The first step in medical imaging is to examine the patient and determine the possible cause of his or her problem. Only after this is done can you decide which imaging study is the most appropriate. A vast number of algorithms or guidelines have been developed, but no consensus exists on the "right" one for a given symptom or disease because a number of imaging modalities have similar sensitivities and specificities. In this text, I have chosen to give you my opinion on the initial study to order in a specific clinical setting.

What should you expect from an imaging examination? Typically, one expects to find the exact location of a problem and hopes to make the diagnosis. Although some diseases present a very characteristic picture, most can appear in a variety of forms depending on the stage. As a result, image interpretation will yield a differential diagnosis that must be placed in the context of the clinical findings.

Examination of images requires a logical approach. First you must understand the type of image, the orientation, and the limitations of the technique used. For example, I begin by mentally stating, "I am looking at a coronal computed tomography (CT) scan of the head done with intravenous contrast." This is important, because intravenous contrast can be confused with fresh blood in the brain.

Next I look at the name and age on the film label to avoid mixing up patients, and it allows making a differential diagnosis that applies to a patient of that age and sex. You would not believe the number of times that this seemingly minor step will keep you from making very dumb mistakes.

The next step is to determine the abnormal findings on the image. This means that you need to know the normal anatomy and variants of that particular part of the body as well as their appearance on the imaging technique used. After this, you should describe the abnormal areas, because it will help you mentally to order a differential diagnosis. The most common mistake is to look at an abnormal image and immediately to name a disease. When you do this, you will find your mind locked on that diagnosis (often the wrong one). It is better to say to yourself something like, "I am going to give a differential diagnosis of generalized cardiac enlargement with normal pulmonary vasculature in a 40-year-old male," rather than to blurt out "viral cardiomyopathy" in a patient who really has a malignant pericardial effusion.

After practicing for 20 years or so, a radiologist knows the spots where pathology most commonly is visualized. Throughout this text, I point out the high-yield areas for the different examinations. Although no absolute rules exist, knowing the pathology and natural history of different diseases will help you. For example, if you are interested in hyperparathyroidism, a film of the hands may be all that is needed, because bone resorption is likely to occur there first. When you have the hand film, the optimal place to look is on the radial aspect of the middle phalanges.

After reviewing the common causes of the x-ray findings that you have observed, you should reorder the etiologies in light of the clinical findings. At this point, you probably think that you are finished. Not so. Often a plethora of information is contained in the patient's film jacket or in the hospital computer information system. This comes in the form of previous findings and histories supplied for the patient's other imaging examinations. Reviewing the old reports has directed me to areas of pathology on the current film that I would have missed if I had not looked into the medical information system. A simple example is a pneumonia that has almost but not completely resolved or a pulmonary nodule that, because of inspiratory difference, is hiding behind a rib on the current examination.

You probably think that you are finished now. Wrong again. A certain number of entities could cause the findings on the image, but you just have not thought of them all. After I have finished looking at a case, I try to go through a set sequence of categories in search of other differential possibilities. The categories I use are congenital, physical/chemical, infectious, neoplastic, metabolic, circulatory, and miscellaneous.