A Primer on CT Scanner X-Ray Tubes

CT scanners are complex diagnostic imaging systems that use X-rays to take pictures of a patient's internal structures and organs. For any type of system to generate X-rays, they must use X-ray tubes. Considering how important the X-ray tubes are to the function of a CT scanner, it's important that you understand what their purpose is, what kind of maintenance is required, and how often they need to be replaced.

The Function of X-Ray Tubes

The X-ray tube is the device that converts electrical input power into X-rays. It does this by accelerating electrons to high energies, causing them to hit a metal target from which the X-rays are emitted. X-rays can only be produced if the X-ray tube is being energized and a significant amount of electrical energy must be transferred to the X-ray tube to achieve this. However, only a small amount of the energy deposited into the X-ray tube is actually converted into X-rays; the majority of the energy turns into heat.

A CT scan takes anywhere from a few seconds to a few minutes to perform depending on what part of the patient's body is being scanned. The CT scanner requires a long continuous exposure time, which means that the X-ray tube must be continually energized throughout that period. As a result, general radiographic X-ray tubes aren't well suited for the use in a CT scanner.

Types of X-Ray Tubes

There are many types of X-ray tubes, but only a few are appropriate for use in CT scanners because they must be continuously energized to create 3D photographs of the patient's body using multiple X-ray images. The biggest difference between CT scanner X-ray tubes and other types of X-ray tubes is that CT scanner tubes are built to withstand the excessive amount of heat that is produced. The following are some of the different types of X-ray tubes:

  • Metal Ceramic X-Ray Tube - Early X-ray tubes were housed in a glass envelope but were eventually replaced by ceramic versions due to the electrical arcing that resulted from the tungsten deposits on the glass caused by vaporization. The metal ceramic X-ray tube uses both ceramic as well as an alloy of chromium and iron. The metal helps prevent electrical arcing, thereby increasing the life of the X-ray tube. A ceramic insulator is used to insulate the tube's high voltage parts, allowing for a more compact design. This also allows higher tube currents and reduces off focus radiation.
  • Maximum Rotalic Ceramic X-ray Tube - The Maximum Rotalix Ceramic (MRC) X-ray tube was designed by Phillips in 1989. It's built with a spiral groove that uses liquid metal alloy as a lubricant. The MRC X-ray tube has a longer tube life as well as a higher output because its rotating anode is cooled directly. The rotating anode, which is noiseless, can also be switched on in the morning and off in the evening. The MRC X-ray tube’s features also help prevent waiting time during and between examinations.
  • Aquilion X-Ray Tube - The Aquilion X-ray tube is a high-capacity multi-slice tube with high heat storage capacity and cooling rate. It's an air cooled tube that uses a straddle bearing structure (something a conventional tube lacks) and is built with a grounded anode.
  • Straton X-Ray Tube - The Straton X-ray tube is unique in that instead of being built with a rotating anode, the entire tube rotates around the anode axis. Because of this, the bearing is located outside of the tube, allowing it to cool down faster. The Straton tube has zero heat storage capacity and can cool down within as little as 20 seconds. It also uses a magnetic deflection coil to shape and control the electron beam within the tube.

Where Are X-Ray Tubes Located?

The X-ray tube is located in the gantry, which is the largest part of the CT scanner and which consists of the the X-ray detectors, the mechanical supports, and the scanner housing. Older CT scanners were built with the X-ray tube affixed near the top of the gantry's interior so that it would face downward toward the patient.

Newer systems are built so that the X-ray tube is built into the rotating components of the scanner, allowing it to rotate along with the detector array 360 degrees. The newest systems use a rotate-fixed ring geometry in which the patient is surrounded by a ring of fixed detectors and the X-ray tube rotates inside of the detector ring.

How Long X-Ray Tubes Should Last

X-ray tubes don't last forever. The more they are used, the more they will degrade over time, resulting in a gradual decrease in performance until they finally need to be replaced. Eventually, the tungsten filament used to supply the electron beam in an X-ray tube will burn out from normal use. The end of its lifespan is determined once it's lost around ten percent of its wire mass. This can occur naturally or at an accelerated pace due to other issues.

Other potential problems that can affect the lifespan of your X-ray tube can include numerous manufacturing deficiencies as well as inactivity (which results in the build-up of gases within the tube vacuum), glass crazing, and slow leaks. However, you shouldn't have to worry about having to replace your X-ray tube too often if you operate your CT scanner properly, since they havean average life span between five and seven years.

Maintenance:

Not only does your CT scanner depend on its X-ray tube to function, but X-ray tubes are very expensive to replace as well. You will want to prolong the lifespan of your X-ray tubes for as long as possible. Although your CT scanner's manufacturer will have specific recommendations for taking care of your CT scanner and its X-ray tubes, here are a few things you can do in general to help extend the lifespan of your X-ray tubes:

  • Warm up the X-ray tube - If your CT scanner hasn't been used for at least two hours, you will need to warm up the X-ray tube before using it. This will help prevent thermal shock, which can damage the X-ray tube. To warm up your X-ray tube, take several exposures, pausing about a minute between each one.
  • Avoid improper heating and cooling - Follow the manufacturer's recommendations for acceptable levels of operation to avoid premature focal track wear or damage as a result of improper heating or cooling. Avoiding improper heating or cooling can also prevent excessive heat transfer, causing bearing failure or rotor body damage.
  • Control the filament boost and current - If the X-ray current is too high, it can cause the tungsten to evaporate from the filament and deposit it into the tube's glass envelope, thereby reducing its lifespan. You can extend the life of the filament by lowering mA stations and slightly lengthening your exposure times.
  • Be careful about rotor operation - The starting and stopping process generates a lot of heat. When the rotor stops, wait at least 30 seconds before initiating the next startup sequence. Avoiding long intervals between spot-films will also help reduce the amount of heat that is generated by stopping and starting.

How To Know If Your X-Ray Tubes Need Replacing:

Once your X-ray tube begins to degrade significantly or starts showing serious issues, replace it as soon as possible. The following are a few obvious signs indicating that you need a new X-ray tube:

  • There's a build up of tungsten deposited on the glass tube window's internal surface.
  • The X-ray tube has begun to produce gas, which means it's no longer a vacuum.
  • When you turn the X-ray tube on and you can hear oil being sucked into the tube insert, it means that the tube has been punctured.
  • The X-ray tube is physically damaged due to improper handling. For example, you can see stress fractures in the tube.
  • Oil is leaking from the X-ray tube, which means there are most likely problems with the tube's bearings, rotor, or housing.

How Much Do X-Ray Tubes Cost?

X-ray tubes are very expensive, which is why careful handling and proper use are so important. There are a lot of factors that contribute to the overall cost, including who the manufacturer is and whether you purchase brand new X-ray tubes from the OEM (original equipment manufacturer) or an after-market supplier, or you purchase a used or refurbished tube.

Costs vary widely for X-ray tubes. Lower end tubes can be obtained for as little as $20,000. However, most high-quality X-ray tubes cost around $150,000 to $200,000 each. The reason X-ray tubes are expensive is that the materials used in their production are very expensive. The design of the tubes varies from one manufacturer to another, so the more complex the design is, the more expensive it will be.

How To Learn More About X-Ray Tubes

Replacing an X-ray tube in your CT scanner is a big investment. Besides taking into consideration your hospital's budget, speak with your service provider to learn more about their construction and their features. This will allow you to make an even more informed decision regarding X-ray tube replacement.