METAL X-RAY TUBE TECHNOLOGY

VARIAN METAL X-RAY TUBE TECHNOLOGY INTRODUCTION Varian Medical Systems has a wide portfolio of metal x-ray tubes that have replaced the glass x-ray tube in many applications. These tubes have advantages over the traditional glass x-ray tube technology as described in this article. Most conventional x-ray tubes are fabricated with a vacuum glass envelope containing a source of electrons (cathode) which are accelerated in a shaped high voltage field colliding on a small area of a target surface(anode) called the “focal spot”. The interaction of the electrons on the target surface produce x-rays. Like any technology, new and advanced designs eventually supersede the old ones; making the old technology less desirable. And so it is with x-ray tube technology. Metal tube technology has advantages in many applications.

WHAT IS A METAL X-RAY TUBE? A metal x-ray tube simply has part of the tube envelope made from metal. If the metal tube has a traditional balanced high voltage (HV) system where the kilovolt (KV) potential is split between the cathode and anode, the metal envelope section is at ground potential between the anode and cathode portions of the tube. Some metal x-ray tubes are “anode grounded” which means the HV potential is developed on the cathode side of the tube only. The remaining portion of the x-ray tube is enclosed in a metal envelope. Likewise, a cathode grounded tube has the metal portion of the envelope on the cathode side and the HV potential is applied to the anode side.

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VARIAN METAL X-RAY TUBE TECHNOLOGY WHAT ARE THE ADVANTAGES OF A METAL TUBE OVER A GLASS TUBE? Higher target heat storage exposure ratings. Since some of the tube current is absorbed into the metal envelope, that extra energy can be added to the target exposure ratings. In a dual ended KV design, that means an approximate extra 15% can be added. In an anode grounded application, up to 30% can be added.

More rugged construction offering greater focal spot stability. Metal is inherently less fragile than glass. Where metal replaces glass, the x-ray tube is more rugged. Anode and cathode assemblies attached to metal components are sturdier and provide a more stable assembly; especially in applications where the tube is moving while making x-rays.

Better flexibility in setting and maintaining inherent filtration. The x-ray beam in a glass tube passes through the glass envelope, oil, and a plastic window before entering into a collimator. These x-ray absorbing materials are called the “inherent filtration” from the tube housing assembly. Although glass thickness is sometimes controlled by grinding the glass, a metal window has a more controlled and uniform thickness than glass. Properly sized metal filters can be placed in the x-ray beam to better control inherent filtration.

Better imaging contrast by reducing off-focus radiation. Off focus radiation is caused by high energy electrons rebounding off the target surface and being re-accelerated onto the target surface in a non-focal spot area. This has the effect of desensitizing the contrast in the imaging process. In a metal tube, a certain percentage of the rebound electrons are absorbed into the metal envelope and reduce the amount of off-focus radiation.

Longer life tubes with high tube current applications. The amount of x-ray from a tube depends on the current applied to the filaments in the cathode assembly. Electrons are emitted from the filament surface from thermionic emission of electrons into the high voltage field. The hotter the filament, the greater number of electrons emitted. To get this thermionic emission, the filaments are at a temperature where metallic atoms are emitted from the filament surface. In a glass tube, these metallic atoms are deposited on the glass envelope between the cathode and anode components.

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FAILURE EXAMPLE: FILAMENT DEPOSIT

FILAMENT DE POS IT

If this deposit is heavy enough, the deposit will absorb and conduct electrons across the surface of the glass jeopardizing the HV stability of the x-ray tube. Not so with a metal x-ray tube. These metallic atoms simply attach to a metal surface that has a defined potential and will never jeopardize the HV stability of the x-ray tube. This phenomenon was the initial motivation for creating a line of metal x-ray tubes at Varian. Metal tubes in chest x-ray applications have been shown to provide triple the lifetime of the glass envelope tubes.

• Better high voltage stability for high target heat applications. As with metal deposits from the filament, so also is the advantage of a metal x-ray tube over a glass tube for metallic deposits from the target surface. Heating and cooling dynamics cause the target track surface to expand and contract, eventually causing small fissures and cracks in the surface as a target surface ages. Small metallic particles are vaporized and deposited on adjacent components.

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FAILURE EXAMPLE: TARGET DEPOSIT

TA R G E T DE POS IT

As described above with filament deposits in a metal x-ray tube, these metallic atoms will deposit on a stable grounded metal surface. Glass crazing is caused by a large temperature gradient between the cathode head, which could reach 500°C, and the glass vacuum envelope. This temperature difference causes localized surface cracking on the hot surface which will lead to severe electrical arcing and will eventually jeopardize the integrity of the vacuum envelope, rendering the tube inoperable. This can be visually detected by crazing of the glass.

• Better cooling flexibility in high average power applications. With all x-ray tubes, the thermodynamics in all of the components determine the tube ratings. With glass tubes, the thermal energy is radiated directly into the adjacent dielectric fluids around the x-ray tube in the housing. With metal tube technology, metallic components provide opportunity for more localized cooling of components. Cooling channels can be designed into the metal tube envelopes to better control the flow of heat. Where certain components may heat more than others, cooling channels can be designed to raise the limiting components cooling ability and hence improve the average power rating of the unit.

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FAILURE EXAMPLE: GLASS CRAZING

GLAS S C R AZING

HOW ARE HV POWER SUPPLIES CONFIGURED FOR METAL X-RAY TUBES? Understanding the current flow through metal x-ray tubes will help the designers of power supplies. In glass tubes, all of the current from the cathode is absorbed into the target. In metal tubes a percentage of the current is absorbed into the metal envelope from rebound electrons off the target surface. In dual ended designs where the KV is divided between the anode and cathode, about 15% of the current goes through the metal envelope. Knowing this, the current monitoring circuits can be designed in the HV generator. Some generators are designed

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VARIAN METAL X-RAY TUBE TECHNOLOGY

with the current monitoring circuits in the cathode side of the generator so all of the current will be monitored. Some generators are designed with the current monitoring in the anode side knowing that a certain percentage is reduced because of the metal tube envelope absorbing some current. In this case, some allow the envelope current to go directly to a controlled ground circuit, while others will route the current from the envelope through a lead back to the low voltage secondary of the generator. Some generators are designed with a balanced current monitoring circuit in both the cathode and anode side. Knowing what current flows from the x-ray tube is important in how these circuits are designed. On anode grounded metal tubes, all of the current goes to the anode grounded circuit. On a cathode grounded metal tube, all of the current goes to the anode.

BEING METAL TUBE-WISE 1. Never energize the power supply when the HV anode plug is disconnected from the

tube. All of the tube current will go directly to the metal envelope and failure of the x-ray tube is certain.

2. The visual view through the port of the housing to the x-ray tube is opaque in a metal center tube. A fluoro indicator must be used to align the tube in the housing properly

3. On the metal center section tube housing, an additional wire connected to a fourth

terminal under the stator cover is brought out in addition to the standard rotor cable with conventional connections. This terminal connects to the metal center section of the insert and should be connected either to a separate ground point or to the anode side of the high voltage transformer rather than directly to ground. This effectively balances the anode and cathode current in the monitoring circuits and subsequent calibration and operation are as for conventional tubes. Safe engineering practices require that the center section lead be connected such that the housing casing or tube does not conduct the center section current. Never tie the center section lead directly to the housing casing or tube stand.

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