Gallstones are hardened deposits of digestive fluid that form within the gallbladder, a small organ located beneath the liver. The gallbladder stores and concentrates bile, a fluid produced by the liver that aids in fat digestion. When bile components crystallize and solidify, they create stones ranging in size from a grain of sand to a golf ball. When gallstones cause symptoms, typically sudden and rapidly intensifying pain in the upper right abdomen, medical imaging is necessary for diagnosis. A computed tomography (CT) scan uses X-rays and computer processing to create detailed cross-sectional images of the body’s internal structures.
How Gallstone Composition Affects CT Visibility
The success of a CT scan in detecting gallstones depends entirely on the stone’s chemical makeup. CT imaging measures how different tissues absorb X-rays, a property quantified by Hounsfield units (HU), which corresponds to tissue density. High-density objects, like bone, appear bright white on a CT scan, while low-density structures, like air, appear black.
The majority of gallstones (approximately 80%) are cholesterol stones, composed mainly of cholesterol monohydrate crystals. Because cholesterol is a soft, organic material, its density is often similar to the surrounding bile, making these stones “radiolucent” or isodense. These stones frequently blend in with the fluid, resulting in low detection sensitivity, sometimes missing up to 61% of stones seen on other imaging modalities.
If a gallstone contains sufficient calcium, however, it becomes radiopaque and easily visible on a CT scan. These calcified stones are typically pigment stones or mixed stones that have accumulated calcium over time. Calcium has a much higher density than bile, causing the stone to appear brightly white (hyperattenuating) on the CT image. If a physician relies solely on a CT scan, only these calcified stones will be reliably found, as they provide the necessary density contrast.
Sonography: The Standard for Gallstone Diagnosis
Due to CT’s limitations in reliably detecting cholesterol stones, abdominal ultrasound (sonography) is considered the initial imaging test of choice for suspected gallstones. Ultrasound uses high-frequency sound waves rather than radiation to create images, a method that is non-invasive and highly accessible. The technique is superior because its effectiveness is independent of the stone’s chemical composition or calcium content.
Ultrasound successfully identifies gallstones with a sensitivity greater than 95%, regardless of their composition. The core principle involves detecting the acoustic shadow that a gallstone casts. As sound waves hit the dense surface of the stone, they are completely reflected, creating a distinct, dark shadow immediately behind the stone on the image. This acoustic shadow confirms the presence of a stone, even if the stone is small or isodense with the bile. Ultrasound also offers the advantages of being more cost-effective and portable.
Identifying Complications with CT Imaging
Even though CT is relatively poor for detecting non-calcified gallstones, it plays an important role when complications are suspected. A CT scan is often ordered when a patient presents with non-specific abdominal pain and the initial diagnosis is uncertain. In these scenarios, the CT scan excels at providing a wider assessment of the entire abdomen and surrounding organs.
CT is effective for diagnosing consequences caused by gallstones, such as acute inflammation or infection of the gallbladder wall (cholecystitis). It can reveal associated findings, including pericholecystic fluid and thickening of the gallbladder wall. The cross-sectional view is also better for identifying complications like gallstone pancreatitis, which occurs when a stone obstructs the pancreatic duct, or for detecting a gallbladder perforation. In these complex cases, CT assesses the extent of the damage and helps rule out other potential causes of abdominal pain.