CT Abdomen - Read Like A Radiologist

Answers - Densities & scan phases

Question 2a. Scroll through the CT abdomen examination (Fig. 2), in which 6 circles have been placed on various structures.
For each circle, note: (1) the estimated HU value, (2) the tissue type, (3) an example of where else you might encounter this tissue type. Use Figure 1 as an aid.

Clinical significance: Recognizing densities is the basis of CT interpretation. A hypodense structure is therefore not always fluid — it can also be fat or parenchymal tissue (HU measurement is then essential).

Important:
The HU values in this overview are average reference values that can vary somewhat in practice. This depends on patient-related factors and on technical aspects such as contrast dose, scan timing, and reconstruction parameters. Small differences can also occur between CT scanners. Therefore, use these values as a guideline, not as an absolute limit.

 

Question 2b
Why is it clinically relevant to differentiate between free fluid and fat?
Think of two clinical scenarios in which this distinction determines the diagnosis and management.

Answer / discussion
Fatty tissue is normally present in the abdomen: mesenteric fat, omentum, and retroperitoneal fat all have relatively low HU values (~–80 to –100) and appear hypodense on CT. Free fluid in the abdominal cavity has an HU value around 0 and, compared with fat, appears slightly less hypodense — free fluid can collect in various parts of the abdominal cavity (around the liver and spleen, in the pouch of Douglas, and in the paracolic gutters).
The distinction is clinically relevant because a cause must always be sought for free fluid, whereas normal fatty tissue requires no intervention. Free fluid (HU ~0) suggests ascites, bile leakage, urine leakage, or chyle — depending on the context.
Scenario 1: polytrauma patient with a 'dark zone' adjacent to the spleen. Is it normal fat or free fluid? At HU ~0–20, it is free fluid — raising suspicion of intra-abdominal bleeding or organ rupture. Immediate action is required.
Scenario 2: patient with liver cirrhosis and a 'dark rim' around the organs. HU measurement ~0–10 confirms ascites — free fluid due to portal hypertension. No acute intervention is needed, but the underlying cause should be treated.

Question 3.
Look at the four CT phases.
a. Which scan phase do you see here? What features do you recognize it by?
b. Note 3 aspects/findings that you can assess much better on a CT examination with intravenous contrast.
c. Provide a clinical question for each phase for which you specifically need that phase.
 

Answer question 3b.

• Organ enhancement — parenchymal organs such as the liver, spleen, kidneys, and pancreas enhance after contrast, allowing you to assess internal structure; abnormalities (lesions, infarcts, necrosis) become visible as areas that enhance abnormally or fail to enhance.
• Vasculature — blood vessels such as the aorta, mesenteric vessels, and portal vein — becomes clearly visible only after contrast, allowing you to assess patency, diameter, and wall abnormalities
• Various pathological processes — inflammation/abscesses and tumors often enhance due to increased blood flow. In active arterial bleeding, contrast extravasation is visible as hyperdense contrast spreading outside the vessel lumen.

Answer question 3c.
Examples of indications per scan phase:

• Non-contrast (native): nephrolithiasis and urolithiasis (outside the abdomen, including fractures and brain hemorrhage). Distinguishing tissue that is inherently high density (e.g., blood, with HU 40-50) from enhancement (to prevent it from being incorrectly interpreted as enhancing tissue).
• Arterial phase: hypervascular tumors/metastases (e.g., HCC, neuroendocrine tumors), active arterial bleeding, abdominal aortic aneurysm (AAA), aortic dissection.
• Portal venous phase: assessment of liver parenchyma (e.g., liver metastases, liver cirrhosis), portal vein thrombosis, bowel wall enhancement (in suspected bowel ischemia), infection/inflammation (abscess formation, lymphadenopathy).
• Late/excretory phase: pathology in the ureter/bladder, enhancement pattern of liver lesions (e.g., washout in HCC)