Quality Control of Gamma Cameras

Quality Control of Gamma Cameras

Objectives

• Understand the importance of quality control (QC) for gamma cameras.

• List and discuss quality control procedures required for the scintillation camera system.

• Analyze QC images.

• Describe QC trigger levels and appropriate actions.

• Recognize the most common QC artifacts.

Quality Assurance

Procedural Quality Assurance

• Examines aspects of quality assurance throughout the entire timeline of a nuclear medicine procedure.

Departmental Quality Assurance

• Examines outcomes and promotes continuous improvement of departmental practices.

Quality Assurance Program

• Overall plan for addressing all quality control and standardization.

• Uncover defects with instrumentation that impact image quality.

• Prevent procedural mistakes.

• Standardize imaging protocols including instrumental acceptance testing, preventative maintenance, and quality control procedures.

  • Includes protocols, acceptable limits, and protocols for abnormal results.

Quality Assurance Programs

Accreditation Agencies

• JCAHO - The Joint Commission

• ACR - American College of Radiology

• IAC/ICANL - Intersocietal Accreditation Commission

• RadSite

• Federal Regulation Agencies

  • NRC - Nuclear Regulatory Commission

  • FDA - Food and Drug Administration

  • DOT - Department of Transportation

  • State Regulators - Radiologic Licensing/Health Department

Routine Quality Control

Characteristics of a Good Quality Control Program

• Consistency in acquisition, analysis, and utilization of results.

• Quantitation of results where possible.

• Action levels that trigger corrective measures.

• Awareness of common problems encountered and how to deal with them.

Daily Quality Control

• Visual Checks

• Motion Touch-Pad

• Background

• Peaking and Window Settings

• Uniformity

Visual Inspection

• Conducted daily and before each patient study.

• Visually check the camera for damage to the collimator.

• Check wires and electrical cords for mechanical or electrical damage.

• If any damage is observed to electrical components, do not use the equipment that day.

Touch-Pad Test

• Testing the motion sensors.

• Lightly tap on each one and ensure it stops and has an audible error.

Background Check

• Operational check with and without collimators with more than one energy window.

• Assesses possible contamination on or around the camera.

• Usually not an image; can be seen while setting up for the uniformity (daily flood).

Peaking Procedure

• Performed daily; verifies alignment of energy window with the gamma ray photopeak.

• Do not peak using the patient as the radiation source, as this can cause too much scatter.

  • Bar Phantoms

  • Performed weekly.

  • Provides semi-quantitative evaluation of spatial resolution and linearity.

Peaking and Energy Window

• Performed daily, verifies alignment of energy window with gamma ray photopeak.

• Some systems are automated.

• Ensure preset energy windows are centered around the energy of the photopeak (e.g., for I-123 centered around 159 keV).

• Peaking Procedure:

  • Can be intrinsic (no collimator) or extrinsic (scatter-free source).

  • Should cover the entire field of view (FOV), though this may not be practical.

  • Peak adjustments should be made every morning based on all radionuclides being used that day.

  • Modern systems may utilize auto-peaking where the peak is split and adjusted until counts on either side are equal.

  • Energy resolution should be compared with manufacturer's guidelines and may fluctuate.

  • Multi-head cameras require greater adjustments due to the need for synchronization among heads.

Uniformity Flood

• Performed daily, verifies the functionality of the gamma camera.

• Can be intrinsic (no collimator) or extrinsic (with collimator).

• Ensure no extraneous radiation sources in the area during testing.

• Measure integral and differential uniformity with the formula: \text{Uniformity} = \frac{\text{max cts/pixel} - \text{min cts/pixel}}{\text{max cts/pixel} + \text{min cts/pixel}}

  • Expected uniformity should be 100%.

Flood Sources

• Introduces various flood sources such as Cobalt-57 sheet sources and 99mTc fluid-filled phantoms.

Uniformity Flood Protocol

• Extrinsic:

  • Place the flood source directly on the collimator.

  • Select daily flood protocol.

  • Check background and peaking/window settings.

  • Acquire an image for 30 million counts.

  • Assess images for non-uniformity and store accordingly.

• Intrinsic:

  • Remove the collimator and follow the same protocol as extrinsic.

NM Instrumentation

• Field Uniformity: Measured across the entire detector surface, including the Useful Field of View (UFOV, 95% of the detector) and the Central Field of View (CFOV, 75% of the UFOV).

Daily Extrinsic Flood Example

• Testing date: 041808 4/18/2008.

• Detector acquisitions:

  • Detector 1: 10072K counts, duration: 529 sec, pixel size: 256x256, phantom: Co-57.

  • Detector 2: 10044K counts, duration: 529 sec, pixel size: 256x256, phantom: Co-57.

Sensitivity

• Performed daily alongside uniformity checks.

• Measures count rate per unit activity (cps/MBq).

Bar Phantoms

• Executed weekly to evaluate spatial resolution and linearity:

  • Spatial Resolution: Ability to accurately see separate sources.

  • Spatial Linearity: Ability of the camera to determine the photon location accurately without displacement.

  • Purpose is to detect gradual long-term deterioration of both spatial resolution and spatial linearity.

Bar Phantom Types

• Lead Bars: Embedded in parallel stripes, with equal width per quadrant.

  • Available dimensions include 2, 2.5, 3, and 3.5 mm.

  • Phantoms for spatial resolution should match system resolution; at least one set should be too small to be visible.

• Hine-Duley Phantom: Checks for intrinsic resolution, spatial resolution, and spatial linearity.

• Parallel Line Equal Space (PLES) Phantom: Measures linearity but is not great for spatial resolution.

• Orthogonal Hole Phantom: Assesses intrinsic resolution and field uniformity.

Off-Peak Energy Windows

• Asymmetric energy windows showing varying uniformity percentage depending on the energy settings.

High-Count Flood

• High count floods (30-120 million counts) should be performed based on manufacturer’s recommendations, generally done weekly, monthly, or quarterly.

  • Quantifies integral uniformity and differential uniformity for both UFOV and CFOV with the following formulas:
    \text{Integral Uniformity} = \frac{\text{max over entire image}}{\text{total counts}} \times 100\%
    \text{Differential Uniformity} = \frac{\text{max over 5-pixel region}}{\text{total counts}} \times 100\%

  • Action Level: 5% for differential uniformity.

High Count Flood and Uniformity Correction

• Purpose is to verify field uniformity and provide uniformity/sensitivity corrections.

• Corrects for non-uniformities in the detector and collimator.

• Usually acquired for 100 million counts, but this can vary by the system.

Nonroutine Quality Control Tests

• Pixel Size Determination: Measures pixel size; can also be checked by FOV and matrix.

• Sensitivity Check: Measured extrinsically with units as cpm/µCi, typically checked quarterly or biannually.

• Collimator Integrity: Assesses for non-parallel collimator holes; examines under varying frequencies based on condition.

• Multiple Window Spatial Registration: Evaluates gamma rays in different energy windows to ensure proper imaging.

Pixel Size Determination Procedure

1. Draw two 10 cm lines at right angles on cardboard.

2. Place a drop of activity at the end of each line.

3. Position on the camera and image using a chux.

4. Generate and measure count profile in pixels (should be < 0.5% between camera heads).

Sensitivity Check Procedure

• Measured separately using a Co-57 sheet source or 1-2 mCi 99mTc.

• Background counts are subtracted to find sensitivity using the formulas provided.

Collimator Integrity Procedure

• Assess symmetry using a 20mCi 99mTc point source

• Acquire images at various counts checking for symmetry across the collimator field.

Multiple Window Spatial Registration Procedure

• Drops of multiple energy radionuclides are placed on a cardboard phantom.

• Each energy window is imaged separately.

• Subtract images should superimpose correctly for verification.

Acceptance Testing for New Systems

• NEMA Standards: Define performance measures and the evaluation methods for gamma cameras.

• Acceptance Testing: Verifies whether a newly installed system meets vendor specifications.

• Benchmarking: Provides initial quality control results for future comparisons.

Typical Performance Measures QC Tests

Recommended Testing Frequency

• Peaking: Daily

• Field Uniformity (99mTc): Daily

• Other Radionuclides: Quarterly

• System Uniformity: Semiannually to Weekly

• Spatial Resolution and Linearity Dead Time/Max Count Rate: Weekly

• Multiple Window Spatial Registration: Quarterly to Annually

Troubleshooting Gamma Cameras

• Common Issues:

  • Moiré patterns, wrong photopeak, PMT malfunction, cracked crystal, mis-calibration, etc.

• Example images illustrating Moiré patterns affecting perception of bar orientation due to improper settings.

Summary

• Gamma cameras are highly complex instruments requiring constant vigilance to ensure correct functionality.

• Quality control is not only about performing tests but also involves evaluating results, understanding them, and taking action based on outcomes.

Gamma Camera QC Math

Window Calculations

• Counts: 99mTc scatter (Compton edge), backscatter, Pb X-ray peak, iodine escape peak.

• Energy FWHM: 140.5 keV

• Energy – LLD, ULD:

  • Centerline calculation for window: \text{energies in the window} = \text{centerline energy} \pm \text{energy in keV}

Example Calculations

• 20% Window Calculation for 99mTc:

  • \text{energies in the window} = 140 \text{ keV} \pm (140 \text{ keV} \times 0.2)

• Window Width: Maximum keV - Minimum keV

• For I-123 (159 keV): \text{Window Width} = 175 \text{ keV} - 143 \text{ keV} = 32 \text{ keV}

Energy Resolution

• Quantitative measurement of photopeak sharpness and extent of peak broadening.

• General causes of poor energy resolution include inconsistencies between PM tubes, crystal breakdown, and poor PM tube to crystal coupling.

FWHM Calculation Example

1. Find the maximum counts at the height of the photopeak.

2. Measure half-max counts and the width at half-max.

3. Determine energy spread: upper limit - lower limit.

Energy Resolution Example

• The acceptable energy resolution for 99mTc determined through multiple measurements, demonstrating ideal ranges for functional outputs of gamma cameras.

Matrix and Pixel Calculations

• Matrix Sizes: 64x64, 128x128, 256x256, 512x512, 1024x1024, and 2048x2048 with implications on sensitivity and resolution ratios.

• Total Pixels: Determined by multiplying height and width in pixels.

Pixel Size Implications

• Recommendation: Pixel size should typically not exceed the intrinsic resolution (~3 mm) to maintain clarity and sensitivity.

Camera Sensitivity Formula

• \text{Sensitivity} = \frac{\text{source cpm} - \text{background cpm}}{\text{source activity in } \mu Ci}

• Example sensitivity calculations with corresponding inputs and outputs.

Conclusion

• Constant monitoring and adjustment of gamma camera parameters and variables are crucial for accurate operations in clinical settings.

• Implementation of structured QC methodologies alongside routine checks serves as a foundation for ensuring quality in nuclear imaging procedures.