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Student Corner: How to Read a Head CT

November 24, 2014

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Head imaging is both a crucial tool in acute medical care, particularly in the setting of trauma, and a very daunting aspect of learning radiology for students. However, as is the case with many clinical skills, a “systematic approach” goes a long way in helping ease the initial challenge of learning how to read and understand head imaging. For this post, we will focus primarily on head CTs because they are more commonly used in emergency departments due to the fact that they are fast, readily available, and highly informative in trauma.

A head CT presents a few unique challenges. The anatomy is subtle and nuanced. The area has numerous pathological possibilities. The pathologies themselves can change over short time periods. There are different types of fluids and soft tissues. In short, the brain is kind of scary.

But, the best way to get over your fears is to face them. And therefore the best way to look at a head CT is to look at it with a plan. The plan in this case is a (surprise!) mnemonic: Blood Can Be Very Bad” and it is detailed below.

Blood

Hemorrhage of blood into the cranial vault is one of the easier things to identify on head CT. Acute hemorrhage is hyperdense (bright) and becomes hypodense (dark) as time goes on. Two of the most commonly encountered types are subdural hematoma and epidural hematoma. Subdural hematomas arise from the bridging veins and are seen as crescent shaped anomalies at the periphery of the cranial vault. Epidural hematomas arise from the middle meningeal artery and are lentiform or lens-shaped because their expansion in limited by suture lines (the dura attaches to the cranium at the suture lines).

Other types of hemorrhage include:

Interparenchymal hemorrhage–can either be traumatic or non-traumatic, occur in the brain matter itself

Interventricular hemorrhage–seen as hyperdense fluid in the ventricles, which are usually black because they are filled with hypodense CSF, can be secondary to other types of hemorrhage or trauma

Subarachnoid hemorrhage–most often due to aneurysm rupture and presents with very acute headache (thunderclap headache), seen as fluid in the subarachnoid spaces.  Subarachnoid is also very common in trauma.

The image below is an example of subdural hemorrhage. The left side of the cranial vault is filled with hyperdense fluid, indicating that this process is acute. Also, note the midline shift that occurs, which is shown by the compression of the ventricles more so on the patient’s left than the right and the movement of brain tissue over to the patient’s right. There is also some extracranial soft tissue swelling on the patient’s left, indicating a possible traumatic process. Extracranial soft tissue swelling can help guide your eyes, so to speak, when looking for pathology.

SDH with midline shift 1

Cisterns

Cisterns are spaces between the pia and subarachnoid meningeal layers that can be filled with CSF. There are numerous cisterns that can be identified on a head CT, but the major ones that you should be familiar with are outlined here on Radiopaedia.

These cisterns can be used to identify increased intracranial pressure or subarachnoid hemorrhage (detailed above). In the setting of increased ICP, these spaces become compressed. In subarachnoid hemorrhage, there is hyperdense blood inside them instead of hypodense CSF.

Brain

The brain tissue itself is composed primarily of grey matter and white matter. You can see the difference between these two types of tissue because grey matter is more dense and therefore appears more bright on CT. The gyri and sulci can also be visualized and they should be generally symmetric.

The pathologies that can be identified in the brain parenchyma include:

Abscesses–areas of focal infection from bacteria or fungi, often seen as round areas of ring-enhancing hypodensity with associated edema; midline shift is also a possible finding depending on the size of the lesion.

Tumors–areas of abnormal growth whose particular appearance is variable depending on type and location; midline shift is also a possible finding depending on the size of the lesion; particularly well visualized on contrast-enhanced CT because the blood-brain barrier is disrupted during tumor development and growth, which allows the contrast to leak into the tumor and make it bright.

Infarction–when the blood supply is cut off from brain tissue it causes swelling (which can result in midline shift) and the area becomes hypodense and loses grey-white differentiation.

The CT image below shows a few interesting things. The most obvious one is the multiple hyperdensities seen in the brain matter. These lesions are most likely calcified and can represent anything from inflammatory reactions to infections to tumors. The other finding is that the gyri are thin and the space between them is much more evident than normal, which represents atrophy of the brain due to old age, dementia or both.

Multiple calcifications 1

Ventricles

For the sake of brevity, we will not go over the normal anatomy of the ventricular system. The key radiological aspects of the ventricles in the brain are their size and symmetry. They are filled with hypodense CSF and their size can increase due to hydrocephalus, or increased accumulation of CSF. Hydrocephalus is either communicating (obstruction at the arachnoid granulations which function to resorb the CSF) or non-communicating (obstruction at any point in the ventricular system, usually at the foramina which connect the different ventricles.

Symmetry comes into play when there is a mass lesion on one side of the brain, which can cause compression of one of the lateral ventricles with or without midline shift.

One other aspect to keep in mind is that enlargement of the ventricles can be due to atrophy of the brain parenchyma itself, a condition known as “hydrocephalus ex-vacuo”. Therefore if the ventricles do indeed look large, the brain parenchyma should be examined, paying close attention to signs of atrophy. If the ventricles are enlarged and the brain matter looks compressed and the sulci lose their normal wavy pattern (a process called “effacement”), hydrocephalus is more likely.

Bone

Skull fractures are a common finding in head trauma and they can be seen on head CT. Fractures are seen as dark lines in the usually bright bones. They must be distinguished from suture lines, which are seen as symmetrical wavy lines across bones. Basilar skull fractures are harder to identify, as the base of the skull has multiple different areas and bones. Radiopaedia has a great example of this here.

One of the things to keep in mind with fractures of the skull is to follow the fracture lines. Fractures often cross into different bones and, especially when looking at the base of the skull, fracture lines can extend much further than you would expect.

The image below shows a painfully obvious frontal sinus fracture, where the the bone fragments actually protrudes back into the brain tissue itself. This view is slightly different from the other images on this post because it is shown in the “bone window”, which is a type of image processing that highlights the hyperdense bones on a CT. It makes fractures much easier to identify (although I’m not quite sure you needed the special window to see this one).

CT head trauma2

 

 

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All in all, it is also helpful to keep a few other concepts in mind.

Symmetry is key in identifying pathologies, since irregularities in the tissues or fluids are almost never symmetrical.

Utilize the bone window, even if you don’t suspect a fracture.

Soft tissue swelling on the outside of the cranial cavity itself can help you identify the principal point of impact in traumatic injuries and help you find underlying pathologies.

Always use a systematic approach because otherwise it is pretty easy to miss subtle pathology.

Hope this was helpful to you all, but don’t take this as a complete manual of how to read a head CT. Always corroborate your reads with a more experienced physician and always attempt to read the image on your own before looking at any published interpretations. Ask other people about tips and tricks that they might have. And finally, read as many as you can!

Author: Jaymin Patel

References/Resources:

University of Virginia tutorial– http://www.med-ed.virginia.edu/courses/rad/headct/

Elsevier Health, How to Read a CT Scan- http://www.elsevierhealth.com.au/media/us/samplechapters/9781416028727/Chapter%2069.pdf

Agrawal A. How to read a CT scan of a patient with traumatic brain injury?. NMJ. 2013; 2(1): 02-11.

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Student Corner: How to Read a Chest X-Ray

August 25, 2014

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In these “Student Corner” pieces, we will go over certain aspects of radiology in EM that are of interest to medical students. Topics will include: common (and interesting) case presentations with accompanying imaging, schematics for how to read different types of imaging in various anatomical locations, discussions on what types of imaging to order and when in the EM setting, and others.

In this inaugural edition of the Student Corner, we’ll take a look at how to tackle reading an anterior-posterior chest x-ray.

For starters, it is important to understand that having a “gameplan” for reading any type of image is key when you first start out trying to decipher radiological images. As a reader and interpreter, you must be systematic in your thought process as you analyze the image in front of you. For chest x-rays, there is a classic schematic: ABCDE. Any medical student will tell you that this is not the only time you will see “ABC…” used as a way to quickly memorize something, but at least it’s easy to remember.

Here’s the image we are going to use and let’s start to dissect it using the mnemonic:

Note: For the purpose of keeping this a short piece, we’ll only focus on the anterior-posterior view only.

CXR UL pna Airway

A-Airway

Legend: Red Arrows–trachea; Blue Arrows–carina; Green arrows–L and R main bronchus

The upper airway, including the trachea, carina and both main bronchi, should all be visible on an AP view. Things to look for include deviation of the trachea away from the midline (there is some deviation to the patient’s right in this image, but this is due to the aortic arch, which passes to the left of the trachea as it passes posteriorly in the mediastinum), obstruction due to aspiration of a foreign object and obscuring of the upper airway due to enlarged mediastinal lymph nodes.

Let’s explore tracheal deviation a bit further. Deviation from the midline is not associated with a defect in the trachea itself, but with a force from either the R or L side of the chest cavity that is pulling or pushing the trachea to one side or the other. For example, introduction of air into one side of the chest cavity will cause that lung to collapse due to the loss of negative intrapleural pressure. The collapsed lung will shrink to the size of a ball and “push” the trachea to the opposite side. You can think of the two lungs like bungee cords that put roughly equal force on the trachea in each direction. If one of the cords snaps or is released from where it is attached to, the cord that is still intact will pull the trachea towards one side, resulting in a deviation that will show up on a CXR.

B-Bones

CXR example Bones

Legend: Numbers–ribs; Red Arrow–clavicle; Blue Arrow–medial border of scapula

A CXR offers a good view to look for rib fractures and clavicle fractures. Clavicular fractures are usually easy to spot, as they usually reveal distinct fracture lines in the middle 3rd of the clavicle. Hairline fractures are less common. Rib fractures are sometimes hard to spot, but each rib should be followed across it’s length to look for fracture lines or step-offs (disruptions in the normal curve of the rib) that could indicate a fracture.

The number of ribs is also important to assess because it is an indirect measurement of the volume of the chest cavity. Hyperinflated lungs are usually the result of obstructive disease where the patient is unable to fully expel the air that is inhaled with every breath they take–this increase in residual volume will build up over time and overinflate the chest cavity. This overinflation will result in a greater-than-normal number of ribs being visible on an AP view. Normally, you should expect to see 8-10 ribs on an upright chest X-ray, depending on whether the patient was instructed to exhale or inhale before the picture was taken.

C-Cardiac

CXR Cardiac

 

Legend: Red Dashed Lines–heart borders

This part of the mnemonic involves the heart and surrounding structures. The silhouette of the heart should be identified and the heart borders should be clear. A general rule of thumb is that the heart base should not be wider than 1/2 the total width of the diaphragm. As with a lot of “general rule of thumb”s in medicine, it’s not quite clear whether this has any diagnostic value–for example, if the heart base is indeed 1/2 the width of the diaphragm on CXR, is that really sensitive for cardiomegaly? In any case, it’s something to keep in mind.

The aortic arch and the L pulmonary artery should be visible as two semi-circles above the left atrium. There is a space called the “AP Window” that has the following borders: ascending aortic arch (anterior), descending aortic arch (posterior), L pulmonary artery (inferior), inferior border of aortic arch (superior). The window should be “concave” in the sense that the lateral border should be caved in medially. If it is not, things like mediastinal lymphadenopathy and aorta/pulmonary artery aneurysms are possible.

D-Diaphragm

CXR Diaphragm

 

Legend: Blue Arrow–gastric air bubble; Red Arrow–costophrenic angle

The diaphragm has 3 major characteristics which you look for on CXR. One is the gastric air bubble, which allows you to identify that the stomach is on the left (as opposed to the right, as in situs inversus). Another is the contour of the diaphragm, which should be a “dome” shape. The right side should be a little higher than the left, thanks to the liver. The third is perhaps the most important: the costophrenic angle. It is the lateral point of attachment for the diaphragm and it should be a sharp, triangle-shaped region at either end. The angle should be acute. If the angle is closer to 90 degrees, then one possible explanation is that the lungs are hyperexpanded (perhaps because of COPD) and pushing the diaphragm down into the abdomen. “blunting” of the angle refers to a radio-opaque marking of the angle that usually is indicative of pleural effusion.

E-Everything Else

Everything else is…everything else. Mostly this means the lung parenchyma itself. For this, asymmetry is key. Compare left and right and see whether there is a difference. More on this particular section of the read later.

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Now you should try to read the above x-ray for yourself and type your own version of the read in the comments if you’d like. If not the entire read, then try to identify the pathology in the x-ray and post your answer in the comments. Any questions/comments would also be appreciated.

I’ll post the answer with the “correct” read a bit later on the site.

Author: Jaymin Patel

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