wk5b quality control

Learning Outcomes

• Interpret the meaning of quality control.

• Outline the quality control (QC) checks required for the dose calibrator.

• Describe the quality control checks required for the gamma camera, PET camera, and the CT component of both SPECT/CT and PET/CT cameras.

Quality Control (QC) vs. Quality Assurance (QA)

• Quality Control (QC):     * Defined as all activities designed to determine the level of quality delivered.     * Primary focus is on detecting changes in equipment performance from a baseline condition.     * Requires the meticulous keeping of records.

• Quality Assurance (QA):     * Focussed on planning, documenting, and agreeing on a set of guidelines that are necessary to assure quality.     * Primary Goal: To ensure all procedures are appropriate and all images provide the necessary information for the specific clinical situation.     * Secondary Goal: To minimise patient radiation exposure, limit inconvenience, and optimise costs.

Definition and Importance of Quality Control (QC)

• QC involves routine assessments of equipment performance, including:     * Dose calibrators.     * Gamma cameras.     * PET cameras.     * CT components of hybrid systems.

• Objective: To ensure that findings on a patient scan have their origins in the patient and are not artefacts generated by the gamma/PET camera itself.

• Accuracy: To ensure that the dose displayed on the dose calibrator is accurate.

• Technologist Role: The technologist must be able to recognise artefacts arising from camera malfunctions or errors occurring due to dose calibrator malfunctions.

• Mandatory Frequency: QC must be performed on each system every morning before injecting any patient.

Dose Calibrator QC Checks

• Purpose: Performed to ensure the correct amount of radiation dose is administered to patients.

• Frequency: Performed daily to ensure readings remain constant from day to day.

• Measurement Source: A long-lived radioactive source, typically Cesium-137 ($^{137}Cs$), is used to confirm the constancy of the response.

• Protocol: The source is measured on all radionuclide settings used clinically, and results are recorded to make any drift noticeable.

Dose Calibrator QC Schedule

• High Voltage:     * Frequency: Daily.     * Purpose: To check constancy and ensure the correct operating voltage.

• Zero Adjustment:     * Frequency: Daily.     * Purpose: To check that the readout is at $0$ when no activity is present.

• Background/Contamination Check:     * Frequency: Daily.     * Purpose: To detect any contamination.

• Constancy:     * Frequency: Daily.     * Purpose: To check the stability and reproducibility of the ionisation chamber, electrometer, and calibrator nuclide settings.

• Accuracy:     * Frequency: Yearly.     * Purpose: To check the accuracy of the activity reading.

Gamma Camera Quality Control

• Many factors within a gamma camera contribute to the final image; a failure in any single factor will affect image quality.

• Schedule Dependencies:     * The manufacturer's specifications.     * The needs of the individual clinical department.     * National and International standards.

Ideal QC Schedule

• Daily:     * Visual Inspection.     * Background Check.     * Energy Window/Peak.     * Uniformity Flood.     * CT Tube Warm-up.

• Weekly:     * High Count Uniformity Flood.     * Spatial Resolution.     * Linearity.

• Monthly or Longer:     * Centre of Rotation (COR).     * CT Calibrations.

Specific Gamma Camera QC Procedures

Visual Inspection

• Mechanical damage check.

• Electrical defects check.

• Touch pad test.

• Background check conducted without the collimator on.

Sources Used for QC

• Point Source:     * $10 - 20\,MBq$ of Technetium-99m ($^{99m}Tc$).     * Source positioned at least $4 \times$ the diameter of the crystal away from the crystal.     * Provides a uniform signal across the detector.     * Acquire image for $3 - 5 \text{ million counts}$.     * Placement varies by manufacturer (Siemens uses a small plastic container; GE uses a needle cap or syringe).     * The source must not be fragmented.

• Sheet Source:     * A solid sheet source.     * Uses a long-lived radionuclide like Cobalt-57 ($^{57}Co$).     * $^{57}Co$ Specifications: $T_{1/2} = 271\text{ days}$; energy of $120\,keV$ gamma.

• Flood Tank:     * A Perspex tank filled with water.     * Approximately $500\,MBq$ of $^{99m}Tc$ is added.     * Must be mixed thoroughly and cleared of air bubbles.

Photopeak Setting

• Performed daily, usually automatically by a pre-set program.

• Goal: Verify that the camera is properly peaked (the photopeak of the radionuclide coincides with the preset photopeak energy window).

• Usually performed with the same point source used for uniformity checks.

• Consequences of Incorrect Settings: Degraded uniformity, reduced sensitivity, and increased scatter contribution to the image.

• Causes of Photopeak Drift: Variations in temperature, high voltage changes, and photomultiplier tube (PMT) drift.

Uniformity (Daily Flood)

• Performed daily.

• Checks if the detector response to a uniform irradiation is uniform within defined limits.

• Requires both visual and quantitative checks.

• Slight variances in camera components (PMTs, electronics) add up to determine overall uniformity.

• Images are saved to monitor changes over time and for uniformity correction.

• Types:     * Intrinsic Uniformity: Performed with the collimator removed, usually using a point source (approx. $18\,MBq$).     * Extrinsic Uniformity: Performed with the collimator on, using a sheet source or flood tank.

• Causes of Non-Uniformity:     * Energy window not peaked properly.     * PMT malfunction (seen as non-functioning tubes in images).     * Cracked crystal (visible as distinct breaks in the flood image).     * Software errors.     * Collimator damage (visible in extrinsic floods as septal damage).     * Poor source preparation (e.g., inadequate mixing of a flood tank).

High Count Flood

• SPECT Sensitivity: SPECT images are highly sensitive to small degrees of non-uniformity in the flood field that might not affect planar images.

• Non-Uniformity Amplification: A $5\%$ non-uniformity in planar images can increase to a $35\%$ non-uniformity in SPECT (which is unacceptable).

• Failure Consequence: Results in "Bullseye" or Ring Artefacts (concentric rings of alternating high and low count densities in transaxial images).

• Frequency: Weekly or monthly.

• Count Requirement: $50 - 100 \text{ million counts}$ (compared to the $10 - 15 \text{ million}$ used for daily floods).

• Method: Can be intrinsic or extrinsic; data used for uniform correction.

Centre of Rotation (COR)

• Definition: The single point around which detectors rotate. Ideally, this should match the centre of projections recorded by detectors at all angles.

• Critical Alignment: The electronic COR must align with the mechanical COR for accurate image generation.

• Causes of Shift: Electronic malfunction (most common) or mechanical malfunction.

• Consequences of Misalignment:     * Image distortion.     * Loss of resolution.     * Ring artefacts.     * Specific Visuals: Small misalignment causes blurred images; large misalignment results in a "doughnut" appearance.

• Frequency: Monthly on SPECT systems (less frequent on newer models).

• Protocol:     * Performed for each camera configuration ($76^\circ$, $90^\circ$, and $180^\circ$).     * Performed for each collimator used (LEHR, medium energy, high energy).     * Uses $1 - 5 \times$ $^{99m}Tc$ point sources of very similar activities.     * Software programs correct for pixel misalignment.

Spatial Resolution

• Definition: The ability of a system to accurately depict two separate events as two separate entities in space, time, or energy.

• Spatial Resolution Metric: The smallest distance between two radioactive sources where they can still be defined as two separate sources. If too close, the system resolves them as one irregular object.

• Frequency: Monthly (or longer for newer cameras).

• Method: Extrinsically (most common) or intrinsically using a quadrant bar phantom.

• Causes of Loss: Septal penetration or increasing source distance from the collimator face.

• Quadrant Bar Phantom Specifications:     * Divided into 4 quadrants.     * Each quadrant has lead strips in a linear pattern.     * Each quadrant is oriented at $90^\circ$ to the adjacent quadrant.     * Lead strips are spaced at different distances in each quadrant.     * One quadrant's strips are specifically positioned too close for the camera to resolve.

Linearity

• Definition: The ability of the camera to accurately determine photon position without displacement relative to the actual entry point.

• Principle: A straight-line source of gamma rays should yield a straight line in the image.

• Measurement: Uses a PLES (parallel line equal spacing) phantom or a quadrant bar phantom.

• Success Criteria: Lines must be straight and unbroken.

PET Quality Control

• Visual inspection of the camera.

• Synchronisation of clocks in the department and the camera system.

• Temperature checks: The sensitivity of the PET camera changes with variations in temperature.

• Camera Checks:     * Constancy of detector functionality.     * Blank scan: Performed to look for evidence of defective detectors.

• Calibration Checks (Less regular):     * Involves updating and refining calibration values.     * Uses Germanium-68 ($^{68}Ge$) sources.     * Sources can be inbuilt (rod source) or external (phantom or point source).

CT Quality Control for Hybrid Systems

• Tube Conditioning (Tube Warm-up): Prevents tube cracking and electrical arcing.

• Automatic Function Checks (Air Calibration): Ensures the response to x-ray flux is uniform among the detectors.

• CT Number QC:     * Performed with a water-filled phantom.     * Ensures the CT number for water is accurate.     * Includes calculation of noise.     * Involves visual inspection for artefacts.