Fluid-sensitive sequences, such as short tau inversion recovery (STIR) and T2-weighted (T2-W) (especially fat-suppressed (FS) sequences), characterize cartilage architecture better than T1-W sequences (Fig. 2.3). On these fluid-sensitive sequences, signal intensity of physeal and articular cartilage is higher than that of epiphyseal cartilage, probably due to increased cellularity and free water in the growth plate cartilage compared with epiphyseal cartilage. Because the physeal cartilage has longer T2 values, increasing the echo time (TE) enhances the contrast between epiphyseal and physeal cartilage. 3D dual-echo steady-state imaging (3D-DESS) is also useful for imaging cartilage, improving the accuracy of cartilage thickness assessment (Fig. 2.4). Regardless of the sequence employed, adequate spatial resolution is essential for satisfactory imaging of cartilage.

With development, the cartilage changes in several ways. Epiphyseal cartilage evolves from its infantile appearance of uniform intermediate signal on T1-W images and moderate hyperintensity on fluid-sensitive sequences [12]. As ossification begins, fluid-sensitive sequences demonstrate patchy, heterogeneous (more hyperintense) areas, likely due to increased free water. These are most evident at the posterior distal femoral condyle and the trochlea of the distal humerus, but are also seen elsewhere [14] (Fig. 2.5). Cartilage at weight-bearing areas (e.g., the acetabulum and the distal and proximal femur) develops decreased T2 signal as the child begins walking, probably as water is displaced from cartilage [15] (Fig. 2.6).

The physeal cartilage at both the primary physis and at the peri-epiphyseal secondary physis has specific architecture that yields a trilaminar appearance on MRI (Figs. 2.7 and 2.8). The portion that abuts the metaphysis, the zone of provisional calcification, is densely mineralized and thus markedly hypointense on all sequences. The remainder of the physeal cartilage, like the articular cartilage, resembles unossified epiphyseal cartilage on T1-W images and is predominantly hypointense. However, on fluid-sensitive sequences, this portion of the physis (the germinal and proliferative zones as well as part of the hypertrophic zone) is brighter than epiphyseal cartilage, yielding the second identifiable layer [13]. The physis that surrounds the ossification center in the epiphysis, the secondary physis, has similar architecture and signal intensity but is thinner.

Epiphyseal ossification begins in a single ossification center (as at the distal femur), in two centers (as at the proximal humerus), or in several centers (as at the trochlea of the distal humerus). The contour may be irregular. Normal signal intensity at MRI helps differentiate multiple ossification centers from fragmentation of a single center. If there is a single focus of ossification, ossification begins in the center of the epiphysis. Initially spherical, as endochondral bone forms, the shape changes to hemispheric, with the flat portion adjacent to the primary physis [16]. Although epiphyseal marrow is initially, like the adjacent metaphysis, hematopoietic, within a few months it becomes uniformly fatty (see Chap. 25).

Development of carpal bones, tarsal bones, and apophyses is similar to that of epiphyses. They therefore also demonstrate conversion to fatty marrow much sooner than do the metaphyses of long bones.

In very young children, the primary physis is flat and smooth, but with growth its contour begins to undulate [8]. It should still have uniform thickness, unless it has been damaged (e.g., after fracture or as a result of chronic stress in athletes). The physis thins with maturation and eventually fuses and disappears, sometimes leaving a very thin hypointense linear scar. Physeal fusion usually begins in the center of the physis and spreads to the periphery. The exception to this is at the distal tibia, where the anteromedial physis fuses first (at Kump’s bump) and fusion progresses posterolaterally.

Epiphyseal cartilage is initially perfused by vessels within cartilage canals; these gradually become smaller and less numerous. Vascularization is most readily appreciated after administration of intravenous gadolinium, on fat-suppressed T1-W images. The vascular channels of the cartilaginous epiphysis exhibit linear and punctate areas of high signal intensity (the punctate areas are vascular channels viewed on end). Before the age of 18 months, some channels cross from metaphysis to epiphysis, and vascular canals are arrayed parallel to each other and to the long axis of the bone. As the secondary ossification center develops, the orientation of the vascular channels changes, so that they are arrayed in a spoke-like fashion around the ossifying epiphysis [9] (Fig. 2.9). During most of childhood, metaphyseal and epiphyseal blood flow are separate, with important consequences for trauma as well as spread of infection and tumor.

Gadolinium-enhanced sequences allow definition of the fibrovascular cuff and the primary spongiosa of new bone formation; it also delineates the posterior metaphyseal stripe, which is a subperiosteal band of fibrovascular tissue that contributes to growth (Fig. 2.10). In older patients, specialized MRI techniques such as delayed gadolinium-enhanced MRI (dGEMRIC) and T2 mapping allow visualization of the four unique zones of articular cartilage: superficial or tangential, middle or transitional, deep or radial, and calcified [11]. In higher field-strength magnets, 3D isotropic techniques can be applied to cartilage imaging [17].