Positron emission tomography (PET) combined with anatomical imaging modalities such as computed tomography (PET/CT) or magnetic resonance imaging (PET/MRI) are widely used in the assessment of oncological, neurological and cardiac diseases. The big advantage of these hybrid PET devices over stand-alone imaging is their ability to assess functional processes and anatomical information in temporal and spatial alignment. Furthermore, the anatomical information gained by CT and MRI can be used to correct the PET data for attenuation and scatter effects inherent to emission tomography applications, thus allowing an accurate quantification of functional and metabolic processes.

Prof. Dr. Clemens Cyran is Professor of Radiology, Section Chief for Hybrid and Molecular Imaging and vice chair at the Department of Radiology, Ludwig-Maximilians-University Munich, Germany. He heads the Laboratory of Experimental Radiology, which focuses on the development and validation of imaging biomarkers of therapy response in oncology, using multimodality hybrid imaging techniques. He is the current President of the European Society for Hybrid, Molecular and Translational Imaging (ESHIMT).

However, hybrid PET/CT and PET/MRI present additional challenges over stand-alone imaging caused by differences of the individual imaging components and the incorporation of anatomical information in the PET data during attenuation and scatter correction. Therefore, profound knowledge of the underlying technical and tracer-specific physiological processes is required for reliable performance and assessment of hybrid imaging examinations. Furthermore, specific attention needs to be paid to identifying and compensating for common imaging artefacts and pitfalls.

A reliable hybrid PET examination starts way before the actual scan, with proper patient information. As functional processes may change depending on patient behaviour and diet, examination-specific instructions need to be closely followed. For example, for 18F-FDG examinations, physical activity before the examination can cause enhanced tracer uptake in muscles, which may obscure pathologically elevated glucose uptake of malignancy, or the carbohydrate content of a meal eaten prior to the examination may alter the uptake of 18F-FDG in the myocardium.

Dr. Ivo Rausch, PhD, is a post doc researcher in the QIMP Team at the Center of Medical Physics and Biomedical Engineering, Medical University of Vienna. He is a member of the ESHIMT research committee, board member of the Austrian Society for Radiation Protection in Medicine (VMSÖ) and member of the Medical Physics working group of the Austrian Society of Nuclear Medicine and Molecular Imaging (OGNMB).

When the patient arrives at the imaging facility, similar care needs to be given to patient preparation. The influence of incorrectly assessed patient parameters (such as body weight and height) or tracer maladministration on image quality and quantitative values such as the standardised uptake values (SUV) has to be accounted for. Additionally, the time interval between tracer administration and PET examination has significant influence on quantitative PET parameters.

One of the most critical parts of a hybrid imaging study is the examination itself. For matters of quality control and reproducibility, standardised imaging protocols are of utmost importance. Proper and comfortable positioning of the patient is essential to avoid truncation and beam hardening artefacts in CT which are propagated into the PET through erroneous attenuation and scatter corrections. In this context, the use of dedicated patient positioning aids may reduce the likelihood of artefacts caused by intra-scan motion between the CT and PET acquisition. The use of contrast agents in CT can lead to the appearance of artificial tracer accumulations in areas with a high contrast agent concentration. Last but not least, the image appearance and quantitative accuracy of the extracted values strongly depend on the applied reconstruction protocol and settings.

The appearance of artefacts can be reduced to a minimum by strictly following proper examination protocols and by ensuring staff have a profound understanding of the underlying mechanisms. However, image artefacts in PET and CT/MRI cannot be completely avoided and frequently present for the first time during image interpretation. Here, the reviewing physician needs to be aware of potential pitfalls and possible artefacts and must crosscheck suspicious and unexpected findings. A good practice therefore is the combined inspection of the attenuation corrected (AC) PET images together with the anatomical images used for attenuation correction and the non-attenuation corrected (NAC) PET. For example, artificial tracer accumulations on AC images introduced by high density structures on the CT images are not visible on the NAC PET, and a corresponding artefact should be present in the AC CT. Similarly, intra-scan motion artefacts can be assessed.

Whole-Body 18F-FDG PET/MRI (A) Attenuation corrected (AC) PET(/MRI) image showing an artificial signal void. (B) NON-AC PET with no visible artefact. (C) MR based AC map showing a signal void caused by a hip implant. This signal void translates into a signal void in the AC PET image.

In summary, pitfalls and artificial findings can be addressed by a profound understanding of the underlying tracer-specific physiological processes and the technical challenges present in hybrid imaging. Therefore, the performance of reliable hybrid imaging examinations requires well-established and close cooperation between the team involved, including physicians, physicists, radiochemists and technologists, as well as continuous education of the participating staff.

LIVE
ESHI Session
(European Society for Hybrid, Molecular and Translational Imaging)
Wednesday, November 25, 17:00–18:00
ESHIMT The importance of identifying technical errors in hybrid imaging: pitfalls and artefacts in PET/CT and PET/MRI

Moderators:
Melvin D’Anastasi; Msida/MT
Johannes Rübenthaler; Munich/DE

  • Artefacts on PET/CT and PET/MRI
    Ivo Rausch; Vienna/AT
  • Pitfalls in FDG-PET/CT
    Jose Luis Vercher-Conejero; Barcelona/ES
  • Beyond FDG: pitfalls and artefacts
    Clemens C. Cyran; Munich/DE
  • Live Q&A: Clinical relevance of misinterpreting pitfalls and artefacts in hybrid imaging. How can we augment diagnostic confidence in hybrid imaging?

FURTHER READING

Davidson T, Lotan E, Klang E et al (2018) Fat necrosis after abdominal surgery: A pitfall in interpretation of FDG-PET/CT. Eur Radiol. 28(6):2264-2272: european-radiology.org/5201

Grootjans W, de Geus-Oei LF, Meeuwis AP et al (2014) Amplitude-based optimal respiratory gating in positron emission tomography in patients with primary lung cancer. Eur Radiol. 24(12):3242-50: european-radiology.org/3362

Kubota K, Fujioka T, Toriihara A, Tateishi U, Saida Y (2017) FDG-PET/CT Pitfalls in Breast Cancer. ECR 2017 / C-1609: myESR.org/171609

Deva R, Gustafson S, Miles K (2017) Pearls and Pitfalls in the Interpretation of PSMA PET/MRI for Prostate Cancer. 2017 ASM / R-0087: myESR.org/1787

Mallia A, Bashir U, Stirling J et al (2016) Artifacts and Diagnostic Pitfalls in PET/MRI. ECR 2016 / C-0773: myESR.org/16773