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Magnetic Resonance scan (MRI)of shoulder is another safe and harmless diagnostic tool. The diagnostic accuracy of the scan is vastly enhanced when one injects a ‘dye’ inside the joint to ‘coat’ the cartilage and ligaments allowing one to understand the insider of the joint better. This technique is called MRI arthrogram. I almost always recommend MRI arthrogram over plain MRI scan especially to identify internal structural changes in the joint. Plain MRI is sometimes done to identify the quality of muscle in a rotator cuff tear.
MRI uses the principle of ‘exciting’ water molecules (hydrogen atom to be precise) using a very high power magnetic field and then ‘hitting’ the excited molecules with radiowaves. When the radiowaves are stopped the molecules return back to their previous ‘excited’ state and release some energy which is mapped. By changing the sequence of ‘excitement’ and ‘hitting’ different levels of tissues details can be revealed. This gives excellent near anatomic pictures of the inside of the body.
Depending on the amount of water content, the ‘shades of grey’ in the picture will vary. Structures with very little
water will be darker in colour. By changing the sequencing of the magnetic field and the duration and rate of ‘hit’ and ‘relaxation’ of the radiowaves, different structures in the body can be ‘highlighted’ for clarity. The use of ‘dye’ allows one to understand the structure of the fine cartilage and the tendon insertion inside the joint.
By using MRI arthrogram, one can diagnose damages to the cartilage rim of the shoulder socket, integrity of the insertion of the biceps tendon (SLAP lesions - Superior Labrum Anterior to Posterior lesions), damage to tendons, fluid collections in and around shoulder tendons and bursa, evidence of pinching of the tendons, arthritis in AC joint, ganglion around nerves etc.
MRI arthrogram is an expensive and time consuming investigation and is done only when it is truly needed.