Virtual Autopsy

This article from the Economist Dec 10th 2009 .

PERFORMING a postmortem on a murder victim can take days, delaying any criminal investigation. Moreover, pathologists sometimes get only one chance to look for clues when dissecting a body. But Anders Persson, director of Linköping University’s centre for medical image science and visualisation in Sweden, hopes to change that. Along with his colleagues Thomas Rydell and Anders Ynnerman of the Norrköping Visualisation Centre, they have created a virtual autopsy system.

The body needing to be examined is first scanned using a computed tomography (CT) machine, a process which takes about 20 seconds and creates up to 25,000 images, each one a slice through the body. Different tissues, bodily substances and foreign objects (such as bullets) absorb the scanner’s X-rays in varying amounts. The software recognises these and assigns them a density value. These densities are then rendered with the aid of an NVIDiA graphics card, of a type used for high-speed gaming, into a 3-D visualisation of different colours and opacities. Air pockets are shown as blue, soft tissues as beige, blood vessels as red and bone as white. A pathologist can then peel through layers of virtual skin and muscle with the click of a computer mouse.

To make the process easier, Dr Persson and his colleagues have also created a virtual autopsy table. This is a large touch-sensitive LCD screen which stands like a table in an operating room, displaying an image of the body. Up to six people can gather around the table and, with a swipe of a finger, remove layers of muscle, zoom in and out of organs and slice through tissue with a virtual knife.

The Swedish police have already used the researchers’ virtual-autopsy technology to investigate nearly 350 cases. It has proved capable of detecting crucial but difficult-to-spot pieces of evidence, such as the angle of a bullet’s trajectory, air pockets in the wrong place in the body and bone fractures in a burns victim. The virtual autopsy can also be used to determine the cause of death in a few hours. And unlike a physical autopsy it does not alter evidence, enabling investigators to revisit a cadaver for additional clues if necessary. Television detectives everywhere will have them soon.

An Economist’s Insight

Roger Koppl was been banging on about knowledge monopolies for the last year or so. I’ve heard comments to the effect: “What business is it of an economist meddling in the world of forensics?” Roger makes the point crystal clear in a recent posting of his. I quote the punchline:

The issue is not getting the right people in the job, but giving them the right incentives, which is another lesson economists can appreciate.

What’s Wrong With CSI

I’ve had several requests to make this recent article available despite it still being freely available on the Forbes site (I know there have been some annoying pop-up advertisement windows.) Anyway, check out Roger’s co-authored article for EPISTEME on this topic: Epistemics for Forensics.


Forensic evidence doesn’t always tell the truth.

Forensic evidence is foolproof, right? It’s how those clever cops on CSI always catch the killer. DNA evidence springs innocent men from prison. Fingerprints nab the bad guys.

If only forensics were that reliable. Instead, to judge by the most comprehensive study on the reliability of forensic evidence to date, the error rate is more than 10% in five categories of analysis, including fiber, paint and body fluids. (Meaning: When the expert says specimen X matches source Y, there’s a 10% probability he’s wrong.) DNA and fingerprints are more reliable but still not foolproof. The 1995 study, in the Journal of Forensic Sciences, looked at proficiency tests labs take to see whether their work is sound.

More recent studies have also shown problems. Though a 2005 study in the Journal of Criminal Law & Criminology suggests a fingerprint false-positive rate a bit below 1%, a widely read 2006 experiment shows an alarming 4% false-positive rate.

Yet the public sees errors as gross anomalies. Like the time the FBI wrongly linked an Oregon attorney named Brandon Mayfield to the 2004 Madrid commuter train bombing that killed 200 people. The FBI had claimed a 100% match between fingerprints found at the scene and Mayfield, who was held for two weeks in federal custody. When the Spanish National Police got the real perpetrator, an Algerian named Ouhnane Daoud, the FBI had to admit its mistake. Mayfield accepted a $2 million settlement from the government.

Another debacle, this time involving DNA testing: Josiah Sutton served four and a half years for rape after the Houston Crime Lab tied him to crime-scene DNA. The lab was later found to be rife with problems, including a leaky roof that let rainwater contaminate evidence. Sutton was proclaimed innocent in 2004 and awarded $118,000 in reparations the next year.

Forensic evidence is also used in white-collar cases. In the Martha Stewart trial, government forensic scientist Larry F. Stewart (no relation to Martha) testified that he had performed incriminating tests on the famous “@60″ written next to “ImClone(nasdaq: IMCLnews - people )” on a worksheet used to record Martha Stewart’s securities positions. The notation suggested that she had a long-standing agreement to sell ImClone should the price hit $60, which would have cleared her of insider trading. Larry Stewart said his tests showed the potentially exonerating notation was made with different ink, thereby suggesting that it might have been added later.

When the court learned that someone else performed the tests he claimed to have done himself, Larry Stewart was tried for perjury. The tester had done an incomplete job, failing to compare the “@60″ ink to all the other ink on the page. (Larry Stewart was acquitted.)

How can we preserve the usefulness of forensic evidence while protecting the public when it breaks down? The core problem with the forensic system is monopoly. Once evidence goes to one lab, it is rarely examined by any other. That needs to change. Each jurisdiction should include several competing labs. Occasionally the same DNA evidence, for instance, could be sent to three different labs for analysis.

This procedure may seem like a waste. But such checks would save taxpayer money. Extra tests are inexpensive compared to the cost of error, including the cost of incarcerating the wrongfully convicted. A forthcoming study I wrote for the Independent Institute (a government-reform think tank) shows that independent triplicate fingerprint examinations in felony cases would not only eliminate most false convictions that result from fingerprint errors but also would reduce the cost of criminal justice if the false-positive error rate is more than 0.115%, or about one in a thousand.

Other reforms should include making labs independent of law enforcement and a requirement for blind testing. When crime labs are part of the police department, some forensic experts make mistakes out of an unconscious desire to help their “clients,” the police and prosecution. Independence and blind testing prevent that. Creating the right to a forensic expert for the defense would help restore the imbalance in scientific firepower that too often exists between prosecution and defense. Private labs are subject to civil liability claims and administrative fines, giving them financial incentives to get it right.

Roger Koppl is a professor of Economics and Finance at Fairleigh Dickinson University’s Silberman College of Business and a Director of the Institute for Forensic Science Administration.