Magnetic resonance imaging

The phenomenon of nuclear magnetic resonance (NMR) was first investigated nearly 50 years ago [39]; since then it has become a standard tool in chemistry and in the last two decades it has served as the basis for a remarkably powerful and flexible medical imaging technique [40]. The intense enthusiasm for and the rapid introduction of MRI into the clinical environment stem from the abundance of diagnostic information present in MR images. Although the image format is similar to CT, the fundamental principles are quite different; in fact, an entirely different part of the atom is responsible for the image formation. In MRI, it is the nucleus that provides the signal used in generating an image. We note that this differs from conventional diagnostic radiology in which the electrons are responsible for the imaging signal. Furthermore, it is not only the nucleus of the atom but also its structural and biochemical environment that influence the signal.

The musculoskeletal system is ideally suited for evaluation by MRI since different tissues display different signal intensities on Tj- and T2-weighted images. The images displayed may have low signal intensity, intermediate signal intensity or high signal intensity. Low signal intensity may be subdivided into: (i) signal void (black); and (ii) signal lower than that of normal muscle (dark). Intermediate signal intensity may be subdivided into: (i) signal equal to that of normal muscle; and (ii) signal higher than muscle but lower than subcutaneous fat (bright). High signal intensity may be subdivided into: (i) signal equal to normal subcutaneous fat (bright); and (ii) signal higher than subcutaneous fat (extremely bright). High signal intensity of fat planes and differences in signal intensity of various structures allow separation of the different tissue components including muscles, tendons, ligaments, vessels, nerves, hyaline cartilage, fibrocartilage, cortical bone and trabecular bone. For instance, fat and yellow (fatty) bone marrow display high signal intensity on Tj-and intermediate signal on T2-weighted images; hematoma displays relatively high signal intensity on both Tj and T2 sequences. Cortical bone, air, ligaments, tendons and fibrocarti-lage display low signal intensity on Tj- and T2-weighted images; muscle, nerves and hyaline cartilage display intermediate signal intensity on Tj- and T2-weighted images. Red (hematopoietic) marrow displays low signal on Tj- and low-to-intermediate signal on T2-weighted images. Fluid displays intermediate signal on Tj- and high signal on T2-weighting. Most tumors display low-to-intermediate signal intensity on Tj-weighted images and high signal intensity on T2-weighted images. Lipomas display high signal intensity on Tj- and intermediate signal on T2-weighted images.

Traumatic conditions of the bones and tissues are particularly well suited to diagnosis and evaluation by MRI. Some abnormalities, such as bone contusion or trabecular microfractures not seen on radiography and CT, are well demonstrated by this technique. Occult fractures, which can be missed on conventional X-ray films, become obvious on MR imaging.

The value of MRI for the imaging of musculoskele-

Fig. 6.2.13 MRI of an injured popliteofibular ligament.

tal conditions such as tumors and osteonecrosis became apparent almost immediately after the introduction of this modality in the early 1980s. Many advances have been made in MRI of the knee since its initial application, in 1984, for evaluation of the meniscus, and MR examination is now routinely used to assess a wide spectrum of internal knee derangement and articular disorders (Fig. 6.2.13) [41 ]. A non-invasive modality, MR has replaced conventional arthrography in the evaluation of the menisci and the cruciate ligaments and has decreased both the morbidity and the cost associated with arthroscopic examinations that yield negative results [42,43]. The decrease in the cost of MR knee studies has also contributed to their acceptance by the orthopedic community as a non-invasive replacement for arthrography and non-therapeutic arthroscopy. With MR imaging, the anatomic and pathologic definition of soft tissue, ligaments, fibrocartilage and articular cartilage is superior to that seen with CT. Fast spin echo imaging, used in conjunction with fat suppression MR techniques, has extended the sensitivity and specificity of MR for the detection of articular cartilage injuries. Additional advantages of MR imaging are multiplanar and thin-section capability and the ability to evaluate subchon-dral bone and marrow. As a result, MR imaging is recommended instead of CT for the evaluation of bone contusion and occult knee fractures, including tibial plateau fractures of the knee. MR has supplanted nuclear scintigraphy in the characterization of os-teonecrosis and, furthermore, can be used to assess the integrity of the overlying articular cartilage surfaces. MR imaging is unique in its ability to evaluate the internal structure as well as the surface of the meniscus. With conventional arthrography, intra-articular injection of contrast media permits visualization of surface anatomy but does not allow delineation of fibrocarti-lage structure or subchondral bone.

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