chapter 13- digital radiography

3 elements of digital radiography

capture

coupling

collection

that in which the x-ray is captured

capture element

that which transfers the x-ray generated signal to the collection element

coupling element

examples of a collection element

photodiode

charge-coupled device (CCD)
a thin-film transistor (TFT)p

photodiode and CCD collect

light

TFT collects

electrons

capture element of CR

BaF PSP

coupling element of CR

lens/fiber optics

collection element of CR

photodetector

capture element of SPR

Nal/Csl

coupling element of SPR

none

collection element of SPR

photodetector

capture element of indirect DR

Csl, GdOS

coupling element of indirect DR

fiber optics

contact layer

collection element of indirect DR

CCD/CMOS

TFT

capture element of direct DR

a-Se

coupling element of direct DR

none

collection element of direct DR

TFT

Used in CT to help with patient positioning image volume

scanned projection radiography

how is scanned projection radiography obtained in CT?

maintaining the energized x-ray tube

early versions of scanned projection radiography had

patient motion blur

scatter

new versions of scanned projection radiography are used with

digital radiographic tomosynthesis

The light-sensing element for most digital cameras, silicon based

charge coupled device (CCD)

3 principal advantageous imaging characteristics of CCD

sensitivity

dynamic range

size

ability to detect and respond to low levels of light

sensitivity

ability of the CCD to respond to a wide range of light intensity, from very dim to very bright

dynamic range

size of CCD is (large/small)

very small

when CCD responds to low level of exposure this means

lower patient dose

CCD contrast (related/unrelated) to IR exposure

unrelated

indirect DR process by which x-rays are
converted first to light and then to an electronic signal.

Csl/CCD

CCD are tiled, so exposure time is

short

because Light from a CsI phosphor is transmitted through fiber-optic bundles to the CCD array

high x-ray capture efficiency
good spatial resolution

Because of the edges of each CCD, pixel values must be

interpolated at interfaces

an indirect DR process by which x-rays are converted first to light and then to an electronic signal.

Csl/a-Si

collection element of Csl/a-Si

silicon and sandwiched as TFT

Amorphous silicon is __ that can be

fluid painted

spatial resolution in DR is

pixel limited

Large portion of the face of the pixel is covered by

electronic components and wires that are not sensitive to the light emitted by the CsI

percentage of pixel face sensitive to x-rays

fill factor (80%)

how much of beam in (CsI/a-Se) beam does not contribute to image

20%

what happens to spatial resolution when pixel size decreases?

increases

why when pixel size decreases, patient dose increases?

fill factor is reduced, but x-ray intensity must be increased to maintain signal strength

a direct DR process by which x-rays are converted directly into an electronic signal

amorphous selenium (a-Se)

incoming x-ray beam interacts with a-Se. producing a

charged pair

charged pair from a-Se is stored by capacitor until signal is read by

TFT

(making a-Se capture and coupling element)