PET/CT image quality

PET/CT Image Quality

Objectives

  • Upon Completion of this section, the learner will be able to:

    • Explain PET/CT acquisition modes.

    • Describe PET/CT scanner quality control.

    • Interpret PET/CT quality control results.

    • Analyze artifact images to determine underlying cause.

    • Prevent or resolve PET/CT image artifacts.

    • Describe quantitative analysis.

Data Acquisition for PET

  • Two-dimensional PET:

    • Comparable to SPECT.

    • Rejects oblique photons.

  • Three-dimensional PET:

    • Interplane septa are removed from the scanner.

    • Data is accepted from all planes leading to fourfold to eightfold improvement in sensitivity.

3D vs 2D Imaging

  • 3D Imaging (Volume Imaging):

    • Removes septa.

    • Offers significant increases in system sensitivity and scatter.

    • Requires a 3D reconstruction algorithm since it cannot be separated into planes.

  • 2D Imaging (Multiple Ring Imaging):

    • Add septa to reduce off-plane photons.

    • Each plane is processed independently using a 2D algorithm.

Dynamic/List-Mode Acquisition

  • Detectors surround the patient to collect simultaneous data.

  • Allows for dynamic studies with time frames as short as a few seconds.

  • Advantages over conventional methods:

    • Higher data storage efficiency.

    • Higher temporal resolution.

    • More flexible data manipulation.

    • Better spatial and contrast resolution.

  • Whole-body studies:

    • Computer-controlled bed positions typically overlap 1/4 to 1/3 of the axial Field of View (FOV).

    • Data from different bed positions are stitched together to form a single image.

List Mode Data Acquisition

Quality Indicators

  • Background Measurements:

    • Stress: $0.000755$

    • Rest: $0.00022$

  • Peak to Plateau Ratio:

    • Stress: $15.64543$

    • Rest: $11.84708$

  • Peak Time:

    • Stress: $31$, Rest: $31$

  • Sample Data (ml/g/min):

    • Myocardial Blood Flow (MBF):

    • Stress: $3.16$ (LAD), $1.19$ (LCX), $1.41$ (RCA)

    • Rest: $0.75$ (LAD), $0.81$ (LCX), $0.69$ (RCA)

    • Global Results:

    • Average values under stress and at rest.

    • Ratios calculated for various coronary arteries (e.g., $4.64$ for LAD).

Data Acquisition for Whole Body PET

  • Acquisition Times:

    • Total-body PET scans can take significantly longer, ranging from 20-60 minutes depending on parameters.

Normalization and Data Corrections

  • Purpose of Normalization:

    • Corrects for variations across the 10,000-20,000 detector elements.

    • A rod source demonstrates uniformity during the calibration process.

  • Correction for Scattered Radiation:

    • Scattered radiation results in fuzzy images, combined with CT to compute images.

    • 2D imaging helps reduce scatter.

Blank Scan

  • Conduct a blank scan daily; it remains constant and is performed using a rod source.

    • Transmission scan occurs before radiotracer injection to avoid patient movement.

Dead Time Correction

  • Exhibits:

    • Dead time and pile-up during high counting rates.

    • Some systems apply a global correction, while others apply corrections to individual pairs of detectors, particularly with short-lived radiotracers (e.g., O-15).

Performance Characteristics

  • Noise Equivalent Counting Rate (NECR):

    • Proportional to the square of the signal-to-noise ratio.

    • Decreases at high activities due to random coincidences.

    • Useful for scanner performance comparisons.

Dual Modality Imaging

  • Types:

    • Conventional anatomical imaging (X-ray, ultrasound, CT, MRI)

    • Functional/metabolic imaging (PET).

  • PET/CT and PET/MR systems:

    • Combine PET and CT, allowing for faster imaging and reduced noise.

Attenuation Correction in PET

  • Definition:

    • Attenuation refers to loss of detection of true coincidence events due to absorption or scattering.

  • Importance of Attenuation Correction:

    • Prevents significant artifacts in whole-body PET scans.

  • Artifacts Due to Lack of Correction Include:

    • Prominent activity at edges due to lack of attenuation.

    • Distorted images from intense activity areas (e.g., urinary bladder).

    • Diffuse activity in low attenuation tissues (e.g., lungs).

Implementing Attenuation Correction

  • Techniques:

    • Use of a rod source for scans.

  • Results of Attenuation Correction

    • Compare corrected vs non-corrected images:

    • Non-corrected: misinterpretations in areas due to increased surface activity.

Quality Control in PET

  • Purpose:

    • Verify operational integrity of the system, detectors, and acquisition electronics.

    • Ensure consistent high image quality and minimize artifacts.

  • Daily QC:

    • Conduct visual inspections of sinograms for streaks and inconsistencies compared to reference scans.

  • Blank Scan:

    • Employed to generate attenuation correction factors and provide QA data.

Quality Assurance Requirements

  • Well-defined measurements that are quick, sensitive to failure modes, and preferably quantitative.

System Calibration and Correction

  • Calibration Purpose:

    • Convert image counts/pixel to activity/volume.

    • Phantom with known activity concentration to validate calibration monthly or after maintenance.

  • Normalization (System Correction):

    • Correct for variations in detector efficiency.

Periodic Quality Control Tests

  • Frequency of tests to be conducted based on function:

    • Blank scan:

    • Before patient imaging.

    • Real incidence with known sources:

    • Daily checks to monitor anomalies.

    • Calibration of the camera:

    • Every six months post-normalization.

Artifacts in PET Imaging

  • Types of Artifacts:

    • Partial volume averaging, motion, metal, truncation, beam hardening, misregistration.

  • Partial Volume Averaging:

    • Results from similar tissues average as one unit; reduce with thinner slices.

  • Beam Hardening:

    • Can create streak artifacts affected by mAs, kVp, and contrast agents.

Motion Artifact

  • Known as ghosting artifact, resulting from patient movement.

Quantitative Analysis of SUV

  • Standard Uptake Value (SUV):

    • Ratio for determining normal vs abnormal uptake levels based on an area of interest (ROI).

  • SUV Equation:

    • $SUV = \text{Mean ROI activity (mCi/mL)} \bigg/ \frac{\text{Administered activity (mCi)}}{\text{Body weight (g)}}$

    • SUV > 2.5 raises malignancy suspicion.

Tumor Parameters Evaluated in PET

  • Metabolic Tumor Volume (MTV), Total Lesion Glycolysis (TLG).

    • TLG is the product of MTV and average SUVmax.

Radiation Safety in PET/CT

  • Regulations:

    • 10 CFR 19, 10 CFR 20, 10 CFR 35 for radiopharmaceuticals, and shipment protocols.

  • Disposal of PET Waste:

    • Waste can often be decayed over ten half-lives.

  • Signage Requirements:

    • Ensure appropriate caution signage is displayed.

  • Shielding Measures:

    • Required for PET/CT rooms for radiation safety.

Minor Spills and Surveys

  • Procedures for minor spills include clearing the room and monitoring hands.

  • CT patient dose assessed with CTDI and MSAD measurements.

Sources of Radiation Exposure

  • Involves preparation, injection, residual activity, uptake, and CT emission sources.