## 6 Alternative Methods of Articular Cartilage Diagnosis
### MRI with Advanced Imaging Techniques  
One of the most relevant imaging modalities for assessing AC is MRI, which has high soft tissue resolution [19]. Advanced techniques in the form of dGEMRIC, T1\(\rho\)- or T2-mapping are used to assess the structural and biomechanical properties of the AC before morphological changes occur [331]. Their aim is to detect AC damage at a stage when the changes that have occurred are potentially reversible and there will be no loss of cartilage tissue [331]. Figure 5: Radiographic image of: (**A**)—healthy knee, (**B**)—knee with degenerative changes. Figure 6: Arthroscopic image of: (**A**)—healthy knee, (**B**)—knee with degenerative changes. The dGEMRIC method, or delayed Gadolinium-Enhanced MRI of Cartilage, is a technique that uses a contrast agent containing negatively charged gadolinium (Gd-DTPA\({}^{2-}\)), which diffuses into the AC in a proportion inverse to the GAG concentration [32]. In areas where GAG concentrations are low, gadolinium accumulates in greater amounts, resulting in a shortening of the T1\(\rho\) relaxation time, making an assessment of AC quality possible [333]. Reduced GAG content and reduced relaxation time due to contrast administration are typical of early degenerative changes [334]. This diagnostic method is considered particularly useful for evaluating the cartilage in the knee and hip. T1\(\rho\)-mapping is a technique based on measuring relaxation in the transverse plane in the presence of a blocking spin, making the method extremely sensitive to changes in proteoglycan content [335]. In AC with degenerative changes, the proteoglycan content decreases, resulting in a prolonged relaxation time [31]. This method has been shown to detect early biomechanical changes in AC before they are visible on standard MRI sequences, hence it shows promise in assessing the progression of OA as well as monitoring the effectiveness of therapy [335]. Another advantage of this method is that it is non-invasive and can be performed without the use of contrast, making it more patient friendly. T2-mapping is a technique for assessing the orientation and integrity of collagen fibres, as well as the water content of the AC [16]. Collagen fibres in healthy AC are characterised by an ordered structure, which influences the characteristic rather low values of the T2 relaxation time. An increase in T2 values may indicate characteristic degenerative changes, such as disorganisation of the collagen structure or an increase in hydration [16; 336]. The use of T2-mapping allows subtle structural changes to be detected before morphological defects appear [336]. The benefits of this method are its non-invasiveness and the lack of a need for contrast. A modern imaging technique that combines MRI with the analysis of mechanical waves introduced into the tissue to measure its mechanical properties, such as stiffness, is Magnetic Resonance Elastography (MRE) [337]. The use of MRE enables the assessment of AC elasticity and viscosity, which are among the key properties in the diagnosis and monitoring of OA [337]. [PERSON] et al. [338] used MRE to assess the mechanical properties of hyaline cartilage in vitro, confirming the potential of this method to directly measure the mechanical properties of the tissue. In vitro studies in animal models have also shown the ability to distinguish healthy AC from cartilage with pathological changes, given the differences in stiffness maps [339]. One significant limitation of using this method to assess AC is its limited thickness, thus requiring high spatial resolution as well as precise signal synchronisation [337]. MRE may also find application in monitoring the effectiveness of AC regenerative therapies [339]. However, the answer to this limitation lies in specially developed high-resolution MRE protocols, as well as the use of special mechanical transducers that generate waves at AC-adapted frequencies [337].