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A MRI Physicist's Notes On Element Density

At ScanMed, we are often asked about the performance comparison between our 8 element coils and 16 element coils. We refer to the element of a coil as being the basic signal generating component of a coil or coil array. A 16 element coil will interface with a 16 channel system with a one-to-one correspondence between the two and typically improve the performance of a similar sized or configured coil with only 8 elements – and yes, an 8 element coil can operate on a 16 channel system by utilizing only 8 of the system channels.

The ScanMed wrist versus wrist/hand coil is a classic example of the impact of element density on performance. In the 8 element configuration, all 8 of the elements surround the wrist and are optimized in length to cover ONLY the wrist bones and joints regions. In the 16 element version within the same general housing shape (hence similar coupling to the tissues), there are 8 elements surrounding the wrist and 8 more surrounding the hand through the first joints of the fingers. However, in the 16 element version, each set of 8 elements must be made longer to overlap with the other set in order to provide a homogeneous imaging volume. These longer elements couple to more tissue, generating more noise than the shorter elements of the 8 channel wrist. So, in this example, the SNR of the wrist is actually better from the 8 element coil than the similar 16 element wrist/hand coil!

“It is all about the element density and size relative to the target imaging volume,” states Dr. Jones, “Translation: If a 16 element coil’s geometry is such that only half of those elements are surrounding the target volume, then only 8 elements are effecting the performance within that volume.”

Be careful on coil selections. Ask how many ELEMENTS are in the coil, not how-many-channel-scanner it can operate on. We have seen many an 8 channel or quadrature array coil that outperforms 16 element coils in the same anatomy due to the 16 element coil’s inferior design compared to a better designed quadrature coil. Proof is in the testing. Make certain that your 8 element coil provides a better SNR than the old one before accepting the purchase order!

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