digital radiography

what is the basic sequence of direct conversion?

x-rays → electronic signal → digital

what is the basic sequence of indirect conversion?

x-rays → light → electronic signal → digital

between direct conversion and indirect conversion, which system is used more commonly?

indirect conversion

between direct conversion and indirect conversion, which system uses a scintillator?

indirect conversion

what is the main function of scintillation?

converts x-rays into light

what are the two types of systems used for indirect conversion?

charge-coupled device (CCD) and thin film transistor (TFT)

which layer(s) consist of the charge-coupled device (CCD)?

scintillation

which layer(s) consist of the thin film transistor (TFT)?

scintillation and photodiode

what is the thin film transistor (TFT) most commonly used for?

angiography and fluoroscopy

what are two advantages of using indirect conversion?

cost-effective and easier to repair and replace parts

what is a disadvantage of using indirect conversion?

extra conversion reduces spatial resolution

what material is a charge-coupled device (CCD) made of?

silicon

what are the 3 characteristics of a charge-coupled device (CCD)?

• very sensitive to light (from scintillator)

• wide dynamic range (many shades of gray)

• small size

what is fill factor?

percentage of the actual pixel size that is sensitive to the x-rays

as fill factor gets larger, resolution increases or decreases?

increases

the higher the fill factor, the _________ the resolution

higher

what is the main function of a storage capacitor?

temporarily holds the charge

which imaging procedures is the direct method most commonly used in?

mammography and angiography

which layer(s) are included in the direct method?

photoconductor (amorphous selenium), thin film transistor (TFT), analog-to-digital converter

what are two advantages of using direct conversion?

higher spatial resolution and less steps

what are digital images?

a matrix of pixel values

what produces a small signal and appears white on a radiograph?

bone

what produces a medium signal and appears grey on a radiograph?

soft tissue

what produces a large signal and appears black on a radiograph?

air

is bone radiopaque or radiolucent?

radiopaque

is air radiopaque or radiolucent?

radiolucent

what is a pixel?

smallest visible unit of a digital image that determines spatial detail of an x-ray image

what is a bit?

• smallest unit of digital data

• how many brightness values each pixel can have

• how many shades of gray it can take on

what is bit depth?

pixel’s range of possible shades of gray

as bit depth increases, contrast resolution…

increases

high contrast resolution means more or less shades of gray?

more

as pixels increase, matrix…

increases

what is resolution?

detail of structure on the image

as pixels increases, resolution…

increases

true or false: no DR imaging system can resolve an object smaller than the pixel size

true

what is an image matrix?

layout of rows and columns of pixels

as matrix size increases, spatial resolution…

increases

what is spatial frequency?

describes how often image details repeat per unit distance in an image

what is spatial frequency measured in?

line pairs per mm

when imaging larger structures, would we want an increase or decrease in spatial frequency?

decrease

when imaging smaller structures, would we want an increase or decrease in spatial frequency?

increase

what does a modulation transfer function help describe?

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

what does a modulation transfer function allow an imaging system to display?

objects of different sizes onto an image

what does a modulation transfer function (MTF) of 0 indicate?

no information is transferred

what does a modulation transfer function (MTF) of 1 indicate?

faithful representation of the object

what is a dynamic range?

range of values over which a system can respond

what is a dynamic range also known as?

gray scale range or number of shades of gray

does an abdomen have a lower or higher dynamic range?

higher

higher dynamic range → [less/more] shades of gray → [low/high] contrast → [less/more] contrast resolution → [narrow/wide] latitude

more, low, more, wide

do extremities have a lower or higher dynamic range?

lower

smaller dynamic range → [less/more] shades of gray → [low/high] contrast → [less/more] contrast resolution → [narrow/wide] latitude

less, high, less, narrow

what is contrast resolution?

an imaging system’s ability to distinguish between similar tissues

list the 6 image quality factors

brightness, contrast, resolution, distortion, exposure index, noise

what is brightness?

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

brightness replaces…

density

is the term ‘density’ used in relation to film or digital?

film

is the term ‘brightness’ used in relation to film or digital?

digital

what is contrast?

difference in brightness between light and dark areas of an image

what are the three methods used to control scatter radiation?

• close collimation

• usage of grids

• optimal kV

in film imaging, what determines spatial resolution?

focal spot size

in digital radiography imaging, what determines spatial resolution?

pixel size

what is noise?

a random disturbance that obscures or reduces clarity of the image

what is clarity?

visibility of detail

what is the signal to noise ratio (SNR)?

the ratio of useful x-ray signal to unwanted noise in an image

the number of x-ray photons that strike the detector is considered the…

mAs

besides noise, what is another factor that affects the final image?

signal

as noise increases, contrast resolution…

decreases

would a low or high signal to noise ratio (SNR) be more desirable?

high

what does a low signal to noise ratio (SNR) demonstrate on a radiograph?

a grainy image

as mAs is increased, the signal to noise ratio (SNR) is…

increased

what two things does low signal to noise ratio (SNR) cause?

scatter radiation and electronic noise

what is electronic noise?

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

can we control electronic noise?

no

with direct digital radiography (DDR), what is the patient dose reduction percentage range?

20-50%

with indirect digital radiography (DDR) from film, what is the patient dose reduction percentage range?

30-50%

what is detective quantum efficiency (DQE)?

the ability of a material to absorb x-ray energy

as detective quantum efficiency (DQE) increases, patient dose…

decreases

which electronic conductor has the highest detective quantum efficiency (DQE)?

selenium

what is the minimum percentage of the imaging plate that should be exposed to x-rays to ensure an accurate exposure indicator reading?

30%