Module 2 Flashcards
3 scanning methods
Localizer, conventional or serial, helical
What scan is obtained with a stationary X-ray tube and the patient table in motion?
Localizer
Advantages of helical scanning
Faster studies, more coverage in a breath hold, no missed anatomy, less contrast agent needed, arbitrary slice positioning, ideal data for post processing.
In a helical scan what determines the path of a X-ray beam?
Thickness of the X-ray beam and the speed of the patient table. The thickness of the X-ray beam is defined by the pre patient collimation.
What determines the thickness of a slice in a multi row detector scanner and a single row detector scanner?
Single row the thickness equals the slice thickness of the final image. Multi row the thick X-ray beam is divided over the number of rows of detectors ring used. Speed of pt table through the gantrydefineshow compressed or stretched the helical path is.
What is interpolation?
The attenuation info required to reconstruct an image must come from projections in single slice planes. In helical scans the attenuation information is not collected within single planes. The problem is fixed by running a math calculation called interpolation on helical data. This calculation is used to divide helical data into raw data files for individual planar slices.
Image reconstruction for conventional and helical scans
Detectors measure attenuations and electrical signals are amplified and digitized. Digitizing process assigns an integer( ct number). Raw Data ofurther processed or reconstructed in the scanners array processor to form a final ct image. The next step is convoluted or filter back projection to reconstruct a ct image from raw data.
What is back projection?
Back projection is a calculation used for reconstructing the final ct images from the raw data by adding together the attenuation information collected from all the projections.
The filter applied to the raw data during image reconstruction is called?
Kernel or reconstruction filter.
What is convolution?
It is a reconstruction filter that is applied to raw data during image reconstruction. It enhances important characteristics before back projection.
What are the two basic types of reconstruction filters?
Sharp improves the definition of the edges of structures but doesn’t minimize the grainy or speckled appearance. Smooth minimizes the grainy appearance of noise in the image so that contrast between tissues can be better distinguished but it doesn’t improve upon the edge definition.
When is sharp and smooth filters used?
Sharp high contrast regions musculoskeletal bone area. Smooth low contrast areas and brain more soft tissue.
What is iterative reconstruction?
It is a complex cycle of correction calculations applied during the image reconstruction process to reduce image noise. Greatly reduces dose. It requires tremendous computing power from the array processor.
What is a cone beam?
When a very thick X-ray beam is used 20 mm, the beam is no longer flat like a fan but 3 dimensional like a cone.
Cone beam artifact?
Structures in the center are not affected but the periphery are not properly reconstructed with filter back projection. It causes spoke or star like appearance around the edges. Ribs often has this. It is fixed by cone beam reconstruction. Used when more than 4 slices on a signal rotation.
