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MRI is a type of diagnostic test that can create detailed images of nearly every structure and organ inside the body. MRI uses magnets and radio waves to produce images on a computer. MRI does not use any radiation. Images produced by an MRI scan can show organs, bones, muscles and blood vessels.
- What is MRI?
- How does MRI work?
- Where else is this effect useful?
- What methods are used to make MRI work even better?
- Consider the following… fMRI
Magnetic Resonance Imaging (MRI) is one way for healthcare professionals to look inside your body and see what is going on inside it without having to cut open your body. While there are lots of different ways to take pictures inside your body such as x-rays, computerized tomography (CT) scans, ultrasounds and so on, MRIs produce far more detailed ...
An MRI takes pictures of places in your body that contain water, and the detail in these images comes from the ways that different tissues interfere with the electromagnetic waves coming from water molecules. The idea of water releasing electromagnetic waves may seem pretty exotic, but it turns out that most molecules do it all the time---the signals that they emit are just so tiny that you’d only notice them if you went looking for them. An MRI is just a device that first excites water molecules into releasing waves, and then records the locations of those waves with high accuracy.
Your body is pretty much entirely made of water. Blood vessels, lymph nodes, and even solid bones are soaked with water molecules, each of which contains two hydrogen atoms. At the center of each hydrogen atom sits a nucleus consisting of a single proton, which can be visualized as a tiny bar magnet with a “north” and “south” pole. Just like the “north” and “south” poles of a needle on a compass tend to align with the magnetic poles of the earth, in the presence of strong magnetic fields each proton in water twists its orientation so that it aligns with the field. When health care providers first turn on the MRI machine, a very strong, constant magnetic field forms that remains in place for the duration of the measurement, and this super-strong field makes all the protons try to line up with the poles of the field. This lining-up doesn’t mess up any of the chemical properties of the tissues, so your body continues to function normally while the doctor makes the measurement.
But while this really strong constant magnetic field makes all the protons want to line up, the MRI machine intentionally disrupts this field by sending a brief pulse of an additional, weaker electromagnetic field. This weaker pulse points in a different direction than the constant magnetic field, and so it disrupts the protons so that they become misaligned with the constant field. After the pulse ends the protons are left askew, but then they gradually re-align with the original constant field. You can think of it as the tiny jiggle that occurs in a compass needle when a weak magnet passes by. The compass normally points north, but the weak magnet causes the compass needle to jiggle slightly.
However, unlike the needle of a normal-sized compass, the direction that the protons can align has single, well defined levels in a manner very similar to the different energy levels of electrons around an atom’s nucleus. Just as electrons in atomic energy levels can absorb and re-emit photons when changing energy levels, the gradual realignment of the nuclear magnetic spin results in the emission of low-energy, radio frequency photons. The time and amount of re-alignment changes based on the thickness and hardness of the tissue where the water molecules are sitting, and so carefully monitoring of the arrival of re-emitted photons in the MRI’s detectors allows the locations and shapes of different tissues to be identified.
MRI uses the same physical effect as Nuclear Magnetic Resonance (NMR) spectroscopy, in which the identity of an unknown compound (like a potential new drug) may be identified by the resonant properties (the jiggling of protons) of the atoms that comprise it. In fact, the only reason that the technique is called MRI and not NMR is because it premiered during the Cold War, during which patients were hesitant to undergo any sort of “nuclear” treatment!
NMR spectroscopy was originally developed to help chemists who had created strange compounds that they couldn’t identify. In the technique (and just as in MRI), an unknown sample is placed in a static magnetic field, briefly excited with radio-frequency photons (light), and then allowed to re-emit those photons. NMR works because the characteristic frequency of the re-emitted photons varies very slightly based on the structure of the molecule. A proton all by itself may absorb and reemit 900 MHz photons, but when it gets near other charges (such as in a large hydrocarbon chain), the magnetic field around it is gets twisted and distorted and so its resonant frequency may shift to something like 906 MHz. This means that NMR may be used to generate “spectra” corresponding to the amount of resonance at various frequencies, which in turn reveals details of the structure of molecules. So if a chemist looks at the NMR spectrum of her unknown sample and sees a huge peak near 906 MHz, then she knows that her sample probably has at least one hydrocarbon chain somewhere on it.
Generally, using stronger stationary magnetic fields results in nicer MRI images. Because the water molecules in the body are warm, they are constantly jiggling around and colliding with one another. This jiggling tends to knock the alignment of protons in random directions, and so if the stationary magnetic field is too weak, these thermal forces will prevent protons from lining up, resulting in a dimmer MRI image.
The images get even better when the radio waves are applied multiple times, with the images from each subsequent re-emission merged together to yield a final, combined image. It’s like taking the same picture multiple times on your camera and blending them together in your favorite image editor to get a better exposed image. The main limitation of this method is ensuring that the patient lies still long enough that the image doesn’t get blurry!
We’ve all seen news articles describing how different parts of the brain become active during tasks like eating or talking. These striking images of brains owe their clarity to yet another modification of MRI, known as Functional MRI (fMRI).
Some bodily processes actually change tissues in ways that are noticeable on an MRI. For example, when tissues stretch or swell, the distribution of protons in that part of the body can change enough that a detectable change will occur in the MRI signal coming from that part of the body. This means that MRIs can be used to create movies that reveal details of events over time in a patient’s body. The simplest case involves imaging moving structures like the heart or lungs, which can help pinpoint abnormal valves or blood vessels that wouldn’t stand out in a still image. In a recently-developed fMRI, information about the changing distribution of oxygen in the brain is generated based on the unique magnetic properties of blood containing oxygen versus blood without oxygen. In oxygenated blood, the electrons from the oxygen molecules tend to block applied magnetic fields, effectively screening the hydrogens in water molecules from the applied magnetic field and decreasing the rapidness with which they will align with it. Deoxygenated blood does not have this screening effect, and so the protons align much faster---leading to more radio-frequency photons visible to the MRI detector. Because the changing distribution of oxygenated blood in the brain is known to correlate with neural activity, fMRI can be used to image the parts of a patient’s brain that become active and inactive during various tasks. This makes fMRI a very useful tool for neuroscientists and psychologists.
Magnetic resonance imaging (MRI) is a medical imaging technique that uses a magnetic field and computer-generated radio waves to create detailed images of the organs and tissues in your body. Most MRI machines are large, tube-shaped magnets.
Jan 1, 2002 · Magnetic resonance imaging (MRI) uses the body's natural magnetic properties to produce detailed images from any part of the body. For imaging purposes the hydrogen nucleus (a single proton) is used because of its abundance in water and fat.
Magnetic resonance imaging ( MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves to generate images of the organs in the body.
