Chapter 22: Digital Radiography/Flat-Panel Detector Systems

Auto-Detection

technology that works by sensing the presence of radiation during the first 1-2ms of the x-ray exposure

charge-coupled device (CCD)

is a photodetector, a device that is capable of converting visible light into an electric charge and storing it in a sequential pattern

CMOS (complementary metal-oxide semiconductor)

image sensor which convert light to electrons; these are more susceptible to noise, and have lower light sensitivity; also, many of the photons hitting this chip hit the transistor instead of the photodiode

Dexel

once a DEL receives exposure and collects signal, it can be referenced as this, due to its bit depth characteristics

Fill Factor

the sensing area compared to the non-sensing area; can be expressed as %

Flat-Panel detector

term used to describe both the indirect amorphous silicon and the direct amorphous selenium plates that are being used in some digital systems

integral detectors

panels that are built into the design of a table

Photoconductor

used by direct detectors, capable of converting light into an electronic signal

Photodetector

converts light photons to an electronic signal in indirect conversion detectors

Portable Detectors

panels that are mobile and can be carried like a cassette

Scintillator

used by indirect detectors, converts incoming x-ray photons to light

Thin Film Transistor (TFT)

used in both IDR and DDR; electronic readout of signal; arranged in an array of detector elements

Trigger Panel

at the start of the exposure, the grid oscillates and the TFT begins the Acquistion of electrons, which are created during the irradiation process.

In DR what is the scintillator constructed of?

Caesium Iodide (Csl). In the form of needle-like crystals to minimize scatter

Name the 3 types of direct digital systems.

charged coupled devices, direct and indirect panel

What is the difference between fluorescence and phosphorescence?

In both, the object absorbs photons at one frequency and re-emit them, usually at a different frequency. When the energy is absorbed, fluorescent dyes go to an unstable state, so that the energy is re-emitted immediately. Phosphorescent ones enter a semi-stable state, which they can hold on to for a while, emitting the energy later

The methods used in DR (indirect Capture) to convert x-rays to electric charges.

The indirect method uses a scintillator that produces light- that then produces electric charges

In DR that uses indirect conversion of x-rays, how is the light, produced in the scintillator, converted to electric charges?

by using an amorphous silicon photodiode array or a CCD (charged coupled device). A photodiode is a semiconductor device that converts light into current.Thin-film transistor (TFT) arrays may be used in both direct and indirect-conversion detectors.

Direct detectors produce an image with less electronic noise. Why?

The x-rays are converted directly to electronic charges. The absence of additional steps results in a much more efficient conversion of x-rays to a digital image.

What is a diode?

An electronic component that allows the current to flow in one direction only

What is a photodiode in respect of DR?

is a semiconductor device that converts light into current. Each one represents 1 pixel

What is the final stage in producing the image in DR?

The charge of each pixel is read out digitally by low-noise electronics and sent to an image processor

Describe a direct flat panel system.

x-ray distribution itself is converted into energy that is stored as a DICOM file

Advantages to a direct system

no cassettes, immediate viewing, can be portable

Disadvantages to a direct system

cost, can't be retrofitted into existing equipment

PSP Plate

also known as an image plate, a plate can be used to record a two- dimensional image of the intensity short-wavelength electromagnetic radiation. The device to reach such a plate is known as a phosphor imager

What is teleradioraphy?

1. Transmission of patient's images form one location to another for the purpose of sharing studies with other physicians

2. used through internet, phone, WAN and LAN. It allows radiologist to provide services without being at the location

Does digital radiology have better spatial resolutions than an analog system?

No

What is exposure latitude?

Extent to which a film/image plate of flat panel can be over/under exposed and still achieve an acceptable image

Does digital have more or less exposure latitude than analog systems?

more, you can use a wider range of exposure settings and still have a quality image

What is creep?

exposure factors can be higher than necessary and still create a diagnostic image

What limitations does CR have?

inefficient x-ray detection, low spatial resolution, and multiple steps required for the technologist

DR systems that were introduced in ____ eliminated cassettes and have reduced the number of steps required to perform exams.

1995

There are a number of detector configurations used in DR that employ either ______ conversion or ________ conversion.

direct, indirect

Indirect conversion detectors are used in a ___ step process.

two

What are the two steps to process indirect detectors?

Fist step involves converting incoming x-ray photons to light first, which is accomplished through the use of a scintillator. The second step involves a conversion of light photons to an electronic signal using a photodetector.

Indirect DR systems include either a __________________ or ___________________ with a __________________ array.

charge-coupled device (CCD), amorphous silicon, thin film transistor (TFT)

T/F: Direct DR systems are capable of directly converting incoming x-ray photons to an electronic signal.

True

What are two types of indirect DR detectors?

flat-panel TFT and Charge-coupled device (CCD)

T/F: Indirect TFT digital detectors utilize amorphous silicon. Amorphous silicon cannot directly convert x-rays into an electric charge, but it does work as a light detector (photodiode) to capture fluorescent light.

True

T/F: Amorphous silicon requires a scintillator, such as cesium iodide, or a rare-earth intensifying screen composed of gadolinium oxysulfide.

True

Scintillators emit light _____________, which can reduce spatial resolution.

isotropically

T/F: The fill factor has an indirect relationship with both the spatial resolution and the contrast resolution.

False, Direct

Detectors with ____ fill factor will yield higher spatial and contrast resolution, as compared to detectors wit ha low fill factor.

high

Currently, most systems are at the smallest DEL possible, because trying to have a smaller pixel pitch will ________ the electronics and lower the fill factor, resulting in less sensitivity

increase

A gadolinium oxysulfide or a cesium iodide screen is used as a _____________, and the light produce from x-ray interactions must be optically coupled to the CCD sensor chip by lenses or fiber optics.

scintillator

The active layer in the imaging plate is amorphous selenium, which is a __________ with excellent x-ray photon detection ability.

semiconductor

DR image processing can be described as a two-step process that invoices ________________ and ________________.

pre-processing and post-processing

When is system calibration performed?

during pre-processing

What is the last step in the DR imaging process?

image display

What are some dangers to DR panels?

dropping, fluids, patient weight, etc

T/F: Direct DR systems are capable of directly converting incoming x-ray photons to an electronic signal. These systems use amorphous selenium and a TFT.

True