Time-domain measurement of EM pulses in MRI using a spectrum analyzer

MRI Safety

The measurement of ElectroMagnetic (EM) pulses generated by a MRI apparatus inside phantoms is of key importance for MRI safety assessment through the measurement of the Specific Absorption Rate (SAR). SAR is related either to the local temperature increase of the phantom or to the RMS value of the local Electric (E) field. Indeed, E field is much more easy to assess thanks to the use of appropriate non-interfering E-field probes.

Due to the Ultra Narrow Band (UNB) nature of EM pulses used in MRI (10 to 700 kHz), Digital Sampling Oscilloscope (DSO) are not well suited to measure the temporal shape of these EM pulses. Indeed, the wide bandwidth of DSO leads to a poor Signal-to-Noise Ratio (SNR) owing to the integration of the noise spectral density over the huge DSO bandwidth in comparison to the one of the useful signal: the ratio between these two bandwidths is typically of the order of 1000.

In fact, an Automatic Spectrum Analyzer (ASA) constitutes a better choice for measuring UNB EM pulses

Experimental Setup

The optimal configuration of the experimental setup is shown in the figure above with the following parameters for the ASA:

• “Zero span” configuration,

• use of an Arbitrary Waveform Generator (AWG) to trig both emission of MRI EM pulses and data acquisition with the ASA,

• Central frequency set on the MRI frequency of operation (~ 128 MHz for a 3T MRI)

• RBW corresponding to the spectral bandwidth of the EM pulses,

• Sweep time equal to 2 to 3 periods of the EM pulses,

• Number (AVG) of EM pulses over which the signal is averaged of a few dozens.

Practical Application

The configuration described above has been used for E-field measurement inside a PolyVinylPyrrolidone (PVP) liquid phantom placed in the bore of a preclinical 7T MRI. The non-interfering E-field probe used for the measurement of horizontal E-field component Ex at 300 MHz is a probe eoProbe ET5-bio associated with its optoelectronic converter eoSense MF-1S. For a RARE 2D MRI sequence, the raw signal delivered by the ASA is shown opposite.

The measurement dynamics is slightly above 30 dB with a noise floor at -67 dBm for a RBW equal to 200 kHz. Thanks to the calibration of the E-field probe in the same PVP liquid phantom, we get its antenna factor AF which value is 10 000/m.

E Field & SAR Assessment

With the knowledge of the antenna factor of the E-field probe, the actual RMS value of the E-field component Ex can be easily calculated and leads to the temporal curve opposite.

The peak value of the E-field strength reaches 385 Vrms/m. From that temporal evolution of the E field component Ex, its contribution to the SAR can be straightforwardly assessed by calculating its RMS value over exactly one period: a value of 3 W/kg is thus obtained.

Key features of the Non-Magnetic E-field Probe for MRI safety

• single probe to cover all existing and future MRI machines: 23.4 MHz (0.55T) → 500 MHz (11.7T)

• ultra high orthogonal E field components rejection ratio: > 50 dB

• ultra high spatial resolution: < 1 mm

• high measurement reproducibility: < 0.2 dB

• ultra compact non-magnetic electric field probe: ∅ = 4 mm

• full dielectric, high permittivity, non-magnetic electric field probe: εr ~ 30

• high sensitivity: < 0.32 Vrms/m√Hz noise floor

• ultra high dynamic range: > 130 dB

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