As doctors increasingly order magnetic resonance (MR) imaging, it becomes important to examine the value and quality of the images. MRI shows more detail about tissue structures and tumors than other imaging techniques, such as CT, PET, or ultrasound. However, one of the most important parts of the MRI process are the coils. Coils are what transmits signals from the patients to the computer to create the images. There have been many improvements in coils since the first MRI was performed, and many of these improvements have been influenced by technological advances. So, what are coils and how have they changed over time?
What Are Coils?
Every MRI machine has three basic parts inside of it: the main magnet, gradient coils, and radio frequency (RF) coils.1,2,3 The magnet generates the magnetic field that allows the test to be performed. This makes up the outer ring of the doughnut shaped machine. The gradient coils alter the frequency, phase, and slice. These sit inside the magnet but around the RF coils. The RF coils broadcast signals to and from the patient. RF coils are inside the gradient coils, circling the hole, or bore, of the machine. The patient has additional coils, referred to as surface coils, near their body to help create the image.
The First Coils
When MRI was first introduced, the only RF coil was the one inside the machine. As MRI progressed, researchers worked to improve RF coils and realized that a small region of interest (ROI) could be imaged by a small RF coil.2 This led to the creation of separate coils. These tools were able to be placed on or around the patients body and allowed for a high signal-to-noise ratio (SNR.) Signal-to-noise ratio represents the relationship between how strong the signal is compared to variations in intensity due to noise. The signal has to be significantly stronger than the noise. The surface coils eliminate the noise from outside the ROI, thus creating a high SNR. Soon after, researchers started creating coils that contained more loop elements which were arranged in arrays. The loop elements each received the signal and were fed into separate receive-channels. To maximize SNR, each element has to be weighted differently. These phased-array coils quickly became the gold standard.
Advancements in RF Coils
Array coils allow high SNR images over a larger ROI.1,2 Initially, array coils had three segments that could be moved and positioned over each area of the spine. Now, array coils have gradually increased the number of channels included. As technology advances and engineers learn to tune and weigh each element more easily, the SNR becomes higher. The higher the density the more likely it is that a clear image can be obtained. Because each element obtains its own readings, artifacts (obstructions on a MR image that results from sound interference) can be cleared through comparing readings. Technological advances have also made it possible for channel increases, since what used to be done mostly by humans can now be done in milliseconds by the computer, allowing for faster scan times. Array coils are engineered in a way that prevents overlap of and interference from nearby elements, creating a higher SNR.
Early coils were heavy and rigid, causing problems for technologists and patients. Coil designs were developed for different, commonly imaged body parts, like the head or shoulder.3 However, adults are not one uniform size. Heights and weights affect how well the image can be obtained with one of these one-size, rigid coils. Add the vast difference in size from a child and an adult, and rigid coils became huge impediments. The need arose for more flexible coils.
Engineers started moving away from stiff coils that clipped into place on the table to answer the call for better, denser coils. The next generation of coils were created as a mix of fabric and stiff elements. The fabric allowed the coil to be bent and braced around the part of the body being imaged. The encasing material wasn't the only change, however. There was also a higher density of elements inside the coils. These coils also marked the beginning of multi-use coils. The flexible multi-use coils were lightweight and easily moved by technologists.4,5,6 Technologists can avoid positioning the patient multiple times throughout the course of the scan. The ability to brace the coil closer to the skin boost the signal emitting from the patient's body, allowing for a higher SNR.
The newest advances have come from making coils that are durable not only due to the cloth encasing but also due to the pliable material used for the inside of the coils, the wires.7,8 For the most part, the most recent coils have little to no stiff outer casing. Coils are becoming increasing lighter, and patients aren't feeling the weight of the old coils. At the same time, technologists can position the coil directly against the patient, which leads to higher SNR and better resolution imaging.
Ana Claudia Arias, a physicist working with a team at University of California-Berkeley, has been working on coils that are screen-printed on a variety of materials.7,8 On average, her team has seen 80-93% SNR depending on the material. The coils they have created do have a slightly lower SNR, but, because they conform to the patient, they provide similar or better image quality compared to the control coils. Because they are printing their own coil designs, the team has found they can easily adjust the geometry of the coils and switch between materials. Additionally, the team has found their process produces extremely lightweight coils that can be integrated into a baby blanket. This could answer one of the current safety issues of small child MRI: the weight of the coil injuring the child.
MRI coils are necessary to produce the images that radiologists analyze to diagnose patients. The quality, density, and durability are equally important to higher signal-to-noise ratio and increase image resolution. The advancements made in the field have led to more lighter and safer coils. Technology improvements, like widespread screen-printing access, will lead to even more clear and accurate imaging processes.
1. Daniel J. Bell, et al. "Radiorequency coils." Radiopaedia.org. Web. 16 October 2018. <https://radiopaedia.org/articles/radiofrequency-coils-1>.
2. Bernhard Gruber, et al. "RF coils: A practical guide for nonphysicists." JMRI. 13 June 2018. Web. 16 October 2018. <https://onlinelibrary.wiley.com/doi/full/10.1002/jmri.26187>.
3. Center for Diagnostic Imaging. "I'm getting an MRI, so What's a Coil?" mycdi.com. 13 January 2016. Web. 16 October 2018. <https://www.mycdi.com/viewpoints/im_getting_an_mri_so_whats_a_coil_103>.
4. "FDA Clears Lightweight MRI Coils." itnonline.com. 11 April 2011. Web. 16 October 2018. <https://www.itnonline.com/content/fda-clears-lightweight-mri-coils>.
5. "Multi-Purpose, Flexible Coils Enhance Image Quality of Existing MRI Machines." itnonline.com. 30 April 2015. Web. 16 October 2018. <https://www.itnonline.com/article/multi-purpose-flexible-coils-enhance-image-quality-existing-mri-machines>.
6. Kiaran P. McGee, et al. "Characterization and evaluation of a flexible MRI receive coil array for radiation therapy MR treatment planning using highly decoupled RF circuits." Phys. Med. Biol. 13 April 2018. Web. 16 October 2018. <http://iopscience.iop.org/article/10.1088/1361-6560/aab691>.
7. Usha Lee McFarling. "Electronics 'like a second skin' make wearables more practical and MRIs safer for kids." statnews.com. 15 November 2017. Web. 16 October 2018. <https://www.statnews.com/2017/11/15/safer-mri-printable-electronics/>.
8. Joseph R. Corea, et al. "Screen-printed flexible MRI receive coils." Nature Communications. 7:10839. 10 March 2016. Web. 17 October 2018. <https://www.nature.com/articles/ncomms10839>.