Medical School · Year 1 · Anatomy Thorax Abdomen · includes a quiz and discussion video
Lecture 14: Abdominal Imaging
Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen
Learning Objectives
By the end of this lecture, students will be able to:
- Describe imaging modalities used for abdominal structures
- Identify normal anatomical structures on abdominal radiographs
- Recognize normal anatomy on CT and MRI of the abdomen
- Identify key anatomical landmarks on ultrasound
- Apply systematic approaches to abdominal image interpretation
- Correlate imaging findings with common pathologies
Imaging Modalities for the Abdomen
Modern abdominal imaging encompasses several complementary modalities, each with distinct strengths and limitations. Plain radiographs (X-rays) remain valuable for detecting bowel obstruction, free intraperitoneal air, and calcifications, though they provide poor soft tissue detail. Computed tomography (CT) serves as the workhorse for most abdominal pathology and trauma evaluation, offering excellent anatomical detail at the cost of radiation exposure and potential contrast-related risks. Magnetic resonance imaging (MRI) provides superior soft tissue contrast for liver, pancreas, biliary, and pelvic organ evaluation, but requires longer acquisition times, higher costs, and is contraindicated in patients with certain metallic implants. Ultrasound excels at evaluating the gallbladder, kidneys, aorta, and obstetric conditions without radiation exposure, though image quality depends on operator skill and can be limited by bowel gas or patient habitus. Fluoroscopy permits dynamic evaluation of gastrointestinal motility through contrast studies, though its use has declined with advances in other modalities.
Selecting the appropriate imaging modality requires matching the clinical question to the strengths of each technique. For acute abdominal presentations, CT typically provides the most comprehensive evaluation. Right upper quadrant pain suggestive of biliary disease warrants ultrasound as the initial study. Suspected renal colic calls for non-contrast CT, which detects stones with high sensitivity. Characterization of liver lesions benefits from MRI's superior tissue contrast. During pregnancy, ultrasound serves as the primary modality, with MRI available when additional imaging is necessary.
<image>Panel A: Conventional radiograph of the right upper abdomen with the liver appearing as a homogeneous gray silhouette against the darker lung field above and air-filled bowel below. Panel B: Axial CT slice showing the liver in shades of gray with portal vessels visible as branching structures that appear darker when unenhanced or brighter when contrast-enhanced. Panel C: MRI image with detailed soft tissue contrast allowing differentiation between liver parenchyma, vessels, and adjacent structures. Panel D: Ultrasound image with the liver appearing as a medium-gray homogeneous structure with scattered echogenic dots representing portal triads.</image>
Abdominal Radiograph Interpretation
Standard Views and Technique
The plain abdominal radiograph, despite its limitations, remains a rapid and accessible initial study for many presentations. Standard views include the supine (AP) projection for routine abdominal assessment, the upright (erect) view to evaluate air-fluid levels and free air beneath the diaphragm, and the left lateral decubitus position as an alternative for detecting free air when the patient cannot stand. The upright view proves particularly valuable because free intraperitoneal air rises to collect beneath the diaphragm, where it appears as a crescent of lucency separating the liver from the hemidiaphragm.
Systematic Approach
A disciplined systematic approach prevents overlooking important findings. Evaluation begins with the gas pattern, noting that gastric gas normally resides in the left upper quadrant, small bowel gas appears centrally with visible valvulae conniventes (complete mucosal folds that traverse the entire bowel width), and colonic gas occupies the periphery with haustra (incomplete folds). Solid organ assessment includes the liver shadow in the right upper quadrant, the spleen in the left upper quadrant (often not visible on normal studies), and the kidneys outlined by adjacent retroperitoneal fat with visible psoas shadows. Skeletal structures including the spine, pelvis, and lower ribs should be examined for abnormalities. Soft tissue evaluation encompasses the psoas muscle margins, flank fat stripes, and bladder shadow. Finally, abnormal calcifications may indicate kidney stones, gallstones (though only 10-15% are sufficiently calcified to be radiographically visible), vascular calcification, or pancreatic calcification from chronic pancreatitis.
<image>Panel A: Normal supine abdominal radiograph showing the stomach gas bubble as a dark lucent area in the left upper quadrant beneath the left hemidiaphragm. Panel B: Small bowel loops containing gas in the central abdomen with thin white lines representing valvulae conniventes crossing the full bowel diameter and large bowel gas outlining the colonic frame peripherally with haustra as incomplete indentations. Panel C: The liver as a homogeneous gray density in the right upper quadrant with both kidneys faintly outlined as ovoid soft tissue densities in the retroperitoneum. Panel D: The psoas muscle shadows as linear densities extending from the lumbar spine bordering the kidneys medially with the vertebral column running vertically through the center.</image>
Abnormal Radiographic Findings
Bowel Obstruction
Small bowel obstruction produces a characteristic pattern of dilated proximal loops (exceeding 3 cm in diameter) with visible valvulae conniventes, central distribution, and absence of distal colonic gas. The upright view demonstrates multiple air-fluid levels at different heights within the same loop, a finding highly suggestive of mechanical obstruction. Large bowel obstruction manifests as colonic dilation exceeding 6 cm (or cecal diameter exceeding 9 cm, which raises concern for impending perforation), with haustra visible along the dilated segments in a peripheral distribution. The transition point, where dilated bowel meets decompressed bowel, may sometimes be identified.
Pneumoperitoneum and Calcifications
Free intraperitoneal air (pneumoperitoneum) indicates perforation of a hollow viscus and constitutes a surgical emergency. This finding is best appreciated on the upright chest or abdominal radiograph, where air collects beneath the right hemidiaphragm as a crescent-shaped lucency. The left lateral decubitus position provides an alternative when the patient cannot stand, with air visible along the lateral abdominal wall. Calcifications on abdominal radiograph may localize to the right upper quadrant (calcified gallstones), flanks (renal stones), pelvis (bladder stones or benign phleboliths), vascular structures (atherosclerosis or aortic aneurysm), or the pancreatic region (chronic pancreatitis).
<image>Panel A: Small bowel obstruction with multiple dilated central loops exceeding 3 cm in diameter and valvulae conniventes visible as complete transverse lines. Panel B: Accompanying upright view demonstrating multiple air-fluid levels at staggered heights within the obstructed bowel. Panel C: Pneumoperitoneum on an upright view with a crescentic collection of free air appearing black beneath the right hemidiaphragm clearly separating the liver shadow from the diaphragm. Panel D: Various calcification patterns including a staghorn calculus outlining the renal collecting system, calcified gallstones as small rounded opacities in the right upper quadrant, and curvilinear vascular calcification outlining an ectatic abdominal aorta.</image>
CT Imaging Principles
Technical Considerations
Computed tomography provides excellent cross-sectional anatomical detail and serves as the primary imaging modality for most acute abdominal presentations. Intravenous contrast enhancement dramatically improves soft tissue differentiation and vascular visualization. Different contrast phases optimize visualization of specific structures: non-contrast imaging precedes contrast administration and best demonstrates stones and acute hemorrhage; the arterial phase at 25-30 seconds post-injection highlights arteries and hypervascular lesions; the portal venous phase at 60-70 seconds provides optimal evaluation for most abdominal pathology; and delayed imaging at 3-5 minutes opacifies the ureters and bladder for urinary tract assessment.
Image interpretation employs different window settings to optimize visualization of specific tissues. Soft tissue windows suit most organ evaluation, while dedicated liver windows enhance detection of hepatic lesions. Lung windows applied to the lower chest included on abdominal scans reveal pulmonary pathology, and bone windows permit skeletal assessment.
<image>Panel A: Non-contrast CT phase at an axial level through the upper abdomen with the kidneys appearing homogeneous gray and calcifications appearing bright white. Panel B: Arterial phase with the aorta and its branches appearing intensely bright while the liver remains relatively unenhanced. Panel C: Portal venous phase showing homogeneous liver enhancement and the portal vein appearing bright representing the standard phase for most abdominal evaluation. Panel D: Delayed phase with contrast opacifying the renal collecting systems and ureters appearing as bright white linear structures descending from the kidneys.</image>
Normal CT Anatomy by Level
Understanding normal cross-sectional anatomy at different vertebral levels provides the foundation for identifying pathology. In the upper abdomen at the T12-L1 level, the liver occupies much of the right side with homogeneous parenchymal density. The spleen lies posterolaterally on the left, with density similar to the liver. The stomach contains variable amounts of gas and fluid in the left upper quadrant. The pancreas lies anterior to the spine, with the splenic vein running posterior to the pancreatic body and tail. The kidneys occupy retroperitoneal positions, and the great vessels appear midline with the aorta anterior to the spine and the IVC to its right.
At the mid-abdominal level (L2-L4), small bowel loops occupy the central abdomen while colonic segments frame the periphery. The kidneys remain visible with upper poles at the L1-L2 level. The psoas muscles create bilateral elongated soft tissue structures along the vertebral bodies, with the ureters coursing anteriorly. The aorta bifurcates into common iliac arteries at the L4 level, approximately at the umbilical plane.
In the lower abdomen and pelvis (L5 and below), the cecum occupies the right lower quadrant and the sigmoid colon the left lower quadrant. The bladder appears as a fluid-density structure in the midline posterior to the pubic symphysis. The iliac vessels course bilaterally, and the iliopsoas and obturator muscles define the pelvic sidewalls.
<image>Panel A: Axial CT at T12-L1 showing the liver as homogeneous gray on the right, spleen on the left, stomach containing dark gas, and pancreas shaped like a horizontal comma anterior to the spine. Panel B: Same level showing both kidneys retroperitoneally with brighter cortex and darker medulla, the aorta as a bright circular midline structure, and the IVC slightly ovoid to the right. Panel C: Axial CT at L3 demonstrating small bowel loops centrally with thin walls, ascending and descending colon in the lateral gutters, and psoas muscles bilaterally paravertebral. Panel D: Axial CT at the pelvic level displaying the bladder as a midline dark fluid-density structure, iliac vessels bilaterally bright with contrast, and the muscular pelvic sidewalls.</image>
CT Signs of Common Pathology
Several abdominal conditions demonstrate characteristic CT findings. Acute appendicitis manifests as a dilated appendix exceeding 6 mm in diameter, often with an appendicolith (a small calcified density within the lumen), surrounding fat stranding (hazy increased density in the periappendiceal fat), and adjacent fluid. Bowel obstruction appears as dilated loops proximal to a transition point where caliber abruptly changes, with decompressed bowel distally. The small bowel feces sign, representing particulate matter in obstructed small bowel, strongly suggests mechanical obstruction.
Acute pancreatitis produces pancreatic enlargement with indistinct margins, surrounding fat stranding, and variable fluid collections. Necrotic pancreatic tissue fails to enhance with contrast administration. Abdominal aortic aneurysm appears as aortic dilation exceeding 3 cm, often with mural thrombus creating a crescent of intermediate density between the contrast-enhanced lumen and the calcified outer wall. Rupture manifests as retroperitoneal hemorrhage, appearing as high-density material in the retroperitoneal spaces.
<image>Panel A: Acute appendicitis on CT showing a dilated appendix measuring greater than 6 mm with a bright round appendicolith within its lumen and surrounding hazy gray fat stranding. Panel B: Small bowel obstruction with multiple dilated proximal loops containing air-fluid levels, a transition point marked with an arrow, and decompressed distal loops. Panel C: Acute pancreatitis with an enlarged edematous pancreas showing indistinct margins and surrounding retroperitoneal fat stranding as hazy gray infiltration. Panel D: Abdominal aortic aneurysm measuring 6 cm with the aortic lumen bright with contrast surrounded by a crescent of gray mural thrombus and an outer ring of calcification.</image>
MRI of the Abdomen
Magnetic resonance imaging offers superior soft tissue contrast without ionizing radiation, making it particularly valuable for hepatic and pelvic imaging. The modality characterizes tissue based on different signal characteristics across sequences. T1-weighted images display fat as bright (high signal) and fluid as dark (low signal), providing excellent anatomical detail and detecting hemorrhage of certain ages. T2-weighted images show fluid as bright, highlighting pathological processes such as edema, inflammation, and cysts. Fat-suppressed T2 sequences selectively darken fat while maintaining bright fluid signal, accentuating areas of inflammation. Diffusion-weighted imaging (DWI) detects restricted water diffusion, which appears bright and helps identify tumors, abscesses, and acute infarcts.
MR cholangiopancreatography (MRCP) represents a specialized heavily T2-weighted sequence that renders bile and pancreatic juice extremely bright against a dark background, creating a non-invasive cholangiogram. This technique beautifully demonstrates the biliary tree and pancreatic duct, revealing stones as dark filling defects, strictures as areas of narrowing, and anatomical variants. MRCP has largely replaced diagnostic ERCP, reserving the invasive procedure for therapeutic interventions.
<image>Panel A: T1-weighted axial MRI of the upper abdomen showing the liver in medium gray with darker vessels, bright white subcutaneous and intraperitoneal fat, and dark fluid in the gallbladder. Panel B: T2-weighted image at the same level where fluid appears bright white including the gallbladder while solid organs appear intermediate gray. Panel C: Coronal MRCP reconstruction showing the biliary tree and pancreatic duct as bright branching structures against a dark background with the common bile duct descending vertically and hepatic ducts branching superiorly. Panel D: The pancreatic duct coursing horizontally with the gallbladder appearing as a bright oval reservoir on the MRCP reconstruction.</image>
Ultrasound of the Abdomen
Ultrasound provides real-time imaging without radiation, portability, and excellent differentiation of fluid-filled from solid structures. The technique relies on tissue echogenicity—the degree to which structures reflect sound waves. Liver parenchyma typically serves as the reference standard, with structures described as hyperechoic (brighter than liver), hypoechoic (darker than liver), isoechoic (same as liver), or anechoic (completely black, as with simple fluid).
Right upper quadrant ultrasound comprehensively evaluates the liver, gallbladder, and biliary system. The normal liver appears homogeneous with scattered echogenic portal triads. The gallbladder appears as an anechoic (black, fluid-filled) pear-shaped structure with a wall measuring less than 3 mm. Gallstones appear as echogenic (bright) foci that cast posterior acoustic shadows—dark bands extending deep to the stone created by sound wave absorption—and move with patient repositioning. The common bile duct measures less than 6 mm in diameter, or up to 10 mm in patients who have undergone cholecystectomy.
Renal ultrasound demonstrates the cortex as slightly hypoechoic relative to liver, with the central renal sinus (containing fat and collecting structures) appearing echogenic. The abdominal aorta normally measures less than 3 cm; dilation beyond this threshold indicates aneurysmal disease.
<image>Panel A: Normal right upper quadrant ultrasound showing the liver as homogeneous medium-gray parenchyma and the gallbladder as a black anechoic pear-shaped structure with the portal vein as an anechoic tube with echogenic walls. Panel B: Cholelithiasis with two bright echogenic round foci within the gallbladder lumen casting dark posterior acoustic shadows extending as black bands deep to the stones. Panel C: Normal kidney in longitudinal ultrasound view with medium-gray cortex and bright white central sinus from echogenic fat with measurement calipers showing normal dimensions. Panel D: The abdominal aorta in transverse ultrasound view as a circular anechoic structure with measurement calipers displaying a normal diameter of 2.1 cm.</image>
Ultrasound Pathology
Cholecystitis demonstrates the combination of gallstones with wall thickening exceeding 3 mm, pericholecystic fluid (appearing as a thin anechoic rim surrounding the gallbladder), and the sonographic Murphy's sign—pain elicited when the ultrasound transducer applies pressure directly over the gallbladder during inspiration. This combination of findings provides high specificity for acute cholecystitis.
Hydronephrosis appears as dilation of the renal collecting system, with the normally echogenic renal sinus replaced by anechoic (black, fluid-filled) dilated calyces and pelvis. Severity is graded from mild (Grade I, slight pelvic dilation) to severe (Grade IV, marked dilation with cortical thinning). Bedside aortic ultrasound readily identifies abdominal aortic aneurysms, measuring the vessel in both transverse and longitudinal planes. Mural thrombus appears as echogenic material layering along the wall, distinct from the anechoic patent lumen.
<image>Panel A: Acute cholecystitis on ultrasound with a gallstone as a bright echogenic focus casting posterior shadowing and the gallbladder wall measuring 5 mm with thickening. Panel B: Thin rim of dark pericholecystic fluid surrounding the gallbladder indicating inflammation. Panel C: Moderate hydronephrosis with the renal pelvis and calyces appearing as interconnected anechoic black fluid-filled spaces replacing the normal echogenic sinus with thinned overlying cortex. Panel D: Abdominal aortic aneurysm measuring 5.5 cm in transverse diameter with the patent lumen appearing anechoic centrally and echogenic mural thrombus creating a gray crescent along the posterior wall.</image>
Contrast Studies
Fluoroscopic contrast studies permit dynamic evaluation of the gastrointestinal tract. Barium swallow examines the esophagus, demonstrating strictures, webs, diverticula, and motility disorders in real-time. Upper GI series extends evaluation to the stomach and duodenum, identifying ulcers (appearing as contrast-filled outpouchings from the mucosal surface), masses (appearing as filling defects), and obstruction. Small bowel follow-through tracks contrast through the small intestine, proving valuable for Crohn's disease (demonstrating strictures, fistulae, and the characteristic "string sign") and partial obstruction.
Barium enema evaluates the colon, though this study has been largely replaced by colonoscopy for mucosal evaluation and CT colonography for structural assessment. When perforation is suspected, water-soluble contrast replaces barium, which causes severe peritonitis if it leaks into the peritoneal cavity.
<image>Panel A: Lateral view of a barium swallow with the esophagus opacified as a white contrast-filled tube descending through the mediastinum with normal smooth mucosal contours and the gastroesophageal junction visible. Panel B: Upper GI series with the stomach as a contrast-filled sac demonstrating rugal folds as linear filling defects and the duodenal bulb and C-loop visible. Panel C: Barium enema with the entire colon opacified and haustra creating regular scalloped indentations along the colonic walls. Panel D: The hepatic and splenic flexures visible as turns in the contrast column with the rectum appearing as the most distal opacified portion.</image>
Systematic Approach to Abdominal Imaging
Regardless of modality, a systematic review ensures complete evaluation. The checklist approach begins with confirming patient identification and reviewing the clinical indication. Technical adequacy assessment ensures appropriate exposure, positioning, and phase of contrast. Solid organ evaluation encompasses the liver, spleen, kidneys, and pancreas. Hollow organ assessment includes the entire gastrointestinal tract from stomach to rectum, the gallbladder, and the bladder. Vascular structures require evaluation of the aorta, IVC, and portal venous system. Lymph node stations in the retroperitoneum, mesentery, and pelvis should be assessed for enlargement. The peritoneal cavity is examined for free fluid or free air. The retroperitoneum requires separate evaluation for masses, fluid, or adenopathy. Bones visible on the study are assessed for lesions, and the soft tissues including the abdominal wall are examined.
Key diagnostic questions to answer include: Is there obstruction? Is there free air? Is there free fluid? Are solid organs normal in size and character? Are there masses? Is there inflammation?
<image>Panel A: Annotated CT image with the liver, spleen, and kidneys outlined in green as solid organs and the stomach, small bowel, and colon traced in yellow as hollow viscera. Panel B: The aorta and IVC highlighted in red as major vessels with retroperitoneal lymph node stations indicated with blue circles. Panel C: The peritoneal cavity shaded in light purple with the vertebrae and visible ribs outlined in white for bone assessment. Panel D: The anterior abdominal wall musculature traced in orange as soft tissues with a checklist appearing alongside marking each category of the systematic review.</image>
Clinical Algorithms and Applications
For the acute abdomen, imaging follows a structured algorithm. An upright chest radiograph rapidly screens for pneumoperitoneum. CT of the abdomen and pelvis with intravenous contrast serves as the primary study for most acute presentations. Ultrasound takes precedence when right upper quadrant pain suggests biliary disease, in pregnancy, and in pediatric patients.
Renal colic evaluation employs non-contrast CT (stone protocol), which detects ureteral stones with greater than 95% sensitivity. Secondary signs including hydronephrosis and perinephric fat stranding suggest acute obstruction even when stones are difficult to visualize directly.
Trauma evaluation begins with FAST (Focused Assessment with Sonography in Trauma) ultrasound at the bedside, rapidly screening for free fluid in the hepatorenal space (Morison's pouch), splenorenal recess, pelvis, and pericardium. Hemodynamically stable patients requiring detailed evaluation proceed to CT, which identifies solid organ injuries, active hemorrhage, and associated injuries.
Cancer staging employs CT for initial assessment, PET-CT for metabolically active tumors, and MRI for detailed characterization of hepatic metastases and pelvic malignancies.
<image>Panel A: Acute abdomen workup flowchart showing upright chest X-ray for free air then CT for most patients with a branch to ultrasound first for RUQ pain, pregnancy, or pediatric patients. Panel B: Non-contrast CT of renal colic showing a bright white stone in the left ureter with proximal hydronephrosis and gray perinephric stranding. Panel C: FAST ultrasound with four views (subxiphoid cardiac, RUQ, LUQ, and pelvic) demonstrating anechoic free fluid as dark stripes between organs. Panel D: CT staging scan with labeled primary tumor and metastatic deposits.</image>
Summary
Plain abdominal radiographs remain useful for detecting bowel obstruction, pneumoperitoneum, and calcifications, though they provide limited soft tissue detail. CT serves as the primary imaging modality for most acute abdominal pathology, with different contrast phases optimizing visualization of specific structures. MRI provides superior soft tissue contrast for liver characterization and pancreaticobiliary imaging through MRCP. Ultrasound serves as the first-line study for gallbladder disease, renal assessment, and aortic screening, with the advantages of real-time imaging, portability, and absence of radiation. A systematic approach to image interpretation—evaluating solid organs, hollow viscera, vessels, lymph nodes, peritoneum, retroperitoneum, bones, and soft tissues—ensures complete evaluation and prevents missed findings. Modality selection should match the clinical question, patient factors, and the strengths of each imaging technique.
Key Terms
Pneumoperitoneum: Free air within the peritoneal cavity, typically indicating hollow viscus perforation and requiring urgent surgical evaluation.
MRCP (Magnetic Resonance Cholangiopancreatography): A specialized heavily T2-weighted MRI sequence that non-invasively visualizes the biliary tree and pancreatic duct as bright structures against a dark background.
Murphy's sign (sonographic): Pain elicited when the ultrasound transducer applies pressure directly over the gallbladder during inspiration, suggesting acute cholecystitis when positive.
FAST (Focused Assessment with Sonography in Trauma): A rapid bedside ultrasound protocol that screens for free fluid in the peritoneal cavity and pericardium in trauma patients.
Hounsfield units: The quantitative scale used to measure tissue density on CT, with water defined as 0, air as approximately -1000, and dense bone as +1000 or greater.
Posterior acoustic shadowing: A dark band appearing deep to a highly echogenic structure on ultrasound (such as a gallstone or calcification), caused by absorption and reflection of sound waves at the tissue interface.
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