What Is Magnetic Resonance Imaging?

Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body in both health and disease. MRI scanners use strong magnetic fields, radio waves, and field gradients to generate images of the inside of the body.

MRI does not involve x-rays, which distinguishes it from computed tomography (CT or CAT). While the hazards of x-rays are now well-controlled in most medical contexts, MRI can still be seen as superior to CT in this regard. MRI can often yield different diagnostic information compared with CT.

There can be risks and discomfort associated with MRI scans. Compared with CT, MRI scans typically: take far more time, are louder, and usually require that the subject go into a narrow, confined tube. In addition, people with some medical implants or other non-removable metal inside the body may be unable to safely undergo an MRI examination.

MRI is based upon the science of nuclear magnetic resonance (NMR). Certain atomic nuclei can absorb and emit radio frequency energy when placed in an external magnetic field. In clinical and research MRI, hydrogen atoms are most-often used to generate a detectable radio-frequency signal that is received by antennas in close proximity to the anatomy being examined. Hydrogen atoms exist naturally in people and other biological organisms in abundance, particularly in water and fat.

For this reason, most MRI scans essentially map the location of water and fat in the body. Pulses of radio waves excite the nuclear spin energy transition, and magnetic field gradients localize the signal in space. By varying the parameters of the pulse sequence, different contrasts can be generated between tissues based on the relaxation properties of the hydrogen atoms therein.

Since its early development in the 1970s and 1980s, MRI has proven to be a highly versatile imaging technique. While MRI is most prominently used in diagnostic medicine and biomedical research, it can also be used to form images of non-living objects. MRI scans are capable of producing a variety of chemical and physical data, in addition to detailed spatial images.

MRI is widely used in hospitals and clinics for medical diagnosis, staging of disease and follow-up without exposing the body to ionizing radiation.

Medical Uses Of Magnetic Resonance Imaging

fmriMRI has a wide range of applications in medical diagnosis and over 25,000 scanners are estimated to be in use worldwide. MRI affects diagnosis and treatment in many specialties although the effect on improved health outcomes is uncertain.

Since MRI does not use any ionizing radiation, its use is generally favored in preference to CT when either modality could yield the same information. (In certain cases, MRI is not preferred as it can be more expensive, time-consuming, and claustrophobia-exacerbating).

MRI is in general a safe technique, although injuries can occur as a result of failed safety procedures or human error. Contraindications to MRI include most cochlear implants and cardiac pacemakers, shrapnel and metallic foreign bodies in the eyes. The safety of MRI during the first trimester of pregnancy is uncertain, but it may be preferable to other options. The sustained increase in demand for MRI within the healthcare industry has led to concerns about cost effectiveness and overdiagnosis.

Neuroimaging

Magnetic resonance imaging is the investigative tool of choice for neurological cancers, as it has better resolution than CT and offers better visualization of the posterior fossa. The contrast provided between grey and white matter makes it the best choice for many conditions of the central nervous system, including demyelinating diseases, dementia, cerebrovascular disease, infectious diseases and epilepsy.

Since many images are taken milliseconds apart, it shows how the brain responds to different stimuli; researchers can then study both the functional and structural brain abnormalities in psychological disorders. MRI is also used in mri-guided stereotactic surgery and radiosurgery for treatment of intracranial tumors, arteriovenous malformations and other surgically treatable conditions using a device known as the N-localizer.

One advantage of MRI of the brain over computed tomography of the head is better tissue contrast, and it has fewer artifacts than CT when viewing the brainstem. MRI is also superior for pituitary imaging. It may however be less effective at identifying early cerebritis.

Cardiovascular

Cardiac magnetic resonance imaging is complementary to other imaging techniques, such as echocardiography, cardiac CT and nuclear medicine. Its applications include assessment of myocardial ischemia and viability, cardiomyopathies, myocarditis, iron overload, vascular diseases and congenital heart disease.

Liver And Gastrointestinal Imaging

Hepatobiliary MR is used to detect and characterize lesions of the liver, pancreas and bile ducts. Focal or diffuse disorders of the liver may be evaluated using diffusion-weighted, opposed-phase imaging and dynamic contrast enhancement sequences. Extracellular contrast agents are widely used in liver MRI and newer hepatobiliary contrast agents also provide the opportunity to perform functional biliary imaging.

Anatomical imaging of the bile ducts is achieved by using a heavily T2-weighted sequence in magnetic resonance cholangiopancreatography (MRCP). Functional imaging of the pancreas is performed following administration of secretin. MR enterography provides non-invasive assessment of inflammatory bowel disease and small bowel tumors.

MR-colonography can play a role in the detection of large polyps in patients at increased risk of colorectal cancer.

Functional MRI

Functional MRI (fMRI) is used to understand how different parts of the brain respond to external stimuli or passive activity in a resting state. Blood oxygenation level dependent (BOLD) fMRI measures the hemodynamic response to transient neural activity resulting from a change in the ratio of oxyhemoglobin and deoxyhemoglobin.

Researchers use statistical methods to construct a 3D parametric map of the brain which indicates the regions of the cortex that demonstrate a significant change in activity in response to the task. fMRI has applications in behavioral and cognitive research, and in planning neurosurgery of eloquent brain areas.

Oncology

MRI is the investigation of choice in the preoperative staging of rectal and prostate cancer, and has a role in the diagnosis, staging, and follow-up of other tumors.

How Magnetic Resonance Imaging Works

medical magnetic resonance imaging scanner

Medical MRI scanner. Credit: Jan Ainali – CC BY 3.0,

To perform a study, the person is positioned within an MRI scanner that forms a strong magnetic field around the area to be imaged. In most medical applications, protons (hydrogen atoms) in tissues containing water molecules create a signal that is processed to form an image of the body.

First, energy from an oscillating magnetic field is temporarily applied to the patient at the appropriate resonance frequency. The excited hydrogen atoms emit a radio frequency signal, which is measured by a receiving coil. The radio signal can be made to encode position information by varying the main magnetic field using gradient coils.

As these coils are rapidly switched on and off they create the characteristic repetitive noise of an MRI scan. The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state. Exogenous contrast agents may be given intravenously, orally or intra-articularly.

MRI requires a magnetic field that is both strong and uniform. The field strength of the magnet is measured in teslas – and while the majority of systems operate at 1.5 T, commercial systems are available between 0.2–7 T.

Most clinical magnets are superconducting magnets, which require liquid helium. Lower field strengths can be achieved with permanent magnets, which are often used in “open” MRI scanners for claustrophobic patients. Recently, MRI has been demonstrated also at ultra-low fields, i.e., in the microtesla-to-millitesla range, where sufficient signal quality is made possible by prepolarization (on the order of 10-100 mT) and by measuring the Larmor precession fields at about 100 microtesla with highly sensitive superconducting quantum interference devices (SQUIDs).

Contrast

Image contrast may be weighted to demonstrate different anatomical structures or pathologies. Each tissue returns to its equilibrium state after excitation by the independent processes of T1 (spin-lattice) and T2 (spin-spin) relaxation.

Examples of T1 weighted, T2 weighted and PD weighted MRI scans

Examples of T1 weighted, T2 weighted and PD weighted MRI scans. Credit: Kieran Maher

To create a T1-weighted image, magnetization is allowed to recover before measuring the MR signal by changing the repetition time (TR). This image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions and in general for obtaining morphological information, as well as for post-contrast imaging.

To create a T2-weighted image, magnetization is allowed to decay before measuring the MR signal by changing the echo time (TE). This image weighting is useful for detecting edema and inflammation, revealing white matter lesions and assessing zonal anatomy in the prostate and uterus.

Contrast Agents

MRI for imaging anatomical structures or blood flow do not require contrast agents as the varying properties of the tissues or blood provide natural contrasts. However, for more specific types of imaging the most commonly used intravenous contrast agents are based on chelates of gadolinium.

In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions are rare, occurring in approx. 0.03–0.1%. Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses—this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo contrast-enhanced CT.

Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illness, nephrogenic systemic fibrosis, which may be linked to the use of certain gadolinium-containing agents.

The most frequently linked is gadodiamide, but other agents have been linked too. Although a causal link has not been definitively established, current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential, and that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.

In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released. Recently, a new contrast agent named gadoxetate, brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: this has the theoretical benefit of a dual excretion path.

McRobbie, Donald W.
MRI from picture to proton
Cambridge, UK ; New York: Cambridge University Press. ISBN 0-521-68384-X

Top Image: keiichiro shikano/Flickr

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