Wilhelm Roentgen discovered the x-ray in 1895 and won the very first Nobel Prize in physics (1901) for this discovery.¹ Since that time, radiography has been used in medicine to image the chest structures. Ionizing radiation has its disadvantages, however, as Thomas Edison learned when his laboratory assistant, Clarence Daley, became the first scientist to die of radiation exposure in the United States.¹ By the 1940s, ultrasound was being used as a way to image the body using nonionizing radiation.¹ Computed tomography (CT) was developed in the 1970s, and magnetic resonance imaging (MRI) in the 1980s.¹ All these methods have been used to image the chest, and each offers distinct advantages and disadvantages for the patient and the clinician.

Most imaging machines still use film to capture the image, but newer systems are using digital formats. In either case, the basic methods are similar: x-rays are generated when the anode of an x-ray tube is bombarded with electrons from the cathode of the tube. The collision gives off energy in the form of x-radiation, which travels out of the tube and through the patient, then hitting the imaging cassette. The cassette contains film or a digital imaging sensor. The image is then processed by a digital processor or a film developer (Figure 11-1).

The film turns black (is exposed) if the x-rays pass through the patient and reach the film surface. If the x-rays are reflected or absorbed by the patient, they do not reach the film, and the resulting image stays white (unexposed). The patterns and shades of light and dark on the film reflect the differing densities present in the part of the human body being imaged.

The darkest images on a film (also called radiolucent areas) represent pockets of air within the body. Fat is denser than air, and it produces a dark-gray image. Muscle and other soft tissues are more dense and produce a much lighter gray image. Finally, bone is the most dense natural substance in the body and produces a white image. Metallic objects are even more dense (also called radiopaque) than bone and produce a pure white image (Figure 11-2).

Figure 11-2, A, demonstrates all the different densities. This lateral view of the knee shows metal from a total knee replacement, along with bone, muscle, fat, and on either side of the patient’s knee, air.²

The x-ray image is a summation of all the densities that the x-rays have passed through. The different layers of tissue that are on top of one another are flattened into a single two-dimensional image. Sometimes the various densities lie next to one another and are easily distinguished in the image. Other times the two densities overlap one another and are blurred together in the image. For this reason, the patient is usually positioned so that two or more images can be taken at a right angle to one another. This allows structures that are overlapping in one orientation to be seen side-by-side with the other orientation.1,3

The Radiograph and Its Evaluation

The process for evaluating a radiograph can be broken down into several stages. The initial evaluation should verify all of the technical details of the scan; subsequent stages look at various aspects of the anatomy imaged.

Initial Evaluation

The initial stage is when the technical details of the images are noted:

 Correct patient identifying information

 Correct body part imaged

 Body part imaged with two projections

 Quality of the exposure of the film

Before proceeding with any further evaluation, the first step is to check that the information identifying the patient has been properly provided on the image and that the image is indeed of the patient in question. Identifying information usually consists of the patient’s full name, date of birth, and if possible, their sex and social security number. Failing to verify this basic information can lead to medical errors in one or multiple patients.

Next, check that the part of the body in question is indeed the part that was imaged and that it was done in at least two projections oriented 90 degrees apart (for example, one view from the front and one from the side). The left and right sides of the patient should be indicated on the films so that they can be viewed in proper orientation. A marker should also indicate the patient’s position at the time of the radiograph. For instance, a frontal chest radiograph can be taken with the patient standing, sitting, or lying down.

Finally, check the overall exposure of the film. If all the structures are too dark, the film has been overpenetrated. It should be retaken using less radiation. If all the structures are too light, the film has been underpenetrated and should be retaken using more radiation.

Evaluation for Pathology

Overview

After verifying the technical aspects of the images, the diagnostic evaluation can begin. It will help to first review the different projections that have been taken (Table 11-1

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