24.2 Interventional Procedures (Embolization, Angioplasty, Stenting) & C-Arm Radiation Safety

24.2 Interventional Procedures & C-Arm Radiation Safety

Vascular Interventional Procedures

Therapeutic vascular interventional radiology utilizes image guidance to restore arterial patency, occlude bleeding vessels or tumor vascular supply, or place intravascular filter devices.

1. Percutaneous Transluminal Angioplasty (PTA) & Vascular Stenting

• PTA Mechanism: An angioplasty balloon catheter is navigated over a guidewire across an arterial stenosis. The balloon is inflated to high pressures ($6\text{--}20\text{ atm}$) using a handheld inflation device filled with a 50:50 mixture of non-ionic contrast media and normal saline. Contrast allows fluoroscopic visualization of balloon expansion and waist resolution. High-pressure inflation causes controlled stretching of the tunica media, cracking of atheromatous plaque, and expansion of the vessel lumen.
• Vascular Stenting: Indicated when PTA yields incomplete expansion ($> 30%$ residual stenosis), flow-limiting arterial dissection, or severe elastic vessel recoil.
  • Balloon-Expandable Stents (Stainless steel or Cobalt-chromium): Mounted directly on an angioplasty balloon and expanded by balloon inflation. Feature high radial strength and precise placement accuracy; ideal for short, rigid ostial lesions (e.g., renal artery origins or common iliac stenoses).
  • Self-Expanding Stents (Nitinol alloy): Constrained inside a outer delivery sheath and automatically expand upon sheath retraction. Feature shape memory and superelastic flexibility; preferred in compressible, mobile peripheral vessels subject to external anatomical deformation (e.g., superficial femoral artery in the Adductor canal).
  • Drug-Eluting Stents (DES): Coated with antiproliferative drugs (Paclitaxel or Sirolimus) that slowly elute into the arterial wall to inhibit smooth muscle cell proliferation (neointimal hyperplasia) and prevent restenosis.

2. Transcatheter Arterial Embolization (TAE) & Chemoembolization (TACE)

Embolization involves deliberate therapeutic occlusion of blood vessels to arrest traumatic bleeding, devascularize tumors, or eliminate arteriovenous malformations (AVMs).

• Classification of Embolic Agents:
  • Temporary Embolic Agents: Gelfoam (absorbable gelatin sponge slurry); occludes vessels for $2\text{--}6\text{ weeks}$, allowing tissue healing and spontaneous vessel recanalization. Primary choice for acute traumatic pelvic hemorrhage or lower GI bleeding.
  • Permanent Particulate Agents: Polyvinyl Alcohol (PVA) particles and calibrated gelatin microspheres ($100\text{--}900\mu\text{m}$ sizes); used for permanent capillary bed occlusion in hypervascular tumors (uterine fibroid embolization, bronchial artery embolization).
  • Mechanical Occluders: Microcoils (platinum or stainless steel with Dacron fibers) and Vascular Plugs; promote rapid thrombosis in target arterial trunks.
  • Liquid Embolic Agents: Onyx (ethylene-vinyl alcohol copolymer) and N-butyl cyanoacrylate (NBCA glue); polymerize upon contact with blood, filling complex AVM niduses.
• Transcatheter Arterial Chemoembolization (TACE): Primary non-surgical treatment for unresectable hepatocellular carcinoma (HCC). A microcatheter super-selectively engages tumor feeding branches of the hepatic artery. A emulsion of chemotherapeutic drugs (Doxorubicin, Cisplatin) combined with Lipiodol (ethiodized oil) is injected, followed by embolic particle delivery. Lipiodol acts as a targeted drug carrier that lodges selectively in HCC microvasculature.

3. Catheter-Directed Thrombolysis & Mechanical Thrombectomy

• Pharmacological Thrombolysis: Direct intra-thrombus delivery of recombinant tissue plasminogen activator (r-tPA / Alteplase) via a multi-sidehole catheter over $12\text{--}24\text{ hours}$ to dissolve acute arterial or deep venous thrombi.
• Mechanical Thrombectomy: Utilizes aspiration catheters or rheolytic thrombectomy devices to physically fragment and aspirate acute thrombi in acute limb ischemia or pulmonary embolism.

4. Inferior Vena Cava (IVC) Filter Placement

• Primary Indication: Prevention of fatal pulmonary embolism (PE) in patients with documented lower extremity deep vein thrombosis (DVT) who have an absolute contraindication to anticoagulation (e.g., active GI bleed, recent neurosurgery) or failure of anticoagulation.
• Deployment Location: Deployed in the infrarenal IVC, immediately below the lowest renal vein ostium (verified via pre-deployment cavogram) to prevent renal vein thrombosis.
• Filter Types: Retrievable (optionally removed within weeks/months once PE risk subsides) vs Permanent.

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Non-Vascular Interventional Procedures

1. Percutaneous Nephrostomy (PCN) & Biliary Drainage (PTBD)

• Percutaneous Nephrostomy (PCN): Placement of a catheter into the renal pelvis/calyx under combined ultrasound and fluoroscopic guidance to relieve hydronephrosis caused by obstructive ureteral calculi or pelvic malignancies.
• Percutaneous Transhepatic Biliary Drainage (PTBD): Insertion of a drainage catheter through the liver parenchyma into the biliary tree to relieve obstructive jaundice caused by cholangiocarcinoma or pancreatic head carcinoma.

2. Percutaneous Biopsy & Abscess Drainage

• Percutaneous Core Biopsy / FNA: Image-guided core needle or fine-needle aspiration under CT or fluoroscopy to obtain diagnostic histopathological tissue.
• Abscess Drainage: Percutaneous placement of a multi-hole pigtail drainage catheter into infected fluid collections using the Seldinger technique, followed by continuous gravity or bulb suction drainage.

3. Vertebroplasty & Kyphoplasty

Interventional treatments for painful osteoporotic vertebral compression fractures.

• Vertebroplasty: Percutaneous transpedicular placement of a large-bore needle into the fractured vertebral body, followed by direct injection of polymethylmethacrylate (PMMA) radiopaque bone cement.
• Kyphoplasty: Introduces an inflatable balloon tamp into the vertebral body prior to cement injection. Inflation restores lost vertebral body height and creates a cavity, allowing lower-pressure PMMA cement delivery and significantly reducing cement extravasation risks.

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Fluoroscopic & C-Arm Radiation Safety in IR

Interventional procedures often require extended fluoroscopy times and high-dose DSA acquisition runs, exposing patients and interventional staff to significant radiation doses.

                    [ Image Detector / Flat Panel (OVER) ]
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                              (PATIENT — Primary Scatter Source)
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                    [ X-Ray Tube under Table (UNDER) ]

1. Scatter Radiation Dynamics & C-Arm Geometry Optimization

• Primary Scatter Source: The patient is the single greatest source of scatter radiation to interventional operating personnel. Maximum scatter intensity occurs at $90^\circ$ to the incident beam and on the X-ray entrance side of the patient.
• Optimal C-Arm Geometry:
  • Position the X-ray tube UNDER the patient table and the Image Intensifier (II) or Flat Panel Detector (FPD) OVER the patient as close to the patient's anterior surface as possible.
  • Physical Rationale: Placing the tube under the table directs primary backscatter downward toward the floor and lower body where protective table-mounted drapes absorb it. If the X-ray tube is positioned over the table, intense backscatter bounces upward directly toward the operator's unshielded head, eyes, and neck.

2. Protective Shielding Requirements for Interventional Staff

• Personal Lead Aprons: Minimum $0.5\text{ mm}$ lead equivalent (Pb eq) wrap-around lead or composite aprons are mandatory for interventional staff.
• Thyroid Shields: Minimum $0.5\text{ mm}$ Pb eq protecting the sensitive thyroid gland.
• Lead Protective Eyewear: Minimum $0.75\text{ mm}$ Pb eq with protective side shields to prevent radiation-induced lens opacities and cataracts.
• Structural Equipment Shields: Ceiling-suspended lead acrylic shields (positioned between operator and patient) and table-side lead skirts mounted to the table side-rail.

3. Operational Dose Reduction Protocols

• Pulsed Fluoroscopy: Using low pulse rates (e.g., $7.5\text{ or }15\text{ pulses/second}$ instead of continuous $30\text{ fps}$ fluoroscopy) reduces total radiation dose by $50%\text{--}75%$ while maintaining adequate catheter tracking visualization.
• Tight Collimation: Collimating to the immediate area of interest reduces total patient scatter volume and improves image contrast.
• Last Image Hold (LIH): Utilizing static LIH frames for anatomical review instead of reactivating live fluoroscopy.

4. Radiation Skin Injury Monitoring & Air Kerma Thresholds

Interventional fluoroscopy can deliver high localized skin doses, potentially causing deterministic radiation injuries.

• Dose Metrics:
  • Reference Air Kerma (AK): Expressed in Gray (Gy); estimates total incident dose at the patient entrance skin reference point.
  • Dose Area Product (DAP): Expressed in $\text{Gy}\cdot\text{cm}^2$; reflects total radiation energy delivered to the patient.
• Deterministic Radiation Skin Injury Thresholds:
  • Peak Skin Dose $> 2\text{ Gy } (2000\text{ mGy})$: Threshold for early transient skin erythema. Requires mandatory procedural documentation and patient clinical follow-up.
  • Peak Skin Dose $> 5\text{ Gy}$: Main erythema and temporary epilation (hair loss).
  • Peak Skin Dose $> 10\text{ Gy}$: Dry and moist skin desquamation.
  • Peak Skin Dose $> 15\text{ Gy}$: Severe dermal necrosis, tissue breakdown, and non-healing radiation skin ulcers.

Summary Table: Interventional Radiology Procedures & Radiation Safety