The advent of the multidetector CT (MDCT) scanner has increased the applicability of this imaging technique for the assessment of the musculoskeletal system. This technology allows for the acquisition of large data set in the axial plane that can be reconstructed in multiple planes of imaging with the use of multiplanar reconstruction (MPR) algorithm.
Any anatomical part in the human body can now be scanned in the axial plane and the anatomical information can later be reconstructed in the sagittal, coronal, or any orthogonal plane desired in order to better assess complex anatomical structures such as the joints of the axial skeleton and the spine (1).
CT provides poor contrast resolution to evaluate the musculoskeletal system. Since the relative soft-tissue density of cartilage, tendons, and muscle is similar, we cannot resolve adequate soft-tissue differences between these structures. For example the articular cartilage can only be assessed with CT after a positive contrast is introduced in the joint space such as is the case with CT arthrography. CT however provides a superb spatial resolution that allows for the accurate evaluation of fine soft-tissue and bone trabecular details.
CT images may be displayed with various windows suitable to resolve different structures. Bone window images provide the highest resolution of compact and cancellous bone. Soft- tissue window offers moderate resolution of muscle, tendon, ligament, fat, cartilage, and neural structures.
The good resolution and enhanced contrast of MRI for soft-tissue structures, together with its direct multiplanar imaging capability, make it a superb modality for evaluating all the principal constituents of the musculoskeletal system. Although a technical discussion of the physics of MRI is beyond the scope of this chapter, the physiatrist should know the normal and abnormal MRI appearance of various tissues to be able to look at an MR image with confidence and explain the findings to a patient. The MRI signal intensity of any tissue primarily reflects its proton density, its T1 relaxation time, and its T2 relaxation time. Various techniques, including manipulating the repetition time (TR) between the application of radiofrequency pulses or the echo time (TE) between the radiofrequency pulse and the recording of a signal (i.e., echo) produced by the tissue, can emphasize the proton density, T1 relaxation time, or T2 relaxation time features of any tissue (2). The TR and TE are expressed in milliseconds. The most commonly used technique is spin echo, in which short TR and TE will emphasize the T1 relaxation time of a tissue, the so- called T1-weighted image. In general, an image is said to be T1 weighted if TR is less than 1,000 ms and TE is less than 30 ms (e.g., TR = 500 ms, TE = 20 ms). A T2-weighted image generally is accomplished with a TR longer than 1,500 ms and a TE greater than 60 ms (e.g., TR = 2,000 ms, TE = 85 ms). Proton density images are obtained with a long TR and a short TE (e.g., TR = 2,000 ms, TE = 20 ms).
FIGURE 6-1. Normal shoulder MR images. A: An axial scout film with cursors displays the oblique coronal planes parallel to the plane of the scapula, which allow optimal visualization of the supraspinatus. B: An oblique coronal image demonstrates the supraspinatus muscle belly (SsB), supraspinatus tendon (SsT), subacromial-subdeltoid fat plane (FP), acromioclavicular joint (ACJ), deltoid muscle (D), articular cartilage of humeral head and glenoid (AC), glenoid (G), and humeral head (H). C: An axial image displays humeral head (H), glenoid (G), glenoid labrum (L), the inferior glenohumeral ligament (IGL), deltoid muscle (D), subscapsularis tendon (ScT), and biceps tendon (BT). D: A sagittal section demonstrates good resolution of the coracoacromial ligament (CAL) extending from the coracoid process (CP) to the acromion; coracoid process (CP), supraspinatus (SS), infraspinatus (IS).
Most normal tissues demonstrate similar signal intensities on both T1- and T2-weighted images. Compact bone, fibrocartilage, ligament, tendon, and the rapidly flowing blood within the blood vessel typically produce very low signal intensity, referred to as a signal void, and appear black both on T1 and T2 (Fig. 6-1). Muscle demonstrates moderately low signal intensity and appears dark gray. Peripheral nerves demonstrate slightly higher signal intensity than muscle because of the fat content of their myelinated fibers. Hyaline cartilage produces
moderate signal intensity and appears light gray. Fat produces very high signal intensity and appears bright on T1 and T2. Because fat is frequently situated adjacent to ligaments and tendons, it can provide a high-contrast interface for evaluating the integrity of these structures. Adult bone marrow also shows high signal intensity because of its high fat content. Most normal body fluids that are not flowing show low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.
Pathologic processes such as tumor, infection, and abnormal fluids (e.g., edema, joint effusion) show intermediate signal intensity on T1-weighted images and become very hyperintense on T2-weighted images. Pathologic calcifications demonstrate very low signal intensity on both T1-and T2-weighted images.
The direct multiplanar imaging capability of MRI is particularly useful in evaluating obliquely oriented musculoskeletal structures such as the supraspinatus tendon, the cruciate ligaments, and the lateral collateral ligaments of the ankle.
MRI has proved useful in evaluating traumatic, degenerative, inflammatory, and neoplastic pathology of the limbs and spine. It is useful in detecting acute or chronic traumatic injuries and degenerative conditions involving bones, muscles, tendons, ligaments, fibrocartilage, and nerves. Bone pathology, particularly well detected by MRI, includes contusions, osteochondral injuries, stress fractures, marrow replacement by neoplastic cells, and ischemic necrosis. Muscle lesions that MRI is especially sensitive at identifying include strain or contusion, complete rupture, compartment syndrome, myopathies, and atrophy (3). Tendon conditions well depicted by MRI include partial and complete tear, tendinitis, and tenosynovitis. MRI is also very sensitive for detecting partial or complete ligament tears. Fibrocartilaginous injuries or diseases well delineated by MRI include pathology of the menisci, the glenoid labrum, the triangular fibrocartilage of the wrist, and the intervertebral disc. Nerve entrapments well visualized by MRI include spinal nerve encroachment by disc disease or spinal stenosis and carpal tunnel syndrome (CTS) or other entrapment syndromes. Enhanced imaging of normal and injured peripheral nerves can be obtained using a short-tau inversion recovery (STIR) excitation-emission sequence due to the increased sensitivity to free water content associated with tissue edema using this MR recording protocol. The increased signal generated by injured nerves using STIR pulse sequences probably reflects an increase in free water content of the nerve due to altered axoplasmic flow, axonal and/or myelin degeneration, and endoneurial or perineurial edema due to a breakdown in the blood-nerve barrier (4). MRI imaging of denervated skeletal muscle shows increased MR signal using the STIR protocol when there is significant muscular weakness and well-defined changes indicating muscle denervation on needle electromyography (5).
Osteomyelitis causes a reduction in bone marrow signal intensity on T1-weighted images because of the replacement of normal fatty marrow by inflammatory exudate. In T2-weighted images, these areas of active infection become hyperintense.
MRI has particular value in evaluating both bone and soft-tissue neoplasms. Most of them demonstrate moderately low signal intensity on T1-weighted images and very high signal intensity on T2-weighted images.
Emphasis will now be directed to the application of imaging modalities to common regional pathologic conditions of the musculoskeletal system. Particular focus will be given to MRI because of its superb soft-tissue imaging capabilities and its rapidly expanding diagnostic applications.
The important role played by radiology in the diagnosis of diseases has come at the expense of increased radiation exposure to the general population. With the advent of new technology, such as Positron Emission Tomography (PET) studies and MDCT technology, there has been a sharp increase in the number of radiographic examinations performed to the general population and as a consequence an increase in the cumulative radiation dose to the individual patient and the general population. It is an expected outcome for increase in radiation exposure to lead to an increased rate of malignancy. Therefore, increased awareness is needed in issues concerning radiation safety.
Ionizing radiation, especially at high doses, is known to increase the risk of developing cancer. It is estimated that medical exposure might be responsible for 1% of cancer diagnosis in the United States. This rate is expected to increase in the coming years due to the increased number of examinations performed today.
The scientific measurement for the effective dose of radiation is the millisievert (mSv). The background radiation dose for the average person in the United States is about 3 mSv. This is secondary to cosmic radiation and naturally occurring radioactive materials. By comparison, the effective radiation dose for a spinal CT is equivalent to 6 mSv or 2 years of natural background radiation. Radiation exposure is particularly important in pregnant women and pediatric patients due to the cumulative life effect of radiation exposure at a younger age. In nuclear medicine examinations, special precautions are needed. Some of the radiopharmaceuticals used in nuclear medicine can pass into the milk of lactating women (6).
The relative radiation level (RRL) is a radiation measurements used to calculate effective dose. This is the dose used to estimate population total radiation risk associated to an imaging procedure. This takes into account the sensitivity of different body organs and tissues. This estimate cannot assess the specific risk of an individual patient.
Every effort should be made to order examination, which is best indicated to address the clinical concern of the patient. To aid in this regard, the American College of Radiology (acr. org) has established guidelines for the appropriate use of imaging to answer specific clinical questions. The appropriateness criteria can be of help when deciding which imaging study to order to answer a clinical question.