High resolution MR imaging of the human eye and orbit at 7T using a dedicated 6-channel eye coil - impact of an optoacoustic triggered MR protocol on imaging quality

Abstract

Purpose: Imaging of the human eye and orbit in vivo at 7 Tesla is a challenging but promising MRI application. Apart from development of special eye coils and the subsequent SNR gain, the major challenge with obtaining high-resolution images is the reduction of artifacts due to eye and eye lid movements during scanning. The purpose of this study was to determine the capability of an in-house developed 6 channel transceiver coil array in combination with patient friendly MR protocol and trigger scheme for reduction of motion artifacts.

Methods: MR imaging (T1 weighted 3D FLASH, 2D T2 weighted turbo spin-echo sequences) was performed in healthy volunteers (n=17, mean age 34±11 years, range 24-64 years) and in patients with intraocular masses (n=5, mean age 57±6years, range 50-68 years). All protocols included an optoacoustic trigger scheme, which consisted of an acquisition window followed by a pause to allow for blinking. Subjects were asked to fix the gaze of the open eye for 3 s on a cross projected on a whiteboard outside the scanner. Fixation period was followed by a rest period of 3 s that allowed for blinking. MR data acquisition was interrupted during these rest periods. Subsequently, re-fixation was conducted after the acoustic indication via a stethoscope-style pneumatic headphone, which turned up 1 s in advance of data acquisition restart. The alternating gaze fixation-resting processes were repeated until the end of each scan series.

Results: Volunteers and patients involved in this study tolerated all examinations well with no relevant adverse events. In vivo scout imaging using relatively low spatial resolution revealed rather uniform signal intensity for sagittal and transversal views of the eye. The proposed optoacoustic triggering protocol increases patient compliance significantly and reduces movement artifacts for 7T imaging within clinical acceptable scan time for each used protocol resulting in a total examination time of less than 25 minutes. The coil/protocol combination supports clinical imaging with a spatial resolution as good as (0.2 x 0.2 x 1.0) mm3.

Conclusions: The six-channel coil array in combination with the dedicated optoacoustic triggering scheme supports the translation from promising ex vivo ultrahigh field MRI application and results into clinical practice.

Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 589 melanoma • 629 optic nerve