Magnetic Resonance Imaging – MRI Machine
MRI is an effective tool in the diagnosis of virtually all bone-related diseases. This is due to the high precision of the imaging technology, which is able to detect variations in bone density, early warning signs of tumours, and complications arising from fractures. The technique is also entirely non-invasive, and has significantly decreased the need for explorative surgery and biopsies.
How MRI Works
MRI relies on the variation in response of different cells in the body to magnetic fields. Taking the simple case of hydrogen as an example, the atom may be thought of as a tiny bar magnet, spinning rapidly about its own axis, due to an intrinsic quality of the proton known as spin. A classical analogy to this is to think of the spinning protons as small current loops. If these current loops are placed within an external magnetic field, the loops will be subjected to a twisting force known as torque.
The bones in the body consist of many different substances, each of which is made up of different atomic constituents. When the bone in question is placed in the magnetic field of an MRI scanner, different atoms will respond in different ways. It is this variation in response to magnetic fields that allows the imaging technique to differentiate between areas of different bone density for example.
To understand how MRI is able to contrast different types of biological tissue, we imagine that before the bone is examined, the magnetic moments of its atoms are pointing in random directions. This is because there is no preferential direction in the absence of external fields. Once the magnetic field is switched on, the magnetic moments of some of the atoms in the bone will align with the field. This process is known as spin lattice relaxation.
In reality, despite the strong magnetic field of MRI scanners, a very small fraction of the atoms respond to the field, due to the fact that thermal influences between atoms far outweigh the external magnetic forces. However, one can ascertain a great deal from the small number of atoms that do respond to the field, and through great amounts of empirical data begin to differentiate between benign and malignant tumours, for instance.
Another way in which one can draw distinctions between different cells is to measure the time it takes for magnetic moments in the bone to return to randomized orientation after turning off the external field. This process is known as spin-spin relaxation. This measurement technique, in combination with spin lattice relaxation described above, can non-invasively map out large areas in the body affected by Bone Diseases.
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