Quantitative MR Imaging: Technical Aspects
Yang Xia

Synopsis

Quantitative MRI in musculoskeletal tissues is challenging. Our technical ability to accurately measure and reliably interpret MRI parameters in musculoskeletal tissues can be influenced by the complexity of our specimens (human patient, animals, ex vivo specimen), instrumentation, experimental details, and data-analysis. This talk examines a number of these issues and their impact on the robustness of quantitative MRI, using the examples mainly from articular cartilage and its degradation process that leads to osteoarthritis. A brief comparison between articular cartilage and other musculoskeletal tissues (tendon, nasal cartilage, meniscus, and bone) will also be given.

Target Audience

Radiologists, technicians, and scientists who aim to improve the sensitivity and specificity of MRI parametric mapping in arthritis.

Outcome/Objectives

This presentation will illustrate the complex influences of various technical issues on the sensitivity and specificity of MRI parametric mapping towards early detection of cartilage diseases.

Abstract

Development of MRI methods that are sensitive and specific to early changes in the molecular composition and structure in cartilage is essential for the early detection of cartilage degradation. Compared with the intensity-based approaches in MRI, parametric mapping (e.g., T1, T2, T1ρ, magnetization transfer, and diffusion) has demonstrated superior quantitative sensitivity to the molecular changes associated with osteoarthritic progression. The improved sensitivity and specificity of these MRI parameters may be attributed to the fact that they reflect the characteristic dynamics of water and macromolecules, and their interactions 1.

However, these tissue characteristics are also subject to manipulation by complex technical issues 2 in MRI experiments, including the complexity of our specimens (human patients, animals, ex vivo specimen), instrumentation, experimental details, and data-analysis. These non-pathologic variations can approach or exceed differences seen between normal and diseased tissue, so that these MRI parameters remain non- or less specific for the purpose of early disease detection.

This presentation examines a number of these technical issues and their impact on the robustness of quantitative MRI, using the examples mainly from articular cartilage and its degradation process that leads to osteoarthritis. Some of these technical issues include:

  • In human MRI, check into the immediate-before activities of a patient;
  • In animal models, there is no perfect model that mimics all aspects of osteoarthritic degradation process 3;
  • In ex vivo MRI, pay attention to the way that the specimens are harvested, and the storage condition and storage solution; and use fresh tissue as much and as quick as possible;
  • Two unique structural characteristics in articular cartilage are (a) the depth-dependent properties of articular cartilage, and (b) the topographical variations of cartilage properties over a joint surface. Any of the two characteristics can mask the change of the MRI parameter due to tissue degradation;
  • Any MRI experiment has numerous technical factors that need to be considered thoroughly and optimized meticulously, from experimental parameters and data analysis;
  • The experimental optimization is extremely critical for any quantitative measurement of relaxation and diffusion in MRI of cartilage. Because these parameters are calculated based on the signal decay of some weighting function, many unintentional factors in the experiment can also cause the decay of the FID signal, which would be mistakenly considered as being due to relaxation or diffusion;
  • Since cartilage is a 3D layer of tissue with a finite thickness, and since early degradation is localized in cartilage (both dept-wise and topographically), high resolution in MRI of cartilage is not a luxury, but a must 4.

Acknowledgements

This work is supported in part by a R01 grant from the National Institutes of Health (AR069047).

References

1. Xia Y. 2013. MRI of articular cartilage at microscopic resolution. Bone and Joint Res 2:9-17.

2. Zheng S, Xia Y. 2017. The Influence of Specimen and Experimental Conditions on NMR and MRI of Cartilage. In: Xia Y, Momot KI editors. Biophysics and Biochemistry of Cartilage by NMR and MRI: The Royal Society of Chemistry; pp. 347-372.

3. Little CB, Zaki S. 2012. What constitutes an "animal model of osteoarthritis"--the need for consensus? Osteoarthritis Cartilage 20:261-267.

4. Xia Y. 2007. Resolution 'scaling law' in MRI of articular cartilage. Osteoarthritis Cartilage 15:363-365.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)