contrast resolution

contrast resolution

•Is the ability to detect a structure that varies only slightly in density from its surroundings (“sensitivity” of the system)

How much can CT differentiate an objection within x contrast variation

0.5%

Low contrast resolution

is desired in CT for most examinations in order to see a density difference between the object and its background

•CT has the x superior contrast resolution in comparison to the other imaging modalities

most

FACTORS AFFECTING CONTRAST RESOLUTION

•mAs/Dose

•Pixel Size

•Slice Thickness

•Reconstruction Algorithm

•Patient Size

•Window Settings

how does mAs/ Dose affect the contrast

it is a direct relationship. It affects the SNR and contrast resolution

⬆ mAs

•⬆ the number of photons, therefore more photons to create the image, ⬆ SNR

what is the downside of increasing mAs

directly related to the patient dose

how does pixel size affect the contrast

size is proportional to the quality of the image

Fewer photons per pixel =

increase in noise and therefore a decrease in contrast resolution

slice thockness has a x relationship between slice thickness and number of photos availble to produce a iamge

linear

Thicker slices allow more photons to  reach the detectors; better SNR but

lower spatial resolution

contrast we want what in slice thickness

bigger

spatial resolution we want what in slice thickness

thinner

bigger patients cause

decrease SNR (more noise) and less contrast resolution

slimmer patients cause

•Less scatter produced with slimmer patients (more photons reach detectors) (also improves contrast resolution)

window level

set to the near average density of the object. think of it as changing the anatomy to have the optimal level in the HU scale.

window width

must be narrow enough so that small groups of HU are represented in shade of gray displayed.

what was done here

they changed the window settings to further see the pathology