Interpreting Chest XR Flashcards
What is the colour (density) of each of the following on a CXR ?
- Air
- Fat
- Soft tissue/Muscle
- Bone
- Metal
- Air = Black
- Fat = Grey
- Soft tissue/Muscle = Grey/white
- Bone = White
- Metal = Bright white
What is the first thing you should do when interpreting a CXR and why ?
Check the patient’s name, date of birth and hospital number
This is because attaching a CXR image to a patient file on PACS requires someone to type an entry into a computer. This could be subject to human error, so it is essential to make sure that the CXR you are looking at belongs to the correct patient.
Why is it important to also note the sex of the person on CXR?
As some diseases are specific to males or females
What is the second thing you should do when interpreting a CXR and why ?
Check the date and time the CXR was taken
- Some patients have multiple CXRs on PACS, so it’s easy to load the wrong image.
What is the third thing you should do when interpreting a CXR and what 4 steps does this involve ?
Make a technical assessment, by checking the:
- Side marker
- Projection
- Degree of inspiration
- Centering (is the CXR rotated?)
Why should you check the side marker when making a technical assessment of a CXR?
For example:
The side marker shows that this CXR has either been incorrectly labelled or the patient has dextrocardia.
You could make the distinction by palpating the apex beat.
Dextrocardia would affect the expected position of the PICC line that has been placed in this patient, hence important to check side markers.
What position is the best quality CXR’s taken in ?
A PA projection, taken with the patient erect, facing away from the x-ray source.
What can be assumed about the position of a CXR projection?
That a PA projection has been used unless the image is annotated otherwise
Why is a PA CXR projection best ?
- CXR’s taken PA are usually well inspired.
- The scapulae only minimally overlap the lungs.
- Heart size can be reliably assessed.
When may an AP projection CXR be used and why ?
- Sick patients may be unable to stand. They are often x-rayed sitting up in bed, using an AP projection.
- These CXR images are annotated ‘AP’ by the radiographer
What are the problems associated with AP projection CXR ?
- AP exams exaggerate heart size e.g. the heart looks large on this young male’s CXR, but the projection is AP so the observation is unreliable and should be discounted.
- Note that on AP exams, the scapulae overlap the lungs, and can simulate or mask disease there. The medial edge of the left scapula is arrowed.
- It can be difficult for the patient to take an adequate inspiration
When may a CXR projection be taken in the supine position ?
- Some patients are too sick to sit up, and are x-rayed supine.
- These CXR images are annotated ‘supine’ by the radiographer.
What are the problems associated with a supine CXR ?
- Pneumoperitoneum due to bowel perforation is diagnosed using an erect CXR, as gas rises to accumulate below the diaphragm. CXRs obtained supine do not reliably show pneumoperitoeum, as gas does not move to the diaphragm.
- A pneumothorax also looks different on a supine CXR, as pleural air is seen anteriorly adjacent to the heart, rather than at the lung apex.
When would a lateral projection CXR usually be taken and do we need to worry about them at this stage of learning ?
- They are performed infrequently, usually at the suggestion of a radiologist.
- Lateral CXRs are challenging to interpret, so you shouldn’t worry too much about them.
How do you determine if a CXR is adequately inspired or not ?
- On an adequately inspired CXR, 6 anterior rib ends should be visible above the left diaphragm (blue line).
- Note - the costal cartilages which unite the ribs to the sternum are not visible on a CXR.
Why is it important to check that a CXR is adequately inspired ?
Because poorly inspired CXRs exaggerate heart size and basal lung markings, simulating heart disease or lung base infection.
e.g. this CXR has only 5 anterior rib ends visible above the diaphragm. As a consequence, the basal lung markings are crowded together, simulating infection when none is actually present.
How do check that a CXR is not rotated ?
Measure the distances from the medial ends of the clavicles to a thoracic spinous process
- On a well centred CXR, the distance is the same on both sides (top pic)
- If the distances are not the same, the CXR is rotated. (bottom pic)
Is a little rotation on a CXR acceptable ?
Yes - it is only grossly roated CXR’s that it becomes a problem
What are the problems with grossly rotated CXR’s ?
They can exaggerate heart size, or simulate a hilar or mediastinal mass.
Rotation also causes lung density to become unequal, leading to misdiagnosis of lung disease. The increased left lung whiteness shown in pic is all due to rotation.
Here are rotated and well centred CXRs from the same patient. Check the relationship between the clavicles and spinous processes to confirm this.
Both x-rays are actually normal, but if you didn’t realise the first CXR was rotated, you could easily misdiagnose left lower lobe pneumonia with displacement of the heart to the right
When examining the heart what should you assess ?
- Measure heart size
- Check the hearts position
How do you measure the heart size on a CXR and state the normal heart size
You should measure the heart and thorax at their widest points to calculate the CTR.
A normal heart measures 50% or less of the cardiothoracic ratio (CTR). (e.g. the pic shown)
Why should you measure the heart size on a CXR ?
Becauase cardiac enlargement is a common sign of heart disease
e.g. This failing heart measures well over 50% of the cardiothoracic ratio
When checking the position of the heart on CXR what is the normal position ?
1/3rd of the heart should lie to the right of the midline with 2/3rds of the heart to the left
Why should you check the heart position on a CXR ?
Chest diseases that alter thoracic volume cause the heart to shift, changing the proportions seen on either side of the midline.
e.g. in the pic shown the left tension pneumothorax opposite increases left thoracic volume, shifting the heart to the right and reversing the proportions seen on either side of the midline.
What are the hila formed by and what is the normal relationship between the right and left hila ?
- Hilar shadows are formed from multiple superimposed pulmonary arteries and veins.
- The normal right hilum lies about 1.5cm below the left hilum.
- The right and left hila have different shapes due to different configurations of their pulmonary vessels, but they should be of similar overall size and density
What should Any hilar enlargement or increase in density should raise suspicion of?
Hilar lymph node enlargement, or lung cancer (as shown in pic)
When examining the mediastinum what should be checked and how should they appear ?
- The trachea should be central, superimposed on the thoracic spine.
- The aortic arch which protrudes above the left hilum
- The descending aorta. The aortic arch merges inferiorly with the descending aorta, the outline of which is usually visible adjacent to the spine.
Why should you check that the trachea is central ?
Chest diseases that alter thoracic volume, such as this tension pneumothorax, cause tracheal shift.
Why should you check that the descending aorta is visible ?
Because the descending aorta lies in the posterior mediastinum, close to the left lower lobe ==> Its CXR outline is obscured when the left lower lobe is diseased, as shown here.
When checking the diaphragms what should be assessed?
- Diaphragmatic position = The normal right diaphragm lies about 1.5cm above the left.
- The Costophrenic recess = normal costophrenic recesses contain a dark flange of lung with sharply pointed edge
- Check for gas below the diaphragm = It is common to see gas within the stomach or splenic flexure below the left diaphragm • It is unusual to see gas below the right diaphragm
Why is diaphragmatic position important ?
- Chest diseases that alter thoracic volume may cause a shift in diaphragmatic position, as in this case of left lower collapse, where reduced lung volume has caused the left diaphragm to rise above that on the right.
- Note The heart has also shifted to the left.
Why check the costophrenic recesses ?
- Pleural fluid gravitates to the lung bases.
- The earliest CXR sign of a pleural effusion is obliteration of the normally sharp costophrenic angle, as shown here.
Why check for gas under the diaphragms ?
Gas under the right hemidiaphragm should raise suspicion of bowel perforation and pneumoperitoeum, as shown here.
Note - The term pneumoperitoneum refers to the presence of air within the peritoneal cavity. The most common cause is a perforation of the abdominal viscus—most commonly, a perforated ulcer, although a pneumoperitoneum may occur as a result of perforation of any part of the bowel
When checking both lungs how should you split up the lung areas and why ?
- Lung density varies from apex to base, due to chest shape and overlying soft tissues
- But the density of the lung Apices, Upper, Mid and Lower zones should be the same between right and left
Why divide the lungs into zones ?
Lung opacification may be subtle but by dividing the lungs into zones, then comparing right with left, it is obvious that the right lower zone is denser, in this case due to middle lobe pneumonia.
What are the review areas on CXR and why do they specifically need to be checked when interpreting a CXR?
Review areas are places where CXR pathology is commonly missed.
Before deciding a CXR is ‘normal’, you must check for:
- apical lung disease
- left lower lobe disease superimposed on the heart
- small pleural effusions
- free intraperitoneal gas
- shoulder pathology
Use this example to understand the importance of reviewing the review areas on a CXR.
This CXR looks superficially normal, however check the review areas!
There is a small left pleural effusion due to an apical pneumothorax.
So this CXR actually has two review area abnormalities (a small pleural effusion and an apical pneumothorax)
Appreciate the normal silhouettes on CXR (basically the heart and diaphragm borders)
Appreciate the silouette sign in practice
What does the CXR shown indicate ?
The CXR on the right shows right lower lobe (RLL) pneumonia. Note that as expected the pneumonia obscures the silhouette of the right diaphragm but not the heart.
Localise the lung disease shown
- The inferior margin of an opacified right upper lobe is often sharply defined as shown below, due to the horizontal fissure along its inferior margin. This is a useful way of localising RUL disease on a CXR.
- Conversely, the superior margin of an opacified right middle lobe is often sharply defined by the horizontal fissure
Believe this one shows RUL disease
What is the correct positioning of a central venous line ?
Correct central venous line tip position is within the SVC, just proximal to the right atrium. (blue arrow)
What is the correct positioning of a NG tube ?
Correct NG tube tip position is below the diaphragm, projected within the stomach. (green arrow)
What is wrong with the positioning of the line shown in the CXR?
This right subclavian venous line has entered the internal jugular vein.
What is wrong with the positioning of the line shown in the CXR?
This nasogastric tube has passed into the right main bronchus.
What is wrong with the positioning of the line shown in the CXR?
This right pneumothorax would not resolve. CXR showed it was because the intercostal chest drain lay outside the pleural space.
