The search for reliable means to investigate structures has coincided with advances in construction. As building technologies evolve, the search for better/more efficient investigative (structural investigation) methodologies also evolves. Most technologies used for structure inspection had their beginnings in other fields but were adapted to construction long after their discovery or development. X-rays are one such development.
Discovery of X-rays
In 1870 Sir William Crookes found that passing an electric current through a glass vacuum bottle with wires embedded at each end produced a purple light inside the bottle and a green glow outside (Wisehart, 1928). This phenomena was simply another unexplained scientific curiosity until Wilhelm Conrad Roentgen, at the University of Wurzburg in Germany, began experimenting with several types of vacuum tube, including the “Crookes tube” in the early 1890s (Nitske, 1971).
There are two common stories about the discovery of x-rays by Roentgen. The first is that while running experiments, Roentgen accidentally left a Crookes tube on and placed it on top of a pile of materials which included a book he had been reading. Inside the book was an antique metal key used as a bookmark. An unexposed photographic plate was under the book. After later use of the photographic plate outside and its subsequent development, the ghost image of the metal key was visible (Wisehart, 1928).
The second, more accepted, story places Roentgen in his darkened lab, using a Crookes tube, covered with a cardboard tube that blocked all visible light. In spite of the presence of the cardboard tube, when an electrical current was run through the Crookes tube, a barium-platinocyanide-coated screen began to glow across the room (Nitske, 1971; Brown, 2002). Roentgen found that when he held objects between the cardboard-covered tube and the screen, that varying amounts of energy were recorded on the screen, and in one image he saw the bones of his hand.
After the initial discovery, however it was made, it is known that Roentgen initiated seven weeks of detailed, meticulous experiments designed to validate his initial discovery and determine the nature of these previously unknown “x- rays” that could penetrate opaque matter. He discovered that x-rays cast shadows, like visible light, but questioned whether they obeyed the same reflection and refractions laws (Roentgen, 1896). He was able to record them on both photographic plates and fluorescent screens (Nitske, 1971; Wiseman, 1928). These experiments, including the first human x-ray (a hand, reported to be his wife’s, Figure 1) were done in isolation, with Roentgen sometimes even sleeping in his laboratory (Wisehart, 1928; Bleich, 1960).
Roentgen formally announced his discovery of x-rays in December of 1895. His first announcement was originally sent to the Wurzburg Physical-Medical Society (Roentgen, 1896). By January 1896, he made his first public presentation to the same organization and had also sent the report to colleagues across Europe. By mid-January, newspapers in the U.S. had reported on the new discovery, with headlines such as “New Light Sees Through Flesh to Bones” (University of Pennsylvania, 1993), and cartoons such as the one in Figure 2, reproduced from Life magazine (Life, 1896; Lang and Middleton, 1997).
The uses of this discovery quickly became obvious to the general public. Since the equipment necessary to create and record or measure the rays was easily available, it was rapidly put to use. For example, the two most familiar modern uses of x-rays that almost everyone has experienced are medical x-rays and baggage inspection at airports. Within a few months after the announcement of their discovery, x-rays were being used on human subjects to identify problems such as bone fractures and kidney stones. In June of 1896, only six months after the announcement of their discovery, x-rays were being routinely used by physicians to find and remove bullets from wounded soldiers (University of Pennsylvania, 1993). And as early as 1897, European Customs Inspectors were using x-ray inspection of baggage at the Brussels railroad station (Bossi et al, 1985). Innovative uses quickly followed, when in 1898 Dr. Charles Leonard produced radiographs of a Peruvian mummy (Lang and Middleton, 1997).
While credit is generally given to Roentgen for the discovery of x-rays, it is interesting to note that Nikola Tesla conducted experiments on visible and invisible rays long before Roentgen presented his findings in 1895 (Cheney, 1981). Research into Tesla’s work in the 1940s found that a photograph of Mark Twain taken by Tesla using a Geissler tube also showed the adjusting screw of the camera lens. This, in fact, was likely the first x-ray taken in the U.S.
In the U.S., Thomas Edison took up the investigation in an attempt to develop x- ray equipment that could be widely used. He eventually developed a hand-held fluoroscope but was unable to develop a commercial x-ray lamp, his true dream for this technology (Josephson, 1959). Other investigators quickly jumped on the bandwagon and throughout the early decades of the 20th Century, x-rays were being used widely not only for medical purposes, but also for a variety of industrial uses such as steel manufacture, foundry practices, railroading, and the production of electrical equipment. X-rays were even used to fit shoes, find grit in chocolates and sort fresh eggs (Wisehart, 1928; Bleich, 1960).
Unfortunately, the radiation hazard present in many of these uses, especially medical uses, was generally unrecognized at this time. Thomas Edison damaged his eyes from exposure to x-rays (Josephson, 1959). As late as 1928, a discussion of medical uses showed that often the patient was in front of an x-ray tube for extended periods while doctors watched the movements of the digestive organs or a beating heart (Wisehart, 1928). However, as early as 1896, Nikolas Tesla had lectured on safety when operating x-ray equipment (Cheney, 1981). He also reported his observations on the dangers of x-rays.
Today, the potential hazards associated with the use of x-ray technology are well known. Industrial applications are controlled so that radiation exposure is minimized, medical applications carefully monitor exposure of the patient, and all states have requirements that control the use of any radioactive substances or x-ray source. In spite of potential radiation hazards, uses of x-rays have continued to expand since their discovery. X-rays have been invaluable in solving esoteric problems, including analyzing ancient Egyptian artifacts and detecting forgers of currency and art (Lang and Middleton, 1997).
The Physics of X-Rays
What, exactly, was it that Roentgen discovered in 1895? These invisible rays that penetrated through solid matter had lead to a great many science-fiction-type speculations. For instance, lead-lined underwear was developed to forestall peeping toms when the eventual x-ray glasses would be developed (University of Pennsylvania, 1993).
It turned out that x-rays are simply another form of electromagnetic radiation, similar to light and heat. But if these x-rays are simply electromagnetic radiation, why do they penetrate matter? The answer lies in an understanding of all forms of electromagnetic radiation.
In the 1860’s, James Clerk Maxwell produced the four equations that define electrodynamics. These remarkable equations brought together for the first time the study of electricity, magnetism, and light. Maxwell showed that all types of electromagnetic radiation were simply mutually perpendicular, fluctuating electric and magnetic fields. This radiation is characterized by both a wavelength (λ) and a frequency (f), related to the speed of light in a vacuum as:
where c is equal to 3 x 108 meters/second. This equation shows that the wavelength of light is inversely proportional to its frequency. Figure 3 shows the types of electromagnetic radiation as both a function of their wavelength and frequency. Note that radio and infrared radiation have wavelengths longer than that of visible light with lower energy for individual photons, while x-rays and gamma rays have shorter wavelengths (and thus greater frequencies), and have greater amounts of photon energy.
This energy relationship is defined as:
where h is Planck’s constant, and the energy (E) of individual photons is measured in electron-volts (eV). The smaller the wavelength (and thus greater the energy), the more likely the radiation is to penetrate matter. If you hold your hand up to a bright light, you can see some light coming through the edges, and they seem translucent. But even low energy x-rays easily penetrate human bodies, while high energy x-rays and gamma rays can only be stopped by several feet of concrete or a few inches of lead (EPA website). This differential absorption is what makes x-rays useful for investigations. X-rays of a given wavelength (or small range of wavelengths), may easily penetrate a wood beam, but are preferentially absorbed by the nails attaching the beam to the rest of a structure. Thus the final image (produced either on film or a fluorescent screen) will have light areas corresponding to less-dense material, where most of the energy is transmitted, and darker areas corresponding to heavier materials, where most of the energy is absorbed.
X-rays are subdivided by wavelength, with soft (or lower energy) x-rays having wavelengths around 1 to 10 Angstroms (one Angstrom is 10-10 meter) while very hard x-rays have a wavelength around 10-3 Angstroms. The x-rays used for this research are soft x-rays, produced at 150 kV, with wavelengths of highest intensity at about 0.13 Angstroms. This energy is approximately equivalent to that produced by medical x-ray equipment, which typically produces x-rays at 40 to 140 kV. As such, safety concerns using the x-ray source are minimized compared to radioactive sources of x-rays.
The intensity of a beam of x-ray photons is a measure of the energy per unit time per unit area produced by the beam of x-rays (for example in watts/square meter), and can be calculated in a variety of ways. The intensity of an x-ray beam obeys an inverse-square law. That is, as the distance from the x-ray source is doubled, the intensity of the beam is reduced by a factor of four. This allows for simply increasing the distance between people and the x-ray source as a means of increasing safety when operating equipment in the field.
Closely related to the intensity of radiation is the idea of a radiation dose, or units of radiation exposure, usually defined as the energy deposition per gram of absorber (such as human tissue). Both the RAD (Radiation Absorbed Dose) and REM (Roentgen Equivalent Man) are units typically used in the U.S. to define appropriate limits for exposure to radiation. The RAD is equivalent to 100 ergs of energy per gram of absorber. The REM is equal to the RAD multiplied by QF (the Quality Factor). This factor accounts for the relative biological effectiveness of different types of radiation (including alpha and beta radiation, which are not electromagnetic radiation). The QF for x-rays is one, however, so the units are equivalent for our discussion.
How does this relate to someone using x-rays to investigate a structure? The Whole Body Occupational Dose limits for an adult are set by the Environmental Protection Agency (EPA) as 5 REM (5,000 milli-REMs or mREM) per year. For comparison, a typical adult living in Colorado (at an elevation between 5,000 and 6,000 feet), married, that takes at least 3 airline flights per year and watches TV receives a typical dosage of about 400-450 mREM per year (EPA website). A dental x-ray is usually 2 to 3 mREM, while the total exposure of each technician during this course of experiments totaled less than 15 mREM over 8 months. Monitoring radiation is discussed in the safety procedures section.
Portable X-ray Equipment
To produce the digital radiographs used in this research, two portable x-ray systems were used. These systems used the same x-ray source but had different imaging systems. One imaging system, the RTR-4TM imaging system from SAIC®, produces real-time digital radiographs, with technology somewhat equivalent to a digital camera. The other imager, the EPIX Digital Imaging System by Logos Imaging, uses a reusable plate which creates fluorescence when x-rays impinge on the surface, similar to x-ray film. This imager plate is then scanned for 3 to 7 minutes for the digital radiograph to be viewed. The x-ray source, the Golden Engineering XR200®, and both types of imagers are discussed in detail below.
XR200® X-ray Source
The source used for this study is an XR200® x-ray source, manufactured by Golden Engineering, Inc. This source is a pulsed source, producing x-ray pulses of short duration (60 nanoseconds each) and minimal dose (3.1 milliroentgens for each pulse at a distance of 12 inches from the front of the unit), with energy up to 150 kV. The aperture size is 1/8 inch (3 mm), and the beam produced by this source has a 40° beam angle, so that x-rays taken about two feet from the source have a spread equal to the width of the imager. For each x-ray, the number of pulses can be set from 1-99. One to two pulses are required to penetrate paper and four to twenty are typically used to penetrate most wood walls.
The battery-operated source is quite portable and easy to use in the confined spaces of historic buildings. It is 4.5 inches wide, 7.5 inches tall, and 12.5 inches wide, and weighs 12 pounds. It is powered by a 14.4 volt DeWalt® removable, rechargeable nickel-cadmium battery and can, therefore, be used in buildings with no source of electrical power. The base of the source unit has a threaded tripod mount that can be used with a standard photographic tripod.
Safety issues are always a concern when using x-rays. This unit has a variety of safety features. It should be understood that the unit itself is NOT radioactive! X-rays are only produced when pulses are generated, due to the introduction of an electrical potential across the vacuum tube (just as light is only produced when the electricity is turned on for a fluorescent fixture). The low dose of each pulse and the ability to create a specific number of pulses allow for an individual to work with the minimum amount of energy necessary to accomplish the investigation. Leakage from the unit while it is working is limited to 10 milliroentgens per 100 pulses on the sides of the unit, three inches from the center, and three milliroentgens per 100 pulses two inches behind the unit. Since x-ray radiation has an inverse-squared relationship between energy and distance, individuals standing in a safety zone more than 10 feet behind the unit when it is working are protected. The final safety feature is the key to the unit. The XR200® will only work when the key is inserted in the top. This allows the operator to always have the key in his or her possession so that the unit will not accidentally discharge while shots are being set up.
The RTR-4TM portable digital x-ray imaging system (Figure 4) was manufactured by SAIC® (Science Applications International Corporation). This system is a fully digital imaging system that includes its own image modification tools. The system is composed of a control unit (which can be a laptop computer), the imager, and cables which connect the imager to the controller. The imager is a compact, solid state camera with an 8.0- inch by 10.7-inch field of view. It measures 7 by 11.75 by 13.25 inches, and weighs 10 pounds. It is typically mounted on a tripod or placed directly against the surface of the object of interest, opposite from the source. As with all portable x-ray systems, access to both sides of the object of interest is required. The imager’s electro-optical system captures the images and transmits them to the control unit, where they are stored as TIF images. Individual images are 304 kb in size, containing up to 65,535 (16-bit) pixels.
This imaging system generates digital radiographs in real time so that the images are essentially instantly available for viewing. It is easy to move the imager when the area of concern is not included in the original image. Similarly, it is easy to shift the imager along an object (say a beam or wall) to make sequential radiographs. The need for cables to connect the imager to the control unit can limit the ability to investigate hard-to-reach areas of a building (where it is not possible to extend the cables around a long wall for example). However, a cordless option, not used in this research, is available which could address this limitation.
The images, since they are TIF files, can be manipulated by any standard photographic-enhancement software. However, the control unit (or the software that is included for the laptop) includes a package that can also be used to enhance the images so that subtle details of the x-ray can be seen. This software includes not only the standard image-enhancement techniques (such as image sharpening and contrast stretching), but also features designed to assist ￼￼specifically with x-ray enhancement (such as edge detection algorithms and the ability to transmit all the grey tones of the x-ray into a full spectrum of colors).
EPIX Scanner and Imaging Plates
The second imaging system used in this research is the EPIX Digital Imaging System manufactured by Logos Imaging (Figure 5). This system is composed of the EPIX imaging plates, the EPIX scanner, and a laptop with software to import and save the scanned image. The imaging plates are reusable, photo- stimulatable phosphor imaging surfaces, either 8-inches by 10-inches, or 8-inches by 17-inches in size. To establish a reference point on the radiograph, a metal key should be taped to the plate cover to indicate the orientation of the plate against the object. The plates are composed of flexible plastic sheets coated with a very thin layer of tiny storage phosphor crystals bonded together. X-ray images are created on these imaging plates as the phosphor crystals capture the energy of x-rays passing through the object of study (Figure 6). This energy is stored in the crystals, and released by the process of scanning. However, the scanning process does not completely erase the image from the plate so the plates need exposure to light (usually about two minutes in the direct light) before they can be used again without seeing a ghost image from the previous shot. With care to keep the imaging plates stored out of the light and in their cases (to avoid scratching the coating), these plates are reusable indefinitely.
The second component of the EPIX Digital Imaging System is the EPIX scanner. This machine, affectionately referred to as the “bread-maker”, is 15.5 inches high, 19.4 inches wide and 10.8 inches deep, and weighs 32 pounds. To mount the imaging plates and insert them in the scanner, two carousels (one for each imaging plate size) are available. After exposure, the imaging plate is mounted on the cylindrical carousel (with care not to expose the photosensitive surface to much light) and inserted into the scanner. The scanner uses red laser light to cause the crystals to release their stored energy, which is released as blue light captured by the scanner. The scanning process can capture the image at either high or low resolution. The high resolution image, which takes about 7 minutes to process, is 300 DPI. The scanned image of an 8-inch by 17-inch plate at high resolution is about 24 MB. The low resolution image is half that and takes half the time to process. A newer version has been introduced since this research was conducted that cuts the processing time to 55 seconds. The laptop and software associated with the EPIX system capture the image as a TIF file. The EPIX software also allows for editing images.
Comparisons Between the SAIC® and Logos Radioscopy Systems
These two imaging systems both produce high quality digital radiographs, are portable and have image editors which can improve and enhance digital image features as part of their software. Each also has advantages and disadvantages, depending on the application. With any radioscopy work, proper setup of the equipment in the field is paramount and key to generating useful images. The setup is also the most time-consuming aspect of field work. With setup being roughly the same for both systems (based on access and the logistics of the site), the following observations have been made when comparing the two systems.
The SAIC® system produces radiographs essentially in real time so more radiographs can be made in a short time relative to the Logos Imaging system, with quick adjustments to maximize the imaged area. However, the resolution of the image is not as high as those using the Logos Imaging system. Also, the imager requires a source of power and placement of the relatively bulky imager with the necessary cables to the control unit can restrict its use in some locations.
The Logos Imaging system can be used in areas with no close source of power since the imaging plates are not connected to a control unit or the battery- operated source. The scanner and laptop controller can be located away from the setup, although this will increase the total processing time. The thin, flexible imaging plates inside their protective covers can usually be easily attached by means of thumbtacks or duct tape to relatively inaccessible areas. However, each shot requires that the imaging plate be attached on the back side of the object, exposed to x-rays, scanned, and then erased with strong light. This process is somewhat time-consuming and, therefore, allows for fewer radiographs to be generated in a given amount of time, relative to the SAIC system. However, with multiple image plates in use and a field assistant, this can be minimized. In many cases, a field assistant is beneficial simply because of overall productivity with setup, documenting setup and test parameters, and processing the images.
In general, if access to the back side of the object and location of power are not limitations in the field, and the contrast between materials being investigated is known to be good (say metal fasteners in a wood wall), the SAIC® system was found to be more efficient. If access to the back side of the object or power could be a limitation, or historic construction details are desired (with possibly minimal contrast between materials, such as mortise and tenon joints or wooden pegs used to join wood members), the Logos Imaging system was found to offer better resolution.