Image Analysis 1 and 2

the purpose of image analysis

to teach how to evaluate projections for acceptability, determine how to improve positioning and technical skills before repeat, continue to improve

histogram x axis

pixel gray shade values

histogram y axis

number of pixels of each gray shade

histogram represents the

subject contrast in the remnant radiation

exposure indicator

readings that denote the amount of radiation intensity (quantity of photons) that struck the IR

why doesn’t exposure indicator measure patient dose

it does not take into account energy level of the x-rays

EI reading is read

by the computer at the midpoint of the defined VOI

resolution

quality of the displayed projection is defined by how well details in the VOI have been resolved

a projection is evaluated for how visible the details are defined by the factors of

brightness, contrast, noise, spatial resolution, magnification, shape distortion

intensity refers to teh

total quantity of x-ray photons that expose the patient and IR

controlling factor for intensity and IRE

mAs

contrast

the ratio of percentage of difference between two adjacent brightness levels

high contrast

black and white

low contrast

very gray

subject contrast is controlled by

kVp

subject contrast is the

difference in radiation intensity in the remnant beam that demonstrates the degree of differential absorption resulting from differing absorption characteristics of the body structure

adequate penetration

demonstrated when the cortical outlines of the densest and thickest bony structures of interest are seen

if cortical outlines are not seen, how do you fix it

increase kVp

saturation

demonstrated as loss of contrast resolution where structures in VOI demonstrate pitch black, overwhelmed digital system

quantum noise is decreased by

increasing the IRE by increasing kVp/mAs

increasing kVp by factor of 15%

doubles IRE

if contrast resolution is adequate

mAs should be adjusted to offset quantum noise

signal to noise ratio (SNR)

used to compare one exposure to the next, ratio between desired signals from the undesired signals that are used to create an image

the higher the SNR is there were more

desired signals than undesired

contrast resolution

degree of difference in brightness levels between adjacent tissues on the displayed image

the higher the contrast resolution

the greater the gray shade difference

subject contrast

magnitude of signal difference in the remnant beam (the xray beam that exits the patient)

contrast resolution is primarily controlled by

bit depth

dynamic range

total span of x-ray exposure values that an image receptor can capture and faithfully represent

dynamic latitude

margin of error allows in technical exposure factors (kVp/mAs) while still producing an acceptable image quality

bit depth

the number of computer bits assigned to store the signal value of an individual pixel (or detector element, DEL)

bit depth determines the number of

gray levels each pixel can display using the formula

overexposure means that the photon intensity reaching the IR is

higher than that required to produce an acceptable projection and an excessive radiation dose was delivered to the patient