Radiography

We will broadly define radiography as imaging by ionizing radiation.  Some of this radiation is not actually photonic - neutrons, for example - but we include it in our coverage because most means of transforming images made by non-photonic radiation into electronic form involve photonic intermediaries.  All applications - medical, industrial, scientific - are included.  Most of the technology is similar and as imaging becomes more universally digital, the distinctions continue to shrink.

The Terminology of Radiography

Each of the major subdivisions of radiography has adopted slightly different terminology for the same things.  This is not to say that these terms are always internally consistent or universally accepted within each of the subdivisions, however.  Thus, all the definitions here must be taken with advice against instant acceptance.

Medical Imaging - In medical x-ray imaging, "radiography" has conventionally meant static x-rays taken on film.  "Fluoroscopy" has meant dynamic imaging, first by direct viewing of fluorescent screens in darkened rooms and then by ever more sensitive and complex apparatus utilizing x-ray image intensifiers.  Early on there was some overlap when both multiple images taken in rapid sequence on large sheets of radiographic film and similar rapid image sequences taken through an image intensifier were both designated "spotfilms".  As digital image processing has increasingly taken over diagnostic imaging, the use of the word "digital" has proliferated so that now "digital radiographs" can be laser-scanned large film, high-resolution images from image intensifiers, or direct digital images from CCD arrays or large-area panel imagers.  Since much of the newer direct digital equipment can take images in rapid sequence - some even at fluoroscopic rates - the boundary gets grayer and grayer.

Overall, the most common purposes of medical imaging are the visualization of structures in the body for either "diagnostic" or "interventional" procedures or for verification after procedures like hip replacements.  Few quantitative measurements are made from x-ray images except for some geometric studies of, for example, the head ("cephalometry") in preparation for orthodontia or for bone densitometry.  Expect, however, an increase in x-ray images taken specifically to facilitate computer-assisted diagnosis.  High-energy verification images for radiation therapy (portal imaging) are becoming more common: soon, the electronic versions of these images will provide dose estimates as well.

Industrial Imaging - Industrial radiographs were film and real-time industrial x-ray imaging was "radioscopy".  "Fluoroscopy" is becoming more common in industry as the medical and industrial equipment and techniques increasingly overlap.  Most of the differences in technique result from the relatively higher x-ray energies required to x-ray industrial objects and from the freedom from restrictions on patient dose.  The "intensifying screen" used with film in diagnostic imaging is, for instance, a phosphor sheet while the high-energy industrial counterpart is a lead foil.  Imaging without intensifying screens is also common in industry while nearly prohibited in medical use.  Special geometries are also common in industrial work.  "Laminography" is widely used in printed circuit board inspection.  The medical equivalent is "tomography" but there the slicing is rarely performed in real time.  Industrial inspection often requires resolution beyond the capability of the x-ray tubes used in medicine so "micro-focus" tubes with focal spots down to just a few microns are utilized.

Industrial x-ray imaging falls in two relatively separated application categories defined by who (or what) examines the images.  Currently, most of the examination is performed by highly trained radiographers able to squeeze the last bit of contrast out of very dense images.  The objects of their inspection are new castings and welds and pipe and all varieties of objects and structures that deteriorate with time.  In a few manufacturing settings - tire production or baggage inspection, for example - real-time imaging is applied.  But, even here, human eyes usually do the analysis.  More frequently, now, these real-time imagers are connected to computers that perform automated image analysis and make the go/no-go decisions without human help.  This is "x-ray machine vision".  

Scientific Imaging - Most scientific x-ray images are not of real objects.  The two largest applications are "autoradiography", in which a radioactive distribution from (typically) an electrophoresis plate is placed in contact with x-ray film or "diffractometry" in which a thin x-ray beam illuminates a small crystal or powder and the diffracted beams are captured.  Specimen imaging using both film and electronic means is also common.  "X-ray microscopy" is available using micro-focus tubes.  Another common scientific x-ray technique, "x-ray fluorescence" uses no images at all.

New Trends - This is by no means the end.  In both medical and industrial applications there are now "reverse-geometry" systems in which the x-ray source is a large plane and the detector is a point.  Also in every filed, there is growing interest in "cone-beam" computed topography in which the usual x-ray apparatus is rotated about an object for very fast volume acquisitions.  Finally, "3-D imaging", in which the data from a few plane images is manipulated digitally to provide a volume reconstruction is under wide investigation.

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