The oldest indirect-conversion digital radiography systems used charge-coupled devices (CCDs) to acquire the digital image. These devices are still being used in a variety of image capture applications. In a CCD system, x-ray photons interact with a scintillation material and the signal is transmitted by lenses or fiber optics to the CCD. During the transmission process, the lenses reduce the size of the projected visible light image and transfer the image to one or more small capacitors that convert the light into an electrical charge. This charge is stored in a sequential pattern and released line by line and sent to an analog-to-digital converter (ADC) (Figure 6-1).

Structure and Function

A photosensitive receptor and electronics embedded into a substrate material in a silicon chip make up a CCD. Incident light from a scintillator strikes the detector and electron/hole pairs are produced in the silicon. The number of electron/hole pairs is related to the amount of light that is absorbed. The electrons are held by electrostatic forces in the array until the charge is read out to form the image. The chip is made up of a polysilicon layer, a silicon dioxide layer, and a silicon substrate (Figure 6-2). The polysilicon layer is coated with a photosensitive material and contains the electronic gates. The silicon dioxide layer is an insulator and the silicon substrate contains the charge storage area.

Each pixel or detector element (del) contains three electrodes that hold the electrons in a electrical potential well. The dels are formed by voltage gates that, at readout, are opened and closed like gates to allow the flow of electrons. To collect the charge on the silicon chips, the voltage sign is changed on the electrodes within each del, moving the electrons by rows down the columns until the readout row is reached. This process is commonly known as the “bucket brigade scheme” (Figure 6-3). Once the readout row is reached, the data are sent sequentially to the sense amplifier and are then digitized. The readout process requires that the movement of the electrons is timed, and multiple voltage changes occur; however, the process is very quick. There can be issues with overfill in the dels, which causes excess electrons to spill out of the del into an adjacent del. This is handled by building overflow drains into the array to reduce or prevent this “blooming” effect.

CCD Applications

CCDs are used in several applications in radiology including digital fluoroscopy, stereotactic breast biopsy, digital mammography, and general radiography. In digital fluoroscopy, the CCD is a great replacement for the television (TV) pickup tube in an image intensifier because the CCD size (2 to 5 cm) is closely matched with the output phosphor of an image intensifier (1 inch). The CCD readout is linear and has a greater dynamic range than a TV tube; because oversaturation is almost zero, blooming is greatly reduced, shortening the recovery time from one image to another. CCDs are much more compact than TV tubes and CCDs are not susceptible to electron scanning errors the way that TV tubes are.

For stereotactic breast biopsy applications, a single CCD detector can be used because the typical field size is 50 mm × 50 mm. However, most current systems use a 2.5 cm × 2.5 cm chip that results in some demagnification. Demagnification is the process of reducing the phosphor output image to the size of the active area of the CCD chip. The demagnification uses lenses, optics, and mirrors to accomplish this task. Because of the low dose, noise is not much of a factor. In digital mammography, early applications used CCDs, but these applications are being replaced by flat-panel systems.

In general radiography, CCDs may be tiled in a 16 × 12 array (Figure 6-4) to a single CCD. CsI is common as the scintillation material. Another way CCDs are used is in a whole-body scanner, where 12 CCD cameras are coupled to gadolinium sulfur dioxide scintillators with tapered fiber optics. The tube and receptor array are housed in a C-arm type of mount and the body is scanned by a moving fan-beam (slot-scanning device) down the length of the table. The scan can be any length up to 180 cm. The table is actually a stretcher and is largely used for trauma applications and forensic pathology.

Developed by NASA, complementary metal oxide semiconductor (CMOS) systems use a scintillator that, when struck with x-ray photons, convert the x-rays into light photons and store them in capacitors (Figure 6-5). Each pixel or detector element has its own amplifier, which is switched on and off by circuitry within the pixel, converting the light photons into electrical charges.

Voltage from the amplifier is converted by an ADC. This system is highly efficient and takes up less fill space than CCDs. CMOS is a semiconductor. A semiconductor is a solid chemical element or compound that conducts electricity under some conditions but not others, making it a good medium for the control of electrical current. Its ability to conduct current varies depending on the amount of current or voltage it receives or on the intensity of radiation by x-rays. Impurities (also known as dopants) are added to semiconductors because the materials semiconductors are made from do not conduct electricity very well by themselves. Thus they are “doped” with impurities that make them highly conductive.

Typical semiconductor materials are antimony, arsenic, boron, carbon, germanium, selenium, silicon, sulfur, and tellurium. Silicon is the most popular and is the base material of most integrated circuits. Common impurities added to the silicon are gallium arsenide, indium antimonide, and oxides of most metals. When doped, the semiconductor becomes a full-scale conductor or becomes an N-type transistor (extra electrons with a negative charge) or positive charge carriers (P-type transistors). In most semiconductors of the CMOS kind, both types of transistor are used so that they form a gate that can be controlled electrically. Although CMOS transistors use little to no power when they are not in use, if the direction of the current changes rapidly, the transistors become hot and this slows down the microprocessors.

Like CCD technology, CMOS image sensors convert light into electrons. The electrons are stored in capacitors located within the pixel. During readout, the charge is sent across the chip and read at one corner of the array. This is typically done using several transistors at each pixel that amplify the charge and send it through wires to the array corner. An ADC turns each pixel’s value into a digital value.

There are some noticeable differences between CCD and CMOS sensors.

Despite their differences CCD and CMOS technologies both provide excellent image capture capabilities; also for both, their usefulness depends on the type of application. Each has strengths unique to those applications and neither is superior to the other.

Current CMOS designers are working diligently to increase image quality and CCD designers are seeking to decrease power requirements and pixel sizes. Both of these technologies continue to be improved year after year because of their wide use in many industries. Medical imaging will continue to see growth and new applications from both CCD and CMOS detectors.