chapter 21: medical image descriptors

Anatomic tissues absorb and transmit x-rays differently based on their composition

differential absorption

__ absorbs more xrays than muscle

bone

the primary x-ray beam loses some of its energy (number of photons) as it interacts with anatomic tissue.

attenuation

attenuation composed of

absorption

scattering

the energy of the primary beam is deposited within the atoms comprising the tissue.

absorption

complete absorption of the incoming photon

photoelectric effect

X-ray photon passes through the anatomic part without interaction with the atomic structures

transmission

composed of transmitted and scattered radiation

remnant or exit radiation

scatter radiation reaching the image receptor creates unwanted exposure called

fog

exit radiation reaching the IR creates the __ invisible at this point

latent image

once image is processed, it becomes the __ and is visible

manifest image

increased brightness or white areas on the displayed image

absorbed radiation

creates decreased brightness or black areas on the displayed image

transmitted radiation

fidelity (trueness) with which the anatomy you are imaging is captured on the image receptor

image quality

If the image reproduces structure and tissues well

high quality image

descriptors of image quality

spatial resolution

contrast resolution

noise

artifacts

visibility of anatomic structure

brightness

contrast

accuracy of structural lines

spatial resolution

distortion

digital image is evaluated by the amount of ___ of display monitor

brightness or luminance

a film image is evaluated by the amount of __ after processing

density or overall blackness

in film screen systems the primary factor affecting density is the

quantity of radiation reaching IR

in digital systems, the quantity has less of an effect because of

computer processing

radiograph must exhibit differences in __ in order to differentiate among anatomic tissues

brightness levels or densities (image contrast)

the range of brightness levels is a result of tissues’

differential absorption of x-ray photons

radiographic contrast =

image receptor contrast x subject contrast

image receptor contrast is selectable with __ and depends on

post processing

bit depth and window level/width

most important factor for radiographic contrast

post processing

Anatomic details must be accurately recorded and with the greatest amount of

sharpness

Ability to image small objects with high subject contrast, such as a

breast microcalcification or a calcified lung nodule

refers to the smallest object that can be detected in a
digital image

spatial resolution

All radiographic images have some degree of

unsharpness

greater resolution=

less unsharpness

___ pixel and focal spot size= better spatial resolution

smaller

Ability to distinguish anatomical structures of similar subject contrast

contrast resolution

contrast resolution improves at

lower kVp

Random fluctuation of x-ray interaction on the image receptor

noise

low noise = better image quality because it improves

contrast resolution

principle controller of noise, too few x-ray photons reached IR

quantum mottle

reduce quantum mottle

high mAs, low kVp, slower IR

fast IR have

high noise and low contrast resolution

low noise accompanies

slow IR and high contrast resolution

geometric factors

magnification

distortion

focal spot blur

All objects on the radiograph are larger than they really are

magnification

magnification controlled by

SID and OID

increasing magnification decreases

resolution

when you use magnification as a tool

magnification radiography

As SID increases, magnification

decreases

as OID increases, magnification

increases

magnification formula

SID/SOD

magnification factor

image size/object size

object appears longer than it really is

elongation

object appears shorter than it really is

foreshortening

amount of foreshortening increases when

angle increases

elongation and foreshortening can occur because of improper alignment of

CR, anatomic part, and/or IR

why does object thickness increase distortion?

thick object increases OID

Misrepresentation in the image of the actual spatial relationships between objects

spatial distortion

objects more __ to central ray cause more significant spatial distortion

lateral

why do you need at least 2 views?

single image not enough to get a true representation of a 3d object

occurs because focal spot is not a point

focal spot blur

the image you want

umbra

the blurry edge around details in the radiograph

penumbra

penumbra is also called

edge gradient

focal spot blur

focal spot size controlled by

line focus principle

2 kinds of focal spot size

actual focal spot size

effective focal spot size

focal spot size is a major controller of image resolution because it controls

unsharpness

if all xrays originate from a single point, the image will consist of nothing but

umbra

as a focal spot consists of many points, there will be

many shadows cast from focal spot

where shadows overlap is called __

where they dont overlap is __

umbra

penumbra

penumbra and focal spot blur formula

(FSS x OID)/SOD

Varying intensity actors the x-ray field in the anode-cathode direction caused by attenuation of x-rays by the anode

heel effect

focal spot blur is small on __ and large on __

anode

cathode

a result of the absorption characteristics of the anatomic tissue radiographed and the quality of the x-ray beam.

subject contrast

describes the ability of an imaging receptor to distinguish between objects having similar in subject contrast

contrast resolution

number of different shades of gray that can be stored and displayed in a digital image

gray scale

the range of densities visible on film

scale of contrast

resulting from great differences in radiation absorption between tissues that vary greatly in composition

higher contrast

resulting from fewer differences in the radiation absorption for tissues that are more similarly composed.

lower contrast

thorax is an anatomci area of

high subject contrast, long scale

thick/thin pts absorbs more radiation

thicker

bone/air absorbs more radiation

bone

low/high kvp results in more absorption

lower kVp

usually the cause of motion blur

patient motion

how to control voluntary motion

communication and immobilization

how to control involuntary motion

short exposure time

to reduce motion blur

use short exposure time

proper instructions/restrain

large SID

small OID