Q: How does an MRI machine work?
Dr. Jeff Hersh: Magnetic resonance imaging explained
By Dr. Jeff Hersh
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Q: How does an MRI machine work?
A: Magnetic resonance imaging, MRI, allows healthcare providers to "see" inside the body without surgery.
Let's start with the basics: an atom's nucleus contains protons and neutrons (they are made up of quarks, but that goes beyond today's discussion) that have magnetic spins. When there is an even number of neutrons and protons, there is an up spin for every down spin, so there is no net magnetic moment. For atoms that have an odd number of protons or neutrons there is a net magnetic moment (think of this like the magnetic field of the earth spinning on it axis).
If an atom whose nucleus has a net magnetic moment is placed in a strong magnetic field it will align (either up or down) with the field. Envision this as a spinning top rotating on a table aligning with gravity (if the top is symmetric it can spin with either end up).
If the atom is "bumped" it will wobble and then return to its aligned state; think of this as flicking a spinning top with your finger to make it wobble and then allowing it to return to its vertical orientation. The time it takes a wobbling top to return to its vertical orientation depends on the speed it is spinning, how hard it was flicked, the properties of the top (weight, shape, etc.) and the strength of the gravitational field.
Similarly, the time it takes a bumped net magnetic moment atom in a magnetic field to relax (go back to its initial state) depends on the molecule it is in, the body tissue that molecule is in (and its local characteristics), the strength of the magnetic field and other factors.
Over 90 percent of the atoms in the human body are carbon (almost all of which is carbon 12 with six protons and six neutrons), oxygen (with eight protons and eight neutrons) and hydrogen (with one proton and no neutrons). Therefore, hydrogen is the most common atom in the body with a net nuclear magnetic moment and is utilized in most MRI scans (other atoms are occasionally used, such as carbon 13 with six protons and seven neutrons, but a discussion of those is beyond today's column).
The superconducting MRI magnetic creates a field inside the scanner that is 10,000 to 30,000 times stronger than earth's magnetic field. This aligns the hydrogen atoms up or down the field's axis, with a small preponderance up. The very large number of hydrogen atoms makes this small net up alignment sufficient to yield the desired image.
Next, we must identify where the particular hydrogen atom is; this is done by applying gradient magnetic fields to slightly change the net magnetic field. Think of this as an extra magnetic field in the x, y and z directions, so that the initial magnetic state is slightly altered.
The final step is to "bump" the atom; this is done by sending a radio frequency (RF) energy pulse. This causes the hydrogen atom to wobble, and as it relaxes it gives off the absorbed energy that created its wobble.
This energy (which depends on the core magnetic field strength, the gradient field and the RF pulse) is detected, as is the time it takes for the "relaxation" to occur. All this information is then decoded to create the MRI image. In a typical MRI scan, multiple different sequences of RF pulses are applied (think of this like different sequential flicking of a spinning top), hence MRI scans may take several minutes to an hour (depending on the specific study).
MRI can differentiate between different types of soft tissues, and hence gives the best pictures to evaluate certain brain diseases (like multiple sclerosis or certain brain tumors), certain joint diseases (although X-rays can see bone well to identify fractures, MRI can identify ligaments, cartilage and other tissues) and many body organs (such as breasts, prostate, abdominal organs including liver, spleen, kidneys and others).
Since an MRI only uses magnetic fields and RF pulses (which are non-ionizing radiation), there is no known biological hazard associated with this technology. However precautions must be taken since very strong magnetic fields are utilized; metal objects can be attracted by the magnet so nothing metal should be brought into the scanner room.
If the patient has metal inside them (a pace maker, artificial joint, fragments of metal from an old injury, surgical clips, etc.), they may not be a candidate for an MRI, or special assessment (considering how long the metal has been inside them, where it is, etc.) may be required to balance any potential harms vs. the anticipated benefit of obtaining the scan.
The technology of MRI continues to advance; modern machines have wider bores (so they are not so claustrophobic-feeling), are quieter, are able to "see" more and more detail and are able to differentiate functional differences (rather than just being able to look at the patient's anatomy). The MRI's role promises to greatly expand in the coming years, improving patient care and outcomes.
Jeff Hersh, Ph.D., M.D., can be reached at DrHersh@juno.com.