SAR and B1+rms are two radio-frequency (RF) exposure metrics used in magnetic resonance imaging (MRI).
SAR – the Specific Absorption Rate – is the RF power transmitted (in watts – W) into the patient divided by the patient weight (in kilograms – kg). SAR has units of W/kg. The allowable SAR is limited to the Normal Mode with a whole body SAR of 2 W/kg, or the First Level Controlled Mode to 4 W/kg. These limits apply to the in-built body coil. Different limits apply for dedicated transmit head or local transmit coils.
You can use the SAR and B1+rms calculator spreadsheet to investigate the effect of changing patient weight and height, sequence type (Turbo spin echo – TSE, spin echo -SE and gradient echo- GRE), RF flip angle, RF pulse type, number of echoes and slices, TR, and even scanner magnetic field strength B0 on the whole body SAR. Hyperechoes only apply for TSE sequences.
You can read more about SAR in MRI from Picture to Proton (3rd edition) section 20.2.
B1+rms is a measure of a time-weighted average RF magnetic field exposure, calculated over 10 second intervals, measured in micro-tesla (uT). It is independent of the patient characteristics, but depends upon flip angle, pulse type, number of echoes, slices, and TR. You can use the SAR and B1+rms calculator to investigate the influence of these parameters on B1+rms.
B1+rms is useful in determining compliance with implant scanning conditions. The Fixed Parameter Option (basic) – FPO:b – is a mode of scanning with RF limits given in terms of B1+rms and peak B1 value – these are shown on the calculator.
Also shown on the calculator are:
Specific Energy Dose (SED): the RF energy absorbed per kg. This is the SAR multiplied by scan time – also shown in minutes and seconds. The total absorbed energy in kilo-joules (kJ) is the SED times patient weight.
Duty cycle is the fraction of the the pulse sequence for which the RF pulse is on. It is used in the calculations.
Notes on the use of the SAR and B1+rms spreadsheet.
The small italic numbers indicate the maximum or minimum values permitted for various parameters. The chosen parameter values will not change automatically but must be entered manually.
The dropdown menus (for B0, sequence, RF type, matrix, NSA and hyperechoes) may not work on all devices. Values may be typed in. Allowable values are shown on the menus sheet. The sheet does not yet work on IOS devices. unless you have the Zoho Sheet app.
When selecting SE or GRE as sequence type, the number of echoes needs to be set manually to 1.
None of your data entered will be saved.
Please remember, this is an educational tool only and is not to be used for choosing parameters for human scanning.