MRI Scan: How It Works, What to Expect & Preparation

What Is an MRI Scan and How Does It Work?

An MRI (magnetic resonance imaging) scan is a medical imaging technique that uses a powerful magnetic field and radio waves to create detailed cross-sectional images of the body's internal structures. Unlike X-rays or CT scans, MRI does not use ionizing radiation, making it a safe option for examining soft tissues, the brain, spine, joints, and internal organs.

MRI technology represents one of the most significant advances in medical diagnostics over the past few decades. The technique was developed in the 1970s and 1980s and has since become an essential tool for physicians in diagnosing a wide range of medical conditions. The name "MRI" stands for Magnetic Resonance Imaging, and you may also hear it called MR imaging, MR scan, or nuclear magnetic resonance (NMR) imaging, though the latter term is rarely used today in clinical settings.

The fundamental principle behind MRI is that hydrogen atoms, which are abundant in the human body (mainly in water and fat), behave like tiny magnets. When you lie inside an MRI scanner, the powerful magnetic field temporarily realigns these hydrogen atoms. Radio waves are then pulsed through your body, causing these atoms to produce faint signals. A computer processes these signals to create remarkably detailed images of your internal structures, allowing physicians to visualize organs, blood vessels, bones, and soft tissues with exceptional clarity.

What makes MRI particularly valuable is its ability to distinguish between different types of soft tissue with far greater precision than other imaging methods. While X-rays excel at showing bones and CT scans provide good general images, MRI can reveal subtle differences between healthy and diseased tissue, making it invaluable for detecting tumors, inflammation, injuries, and abnormalities in organs like the brain, heart, and liver. The images can be captured in any plane or direction, giving physicians a comprehensive three-dimensional understanding of the area being examined.

The physics of magnetic resonance imaging

The MRI scanner contains an extremely powerful magnet, typically between 1.5 and 3 Tesla in strength. To put this in perspective, a typical refrigerator magnet is about 0.005 Tesla, meaning an MRI magnet is 300 to 600 times stronger. This powerful magnetic field is always active, which is why metal objects are strictly prohibited in the MRI room.

When you enter the scanner, the magnetic field causes the hydrogen atoms in your body to align in the same direction, like tiny compass needles pointing north. Radiofrequency pulses are then applied, which briefly knock these atoms out of alignment. As the atoms return to their original position, they release energy in the form of radio signals. Different tissues release energy at different rates, which is how the MRI distinguishes between muscle, fat, fluid, bone, and other structures.

Types of MRI scans

There are several specialized types of MRI examinations, each designed to provide specific information:

• Standard structural MRI: Creates detailed anatomical images of organs and tissues
• Functional MRI (fMRI): Measures brain activity by detecting changes in blood flow
• MR angiography (MRA): Visualizes blood vessels without invasive catheterization
• Cardiac MRI: Specialized imaging of the heart's structure and function
• Diffusion-weighted imaging (DWI): Particularly useful for detecting strokes and characterizing tumors
• MR spectroscopy: Analyzes the chemical composition of tissues

How Should You Prepare for an MRI Scan?

Preparing for an MRI requires removing all metal objects including jewelry, watches, and piercings before entering the scanner room. You must inform staff about any implants, pacemakers, or metal in your body. For head and neck scans, avoid makeup and hair products that may contain metal particles. You'll receive specific instructions from your healthcare facility about eating, drinking, and medications before the scan.

Proper preparation for an MRI scan is essential for both your safety and the quality of the images obtained. The preparation requirements can vary depending on which part of your body is being examined and whether contrast material will be used. Your healthcare facility will provide specific instructions tailored to your particular scan, but understanding the general principles will help you prepare effectively.

The most critical preparation step involves removing metal objects and informing staff about anything metallic in or on your body. The MRI's powerful magnetic field can attract ferromagnetic metals with tremendous force, potentially causing serious injury. Additionally, metal objects can interfere with the imaging process, creating artifacts that obscure important details in the final images. Even small items like hairpins, dental retainers, or body piercings can affect image quality.

Before your appointment, take time to review your medical history and make a list of any implants, medical devices, or previous surgeries that may have left metal in your body. This information is crucial for the radiology team to assess whether MRI is safe for you and to adjust the imaging parameters if necessary.

Items that must be removed before entering the MRI room

Before your scan, you will need to remove and leave the following items outside the examination room:

• Jewelry: Rings, necklaces, bracelets, earrings, and body piercings
• Watches and fitness trackers: All electronic wearables
• Mobile phones and electronic devices: Can be damaged and interfere with the scan
• Credit cards and key cards: The magnetic field will erase the data
• Keys and coins: Any loose metal objects in pockets
• Hearing aids: Must be removed before entering the room
• Glasses: Metal frames should be removed
• Dentures with metal components: Discuss with your radiologist
• Wigs with metal clips: Remove or inform staff
• Underwire bras: You'll typically change into a hospital gown

Medical implants and devices to report

It is absolutely essential to inform the MRI staff about any implanted medical devices or metal in your body. While many modern implants are designed to be MRI-compatible, some are not safe in the magnetic field:

Preparing for head and neck MRI scans

If you're having an MRI of your head or neck, additional preparation is important. Avoid using makeup, hair gel, hairspray, or other hair products on the day of your scan. Many cosmetic products contain metallic particles that can interfere with the imaging and create artifacts on the pictures. This includes foundation, mascara, eye shadow, and some sunscreens. Wash your face and hair thoroughly if you've applied any of these products.

Discussing anxiety and claustrophobia

If you experience anxiety about enclosed spaces or medical procedures, contact your healthcare provider before your appointment. Many people feel nervous about MRI scans, and there are several options available to help you through the experience. Some patients find that simply knowing what to expect reduces their anxiety significantly, while others may benefit from mild sedation.

Options for managing anxiety include mild anti-anxiety medication taken before the scan, having a companion present in the room (they will also be screened for metal), using an open MRI machine if available, listening to music through MRI-compatible headphones, or using relaxation techniques. The MRI technologist can also provide reassurance through the intercom throughout the procedure.

What Happens During an MRI Scan?

During an MRI scan, you lie on a motorized table that slides into a large cylindrical scanner. You'll wear headphones to protect your ears from the loud sounds and can communicate with the technologist through an intercom. You must remain still during each image sequence (15 seconds to several minutes each), though you can move slightly between sequences. The entire scan typically takes 15 to 60 minutes.

Understanding what happens during an MRI scan can help reduce anxiety and ensure you're well-prepared for the experience. The procedure follows a standard sequence, though specific details may vary depending on which body part is being examined and your healthcare facility's protocols.

When you arrive for your MRI, you'll first complete a safety screening questionnaire that asks about implants, metal in your body, and other safety considerations. A radiographer or MRI technologist will review this with you and answer any questions. You'll then be asked to change into a hospital gown or scrubs provided by the facility. This ensures no metal from your clothing interferes with the scan. Most facilities have secure lockers where you can store your belongings.

Once you're ready, you'll be guided into the scanning room, which is specially shielded to prevent the magnetic field from affecting equipment outside the room and to block external radio signals from interfering with the images. The MRI machine itself looks like a large tube or doughnut shape. The central opening, called the bore, is where you'll lie during the scan. Modern MRI machines have wider, shorter bores than older models, making the experience more comfortable for many patients.

Positioning and equipment

You'll lie down on a padded table that can slide in and out of the scanner. The technologist will help position you correctly for the specific scan being performed. For head and brain scans, a special frame or coil is placed around your head to capture the clearest images. Similarly, other body parts may require specialized coils to be positioned near the area being examined.

You'll be given headphones or earplugs, which serve two purposes: protecting your ears from the loud sounds the scanner makes and allowing you to listen to music or hear instructions from the technologist. You'll also receive an emergency call button or squeeze ball that you can use at any time if you need to communicate with the staff or if you want to stop the scan.

For most scans, the table will slide into the scanner so that the body part being examined is in the center of the magnetic field. Depending on what's being scanned, your head may be outside the scanner (for knee or foot scans, for example), which many patients find more comfortable.

The sounds of MRI

MRI machines produce a variety of loud sounds during the scanning process, including clicking, thumping, knocking, and buzzing noises. These sounds are produced by the gradient coils inside the machine as they rapidly switch on and off to create the images. The noise level can reach 100 decibels or more, which is why hearing protection is essential.

Each sequence of sounds corresponds to a different type of image being captured. The sounds may change in rhythm and intensity throughout the scan as different sequences are run. Many patients find that listening to music helps distract them from the noise and makes the time pass more quickly.

Staying still during the scan

The most important thing you can do during your MRI is to remain as still as possible. Even small movements can blur the images and may require sequences to be repeated. Each image sequence typically lasts between 15 seconds and several minutes. Between sequences, there are brief pauses during which you can move slightly if needed, swallow, or adjust your position.

The technologist will keep you informed through the intercom, letting you know when each sequence is about to begin and how long it will last. They may also give you specific instructions, such as holding your breath for certain sequences, particularly during abdominal or cardiac imaging.

Duration of the procedure

The total time for an MRI scan varies considerably depending on which body part is being examined and how many different sequences are needed. A simple scan of a single joint might take 20 to 30 minutes, while a comprehensive examination of the brain or abdomen could take 45 to 60 minutes or longer. If contrast material is used, additional time is needed for the injection and extra imaging sequences.

When Is MRI Contrast Needed and Is It Safe?

MRI contrast agents, usually gadolinium-based compounds, are injected intravenously to enhance the visibility of certain tissues, blood vessels, and abnormalities. Contrast is typically used when detailed imaging of tumors, inflammation, blood vessels, or the brain is needed. Gadolinium contrast is generally very safe, with serious adverse reactions being rare. However, patients with severe kidney disease should inform their doctor, as certain contrast agents may not be suitable for them.

Contrast agents play an important role in many MRI examinations by improving the visibility of specific structures or abnormalities that might otherwise be difficult to see. When contrast is used, it creates a more dramatic difference between different tissues, making it easier for radiologists to identify areas of concern such as tumors, infections, or blood vessel problems.

The most commonly used MRI contrast agents are based on gadolinium, a rare earth metal. Unlike the iodine-based contrast used in CT scans, gadolinium contrast rarely causes allergic reactions. The contrast is injected through a small needle into a vein, usually in your arm, during the MRI examination. You may feel a cool sensation as the contrast enters your bloodstream, but this is temporary and typically not uncomfortable.

After the contrast is injected, additional image sequences are captured. The gadolinium temporarily changes the magnetic properties of nearby tissues, making certain structures appear brighter on the images. This enhanced visibility is particularly valuable for detecting and characterizing brain tumors, identifying areas of inflammation, evaluating blood vessels, and assessing whether cancer has spread to other areas.

Common uses for contrast-enhanced MRI

Your doctor may order an MRI with contrast for various reasons, including:

• Brain imaging: To detect tumors, multiple sclerosis lesions, infections, or blood vessel abnormalities
• Spine imaging: To identify infections, tumors, or post-surgical changes
• Musculoskeletal imaging: To evaluate joint inflammation or soft tissue tumors
• Breast MRI: For cancer detection and staging
• Liver imaging: To characterize liver lesions or tumors
• Cardiac MRI: To assess heart muscle damage or inflammation

Safety considerations for gadolinium contrast

For the vast majority of patients, gadolinium-based contrast agents are very safe. However, there are some important considerations:

Patients with severely impaired kidney function may be at risk for a rare condition called nephrogenic systemic fibrosis (NSF), which can occur when gadolinium is not properly eliminated from the body. For this reason, kidney function is typically checked before administering contrast to patients with known kidney disease or risk factors for kidney problems. If you have kidney disease, your doctor will carefully weigh the benefits and risks of using contrast.

Some patients may experience mild side effects after gadolinium injection, including headache, nausea, or a feeling of coldness at the injection site. These effects are typically brief and resolve without treatment. Severe allergic reactions to gadolinium are extremely rare, occurring in fewer than 1 in 10,000 patients.

Recent research has shown that small amounts of gadolinium may remain in the body, particularly in the brain, after contrast-enhanced MRI. While the long-term significance of this finding is still being studied, no adverse health effects have been definitively linked to these trace deposits. Your healthcare provider can discuss the current understanding of this issue with you if you have concerns.

What Should You Expect After an MRI Scan?

After a standard MRI scan without sedation, you can return to your normal activities immediately with no recovery time needed. If you received sedation, you should not drive for the rest of the day and may need someone to accompany you home. Results are typically sent to your referring doctor within 1-2 weeks, though urgent findings may be communicated sooner.

One of the significant advantages of MRI is that it requires no recovery time for most patients. Once the scan is complete, you can get dressed, collect your belongings, and leave the facility. There are no restrictions on eating, drinking, or activities after a standard MRI examination. The magnetic field has no lasting effects on your body, and any mild discomfort from lying still during the scan will resolve quickly.

If contrast was used during your scan, you'll be encouraged to drink plenty of fluids over the next 24 hours to help your kidneys flush the gadolinium from your body. Most people eliminate contrast completely within 24 hours if they have normal kidney function. You may notice your urine appears slightly different in color, which is normal and temporary.

If you received sedation

Patients who received sedation to help them remain calm during the scan will need additional monitoring before leaving the facility. The effects of sedation can take several hours to wear off completely. During this time, you should not drive, operate machinery, make important decisions, or consume alcohol. It's essential to arrange for someone to drive you home and to have someone stay with you for several hours after the procedure.

Receiving your results

After your scan is complete, a radiologist will analyze the images and prepare a detailed report. This process typically takes 1-2 weeks for routine examinations, though the timeline can vary depending on the complexity of the scan and the healthcare facility's workload. For urgent cases, results may be available much sooner.

The radiologist's report is sent to your referring doctor, who will discuss the findings with you. You may receive results through a phone call, at a follow-up appointment, through a patient portal, or by mail. If you haven't heard about your results within the expected timeframe, don't hesitate to contact your doctor's office.

If the radiologist identifies any urgent or unexpected findings, they will typically contact your doctor immediately so that appropriate follow-up care can be arranged promptly.

Is MRI Safe During Pregnancy and Breastfeeding?

MRI is generally considered safe during pregnancy because it doesn't use ionizing radiation. However, it's typically avoided during the first trimester unless medically necessary. Gadolinium contrast agents are usually avoided during pregnancy as they can cross the placenta. For breastfeeding mothers, MRI is safe, and while gadolinium does pass into breast milk in small amounts, current guidelines suggest breastfeeding can continue normally after contrast.

Pregnancy raises special considerations for any medical imaging procedure. Unlike X-rays and CT scans, MRI does not use ionizing radiation, which is known to potentially harm a developing fetus. This makes MRI one of the safer imaging options during pregnancy when imaging is medically necessary. However, as with any medical procedure during pregnancy, the benefits must be carefully weighed against potential risks.

During the first trimester, when the baby's organs are forming, MRI is generally avoided unless the potential benefits clearly outweigh the theoretical risks. This is primarily a precautionary measure, as there is no definitive evidence that MRI causes harm to the developing baby. However, the fetus is exposed to the magnetic field and radio waves, and while extensive research has not identified any adverse effects, some caution is warranted during this critical developmental period.

After the first trimester, MRI is considered a valuable diagnostic tool when other imaging methods are insufficient or when avoiding radiation is important. MRI can be particularly useful during pregnancy for evaluating maternal conditions, assessing fetal abnormalities detected on ultrasound, or diagnosing conditions that affect pregnancy management.

Gadolinium contrast during pregnancy

Gadolinium contrast agents are typically avoided during pregnancy unless absolutely necessary for diagnosis that will affect pregnancy management. Gadolinium crosses the placenta and enters the fetal circulation, and the fetus may have limited ability to excrete it. While no definitive harm has been established, the precautionary principle suggests avoiding contrast when possible.

If contrast is deemed essential, your healthcare team will discuss the specific risks and benefits with you. In some cases, the diagnostic information provided by contrast-enhanced MRI may be crucial for making important decisions about your care or your baby's care.

MRI and breastfeeding

For breastfeeding mothers, MRI itself is completely safe and does not affect breast milk in any way. The magnetic field has no lingering effects and does not affect your ability to breastfeed.

If gadolinium contrast is used, a very small amount (less than 0.04% of the dose) passes into breast milk. Of this small amount, the baby absorbs less than 1% from the gastrointestinal tract. Based on current evidence and guidelines from organizations like the American College of Radiology, breastfeeding can continue normally after gadolinium administration without the need to pump and discard milk. However, if you prefer to be extra cautious, you may choose to pump and discard milk for 12-24 hours after receiving contrast.

How Can Children Prepare for an MRI Scan?

Children often benefit from extra preparation before an MRI to reduce anxiety and help them stay still during the scan. Age-appropriate explanations, practice sessions, and the option to bring a comfort item can help. Young children or those who cannot remain still may need sedation or general anesthesia, which will be arranged and monitored by specialized pediatric staff.

MRI examinations can be challenging for children due to the need to remain still for extended periods in an unfamiliar, enclosed, and noisy environment. However, with appropriate preparation and support, many children can successfully complete MRI scans without sedation. The key is to reduce anxiety through preparation, provide comfort and reassurance during the scan, and have appropriate options available if additional help is needed.

Preparation should begin several days before the scheduled MRI. Use age-appropriate language to explain what will happen, emphasizing that the scan is not painful and that the machine takes special pictures of the inside of the body. Avoid using words like "shot" or "cut," which might cause unnecessary worry. Children's hospitals and radiology departments often have child-friendly educational materials, videos, or coloring books that can help explain the process.

Consider practicing at home by having your child lie still for increasing periods of time while listening to music through headphones. This simulates the MRI experience and helps them understand what will be expected. You might also practice deep breathing or relaxation techniques that they can use during the actual scan.

During the MRI with a child

In most pediatric MRI settings, a parent or caregiver is allowed to stay in the scanning room, which can provide significant comfort and reassurance. The parent must also be screened for metal and will wear hearing protection. Having a familiar face nearby can help the child feel safe and more willing to cooperate.

Many facilities allow children to bring a comfort object such as a favorite stuffed animal or blanket, as long as it contains no metal. Some facilities also have screens that allow children to watch movies during the scan, which can be an excellent distraction.

Sedation for pediatric MRI

For young children (typically under 5-7 years) or those who are unable to remain still due to medical conditions, anxiety, or developmental considerations, sedation may be necessary to obtain quality images. Sedation can range from mild oral medication that helps the child relax to deeper sedation or general anesthesia administered by an anesthesiologist.

If sedation is needed, you'll receive specific instructions about fasting beforehand and what to expect after the procedure. Sedated children are monitored throughout the scan and during recovery. You'll need to plan for your child to rest after the procedure and may need to stay at the facility until the sedation effects have worn off sufficiently.