Ch 2 Image Formation

Front 1

digital image

Back 1

numerical representations of an object that can be recognized and processed by a computer

Front 2

1st step in generating digital image

Back 2

is to divide object onto a grid of small regions. each region is known as picture element or pixel

Front 3

2nd step in generating digital image

Back 3

sample the object in order to differentiate structures within object then assign various shades of gray to pixels

Front 4

Projection

Back 4

a view of the anatomical cross section from a particular vantage point

3rd gen vantage point: xray tube at any given point

4th gen vantage point: detector

Front 5

3 types of scanning methods

Back 5

1- localizer

2- conventional or serial CT

3- helical CT

Front 6

Localizer

Back 6

scout image, stationary tube and pat table in motion

limited resolution due to size and spacing of detector elements but better tissue contrast than XR due to pre/post collimation

the localizer allows us to prescribe the cross sectional slices for scan

Front 7

conventional or serial CT

Back 7

xray tube rotates around patient while table remains stationary

to obtain next slice table moves to appropriate spot and next slice is taken

assigning gray levels to each pixel in scan is more difficult than localizer

3rd gen: each ray of X-ray fan beam strikes a single detector element and each set of rays with tube in single position is a projection

4th gen: a projection is a set of rays striking a single sector as tube travels around gantry.

Front 8

CT Number

Back 8

the value for each ray is proportional to X-ray photons measured by each detector

Front 9

Raw Data

Back 9

the data measured at each projection

ex if there are 750 detectors and 1500 projections acquired how many samples of info comprising the raw data in each slice?

750 x 1500 = 1125000 samples

Front 10

Helical Scan

Back 10

Spiral or volumetric CT

xray tube remains on for extended period of time rotating around pt while table is fed through gantry. anatomy is scanned during the continuous tube rotation rather than a single rotation only

scanner must be continuous rotation type and also have high heat capacity with rapid cooling capabilities\

very difficult to assign gray levels to image because acquired data does not fall within a single plane and that data needed to reconstruct must be all on same plane

Front 11

Advantages of Helical Scans

Back 11

-no misregistration of slices

-gapless scanning

-less contrast

-arbitrary slice positioning

-facilitates post processing

Front 12

cine mode

Back 12

anatomical movie -not static images

Front 13

Image Reconstruction

Back 13

most CT scanners use a filtered back projection algorithm to reconstruct and form final image

Front 14

filtered back projection

Back 14

simply a summation technique of the attenuation profiles collected form all the projections

two steps:

1- applying a filter to raw data

2- and back projection

the reconstruction filter or kernel is used to ensure accurate reconstruction of the anatomy. it also allows for enhancement of either spatial resolution or low contrast resolution

Front 15

convolution

Back 15

the process of applying filtration to the data

Front 16

Post Processing Techniques

Back 16

-retrospective reconstruction

-multiplanar reconstruction

-3D shaded surface display

-3D volume rendering

-maximum intensity projection (MIPS)

Front 17

Retrospective Reconstruction

Back 17

requires the raw dad which is lost once that portion of the raw data storage device is overwritten