Digital: Lesson 6 - CCD/CMOS Image Capture

Reducing the image size involves several components that contribute to the quality of the image:

-The scintillator -Light collection component -Noise

Structure of a CCD:

A CCD is made up of a photosensitive receptor and electronics embedded in a substrate material in a silicon chip

Stereotactic Breast Biopsy

A single CCD detector can be used because the typical field size is 50mm × 50 mm. -Most current systems use a 2.5 cm x 2.5 cm chip that results in some demagnification. Demagnification -Lenses, optics, and mirrors are used in the process. -Noise is not as much of a factor because of the low dose. Newer digital mammography systems are replacing CCD with flat-panel technology.

Amplification Noise

Amplification noise 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. -Software is used to correct the statistical variation caused by inoperable pixels.

Advantages of CCD:

CCDs are simple and modular. -Easy to repair, replace, and upgrade CCDs are relatively cheaper than TFT-based flat-panel systems.

General Radiography

CCDs may be tiled in a 16 x 12 array to a single CCD. -CsI is a common scintillator material. Whole-body scanners are used in trauma and forensic pathology applications. -12 CCD cameras are coupled to gadolinium sulfur dioxide scintillators with tapered fiberoptics. -The tube and receptor array are housed in a C-arm type mount. -A fan-beam (slot scanning device) moves down the length of the table (stretcher). The scan can be any length up to 180 cm.

How do Complementary Metal Oxide Silicon (CMOS) work?

CMOS image sensors convert light to electrons that are stored in capacitors located within each pixel.

CMOS

CMOS is a semiconductor, a solid chemical element or compound that conducts electricity under some conditions but not others. Its ability to conduct varies 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 a semiconductor to make them highly conductive.

Comparison of CCD and CMOS Technology

CMOS sensors are more susceptible to noise. Light sensitivity of a CMOS chip is lower than a 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 less expensive to manufacture. They tend to have lower quality, lower resolution, and lower sensitivity, but the technology is improving. Pixel fill factor is greater with CCDs than CMOS.

Complementary Metal Oxide Silicon

CMOS systems were developed by NASA -Uses a scintillator that, converts the x-rays into light photons and stores them in 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.

Disadvantages of CCD:

Demagnification issues reduce DQE

Function of a CCD:

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.

Image-Related Characteristics

It is not currently practical to manufacture a CCD chip larger than 5 cm x 5 cm. -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.

"Dark" or current noise

Occurs when the CCD chip operates without radiation stimulation. When the temperature rises and electrons are moved into the bulk silicon layer or depletion area, unnecessary information is added to the image.

Noise

Several types of noise are associated with CCD technology. The most common three are: -Statistical noise -"Dark" or current noise -Amplification noise

Digital Fluoroscopy

The CCD is a great replacement for the television pick-up tube in an image intensifier. -The CCD size (2-5 cm) is closely matched with the output phosphor of an image intensifier (1 in). -The CCD readout is linear and has a greater dynamic range than a TV tube. Oversaturation is almost zero. -Blooming is greatly reduced, shortening the recovery time from one image to another. CCDs are more compact than TV tubes.

Image-Related Characteristics

The CCD is built to be as efficient as possible. The amount of electrons produced relative to the incident light from the scintillator is the quantum efficiency of the CCD. Quantum efficiency affects detective quantum efficiency (DQE). ***DQE is the ratio of output signal to signal-to-noise ratio. The spectrum sensitivity must match the spectral output of the phosphor.

What is a Charge-Coupled Devices?

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

Importance of Optics

The optics in a CCD are critical contributors to the quality of the digital image. 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. Design of optics is crucial in imaging performance. -Issues - Geometric distortion such as light scattering, lens flaws or optics flaws.

The Scintillator

The two main types of phosphors are structured and unstructured (turbid). -CsI is considered structured because of its needle-like crystals. -Gd2O2 is considered unstructured because of its powder-like grains.

T or F: This system is highly efficient and takes up less fill space than CCDs.

True

T or F: Some manufacturers add thermoelectric cooling devices that drop the temperature as much as 40F lower than room temperature.

True A 40F drop can reduce dark noise by a factor of 100.

The more bad pixels, the _________ the detector efficiency.

Worse The cheaper the CCD, the more defects.

How does a Charged-Coupled Device work?

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 One or more small capacitors convert the light into an electrical charge. This charge is stored in a sequential pattern and released line by line.

Statistical noise

is created by lack of light photons from the scintillator; this is known as quantum mottle. -If there are 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.

How does a ADC work?

turns each pixel's value into a digital value.

CCD Applications

used in several applications in radiology, including: -Digital fluoroscopy -Stereotactic breast biopsy -Digital mammography General radiography