Unit 3 - CR and DR

What does PSP stand for?

Photostimulable phosphor

What does the barcode on a PSP cassette do?

Matches image information with patient identifiers

What does the protective layer of a PSP plate do?

Protects the phosphor layer

What material is in the PSP active layer?

Barium fluorohalide

What is the function of the reflective layer?

Sends light in a forward direction

What is the function of the conductive layer?

Absorbs and reduces static electricity

What does the color layer do?

Absorbs stimulating light and reflects emitted light

What is the function of the PSP support layer?

Provides semi-rigid support

What is the function of the backing layer?

Protects the back of the plate

What happens when x-rays interact with the PSP phosphor layer?

Electrons in the phosphor layer are excited

What happens when excited electrons return to their original state?

They give off light

What are the two types of PSP image-reader scanning?

Point scan and line scan

What does the photodetector do in a PSP reader?

Collects, amplifies, and converts visible light into an electrical signal

What does the ADC do to the electrical signal?

Digitizes the electrical signal

What does ADC sampling do?

Reduces a continuous analog signal to a discrete digital signal

What is sampling frequency?

How often sampling is performed

What is sampling pitch?

The distance between sampling points

For better spatial resolution, should sampling pitch be large or small?

Small

When does sampling occur relative to digitization?

Before digitization

How can sampling improve spatial resolution?

Increase sampling frequency and decrease sampling pitch

How is a PSP imaging plate erased?

It is flooded with bright white light to remove remaining trapped electrons

When is automatic PSP erasure performed?

After each reading

Why should PSP plates be manually erased weekly?

To remove background radiation and scatter

What primarily determines PSP spatial resolution?

Sampling frequency

What factors affect PSP spatial resolution?

Sampling frequency, phosphor thickness, laser diameter, translation speed, and sampling pitch

Does a thinner or thicker phosphor layer improve spatial resolution?

Thinner phosphor layer

Does a smaller or larger laser beam diameter improve spatial resolution?

Smaller laser beam diameter

Does higher or lower sampling frequency improve spatial resolution?

Higher sampling frequency

What can incorrect part selection at the workstation cause?

Image degradation

What is kVp most important for in digital imaging?

Penetration of the body part

What can insufficient mAs cause?

Insufficient x-ray photons/phosphor stimulation, resulting in quantum mottle

Why is grid selection especially important with PSP?

Parallel grid lines and the scanning laser can produce a moiré pattern

Is collimation performed pre-exposure or post-exposure?

Pre-exposure

Is shuttering performed pre-exposure or post-exposure?

Post-exposure

What are examples of PSP image-plate artifacts?

Cracks, static/hair, and backscatter

What can cause a PSP processing artifact?

Choosing the incorrect processing parameter or body part

What causes intermittent line patterns from a PSP reader?

Electronic malfunction in the reader

What causes parallel white lines on a PSP image?

Dirt, dust, or scratches

What can cause the appearance of a double exposure?

Overexposure, insufficient white-light erasure, or two imaging plates stuck together

What are common PSP operator errors?

Insufficient collimation, backward cassette, quantum mottle, poor contrast, and physical artifacts

What are the two types of flat-panel x-ray conversion?

Indirect conversion and direct conversion

What is the first step of indirect conversion?

X-rays hit the scintillator, which gives off light

What are the two types of indirect-conversion scintillators?

Gd2O2S and CsI

What does Gd2O2S stand for?

Gadolinium oxysulfide

What type of structure does Gd2O2S have?

Unstructured or turbid

What is an advantage of Gd2O2S?

Rugged design

What is a disadvantage of Gd2O2S?

Too much light spread

What does CsI stand for?

Thallium-doped cesium iodide

What type of structure does CsI have?

Structured layer of crystalline needles

Why does CsI have a higher DQE?

Its structured crystalline needles reduce light spread

In indirect conversion, what converts light into electric charge?

A photodetector made of amorphous silicon (a-Si)

What material is used as the photoconductor in direct conversion?

Amorphous selenium (a-Se)

What happens during direct conversion?

X-ray photons are absorbed and directly converted into an electric signal

What does TFT stand for?

Thin-film transistor

What is the function of a TFT?

Isolates each pixel and sends electric charges to the image processor

What sensing device does indirect conversion require?

A photodiode

What does direct conversion use to convert x-rays into an electric signal?

A photoconductor

How is flat-panel detector performance measured numerically?

Spatial resolution and DQE

How can flat-panel detector performance be evaluated observationally?

Using low-contrast objects in a contrast-detail phantom

What is a dead pixel?

Loss or misrepresentation of image information due to device defects

What can cause dead pixels?

Dust, scratches, static discharge, chemical corrosion, or interactions between materials

What happens to dead pixels as a detector ages?

They increase

What is the acceptable standard for dead pixels?

Less than 0.1%–0.2%

What is interpolation?

A method of filling in dead pixels using surrounding pixels as a guide

What is another name for gain calibration?

Flat fielding

What does gain calibration correct?

Detector flaws by creating a defect mask to remove unwanted densities

What can cause image lag?

Rapid image acquisition, overexposure, and lack of beam attenuation

How can image lag be reduced?

Increase time between exposures, use good collimation, and use appropriate technical factors

What software correction helps correct image lag?

Offset correction

What does CCD stand for?

Charge Coupled Device

Is CCD a direct- or indirect-conversion system?

Indirect conversion

What connects the light image to a CCD?

Lenses or fiber optics

What does demagnification mean in CCD imaging?

Reducing the visible-light image so it can be transferred to the small CCD chip

What happens to light when it reaches the CCD?

It is converted into electric charge

What determines the number of electron-hole pairs produced in CCD silicon?

The amount of light absorbed

What scintillators are used with CCD systems?

Gd2O2S and CsI

Which scintillator is preferred for CCD systems?

CsI

Why is CsI preferred?

Its crystalline needles allow greater x-ray detection with almost no light spread, improving spatial resolution

What does the scintillator material determine?

How many x-ray photons are absorbed and how much light is produced

What is the purpose of CCD lenses and optics?

To focus light onto the CCD chip

What is quantum efficiency in a CCD?

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

Is quantum efficiency the same as DQE?

No; it is not the same as DQE, but it affects DQE

What causes statistical noise in a CCD?

Lack of light photons from the scintillator

What is dark-current noise?

Noise produced when the CCD operates without x-rays

What causes amplification noise?

Variation in CCD chip response and pixels going bad

What are applications of CCD systems?

Digital fluoroscopy, stereotactic breast biopsies, trauma/forensic scanning, and general radiography

What does CMOS stand for?

Complementary Metal Oxide Semiconductor

Who created CMOS?

NASA

Does CMOS use scintillators?

Yes

What does CMOS convert x-ray photons into first?

Light photons

Where are light photons stored in a CMOS detector?

In a capacitor

What does each CMOS pixel have?

Its own amplifier

What does the CMOS pixel amplifier do?

Converts light photons into electric charges when switched on and off

Where is the ADC located in CMOS?

Within the pixel

Why is CMOS efficient?

The ADC is located within the pixel

What is a semiconductor?

A material that conducts electricity under some conditions but not others

What are dopants?

Materials added to semiconductors to make them full-scale conductors

What are the two semiconductor types listed in the study guide?

N-type and P-type

Which semiconductor types does CMOS use?

Both N-type and P-type

What is the indirect-conversion sequence?

X-rays → light → electric signal → digital