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.
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| The coil restraint shim is put into position before the 32-channel coil. |
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| Coil restraint shim in position. |
Hi,
ReplyDeleteOur scanner and head coil is the same as the one on the picture. Is this issue relevant to for those who use a custom head restraint or is it a general issue which will exhibit itself when participants are restrained with foam pads as well? I am not sure what you mean by "custom head restraint" here so I needed to ask. When we tried the head coil and placed it in place (without running a scan) and tried wiggling it in the direction towards (inside) the bore we found that there was room for motion, but that much motion done by a participant's head would already lead to lots of other problems apart from the movement of the coil. Would the vibrations of the scanner affect things that much? And what kind of tests can be do to check if this issue is indeed there.
Hi Tamer, we came across the issue first in relation to custom printed head restraint but yes, in principle it can happen with any sort of head restraint, even foam. The first part of the problem is that pushing the coil all the way to the back of the bed where it is least likely to move will place some strain on the front connectors. If the subject's head motion is well coupled to the upper part of the coil, e.g. because you have a lot of foam packing, then those front connectors can wiggle. Generally, however, most subjects can still move their head vs the foam packing and not move the coil. The anti-slip pads on the bottom of the coil also reduce the likelihood of the coil moving. So the bottom line is that the better you restrain the subject's head, the greater the concern that the coil might now move instead.
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