MRI

1. INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
Topic: Magnetic Resonance Imaging (MRI)
Course: Physics of Radiation Generators (PH40008)
Submitted By: Aftab Ahmad
Department of Physics
IIT Kharagpur

2. INTRODUCTION
 MRI is a non-invasive method for imaging internal structures with in the body which
uses non-ionizing electromagnetic radiations & employs radio-frequency radiations in
the presence of carefully controlled magnetic fields to produce high quality cross
sectional images of the body in any plane.
 Steps Involved:
Placing the patients in the magnet.
Sending Radio frequency waves by coils.
Transmitter Turned off.
Patients Re-emit Radio-waves.
Re-emmited Radio-waves received.
Received radio-waves are used for image reconstructions.

3. MRI MACHINE & OPERATING CONSOLE

4. MAIN COMPONENTS OF MRI

5. GRADIENT COILS
 Gradient coils are used to produce
deliberate vibrations in the main
magnetic field.
 There are usually three sets of
gradient coils, one for each direction.
 The vibration in the magnetic field
permits localization of image slices as
well as phase encoding & frequency
encoding.
 The set of gradient coils for z-axis are
Helmholtz coils but for x & y-axis are
paired saddle coils.

6. STATIC MAGNETIC FIELDS COILS
 There are three Methods to generate Magnetic Fields as follows:
 Fixed Magnet 2. Resistive Magnet 3. Superconducting Magnet
 Fixed Magnets & Resistive magnets are generally restricted to the field strength below 0.4 Tesla.
 Superconducting Magnets are used for high resolution imaging system.
 The Superconducting magnets are large & complex.
 They need the coils to be soaked in liquid helium so that very large currents can be used to
produce strong magnetic field.
 To maintain sufficient liquid helium within the magnet to cool the superconducting wire, liquid
helium maintained within a vacuum insulated cryostat.

7. BLOCK DIAGRAM OF MRI

8. PRINCIPLE OF MRI
 MRI makes use of magnetic properties of certain atomic nuclei.
 Body has many such atoms that can act as a good MR nuclei (1H, 12C, 9F,
23Na).
 Hydrogen nucleus (single proton) present in all water molecules, &
therefore in all body tissues.
 Hydrogen nuclei is not only positively charged but also has a magnetic
spin.
 MRI utilises this magnetic properties of protons of hydrogen to produce
images.
 The hydrogen nuclei can be rotated using radio-waves & they
subsequently oscillate in a magnetic field while returning to equilibrium.
 Simultaneously they emit radio signal which are detected using coils to
produce highly detailed images.

9. HOW PROTONS OF HYDROGEN ATOM BEHAVE?

10. LARMOUR EQUATION
• This equation states that the Precession
frequency becomes higher when the magnetic
field strength increases.
where,
fo is the precession frequency in (Hz or MHz).
Bo is the strength of the external magnetic field in (T).
γ is the Gyromagnetic ratio.
• The value of γ for Proton is 42.5MHz/T
fo = γBo

11. MECHANISM INVOLVED

13. MAGNETIZATION VECTOR
 MRI signals rely on the magnetization
vector M.
 Vector M has a Mz & Mxy component.
 Signal is obtained from the Mxy component
of the vector M.
 Signal intensity is dependent on Mxy
Magnitude.

14. HOW DO WE KNOW WHERE THE SIGNAL IS COMING FROM?
 Its Magnetization Gradients.
 Magnetization Gradients allow each point in space to be distinguishable.
 Like placing an xyz coordinate system on the imaged object.
 Without magnetization gradients, there is no way to determine where the data
came from in space, Called spatial encoding.
 Three types of gradients
• Slice selection – along the z-axis
• Phase encoding – along x-axis
• Frequency-encoding – along y-axis

15. NET MAGNETIZATION
 The longer as well as stronger the
RF pulse is applied, The bigger the
deflection of the net magnetic field
that is the bigger the angle α.
 It can reach 90˚ or even 180˚, the
bigger α, the longer it takes to
recover when the RF is turned OFF

16. SPIN-LATTICE RELAXATION TIME T1
 For the nuclei to return to their initial energy states by emitting energy (the MR
signal), the excited spin system must be exposed to an electromagnetic field
oscillating with a frequency at or close to the Larmor frequency.
 This process can occur by the nuclei being `stimulated' by surrounding nuclei and is
assumed to occur in a simple exponential manner.
 T1 corresponds to the time required for the system to return to 63% of its equilibrium
value after it has been exposedto a 90°pulse.

17. T1 RELAXATION

18. DEPHASING
 The contribution of all the
spins precessing around the
external magnetic field B0
produces a net
magnetization M0.
 When a 90 degree RF pulse
is applied, this net
magnetization is tipped
onto the xy-plane.
 Dephasing of the spins
results in a quick decrease
of the net magnetization in
the xy-plane. The dephasing
is exponential and
characterized by T2.

19. SPIN-SPIN RELAXATION TIME T2
 Immediately after a pulse is applied, all of the nuclei precess around the magnetic field in
phase. As time passes, the spins begin to diphase & so the observed signal decreases. They
do so according to the equation,
 T2 values are 40-200ms depending on the tissue .
 T2is approximately ten times smaller than T1.
 Different scan sequences show up differences in these relaxation times generating what are
referred to as T1, T2 or Proton density (the concentration of protons) weighted images.

20. T2 RELAXATION

21. USES OF MRI
 There are two uses for MRI:
Diagnostic Research
• Find unhealthy tissue in the body • Neuroscience
• Locate tumors • Images & Disorders relationship
• Bone damage • Cancer
• Assess condition of tissue • Analysis of Brain Functioning
• Surgery planning

22. ADVANTAGES OF MRI
Non ionizing radiations.
Variable thickness in any plane.
Better contrast resolution.
Detailed image of soft tissue structures.
Non invasive

23. DISADVANTAGES OF MRI
 MRI equipment is expensive to purchase, maintain, and operate.
 Better technological expertise is required for utilization of MRI
than for most other imaging modalities.
 Patient throughput is slow compared with other imaging
modalities.
 Dangerous for patients with metallic devices implanted with in
the body.
 Patient movements during scanning may cause image artefacts.