The MRI scan “sees” the spine by using a large magnet that stimulates (excites) the hydrogen atoms in the vertebrae (bony building blocks of the spine), spinal sac (contains the spinal cord, nerves and spinal fluid), supporting muscles and ligaments. Because the human body is mostly comprised of water (which is 2 parts hydrogen and 1 part oxygen, or H2O), an accurate picture of the spinal anatomy can be attained.
The strong magnets necessary to accomplish the excitation of the body’s water molecules (H2O) are constructed so that the patient is completely surrounded by the magnetic field. Basically, the patient must fit into the magnet and be completely encompassed by it.
When the scanning starts, there are multiple simultaneous processes occurring.
- The water molecules that make up most of the human body can be thought of as very tiny bar magnets, with North and South poles.
- Inside the MRI magnet, all of the body’s water molecules, which were previously randomly arranged, now line up so they are facing either North or South.
- All of the noise generated within the magnet housing is from mechanical devices called gradients.
- The gradients emit an FM radio signal, which tips the lined-up water molecules away from North or South.
- When the gradient is turned off (no noise) the molecules pop back to North or South and the energy required to previously tip them is given off as another FM radio wave which is then “detected” by a listening device associated with the noisy gradients.
- A computer analyzes this new FM radio wave and digital images are constructed that represent the studied anatomy.
There is no radiation with an MRI scan, and the scanning is painless.
If post-operative scarring, or an infection or spinal tumor is suspected, contrast material may be injected into a vein. The contrast agent tends to accumulate and outline tissues with abnormal vessels (scar tissue, infection and tumor tend to have abnormal vasculature). For a patient who has already had spine surgery, the contrast agent has traditionally helped in differentiating a recurrent or remaining disc herniation from scar tissue.