Principles of NMR imaging system

1. Principles of NMR imaging system
By Shruti Varode
PRN – 20210902023
MSc Medical Biotechnology
A presentation on
School of Bioengineering and Biosciences
Submitted to
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2. NMR
• 1H
• 13C
NMR results from
specific magnetic
properties of certain
atomic nuclei.
Applications of NMR
• NMR spectroscopy
• Magnetic Resonance Imaging (MRI)
Fig 1 Introduction to NMR
Fig 2 Nuclei used in NMR Fig 3 Applications of NMR
2
Atomic nuclei
Nuclear
a strong constant
magnetic field
+ a weak oscillating
magnetic field
Magnetic
produce an
electromagnetic
signal near
resonance
Resonance

3. Fig 4. Maxwell’s Right Hand Thumb Rule - Kumari, S., & Anand, B. (n.d.-b).
State and explain Maxwell s right hand thumb rule - Tutorix.
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4. Fig 6. Nuclei in Magnetic field -https://www.chem.ucalgary.ca/courses/351/Carey5th/Ch13/ch13-nmr-1.html
Longitudinal magnetization-
When placed in an external
magnetic field, the proton’s
spin can either align with the
field or flip to orient itself
against the field.
The nuclear spin
is a fixed
property of each
nucleus and may
be either an
integer or a half-
integer.
Fig 5. Nuclear Spin - Organic Chemistry, L. G. Wade. Jr. Prentice-Hall:
New York, 1987. https://doi.org/10.1021/ed065pA169.1
Gibney, E. Proton's magnetism measured with greatest precision yet.
Nature (2014). https://doi.org/10.1038/nature.2014.15310
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5. Larmor Equation
• The Larmor or precessional frequency in MRI refers to the rate of
precession of the magnetic moment of the proton around the
external magnetic field. The frequency of precession is related to
the strength of the magnetic field.
• The precessional frequency of nuclei of a substance placed in a
static magnetic field is calculated from the Larmor Equation:
Fig 7. Precession in Atomic Nucleus –
Wikipedia contributors. (2022b, September 26). Larmor precession. Wikipedia.
Retrieved October 11, 2022, from https://en.wikipedia.org/wiki/Larmor_precession.
Jones, J. Larmor frequency. Reference article,
Radiopaedia.org. (accessed on 10 Oct 2022)
https://doi.org/10.53347/rID-5855
ω = γB
• ω is the Larmor frequency in MHz
• γ is the gyromagnetic ratio in MHz/tesla
• B is the strength of the static magnetic field in
tesla
At 1 tesla, the Larmor
frequency of hydrogen
nucleus/ proton is 42.58
MHz
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6. Transverse magnetization-
If we apply a short radio-
frequency wave (Rf pulse)
at right angle to magnetic
field, the nuclei will flip
down to X-Y plane in
unison.
The rotating magnetization
vector induces a current.
After we remove the Rf
pulse, the oscillating signal
gives a sine wave with
decreasing magnitude as
the nucleus realigns with
the magnetic field. This
wave is called Free
Induction Decay or FID.
Fig 8. How does MRI work?
Hoffman, R. (2022, March 8). What is NMR? Retrieved October 11, 2022,
from http://chem.ch.huji.ac.il/nmr/whatisnmr/whatisnmr.html
[Doctor Klioze]. (2013, November 5). MRI: Basic Physics & a Brief History.
YouTube. Retrieved October 11, 2022, from
https://www.youtube.com/watch?v=djAxjtN_7VE
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7. Fourier transformation is used to convert the
frequency information contained in the signal from
each location in the imaged plane to corresponding
intensity levels, which are then displayed as shades
of gray in a matrix arrangement of pixels. By
varying the sequence of RF pulses applied &
collected, different types of images are created.
[Doctor Klioze]. (2013, November 5). MRI: Basic Physics & a Brief History.
YouTube. Retrieved October 11, 2022, from
https://www.youtube.com/watch?v=djAxjtN_7VE
T2
Spin-spin
relaxation
Protons or
spins repel
each other and
move apart
No net energy
transfer
T1
Spin-Lattice relaxation
High energy protons
fall back into low
energy state, more
longitudinal
magnetization
Net energy transfer
Tissue can be characterized by two different
relaxation times – T1 and T2.
T1 (longitudinal relaxation time) is the time constant
which determines the rate at which excited protons
return to equilibrium. It is a measure of the time
taken for spinning protons to realign with the
external magnetic field.
T2 (transverse relaxation time) is the time constant
which determines the rate at which excited protons
reach equilibrium or go out of phase with each
other. It is a measure of the time taken for spinning
protons to lose phase coherence among the nuclei
spinning perpendicular to the main field.
Fig 9. T1 and T2 type of MRI 7

8. T1 and T2 weighted images
In general, T1- and T2-weighted images can
be easily differentiated by looking the CSF.
CSF is dark on T1-weighted imaging and
bright on T2-weighted imaging.
Fig 10. https://case.edu/med/neurology/NR/MRI%20Basics.htm
Repetition Time (TR) is the
amount of time between
successive pulse sequences
applied to the same slice.
Time to Echo (TE) is the time
between the delivery of the
RF pulse and the receipt of
the echo signal.
In general, T1- and T2-weighted images can
be easily differentiated by looking the CSF.
CSF is dark on T1-weighted imaging and
bright on T2-weighted imaging.
Tissue T1-Weighted T2-Weighted
CSF Dark Bright
White matter Light Dark Gray
Cortex Gray Light Gray
Fat (within bone
marrow)
Bright Light
Inflammation
(infection,
demyelination)
Dark Bright
T1 T2
TE
(Time to Echo)
short long
TR
(Repetition
Time)
short long
Fig 10. Brain MRI
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9. 9
Conclusion
What we
studied
Introduction
to NMR
Right Hand
Thumb Rule
Nuclear spin
Longitudinal
and
transverse
magnetization
Larmor
frequency
Free
Induction
Decay
MRI –
T1 and T2
weighted
images
Fig 11. Conclusion

10. Thank you !
Any questions ?
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