Improving signal-to-noise ratio in transcranial magnetic resonance guided focused ultrasound

Signal-to-Noise Ratio (SNR) can be increased in Magnetic Resonance Imaging (MRI) using Radio Frequency (RF) coils. Of all the options for increasing SNR, coils provide the greatest gains for the dollars spent. Coils for 1.5 and 3 Tesla MRI systems consist of conductive loops that are tuned to resonate at the fundamental frequency associated with the field strength of the MRI scanner. These coils are the transducers between the MR signal and the system electronics and are sensitive to the magnetic fields of the MR signal via Faraday’s Law of Maxwell’s equations. Sensitivity of a coil to the signal in the imaging sample depends primarily on the geometry and position of the loop with respect to the sample. To achieve the highest SNR requires the coil to be in close proximity to the sample of interest, small enough that it is not sensitive to regions outside the region of interest and large enough to pick up the signal at the depth of interest. Other factors such as dielectric or conductive material loading of the coils can significantly affect the tuning and function of a coil. Custom coils designed for a specific application typically provide much higher SNR than commercial coils that are designed for general purpose imaging of a broad range body habitus. Coils that provide even small gains in SNR provide significant imaging improvement. For example, a specific-purpose (SP) coil that can provide a 40% SNR improvement over a general-purpose (GP) coil, can achieve the same image quality as the GP coil in half the imaging time. Similarly, an SP coil that can provide a factor of 2 improvement in SNR over a GP coil can achieve the same image quality 4 times faster than the GP coil. SNR can be used to improve image quality, temporal and spatial resolution, and enable or improve imaging functionality such as temperature measurement accuracy, Diffusion Tensor Imaging (DTI), and MR Acoustic Radiation Force Imaging (MR-ARFI).