Measurements in Interventional Radiology

Wires, catheters, and devices often have many measurements associated with them. These can often be confusing as each type of device uses its own measurement with its own unit. This post goes over some of the basic measurements one may encounter in the interventional radiology suite.

Needle Measurements

Needles, specifically hypodermic needles, are measured in a unit termed "gauge" (abbreviated G).  Unlike other measurements, a higher gauge number indicates a smaller needle. The gauge refers to the outer diameter. Typical needles encountered in IR include a 21 G needle to do micro puncture access, a 19 G needle to do direct access or transjugular liver biopsies, and 18 G needles to draw up medications. As an example, a 21 G needle has an outer diameter of 0.03225 in (0.8192 mm) and an inner diameter of 0.02025 in (0.514 mm). For more detailed measurements, see this needle gauge chart


Wire Measurements

Guide wires are typically measured in inches of thickness. Using the example above for a 21 G needle, one can see that an 0.018 inch wire can pass through a 21 G needle, because 0.018 inches is less than the inner diameter of 0.02025 inches. Similarly, an 0.035 inch guide wire can pass through a 19 G needle. For more detailed comparisons of wires, use this comparison chart to compare up to 5 wires at a time. 


Catheter Measurements

Catheters are often measured using the French scale, often abbreviated "Fr". A catheter of 1 French has an external diameter of 1/3 millimeter (mm). Therefore, a 3 Fr catheter has an external diameter of 1 mm. 

Source: "French catheter scale" by Glitzy queen00. Licensed under CC BY-SA 3.0 via Wikimedia Commons

Sheath Measurements

A sheath is used to stabilize access within a vessel. While also using the French system for catheters described above, the designation here indicates the largest French catheter the sheath will accept. So, a 6 French sheath will accept up to a 6 French catheter.

Other devices such as coils or TIPS shunts have measurements specific to them, which should be carefully considered before use. 
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