Abdominal Radiography: Anatomical Landmarks, Patient Positioning, and Contrast Procedures

Anatomical Landmarks and Patient Positioning for Abdominal Radiography

• Pubic Symphysis Positioning:

  • Direct palpation of the pubic symphysis is no longer practiced in modern clinical radiography culture.

  • The greater trochanter of the femur is the anatomical landmark that directly aligns horizontally with the pubic symphysis.

  • To ensure inclusion of the pubic symphysis on an abdominal or pelvic radiograph, palpate the greater trochanter of the femur.

  • Leg Positioning Technique: Internally rotate the patient's entire leg (including knee, foot, and hip) with hands placed flat.

• Iliac Crest and Central Ray Centering:

  • The top of the ilium is defined as the iliac crest, which corresponds anatomically to the level of the patient's waist/hips.

  • Full-Length Abdomen Centering: Position the central ray ($CR$) at the level of the top of the iliac crest to capture the complete length of the abdomen on a supine image.

Upright Abdominal Radiography Considerations

• Central Ray Centering for Upright Images:

  • For upright abdominal projections, center the central ray $2\text{ inches}$ above the top of the iliac crest.

  • Procedure: Palpate the iliac crest and position the central ray exactly $2\text{ inches}$ superior to it.

• Diagnostic Rationale for Superior Centering:

  • Superior centering ($2\text{ inches}$ above the crest) is required on upright abdominal views to ensure complete visualization of the diaphragms.

  • Inclusion of the diaphragm on the image is essential for evaluating free intraperitoneal air.

• Anatomical Variations in Torso and Limb Lengths:

  • Torso and leg proportions vary significantly between individual patients.

  • Two patients of identical height can possess completely different body proportions (e.g., one patient with long legs and a short torso versus another with a long torso and short legs).

  • Applying blanket or unadjusted positioning without individual landmark palpation leads to anatomical clipping due to structural variations.

Diagnostic Projections: Upright vs. Left Lateral Decubitus

• Upright Abdomen Objectives:

  • Upright abdominal projections are primarily performed to assess for air-fluid levels and intraperitoneal free air.

  • Required Upright Duration: The patient must remain in an upright position (standing, or sitting in a bed, chair, or wheelchair) for $3\text{ to }5\text{ minutes}$ prior to exposure to allow free gas to rise and fluid levels to settle.

• Left Lateral Decubitus ("Left Lateral Cue") Protocol:

  • If a patient is physically unable to stand or sit upright, a decubitus projection is performed.

  • Difference Between Chest and Abdomen Decubitus Views:

    • Chest decubitus projections may be performed as either a right lateral decubitus or a left lateral decubitus.

    • Abdominal decubitus projections are performed exclusively as a left lateral decubitus.

  • Rationale for Left Lateral Decubitus: Positioning the left side down prevents free air from being obscured by gastric gas bubbles and allows free intraperitoneal air to rise and be easily visualized against the homogeneous density of the liver on the right side.

Free Air and Abdominopelvic Cavity Anatomy

• Definition of Free Air:

  • Free air refers to gas situated outside of its normal anatomical cavities (such as air outside the pleural cavities in the chest, or outside the lumen of the gastrointestinal tract within the abdominal and pelvic cavities).

• Visualization of Abdominal Organs:

  • Plain radiographs do not demonstrate all visceral organs clearly.

  • Hollow organs are not inherently radiopaque and require administration of contrast media to be visualized.

• Abdominopelvic Cavity Divisions and Diagnostic Modalities:

  • The abdominopelvic region is divided into the abdominal cavity and the pelvic cavity.

  • Gallbladder Procedures: Historical fluoroscopic procedures involved oral administration of a fatty contrast drink. The high-fat content stimulated gallbladder contraction, rendering the gallbladder visible as a dense, round white structure on X-ray.

  • Routine plain film contrast studies of the gallbladder have been largely replaced by modern imaging modalities including Ultrasound, Computed Tomography (CT), and Magnetic Resonance Imaging (MRI).

  • Surgical removal of the gallbladder is termed a cholecystectomy (historically referenced in context as a polysuspectomy).

Abdominal Organs, Contrast Examinations, and Special Procedures

• Anatomical Layout of Abdominal Organs:

  • Small and Large Intestines: Positioned centrally and peripherally within the abdominal cavity.

  • Spleen: Located in the left upper quadrant inferior to the stomach. The normal spleen is not routinely visible on plain X-rays unless significantly enlarged (e.g., secondary to infectious mononucleosis / mono). Splenic pathology is evaluated primarily via CT, MRI, or ultrasound.

  • Pancreas: Situated transversely between upper abdominal regions.

  • Kidneys: Located posteriorly in the middle of the abdomen, with the right kidney positioned slightly higher relative to the left kidney in this anatomical overview.

• Fluoroscopic Contrast Examinations:

  • Barium Enema: Fluoroscopic examination of the large intestine using barium sulfate.

  • Gastrografin ("Gastropan") Enema: Examination using water-soluble contrast media when bowel perforation is suspected.

  • Upper GI (UGI) with Small Bowel Follow-Through: Radiographic assessment of the stomach and small bowel sequence.

• Gynecological Procedures (Hysterosalpingogram):

  • A Hysterosalpingogram (HSG) is an OB/GYN procedure performed using radiopaque contrast media to evaluate the uterine cavity and check for fallopian tube blockages.

Questions & Discussion

• Question: How does contrast administration in a hysterosalpingogram (HSG) assist with or enable pregnancy?

• Answer: The therapeutic benefit is not caused by the chemical composition of the contrast agent itself, but rather by the mechanical force and hydrostatic pressure of pushing the liquid contrast through the uterus and fallopian tubes, which can flush out minor blockages or mucus plugs.

Peritoneal Structure, Psoas Muscles, and Technical Factors

• Peritoneal Sac:

  • The abdominopelvic cavity is lined by a double-walled membranous sac (peritoneum) that holds internal organs in place.

• Psoas Muscle Anatomy:

  • The psoas muscles (spelled $P\text{-}S\text{-}O\text{-}A\text{-}S$) are essential muscular landmarks on abdominal radiographs.

  • Anatomical Origin and Path: The psoas muscle originates bilaterally along the lumbar spine from the level of $L1$ down to the ilium, extending along vertebrae $L1, L2, L3, L4, \text{and } L5$.

  • Radiographic Visualization: The outline of the right-sided psoas muscle is commonly visualized extending diagonally downward toward the pelvis.

• Exposure Factors ($kVp$ and Density):

  • The abdomen contains variable subject densities: organs, gas/air, muscle, and bone.

  • Exposure techniques (mAs) vary depending on tissue mass and patient habitus, while peak kilovoltage ($kVp$) remains standardized to maintain optimal subject contrast.

Positioning Criteria, Respiration, and Pathological Evaluation

• Respiration Protocol:

  • Abdominal radiographs must always be acquired on full expiration.

  • Full expiration elevates the diaphragm, reduces intra-abdominal compression, and prevents respiratory motion artifact.

• Motion Management:

  • Radiographers must minimize both voluntary motion (patient body movement) and involuntary motion (peristalsis) through proper patient instructions and exposure controls.

• Standard Routine Abdominal Series:

  1. Supine Abdomen (KUB / Flat)

  2. Upright Abdomen

  3. Left Lateral Decubitus

• Collimation and Field Coverage:

  • The inferior border of an abdominal radiograph must include the pubic symphysis.

  • Clipping lateral excess soft tissue on larger patients is acceptable provided all intra-abdominal viscera, kidneys, ureters, and bladder are included.

  • Broad soft tissue coverage must be maintained when evaluating abdominal wall hernias (such as an ascending large bowel herniation through abdominal muscle wall causing intestinal obstruction). Intestinal obstruction represents a bowel pathology requiring full abdominal coverage rather than a targeted renal or bladder examination.