Digital imaging quality

what type of signal is XR

analog

short summary of how a digital image is made

an analog signal is accepted by a matrix of DELS, each assigned a brightness number based on attenuation, and an image is digitized

exposure to the IR is expressed in

mR

to produce a quality image, the IR must receive

adequate exposure

what does DAP stand for

dose area product

Dose area product

an indicator of exposure

measures the dose to a patient using exposure factors x field size

• measured with a meter in the collimator

exposure indicators

indicates if there was adequate exposure to the IR

EI# - direct relationship

S# (fuji) - indirect relationship

exposure indicator errors

extraneous exposure information (scatter)

exposure field recognition error

unexpected material in the field

collimation margins not detected

extreme over or under exposure

delay in processing

criteria for digital image quality

brightness

contrast

SNR

spatial resolution

shape distortion

size distortion

artifacts

image brightness

the measurement of the luminance of a monitor calibrated in units of candela per square meter on a monitor soft copy

inherent brightness

product of technical factors

display brightness

monitor controls

luminance QC requirements

monitor luminance must be consistent from the center to the edges

• may vary up to 20%

luminance must be consistent with monitors in the same room

• may vary up to 10%

optimal brightness

shades of gray present in the LUT for that exam

• conveys the maximum amount of information

contrast is determined by

the look up table and processing algorithms

dynamic range

range of exposures that may be captured by a detector

gray scale

the number of shades of gray available to the system

determined by bit depth

bit depth calculation

2ⁿ

receptor contrast

fixed characteristic of the receptor

most are a linear response

algorithm

a concise set of instructions for a single specific task

examples of algorithms

soft tissue

lung

high resolution

edge enhancement

smoothing

data recognition

determines the borders for image histogram formation

identifies a region of interest

• data outside of the region is subtracted

what percent of the plate must be covered to prevent a histogram error

30%

re-scaling

the computer creates a histogram of the image taken

compares it to the histogram in the LUT for that exam

computer re-scales the image to more accurately represent the histogram in the LUT

signal

desirable diagnostic information within remnant beam

noise

random superimposed signals

• scatter

  • electronic static

a high SNR indicates

little noise in the image

DQE stands for

detective quantum efficiency

DQE

expression of a systems potential speed class

what causes a higher DQE

a higher fill factor

how does pixel size impact spatial resolution

smaller pixels, larger matrix, increases resolution

pixel size calculation

pixel size = FOV / matrix

how does pixel pitch impact spatial resolution

want a smaller pixel pitch

• less space from center of one pixel to another

how does sampling frequency impact spatial resolution

higher sampling frequency = better resolution

bandwidth

the frequency of the current carrying the signal

Nyquist Frequency

expresses the max spatial resolution of a system

what are the primary controlling factors of spatial resolution with DR

matrix, pixel size, and pixel pitch

what determines Nyquist frequency with CR

sampling frequency

what determines Nyquist frequency with DR

DEL size/spacing

how does light spread impact spatial resolution with CR

decreases resolution

how does scanning speed impact spatial resolution with CR

faster scanning speed = better resolution

how does laser light size impact spatial resolution with CR

thinner laser = better resolution

what are most sampling frequencies in CR systems

10 lp/mm

results in sharpness of 5 lp/mm in the image

what is an MTF of 1

all information in the body part is recorded in the image

MTF stands for

modulation transfer function

MTF

ability of the IR to reproduce image contrast from subject contrast at various spatial frequencies

how accurately the system can transfer anatomical information into the image

MTF is a measurement of

recorded detail, sharpness, and resolution

MTF calculation

information in image / information in body part

total penumbra =

absorption penumbra + geometric penumbra +

what shape produces the most penumbra

spheres

what are methods used to measure spatial resolution graphically

edge spread function

line spread function

point spread function

edge spread function

measures response of imaging system to sharp edge

image reproduces transition from low intensity area to high intensity area

sheet of lead is placed on the cassette and exposed

intensity readings are taken at the border between black and white areas and plotted on the graph

line spread function

plots density readings

MTF can be obtained using “Fourier Transformation” math

point spread function

pinhole camera is used to create a black dot in the center of the film

a microdensitometer is used to take readings

values are plotted on the graph comparing density to distance from the dot

point spread function graph look for better resolution

narrow peak = better resolution

artifact

the appearance in image of anything that is not a part of the patient’s anatomy

heat blur

a CR artifact where blurring of an image occurs when the IR is exposed to intense heat prior to processing

improper brightness

CR artifact caused by a histogram error when the tech selects the incorrect view

most common scanner malfunction

laser jitter

laser jitter

malfunctioning of the transport system or malfunction of the laser beam

scanner malfunction examples

laser jitter

skipped scan lines

missing pixels

distorted image

dust and debris on mirror or light collecting objects

problem with the digitizer

communication errors

foreign objects within the plate

variables affecting contrast

kVp

generator type

filtration

field size

part thickness

pt condition

scatter

OID

pt motion

variable affecting noise

insufficient mAs

insufficient or excessive kVp

Large field size increases scatter

part thickness

artifacts

grid lines or grid cut off

increased OID reduces scatter

motion can generate false image

poor positioning

noise from electronic components

variables affecting recorded detail

focal spot size

• primary factor

anode angle

SOD/OID ratio and relationship to SID

positioning as it affects SOD/OID

motion

• primary enemy of sharpness

exposure time

• shorter time = less chance of motion

matrix

pixel size

pixel pitch

sampling frequency

contrast is primarily controlled by

the look up table

% of exposure for mottle

less than 50% of optimal IR exposure