title: Ophthalmic magnetic resonance imaging at 7 T using a 6-channel transceiver radiofrequency coil array in healthy subjects and patients with intraocular masses.
abstract: OBJECTIVE
  This study was designed to examine the feasibility of ophthalmic magnetic resonance imaging (MRI) at 7 T using a local 6-channel transmit/receive radiofrequency (RF) coil array in healthy volunteers and patients with intraocular masses.
  
  
  METHODS
  A novel 6-element transceiver RF coil array that makes uses of loop elements and that is customized for eye imaging at 7 T is proposed. Considerations influencing the RF coil design and the characteristics of the proposed RF coil array are presented. Numerical electromagnetic field simulations were conducted to enhance the RF coil characteristics. Specific absorption rate simulations and a thorough assessment of RF power deposition were performed to meet the safety requirements. Phantom experiments were carried out to validate the electromagnetic field simulations and to assess the real performance of the proposed transceiver array. Certified approval for clinical studies was provided by a local notified body before the in vivo studies. The suitability of the RF coil to image the human eye, optical nerve, and orbit was examined in an in vivo feasibility study including (a) 3-dimensional (3D) gradient echo (GRE) imaging, (b) inversion recovery 3D GRE imaging, and (c) 2D T2-weighted fast spin-echo imaging. For this purpose, healthy adult volunteers (n = 17; mean age, 34 ± 11 years) and patients with intraocular masses (uveal melanoma, n = 5; mean age, 57 ± 6 years) were investigated.
  
  
  RESULTS
  All subjects tolerated all examinations well with no relevant adverse events. The 6-channel coil array supports high-resolution 3D GRE imaging with a spatial resolution as good as 0.2 × 0.2 × 1.0 mm, which facilitates the depiction of anatomical details of the eye. Rather, uniform signal intensity across the eye was found. A mean signal-to-noise ratio of approximately 35 was found for the lens, whereas the vitreous humor showed a signal-to-noise ratio of approximately 30. The lens-vitreous humor contrast-to-noise ratio was 8, which allows good differentiation between the lens and the vitreous compartment. Inversion recovery prepared 3D GRE imaging using a spatial resolution of 0.4 × 0.4 × 1.0 mm was found to be feasible. T2-weighted 2D fast spin-echo imaging with the proposed RF coil afforded a spatial resolution of 0.25 × 0.25 × 0.7 mm.
  
  
  CONCLUSIONS
  This work provides valuable information on the feasibility of ophthalmic MRI at 7 T using a dedicated 6-channel transceiver coil array that supports the acquisition of high-contrast, high-spatial resolution images in healthy volunteers and patients with intraocular masses. The results underscore the challenges of ocular imaging at 7 T and demonstrate that these issues can be offset by using tailored RF coil hardware. The benefits of such improvements would be in positive alignment with explorations that are designed to examine the potential of MRI for the assessment of spatial arrangements of the eye segments and their masses with the ultimate goal to provide imaging means for guiding treatment decisions in ophthalmological diseases.
authors.affiliation: From the *Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck-Center for Molecular Medicine; †MRI.TOOLS GmbH, Berlin; ‡Department of Ophthalmology, University of Rostock, Rostock; §Institute for Diagnostic Radiology and Neuroradiology, University of Greifswald, Greifswald; and ∥Experimental and Clinical Research Center (a joint cooperation between the Charité Medical Faculty and the Max Delbrueck Center for Molecular Medicine Campus Berlin Buch), Berlin, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Graessl
authors.forename: Maximilian
authors.initials: M
authors.lastname: Muhle
authors.forename: Michael
authors.initials: M
authors.lastname: Schwerter
authors.forename: Jan
authors.initials: J
authors.lastname: Rieger
authors.forename: Celal
authors.initials: C
authors.lastname: Oezerdem
authors.forename: Davide
authors.initials: D
authors.lastname: Santoro
authors.forename: Darius
authors.initials: D
authors.lastname: Lysiak
authors.forename: Lukas
authors.initials: L
authors.lastname: Winter
authors.forename: Fabian
authors.initials: F
authors.lastname: Hezel
authors.forename: Sonia
authors.initials: S
authors.lastname: Waiczies
authors.forename: Rudolf F
authors.initials: RF
authors.lastname: Guthoff
authors.forename: Karen
authors.initials: K
authors.lastname: Falke
authors.forename: Norbert
authors.initials: N
authors.lastname: Hosten
authors.forename: Stefan
authors.initials: S
authors.lastname: Hadlich
authors.forename: Paul-Christian
authors.initials: PC
authors.lastname: Krueger
authors.forename: Soenke
authors.initials: S
authors.lastname: Langner
authors.forename: Oliver
authors.initials: O
authors.lastname: Stachs
authors.forename: Thoralf
authors.initials: T
authors.lastname: Niendorf
mesh_terms: D000328:Adult; D000368:Aged; D004867:Equipment Design; D005123:Eye / Q000033:anatomy & histology / Q000473:pathology*; D005134:Eye Neoplasms / Q000175:diagnosis*; D005240:Feasibility Studies; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods / Q000706:statistics & numerical data; D008297:Male; D008875:Middle Aged; D019047:Phantoms, Imaging; D012016:Reference Values; D059629:Signal-To-Noise Ratio; D055815:Young Adult