X Rays

Regular x-rays (plain x-rays) account for about 80% of imaging examinations. X-ray examinations, or plain x-rays, are made by an x-ray beam passing through the patient. The x-rays are absorbed in different amounts by the various tissues or materials in the body. Most of the beam is absorbed or scattered. This represents deposition of energy in the tissue but does not cause the patient to become radioactive or to emit radiation. A small percentage of the incident radiation beam exits the patient and strikes a detector.

The classic imaging receptor is a film/screen combination. The x-ray beam strikes a fluorescent screen, which produces light that exposes the film, and then the film is developed. Newer systems are called computed radiography or digital radiography. In computed radiography, the x-rays strike a plate that absorbs the x-rays and stores the energy at a specific location. The plate is then scanned by a laser, which releases a point of light from the plate. The location is detected and stored in a computer. In digital radiography detector systems, the x-ray hits a detector and then is converted to light immediately. Once either type of image is stored in the computer, it can be displayed on a monitor for interpretation or transmitted to remote locations for viewing.

Four basic densities, or shades, are visible on plain films. These are air, fat, water (blood and soft tissue), and bone. Air is black or very dark. On x-rays, fat is generally gray and darker than muscle or blood. Bone and calcium appear almost white. Items that contain metal (such as prosthetic hips) and contrast agents also appear white. The contrast agents generally used are barium for most gastrointestinal studies and iodine for most intravenously administered agents.

Remember that standard or plain x-rays are two-dimensional presentations of three-dimensional information. That is why frontal and lateral views are often needed. Without these, mistakes can easily be made. You must remember that an object visualized on a specific view is somewhere in the path of the x-ray beam (not necessarily in the patient). If an object projects outside the patient on any view, it is outside the patient. However, even if an object projects within the patient on two orthogonal views, it can still be located outside the patient. Each additional view needed to make a diagnosis requires an additional x-ray exposure and therefore adds to the patient's radiation dose.

The terminology used to describe images is usually quite straightforward. Chest and abdominal films are referred to as upright or supine, depending on the position of the patient. In addition, chest x-rays are usually described as posteroanterior (PA) or anteroposterior (AP). These terms indicate the direction in which the x-ray beam traversed the patient on its way to the detector. PA means that the x-ray beam entered the posterior aspect of the patient and exited anteriorly. AP means that the beam direction through the patient was anterior to posterior. A left lateral decubitus view is one taken with the patient's left side down.

Position is important to note, because it can affect magnification, organ position, and blood flow and therefore significantly affect image interpretation. For example, the heart appears larger on AP than on PA images because on an AP projection, the heart is farther from the detector and is magnified more by the diverging x-ray beam. It also appears larger on supine than on upright images because the hemidiaphragms are pushed up, making the heart appear wider. Portable chest images are taken not only in the AP projection but also with the tube closer to the patient than on upright films. This magnifies the heart even more.

Use of contrast agents permits visualization of anatomic structures that are not normally seen. For example, intravenous or intra-arterially injected agents allow visualization of blood vessels. If imaging is done with standard format, the blood vessels appear white. Digital imaging allows subtraction or removal of unwanted structures, such as the bones, from an image. Often the computer manipulation is done in such a way that the arteries may appear black instead of white, although this usually does not present a problem in interpretation.

Contrast agents are used to fill either a hollow viscus (such as the stomach) or anatomic tubular structures that can be accessed in some way (such as blood vessels, ureter, and common bile duct). When you see an abnormality on one of these studies, you must determine whether the location is intraluminal, mural, or extrinsic. This usually requires seeing the abnormality in perpendicular views. Unless you are careful about this determination, you will make errors in diagnosis.

Contrast agents instilled orally, rectally, or retrograde into the ureter or bladder incur little or no risk unless aspiration or perforation occurs. With the intravenously or intra-arterially administered agents, a small but real risk of contrast reaction exists. This is something that you should consider before ordering an intravenous pyelogram or a contrast-enhanced CT scan. About 5% of patients will experience an immediate mild reaction, such as a metallic taste or a feeling of warmth; some experience nausea and vomiting, wheeze, or get hives as a result of these contrast agents. Some of these mild reactions can be treated with 50 mg of intramuscular diphenhydramine (Benadryl). Because contrast agents also can reduce renal function, they should not generally be used in patients with compromised renal function or multiple myeloma.

A small number (about 1 in 1000) patients have a severe reaction to intravascular contrast. This may be a vasovagal reaction, laryngeal edema, severe hypotension, an anaphylactic-type reaction, or cardiac arrest. A vasovagal reaction can be treated with 0.5 to 1.0 mg of intravenous atropine. The most important initial therapeutic measures in these severe reactions are to establish an airway, ensure breathing and circulation, and give intravenous fluids. Other drugs obviously also may be necessary. The risk of death from a study using intravenously administered contrast agents is between 1 in 40,000 and 1 in 100,000.