chapter 6 -- charge-coupled device/complementary metal oxide semiconductor image capture

charge-coupled devices (CCD)

• the oldest indirect conversion digital radiography system used to acquire a digital image

  • x-ray photons interact with a scintillation material, and the signal is transmitted by lenses or fiberoptics 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 ADC

structure and function of CCD

• made of a photosensitive receptor and electronics embedded in a substrate material in a silicon chip

• x-rays strike on scintillator, converted into light

• light strike on the chip, converted into electrical signals

• incident light from a scintillator strikes the detector, and electron—hole pairs are produced in the silicon

  • the amount of electron-hole pairs is related to the amount of light absorbed

• the electrons are held by electrostatic forces in the array until the charge is read out to form the image

• the detector element (del) contains three electrodes that hold the electrons in an electrical potential well

  • bucket brigade scheme

• when the readout row is reached, data is sent sequentially to the sense amplifier and digitalized

  • the readout process requires that the movement of electrons is timed

  • multiple voltage change occur very quickly

• overfill of the dels will cause a blooming effect

bucket brigade scheme

• the dels are formed by voltage gates that, at readout, are opened and closed like gates to allow flow of electrons

• to collect the charge on the silicon chips, the voltage sign is changed on the electrodes with each del, moving the electrons by rows down the columns until the readout row is reached

blooming effect

• excess electrons spill out of the del into an adjacent del

• overflow drains are built into the array to reduce or prevent this effect

silicon chip

this chip is made up of a polysilicon layer, a silicon dioxide layer, and a silicon substrate

• the polysilicon layer is coated with a photosensitive material and contains the electron gates

• the silicon dioxide layer is an insulator

• the silicon substrate contains the charge storage area

image-related characteristics

• size:

  • it is not practical to manufacture a CCD chip larger than 5 × 5 cm — cost is high

  • most range from 2 to 4 cm

  • the image must be matched to the size of the CCD

  • CCD technology uses lenses or fiberoptics so that the image matches the receptor size

• reducing the image size involves several components that contribute to the quality of the image

  • the scintillator, light collection component, noise

• the scintillator and the way it is contructed will determine

  • how many of the incident x-ray photons are abosrbed

  • how much light is produced

  • the wavelength or color of the light

• CCD optics are critical contributors to digital image quality

• lenses or fiberoptics focus light onto the CCD chip

• the efficiency of the transportation of light to the chip is important because of the noise created

  • the more light that is sent to the chip without creation of noise, the more efficient the system is

• the design of optics is also crucial in imaging performance

  • issues arise with geometric distortion such as light scattering or lens or optic flaws

• the amount of electrons produced relative to the incident light from the scintillator is the quantum efficiency of the CCD

scintillator

phosphors are structured or unstructured (turbid)

structured scintillator

• CsI — due to its needle-like crystals

  • CsI needles focus light onto a very narrow area, reducing light spread and allowing the use of thicker scintillators minimizing the loss of spatial resolution

unstructured scintillator

• Gd₂O₂ — due to its powder-like grains

  • turbid phosphors produce more light spread, resulting in decreased efficiency of the detector

quantum efficiency

• represents the absolute efficient of the light collection and signal created in the chip

• affects detective quantum efficiency

noises associated with CCD technology

• quantum noise (statistical noise)

• “dark” or current noise

• amplification noise

noise

the random variation of brightness or color information in digital images that creates a grainy, speckled, or mottled texture interference

signal

in medical and digital imaging, refers to the amount of useful, informative data captured by the imaging system

quantum noise

• statistical noise

• is created by lack of light photons from the scintillator; this is known as quantum mottle

  • is caused if there is not enough x-ray photons striking the scintillator

  • if too few light photons are produced by the scintillator, then too few electrons will be generated, and the output signal will be noisy

dark noise

• occurs when the CCD chip operates without radiation stimulation

  • when temperature rises electrons are moving into some areas

  • at readout, unnecessary information is added to the image

  • some manufacturers add thermoelectric cooling devices that drop the temperature as much as 40 degrees F, lower than room temperature

  • can be as much as 10,000 electrons/pixel per second

  • A 40 degree F drop can reduce by a factor of 100

amplification noise

• most commonly referred to as readout noise or read noise

• represents the electronic noise generated during the physical process of converting a pixel’s stored electrical charge into a readable analog voltage signal

• is common to digital systems, and there are no CCDs without it

• the response of each CCD chip will vary across the array

  • some pixels may not work at all; or work differently

  • software is used to correct the statistical variation caused by inoperable pixels

• the more bad-pixel, the worse the detector efficiency

  • the cheaper the CCD, the more defects

CCD applications

• used in several applications in radiology:

  • digital fluoroscopy, stereotactic breast biopsy, digital mammography, general radiography, dental, others

advantages/disadvantages of CCD

• CCDs are simple and modular

  • easy to repair, replace, and upgrade

• CCDs are relatively cheaper than TFT- based flat-panel systems

• demagnification issues reduce DQE

  • decreases image quality

CMOS

complementary metal oxide semiconductor

• systems were developed by NASA

  • uses a scintillator that converts x-rays into light photons and stores them into capacitors

  • each pixel 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 also located within the pixel

• this system is highly efficient and takes up less fill space than CCDs

• a semiconductor

• when doped, the semiconductor becomes a fill-scale

  • transistors use little to no power when not needed

• image sensors convert light to electrons that 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

  • at each pixel, several transistors amplify the charge and send it through wires to the array corner

• an ADC turns each pixel’s value into a digital value

semiconductors

a solid, chemical element or compound that conducts electricity under some conditions but not others

• a good medium for control of electrical current

• conduction ability varies according to the amount of current or voltage it receives or on the x-ray intensity

do not conduct electricity well on their own

• impurities, or dopants, added to increase conductivity

  • common dopants added to silicon are:

    • gallium arsenide, indium antimonide, and oxides of most metals

materials: antimony, arsenic, boron, carbon, germanium, selenium, silicon, sulfur, and tellurium

• silicon is the most popular and is the base material of most integrated circuits

comparison of CCD and CMOS technology

• CCD sensors are more susceptible to noise

• light sensitivity of a CMOS chip is lower than CCD

  • in a CMOS, the pixel is surrounded with transistors, and many of the light photons hit these rather than the photodiode

• CMOS uses very little power

  • CCDs may use as much as 110x more power

• CMOS is very inexpensive to manufacture

  • CMOS ships tend to have lower quality, lower resolution, and lower sensitivity, but the technology is improving

• pixel fill factor is greater with CCDs than CMOS

in digital fluoroscopy

• with II:

  • video system: mirror directing light; video/camera system for delivering image or recording

  • CCD photodetector is attached to output screen and converts exit light photons into electrical signal and send them to the monitor

• without II:

  • TFT-flat panel uses:

    • under mag mode, no patient dose increasing

    • indirect (CsI) is more common — at low mA, better signal

    • CMOS flat panel detector is catching up (image quality is getting better)