PET and CT Image Quality and Artifacts

PET and CT Image Quality and Artifacts

Introduction

• All images are courtesy of Siemens HSC, Knoxville TN unless otherwise noted. This document discusses the quality of PET images and associated artifacts, aiming to provide comprehensive insights into their influencing factors and the troubleshooting processes for maintaining image quality.

PET Image Quality

Quality Appreciation

• The assessment of PET image quality involves subjective interpretations; however, there are generally accepted criteria that contribute to the standards of good PET images, acknowledging the phrase "beauty is in the eye of the beholder".

• Image courtesy of University of Erlangen, Nuclear Medicine, Germany (Prof. Dr. T. Kuwert).

Factors Influencing Image Quality in PET

Patient Factors

• Size: Body size influences the distribution of the radiotracer, impacting image quality.

• Preparation: Proper preparation is crucial for optimal imaging results, aligning patient protocols with imaging requirements.

• Uptake: Varies based on physiological conditions and can lead to differences in image quality.

• Therapy: Ongoing treatments can alter tracer distribution and metabolism.

• Excretion and Contamination: Approximately 30-40% of administered tracer can be excreted, affecting background noise and clarity of images.

Technologist/Physician Factors

• Poor Injection Technique: Inaccurate administration can lead to infiltrated doses that distort the images.

• Poor Patient Positioning: Improper alignment or motion during scanning affects clarity.

• Reconstruction Issues: Flawed algorithms or techniques can yield inaccurate images.

• Incorrect SUV Parameters: Incorrectly calculated Standardized Uptake Values (SUV) can misrepresent tracer uptake.

Hardware/Software Factors

• Detector Failures: Compromised detectors can produce erroneous images.

• Cross-wired Detectors: Incorrect wiring leads to image artifacts.

• Bad Coincidence Processor: Can misinterpret the timing of detected events, leading to inaccuracies.

• Bad ACS Boards: Detector calibration issues resulting in poor image quality.

• Incorrect Gantry Offsets: Faulty alignment of PET and CT imaging can cause image misregistrations and loss of accuracy.

Normal/Expected Image Quality

Definition of Normality

• Variability: Normal uptake patterns differ significantly amongst various isotopes and pharmaceuticals across diverse anatomical regions such as oncology, cardiology, and neurology.

• ** Factors Affecting Distribution**: Multiple variables (patient health, environmental conditions, equipment performance) can cause challenges in achieving consistent image quality.

Specific Normal Distribution Examples

FDG - Oncology

• Uptake in various organs:

  • Lungs: Low uptake

  • Mediastinum: Low uptake

  • Liver: Low uptake

  • GI Tract: Variable activity based on section

  • Urinary Tract: Excretes FDG

  • Muscular System: Low uptake in a relaxed state.

• Influence of Isotope: Understanding that the distribution of FDG and Ga68 Dotatate differs based on physiological processes under study.

Brain Image Quality

• Normal gray matter in the brain should present with high uptake.

• Positive and negative scans for amyloid plaque tracer uptake visual comparison.

Patient Preparation Factors Impacting Image Quality

Attributes Prior to Scanning

• Diet: Elevated blood glucose levels (e.g., BGL = 210 vs. BGL = 110) can significantly affect imaging quality. High-carbohydrate diets can lead to increased bowel uptake. Adjustments may be required 1-2 days prior to scanning.

• Exercise: High-intensity exercise can mask liver lesions and cause erroneous high muscle uptake. Example: A patient repeated imaging after halting vigorous exercise.

• Medications: Both chemotherapy and radiation therapy can impact FDG distribution. Before and after comparisons are critical.

• Hydration: Proper hydration enhances the target/background ratio; dehydration leads to higher tracer concentrations in the urinary tract, affecting image quality.

Troubleshooting Patient Preparation Issues

• Image quality issues often trace to individual patient physiological characteristics and can typically be prevented with thorough histories and appropriate instructions prior to scanning.

Uptake Phase IQ Issues

Background Factors

• Brown Fat: Cold environments can induce brown fat uptake; anxiety in patients may reproduce similar patterns. Intervention measures include maintaining warmth during scans (e.g., blankets, sedatives).

• Pet Motion: Patient directives to remain still during uptake phase are critical, as muscle activity can misrepresent tracer uptake.

• Timing: Timing of imaging can markedly affect results, whether scanning early (high random count noise) or late (decay of tracer impacting statistics).

Artifacts from Implants and Oral Contrast

Metal Implants

• Attenuation corrections from CT data are necessary as artifacts will not manifest on uncorrected PET images.

• Example: Metal plates can cause significant artifacts but are corrected in AC images.

Oral Contrast

• Introduction of oral contrast agents can artificially enhance counts, necessitating careful interpretation of PET images.

Miscellaneous Patient IQ Issues

• Excretion Pathways: Importance of bladder emptying to prevent high pelvic activity concentrations. Dehydration effects and activity scatter correction factors.

• Contamination: Urine trace contamination can lead to false positives; correct management protocols are essential.

Technologist/Physician IQ Factors

Injection Handling

• Infiltrated Doses: Risks associated with infiltrated doses impacting PET image display and quantification; issues may arise from improper injection technique and dosage calculations.

Route of Administration

• Importance of adherence to appropriate administration pathways to minimize imaging artifacts.

Timing of Intervention

• Timing of radiopharmaceutical injection directly connected to the positioning of scans can induce iodine enhancements and variability in imaging.

Patient Positioning and Motion

Positioning Protocols

• Detailing upper and lower body positioning impacts image acquisition quality in PET scans.

Motion Artifacts

• Instruction to minimize patient movement is paramount; blurring caused by motion yields poor quality images.

Reconstruction Techniques

Influence of Algorithms on Image Quality

• Comparison of conventional and advanced HDR reconstruction techniques, emphasizing the distinction in image sharpness, noise, and overall quality.

• Iterations and subsets in reconstruction parameters can greatly influence the image outputs, with implications on quantification and interpretation of results.

Quantification Pitfalls

• Incorrect dose information can significantly downgrade image value (e.g., comparing 54 mCi vs. 11 mCi dosages and the resultant SUVmax). It's imperative to validate dose entries pre-image acquisition.

System Factors (Hardware and Software)

Daily Quality Checks

• Establish routine checks to ensure the system’s readiness and accuracy; factors include calibration of ECF, normalization of detector responses, and sinogram inspection.

• Documentation of any faults (detector failures, cross-wired detectors, processing errors) that may impede accurate results.

Common CT Artifacts

Description of Artifacts

• Clear distinction among artifacts such as streaks, dark bars, and rings, and their causation from improper scanner utilization, physical phenomena, or maintenance issues.

Beam Hardening Effects

• Artifacts due to beam hardening lead to streak appearances in images; implementing adaptive filters or algorithms can mitigate these effects.

Partial Volume Effects

• Analyze the consequences of scanning parameters (slice thickness vs. anatomy) as thicker slices may introduce blurriness or false findings. Corrective measures involve selecting thinner slices.

Protocol Selection

• Importance of selecting the appropriate protocol corresponding to the specific anatomical region or studying requirements, ensuring adherence to all procedural standards for successful imaging outcomes.