Elsevier

NeuroImage

Volume 186, 1 February 2019, Pages 399-409
NeuroImage

Lateral geniculate nucleus volumetry at 3T and 7T: Four different optimized magnetic-resonance-imaging sequences evaluated against a 7T reference acquisition

https://doi.org/10.1016/j.neuroimage.2018.09.046Get rights and content

Highlights

  • The LGN volume can be determined with a reliability never achieved before.

  • The LGN volume can be determined with a spatial resolution never realized before.

  • All methods are compared between 3 T and 7 T.

Abstract

Purpose

The lateral geniculate nucleus (LGN) is an essential nucleus of the visual pathway, occupying a small volume (60–160 mm3) among the other thalamic nuclei. The reported LGN volumes vary greatly across studies due to technical limitations and due to methodological differences of volume assessment. Yet, structural and anatomical alterations in ophthalmologic and neurodegenerative pathologies can only be revealed by a precise and reliable LGN representation. To improve LGN volume assessment, we first implemented a reference acquisition for LGN volume determination with optimized Contrast to Noise Ratio (CNR) and high spatial resolution. Next, we compared CNR efficiency and rating reliability of 3D Magnetization Prepared Rapid Gradient Echo (MPRAGE) images using white matter nulled (WMn) and grey matter nulled (GMn) sequences and its subtraction (WMn-GMn) relative to the clinical standard Proton Density Turbo Spin Echo (PD 2D TSE) and the reference acquisition. We hypothesized that 3D MPRAGE should provide a higher CNR and volume determination accuracy than the currently used 2D sequences.

Materials and methods

In 31 healthy subjects, we obtained at 3 and 7 T the following MR sequences: PD-TSE, MPRAGE with white/grey matter signal nulled (WMn/GMn), and a motion-corrected segmented MPRAGE sequence with a resolution of 0.4 × 0.4 × 0.4 mm3 (reference acquisition). To increase CNR, GMn were subtracted from WMn (WMn-GMn). Four investigators manually segmented the LGN independently.

Results

The reference acquisition provided a very sharp depiction of the LGN and an estimated mean LGN volume of 124 ± 3.3 mm3. WMn-GMn had the highest CNR and gave the most reproducible LGN volume estimations between field strengths. Even with the highest CNR efficiency, PD-TSE gave inconsistent LGN volumes with the weakest reference acquisition correlation. The LGN WM rim induced a significant difference between LGN volumes estimated from WMn and GMn. WMn and GMn LGN volume estimations explained most of the reference acquisition volumes' variance. For all sequences, the volume rating reliability were good. On the other hand, the best CNR rating reliability, LGN volume and CNR correlations with the reference acquisition were obtained with GMn at 7 T.

Conclusion

WMn and GMn MPRAGE allow reliable LGN volume determination at both field strengths. The precise location and identification of the LGN (volume) can help to optimize neuroanatomical and neurophysiological studies, which involve the LGN structure. Our optimized imaging protocol may be used for clinical applications aiming at small nuclei volumetric and CNR quantification.

Introduction

The LGN is the main thalamic node of the visual pathway between the retina and the visual cortex (Guillery et al., 2001; Leventhal et al., 1981; RW, 1988; Sherman and Guillery, 2002) and is involved in visual processing but also in control of visual spatial attention (Schneider and Kastner, 2009). Located superiorly to the hippocampus and medially to the optic radiation (Gray, 1918), each LGN consists of retinotopically organized (two) magnocellular and (six) parvocellular neuronal layers (Guillery, 1979; Hickey and Guillery, 1979) which process complementary information of visual stimuli. In healthy adults, it has been shown by functional magnetic resonance imaging (fMRI) that the LGN exhibited significant attentional enhancements during an attention task (Poltoratski et al., 2017). However, the LGN was defined functionally, and thus no segmentation of the LGN was performed which makes it difficult to attribute fMRI signal changes solely to the LGN in this study. The LGN volume is altered in ophthalmologic and neurodegenerative pathologies of afferent and efferent visual systems such as albinism, amblyopia, and glaucoma (Barnes et al., 2010; Lee et al., 2014; Mcketton et al., 2014).

To determine structural and volumetric variability or subtle anatomical alterations of the LGN, precise visualization of its morphology is needed. However, its small size and its deep brain localization make in vivo evaluation challenging (Andrews et al., 1997). To depict morphological LGN changes and to measure its volume in-vivo, proton density weighted (PDw) magnetic resonance imaging (MRI) 2-dimensional (2D) acquisitions have been applied (Bridge et al., 2008; Horton et al., 1990; Kitajima et al., 2015; Lee et al., 2014; Mcketton et al., 2014). Moreover, functional imaging at field strengths of 3 T (3T) and 7 T (7T) applying visual stimuli varying in space, time, luminance, or color revealed subdivisions of the functional maps in the LGN (Denison et al., 2014).

In other studies, MPRAGE sequences have depicted various intra-thalamic structures including the LGN (Dai et al., 2011; Fujita et al., 2001; Kitajima et al., 2015; Korsholm et al., 2007; Li et al., 2012; Wang et al., 2015). The different myelin concentrations of thalamic nuclei and the fact that myelin influences T1 contrast allowed improvements of MPRAGE contrast of the LGN by optimizing inversion times (TI) to null either with white matter (WMn) (Sudhyadhom et al., 2009; Vassal et al., 2012), grey matter (GMn) (Bender et al., 2011; Magnotta et al., 2000; Wyss et al., 2014, 2016) or both (Tourdias et al., 2014). For example, Tourdias et al. (2014) applied WMn and GMn, and found that WMn improved the contrast between the thalamus and the surrounding tissues, and also revealed best intrathalamic contrast. The above mentioned studies optimized their sequences for the maximal contrast between WM and GM. Yet, the accuracy of simulations reported in these studies is limited not only by the model assumptions, but also by the variability of the tissue properties, by the lack of estimation or measurements of the LGN's relaxation times, and by spin density (Deichmann et al., 2000).

Previous MRI studies, however, lack sufficient contrast for accurate LGN volume quantification or use intricate processing methods and thereby limit clinical and neuroscientific applicability of LGN imaging (Yu et al., 2015). Therefore, a fast, clinically as well as neuroscientifically implementable MRI method to investigate LGN morphology is needed. Here, we present a 3D isotropic high-resolution reference acquisition for LGN volume determination with optimized CNR at 7T. In addition, we recorded fast MRI sequences using an isotropic high-resolution MPRAGE with a TI that nulled either WM or GM signals (i.e. WMn and GMn, respectively). Next, WMn-GMn were tested on volume rating reliability and CNR (efficiency) against the clinical standard (PD 2D Turbo-Spin-Echo) and the reference acquisition.

Section snippets

Subjects

Prospectively, 31 healthy subjects (mean age 31 and standard deviation (STD) 7.0 years, range: 18–49 years, 18 women) gave informed written consent to attend this project accepted by the ethics committee, Canton Zurich, Switzerland. To be included, subjects had to be older than 18 years, with no history of neurologic, psychiatric or ophthalmologic diseases affecting the LGN. On 3T and 7T MRI Scanners (Philips Healthcare, Best, the Netherlands) with 2 channel transmit and 32-channels receive

Results

Sample characteristics evaluations showed that volume and CNR of the 31 subjects and of the subgroup of 20 subjects were normally distributed for all sequences on both scanner (Shapiro-Wilk tests: all p > 0.05). Only the CNR of the PD-TSE at 7T had a distribution over the subjects significantly different for normal distribution (p = 0.04 Shapiro-Wilk test corrected for multiple comparisons). Simulations of the signal and contrast of GM and WM for MPRAGE sequences are shown in Supplementary

Discussion

This study aimed at determining an MR sequence that assesses the LGN volume more accurately, when compared to the currently used 2D sequences and to a reference acquisition. We found that 3D MPRAGE imaging of the LGN resulted in a higher CNR and volume determination accuracy. Our results indicate that reliable LGN volume quantification can be achieved by short scanning on standard 3T scanners.

MPRAGE improved imaging of the LGN (Yu et al., 2015) and revealed detailed intra-thalamic structures (

Conclusion

We obtained LGN images with high contrast by optimizing T1-contrast of MPRAGE scans and by correcting for motion. Based on this procedure, we provided reliable LGN volume quantification. Measuring LGN volume with 0.75 mm isotropic protocols with the short duration of about 15 min (using GMn – WMn) might thus be clinically and neuroscientifically relevant for diseases affecting the visual pathway or for further insight into the structural and functional connectivity of the LGN. The

Acknowledgment

We would like to acknowledge the King Abdulaziz University and The Saudi Arabian Cultural Office in Paris, France, for funding part of this Project.

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