Mri what is it
Mri what is it
MRI: The basics
Author: Jana Vasković MD • Reviewer: Dimitrios Mytilinaios MD, PhD
Last reviewed: July 06, 2022
Reading time: 17 minutes
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Back in 1632, the famous painter Rembrandt made an oil painting called The Anatomy Lesson of Dr. Nicolaes Tulp. Seven doctors gathered around a cadaver studying anatomy. Luckily, today we have radiological anatomy as a whole new discipline that enables doctors to visualize inner body structures by using technology to produce images of the specific body regions and organs.
Radiology today uses three modes of image production:
The subject of interest in this specific article is the method that uses the emission of energy as a modality of image production, and that is magnetic resonance imaging (MRI).
What is MRI and how does it work?
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 creates images by distinguishing between the nuclear magnetic properties of various tissues, a property that makes MRI capable for very precise tissue differentiation. MRI utilizes no ionizing radiation that could damage the tissue, but instead it produces images by using a magnetic field and radio waves.
| Basis | Creation of 2D and 3D images by distinguishing between the nuclear magnetic properties of various tissues |
| Energy | Magnetic fields, radio waves |
| T1 Weighted Images | High signal for fat, high signal for contrast substances (Gadolinium), low signal for water |
| T2 Weighted Images | Low signal for fat, low signal for contrast substances, high signal for water |
| Fluid Attenuated Inversion Recovery | Similar to T1 weighted images |
| Proton Density | High signal for fat but lower than in T1, intermediate signal for water but lower than T2 |
| Scanner Types | Open, closed |
| Contraindications | Metal implants, pregnancy, allergies to contrast substances, kidney disease |
| Advantages | Safe (no ionizing radiation), excellent ability for soft tissue differentiation, multiplanar imaging, image quality not degraded by bone or air |
Since it is clear that an MRI image is made after interaction between a specific tissue and an MRI machine that uses some kind of physical mechanisms, let’s define and explain both aspects of that interaction:
When it comes to the tissue properties, it is all about protons. It is known that protons behave like magnet bars, which means they have one positive and one negative pole, and that makes them responsive to external magnetic fields. And since the human body consists mostly of water and fat molecules, that gives it a huge amount of hydrogen (H+) as a source of protons that is needed for interaction with specific radio-waves of the MRI scanner. So, in this manner of speaking, our body composition actually makes the MRI capable of mapping the location of water and fat in the body.
To fully understand the MRI technical background, let’s clear up few more things that are important about protons. Each proton spins around its axis, like a ballerina doing a pirouette. While it spins, it constantly changes the »phase».
When hydrogen protons are exposed to a strong magnetic field, such as the one of the MRI scanner, most of them will align with that field.
Afterward, the MRI hits protons with a radio wave pulse that gives them the energy to start rotating in the clockwise direction until full 180 degree rotation, when they realign with the magnetic field but in the opposite direction.
Since this radio wave is transmitting energy to protons (excites them), once it is turned off, protons relax and realign with the external magnetic field once again, releasing electromagnetic energy along the way.
The density of protons is one of the factors that defines the contrast and illumination of the final image. The more protons one specific tissue has, the final image will be lighter, and tissues with less protons will give darker image. That is the key of forming contrast resolution of a final image so each tissue can be differentiated based on the number of protons it has.
Besides the density of the protons, a couple of more factors contribute to forming of the final MRI image and they are related to the aforementioned relaxation of protons. Relaxation consists of two processes, called T1 relaxation time and T2 relaxation time.
So, MRI is capable to detect the relaxation energy and differentiates the tissues based on how quickly they release that energy after the radio wave pulse is turned off. Combination of getting the image in T1 and T2 time gives a complete overview about the density of different tissues.
After MRI builds a map of the tissue types of the scanned area, a computer that is connected to the scanner integrates all of the information using specific mathematical formula, and finally produces 2D and 3D images of the tissue. After comparing the properties of emitted signals from tissue that is filmed to the values of the normal tissue, it is easier to conclude if those tissues are undergoing some pathological process.
What are the types of MRI?
An MRI sequence is a particular setting of radiofrequency pulses and gradients, resulting in particular image appearance. The most common MRI sequences are T1-weighted, T2-weighted, Fluid Attenuated Inversion Recovery (FLAIR) and Proton Density weighted image (PD).
To fully understand the sequences, let’s define a couple of terms used in radiology:
Combining different values of the Repetition Time and Time to Echo defines the upper mentioned MRI sequences. So, in this manner of speaking, we can state the following:
In general, T1 and T2-weighted images can easily be differentiated by the look on the cerebrospinal fluid. On T1-weighted images, cerebrospinal fluid is dark, while on T2-weighted images is bright.
For this reasons, PD sequence is very useful within the following clinical procedures:
Types of MRI scanners
When it is about physical properties of a scanner, we can talk about closed or open MRI. A regular MRI scanner, or closed MRI, is a large tube surrounded by a magnet. The patient is placed on the sliding table and then slid into the tube.
For claustrophobic or overweight individuals, more convenient type of scanner is an open MRI, but it produces images of a slightly less quality than the closed variant.
Indications?
MRI has a wide range of indications when it comes to medical diagnosis. Generally speaking, it is the method of choice when the staging of the majority of cancers (prostate, breast, lung etc) is required.
To be more specific, let’s list the usage of MRI by organ or system:
Contraindications?
As much as being familiar with indications for MRI, knowing the contraindications can literally save someone’s life. Here is the must-know list:
The rest of the contraindications are relative and they refer to the contrast agents and not the MRI itself, so the MRI can be used as the method of diagnostics but without using the contrast agents. These are:
Advantages over other techniques?
Advantages over other imaging modalities are numerous. MRI is mostly compared to CT, ultrasound and PET scan.
First of all, unlike the ultrasound, it can produce images through the entire section of the body while the presence of the bone or air will not degrade the quality of the image. MRI spectroscopy provides important clinical benefits through the in vivo characterization of chemical composition and metabolic activity.
But since MRI is mostly compared to CT, let’s be more specific about that.
Highlights
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. Because it’s mostly non-invasive, it gives a full image of a patient’s inner state to the clinician without performing complex procedures, and because it utilizes no ionizing radiation, it guarantees safety to the patient.
The capability of understanding how MRI works is very important for future clinicians. Here’s why:
MRI is the most commonly performed diagnostic test after the CT and X-rays, so mastering your knowledge in each field of that radiological triad is ‘a must’ that will benefit your studying process and make your life as a future clinician a lot easier.
Sources
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MRI: What You Need to Know
In this Article
What Is an MRI?
Magnetic resonance imaging (MRI) is a test that uses powerful magnets, radio waves, and a computer to make detailed pictures of the inside of your body.
Your doctor can use this test to diagnose you or to see how well you’ve responded to treatment. Unlike X-rays and computed tomography (CT) scans, MRIs don’t use the damaging ionizing radiation of X-rays.
How Do Doctors Use MRIs?
An MRI helps a doctor diagnose a disease or injury, and it can monitor how well you’re doing with a treatment. MRIs can be done on different parts of your body. It’s especially useful for looking at soft tissues and the nervous system.
An MRI of the brain and spinal cord can help find many things, including:
An MRI of the heart and blood vessels looks for:
An MRI of the bones and joints looks for:
MRIs can also be done to check the health of these organs:
A special kind of MRI called a functional MRI (fMRI) maps brain activity.
This test looks at blood flow in your brain to see which areas become active when you do certain tasks. An fMRI can detect brain problems, such as the effects of a stroke, Alzheimer’s, or a brain injury. It can also be used for brain mapping if you need brain surgery for epilepsy or tumors. Your doctor can use this test to plan your treatment.
What Are the Risks of MRI?
Pregnant women shouldn’t get an MRI during their first trimester unless they absolutely need the test. The first trimester is when the baby’s organs develop. You also shouldn’t get contrast dye when you’re pregnant.
Don’t get contrast dye if you’ve had an allergic reaction to it in the past or you have severe kidney disease.
Certain people with metal inside their body can’t get this test, including those with:
How Should You Prepare for an MRI?
It’s a good idea to fully understand the reasons for the MRI. You can start by asking your doctor questions like:
Before your MRI, let your doctor know if you:
No metal is allowed in the MRI room, because the magnetic field in the machine can attract metal. Tell your doctor whether you have any metal-based devices that might cause problems during the test. These can include:
If you have tattoos, talk with your doctor. Some inks contain metal.
On the day of the test, wear loose, comfortable clothing that doesn’t have snaps or other metal fasteners. You might need to take off your own clothes and wear a gown during the test.
Remove all of these before you go into the MRI room:
If you don’t like enclosed spaces or you’re nervous about the test, tell your doctor. You may be able to have an open MRI or get medicine to relax you before the test.
What Kind of Equipment Is Used in an MRI?
A typical MRI machine is a large tube with a hole at both ends. A magnet surrounds the tube. You lie on a table that slides all the way into the tube.
In a short-bore system, you are not totally inside the MRI machine. Only the part of your body that’s being scanned is inside. The rest of your body is outside the machine.
What Happens During an MRI?
Before some MRIs, you’ll get contrast dye into a vein in your arm or hand. This dye helps the doctor more clearly see structures inside your body. The dye often used in MRIs is called gadolinium. It can leave a metal taste in your mouth.
You will lie on a table that slides into the MRI machine. Straps might be used to hold you still during the test. Your body might be completely inside the machine. Or, part of your body may stay outside the machine.
The MRI machine creates a strong magnetic field inside your body. A computer takes the signals from the MRI and uses them to make a series of pictures. Each picture shows a thin slice of your body.
You might hear a loud thumping or tapping sound during the test. This is the machine creating energy to take pictures inside your body. You can ask for earplugs or headphones to muffle the sound.
You might feel a twitching sensation during the test. This happens as the MRI stimulates nerves in your body. It’s normal, and nothing to worry about.
The MRI scan should take 20-90 minutes.
What Happens After an MRI?
You can usually go home after an MRI and get back to your normal routine. If you had medicine to help you relax, you’ll stay in the imaging center until you’re fully awake. You’ll also need someone to drive you home.
What Are the Possible Side Effects From an MRI?
Contrast dye helps your doctor better see what’s going on, and for most people it doesn’t cause any problems. But it can cause an allergic reaction in some people. There are steps doctors take to treat them.
Your doctor likely won’t use it on you if you’re pregnant, even though there’s no evidence it could harm a fetus. Your doctor may check your kidney function before the test. People with severe kidney disease are at risk of a rare disease called nephrogenic sclerosing fibrosis.
There’s a chance some of the dye may stay in your body and build up in the brain and other organs. It’s not clear yet if this buildup is harmful. The FDA hasn’t restricted its use.
How Do You Get MRI Results?
A specially trained doctor called a radiologist will read the results of your MRI and send the report to your doctor.
Your doctor will explain the meaning of your test results and what to do next.
Show Sources
American Academy of Orthopaedic Surgeons: «X-Rays, CT Scans, and MRIs.»
FDA: «MRI (Magnetic Resonance Imaging).»
Mayo Clinic: «MRI,” “MRI: Why it’s done.”
Stanford Health Care: “After the Examination.”
National Institute of Biomedical Imaging and Bioengineering: «Magnetic Resonance Imaging (MRI).»
My.ClevelandClinic.org: “MRI Imaging in Multiple Sclerosis.”
Cancer Today: “Concerns Raised about MRI Contrast Dye.”
NYU Langone Health: Diagnosing Vertigo.”
Harvard Medical School: «What you need to ask before getting an imaging test.»
National Heart, Lung, and Blood Institute: «Cardiac MRI.» «Chest MRI.»
MRI for Cancer
MRI (also known as magnetic resonance imaging, magnetic resonance, MR, and nuclear magnetic resonance [NMR] imaging) helps doctors find cancer in the body and look for signs that it has spread. MRI also can help doctors plan cancer treatment, like surgery or radiation. MRI is painless and you don’t have to do anything special to get ready for this test. But, it’s very important to tell your doctor and the technologist (the person who does the test) if you have any metal in your body.
What does MRI show?
MRI creates cross-section pictures of your insides. But MRI uses strong magnets to make the images – not radiation. An MRI scan takes cross-sectional slices (views) from many angles, as if someone were looking at a slice of your body from the front, from the side, or from above your head. MRI creates pictures of soft tissue parts of the body that are sometimes hard to see using other imaging tests.
MRI is very good at finding and pinpointing some cancers. An MRI with contrast dye is the best way to see brain and spinal cord tumors. Using MRI, doctors can sometimes tell if a tumor is or isn’t cancer.
MRI can also be used to look for signs that cancer may have metastasized (spread) from where it started to another part of the body.
MRI images can also help doctors plan treatment such as surgery or radiation therapy.
(A specific kind of MRI can be used to look inside the breast. Learn more about breast MRI.)
How does MRI work?
An MRI scanner is a long cylinder or tube that holds a large, very strong magnet. You lie on a table that slides into the tube, and the machine surrounds you with a powerful magnetic field. The machine uses a powerful magnetic force and a burst of radiofrequency waves to pick up signals from the nuclei (centers) of hydrogen atoms in your body. A computer converts these signals them into a black and white picture.
Contrast materials can be put into the body through a vein to clearer images. Once absorbed by the body, the contrast speeds up the rate at which tissue responds to the magnetic and radio waves. The stronger signals give clearer pictures.
How do I get ready for an MRI?
MRI scans are most often done on an outpatient basis, so you don’t have to be in a hospital to get one.
You don’t usually need to follow a special diet or do anything to get ready for an MRI, but follow any instructions you are given.
If being in a small, enclosed space is a problem for you (you have claustrophobia), you might need to take medicine to help you relax while in the scanner. Sometimes talking with the technologist or a patient counselor, or seeing the MRI machine before the test can help. In some cases, you can arrange to have an open MRI which allows more space around your body (see the next section).
Sometimes a contrast material is used for MRI imaging. You may have to swallow the contrast, or you may have an intravenous (IV) catheter put in a vein in your arm so the contrast can be given into your bloodstream. The contrast material used for an MRI exam is called gadolinium. (This is not the same as the contrast dye used in CT scans.) Let your doctor and the technologist know if you have any kind of allergies or have had problems with any contrast used in imaging tests in the past.
If you have any of these implants, you should not even enter the MRI scanning area unless told to do so by a radiologist or technologist who knows you have:
Also be sure the technologist knows if you have other permanent metal objects, such as surgical clips, staples, screws, plates, or stents; artificial joints; metal fragments (shrapnel); tattoos or permanent makeup; artificial heart valves; implanted infusion ports; implanted nerve stimulators; and so on.
You may need to have an x-ray to check for metal objects if there’s any doubt.
What is it like having an MRI?
You may be asked to undress and put on a gown or other clothes without zippers or metal. Be sure to remove any metal objects you can, like hair clips, jewelry, dental work, and body piercings. Before the scan, the technologist will ask you if you have any metal in your body.
You will lie down on a narrow, flat table. The technologist may use straps or pillows to make you comfortable and help keep you from moving. The table slides into a long, narrow cylinder. The part of your body that’s being scanned will be in the center of the cylinder. The scanned part of your body may feel a little warm during the test, this is normal and nothing to worry about.
You’ll be in the exam room alone, but you can talk to the technologist, who can see and hear you at all times.
The test is painless, but you have to lie inside the cylinder with its surface a few inches from your face. It’s important to stay very still while the images are being made, which can take a few minutes at a time. You may be asked to hold your breath during certain parts of the test. Tell the technologist if you need to move or take a break.
The machine makes loud, thumping, clicking, and whirring noises, much like the sound of a washing machine, as the magnet switches on and off. You may be given earplugs or headphones with music to block noise out during the scan.
Special, open MRI machines that are less restrictive may be easier for some people. These machines replace the narrow cylinder with a larger ring. This design lessens the banging sound and the feeling of lying in an enclosed space. But the machine doesn’t create as strong a magnetic field, and the pictures may not be as clear or detailed as they are with standard MRI. Sometimes, this can lead to getting rescanned on a standard MRI machine.
How long does an MRI take?
MRI scans usually take between 45 and 60 minutes, but can sometimes take up to 2 hours. After the test, you may be asked to wait while the pictures are checked to make sure that they are clear and show all of the body part. If not, more pictures may be needed.
What are the possible complications of an MRI?
People can be hurt in MRI machines if they take metal items into the room or if other people leave metal items in the room.
Some people become very uneasy and even panic when lying inside the MRI scanner.
Some people react to the contrast material. Such reactions can include:
Be sure to let your health care team know if you have any of these symptoms or notice any other changes after you get the contrast material.
Gadolinium, the contrast material used for MRI, can cause a special complication when it’s given to patients on dialysis or who have severe kidney problems, so it’s rarely given to these people. Your doctor will discuss this with you if you have severe kidney problems and need an MRI with contrast.
Small amounts of gadolinium can stay in the brain, bones, skin and other parts of your body for a long time (several months to years) after the test. It’s not known if this might have any health effects, but so far, studies haven’t found any harmful effects in patients with normal kidneys.
What is an MRI (Magnetic Resonance Imaging)?
By Tanya Lewis published 12 August 17
Magnetic resonance imaging (MRI), also known as nuclear magnetic resonance imaging, is a scanning technique for creating detailed images of the human body.
The scan uses a strong magnetic field and radio waves to generate images of parts of the body that can’t be seen as well with X-rays, CT scans or ultrasound. For example, it can help doctors to see inside joints, cartilage, ligaments, muscles and tendons, which makes it helpful for detecting various sports injuries.
MRI is also used to examine internal body structures and diagnose a variety of disorders, such as strokes, tumors, aneurysms, spinal cord injuries, multiple sclerosis and eye or inner ear problems, according to the Mayo Clinic. It is also widely used in research to measure brain structure and function, among other things.
«What makes MRI so powerful is, you have really exquisite soft tissue, and anatomic, detail,» said Dr. Christopher Filippi, a diagnostic radiologist at North Shore University Hospital, Manhasset, New York. The biggest benefit of MRI compared with other imaging techniques (such as CT scans and x-rays) is, there’s no risk of being exposed to radiation, Filippi told Live Science.
What to expect
During an MRI, a person will be asked to lie on a movable table that will slide into a doughnut-shaped opening of the machine to scan a specific portion of your body. The machine itself will generate a strong magnetic field around the person and radio waves will be directed at the body, according to the Mayo Clinic.
A person will not feel the magnetic field or radio waves, so the procedure itself is painless. However, there may be a lot of loud thumping or tapping noises during the scan (it may sound like a sledgehammer!), so people are often given headphones to listen to music or earplugs to help block the sound. A technician may also give instructions to you during the test.
Some people may be given a contrast solution by intravenous, a liquid dye that can highlight specific problems that might not show up otherwise on the scan.
Young children as well as people who feel claustrophobic in enclosed places may be given sedating medication to help them relax or fall asleep during the scan because it is important to stay as still as possible to get clear images. Movement can blur the images.
Some hospitals might have an open MRI machine that is open on the sides rather than the tunnel-like tube found in a traditional machine. This may be a helpful alternative for people who feel afraid of confined spaces.
The scan itself may take 30 to 60 minutes, on average, according to the American Academy of Family Physicians.
A radiologist will look at the images and send a report to your doctor with your test results.
How it works
The human body is mostly water. Water molecules (H2O) contain hydrogen nuclei (protons), which become aligned in a magnetic field. An MRI scanner applies a very strong magnetic field (about 0.2 to 3 teslas, or roughly a thousand times the strength of a typical fridge magnet), which aligns the proton «spins.»
The scanner also produces a radio frequency current that creates a varying magnetic field. The protons absorb the energy from the magnetic field and flip their spins. When the field is turned off, the protons gradually return to their normal spin, a process called precession. The return process produces a radio signal that can be measured by receivers in the scanner and made into an image, Filippi explained.
Protons in different body tissues return to their normal spins at different rates, so the scanner can distinguish among various types of tissue. The scanner settings can be adjusted to produce contrasts between different body tissues. Additional magnetic fields are used to produce 3-dimensional images that may be viewed from different angles. There are many forms of MRI, but diffusion MRI and functional MRI (fMRI) are two of the most common.
Diffusion MRI
This form of MRI measures how water molecules diffuse through body tissues. Certain disease processes — such as a stroke or tumor — can restrict this diffusion, so this method is often used to diagnose them, Filippi said. Diffusion MRI has only been around for about 15 to 20 years, he added.
Functional MRI
In addition to structural imaging, MRI can also be used to visualize functional activity in the brain. Functional MRI, or fMRI, measures changes in blood flow to different parts of the brain.
It is used to observe brain structures and to determine which parts of the brain are handling critical functions. Functional MRI may also be used to evaluate damage from a head injury or Alzheimer’s disease. fMRI has been especially useful in neuroscience — «It has really revolutionized how we study the brain,» Filippi told Live Science.
Magnetic Resonance Imaging (MRI)
What Facts Should I Know about MRI?
History of MRI
Working independently, Felix Bloch of Stanford University and Edward Purcell of Harvard University made the first successful nuclear magnetic resonance experiment to study chemical compounds in 1946. Dr Bloch and Dr Purcell were awarded the Nobel Prize for Physics in 1952. In the early 1980s, the first «human» magnetic resonance imaging (MRI) scanners became available, producing images of the inside of the body. Current MRI scanners produce highly detailed two-dimensional and three-dimensional images of the human anatomy.
What is the medical definition of MRI?
What is an MRI scan used to diagnose?
What Are the Risks of MRI?
MRI is a very safe procedure. The strong magnetic field itself does not hurt people, unless they have certain types of metal implanted in their body. The magnetic field can cause certain types of metal to move, which could potentially cause an injury.
What Is the Preparation for MRI?
Typically, all metal and electronic devices (watches, jewelry, cellular phones, and credit cards) must be removed from one’s clothing and body before the exam. This protects valuables from the effects of the MRI machine.
SLIDESHOW
What Happens During the MRI Procedure?
The study may take place on either an open scanner or a closed scanner. For an open scanner, a person lies on a table face up, and the table slides under the magnet from the side. For a closed scanner, which looks like a tube, a person lies on the table face up and goes in either head-first or feet-first, depending on what part of the body is being scanned.
The MRI scan is performed inside a large magnet, and the person lies on the table in the center. During the procedure, the machine scans the body by turning small magnets on and off. Radio waves are sent into the body. The machine then receives returning radio waves and uses a computer to create pictures of the part of the body being scanned. The radio waves used in the procedure are safe and are similar to the radio waves used in a car radio.
What Happens After the MRI Procedure?
If a contrast injection is used, the IV is removed from the arm before the person goes home. No side effects from the scan or the contrast injection should occur.
In the rare circumstance that sedation is needed, that person is sent home once awake and alert. For those people who receive sedation, someone must drive them home. No aftereffects occur from having an MRI.
A radiologist is a medical doctor trained to interpret various imaging studies. The radiologist interprets the results of the scan, and the results are then sent to the doctor. How quickly the doctor receives the report depends on the imaging center where the study is performed.