How to read an abdominal CT
Computed tomography (CT or CAT scan) is one of the most commonly used medical imaging procedures in clinical practice, along with radiography (x-ray) and magnetic resonance imaging (MRI). When the pathological process in the abdominal cavity is suspected, the x-ray and CT scans are the methods of choice because they are fast, cheap, widely available, non-invasive and they can be performed without special preparations of the patient.
CT is an imaging technique that uses x-ray beams to produce images that depict the tissues in two and three dimensions based on their density. The density of the structures is expressed in the Hounsfield unit (HUs) and it reflects the level of absorption of x-ray beams by certain structures in the body. It ranges from +1000 (hyperdense), 0 (isodense), to -1000 (hypodense).
|Definition||Computed tomography is a computerized, x-ray based, imaging procedure that generates cross-sectional images or 'slices' of the body|
|Mechanism||The mechanism is based on a quickly rotating narrow beam of x-rays directed towards a patient that produces signals that are processed by the machine's software.|
Hyperdense - bright structures (usually bones)
Hypodense - dark structures (usually air)
Isodense - gray structures (fluids)
This article will explain how to read an abdominal CT scan on the concrete examples of the CT images of the abdomen.
- Orientation and approach
- Abdominal CT anatomy
Orientation and approach
Before getting into the detailed description of the structures that are visible on the abdominal CT, it is important to know how to orientate with a CT scan. The axial scans (cross-sectional images) are most commonly used in clinical practice, but when needed, the CT can produce other views (sagittal or coronal). For axial scans, you can imagine that you're looking at the person through their feet while facing the person directly (your left is the right side on the scanned patient).
The CT machine provides a series of these cross-sectional scans that can be rendered into a 3D image. The CT machine also allows you to 'window' different structures on the scan. Windowing refers to the contrast adjustment in order to change the appearance of the picture to highlight particular structures. In order to examine the abdominal structures, you can switch between the 'bone window' and 'soft tissue window', depending on which structures we want to observe. To get the full picture of abdominal anatomy, the examiner needs to move through the slices from cranial (proximal) to caudal (distal) while examining structures from their beginning to end.
When examining the abdomen through the CT scan it is important to use a systematic approach. This means that the examiner needs to be focused on one organ at the time and that every abdominal organ needs to be evaluated individually and thoroughly. Additionally, when a patient comes with a specific clinical presentation (i.e. bone fracture) the radiologist should not focus only on one structure, but rather on the entire scan. These techniques minimize the possibility of missing out on small structural changes and incidental findings that the patient may have. One of the recommended approaches includes the following steps:
- Examine anatomical borders of the region.
- Identify the scan level with anatomical landmarks; diaphragmatic vault (T10-T11), celiac trunk (T12), superior mesenteric artery (L1), renal arteries (L2-L3) and aortic bifurcation (L4).
- Analyze the peritoneal cavity and retroperitoneal space.
- Review the organs by dividing them into hollow and solid.
Abdominal CT anatomy
The examiners usually start their CT scan examination by looking at the most lateral structures and then proceeding towards the midline. When looking at the abdomen, we should start by looking at the outer line which represents the skin and subcutaneous tissue of the abdominal wall. Medial to this line, we can identify the two groups of muscles of the abdominal wall: anterolateral and posterior muscles. In the anterolateral compartment, the three muscle layers (external oblique, internal oblique and transversus abdominis muscles) can be identified together with the rectus abdominis muscle and its sheath. The posterior group includes the latissimus dorsi, psoas major, iliacus, erector spinae, quadratus lumborum and psoas minor muscles. These muscles are embedded around the hyperdensity of the lumbar vertebra.
If the integrity of the abdominal wall is preserved the outer line should be entirely smooth.
Anteriorly, at the midline, the linea alba can be seen as a thin band that holds both halves of the rectus abdominis muscle together. If there is a widening of the gap between the rectus abdominis halves, then think about whether it’s a hernia or a simple stretching of the linea alba (diastasis recti). A special focus should be directed towards the rectus sheath; blunt trauma can cause inferior epigastric arteries tearing and bleeding. As there is no posterior layer of the rectus sheath below the arcuate line, the blood easily gets in contact with the peritoneum and can cause peritonitis.
After the abdominal wall, the examination should be continued by looking at the 'bone window' that highlights the bony structures. When looking at the proximal abdomen, the ribs on both sides can be spotted. More importantly for this region, the lumbar vertebrae and the corresponding vertebral spinous processes should be examined in detail. The right positioning of the vertebrae at the midline and their smooth hyperdense appearance on the scan indicates that their anatomy is intact. Any changes in these features can indicate bone pathology (fractures, dislocations, osteolytic and osteoblastic lesions).
Occasionally, the thoracic structures can be spotted on the abdominal CT (e.g. heart and the base of the lungs). This is mainly because the radiologists need to make sure to examine the superiormost aspects of the abdominal cavity. When looking at the peritoneal cavity, the examination should start with the bases of the lungs and proceed downward by looking through the 'lung window'. While skimming down the peritoneal cavity the examiner should pay close attention to the abnormal collections of air, especially around the hollow organs. In case of the presence of extensive darkness suppressing the organs against the abdominal wall, the pneumoperitoneum should be suspected. Next, the peritoneal formations in the abdomen (greater and lesser omenta, mesentery and recesses) should be identified. The greater omentum extends from the greater curvature of the stomach and duodenum to the transverse colon, while the lesser omentum extends between the stomach and the liver. The mesenteries are located between the intestines and the abdominal wall. They can be divided into the mesentery proper (for small intestine), transverse mesocolon (for transverse colon), sigmoid mesocolon (for the sigmoid). In the abdominal cavity there are several peritoneal recesses (folds of peritoneum) located around the duodenum. These include the superior and inferior duodenal, retroduodenal, paraduodenal, duodenojejunal and mesentericoparietal recesses.
Further, the intraperitoneal spaces should be examined for the presence of fluids and solid masses. Proceed by defining the retroperitoneal space. Anatomically, it’s the space between the peritoneum and transversalis fascia that covers the muscles of the posterior abdominal wall. Normally, peritoneum nor the fascia are visible on CT. Therefore, focus on the mentioned muscles to define the posterior retroperitoneal border, and for the approximate anterior border use the anterior kidney surface (renal fascia).
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When examining the solid organs, it is of great importance to check their size, shape, homogeneity and abnormal areas of density. Look for an increase in organ size or abnormal shape. Such disturbances can occur due to hemorrhage, edema, mass effect of tumor or hematoma, displacements of vessels or adjacent organs. Additionally, the contrast agents can be applied to check the blood flow and thus evaluate the blood supply of the organs. Lack of perfusion leads to irreversible tissue death (necrosis, infarction).
The liver is usually the first explored solid organ as it is the most prominent structure on the abdominal CT. The normal density of the liver is usually slightly brighter than muscles and spleen. The parenchyma of the liver should be solid and homogeneous. If the intrahepatic biliary ducts are visible without contrast agents, that’s always a pathological finding (i.e. inflammation of the ducts - cholangitis). Extrahepatic ducts are normally visible and should be inspected for dilation and obstruction. To examine the hepatic and portal vessels the contrast needs to be applied. With this, the hypervascularized lesions (i.e. tumors) can be discovered.
The gallbladder is filled with liquid (bile) and is hypodense compared to the liver. We should pay attention to the presence of gallstones, which can appear as hyperdense masses (like bone).
The most important feature of the spleen on the CT scan, is the shape of the organ and integrity of its capsule. The size of the spleen can vary among individuals, so the certain sign of enlargement is not the organ's size, but instead the shape of the spleen. The normal spleen has a concave visceral surface; when enlarged, this surface inverts and becomes convex.
The pancreas and suprarenal glands are often analysed together, as they lie in the same transverse plane. The borders of the pancreas should be clear; bumpy borders or intraparenchymal masses may indicate tumors or acute pancreatitis. Examine pancreatic tissue for any signs of solid, liquid, or cystic masses. Then focus on pancreatic duct which is normally thin and hypodense and look for duct dilation or obstruction. Check if the suprarenal glands contain irregular masses which may represent various tumors (e.g. pheochromocytoma).
The kidneys appear elliptical in shape. You’ll notice the hyperdense pyelocalyx system (renal sinus) surrounded by hypodense kidney parenchyma. Check if there are hyperdense signals from the renal sinus indicating the presence of kidney stones.
There is no way to differentiate the cortex and medulla except if you’re using contrast to examine the flow through the renal blood vessels. This way you’ll clearly see a highly vascularized cortex (90% of renal blood supply) or any other hyper-supplied mass (i.e. tumors).
The methods of choice of the examination of the hollow organs are usually the endoscopy and radiography with barium contrast. The CT scan can also be indicated in some cases that are hardly visible on the previously mentioned methods. Some of these indications include: inflammatory processes (i.e. appendicitis) and intramural lesions (e.g. ulcers, polyps).
The hollow organs of the abdomen include the stomach and intestines. While analyzing the stomach and intestines, focus on the following features;
- The walls and lumens of every part of the tube.
- The patency of the lumen. If any mass obstructs the intestine, the intestines will appear dilated proximal to the mass and collapse distal to it.
- The distribution of the loops of the small intestine. The change in the distribution of the loops can indicate certain abnormalities, such as torsion, incarceration and invaginations.
- The integrity of the large intestine haustra. Shape of haustra can be altered in many diseases, such as ulcer colitis (complete loss - lead pipe sign), Chrone’s disease (segmental ulcerations) and colorectal carcinoma (apple core sign).
Finally, through CT scans the major blood vessels of the abdomen can be examined. These vessels usually include the abdominal aorta and/or inferior vena cava (IVC). The abdominal aorta runs retroperitoneally, anterior to the vertebral bodies, from the aortic orifice on the diaphragm (T12) to the L4 vertebra. It terminates by dividing into two terminal branches (left and right common iliac arteries). The inferior vena cava is located on the right side of the aorta. It runs from the confluence of the common iliac veins (L5) to the inferior vena cava foramen of the diaphragm (T8).
Primarily, we should check if their diameters are normal. Dilation of the vessels indicates an abdominal aorta aneurysm (urgent medical condition) or IVC thrombosis.