How Ultrasound Works Inside the Brain

A familiar diagnostic tool also has other uses, including in the brain.

Key points

• Successful detection of enemy submarines paved the way for the use of ultrasound in medical diagnosis.

• Low-intensity ultrasound’s gentle warming is used to treat damaged muscles

• A high-intensity focused beam may become a treatment option for Parkinson’s disease.

• As with deep brain stimulation, attempts will be made to establish a wider use in psychiatry.

Medical ultrasound is widely known. Cardiologists use it to check the heart is pumping normally. Ophthalmologists use it to check for detached retinas and for tumors. But its most familiar application is probably in obstetrics. Ultrasound scans display the fetus in the womb. They enable early identification of gender and of various possible abnormalities and they allow the obstetrician to check that the fetus is growing normally. In the United States, a pregnant woman might receive 3, 4, or even 5 ultrasonograms in the course of her pregnancy. Since there are over 5 million pregnancies each year in the US alone, we can assume that many people are familiar with ultrasonography.

Whilst medical ultrasound is widely known, its military origins are not. At the end of the 19th century, Francis Galton, an English scientist, discovered sound waves beyond the range of human hearing. Dogs could hear them but humans could not. The search for practical applications began in 1915, during the First World War. It had nothing to do with health care. German submarines were becoming a menace to Allied shipping. A Russian émigré in Paris, Constantin Chilowsky, suggested that the submarines could be located by sending ultrasonic waves into the sea. When they struck the submarine, the waves should be reflected back: the echo effect. The French government asked the eminent physicist Paul Langevin to investigate. He set to work, using a quartz crystal to generate the waves in a large water tank containing small fish. The experiment was successful. Gradually, scientists learned how the properties of ultrasonic waves varied with wavelength and intensity. Striking a hard surface, they could be reflected, like sounds echoing in a tunnel. During the Second World War, reflected ultrasound became "Sonar," and was successfully used to detect enemy submarines.

Langevin had also noticed that small fish swimming in the tank died.

From Ocean to Clinic

Inspired by wartime experiences, in the 1940s and 1950s medical interest in ultrasound flourished. Radiologists, neurologists, oncologists, and, later, gynecologists and obstetricians all experimented with using it diagnostically. There were successes and failures. Attempts to locate brain tumors failed, partly because the skull was too hard and impenetrable. Despite initial professional skepticism, by the 1970s, ultrasonic scanning was becoming routine in obstetrics and gynecology.

Ultrasonic waves don’t only reflect. The death of the fish in Langevin’s experiment was due to something else: The waves produced heat. Some medical practitioners began to use ultrasound therapeutically. In the 1940s, an excess of postwar enthusiasm led to unrealistic claims for ultrasound treatments. But growing understanding of how the properties of ultrasonic beams depend on frequency and intensity paid off. Damage to muscles could be treated using the warming effects of low intensity ultrasound. This led to its now well-established use in physiotherapy. In the 1950s, William Fry at the University of Illinois showed that a focused high-intensity beam could surgically destroy affected tissue. Fry and colleagues then began treating patients with Parkinson’s disease. They treated 50 patients successfully, and then stopped.

Treating Parkinson’s Disease

Parkinson’s disease involves deterioration in a specific area of the brain. Its most familiar symptoms are slow and involuntary movements, and muscular tremors. But at an advanced stage it can give rise to depression, cognitive dysfunction, and behavioral disorders. There is no known cure. At the root of the problem is a lack of dopamine, a neurotransmitter which plays a vital role in passing instructions from the brain to the muscles.

The Illinois researchers stopped their work on Parkinson’s disease because of a simpler pharmaceutical alternative. In the 1960s, clinical trials found the drug Levodopa effective in treating disease symptoms. Administered orally, Levodopa is converted to dopamine in the brain. Approved by the FDA in 1970, Levodopa was soon used widely. Nevertheless, it was no miracle drug. It isn’t always effective, and used over a long period of time, its effects can fade. In the late 1990s, the search for alternative treatment options led to approval of deep brain stimulation (DBS). DBS uses electrodes surgically implanted in the brain to stimulate and reactivate failing regions. It’s not known how exactly it works, although today more than 200,000 people benefit from it. However, because the procedure is invasive, it involves a degree of risk.

A non-invasive alternative is now entering the clinic. It’s ultrasound again, but at high intensity and highly focused. It is known as Focused Ultrasound (FUS). FUS is a technologically-updated version of what Fry once tried:

By converging ultrasound energy at precise focal points, FUS can destroy tissue or open the blood-brain barrier without a single scalpel cut... It's like using a magnifying glass to focus beams of light on a point and burn a hole in a leaf.

Magnetic Resonance Imaging (MRI) is used to direct the beam to the target spot. In 2016, an FUS device, Insightec’s ‘Exablate Neuro’, received FDA approval for the treatment of essential tremor. In 2018, it was approved for treating tremor-dominant Parkinson’s disease, and subsequently for patients with other Parkinson’s symptoms.

Between Neurology and Psychiatry

In 1999, it was suggested that DBS could also be used to treat psychiatric disorders such as OCD. While hopes were high, its use in psychiatry has remained limited. This might be due to a lack of belief in the biology of psychiatric disorders. It might also be due to ethical issues traditionally associated with neurosurgery. Since FUS involves no invasive surgery, however, ethical doubts may be less. In 2021, South Korea approved it for OCD treatment, although the FDA has not followed suit in the United States. Whether it becomes a more widely available treatment option in psychiatry than DBS remains to be seen.

Note: This article originally appeared on Psychology Today.