Flat-panel detectors are a class of

solid-state Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their use ...

x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

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 inclu ...

devices similar in principle to the image sensors 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 intensifier An X-ray image intensifier (XRII) is an image intensifier that converts X-rays into visible light at higher intensity than the more traditional fluorescent screens can. Such intensifiers are used in X-ray imaging systems (such as fluoroscopes) ...

s (IIs) in

fluoroscopy Fluoroscopy () 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 physician to see the internal structure and function ...

equipment.

Principles

Resolution in direct and indirect x-ray detectors

X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nbs ...

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 Gadolinium oxysulfide ( Gd2 O2 S), also called gadolinium sulfoxylate, GOS or Gadox, is an inorganic compound, a mixed oxide-sulfide of gadolinium. Uses The main use of gadolinium oxysulfide is in ceramic scintillators. Scintillators are used ...

or cesium iodide, which converts the x-rays into light. Directly behind the scintillator layer is an

amorphous silicon Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs. Used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells, it is deposited in thin films ont ...

detector array manufactured using a process very similar to that used to make LCD televisions and computer monitors. Like a

TFT-LCD A thin-film-transistor liquid-crystal display (TFT LCD) is a variant of a liquid-crystal display that uses thin-film-transistor technology to improve image qualities such as addressability and contrast. A TFT LCD is an active matrix LCD, in con ...

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 point in an all points addressable display device. In most digital display devices, pixels are the sm ...

each containing a

thin-film transistor A thin-film transistor (TFT) is a special type of field-effect transistor (FET) where the transistor is thin relative to the plane of the device. TFTs are grown on a supporting (but non-conducting) substrate. A common substrate is glass, becaus ...

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

Direct FPDs

Direct conversion imagers utilize

photoconductor Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation. W ...

s, such as amorphous

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

(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 closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group ...

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

(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. 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

s 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 (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

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