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Nature Methods | Brief Communication

Functional ultrasound imaging of the brain

Journal name:
Nature Methods
Volume:
8,
Pages:
662–664
Year published:
DOI:
doi:10.1038/nmeth.1641
Received
Accepted
Published online

We present functional ultrasound (fUS), a method for imaging transient changes in blood volume in the whole brain at better spatiotemporal resolution than with other functional brain imaging modalities. fUS uses plane-wave illumination at high frame rate and can measure blood volumes in smaller vessels than previous ultrasound methods. fUS identifies regions of brain activation and was used to image whisker-evoked cortical and thalamic responses and the propagation of epileptiform seizures in the rat brain.

Subject terms:

  • Imaging•
  • Neuroscience

At a glance

Figures

  1. Principles for performing fUS in the rat brain.
    Figure 1: Principles for performing fUS in the rat brain.

    (a) Schematic setup depicting the ultrasonic probe, cranial window and a schema of a coronal slice from the rat brain. The principles of ultrasound imaging are schematized. (b) fUS is performed by emitting 17 planar ultrasonic waves tilted with different angles into the rat brain. The ultrasonic echoes produce 17 images of 2 cm × 2 cm (amplitudes are in decibels, dB). Summing up these images results in a compound image acquired in 1 ms. The entire fUS sequence consists of acquiring 200 compound images in 200 ms. (c) Temporal variation s(t) of the backscattered ultrasonic amplitude in one pixel (normalized by the maximum amplitude). The blood signal sB is extracted by applying a high-pass filter (same scale in the two graphs). (d) Frequency spectrum of sB (top left). Two parameters are extracted from this spectrum: the central frequency fD, which is proportional to the axial blood velocity with respect to the z axis and gives rise to the axial velocity image (below left); and the intensity (power Doppler), which is proportional to the cerebral blood volume and gives rise to the power Doppler image (right). fUS is based on power Doppler images. Scale bars, 2 mm.

  2. Applications of fUS imaging.
    Figure 2: Applications of fUS imaging.

    (a–c) fUS imaging of task-evoked brain activation in the rat brain. (a) Power Doppler (PD) fUS images are acquired every 3 s during whisker stimulation (probe in the coronal plane, stereotaxic coordinates β −2.5 mm). The whisker stimulation pattern (red line) consisted of 32 s on and 64 s off repeated 10 times (7 cycles shown). The PD is plotted in percentage relative to the baseline (n = 6). (b) Representative example of an activation map obtained when stimulating the left whiskers. We calculated maps as the correlation coefficient between the power Doppler signal and the stimulus pattern. S1, primary somatosensory barrel cortex; VPM, ventral posterior medial nucleus. S1 and VPM regions were delineated from a rat brain atlas. (c) Representative example of an activation map obtained when stimulating a single whisker. (d–g) fUS imaging of transient brain activity in a rat model of epilepsy. (d) Schematic setup for the imaging of epileptiform seizures. We injected 4-AP focally in the cortex and implanted cortical electrodes for EEG recordings (n = 4). (e) Spatiotemporal spreading of epileptiform activity for two selected ictal events. The power Doppler signal (in percentage relative to baseline) is superimposed on a control power Doppler image. (f) Comparison between electrical recordings (EEG; green line) and the power Doppler signal (PD, blue line) at the site of 4-AP injection. The two events in the shaded region are zoomed in on the graph at right. (g) Maps of the propagation delay of blood volume changes from the focus to other regions (propagation delay in seconds is color coded following the legend on the right: onset is indicated in blue; and blue to red indicates delay increases). Arrows represent the direction of propagation. Scale bars, 2 mm.

References

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Author information

Affiliations

  1. Institut Langevin, Ecole Supérieure de Physique et de Chimie Industrielles Paris Tech, Centre National de la Recherche Scientifique (CNRS) UMR7587, Institut National de la Santé et de la Recherche Médicale (INSERM) U979, Université Paris VII, Paris, France.

    • Emilie Macé,
    • Gabriel Montaldo,
    • Mathias Fink &
    • Mickael Tanter

  2. Centre de Recherche Institut du Cerveau et de la Moelle Epinière, INSERM UMRS 975, CNRS UMR7225, Centre Hospitalier Universitaire Pitié Salpêtrière, Paris, France.

    • Ivan Cohen &
    • Michel Baulac

Contributions

M.B., M.F. and M.T. conceived and initiated the project; M.T. supervised the project. E.M., G.M. and I.C. designed and performed experiments; E.M., G.M. and M.T. wrote the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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Supplementary information

PDF files

  1. Supplementary Text and Figures (572K)

    Supplementary Figures 1–3 and Supplementary Notes 1 and 2

Movies

  1. Supplementary Video 1 (3M)

    Video showing changes in cerebral blood volume during induced epileptiform activity (4AP injection) imaged by fUS. The epileptiform activity is recorded during 1 hour with a short control time at the beginning (baseline). The video displays the variation in Power Doppler relative to the baseline in a color scale ranging from - 50% (blue) to + 50% (red), superimposed on a control Power Doppler image. fUS acquisitions are performed every 3s. The injection site is represented by a green circle and the time relative to injection is displayed in the video. The blood volume response to onset and propagation of hyperactivity is visualized from the injection site to other brain regions.

Zip files

  1. Supplementary Software (5M)

    Implementation of fUS. A guide describes ultrasound sequences and signal processing. Code and data example are also provided.

Additional data