Magnetic Resonance Imaging (MRI) in Orthopaedics

• Magnetic resonance imaging produces cross-sectional images of any body part in any plane.
• It yields superb soft-tissue contrast, allowing different soft tissues to be clearly distinguished, e.g. ligaments, tendons, muscle and hyaline cartilage.
• Another big advantage of MRI is that it does not use ionising radiation.

MRI Physics 

The patient’s body is placed in a strong magnetic field (between 5 and 30,000 times the strength of the earth’s magnetic field).

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The body’s protons have a positive charge and align themselves along this strong external magnetic field.

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The protons are spinning and can be further excited by radio frequency pulses, rather like whipping a spinning top.

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These spinning positive charges will not only induce a small magnetic field of their own, but will produce a signal as they relax (slow down) at different rates.

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A proton density map is recorded from these signals and plotted in x, y and z coordinates.

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Different speeds of tissue excitation with radio frequency pulses (repetition times, or TR) and different intervals between recording these signals (time to echo, or TE) will yield anatomical pictures with varying ‘weighting’ and characteristics.

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MR Image: A map of proton concentration through a slice of the body.

• A short TR and short TE will result in a T1 weighted image 
• A long TR and long TE will result in a T2 weighted image 

T1-weighted Images

1. Fluids are very dark
2. Water-based tissues are mid-grey and 
3. Fat-based tissues are very bright.

• Have a high spatial resolution and provide good anatomical-looking pictures, so they are known as the ‘ANATOMY SCANS’. 

T2-weighted Images

1. Fluids have the highest intensity (Bright), and
2. Water- and fat-based tissues are mid-grey. 

• Abnormal collections of fluid are bright against the darker normal tissue, so they are known as ‘PATHOLOGY SCANS’.

Structures with little water or fat, such as cortical bone, tendons, and ligaments, remain dark in all types of sequences.

• Proton density (PD) images are also described as ‘balanced’ or ‘intermediate’ as they are essentially a combination of T1and T2 weighting and yield excellent anatomical detail for orthopaedic imaging.

• Fat suppression sequences allow highlighting of abnormal water, which is particularly useful in orthopaedics when assessing both soft tissue and bone marrow oedema. 

Intravenous Contrast

• Enhancement by intravenous contrast relies on an active blood supply and leaky cell membranes.
• Areas of inflammation and active tumour tissue will be highlighted.
• Gadolinium compounds are employed as they have seven unpaired electrons and work by creating local magnetic field disturbances at their sites of accumulation. 

Clinical Applications 

• Magnetic resonance imaging is becoming cheaper and more widely available. Its excellent anatomical detail, soft-tissue contrast and multi-planar capability make it ideal for non-invasive imaging of the musculoskeletal system.
• The MULTI-PLANAR CAPABILITY provides accurate cross-sectional information and the axial images in particular will reveal detailed limb compartmental anatomy.
• The EXCELLENT SOFT-TISSUE CONTRAST allows identification of similar density soft tissues, for example in distinguishing between tendons, cartilage and ligaments.
• By using combinations of T1W, T2W and fat suppressed sequences, specific abnormalities can be further characterized with tissue specificity, so further extending the diagnostic possibilities. 

1. In orthopaedic surgery, MRI of the hip, knee, ankle, shoulder and wrist is now fairly commonplace.
2. It can detect the early changes of bone marrow oedema and osteonecrosis before any other imaging modality. 
3. In the knee, MRI is as accurate as arthroscopy in diagnosing meniscal tears and cruciate ligament injuries.
4. Bone and soft-tissue tumours should be routinely examined by MRI as the intraosseous & extra-osseous extent and spread of disease, as well as the compartmental anatomy, can be accurately assessed. 
5. Additional use of fat suppression sequences determines the extent of peri-lesional oedema and intravenous contrast will demonstrate the active part of the tumour.
6. Intravenous contrast is used to distinguish vascularized from avascular tissue, e.g. following a scaphoid fracture, or in defining active necrotic areas of tumour, or in demonstrating areas of active inflammation.
7. Direct MRI arthrography is used to distend the joint capsule and outline labral tears in the shoulder and the hip. 

• In the ankle, it provides the way to demonstrate anterolateral impingement and assess the integrity of the capsular ligaments.

Limitations

Despite its undoubted value, MRI (like all singular methods of investigation) has its limitations and it must be seen as one of a group of imaging techniques, none of which by itself is appropriate in every situation.

1. Conventional radiographs and CT are more sensitive to soft-tissue calcification and ossification, changes which can easily be easily overlooked on MRI
2. It is contra-indicated in patients with pacemakers and possible metallic foreign bodies in the eye or brain, as these could potentially move when the patient is introduced into the scanner’s strong magnetic field.