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Radiology Basics

Ultrasound and CT e-learning for junior doctors and students

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Fundamental Principles of IR

The fundamental principles underpinning interventional radiology (IR) techniques

  • What is the Seldinger technique and why is it important in interventional radiology?
  • Why is it important to select the right tools important in interventional radiology?
  • What are angioplasty balloons and how are they used in interventional radiology?
  • What are stents, stent grafts, and drains, and how are they used in interventional radiology?
  • What are embolic agents and how are they used in interventional radiology?

Getting access

One of the fundamental parts of IR is getting safe access to a blood vessel or organ. Ultrasound guidance is usually used to access vessels, transfemoral being the most common approach.

Femoral access is gained by identifying the common femoral artery overlying the head of the femur on ultrasound. Then ‘cutting down’ by making a small incision in the groin area. Under careful visualisation with ultrasound a needle is then inserted into the femoral artery with position confirmed by ultrasound and flashback of pulsatile arterial blood. A wire is then passed down the lumen of the needle to secure the position in the artery before changing out the needle for a sheath. This is the Seldinger technique, the method used for nearly all procedures in IR including vascular procedures like angiography, angioplasty, and stenting and non-vascular procedures like cholangiograms and nephrostomies.

Transfemoral access is often preferred when the target vessels are located in the legs or abdomen or when a large lumen is required.

Access to other visceral organs may be obtained using other methods, like radial artery access or non-vascular transcutaneous direct organ access.

Seldinger technique

The Seldinger technique is a medical procedure used to access blood vessels or other hollow organs in the body, such as the urinary tract or gastrointestinal tract. It was first described by Swedish radiologist Dr. Sven-Ivar Seldinger in 1953.

The technique involves the use of a needle to puncture the skin and gain access to the vessel or organ of interest. A guidewire is then passed through the needle and into the vessel or organ. The needle is removed, and a sheath or catheter is then passed over the guidewire into the vessel or organ. The guidewire is removed, and medical instruments can be passed through the sheath for various diagnostic or therapeutic purposes, such as performing angiography, delivering medications or contrast agents, or draining fluids.


Needle diameter is measured in gauges. The lower the gauge, the larger the diameter of the needle with a colour corresponding to each size.

21-, 19-, and 18-gauge needles are often used for arterial and venous access. 22- and 21-gauge needles can be used for small targets (eg: small abscesses, renal calyces, and small vessels). Larger needles (eg: 18 gauge and under) can be used for bigger targets (eg: large abscesses, larger vessels that are easily accessible) and biopsies.

Single-wall: Hollow core needles with a single bevelled edge are most commonly used.

Trocar: consists of an outer cannula, an inner stylet, and a sharp tip (three sided).

Chiba: Two-part high-gauge needle system with a bevelled edge.


Guidewires used in interventional radiology are designed to navigate through the complex anatomy of blood vessels to access and treat specific areas of the body.

Standard access guidewire

A simple short wire; used for access and often quickly exchanged.

Rail wire

Stiff, good for maintaining access.

J-tip guidewires

These guidewires have a pre-formed J-shaped tip that is designed to facilitate navigation through small or tortuous vessels.

Hydrophilic guidewires

These guidewires have a hydrophilic coating that makes them easier to navigate through blood vessels. They are particularly useful for accessing vessels that are narrow or have a lot of twists and turns.

Microcatheter guidewires

These guidewires are used in conjunction with microcatheters, which are small, flexible tubes that are used to access and treat small blood vessels. They are typically smaller in diameter than standard guidewires and are designed to navigate through very small or delicate vessels.

Wire diameter is measured in inches. 0.018 inch and 0.035 inch diameter wires are most commonly used.


A catheter in interventional radiology is a thin, flexible tube that is inserted into a blood vessel or other body cavity to allow for the injection of contrast dye, medications, or other diagnostic or therapeutic materials.

Catheter and sheath diameters are measured in French (F) units.

1 French = 1/3 mm = 0.013 inch


Balloons used in interventional radiology are inflatable devices that are used to open up narrowed or blocked blood vessels. They can also be used to compress bleeding vessels in specific areas of the body.

Angioplasty balloons

Angioplasty balloons are used to widen narrowed or blocked blood vessels. The balloon is inflated inside the vessel, which pushes the vessel walls outward and improves blood flow. After the balloon is deflated and removed, a stent may be placed to help keep the vessel open.


A stent is a medical device used in interventional radiology to treat various conditions, such as blocked or narrowed blood vessels or organs. Common causes of such blockages are atherosclerosis in blood vessels or tumours, stones and strictures in other organs. A stent is a small, expandable tube that is inserted into a blocked or narrowed lumen to help hold it open and improve blood flow or function.

Stents can be made from a variety of materials, such as metal, plastic, or fabric, and come in different shapes and sizes depending on the intended use.

A self-expanding stent: This type of stent is designed to expand on its own once it’s deployed into the target area. It’s made of metal or polymer, with a mesh-like structure that can be compressed into a smaller diameter for delivery through a catheter or other minimally invasive means. Once in place, the stent can expand to a predetermined size and exert a constant radial force against the vessel wall, helping to keep it open.

Balloon-expandable stents: a small balloon is inflated inside the stent, which expands the mesh and presses it against the walls of the vessel. This locks the stent in place and helps to hold the artery open.

A stent graft is a medical device that is used to treat a variety of conditions, including aneurysms, or weakened areas in blood vessels that can bulge and potentially rupture. It is a tubular structure made of a metal mesh frame covered with a synthetic fabric.


Drains are thin, flexible tubes that are inserted through the skin and into a specific area of the body to remove excess fluid, blood, or pus. They are commonly used in interventional radiology procedures to help manage a variety of conditions, such as abscesses, cysts, and fluid build-up after surgery.

Common Drain Types

Percutaneous Drain: An ultrasound-guided drainage tube that is inserted through the skin into an abscess or cyst to drain the fluid. This is typically done under local anaesthesic.

Pigtail Catheter: A small, flexible tube that is inserted through a small incision or puncture wound to allow drainage of fluid or blood. The tip has a characteristic curly shape much like the tail of a pig.

Chest Drain: A tube inserted through the chest wall to drain fluid or air from the chest cavity.

Nephrostomy Tube: A tube inserted through the skin into the kidney to drain urine in cases of obstruction or infection.

Placement and Management of Drains

Drains are typically placed by interventional radiologists using imaging techniques such as ultrasound, computed tomography (CT) scans, or plain radiographs (X-rays) to guide the placement of the tube. Once the tube is in place, it is secured to the skin and connected to a drainage bag or system.

Managing the drains includes monitoring the amount and quality of fluid draining from the site, checking for infection or other complications, and changing the drainage bag or system as needed. Most patients with drains will require regular follow-up to ensure the tube is properly functioning and to monitor for any complications. Overall, drains are an important tool for interventional radiologists to manage a variety of medical conditions.

Embolic Agents

Embolic agents are substances used to block or occlude blood vessels, either to control bleeding, vascular malformations or to treat tumours. Interventional radiologists use these agents to target specific blood vessels during procedures such as embolisation or chemoembolisation.

Some embolic agents are particulate, meaning they are made up of tiny beads that can be injected through a catheter into the blood vessels. The beads will then lodge in the vessel, obstructing blood flow and depriving the targeted area of oxygen and nutrients. Examples of particulate embolic agents include polyvinyl alcohol (PVA) particles and microspheres.

Another type of embolic agent is liquid embolic material, such as glue (cyanoacrylate glue). This type of agent is typically used in the treatment of arteriovenous malformations (AVMs) and aneurysms. When injected into the affected vessel, it polymerises and hardens, forming a plug or seal to block blood flow.

Other embolic agents include coils, which are small metal wires that can be placed into an aneurysm or blood vessel to prevent rupture or bleeding.

Embolic agents can also be combined with chemotherapy drugs to treat tumours, a technique known as chemoembolisation.

It is important for interventional radiologists to carefully select and use the appropriate embolic agent for each particular case, taking into account factors such as the size and location of the targeted vessel, the nature of the lesion or disease being treated, and the patient’s overall health and medical history.


  • The Seldinger technique is fundamental to gaining access to body structures.
  • Selecting the right tools for the task is an important skill for interventional radiologists.
  • Angioplasty balloons are used to widen narrowed or blocked blood vessels.
  • Stents and stent grafts help keep vessels open.
  • Drains are thin tubes used to remove excess fluids from the body.
  • Embolic agents (PVA, microspheres, glue or metal coils) are used to occlude blood vessels.
  • Embolic agents can be combined with chemotherapy drugs for chemoembolisation.