Synopsis

Keywords: Musculoskeletal: Cartilage, Musculoskeletal: Joints

Quantitative MRI (qMRI) is a rapidly advancing field in musculoskeletal imaging that allows for more accurate and objective assessment of tissue structure and composition. QMRI techniques can provide measurements of parameters such as T1, T2, T1rho and T2* relaxation times, magnetization transfer ratio (MTR), gagCEST, and diffusion properties. These measurements can be used to detect subtle changes in tissue microstructure. QMRI is particularly useful in evaluating degenerative cartilage disease, bone health, and muscle function. The ability to quantify tissue changes over time can also aid in monitoring disease progression and evaluating the effectiveness of treatments.

Introduction

Quantitative MRI (qMRI) offers several benefits to musculoskeletal (MSK) imaging:
→ qMRI provides objective and reproducible measurements of various tissue parameters, such as T1 and T2 relaxation times, diffusion coefficients, and magnetization transfer ratios. These measurements can be used to detect subtle changes in tissue (micro)-structure and composition, which may be not visible in conventional MRI.
→ qMRI can detect early changes in tissue microstructure and composition that may precede the development of MSK disorders. For example, qMRI has been used to detect changes in articular cartilage that may indicate the onset of osteoarthritis.
→ qMRI can be used to monitor changes in tissue structure and composition over time, allowing for the tracking of disease progression and treatment response.
→ qMRI can provide additional information to aid in the diagnosis and treatment planning of MSK disorders. For example, qMRI can differentiate between different types of soft tissue tumors, which may have different treatment options.
→ qMRI can reduce variability between different MRI scanners and imaging protocols, as well as increase the sensitivity of MRI to subtle changes in tissue structure and composition.

Overall, qMRI offers several benefits to MSK imaging and has the potential to improve the diagnosis, monitoring, and treatment of MSK disorders.

In this lecture, the most popular qMRI methods will be discussed, with the special focus on the target structure sensitivity and their clinical potential.

T2 mapping

T2 mapping is a technique used in musculoskeletal MRI to evaluate the tissue properties of cartilage, menisci, tendons, and ligaments. This quantitative method measures the transverse relaxation time (T2) of the protons in the tissue. Typically, during a T2 mapping sequence, a series of images are acquired with different echo times (TE), and the signal intensity is measured for each TE. By fitting a curve to the signal intensity data, the T2 value can be calculated for each pixel in the image. In musculoskeletal MRI, T2 mapping is commonly used to assess the health of articular cartilage in joints such as the knee or shoulder. Healthy cartilage has a low T2 value, indicating that it has a high water content and a smooth surface. In contrast, damaged or degenerated cartilage has a higher T2 value, reflecting an increased hydration and surface irregularities. T2 mapping can also be used to evaluate other tissues in the musculoskeletal system, such as the menisci, tendons, and ligaments. By providing quantitative measurements of tissue properties, T2 mapping can help clinicians make diagnoses that are more accurate and guide treatment decisions for patients with musculoskeletal injuries or conditions.

T1rho mapping

T1rho mapping is another quantitative technique used in musculoskeletal MRI to evaluate the biochemical properties of cartilage, tendons, and ligaments. T1rho mapping is in some ways similar to T2 mapping, but uses low amplitude spin-lock RF pulses after the excitation pulse. It is based on the concept of spin-locking, which involves applying a constant electromagnetic field to the tissue, causing the protons to precess around a different axis. During a T1rho mapping sequence, a series of spin-locking pulses with different durations are applied to the tissue, and the signal intensity is measured for each pulse. By fitting a curve to the signal intensity data, the T1rho value can be calculated for each pixel in the image. In addition to cartilage, T1rho mapping can also be used to evaluate other tissues in the musculoskeletal system, such as tendons and ligaments. T1rho probes the slow motion interactions between motion-restricted water molecules and their local macromolecular environment. Some comparisons with traditional imaging methods such as T2 mapping suggest that T1rho has higher sensitivity to PG depletion in cartilage, although this is not definitively established for human in vivo studies with clinically feasible protocols.

Magnetization Transfer

Magnetization transfer (MT) is a technique used in musculoskeletal MRI to enhance the contrast between different tissues, particularly between water and macromolecules, such as collagen and proteoglycans. During an MT sequence, a radiofrequency pulse is applied to a pool of macromolecules in the tissue, causing the protons in the macromolecules to exchange magnetization with the surrounding water protons. This transfer of magnetization from the macromolecules to the water protons reduces the signal intensity of the water in the image, leading to improved contrast between the macromolecules and the water. In musculoskeletal MRI, MT is often used to enhance the visualization of structures such as tendons, ligaments, and cartilage. In cartilage, for example, the proteoglycans are highly concentrated in the extracellular matrix and are responsible for maintaining the cartilage's mechanical properties. By selectively suppressing the signal from water molecules and highlighting the signal from macromolecules, MT can provide a more detailed picture of the proteoglycan distribution in cartilage. MT is a powerful tool for identifying early changes in the composition of musculoskeletal tissues, such as cartilage degeneration.

Diffusion MRI

Diffusion-weighted imaging (DWI) is a technique used in musculoskeletal MRI to evaluate the microstructural integrity of tissues, particularly in cartilage. A DWI sequence uses a series of magnetic field gradients, which cause the protons to diffuse along the direction of the gradient. By measuring the rate of proton diffusion, known as the apparent diffusion coefficient (ADC), information about the tissue's microstructure can be obtained. In musculoskeletal MRI, DWI is often used to evaluate tendons, where changes in the microstructure, such as inflammation, fibrosis, or degeneration, can affect the rate of proton diffusion. DWI can also be used to evaluate muscles, where changes in the fiber structure, such as atrophy or hypertrophy, can alter the rate of proton diffusion.

gagCEST

Glycosaminoglycan Chemical Exchange Saturation Transfer (gagCEST) is a type of MRI technique used to assess the glycosaminoglycan (GAG) content of cartilage in the musculoskeletal system. During a gagCEST sequence, a radiofrequency pulse is applied to selectively saturate the protons in the GAGs of the cartilage. The protons then exchange magnetization with the water protons, which can be detected in the image. By measuring the difference in signal intensity between the saturated and unsaturated protons, the concentration of GAGs in the cartilage can be quantified. GAGs are an essential component of cartilage, providing the tissue with its compressive strength and elasticity. In many musculoskeletal conditions, such as osteoarthritis, there is a loss of GAGs in the cartilage, which can lead to joint pain and stiffness. By providing a non-invasive measure of GAG concentration, gagCEST can help clinicians monitor the progression of cartilage degeneration and evaluate the effectiveness of interventions aimed at preserving or restoring the tissue's health. gagCEST is a relatively new technique, and its clinical application is still being explored. However, preliminary studies have shown promising results for its use in detecting early changes in cartilage health and evaluating the effectiveness of treatments aimed at preserving cartilage function

Sodium MRI

Sodium MRI is a specialized MRI technique used to evaluate the concentration of sodium ions in tissues in the musculoskeletal system, such as bone, cartilage, and muscle. Sodium ions play an important role in maintaining the function of many tissues in the body, and changes in sodium concentration can indicate underlying disease or injury. In musculoskeletal MRI, sodium MRI can provide information about the health of bone, cartilage, and muscle tissue that is not visible with other MRI techniques. By measuring the intensity of the sodium signal along with the reference tubes, the concentration of sodium ions in the tissue can be quantified. Preliminary studies have shown promising results for its use in detecting early changes in bone and cartilage health, as well as in evaluating the effectiveness of treatments aimed at preserving or restoring tissue function.

Acknowledgements

Funding support provided by the Austrian Science Fund (FWF) KLI 917. The financial support by the Austrian Federal Ministry for Digital and Economic Affairs and the National Foundation for Research, Technology and Development is gratefully acknowledged