Gamma Camera Instrumentation and Quality Control Notes

Key Definitions in Nuclear Medicine Instrumentation

• Spatial Resolution: The specific ability of the gamma camera system to distinguish between two adjacent points within an organ or phantom.

• Energy Resolution: The ability of the camera to discriminate between pulses (photons) of different energy levels. This is crucial for separating the primary photopeak from scattered radiation.

• Temporal Resolution: The minimum period required for the acquisition of counts; essentially the system's ability to handle high count rates over time.

• Sensitivity: The capacity of the camera to detect photons generated by the radiopharmaceutical.

  • Low Sensitivity: The camera detects only a small fraction of the generated photons.

  • High Sensitivity: The camera detects a larger fraction of the generated photons.

  • Structural Influence: Sensitivity is increased by using a smaller number of larger holes in the collimator separated by thin septa.

• Full Width Half Maximum (FWHM): A common concept in nuclear medicine used to quantify resolution. A smaller $FWHM$ indicates better spatial resolution. It is measured on a graph of relative detected events ($N$) against photon energy ($keV$) or location (pixels).

Gamma Camera Components and Functionality

• Collimator: This component defines the lines of acceptance for photons. It prevents non-perpendicular (oblique) photons from reaching the detector crystal.

• Scintillation Crystal: Typically made of Sodium Iodide ($NaI$). When an incident photon hits the crystal, it causes a scintillation, which is the release of visible light photons.

• Lucite Light Pipe: Acts as optical fibers to transmit the light signal from the scintillation crystal to the Photomultiplier Tubes ($PMT$).

• Photodetector Array: Consists of a hexagonal arrangement of $30$ to $90$ Photomultiplier Tubes ($PMT$).

  • Photocathode: Detects the scintillation (light) and emits photoelectrons.

  • Dynodes: Electrons hit these to create a cascade effect, multiplying the number of electrons (amplification).

  • Anode: The point where the final amplified signal hits.

• Pulse Height Analyser (PHA) Circuitry: Determines the specific location ($x-y$ coordinates) and the energy of the incident photon.

• Pre-amplifier: Responsible for converting the electric current from the anode into a voltage signal.

• Correction Circuitry: Applies necessary adjustments for energy, spatial positioning, and uniformity to the final image.

• Computer: Processes signals and manages the digital matrix for image display.

Matrix Acquisition and Digital Resolution

• Anger Logic: The system calculates positions based on the distribution of light detected by the $PMT$ array.

• Digital Matrix Creation:

  • Step A: Calculated positions of a number of events are identified.

  • Step B: A matrix (e.g., $6 \times 6$) is superimposed onto these events to see which pixel each event fits into.

  • Step C: The number of events (dots) in each pixel is recorded and assigned a specific shade of grey or color.

• Resolution and Matrix Size:

  • A matrix cannot resolve points that are separated by less than $1$ pixel.

  • A higher-density matrix (e.g., $30 \times 30$ pixels) offers better resolution than a lower-density matrix (e.g., $10 \times 10$ pixels).

Gamma Camera Quality Control (QC)

• Definition of QC: A process involving equipment inspection to ensure satisfactory quality in all aspects. It comprises procedures to ensure products or services adhere to defined performance criteria.

• Photopeak and Energy Window:

  • The energy window must be centered on the photopeak (e.g., for $^{99m}Tc$ in air or a patient).

  • An off-center energy window can cause non-uniformity, appearing as "hot" or "cold" $PMT$s.

• Uniformity Flood (Intrinsic vs. Extrinsic):

  • Intrinsic Flood:

    • Setup: No collimator used.

    • Source: $^{99m}Tc$ point source ($18.5\,MBq$) in less than $1\,ml$.

    • Placement: Source placed at least $5$ times the detector width away from the camera face.

    • Frequency: Weekly or Monthly.

    • Count Requirement: $5$ to $30$ million counts.

  • Extrinsic Flood:

    • Setup: Collimator remains on.

    • Source: $^{57}Co$ flood source ($185-555\,MBq$).

    • Placement: Source placed directly on the detector.

    • Frequency: Daily.

    • Count Requirement: $10$ to $15$ million counts.

• Uniformity Results: Results are analyzed for the Full Field of View ($FFOV$) and Useful Field of View ($UFOV$). Trends should be monitored; an increase in percentage non-uniformity requires service personnel.

• Spatial Linearity: Performed monthly focusing on visual inspection of lines in a phantom. Lines should appear straight and unbroken. Requires $5-10 \times 10^6$ counts per image.

• Spatial Resolution (Bar Phantom): Performed monthly using a bar phantom. The quadrant with the closest bar spacing that can be distinguished measures the system resolution. Resolution decreases as distance from the camera increases.

• Intrinsic Spatial Resolution (Line Point Spread Function): Performed at installation or annually. Typical values are $3$ to $4\,mm$.

• SPECT Centre of Rotation (COR): Performed monthly (or weekly after installation). Corrects for misalignment between the electronic and mechanical centers of rotation.

• SPECT Uniformity:

  • Statistical variations can lead to "ring" or "bull's-eye" artifacts in transverse images.

  • Phantom: $^{99m}Tc$-filled cylinder ($20\,cm$ diameter by $20\,cm$ length).

  • Settings: $370\,MBq$, $64$ projections, and at least $40$ seconds per projection.

• Daily Maintenance Checks:

  • Visual and safety checks.

  • Background radiation test.

  • $CT$ Air Phantom (for hybrid systems, manufacturer dependent).

Collimators: Principles and Types

• Core Concepts:

  • Dimensions: Determined by hole diameter/bore, septum thickness, and hole length.

  • Hole/Bore Impact: For the same bore length, smaller diameters result in higher resolution. For the same hole diameter, longer bores result in higher resolution.

  • Septal Thickness: Thicker septa are required to block high-energy and medium-energy photons.

  • The Trade-off: There is a fundamental "trade-off" between sensitivity and spatial resolution. Greater sensitivity (accepting more photons) leads to decreased spatial resolution.

• Collimator Categories and Uses:

  • Low Energy All Purpose (LEAP) / Low Energy General Purpose (LEGP): Used for studies with low count rates or where high counts are needed quickly (e.g., dynamic renal or bone flow). High sensitivity, low resolution.

  • Low Energy High Resolution (LEHR): Commonly used for $^{99m}Tc$ studies when high resolution is required. Lower sensitivity than $LEAP$.

  • Medium Energy All Purpose (MEAP): Used for medium-energy isotopes like $^{67}Ga$ or $^{123}I$. Features thicker septa to prevent penetration.

  • High Energy General Purpose (HEGP): Used for high-energy isotopes like $^{131}I$. Features very thick septa.

  • Pinhole: Used to magnify small organs (thyroid, parathyroid, pediatric hips) over a large area of the detector to improve spatial resolution.

  • Diverging: Used to minimize an organ onto a small detector (e.g., on portable cameras with small fields of view).

  • Converging: Used to magnify small organs to a lesser extent than a pinhole.

  • Fanbeam: Specifically used for Brain $SPECT$ with $^{99m}Tc$. Holes focused toward a focal line parallel to the axis of rotation for improved resolution.

  • Slant-hole: Used to separate overlapping organs (e.g., heart and spleen).

Dose Calibrator Instrumentation and Operation

• Main Components:

  • Detector (Ionization Chamber).

  • High voltage source.

  • Amplifier / Electrometer.

  • Display unit/Power supply.

  • Plastic sleeve, syringe holder, and vial holder within lead shielding.

• Operation Mechanism:

  1. The sample (syringe or vial) is placed in the calibrator.

  2. An electrical potential exists between the electrodes (anode and cathode).

  3. Radioactivity causes ionization of Argon gas inside the sealed chamber.

  4. Positively charged gas molecules flow to the cathode; negatively charged molecules flow to the anode.

  5. This creates an ionization current proportional to the radioactivity of the isotope.

  6. An electrometer quantifies this current and displays it in Curies ($Ci$) or Becquerels ($Bq$).

• Correction Factors: Adjustable resistors or digital calibration factors ensure the device reads correctly for different isotopes with varying energies.

Dose Calibrator Quality Control (QC)

• Constancy:

  • Method: Use a long-lived reference source (e.g., $^{57}Co, ^{60}Co, ^{226}Ra, ^{137}Cs$).

  • Frequency: Daily (recorded at $24$-hour intervals).

  • Tolerance: Readings should agree within $\pm 10\%$.

• Linearity:

  • Method: Measure a high-dose $^{99m}Tc$ source (approx. $37\,GBq$) at $12$-hour intervals over three consecutive days.

  • Frequency: Annually.

  • Tolerance: Deviation greater than $10\%$ from the expected decay curve is significant.

• Accuracy:

  • Method: Calibration using a traceable national standard (e.g., certified source from ANSTO).

  • Frequency: Annually.

• Geometry Independence:

  • Method: Measure a sample (e.g., $^{99m}Tc$) starting at $1\,ml$ and increasing volume to $2, 3, 4, 5, 10\,ml$ in a syringe or vial.

  • Frequency: At installation and after major changes in sample geometry.

  • Tolerance: Readings should not vary by more than $\pm 2\%$.

Common Nuclear Medicine Isotopes

• $^{99m}Tc$: Low energy ($140\,keV$), most widely used.

• $^{131}I$: High energy.

• $^{123}I$: Medium energy.

• $^{67}Ga$: Medium energy.

• $^{201}Tl$: Low energy.

Questions & Discussion

• Exit Question: Explain the difference in both collimator and radioactive source used for intrinsic and extrinsic flood acquisition.

  • Response: For Intrinsic flood acquisition, no collimator is used, and a small volume (< 1\,ml) point source of $^{99m}Tc$ ($18.5\,MBq$) is placed at a distance. For Extrinsic flood acquisition, the collimator is left on, and a large flat $^{57}Co$ flood source ($185-555\,MBq$) is placed directly on the detector face.