rad quality- spatial resolution and distortion

spatial resolution

the degree of geometric sharpness or accuracy of the structural lines actually recorded in the image

synonyms terms for spatial resolution

definition, sharpness, recorded detail, detail, umbra

penumbra

poor detail; bad spatial resolution; unsharpness around the boundaries of an image

umbra

good detail; the area of complete or true shadow formed when an object fully block the radiation source

spatial resolution is measured in

line pairs per millimeter (lp/mm)

limit of most human visual acuity

5 lp/mm

point spread function (PSF)

determined by complex mathematical measurement of the images "speed/blur" from a single point

line spread function (LSF)

determine by complex mathematical measurement of how much "spread/blur" occurs after a line is imaged

edge spread function (ESF)

determine by complex mathematical measurement of how much "spread/blur" occurs around the edge of a structure in an image

MTF

modulation transfer function

modulation transfer function

measures the accuracy of an imaging system by comparing the image to the actual object on a scale of 0 to 1

best qualitative assessments of spatial resolution

presence of bony trabeculation and an absence of motion

factors affecting spatial resolution

geometry, digital image receptor factors, digital image factors, digital display factors, motion

geometry

broken down into FSS, SID, and OID

focal spot size (FSS)

area on the anode of the xray tube that is struck with electrons from the filament

SID

distance from the source of xray photons to the image receptor

OID

distance from the object being imaged to the IR

geometric unsharpness

amount of penumbra that is result of the geometric factors of the radiographic equipment and setup; measured in mm

Detector Element (DEL)

the smallest radiation-sensitive component of a digital image receptor

pitch

the distance from the end to end within a DEL

fill factor

the sensing area of the DEL compared to the non-sending area of the DEL

pixel

picture element; the smallest unit of the digital image

pixel pitch

the distance from the center of one pixel to the center of the adjacent pixel

matrix

grid (row and columns) of pixel making up digital image

pixel density

the number of pixels per given area

how is spatial resolution altered as affecting factors are adjusted

more lp/mm -> higher spatial frequencies -> higher spatial resolution

state most detrimental geometric factor affecting spatial resolution

OID

most detrimental factor affecting spatial resolution overall

motion

3 strategies for reducing exposure time in suggested order

increase mA

decrease SID

increase kVp

effect of increasing mA

reduce exposure time

effecting of decreasing SID

reduce exposure time, decreased spatial resolution, increase magnification

effect of increasing kVp

reduce exposure time, decreased contrast

inverse square law

the intensity of radiation is inversely proportional to the square of the distance from the source of radiation

exposure maintenance formula

direct relationship between mAs and distance: mAs1/mAs2=D1^2/D2^2

15% rule

Changing the kVp by 15% has the same effect as doubling the mAs or reducing the mAs by 50%

do mA, time, and kVp affect spatial resoltuion

no

geometrical factors

SID, OID, pt. thickness, FSS

digital receptir factors

DEL size, pitch, and fill factor

direct relationship with spatial resoltuion

SID, full factor, matrix size, and pixel density

indirection relationship with spatial resolution

OID, pt. thickness, FSS, DEL size, pitch, pixel pitch, pixel size, motion

distortion

misrepresentation of the size or shape of the structure being examined

two types of disortion

size and shape

another name for size distortion

magnification

factors affecting size distortion

SID and OID

relationship between SID and size disortion

indirection; greater the SID, the smaller the magnification

relationship between OID and size disortion

direct; the greater the OID, the greater the magnification

two types of shape disortion

elongation and foreshortening

how does elongation occur

occurs when the tube or image receptor is not properly aligned

how does foreshortening occur

occurs when the anatomical part is not properly aligned

routine relationship between CR, anatomical part, and IR

anatomical part and IR are parallel to each other; CR is perp to the anatomical part and IR

why will distortion never be eliminated

SID and OID always exist; divergence of the beam

define angulation

direction and degree the tube is move from its normal postion

describe how angulation can be used to radiographers advantage

causes controlled amount of shape disorition to avoid superimposition and demonstrate the anatomy

examples when angulation is used to radiographers advantage

AP coccyx, Axial clavicle, Lateral L5-S1 spot

relationship between magnification and spatial resolution

indirect

synonymous terms for magnification

zoom, enlargement, scale factor

synonymous terms for spatial resolution

image resolution, detail resolution, pixel density, resolving powerg

Magnification factor formula

SID/SOD or image size/object size

Magnification Factor percentage formula

SID/SOD - 1 x 100 or image - object/object x 100