- All imaging is based on the resonance equation.
n = g Bo Where:
- g = gyromagnetic ratio
n = resonance frequency
Bo = magnetic field
Where is the image?
- Magnetic Field Gradient
- A variation in the magnetic field with respect to position.
A one-dimensional magnetic field gradient is a variation with respect to one direction, while a two-dimensional gradient is a variation with respect to two.
A 1-D gradient in Bo along +x axis.
Symbols for magnetic field gradient in the x, y, and z directions are Gx, Gy, and Gz.
- Frequency Encoding
- Isocenter - A point in the center of the magnet where (x,y,z) = 0,0,0
The magnetic field at the isocenter is Bo and the resonant frequency is no.
When a linear magnetic field gradient applied to our hypothetical head...
The resonance frequency in the presence of a magnetic field gradient.n = g ( Bo + x Gx ) = no + g x Gx x = (n - no ) / (g Gx )
- Back Projection Imaging
- One of the first forms of magnetic resonance imaging.
An object is first placed in a magnetic field.
1-D field gradient is applied at several angles, and the NMR spectrum is recorded for each gradient.
A second spectrum is recorded with the gradient now at a one degree angle q to the +X axis.Process repeated for 0o < q <359o.
Data is backprojected through space in computer memory.
Background intensity is suppressed ...
The actual backprojection scheme is called the inverse radon transform.Backprojection imaging sequence for XY-plane image ...
frequency encoding gradient = Gf.
Gy = Gf Sin 
Gx = Gf Cos Why was Gz applied?
- Slice Selection
- Selection of a slice in a cube.
How?Frequency content of a 90o pulse is shaped as a sinc function. (RE only)
Distribution of rotation angles.
Solution: 90o sinc pulse as square frequency distribution.
Tomographic backprojection imaging
The backprojection imaging technique is highly educational but rarely.
Copyright © 2000 J.P. Hornak.
All Rights Reserved.
All Rights Reserved.