# Abdominal Imaging ## Year 4: Radiology Elective --- ## Learning Objectives By the end of this seminar, students will be able to: 1. Apply a systematic approach to abdominal radiograph interpretation including assessment of bowel gas patterns, soft tissues, bones, and calcifications 2. Identify imaging findings of bowel obstruction including small bowel obstruction, large bowel obstruction, and volvulus on radiography and CT 3. Evaluate hepatobiliary imaging findings using ultrasound, CT, and MRI for focal liver lesions, biliary disease, and acute cholecystitis 4. Recognize acute abdominal pathology including appendicitis, pancreatitis, and diverticulitis on cross-sectional imaging 5. Select appropriate imaging protocols for urinary tract evaluation including renal stones, masses, and urothelial pathology 6. Apply imaging appropriately in abdominal trauma and vascular emergencies including aortic aneurysm and mesenteric ischemia --- ## I. Abdominal Radiograph Interpretation Systematic evaluation of abdominal radiographs follows a structured approach that ensures comprehensive assessment of all visible structures. The bowel gas pattern is evaluated first, noting the location and caliber of gas within the stomach, small bowel, and colon, and identifying any abnormal distribution. Soft tissue structures including the liver, spleen, kidneys, and psoas muscles are assessed for size, contour, and any focal abnormalities. Bones of the spine and pelvis are examined for degenerative changes, fractures, and lytic or blastic lesions. Calcifications are identified and characterized, including vascular calcifications, potential urinary tract stones, and calcified lymph nodes or masses. Normal bowel gas distribution follows predictable patterns that serve as reference for identifying pathology. The gastric bubble appears as a collection of air in the left upper quadrant beneath the left hemidiaphragm. Small bowel gas should be minimal in normal fasting patients and, when present, occupies the central abdomen with visible valvulae conniventes that extend across the full lumen diameter. Colonic gas is peripherally distributed and demonstrates haustra, which are incomplete folds that do not span the entire lumen width. Rectal gas is normally present in variable amounts and should not be mistaken for abnormality. Abnormal gas patterns provide critical diagnostic information about bowel pathology and abdominal emergencies. Dilated small bowel loops measuring greater than three centimeters suggest mechanical obstruction or paralytic ileus. Dilated colon measuring greater than six centimeters, or cecum greater than nine centimeters, indicates large bowel obstruction or toxic megacolon. Multiple air-fluid levels on upright films suggest intestinal obstruction. Pneumoperitoneum, or free intraperitoneal air, indicates bowel perforation and appears as lucency beneath the diaphragm on upright films or outlining the falciform ligament on supine radiographs. Classic radiographic signs help identify specific abdominal pathologies with characteristic appearances. Rigler sign demonstrates air on both sides of the bowel wall, indicating pneumoperitoneum. The football sign describes a large oval lucency in the abdomen representing massive pneumoperitoneum outlining the peritoneal cavity. String of pearls sign shows small trapped air bubbles in a row within fluid-filled obstructed small bowel. The coffee bean sign is produced by the apposed walls of a dilated sigmoid colon in sigmoid volvulus, with the apex pointing toward the right upper quadrant. Panel A: Normal abdominal radiograph demonstrating gastric bubble, minimal small bowel gas, and peripheral colonic gas pattern. Panel B: Upright abdominal radiograph showing free air under the right hemidiaphragm indicating pneumoperitoneum. Panel C: Small bowel obstruction with multiple dilated loops and string of pearls sign. Panel D: Coffee bean sign of sigmoid volvulus with massively dilated sigmoid loop. --- ## II. Bowel Obstruction Small bowel obstruction presents with dilated small bowel loops, defined as diameter greater than three centimeters, proximal to a point of transition and decompressed bowel distally. Air-fluid levels at multiple heights within a single loop of dilated bowel, seen on upright or decubitus radiographs, suggest mechanical obstruction rather than ileus. The most common causes include adhesions from prior surgery, hernias containing incarcerated bowel, and tumors causing luminal obstruction. CT is the definitive imaging modality for confirming obstruction, identifying the transition point, determining the cause, and detecting complications including closed loop and strangulation. Large bowel obstruction produces colonic dilation proximal to an obstructing lesion, most commonly caused by colorectal carcinoma in adults. The competence of the ileocecal valve determines whether small bowel also dilates in large bowel obstruction. A competent valve prevents decompression into the small bowel, resulting in progressive cecal distention with risk of perforation when diameter exceeds nine to twelve centimeters. An incompetent valve allows air to reflux into the small bowel, producing combined large and small bowel dilation that may mimic diffuse ileus. Volvulus occurs when a segment of bowel twists around its mesenteric axis, producing obstruction and potentially compromising blood supply. Sigmoid volvulus is most common in elderly, institutionalized, or psychiatric patients and classically produces a massively dilated sigmoid colon extending from the pelvis toward the right upper quadrant. Cecal volvulus occurs when an abnormally mobile cecum twists on its axis, producing a dilated air-filled structure in the left upper or mid abdomen away from its normal right lower quadrant location. CT demonstrates the characteristic whirl sign at the point of mesenteric twist in both types of volvulus. CT findings in bowel obstruction provide information beyond that available on radiographs and guide surgical decision-making. The transition point is identified as the junction between dilated proximal and decompressed distal bowel, and the cause is often visible at this location. Small bowel feces sign, where particulate matter resembling fecal material appears in the small bowel, suggests chronic or low-grade obstruction. Closed loop obstruction, where a segment of bowel is obstructed at two points, represents a surgical emergency due to high risk of strangulation. Signs of strangulation include bowel wall thickening, mesenteric haziness, decreased wall enhancement, and pneumatosis intestinalis. Panel A: CT demonstrating small bowel obstruction with dilated proximal loops and decompressed distal bowel at the transition point. Panel B: Large bowel obstruction from obstructing colon cancer with dilated proximal colon and collapsed distal rectum. Panel C: Sigmoid volvulus on CT showing whirl sign at the point of mesenteric twist. Panel D: Closed loop small bowel obstruction with U-shaped dilated loop and mesenteric vessel convergence. --- ## III. Acute Appendicitis Ultrasound serves as the initial imaging modality for suspected appendicitis in children and young women to minimize radiation exposure while maintaining diagnostic accuracy. The normal appendix is compressible and measures six millimeters or less in diameter, while an inflamed appendix is non-compressible and dilated beyond six millimeters. The target or bull's eye sign represents the layers of the thickened appendiceal wall seen in cross section. An appendicolith, when present, appears as an echogenic focus with posterior acoustic shadowing and is seen in approximately one-third of cases. Computed tomography provides the highest diagnostic accuracy for appendicitis and is the preferred modality in adults and when ultrasound is non-diagnostic. The inflamed appendix measures greater than six millimeters in diameter with wall thickening and mucosal enhancement after contrast administration. Periappendiceal fat stranding, the hazy increased attenuation of the surrounding mesenteric fat, indicates inflammation extending beyond the appendiceal wall. An appendicolith appears as a calcified density within the appendiceal lumen and is present in approximately twenty-five percent of cases. Complications of appendicitis alter management and require identification on imaging for appropriate treatment planning. Perforation is suggested by discontinuity of the appendiceal wall, adjacent abscess formation, or extraluminal air or appendicolith. Periappendiceal abscess appears as a rim-enhancing fluid collection that may require percutaneous drainage before definitive surgical management. Phlegmon represents inflammatory tissue without drainable collection and typically necessitates initial antibiotic management. Complications are more common in elderly patients and those with delayed presentation. Several conditions mimic appendicitis clinically and must be distinguished on imaging to avoid unnecessary surgery. Mesenteric adenitis presents with enlarged mesenteric lymph nodes, often associated with viral illness, in the setting of a normal-appearing appendix. Ovarian pathology including hemorrhagic cysts, ovarian torsion, and tubo-ovarian abscess presents with right lower quadrant pain in women. Crohn disease involving the terminal ileum may produce right lower quadrant pain, diarrhea, and imaging findings of bowel wall thickening. Epiploic appendagitis appears as an oval fat-density lesion with surrounding inflammation, typically along the sigmoid colon but occasionally in the right lower quadrant. Panel A: Ultrasound showing non-compressible dilated appendix measuring greater than six millimeters with target sign. Panel B: CT demonstrating appendicitis with dilated fluid-filled appendix, wall enhancement, and periappendiceal stranding. Panel C: CT showing perforated appendicitis with periappendiceal abscess and extraluminal appendicolith. Panel D: Epiploic appendagitis mimicking appendicitis with oval fat-density lesion and surrounding inflammatory changes. --- ## IV. Hepatobiliary Imaging Ultrasound serves as the primary screening modality for hepatic and biliary evaluation, offering real-time imaging without radiation and excellent detection of biliary calculi. Normal liver parenchyma demonstrates homogeneous echotexture slightly more echogenic than normal renal cortex. Focal liver lesions are characterized by their echogenicity relative to surrounding parenchyma, with simple cysts appearing anechoic with posterior acoustic enhancement. Biliary evaluation assesses gallbladder wall thickness, presence of calculi, and caliber of the common bile duct, which normally measures less than six millimeters or up to one millimeter per decade of life in older adults. Focal liver lesions are characterized by their imaging features to distinguish benign from malignant entities. Simple hepatic cysts appear as sharply marginated anechoic lesions on ultrasound and demonstrate water attenuation on CT without enhancement. Hemangiomas, the most common benign hepatic tumors, demonstrate peripheral nodular enhancement with progressive centripetal fill-in on multiphasic CT and MRI. Focal nodular hyperplasia appears as a well-defined lesion with arterial enhancement and a characteristic central scar. Hepatocellular carcinoma demonstrates arterial phase hyperenhancement followed by washout on portal venous or delayed phases, a pattern essentially diagnostic in patients with cirrhosis. The biliary system is evaluated with complementary modalities depending on the clinical question. Ultrasound detects gallstones with high sensitivity as echogenic foci with posterior acoustic shadowing and mobile within the gallbladder lumen. Magnetic resonance cholangiopancreatography provides non-invasive visualization of the biliary tree and pancreatic duct, demonstrating stones as filling defects within T2-bright bile. Hepatobiliary iminodiacetic acid scan confirms acute cholecystitis when the gallbladder fails to fill after radiotracer administration, indicating cystic duct obstruction. Endoscopic retrograde cholangiopancreatography provides both diagnostic imaging and therapeutic intervention for biliary obstruction. Acute cholecystitis imaging findings reflect gallbladder inflammation, typically resulting from cystic duct obstruction by an impacted gallstone. Ultrasound demonstrates gallstones, gallbladder wall thickening greater than three millimeters, pericholecystic fluid representing inflammatory exudate, and a positive sonographic Murphy sign. The sonographic Murphy sign is elicited when pressure applied with the transducer over the gallbladder fundus produces focal tenderness. CT findings include gallbladder distension, wall thickening with enhancement, pericholecystic stranding, and hyperemia of the adjacent liver parenchyma. Complications including gangrenous cholecystitis, perforation, and emphysematous cholecystitis with intramural gas must be identified because they require urgent surgical intervention. Panel A: Ultrasound showing gallstones with posterior acoustic shadowing within a normal gallbladder. Panel B: Hepatic hemangioma on multiphasic CT demonstrating peripheral nodular enhancement with centripetal fill-in. Panel C: MRCP showing common bile duct stone as filling defect within the dilated distal duct. Panel D: Acute cholecystitis on CT with gallbladder wall thickening, pericholecystic fluid, and adjacent hepatic hyperemia. --- ## V. Pancreatic Imaging Acute pancreatitis is diagnosed primarily by clinical and laboratory criteria, but CT plays an essential role in assessing severity and detecting complications. The normal pancreas enhances homogeneously after contrast administration, while acute pancreatitis produces enlargement with heterogeneous enhancement and peripancreatic inflammatory changes. Peripancreatic fat stranding and fluid collections indicate the extent of inflammatory involvement. Necrotizing pancreatitis is identified by areas of pancreatic parenchyma that fail to enhance, indicating devitalized tissue that may become infected. The CT severity index provides a standardized method for grading acute pancreatitis and predicting clinical outcomes. Points are assigned based on pancreatic appearance, ranging from normal to multiple fluid collections, and the extent of pancreatic necrosis from none to greater than fifty percent. Higher scores correlate with increased morbidity and mortality, longer hospitalization, and higher rates of complications requiring intervention. The modified CT severity index incorporates extrapancreatic complications to improve prognostic accuracy. Pancreatic fluid collections evolve over time and are classified according to their content and wall characteristics. Acute peripancreatic fluid collections develop within the first four weeks and lack a well-defined wall. Pseudocysts develop after four weeks and demonstrate a well-defined fibrous capsule surrounding homogeneous fluid without solid components. Acute necrotic collections contain both fluid and solid necrotic material within the first four weeks. Walled-off necrosis develops after four weeks when necrotic collections become encapsulated, and these may require drainage or necrosectomy if infected. Pancreatic masses require careful imaging evaluation to differentiate benign from malignant lesions and assess resectability. Pancreatic ductal adenocarcinoma typically appears as a hypoenhancing mass relative to normal pancreatic parenchyma, often with upstream pancreatic duct dilation. Vascular involvement including encasement or occlusion of the superior mesenteric artery, celiac axis, or portal vein determines surgical resectability. Metastatic disease to liver, peritoneum, or distant lymph nodes precludes curative resection. Cystic pancreatic lesions include mucinous neoplasms with malignant potential and serous cystadenomas that are almost always benign, distinguished by imaging characteristics and cyst fluid analysis. Panel A: CT showing acute pancreatitis with enlarged pancreas, peripancreatic stranding, and acute fluid collections. Panel B: Necrotizing pancreatitis demonstrating areas of non-enhancing pancreatic parenchyma. Panel C: Pancreatic pseudocyst with well-defined wall and homogeneous internal fluid content. Panel D: Pancreatic adenocarcinoma presenting as hypoenhancing mass with upstream duct dilation and vascular involvement. --- ## VI. Renal and Urinary Tract Renal ultrasound provides initial evaluation of the kidneys and collecting systems without radiation, with particular utility in pediatric patients and pregnant women. Normal kidneys measure approximately ten to twelve centimeters in length and demonstrate a distinct corticomedullary junction with echogenic renal sinus fat centrally. Hydronephrosis appears as dilation of the renal pelvis and calyces, graded from mild to severe based on extent of parenchymal thinning. Renal masses are characterized as solid or cystic, with simple cysts demonstrating anechoic content, imperceptible wall, and posterior acoustic enhancement. CT urography is the comprehensive imaging protocol for evaluating the entire urinary tract, with phases optimized for different diagnostic questions. Non-contrast imaging detects urinary calculi, which appear as high-attenuation foci within the collecting system, ureter, or bladder. The corticomedullary phase obtained during early contrast administration optimizes detection of renal masses and characterizes their enhancement pattern. Nephrographic phase imaging demonstrates homogeneous parenchymal enhancement and is most sensitive for detecting small renal lesions. Excretory phase images opacify the collecting systems and ureters, enabling detection of urothelial abnormalities and filling defects. Renal calculi are optimally evaluated with non-contrast CT, which detects essentially all stone compositions with high sensitivity and specificity. Stone attenuation measured in Hounsfield units provides information about composition, with calcium-containing stones measuring greater than four hundred Hounsfield units. Stone size and location guide management decisions, with ureteral stones greater than five millimeters less likely to pass spontaneously. Secondary signs including hydronephrosis, perinephric stranding, and periureteral soft tissue at the level of the stone indicate acute obstruction. Uric acid stones demonstrate lower attenuation than calcium stones and may be amenable to dissolution therapy. The Bosniak classification system guides management of cystic renal masses based on imaging characteristics. Category I cysts are simple with thin walls, no septa or calcifications, and water attenuation, representing benign lesions requiring no follow-up. Category II includes minimally complex cysts with thin septa, fine calcifications, or homogeneous high-attenuation content. Category IIF designates cysts requiring follow-up imaging due to more complex features but still likely benign. Category III cysts are indeterminate with thickened walls, enhancing septa, or nodularity requiring surgical resection or biopsy. Category IV lesions demonstrate unequivocal enhancement indicating solid malignant components and require surgical treatment. Panel A: Renal ultrasound showing moderate hydronephrosis with dilated calyces and preserved parenchymal thickness. Panel B: Non-contrast CT demonstrating obstructing ureteral stone with proximal hydroureter and perinephric stranding. Panel C: CT urography excretory phase showing normal bilateral collecting systems and contrast-filled ureters. Panel D: Complex cystic renal mass with thickened enhancing septa representing Bosniak category III lesion. --- ## VII. Vascular Emergencies Abdominal aortic aneurysm is defined as focal dilation of the aorta to greater than three centimeters and is most commonly infrarenal in location. Ultrasound provides effective screening and surveillance for known aneurysms, measuring the maximum anteroposterior and transverse diameters. CT angiography offers comprehensive evaluation of aneurysm extent, relationship to branch vessels, and presence of complications. Findings concerning for rupture include retroperitoneal hematoma, high-attenuation crescent within the thrombus representing acute intramural hemorrhage, and the draped aorta sign where the aneurysm conforms to the vertebral body contour. Mesenteric ischemia results from inadequate blood supply to the intestines and may be arterial or venous in origin. Acute arterial occlusion, caused by embolism or thrombosis of the superior mesenteric artery, produces sudden severe abdominal pain often out of proportion to physical findings. CT angiography demonstrates thrombus or embolic occlusion within the mesenteric vessels and ischemic changes in the bowel wall. Bowel findings progress from wall thickening and decreased enhancement to paper-thin wall, pneumatosis intestinalis, and portomesenteric venous gas in advanced ischemia. Mesenteric venous thrombosis produces bowel edema with thickened wall, engorgement of mesenteric vessels, and filling defect within the superior mesenteric vein or portal vein. Gastrointestinal bleeding evaluation with CT angiography detects active extravasation when the rate of hemorrhage exceeds the detection threshold of approximately 0.3 to 0.5 milliliters per minute. Active bleeding appears as contrast material extravasating from a vessel into the bowel lumen, mesentery, or peritoneal cavity that increases in volume between arterial and delayed phases. The sentinel clot sign identifies the bleeding source by locating the highest attenuation clot closest to the hemorrhage origin. CT angiography also identifies the underlying cause of bleeding, including aneurysms, tumors, and vascular malformations, which guides therapeutic planning. Findings of active hemorrhage on CT angiography guide triage to intervention versus conservative management. Active extravasation with hemodynamic instability typically requires urgent intervention, either endovascular embolization or surgical exploration depending on the source. The location and etiology of bleeding determine whether angiographic embolization, endoscopic therapy, or surgery provides the optimal treatment approach. Tagged red blood cell scintigraphy offers higher sensitivity for detecting intermittent slow bleeding but lower spatial resolution than CT angiography for localizing the bleeding source. Panel A: CT angiography showing infrarenal abdominal aortic aneurysm with mural thrombus and calcification. Panel B: Ruptured aortic aneurysm with high-attenuation crescent sign and retroperitoneal hematoma. Panel C: Acute mesenteric ischemia with SMA thrombus and adjacent pneumatosis intestinalis. Panel D: Active gastrointestinal bleeding with contrast extravasation into the bowel lumen and sentinel clot. --- ## VIII. Trauma Imaging The Focused Assessment with Sonography for Trauma, or FAST, examination provides rapid bedside detection of hemoperitoneum and hemopericardium in hemodynamically unstable trauma patients. Four standard views evaluate the hepatorenal space or Morrison's pouch, the splenorenal space, the pelvis, and the pericardium. Free fluid appears as anechoic collections in dependent peritoneal recesses, with as little as two hundred milliliters detectable by experienced operators. The extended FAST or e-FAST adds evaluation for pneumothorax and hemothorax. A positive FAST in an unstable patient may prompt immediate operative intervention without additional imaging. Computed tomography serves as the definitive imaging modality for comprehensive trauma evaluation in hemodynamically stable patients. Whole-body CT protocols cover the head, cervical spine, chest, abdomen, and pelvis in a single rapid acquisition. Intravenous contrast is essential for detecting solid organ injuries, vascular abnormalities, and active bleeding. CT detects injuries occult on physical examination and FAST, including contained vascular injuries, solid organ lacerations without significant hemoperitoneum, and bowel or mesenteric injuries. Solid organ injury grading follows the American Association for the Surgery of Trauma organ injury scale, which standardizes injury description and guides management. Grade I injuries include subcapsular hematomas involving less than ten percent of surface area and superficial lacerations less than one centimeter deep. Grade II includes larger hematomas and deeper lacerations. Grade III involves major lacerations greater than three centimeters or parenchymal involvement greater than fifty percent. Grade IV injuries involve active bleeding, and grade V describes shattered organs or complete vascular disruption. Higher-grade injuries are more likely to require operative or angiographic intervention. Specific injury patterns characterize trauma to different abdominal organs and influence management decisions. Liver lacerations appear as linear hypoattenuating regions through the parenchyma, with active bleeding demonstrating contrast extravasation. Splenic injuries are particularly concerning due to the organ's vascularity, and delayed splenic rupture can occur days after initial trauma. Renal injuries may involve parenchymal laceration, collecting system injury with contrast extravasation, or vascular pedicle injury. Bowel and mesenteric injuries are suggested by free fluid without solid organ injury, mesenteric hematoma or infiltration, bowel wall thickening or discontinuity, and free air. Panel A: FAST examination showing free fluid in Morrison's pouch between liver and right kidney. Panel B: CT demonstrating grade IV splenic laceration with active contrast extravasation. Panel C: Liver laceration extending through multiple segments with perihepatic hematoma. Panel D: Mesenteric injury with hematoma and adjacent bowel wall thickening suggesting hollow viscus injury. --- ## IX. GI Tract Imaging CT enterography optimizes evaluation of the small bowel through administration of oral contrast that distends the lumen and enables detection of mucosal and wall abnormalities. Neutral oral contrast agents, typically low-density solutions, provide optimal visualization of enhancing mucosal lesions that would be obscured by positive oral contrast. The examination is indicated for known or suspected inflammatory bowel disease, obscure gastrointestinal bleeding, and evaluation of small bowel tumors. Multiphasic imaging with enteric phase timing demonstrates mural enhancement patterns that characterize various small bowel pathologies. Crohn disease produces characteristic imaging findings that enable diagnosis, assessment of disease extent, and detection of complications. Active inflammation manifests as wall thickening greater than three millimeters, mucosal hyperenhancement, and stratified enhancement pattern. Mesenteric changes include engorgement of the vasa recta, termed the comb sign, and fibrofatty proliferation of mesenteric fat. Skip lesions with intervening normal bowel are characteristic of Crohn disease, distinguishing it from ulcerative colitis. Complications including strictures causing obstruction, fistulas connecting bowel loops or extending to other organs, and abscesses require identification for treatment planning. Acute diverticulitis is diagnosed on CT by findings of inflamed diverticula with pericolonic inflammatory changes. The affected segment demonstrates wall thickening, pericolonic fat stranding, and inflamed diverticula with or without intraluminal or extraluminal air. The sigmoid colon is most commonly affected given the high prevalence of diverticulosis in this location. Uncomplicated diverticulitis is managed medically, while complications including abscess, perforation with free air, and fistula formation may require drainage or surgical intervention. CT also excludes alternative diagnoses including colon cancer, which should be considered particularly in patients without known diverticulosis. Fluoroscopic examinations of the gastrointestinal tract provide dynamic evaluation complementing cross-sectional imaging. Esophagram with barium or water-soluble contrast evaluates dysphagia, suspected stricture, and postoperative leak. Upper gastrointestinal series examines the esophagus, stomach, and duodenum for ulcers, masses, and anatomic abnormalities. Small bowel follow-through provides overview of small bowel anatomy, though CT and MR enterography have largely replaced this examination. Contrast enema evaluates for obstruction, stricture, and anastomotic leak, with water-soluble contrast used when perforation is suspected. Panel A: CT enterography showing Crohn disease with terminal ileal wall thickening, enhancement, and mesenteric comb sign. Panel B: Acute diverticulitis with sigmoid wall thickening, pericolonic stranding, and inflamed diverticulum. Panel C: Perforated diverticulitis with extraluminal air and pericolonic abscess. Panel D: Barium esophagram demonstrating smooth distal esophageal stricture with bird-beak appearance in achalasia. --- ## X. Choosing the Right Study Selection of imaging for acute abdominal pain depends on pain location, clinical presentation, and patient factors including age and pregnancy status. Right upper quadrant pain suggests biliary disease and is best initially evaluated with ultrasound to assess for gallstones, cholecystitis, and biliary dilation. Right lower quadrant pain raises concern for appendicitis and is evaluated with CT in adults or ultrasound in children and young women. Left lower quadrant pain suggests diverticulitis and is evaluated with CT. Diffuse or poorly localized pain typically warrants CT with intravenous contrast to evaluate for a broad differential diagnosis. Specific clinical questions dictate optimal imaging modality and protocol to maximize diagnostic yield while minimizing risk. Gallstone disease is evaluated with ultrasound, which has near-perfect sensitivity for gallbladder stones and detects signs of cholecystitis. Renal stones are detected with non-contrast CT, which identifies stones of all compositions and demonstrates secondary signs of obstruction. Appendicitis evaluation uses CT with intravenous contrast in adults for highest accuracy or ultrasound in pediatric and pregnant patients. Bowel obstruction is evaluated with CT, which identifies the transition point, determines the cause, and detects complications. Pediatric patients require special consideration to minimize radiation exposure while maintaining diagnostic accuracy. Ultrasound serves as the primary modality for many indications including appendicitis, pyloric stenosis, and intussusception. When appendicitis is suspected, ultrasound should be performed first, with CT reserved for cases where ultrasound is non-diagnostic and clinical suspicion remains high. Intussusception is diagnosed sonographically by the target or doughnut sign representing layers of telescoped bowel, and may be treated with air or contrast enema reduction. Pyloric stenosis demonstrates muscle thickness greater than three millimeters and channel length greater than fourteen millimeters on ultrasound. Imaging in pregnancy balances the need for accurate diagnosis against fetal radiation exposure and the safety of contrast agents. Ultrasound is safe throughout pregnancy and should be the first-line modality when it can answer the clinical question. MRI without gadolinium is considered safe at all gestational ages and provides excellent soft tissue evaluation. CT should be performed when clinically indicated for maternal diagnosis, as the diagnostic benefit typically outweighs the small theoretical fetal risk. Gadolinium-based contrast should be avoided in pregnancy unless essential for diagnosis, as it crosses the placenta and has not been adequately studied in pregnant populations. Panel A: Algorithm for imaging approach to acute abdominal pain based on pain location and patient factors. Panel B: Pediatric appendicitis on ultrasound with dilated non-compressible appendix and appendicolith. Panel C: Pyloric stenosis on ultrasound demonstrating thickened muscle and elongated channel. Panel D: MRI of appendicitis in pregnant patient showing dilated appendix with periappendiceal inflammation. --- ## Summary Abdominal radiograph interpretation follows a systematic approach evaluating bowel gas pattern, soft tissues, bones, and calcifications. Normal gas distribution includes gastric bubble, minimal central small bowel gas, and peripheral colonic gas. Abnormal patterns including dilated loops, air-fluid levels, and pneumoperitoneum indicate obstruction or perforation. Classic signs such as Rigler sign, coffee bean sign, and string of pearls provide specific diagnostic information. Bowel obstruction evaluation with CT identifies the transition point, determines etiology, and detects complications including closed loop and strangulation. Small bowel obstruction is most commonly caused by adhesions, while large bowel obstruction usually results from malignancy. Volvulus produces characteristic imaging with the whirl sign at the point of mesenteric twist. Appendicitis demonstrates dilated non-compressible appendix greater than six millimeters with periappendiceal stranding. Hepatobiliary imaging utilizes ultrasound for initial evaluation and CT or MRI for lesion characterization. Focal liver lesions are distinguished by enhancement patterns, with hepatocellular carcinoma showing arterial enhancement and washout. Biliary disease evaluation includes ultrasound for stones, MRCP for ductal anatomy, and HIDA for acute cholecystitis. Pancreatic imaging assesses severity of pancreatitis and characterizes masses with multiphasic technique. Urinary tract evaluation uses non-contrast CT for stones and CT urography for comprehensive assessment. The Bosniak classification guides management of cystic renal masses. Vascular emergencies including aortic aneurysm rupture and mesenteric ischemia require urgent CT angiography. Trauma imaging begins with FAST followed by CT for comprehensive evaluation, with injury grading guiding management decisions. --- ## Key Terms **Transition Point**: Junction between dilated proximal and decompressed distal bowel in mechanical obstruction, identifiable on CT to determine the level and often the cause of obstruction. **Pneumatosis Intestinalis**: Air within the bowel wall, an ominous finding when associated with portal venous gas, suggesting bowel ischemia and necrosis. **MRCP**: Magnetic resonance cholangiopancreatography, a non-invasive technique for visualizing the biliary tree and pancreatic duct using heavily T2-weighted sequences. **Bosniak Classification**: Standardized system for categorizing cystic renal masses based on imaging features to guide management decisions from benign neglect to surgical intervention. **FAST Examination**: Focused Assessment with Sonography for Trauma, a rapid bedside ultrasound protocol detecting free fluid in the abdomen and pericardium in trauma patients. **Sentinel Clot Sign**: High-attenuation clot located nearest to the bleeding source on CT, helping to identify the origin of gastrointestinal hemorrhage. **CT Enterography**: Optimized CT technique for small bowel evaluation using neutral oral contrast to distend the lumen and enable detection of mucosal and wall abnormalities. **Extravasation**: Active leakage of contrast material from a vessel indicating hemorrhage, appearing as high-attenuation material that increases between arterial and delayed phases. --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*