The forensic examination of an accident or crime scene can involve the direct visual observation of the surroundings. As well, evidence may be present or even hidden within objects. For detection of the latter, more specialized scanning technologies are necessary.

X rays are electromagnetic waves in the 10 10 ⁸ to ¹¹ meter (3 10 ¹⁶ to 3 10 ¹⁹ Hz) range of the spectrum. (Alternatively, x rays can, like all electromagnetic waves, be conceived of as particles termed "photons.") Because x rays have more energy than visible light, they can pass through solid objects that are otherwise opaque. However, they do not, in general, pass through them as if they were almost transparent, as air is to visible light; rather, when x rays encounter materials of different densities and compositions, they are absorbed and deflected from their original straight-line paths (scattered) to different degrees. This allows x rays to be used for imaging the interiors of many objects. The two commonest commercial applications of x-ray scanning technology are medical imaging of the interior of the body and security scanning of baggage and cargo. X rays also have a place in forensic science.

Projection radiography (also termed transmission imaging or fluoroscopy), discovered in 1895, is the oldest and simplest form of x-ray scanning. In projection radiography, a beam of x rays is directed at an object behind which a detector or x-ray sensitive surface (i.e., electronic-device array or photographic film) is placed. Volumes of different absorptive properties in the object absorb and scatter the incident x rays to different degrees, causing an x-ray shadow to be cast on the detecting surface. This shadow pattern is the x-ray image.

There are two essential limitations on projection radiography. Firstly, it can readily resolve only structures that contain strong x-ray absorption contrasts. In human beings, this means that the soft tissues are difficult, or impossible, to image. Secondly, all three-dimensional structure in the x-rayed object is collapsed or flattened onto the image plane, destroying information. Nevertheless, because of their speed, simplicity, and economy, projection-type x-ray systems are still commonplace in hospitals, and standard in security systems that examine cargo, baggage, and other inanimate objects.

Computed tomography (CT, also known as computerized axial tomography, CAT) was first made commercially available in the mid-1970s. CT combines projection radiography with computer processing to recover the three-dimensional information that is lost in a traditional two-dimensional x-ray. In a CT scanner, the object to be scanned (e.g., person or baggage item) is placed in a cylindrical or doughnut-shaped device. Inside the cylinder or doughnut is an x-ray source that is mechanically rotated entirely around the object. Also, the cylinder or doughnut is lined with detectors that measure the x rays that pass through the scanned object at all angles. By collating all the information that is gathered during a full revolution of the x-ray source, a computer can form a three-dimensional model of the irradiated volume of the object. This information can then be presented to the user on a video screen in any desired form; most commonly, a thin slice of the object is modeled, with the details of its structure imaged as a black-and-white cross section. To examine more of the object, the user looks at multiple slices.

"Backscatter" consists of waves that are reflected back from an obstacle. In backscatter imaging, x rays are beamed at a target object and a sensor co-located with the beam source records reflected (backscattered) waves. Since denser objects tend to create more backscatter, backscatter x-ray systems create a density-contrast image that reveals different information about objects' interiors than does transmission imaging.

Using specially constructed sensors it is possible to acquire transmission-type x-ray information that can be formed into stereoscopic images (that is, a left-eye, right-eye image pair that the user's brain combines into a three-dimensional impression). Because such an image has apparent depth but cannot be rotated, it is sometimes referred to as "2 1/2 dimensional."

While still in its infancy as an examination technique, stereoscopic x-ray scanning is quicker and cheaper than CT scanning, as it requires less computation and does not need to rotate the x-ray source around the object being scanned. Its limitations are that it provides neither fully rotatable three-dimensional knowledge of an object nor density data, both of which are provided by CT scanning.

The atomic orderliness of a substance affects the way in which x rays are diffracted (i.e., forced to mutually interfere) when passed through it. By recording the scattering patterns characteristic of specific compounds (e.g., drugs), and comparing these templates to patterns observed when scanning objects, a substance-specific detection system can be devised. This technique is now in the early development stage, and is not ready for deployment.

Several other techniques for imaging object interiors exist, including ultrasound, positron emission tomography (PET), nuclear magnetic resonance (NMR) imaging, nuclear quadrupole resonance (NQR) scanning, and neutron emission analysis. All, like x-ray scanning, have security, medical, or scientific applications; the question of which technique is best for any given application is decided based on physics (i.e., which imaging modalities can do a particular job) and, if more than one technique is usable for a given task, on economics (i.e., which imaging modality yields the minimum acceptable image quality for the least cost). For enhanced efficacy, airport security systems are now being planned that will combine complementary techniques to increase the probability of weapon or contraband detection. Such a system might combine x-ray scanning for suspicious-object detection with neutron emission analysis for chemical identification.

SEE ALSO Bomb detection devices.