24.1 Vascular Access, Seldinger Technique & Digital Subtraction Angiography (DSA)

24.1 Vascular Access, Seldinger Technique & Digital Subtraction Angiography (DSA)

Introduction to Interventional Vascular Access

Interventional Radiology (IR) combines high-resolution diagnostic fluoroscopy with minimally invasive therapeutic techniques. Central to diagnostic arteriography and endovascular therapy is obtaining safe, predictable, and reproducible percutaneous entry into the vascular system. The radiologic technologist (RadTech) plays a crucial operational role in sterile field preparation, vascular hardware management, monitoring automatic high-pressure injectors, and executing digital subtraction angiography (DSA) post-processing protocols.

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Percutaneous Vascular Access: The Seldinger Technique

Introduced in 1953 by Swedish radiologist Sven Ivar Seldinger, the Seldinger technique revolutionized vascular intervention by replacing traumatic surgical vessel cutdowns with percutaneous needle puncture. It is the universally accepted standard method for arterial, venous, and non-vascular percutaneous lumen access.

The Standard 6-Step Percutaneous Access Protocol

1. Step 1: Needle Puncture of the Target Vessel An access needle (typically an 18-gauge thin-wall needle or a 21-gauge micro-puncture needle) with an internal sharp stylet is inserted through a small skin nick. The needle is angled at $30^\circ$ to $45^\circ$ relative to the skin surface and advanced directly into the anterior wall of the target blood vessel.

2. Step 2: Removal of Inner Stylet & Verification of Blood Return The sharp inner stylet is carefully withdrawn while maintaining the needle position. Free, brisk, pulsatile flow of bright red blood confirms proper intraluminal positioning within an artery (or steady non-pulsatile dark blood return for venous access). If blood flow is sluggish or absent, the needle tip may reside subintimally or beyond the posterior wall.

3. Step 3: Guidewire Insertion Through Needle A flexible, atraumatic guidewire (typically $0.035\text{ inch}$ or $0.018\text{ inch}$ diameter) is gently passed through the needle lumen into the vessel under fluoroscopic visualization. The wire must advance smoothly without any tactile resistance. Rule: Never force a guidewire against resistance, as this can cause vessel dissection, intima perforation, or arterial spasm.

4. Step 4: Needle Removal Over the Guidewire While keeping the guidewire firmly anchored at the skin surface to prevent accidental wire advancement or dislodgement, the puncture needle is completely withdrawn over the wire and removed from the field. The external wire is immediately wiped with a wet heparinized gauze sponge to remove dried blood clots.

5. Step 5: Sheath or Catheter Advancement Over Guidewire A tapered vascular sheath (fitted with an internal vessel dilator) or a diagnostic angiographic catheter is threaded over the proximal end of the guidewire. The sheath-dilator assembly is advanced through the skin track into the vessel lumen using a gentle twisting motion.

6. Step 6: Removal of Guidewire & Vessel Dilator Once the vascular sheath is securely positioned within the vessel, the internal dilator and guidewire are removed together, leaving the hemostatic sheath in place. The side-port valve of the sheath is aspirated to remove any air micro-bubbles or clot fragments and immediately flushed with heparinized saline solution.

Summary Table: Steps of the Standard Seldinger Technique

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Arterial Access Sites & Anatomical Landmarks

1. Common Femoral Artery (CFA) — Primary Access Site

• Anatomical Boundaries: The CFA is the direct downward continuation of the external iliac artery below the inguinal ligament. It bifurcates into the superficial femoral artery (SFA) and profunda femoris artery.
• Ideal Fluoroscopic Target: The puncture site must be positioned over the inner third of the femoral head as identified on fluoroscopy.
• Advantages: High vessel caliber ($7\text{--}10\text{ mm}$ diameter), superficial depth, and firm posterior bony backing provided by the femoral head, which allows effective manual compression to achieve post-procedure hemostasis.
• Puncture Hazards:
  • High Puncture (Above Inguinal Ligament / Femoral Head): Enters the external iliac artery above the retroperitoneal reflection. Compression against bone is impossible, leading to occult, life-threatening retroperitoneal hemorrhage.
  • Low Puncture (Below CFA Bifurcation): Puncturing the SFA or profunda femoris increases the risk of pseudoaneurysm formation, arteriovenous (AV) fistula, thrombosis, and hematoma.

2. Radial Artery — Transradial Access (TRA)

• Clinical Trend: Transradial access has become the preferred approach for coronary angiography and selected peripheral interventions.
• Advantages: Eliminates retroperitoneal bleeding risks, drastically lowers access-site hematoma rates, permits immediate patient post-procedure ambulation, and reduces hospital stay costs.
• Prerequisite Assessment (Modified Allen Test / Barbeau Test): Prior to radial cannulation, patency of the dual palmar arterial arch must be verified to ensure collateral blood supply to the hand via the ulnar artery if radial artery occlusion occurs. Barbeau test utilizes pulse oximetry on the thumb during radial artery compression to confirm collateral flow.

3. Brachial & Axillary Arteries — Secondary Access Sites

• Utilized when severe aortoiliac occlusive disease, Leriche syndrome, or anatomical tortuosity precludes lower extremity access. Punctures carry a higher risk of median nerve injury, brachial artery thrombosis, and compartment syndrome due to smaller vessel diameter and lack of bony compression backing.

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Angiographic Equipment & Catheters

1. Guidewires

Guidewires serve as the foundational structural track over which catheters, sheath dilators, balloons, and stents are navigated.

• Tip Configurations:
  • J-Tip ($1.5\text{--}3.0\text{ mm}$ radius curve): The curved J-tip deflects off vessel walls, preventing accidental subintimal dissection or intima scoring in tortuous, diseased, or aneurysmal vessels.
  • Straight Tip: Used for selective probing and crossing tight arterial stenoses or calcified branch origins.
• Core Materials & Coatings:
  • Hydrophilic Polymer Coating (Glidewire): Becomes extremely slick when hydrated with saline, reducing friction inside tortuous vessels. Must be kept continuously wet on the back table to prevent friction lock.
  • Nitinol (Nickel-Titanium Alloy): Superelastic core providing maximum kink-resistance and shape memory.
  • Stainless Steel Core: Provides high vertical column strength and tactile pushability for delivering heavy interventional equipment.
• Standard Diameters: $0.035\text{ inch } (0.89\text{ mm})$ for general peripheral and visceral diagnostic imaging; $0.014\text{ inch } (0.36\text{ mm})$ for microvascular, renal, and coronary interventions.

2. Vascular Sheaths

Vascular sheaths are outer protective tubes inserted into the access site to shield the arterial wall from friction trauma during repeated catheter exchanges. Key components include an integrated hemostatic valve at the hub to prevent backbleeding and a side-port channel for continuous heparinized saline infusion or direct invasive blood pressure monitoring.

3. Diagnostic Angiographic Catheters

Catheters are sized according to the French scale (Fr), where $1\text{ Fr} = \frac{1}{3}\text{ mm}$ outer diameter (e.g., a $6\text{ Fr}$ catheter has an outer diameter of $2.0\text{ mm}$):

• Pigtail Flush Catheter: Features a circular coiled tip with multiple side-holes located proximal to the end-hole. Designed for high-volume non-selective injections (e.g., thoracic or abdominal aortography). The side-holes disperse contrast radially, preventing high-velocity fluid jets that could cause catheter recoil ("whipping") or traumatic aortic wall dissection.
• Cobra Catheter (C1, C2, C3): Angled tapered tip used for selective cannulation of visceral arteries arising from the abdominal aorta (renals, celiac trunk, superior mesenteric artery).
• Judkins Left (JL) & Judkins Right (JR): Pre-shaped specialized catheters designed for selective engagement of left and right coronary artery ostia.
• Simmons / Sidewinder Catheter: Reverse-curve catheter designed for selective catheterization of branch vessels in tortuous aortic arches or acute-angled visceral take-offs.

4. Automatic High-Pressure Injectors

Automatic injectors deliver high-density non-ionic contrast media at exact flow rates and pressures beyond human hand-injection capability.

• Injector Control Parameters:
  • Flow Rate (mL/sec): Volume of contrast delivered per second (e.g., $20\text{--}25\text{ mL/sec}$ for abdominal aortography; $3\text{--}5\text{ mL/sec}$ for selective renal angiography).
  • Total Volume (mL): Total contrast dose delivered during an injection run (e.g., $30\text{--}50\text{ mL}$ for aortography).
  • Pressure Limit (PSI): Maximum allowable pounds per square inch (typically set to $800\text{--}1200\text{ PSI}$) to prevent catheter hub bursting or lumen rupture.
  • Linear Acceleration / Rise Time: Time taken to reach peak flow rate (e.g., $0.2\text{--}0.5\text{ sec}$); gradual acceleration prevents sudden catheter recoil.
  • Syringe Heating Sleeve: Warms viscous contrast to body temperature ($37^\circ\text{C}$), which reduces contrast viscosity by approximately 50%, enabling higher flow rates through thin-walled diagnostic catheters.

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Digital Subtraction Angiography (DSA) Principles

Digital Subtraction Angiography (DSA) is a computer-assisted fluoroscopic technique that isolates contrast-filled blood vessels by mathematically eliminating background anatomical structures (bone and soft tissue).

[Pre-Contrast Mask Image]  --->  (Inverted Pixel Values)
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[Live Contrast Image]      --->  [Subtracted Image] (Vessels Only)

The DSA Subtraction Algorithm

1. Mask Frame Acquisition: Prior to contrast arrival, a baseline pre-contrast radiograph (the mask frame) is acquired under fluoroscopy. This frame contains all background skeletal, dental, and soft-tissue structures.
2. Contrast Injection Frames: Contrast medium is injected via an automatic injector, and serial high-resolution radiograms are recorded at frame rates of $1\text{--}8\text{ frames/sec}$ across arterial, capillary, and venous phases.
3. Mathematical Subtraction: The digital matrix values (logarithmic attenuation values) of the pre-contrast mask image are subtracted pixel-by-pixel from the live contrast-enhanced frames. Unchanged background structures equal zero density and disappear, leaving only contrast-opacified vascular lumens displayed in high contrast against a neutral background.

Advanced DSA Post-Processing Functions

• Roadmap Function: A DSA image exhibiting peak vascular opacification is captured and stored as a baseline map. This image is inverted and displayed as a semi-transparent live overlay on low-dose real-time fluoroscopy. The operator can navigate microcatheters and guidewires through intricate vascular branches without administering additional contrast injections.
• Pixel Shift (Re-registration): Involuntary patient motion (respiration, swallowing, peristalsis) between mask acquisition and contrast arrival causes misregistration artifacts (unsubtracted bright or dark bone edges obscuring vessels). Pixel shift allows the technologist to manually or automatically shift the mask matrix along sub-pixel X and Y axes to realign bone boundaries and eliminate motion artifacts.
• Re-Masking: Selecting a different pre-contrast frame from the acquired frame sequence closer to the contrast arrival time to eliminate motion displacement artifacts.