How MRI Technology Works
Have you ever wanted to impress others by your mastery of a wide variety of complex subjects, but you'd like to be able to do this without the actually doing the hard work necessary to truly understand the subject? Well, if so, you're in luck, because here's a handy guide.
How an MRI Works
(Cocktail Party Discussion Edition)
The Physics
Magnetic Resonance Imaging is possible because some nuclei of the atoms contained in various tissues of the body have an electrical charge. This means that they can be influenced both by magnetic fields and by high frequency radio waves.
By happy coincidence, the nucleus of the Hydrogen atom has this property, and it's also by far the most abundant element in the body, by virtue of being present (twice) in every molecule of water (H2O).
It turns out that these nuclei, when in the presence of a magnetic field, will absorb a small amount of energy from a radio frequency (RF) transmitter, and then will re-emit that energy after the RF transmitter is turned off, analogous to the way that some fluorescent materials will 'glow in the dark' after the lights are turned off. And just as there are different types of fluorescent materials that can produce different shades of color, so also the Hydrogen that is contained in different tissues will re-emit its RF energy in different ways. More importantly, diseased tissue often behaves differently from normal tissue.
Just as the microscope is a tool that enables us to see physical properties that were previously unknown, the MRI machine enables us to discover properties of tissue that were invisible to us before its invention. The hitherto-invisible properties of tissues that are now visible with the MRI are called T1 and T2. When you have physicists naming things, that's the kind of name you end up with.
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| T1 Image |
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T2 Image |
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Proton Density Image |
In addition to these images that are based on the differences of T1 and/or T2, the MRI also can make images that are based on principles that are not unique to the MRI, such as motion within the body. The MRI can also make images that correspond very closely to the way the body appears if you were to actually cut it open and photograph it in black and white, except without the mess and the pain. Images based on a combination of these factors are often taken during a typical examination.
The Images
The MRI creates images in 3 dimensions, like a CT scanner. Since we can't visualize three dimensions on the screen of a computer or on a sheet of paper, the images are formatted in a series of 'slices'. These slices can be in any orientation or in more than one orientation.
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| Sagittal Lumbar Spine |
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Sagittal Brain, set-up of additional Axial images |
A conventional x-ray creates images in 2 dimensions,
with the image data in the 3rd dimension piled up on top of
itself. This is still useful if, for
example, you're trying to diagnose a broken bone, but it's not so good if you
are trying to describe complex shapes or to locate a tumor. This ability to visualize human anatomy in 3 dimensions is an important feature of the MRI.
But an even more important feature of MRI is that the signal differences between one type of tissue and another are readily apparent; there is high contrast between the kinds of tissue that the doctor is trying to differentiate. When the doctor orders an MRI scan, he usually chooses that instead of CT, X-ray, Ultrasound, or other tests primarily because of the high contrast that MRI provides between healthy and normal tissue.
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Axial T1 Liver image
Note: Spinal Vertebra near center |
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Sagittal Cervical Spine PD on left, T1 image on right |
Other advantages of MRI compared to other imaging technologies include:
- No ionizing radiation is used; there is no known harm in having an MRI exam
- The physical structure as well as chemical environment can be imaged
- MRI can image more than one physical property, using the same hardware
- Relatively tiny structures can be visualized
- Images are easily stored, manipulated, and transferred because they are in digital format
- Because the machine is software controlled, it's very flexible and offers a lot of control to the operator. One machine can image the whole body.
- Blood vessels and blood flow can be imaged
- Brain functions can be measured
- Some things can be seen with an MRI that cannot be seen any other way
Bonus cocktail party talking point: using the computer power of the MRI, it's possible to make pictures of a person 'without their clothes on'. Since examinations are never done on a head-to-foot basis, the lurid aspects of this feature are not likely to become widely used. Medical images are very closely controlled; you need not fear that your MRI study will be circulating on the internet.
Bonus cocktail party talking point #2: Many of you may be thinking, "Has a couple ever been imaged in an MRI while engaged in sex?" Others may be thinking, "Ugh! Who would think of that? Who would do that?" Still others may be thinking, "Where can I see these images?" It was necessary to find a relatively small and acrobatic couple to fit in the machine, but you can find it on the internet. Note to employees of Professional Imaging Services; don't waste your time looking at these images at work!
The Hardware
The MRI machine itself is a massive, complex device, comprising in several ways the most advanced technology that mankind can build. Leading-edge technology includes:
- Superconducting magnet design
- Software architecture
- Antenna design
- Multichannel data acquisition
- Low noise amplifiers
- High power amplifiers
The main components of the MRI are:
Magnet
The most common MRI magnets in the USA are electromagnets that use special superconducting wire. That means that, as long as the wire is kept very cold, the wire carries electrical current without resistance. This creates a very strong and stable field, both of which are important.
Superconducting magnets must be kept cold, which is usually done by the use of special ultra-low temperature refrigeration systems and a reservoir of liquid helium.
There has been ongoing controversy over the optimum field strength for an MRI magnet. MRI magnets range in strength from just under 0.2 Tesla to 3 Tesla. Some MRI examinations are less demanding of the machine's capabilities than others, however, and low field machines can do very good examinations for many studies.
"Open" MRI magnets are less constricting for the patients (although some of the open magnets are also quite small, too) and are usually permanent magnets, less powerful and generating images that, for equivalent image quality to their high-field cousins, take longer for the same examination.
RF Transmit and Receive System
During image acquisition, the gradient magnetic fields are switched off and on, making the characteristic noise of MRI, The RF transmit and receive system is also active, but silent, during this time.
The RF transmitter in high-field MRIs is extremely powerful and could be used in a small radio station if necessary. The transmitter sends very brief, well-controlled pulses to the RF coils that surround the patient, and then the pulses are disabled. Some milliseconds later, the energy absorbed by the body is released in the form of RF pulses that are received by a coil (often a separate coil) for processing by the co0mputer.
The antennas used in MRI are some of the most advanced in the world, and contain amplifiers so noise-free that they are capable of measuring the noise generated by the tissue itself and by the molecular motion within the metal of the coil.
Computer System
The MRI computer and associated software are sophisticated, but ironically, not the most demanding part of the system design. A faster computer will not generally allow faster imaging, since the physical properties of the nuclei mean that the data acquisition system must "wait" for the nuclei to release their energy.
The MRI computer does have complex tasks coordinating the various components of the device with great accuracy and in handling vast amounts of incoming data.
The monitors used for displaying medical images are usually very costly, because their brightness, contrast, resolution, and accuracy are critical in the ability of the radiologist (who reads the images) to diagnose disease.
The MRI machine can display thousands of varying shades of black-gray-white, but the human eyes and brain can only discern around 150 discrete gray scale levels. Therefore, the machine has display controls called Window and Level to limit the displayed gray levels of the image to the areas of anatomy where the area of interest is located. This is manually adjusted on an image-to-image basis in order to optimize the image appearance. Usually the doctor isn't interested in the air around the patient (although the machine images that, too, and makes it black), and usually the doctor is not interested in the fat just under the skin (although the machine images that, and usually makes it white).
Gradient Field System
"I had an MRI. What makes that awful banging noise?" Sometimes patients speak about the traumatic ordeal of undergoing an MRI, forgetting that it has largely replaced the practice of exploratory surgery. Nonetheless, it is true that some machines make enough noise that hearing protection is required for the patient.
The noise that the MRI makes comes not from any large moving parts, but from additional magnetic fields that either add to or subtract from the main field and are switched on and off rapidly during the exam. The coils that produce these gradient fields are placed under extreme torque as they are cycled, and their vibration is the source of the noise.
The gradient fields were not used in the very first MRI machine, but were added by a later researcher. They enable the MRI to acquire data hundreds of times faster than it would otherwise be possible.
A Few Buzzwords Sure To Impress
Axial A plane that is perpendicular to the long axis of the body. In a tree, the axial plane would show all the growth rings of the tree.
Coronal A plane that, in the body, runs from head to foot and sideways; that is, from one shoulder to the other.
Cryogen Supercold liquids that are used in MRI for keeping the magnet wires at a temperature of nearly absolute zero (-460° F).
Hertz A rate of oscillation. One Hertz is equal to one cycle per second.
Homogeneity Uniformity Modern MRI magnets have a maximum inhomogeneity of the strength of their magnets of just a few parts in a million, or less than 1/1,000 of 1%.
Resolution The smallest object that can be seen in an image. An important, but not the only important, measure of image quality.
Resonance Physical property in which an abnormally high response is given in response to a given stimulus; akin to reverberation. In MRI, a specific RF transmit frequency results in a relatively robust echo signal returning from the body.
Sagittal A plane that extends through the long axis of the body, head to foot, and aligns with the sagittal suture of the skull. The sagittal suture of the skull is the 'seam' that runs from the front (nose) of the head to the back of the head.
Tesla The 19th century experimenter and eccentric and also the primary unit of magnetic field intensity. A 3-Tesla magnet (the most powerful in common use) has a field of about 60,000 times the background magnetic field intensity of the earth.
Voxel Short for Volume Element, a 3-dimensional pixel. A pixel with thickness coming in and out of the page.
Fun Facts
The MRI can detect and display changes in the blood flow in the brain as different parts of the brain are used to process information, This data can be used as a high-tech lie detector (a term that the researchers in this area do not use). Early studies at this time indicate that it really works quite well for certain applications, and it's expected that someday there will be "Truth Centers" available to scan brains of alleged liars.
The gradient coils used in an MRI act very much like speakers. If you were to hook up an MP3 or CD player to the input of the gradient coil amplifiers, you could listen to Willy Nelson on the MRI magnet. It may not make a very good image, though. Waylon Jennings CDs might make better images.
The RF transmitter used in the MRI to energize the nuclei is quite powerful and can slightly heat the patient. The MRI is essentially microwaving the patient, but the energy levels absorbed are far lower than in your microwave oven, so it would not work well for warming up a plate of lasagna. The FDA regulates the amount of heat that can be deposited in the patient, and although there is no known harm to this experience, it's not unusual for patients to notice that they are getting warm during the examination.
The FDA also regulates the background magnetic field that the public can be exposed to. There is no evidence that humans cannot tolerate high background magnetic fields, but some electronic devices do not tolerate it well.
Magnetic fields will not heal you any more effectively than a placebo. On the positive side, a placebo works surprisingly well.
Liquid helium, used in superconducting magnets, costs about the same per liter as champagne. Liquid nitrogen is also used in some magnets, and it costs about the same per liter as beer. Helium occurs naturally in just a few places in the world, including the USA. Since it's produced mainly in the USA and is very expensive to store and transport, liquid helium is extremely costly outside the USA.
If a superconducting magnet becomes warm enough, the wires will lose their superconducting properties, and an 'avalanche effect' occurs in which the entire stored energy of the magnet is changed to heat instantly, which changes the liquid helium into a very large amount of gaseous helium. This is called a quench, and it's considered a bad thing.
During the late 1930's the USA refused to sell gaseous helium to Nazi Germany, leading them to use the flammable Hydrogen in the Hindenburg.
X-rays are used for screening airport luggage. One of the limitations of x-ray in this application is that it cannot distinguish between various common liquids. An MRI can easily distinguish between a bottle of wine, water, gasoline, and toothpaste. An MRI can distinguish between water and ice cubes. An MRI can also be set up to automatically detect the presence of explosive compounds and to indicate where in the suitcase those compounds are located. An MRI magnet also will magnetize sensitive mechanical devices such as watches and camera shutters, and will erase the data on the back of a credit card.
The MRI magnet can pull metal objects towards itself, especially large ones, at incredible force. A few people have been killed by high speed metal objects flying into an MRI magnet. If an object such as a floor scrubber, pallet jack, or oxygen cylinder gets pulled into the magnet, it's sometimes necessary to spend thousands of dollars to deenergize the magnet and repair it.
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