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Tuesday, February 16, 2021

Restraining the 32-channel coil

There has been a move towards custom head restraint in recent years. These devices are tailored to fit the subject in such a way that any movement of the head can be transmitted to the coil. It is therefore imperative to make sure that the RF coil is also well restrained.

On Siemens Trio and Prisma scanners, the 32-channel head coil is a special case. It was designed independent of the standard head coils. Restraint on the bed is thus a bit of an afterthought. Sticky pads on the base of the coil are designed to prevent movement through friction, but there are gaps on all four sides and no specific mechanism - slots, grooves, etc. - to lock the coil into a particular position. On my Trio, I was in the habit of putting the 32-channel coil all the way back to the frame of the bed, assuming that the most likely direction of motion from a subject would be backwards. Problem solved, right? No. By putting the coil all the way back, when using custom head restraint I actually put stress on the front two coil cables and this led to intermittent receive RF artifacts. A more refined fix was necessary.

My engineer built a simple frame (see photos below) that fits snugly into the rear portion of the bed frame and forces the coil onto protrusions that hold the standard (12-channel) coils properly. It also shims out the left and right gaps so there is no chance of side to side motion, either. With this device in place, the coil can only go one way: up. 

There has been some debate in the literature about the utility of custom head restraint for motion mitigation, with one group finding benefits while another found it made things worse. I note that both groups were using 32-channel coils on a Prisma, so proper head coil restraint may be a reason for different outcomes. I am now working on a fix for Prisma scanners and will do a separate post on the solution once it's been tested. (ETA April-May.) Until then, if you use a 32-channel coil on any Siemens scanner, my advice is to use additional restraint and make sure your coil is in a reliable, stable position. 


The coil restraint shim is put into position before the 32-channel coil.

Coil restraint shim in position.