Compensating severe non-linearities caused by high-density objects.



The problem of metal and high-density objects
 
Metallic materials such as protheses, dental fillings, surgical clips, and electrodes produce streak and star pattern artifacts in the CT image. In medical applications they usually appear together. Streaks across the image are typically caused by patient motion and/or small errors of scanner geometry parameters. Streaking may be absent for stationary objects.

The reconstruction of the same data with MAR applied.

The reconstruction of the same data with MAR applied.

Typical dental slice (real patient) with massive metal parts (crowns) within field of view.

Typical dental slice (real patient) with massive metal parts (crowns) within field of view.



MAR


Exxim has developed and implemented an iterative algorithm for 3D reconstruction, which mitigates non-linearities caused by high-density and metal objects. In Cobra it is called MAR (metal artifact reduction) mode. This method is approximately only 2.5 times slower than standard Feldkamp.

 
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Describing a reconstruction task

Within the COBRA reconstruction package, all input, output and intermediate data are kept on the hard disk. The user defines and controls the reconstruction task through a parameter file, a text file with extension .xxm. This file contains tags and assigned values, and describes the scanner hardware setup, input and output data characteristics and representation, and the chosen reconstruction method. Some of the reconstruction options are only accessible through SDK API’s. These are related to synchronization and control of in-line reconstruction (recon concurrent with acquisition), and the setup of COBRA’s working folders.
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Tags defining MAR parameters

SAMARATAG_HIGHCONTRASTLEVEL

SAMARATAG_HIGHCONTRASTLEVEL

Defines: Behavior of SAR method (part of SAMARA). SAMARA stands for Streak Artifact and Metal Artifact Reduction Algorithm. It is an iterative algorithm for 3D reconstruction, which is more tolerant against input data imperfections than the standard Feldkamp method.
Value type: integer (0-255)
Default value: 0
SAMARATAG_LOWCONTRASTLEVEL

SAMARATAG_LOWCONTRASTLEVEL

Value type: integer (0-255)
Default value: 0
SAMARATAG_HIDENSLEVEL

SAMARATAG_HIDENSLEVEL

Value type: integer (0-255)
Default value: 0
SAMARATAG_TOOTHDENSLEVEL

SAMARATAG_TOOTHDENSLEVEL

The tag is used to set a density level threshold. Densities between SAMARATAG_TOOTHDENSLEVEL and SAMARATAG_HIDENSLEVEL do not get negative corrections.
Value type: integer (0-255)
Default value: 0
SAMARATAG_SMALLDETAILSIZE

SAMARATAG_SMALLDETAILSIZE

Defines: Behavior of MAR method (part of SAMARA). The tag is used to set an object size threshold. Small dense objects are considered not causing non-linearity.
Value type: integer (0-255)
Default value: 0
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SEE ALSO

Full list of COBRA parameters
Download COBRA User Manual
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