Physics exam 3

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Digital image quality is

the fidelity with which the anatomy under investigation is rendered on the IR

spatial resolution is

ability to image small objects that have high subject contrast

as pixel size decreases, spatial resolution

increases

Bennett’s definition of spatial resolution

ability of a system to resolve spatial frequency

spatial frequency is

the area you take up or exist in so it requires a pixel inside and outside an area

contrast resolution is

ability to distinguish anatomical structures of similar subject contrast

Bennett’s definition of contrast resolution

ability of system to show it to me

noise is

random fluctuation of x-ray interaction on the IR

Quantum mottle decreases as

mAs increases, kVp decreases, and focal spot size increases

Why causes quantum mottle to decrease overall

more photons

Bennett’s definition of noise

inability to accurately draw the edges

A Fast IR has

high noise and low contrast resolution

A slow IR has

low noise and high contrast resolution

Spatial resolution is limited to

pixel size

DeL stands for

detector elements

What are the radiographic quality Image receptor factors

pixel size

dynamic range

intensity response

signal to noise

post processing

what are the radiographic quality geometric factors

magnification

distortion

focal spot blur

how often does magnification occur

every exam

why does magnification occur

because the beam diverges from a point source

equation for magnifcation factor

MF= Img size/Obj size

Magnification facotr alternative equation

MF= SID/SOD

The numerator of the magnification factor equation is always

positive and higher

For every inch of OID, it requires how much of an increase in SID

7”

If the part is angled, what distortion occurs

foreshortening

if the tube is angled, what distortion occurs

elongation

if the IR is angled against the tube, what distortion occurs

elongation

If the IR is angled to the part, what distortion occurs

foreshortening

distortion depends on what three things

Object thickness(OID)

Object position

Object shape

are thick objects more distorted than thin

yes

when outside the center of central ray what happens to distortion

increases

focal spot blurring occurs more toward what side of the tube

cathode

what are the subject factors

subject contrast

part thickness

what help display subject contrast

capabilites of the IR, the system, and the monitor

as part thickness increases

scatter increases and technique increases

what is the effective atomic number

collective atominc numbers of stuff

when the effective atomic number is high, subject contrast is

high

Iodine Z#

53

Barium Z#

56

the more the shape coincides with the x-ray beam shape

contrast increases

best solution to involuntary motion

short exposure time

best solution to voluntary motion

good pt instructions

if good pt instructions isnt an option you choose

short exposure time

exit skin exposure is how much of the entrance skin exposure

10%

pixel size affects pt

relative dose

the smaller the pixel size, it rquires what

increase in pt dose so more photons hit the pixels

increases in Pt dose are due to increases in

pt thickness

field size

mAs

Tiem

kVp

Voltage ripple

increase in magnification is due to increases in

pt thickness

OID

increase in focal spot blur is due to increases in

pt thickness

focal spot size

OID

increase in motion blur is due to increases in

pt thickness

OID

Time

Voltage ripple

increase in absorption blur is due to increases in

pt thickness

three main factors in scatter radiation are

kVp

Field size

Pt thickness

2 main tools for controlling scatter

beam restriction

grids

scatter does what to contrast

decreases

can scatter change the shape of an object

yes

kVp effects the levels of

scatter production

why is low kVp not the solution to scater

it would increase pt dose due to an increase in mAs

collimation has what affect on quality

decreases pt dose and increases contrast

as field size decreaes, scatter

decreases

as field size decreases, contrast

increases

as pt thickness increases, scatter

increases

with an increased pt thickness, more x-rays undergo multiple scattering which in turn causes an increase in

average angle of scatter

putting a pt prone increases

compression

beam restrictors are

aperture diaphragm

cones & cylinders

variable aperture collimator

What are the grid types

parallel

crosses

focused

moving

what are the 2 reasons to use beam restriction

only tissue examined is exposed

large field sizes produce more scatter and decrease contrast

radiolysis of water is

x-rays scission water molecules into hydrogen peroxide

the aperture diaphragm is designed for the opening to be 1cm ______ on all sides of film

smaller

what uses aperture diaphragm

fixed chest rooms

trauma machines

dental

why is aperture diaphragm least used

most restrictive and simplest

cones and cylinders are modifications to what type of beam restriction

aperture diaphragm

the position and size of the distal end of the cone or cylinder determines

field size

cones have a fixed

SID

when does cone cutting occur

when the center of central ray is not in center of cone or cylinder

collimator is what type of beam restriction

variable aperture diaphragm

what is the most common beam restriction

collimator box

how many field sizes does a collimator box have

infinite

does beam restriction prevent scatter production

yes

the first stage entrance shutters control

off focus radiation

positive beam limitation was instated by the FDA why

techs wouldnt collimate

PBL was mandated in what year

1974

PBL was resended in what year

1994

do grids prevent scatter production

no

grid ratio equation

H/D

in the grid ratio equation, H stands fpr

height of the grid strip

in the grid ratio equation, D stands for

how far the strips are from one another

a 5:1 grid has how much cleanup

87%

a 16:1 grid has how much cleanup

97%

Grid frequency is

the number of grid strips per cm or inch

High frequency grids show less distinct what

grid strips

how is grid frequency increased

adding lead strips and thinning interspace materials

Does grid strip size ever change

no

what are the interspace materials made of

aluminum or plastic/carbon fiber

aluminum is perferred why

less visible grid lines and nonhygroscopic

what is the disadvantage of aluminum interspace material

absorption of primary beam which is compensated with an increase in mAs

The contrast improvement factor equation is

Image contrast w grid/Image contrast w/o grid

the numerator of the CIF equation is always

greater than 1

the range for the CIF for most grids is usually

1.5-2.5

The bucky factor determines

the amount of technique required to penetrate (?)

what is the ascending order of grid ratios

0,5:1,6:1,8:1,10:1,12:1,16:1

what is the ascending order of bucky factors

1,2,3,4,5,5,6,