By Alexander Eitel

Some thought of it as a mission impossible. Hot liquid metal in vacuum? Yet when liquid metal bearing tubes became a reality, they revolutionized rotary anode X-ray tubes. This advance in X-ray tube technology brought a host of benefits: enhanced clinical workflow, no wait times for tube cooling, hard filtering, dose savings, tripled tube life, quiet operation, and immediate start up.

The limitations of traditional rotating anode tubes have been understood for nearly a century. The group that first commercialized rotating anode tubes, led by Dr. Albert Bouwers in the Netherlands, had already noticed a persisting problem in the late 1920s. In a rapid patient sequence, the use of heat radiation to cool rotating anodes suspended by ball bearings in a vacuum left substantial residual heat in the anode. In addition, the absence of intermediate layers in the vacuum brought the metallic bearings in direct atomic contact under pressure.

Hot ball bearings in a vacuum tend to fret after a few minutes of rotation. Metal coating, typical with lead or silver, prevents the worst wear, but cannot allow for keeping the rotor operating at top speed all day. A second pitfall is the noise caused by the metal interlayers and bearings during operation. In addition, as the prep time scales with the fourth to fifth power of the anode diameter, small anodes are a must for ball bearing tubes in radiography and angiography applications, which severely limits the possible X-ray output. Hard X-ray filtering for dose saving is hardly feasible. Finally, ball bearing failure and its impact on high voltage stability is a primary cause of tube failure.

Stationary anode tubes dissipate heat directly through bulk metal. The cooling rate also declines with temperature, but far less steeply. However, the permissible pulse power of stationary tubes is about two orders of magnitude smaller than for rotating anode tubes.

A new approach: liquid metal
Years after Bouwers developed a rotating anode tube, one major vendor started a product development project aimed at revolutionizing X-ray tube technology and harvesting the best of the both rotating and stationary anode tubes. By 1989, the first angiography tube with a spiral groove bearing, also called a liquid metal bearing, was integrated in a system. The term spiral groove bearing reflects the fishbone pattern on the surfaces of the bearing members, while liquid metal bearing refers to the liquid metal on which the rotating member slides without touching the stationary member. In these tubes, oil or water directly cools the anode.

How liquid metal bearings deliver cool operation, quiet and tube longevity
Use of liquid metal bearings sidestepped the pitfalls that shortened the life of tubes with traditional ball bearings. The liquid metal bearing of a rotating anode X-ray tube typically has four individual load-carrying components to take radial forces, gyroscopic moments, and thrust loads. The liquid metal fills a thin gap between stationary shaft and rotating sleeve. As soon as the rotor drive overcomes sticking friction, the rotation of the sleeve forces the liquid to separate the members. Capillary forces prevent leakage of fluid into vacuum. The bearing members touch each other for only seconds during start and landing.

The design of liquid metal bearing tubes results in several clinical and workflow benefits:

• Large surfaces of the bearings and a gap filled with metallic liquid guarantee excellent heat conduction as well as conduction of tube current. Because the tube doesn't heat up as easily as metal bearing tubes, there is no need to wait between exams for the tube to cool down, even in demanding applications that require many scans or ultra-high image quality.
• Liquid metal bearing tubes are ready instantaneously, after the first ramp up in the day. Because the anode is spinning continuously, no preparation time is necessary for the anode to speed up, liquid metal bearing tubes also are well suited for emergency situations.
• The absence of wear during rotation allows for constant high-speed rotation through the entire working day, allowing smoother workflow and promoting long life.
• Audible bearing noise is absent, which promotes a pleasant scanning experience for patients.
• There is abundant primary photon flux available with the enhanced tube power. Hard filtering becomes feasible, e. g. with an extra slab of copper, that is eliminating excessive patient dose from the soft portion of the X-ray spectrum.

The current liquid metal bearing was introduced in 2007 with the advent of the straddle-type bearing for premium tier CT. This state-of-the-art bearing is suspended on both ends of the axis to enable extreme rotor speed while operating below the intrinsic resonance of the rotor and suspension system. That guarantees stability and low vibration. However, special measures are required to encapsulate the liquid metal. The cooling rate could be further enhanced, which allows to omit a graphite back of the anode disk.

Assessing tube power
To assess tube power, the International Electrotechnical Commission (IEC) uses nominal CT anode input power. This performance metric conveys the level of instantaneous power that the tube can bear for a typical loading time in CT of four seconds in an endless cycle with a typical cycle time of ten minutes. Nominal CT anode input power serves as a useful metric when comparing tube performance.

Today's liquid metal bearing tubes are precision components. Their ability to run continuously without overheating supports the clinical need for longer and more complex exams, and their reliability and long life contribute to departmental workflow and lower total cost of ownership compared to ball bearing X-ray tubes.

About the author: Alexander Eitel, is the head of marketing and business development for Dunlee.