A CT scan of the abdomen shows detailed images of the abdominal organs using X-rays. A CT scan creates a 2D cross-sectional “slice” at one location within the body. These slices are then collected at multiple contiguous locations and combined digitally into a detailed 3D image. A CT scan is used to locate blood collections and tumours, identify bone or muscle diseases, examine organs, diagnose specific illnesses, and monitor treatment.
What are the coronal, sagittal, and axial planes?
- Coronal or frontal view presents images from the front to the back of your body.
- Sagittal or side view provides lateral images from the left to the right sides of the body.
- Axial or transverse view creates images from the top to the bottom of the body.
What is Contrast Enhanced CT (CECT) ?
Some CT scan examinations require a contrast agent.
Literally, contrast is the ability to distinguish two objects.
In medical imaging, it allows adjacent substances or tissue to be distinguishable and visualized.
Thus, it is the ability to distinguish target tissue from the surrounding structures.
A contrast agent a liquid compound of iodine.
It works by making structures with contrast appear whiter.
The addition of IV contrast media increases the density and thus attenuation of the blood with which it mixes.
In a CT scan with contrast, an abnormality may become more obvious either whiter (enhancing/hyperdense) or darker (hypodense) than the surrounding tissues.
There are three ways to administer a contrast material for a CT Scan:
- Intravenously (IV): The contrast agent is injected into the body to highlight areas of increased blood flow, like blood vessels or organs (e.g. liver, kidneys or spleen).
- Orally: The contrast agent is swallowed and helps CT scans better delineate areas of the esophagus, stomach and intestines.
- Rectally: The contrast agent is inserted per rectum to improve CT images in the large intestines and pelvic organs.
Intravenous contrast is the most commonly used contrast agent.
A CT scan with contrast allows multiphase imaging technique, which takes images at specific points in time, for instance as the contrast reaches particular blood vessels or moves through specific organs like kidneys and ureters.
Hounsfield scale/unit or CT Number in CT Scan
The Hounsfield scale is a quantitative scale used in CT (computed tomography) scans to describe the radiodensity of various tissues and materials. Named after Sir Godfrey Hounsfield, the scale measures the attenuation of X-rays by different substances.
The Hounsfield scale measures tissue radiodensity. It assigns a common scale to express how different tissues in your body absorb the radiation from the CT scan. This process of absorption is called attenuation.
Hounsfield units (HU) or CT number are numbers on a scale that express the radiodensity of different tissues and organs of the body, relative to water. Hence the water has been assigned a value of 0 (zero). The lower the number on the Hounsfield scale, the less dense the tissue is. The higher the number, the more dense the tissue is.
| Tissue/Organ | CT number (HU) |
|---|---|
| Bone | 300 – 1900 |
| Liver | 40 – 60 |
| White mater | -20 to -30 |
| Grey mater | -37 to -45 |
| Blood | 40 |
| Muscle | 10 – 40 |
| Kidney | 30 |
| CSF | 15 |
| Water | 0 |
| Fat | -50 to 100 |
| Air | -1000 |
Common abnormalities in a CT scan of the abdomen
Common findings in an abdominal CT scan can vary widely depending on the patient’s symptoms and medical history. Following are some frequently observed abnormalities organ-wise:
LIVER
Lesions: Such as cysts, hemangiomas, or tumors (e.g., hepatocellular carcinoma, metastasis).
Fatty Infiltration: This is indicated by a lower liver density compared to the spleen.
Cirrhosis: Characterized by a nodular liver surface and signs of portal hypertension.
GALLBLADDER and BILIARY SYSTEM
Gallstones: Appearing as hyperdense (bright) structures within the gallbladder.
Cholecystitis: Gallbladder inflammation, often with wall thickening and pericholecystic fluid.
PANCREAS
Pancreatitis: Inflammation, often with gland enlargement, peripancreatic fluid, and fat stranding.
Pancreatic Masses: Such as adenocarcinoma, which may cause ductal dilation.
SPLEEN
Splenomegaly: Enlargement of the spleen, that can be due to various causes like infections or hematologic disorders.
Splenic Lesions: Including cysts, infarcts, or tumors.
KIDNEYS and ADRENAL GLANDS
Renal Stones: Hyperdense structures within the renal pelvis or ureters.
Hydronephrosis: Dilation of the renal pelvis and calyces due to obstruction.
Renal Masses: Such as renal cell carcinoma.
Adrenal Lesions: Including adenomas or pheochromocytomas.
BOWEL
Bowel Obstruction: Dilated loops of the bowel with air-fluid levels.
Inflammatory Bowel Disease: Wall thickening, fat stranding, and complications like abscesses.
Appendicitis: Enlarged appendix with surrounding fat stranding and possibly an appendicolith.
BLOOD VESSELS
Aneurysms: Enlargement of the aorta or other major vessels.
Thrombosis: Blood clots within the vessels, such as the portal vein or inferior vena cava.
PERITONEAL CAVITY
Ascites: Free fluid within the peritoneal cavity.
Peritoneal Carcinomatosis: Multiple small nodules or masses on the peritoneal surfaces.
Thus, CT findings help diagnose a wide range of conditions and guide further management.
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