5.3 Quality Control of Gamma Cameras

Quality Control of Gamma Cameras

Quick Answer: Quality Control (QC) ensures the gamma camera operates accurately and reliably. The most critical daily test is flood field uniformity, ensuring the camera sees a uniform source of radiation equally across its field of view (FOV). Weekly tests verify spatial resolution and linearity using bar phantoms.

For the ARRT exam, understanding the distinction between intrinsic and extrinsic testing, the specific sources used, and the quantitative limits for QC failure is mandatory.

Flood Field Uniformity

Uniformity evaluates the camera's ability to produce an image of uniform density when exposed to a uniform source of radiation. Over time, PMTs can drift in voltage, or the crystal can degrade, leading to "hot" (hyperactive) or "cold" (hypoactive) spots on the image.

Uniformity testing is performed daily before any patient imaging occurs. It can be performed in two ways: intrinsically or extrinsically.

Intrinsic Uniformity

• Collimator: OFF.
• Source: A small Tc-99m point source (typically ~10-20 MBq or a few hundred microcuries).
• Setup: The source must be placed at a distance of at least 5 times the largest dimension of the detector (usually the diagonal) from the uncollimated crystal. This ensures the gamma rays arrive relatively parallel and uniformly across the FOV.
• Purpose: Tests the crystal and the electronics (PMTs, PHA) without the influence of the collimator.

Extrinsic Uniformity

• Collimator: ON (usually LEHR).
• Source: A Co-57 sheet source (a flat, radioactive source that covers the entire FOV).
• Setup: The sheet source is placed directly on the collimator face.
• Purpose: Tests the entire system, including the integrity of the collimator (checking for defects, dents, or manufacturing flaws).
• Note: Co-57 is used because its principal gamma energy (122 keV) is very close to Tc-99m (140 keV), and its long half-life (271 days) means the source only needs to be replaced every 1-2 years.

Quantifying Uniformity

Modern cameras use computers to analyze the flood image and calculate uniformity numerically. The calculation compares the pixel with the highest counts (Max) to the pixel with the lowest counts (Min).

There are two main metrics evaluated:

1. Integral Uniformity: Evaluates the variation over the entire field of view. It finds the maximum and minimum pixels anywhere in the useful FOV.
2. Differential Uniformity: Evaluates the rate of change over small, local areas (typically comparing any 5 consecutive pixels). This is often more visually sensitive to rapid changes in density.

Tolerance: For most clinical systems, both integral and differential uniformity should be less than 3% to 5%. If the value exceeds 5%, the camera should not be used for patient imaging until serviced or recalibrated (e.g., PMT tuning).

Spatial Resolution and Linearity

While uniformity is tested daily, spatial resolution (ability to separate distinct objects) and linearity (ability to draw a straight line straight) are typically tested weekly.

These parameters are evaluated using a lead bar phantom. The phantom consists of lead bars embedded in plastic. A uniform radiation source is placed behind the phantom.

Types of Bar Phantoms

1. Four-Quadrant Bar Phantom: The most common. It has four sections, each with a different bar width and spacing. You can only evaluate resolution accurately in one quadrant at a time, so the phantom often needs to be rotated 90 degrees and imaged multiple times.
2. Parallel-Line Equal-Space (PLES): All bars are the same width and spacing across the entire phantom. Two images at 90 degrees evaluate the whole FOV.
3. Orthogonal Hole Pattern (OHP): An array of holes in a lead sheet.

Evaluating the Image

• Resolution: Is determined by the smallest set of bars the camera can clearly distinguish (resolve). If you can see the 2 mm bars today, but next week they are blurred, resolution has degraded.
• Linearity: Is evaluated by looking at the lines of the bars across the FOV. They should appear perfectly straight. If the lines appear wavy, curved, or "barrel" shaped, there is a failure in the positioning logic, requiring immediate corrective action.

Corrective Action Protocols

If a daily uniformity flood fails (e.g., a visible "bullseye" artifact, cold tubes, or quantitative values >5%):

1. Check the obvious: Is the wrong energy window set? Is the source cracked or placed incorrectly?
2. Perform a "tune" or PMT calibration if the software allows.
3. If the failure persists, the camera must be taken out of service, and an engineer must be called. Document the failure and actions in the QC log.