Technology
Varian: Early Pioneers
In the early 1990s, researchers at Varian's Ginzton Technology Center began development work to bring together several new technologies combining speed, image quality, compactness and ease of use. In 1998, Varian became the first and only company in the world to deliver amorphous silicon flat panel systems capable of both fluoroscopic and radioscopic digital imaging. Varian has continued to refine and improve our products and competencies in real time flat panel image receptor technology. Varian has over 20,000 receptors installed in various applications spanning the medical and industrial imaging markets.
Real-time X-Ray Imaging Systems
Varian's PaxScan™ products meld the best in amorphous silicon sensor panels, radiation-converting materials, low-noise analog and high-speed digital electronics, custom ASIC control and processing electronics, and compact packaging. As new materials, processes, circuits and techniques emerge which promise improvements in performance or extensions to new applications, they will become part of our imaging technology. Today, our technology includes:
Amorphous Silicon Sensor Panels
The purpose of the sensor panel is to accumulate charge generated by the absorption of x-rays and to provide it row by row during scanning to the charge amplifiers. The charge storage device is a capacitor in photoconductor imagers or a photodiode in panels used with scintillators. The switch used to permit the charge to flow out can be a single diode, a diode pair or a thin-film transistor. All possible combinations of these storage devices can be made to work but each has a specific set of advantages and disadvantages. Varian products use the photodiode TFT combination because of its simplicity of use, commercial availability and flexibility of design.
Photomicrograph of an amorphous silicon sensor panel
In the array pictured here, the switch is a thin-film transistor (TFT) much like the switches used in active-matrix liquid crystal displays. An important goal in panel design is maximizing the area of the imager that is taken up by the photodiode (a "high fill factor") so that a minimum amount of arriving light is wasted. The signals are carried by thin metal lines. The pixel center-to-center distance in this sensor panel is 127 microns and the fill factor is 35%. Panels with higher fill factor are used in the current Varian products.
Circuit diagram of an amorphous silicon sensor panel
In operation, the photodiodes are reverse-biased by an external voltage applied to them all. While the TFT switches are off, charge generated by light from the scintillator accumulates on the diodes. When readout is wanted, a row line is energized to turn on the switches in that row. The charge from all of the photodiodes in the selected row flows out through all of the data lines simultaneously. In large arrays, this produces several thousand signals that must all be read at the same time. Varian has developed a custom 128-channel low-noise device with high charge capacity for this purpose to accommodate the wide dynamic range of amorphous silicon sensor panels.
X-ray Conversion Methods
Three common methods to convert incoming x-rays into charge for electronic readout can be implemented in amorphous silicon. They are the Intrinsic, the Photoconductor, and the Scintillator methods. Each method has its performance advantages and disadvantages and each has certain limitations on its use in practical x-ray imagers. In all three methods, the charge is accumulated for a frame period before being read out. Gamma cameras, in contrast, count each x-ray photon as it arrives. That technique is generally not used for x-ray imaging because the x-ray photon arrival rates are too high to permit counting.
The Intrinsic Method
Arriving x-rays are captured by the amorphous silicon diode where hole-electron pairs are generated. An applied bias separates the charge to prevent recombination. Because a charge pair is generated for about each 5 electron volts of x-ray energy, the signals are high. Unfortunately, the x-ray absorption of silicon is very low so the photodiode needs to be 10 to 20 mm thick. Fabricating such devices of amorphous silicon is not feasible. Intrinsic devices have been made from crystalline silicon but only arrays of one or two lines are practical and even these are expensive.
The Photoconductor Method
Photoconductive materials with higher x-ray absorption than silicon can be coated on an array of conductive charge collection plates each supplied with a storage capacitor. These also produce hole-electron pairs when x-rays are absorbed but the charge generated must be stored out of the layer to avoid lateral crosstalk. The applied field not only separates the charge but directs it towards the collector plate directly below to maintain image sharpness. Currently, the only photoconductor in production, selenium also has relatively low x-ray absorption and requires about 50 electron volts to produce a hole-electron pair. These restrict both the minimum dose needed and the size of the signal generated. Other materials with lower energy requirements and higher x-ray absorption are under development.
The Scintillator Method
A scintillator is a compound that absorbs x-rays and converts the energy to visible light. A good scintillator yields many light photons for each incoming x-ray photon; 20 to 50 visible photons out per 1kV of incoming x-ray energy are typical. Scintillators usually consist of a high-atomic number material, which has high x-ray absorption, and a low-concentration activator that provides direct band transitions to facilitate visible photon emission. Scintillators may be granular like phosphors or crystalline like cesium iodide.
Structure of a phosphor scintillator
Phosphors are materials which glow when exposed to x-rays. For maximum brightness, the phosphors use in x-ray imaging are made of rare-earth oxysulfides doped with other rare earths. The most common are gadolinium and lanthanum oxysulfides doped with terbium. These typically emit blue to green light which is well-matched to film sensitivity. Various grain sizes and chemical mixtures are used to produce a variety of resolution and brightness varieties. In use, these are mixed with a glue binder and coated on to plastic sheets. These were designed to be pressed against x-ray film to improve sensitivity but they may also be pressed against arrays of amorphous silicon photodiodes to make electronic x-ray detectors with sensitivity at least as good as that of film. Tens of electron volts are needed to produce each visible photon in a phosphor screen and x-ray absorption is good. Light scatter can be a problem if the layers must be thick to stop higher-energy x-rays.
Structure of a cesium iodide scintillator
For a better combination of resolution and brightness, cesium iodide is used. CsI has the useful property that it grows as a dense array of fine needles (10 to 20 micrometers in diameter) under the proper evaporation conditions. This produces crystals which act as light pipes for the visible photons generated near the input side of the layer allowing very thick (up to 1 mm) layers to be used with excellent retention of resolution. Because cesium has a high atomic number, it is an excellent x-ray absorber so this material makes very efficient use of the incoming x-rays. About 20-25 electron volts are needed to generate each light photon. When doped with thallium, CsI emits at about 550 nm, just at the peak of the spectral sensitivity of amorphous silicon. The combination of CsI and amorphous silicon has the highest DQE of all materials in production today.
Real-time X-Ray Imaging Systems
In the past, real-time x-ray imaging (fluoroscopy or radioscopy) has usually involved a television camera combined with a device to convert incoming x-rays into light visible to the camera. Until recently, cameras with image tubes were common but new systems use CCD models almost exclusively. CCDs (and other related solid-state imaging devices) have advantages over image tubes in stability, geometric accuracy, signal uniformity and size but these advantages are substantially surrendered when the x-ray conversion facility is added. Sensor-panel imagers bring these advantages back supplemented with a few of their own. The illustrations show why.
 | CCD with X-ray Image Intensifier This combination provides real-time imaging with low x-ray flux over reasonably large areas. Geometric distortion and susceptibility to image burn are high. Because the intensifier relies on electron acceleration for gain, it is susceptible to external magnetic fields and requires high voltage. |
 | Lens-coupled CCD Since the optical collection efficiency of the lens is very low, this combination requires high flux or an intensified camera for real-time operation. The mirror moves the camera from the primary x-ray beam. Changing the field of view or the energy band is as easy as changing the converter screen. |
 | CCD with Fiber-optic Reducer This combination provides a simple solution for small areas. Geometric distortion and uniformity are good. Various screens provide adaptability to various energy bands. At higher energies, a right-angle reducer may be needed to move the camera from the primary beam. |
 | Sensor-panel Imager The simplicity of this imager avoids most of the opportunities for degradation in other imagers. Dynamic range, contrast, and geometry are improved. Converter selection is provided. At higher energies, only the scan and read electronics need be positioned out of the primary beam. |
Digital Imaging: Technology: Real Time Imaging
| Flat Panel Imager | CCD Optically-Coupled | CCD Fiber Optic | CCD X-ray Image Intensifier | |
|---|---|---|---|---|
| Area Covered | Large | Large | Small | Moderate |
| Sensitivity | High | Low | Moderate | High |
| Dynamic Range | High | Moderate | Moderate | Moderate |
| Contrast | High | Moderate | Moderate | Low |
| Geometric Accuracy | Very High | Moderate | Moderate | Low |
| Stability | High | High | High | Moderate |
| Radiation Resistance | Very High | High (with mirror) | Moderate | Moderate |
| Pixels per Image | High- Very High | Low-High | Low-High | Low-High |
| Magnetic Field Resistance | Very High | Very High | Very High | Very Low |
| Compactness | High | Low | Moderate | Low |
| Low-Voltage Operation | Yes | Yes | Yes | No |
| Electronic Zoom | Yes | Rare | Rare | Yes |
Contact X-Ray Products Headquarters
Tel: 801.972.5000
Fax: 801.973.5050
E-Mail: xray.info@varian.com
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