FAQ about CMOS X-ray Detectors

Complementary metal oxide silicon. This is the material that computer memory chips are made from, as well as most integrated circuitry. Three engineers from NASA’s Jet Propulsion Laboratory at California Institute of Technology in Pasadena invented the CMOS "active pixel sensor" in 1995.

What are the factors that determine image quality?

Spatial resolution is the most important quality.

Fill factor, which is the actual active area of the surface of the pixel.

Dynamic range of the technology, minimum of 12 bits.

Achieving a high signal to noise ratio.

Optimizing the proper radioscopic technique.

Quality of the radiation source.

What advantages do CMOS detectors have over amorphous silicon detectors?

Durability, temperature range, fill factor, sensitivity, off-axis detection, less image blooming, simple calibration, and cost.

Why is the CMOS detector more durable than other detectors?

The active pixel sensor technology used in CMOS detectors allows all the electronic control and amplifier circuitry to be located at each pixel instead of being delicately wired to circuitry at the edge of the detector array. Also, because of the microscopic thin-film-transistor circuitry at the pixel level, CMOS detectors have less mass making them more tolerant to shock.

What about the temperature range?

All digital detectors have electronic noise that increases with temperature, but CMOS has far less of this characteristic compared to amorphous silicon. Amorphous silicon panels require re-calibration with as little as a 10°F temperature change. CMOS detectors require little or no re-calibration across a broad operating range of 32° to 110° F.

What is fill factor, and why is it important?

The fill factor of a detector is the percentage of the surface area that is active - capable of detecting photons. Envision’s CMOS detectors have fill factors in excess of 90%, compared to amorphous silicon detectors at about 60%. The higher the fill factor the more sensitive the detector.

How sensitive are these CMOS detectors?

From 10 to 100 times more sensitive than x-ray film. Envision’s Mini™ detectors capture a complete x-ray image in as little as a milli-second up to three seconds maximum. The EnvisionScan™ detectors capture a single line of an x-ray image in 1 to 80 milli-seconds; these lines are then automatically added together to form a complete image.

What is off-axis detection, how is this achieved, and what are the benefits?

Off-axis detection means that the design of the EnvisionScan™ detector protects the CMOS pixels from direct x-ray radiation.

The EnvisionScan™ panel’s CMOS line array detector is shielded from radiation by a thick layer of lead and/or tungsten. A built-in slot collumator directs the radiation to the ends of micro fiber optic couplers which are coated with scintillating material. The fiber optic couplers then direct the light photons created by the scintillator to the detector pixels in a path perpendicular to the radiation beam.

Three major benefits result from this unique detector design: elimination of scatter (undesired signal), reduced direct hits to the detector (radiation noise), and extended life of the detector.

This is a big advantage CMOS has over the other technologies. This allows for a very high signal to noise ratio due to the fact that the electronics are shielded from the primary beam of radiation. The reduction of radiation in the electronics is the reason that CMOS works better in the higher energy applications.

Why does a CMOS detector have less image blooming than other detectors?

In conventional detectors, image blooming occurs when individual pixels in the detector are over-driven by direct radiation to the pixels. This happens because the pixels deliver their signals to the electronic amplifiers in a "bucket brigade" along each row and column. When one or more pixels are over-driven, the signals from all other pixels in that row and column can be affected causing blooming or streaks in the observed x-ray image. CMOS detectors eliminate this problem because each pixel is amplified separately at each pixel and as a result their signals do not affect signals from adjoining pixels.

What type of x-ray source is used with CMOS detectors?

Mini™ detectors work with any x-ray source: pulse, rectified, or constant potential. The scintilator material in the Mini™ detectors works best with energies in the range of 20 to 160 Kev, and with current from microamps to 30 milliamps.

EnvisionScan™ detectors require constant potential x-ray sources in the range of 20 Kev up to 300 Kev at any milliamp level. These detectors can also be built to accommodate higher energies from 450 Kev to 2 Mev.

What is spatial resolution, and how do digital detectors compare?

Spatial resolution is the size of the smallest detail that can be seen with an imaging system, and is primarily limited by the size of the pixels in the detector.

Envision's Mini™ CMOS detectors have a pixel size of 39 microns (0.0015"), producing the highest spatial resolution currently available in the digital x-ray industry, without geometric magnification. With geometric magnification (using a micro-focus x-ray source) spatial resolution of a few microns can be achieved.

EnvisionScan™ CMOS line array detectors have a pixel size of 80 microns (0.0031"), producing approximately 30% better spatial resolution than amorphous silicon or selenium panels, without geometric magnification. With geometric magnification (using a micro-focus x-ray source) spatial resolution of a few microns can be achieved.

For comparison: the best linear diode arrays (LDA’s) have a spatial resolution of 250 microns, the best CCD-based imagers have a spatial resolution of 160 microns, the best amorphous silicon panels have a spatial resolution of 127 microns, and the best medical flat panels have a spatial resolution of 100 microns.

How long does it take to get an x-ray image?

With a Mini™ system, objects are typically exposed with radiation for 1/2 second up to three seconds, then the data is transferred to the workstation computer for correction and the image is presented on the computer display about 10 seconds after the exposure. If multiple images are desired, the views are automatically sequenced every 15 to 18 seconds.

With an EnvisionScan™ system, objects are scanned by the panel at rates of up to 1.6 inches per second at 80 micron resolution. At lower resolutions, for example 1mm, scanning can be done at rates up to 39 inches per second. Un-corrected images can be scrolled on to the computer screen as the data is captured, or displayed with correction a few seconds after the scanning is completed.

Is the software DICOM/DICONDE compliant?

The latest Mini™ and 4x4™ system software is compliant

What is CR Technology? CR is technology that reads a storage phosphor screen with a laser.

What is DR Technology? DR is often referred in two different meanings.

A) Direct Radioscopy is where the detector creates the signal without the use of a conversion screen.

B) Digital Radioscopy is more of a general term that refers to digital panel technologies.

Are CMOS Panels fragile?

No, CMOS technology is inherently robust by design. Each pixel is TFT technology that does not have the substrate deposited on glass as in the amorphous panels.

What about the movement system in the EnvisionScan™ Panels?

The movement system incorporates a precision rack and pinion technology that has been proven in the automotive industry since the early thirties.

What are the cost comparisons to the EnvisionScan™ technology?

The basic EnvisionScan™ systems start at $72,420 with the software and workstation, and can be configured to customers needs. The other flat panel technologies start with the basic systems around $145,000.

Is the annual recurring cost with the CR system much less than the other technologies?

No, the up front cost of the phosphor screens will always be an issue and the cost to replace them due to normal wear and tear.

Is the CR system more portable that the EnvisionScan™ Panel?

In cases where field applications have limited access and power is not convenient, a phosphor screen has the advantage.

Are the EnvisonScan™ Panels risky to move for fear of breaking them?

One of the biggest advantages of the CMOS technology is that it is inherently robust. EnvisionScan™ Panels have been hardened to withstand the everyday rigors of the x-ray process.

Does Digital Radioscopy eliminate a workflow step in processing images?

Yes, it takes the processing and reading film or phosphor screens out of the equation by producing instant results at the point of inspection.

How does CMOS acquire an image?

The specimen is placed between the x-ray source and the detector. The source is energized and the CMOS detector is started by the press of a button. The image is captured in a few seconds and displayed on the monitor at the point of inspection seconds after the acquisition time.

How does the CR system acquire an image?

The exposure occurs basically the same as the CMOS technology. The difference is the CR technology utilizes a storage phosphor plate in a cassette much like the film cassette instead of a panel. The plate is positioned like a film cassette would be placed and the exposure occurs. The plate is then removed and taken back to the darkroom and placed in a reader. The reader has a laser scanner that reads down the information from the storage phosphor plate and displays the information on a monitor. This is interpreted for the results. The biggest advantage this technology has over film is the storage phosphors are reusable and the exposure time is usually much less than film.

Does CMOS produce the highest quality image on the market?

CMOS produces the best images available from digital panel technology. CR technology also produces very high quality images, however there are still the same steps to follow as with x-ray film.