Processing and Characteristics

Grayscale bit depth

controls the number of shades of gray available within each pixel

is inherent to each system (can’t change)

calculating bit depth

2^power of the bit depth = number of shades of gray available

higher bit depth =

better contrast resolution

spatial resolution is also called

detail

definition

resolution

spatial resolution

ability to distinguish adjacent structures as separate disctinct structures

expressed in line pairs per mm

what controlls spatial resolution

matrix size and how many pixels are displayed by the monitor

lp/mm and detail

more lp/mm = more detail

b

how does focal spot size affect spatial resolution

smaller FSS = increased resolution

how does pixel size affect spatial resolution

smaller pixel size = increased resolution

how does pixel pitch affect spatial resolution

smaller pixel pitch = increased resolution

how does sampling frequency affect spatial resolution

high frequency = increased resolution

how does the phosphor layer affect spatial resolution

small, columnar crystals = increased resolution

how does laser beam size affect spatial resolution

smaller beam = more resolution

how does the speed of the IR passing through the reader affect spatial resolution

faster = increased resolution

contrast resolution

ability of an imaging system to distinguish and display a range of attenuation coefficients fom different tissues within the body

• ability to reproduce subject contrast

• expressed as varying grayscale levels

film vs digital for spatial and contrast resolution

film has superior spatial resolution

digital has superior contrast resolution

histogram

graphic representation of the frequency of each pixel intensity

shape is characteristic of the exam and view

histogram analysis

the computer compares the image histogram to the histogram of the way it should be stored in the look up table then rescales it

dynamic range

range of exposure intensities that the IR can respond to and use to aquire image data

values of interest

the region of pixel values that should be included in an image

dynamic range of digital recetors

responsive to a wide variety of exposure intensities

long linear line on the graph

film screen dynamic range on the graph

curved line

exposure latitude

range of exposure values an IR is able to produce

dependent on IR

detector response to exposure is linear

film vs digital exposure latitude

digital has a wider exposure latitude

what system is more sensitive to scatter

digital

exposure indicators

the amount of exposure received by the IR

requires adequate exposure to read out

underexposure = quantum mottle

overexposure = poor contrast

deviation index

difference between actual and ideal exposure for specific body part

factors that cause exposure deviation

prosthesis

gonadal shielding

improper collimation

unexpected body part in image

Nyquist theorem

for an adequate representation of the original signal, you must sample each wave form twice

each pixel must be sampled twice

Nyquist frequency

the highest spatial frequency that a digital detector can record and is determined by the sampling frequency

spatial resolution and the Nyquist theory

spatial resolution = ½ the Nyquist frequency

to produce a resolution of 5 lp/mm you must sample 10 lp/mm

pixel pitch

physical distance between pixels

measured from center to center

aliasing

fold-over or biasing that causes mirroring of the image

wrap around image produced by two superimposed images

looks similar to the Moire effect

noise

any type of signal interference with a digital image

signal to noise ration

the percentage of useful signal to non-useful destructive noise contained within the acquired image data

as SNR decreases, graininess increases

contrast to noise ratio

measuring the difference between tissue and background noise

typically used to enhance anatomy

electronic interference that causes noise

snow

noise power spectrum

higher NPS = higher noise

detective quantum efficiency

overall efficiency of with which a detector converts incoming x-ray exposure to useful output image

AKA speed class

ideally want 100% or DQE of 1

DQE of digital systems best to worst

indirect capture

direct capture

CR

film

what system has less pt dose

indirect capture

TFT size and DQE

larger area of TFT = increased DQE

moculation transfer function

ability of a system to record available spatial frequencies

a way to quantify the contribution of each system component to overall efficiency of the system as a whole

perfect system MTF = 100%

ideal expression of image resolution

modulation transfer function

diagnostic yield

images are assumed to be taken using the established depatment procedures and protocols in accordance with standards of care

diagnostic efficacy

faithful representation of the patient on the radiograph

image fidelity

all info on image should clearly represent the patient

image quality factors

geometric quality

visibility quality

geometric quality

remains the same with various imaging systems

• Focal spot size

• SID

• OID

• motion

visibility quality

enables viewer to visualize details

requires appropriate visualization and contrast

requirements for viewing monitors

QC monitor must be 1K

diagnostic reading monitor must be 2K or 1600×1200

mammo reading monitor must be 4K

preprocessing

usually automatic

prepares raw digital data for analysis

electronic calibration to reduce pixel differences

purposes is to produce artifact free images

postprocessing

requires intervention

anything that can be sone to image after it is aquired

purpose is to optimize the apperance of the image for interpretation

types of preprocessing

flatfielding

signal interpolation

flatfielding

results in a uniform response

standardizes images across dataset

offest images and gain images

offset images

dark current or offset correction to subtract the bacground signal from detector

generated many times a day

gain images

corrects for non-uniform pixel responses

generated every few months

signal interpolation

corrects by averaging any defective pixels

fills in the signal for defective areas by removing artifacts from the final image

types of postprocessing

window width

window level

shuttering

stitching

annotation

magnification

image flip

image inversion

subtraction

pixel shift

region of interest

brightness

the apperance of the image on the display monitor

allows interest to be visualized

window level

adjusts brigtness

window width

adjusts contrast

edge enhancement

AKA high pass filtering

increases contrast

edges of the structure are enhanced

can be a noisy image

smoothing

AKA low pass filtering

averages pixel swith surrounding pixels

used for viewing small structures

reduces noise and contrast

shuttering

used to blacken out white collimated borders to reduce veiling lgare

pixel shift

misregistration of image

occurs when pt moves durig serial imaging

region of interest

area of anatomical structure on a reconstructed digital image

defined by the operator

dead pixels

non-funct5ioning or poor connections between pixels can cause artifacts

image lag

detector does not clear signal from previous image

data drop

extreme over exposure causes data points to be dropped and not included in the reconstruction

detector saturation

photon starvation

detector saturation

type of data drop that involes areas or regions

photon starvation

type of data drop that is due to inadequate exposure

advantages of digital

better contrast resolution

adjustable image processing

faster acquisition and readout

image access remotely and in multiple locations