Adjusting For B1+Rms Or Sar Below Normal Mode - Boston Scientific ImageReady MRI Mode D'emploi

ImageReady™ MRI Guidelines for Boston Scientific Deep Brain Stimulation Systems

Adjusting for B1+rms or SAR Below Normal Mode

Some pulse sequences may exceed the implant safety limits for the Boston Scientific DBS System. The below
guidelines will enable lower B1+rms or SAR levels to be achieved. If, at any point prior to completing the full workflow,
an acceptable B1+rms or SAR level has been achieved, no further parameter adjustments are necessary. If adjusting
or checking B1+rms is possible on the scanner, this is likely to be a preferable option over adjusting or checking SAR
because B1+rms tends to be less restrictive and more accurate.
Once a sequence has been optimized for reduced B1+rms, saving the parameters for the sequence locally may be
helpful for use with other patients with similar implants.
Note: Some scanners provide the user with an updated estimate of B1+rms or SAR while the user changes the
sequence parameters. If a scanner does not provide this information in real time, one option is to initiate a scan
each time after changing a parameter. At the time of a sequence initiation, the scanner should provide the new
adjusted B1+rms or SAR level with the chosen parameters.
• If the scanner provides an 'implant option,' this option can be utilized to input scan conditions.
• If the scanner does not provide an 'implant option,' many pulse sequences under Normal Mode, especially in
the gradient Echo family, have low B1+rms or SAR levels without any modifications.
• If the required pulse sequence exceeds the implant B1+rms or SAR limit, the RF pulse type may be set to 'Low
SAR' if this option is available on the scanner. 'Low SAR' is available on most scanners and helps to reduce
B1+rms or SAR without affecting image quality.
• If the 'Low SAR' option is unavailable or the B1+rms or SAR levels still exceed the manufacturer limits after
setting the RF pulse type to 'Low SAR,' two additional options that can help reduce RF exposure are listed
below. These two options are trade‑offs and can be exercised together to achieve a good common trade‑off.
○ Increasing TR. In some cases, 20%, e.g., from 2500 ms to 3000 ms could be sufficient, but this could be
increased by 100% if need be e.g., from 550 ms to 1100 ms.
▪ Choose this option when reducing the number of slices is not acceptable.
▪ Avoid this option in T1‑SE sequences as this impacts contrast.
▪ Also avoid this option if longer scan time is not acceptable.
○ Reducing number of slices.
• If B1+rms or SAR levels still exceed the implant limit, reducing RF can still be achieved with:
○ Reducing flip‑angle (alpha), reducing refocusing flip angle, or using fewer RF saturation bands.
○ Reducing number of echoes (echo train length/ turbo factor/ shot factor).
12 References
McRobbie, et al. "MRI from Picture to Proton." 2007. Cambridge university press.
Faulkner W.. "New MRI Safety Labels & Devices, B1+rms as a Condition of Use." SMRT Signals, Feb 2016 V5, Nol.
https://www.ismrm.org/smrt/E‑Signals/2016FEBRUARY/eSig_5_1_hot_2.htm
Franceschi A.M. et al. "Optimized, Minimal Specific Absorption Rate MRI for High‑Resolution Imaging in Patients with Implanted Deep Brain Stimulation
Electrodes." AJNR Am J Neuroradio1. 2016 Nov; 37(11): 1996‑2000.
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