Introduction
We fall ill or get injured from time to time but when was the last you visited your doctor and was totally pleased with the way you were diagnosed. You probably wished your doctor could precisely view inside your body and pinpoint where and how much damage has been caused to your tissues. We have seen and heard of tiny capsules carrying microcameras which when injected can travel through the blood stream and relay back detailed pictures of the human anatomy. Non-invasive examination of the interior of human body has always been a challenge to the medical community, scientists and engineers alike.

Brief History of Medical Imaging
X-rays which were discovered in the late 19th century by Roentgen was the first step in the direction of studying our interior. Unfortunately, due to ionizing radiation X-rays constituted health risks, but modern techniques are much more safe and sophisticated. Computer Tomography (CT) was a huge breakthrough earning its inventors Hounsfield and Cormack the Nobel Prize for medicine and physiology in 1979. X-ray CT was unique in producing tomographic images or slices of the living human body for the first time and with a higher contrast than achievable by conventional planar techniques. Nuclear magnetic resonance (NMR) which was discovered in the mid 40s offered tremendous potential for a number of medical applications most of which were not immediately apparent at that time. NMR is a non ionizing technique which employs low intensity radio frequency electromagnetic waves to study objects placed in a strong magnetic field. NMR techniques can be truly deemed non-invasive as neither the radiation employed nor the strong magnetic field have been proven harmful to living organisms. Ultrasound was developed in the 1950s following the development of SONAR in WWII. Ultrasound does not involve any ionizing radiation and its ability to image in real time offered the possibility of safe non-invasive imaging.

Magnetic Resonance Imaging (MRI)
The initial concept for the medical application of NMR originated with the discovery by Raymond Damadian in 1971 that certain mouse tumors displayed elevated relaxation times compared with normal tissues in vitro. This exciting discovery opened the door for a completely new way of imaging the human body where the potential contrast between the tissues and disease was many times greater than that offered by X-ray technology and ultrasound. In the early days, this technique was called NMR imaging but as its medical potential grew in the 1970s, the word 'nuclear' was dropped from the title so as not to frighten or alienate patients.

In the last decade, MRI has evolved into one of the most powerful techniques in diagnostic clinical medicine and biomedical research. The level of detail is extraordinary compared to any other imaging modality. MRI is the method of choice for the many types of injuries and conditions because of the incredible ability to tailor the exam to the particular medical question being asked. MRI provides an unparalleled view inside the human body. By changing examination parameters, the MRI system can cause tissues in the body to take on different appearances. This is very helpful to the radiologist (who reads the MR image) in determining if something seen is normal or not. MRI systems can also image flowing blood in virtually any part of the body. This allows to perform studies that show the arterial system in the body.

Principles of MRI
MRI involves an amazing combination of advanced science and engineering, including the use of superconductivity, cryogenics, quantum physics, digital and computer technology and all within the radiology department of your local hospital. It is an imaging method based principally upon the sensitivity to the presence and properties of water which makes up 70 to 90% of most tissues. The properties and the amount of water in tissue can alter dramatically with disease and injury which makes MRI very sensitive as a diagnostic technique. MRI detects subtle changes in the magnetism of the body's water protons. Much like X-ray CT, MRI is a method of obtaining images of the body in thin slices. It measures the characteristics of the hydrogen nuclei of water and nuclei with similar chemical shifts modified by chemical environment across the slice. Advanced MR can be used not just to image anatomy (Fig. 1) and pathology but to investigate organ function to probe in vivo chemistry and even to visualize brain thinking. MRI is truly a powerful modality.

The prinicipal components of the MR system as shown in Fig. 2 are as:
· A magnet (heart of MR system) that produces a strong and constant magnetic field.
· Radiofrequency coils that excite and detect the MR signal.
· Magnetic field gradients which localize the MR signals.
· A computer system for scanner control, image display and archiving.
· Patient handling system.

New Frontiers
In the early days, long scanning times and the need for patients to remain motionless tended to limit the clinical usefulness of MRI. However technical improvements which make ultra high speed imaging possible are now finding their way into clinical use. High speed scanning will open up a whole range of applications, such as real time imaging.

Future developments are likely to see MRI used for real time imaging of cerebrovascular processes and the management of neuropsychiatric disorders such as schizophrenia and Alzheimer's disease. MRI has had major impact on neurology, particularly in the field of brain mapping. Functional MRI may enable a better understanding of the relationship between neuropsychiatric dynamics and mental illness. New techniques to overcome the problem of fetal motion are being developed so MRI may have a role in field of obstetrics. The development of interventional MRI is still in the early stages. Research is currently under way to enhance MRI's capabilities in interventional radiology and image-guided therapy.

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