Evaluating Thermal Load When Choosing an X-ray Tube
Achieving sufficient power in the photon beam for high resolution imaging, especially with small focal spots, is irrevocably tied to the thermal dynamics inside of the X-ray tube. When the incident beam is concentrated on a smaller area on the anode, it results in a higher thermal load, which could easily melt or cause significant damage to the anode as well as damage other parts of the tube. In addition, sudden transient temperature changes, such as those that may occur when the anode track is heated, could severely damage the tube.
Replacing an X-ray tube can cause significant CT system downtime, hampering an imaging department’s workflow and possibly affecting the ability to provide timely patient care. Failed tubes also damage the reputation of system manufacturers. For these reasons, manufacturers need to evaluate thermal load when choosing X-ray tubes for their CT systems.
This evaluation is not as simple as one might expect. The earliest measurements of the heat capacity of an anode used Mega Heat Units (MHU), which has since been eradicated from the compendium of the International Electronic Committee (IEC) because it does not give an accurate depiction of clinical performance.
Heat Dissipation is an Important Consideration
Relying on MHU measurements to evaluate tubes ignores that modern liquid metal bearing (LMB) tubes also have modern methods of heat dissipation:
- LMB are frictionless, so no additional heat is generated inside of the bearing through friction.
- Liquid metal is a very good heat conductor, meaning that heat can be conducted away from the tube. In traditional ball-bearing tubes, heat is only dissipated via convection.
- LMB technology enables the introduction of active cooling into the tube that can meaningfully reduce the thermal load of the anode body.
Comparing LMB Tubes and Ball Bearing Tubes
Both the IEC-based method, which uses nominal CT anode input power and the CT Scan Power Index, and the Dunlee method, which assesses effective heat content and maximum anode heat dissipation, take into account the cooling behavior of tubes in between scans. However, the IEC method doesn’t allow an “apples-to-apples” comparison for manufacturers considering both LMB and non-LMB tubes. In contrast, Dunlee’s effective anode heat content method enables comparison of the thermal performance of LMB tubes against ball bearing tubes with large anodes.
Learn More
To learn more about Dunlee’s effective anode heat content measurement and how Dunlee designs tubes to achieve the high spatial resolution and high uptime that hospitals need, visit us at RSNA 2026, booth 3117.
