Abstract:
MRI abbreviated as Magnetic Resonance Imaging, is a scan/technique that makes use of string magnetic fields and radio waves to produce detailed images of the moieties inside the body. The MR image that has been generated relies on the water proton (from hydrogen), the spin –lattice and spin relaxation time. As it makes use of powerful magnets, the protons aligns themselves accordingly. It is their orientation that is important. Voxel is the portmanteau for “volume” and “pixel”. As the MRI machine scans the organ millimeter by millimeter, voxels are formed to enclose the signals that have been created by the interaction of proton and magnetic waves. There are several advantages of this device/technique in diagnosis one of it being, that it is non-invasive in nature, however its stands a major drawback of being very expensive as compared to other diagnostic techniques.
Introduction:
Magnetic Resonance Imaging is an imaging paradigm that makes use of non- ionizing radiations to create images for diagnostic purposes. The test involves the use of powerful magnets, radio waves and a computer to produce an intricate image of the internal body parts. MRI does not involve the use of X-rays or the use of ionizing radiations, thus making it a suitable choice over CT scan and PET scans for diagnostic purposes. However people with certain implants or any other non-removable metals inside their body are unable to go through MRI as it involves the use of magnetic fields. Supported by the improvements in the hardware and advances in the pulse sequence design, with high field super conducting magnets and coils the image quality has been dramatically improved since its discovery. Overall, it is one of the most reliable and safe technique for diagnosis owing to its non-invasive nature.
History:
The first fundamental study was done in 1938 by Isidor Issac Rabi when he passed a beam of molecules through the magnetic field, and the molecules emitted radio waves of specific frequencies. The work was further carried forward by Felix Bolch and Edward Purcell. It was until 1971 when Raymond Damadian suggested that the relaxation times could be used to distinguish between a normal and an abnormal cell. It was in 1973 when Paul Lauterbeur and Peter Mansfiel demonstrated that nuclear magnetic resonance can help in accounting the spatial distributions of the spins followed by image reconstruction using Fourier transformation. All this ultimately ended up in constructing a non-invasive approach for image production, and the first human MR images were produced in 1977.
The working:
MRI is based upon the principle of nuclear magnetic resonance. This sophisticated technology deals with the excitation and the changes in the rotational axis of the protons found in the living tissues. It is the spin echo nuclear magnetic resonance measurement that is used to distinguish between a normal and an abnormal cell. In general, magnetic resonance measurements cause no obvious deleterious effect on the tissue. The Human body is large pool of organic matter and 70 % of it is made up of water. Both of them have hydrogen in common. It is the magnetic spin of the hydrogen atom mediated by its single proton, on which the entire concept of MRI relies. As a spinning charged particle it produces a magnetic field (magnetic moment). The nucleus itself does not spin but it is by the virtue of its constituent parts that induces a magnetic moment.
Under the application of the strong external magnetic field produced by the primary superconducting magnets, the nucleus aligns itself either in parallel or perpendicular axis to the external magnetic field which is known as the longitudinal magnetization. A greater proportion of the hydrogen protons aligns themselves parallel to the field indicating that they are in a low energy state and while those aligning themselves in the perpendicular direction indicate a high energy state. The nucleus has its own angular momentum and so it will rotate. The velocity of the rotation around the direction of the field strength is further elucidated by Larmor equation. Further the radio frequency magnetic field is applied perpendicular to the primary magnetic field. The RF magnetic field is pulsated. The absorption of the energy by the nucleus causes a transition from higher to lower energy levels and vice versa upon relaxation. Thus, the energy that has been absorbed by the nuclei induces a voltage that can be suitably detected. The energy that resulted in transition between energy levels is the energy difference between the two nuclear spin states and this depends upon the strength of the magnetic field produced by the primary magnets. By applying RF pulses, multiple “free -induction decay” (FID) are obtained, which is further averaged to get a proper signal to –noise ratio. The FIDs is a time –domain signal. They are generated as a result of contributions made by several different nuclei within the system that is being studied. It is the FIDs that are mathematically resolved by Fourier transformation into an image spectrum that provides biochemical information.
As MR examination involves pulses, and different tissues have different relaxation times. The first time taken T1 is for the magnetic vector to return back to its resting stage and T2 is the time needed for the axial spin to return to its resting stage. As different tissues differ by such, they can be
identified separately. So upon considering the relaxation time between the normal and the abnormal tissue, it has been observed that there is a significant increase in the relaxation time suggesting that there is a significant decrease in the degree of ordering of the intracellular water in the abnormal tissue. This indicates that the motional freedom of abnormal tissue water is high.
The concept revolving around the relaxation time is so efficient that it can further help in distinguishing between a benign tumor and a malignant tumor. So when the radiofrequency supply is switched off the magnetic vector would return back to its resting state and this would cause a signal to be emitted out, and it is this signal that is used for the actual creation of the images. Cross sectional images are developed when the intensity of the received signal is plotted on a grey scale (the conventional ones).
The Nobel –duel:
The Nobel Prize of 2003 was of great controversy, when the Nobel Prize in Physiology or medicine was awarded to Paul Lauterbur and Sir Peter Mansfield for their discoveries concerning the magnetic resonance. The development of MRI was a result of work by thousands of contributors and Raymond Damadian was one of the most important ones as it was solely due to his efforts made in understanding and developing the concept of relaxation time that enabled to go from the test tube NMR of chemicals to the MRI of human tissues.
Dr. Raymond Damadian
The committee allows 3 recipients for any given area of scientific endeavor. However Dr. Damadian was excluded from the Nobel sharing. It has been speculated that the issue that may have led to this injustice was due to Damadian’s religious views and his aggressive development of the business revolving around MRI. Damadian’s Sir.Peter Mansfield Dr.Paul Lauterbeur omission is disservice to the entire scientific community. Awarding Lauterbeur and Mansfield does not lead to any disagreement but there has to be some concrete and valid scientific ground to discard Damadian from sharing his Nobel Prize irrespective of his personality.
The future that it beholds:
The developing world is being benifitted by the advances in the MRI, especially in terms of its technicalities. Much of the focus has been on building high strength magnets to improve the polarization of the protons and increase the overall signal to create better images. Advaances in the hardware have led to increase in sophistication and now 7T MRIs are available for clinical purposes. The images produced are black and white but researchers have now figured out a way to add equivalent colour to the MRI. One such advancement include MRI dynamic colour mapping that helps to measure the motion of soft tissues. Using this technique, the kinetics of orbital tissues such as muscle, optic nerve, orbital fat, etc. can be determined. However there is still a lot to be discovered and intervened.
Raghav Worah
Department of Biochemistry and Biotechnology
St. Xavier’s College, Ahmedabad
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