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3 N S N S N Set of equally spaced parallel lines indicates homogeneous field No. of lines within a unit area of cross section [in plane perpendicular] Is the field strength/ Intensity of magnetic field Area of cross section Inhomogeneous Fields Presence of a Magnetic field is pictorially depicted by a vector-line pointing along the direction in which an isolated North pole would move It is important to realize the difference between two poles forming magnet pole faces and two poles making up the dipole with a dipole moment Increasingly More intense homogeneous Fields
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Nuclear Magnetic Resonance
ANIMATED ILLUSTRATIONS
MS Powerpoint Presentation Files
Uses Animation Schemes as available in MS XP or MS
2003 versionsA class room educational material
http://ugc-inno-nehu.com/links_from_web.html
Relaxation and In-homogeneities
Longitudinal and transverse Relaxation Slide # 2Description, depiction and definitions in Field In-homogeneities Slide # 3
In-homogeneities and NMR line widths Slide # 4Elements of NMR Imaging Slide # 5
Isochromats and spin evolutions in inhomogeneous fields Slide # 6Field-Gradient description for In-homogeneities Slide # 7
Set to Video conversion on 17th Dec. 2009
2
Randomization in XY plane
I h NET Magnetization
Transverse T2
Relaxation
Relaxation Longitudinal and transverse
Longitudinal T1 Relaxation
π/2 pulse
3
N
S
N
S
N
Set of equally spaced parallel lines indicates
homogeneous field
No. of lines within a unit area of cross section [in plane perpendicular]
Is the field strength/ Intensity of magnetic
field
Area of cross section
Inhomogeneous Fields
Presence of a Magnetic field is pictorially depicted by a vector-line pointing along the direction in which an isolated North pole would move
It is important to realize the difference between two poles forming magnet pole faces and two poles making up the dipole with a dipole moment
Increasingly More intense homogeneous Fields
4
H0
H0 - ΔH
H0 - ΔH
180°pulse
180°pulse
90°Pulse
90°Pulse
Thermal equilibrium Boltzmann population distribution
H0
Net Magnetization only along H0 and no magnetization in plane perpendicular (zero component)
Magnetization has non-zero component in XY-plane…
Tilted to XY plane
Homogeneous Field
Inhomogeneous Field
5
If the spatial location within the sample where a particular filed is present can be encoded along the frequency axis, then it is spatial resolution which results.
Elements of NMR imaging
Precession frequency would be proportional to the strength of the filed at the point where the spin is located
6
6 spins/all precess with same frequency
Single isochromat of 6 spinsAll the 6 Superposed spins result in
5 IsochromatsEach with a bunch
of spins
1 3 542
Multitude of isochromats and resultant vector
evolution
Degree of field homogeneity is represented by set of parallel lines of force
in homogeneous fieldThese 6 spins placed closer
Weakest fieldStrongest field
Lowest precession frequency
Highest precession frequency
Vector Resultant
7
Equal number of lines pass through fixed area of cross section along the length
3 lines inside
6 lines inside
Linear Field Gradient along z-axis.
Along the length same nmr frequency for the sample
NMR frequency will vary linearly along the length.
In the Pulsed Field Gradient PFG and Magnetic Resonance Imaging MRI techniques calculated field gradient are externally superposed.
The inherent unwanted and incidental inhomogeneities are reduced by Shimming and sample spinning
The End
