Chapter 6 CCD / CMOS Image Capture

CCD structure & function

- light sensitive silicon chip - Light from scintillator hits chip, & electron-hole pairs are produced (light to energy conversion) - Electrons = held in silicon until charge is read out to form the image

Inhibits

An increase in temperature ____________ the flow of electricity

Typical semiconductor materials

Antimony, arsenic, boron, carbon, silicon (most popular)

Very quickly

Bucket brigade scheme (readout) requires that the movement of the electrons be carefully timed. This process occurs:

Readout

Bucket brigade scheme is also known as what?

110

CCDs may use as much as ___________ times more power

Complimentary metal oxide silicon

CMOS

CCDs

Designers are decreasing power requirements and pixel sizes

CMOS

Designers are working to increase quality

CMOS

Developed by NASA, uses scintillator that converts x-rays into light photons when struck. Each pixel has own amplifier (converts light photons into electrical charges & stores within capacitors)

CCDs are used

Digital fluoro, stereostatic breask biopsy, digital mams, general radiography

Adjacent

Excess electrons spill out of the DEL into an __________________ DEL.

Common dopants added to silicon

Gallium arsenide, indium antimondie

2, Structured & unstructured (turbid)

How many main types of scintillators are there? and what are they?

Be swept away from the junction

If an electrical potential is placed on the n side, both the electrons and holes will do what? (no electrons will be available at the junction to form a current)

Dopants

Impurities, also known as _______________ are added to the semiconductor to make them highly conductive.

Readout

In CCD, electrons are held in a well until when?

Light

In CCD, the amount of electron-hole pairs is released to the amount of __________ absorbed.

An electrical potential well

In CCD, where are the electrons held on the detector element?

CMOS

Is a semiconductor, a solid chemical element or compound that conducts electricity under some conditions and inhibits electricity under others. - A good medium for the control of electrical current - Ability to conduct varies on amount of current received

Amplification noise

Is common to digital systems, and there are no CCDs without it. Response of each CCD chip will vary across array (some pixels may not work at all, gain calibration). The more bad pixels, the worse the detector efficiacy

CsI

Is considered structured (needle-like crystals)

Gd2O2S

Is considered unstructured (powder-like grains)

Statistical noise

Is created by lack of light photons from scintillator; this is the cause of quantum mottle. Not enough x-ray photons striking scintillator, output signal = noisy.

DQE

Measurement of how efficiently a system converts an x-ray input signal into a useful output image.

N-type

Negative carrier, these are the electrons

"dark" / current noise

Occurs when the CCD chip operates without radiation stimulation. Temp fluctuation can = released electrons. Unnecessary info added to image at readout.

Zero

Over saturation is almost:

Blooming effect

Overfull of the DELs will cause what?

P-type

Positive carrier, these are the holes

Detective

Quantum efficiency effects _____________ quantum efficiency of the system

Absolute efficiency

Quantum efficiency represents the ________________ _______________ of the CCD chip

Become hot and slow down

Rapid changes in the direction of the electrical current will cause the CMOS to do what?

Light

Scintillators always produce what?

False

Semiconductor materials do conduct electricity well on their own. True or false?

CMOS

System more susceptible to noise, uses less power, less expensive to manufacture, more sensitive to light, tend to have lower quality, lower resolution, and lower sensitivity (the technology is improving)

CCDs

System that is more sensitive to x-ray, simpler and modular, cheaper than TFT-based flat-panel DR

Both

System that provides excellent image capture capabilities and their usefulness depends on the type of application

P-N junction

The "gate" where the type charge carrier is placed in contact with the p-type charge carrier

Statistical, "dark"/current, amplification

The 3 most common types of noise associated with CCD

True

The CCD is built to be as efficient as possible. True or false?

Wavelength

The CCD must be sensitive to _______________ (color of light the phosphor will emit)

TV tube cameras

The CCD replaces what device that was used in the past? (more compact & sensitive to light)

Quantum efficiency

The amount of electrons produced relative to the incident light from the scintillator is the ______________ ______________ of the CCD

True

The efficiency of the transportation of light to the chip is very important. True or false?

Less

The less sensitive the CCD to the light spectrum of the scintillator, the _________ effective the CCD

More

The more light that is sent to the chip without creation of noise, the ______________ efficient the system is.

Charge-coupled devices

The oldest indirect conversion digital radiography system used to acquire a digital image

Critical contributors

The optics in a CCD are ______________ _________________ to the quality of the digital image

How many incident x-ray photons are absorbed, amount of light produced by scintillator, wavelength

The scintillator and the way it is constructed will determine:

Spectral output

The spectrum sensitivity must match the ___________ ___________ of the phosphor

CCDs

Versus CCDs and CMOS, pixel fill factor is greater with which one?

Bucket brigade scheme

Voltage gates are opened & closed at readout to allow the flow of electrons. Electricity is flowing and image formation is being sent from the detector (1 line at a time) to the ADC then on to the comp. workstation

Overflow drains

What are built into the detector to prevent blooming?

Imaging system as a whole

What does DQE refer to?

Migrate toward the junction, & move across it.

When an electric potential is placed on the P side of the junction, the electrons and holes do what?

Full-scale conductor

When doped, the semiconductor becomes a ________-_________ ______________ with extra electrons becoming negative or positive charge carriers

CCD

X-ray photons interact with a scintillation material, and the light is transmitted by lenses or fiber optics - light converted to electricity, where it is stored in capacitors - Charge = released line by line, & sent to ADC