3D printed holographic lenses focus ultrasound for low-cost brain imaging

Researchers at the Polytechnic University of Valencia in Spain 3D printed acoustic holographic lenses for focusing ultrasonic sound waves in the brain. Running computer simulations, the team found acoustic fields with spatial distributions matching the structures of the central nervous system. The 3D printed lenses are engineered to produce these fields that can effectively target brain regions. Better brain imaging and even drug delivery with ultrasound could be made possible by these lenses.

Imaging the brain

Proving a non-invasive window through the skull, brain imaging deals with the structure, function and pharmacology of the nervous system. It is essential for diagnosing tumours, metabolic diseases and lesions. Major techniques to map the human brain include magnetoencephalography, gamma cameras, light-sheet microscopy and electrocorticography

Widely used in diagnostics and soft tissue imaging, ultrasounds are not commonly used for brain imaging. This is due to the inability to precisely control focused ultrasound into the central nervous system. Refraction and attenuation of the skull produces strong phase aberrations, preventing ultrasound from focusing on brain tissues. An attempt to circumvent this problem is using phased arrays to control the incoming ultrasound to correct for aberrations on penetrating the skull. However, this method is costly and also has low resolution.

Penetrating the skull

The Polytechnic University of Valencia’s 3D printed holographic acoustic lens is made from a block of plastic with varying voxel sizes. Ultrasonic waves diffract differently based on the voxel used. Interference of these diffracted waves creates a hologram focusing onto a 3D target volume inside the brain. 

Hologram generation process. (a) CT+MRI tomographic images. (b) Selected target (red volume) acting as a virtual acoustic source and holographic recording surface (blue area). (c) Lens design using the TR back-propagated field. (d) Forward propagation from the holographic lens (red area) to the target tissue (blue volume). Image via Polytechnic University of Valencia.

The study started with producing a computer models of the skull and brain. The researchers extracted the geometry and acoustic properties of the human skull from open source CAT scans. Data from MRI were also gathered to examine soft tissue information from the brain. A realistic brain replica, also known as a skull phantom, was 3D printed according to the computer model. 

After modelling the skull and brain, the team then devised a method to bend ultrasonic waves inside the skull. Three types of holographic focusing with increasing complexity were used: the first focuses the wave to a point, the second results in a curved wave path and the third guides waves to fill the entire right hippocampus.

The researchers then ran computer simulations of the above types of holographic focusing. Sound waves required to create an ultrasonic hologram within the brain were modelled. This was done by back propagating virtual acoustic fields in the brain to a point inside the skull. Calculating the phase and amplitude of the source waves needed, the scientists then designed holographic lenses to produce such waves with specified properties.

Holographic lenses were then 3D printed and tested with the skull phantom. Compared to the computer simulations, the data they obtained from tests with the skull phantom were in great agreement with the theory.  

By generating complex patterns, the 3D printed lenses have successfully helped refocusing the acoustic beam upon penetrating the skull. The beam can then effectively target brain regions such as the hippocampus, hence produce clearer images.

The study will hopefully lead to low cost therapy and imaging of the central nervous system. The holographic lenses may also be capable of altering nerve activity through targeted delivery of ultrasound waves, enabling ultrasound-triggered neuromodulation. The 3D printed holographic lenses could also have implications for new drug delivery techniques. In particular, this ultrasound method can potentially open the blood-brain barrier, which typically blocks therapeutic drugs in treating Alzheimer’s

Experimental setup showing the block diagram and skull phantom inside the water tank with the ultrasonic source and the acoustic hologram at the bottom. Image via Polytechnic University of Valencia.

Holograms to Focus Arbitrary Ultrasonic Fields through the Skull is published at Phys. Rev. Applied 12. It is co-authored by Sergio Jiménez-Gambín, Noé Jiménez, José María Benlloch, and Francisco Camarena.

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