Magnetic Resonance Imaging ( M R I )

Magnetic Resonance Imaging ( M R I )

Magnetic resonance imaging (MRI) uses the body's natural magnetic properties to produce detailed images from any part of the body. For imaging purposes the hydrogen nucleus (a single proton) is used because of its abundance in water and fat. The hydrogen proton can be likened to the planet earth, spinning on its axis, with a north-south pole. In this respect it behaves like a small bar magnet. Under normal circumstances, these hydrogen proton “bar magnets” spin in the body with their axes randomly aligned. When the body is placed in a strong magnetic field, such as an MRI scanner, the protons' axes all line up. This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner. MRI scanners come in different field strengths, usually between 0.5 and 1.5  tesla.

 When additional energy (in the form of a radio wave) is added to the magnetic field, the magnetic vector is deflected. The radio wave frequency (RF) that causes the hydrogen nuclei to resonate is dependent on the element sought (hydrogen in this case) and the strength of the magnetic field. The strength of the magnetic field can be altered electronically from head to toe using a series of gradient electric coils, and, by altering the local magnetic field by these small increments, different slices of the body will resonate as different frequencies are applied. When the radio frequency source is switched off the magnetic vector returns to its resting state, and this causes a signal (also a radio wave) to be emitted. It is this signal which is used to create the MR images. Receiver coils are used around the body part in question to act as aerials to improve the detection of the emitted signal. The intensity of the received signal is then plotted on a grey scale and cross sectional images are built up. Multiple transmitted radiofrequency pulses can be used in sequence to emphasise particular tissues or abnormalities. A different emphasis occurs because different tissues relax at different rates when the transmitted radiofrequency pulse is switched off. 

The time taken for the protons to fully relax is measured in two ways. The first is the time taken for the magnetic vector to return to its resting state and the second is the time needed for the axial spin to return to its resting state. The first is called T1 relaxation, the second is called T2 relaxation. An MR examination is thus made up of a series of pulse sequences. Different tissues (such as fat and water) have different relaxation times and can be identified separately. By using a “fat suppression” pulse sequence, for example, the signal from fat will be removed, leaving only the signal from any abnormalities lying within it. Most diseases manifest themselves by an increase in water content, so MRI is a sensitive test for the detection of disease. The exact nature of the pathology can be more difficult to ascertain: for example, infection and tumour can in some cases look similar. A careful analysis of the images by a radiologist will often yield the correct answer.

There are no known biological hazards of MRI because, unlike x ray and computed tomography, MRI uses radiation in the radio frequency range which is found all around us and does not damage tissue as it passes through. Pacemakers, metal clips, and metal valves can be dangerous in MRI scanners because of potential movement within a magnetic field. Metal joint prostheses are less of a problem, although there may be some distortion of the image close to the metal. MRI departments always check for implanted metal and can advise on their safety.

A magnetic resonance imaging (MRI) scan is a common procedure used by hospitals around the world. An MRI Scan utilizes a strong magnetic field and radio waves to create detailed images of the organs and tissues within the body. The development of MRI revolutionized the medical world. Since its discovery, doctors and researchers have developed the use of MRI scans to not only assist in medical procedures but also aid in research. MRI scans are a non-invasive and painless procedure. Unlike X-rays and CT scans, MRI does not ionize radiation, which is potentially harmful to the patient. MRI is an acronym for magnetic resonance imaging. 

The magnets used in an MRI scan need to be constantly cooled to a temperature of absolute zero (-459.67°F). Liquid helium is traditionally used to cool the magnets. An upright MRI scanner has now been created, enabling patients to feel less claustrophobic. An MRI scan uses a large magnet, radio waves, and a computer to create a detailed cross-sectional image of the patient's internal organs and structures. The scanner itself will resemble a large tube with a table in the middle, allowing the patient to slide into the tunnel. An MRI scan differs from CT scans and X-rays because it does not use ionizing radiation, which can be potentially harmful to a patient. MRI scan work - An MRI scanner contains two powerful magnets, which represent the most critical part of the equipment. The human body is largely made of water molecules, which each consists of smaller hydrogen and oxygen atoms. At the centre of each atom lies an even smaller particle called a proton, which serves as a magnet and is sensitive to any magnetic field. Normally the water molecules in our bodies are randomly arranged, but upon entering an MRI scanner, the first magnet causes the body's water molecules to align in one direction, either north or south. The second magnetic field is then turned on and off in a series of quick pulses, causing each hydrogen atom to alter their alignment and then quickly switch back to their original relaxed state when switched off. This creates a knocking sound inside the scanner and is a result of the gradient coils being switched on and off. When electricity is passed through the coil, a magnetic field is created and the coil vibrates, which accounts for the noise you hear.

 Although the patient cannot feel these changes, the scanner can detect them, and in conjunction with a computer, can create a detailed cross-sectional image for the radiologist. The development of the MRI scan represents a huge milestone for the medical world, as doctors, scientists and researchers are now able to examine the insides of the human body accurately using a non-invasive tool. Where an MRI scan is used: Abnormalities of the brain and spinal cord, Tumors, cysts, and other abnormalities in various parts of the body, Injuries or abnormalities of the joints, such as back pain, Certain types of heart problems, Diseases of the liver and other abdominal organs, Causes of pelvic pain in women (e.g. fibroids, endometriosis), Suspected uterine abnormalities in women undergoing evaluation for infertility.