A 20-channel coil for improved magnetic resonance imaging of the optic nerve.

Optic neuritis is characterized by inflammation of the optic nerve, which causes a loss of vision due to swelling and destruction of the myelin sheath covering the optic nerve (1). The most common etiology is Multiple Sclerosis (MS). Nearly 50% of patients with MS will develop optic neuritis and 20-30% of the time optic neuritis is the presenting sign of MS (2). Early diagnosis can decrease the risk of developing negative symptoms from optic neuritis (3, 4). In MRI the presence of lesions, which indicate demyelization of the white matter, is the strongest predictor for clinically definite MS. Other methods of determining optic neuritis include a funduscopic exam, which in some cases does not give an adequate diagnosis. MRI of the optic nerve, which is usually performed with a standard head coil such as the clinical 12-channel coil available on the Siemens TIM Trio 3T MRI scanner, is subject to several limitations such as artifacts from eye movement (5). Many limitations could be overcome using a coil built specifically to image the optic nerves (6-8). For example, increased SNR obtained using a dedicated optic nerve coil would enable reduced field-of-view (FOV) DTI images as well as faster scans through parallel imaging techniques to minimize patient movement artifacts. Phased array receiver coil designs have been increasingly applied in MRI (9-13). Phased arrays are advantageous since multiple channels can provide an increase in signal sensitivity and imaging acceleration (14). A single surface coil typically has sufficient signal to noise ratio (rSNR) at the surface of a sample; however, at greater depths the SNR can suffer greatly compared to a traditional volume coil. Use of a phased array coil can compensate at the greater imaging depths and, if utilized correctly, can provide similar or greater SNR values compared to a volume coil (15). It has also been discovered that one of the factors that improves accelerated imaging is increasing the number of coils surrounding a volume of interest. This is due to the increased number of different coil sensitivity profiles. The increased parallel imaging performance can aid in the use of multiple parallel imaging techniques (16, 17). Furthermore, in addition to providing increased resolution and/or decreasing imaging time, the increase in SNR can enable diffusion tensor imaging (DTI) (18, 19), enhance temperature measurements (20), and fundamentally improve clinical diagnosis and patient treatment planning. In this study, an RF phased array coil was built to image the optic nerve from the orbits to the optic chiasm for the purposes of visualizing optic neuritis that typically manifests in Multiple Sclerosis patients. As opposed to previous designs, which are limited by the number of coils or generalized to imaging all of the head (9-11) we present a dedicated 20-channel coil for imaging of the optic nerve. This coil achieves improved SNR for higher resolution, faster imaging times and useful DTI performance.