25.3 Ultrasound-Guided Interventions, MSK & Superficial Masses

Ultrasound-Guided Interventional Technique

Ultrasound's chief advantage for guidance is real-time visualization: the operator watches the needle tip advance toward the target and steers around vessels, bowel, and pleura — all with no ionizing radiation and no contrast. Before any procedure, verify the order, obtain informed consent (risks include bleeding, infection, and injury to adjacent structures), perform a time-out confirming patient, site, and procedure, and review coagulation status and anticoagulant medications when relevant.

Two guidance approaches exist:

• Freehand technique: the operator holds the transducer in one hand and the needle in the other. It is flexible — the needle angle can be adjusted continuously — but demands hand-eye coordination and constant attention to keeping the needle in the imaging plane
• Needle guide: a bracket clipped to the transducer constrains the needle to a fixed angle displayed as an on-screen guideline. It improves accuracy for deep or small targets and for less experienced operators, at the cost of some flexibility

Sterile technique is mandatory: a sterile probe cover (with gel inside the cover for coupling), sterile gel on the skin, a sterile field, and skin antisepsis. Local anesthesia (for example, 1% lidocaine) is infiltrated along the planned track under direct visualization. Color Doppler maps vessels in the path before the needle is advanced. The needle tip — never the shaft — must be positively identified at the target before sampling or deployment; gentle tissue motion or a tiny injection of saline or anesthetic can confirm tip position.

Fine-Needle Aspiration vs. Core Needle Biopsy

Typical applications: thyroid nodules are sampled by FNA with several passes under ultrasound guidance; breast masses generally undergo core biopsy so hormone receptors and tumor grade can be assessed; liver lesions may be biopsied by either method, with Doppler used to avoid crossing large vessels; lymph nodes require core samples when lymphoma is suspected because architectural pattern matters. On-site cytopathology review, when available, confirms specimen adequacy before the patient leaves.

Drainage Procedures

• Paracentesis: scan the lower abdomen and flank to find the largest dependent pocket of ascites. Mark the skin site, document skin-to-fluid depth and pocket depth, and use Doppler to avoid the inferior epigastric vessels (which course along the posterior rectus sheath) and any varices. A left lower quadrant or midline infraumbilical site is common
• Thoracentesis: with the patient upright (or decubitus), identify the effusion in a costophrenic sulcus, confirm diaphragm position throughout the respiratory cycle, and mark an entry site in the intercostal space above the rib — the intercostal artery, vein, and nerve run in the groove along the rib's inferior margin. Confirm the pocket is large enough to avoid lung puncture, and never advance the needle deeper than the measured pocket
• Abscess drainage: after confirming a drainable collection, most catheters are placed by the Seldinger technique — access needle, guidewire coiled within the cavity, serial dilators, then a pigtail drainage catheter secured at the skin. Output is recorded, and the catheter is flushed per protocol

Intraoperative ultrasound uses a sterile-draped transducer directly on an exposed organ — most often the liver — to localize sub-centimeter tumors, define their relationship to hepatic and portal veins, and guide resection margins or ablation probe placement.

Complications to counsel about and monitor for include bleeding (the most common), infection, pneumothorax after thoracic or subdiaphragmatic procedures, and vasovagal reactions. Post-procedure care includes vital signs, puncture-site checks, and a brief observation period scaled to the organ biopsied (longer for liver and kidney); patients receive written precautions and instructions for when to seek care.

Musculoskeletal Applications

Musculoskeletal sonography uses high-frequency linear transducers (10 MHz and above), generous gel or a stand-off pad for superficial structures, side-by-side comparison with the contralateral limb, and dynamic imaging through range of motion.

• Rotator cuff tears: the supraspinatus is examined in the Crass or modified Crass position — extension, adduction, and internal rotation, with the hand behind the back or resting on the ipsilateral hip. A full-thickness tear appears as an anechoic or hypoechoic defect through the tendon, nonvisualization of tendon fibers with deltoid sagging against the humeral head, and flattening of the bursal contour; partial-thickness tears are focal hypoechoic defects on the bursal or articular surface
• Tendon rupture: complete rupture shows a fiber gap, often filled with fluid or granulation tissue, and dynamic scanning demonstrates the torn ends separating with motion
• Joint effusion: anechoic or complex fluid distends the joint recess (for example, the suprapatellar pouch of the knee), is compressible and displaceable, and can be targeted for aspiration
• Foreign body localization: wood, glass, and metal are echogenic with posterior shadowing or comet-tail artifact; ultrasound excels at finding radiolucent objects like wooden splinters and marking the skin directly over them for removal

Superficial Masses and the Abdominal Wall

The swirling debris sign — gently compressing the mass and watching internal echoes shift in real time — is a favorite exam clue for abscess and for complicated cysts.

For the abdominal wall, ultrasound evaluates hernias dynamically: inguinal, femoral, umbilical, and incisional hernias are scanned at rest and with the Valsalva maneuver, which pushes contents through the fascial defect and demonstrates reducibility and the neck width. An abdominal wall endometrioma appears as a solid, hypoechoic, sometimes vascular mass within a cesarean-section or laparoscopic scar in a woman of reproductive age, with pain and swelling that wax and wane with the menstrual cycle. Desmoid tumors and rectus sheath hematomas round out the differential for solid and fluid abdominal wall lesions.