MRI is a non invasive method used to render images of the inside of an object. It is mainly used in medical imaging to demonstrate pathological or other physiological alterations of living tissues. It is also used for detecting rock permeability to hydrocarbons and as a non-destructive testing method to characterize the quality of products such as produce and timber.
In the early years MRI was referred to as nuclear magnetic resonance imaging – NMRI. Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance. The word nuclear has been associated with ionizing radiation exposure and that is not used in MRI. So the prevent patients from making a negative association between MRI and ionizing radiation the word was removed.
Paul Lauterbur who was one of the inventors of MRI originally named the technique zeugmatography, a Greek term meaning ‘that which is used for joining’; it referred to the interaction between the static and the gradient magnetic fields necessary to create an image. It was also formerly referred to as magnetic resonance tomography.
MRI uses non-ionizing radio frequency signals to acquire its images and is best suited for non-calcified tissue. The MRI scanners can generate multiple two-dimensional cross-sections of tissues and three-dimensional reconstructions. It can generate cross-sectional images in any plane including oblique plane. Medical MRI mostly relies on the relaxation properties of excited hydrogen nuclei in water and lipids.
There are three major components to an MRI scanner: a static magnetic field, an RF transmitter and receiver, and three orthogonal, controllable magnetic gradients.
Magnet – the magnet is the largest and most expensive part of the scanner. The remainder of the scanner is built around the magnet. The precision of the magnet is just as important as its strength. Three types of magnets are normally used in this system –
1. Permanent magnet - These are conventional magnets made from ferromagnetic materials like steel. They have limited field strength and have limited stability and precision. There are also certain safety issues as the magnetic field cannot be removed in case of entrapment.
2. Resistive electromagnet – a solenoid wound from copper wire is an alternative to a permanent magnet. Field strength and stability are poor. The electromagnet requires considerable electrical energy during operation which can make it expensive to operate.
3. Superconducting electromagnet - by building an electromagnet from superconducting wire it is possible to develop extremely high field strengths with very high stability. These magnets are costly and the cryogenic helium is expensive and difficult to handle. These are most commonly found in MRI scanners today.
A recent development in MRI technology has been the development of sophisticated multi – element phased array coils which are capable of acquiring multiple channels of data in parallel and this parallel imaging technique uses unique acquisition schemes that allow for accelerated imaging, by replacing some of the spatial coding originating from the magnetic gradients with the spatial sensitivity of the different coil elements.
The first MRI exam was performed on a human being on
The basic design of an MRI machine is a giant cube. The cube in a typical system might be 7 feet tall and 7 feet wide by 10 feet long. A horizontal tube runs from front to back through the magnet. The patient slides into the bore on a special table lying on his or her back. The scan begins when the once the body part to be scanned is in the exact center or isocenter of the magnetic field. The MRI system goes through a patient’s body point by point and builds up a 2-d or a 3-d map of tissue types. 2-d or 3-d images are then created by the integration of all this information.
MRI provides an unparalleled view inside the human body and the level of detail that we can see is extraordinary compared with any other imaging technique. It helps diagnose many types of injuries and conditions because of the incredible ability to tailor the exam to a particular medical question. MRI can also image flowing blood in virtually any part of the body, so studies can be performed based on the arterial system in the body.
MRI systems do not use ionizing radiation and have a very low incidence of side effects. Another major advantage of MRI is its ability to image in any plane. MRI is ideal for:
Diagnosing multiple sclerosis, Tumors of the pituitary gland and brain Infections in the brain, spine or joints. Visualizing torn ligaments in the wrist, knee and ankle and shoulder injuries. Diagnosing tendonitis, evaluating masses in the soft tissues of the body. Evaluating bone tumors, cysts and bulging or herniated discs in the spine. Diagnosing strokes in their earliest stages.
Although MRI is ideal for diagnosing and evaluating many conditions it has a few drawbacks as well.
People who have pacemakers and who are too big cannot be scanned for MRI.
MRI machines can also generate claustrophobic experience to some people. The machine makes a tremendous amount of noise during scanning, which sounds like a continual, rapid hammering. Patients are given ear plugs to muffle the sound. It requires the patient to hold very still for extended periods of time. The slightest movement of the part being scanned can cause very distorted images. Orthopedic hardware in the area of the scan cause severe distortions on the images. The MRI systems are very expensive to purchase and so the exams are also expensive.
The future of MRI is limitless as the technology is still in its infancy. It has been in widespread use only since the past 20 years. Small scanners for imaging specific body parts are being developed. A scanner can simply be placed on your arm, knee or foot and functional brain mapping is helping researchers understand how the brain works.
MRI represents a break through in medical diagnostic and research. More than 60 million investigations are performed each year. In 2003 this imaging technique was awarded the noble prize in physiology or medicine.
It uses radio frequency waves and a strong magnetic field rather than x-rays to provide remarkably clear and detailed pictures of the internal organs and tissues. The technique has proven very valuable for the diagnosis of a broad range. It includes pathologic conditions in all parts of the body like cancer, heart and vascular diseases, stroke and joint and musculoskeletal disorders.
The conventional MRI unit is like a closed cylinder magnet in which the patient has to lie still for some time and is truly enclosed. The ‘short-bore’ systems are wider and shorter and do not fully enclose the patient. The patient is placed on a sliding table and positioned comfortably for the MRI exam. Then the individual MRI sequences are performed and the patient is able to communicate with the radiologist or technologist at any time using an intercom. The exam normally takes 15 to 45 minutes depending on the number of images that are required. A contrast material like gadolinium may be used to enhance the visibility depending on the part of the body which has to be examined.
IBM has already announced that researchers at its labs have demonstrated MRI techniques to visualize nano scale objects. This brings the MRI capability to the nanoscale level for the first time and also crosses a major milestone in a quest to build a microscope that could see individual atoms in 3-d. using magnetic resonance force microscopy – MFRM, they have demonstrated 2-D imaging of an object as small as 90 nanometers. This could help in better understanding of how proteins work. It would also allow scientists to study the atomic structures of molecules. It offers a sensitivity upto 60,000 times better than current magnetic resonance imaging technology. MFRM uses what is known as force detection to overcome the sensitivity limitations of the conventional MRI.
Functional MRI measures signal changes in the brain that are due to changing neural activity. The brain is scanned at a low resolution but at a rapid rate. The lack of harmful effects on the patient and the operator make MR well suited for interventional radiology. The images are produces by an MRI scanner and then are used to guide minimally invasive procedure. Because of its superior imaging of soft tissues MRI is now being utilized to specifically locate tumors within the body in preparation for radiation therapy treatments. MRA – magnetic resonance angiography is used to generate pictures of the arteries in order to evaluate them for stenosis and aneurysms. It is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs. MRV - Magnetic resonance venography is a similar procedure used to image veins.
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