image quality and spatial resolution

SPATIAL RESOLUTION •Also known as

detail resolution

unit for spatial resolution

pairs/mm

spatial resolution for CT

1 lp/mm

spatial resolution for digital radiography

5 lp/mm

CT spatial resolution is x in comparison to digital radiography

inferior (low)

FACTORS AFFECTING SPATIAL RESOLUTION

•Slice Thickness

•Pixel Dimension (and Sampling Frequency)

•Patient Motion

•Reconstruction Algorithm (to be discussed after “Contrast Resolution”)

•Thinner slices have x spatial resolution; thicker slices require more x

better, flattening

•Voxel size plays a role in x; affects spatial resolution

volume averaging

•Thicker slices =

↓ spatial resolution

•Thicker slices will increase the likelihood of excluding x (more volume averaging)

small objects

•When decreasing the pixel size, the number of pixels x

increases

•A larger matrix will contain x individual pixels

smaller

what are the elements of image quality

1. pixel

2. matrix

3. voxel

•Decreased pixel size = increased matrix size =

increased spatial resolution

•x voxel sections = increased spatial resolution

Thinner

•1024 x 1024  matrix has x spatial resolution than 512 x 512 matrix

better

Matrix size and DFOV selection determines

the spatial resolution

Smaller pixels =

better spatial resolution

partial volume effect

Smaller pixels will decrease the amount of volume averaging 

Smaller pixels will x the amount of volume averaging 

decrease

Sampling theorem

“because an object may not lie entirely within a pixel, the pixel dimension should  be half the size of the object to increase the likelihood of that object being resolved”

DFOV determines how much x will be used to reconstruct the image

raw data

•Increasing the DFOV, x size of pixel in the image

increases

•x scan times may help with involuntary and overt patient motion

Shorter

•Blurriness in image x spatial resolution

reduces

PARAMETER	INCREASE	DECREASE
Detector Aperture Width	Decreased resolution	Increased resolution
Sampling Rate	Increased resolution	Decreased resolution
Number of Detectors	Increased resolution	Decreased resolution
Field Size	Decreased resolution	Increased resolution
Slice Collimation	Decreased resolution	Increased resolution
Pixel Size	Decreased resolution	Increased resolution
Algorithm	Increased resolution	Decreased resolution

.

what is happening in this image, explain

this is where volume averaging happens, it is because it is between 2 images and need to figure out what it is. to fix this, the slice thickness needs to be smaller