Studies reaffirm safety of magnetic resonance imaging

January 15, 2010

Results of a published case series reaffirm the safety of magnetic resonance imaging studies in patients with miniature glaucoma shunt.

Genova, Italy-Results of a published case series reaffirm the safety of magnetic resonance imaging (MRI) studies in patients with a miniature glaucoma shunt (Ex-PRESS, Optonol) implanted as well as previous evidence that MRI interpretation of the brain is not affected by any artifact from the stainless steel implant.

As new information, the study shows the drainage device has minimal effect, if any, on orbital images and that although MRI interpretation of the optic nerve may be affected by artifacts caused by the shunt, the quality of the MRI image is generally sufficient for clinical evaluation, said Carlo E. Traverso, MD.

"Proof of the safety of the shunt with MRI examination was required for its FDA and CE approval," said Dr. Traverso, director, Clinica Oculistica, Department of Neurosciences, Ophthalmology, and Genetics (DiNOG), University of Genova, Italy. "However, since this implant has gained substantial popularity in clinical practice and considering that it is generally implanted in older individuals, it is more and more likely that patients with the shunt will undergo MRI studies of the optic nerve and brain to assess for a variety of nonophthalmologic conditions.

The study [Am J Ophthalmol. 2009;14:907-911] included seven eyes of five patients with the shunt implanted for management of primary open-angle glaucoma. All patients had the shunt positioned at the limbus; the implantation was under a conjunctival flap in two eyes and under a scleral flap in five eyes. Time since implantation ranged from 1 month to 24 months (mean, 13). The five patients included three men and two women and they had a mean age of about 70 years.

The MRI was performed with T1-weighted, axial, and coronal T2-weighted imaging with fat saturation and whole brain PD/T2-weighted MRI without any change in the standard MRI protocol.

Image quality

Determination of the effect of the shunt on image interpretation was made by two independent evaluators, a radiologist and a neuroradiologist. The quality of the images for clinical purposes was rated using a 5-point system where 1 represents optic nerve margins unidentifiable/brain details altered and 5 denotes excellent visualization of the optic nerve/brain.

The shunt clearly was visible on all MRI images, and the ratings of the two assessors agreed for all images with the exception of a single optic nerve study where there was a 1-point difference in their scores. For the optic nerve studies, one image was assigned a score of 2; three received a score of 3, and the remaining three received a score of 4 (mean score 3.3)

"The score of 2 denotes that the imaging is not suitable for clinical purposes whereas according to the definitions of the scoring system, the visualization of the optic nerve was sufficient for clinical evaluation in six of the seven [images]," Dr. Traverso said. "Although the imaging in this study was performed without any modification to the standard MRI protocol, optimization of the pulse sequence for optic nerve imaging in eyes with [a miniature glaucoma shunt] may minimize the effect of artifacts from this device. Further studies in a larger number of patients would be helpful."

The brain imaging was of good quality with a score of 4 assigned to the images in four patients and studies of the fifth patient received a score of 5.

Mean signal intensity values of fellow eye optic nerve images also were compared in the three patients with the shunt unilaterally implanted and showed no significant difference between eyes with and without the shunt.

All patients underwent a complete ophthalmologic examination before and 1 month after the MRI studies, and there were no changes in IOP or in any other clinical features following the imaging.