Radio waves of a predetermined frequency are then used to excite these already agitated protons. The latter action induces a “resonant” current within a coil which is the MRI signal. This signal has certain information contained within it, namely strength which varies according to the chemical composition of the tissue from which it emanates and spatial information, namely from where in the body it originates from – rather like co-ordinates in a map. This signal is then used to create the MRI image by the computer using the co-ordinate and strength information and an image is recreated by the computers using different shades of grey to denote strength of the signal and co-ordinate onto a grid rather like making a cross stitch picture on a piece of cloth.
The advantage of this technique is that it is completely harmless to the human body. Even pregnant mothers and unborn babies can be scanned due to this, though scanning in early pregnancy is avoided. Only patients with implants such as cardiac pacemakers cannot be scanned as these devices may be damaged. All implants are not however deterrents to doing a scan. Besides, the information got out of such a scan is immense as certain diseases can be visualized only through an MRI scan.
Any part of the body can be scanned by MRI. The technique is especially helpful in scanning the brain and spine. Tumors, infection, congenital abnormalities, backache etc. are situations in which MRI is extremely helpful. In the scanning of the musculoskeletal system and the joints, MRI is an established mode of diagnosis and it helps in evaluating joint problems. It is the only way that doctors can look at the ligaments and MRI shows injuries to ligaments, cartilage and bone very well. Initial problems of scanning the chest, heart and the abdomen were there as these parts inherently move due to respiration or a beating heart resulting in freezing motion. This has largely been overcome in present generation scanners that are extremely fast and can freeze heart or respiratory motion. Therefore it is possible to scan the heart, look for viable heart muscle in heart attack patients, or look at congenital defects in the heart. Besides, emerging applications of MRI are in looking for congenital defects in the unborn fetus, looking at abdominal problems like gallbladder stones etc.
MRI can give chemical compositional information of abnormal lesions in the body for example, helping doctors to predict if a lesion is cancerous or non cancerous. This is called MR spectroscopy and is available in the high end machines. Functional information of the brain is also possible in high end MRI machines. It is possible to map out many functional areas of the brain such as the areas for motor function, areas for sight, language etc. This can be of immense help when planning brain surgery.
MRI therefore is an extremely useful medical diagnostic tool that has revolutionized approach to treatment of many diseases.