One of the most commonly requested tests in medicine is the chest x-ray, and while many of us receive extensive training in interpretation of the AP or PA view, most students could use a little help when it comes to the lateral film. By understanding a few basic anatomic landmarks and examination techniques, you can increase the accuracy and efficiency with which you read chest films; and the more proficient you are with a lateral film, the less likely you are to order an unnecessary CT. Below are 10 tips to help you on your way.
1. Check the Basics. Approach the lateral chest film just as you would any x-ray: check name, age, date, film quality, and tubes, wires, etc. Also check for old films for comparison.
2. Locate the Patient's Arms. If they are not straight up, then the lung apices will be obscured.
3. Important landmarks:
- Fissures: Oblique fissure begins at T4/5 and runs anteriorly to pass through the hilum. The left oblique is steeper than the right. The right horizontal fissure runs from the hilum anteriorly, and there is no left horizontal fissure.
- Vertebral bodies: these should become less dense as you proceed down the spine. If they remain opaque then think about basilar pneumonia, mass or pleural disease.
- Diaphragm: see # 5 for a discussion on locating the hemidiaphragms.
- Find the Scapula: Now disregard them.
4. Examine Three Important Spaces:
- Retrotracheal space (Raider triangle): Space between posterior wall of trachea, anterior surface of spine, and superior wall of aorta. Look for nodules here, which can be difficult to pick up on the frontal view.
- Retrosternal space: Area between sternum and ventral cardiac surface. A normal-sized heart contacts the lower third of the sternum, but an enlarged right atrium or ventricle can cause obliteration of this space. The retrosternal space can be enlarged in emphysema.
- Retrocardiac space: Area behind the heart. In this space you may see basilar consolidation, effusion or masses that are otherwise lost in an underpenetrated frontal film.
5. Consider Magnification and Beam Divergence. If you order a lateral film and do not specify a side, a left lateral will usually be taken. In a left lateral radiograph, the right hemithorax is farther away from the film than the left hemithorax. Consequently, right-sided structures appear magnified. Beam divergence causes the right hemithorax to project more anteriorly, posteriorly, superiorly, and inferiorly than the left hemithorax. Understanding these principles can help you locate important structures. For example, you can differentiate the right hemidiaphragm from the left hemidiaphragm by locating the right posterior ribs (which will appear slightly magnified) and the right posterior costophrenic angle (which will project posterior to the left posterior costophrenic angle in a true left lateral film). The principles are reversed for a right lateral, so make sure you know what you ordered. The side of interest should be the side closest to the film.
6. Understand the Mediastinum. Often considered the most confusing part of the lateral chest x-ray, the mediastinum is divided into three compartments:
- Anterior compartment: between sternum and ventral cardiac surface.
- Middle compartment: between ventral cardiac surface and anterior surface of spine.
- Posterior compartment: area behind anterior surface of spine.
7. Anterior Mediastinal Pearl: Anterior mediastinal masses are caused by the "Big White and 5 T's":
- Big White = Big Heart
- 5 T's = Thyroid, Thymus, Teratoma, Thoracic aorta, and Terrible lymphoma.
8. Middle Mediastinal Pearl: Enlarged lymph nodes are the most frequent cause of a middle mediastinal mass. But if you see an air fluid level, think hiatal hernia.
9. Posterior Mediastinal Pearl: In younger patients, most posterior mediastinal masses are from the nerves or their coverings (neurofibroma, meningocele, etc.). In older patients, aortic aneurysms, multiple myeloma, and metastatic spinal disease are more common.
10. Look for the Black Hole: Identify the left upper lobe bronchus (see diagram below), otherwise known as the "black hole" on a lateral chest film. Put your finger immediately anterior to the black hole — that's the right pulmonary artery (even if you don't see it)! Put your finger immediately above the left upper lobe bronchus — that's the left pulmonary artery! The carina will be approximately 2 ½ cm above the black hole. There is much to learn from this area of the mediastinum. Refer to Proto's work below for more information.
With these pearls in your pocket, you're ready to follow the advice of Dr. Benjamin Felson, father of chest radiology: "Show off what you know. Brag a little. Speak up in class. Tell your spouse or sweetheart; tell your colleagues; don't bother to tell your friends — you won't have any."
1. Gaber et al. Lateral chest x-ray for physicians. Journal of the Royal Society of Medicine. 2005;98(7): 310-312.
2. Goodman L. Felson's principles of chest roentgenology: a programmed text. Philadelphia: W.B. Saunders; 1999.
3. Muma L. Basics of chest x-ray interpretation: a programmed study. Available at: http://www.usfca.edu/fac-staff/ritter/chestxra.htm. Accessed June 2004.
4. Proto A, Speckman J. The left lateral radiograph of the chest. Medical Radiography and Photography. 1979;55(2):30-74.
Figure used by permission of authors of "Basics of Chest X-ray Interpretation: A Programmed Study" http://www.usfca.edu/fac-staff/ritter/chestxra.html
Brian Sorensen, EMRA MSC 2003-04
John Anderson, MSIII
University of Colorado
EMRA Medical Student Council