W4 - Image Recon and Display

Algorithm Definition 

a precise set of steps to be performed in a specific order to solve a problem; the basis for most computer programming

DAS (Data Acquisition System)

where all of the acquired data created during scanning is preprocessed (x-ray → digital signals)

x-ray to image process

Photon is measured, corrected, quantified, and amplified → processed scan data, aka “raw data” → the reconstruction computer performs the calculations required to convert the analog data to a grey shade → image data

thing to note about reconstruction computer and raw data

the raw data is only stored for 24-48hrs, thus further recon is impossible once the raw data has been deleted 

Other names for Recon Processing techniques

• algorithm, convolution fiter, FC filter, simply filter, kernel (Siemens)

• all are the process of applying a filter to an attenuation profile

CT Reconstruction Methods

• filtered back projection (FBP)

• iterative reconstruction (IR)

Pros and Cons of filtered back projection (FBP)

• Pros:

  • very fast recon speed, thus preferred for trauma scanning

• Cons:

  • star pattern streaking artifacts, however eliminated with the filter function

  • high noise generated during data processing, thus mAs needs to be increased → increased pt dose

Filtered back projection uses which theory to supress noise?

the Fourier Theory

Pros and Cons of iterative reconstruction (IR)

• Pros:

  • model-based algorithms (MBIR) has increased image accuracy, recon speed and dose reduction

  • improved IQ (less noise)

• Cons:

  • plastic-y images as per seasoned rads

  • slower recon time compared to FBP

Types of Iterative Recons

• ASIR

• MBIR = compares simulated data to an ideal model data set

Interpolation Definition

a mathematical method of estimating the value of an unknown function using the known value on either side of the function

Significance of Interpolation Algorithms 

used to acquire true axial images, as with helical acquisitions, the data is a stretched ‘slinky’

Most commonly used Algorithms

• 360-degree linear interpolation algorithms 

• 180-degree linear interpolation algorithms

CT Image display and Raw data

image display uses image data, raw data is not utilized 

DAC

changes the digital signal from the computer memory back to an analog format so the image can be displayed

Common monitors used for CT Image Display

LCD or LED

Window width vs. Window level

• width = the range of pixel values (grey shades)

• level = the center of widow width and always set at the avg. attenuation of the tissue of interst 

window setting eg.

1500 grey shades, divide 1500 by 2 as each half will be applied to the + (white) and the - (dark) from the window level 

Display Function/Tools

• ROI

• Distance Measurements

• Reference Image

• Image Magnification

ROI

• an area on the image defined by the operator; defining a region of interest is the first step in a number of image display and measurement functions

• frequently used to measure the attenuation of a region

Distance Measurements common use 

reporting the size of an abnormality 

Reference Image

displays the slice lines in corresponding locations on the scout image; aids in localizing slices according to anatomic landmarks

Image Magnification

• pixel size is expanded by using only the image data

• NOT the same as decreasing the FOV

The Quantitative aspect of the HU

each photon that interacts at the detector is quantified based on the energy remaining compared to the energy applied at the tube

HU and beam relationship

inverse relationship

Post-processing Functions

• multiplanar recons

• Post and Retrospective Recons

• surface shaded display (SSD)

• virtual reality technique (VRT)

Multiplanar Recons

• a routine aspect of image recon

• process raw data into multiple planes, typically coronal and sagittal, and additions to true axials

Pre vs. Retrospective Recons

• both uses raw data

• Pre is done during the scan

• Retro is done after the scan, manually

SSD

• often required for orthopedic CT reconstructions

• a simple and fast 3D reconstruction that can display a limited range of attenuation values

VRT

• allows multiple attenuation ranges to be displayed within a single model,

• bone and ST can be evaluated