We have demonstrated two methods for reliably identifying MGB based solely on structural data' The f

1
MO85
Identification of the auditory thalamus using
multi-modal structural analyses J. T. Devlin1, E.
Sillery1, H. Johansen-Berg1, T. E. J. Behrens1,
P. M. Matthews1, D. A. Hall2, and D. R.
Moore2 1Centre for Functional Magnetic Resonance
Imaging of the Brain, University of Oxford, U.
K. 2MRC Institute of Hearing Research, University
Park, Nottingham, U.K.
Objective
Manual identification of MGB
Automatic identification of MGB
Functional investigations of the ascending
auditory system in humans are limited due to
difficulty identifying the medial geniculate body
(MGB) from surrounding thalamic nuclei. Here we
use high resolution proton-density and
diffusion-weighted structural images to develop a
highly reliable, accurate method to identify MGB
in individuals.

• In the PD, but not the T1 scan, MGB was distinct
  from the lateral geniculate nucleus (Figure 2),
  allowing independent manual identification by two
  authors (JTD, HJB).
• To identify MGB, the following method was used
• 1. Find the coronal section showing
• the substantia nigra (SN) meeting
• at the midline, just inferior to the
• third ventricle (V3).
• 2. Move 6-10mm posteriorally until the
• LGN appears, a tear-dropped shaped
• high intensity region superior to the body
• of the hippocampus and inferolateral to
• the majority of the thalamus (Th).

• An alternate, unbiased method is to separate the
  medial and lateral geniculate nuclei based on
  their distinct connectivity profiles
• 1. An ROI was defined in standard
• space for each hemisphere to
• conservatively encompass both LGN
• and MGB x 10 to 26, y 22 to
• 30, z 2 to 10.
• 2. Probabilistic tractography 6 was run
• from each voxel in the mask. The
• resulting connectivity profiles were
• entered into a cross-correlation matrix
• and the cells of the matrix were sorted
• to bring similar items together,
• yielding clusters of voxels with similar
• connectivity 7.

Background
LV
Y 28
HG
Th
Th

• Unlike other sensory systems, substantial
  processing of auditory signals occurs
  subcortically, before ever reaching the cortex.
  Sounds first reach the nervous system via the
  hair cells of the cochlea whose movements
  transduce the acoustic energy into a neural
  signal carried by the auditory nerve into the
  brain. Information arrives at the cochlear
  nucleus of the brainstem which projects to the
  brainstem and inferior colliculus (IC). The IC
  connects to the medial geniculate body (MGB) of
  the thalamus, which in turn projects to primary
  auditory cortex (PAC). Together, the IC, MGB,
  and PAC constitute three of the main regions of
  the ascending auditory system (Figure 1).
  Imaging these subcortical structures presents a
  challenge due to the effects of pulsatile motion,
  the small size of the relevant nuclei, and the
  difficulty identifying these regions
  anatomically. Here we address the latter problem
  using multi-modal structural imaging techniques
  to reliably identify MGB in individuals.
• Figure 1 A schematic diagram of the ascending
  auditory pathway. IC inferior colliculus, MGB
  medial geniculate body, HGHeschls gyrus (site
  of primary auditory cortex). Note neither
  contralateral nor top-down projections are shown.

Pu
V3
IC
Hi
SN
Interpeduncular fossa
Y 22
LV
HG
V3
Th
LGN
PC
Hi
MGB
SN
Right hemisphere
Y 28
Abbrevs HG Heschls gyrus, Hi body of the
hippocampus, IC internal capsule, LGN lateral
geniculate nucleus, LV lateral ventricle, MGB
medial geniculate nucleus, PC posterior
commissure, Pu putamen, SN substantia nigra,
Th thalamus, V3 third ventricle
Left hemisphere
HG
HG
MGB MGB
IC
Method

• Grey matter typically has 20 higher proton
  density than white matter 1, suggesting that
  grey-white contrast could be improved in a
  proton-density (PD) scan relative to T1-weighted
  images 2
• In addition, connectivity patters differ across
  thalamic nuclei, suggesting that MBG might be
  identified according to its connectivity profile
  3
• Five neurologically normal volunteers (3F, 2M)
  participated in two separate scanning sessions
• 1. 5-10 high resolution proton-density (PD)
  scans using a
• fast spin echo protocol (coronal acquisition,
  800?m2 in-
• plane resolution, 2mm slice thickness, TR 6
  sec,
• effective TE 9.5 mse), implemented on a 1.5T
• Siemens Sonata.
• 2. Three diffusion-weighted scans using an EPI
  protocol
• (TR 15sec, TE106.2ms, b-value1000s/mm2, 60
• directions, 5-10 non-diffusion weighted
  images, 1.9mm2
• in-plane resolution, 2.5mm slice thickness)
  optimized
• for tractography 4 implemented on a 3T
  Varian
• scanner. Data collection was cardiac gated to
  reduce
• pulsatile motion artefacts.

Discussion
We have demonstrated two methods for reliably
identifying MGB based solely on structural data.
The first relies on differences in proton density
between grey and white matter 1 while the
second is based on the distinct connectivity
profiles of the medial and lateral geniculate.
In both cases, data acquisition required less
than one hour using commonly available pulse
sequences on standard hardware, a clear advantage
over approaches that rely on either extremely
long acquisitions (e.g. 13 hours, 8) or very
high fields (4-8T, 8, 9). These data also
provide further validation for using DWI to
structurally segment separate anatomical regions.
Previously Johansen-Berg et al. 7 showed that
this method produced excellence correspondence
between DWI and functional parcellation of SMA
and pre-SMA. Here we show that the same
technique also matches visually identified
anatomical borders. The ability to reliably
identify MGB anatomically will facilitate
functional studies of the ascending auditory
system such as those investigating spatial
localisation 10 or laterality effects 11.
These methods will complement, extend, and
objectify further functional characterisations of
this increasingly interesting nuclear group.
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