MRI & Medical Imaging

An MRI system relies on two kinds of signal working together: an RF carrier at the Larmor frequency, 42.58 MHz per tesla (63.9 MHz at 1.5 T, 127.7 MHz at 3 T), that excites the spins, and arbitrary waveforms that shape the RF envelope and drive the gradient coils. Figure 1 shows the RF side, a sinc envelope modulating the carrier. Because MRI is performed on people, the bench also confirms that delivered RF power stays within safe limits, which is why these labs pair waveform generation with calibrated RF power sensing.

The demands are specific. Gradient and RF events have to land in a fixed relationship across the whole sequence, so timing has to be tight and repeatable from shot to shot. The envelopes have to be smooth and faithful, because a distorted RF pulse rotates the spins imperfectly and a noisy gradient blurs the encoding, both of which show up in the image. And the sequences are long, so the generator has to hold the full pattern rather than loop a short fragment. These requirements apply together, on the same channels: clean amplitude, fine timing, multi-channel coordination and deep memory.

Precise, low-jitter timing. The boundary between excitation, encoding, and readout is defined in time. Low trigger jitter and fine timing resolution let each event land where the sequence expects it, shot after shot, so the encoding stays consistent and the emulated returns stay coherent.

Arbitrary pulse shaping with clean amplitude. The point of an arbitrary waveform generator is the shape. High vertical resolution keeps amplitude steps small and the envelope faithful through the gradient amplifier or the RF modulator, so the spins see the profile the sequence designed rather than a quantized approximation of it.

Deep waveform memory. Imaging sequences are long and rarely repeat a short pattern. Deep memory lets the generator hold a full multi-step sequence and play it through without looping, which matters for faithful sequence reproduction and for emulating a realistic stream of returns.

Which models fit

For gradient and RF envelope work where amplitude fidelity is the priority, start with the 16-bit Model 675 or Model 685 (Figure 3). Sixteen bits give 65,536 levels across the output range, which keeps the envelope finely graded through the modulator or gradient amplifier. The Model 675 is the gradient-drive candidate: 6 Vpp into 50 Ω, or 12 Vpp with the HV option, and the amplitude doubles into a high-impedance input, to 24 Vpp (±12 V) with the HV option. Where a gradient amplifier specifies a ±10 V analog input, the 675 with the HV option covers it directly. Its 1.2 GS/s sample rate and 400 MHz sine range also cover the RF carrier at 1.5 T and 3 T. The Model 685 adds sample rate, 6.16 GS/s (12.32 GS/s in RF mode, with carriers to 6 GHz), for envelope and emulation work above that.

Where the work calls for the highest sample rate, the faster edges of short pulses or the emulation of higher-bandwidth returns, the Model 686 fits: 20 GS/s at 14 bits. The three models use different synchronization arrangements: a QSFP sync bus on the 686, an Infiniband 4X sync bus on the 685, and no multi-unit bus on the 675. A mixed bench therefore shares one timebase through a common external reference clock, for example 10 MHz, which is inside every model's reference input range (5 MHz to 100 MHz on the 675, 5 MHz to 200 MHz on the 685, 5 MHz to 500 MHz on the 686). Trigger the units from one source and measure the residual offset between them once at setup. Table 1 lists the datasheet figures for the three models.

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