and Claudio Chillemi2
(1)
Sapienza University of Rome, Latina, Italy
(2)
Latina, Italy
Cartilage, Compositional MR Imaging
With the compositional cartilage imaging, an advanced MR tool, it is possible to detect biochemical changes that anticipate morphological cartilage interruption. Each compositional technique is based on the characterization of macromolecular content and organization of the chondral layer. In particular, two aspects are mainly considered: first, the collagen fiber network of the extracellular matrix, extremely arranged in the space for its tensile and shear straight properties, and, second, matrix content of GAGs that is negatively charged due to the presence of carboxyl and sulfate groups. Most validated techniques are the following:
T2 mapping: T2 mapping is based on the differences in T2 times of dissimilar zones and organization of the collagen matrix. Ordinarily with MR imaging, when a good contrast between fluid and cartilage is present, it is possible to give a subjective assessment of cartilage T2 changes, whereas T2 mapping provides numerical data by creating a color scale map representing the variations in relaxation time within cartilage. The areas with higher T2 than that of normal cartilage represent zones of early disruption and degeneration. Another application of T2 maps could be the assessment of the achievement of cartilage repair.
dGEMRIC: The dGEMRIC technique is based on the interaction between Gd-DTPA2− and negatively charged GAGs. After the intravenous administration of Gd, this is more concentrated in areas with little content of GAGs, and this is observable on a T1-colored map, allowing a quantitative evaluation of GAGs. After administration of Gd, the joint is moved for at least 15 min, to allow the infiltration of the Gd in the chondral clefts.
T1ρ: T1ρ measures the interactions between motion-constrained water molecules and their local macromolecular environment, so this technique may provide an ideal measure of cartilage composition in collagen and GAGs and a depletion of those may result in higher T1ρ values than normal cartilage, especially in the first stage of OA. However, T1ρ measure is not specific for GAGs depletion, and a higher T1ρ may imply other macromolecules reducing or collagen fiber “mal-orientation.” Although this technique is disposable as add-on pack in most MRI scanners and several studies tested its performance in the early stage of the OA, there are few studies about the performance of the technique in the evaluation of chondral implant and repair, and this is the theoretically ideal field of employment (low specificity with highest sensitivity).
Sodium imaging: On the basis of mentioned negative charge of GAGs, sodium occurs naturally in the cartilage matrix as positive ion. This promising technique may hypothetically map the depletion of GAGs but is actually limited by the requirement of special hardware and low SNR of the images obtained.
Diffusion-weighted imaging: The diffusion of water is higher in cartilage when the strict organization of collagen fibers is intact; an increase in ADC, although with low quality images, may represent a first stage of chondral layer disruption, but, at the date of this publication, further studies are necessary to improve the acquisition parameters and technique.
Cartilage, Semiquantitative Morphologic Assessment
Chondral defects and bone remodeling during OA process must be assessed in clinical routine using Outerbridge grading system, but in some patients, especially young athletes, or in studies in which the postoperative success has to be estimated before and after surgery, the use of semiquantitative scoring methods such as those known as WORMS, BLOKS, and MOAKS (see lemmas) must be assessed. Such scoring systems are based on the evaluation of chondral deficiency in conjunction with those of the menisci, subchondral bone, and synovial inflammation.
MRI Techniques and Sequences
For the correct assessment of those scores, it is crucial to pay attention on the imaging protocol used; an optimal contrast between the cartilage and articular fluid and a fine BML estimation and meniscal lesions must be evaluated by dedicated protocols including two- or three-dimensional techniques. The basic protocol provides at least a PD or intermediate-weighted FS sequences in all planes plus one non-FS T1 weighted (coronal preferred). These sequences enable to find also loose bodies and to assess the quality of subchondral bone (granulation tissue vs. sclerosis). If 3D technique were available, a GRE or SSFP must be included. When a chondral defect is suspected on images acquired on low-field articular-dedicated scanner (<0.7 Tesla), we suggest to repeat the exam on a high-field scanner (≥1.5 Tesla), because of the lack of correct contrast between the cartilage and fluid with low-field magnets, which may cause false-positive results. The following are the dedicated sequences for cartilage evaluation:
Fat saturation techniques: These are commonly used but time consuming and sensitive to motion artifacts. It may be replaced by volumetric gradient-recalled echo techniques.
2D PD or FS TSE: This is generally used in the clinical routine. The use of intermediate echo times of 30–60 ms may be helpful, in order to avoid magic-angle artifact and to obtain more contrast between cartilage and articular fluid. Partial volume effects must be considered because of anisotropic acquisition.
3D FSE: This is recently introduced to outperform 2D techniques limits. 3D FSE imaging permits to perform high-quality multiplanar reformations for cartilage segmentation and to evaluate menisci, ligaments, and subchondral damages. An example of 3D FSE is the SPACE (Siemens vendor)/CUBE (GE vendor)/VISTA (Philips vendor) acquisition method, that is, a long acquisition technique with poor cartilage-to-fluid contrast.
3D SPGR: This is the ideal acquisition technique for the assessment of cartilage morphology, especially when a segmentation is required. The sequence may be performed when a long acquisition time is possible (sensitivity to susceptibility artifacts) and during the acute phase of a traumatic chondral lesion when articular fluid is copious (contrast between cartilage and fluid).
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