Magnetic Resonance Image and k-space

Spatial frequency, image resolution and contrast

Denis Hoa

Most MR image information (contrast and general shape) is contained in the center of k-space. Low-spatial-frequency data have the highest amplitude, giving the greatest changes in gray levels (contrast). However, these changes spread over in the image and only give the general shape of organs.
We can see below the resulting images of inverse 2D transform performed on data at the center of k-space. Image is contrasted but blurry.


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High-spatial-frequency data have a lower amplitude. They don't have effect on contrast or general shape but sharpens the image as they encode the edges (rapid changes of image signal as a function of position). Thus, the farther from the center of k-space the data are collected, the higher is the spatial-frequency information and the better the spatial resolution will be.
We can see below the resulting images of inverse 2D transform performed on data at the periphery of k-space. As they have a low intensity, click on Enhance contrast to see the effect of high-spatial-frequency information on image.



In order to make visible the effect of low and high spatial frequencies, you can choose in the following experiment the frequencies you want to keep to reconstruct the image. The low spatial frequencies are at the center of k-space while the high spatial frequencies are at its periphery.
Then, move the blue borders to change the extent of k-space data to be used : this allows for selecting the lowest or highest spatial frequencies.
If the image seems too dark (particularly if you select high spatial frequencies), click the Amplify button to enhance image contrast.