Contrast Injector Pressure Limit: What psi and Bar Tell You

psi versus bar: reading the numbers

Pressure on injector datasheets is given in bar or psi, and 1 bar is about 14.5 psi. The ulrich medical CT motion has a maximum system pressure of 17 bar (246.6 psi), the CT motion SPICY 22.4 bar (325 psi) and the Max MRI injectors 14 bar (203.1 psi). When you compare injection pressure bar figures with psi figures from other sources, convert them first.

Always read the figure as a maximum system pressure, the limit the system is built and monitored to. In use the pressure is usually well below it.

What determines the pressure you actually need

Pressure rises with flow rate, with the viscosity of the contrast medium and with the resistance of the path: narrow or long tubing, small-gauge cannulas, kinks and connectors. Halving the viscosity or increasing the cannula size reduces the pressure needed for the same flow; warming contrast to body temperature lowers its viscosity, as explained in our article on warming contrast media.

This is why high pressure contrast injection is needed for some protocols: with high flow rates and viscous media, a system with only a low ceiling may reach its limit and the injector will reduce flow or stop. The higher ceiling of CT motion SPICY supports high flow rates and highly viscous contrast media, for example in cardiac and extremity examinations, according to the manufacturer.

Temperature is the easiest variable to control. A bottle taken from a cold store may need noticeably more pressure at the same flow rate than one warmed to body temperature, and a department that has frequent pressure alarms in winter or after a long idle period should check its warming routine first.

• Flow rate: higher flow needs more pressure
• Viscosity: thicker media, and cold media, need more pressure
• Cannula gauge and length: small and long lines add resistance
• Kinks, closed clamps and poor connections raise pressure suddenly

Pressure limiting and power injectable catheters

Pressure limiting protects the patient and the equipment. If the pressure approaches the programmed limit, the injector reduces flow to stay within it or stops with an alarm. The limit you set must never exceed the rating of the weakest component in the line, which is often the cannula or the central line.

A power injectable catheter or cannula is labelled by its manufacturer for use with power injectors and states a maximum flow rate and pressure. Check that label before programming a high flow rate, and match the flow in your protocol to the rated catheter. Choice of access is a clinical decision; the technologist's job is to ensure the programmed limit is compatible with the labelling.

Choosing an injector by pressure

A higher ceiling is useful if you perform CT angiography or cardiac scans with high flow, if you use viscous media or if you work through small cannulas. It is less relevant for departments that mostly do routine body scans at moderate rates. Review your five most demanding protocols and note flow rate, cannula and contrast; see our protocol article.

Container size is another factor: CT motion works with containers up to 500 ml, SPICY with up to 1,000 ml. A comparison of the two models is in the CT motion SPICY overview.

Ask the supplier how the injector reports pressure in use: some systems display the live pressure on the terminal and log alarms, which helps to troubleshoot repeated limit events.

Ask Beckfinity

If you are unsure which pressure rating your protocols require, Beckfinity specialists can review your protocol list with you and propose a configuration. Contact us for a consultation or quote.