5 Chest Radiography

Andrew O. Zurick, III, Park W. Willis, IV

Imaging techniques are central to the evaluation and management of patients with known or suspected heart disease. While technological advances in recent years have produced a broad spectrum of diagnostic imaging studies, each with advantages and appropriate clinical applications, it is necessary for clinicians treating patients with cardiovascular diseases to understand the applications and limitations of the available methodologies and use them effectively, but efficiently. This chapter focuses on technological aspects of cardiac and more specifically chest radiography and how this common imaging modality can still provide very useful information in the evaluation of patients with cardiovascular diseases.

Technical Aspects

Roentgenology is the science of both ionizing and nonionizing radiation modalities for the diagnosis and treatment of disease. Wilhelm Conrad Roentgen, a German physics professor who initially discovered x-rays in 1895, discovered that he could film his thumb and forefinger and their bones on a screen through the use of cathode rays. X-rays form part of the continuum of electromagnetic radiation, exhibiting both electrical and magnetic forces. They are typically generated by passing a current across a diode resulting in the generation of electrons, which are subsequently aimed at a metal anode that then gives off x-rays. The remarkable property of x-rays is their differential ability to penetrate through different types of matter, many of which are otherwise opaque to visible light.

X-ray beam projection determines resolution and magnification. As x-rays emerge from the x-ray tube, divergence occurs. When x-rays are captured by film, geometric distortion results as a function of the distance of the x-ray beam from the midline and the distance of the object from the film. The farther an object from the x-ray source, the less geometric distortion occurs; however, this greater distance also necessitates added energy to penetrate the object and expose the film. Ideally, the farther an object is from the x-ray tube, the more parallel the x-rays are that penetrate it. However, an object closer to the x-ray source will require greater x-ray divergence to cover the area of interest. Overall, resolution is improved by increasing the distance between the object and x-ray source at the expense of increased patient radiation exposure. Standard chest x-ray (CXR) examinations are obtained with a source-to-image distance of 6 feet.

An x-ray image of an object will only occur if there is a difference in the transmission of x-rays between the surrounding medium and the object of interest. Shades of gray or “contrast” result from different amounts of x-ray absorption between the surrounding medium and object of interest. The differential density of myocardium, blood, vascular tissue, and the surrounding air-filled lung allows distinction of these structures in CXRs. Thus, the CXR provides a means to assess the heart, the great vessels and the pulmonary veins, the lung fields, and the mediastinum. CXRs can be difficult to interpret, since imaging technique, body size, age, and other factors can all impact image quality.

Safety Considerations

The risks of a single CXR are minimal. However, it is important to understand that even low-level, environmental radiation exposure (to sunlight) has effects on biologic systems, including resultant cell death by apoptosis. Because ionizing radiation has a dose-dependent effect, it is important to minimize unnecessary radiation exposure. The risks of medical radiation exposure include development of malignancy and/or genetic alteration. Lifetime cancer risk has been estimated to increase 0.5% to 1.4%, based on lifetime risk estimates for a general population, following 10 rad of x-ray or gamma radiation received by the whole body.

Chest Radiology—Normal Anatomy

The CXR can be very useful in detecting abnormalities in the structure of the heart and great vessels. To do so requires an understanding of the cardiac and vascular structures normally seen in a CXR. In the standard posteroanterior projection (Fig. 5-1

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