Knee MRI, with its high soft tissue contrast is one of the main imaging tools to depict knee joint pathology. MRI allows accurate imaging of intra-articular structures such as ligaments, cartilage, menisci, bone marrow, synovium, and adjacent soft tissue.
Knee exams require a dedicated extremity coil, providing a homogenous imaging volume and high SNR to ensure best signal coverage. A complete knee MR examination includes for example sagittal and coronal T1 weighted, and proton density weighted pulse sequences +/- fat saturation, or STIR sequences. For high spatial resolution, maximal 4 mm thick slices with at least an in plane resolution of 0.75 mm and small gap are recommended. To depict the anterior cruciate ligament clearly, the sagittal plane has to be rotated 10 - 20° externally (parallel to the medial border of the femoral condyle). Retropatellar cartilage can bee seen for example in axial T2 weighted gradient echo sequences with Fatsat. However, the choice of the pulse sequences is depended of the diagnostic question, the used scanner, and preference of the operator.
Diagnostic quality in knee imaging is possible with field strengths ranging from 0.2 to 3T. With low field strengths more signal averages must be measured, resulting in increased scan times to provide equivalent quality as high field strengths.
More diagnostic information of meniscal tears and chondral defects can be obtained by direct magnetic resonance arthrography, which is done by introducing a dilute solution of gadolinium in saline (1:1000) into the joint capsule. The knee is then scanned in all three planes using T1W sequences with fat suppression. For indirect arthrography, the contrast is given i.v. and similar scans are started 20 min. after injection and exercise of the knee.
Frequent indications of MRI scans in musculoskeletal knee diseases are:
e.g., meniscal degeneration and tears, ligament injuries, osteochondral fractures, osteochondritis dissecans, avascular bone necrosis and rheumatoid arthritis.

See also Imaging of the Extremities and STIR.

MRI of the shoulder with its excellent soft tissue discrimination, and high spatial resolution offers the best noninvasive way to study the shoulder. MRI images of the bone, muscles and tendons of the glenohumeral joint can be obtained in any oblique planes and projections. MRI gives excellent depiction of rotator cuff tears, injuries to the biceps tendon and damage to the glenoid labrum. Shoulder MRI is better than ultrasound imaging at depicting structural changes such as osteophytic spurs, ligament thickening, and acromial shape that may have predisposed to tendon degeneration.
A dedicated shoulder coil and careful patient positioning in external rotation with the shoulder as close as reasonably possible to the center of the magnet is necessary for a good image quality. If possible, the opposite shoulder should be lifted up, so that the patient lies on the imaged shoulder in order to rotate and fix this shoulder to reduce motion during breathing.
Axial, coronal oblique, and sagittal oblique proton density with fat suppression, T2 and T1 provide an assessment of the rotator cuff, biceps, deltoid, acromio-clavicular joint, the glenohumeral joint and surrounding large structures. If a labral injury is suspected, a Fat Sat gradient echo sequence is recommended. In some cases, a direct MR shoulder arthrogram with intra-articular injection of dilute gadolinium or an indirect arthrogram with imaging 20 min. after intravenous injection may be helpful.

See also Imaging of the Extremities.

Nitroxide radicals (or nitroxyl spin labels) are stable organic compounds with theoretical potential for use as a paramagnetic MRI contrast agent. Similar to gadolinium they have an unpaired electron, a property that provides enhancement in T1 based MRI, and a comparable pharmacokinetic. Depending on their structure and chemical bonding, different nitroxides formula may have the potential for use as cardiovascular imaging agents, to enhance the MR imaging on joints (e.g., dendrimer-linked nitroxides have a strong affinity for cartilage), to evaluate brain tumors and infarction, and as a contrast enhancement agent of body/abdominal NMR imaging. Nitroxides are rapidly enzymatically reduced in tissues to products that do not enhance the NMR signal, which can be a problem for MR imaging. In animal experiments with EPRI (electron paramagnetic resonance imaging), tissue redox studies show differences between tumors and normal tissues, which reflect their respective redox status consistent with the reduction/clearance of nitroxides.

The definition of a scan is to form an image or an electronic representation. The MRI scan uses magnetic resonance principles to produce extremely detailed pictures of the body tissue without the need for X-ray exposure or other damaging forms of radiation.
MRI scans show structures of the different tissues in the body. The tissue that has the least hydrogen atoms (e.g., bones) appears dark, while the tissue with many hydrogen atoms (e.g., fat) looks bright. The MRI pictures of the brain show details and abnormal structures (brain MRI), for example, tumors, multiple sclerosis lesions, bleedings, or brain tissue that has suffered lack of oxygen after a stroke. A cardiac MRI scan demonstrates the heart as well as blood vessels (cardiovascular imaging) and is used to detect heart defects with e.g., changes in the thickness and infarctions of the muscles around the heart. With MRI scans, nearly all kind of body parts can be tested, for example the joints like knee and shoulder, lumbar, thoracic and cervical spine, the pelvis including fetal MRI, and the soft parts of the body such as the liver, kidneys, and spleen. The MRI procedure includes three to nine imaging sequences and may take up to one hour.

See also Lumbar Spine MRI, MRI Safety and Open MRI.