This is the third video in this series on interpreting chest x-rays, and the topic is the assessment of an x-ray's technical quality. As you might guess, the learning objectives are first to be able to assess the technical quality of a chest X-ray, and second to understand how specific reductions in quality impact accuracy of an X-ray's interpretation. The first thing to discuss is a quick comparison of films produced by the PA and AP views.
In short, the superiority of PA over AP films is almost solely due to improved technical quality. To understand why this is, all we need to do is to take a quick look at once again at how they are taken. The PA film is taken of an ambulatory patient who can be easily positioned however necessary and is done in the controlled setting of the radiology department.
In contrast, the AP film is taken of a sick patient in his or her hospital room. The patient may or may not be able to be positioned in an optimal way due to illness or immobility and may not even be cooperative. There may be lines and tubes that the radiology technician needs to work around, maybe an attached IV pole or overlying telemetry leads.
There may not be enough room for the x-ray machine to be properly spaced and positioned. Overall, AP films are taken in an uncontrolled environment, at least uncontrolled from the standpoint of the technician. As a consequence, the quality of an AP film is almost always inferior to that of a PA film taken of the same patient.
Therefore, this video will be predominantly relevant to AP films. So what are the specific factors affecting technical quality? They are rotation of the patient, inadequate inspiration, and suboptimal penetration. I'll review each one at a time. What are the ways in which the patient can be rotated relative to the x-ray beam or film plate?
Like all three-dimensional objects, patients can be rotated in three different ways. each around one axis. The patient can be rotated side to side, as if he or she is leaning to a side.
The patient can be pitched forward or backwards, or he or she can be turned such that one side is displaced forward while the other side is displaced backwards. Each one of these rotational displacements leaves evidence on the x-ray and also impacts the reliability of the x-ray interpretation. The easiest form of rotation to spot and the one that has the smallest impact on technical quality is that around what I've labeled the x-axis here.
Be aware that these specific axis designations are only my own convention and are not standardized. As you might imagine, the consequence of rotation around this axis is that the patient looks crooked on the film. In general, this type has minimal impact on the accuracy of interpretation. However, there are two consequences. First, one or both of the costospheric angles may not be visible.
And second, the gastric air bubble and intraperitoneal free air, if present in the patient, may not be visible. The second type of possible rotation, which is around what I've labeled the Y-axis, isn't usually a consequence of the patient being rotated relative to both the beam and the film plate, but is rather a consequence of the x-ray beam being rotated relative to both the patient and the film plate. That is, instead of the beam of x-ray photons being directed perpendicularly to the frontal plane of the patient's chest, they are directed upwards.
Although not common, when this happens it can be due to either human error in judging the angle when the x-ray source is positioned, it can also be due to constraints with the patient and the room. For example, if a patient is not clinically stable enough to be sat completely upright, but a piece of equipment suspended from the ceiling prevents the x-ray source from being positioned above the patient. To understand how the x-ray is affected by this, we need to review the relationship between the clavicle and the lung apex. Here in green is the outline of the lung as seen from the side. You usually cannot actually see the clavicles on lateral film because of the superimposed arms, but here in purple is the approximate location of their medial ends.
We'll put an undeveloped film within the x-ray plate behind the patient, and then fire some x-rays. As I'll discuss in a future video, the x-rays aren't truly parallel as this picture suggests, but they're close enough to parallel for this demonstration. If the shadow of the medial end of the clavicle is projected straight back, we would obviously expect it to fall here on the film.
But notice that a tiny portion of the lung apex actually extends superiorly above the level of the clavicle. Therefore, when we look at a normal AP film, once we've identified the location of the clavicles, we can see a small portion of the lung apices above them. This is normal. But what happens if the x-rays are not directed perpendicularly to the patient and the film? In this case, shadow produced by the clavicle and that produced by the lung apex line up differently.
So when we look at the consequent AP film and identify the clavicles and the lung apices, we can see that neither lung apex is visible above the medial clavicle. You may have noticed that I've placed the term AP view in quotes because this is not truly an AP view, but rather is something called a lordotic view, or occasionally called an apical lordotic view. There are occasions in which this type of view is actually desirable, for example when trying to identify lung pathology that would normally be obscured by the clavicle shadow, however in the hospitalized patient, this type of view is more often unintentional.
What are the consequences of a lordotic view? As I mentioned, there is an improved view of the lung apices, but there is a diminished view of the bases and costaphragmic angles, there is distortion of the cardiac silhouette, and the lung volumes appear falsely low. If I return to the previous lordotic film, hopefully you can appreciate the marked difference in the appearance of the cardiac silhouette and lung bases. as well as the fact that the overall lung volumes appear quite low. It would be relatively easy for an inexperienced clinician to make erroneous diagnoses based on this film.
Returning to our patient's room, the last type of rotation is that around the Z-axis. Unless otherwise specified, when a clinician or technologist refers to rotation on a chest film, this is almost always the type of rotation to which he or she is referring. In this case, we need to review the relationship between the clavicle and vertebral spinous processes.
Here's a schematic of an axial cross-section through the upper thorax with anterior on top and posterior below. In this case, I'll place the x-ray plate in front for a PA film, although the following principle applies just the same to AP films. We'll have our nearly parallel x-rays coming in. Consider these three shadows, or more specifically one shadow caused by the spinous process of the vertebral body and the two medial edges of the clavicle.
We can take the PA film that's produced here and zoom in a little. Let's identify the clavicles and then the spinous processes. You'll notice that a line drawn that connects the spinous processes bisects the distance between the medial clavicular ends.
Therefore, we know that the patient was not rotated around the z-axis. What happens if the patient is rotated in such a way? Let's indicate again where the shadows and shadow edges get cast.
On the consequent x-ray, identify the clavicles, then the spinous processes, and we see that the processes no longer bisect the distance between clavicles. Therefore, this patient is rotated. As a rule, the spinous processes will be closer to the clavicle on the side that is rotated forward. So what is the consequence of z-axis rotation?
The size and shape of the cardiac silhouette, mediastinum, and or hilum may all be distorted. The next factor that can affect technical quality is an inadequate inspiration. In a patient with normal lung volumes, on a chest x-ray taken during full inspiration, 9 to 10 posterior ribs should be visible.
Let's count them in this example. Remember that the posterior ribs are the ones that are roughly horizontal and are usually easier to see. Here's the first rib, which is normally much smaller than the others and angled a bit differently, which is why it looks different on the x-ray, and is the only rib in which the anterior end is typically easier to identify. Then the second rib, then the third, and the fourth, and so on, all the way down until we reach number 10. Since there are 10 ribs showing here, we would say that this person has given adequate inspiratory effort. Although less commonly done, some clinicians assess inspiration using the anterior ribs.
In this case, 6 to 7 of them should be visible, with the 7th appearing to pierce the diaphragm. Here are the patient's anterior ribs. You might wonder why it's important for the patient to have a full inspiration during the x-ray. To understand this, Let's compare two films. Here is the first.
We count the posterior ribs and see eight of them. The technologist was concerned that the patient had not fully inspired, so appropriately repeated the film. And here's the repeat in the same patient.
Let's count posterior ribs again, and now we see 10. Notice how dramatically different these two films appear. The consequences of inadequate inspiration are that the lung volumes appear falsely low, the lung markings appear falsely prominent, which can give the false appearance of pulmonary edema, and the cardiac silhouette and mediastinum may appear falsely enlarged. Take a look at those two films again to see if you can appreciate those changes in the one on the left. One important side note is that while it is perfectly appropriate to refer to the film on the right as demonstrating a good inspiratory effort, I advise you to avoid referring to the film on the left as a poor inspiratory effort without having another good quality film in the same patient against which to compare it.
That's because a variety of lung diseases can result in low lung volumes which can have the identical appearance to a poor inspiratory effort. I remember a patient from my residency who once had several years of serial chest x-rays, every one of which had been interpreted as poor inspiratory effort, before he was ultimately diagnosed with restrictive lung disease based on pulmonary function testing. Had those x-rays instead been interpreted as low lung volumes, which would have been more appropriate, the diagnosis may have been made sooner.
The final issue that can diminish the technical quality of a film is suboptimal penetration. If you recall from the first video in this series, I discussed how exposure duration can impact image brightness on a film. I simplified things a bit for that introduction because it's not just the duration of an exposure that impacts the image, there are other physical factors that determine whether a film has a relatively low exposure or relatively high one.
In addition to the duration, There is also the energy of the photons in the X-ray beam, as well as the source-to-image distance. The radiology technician alters these factors using three parameters at the time of image acquisition. Milliamp seconds, abbreviated MAS, which controls the total quantity of photons produced.
Kilovolt peak, abbreviated KVP, which indirectly controls the energy of the photons produced. And the source-to-image distance, abbreviated SID, which controls the fraction of produced photons which actually strike the film. The greater the distance, the greater the number of photons that will miss the target. For PA films, the standard distance is 6 feet, but for portable films this may need to be adjusted depending upon the situation.
The specific combination of parameters will result in a specific amount of brightness, sometimes referred to as optical density, and the amount of contrast. The technologist has a good deal of control in varying these parameters in different situations, such as extreme body habitus or an unusual SID. However, in general, they should be kept as consistent as possible from one exam to the next for the same patient. Otherwise, clinicians would not be able to tell if changes that occurred from one x-ray to the next were due to an actual change in clinical status, such as increasing or decreasing pulmonary edema, or were due to difference in radiographic technique. When it comes to assessing a film's exposure, in practice almost all people who are neither radiologists nor radiology technicians use the terms exposure and penetration interchangeably in a technically imprecise manner to describe a film's contrast and or overall brightness.
Here's a good quality film where the specific combination of brightness and contrast allow for the best balance of identifying various types of chest pathology. Here is the same patient in which the film is too bright. And finally, in this case the overall brightness is technically normal, but the film is too much contrast, so the heart looks too bright, and the lungs look too dark. The imperfections in the middle and right films are frequently referred to as underexposed or underpenetrated, though both terms are better applied to the left image than the right.
For digitally processed images, Excessive brightness can occasionally be corrected when viewing the image, but improvements with contrast problems are usually very limited since critical detail was not captured at the time of image acquisition. Is there a way to assess penetration that is more precise than gestalt? There is. Let's focus in on the good quality film. The penetration of a PA or AP film is considered good when the outlines of the thoracic vertebral bodies are just barely visible behind the heart.
I find it easiest to look for this by focusing on the intervertebral spaces. And here they are. The consequences of suboptimal penetration.
Excessive brightness leads to falsely prominent pulmonary markings, while diminished brightness leads to falsely diminished pulmonary markings. And either excessive or diminished contrast causes all kinds of fine or subtle details to be lost. This most significantly causes falsely diminished pulmonary markings and obscuring of both pulmonary nodules and pneumothoraces.
In my last minute, I'll summarize how to assess technical quality. First, Look for rotation around the three axises. That includes ensuring the patient is not crooked on the film, that the lung apices are visible above the clavicles, and that the vertebral spinous processes bisect the distance between the medial ends of the clavicles. Second, check for adequate inspiration.
9 to 10 posterior ribs should be visible. Finally, check exposure or penetration by identifying the thoracic vertebrae behind the That's the end of this video on how to assess the technical quality of a chest x-ray. In the next video in this series, I'll be shifting to discussion of specific pathologies, starting with the airways, bones, and soft tissues.