Digital Radiography Lec

digital radiography (DR)

cassette-less system

direct and indirect conversion

direct digital radiography DDR

x-rays —> electronic signals to digital

indirect digital radiography IDR

xrays —> visible light —> electronic signals to digital

is direct or indirect more common?

indirect because it is more cost effective (lower spatial resolution though because extra conversion)

difference between direct vs indirect?

direct does not use scintillator (converts xr to light)

indirect conversion can use which two devices?

CCD charge-coupled device (scintillation layer)

TFT thin film transistor (scintillation and photodiode layer)

thin film transistor used for?

angiography and FL

CCD

charge coupling device

used in indirect digital radiography

light sensing element

made of silicon

why is silicon used for CCD?

very sensitive to light

wide dynamic range

small size

fill factor

percentage of the actual pixel size that is sensitive to xrs

bigger fill factor desired

INC fill factor = ? resolution

INC fill factor = INC resolution

direct digital radiography layers

photoconductor (amorphous selenium)

thin film transistor TFT

analog to digital converter

does indirect or direct have higher resolution?

direct has higher resolution bc less steps but costs more money

digital images are a ?

matrix of picture element (pixels) values

each pixel is assigned what?

a shade of gray

bone

small signal

radiopaque

soft tissue

medium signal

gray

air

large signal

radiolucent

pixel

picture element

smallest visible unit of a digital image that determine spatial detail of an xray image

bit

smallest unit of digital data

How many brightness values each pixel can have OR how many shades of gray each pixel can take on

bit depth

pixel’s range of possible shades of gray (10, 12, 16 are common)

bit depth formula

2^n

INC bit depth = ? shades of gray = ? contrast resolution

INC bit depth = INC shades of gray = INC contrast resolution

INC pixels = ? matrix = ? resolution

INC pixel amount = INC matrix size = INC resolution

bit depth and contrast resolution related how?

directly proportional

No DR imaging system can resolve an object ?

No DR imaging system can resolve an object smaller than the pixel size

pixel size too big, object size too small

only one shade of gray per pixel

matrix size and spatial resolution relationship?

directly proportional

common matrix sizes

1024x1024, 2048x2048, 4096x4096

spatial frequency

describes how often image details repeat per unit distance in an image / how often image details change from light and dark over a given distance

spatial frequency measured how?

line pairs / mm

when do we want low spatial frequency?

larger structures, soft tissue outlines

when do we want high spatial frequency?

finer detail; looking for trabeculae detail, extremities, smaller bones

mtf

modulation transfer function

describes how well an imaging system preserves detail or contrast at different spatial frequencies

Ability of an imaging system to display objects of different sizes onto an image

mtf scale 0-1

1 = perfect representation

0 = no representation

dynamic range

range of values over which a system can respond

AKA gray scale

larger dynamic range

• More shades of gray (long scale)

• Low contrast

• Better contrast resolution 

• Wide image latitude (margin of error for an image)

smaller dynamic range

• Less shades of gray (short scale)

• High contrast 

• Less contrast resolution 

• Narrow image latitude

image quality factors

brightness

contrast

resolution

distortion

exposure index

noise

brightness

intensity of light that represents the individual pixels in the image on the monitor

brightness replaces density

contrast

difference in brightness between light and dark areas of an image

scatter radiation control

• Closely collimate (hardens the beam, filters out low energy xr)  

• Use grids (absorb scatter radiation before reaching IR) 

• Optimal kV

what determines spatial resolution in both film and digital ?

film = focal spot size

digital = pixel size

SNR

signal to noise ratio

high snr desired

mAs and SNR have direct relationship

INC mAs = ? SNR

INC mas = INC snr (direct relationship)

signal

number of xr photons that strike the detector

determined by mAs

noise

Other factors that negatively affect the final image

low snr makes what kind of image?

grainy image - quantum mottle bc not enough xr photons reaching IR

INC mas to fix !

electronic noise

inherent noise in the electronic system, power fluctuations, non-uniformity of detector

detective quantum efficiency

the ability of a material to absorb xr energy ; how efficient a IR absorbs xr signal and produces signal

DQE and pt dose relationship?

inverse

inc dqe = dec pt dose

30% rule:

A minimum of 30% of the IR must be exposed for an accurate exposure index

pt dose from film to DDR?

pt dose from film to IDR?

film to DDR = reduce dose 20-50%

film to IDR = reduce dose 30-50%

highest DQE of electronic conductors?

selenium