A man was shot in the head during a violent protest in South Africa, but survived. The victim sued in 2014, saying a cop had fired the round.

But the forensic evidence proved a challenge. The bullet was lodged behind the man’s right eye and extremely close to the brain—so close that removing it would endanger the 28 year old.

A team of ballistics and chemistry experts have now demonstrated exacting computed tomography (CT) methodology about how they determined whether the shot was fired by a police officer, as described in the Journal of Forensic Sciences this week.

Miniscule measurements down to fractions of a millimeter were vital to the determination. Simple 2-D X-rays may not be able to produce that level of detail, according to the experts.

Early on, a series of CT scans were taken of the man’s head.

The tool used was a Siemens Somaton Emotion 16, and the images were inspected by the man’s medical doctor, using Terracon Software.

The determination was that the bullet within the skull had a diameter between 9.03 and 9.19 mm at the base, and a total length somewhere between 16.9 and 17 mm, according to the paper. Those measurements indicated that it was in line with the general dimensions of a 9 mm Luger bullet, according to the man’s doctor.

In South Africa, the 9 mm Luger bullet, full-metal jacket, round-nose and weighing 115 grams, are the standard-issue ammunition for police.

The state disagreed with the conclusion, however, and started the process of independent assessment.

The experts did their own measurements and scans without the case’s context—or the importance of a fraction of a millimeter in either direction.

The experts also took 15 standard issue bullets from the same manufacturing batch. Those cartridges were measured using a calibrated Vernier caliper, and also CT scans, to determine the accuracy of the imaging that would be applied to the object within the man’s head.

The challenges included possible bullet deformation, the proximity between .357 rounds and their 9 mm counterparts, the variation even within the same brand of ammunition coming off the same assembly line—and even the differences within 9 mm designations.

For instance, the 9 mm Luger is actually 9.02 mm in diameter.

Their results indicated that the 9 mm Luger rounds used by cops could have diameters ranging from 8.95 mm to 9.03 mm.

The team crunched the many images, and measurement estimates, using a length-to-diameter ratio.

The forensic scientists determined that the bullet trapped in the man’s head could be a wide range of 9 mm varieties, and even .38 Special or .357 round types that were lodged in the man’s head.

But the calculations exclude the specific type of police ammunition—at a 99 percent confidence interval, they add.

Although the imagery is not exact enough to identify a specific caliber and ammunition type yet, the cop’s rounds were outside the range, they conclude.

“The results confirmed that a specific type of bullet, a 115-gram 9 mm (full metal jacket, round-noise) could successfully be excluded from being involved in a shooting based on CT scan qualitative aspects regarding bullet shape that were combined with length, base and ratio measurements of a bullet in a living person,” according to the paper.

The value of the hyper-detail provided by CT scans has been advocated by the National Institutes of Justice in recent publications. Some studies, including a New Mexico investigation of strangulation deaths, indicates autopsies could be much more revealing with the added nuances of the multi-dimensional visualizations.