New brain scanning technology provides “accurate” measure of THC impairment

A new brain imaging procedure could prevent cannabis consumers from being penalised on the roads and in the workplace, by accurately measuring THC impairment.

Researchers at Massachusetts General Hospital (MGH) have found a non-invasive brain imaging procedure to be a reliable way of determining whether someone is impaired as a result of THC consumption.

The technique uses imaging technology known as functional near-infrared spectroscopy (fNIRS) to measure brain activation patterns that correlate to impairment from THC intoxication. 

The procedure could have significant implications for improving road and workplace safety.

The increased use of cannabis, medically and recreationally, through legalisation in countries across the globe, has created the urgent need for a method that can distinguish between impairment and mild intoxication from THC.

Lead author Jodi Gilman, PhD, investigator in the Center for Addiction Medicine, MGH, commented: “Our goal was to determine if cannabis impairment could be detected from activity of the brain on an individual level. This is a critical issue because a ‘breathalyser’ type of approach will not work for detecting cannabis impairment, which makes it very difficult to objectively assess impairment from THC during a traffic stop.”

THC impairment

THC has been shown in past studies to impair cognitive and psychomotor performance essential to safe driving, a factor thought to at least double the risk of fatal motor vehicle accidents.

The challenge for scientists, however, is that the concentration of THC in the body does not correspond well to functional impairment. 

One reason is that people who use cannabis often can have high levels of THC in the body and not be impaired. 

Another is that THC can remain in the bloodstream for weeks after the last cannabis use, well beyond the period of intoxication. Hence the need for a different method to determine impairment from cannabis intoxication.

In the UK any driver who is stopped by the police can expect to be swabbed and if THC is identified, a blood test is enough to secure a conviction. 

This means that anyone who has consumed cannabis within the last few days – or has been subject to passive smoking – may be over the zero-THC limit and at risk of prosecution, regardless of whether there is evidence of impairment.

Although patients who hold a legal prescription have a right to a medical defence, this is not always taken into account and those who are unable to afford one risk being criminalised and having their licences removed.

The study

In the study, 169 cannabis consumers underwent fNIRS brain imaging before and after receiving either oral THC or a placebo. 

Participants who reported intoxication after being given oral THC showed an increased oxygenated hemoglobin concentration (HbO) – a type of neural activity signature in the brain – compared to those who reported low or no intoxication.

“Identification of acute impairment from THC intoxication through portable brain imaging could be a vital tool in the hands of police officers in the field,” explains senior author and principal investigator A Eden Evins, MD, MPH, founding director of the Center for Addiction Medicine. 

“The accuracy of this method was confirmed by the fact that impairment determined by machine learning models using only information from fNIRS matched self-report and clinical assessment of impairment 76 per cent of the time.”

The study cites considerable advantages for using fNIRS technology in roadside drug testing. 

These include the feasibility of inexpensive, lightweight, battery-powered fNIRS devices that allow data to either be stored on wearable recording units or transmitted wirelessly to a laptop. The fNIRS technology could be incorporated into a headband or cap, and thus require minimal set-up time.

“Companies are developing breathalyser devices that only measure exposure to cannabis but not impairment from cannabis,” added Gilman. 

“We need a method that won’t penalise medical marijuana users or others with insufficient amounts of cannabis in their system to impair their performance. While it requires further study, we believe brain-based testing could provide an objective, practical and much needed solution.”

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