Magnetic Resonance Imaging (MRI)
Over the last 20 years MRI has become one of the basic imaging tools employed by the medical profession. It has a unique ability to visualise all soft tissue and bony structures within the body with a combination of excellent spatial and contrast resolution.
An MRI scan involves the patient being exposed to a very homogeneous magnetic field. Radiofrequency pulses are employed to stimulate the area of the body under investigation and information returned from that region is used to build up an image of the area that is being investigated. In its simplest form it is a map of hydrogen ion concentrations within the body.
Most MRI scanners will employ a superconducting high field magnet. Most superconducting magnets are circular in construction leaving a central bore within the magnet. It is this central bore that is used for imaging. Usually a surface coil is placed over the part of the body that is going to be studied and the patient is then introduced into the bore of the magnet on a moving table top.
Once positioned all the patient has to do is to remain stationary. Conventionally most studies are completed in a modern high field magnet in between 10 and 30 minutes. The vast majority of musculo-skeletal studies are undertaken without the use of intravenous or intra-articular contrast agents but occasionally for some specific studies an intravenous or intra-articular injection may be required.
There are few contraindications to MRI and the majority of these relate to possible ferromagnetic objects within the body. Almost all orthopaedic implants are non-ferromagnetic and perfectly safe in all MRI scanners. Prior to any MRI study a patient will be required to complete a questionnaire relating primarily to possible contraindications to a study. Possible contraindications will include metal foreign bodies within the eye, cardiac pacemakers and intracranial aneurysm clips.
Some of the common indications for MRI in musculo-skeletal disease are explained below.
MRI will commonly be used for assessment of the rotator cuff. It not only visualises the tendons of the rotator cuff but will clearly define all related musculature. It will clearly define the state of the major articulations of the shoulder girdle including both the gleno-humeral and acromio-clavicular joints.
MRI will demonstrate the state of the hip joint with the identification of early arthritis or traumatic change in the soft tissues adjacent to the hip joint including local subtle change such as labral pathology. It will identify developing abnormalities in the underlying osseous structures including the development of avascular necrosis. It will define the muscles and tendons adjacent to the hip joint. In many older patients it is helpful in defining the state of the tendons around the hip joint including gluteus medius which can be a common source for undiagnosed lateral hip pain in the elderly. In younger patients often those with significant sports related injuries it is able to define even subtle change developing in relation to the origins of many of the major muscle groups within the thighs as well as local muscle pathology. It is, therefore, a very helpful tool in the evaluation of hip, groin and thigh pain in all major age groups.
The ability of MRI to demonstrate different types of soft tissue as well as bone makes it unique for the evaluation of knee problems. Its ability to define soft tissue enables it to demonstrate change within the cartilage and ligaments within the knee joint as well as ligaments and musculature which maintain the function of the joint. It is one of the basic imaging tools for the evaluation of knee pain and acute injuries in the younger patient as well as defining cause for knee pain in both children and the older patient.
Foot & Ankle
As with the knee joint the ability to demonstrate bone and different forms of soft tissue means that MRI can be very sensitive in defining abnormalities developing within the ankle and subtalar joint including arthritis and osteochondritis as well as defining both injuries and more chronic tendon problems developing in relation to the tendons which pass close to the ankle and subtalar joints.
MRI has become the basic tool for the investigation of all spinal pathology. It is the unique ability to demonstrate bone, the soft tissues of the spinal cord and differentiation of cord from surrounding fluid (CSF) that enables MRI to identify all spinal pathology. Its basic role is in the evaluation of problems relating to intervertebral disc disease (including slipped disc) as well as problems with the adjacent joints (interfacetal joints) within the spine. It will define with great accuracy the location of any compression to the underlying nerves or spinal cord. Demonstration of the contents of the bony neural canal including the spinal cord and surrounding CSF enables it to identify virtually all spinal pathologies.
All joints and adjacent soft tissues within the body can be visualised with MRI. Developments with both the main field magnet and surface coils have resulted in MRI being able to clearly demonstrate even the smallest structures within the body. It is used frequently to evaluate elbow and wrist joints as well as investigation of disease in relation to the smaller structures of the hands and feet. Abnormalities within muscle groups including muscle strains and tears and focal swellings are all suitable for evaluation by MRI.