MRI – Types, Indications, Patients Safety, Procedure

MRI

MRI (Magnetic resonance imaging) 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, electric field gradients, and radio waves to generate images of the organs in the body. MRI does not involve X-rays and the use of ionizing radiation, which distinguishes it from CT or CAT scans. Magnetic resonance imaging is a medical application of nuclear magnetic resonance (NMR). NMR can also be used for imaging in other NMR applications such as NMR spectroscopy.

Types of MRI

The following are common magnetic resonance imaging examinations:

  • Functional MRI (fMRI)
  • Breast scans
  • Magnetic resonance angiography (MRA)
  • Magnetic resonance venography (MRV)
  • Cardiac MRI

Indications/Uses of MRI

MR imaging of the body is performed to evaluate

Physicians use an MR examination to help diagnose or monitor treatment for conditions such as

Contra-Indications of MRI

The following are some items that might be contraindicated

  • Foreign bodies from trauma, mechanical heart valves, surgical implants, plates, screws, staples and clips, and prosthetics that contain metal
  • Pacemakers, cochlear implants, drug infusion ports, insulin pumps, deep-brain stimulators, and other electrical devices
  • Metal tooth implants and fillings
  • Accessories such as keys, glasses, piercings, jewelry, hairpins, pagers, watches, wallets, identification badges, and pens
  • Oxygen tanks, carts, chairs, IV poles, and other medical equipment
  • Implanted electric and electronic devices are a strict contraindication to the magnetic resonance imaging, and in particular:
  • heart pacemakers (especially older types)
  • insulin pumps
  • implanted hearing aids
  • neurostimulators
  • intracranial metal clips
  • metallic bodies in the eye

Benefits of MRI

  • MRI is a noninvasive imaging technique that does not involve exposure to ionizing radiation.
  • MR images of the soft-tissue structures of the body—such as the heart, liver, and many other organs— is more likely in some instances to identify and accurately characterize diseases than other imaging methods. This detail makes MRI an invaluable tool in the early diagnosis and evaluation of many focal lesions and tumors.
  • MRI has proven valuable in diagnosing a broad range of conditions, including cancer, heart and vascular disease, and muscular and bone abnormalities.
  • MRI enables the discovery of abnormalities that might be obscured by bone with other imaging methods.
  • MRI allows physicians to assess the biliary system noninvasively and without contrast injection.
  • The contrast material used in MRI exams is less likely to produce an allergic reaction than the iodine-based contrast materials used for conventional x-rays and CT scanning.
  • MRI provides a non-invasive alternative to x-ray, angiography, and CT for diagnosing problems of the heart and blood vessels.

Risks Factors of MRI

Are there risks?

Although MRI does not emit the ionizing radiation that is found in x-ray and CT imaging, it does employ a strong magnetic field. The magnetic field extends beyond the machine and exerts very powerful forces on objects of iron, some steels, and other magnetizable objects; it is strong enough to fling a wheelchair across the room. Patients should notify their physicians of any form of medical or implant prior to an MR scan.

When having an MRI scan, the following should be taken into consideration:

  • People with implants, particularly those containing iron, — pacemakers, vagus nerve stimulators, implantable cardioverter- defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, and capsules from capsule endoscopy should not enter an MRI machine.
  • Noise—loud noise commonly referred to as clicking and beeping, as well as sound intensity up to 120 decibels in certain MR scanners, may require special ear protection.
  • Nerve Stimulation—a twitching sensation sometimes results from the rapidly switched fields in the MRI.
  • Contrast agents—patients with severe renal failure who require dialysis may risk a rare but serious illness called nephrogenic systemic fibrosis that may be linked to the use of certain gadolinium-containing agents, such as gadodiamide and others. Although a causal link has not been established, current guidelines in the United States recommend that dialysis patients should only receive gadolinium agents when essential and that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.
  • Pregnancy—while no effects have been demonstrated on the fetus, it is recommended that MRI scans be avoided as a precaution especially in the first trimester of pregnancy when the fetus’ organs are being formed and contrast agents if used, could enter the fetal bloodstream.
  • Claustrophobia—people with even mild claustrophobia may find it difficult to tolerate long scan times inside the machine. Familiarization with the machine and process, as well as visualization techniques, sedation, and anesthesia, provide patients with mechanisms to overcome their discomfort. Additional coping mechanisms include listening to music or watching a video or movie, closing or covering the eyes, and holding a panic button. The open MRI is a machine that is open on the sides rather than a tube closed at one end, so it does not fully surround the patient. It was developed to accommodate the needs of patients who are uncomfortable with the narrow tunnel and noises of the traditional MRI and for patients whose, size or weight make the traditional MRI impractical. Newer open MRI technology provides high-quality images for many but not all types of examinations.
  • The MRI examination poses almost no risk to the average patient when appropriate safety guidelines are followed.
  • If sedation is used, there are risks of excessive sedation. However, the technologist or nurse will monitor your vital signs to minimize this risk.
  • Although the strong magnetic field is not harmful in itself, implanted medical devices that contain metal may malfunction or cause problems during an MRI exam.
  • Nephrogenic systemic fibrosis is currently a recognized, but rare, a complication of MRI believed to be caused by the injection of high doses of gadolinium-based contrast material in patients with very poor kidney function. Careful assessment of kidney function before considering a contrast injection minimizes the risk of this very rare complication.
  • There is a very slight risk of an allergic reaction if contrast material is injected. Such reactions are usually mild and easily controlled by medication. If you experience allergic symptoms, a radiologist or other physician will be available for immediate assistance.
  • Manufacturers of intravenous contrast indicate mothers should not breastfeed their babies for 24-48 hours after contrast medium is given. However, both the American College of Radiology (ACR) and the European Society of Urogenital Radiology note that the available data suggest that it is safe to continue breastfeeding after receiving intravenous contrast.
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Patient safety tips prior to the procedure of MRI

EAT/DRINK

You may eat, drink and take medications as usual.

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 canceled upon your arrival until further information can be obtained.

  • Pacemaker
  • Pregnancy
  • Claustrophobia
  • History of kidney problems
  • Skin tattoos
  • Neurostimulators (TENS-unit)
  • Implanted drug infusion device (i.e., insulin pump)
  • Exposure of metal fragments to your eye
  • Artificial heart valves
  • Aneurysm clips
  • Cochlear implants
  • Metallic implants and prosthesis
  • Vascular stent or stent-graft
  • History as a metal worker
  • Shrapnel or bullet wounds
  • Dorsal column stimulators
  • Allergy to iodine, or gadolinium
  • History of diabetes
  • Other conditions you believe to be relevant
  • jewelry, watches, credit cards, and hearing aids, all of which can be damaged
  • pins, hairpins, metal zippers, and similar metallic items, which can distort MRI images
  • removable dental work
  • pens, pocket knives, and eyeglasses
  • body piercings

How should I prepare for the procedure?

You may need to wear a hospital gown. Or, you may be allowed to wear your own clothing if it is loose-fitting and has no metal fasteners.

Guidelines about eating and drinking before an MRI vary between specific exams and facilities. Unless you are told otherwise, take food and medications as usual.

Some MRI exams use an injection of contrast material. You may be asked if you have asthma or allergies to iodine contrast material, drugs, food, or the environment. MRI exams commonly use a contrast material called gadolinium. Gadolinium can be used in patients with iodine contrast allergy. A patient is much less likely to be allergic to gadolinium contrast than to iodine contrast. However, even if the patient has a known allergy to gadolinium, it may be possible to use it after appropriate pre-medication.

Tell the technologist or radiologist if you have any serious health problems or recently had surgery. Some conditions, such as severe kidney disease, may require the use of specific types of gadolinium contrast that are considered safe for patients with kidney disease. You may need a blood test to determine whether your kidneys are functioning normally.

Women should always tell their doctor and technologist if there is a chance they are pregnant. MRI has been used since the 1980s with no reports of any ill effects on pregnant women or their unborn babies. However, the baby will be in a strong magnetic field. Therefore, pregnant women should not have an MRI in the first trimester unless the benefit of the exam clearly outweighs any potential risks. Pregnant women should not receive gadolinium contrast unless absolutely necessary.

If you have claustrophobia (fear of enclosed spaces) or anxiety, you may want to ask your doctor to prescribe a mild sedative prior to your exam.

Leave all jewelry and other accessories at home or remove them prior to the MRI scan. Metal and electronic items can interfere with the magnetic field of the MRI unit, and they are not allowed in the exam room. They may cause burns or become harmful projectiles within the MRI scanner room. These items include:

  • jewelry, watches, credit cards and hearing aids, all of which can be damaged
  • pins, hairpins, metal zippers and similar metallic items, which can distort MRI images
  • removable dental work
  • pens, pocket knives and eyeglasses
  • body piercings
  • mobile phones, electronic watches and tracking devices.
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In most cases, an MRI exam is safe for patients with metal implants, except for a few types. People with the following implants may not be scanned and should not enter the MRI scanning area without first being evaluated for safety:

  • some cochlear (ear) implants
  • some types of clips used for brain aneurysms
  • some types of metal coils placed within blood vessels
  • some older cardiac defibrillators and pacemakers

Tell the technologist if you have medical or electronic devices in your body. These devices may interfere with the exam or pose a risk. Many implanted devices will have a pamphlet explaining the MRI risks for that particular device. If you have the pamphlet, bring it to the attention of the scheduler before the exam. MRI cannot be performed without confirmation and documentation of the type of implant and MRI compatibility. You should also bring any pamphlet to your exam in case the radiologist or technologist has any questions.

If there is any question, an x-ray can detect and identify any metal objects. Metal objects used in orthopedic surgery generally pose no risk during MRI. However, a recently placed artificial joint may require the use of a different imaging exam.

Tell the technologist or radiologist about any shrapnel, bullets, or other metal that may be in your body. Foreign bodies near and especially lodged in the eyes are very important because they may move or heat up during the scan and cause blindness. Dyes used in tattoos may contain iron and could heat up during an MRI scan. This is rare. Tooth fillings, braces, eyeshadows and other cosmetics usually are not affected by the magnetic field. However, they may distort images of the facial area or brain. Tell the radiologist about them.

Infants and young children often require sedation or anesthesia to complete an MRI exam without moving. This depends on the child’s age, intellectual development and the type of exam. Sedation can be provided at many facilities. A specialist in pediatric sedation or anesthesia should be available during the exam for your child’s safety. You will be told how to prepare your child.

Some facilities may have personnel who work with children to help avoid the need for sedation or anesthesia. They prepare children by showing them a dummy MRI scanner and playing the noises they might hear during the exam. They also answer any questions and explain the procedure to relieve anxiety. Some facilities also provide goggles or headsets so the child can watch a movie while the scan is being done. This helps the child stay still and allows for good quality images.

Other Tips of MRI

  • Please leave your valuables at home, including jewelry, to prevent it from being lost or stolen, for they have to be removed prior to entering the scan room.
  • Please let us know if you need interpreting services, this can be arranged for you.
  • Please bring a list of your current medications.
  • If you have claustrophobia, your doctor may prescribe an oral medication for you to take with you for your MRI appointment.
  • cochlear (ear) implant
  • some types of clips used for brain aneurysms
  • some types of metal coils placed within blood vessels
  • nearly all cardiac defibrillators and pacemakers

What happens during an MRI?

rxharun.com/Block diagram of an MRI imaging system.

MRI may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your doctor’s practices.

Generally, MRI follows this process

  • You will be asked to remove any clothing, jewelry, eyeglasses, hearing aids, hairpins, removable dental work, or other objects that may interfere with the procedure.
  • If you are asked to remove clothing, you will be given a gown to wear.
  • If you are to have a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast dye.
  • You will lie on a scan table that slides into a large circular opening of the scanning machine. Pillows and straps may be used to prevent movement during the procedure.
  • The technologist will be in another room where the scanner controls are located. However, you will be in constant sight of the technologist through a window. Speakers inside the scanner will enable the technologist to communicate with and hear you. You will have a call button so that you can let the technologist know if you have any problems during the procedure. The technologist will be watching you at all times and will be in constant communication.
  • You will be given earplugs or a headset to wear to help block out the noise from the scanner. Some headsets may provide music for you to listen to.
  • During the scanning process, a clicking noise will sound as the magnetic field is created and pulses of radio waves are sent from the scanner.
  • It will be important for you to remain very still during the examination, as any movement could cause distortion and affect the quality of the scan.
  • At intervals, you may be instructed to hold your breath, or to not breathe, for a few seconds, depending on the body part being examined. You will then be told when you can breathe. You should not have to hold your breath for longer than a few seconds.
  • If contrast dye is used for your procedure, you may feel some effects when the dye is injected into the IV line. These effects include a flushing sensation or a feeling of coldness, a salty or metallic taste in the mouth, a brief headache, itching, or nausea, and/or vomiting. These effects usually last for a few moments.
  • You should notify the technologist if you feel any breathing difficulties, sweating, numbness, or heart palpitations.
  • Once the scan is complete, the table will slide out of the scanner and you will be assisted off the table.
  • If an IV line was inserted for contrast administration, the line will be removed.
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While the MRI procedure itself causes no pain, having to lie still for the length of the procedure might cause some discomfort or pain, particularly in the case of a recent injury or invasive procedure such as surgery.

How does the procedure work?

Unlike conventional x-ray examinations and computed tomography (CT) scans, MRI does not utilize ionizing radiation. Instead, radiofrequency pulses re-align hydrogen atoms that naturally exist within the body while you are in the scanner without causing any chemical changes in the tissues. As the hydrogen atoms return to their usual alignment, they emit different amounts of energy that vary according to the type of body tissue from which they come. The MR scanner captures this energy and creates a picture of the tissues scanned based on this information.

The magnetic field is produced by passing an electric current through wire coils in most MRI units. Other coils, located in the machine and in some cases, placed around the part of the body being imaged, send and receive radio waves, producing signals that are detected by the coils. The electric current does not come in contact with the patient.

A computer then processes the signals and generates a series of images, each of which shows a thin slice of the body. The images can then be studied from different angles by the interpreting radiologist.

Frequently, the differentiation of abnormal (diseased) tissue from normal tissues is better with MRI than with other imaging modalities such as x-ray, CT, and ultrasound.

What are the benefits vs. risks?

Benefits

  • MRI is a noninvasive imaging technique that does not involve exposure to radiation.
  • MR images of the soft-tissue structures of the body—such as the heart, liver and many other organs— is more likely in some instances to identify and accurately characterize diseases than other imaging methods. This detail makes MRI an invaluable tool in early diagnosis and evaluation of many focal lesions and tumors.
  • MRI has proven valuable in diagnosing a broad range of conditions, including cancer, heart and vascular disease, and muscular and bone abnormalities.
  • MRI can detect abnormalities that might be obscured by bone with other imaging methods.
  • MRI allows physicians to assess the biliary system noninvasively and without contrast injection.
  • The MRI gadolinium contrast material is less likely to cause an allergic reaction than the iodine-based contrast materials used for x-rays and CT scanning.
  • MRI provides a non-invasive alternative to x-ray, angiography, and CT for diagnosing problems of the heart and blood vessels.
  • The MRI exam poses almost no risk to the average patient when appropriate safety guidelines are followed.
  • If sedation is used, there is a risk of using too much. However, your vital signs will be monitored to minimize this risk.
  • The strong magnetic field is not harmful. However, it may cause implanted medical devices to malfunction or cause distortion of the images.
  • Nephrogenic systemic fibrosis is a recognized, but rare, complication related to injection of gadolinium contrast. It usually occurs in patients with serious kidney disease. Your doctor will carefully assess your kidney function before considering a contrast injection.
  • There is a very slight risk of an allergic reaction if contrast material is used. Such reactions are usually mild and controlled by medication. If you have an allergic reaction, a doctor will be available for immediate assistance.
  • IV contrast manufacturers indicate mothers should not breastfeed their babies for 24-48 hours after contrast material is given. However, the most recent American College of Radiology (ACR) Manual on Contrast Media reports that studies show the amount of contrast absorbed by the infant during breastfeeding is extremely low. For further information please consult the ACR Manual on Contrast Media and its references.

Metal implants or fragments

Having something metallic in your body doesn’t necessarily mean you can’t have an MRI scan, but it’s important for medical staff carrying out the scan to be aware of it.They can decide on a case-by-case basis if there are any risks, or if further measures need to be taken to ensure the scan is as safe as possible.

For example, it may be possible to make a pacemaker or defibrillator MRI-safe or to monitor your heart rhythm during the procedure. You may need to have an X-ray if you’re unsure about any metal fragments in your body.

Examples of metal implants or fragments include:

  • a pacemaker – a small electrical device used to control an irregular heartbeat
  • an implantable cardioverter-defibrillator (ICD) – a similar device to a pacemaker that uses electrical shocks to regulate heartbeats
  • metal plates, wires, screws or rods – used during surgery for bone fractures
  • a nerve stimulator – an electrical implant used to treat long-term nerve pain
  • a cochlear implant – a device similar to a hearing aid that’s surgically implanted inside the ear
  • a drug pump implant – used to treat long-term pain by delivering painkilling medication directly to an area of the body, such as the lower back
  • brain aneurysm clips – small metal clips used to seal blood vessels in the brain that would otherwise be at risk of rupturing (bursting)
  • metallic fragments in or near your eyes or blood vessels (common in people who do welding or metalwork for a living)
  • prosthetic (artificial) metal heart valves
  • penile implants – used to treat erectile dysfunction (impotence)
  • eye implants – such as small metal clips used to hold the retina in place
  • an intrauterine device (IUD) – a contraceptive device made of plastic and copper that fits inside the womb
  • artificial joints – such as those used for a hip replacement or knee replacement
  • dental fillings and bridges
  • tubal ligation clips – used in female sterilization
  • surgical clips or staples – used to close wounds after an operation

Report that are found in imaging

The standard display of MRI images is to represent fluid characteristics in black and white images, where different tissues turn out as follows:

References

mri