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 multimillion-dollar high-tech imaging equipment. The bulk of all diagnostic imaging studies is still done with this technology. 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 (Fig. 3.1). 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. Mammo-graphy 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 angiogra-pher 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.

The Guy Who Took Care of the Scatter

Gustav Bucky's name is known to radiologists all over the world for his invention of the scatter grid in 1912. After the initial presentation at a medical convention, some colleagues suggested that the images were so good it must be a hoax. Having been forced into emigration by the Nazis, he left Berlin for New York, where he continued his innovative work. With his invention of the grid that is in use in every x-ray machine to this very day he eventually earned the lump sum of $25— ingenuity is definitely not a monetary unit.

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

I Generation of x-Rays

Pictures Screen Film Radiography

Heated cathode filament Rotating anode Focal spot Tube housing (lead) Insulating oil Tube port

Tube filter Collimation Primary radiation

Scattered radiation

Patient

Table top Scatter grid Cassette

Intensifying screen Film

Heated cathode filament Rotating anode Focal spot Tube housing (lead) Insulating oil Tube port

Tube filter Collimation Primary radiation

Scattered radiation

Patient

Table top Scatter grid Cassette

Intensifying screen Film

Fig. 3.1a The figure shows the generation of x-rays, their attenuation due to scatter, and their detection.

b This is a modern digital projection radiography unit used primarily for skeletal work (by Philips Medical Systems).

b the foil of "Bariumplatincyanur" 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.

f Film-screen combinations vary greatly in their x-ray i sensitivity and spatial resolution and thus have to be selected according to the specific imaging problem to be solved. If the depiction of fine detail is important, the required dose is generally higher. If the dose must be kept as low as possible, such as in children, fine detail must often be sacrificed.

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

I Digital Subtraction Angiography (DSA)

Brain Vasculature Radiology

Fig. 3.2 a The arterial vasculature of the brain is very complex. The bony skull is not simple either.

b 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.

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 (see p. 9) 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 summation 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.

f In projection radiography, a decrease in transparency or a i "shadow" (e.g., a tumor) is bright; an increase in transparency (e.g., air in the bowel) is dark.

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 connects 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 air-filed or fluid-filled bowel (for example, in double contrast barium enemas).

To look at the vascular system, for example, in interven-tional 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 (Fig. 3.2).

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

3D Reconstruction

3D Reconstruction

Fig. 3.3 This complete 3D reconstruction of a child's head was performed as a special service to the plastic surgeons: They wanted a precise documentation before surgically approaching a congenital skeletal abnormality. The left part of the image shows the head with surrounding soft tissue and also the finding that worried the patient's parents. What do you make of it? •auoq ibjuojj aip jo ajnjns uejpauj AjossaDDB ue si ajaqi

Fig. 3.3 This complete 3D reconstruction of a child's head was performed as a special service to the plastic surgeons: They wanted a precise documentation before surgically approaching a congenital skeletal abnormality. The left part of the image shows the head with surrounding soft tissue and also the finding that worried the patient's parents. What do you make of it? •auoq ibjuojj aip jo ajnjns uejpauj AjossaDDB ue si ajaqi 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.

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  • SILKE
    How is projection rdiography used?
    4 years ago

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