CT X-ray tube

The CT X-ray tube is one of the more popular, most used and most replaced. The average useful life of a CT tube is sometimes estimated by the number of slices the CT system has performed. Michael Webster, president of Legacy Medical Imaging, Fort Worth TX, says that tube life depends on the type of tube and which system it's used in. "LMI works with a lot of GE systems and have found the life expectancy on a 3.5MHU tube lasts between 120K to 200K slices and a 6.3MHU Proformix tube can last over one million slices," says Webster. Ralph Babcock at Imaging Affiliates says, "The CT tubes today are called on to perform at a much higher level then we though possible 20 years ago. A typical spiral body scan would be performed at 120 kV, 400mA for 15 seconds or more continuous. This means the tube is performing at 48kw continuous and in the course of that 15 seconds, will emit 720kJ of X-ray energy and heat. Ten to fifteen years ago, CT technology was limited to single exposures of lower techniques and much longer wait times between slices. In order to do the necessary power and through-put demands placed on the scanner, the tube must be comprised of very special components that are designed to take the loads and stresses of the demands placed on the tube. CT tube components such as bearings, targets, cathodes and metal-frames must not only be up to the high heat demands of this technology, but also be able to withstand extremely high G forces as the gantry rotates as fast as two to three times per seconds." Babcock went on to say that everything in the CT tube has gotten much bigger to handle this and a lot of the metals used in the target are strategically desired materials that are in high demand throughout the world such as molybdenum and tungsten and have a correspondingly high price to go with that demand. "When we look at the types of procedures that we can do non-invasively that result in quicker and faster diagnosis, we can justify the higher costs for these tubes," says Babcock.




Greg Kramer, president of C&G Technologies, Inc., Jefferson, IN said that prior to multi-slice CT, the industry standard units for tube wear was the "slice". One rotation under X-ray power, producing one image, equaled one "slice". According to Kramer, "1 slice = 1 image. The term slice and image became interchangeable words." Kramer states, "Toshiba counts rotations whereby one 360 degree scan under power, equals the measure of tube wear in 'rotations'. One rotation can equal as many 'images' as the detector can collect in one rotation, i.e. 1,2,4,8,16,32,64, etc." He went on to say that Philips and Siemens use the 'scan-second' measure of tube wear. "This is the number of seconds the tube is under power while scanning," says Kramer. "One scan-second can equal the same number of images as in rotations, or it can mean more since many scanners can do two or more full 360 degree rotations in one scan-second." "Note that none of the above methods take into account the amount of Voltage or Current put into the tube," states Kramer. "Therefore, two tubes with identical amounts of slices, or rotations, or scan-seconds can have very different amounts of true wear on them." Kramer explained that GE uses Amp-Seconds to measure tube wear. The amount of current (amps), times the duration of the scan (seconds), equals Amp-Seconds. GE CT scanners report this in 'mas," a smaller unit the amp-second. 1 amp second = 1000 mas. "Scanning at 250ma for one second = 250mas. The number of images generated depends on the number of detector rows. This method is more of a "true" measure of tube wear," states Kramer He goes further saying, "having said all this, you can see that the terms 'scans' and 'slices are no longer necessarily the same. It would be easier on all of us if the manufacturers would adopt the same method for measuring tube wear, and my preference would be for amp-seconds." Rick Fow, vice president of sales at C&G said that the company sells new tubes from Varian and Dunlee and also sells refurbished tubes. "We put our used tubes though a "process" checking all parts and replacing the oil in the protective covering," says Fow. "This process will add longevity to the life of the tube."

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