Advances In Mri Scanner Receiver Technologies

By Author: Wordsworth White
Total Articles: 73
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In current MRI systems, surface coils are often used for better signal reception. This is because they are sensitive to signals close to the coil, which means that surface coils detect the signal in the part of the body being examined more efficiently. But they also amplify noise signals more efficiently too, so why are they better? This is because their sensitivity to noise signals from the rest of the patient's body, which is further away from the coil, is reduced.

Although they are placed on or around the surface of a patient, they may be optimized to image deep-body structures. The signal from the RF power amplifier drives the transmit coils in such a way that the desired RF magnetic field is produced. It isn't the transmission and reception of radio waves which are used in MRI systems, but actually magnetic induction from the oscillating magnetic fields.

Sometimes the transmit coil (or body coil) is also used as the receive coil. If the transmit and receive coils are different coils, the receive coil must be detuned and the preamplifier blocked, so that the large transmit signal cannot burn them out. The ...
... transmit coil is also separated from the gradient coils by an RF shield.

The RF power received from the RF power amplifier comes in two signals, which have a 90° phase difference. The RF transmit coil converts the power into a circularly polarized RF magnetic field. An RF MRI receiver is used to process the signals from the MRI receiver coils. Most contemporary MRI systems have six or more MRI receivers to process the signals from numerous coils. The signals range from approximately 1MHz to 300MHz, with the frequency range highly dependent on applied-static magnetic field strength. The bandwidth of the received signal is small, usually less than 20kHz, and dependent on the size of the gradient field.

A traditional MRI receiver configuration has a low-noise amplifier followed by a mixer. The mixer combines the signal of interest to a low-frequency IF frequency for conversion by a high-resolution, low-speed, 12-bit to 16-bit analog-to-digital converter (ADC). In this receiver architecture, the ADCs used have relatively low sample rates below 1MHz. Because of the low-bandwidth requirements, ADCs with higher 1MHz to 5MHz sample rates can be used to convert multiple channels by time-multiplexing the receive channels through an analog multiplexer into a single ADC.

With the introduction of higher-performance ADCs, newer MRI receiver architectures are now possible. High-input bandwidth, high-resolution 12-bit to 16-bit ADCs with samples rates up to 100MHz can also be used to directly sample the signals, therefore removing the need for analog mixers in the receive chain.

When dealing with MRI scanner receivers, it is important to take a good look at the signal-to-noise ratio and the bandwidth. In general, a wider bandwidth includes more noise. Decreasing the bandwidth by a factor of 4 results in an increase in the signal-to-noise ratio by a factor of 2 (less noise in the image).

So, when decreasing the bandwidth, a better signal quality is gained.

In the signal-to-noise ratio however, the overall effect of the reduction of the bandwidth is an improvement in the signal-to-noise ratio.
The initial bandwidth of the MRI signal produced by the MRI scanner is a function of the special encoding readout gradient strength, and the chemical shift.

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