Thursday, July 30, 2009

How To Read A Chest X-Ray

Let's say you are trekking in the Himalayas and suddenly become short of breath. Even though you are on your way to Annapurna Base Camp in a remote corner of the world, you would most likely be taken to a local clinic and have a plain film chest x-ray taken of your chest to help determine the etiology of your dyspnea, just like in any other part of the world. This post is not a comprehensive account of how to read a chest x-ray, but rather a collection of tips and tricks that should help one read most plain chest x-ray films.


The Plain Film Chest X-Ray: PA And Lateral Views

While there are many possible views that can be taken of the chest, the two most common are the PA view and the lateral view. The PA view, or Posterior-Anterior view, positions the beam at the patient's back. The radiation then travels through the spine, lungs, heart, and finally skin until it meets the detector or film anterior to the patient. The advantage of a PA film over an AP, or anterior-posterior, view is a reduction in the enlargement of the cardiac silhouette. Since the heart is closer to the film, its shadow is less distorted.

The PA view is often taken simultaneously with a lateral view. The benefit of having two views is that the radiologist is better able to visualize abnormalities in 3-D space with two views. Otherwise, there is a chance that two abnormalities can be in the same sagittal plane, yielding only one shadow on the PA view. The lateral view would distinguish between these two lesions. In other words, the PA view tells you where things are, medial to lateral, while the lateral view tells you where things are in an anterior to posterior sense.


How To Read A Plain Film Chest X-Ray

There is no right or wrong way to read a chest x-ray, but it is beneficial to develop a systematic method of doing so. The first thing to always check is that the film is associated with the correct patient. After doing so, to read a PA view, I utilize a mnemonic called RIP ABCDEFGH. Here is how it works:

Rotation: Check to see that the patient is not rotated. You can look at the clavicles and make sure the vertebral processes line up nicely in between them.
Inspiration: Check to see that you can see about 9 ribs on each side. Less than 8? It is likely poor inspiration.
Penetration: You should be able to see lucencies in the middle of the film representing the intervertebral discs. If there are none, the film is over-penetrated; if they are too well-defined, the film is under-penetrated.

Airway: Trace the lucency from the neck down towards the carina. It should be midline and you should be able to see two bronchi splitting from it.
Bones: Look at the shoulder joint and trace out each rib contour to check for fractures or other abnormalities.
Cardiac Silhouette: Check the right and left heart borders.
Diaphragms: These should be well-defined with no obscuration of their margins.
Empty Space =)
Fields: Look at the lung fields bilaterally and compare. Don't forget the apices.
Gastric Bubble: Check for a lucency in the left upper abdominal quadrant.
Hardware: Make sure the placement of any lines or other hardware is appropriate.

Pretty simple, no? After going through this process several times, it becomes routines. You can then use the lateral view to confirm your findings. Develop a process for evaluating lateral films as well, but in general, they are used to confirm what is seen on the PA view. While simple, learning how to read a chest x-ray is a basic skill all radiologists should know well before moving on to reading more advanced imaging, even in the remote reaches of Nepal =)

For patients, there are several advantages to using x-rays when possible, including lower radiation doses, faster acquisition times, and lower total cost. Often, the cost of x-rays will be covered by the patient's health insurance provider. This can be a significant issue, especially for patients who do not have great access to care.

Saturday, July 18, 2009

How A CT Abdomen/Pelvis Is Performed

CT Scan of the Abdomen (Source: Wikipedia)

The patient was having nonspecific abdominal pain that had not resolved after several days in the hospital. On the suggestion of her kidney doctors, a computed tomography (CT) scan of the abdomen and pelvis with contrast was ordered. The patient desired the exam in order to find out the etiology (origin) of her pain.

When ordering any imaging study, CTs in particular, it is important to consider several things beforehand:
  1. Is this study necessary?
  2. Is this study appropriate?
  3. What are the risks of this study?
  4. What are the benefits of the study?
  5. Will the study reveal new information that will affect the management of the patient?
  6. For hospitalized patients, does this study need to be performed in-house, or can it done on an outpatient basis?
In the case of this patient, the answers to these questions were as follows:
  1. Yes, to determine the etiology of her pain, which could have potentially been serious.
  2. One could argue about whether an ultrasound should have been performed first, but a CT provides a great deal more information, so the CT was preferred.
  3. The major risks include radiation and contrast-induced nephropathy. The risk of radiation is development of a future cancer, but this is tied to cumulative radiation exposure, so one CT by itself is not a huge harm. Contrast-induced nephropathy is potentially quite serious, but in this patient, her kidney function was so depressed that there was not much kidney function to preserve to begin with.
  4. The benefits would be to rule out any potentially urgent, addressable causes of her pain.
  5. Yes, potentially (see #4)
  6. In practical terms, it made sense to do this study as an inpatient, versus having the patient come back for a separate study.
After the order was sent by the primary team, the radiologist has to look over the order to ensure that it is appropriate and then protocol it. The protocol is the specific routine that the scanner will run while acquiring the images. After approval, the patient was taken down to the CT scanner room. Contrast was infused into her via an intravenous (IV) line. Since the area of interest was any soft tissue mass causing the pain, the scanner was programmed to wait until the contrast had entered the venous phase after perfusing through the arteries, roughly 80 seconds after the infusion began. At this point, the scanner spun into action, using its radiation beam and detectors to create the typical cross-sectional images of a CT scan. The scanner only scanned the patient's abdomen and pelvis in order to ensure a limited dose of radiation. After the scan, the patient was returned to her room. The images were sent to the radiologist's Picture Archiving and Communication System (PACS) machine, where she was able to interpret the images minutes after the scan had been taken.

Fortunately for the patient, the scan was negative and she was discharged home. The contrast would be dialyzed off at a later date.