What is magnetic resonance imaging (MRI)?

(MRI of the Brain, MRI of the Spine)

Magnetic resonance imaging (MRI) is a diagnostic procedure that uses a combination of a large magnet, radiofrequencies, and a computer to produce detailed images of organs and structures within the body. Unlike X-rays or computed tomography (CT scans), MRI does not use ionizing radiation. Some MRI machines look like narrow tunnels, while others are more spacious or wider. MRI scans can last from 30 minutes to two hours.

How does an MRI scan work?

The MRI machine is a large, cylindrical (tube-shaped) machine that creates a strong magnetic field around the patient. The magnetic field, along with radio waves, alters the hydrogen atoms' natural alignment in the body. Pulses of radio waves sent from a scanner knock the nuclei in your atoms out of their normal position. As the nuclei realign back into proper position, the nuclei send out radio signals. These signals are received by a computer that analyzes and converts them into a two-dimensional (2D) image of the body structure or organ being examined.

Magnetic resonance (MRI) may be used instead of computed tomography (CT) in situations where organs or soft tissue are being studied, because MRI is better at telling the difference between normal and abnormal soft tissue.

New uses and indications for MRI have contributed to the development of additional magnetic resonance technology. Magnetic resonance angiography (MRA) is a new procedure used to evaluate blood flow through arteries in a noninvasive (the skin is not pierced) manner. MRA can also be used to detect intracranial (within the brain) aneurysms and vascular malformations (abnormalities of blood vessels within the brain, spinal cord, or other parts of the body).

Magnetic resonance spectroscopy (MRS) is another noninvasive procedure used to assess chemical abnormalities in body tissues, such as the brain. MRS may be used to assess disorders such as HIV infection of the brain , stroke , head injury , coma, Alzheimer's disease , tumors, and multiple sclerosis .

Functional magnetic resonance imaging of the brain (fMRI) is used to determine the specific location of the brain where a certain function, such as speech or memory, occurs. The general areas of the brain in which such functions occur are known, but the exact location may vary from person to person. During functional resonance imaging of the brain, you will be asked to perform a specific task, such as recite the Pledge of Allegiance, while the scan is being done. By pinpointing the exact location of the functional center in the brain, doctors can plan surgery or other treatments for a particular disorder of the brain.

Anatomy of the spine

The spinal column, also called the vertebral or spinal canal, is made up of 33 vertebrae that are separated by spongy disks and classified into distinct areas.

• The cervical area consists of 7 vertebrae in the neck.

• The thoracic area consists of 12 vertebrae in the chest area.

• The lumbar area consists of 5 vertebrae in the lower back area.

• The sacrum has 5 small, fused vertebrae.

• The 4 coccygeal vertebrae fuse to form 1 bone, called the coccyx or tailbone.

The spinal cord, a major part of the central nervous system, is located in the vertebral canal and reaches from the base of the skull to the upper part of the lower back. The spinal cord is surrounded by the bones of the spine and a sac containing cerebrospinal fluid. The spinal cord carries sensory and movement signals to and from the brain, and controls many reflexes.

Anatomy of the brain

The central nervous system (CNS) consists of the brain and spinal cord. The brain is an important organ that controls thought, memory, emotion, touch, motor skills, vision, respirations, temperature, hunger, and every other process that regulates our body.

What are the different parts of the brain?

The brain can be divided into the cerebrum, brainstem, and cerebellum:

• Cerebrum. The cerebrum (supratentorial or front of brain) is composed of the right and left hemispheres. Functions of the cerebrum include: initiation of movement, coordination of movement, body temperature, touch, vision, hearing, judgment, reasoning, problem solving, emotions, and learning.

• Brainstem. The brainstem (midline or middle of brain) includes the midbrain, the pons, and the medulla. Functions of this area include: movement of the eyes and mouth, relaying sensory messages (such as hot, pain, and loud), hunger, respirations, consciousness, cardiac function, body temperature, involuntary muscle movements, sneezing, coughing, vomiting, and swallowing.

• Cerebellum. The cerebellum (infratentorial or back of brain) is located at the back of the head. Its function is to coordinate voluntary muscle movements and to maintain posture, balance, and equilibrium.

More specifically, other parts of the brain include the following:

• Pons. A deep part of the brain, located in the brainstem, the pons contains many of the control areas for eye and face movements.

• Medulla. The lowest part of the brainstem, the medulla is the most vital part of the entire brain and contains important control centers for the heart and lungs.

• Spinal cord. A large bundle of nerve fibers located in the back that extends from the base of the brain to the lower back, the spinal cord carries messages to and from the brain and controls many reflexes.

• Frontal lobe. The largest section of the brain located in the front of the head, the frontal lobe is involved in personality characteristics and movement.

• Parietal lobe. The middle part of the brain, the parietal lobe helps a person to identify objects and understand spatial relationships (where one's body is compared to objects around the person). The parietal lobe is also involved in interpreting pain and touch in the body.

• Occipital lobe. The occipital lobe is the back part of the brain that is involved with vision.

• Temporal lobe. The sides of the brain, these temporal lobes are involved in memory, speech, and sense of smell.

What are the reasons for an MRI of the brain or spine?

MRI may be used to examine the brain and/or spinal cord for injuries or the presence of structural abnormalities or certain other conditions, such as:

• Tumors

• Abscesses

• Congenital abnormalities

• Aneurysms

• Venous malformations

• Hemorrhage, or bleeding into the brain or spinal cord

• Subdural hematoma (an area of bleeding just under the dura mater, or covering of the brain)

• Degenerative diseases, multiple sclerosis , hypoxic encephalopathy (dysfunction of the brain due to a lack of oxygen), or encephalomyelitis (inflammation or infection of the brain and/or spinal cord)

• Hydrocephalus , or fluid in the brain

• Herniation or degeneration of discs of the spinal cord

• Help plan surgeries on the spine, such as decompression of a pinched nerve or spinal fusion

MRI can also help to identify the specific location of a functional center of the brain (the specific part of the brain controlling a function, such as speech or memory) to assist in treatment of a condition of the brain.

There may be other reasons for your doctor to recommend MRI of the spine or brain.

What are the risks of an MRI?

Because radiation is not used, there is no risk of exposure to ionizing radiation during an MRI exam.

Due to the use of the strong magnet, special precautions must be taken to perform an MRI on patients with certain implanted devices such as pacemakers or cochlear implants. The MRI technologist will need some information from you regarding the implanted device, such as the make and model number, to determine if it is safe for you to have an MRI. Patients who have internal metal objects, such as surgical clips, plates, screws or wire mesh, might not be eligible for an MRI.

If there is a possibility that you are claustrophobic, then you can ask your physician to provide you with anti-anxiety medication to take prior to your MRI examination. You should plan to have someone drive you home after the MRI.

If you are pregnant or suspect that you may be pregnant, you should notify your health care provider. To date there is no information indicating that MRI is harmful to an unborn child, however MRI testing during the first trimester is discouraged.

A doctor may order a contrast dye to be used during some MRI exams in order for the radiologist to better view internal tissues and blood vessels on the completed images. If contrast is used, there is a risk for allergic reaction. Patients who are allergic or sensitive to contrast dye or iodine should notify the radiologist or technologist.

There may be other risks depending on your specific medical condition. Be sure to discuss any concerns with your doctor prior to the procedure.

How do I prepare for an MRI?

EAT/DRINK : You may eat, drink and take medications as usual for most MRI exams.  There are some specialty MRI exams that require certain restrictions. You will be provided detailed preparations instructions by Johns Hopkins Medical Imaging when you schedule your exam. 

CLOTHING : You must completely change into a patient gown and lock up all personal belongings. A locker will be provided for you to use. Please remove all piercings and leave all jewelry and valuables at home.

WHAT TO EXPECT : Imaging takes place inside of a large tube-like structure, open on both ends. You must lie perfectly still for quality images. Due to the loud noise of the MRI machine, earplugs are required and will be provided.

ALLERGY : If you have had an allergic reaction to contrast that required medical treatment, contact your ordering physician to obtain the recommended prescription. You will likely take this by mouth 24, 12 and two hours prior to examination.

ANTI-ANXIETY MEDICATION : If you require anti-anxiety medication due to claustrophobia, contact your ordering physician for a prescription. Please note that you will need some else to drive you home.

STRONG MAGNETIC ENVIRONMENT : If you have metal within your body that was not disclosed prior to your appointment, your study may be delayed, rescheduled or cancelled upon your arrival until further information can be obtained.

Based on your medical condition, your health care provider may require other specific preparation.

When you call to make an appointment, it is extremely important that you inform if any of the following apply to you:

• You have a pacemaker or have had heart valves replaced

• You have any type of implantable pump, such as an insulin pump

• You have vessel coils, filters, stents, or clips 

• You are pregnant or think you might be pregnant

• You have any body piercing

• You are wearing a medication patch

• You have permanent eye liner or tattoos

• You have ever had a bullet wound

• You have ever worked with metal (for example, a metal grinder or welder)

• You have metallic fragments anywhere in the body

• You are not able to lie down for 30 to 60 minutes.

What happens during an MRI?