MRI Machine Design Lessons: See Inside
|The main magnetic coil is on the outermost element of the system and the largest part. The RF antennas and gradient coils are on the inside, closer to the patient.|
If you watch House or other medical shows on TV, you’ve probably seen an MRI machine in action. It stands for magnetic resonance imaging, and they’re those huge high-tech looking doughnuts where the patient lies inside the hole. This technology has been available to doctors for little more than 30 years, but it has advanced diagnostics tremendously. The need for exploratory surgery and other invasive processes has been reduced due to the ability to image conditions inside the body without harm to the patient.
The main element of an MRI machine is a massive electromagnet coil that fills most of the enclosure. The ability to capture detailed images quickly depends on having a very powerful magnetic field, usually in the range of 1 to 3 Tesla. (1 Tesla = 10,000 Gauss.) Some specialized machines designed for super-detailed images inside a patient’s head can be as high as 7 Tesla. Generating that kind of field requires superconducting coils cooled with liquid helium.
Moreover, the field must be aligned with great precision to obtain accurate images. That kind of field is hugely dangerous when active, because any magnetic metal objects can become projectiles. When a patient is placed inside the magnetic field, protons in hydrogen atoms in water molecules align themselves with the field and spin at a frequency determined by the field strength. John Metellus, magnetic resonance marketing manager for Siemens Medical Solutions , explains the concept:
“When you place a tissue sample containing hydrogen protons in a 1 Tesla magnetic field, the proton starts to spin at 42 MHz. Once all the protons are aligned parallel to the magnetic field, we disrupt this alignment so we can determine what they’re made of. The process of disruption is done by turning on an RF energy pulse, matching the proton’s frequency, which causes the protons to drop to an antiparallel dynamic state. Slowly they come back to their 42 MHz steady state, but depending on their makeup, they’re going to get there at different times, and we measure these relaxation time spans. Depending upon the specific measurement algorithm (i.e., the imaging sequence) used, the image’s contrast will be characterized by the relaxation times of the tissue dependent protons. The ones that are quickest have the most signal, and the weak ones have the least. We match specific frequencies so we know what kind of tissue the proton is embedded in. If there’s a lot of water, it relaxes quicker. If it’s a tumor, it relaxes slower. Bone takes an even longer time. The time it takes for the signal to come back to its 42 MHz frequency gives us the intensity of the signal and its grayscale color on the image, and that’s how we map our picture. The higher the magnetic field, the more image resolution. Increasing the field strength is like increasing the magnification of a lens.
“Inside the magnet, we have the RF antenna (anatomically shaped surface coils), which picks up the MR signal, emitted from the tissue sample, and then we have a gradient coil which helps us find the spatial location where the signal is coming from. The gradient coil is made up of three groupings of coils, oriented on the X, Y, and Z axes. With three axes, you can construct a volume. We have our RF signal, and at the same time, we superimpose this gradient over it. This gives us three orientations to work with. If we want to make a horizontal ‘cut,’ we make the gradient in the horizontal direction a uniform value. If we want to look at the thickness of that volume, we vary the other planes, the Y and Z, and slowly move them around through a 360° change in gradient strength. We control the amount, so we know exactly how the plane is rotating. The only thing that is moving is current inside the coil.
If nothing’s moving, why the noise?
Metellus says MRI machines make a loud acoustic noise because, when you pass current through the gradient coil that lies within the electromagnetic field, the currents are opposing each other, resulting in a sound of varying intensity. Depending on the scanning algorithm, it has a different rhythm. While the sound suggests something is spinning inside the housing, there are no moving parts. In fact, keeping everything still, especially the patient, creates the highest quality images.
The technology has come a long way since the early machines. With weaker magnetic fields, an image could take hours to form. Forming the image takes many cycles of disrupting the magnetic field and gathering data as the protons change state. Using the scanning action of the gradient coils helps build a picture of the volume of tissue in the patient. Those cycles typically require a few milliseconds, so a simple image can take as little as a minute or two, or longer depending in the volume and detail required.
Metellus adds, “We take the signals that we capture through the antenna and send it to a receiver where we do a fast Fourier transform of that raw data. It gives us the spatial location where it found the information inside the magnet. We put all the dots together, and it comes out as an image.”
When applied with human and computer skill , that data becomes a critical key to unlocking medical secrets for millions of patients.
Also read from Control Engineering : Data Acquisition and Digitizers Advance Medical Imaging and Lower Cost Medical Devices? FDA Wants to Help .
|Peter Welander is process industries editor. Reach him at PWelander@cfemedia.com .|