Magnetic Resonance Imaging

Joseph P. Hornak, Ph.D.

Image Presentation

Image Histogram

A plot of the number of pixels with a given data value.

Image Processing

1 Raw or K-Space Data
Typically 256x256 point complex data.
Mx & My or Real (RE) & Imaginary (IM)
signals from the signal digitizer.
16 bits of amplitude resolution.
Mx (RE) My (IM)
2 Smooth data by convolution with a
Lorentzian line shape

(multiply by an exponential cone before
it is Fourier Transformed)

x
RE IM
3 Fourier transform is performed
first in the vertical direction,

{ FT in }
RE IM
4 and then in the horizontal direction.
{ FT in f }
RE IM
5 Once Fourier transformed,
the magnitude is calculated.
{ magnitude }
6 Magnitude data is expanded
to a 512x512 matrix by
either pixel interpolation
or pixel replication.
{ interpolation to 512x512 }

Images are displayed with an 8 bit video display.

There are 256 possible gray levels with which to display the 32768 possible data values from the 15 bits of magnitude information.

A linear look-up table (LUT) is typically used. Here the video intensity between 0 and 255 is set by a linear relationship to the data value.

The width of the data values set to the 256 possible gray levels is called the width or contrast.

The data value assigned the center of the gray scale is referred to as the level or brightness.

Examples of width and level applied to a spin-echo image of the human head.

WindowLevelLUT Image
1153 576
280 860
780 735
320 730
1 470
1 865
WindowLevelLUT Image

Often times less than 256x256 data points are collected to decrease the imaging time.

For example, a 256x192 or 256x128 data matrix can be collected where there are 192 or 128 phase encoding steps.

It is preferable to always have the same size matrix for input into the 2-D Fourier transform.

In the case of a 256x192 or 256x128 acquisition the ends of the matrix are filled with zeroes.

RE IM
{ zero fill }
RE IM

This process is called zero filling.

The process is equivalent to pixel replication to produce a 256x256 image from 256x128 data.

Once filled with zeroes the data is processed as described above.

Imaging Coordinates

Imaging Planes

Copyright © 2000 J.P. Hornak.
All Rights Reserved.