How does the positioning circuit locate an interaction?

• Gamma ray interacts with NaI(Tl) crystal → light

• Light reaches multiple PMTs

• PMTs produce electrical signals proportional to the amount of light received

• Positioning circuit compares the signal intensities

• Uses these to determine the X-Y position of the interaction

• E.g. equal signal from two PMTs → interaction occurred approximately midway between them

What is the function of the PHA?

• Acts as an energy discrimination circuit

• Accepts photons within a selected energy window

• Rejects photons outside this window, particularly lower-energy Compton scatter

• Example for 99mTc: photopeak around 140 keV

• Example window from lecture: 140 ± 10 keV

• Helps prevent scattered photons from degrading the image

What basic characteristics are used to assess scintillation-camera performance?

• Intrinsic spatial resolution

• Collimator resolution

• Detection efficiency/sensitivity

• Energy resolution

• High count-rate performance

• Uniformity is also an important performance/QC consideration

What is spatial resolution and how can it be measured?

• Ability to distinguish small objects that are close together

• Commonly described using:

  • FWHM – Full Width at Half Maximum

  • MTF – Modulation Transfer Function

• Smaller FWHM = better spatial resolution

• MTF describes how well different spatial frequencies/detail are transferred into the image

How do intrinsic and collimator resolution affect overall system resolution?

• Overall system resolution depends on both:

  • Intrinsic resolution

  • Collimator resolution

• These combine to determine the final resolution of the imaging system

• Therefore, poor resolution from either component worsens overall image resolution

What is intrinsic spatial resolution and what affects it?

• Resolution of the camera/detector itself, independent of the collimator

• Mainly limited by:

  • Multiple scattering of photons within detector

  • Statistical fluctuations in distribution of light between PMTs

• As gamma-ray energy increases → intrinsic resolution improves (FWHM decreases)

• As crystal thickness increases → intrinsic resolution worsens (FWHM increases)

What happens to collimator resolution as the source moves away from the collimator?

• Increasing source-to-collimator distance → poorer spatial resolution

• Point-spread function becomes broader

• FWHM therefore increases

• Practical takeaway: position the detector as close to the patient as possible for better resolution

What is gamma-camera sensitivity?

• Ability of the imaging device to effectively use/detect photons from the object

• Usually expressed as counts per second per MBq (cps/MBq)

• Can be measured as:

  • Point sensitivity – point source

  • Line sensitivity – long line source

  • Plane sensitivity – large uniform plane source

What is septal penetration?

• Occurs when gamma photons penetrate through the lead septa rather than being absorbed

• Allows photons travelling in an unwanted direction to reach detector

• Can therefore reduce accurate spatial localisation and degrade image quality

• More of a problem when photons have enough energy to penetrate the septa

The diagram on p. 33 shows a gamma ray passing through the septum into an adjacent collimator hole

What does gamma-camera uniformity mean?

• Ability of the scintillation camera to reproduce a uniform radioactive distribution uniformly

• A uniform source should therefore produce an image with relatively even intensity

• Non-uniformities can appear as areas with artificially high/low counts

• Uniformity corrections improve the consistency of the image

What is dead time and how does it affect gamma-camera performance?

• Dead time: short period after detecting an event when the system is still processing it

• During this period, further events may not be counted

• Causes count losses at high activity/count rates

• Ideal dead time = 1–2 μs

• Clinically ≈ 10–15 μs

• In a paralyzable system, events arriving while the previous signal is being processed are lost

What is pulse pile-up and what problems can it cause?

• Occurs when two events are detected too close together in time

• Signals can overlap and be treated incorrectly

• Can cause:

  • Counting losses – combined pulse may fall outside accepted energy window

  • Mispositioning – event may appear somewhere between the two true locations

• Becomes more important at high count rates

What is the purpose of scintillation-camera QC?

• Ensures images/results remain accurate and reproducible

• Checks performance remains within a predefined acceptable range

• Acceptance testing: performed on new equipment

  • Checks equipment meets specifications

  • Establishes baseline performance

• Routine QC: performed regularly afterwards

  • Ensures equipment continues to perform correctly

How can spatial resolution be assessed?

• Can use quadrant parallel lead bars

• Bar patterns have different:

  • Thicknesses

  • Distances/separations

• Ability to distinguish increasingly fine bars indicates the camera's spatial resolution

• Can assess intrinsic resolution (camera itself) or extrinsic resolution (system including collimator)