Non-invasive neuroimaging techniques
How neurosurgeons can now look at your brain through your eyes
Llewellyn Padayachy, University of Cape Town
For many years scientists have been trying to find a way to measure the pressure in a patient’s brain without having to drill a hole in the person’s skull. Although this remains the most reliable way to measure pressure in the brain, it is invasive, expensive and comes with the risk of infection and bleeding.
Assessing pressure inside the brain is an important part of diagnosing certain neurosurgical conditions. These include brain tumours, cranial deformities, traumatic brain injury and infection.
Several years ago ultrasound imaging technology, which uses an ultrasound probe over the eye, was introduced as a non-invasive method to identify this pressure using static imaging. Although it allows neurosurgeons to assess most cases of pressure inside the brain, static ultrasound imaging does not pick up all the cases.
Our study, to be published soon, has advanced the current static imaging method. Our technique involves analysing a short video clip of the back of the eye to mark pressure in the brain. It is a faster and potentially more accurate way than the existing technique.
There are limited statistics about children with neurosurgical disorders in Africa, but the number of children with hydrocephalus is thought to be quite high. Hydrocephalus is the result of a build up of fluid pressure which compresses the brain and causes the skull to enlarge. Untreated, it could result in death.
A reliable technique to estimate the pressure on the brain therefore needs to be very accurate.
Using sound waves to see the brain
The eye is directly linked to the brain by the optic nerve which sits at the back of the eyeball. It delivers the visual information collected by the retina to the brain. The optic nerve sheath is a balloon-shaped structure. As pressure in the brain builds up, fluid from the brain is forced along this sheath. It dilates this sheath in the same way that a balloon is inflated.
The optic pathway therefore allows us to extract important information from the brain using non-invasive imaging techniques. Recent advances in ultrasound imaging technology have made it a very appealing tool to assess raised pressure inside the skull. The use of ultrasound in neurosurgery is most appealing because it is radiation-free, portable, widely available and relatively cheap.
The way the technique works is that the ultrasound probe is placed over the closed eye allowing us to see the deeper optic structures as they connect with the brain.
The currently used technique involves a snapshot of the optic nerve sheath. The width of the sheath is then compared to other clinical and imaging markers to infer that there was increased pressure in the brain.
How the new technique works
Llewelyn Padayachy explains the improved technique on the eye ultrasound to measure pressure in the brain.
Our study has several differences from the existing static imaging technique. Aside from measuring the changes in the diameter of the sheath to indicate increased pressure, we have developed a dynamic technique that analyses the way the sheath moves as a result of the person’s pulse. This motion was then compared with intracranial pressure, demonstrating a remarkable consistency.
As an initial study we performed the ultrasound measurement on a large cohort of children. Previous studies using the ultrasound technique on children have not compared it to directly measured pressure in the brain. Diagnosing neurological disease in children is notoriously difficult because the symptoms are often quite subtle.
We also identified certain shortcomings in the current “static imaging” technique which resulted in limited accuracy, a limitation described in many other studies.
Although the static technique takes between two to three minutes to collect all the images that are needed, our technique could significantly decrease this time to around 30 seconds to record the information.
It is also the first study of its kind to be conducted on such a large group of patients, with significant results.
The use of non-invasive techniques to measure the pressure inside the brain to diagnose certain neurological conditions has gained much attention recently. These include measurement of blood flow to the brain and the pressure in the ear. But many of these studies have been limited because of inconsistent accuracy.
Making it more accessible
Our goal is to refine the accuracy and improve the simplicity of our technique. By doing this we hope that assessing the pressure inside the skull using this modified technique can be performed at a primary health care level.
This would speed up the diagnosis of raised pressure in the brain associated with certain neurological disorders.
In a resource challenged environment like South Africa, where the average child with a neurological condition is referred to the appropriate centre much later than they should be, an accurate tool that allows early diagnosis would make a substantial difference.
From a neurosurgical perspective, diagnosing increased pressure in the brain earlier would be a useful marker of underlying neurological disease.
This simplified yet effective technique has the potential to change the way we diagnose certain neurological conditions. But more importantly perhaps, this could possibly be done at the level of primary healthcare facilities, such as day hospitals and clinics.
This study is a collaboration between the UCT’s division of neurosurgery and a leading Norwegian research institute. It has received a provisional patent.
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