Tools in Radiology
Projection Radiography
Good old projection radiography remains one of the staples of radiology, although a little over 100 years old. And it is by no means obsolete even in times of multi- million-dollar high-tech imaging equipment. The bulk of all diagnostic imaging studies is still done with this tech- nology. Mammography, a prominent representative of this group, is the only imaging study that has been proven to lower patient mortality significantly if performed correctly and, of course, only in women. The basic technical principle of projection radiography is simple. However, the complete chain of events from generating the x-ray beam to viewing the developed image can be full of surprises to keep even the “pro” busy making sure everything is done properly and the radiograph at hand is a quality product. With insufficient knowledge or lack of experience and care, things can easily derail there are enough catastrophic studies to prove that point.
Generation of X-Rays
A high-voltage current is built up between a cathode and an anode, all of this inside a vacuum tube. The cathode is heated to about 2000 C by a specific heating filament. Electrons are emitted by the cathode, accelerated by the electric field between cathode and anode, and hit the anode with considerable energy, where they induce electromagnetic radiation of the type called x-rays. These
rays are richer in energy the higher the applied voltage.The area where the electrons hit the anode is called the focus. As a lot of heat is generated in the process, the anode consists of a heat-resistant disk covered with tungsten in most cases. The disk rotates quickly to disperse the heat along its circumference, thus forming a focal track. The vacuum tube is surrounded by oil inside a lead-lined housing that features only one small opening for the radiation to escape.
The generated radiation has a spectrum, or spread of energies, only a part of which can be used for imaging. Some of the so-called “soft” or very low-energy rays would be completely absorbed by the body’s soft tissues and thus only increase the dose to the patient without contributing anything to the image. For that reason, they are filtered out, typically by an aluminum or copper sheet. In addition
the radiation exiting the tube housing is also constrained by lead collimators that keep the beam strictly limited to the body area of interest.
Attenuation of X-Rays
X-rays are attenuated as they pass through the patient’s body. Two processes play a role: absorption and scatter. With lower-energy radiation (corresponding to lower exposure voltage) absorption dominates. It correlates well with the atomic number of the irradiated matter. Mammography makes proper use of this characteristic and employs low-energy radiation to detect minute spots of calcium in the breast that may indicate cancer. With high-energy radiation (corresponding to high exposure voltage) scatter is mainly responsible for attenuation. In this process the radiation beam loses energy and is diverted in all directions (scattered). The scattered radiation
increases with irradiated body volume. It is hazardous for patients and their immediate vicinity, i.e., the angiographer standing alongside the patient to work with his or her catheters. When scatter reaches the detector, it causes an unstructured shade of gray that diminishes the contrast of the image. A scatter grid (Fig. 3.1) positioned in front of the detector reduces this “diverted” radiation.
Detection of X-Rays
A variety of detectors can make x-rays visible. The simplest is photographic film; because of the high spatial resolution one can achieve, it is used in nondestructive testing of industrial materials such as alloy car wheels or gas pipelines. To expose film alone an incredible dose of x-rays is necessary, but that does not matter in this instance. Film is much less sensitive to x-rays than to light—any airport security x-ray scan will show you the inside of your camera without significantly damaging your valuable vacation photos, which proves the point. As light exposes film much better, in diagnostic radiology a combination is used of film and intensifying screens that are made of rare earth materials (gadolinium, barium, lanthanum, yttrium). These screens fluoresce when irradiated (just like the foil of “Bariumplatincyanür” that Wilhelm Conrad Roentgen used in his initial experiments) and thus expose the film. Usually the film is sandwiched between two
intensifying screens inside a light-tight cassette.
Some intensifying screens emit the main fraction of their light only after stimulation by a laser beam. These screens are called storage phosphors. After their exposure they are scanned in a read-out system and their information content is immediately digitized. These screens can register a larger bandwidth of radiation intensity, which is why “over- or underexposure” is widely tolerated by
the digital system. The information content of the image and the dose to the patient, however, may be inadequate although the image looks normal at first glance. Another digital detector that is currently becoming popular consists of a layer of cesium iodide crystals on top of an amorphous silicon photodiode panel. The crystals light up when hit by x-rays and their light is then converted into
an electronic charge by the photodiode. This is immediately read out by special electronics. For fluoroscopy (e.g., in small-bowel follow-through or in vascular intervention) image intensifier systems are used. A luminescent layer that covers a large-area cathode absorbs the x-rays. The emitted light liberates electrons in the cathode material. These electrons are focused by electronic lenses and hit a small screen that serves as anode. All this happens inside an evacuated large tube. The resulting very bright image is registered by an external television camera and shown on a viewing monitor. Other digital detectors are used in computed tomography or are being tried out for projection radiography. The resulting signal is always a digital one, permitting
post-processing of images and archiving and image communication with an ease unheard of in analog systems.
Techniques of Exposure
Projection radiography: The usual radiograph is a sum- mation image of the exposed body part. A nodule seen over the lung fields, for example, cannot generally be assigned to the lung, the anterior or posterior chest wall, or even the skin surface, because all these structures are superimposed on each other. Clinical inspection, a little brain work, a lateral projection, a fluoroscopy, or a conventional or computed tomography might help.
Conventional tomography: In conventional tomography, only a single slice of the body (e.g., in the hip joint) is depicted while all others are blurred by motion. During the exposure the x-ray tube and the detector move in opposite directions parallel to the imaging plane. A steel beam con-
nects the two and swivels around a movable axis. The position of the axis marks the body layer that is imaged motion-free—the tomographic plane. By moving the beam axis ventrally or dorsally, other planes can be selected. Conventional tomography is a beautiful but dying art—well-equipped departments continue to use it for special, mostly skeletal, studies.
Fluoroscopy: In a considerable number of diagnostic and interventional examinations, the function and morphology of, for example, hollow organs are first evaluated in real time under fluoroscopy with image intensifier systems. Exposures of specific regions, projections, and findings are then performed separately but often with these same systems. The exposures can be viewed immediately on a monitor.
Contrast Media Examinations
To take a closer look at the gastrointestinal tract, it is filled with iodinated contrast solution or a barium suspension. Iodine and barium have high atomic numbers; they therefore absorb x-rays splendidly and are very visible on the radiograph. Barium suspensions can also be prepared
and instilled to beautifully coat the interior wall of the airfiled or fluid-filled bowel (for example, in double contrast barium enemas). To look at the vascular system, for example, in interventional procedures such as balloon dilations of the arteries, iodinated contrast solution is injected into the vessel. In angiography, subtraction is used to improve the depiction of vessels: the images before contrast are subtracted from the images after contrast administration. The resulting radiographs show only the vascular tree without the anatomical background. This is especially helpful in the
abdomen and the skull base as shown in the figure bellow.
If a precontrast image is subtracted from the image after contrast administration, the bony structures, especially at the skull base, disappear and the visualization of the vascular tree improves considerably.
Image Processing
Rest assured that the chemistry of traditional film processing or the post-processing of digital radiographs is all but trivial. The effects on image quality and patient dose can be tremendous. It is a regular and exciting pastime of experienced radiologists to detect and correct any mistakes that the numerous systems may come up with.
