Combined PET-MR Principles, Strengths, and Weaknesses

The combination of positron emission tomography (PET) and magnetic resonance imaging (MRI) provides a unique clinical imaging tool with significant applications in biomedical research in small animals. PET/MRI systems for use in humans were first introduced in the year 2010.

Other hybrid imaging systems such as CT/PET and SPECT/CT have been widely accepted in clinical practice in the past decade. PET/MR technology has revolutionized clinical imaging in small animals and offers high performance functional imaging solutions. Some medical imaging experts foresee PET/MRI completely replacing PET/CT in the future.

Strengths of PET/MR technology

Advantages of combined PET/MR as compared to conventional imaging methods include:

• Lesser radiation dose from MRI
• Simultaneous multi-modality preclinical imaging
• Excellent soft tissue contrast
• Plenty of tracers available for PET
• Enables good visualization, quantification, and translational studies
• Cryogen-free magnet use highly reduces infrastructure needs

The use of PET/MRI in neurology and neuro-oncology is mainly because of its ability to provide complementary functional, morphological, molecular, and (patho-) physiological information related to the brain.

The key strength of combined PET/MR systems in neurology-related applications is its capability for simultaneous data acquisition. This paves the way for correlation and cross-validation of both spatial and temporal measurements obtained from PET and MR.

Simultaneous acquisition of data helps in understanding the functional, hemodynamic, and metabolic interactions within various neurological disorders. Moreover, using MR data helps enhance PET quantification through motion correction.

Weaknesses of PET/MR imaging tool

Combined PET/MRI technology has some disadvantages compared to other hybrid imaging technologies:

• It requires high initial capital cost
• There is lack of protocol and standardization due to huge variations in MR protocols
• No combined reporting of PET and MR components
• Limited flexibility of combined PET/MR systems
• High acquisition times of up to 60 min

Technical challenges in building PET/MR systems include:

• The addition of PET components to the system in the presence of strong magnetic field from MR
• Working with long optical fibers
• Use of semiconductor light detectors for counting photons

Applications of combined PET/MR

• Combined PET/MR has profound applications in oncology. It allows high-resolution imaging of the four key steps in cancer formation, namely; apoptosis resistance, cancer angiogenesis, tumor proliferation and cancer metastasis.
• PET/MR finds use in several key clinical as well as research applications in fields such as neurology and cardiology.
• Combined PET/MR imaging modality enables simultaneous multifunctional and anatomical imaging in small animals, which greatly impacts biomedical imaging in research and clinical settings.

Future of combined PET/MR

The ability of the PET technique to quantify metabolic and physiological pathways in a non-invasive manner should be maintained in the combined PET/MR modality. There is a pressing need for viable options for correction of scattered radiation and attenuation and these approaches should account for the positioning aids, scanning bed, and RF coils used in the hybrid technique.

One key issue is the inaccuracies related to attenuation factors in MR-based tools. MR-based attenuation correction in neurological applications is problematic, more so around the dense areas in the skull.

References

1. https://www.ncbi.nlm.nih.gov/pubmed/20082526
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4422837/

Further Reading

• All Preclinical Imaging Content
• Micro-CT Principles, Strengths, and Weaknesses
• Micro-MRI Principles, Strengths, and Weaknesses
• Micro-PAT Principles, Strengths, and Weaknesses
• Micro-PET Principles, Strengths, and Weaknesses

Written by

Susha Cheriyedath

Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.