2 Neuroscience2.3 Basic techniques in neuroscience

2.3.1.3 Neuroimaging and its role in understanding brain function

3 min read2 figures

Written by Resident Doctor Committee · Published

Structural imaging

  • Magnetic resonance imaging (MRI) uses a magnetic field and radiofrequency pulses to measure signals from hydrogen nuclei.

  • Computed tomography (CT) shows bone and calcification better than MRI, because calcified tissue strongly attenuates X-rays.

  • MRI's advantages over CT are no radiation, better grey and white matter contrast, direct multiplanar acquisition and earlier detection of ischaemia.

  • CLARITY embeds fixed brain tissue in a hydrogel and removes its lipids, so the intact organ turns transparent and can be imaged in three dimensions at cellular resolution without sectioning.

Weighting

Appearance

Use

T1

White matter bright, cerebrospinal fluid dark

Anatomy, volume, cortical thickness

T2

Water bright

Oedema, demyelination

Diffusion tensor imaging

  • Diffusion tensor imaging (DTI) is an MRI sequence whose signal comes from water diffusion, not from a radiotracer, X-ray attenuation or neuronal magnetic fields.

  • Water diffuses more freely along myelinated axons than across them, and this anisotropic diffusion shows the course of white matter tracts and gives an in vivo index of their integrity.

  • Fractional anisotropy runs from 0, equal diffusion in all directions, towards 1, strongly directional diffusion.

  • Tractography joins estimated directions into streamlines; it models pathways rather than tracing real axons, and crossing fibres break the tensor model.

Each coloured line is a streamline built by joining estimated diffusion directions, so the arching corpus callosum and descending tracts are a modelled reconstruction of white matter, not real axons. Thomas Schultz, CC BY-SA 3.0, via Wikimedia Commons

DTI shows structural wiring, whereas functional MRI shows correlated activity.

Functional MRI and connectivity

  • Blood oxygen level dependent (BOLD) signal rises when active tissue receives more oxygenated blood than it uses, washing out paramagnetic deoxyhaemoglobin.

  • Task-based functional MRI records the task-locked BOLD signal, an indirect vascular response that peaks around 4 to 6 seconds.

  • Resting-state connectivity is carried by slow spontaneous BOLD fluctuations of roughly 0.01 to 0.1 Hz.

  • Functional connectivity is an undirected statistical dependency, usually a correlation, between the activity time courses of spatially separate regions.

In active tissue, blood flow rises more than oxygen use, so paramagnetic deoxyhaemoglobin is washed out and the BOLD signal rises, peaking about 5 seconds after a brief stimulus. Drawn for MRCPsychiatrist

Functional connectivity is neither a structural link nor evidence of causal direction.

Radiotracer imaging

Method

Tracer and detection

Example

Positron emission tomography (PET)

Fluorine-18 or carbon-11 tracer; paired 511 keV photons detected in coincidence

Fluorodeoxyglucose for glucose metabolism; receptor occupancy

Single-photon emission computed tomography (SPECT)

Single gamma photons from a tracer such as iodine-123, counted by a rotating gamma camera

DaTSCAN

  • Reduced striatal dopamine transporter binding on DaTSCAN supports dementia with Lewy bodies.

  • With many antipsychotics, response is associated with roughly 65% dopamine D2 receptor occupancy, and extrapyramidal effects with occupancy above about 80%.

  • PET uses ionising radiation, which limits repeat scans and research in children.

Magnetic resonance spectroscopy

  • Magnetic resonance spectroscopy (MRS) measures chemical concentrations in a single voxel with no injection and no ionising radiation.

  • Nuclei in different molecules resonate at slightly different frequencies, which produces a spectrum of peaks.

Peak

Taken to reflect

N-acetylaspartate

Neuronal integrity

Choline

Membrane turnover

Creatine

Energy compounds; often the reference

Magnetoencephalography

  • Magnetoencephalography (MEG) detects the magnetic fields of postsynaptic currents with millisecond timing and good source localisation, because the skull does not distort them.

  • Its sensors are superconducting quantum interference devices (SQUIDs), kept in liquid helium at about -269 °C inside a magnetically shielded room.

  • MEG records directly, with no reference electrode.

In MEG helium cools the detectors, whereas in MRI it cools the magnet.

Limits of inference

  • Reverse inference, such as assuming fear because the amygdala activates, is weak when a region supports many processes.

  • Imaging findings from research are not standalone diagnostic tests for primary psychiatric disorders.

Revision material for the MRCPsych examinations. Not clinical guidance, and not for use in patient care.