The Ins and Outs of Inflammation

It’s Friday night. After a long work week, you are ready to blow off some steam with your crew at the local TGI Fridays. But before you can even dig into some cheesy mozzarella sticks, you get a sharp pain in your abdomen, bad enough that you nearly drop your 60oz margarita. You keep on hoping the pain will ease off, but it worsens, and before long it’s clear you need to go to the emergency room to find out what is wrong.

grimacing man
“The Vexed Man.” Getty museum, Los Angeles.

In the emergency room, the news does not get any better. You have a fever and labs show an increase in your white blood count: a sign of possible infection or inflammation. The ER doctor orders a CT of your abdomen and pelvis, and the interpreting radiologist confirms the suspicion of acute appendicitis. Then the bank calls: they are repossessing your house! You wish you still had that margarita.

But how does a radiologist come to such a conclusion when interpreting a CT scan? What tells the radiologist that a given body part is the likely source of pain?

Inflammation across the nation

An imaging feature that often directs radiologists to the location of the problem is inflammation. All of us have experienced the effects of inflammation at some point in our lives. The affected area becomes warm, swollen, red, and painful: the so-called four cardinal signs of inflammation. These signs were first grouped together by Roman encyclopedist Celsus around 25 AD, at which time they were known by their Latin names calor (warm), tumor (swollen), dolor (painful), and rubor (red). I like to imagine Celsus and his buddies having some epic, inflammation-themed rap battles using these rhyming terms.

Inflammation is your body’s response to infection or tissue injury, with the goals of repairing damaged tissue and eliminating the underlying cause, if one is present. During an inflammatory reaction, white blood cells and other substances in the blood are directed to the site of concern. This is accomplished in part by dilating adjacent blood vessels and making those vessels “leak,” so that fluid—containing armies of cells and proteins—can directly infiltrate around the affected site.

And now to the imaging

As inflammation increases, we can begin to visualize it on imaging. Fluid leaks into the surrounding tissues and can be seen with multiple imaging modalities, including CT, MRI and ultrasound. The appearance of inflammation differs by modality, but it generally follows the imaging characteristics of fluid. I will focus on CT for two related reasons: first, it is often performed in the emergency department in the setting of acute illness; as a result of this, CT frequently captures images of acute inflammation. CT of the abdomen and pelvis gives us particularly clear and classic examples.

Fat is your friend

Inflammation can occur anywhere, but depending on the adjacent anatomy and tissue structure, it can be more or less evident on imaging. In the abdomen and pelvis, all of us have peritoneal fat—fat which surrounds our abdominal organs—which works to our advantage when looking for inflammation. The typically dark fat becomes brighter on a CT image as fluid infiltrates the small spaces between fat tissue.

inflammation around kidney
Figure 1. CT image at the level of the kidneys. Branching linear structures surrounding the left kidney represent fluid infiltrating the fat planes, and evidence of inflammation. This patient had an obstructing kidney stone (not shown).

Inflammation is nonspecific—i.e. it does not always tell us the precise cause—but it points us toward the problem area. Depending on the tissue or organ involved, additional signs of inflammation may be present. For example, inflammation in the bowel or appendix is often accompanied by thickened bowel walls. Some inflammatory processes show little or no infiltration of the adjacent fat, often because nearby fat is separated from the inflammatory process by some anatomic structure. Infection in the liver, brain, or spine often exhibit little inflammation in the adjacent fat.

In conclusion, I’d like to present a short pictorial essay illustrating some CT manifestations of inflammation.

Inflammation: Friend and Foe of the Radiologist

CT of acute pancreatitis
“Impression: Pancreatitis”
CT image through the upper abdomen with inflammatory changes adjacent to the pancreas in this patient with acute pancreatitis.
CT of acute diverticulitis of the colon
“A Sunday Afternoon of Diverticulitis”
CT image of the upper pelvis shows inflammation adjacent to the colon. Numerous diverticula—small pouches extending from the colon, secondary to areas of colon wall weakness—are present in this region. When diverticula become inflamed or infected, diverticulitis ensues.
CT of terminal ileitis
“Girl with a Terminal Ileitis”
Reconstructed coronal (from the front) CT image of the lower abdomen and pelvis shows inflammation in the right lower abdomen near the very last part of the small bowel: the terminal ileum. We can also see wall thickening and enhancement of the inner mucosal lining, additional evidence of inflammation.
CT of cellulitis
“Whistler’s Mother’s Cellulitis”
CT image of the upper thighs in a patient with cellulitis. Cellulitis is infection of the skin and the tissues immediately beneath it. In most parts of the body, a layer of fat immediately beneath the skin becomes inflamed.

Now you can begin to understand the “signposts” that point the radiologist toward inflammation. Questions? Comments? Please post below!

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