Comprehensive Radiographic and Cross-Sectional Imaging of the Abdomen and Pelvis

Clinical Evaluation and Plain Film Radiography of the Abdomen

• Clinical Presentation and Initial Evaluation:

  • Abdominal pain represents one of the most common chief complaints presenting to the emergency department.
  • Patient medical history and detailed physical examination are essential first steps to formulate a differential diagnosis and guide the selection of diagnostic imaging modalities.
  • For patients presenting with generalized, diffuse abdominal pain, conventional radiography (plain film / placa) is universally established as the initial, first-line imaging modality before advancing to computed tomography (CT), ultrasound, or magnetic resonance imaging (MRI).
  • Conventional radiography is also routinely indicated to monitor and follow up on previously documented imaging findings.

• The Four Standard and Alternative Plain Film Projections:

  • Supine Abdominal Projection (Scout Film / KUB):
    • Patient Position: Supine (lying flat on the examination table facing upward / boca arriba). Tolerated by virtually all patients without requiring a postural substitute.
    • X-ray Beam Geometry: The X-ray source is positioned superiorly and anteriorly, directing the beam vertically downward through the patient to the radiographic cassette located posteriorly.
    • Diagnostic Purpose: Functions as a scout film (scout view) providing a comprehensive overview of the abdominal gas distribution, soft tissue masses, organomegaly, and radiopaque calcifications.
    • KUB Designation: Frequently ordered as a KUB view, which stands for Kidneys, Ureters, and Bladder. A KUB study is an unenhanced (non-contrast) plain abdominal radiograph focusing specifically on identifying nephrolithiasis, ureterolithiasis, and cystolithiasis.
  • Prone Abdominal Projection:
    • Patient Position: Prone (lying face down / boca abajo). Cannot be tolerated by all patients, particularly those with recent abdominal surgery or voluminous abdominal tumors.
    • Alternative / Substitute View: Lateral rectum projection (lateral decubitus or cross-table lateral recumbent view).
    • X-ray Beam Geometry: Identical to the supine view; the beam is directed vertically downward from a superior source to the posterior cassette.
    • Diagnostic Purpose: Because gas rises to the most anatomically posterior structures when the patient is prone, this view preferentially visualizes the rectosigmoid and retroperitoneal colonic segments (ascending colon and descending colon).
  • Upright Abdominal Projection (Abdominal Upright):
    • Patient Position: Standing erect (preferred) or seated upright with an erect torso.
    • Alternative / Substitute View: Left lateral decubitus projection (patient positioned lying on their left side on the examination table).
    • X-ray Beam Geometry: The X-ray tube is placed anteriorly, directing the beam horizontally (parallel to the floor) through the patient to the cassette positioned posteriorly.
    • Diagnostic Purpose: Relies on physical gravity and horizontal beam dynamics to demonstrate air-fluid levels (air-fluid levels) and free extraluminal intraperitoneal air. In the upright position, free air rises to the highest anatomical point beneath the diaphragm. In the left lateral decubitus position, free air migrates between the liver margin and the lateral abdominal wall.
  • Upright Frontal Chest Projection (PA Chest / Upright Frontal Chest):
    • Patient Position: Standing erect or seated with an upright torso.
    • Alternative / Substitute View: Supine chest projection (AP supine chest), in which the beam travels vertically rather than horizontally, sacrificing the ability to identify horizontal air-fluid interfaces.
    • X-ray Beam Geometry: Posteroanterior horizontal beam, directed parallel to the floor across the thoracic cavity.
    • Diagnostic Purpose: Detects free subdiaphragmatic intraperitoneal air, visualizes epigastric or hiatal hernias extending into the thoracic cavity, and identifies secondary thoracic complications of abdominal pathology (such as basal pneumonias or pleural effusions).

• Physics of Air-Fluid Levels:

  • Air-fluid levels can only be visualized when the incident X-ray beam is oriented horizontally (parallel to the floor).
  • A vertical beam (directed perpendicularly toward the floor) cannot depict a horizontal interface between gas and liquid, displaying only mixed or superimposing densities.
  • The specific plain film projections that utilize a horizontal beam and permit detection of air-fluid levels are:
    • Upright abdominal projection.
    • Left lateral decubitus projection.
    • Upright frontal chest projection.

• Image Orientation Conventions and Distinguishing Specific Projections:

  • Lateral Rectum View: Displays the inferior spine (sacrum and coccyx) along the posterior/superior margin. Anterior to the sacrum lies the rectosigmoid junction, which normally contains radiolucent gas. The presence of gas in the rectosigmoid on this view rules out mechanical distal bowel obstruction.
  • Left Lateral Decubitus View: The patient lies on their left side, placing the right anatomical side superiorly (at the top of the radiographic image). By clinical convention, the patient's head is oriented toward the right side of the image frame, and the feet are directed toward the left side. The superior soft tissue shadow corresponds to the liver. Normally, a thin radiolucent band representing preperitoneal fat separates the hepatic margin from the lateral abdominal wall. In cases of hollow viscus perforation (such as a perforated peptic ulcer), free extraluminal gas forms a prominent radiolucent crescent between the liver and the abdominal wall.

Systematic Interpretation of Abdominal Plain Films

• The Four Systematic Review Elements:

  • General bowel gas pattern (location, volume, luminal caliber, and configuration).
  • Extraluminal / free peritoneal air (pneumoperitoneum).
  • Abnormal abdominal and pelvic calcifications.
  • Soft tissue masses and organomegaly (abnormal organ enlargement).

• Terminology: Intestinal Distension vs. Intestinal Dilatation:

  • Distension (Distensión): A normal physiological state in which bowel loops contain adequate volumes of gas and fluid to separate luminal walls and fully expand the lumen within normal caliber thresholds.
  • Dilatation (Dilatación): A distinctly pathological state in which the intestinal walls are stretched beyond standard physiological limits, reflecting an underlying ileus or mechanical obstruction.

• Physiological Gas Distribution and Dimensional Limits (The 3-6-9 Rule):

  • Origin of Intestinal Gas: The overwhelming majority of intra-abdominal gas originates from swallowed atmospheric air (aerophagia occurring during speech, eating, deglutition, and breathing). Only a minimal fraction is generated by intraluminal bacterial fermentation of dietary nutrients. Intraluminal gas is radiolucent (black/dark).
  • Stomach: Almost universally contains a volume of gas and an air-fluid level.
  • Small Intestine: Typically exhibits gas within $2$ to $3$ non-dilated (physiologically distended) loops. The normal internal diameter measures $< 2.5\,\text{cm}$, with an upper absolute physiological threshold of $3\,\text{cm}$.
  • Large Intestine (Colon): Contains a variable volume of gas throughout its course. The normal maximum caliber of the colon measures $< 6\,\text{cm}$.
  • Cecum: Represents the widest physiological segment of the large intestine, with a normal maximum diameter of $< 9\,\text{cm}$.
  • The 3-6-9 Rule Summary: Maximum normal luminal diameters are $3\,\text{cm}$ for the small bowel, $6\,\text{cm}$ for the large bowel (colon), and $9\,\text{cm}$ for the cecum.
  • Rectosigmoid Gas: Gas is almost always present in the rectosigmoid. The complete absence of rectosigmoid gas on supine or prone films strongly suggests a mechanical bowel obstruction.

• Visualizing Normal Large Bowel Anatomy and Stool:

  • Double-Contrast Barium Enema: Serves as a reference model for luminal distension. Barium sulfate opacifies the mucosal lining (radiopaque), while insufflated air distends the lumen (radiolucent). Anatomical segments progress from the cecum in the right iliac fossa, to the ascending colon, right colic flexure (hepatic flexure), transverse colon (which may sag inferiorly toward the urinary bladder in asthenic individuals), left colic flexure (splenic flexure), descending colon on the left, sigmoid colon, and rectum.
  • Radiographic Appearance of Stool (Excreta): Fecal material is identified by a mottled appearance consisting of multiple small, trapped radiolucent gas bubbles dispersed within semi-solid, amorphous soft tissue density, primarily visualized in the colonic lumen.

• Pathological Gas Patterns: Aerophagia:

  • Definition and Etiology: Marked accumulation of gastrointestinal air caused by excessive air swallowing. Precipitating factors include rapid food ingestion, heavy intake of carbonated sodas, chronic gum chewing, or hyperventilation.
  • Clinical Manifestations: Sensation of profound abdominal bloating ("globus"), marked abdominal distension, and dodging/guarding behavior during physical examination where the patient reflexively retracts the abdomen due to tension pain.
  • Radiographic Presentation: Prominent, overlapping, multi-polygonal radiolucent loops throughout the small and large bowel. Although the loops are crowded with excess gas, they remain distended within normal calibers rather than pathologically dilated.
  • Symptom Resolution: Occurs through gastrointestinal gas expulsion via eructation (burping) or flatulence.

• Physiological Air-Fluid Levels:

  • Stomach: Normal upright view demonstrates a prominent air-fluid level in the gastric fundus with a crisp horizontal line separating the superior gas from dependent fluid.
  • Small Intestine: Normal plain films demonstrate up to $2$ to $3$ small air-fluid levels within distended loops.
  • Large Intestine: Air-fluid levels are normally absent or exceptionally scarce in the colon because mucosal absorption of water solidifies fluid into stool.

• Radiographic Differentiation Between Small and Large Intestine:

  • Anatomical Distribution: Small intestine loops are located centrally in the mid-abdomen; the large intestine is arranged circumferentially around the periphery.
  • Valvulae Conniventes (Plicae Circulares): Characteristic of the small intestine. Appear as thin, closely spaced radiopaque transverse mucosal folds that extend completely across the entire width of the intestinal lumen from one wall to the opposite wall.
  • Haustra: Characteristic of the large intestine. Formed by plications that appear as thicker, widely spaced radiopaque folds that do not traverse the full diameter of the colonic lumen, stopping midway across the lumen.

Incidental Non-Pathological Calcifications and Normal Landmarks

• Pelvic Phleboliths:

  • Pathophysiology: Calcified venous thrombi occurring within pelvic venous plexuses. They develop progressively with advancing age and show a higher incidence in females.
  • Radiographic Appearance: Small, rounded, discrete radiopacities located in the true pelvis. They characteristically feature a dense radiopaque outer calcified rim surrounding a distinct radiolucent central core.
  • Clinical Significance: Clinically benign and asymptomatic incidental findings; non-pathological due to extensive collateral venous drainage. Must not be confused with lower ureteral calculi.

• Costal Cartilage Calcifications:

  • Pathophysiology: Physiological, age-related ossification of the costochondral cartilages extending from the ribs.
  • Radiographic Appearance: Amorphous, patchy, linear radiopacities following the anatomical curvilinear arc of the anterior lower ribs as they converge toward the spine and sternum.
  • Forensic Significance: The morphological calcification pattern demonstrates distinct sexual dimorphism, allowing forensic anthropologists and pathologists to assist in sex identification from skeletal remains.

• Soft Tissue Shadows on Normal Abdominal Films:

  • Psoas Major Muscle Shadows: Symmetric, elongated cord-like soft tissue stripes extending diagonally from the lateral margins of the lumbar vertebrae toward the iliac crests. They are visualized radiographically because they are bordered by retroperitoneal fat. Ureters course along the anterior surface of the psoas muscles.
  • Solid Viscus Shadows: The inferior and lateral margins of the liver in the right upper quadrant and the splenic shadow in the left upper quadrant are partially visible when contrasted against adjacent intra-abdominal fat.

Organomegaly and Soft Tissue Masses

• Diagnostic Principles in Plain Film Organomegaly:

  • Conventional radiography has limited soft tissue differentiation because adjacent parenchymal organs share identical water densities. Plain films are limited to five basic radiographic densities (air, fat, soft tissue/water, bone/mineral, metal).
  • Pathological organ enlargement is recognized via two mechanisms:
    • Direct Visualization: Outlining of the organ's contours against adjacent radiolucent retroperitoneal or peritoneal fat layers, or against gas-filled intestinal loops.
    • Indirect Visualization (Mass Effect): The enlarged, heavy organ compresses, displaces, and relocates adjacent flexible, gas-filled bowel loops out of their anatomical compartments.
  • Plain films cannot provide volumetric or dimensional organ measurements; definitive evaluation requires cross-sectional imaging (CT, Ultrasound, MRI).

• Hepatomegaly:

  • Radiographic Findings: Marked caudal and medial expansion of the liver silhouette crossing the midline and reaching down into the pelvic brim. Displaces small bowel loops inferiorly into the pelvis and shunts the transverse colon and stomach far into the left lateral abdomen.
  • Etiology: Commonly associated with end-stage chronic liver diseases, such as hepatic cirrhosis.
  • Anatomical Variant (Riedel's Lobe): A benign, non-pathological anatomical variant consisting of a downward, tongue-like projection of the inferior border of the right hepatic lobe extending toward the right iliac crest. It must not be misinterpreted as true hepatomegaly or an intra-abdominal neoplasm.

• Splenomegaly:

  • Radiographic Criteria: Pathological enlargement of the spleen. The splenic shadow must not extend inferiorly beyond the level of the $12\text{th}$ rib. Extension below the $12\text{th}$ rib confirms splenomegaly.
  • Pathological Effects: The enlarged spleen can reach a volume equivalent to the liver, displacing adjacent loops of the splenic flexure and small bowel inferiorly, while pushing the gastric fundus and body medially toward the midline.
  • Etiology: Frequently observed in hematologic malignancies, such as leukemia.

• Renal Enlargement (Renomegaly):

  • Normal Radiographic Anatomy: The left kidney is situated slightly more cephalad than the right kidney, which is displaced downward by the liver. Normal adult renal bipolar length is approximately $12\,\text{cm}$ (comparable to splenic length). The renal outlines are visualized on unenhanced films due to enclosing perirenal fat.
  • Intravenous Urogram (Excretory Pyelogram / IVP): An unenhanced KUB film serves as the baseline; subsequent IV contrast administration opacifies the renal parenchyma, collecting systems (calyces and renal pelvis), ureters, and urinary bladder.
  • Pathology: Massive renal enlargement (e.g., giant simple renal cysts or polycystic disease) presents as a dense lateral abdominal soft tissue mass that compresses the ipsilateral spleen superiorly and displaces small and large bowel loops across the midline to the contralateral side. Definitive diagnostic characterization requires CT or ultrasound.

• Urinary Bladder Enlargement:

  • Normal Anatomy: Outlined within the true pelvic basin by adjacent radiolucent perivesical fat.
  • Pathology and Mass Effect: Marked distension of the bladder produces a massive, smooth, rounded soft tissue opacity rising from the pelvis that displaces all gas-filled bowel loops superiorly into the mid-abdomen.
  • Etiology: In elderly males, massive urinary retention presenting this radiographic appearance is considered secondary to benign prostatic hyperplasia (BPH) until proven otherwise. Mechanical bladder outlet obstruction leads sequentially to urinary retention, bladder enlargement, bilateral hydroureter, and hydronephrosis.

• Uterine and Pelvic Enlargement:

  • Conventional Film Limitations: The normal non-gravid uterus is not visible on plain abdominal films. It only becomes detectable when markedly enlarged by gravidity, leiomyomas (fibroids), or uterine malignancies, presenting as a pelvic mass outlined against peritoneal fat that displaces pelvic bowel loops.
  • Rectal Gas Trap: Linear radiopacities superimposed within the pelvic soft tissues must not be mistaken for the vaginal canal; they represent fecal material and mucosal folds within the rectum.
  • Preferred Modality: Conventional radiography is contraindicated for primary uterine and adnexal evaluation. Sonography (ultrasound) is the definitive imaging modality of choice for assessing the uterus, endometrium, and ovaries. Pelvic transabdominal ultrasound requires a fully distended urinary bladder (which appears anechoic) to provide an acoustic window, allowing clear visualization of the retrovesical uterine body, endometrial stripe, and vaginal canal.

Advanced Abdominal Modalities and Contrast Protocols

• Computed Tomography (CT) in Abdominopelvic Imaging:

  • Clinical Value: CT acts as the definitive imaging workhorse, clarifying ambiguous findings on plain films. It provides cross-sectional multiplanar imaging, high spatial resolution, and exquisite contrast resolution, easily separating subtle tissue densities that plain film radiography (limited to five densities) cannot distinguish.
  • Impact on Clinical Practice: Widespread CT utilization drastically decreased the frequency of negative exploratory laparotomies, reserving exploratory surgery for specific acute indications.
  • Limitations: Involves substantial exposure to ionizing radiation and carries risks associated with iodinated contrast media.

• Iodinated Intravenous Contrast Media:

  • Administration: Water-soluble iodine compounds injected intravenously to assess vascular structures, active hemorrhage, organ perfusion, and inflammatory conditions.
  • Mild Adverse Reactions: Common, self-limiting reactions include flushing/warmth, nausea, vomiting, metallic taste, pruritus, urticaria, and mild rhinitis.
  • Severe Adverse Reactions: Occur at an incidence of $0.01\%$ to $0.04\%$. Include life-threatening bronchospasm, laryngeal edema, anaphylactic cardiovascular collapse, and death. Centers performing contrast studies must have immediate medical and nursing resuscitation teams available.
  • High-Risk Patient Protocols: Patients with a history of asthma or severe atopic allergies have an increased likelihood of contrast reactions. They require prophylactic premedication with systemic corticosteroids and antihistamines (such as cimetidine) prior to injection.
  • Prior Contrast Reactions: A verified previous reaction to intravenous iodinated contrast does not preclude the safe administration of oral contrast without premedication.
  • Renal Safety Thresholds (Mandatory Pre-Contrast Laboratory Testing):
    • Renal function must be screened prior to ordering intravenous contrast.
    • Heightened clinical caution, radiologist consultation, and risk-versus-benefit determination are required when the estimated glomerular filtration rate is $\text{eGFR} < 30\,\text{mL/min/1.73 m}^2$ ($\text{GFR} < 30$) and the serum creatinine is $> 1.5\,\text{mg/dL}$.

• Gastrointestinal Contrast Media: Barium Sulfate vs. Water-Soluble Iodine:

  • Barium Sulfate Properties: Heavy, radiopaque mineral suspension administered orally in divided, timed sips to track luminal transit, or rectally via enema to coat the distal colonic mucosa.
  • Absolute Contraindications to Barium: Barium is strictly prohibited via intravenous routes. It is absolutely contraindicated in cases of suspected esophageal rupture, hollow viscus perforation, anastomotic breakdown, or gastrointestinal fistulas.
  • Pathology of Extravasated Barium: If barium leaks into the peritoneal cavity through a perforation, it triggers catastrophic, severe chemical peritonitis and chronic granulomatous adhesive reactions.
  • Perforation Protocol: When evaluating suspected gastrointestinal tract perforations or fistulas, water-soluble iodinated contrast must be used instead of barium. Extravasated iodinated contrast is biologically inert and safely resorbed by the peritoneal membrane without inducing inflammatory peritonitis.

• Evidence-Based Use of Intravenous and Oral Contrast by Clinical Scenario:

  • Abdominal Trauma: Administer IV contrast to identify vascular injury, parenchymal lacerations, and active extravasation. Oral contrast is contraindicated due to aspiration risk and unnecessary delay.
  • Acute Appendicitis, Aortic Aneurysm, Aortic Dissection, and Soft Tissue Neoplasms: Administer IV contrast to define vascularity, tissue boundaries, and mural inflammation.
  • CT Colonography: Routinely performed without IV contrast, unless staging a documented malignancy.
  • Ureteral Calculi (Stone Search Study Protocol): Performed without IV contrast. Intravenous contrast enters the renal collecting system and ureters, opacifying the lumen and completely obscuring the high attenuation of calcified stones.
  • Non-Traumatic Abdominal Pain, Inflammatory Bowel Disease, and Intra-Abdominal Abscesses: Administer oral contrast (dilute barium or water-soluble iodine) to distend and opacify bowel loops, differentiating fluid-filled loops from pathological abscesses.

• Summary of Core Abdominopelvic Imaging Modalities:

  • Conventional Radiography (Plain Film): Initial triage modality for undifferentiated acute abdominal pain, suspected mechanical bowel obstruction, and screening for pneumoperitoneum. Inexpensive, rapid, and low radiation, but limited to five basic densities.
  • Computed Tomography (CT): The gold-standard modality for comprehensive abdominal evaluation, trauma assessment, staging, and complex acute abdominal pathology.
  • Ultrasound (Sonography): The first-line modality for evaluating the gallbladder, biliary ductal tree, and female pelvic organs (uterus, adnexa, ovaries). It is the modality of choice for investigating pulsatile abdominal masses to assess for abdominal aortic aneurysms (AAA), evaluating vascular flow via Doppler, detecting ascites, and assessing the male pelvis/prostate without radiation.
  • Magnetic Resonance Imaging (MRI): A high-cost, radiation-free problem-solving modality reserved for resolving equivocal findings from CT/ultrasound, performing fine oncologic staging, characterizing complex hepatobiliary architecture (MRCP), and imaging pregnant or pediatric patients.

Questions and Discussion

• Meaning of the KUB Acronym:

  • Prompt: Inquiring what anatomical organs are denoted by the acronym KUB.
  • Response (Rose): Kidneys, Ureter, and Bladder.
  • Explanation: A KUB view is a specialized plain film centered over the renal beds, ureteral paths, and urinary bladder to evaluate for urinary tract calculi without intravenous contrast enhancement.

• Inclusion of Upright Chest Radiographs in Abdominal Pain Workups:

  • Prompt: Why a posteroanterior upright chest radiograph is routinely included when evaluating a patient presenting with abdominal pain.
  • Response (Danley): Because the esophagus connects directly into the abdominal cavity.
  • Response (Natalia): Because the diaphragm divides the thoracic and abdominal cavities, and an upright chest film captures the upper abdomen beneath the diaphragmatic leaflets.
  • Synthesis: Upright chest radiographs capture the subdiaphragmatic space where free peritoneal air preferentially collects beneath the right hemidiaphragm. Furthermore, abdominal pathologies frequently induce reactive thoracic complications (such as a left-sided pleural effusion from acute pancreatitis, a right-sided pleural effusion from a right subphrenic abscess, or basal atelectasis/pneumonia).

• Thoracic Visualization of Abdominal Hernias:

  • Prompt (Danley): Whether epigastric or high abdominal wall hernias that protrude superiorly can be observed on an upright chest radiograph.
  • Response: Confirmed; diaphragmatic and high abdominal hernias protruding into the thoracic cavity are readily visualized on upright chest radiography.

• Radiographic Views Capable of Displaying Air-Fluid Levels:

  • Prompt: Identifying which specific radiographic projections employ a horizontal beam, permitting the detection of air-fluid levels.
  • Response: Upright abdominal projection, left lateral decubitus projection, and posteroanterior upright frontal chest projection. The supine alternative is excluded because its beam travels vertically downward.

• Image Orientation in Lateral Decubitus Radiographs:

  • Prompt: Inquiring whether the patient's feet and head are always oriented in the same standard directions on lateral decubitus views.
  • Response: By universal convention, the feet are directed toward the left of the image display and the head toward the right. If atypical imaging displays are encountered, the lung bases serve as an anatomical landmark to orient the cephalic aspect.

• Anatomical Correlate of Valvulae Conniventes:

  • Prompt: Identifying the anatomical structure within the small intestine represented by valvulae conniventes on plain films.
  • Response: Plicae circulares. Valvulae conniventes correspond to the plicae circulares (circular folds) described in gross anatomy. Intestinal crypts (crypts of Lieberkühn) are microscopic glandular invaginations that cannot be resolved on conventional radiographs.

• Etymological Derivation of Phleboliths:

  • Prompt: Deconstructing the medical components of the term "phlebolith".
  • Response: "Phlebo" refers to veins (as in phlebotomy) and "lith" refers to stone or calcification; thus, a phlebolith is a calcified venous thrombus.