Page 291 - Cardiac Nursing

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CHAPTER
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R R R R Radiologic Examination of the Chest
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Jon S. Huseby / Denise LeDoux
Th ch es ra di og ra ph y o f th e mo st c om ic st stored and transferred m kaking them more accesssible from aa vari- -
Thee chestt radiogra hphy iis one off the most common diaggnostic
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tools used in the evaluation of cardiovascular disease and the et etyy fof remote viewingg statioonns. Digital images can bee eeaasily ma-
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critically ill. Although a variety of other imaging moddalities are ni nipulatedd byy changingg mmagnification orr rrellative dennsity, which
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av availablee, chest r dadiiography remains fundamental because off its s mayy addd ssubbstantiall information to the examination without ex-
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ready availability in most settings, relatively low cost, and the po p sing the patient too repeated imaging. 3
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ability to interpret films by a iwidde variety of health care Anteroposterior chest radiographs are often taken in cardiac
providers. The most recent advancement in chest radiology has care units (CCU) because it is difficult to put the x-ray tube be-
been the rapid conversion from film-based to digital radi- hind the patient. The x-ray film is therefore placed behind the pa-
ographic images. 1 tient. Because the heart is relatively far away from the x-ray film,
The cardiac care nurse may be the first health care professional its outline is somewhat less distinct and the heart size is magnified.
to see the chest radiograph of a patient in acute distress. Valuable Moreover, the distance between the tube and the patient in CCU
time may be saved if the nurse is able to recognize the presence of is shorter than usual to cut-down x-ray scatter, which also results
an abnormality. Knowledge of chest radiograph interpretation in greater magnification.
and the disease processes that an abnormal film indicate can help The degree of darkness of the x-ray film depends on how
the nurse in understanding disease pathophysiology, thereby al- much x-ray energy traverses the patient and exposes the film.
lowing for better patient care; dual reading of radiographs signifi- This depends on the density of the material through which the
cantly increases diagnostic accuracy and decreases the incidence of x-ray beam passes. The chest has four major types of tissue den-
missed abnormalities. sities through which rays must pass: bone, water, fat, and air. Be-
This chapter is divided into four sections: (1) How x-rays cause bone is the densest of these tissues, fewer and less energetic
work; (2) Interpretation of chest radiographs; (3) Chest film find- x-rays pass through bone. Thus, the shadow on the x-ray film
ings in acute care determining line placement; and (4) Chest film cast by bone is light. (An x-ray image is like a photographic neg-
findings in cardiovascular disease and acute care. ative, with white color indicating lack of exposure and black
color indicating intense exposure.) The lung, which is largely air,
is least dense; therefore, it appears black on a chest radiograph.
Soft tissues and blood are largely water, with similar densities,
HOW X-RAYS WORK between those of bone and air. Fat is usually visibly less dense
than other soft tissues. Thus, a chest radiograph is actually a
X-rays are radiant energy, like light, except that these waves are shadowgraph.
shorter and can pass through opaque objects. They are produced The reason a structure can be outlined is that the shadow of
by bombarding a tungsten target with an electron beam and are one density contrasts with that of an adjacent density. If two
channeled so that a narrow but diverging beam is emitted from structures are of equal density and adjacent to each other, then a
the tube. When an x-ray exposure is taken, the tube is usually single combined shadow results. If two structures of similar den-
aimed so that the rays pass through the subject to the x-ray film sity are in different planes or are separated by a structure of a dif-
in either a posterior to anterior (posteroanterior) or anterior to ferent density, then the two structures are seen on x-ray film sep-
posterior (anteroposterior) direction. Because the x-rays are di- arately. This property of the x-ray shadowgraph is helpful in
verging and subject to reflection (scatter), structures more distant determining where a certain density lies. For example, if a density
from the film are magnified and less distinctly outlined. In gen- on a posteroanterior chest radiograph is inseparable from and
eral, chest radiographs are taken in the posteroanterior direction therefore adjacent to the descending thoracic aorta, then the ob-
because this places the heart, an anterior structure, closer to the server knows that this abnormal density is in the posterior chest;
film, resulting in less magnification and allowing the cardiac out- if the density is inseparable from the right heart border, then the
line to be seen clearly. density is in an anterior position, because the heart is an anterior
When using conventional radiology methods, the chest x-ray structure.
image is recorded on a film that is chemically processed. Com-
puterized digital chest radiology utilizes a special phosphor plate
instead of traditional film. The digital x-ray image is produced by
scanning the phosphor plate with a laser beam that causes light to INTERPRETATION OF CHEST
be released from the phosphor plate. This image is then digitized RADIOGRAPHS
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and converted to an image by computer. The computer image is
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then viewed from monitors and can also be converted to radi- The chest radiog gph is read as though the reader were looking at
ographic film providing a hard copy. Digital images afford many the patient. Traditionally, the x-ray film is placed on a view box or
advantages over traditional chest x-rays. Digital images are easily light box that allows the radiograph to be backlit so it can be
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