Friday, June 22, 2018
Publication Date: 09/1/2009
Archive >  September 2009 Issue >  Special Features: PCB and Assembly > 

X-Ray Imaging: Some need-to-Know Basics
MicroBGA balls.

Electronic technology has given a great boost to real-time x-ray imaging. A tube was developed that could intensify the light from the intensifying screen. Called an "image intensifier," it is a vacuum tube with an input phosphor. Photons are converted to light and the light is converted to electricity by a photo multiplier. The output window phosphor converts the amplified electricity back to visible light. The CCD or Mega Pixel camera captures the output of the image intensifier and the processed image is viewed on a computer monitor. The addition of Mega Pixel cameras and faster lower cost computer image processing has made this technology the standard method of obtaining an image in real time x-ray imaging.

Because the image intensifier degrades with age and use, it is a good idea to have the tube tested by a qualified service technician annually. The tube should be tested for resolution, gain, and contrast. Degradation of the image intensifier is not something that happens overnight; because it is so gradual, the operators will most likely not notice the degradation because they do not have a new image intensifier to compare it against.

One of the greatest advancements in x-ray imaging is the Flat Panel Detector (FPD), which have great dynamic range and contrast when compared to image intensifier-based imaging systems. The FPD still utilizes a phosphor coating or photon-sensitive screen to convert photons to visible light, this visible light is then picked up by photo diodes and transmitted digitally to the image processing computer. There is no doubt that the digital x-ray image detectors will become the standard for x-ray imaging as the prices continue to fall and the ability to quickly capture the images with lower cost computers becomes greater. The main advantage that a FPD has is the ability to increase the gray scale from the image intensifiers 256 shades of gray to a minimum of 4096 (12 bit) shades of gray. Images from Flat Panel Detectors do not have the inherent distortion found in tube-based image detectors, and this allows for far greater accuracy in measurements. Flat panel detectors also have no image distortion; image intensifiers have some distortion at the edges due to the shape of the imaging tube input phosphor.

X-Ray Tubes
There are two basic types of x-ray tubes — open and closed. Open x-ray tubes consist of an x-ray tube (aluminum envelope) with a field replaceable filament, replaceable anode, high-voltage generator, roughing pump, turbo molecular pump and an x-ray control panel. The open style x-ray tubes are ideal for applications that require high power and a very small focal spot.

Closed (sealed) tubes come in two varieties — stationary anode and rotating anode, and the two types have some similarities. All x-ray tubes have an anode and cathode that are enclosed in a glass or ceramic envelope. The envelope contains a vacuum environment that limits arcing at high voltage. Also, all x-ray tubes have windows that allow the x-ray photons to exit the x-ray tube. Sealed Micro focus x-ray tubes (5 to 10µ focal spots) require shorter distances from the x-ray tube to the image intensifier tube to make up for the lack of x-ray output. The lack of unsharpness (penumbra effect), caused by close proximity of the tubes is offset by the small size of the focal spot of the x-ray tube. Remember, micro focus x-ray tubes cannot produce high-energy exposures. They may go to 90kV but the current is limited to 0.1mA. For example, 90kV x 0.1mA = 9 watts of total output energy. Thus the X-ray source and the object being tested must be placed near the image intensifier to yield an image of diagnostic density and quality. The exception to this would be in electronics inspection, where the kV and mA levels required to image electronics are low enough that the image intensifier and x-ray tube can be placed as far as 32-in. and still provide an acceptable image.

All x-ray tubes have focal spots that can be measured; focal spot size is one of the main determining factors of geometric sharpness of the resultant image on the computer monitor. Focal spots are determined by the size of the cathode filament and the angle of the anode target. As a rule, the focal spot size should be determine based on the actual use of the x-ray system, since small focal spots are very expensive and have limited current capabilities (mA). Also in some applications the user is not interested in the small, fine detail and magnification that a 5µ focal spot produces. Remember the golden rule of physics: one watt of power per micron of focal spot. A 5-micron x-ray tube will only allow for 5 watts of power, an industrial x-ray system for inspecting castings may need 1000 watts or more to properly penetrate the sample part.

Filaments Wear Out
Eventually, all x-ray tube filaments will melt or evaporate, causing a short or "open" circuit. Sometimes the metal from the filament evaporates and starts depositing on the glass envelope; this causes the tube to "arc" or short out. When this happens the x-ray system will start to blow fuses or circuit breakers. This arcing usually starts at high-energy exposures (high kV). As more exposures are made. The arcing will occurs at lower and lower exposures. You will sometimes hear a snapping or popping sound from the arcing, along with a sudden brightness of the image on the computer monitor. As soon as you hear or see this happening, shut off the machine and call for service. Unlike image intensifiers, x-ray tubes tend to fail totally (all at once) and do not degrade, with the exception that the focal spot size is always increasing as the tube filament evaporates and the target broadens from heat. Focal spots are important to image quality, the smaller the focal spot, the sharper the image. The focal spots become larger with use and the resulting image becomes less sharp. It is actually possible to have an operational x-ray tube that has to be replaced due to focal spot enlargement from heavy use, even though the x-ray tube still works.
microBGA balls on a cracked circuit board.

When you consider the lowest setting used for radioscopy is about 20kV — because the typical image intensifier will not respond below 20kV — it is wise to remember that this is 20,000 volts of electricity. This is the lowest setting you will ever use with a standard image intensifier, although there are some image intensifiers that will work down in the 5kV range. The higher the kV setting, the greater the penetrating power of the x-ray beam. The x-ray beam is heterogeneous — it is made up of many different wavelengths of energy. The different wavelengths have different penetrating capabilities. The object that is being x-rayed will absorb some of these wavelengths and be penetrated by others. This is what creates the blacks, whites and shades of gray that make up the image. The more wavelengths that penetrate the object, the lighter the image will be — using real time positive imaging. With film, the opposite will be true. If too much voltage is used, the object will appear to be over-penetrated; the image will be washed out. There is an optimum kV level for each area of a sample object. Thicker, more dense parts require a higher voltage than thinner, less dense parts. Too little kV means a part is not being penetrated; the part absorbs all of the x-rays and does not let them through to make an image. KV always controls penetration. Now that we know how the kV works for penetration, let's look at how many x-ray photons we have to work with.

We think of mA (current) as "quantity" of the x-ray photons. A higher the mA setting will provide more x-ray photons to strike the image detector. As the current increases to the x-ray tube cathode, more electrons boil off the filament and more x-ray photons are made. The kV and mA range is dependent on the x-ray generator and the load limit of the x-ray tube.

Image Enhancement Devices
Image Enhancement devices are commonly used on cabinet and industrial x-ray systems. The cost of image enhancement devices has drastically dropped in the last few years as the cost of PC has dropped. Computer based digital image processing is the most prevalent type of image enhancers used today.
Multilayer board, demonstrating the need for different intensities.

There are two types of image enhancement devices; analog and digital. Both types of devices accept a standard RS-170 video input. The analog types of image enhancers were very common in early x-ray systems. Analog image enhancers are typically stand-alone devices that have frame averaging, gain and offset controls. These systems were very limited but better than nothing. While it is still possible to buy this type of image enhancer, they are expensive, especially considering their limited functionality, and they are rarely used today. If you are looking to replace an analog type image enhancer be sure to investigate the computer-based digital image processors.

Early digital image processors were standalone devices that ran on proprietary operating systems. While this allowed for fast processing speeds, functionality was limited when it came to saving and transferring files. Later versions of this type of digital image processor were packed with a large variety of functions.

Today's digital image processing computers are fast when performing the image processing algorithms, operate in real time (28 to 30 frames per second) and offer all of the conveniences of a standard personal computer.

Make Sure It's Legal
Finally, a word of caution. When purchasing x-ray test equipment, do not assume that the seller or manufacturer has followed the CFR 21 1020.40 sub chapter J rules when manufacturing the x-ray system. Some companies will sell systems that are not only illegal but seriously unsafe for the operator and other individuals in the area. Always ask the seller or manufacture to give you a copy of his accession paperwork or his accession number that he received from the FDA/CDRH. If the manufacturer can't produce the number and seems to have no idea what the regulations are, thank him for his time and show him the door. You will never regret not buying a system that is illegal and unsafe.

Legal x-ray systems have been assembled using sound manufacturing practices and backed up by 100 percent quality testing, which is documented.

Contact: Scienscope International Corp., 5751 Schaefer Ave., Chino, CA 91710 909-590-7273 fax: 909-590-7020 Web:

 2) X ray aging

Add your comment:

Full Name:

search login