6.3 PET/CT Quality Control, Attenuation Correction & PACS

PET/CT Quality Control and Data Management

Operating a high-end PET/CT system requires a rigorous quality control (QC) regimen. Because PET is frequently used for quantitative analysis (measuring the exact concentration of tracer in a tumor), the scanner must be exquisitely calibrated. Furthermore, understanding how the CT scan alters the PET data, and how that data is managed post-acquisition, is vital for the technologist.

PET Quality Control

The Daily Blank Scan

Just as a gamma camera requires a daily uniformity flood, a PET scanner requires a daily Blank Scan.

• Procedure: A long-lived radioactive source—typically a solid cylinder of Germanium-68 (Ge-68), which decays to the positron emitter Gallium-68—is placed in the center of the gantry. The system acquires data for a set duration.
• Purpose: The Blank Scan has two primary functions:
  1. Detector Health: It checks for malfunctioning detector blocks or entire rings. The resulting sinogram is visually inspected for "dead" or "hot" streaks, which indicate failing photomultiplier tubes or crystal modules.
  2. Normalization Update: It serves as a daily reference to adjust the system's normalization matrix. Normalization corrects for slight variations in sensitivity between the tens of thousands of individual crystal elements in the scanner. The daily blank scan is mathematically compared against a much longer "reference" blank scan acquired during quarterly or annual calibration to calculate correction factors.

SUV Calibration (Well Counter Calibration)

The Standardized Uptake Value (SUV) is a semi-quantitative metric widely used in oncology to assess tumor metabolism and track response to therapy. It compares the concentration of radioactivity in a region of interest (e.g., a tumor) to the concentration that would be expected if the injected dose were distributed evenly throughout the patient's body.

SUV calculation depends heavily on:

1. Scanner Calibration: The PET scanner must accurately convert the counts it detects into absolute activity concentration (Bq/mL). This requires a periodic Cross-Calibration (or Well Counter Calibration). A phantom is filled with a precisely measured amount of F-18 (measured in the department's dose calibrator), scanned, and the scanner software is adjusted so the measured concentration matches the known injected concentration. This explicitly ties the PET scanner to the specific dose calibrator used to assay patient doses.
2. Technologist Input: If the technologist enters an incorrect patient weight, incorrect assay time, incorrect administration time, or an inaccurate residual dose in the syringe, the calculated SUV will be wrong, potentially altering clinical decision-making.

Attenuation Correction: CT vs. Ge-68

In PET imaging, a 511 keV photon originating deep within the body has a high probability of being attenuated (absorbed or scattered) before reaching the detectors. Because coincidence detection requires both photons to arrive, if one photon is lost, the entire event is lost. Therefore, deep structures appear falsely "cold" without correction.

Historical: Ge-68 Rod Sources

Early stand-alone PET scanners performed a transmission scan using rotating Ge-68 rod sources to map the patient's density. While accurate, these scans were very slow (adding 15-20 minutes to the exam) and generated noisy correction maps.

Modern: CT Attenuation Correction (CTAC)

With the advent of PET/CT, the CT portion is used to generate the density map.

• Mechanism: A fast, low-dose CT scan is acquired. The CT Hounsfield Units (which map tissue density based on x-ray energy, typically 120-140 kVp) are mathematically scaled up to estimate the tissue density for 511 keV gamma photons.
• Advantages: CTAC is extremely fast (seconds instead of minutes), provides a virtually noiseless attenuation map, and simultaneously provides the anatomical reference image.

Artifacts in PET/CT

While CTAC is superior, combining two different imaging modalities introduces unique artifacts, primarily arising from the fact that the fast CT and the slow PET capture different physiological states.

1. Respiratory Motion Misregistration

The most significant artifact in PET/CT occurs at the lung bases and dome of the liver.

• The Problem: The CT is usually acquired rapidly (seconds), often during normal shallow breathing or a breath-hold. The PET is acquired over several minutes per bed position, representing a time-averaged position of the organs.
• The Artifact: If the diaphragm's position on the CT map does not match its average position during the PET scan, the attenuation correction is applied incorrectly. This can cause a high-uptake lesion in the liver dome to be projected into the lung base on the fused image, or it can create an artificial "cold" defect in the liver where the computer incorrectly assumes lung tissue exists (applying too little correction).

2. High-Density Material (Metal) Artifacts

Metallic implants (pacemakers, hip replacements, dental fillings) severely attenuate x-rays.

• The Problem: On the CT scan, this creates severe streak artifacts and artificially high Hounsfield Units.
• The Artifact: When the CTAC algorithm encounters these artificially high densities, it drastically overcorrects the PET data in that region. This results in an intense, false "hot spot" around the metal implant on the attenuation-corrected PET images.
• Solution: Technologists must always review the non-attenuation-corrected (NAC) PET images. A true lesion will be visible on the NAC images, while a metal-induced artifact will be absent or much less prominent on the NAC images.

PACS and DICOM

Once acquired and reconstructed, the massive datasets generated by PET/CT must be stored, viewed, and shared.

• PACS (Picture Archiving and Communication System): This is the hospital's central digital network for storing and retrieving medical images. A PET/CT scan may generate thousands of individual slice images, requiring robust server storage and fast network speeds.
• DICOM (Digital Imaging and Communications in Medicine): This is the universal standard format for medical images. DICOM ensures that images from a GE scanner can be read on a Siemens workstation. Crucially for PET, the DICOM header contains vital metadata (patient weight, injected dose, injection time, half-life) required by the PACS workstation software to calculate and display SUVs on the fly when the radiologist draws a region of interest.
• Display Matrices: While the CT may reconstruct into a high-resolution 512x512 matrix, the PET data, due to fundamental resolution limits and noise, is typically reconstructed into lower resolution matrices, such as 128x128 or 256x256, before fusion.