Reconstruction, prospective vs retrospective, slice thickness and interval, CT numbers

The digital signal leaving the data acquisition system is considered

raw data (scan data). cannot be viewed by the tech.

is the process through this raw data becomes image data or processed data, sometimes called scan data

Reconstruction

Raw data is stored only in the..

CT control console

The image data is sent to

PACS for radiologist interpretation

What are the three steps that contribute to reconstruction?

1. interpolation

2. convolution

3. and filtered back-projection.

an aspect of reconstruction process unique to helical scanning.

The CT images need to be presented as axial sections rather than a spiral. the computer has to reconstruct this data into perfect planar slices from the spiral volume of data. this process is called…

interpolation

the process through which the scan data is mathematically filtered to visualize specific tissue types better.

convolution

There are two forms of convolution filters which are

smoothing reconstruction

and edge-enhancing reconstruction algorithms.

this filter softens the edges of all the anatomical structures. however when looking at the bone this is not especially helpful. standard algorithm will fall under this.

smoothing reconstruction algorithms

also called a bone algorithm. In bone, it is imperative that sharp edges in all of the anatomic structures are visible. 

edge-enhancing algorithm

This is the process through which projections are compiled back onto each other. It is a special mathematical filtration process is used to remove a star-like artifact that would be present through the back projection process. 

Filtered back projection

an image series initiated before scanning the patient. EX: standards and lung recons can be set before initiating the helical scan.

prospective reconstruction

differs because the images are initiated after the scan is finished

retrospective reconstruction

The raw data stored on the computer can be used to retrospectively

reconstruct the data into any algorithm.

thickness of the detector rows

the acquisition slice thickness

the thickness of the images

reconstruction slice thickness

larger slice thickness is more valuable for imaging

larger body parts

smaller slice thickness is more valuable for imaging

smaller body parts such as blood vessels and bones.

smaller slice thickness results in

an increase in image noise and spatial resolution

larger slice thickness results in

lower dose of radiation, less image noise

The slice interval is

the space between slices and can be set by the tech. 

when the slice interval is equal to the slice thickness, it means where one slice ends, the next slice begins results in

no overlapping data and no gaps between the reconstructed data.

slice interval is greater than the slice thickness results..

in a gap between slices

when the slice interval is less than the slice thickness results in

the next slice begins before the previous slice ends.

Each pixel is assigned its CT number. Each CT number has a unit called…

these numbers represent the physical density of the tissues within that pixel.

Hounsfield unit (HU)

Water has a CT number that is equal to

0 HU

Tissues with a physical density greater than water will have a CT number that is…

greater than 0

Tissues with a physical density less than water will have a CT number

less than 0

blood has a HU of

+30

Cerebral spinal fluid has a HU of 

15+

muscle has an HU of

+40

solid bone has a HU of

+1000

fat has an HU of

-50

The Lungs have a HU of approx.

-700

Pure air without tissue will have an HU of 

-1000

Changing the WW and WL does not change the…

CT HU values

The liver shows HU values far less in the spleen than in the liver is an indication of

fatty liver