15.2 CT-Guided Interventional Procedures — Biopsy, Drainage & Aspiration

Why Interventional Procedures Are Tested Here

CT is not only a diagnostic tool — it is a real-time guidance system for percutaneous procedures, and ARRT's "Additional Procedures" focus theme explicitly names image-guided biopsy, drainage, and aspiration alongside vascular and trauma work. These cases test a different skill than routine diagnostic scanning: choosing needle/catheter hardware, planning a safe path, and recognizing complications on the confirmation images taken immediately afterward.

Core Biopsy vs. Fine Needle Aspiration

The two sampling techniques answer different pathology questions.

Core biopsy generally has a higher complication rate than FNA (pooled major-complication rates roughly 5.7% vs. 4.4%, and higher pneumothorax/hemorrhage rates for lung core biopsy specifically) because the larger needle causes more tissue disruption, but it yields architecture that cytology alone cannot provide — for example, distinguishing a well-differentiated tumor from reactive tissue. Many practices now combine both: an FNA pass for rapid on-site cytologic assessment, followed by one or two core passes for definitive histology, all through the same coaxial introducer.

Coaxial Technique

Rather than reinserting a fresh needle through the skin, pleura, or organ capsule for every sample, the coaxial technique places one introducer (guide) needle through the critical boundary once, confirms position with a low-dose scan, and then passes the smaller sampling needle through the introducer's lumen as many times as needed. Benefits:

• Only one puncture through the pleura (for lung), capsule (for liver/kidney), or dura (for spine) regardless of how many samples are taken.
• The introducer stays in place as a stable, reusable channel, reducing cumulative tissue trauma and procedure time.
• If a complication occurs (for example, bleeding at the needle tip), the introducer can be used to deliver a hemostatic agent or coil without another pass.

Guidance Methods: Step-and-Shoot vs. CT Fluoroscopy

Two guidance workflows are used to advance a needle to target:

• Step-and-shoot (intermittent) guidance: The technologist advances the needle a short distance, then acquires a single low-dose confirmation image, repeating until the tip reaches target. This is the lower-dose default for most straightforward biopsies.
• CT fluoroscopy: Near real-time, continuously updated images let the operator watch the needle advance live, which is valuable for moving targets, tight anatomic corridors, or fine adjustments near vessels. The tradeoff is continuous radiation exposure to everyone in the room for as long as the fluoroscopic mode runs — the operator's hand may be in or near the beam, and the technologist should step back from the gantry or use additional lead shielding rather than remaining at the console position used for diagnostic scanning.

Gantry angulation (tilt) and skin-entry grid or laser-guidance systems help plan an oblique needle path that avoids ribs, bowel loops, or major vessels when the target does not sit in a straight vertical line from the skin.

Percutaneous Drainage of Fluid Collections

For an abscess or symptomatic fluid collection, the standard approach is the Seldinger technique:

1. An 18-gauge access needle is advanced under CT guidance into the collection, confirmed by fluid aspiration.
2. A 0.035-inch J-tipped guidewire is passed through the needle and coiled within the cavity.
3. The needle is removed over the wire, and the tract is serially dilated.
4. A pigtail catheter, typically 8 to 14 French, is advanced over the wire and its locking loop is formed inside the collection.

Catheter size is matched to fluid viscosity: an 8- to 8.5-French catheter is usually adequate for thin, low-viscosity fluid, while 10- to 14-French catheters are chosen for thick, particulate, or organizing collections that would occlude a smaller lumen. Undersizing the catheter for viscous contents is a common cause of drainage failure and repeat intervention.

Post-Procedure Confirmation and Complications

Immediately after a lung biopsy, a confirmation CT through the biopsy tract checks for pneumothorax and pulmonary hemorrhage — the two leading complications. Pooled literature places overall pneumothorax rates around 25% for core biopsy and 19% for FNA, with roughly 5-6% of cases needing chest tube placement; risk climbs with larger needle gauge, more aerated lung traversed, emphysema, and deeper lesions. If the immediate scan is negative but the patient later becomes symptomatic, a delayed upright chest radiograph around one hour post-procedure is the standard next step, since a small pneumothorax can enlarge over time. Positioning the patient biopsy-site-down (or otherwise dependent) after the procedure is a common technique to tamponade the needle tract and reduce the chance of a pneumothorax progressing.

Common Traps

• Assuming FNA and core biopsy are interchangeable — the pathologist's question (cytology vs. histology) should drive needle choice, not habit.
• Forgetting that CT fluoroscopy's continuous exposure means the technologist's normal console position is not automatically a safe distance — additional shielding or physical distance is required.
• Selecting an undersized drainage catheter for a thick, organizing abscess, leading to occlusion and a failed drain.
• Skipping the immediate post-biopsy confirmation scan through the needle tract, which is the fastest way to catch a developing pneumothorax before the patient leaves the department.