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Student Corner: Peritonsillar Abscess

July 7, 2015

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Peritonsillar abscess (PTA) is one of the most common head and neck infections that is diagnosed in the emergency department. The common presenting symptoms are a muffled/altered voice, throat pain, fever and odynophagia. A non-contrast CT image of a  particularly severe example of a PTA is shown below.

PTA2

 

The next horizontal cut image is below, with red arrows to highlight the abscess.

 

PTA1 with arrows

One of the more striking aspects of the image is the large degree of airway compression, with the maximum measured diameter of the airway being 2cm. Also, the first image shows that the abscess has two distinct “pockets” that eventually coalesce.

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To backtrack, this particular patient initially presented with symptoms of fever, chills, dysphagia, dysphonia and trismus. On examination, there were thin tonsillar exudates, erythema and deviation of the uvula. A diagnosis of peritonsillar abscess was made without imaging and the patient underwent incision and drainage, given antibiotics and discharge. The above images were taken after the patient returned to the ED several days later with continued, worsening symptoms.

The options for imaging of a soft tissue infection of the head and neck include CT and ultrasound. In the ED setting, ultrasound is becoming more and more utilized as the preferred imaging modality. However, this patient received a CT because they failed therapy. CT is superior to ultrasound in differentiation between peritonsillar abscess and other infections of the oral cavity and pharynx. It also allows clinicians to determine the degree of airway compromise. Other indications for CT imaging in suspected peritonsillar abscess include: uncertain diagnosis, obstructed view through physical exam or suspicion of an associated infection such as peritonsillar cellulitis.

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Overall, peritonsillar abscess is one of the most common soft tissue infection of the head and neck that is encountered in the emergency department. Most of the time, the diagnosis is clinical. Ultrasound is the preferred imaging modality, but CT is useful in a variety of situations as well.

References:

Powell, J. and Wilson, J.A. (2012), An evidence-based review of peritonsillar abscess. Clinical Otolaryngology, 37: 136–145.
Author:  Jaymin Patel
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Student Corner: CT Evaluation of Appendicitis

April 9, 2015

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Appendicitis is commonly encountered in the ER and is the leading cause of surgical emergency in the abdomen. The initial evaluation for a presentation that is concerning for appendicitis often includes history taking and exam, supplemented by labs. The Alvarado Score is a 10 point rating scale that is widely used as a tool to help decide whether or not a patient presenting with abdominal pain requires CT imaging (although it’s overall clinical usefulness is controversial). It is outlined here by MDCalc. According to the rule, a score of greater than 4 warrants CT evaluation and greater than 7 requires immediate surgical consult.  CT scan is a highly sensitive and specific tool in diagnosing appendicitis, however it comes with radiation, cost, and sometimes IV contrast risks.  In the pediatric patient population radiation from CT scans are not as desirable as the long-term consequences have theoretical potential to be deleterious (long discussion…for another post maybe!).

The purpose of this article is to go over characteristics of appendicitis that can be seen on a CT scan. The use of contrast is a long debated point of contention amongst the emergency medicine community and the usual practice varies between institutions. Medscape has a great rundown of the issue here, which notes that the use of contrast may be more beneficial in circumstances where appendicitis is a relatively less likely diagnosis because the contrast better helps characterize other possibilities.  Contrast studies are also more helpful in the patient not expected to have a large amount of intraperitoneal fat.

As usual, it is important to understand the local anatomy when analyzing radiological images of the abdomen. The image below is an example of an axial cut, non-contrast abdominal CT of a patient who came in with abdominal pain concerning for appendicitis. Try to identify the following structures: vertebrae, psoas major, IVC, iliac arteries, small bowel, colon and appendix.

Appy

And below is a labeled version of the same image:

Appendicitis labeled

Key: Blue arrow = bowel gas, ascending colon; Green arrows = small bowel; Purple arrows = L and R Iliac arteries; Yellow arrow = IVC; Red arrow = inflamed appendix

This image contains several signs that indicate that the appendix is inflamed. They include:

  • Diameter greater than 6mm–this usually implies the the appendix has either been twisted or blocked off from the cecum by an appendicolith, which causes inflammation
  • Periappendiceal fat stranding–seen as distinct lines that radiate out from the appendix in the image above, it is caused by inflammation of the appendix causes fluid accumulation around the wall of the appendix which turns the normally hypodense surrounding fat into a hyperdense area; note that the visceral fat around the appendix on the L side of the image looks much different than the visceral fat on the other side of the image
  • Appendiceal wall thickening–normally the wall of the appendix is thin and barely noticeable, but this image shows that the wall is generally thickened and may even be slightly more hyperdense than expected (more below)

Other signs that aid in the diagnosis of appendicitis include:

  • Appendiceal wall enhancement–the wall of the appendix becomes slightly more hyperdense when you compare it to the wall of any other loop of bowel, which is again a product of inflammation; note that this finding is usually more evident on contrast-enhanced CT
  • Abscess–the colon has a large reservoir of commensal bacteria, which can grow and wall off into an abscess if they are trapped in the appendix
  • Appendicolith–a calcified mass that is hyperdense on CT which can be an obstruction between the cecum and the appendix

Overall, CT has a high degree of sensitivity and specificity when used to evaluate the possibility of appendicitis. The clues outlined above, especially when seen together and as a part of a larger clinical picture that fits with appendicitis, are instrumental in confirming the diagnosis.

References:

Ohle R, O’Reilly F, O’Brien KK, Fahey T, Dimitrov BD. The Alvarado score for predicting acute appendicitis: a systematic review.BMC Med. 2011 Dec 28;9:139. doi: 10.1186/1741-7015-9-139. Review. PubMed PMID: 22204638; PubMed Central PMCID: PMC3299622.

Reich B, Zalut T, Weiner SG. An international evaluation of ultrasound vs. computed tomography in the diagnosis of appendicitis.Int J Emerg Med. 2011 Oct 29;4:68. doi: 10.1186/1865-1380-4-68. PubMed PMID: 22035447; PubMed Central PMCID: PMC3215954.

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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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