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Radiological Imaging Technology (brief technology outline)
The following is a brief overview of portions of relevant technology to appreciate the basics of the associated state of the art, - but this outline is in no way complete, neither that the presented information is necessarily generic for all the manufacturers
Preamble
Radiography is the use of ionizing electromagnetic radiation such as X-rays to view objects. Although not technically radiographic techniques, imaging modalities such as PET and MRI are sometimes grouped in radiography because the radiology department of hospitals handle all forms of imaging. Treatment using radiation is known as radiotherapy.
Diagnostic radiography involves the use of both ionising radiation and non-ionising radiation to create images for medical diagnoses. The predominant test is still the X-ray (the word X-ray is often used for both the test and the actual film or digital image). X-rays are the second most commonly used medical tests, after laboratory tests. This application is known as diagnostic radiography. Since the body is made up of various substances with differing densities, X-rays can be used to reveal the internal structure of the body on film by highlighting these differences using attenuation, or the absorption of X-ray photons by the denser substances (like calcium-rich bones). Medical diagnostic radiography is undertaken by a specially trained professional called a diagnostic radiographer in the UK, or a radiologic technologist.
The creation of images by exposing an object to X-rays or other high-energy forms of electromagnetic radiation and capturing the resulting remnant beam (or "shadow") as a latent image is known as "projection radiography." The "shadow" may be converted to light using a fluorescent screen, which is then captured on photographic film, it may be captured by a phosphor screen to be "read" later by a laser (CR), or it may directly activate a matrix of solid-state detectors (DR—similar to a very large version of a CCD in a digital camera). Bone and some organs (such as lungs) especially lend themselves to projection radiography. It is a relatively low-cost investigation with a high diagnostic yield.
Projection radiography uses X-rays in different amounts and strengths depending on what body part is being imaged:
- Hard tissues such as bone require a relatively high energy photon source. Bony tissue and metals are denser than the surrounding tissue, and thus by absorbing more of the X-ray photons they prevent the film from getting exposed as much. Wherever dense tissue absorbs or stops the X-rays, the resulting X-ray film is unexposed, and appears translucent blue, whereas the black parts of the film represent lower-density tissues such as fat, skin, and internal organs, which could not stop the X-rays.
- Soft tissues are seen with the same machine as for hard tissues, but a "softer" or less-penetrating X-ray beam is used. Tissues commonly imaged include the lungs and heart shadow in a chest X-ray, the air pattern of the bowel in abdominal X-rays, the soft tissues of the neck, the orbits by a skull X-ray before an MRI to check for radiopaque foreign bodies (especially metal), and of course the soft tissue shadows in X-rays of bony injuries are looked at by the radiologist for signs of hidden trauma.
- Dental radiography uses a small radiation dose with high penetration to view teeth, which are relatively dense. A dentist may examine a painful tooth and gum using X-ray equipment.
- Mammography is an X-ray examination of breasts and other soft tissues. This has been used mostly on women to screen for breast cancer, but is also used to view male breasts, and used in conjunction with a radiologist or a surgeon to localise suspicious tissues before a biopsy or a lumpectomy.
Computed tomography
Computed tomography or CT scan (previously known as CAT scan, the "A" standing for "axial") uses a high amount of ionizing radiation (in the form of X-rays) in conjunction with a computer to create images of both soft and hard tissues. These images look as though the patient was sliced like bread (thus, "tomography"-- "tomo" means "slice"). The machine looks similar to an MRI machine to many patients, but is not related. The exams are generally short, most lasting only as long as a breath-hold. Contrast agents are often used, depending on the tissues needing to be seen. Radiographers perform these examinations, sometimes in conjunction with a radiologist (for instance, when a radiologist performs a CT-guided biopsy).
Computed Radiography
Although people often make a distinction between the technology that today is commonly called computed radiography (CR) and the family of projection imaging technologies called digital radiography (DR), CR is really a form of DR, in fact, its earliest form. To add to the confusion, CR itself also has a number of different names. In the radiology literature, the technology is referred to, among other things, as digital radiography with storage phosphors (SP), digital luminescence radiography (DLR), photostimulable luminescence (PSL) radiography, and variation of these terms, along with their abbreviations. Regardless of the name used, they all refer to a technology designed to acquire or record projection images made with high-energy electromagnetic radiation, e.g. x-rays, on a reusable detector containing a dispersion of special storage materials.
Present day computed radiography (CR) is based on the use of photostimulable phosphors, which are also known as storage phosphors. The phosphors used are most often in the barium fluorohalide family in powder form and deposited onto a substrate to form an imaging plate or screen. X-ray absorption mechanisms are identical to those of conventional phosphor screens used with film. They differ in that the useful optical signal is not derived from the light emitted in prompt response to the incident radiation, but rather from subsequent emission when the latent image, consisting of trapped charge, is optically stimulated and released from metastable traps. This triggers a process called photostimulated luminescence (PSL) resulting in the emission of shorter wavelength (blue) light in an amount proportional to the original x-ray irradiation. In CR, an imaging plate (IP) containing the storage phosphor is positioned in a light-tight enclosure, exposed to the x-ray image and then read out by raster scanning with a laser to release the PSL. The blue PSL light is collected with a light guide and detected with a photomultiplier tube (PMT). The PMT signal is digitized to form the image on a point-by-point basis.
The broad acceptance of CR has been due to its large dynamic range, digital nature, easy portability and uniqueness rather than its intrinsic image quality. CR based on the use of storage phosphor screens in a cassette is seen to be complementary to rather than directly competitive with integrated readout digital radiography (DR) systems. Flat panels are very close to being ‘perfect’ in terms of quantitative measures of the efficiency in which the x-ray aerial image is transformed to a digital image.
Any image acquisition technology for digital projection radiography must do three functions properly (Fig. 1):
(a) interact with (e.g. absorb) the analogue x-ray aerial image emerging from the exposed object, usually a patient;
(b) produce, and retain sufficiently long, a Latent image corresponding to the aerial image;
(c) convert the latent image into a digital image. This acquisition process is only a small but important piece of imaging system, which, in turn, is embedded in a much bigger chain of events (see Fig. 2)
First, the acquired digital image must be processed to produce a new digital image suitable for human viewing. This optimized image must also be reproduced on some analogue output medium or device (e.g. film from a Laser Printer, soft-copy display) with which a human being can interact. In the case of machine vision (e.g. computer-aided detection or diagnosis [CAD, CADx]), the image processing extracts useful diagnostic information directly from the digital image data without necessarily producing an image as output. There must also be a means to manage the image data, for example, to store them and distribute them among the various components that make up the complete digital imaging system or the PACS (Picture Archiving and Communication System).
In a CR system, the image acquisition stage also uses two components:
(a) the screen, the detector that interacts with x-ray aerial image to produce a (stored) latent image;
(b) the scanner, the readout device that extracts the latent image from the screen and converts it into a digital image.
As explained earlier, in a digital system, additional components are always required to perform the other necessary image related functions. This separation of the five basic functions (: acquire, process, reproduce, store, distribute) are the key advantages of digital systems over their analogue competitors, because the individual stages can be optimized independently.
CR Readers - Flying Spot Scanner
Presently CR systems are of two general types:
(i) cassette-based systems as shown in figure 3(a) where the IP is enclosed in a light-tight cassette for the x-ray exposure, and
(ii) integrated readout systems, shown in figure 3 (b) where the IP's are captive within the readout system, re-circulated and re-used without handling. Both types are flying spot readout system, i.e. a Laser spot is scanned with a mirror over the exposed IP in a point-by-point raster pattern.
Note 1: The flying spot scanner is not the only possible approach, but this is frequently utilized commercially for medical CR systems.
Note 2: These systems are re-usable for many thousands of x-ray exposures:; physical wear, not exposure related change, is the primary indication for screen replacement. Conceptually, a modern storage phosphor screen consists of an active layer coated onto a rigid or flexible support. On a microscopic level, the physical structure of the screens has the phosphor grains suspended in binder material, located above the support plate. The active layer is the site of x-ray absorption, creation and storage of the latent image, and the simulated emission, - while the support (e.g. aluminum, glass, or terephthalate [PET]) provide smooth sturdy surface for the sensitive phosphor layer, contributes to the optical performance, and allows the screen to be handled and transported by the CR scanner. The active layer, the thickness of which is usually adapted to the intended clinical application, contains small irregular phosphor particles (3 - 10 micro meters) suspended in binder material.
In practice, a storage phosphor screen contains a number of additional (manufacture-dependent) layers that are needed to optimize its performance for clinical use. Mechanically, screens must be robust to user and machine handling, so special backing layers and overcoats are frequently used. Electrically, screens must be insensitive to electrostatic discharge (ESD), which can be addressed by using a conductive layer on the screen surface.
Optically, screens must be optimized to enable as much as possible permitting emitted light to escape to the screen surface for its detection, whilst simultaneously reducing the spread of stimulating light (to retain sharpness). These two apparently conflicting goals have produced a number of different solutions. For obvious reasons, we will refrain from outlining the manufacturers choices, to limit the scope of this discussion.
Finally, to maintain high image quality, screens must also be insensitive to x-rays that pass through or around the screen and are backscattered from objects behind it. This has led to the use of lead (Pb), either in the cassette for the screen or on te screen itself (only with rigid supports). As a result, the simple two-layer screen concept becomes, in reality, a complex layer structure incorporating many design trade-offs and tuned to the characteristics of the scanning system.
Beam Scanning
The diode Laser beam is sent through several sub-systems before reaching the CR plate as shown in figure 4(a). The Laser beam is divided (not necessarily equally) into two paths with a beam splitter. The main beam passes to the scanning system; the side beam is sent to a photodiode used to monitor, and with feedback, stabilizes the Laser output intensity. The Laser focussing lens generally of the F/theta¸ design. It has three further functions: (i) to make the focal plane flat so that focus is uniform across the IP, (ii) to convert the constant angular motion of the scan mirror into a constant linear speed at the image plane so that the pixel spacing is constant and (iii) to move the entrance aperture of the lens, i.e. locus defining region where any ray within the angle of acceptance will be imaged by the lens, significantly out from the body of the lens so that the rapidly scanning mirror has space to operate. Either a rotating polygonal mirror driven by a synchronous motor or an oscillating flat mirror driven by a galvanometer can perform the scanning function. The advantage of the rotating polygon mirror is that the transition from one facet to the next performs the flyback, i.e. retrace which occurs at the end of one line as the beam rapidly returns to the start of the next line. This maintains a high laser duty cycle, i.e. the fraction of the time the laser is actually reading out the IP. In contrast, a galvanometer has to be driven in an oscillatory manner by a sawtooth signal and after flyback takes a fixed time to return to stable operation. Its duty cycle is therefore lower than a polygon and at high line rates, the duty cycle decreases even further. Thus the polygon is used for faster scanning speeds but it has two disadvantages which lead to periodic errors that appear as banding artifacts in the subscan direction. (i) The reflective properties of its facets may each differ slightly, which requires a further correction to the laser output. (ii) Unintentional scanning of the beam perpendicular to the scan direction results in an effect called cross-scan error, caused by slight angular shifts between facets. This can be corrected passively using a pair of cylindrical optics, CR, as in any destructive readout, is very sensitive to cross-scan error. If the beam moves one way, there will be reduced signal as it rereads already partially discharged regions of the IP and in the other way, it will steal signal from the next line. Control of cross-scan error to < 1 micro meter is required.
The scan mirror repetitively scans a line and retraces the laser beam thus defining the scan direction. During retrace the laser beam is turned off and restarted just before it is expected to reach the active area of the IP. The laser beam positioning with respect to the previously read out pixels is accomplished with a line start detector, i.e. a photodiode near to the starting point of the scanning. The linear motion of the plate in the subscan direction combined with the laser scan creates a raster pattern that is read out progressively.
Improved technology The above chart shows the preferred attributes of an "ideal" DR system. Changing healthcare needs require tomorrow's diagnostic imaging service provider to rapidly produce the highest quality images, transmit them broadly, display them in alternative ways, and then archive and retrieve them efficiently. New digital radiography image-capture systems are a critical element in this all-digital version. A digitized image can be transmitted electronically for diagnosis via reading on a workstation monitor or for printing on film, as well as for electronic storage. Because of the wide dynamic range inherent in most digital images, a radiologist can adjust the image electronically at a workstation to optimize the view of the desired anatomy. Hardcopy images from digital printers can also be adjusted/ optimized to suit the viewer's preferences.
