What Does Diverticulitis Look Like on a CT Scan?

Diverticulitis on a CT scan shows up as a combination of bowel wall thickening, hazy streaking in the surrounding fat, and visible inflamed pouches (diverticula) along the colon wall. These findings are so characteristic that CT achieves roughly 99% sensitivity and specificity for the diagnosis in experienced hands.¹ But the scan does more than confirm that you have diverticulitis. It also grades how severe it is, flags dangerous complications like abscesses or perforations, and helps doctors decide whether you need antibiotics at home or emergency surgery.

The Three Hallmark Signs

Radiologists look for a cluster of findings rather than a single telltale feature. When all three hallmarks appear together in the right clinical context, the diagnosis is straightforward.

The first and most sensitive sign is thickening of the colon wall. Normally, the colon wall is just a few millimeters thick on CT. In diverticulitis, the inflamed segment swells, and the wall becomes visibly thicker. This finding alone has a sensitivity around 96%.² The thickening tends to be smooth and symmetric rather than lumpy or irregular, which turns out to be an important clue when distinguishing diverticulitis from colon cancer.

The second hallmark is pericolonic fat stranding. This is the hazy, cloudy look in the fat tissue surrounding the inflamed segment of colon. Fat normally appears very dark on CT because it is low-density. When inflammation spreads from the colon wall into the neighboring fat, the fat becomes streaky and lighter in appearance. In diverticulitis, this fat stranding is often “disproportionate,” meaning the amount of haziness in the fat looks more dramatic than the degree of wall thickening would suggest.³ That pattern of more stranding than wall thickening helps point toward diverticulitis rather than other causes of abdominal pain.⁴

The third hallmark is the presence of visible diverticula themselves, the small outpouchings of the colon wall. In one classic study, diverticula were seen on CT in 84% of confirmed diverticulitis cases.⁵ Sometimes an individual diverticulum looks inflamed, appearing as a small pouch with a thickened, enhancing wall or containing a tiny, bright fecalith (a hardened pellet of stool). The presence of both inflamed and non-inflamed diverticula near the thickened bowel segment makes the radiologist’s job easier, because it anchors the diagnosis firmly in diverticular disease.

Uncomplicated Versus Complicated Disease

CT does not just say “diverticulitis yes or no.” It also tells your doctor how bad it is. The distinction between uncomplicated and complicated diverticulitis drives almost every treatment decision that follows, and CT is the tool that makes that distinction.

Uncomplicated diverticulitis is the milder form. On CT, it looks like wall thickening plus fat stranding, with no abscess, no free air outside the bowel, and no fistula connecting the colon to another organ. This category accounts for the majority of cases and is typically treated with rest and sometimes antibiotics at home.⁶

Complicated diverticulitis adds one or more worrisome findings to the picture. Radiologists and surgeons often grade the severity using a classification system called the modified Hinchey classification, which assigns a stage based on what the CT shows. The stages range from localized inflammation confined to the area around the colon up through large abscesses, free perforation, and widespread peritonitis. Higher stages on this scale are strongly linked to needing surgery. In one study, patients with a modified Hinchey score of Ib or higher ended up in the operating room far more often than those with lower scores.⁷ CT-based staging systems like this one, along with the WSES (World Society of Emergency Surgery) classification, are the most commonly used tools for grading diverticulitis severity.⁸

What Complications Look Like on the Scan

When diverticulitis is complicated, specific CT findings tell your doctor exactly what has gone wrong. Each complication has its own appearance, and each one steers treatment differently.

Abscesses

An abscess shows up as a fluid-filled pocket, usually near the inflamed colon but sometimes at a distance in the pelvis or elsewhere in the abdomen. The collection typically has a distinct wall and may contain air bubbles. In one early landmark study, pericolic abscesses appeared in about 35% of confirmed diverticulitis cases, with distant abscesses in another 12%.⁵ The size and location of the abscess on CT directly influence whether a radiologist can drain it by threading a small catheter through the skin under CT guidance, or whether surgery is needed instead.⁹

Perforation and Free Air

Perforation means the colon wall has developed a hole, allowing air or bowel contents to leak into the abdominal cavity. On CT, the telltale sign is extraluminal air, which shows up as tiny bright-white pockets or bubbles sitting outside the bowel, often against the dark background of surrounding tissue. Small amounts of air confined close to the colon wall may be managed without surgery in select patients, but larger amounts of free air floating throughout the peritoneal cavity usually demand urgent operation. In one study of patients with perforated diverticulitis and extraluminal or free air, about a third with free air were successfully managed without surgery, but those who needed operation within 24 hours were more likely to have widespread free intraperitoneal air combined with free fluid.¹⁰ The CT findings associated with surgical management also include the presence of pericolonic fluid collections and bowel obstruction.⁷

Fistulas

A fistula is an abnormal tunnel that forms between the inflamed colon and an adjacent organ, most commonly the bladder (a colovesical fistula). On CT, signs of a fistula include air inside the bladder when there has been no catheter placed, thickening of the bladder wall next to the inflamed colon, and sometimes a visible tract connecting the two structures. Fistulas are present in roughly 14% of diverticulitis cases evaluated by CT.⁵ CT has proven accurate at both detecting and localizing colovesical fistulas, providing useful information for planning surgery.¹¹ That said, fistulas can occasionally be subtle. In one reported case, a colovesical fistula was not identified until a delayed repeat CT scan with rectal contrast was performed days after admission.¹²

Bowel Obstruction

Diverticulitis can cause bowel obstruction through several mechanisms: small bowel loops sticking to the inflamed area, scarring and fibrosis around the colon, or compression by an abscess. On CT, obstruction appears as dilated loops of bowel upstream from the inflamed segment, with a visible transition point where the normal caliber suddenly narrows. Wall thickening in obstruction from diverticulitis tends to be symmetric, moderate, and extend over a longer segment compared to obstruction caused by a tumor.¹³

Telling Diverticulitis Apart From Colon Cancer

This is the question that keeps both radiologists and patients up at night. Diverticulitis and colon cancer can look similar on CT: both cause wall thickening, both occur in the same part of the colon (the sigmoid, in left-sided cases), and both can obstruct the bowel. Misreading one as the other has real consequences in both directions.

Several CT features help separate the two. Diverticulitis tends to involve a longer segment of colon (often over 10 cm), while colon cancer typically creates a shorter, more abrupt area of thickening. Pericolonic inflammation and fat stranding are far more prominent in diverticulitis. In contrast, colon cancer is more likely to show pericolonic lymph nodes and a discrete luminal mass.¹⁴ The presence of nearby non-inflamed diverticula is another helpful clue favoring diverticulitis, as is a mild and homogeneous pattern of wall enhancement rather than the more irregular enhancement seen with tumors.¹⁵

Lymph nodes deserve special attention. Enlarged lymph nodes near the colon show up far more often in cancer than in diverticulitis. In one study, nodes were visible in about 71% of cancer cases but only 15% of diverticulitis cases, making their presence a moderately useful indicator of malignancy.¹⁶ However, the nodes themselves were not significantly different in size between the two groups, so the radiologist is looking for whether nodes are there at all, not how big they are.

When bowel obstruction is present, the odds tilt toward cancer rather than diverticulitis.¹⁵ This is one reason many doctors recommend a follow-up colonoscopy after an episode of diverticulitis, particularly when the CT findings were at all ambiguous or complicated.

Other Conditions That Mimic Diverticulitis on CT

Colon cancer is not the only impostor. Epiploic appendagitis, a condition in which a small fatty tag on the outside of the colon becomes inflamed or loses its blood supply, can produce left-sided abdominal pain and pericolonic fat changes that closely resemble sigmoid diverticulitis on initial clinical assessment.¹⁷ The distinction matters because epiploic appendagitis is self-limiting and never needs surgery or antibiotics.

On CT, the difference is usually clear to an experienced reader. Epiploic appendagitis typically shows a small, oval fatty mass with a distinctive bright ring around it, sitting just outside the colon wall. About 89% of left-sided cases display this “hyperattenuated ring sign.”¹⁸ The colon wall itself stays normal in thickness, and there is no thickened diverticulum. In patients with known diverticulosis who have recurrent pain, recognizing epiploic appendagitis on CT can prevent unnecessary escalation of treatment.¹⁹

Right-Sided Diverticulitis

Most discussion of diverticulitis centers on the left side of the colon, especially the sigmoid, because that is where diverticular disease is most common in Western populations. But diverticulitis can also develop on the right side, particularly in younger patients and in East Asian populations where right-sided diverticula are more prevalent. On CT, right-sided diverticulitis shows the same hallmarks of wall thickening and pericolonic inflammation as left-sided disease.²⁰ The diagnostic challenge is that right-sided diverticulitis can mimic appendicitis, since both affect the right lower abdomen and both produce fat stranding in the same neighborhood. CT is especially valuable in this scenario because it can show the inflamed diverticulum directly on the cecum or ascending colon, rather than on the appendix, preventing unnecessary appendectomy.

Does the Scan Need Contrast?

CT scans for suspected diverticulitis can be done with or without intravenous contrast, and sometimes with rectal contrast as well. The question of which protocol to use has practical implications for speed, cost, and what the scan can detect.

For the basic diagnosis of diverticulitis, a non-contrast CT performs well. A recent multireader study found that non-contrast CT was statistically non-inferior to contrast-enhanced CT for diagnosing diverticulitis, with accuracy around 90%.²¹ This matters in the emergency department, where giving IV contrast adds time and carries a small risk of allergic reaction or kidney strain. However, that same study found that sensitivity for detecting perforation and abscess was meaningfully lower without contrast. So if the clinical suspicion is high for a complicated case, contrast-enhanced CT is the stronger choice.

Rectal contrast, where a water-soluble liquid is administered into the rectum before scanning, has been debated for years. One study concluded that adding positive rectal contrast did not significantly improve diagnostic accuracy for diverticular disease overall, suggesting it can be skipped in many patients.²² That said, rectal contrast can occasionally help identify subtle fistulas or contained perforations that would otherwise be missed.

Radiation dose is another consideration, especially for younger patients who may experience multiple flares over a lifetime. Research suggests that reducing the radiation dose to half the standard level still preserves high diagnostic accuracy and strong agreement between readers.²³ Low-dose CT protocols achieved sensitivity of 95–99% for diagnosing diverticulitis, though their ability to detect complications was somewhat lower.²⁴ For a young person with typical symptoms of uncomplicated diverticulitis, a low-dose non-contrast scan may be the most reasonable trade-off between diagnostic confidence and cumulative radiation exposure.

What Happens After the CT Diagnosis

Once diverticulitis has been diagnosed on CT, you may wonder whether a colonoscopy is still necessary. The traditional recommendation has been to scope the colon about six weeks after recovery to rule out an underlying colon cancer that might have been hidden by the acute inflammation. But the evidence supporting routine colonoscopy in everyone has weakened.

One study found that routine colonoscopy after CT-diagnosed diverticulitis did not appear necessary, and a selective approach targeting patients with a prolonged clinical course or worrisome features like a mass or obstruction on CT made more sense.²⁵ More recent data supports this nuance: the overall risk of a missed colorectal cancer is low for most diverticulitis patients, but patients with complicated diverticulitis and those who are not up to date on cancer screening are the exception and should still get a colonoscopy.²⁶

CT colonography, sometimes called a “virtual colonoscopy,” has also been studied as a follow-up option. In one head-to-head comparison, CT colonography actually found diverticular disease in a slightly higher percentage of patients than standard colonoscopy, patients overwhelmingly found it less painful and less uncomfortable, and about three-quarters who stated a preference chose CT colonography over conventional colonoscopy.²⁷ It does not replace standard colonoscopy for all purposes (biopsies cannot be taken during CT colonography), but for mapping the extent of diverticular disease after a flare, it is a reasonable and better-tolerated alternative.

CT Angiography for Diverticular Bleeding

Diverticular bleeding is a different clinical scenario from diverticulitis. A diverticulum can bleed without being inflamed, and the resulting hemorrhage can be massive. Standard CT for diverticulitis is not designed to catch active bleeding, but CT angiography, which uses a carefully timed contrast injection to image blood vessels during the moment contrast is flowing, can show the exact site of hemorrhage as a bright blush of contrast leaking into the bowel lumen.²⁸

Not every patient with diverticular bleeding will have active extravasation at the time of the scan, so a negative CT angiogram does not mean nothing is wrong. Researchers have looked for features on the scan that predict which patients will show active bleeding when they undergo a subsequent catheter-based angiography procedure. One group found that a wider inferior vena cava on the CT angiogram predicted a positive finding at mesenteric angiography, likely because it indicates greater blood loss and intravascular volume changes.²⁹

Artificial Intelligence Reading CT Scans for Diverticulitis

AI-assisted reading of CT scans is an active area of research, and diverticulitis is one of the conditions where early results are promising. The main challenge AI is being trained on is the same one that gives human radiologists pause: telling diverticulitis apart from colon cancer when the CT appearances overlap.

A deep learning algorithm developed and validated on CT images achieved a sensitivity of about 83% and specificity of about 87% for distinguishing colon cancer from acute diverticulitis, which was comparable to the performance of board-certified radiologists. More interesting was what happened when the AI was used as a support tool alongside human readers. The combined performance improved significantly, with sensitivity rising from roughly 78% to 86% and specificity from about 82% to 91%, along with meaningful drops in both false-positive and false-negative readings.³⁰ The AI is not replacing the radiologist; it is catching cases that humans might second-guess and vice versa.

Separate work has focused on using AI not just for diagnosis but for severity grading, building a pipeline that segments the relevant anatomy on CT, localizes the disease, and classifies it as mild, moderate, or severe.³¹ Automated severity classification could eventually standardize how diverticulitis is graded across hospitals and reduce variability between individual radiologists, particularly in settings where emergency scans are read by less experienced practitioners overnight.