Magnetic Resonance Imaging

Summary

Magnetic resonance imaging (MRI) is a noninvasive form of diagnostic radiography that produces images of slices or planes from tissues and organs inside the body. An MRI scan is painless and does not expose the patient to radiation, as an X-ray does. The images produced are detailed and can be used to detect tiny changes in structures within the body, which are extremely valuable clues to physicians in the diagnosis and treatment of their patients. A strong magnetic field is created around the patient, causing the protons of hydrogen atoms in body tissues to absorb and release energy. This energy, when exposed to a radiofrequency, produces a faint signal that is detected by the receiver portion of the MRI scanner, which transforms it into an image.

Definition and Basic Principles

Magnetic resonance imaging (MRI), sometimes called magnetic resonance tomography, is a noninvasive medical imaging method used to visualize the internal structures and some functions of the body. MRI provides much greater detail and contrast between the different tissues in the body than X-rays or computed tomography (CT) without using ionizing radiation. MRI uses a powerful magnetic field to align the nuclear magnetization of protons of hydrogen atoms in the body. A radio frequency alters the alignment of the protons, creating a signal that is detectable by the scanner. The signals are processed through a mathematical algorithm to produce a series of cross-sectional images of the desired area. Image resolution and accuracy can be further refined through the use of contrast agents.

The detailed images produced are extremely valuable in the detection and diagnosing of medical conditions and diseases. The need for exploratory surgery has been greatly reduced, and surgical procedures and treatments can be more accurately directed by the ability to visualize structures and changes within the body.

Background and History

Magnetic resonance imaging is a relatively new scientific discovery, and its application to human diagnostics was first published in 1977. Two American scientists, Felix Bloch at Stanford University and Edward Mills Purcell from Harvard University, were both independently successful with their nuclear magnetic resonance (NMR) experiments in 1946. Their work was based on the Larmor relationship, named for Irish physicist Joseph Larmor, which stated that the strength of the magnetic field matched the radiofrequency. Bloch and Purcell found that when certain nuclei were in the presence of a magnetic field, they absorbed energy in the radiofrequency range of the electromagnetic spectrum and emitted this energy when the nuclei returned to their original state. They termed their discovery nuclear magnetic resonance: “nuclear” because only the nuclei of certain atoms reacted, “magnetic” because a magnetic field was required, and “resonance” because of the direct frequency dependence of the magnetic and radiofrequency fields. Bloch and Purcell were awarded the Nobel Prize in Physics in 1952.

NMR technology was used for the next few decades as a spectroscopy method to determine the composition of chemical compounds. In the late 1960s and early 1970s, Raymond Damadian, a State University of New York physician, found that when NMR techniques were applied to tumor samples, the results were distinguishable from normal tissue. His results were published in the journal Science in 1971. Damadian filed patents in 1972 and 1978 for an NMR system large enough to accommodate a human being that would emit a signal if tumor tissue was detected but did not produce an image. Paul Lauterbur, a physicist from the State University of New York, devised technology that could run the signals produced by NMR through a computed back projection algorithm, which produced an image. Peter Mansfield, a British physicist from the University of Nottingham, further refined the mathematical analysis, improving the image. He also discovered echo-planar imaging, which is a fast imaging protocol for MRI and the basis for functional MRI. Lauterbur and Mansfield shared the 2003 Nobel Prize in Physiology or Medicine. Some controversy still exists regarding Damadian's exclusion from this honor.

How It Works

The human body is made up of about 70 percent water. Water molecules are made up of two hydrogen atoms and one oxygen atom. When exposed to a powerful magnetic field, some of the protons in the nuclei of the hydrogen atoms align with the direction of the field. When a radio frequency transmitter is added, creating an electromagnetic field, a resonance frequency provides the energy required to flip the alignment of the affected protons. Once the field is turned off, the protons return to their original state. The difference in energy between the two states is called a photon, which is a frequency signal detected by the scanner. The photon frequency is determined by the strength of the magnetic field. The detected signals are run through a computerized algorithm to deliver an image. The contrast of the image is produced by differences in proton density and magnetic resonance relaxation time, referred to as T1 or T2.

An MRI scanning machine is a tube surrounded by a giant circular magnet. The patient is placed on an examination table that is inserted through the tube space. Some individuals experience claustrophobia when lying in the closed space of the scanning tube and may be given a mild sedative to reduce anxiety. Children are often sedated or receive anesthesia for an MRI. Patients are required to remain very still during the scan, which normally takes between thirty to ninety minutes to complete. During the scan, patients are usually provided with a hand buzzer or communication device so that they may interact with technicians. The magnetic field is created by passing electric current through a series of gradient coils. The strong magnetic fields are normally safe for patients, with the exception of people with metal implants such as pacemakers, surgical clips or plates, or cochlear implants, making them ineligible for MRI. During the scanning procedure, patients will hear a loud humming, beeping, or knocking noise, which can reach up to 120 decibels. (Patients are often provided with ear protection.) The noise is caused by the interaction of the gradient magnetic fields with the static magnetic field. The gradient coils are subject to a twisting force each time they are switched on and off, and this creates a loud mechanical vibration in the cylinder supporting the coils and surrounding mountings.

To enhance the images, contrast agents can be injected intravenously or directly into a joint. MRI is being used to visualize all parts of the body by producing a series of two-dimensional images that appear as cross-sections or slices. These slices can also be reconstructed to create three-dimensional (3D) views of the entire body or specific parts.

Applications and Products

Research into the applications of magnetic resonance imaging technology beyond basic image generation has been progressing at a tremendous rate. Although the basic images are immensely valuable to physicians and scientists, the application of scientific principles in the development of specialized scans is reaching far beyond original expectations and benefiting healthcare delivery and patient care.

Functional MRI (fMRI) is based on the changes in blood flow to the parts of the brain that accompany neural activity, and it provides visualization of these changes. This has been critical in detecting the brain areas involved in specific tasks, processes, or emotions. fMRI does not detect absolute activity of areas of the brain but it detects differences in activity. During the scan, the patient is asked to perform tasks or is presented with stimuli to trigger thoughts or emotions. The detection of the brain areas that are used is based on the blood oxygenation level dependent (BOLD) effect, which creates a variation signal, linked with the concentration of oxy-/deoxyhemoglobin in each area. These scans are performed every two to three seconds over a period of minutes at a low resolution and do not often require additional contrast media to be used.

Diffusion MRI can measure the diffusion of water molecules in biological tissues. This is incredibly useful in detecting the movement of molecules in neural fiber, which can enable brain mapping, illustrating connectivity of different regions in the brain, and examination of areas of the brain affected by neural degeneration and demyelination, as in multiple sclerosis. Diffusion MRI, when applied to diffusion-weighted imaging, can detect swelling in brain cells within ten minutes of the onset of ischemic stroke symptoms, allowing physicians to direct reperfusion therapy to specific regions in the brain. Previously, computed tomography would take up to four hours to detect similar findings, delaying cerebral perfusion therapy to salvageable areas.

Interventional magnetic resonance imaging is used to guide medical practitioners during minimally invasive procedures that do not involve any potentially magnetic instruments. A subset of this is intraoperative MRI, which is used during surgical procedures. However, most often images are taken during a break from the procedure in order to track progress and success and further guide ongoing surgery.

Magnetic resonance angiography (MRA) and venography (MRV) provide visualization of arteries and veins. The images produced can help physicians evaluate potential health problems such as narrowing of the vessels or vessel walls at risk of rupture as in an aneurysm. The most common arteries and veins examined are the major vessels in the head, neck, abdomen, kidneys, and legs.

Magnetic resonance spectroscopy (MRS) measures the levels of different metabolites in body tissues, usually in the evaluation of nervous system disorders. Concentrations of metabolites such as N-acetyl aspartate, choline, creatine, and lactate in brain tissue can be examined. Information on levels of metabolites is useful in determining and diagnosing specific metabolic disorders such as Canavan's disease, creatine deficiency, and untreated bacterial brain abscess. MRS has also been useful in the differentiation of high-grade from low-grade brain tumors.

Precise treatment of diseased or cancerous tissue within the body is a tremendous advance in healthcare delivery. Radiation therapy simulation uses MRI technology to locate tumors within the body and determine their exact location, size, shape, and orientation. The patient is carefully marked with points corresponding to this information, and precise radiation therapy can be delivered to the tumor mass. This drastically reduces excess radiation therapy and limits damage to healthy tissues surrounding the tumor. Similarly, magnetic resonance-guided focused ultrasound (MRgFUS) allows ultrasound beams to achieve more precise and complete treatment and the ablation of diseased tissues is guided and controlled by magnetic resonance thermal imaging.

In 2019, scientists developed portable MRI scanners to detect infections and tumors of the soft tissues of hands, feet, and elbows. Moreover, a 3D amplified MRI technique was developed in 2021, which detected pulsating brain movements and helped in a noninvasive visualization of brain disorders, obstructions, and aneurysms.

Careers and Course Work

The most common career choice in the field of magnetic resonance imaging is the MRI technician or technologist, individuals who operate the MRI system to effectively produce the desired images for diagnostic purposes while adhering to radiation safety measures and government regulations. Aspirants can work as nuclear medicine technologists, imaging technologists, and diagnostic radiologists. Researchers and government agencies are exploring potential occupational hazards to personnel because of prolonged and frequent exposure to magnetic fields. Technicians first explain the procedure to patients. Then, they ensure that patients do not have any metal present on their person or in their body and position them correctly on the examination table. Some technicians also administer intravenous sedation or contrast media to the patients. During the scan, the technologist observes the patient as well as the display of the area being scanned and makes any needed adjustments to density or contrast to improve picture quality. When the scan is complete, the technologist will evaluate the images to ensure that they are satisfactory for diagnostic purposes. The MRI training program may result in an associate's degree or certificate. Some programs require prior completion of radiology or sonography programs and core competencies in writing, math, anatomy, physiology, and psychology. Once admitted to an accredited program, students receive training that includes patient care, magnetic resonance physics, and anatomy and physiology. The American Registry of Magnetic Resonance Imaging Technologists requires that students complete one thousand hours of clinical training. To satisfy this clinical training requirement, students are assigned to a specific hospital or are rotated through different hospitals. Becoming an MRI technician can take two to three years.

Diagnostic radiologists are physicians who have specialized in obtaining and interpreting medical images such as those produced by MRI. Becoming a radiologist in the United States requires four years of college or university, four years of medical school, and four to five years of additional specialized training. Various universities like West Virginia University and Boise State University in Idaho offer bachelor's degrees and certificate courses in MRI. Existing professionals can also pursue an online training course.

MRI physicists are specialized scientists with diverse backgrounds covering nuclear magnetic resonance (NMR) physics, biophysics, and medical physics, in combination with basic medical sciences, including human anatomy, physiology, and pathology. They also understand engineering issues involving advanced hardware, such as large superconducting magnets, high-power radio frequencies, fast digital data processing, and remote sensing and control. Industrial MRI physicists often work in research and development for biotechnology companies, or they implement new applications and provide support for equipment already installed in healthcare centers. Academic MRI physicists work in a university laboratory or cooperate with a medical center in clinical research and training. Academic research may involve basic science in MRI spectroscopy, functional imaging, contrast media, or echo-planar imaging.

Social Context and Future Prospects

Magnetic resonance imaging allows physicians to see detailed images of the inside of their patients to diagnose and guide treatment more easily. It provides researchers with valuable insight into the metabolism and physiology of the body. Still, some drawbacks make this scientific advance unavailable to some patients because of their economic circumstances or body shape. 

For patients who do not have health insurance, the price of an MRI scan, which can range from USD$400 to USD$12,000 depending on the body part and type of examination, may be beyond what they can afford. Also, MRI systems have weight and circumference restrictions that make many people unsuitable candidates, limiting their quality of acute care. Typically, the weight limit is 350 to 500 pounds for the examination table, and the diameter of the magnetic tube limits the patients’ maximum size. As people tend to be larger, biotechnology companies are working on scanning systems, such as the upright or open concept scanner, to accommodate people of all sizes. MRIs also cannot differentiate between malignant and nonmalignant tumors. Moreover, researchers found that using gadolinium-based contrast agents (GBCAs) in MRI can occasionally cause allergic reactions.

Modern MRI scans offer patients improved implant imaging, fewer susceptibility challenges, and improved lung imaging opportunities. Twenty-first-century technology allows these image improvements with smaller, more portable, and more easily accessed systems. Hospitals were once limited in the placement of MRI machines, but the smaller, helium-free infrastructure does not require a helium chimney and is better suited for a range of placements.

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