When a nucleus contains either unpaired protons, neutrons, or both, it has angular momentum. This property provides the basis for magnetic resonance imaging (MRI). Because of its abundance in the human body, hydrogen is used for clinical MRI. Within the MR magnet, the atoms are aligned with the magnetic field. The MR signal is generated by subjecting the atoms to radiofrequency (RF) pulsations, and the pulse sequences selected will determine the appearance of the image. The repitition time (TR) is the time between the RF pulses, and the echo time (TE) is the time between the RF pulse and the recording of the signal. T1-weighted images (short TR/short TE) provide the best anatomic detail, but T2-weighted images (long TR/long TE) are often needed to demonstrate pathology. Many unfamiliar artifacts may be encountered on MR images. These can be due to several factors, including inhomogeneities in the magnetic field (for example, those resulting from metallic orthopedic applicances or prior surgery), extraneous RF interference, and motion. Surface coils are commonly used in musculoskeletal MR imaging to improve the quality of the examination by increasing the signal-to-noise ratio over the area of interest.

MRI is rapidly altering the presurgical evaluation for many forms of musculoskeletal pathology, and the indications for MRI will undoubtedly continue to grow. Because of the complexity of this modality and the ability to totally "miss" pathology by the inappropriate choice of imaging plane or pulse sequence, cooperation between the orthopedic surgeon and the radiologist is essential. A close working relationship is required for maximum diagnostic information to be obtained with each examination and for optimal patient care.

Magnetic resonance imaging (MRI) is proving to be an effective means for evaluating the shoulder. The use of a surface coil and high resolution scanning techniques have allowed detailed analysis of the normal anatomy of the shoulder. When evaluating for shoulder pathology, the choice of imaging plane and pulse sequencing will be determined by the suspected pathology. In patients with impingement syndrome, subacromial bursitis, supraspinatus tendinitis, and supraspinatus tendon tear can be differentiated, and the offending component of the subacromial are traumatizing these soft tissues can be identified. MRI is also useful in determining the extent of retraction of the supraspinatus musculotendinous junction and the amount of muscle atrophy in cases of massive, chronic tears. Labral tear or attenuation due to glenohumeral instability can be imaged without the injection of contrast material, and MRI can identify those patients with multidirectional instability. Because of the ability to directly depict bone marrow, MRI is the imaging method of choice for the evaluation of early ischemic necrosis of the humeral head.

Bone infection, or osteomyelitis, is a common medical problem that is often difficult to diagnose. Osteomyelitis is not unusual in diabetic patients with foot ulcers and bedridden patients with decubitus ulcers--populations where diagnosis is especially difficult. Spinal osteomyelitis and septic arthritis require early diagnosis in order to avoid permanent debilitating consequences. Regardless of the clinical setting, imaging plays an important role in establishing the diagnosis and directing treatment. A variety of imaging modalities may be used, including plain radiography, radionuclide imaging, computed tomography (CT), and magnetic resonance imaging (MRI). Decisions regarding the best imaging modality can be challenging and should reflect the location of the suspected infection and associated illness or bony disorders.

Magnetic resonance imaging has had a dramatic effect on the means by which we diagnose ligament injuries. Tears resulting from either acute trauma or overuse can be detected noninvasively, directing appropriate therapy be it conservative or surgical. For the elite athlete, earlier diagnosis leads to earlier intervention, or alternatively, a normal MRI examination can result in an earlier return to play. While MRI is accepted for the diagnosis of certain injuries such as complete tears of the cruciate ligaments of the knee, other injuries, such as partial cruciate ligament tears or tears of the intercarpal ligaments of the wrist, remain controversial.

Coccidioidomycosis, an infection caused by the fungus Coccidioides immitis, rarely affects the hand, but we report an unusual case which mimicked the radiological appearance of an enchondroma in the metacarpal. Curettage and bone grafting in combination with long-term antifungal therapy are necessary for successful treatment of coccidioidomycosis of the hand.

Several new imaging modalities have been found useful in clinical evaluation of patients with rheumatoid arthritis (RA). Magnetic resonance (MR) imaging has proven to be an excellent noninvasive method to evaluate the spine, shoulder, hip, and knees; its use for the evaluation of smaller joints is still being investigated. In patients with RA, MR imaging has been used to evaluate cervical spine instability, rotator cuff tear, osteonecrosis, and osteomyelitis. Patients with RA may have advanced osteoporosis, predisposing to insufficiency fractures. This includes fractures associated with increased activity after hip or knee arthroplasty. Newer methods for measuring the degree of osteoporosis include single photon absorptiometry, dual photon absorptiometry, quantitative computed tomography (CT), and dual-energy projection radiography. It has not yet been determined which of these methods will be most widely used in the future, but quantitative CT and dual-energy projection radiography currently show the most promise. Ultrasonography provides an excellent noninvasive method for the diagnosis of popliteal cysts, and color Doppler sonography can differentiate cyst and popliteal aneurysm. As compared to radiography or conventional CT, high-resolution CT provides an improved method to detect the early changes of RA in the lung parenchyma.

To clarify the choice of imaging parameters for optimal gradient-recalled echo MR scanning of joints, we analyzed the behavior of contrast-to-noise and signal-to-noise ratios for spoiled (i.e., fast low-angle shot [FLASH] or spoiled GRASS) and steady-state (i.e., gradient-recalled acquisition in the steady state [GRASS] or fast imaging with steady precession) techniques at 1.5 T. The analysis is based on tissue characteristics derived from spin-echo measurements of hyaline cartilage and synovial fluid signal in the patellofemoral joints of 11 volunteers. Separate analysis of contrast-to-noise and signal-to-noise ratios for multiplanar (long TR) acquisitions shows that these parameters are each improved compared with single-slice methods. At TRs greater than 250 msec, there is no significant difference in the contrast behavior of FLASH and GRASS. For optimal contrast-to-noise ratio (synovial fluid-cartilage), the best multiplanar sequence (for TE less than 23 msec) is with a short TE and a large flip angle (e.g., 400/9/73 degrees [TR/TE/flip angle]). If a single-scan or three-dimensional technique is desired, than a GRASS sequence at minimal TR and TE and intermediate flip angle (18/9/32 degrees) is best. For optimal signal-to-noise ratio (for both synovial fluid and hyaline cartilage), the best multiplanar sequence uses a short TE and an intermediate flip angle (e.g., 400/9/30 degrees). If a short TR, high signal-to-noise technique is desired, then GRASS (18/9/13 degrees) is superior to FLASH.