Flat-panel detectors are a class of solid-state

x-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

digital radiography Digital radiography is a form of radiography that uses x-ray–sensitive plates to directly capture data during the patient examination, immediately transferring it to a computer system without the use of an intermediate cassette. Advantages incl ...

devices similar in principle to the

image sensors An image sensor or imager is a sensor that detects and conveys information used to form an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of curren ...

used in digital photography and video. They are used in both

projectional radiography Projectional radiography, also known as conventional radiography, is a form of radiography and medical imaging that produces two-dimensional images by X-ray radiation. The image acquisition is generally performed by radiographers, and the images a ...

and as an alternative to x-ray image intensifiers (IIs) in

fluoroscopy Fluoroscopy (), informally referred to as "fluoro", is an imaging technique that uses X-rays to obtain real-time moving images of the interior of an object. In its primary application of medical imaging, a fluoroscope () allows a surgeon to see t ...

equipment.

Principles

Resolution in direct and indirect x-ray detectors

X-rays An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

pass through the subject being imaged and strike one of two types of detectors.

Indirect detectors

Indirect detectors contain a layer of scintillator material, typically either gadolinium oxysulfide or cesium iodide, which converts the x-rays into light. Directly behind the scintillator layer is an amorphous silicon detector array manufactured using a process very similar to that used to make

LCD A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers to display information. Liquid crystals do not em ...

televisions and computer monitors. Like a TFT-LCD display, millions of roughly 0.2 mm

pixels In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, pixels are the sma ...

each containing a thin-film transistor form a grid patterned in amorphous silicon on the glass substrate. Unlike an LCD, but similar to a digital camera's image sensor chip, each pixel also contains a

photodiode A photodiode is a semiconductor diode sensitive to photon radiation, such as visible light, infrared or ultraviolet radiation, X-rays and gamma rays. It produces an electrical current when it absorbs photons. This can be used for detection and me ...

which generates an electrical signal in proportion to the light produced by the portion of scintillator layer in front of the pixel. The signals from the photodiodes are amplified and encoded by additional electronics positioned at the edges or behind the

sensor array A sensor array is a group of sensors, usually deployed in a certain geometry pattern, used for collecting and processing electromagnetic or acoustic signals. The advantage of using a sensor array over using a single sensor lies in the fact that an ...

in order to produce an accurate and sensitive digital representation of the x-ray image.

Direct FPDs

Direct conversion imagers utilize photoconductors, such as amorphous

selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...

(a-Se), to capture and convert incident x-ray photons directly into electric charge. X-ray photons incident upon a layer of a-Se generate electron-hole pairs via the internal photoelectric effect. A

bias Bias is a disproportionate weight ''in favor of'' or ''against'' an idea or thing, usually in a way that is inaccurate, closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individ ...

voltage applied to the depth of the selenium layer draw the electrons and holes to corresponding electrodes; the generated current is thus proportional to the intensity of the irradiation. Signal is then read out using underlying readout electronics, typically by a thin-film transistor (TFT) array. By eliminating the optical conversion step inherent to indirect conversion detectors, lateral spread of optical photons is eliminated, thus reducing blur in the resulting signal profile in direct conversion detectors. Coupled with the small pixel sizes achievable with TFT technology, a-Se direct conversion detectors can thus provide high spatial resolution. This high spatial resolution, coupled with a-Se's relative high quantum detection efficiency for low energy photons (< 30 keV), motivate the use of this detector configuration for mammography, in which high resolution is desirable to identify microcalcifications.

Advantages and disadvantages

Flat-panel detectors are more sensitive and faster than

film A film, also known as a movie or motion picture, is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, emotions, or atmosphere through the use of moving images that are generally, sinc ...

. Their sensitivity allows a lower dose of radiation for a given picture quality than film. For

, they are lighter, far more durable, smaller in volume, more accurate, and have much less image distortion than x-ray image intensifiers and can also be produced with larger areas. Disadvantages compared to IIs can include defective image elements, higher costs and lower spatial resolution. In general radiography, there are time and cost savings to be made over computed radiography and (especially) film systems. In the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

is on course to surpass use of computed radiography and film. In mammography, direct conversion FPDs have been shown to outperform film and indirect technologies in terms of resolution, signal-to-noise ratio, and quantum efficiency. Digital mammography is commonly recommended as the minimum standard for breast screening programmes.

References

External links