tSNR changes across time
I got interested in gradient temperature because of the weird effect that tSNR seemed to increase over time.
Upon posing this effect on Twitter, PracitalfMRI and Ben Poser suggested that it might be due to gradient temperature. So I learned how to track it with as described below.
The gradient temperature
The gradient temperature can be checked on the scanner as follows.
- Options -> Service -> Local Service
- Login (Password is the string after)
- Magnet and cooling
- Cooling Status
The effect of drifting signal and drifting tSNR is not necessarily only caused by the gradient temperature. the gradients can also have an effect on other system components. iIn fact, it’s likely not the heating of the gradients themselves, rather due to eddy-induced warming in other parts of the system (shim plates, cryo shields, RF coil).
For me with a conventional high-res pipeline, the temperature ACS temperature increases up to 25dec C.
Ben Poser suggested that the gradient temperature alone might not be the solely reason for the instable tSNR. Instead, the usage of the gradients can also heat up the magnet itself and the RF coils.
Comment from Lars Kasper
Lars Kasper from ETH Zurich pointed out two additional factors that could play a role in temporally unstable tSNR.
- The acoustic resonances of the gradients are dependent on the temperature of the gradients. Hence, when you are very close to an acoustic resonance at the beginning of the scan, it can happen that the temperature increase during the scan shifts the acoustic resonance further away or closer to the echo-spacing. This can introduce a time-dependent eddy current and affect the tSNR across the fMRI time series.
- The RF-amplifiers (both for transmit and receive, respectively) work the best when they are a bit warmed up already. Hence, it can happen that the cold amplifiers at the beginning of the time series are responsible for a lower tSNR. As they heat up during the fMRI scan, the tSNR improves too.