Hunting for SNR - from Birdcage coils to cryogenically cooled coils for small animal MRI

The RF-coil is the first component where the MR signal is stimulated and received and therefore it is one of the most important components of a MRI-system. The design of properly developed RF-coils is the key to achieve the best clinical, preclinical or experimental result for MRI scientists or clinicians. Demands on high spatial or temporal resolutions require a high signal-to-noise ratio (SNR). For applications on small animals spatial resolutions are required, that are up to a factor 10 higher than for human applications: This fact leads into a decrease of the sensitivity by a factor of the order of 1000. On one hand the SNR can be enhanced by increasing the static magnetic field and commercial animal-scanners up to 21T are available. On the other hand higher SNR can be achieved by optimizing the RF-coil sensitivity and by decreasing the noise contribution of the RF coils and the different receive-chain components of the MRI-system.

The basic principle of volume RF coils is based on magneto static theory and on the general solution of the magnetic field in a source-free region e.g. in spherical coordinates. The approach is the generation of a sinusoidal current density modulation for generating a homogenous transversal B-field inside a cylindrical volume. A continuous sinusoidal or cosine current distribution is an only theoretical model and in praxis this current distribution is approximated by two basic principles. On one hand the current distribution can be approximated by conductive wires placed on non-equidistant angles using a fixed current I=I0. On the other hand the current distribution is approximated on radially equidistant positions of the wires on the cylindrical structure and current I is modulated by sinusoidal or cosine current I0.
The most popular used volume RF-coils are so-called birdcage coils that are based on the approximation of the ideal current distribution on many equidistant points. The basic electrical structure is based on two conductive rings at the front side of the cylindrical surface connected by multiple conductive wires. In terms of a homogenous RF-field inside the volume of interest (VOI) a birdcage coil is a good choice but lacks in most cases with respect to the local sensitivity and SNR.
Local RF coils play an important role when spatial resolution is targeted and high sensitivity is required. This includes single-loop RF-coils and multi-loop coils of various shapes. These coils are generally much smaller than volume coils and have therefore a higher SNR and provide a much higher sensitivity when they are carefully designed. These local coils are optimized for high SNR within a certain VOI and typically do not provide a homogenous sensitivity like a volume birdcage RF-coil.
A major drawback of local RF-coils is the limit with respect to the field of view (FOV) but this limitation can be overcome by combination of multiple surface coils as elements of a large-scale RF-array. The signals detected by the individual coil-elements of the array coil are fed into the separate receiver channels for final signal combination. This approach preserves the primarily high SNR of each coil-element and offers the advantage that the array-coil covers a much larger FOV.
The optimization of room-temperature coils is in most cases limited by theoretical and practical limitations and such changes will lead in an increase of several percent in the sensitivity of the optimized coils. At high magnetic fields (>4T) for small animals or for low field applications on humans (<1T) the observed tissue volumes are of a size that renders the sample noise contributions that is comparable or smaller than the thermal noise contribution. For this case the noise contribution is primarily caused by resistive losses of the RF-coils and the components of the receive-chain. Noise contributions from the sample are the thermal noise, due to the thermal motion of the charge carriers and the so-called dielectric losses due to the electrical field of the RF-field. Decreasing the temperature of living samples is usually not possible, but a reduction of the thermal noise and the resistive losses of the RF receive-chain will produce a significant increase of the SNR. So-called cryogenic coils show a significant increase of the SNR ratio compared to conventional room temperature coils. The results acquired with the MRI CryoProbeTM demonstrate the high performance of cryogenically cooled RF-coils for in-vivo small animal applications. The gain in SNR can be used for higher spatial or temporal resolution or for shorter scan times.