The Chest Radiograph
Mark Mangano
Adrian Shifren
Sanjeev Bhalla
General Principles
The CXR is ubiquitous in medicine and remains among the hardest of diagnostic studies to master. The key to proficiency lies in reviewing all CXRs ordered. Close interaction with radiology staff is invaluable in building ones’ skills and honing in on a specific diagnosis.
Using a constant search pattern will allow for systematic and comprehensive analyses.
Ideally, CXRs should be interpreted without knowledge of the clinical context to allow an unbiased and objective evaluation of the study. However, similar to any diagnostic test, evaluating the CXR in the context of the clinical scenario is very important and allows a focus on specific areas of the study and a detailed search for associated pathologic findings.
At the Mallinckrodt Institute of Radiology (MIR), we try to initially read all CXRs without any clinical history. The clinical information is reviewed after the initial perusal so as to avoid bias at first glance and ensure subtle, clinically relevant issues are not overlooked.
The importance of prior CXRs for comparison cannot be stressed enough. Understanding a finding often relies on knowing whether it is acute, subacute, or chronic. An area of consolidation, for example, could represent a community-acquired pneumonia on a CXR. If the area is stable from 1 year ago, low-grade adenocarcinoma or radiation changes become more likely.
In our daily practice we often rely on old films. They are cheaper and lower in radiation than a CT and often provide greater information.
Initial Assessment
Patient Position and Study Quality
Initial evaluation begins with assessment of patient position and quality of the study.
This evaluation includes assessing the film for:
Rotation
Degree of inspiration
Patient position
Radiation dose
Patient Rotation
A common method of assessing for the presence of rotation is to evaluate the relationship of the medial heads of the clavicles to the spinous processes of the vertebral bodies. When truly straight, each clavicular head will be equidistant to the adjacent spinous process.
If the patient is rotated, the mediastinal borders will be altered. Rotation can be confused for mediastinal widening. Lack of appreciation for patient rotation can result in needless workup of perceived mediastinal changes.
Degree of Inspiration
The degree of inspiration will affect the density of the lungs.
As a general rule, the diaphragm should be crisp and the peak should be rounded.
For those who prefer counting ribs, 10 posterior ribs and 6 anterior ribs should be seen on an inspiratory study.
Patient Position
Erect versus supine positioning is pertinent to verify, as it will alter the interpretation of air–fluid interfaces, blood flow distribution, and caliber of the pulmonary vessels. Cephalization, for example, can be appreciated only on an upright film.
The air–fluid level in the gastric fundus often allows one to understand whether the CXR is upright, supine, or decubitus.
Radiation Dose
Radiation dose of a CXR has become more challenging in the digital era.
As a general rule, the optimally exposed CXR allows visualization of the vertebral bodies and disk spaces through the mediastinal structures and also allows visualization of the pulmonary vessels through the heart and diaphragm.
With new digital techniques, postprocessing allows the technologist to manipulate the image to achieve this same effect. The reader, therefore, must be careful that he/she can see through both the heart and mediastinum but that the image does not look too pixilated. Should pixilation occur, the reader must be aware that an insufficient radiation dose was used.
CXR Views
There are a number of different variations of a CXR that may be obtained to evaluate thoracic pathology. These include the posteroanterior (PA) view, the lateral (LAT) view, the anteroposterior (AP) view, and the lateral decubitus (LD) view. Some centers also make use of end-expiratory (EE) views.
All of these views share the concept of a point source which results in a fan x-ray beam. The result is magnification of structures which are farther from the detector. A good analogy is the shadow created by your hand on a classroom desk from an overhead light bulb. If you lift your hand off the desk, the shadow becomes bigger and fuzzier.
Posteroanterior View
The PA view is acquired with the patient in a standing position during full inspiration. The patient faces the detector, which is in contact with the anterior chest wall. The x-ray beam is directed toward the cassette from a distance of 6 ft, which results in minimal magnification of the heart.
Lateral View
The LAT view is also taken with the patient standing during full inspiration at a distance of 6 ft. The arms are lifted. By convention, the patient’s left side is placed in contact with the detector, and the beam is directed from right to left to reduce magnification of the heart.
LAT views are useful for evaluating lesions behind the heart, diaphragm, or mediastinum that may be hidden on PA views.
The left diaphragm can be differentiated from the right diaphragm on this view by locating the loss of the left diaphragmatic border when in contact with the cardiac silhouette or by locating the right posterior ribs (which will appear magnified and larger since they are farther from the cassette).
It is important to note that magnification is about the same between the PA and LAT views. If a lesion is found on one view, the relationship with a landmark (e.g., aortic arch) can be used to localize it on the other.
Anteroposterior Views
The AP views are usually taken with portable machines and are most often used to image the chest in patients who cannot have formal PA and LAT views, such as intensive care unit or intraoperative patients.
These studies are conducted with the cassette behind the patient, in contact with his/her back. The x-rays are directed from front to back, often at a distance of <6 ft. The patient is often in a sitting or supine position and unable to perform a full inspiration.
AP views often result in increased lung attenuation (from lack of complete inspiration) and increased magnification of mediastinal and cardiac structures (from increased distance between these structures and the cassette).
It is important to understand that magnification of anterior structures occurs in the AP view to prevent inappropriate interpretation of an enlarged mediastinum or cardiac silhouette.
Lateral Decubitus Views
LD views are taken with the patient lying on the ipsilateral side. For example, a left LD is taken with the left side down.
There are four clinical situations in which a decubitus view might be helpful.
When evaluating whether the ipsilateral effusion is mobile
When evaluating whether a contralateral pneumothorax is present
When the contralateral lung has a concomitant pneumonia with an effusion
When the ipsilateral lung collapses normally. If it does not, one might suspect a radiolucent foreign body
End-Expiratory Views
EE views are occasionally used to detect a subtle pneumothorax. The EE view should accentuate a pneumothorax as the EE radiograph will make the lung artificially whiter.
Care must be taken to avoid mixing inspiratory and expiratory images when following a pneumothorax on serial images. The expiratory images will exaggerate the size of the pneumothorax.
General Approach to CXR Interpretation
A systematic approach to the CXR reduces the risk of missed pathology. Subtle pathologies can be missed for two reasons. Only the process that is suspected is noted. Or, an obvious but previously unexpected finding commands the viewer’s attention.
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