Presentation on
Discrimination Based on
Relaxation Rates & Types of
Imaging Sequences
Presented By:
Student ID: 190931
190932
190933
190934
Magnetic Resonance
Imaging System
Discrimination Based on Relaxation Rates
In the magnetic resonance imaging (MRI), the intricate interplay between
relaxation processes and image intensity is of paramount significance.
Relaxation Processes and Image Intensity:
•
Relaxation processes significantly impact magnetization and image intensity.
•
Image intensity depends on relaxation rates.
•
Variation in relaxation rates among biological tissues can serve as a
discrimination factor.
•
Proton Rotation Rates and Water Content:
•
Proton rotation rates vary between tissues based on water content.
•
Tissue types can be characterized roughly by their water content.
•
Example: In mice, brain has ~80% water content, while the liver has 66-70%,
affecting T1 and T2.
Sensitivity of NMR:
•
•
NMR sensitivity is high due to water
concentration changes.
A 15-20% change in water concentration leads to
a 200% change in relaxation rate.
Modulating Pixel Intensities with
Relaxation Rates:
• Relaxation rates are used to control pixel
intensities.
• Relaxation times can be manipulated to affect
magnetization and nuclear signal.
• This is preferred over allowing magnetization to
recover to equilibrium before each projection.
A number of approaches have been attempted to achive this and the three
commonly used techniques are:
1. Saturation recovery.
2. Inversion recovery
3. Spin-echo imaging technique.
Saturation recovery
• In this sequence a series of 90-degree pulses is applied.
• The spacing between pulses, denoted as 't,' is longer than T2 but roughly equal to T1.
• Varying the value of 't' reveals variations in T1 across different sample regions, yielding differences
in image intensity and enabling the creation of a T1map.
• When 't' is much shorter than T2, signal decay between pulses doesn't reach zero, resulting in a
condition known as SSFP (Steady-State Free Precession).
• SSFP can produce high-quality images but poses challenges in separating the contributions of T1
and T2 to image intensity.
• Saturation recovery can depress signals from regions with T1 longer than 't.'
• This property can be leveraged to visualize moving fluids within the imaged slice.
• By keeping 't' short relative to the biological water's T1, permanently residing nuclei are saturated,
yielding small signals, while fresh-flowing fluid generates larger signals, creating brighter regions
in the image.
Questions:
1.
What is the primary purpose of using a saturation recovery pulse sequence in MRI? (2 Mark)
2.
Explain how the saturation recovery technique is employed to visualize fluid movement within
the imaged slice, and how it affects the signals from permanently residing nuclei and freshflowing fluid?(4 Mark)
3.
Why is it significant that relaxation rates are used to modulate individual pixel intensities in
MRI?(2 Mark)
4.
Elaborate on how the variation in relaxation rates among different biological tissues, especially
related to water content, contributes to the sensitivity of NMR and the potential for tissue
characterization in MRI?(4 Mark)
Magnetic Resonance Imaging System
A number of approaches have been attempted to achieve this and the three commonly used techniques
are:
1. Saturation recovery.
2. Inversion recovery
3. Spin-echo imaging technique.
Inversion recovery: An inversion recovery sequence resembles the saturation recovery
sequence in that T1variations in the sample can be exploited to achieve better contrast.
 In inversion recovery first 180 degree RF pulse is applied which flip the magnetization vector in 180
degree i.e. –Z direction.
 There is no magnetization in the x-y plane yet.
 After the 180 degree pulse there is only T1 recovery going on because there is no
component in the x-y plane and therefore no T2 relaxation.
Figure: Pulse sequence in an inversion recovery method.
 The T1 relaxation process would take place twice as long as when the net magnetization
would have been flipped to x-y plane.
 T1 relaxation is allowed to happen for certain time, known as the inversion time (TI) after
that normal SE sequence is applied.
Spin-echo imaging technique: The spin-echo imaging technique is
useful for creating images that are primarily dependent on T2.
 In this technique, it consists in applying a 90° pulse to rotate the magnetization to the X-Y
plane.
 This is followed by periodically flipping X-Y magnetization through 180° by means of a 180°
RF pulse.
 Spin echo sequence applies a 180° refocusing pulse half way between 90° pulse and
measurement.
Figure: Pulse sequence employed in spin-echo imaging technique
 This pulse eliminates phase differences due to artifacts, allowing measurement of pure T2.
 Due to spin-spin relaxation, successive echoes progressively decrease in size.
 The contributions to the echoes from those regions with a short T2 will decrease faster
than the contributions from regions with a long T2.
 For example, long T2 regions will show up brightly whereas short T2 regions will appear
black, on the NMR image.
Question:
1.
2.
3.
4.
How is a spin echo generated? (2)
What is the inversion recovery pulse sequence?(2)
Difference between inversion recovery sequence and spin echo imaging technique?(4)
Explain the inversion recovery techniques for MRI?(4)
Imaging sequences
Magnetic resonance imaging (MRI) is based on the principles of
nuclear magnetic resonance (NMR), a spectroscopic technique
used to obtain microscopic chemical and physical information
about molecules. Collection of images in MRI slices can be an
example of image sequencing.
MRI is based on the absorption and emission of energy in the
radiofrequency (RF) range of the electromagnetic spectrum.
Two dominated areas of NMR imaging system:
(i) the extension and improvement in tissue differentiation
(ii) the possibilities for shortening the required scan time.
Image in NMR System
An NMR system would involve two process for imaging
 spatially resolved information is encoded into a measurable signal
 the spatially encoded signal is subsequently decoded to produce an image
The spatial encoding process is achieved by acquiring NMR signals under the influence of three
orthogonal magnetic field gradients. There are many ways in which a gradient can influence and
interact with a spin system
Principle of Spatial Encoding in NMR Imaging
 The pulse sequence starts with the application of an RF
pulse which is at the Larmor carrier frequency but
modulated as shown with a sin C like function.
 The sin c modulation causes the bandwidth of the RF to be
restricted to within several kHz about the Larmor frequency.
 with the RF, a gradient (G-Slice) is applied to energize the
z-gradient coil.
 Because of the finite bandwidth of the RF pulse, only those
spins in the object within a thin slab or slice perpendicular
to the z direction experience RF radiation matching their
Larmor frequency.
Fig: Generic pulse sequence used in
magnetic resonance imaging
 The spins must be encoded spatially within the slice, i.e. along the x and y directions.
 The second gradient G-frequency does such encoding along the x direction.
 The second pulse is applied at the desired time of the signal measurement when it effectively
causes the net Larmor frequency to vary linearly with position along the x-axis.
 The final gradient G-phase, performs encoding along the y direction.
 A separate phasing coding gradient amplitude is applied during each cycle of the MR image
acquisition, with each cycle corresponding to a separate exponential function.
 Typically, a 128 phase encoding is used in standard imaging
The Spin Echo Sequence Family
 Turbo Spin Echo (TSE), used by Siemens and Philips
 Fast Spin Echo (FSE), used by General Electric
 Rapid Acquisition and Relaxation Enhancement (RARE)
 Turbo Inversion Recovery (TIR)
 Turbo Inversion Recovery Magnitude (TIRM)
 Fluid Attenuated Inversion Recovery (FLAIR)
 Half Fourier Acquired Single Shot Turbo Spin Echo (HASTE).
 (HASTE).
Questions
1. What are the process of imaging in NMR systems? (2 Mark) –
Slide 13
2. Principal of Spatial Encoding in NMR System? ( 4 Mark) –
Slide 14 & 15
3. Describe the spin echo sequence family? (4 Mark) - Slide 16
4. What is image sequencing? (2 Mark) – Slide 12
Gradient Echo Sequence
Gradient echo sequences (GRE) are an alternative technique
to spin-echo sequences, differing from it in two principal
points: utilization of gradient fields to generate transverse
magnetization. flip angles of less than 90°.
Figure 1: Gradient Echo Sequence
Gradient Echo Sequence
A
spin
echo (SE)
is
produced
by
pairs
of
radiofrequency (RF) pulses, whereas a gradient echo
(GRE) is produced by a single RF pulse in
conjunction with a gradient reversal.
Figure 2: No Gradient VS Gradient
Echo Sequence
The advantages of low-flip angle excitations and
gradient echo techniques are faster acquisitions,
new contrasts between tissues and a stronger MR
Figure 2: Gradient Echo Sequence
signal in case of short TR.
Gradient Echo Sequence Family
 Turbo Spin Echo (TSE)
 Fast Spin Echo (FSE)
 Rapid Acquisition and Relaxation Enhancement (RARE)
 Turbo Inversion Recovery (TIR)
 Turbo Inversion Recovery Magnitude (TIRM)
 Fluid Attenuated Inversion Recovery (FLAIR)
 Half Fourier Acquired Single Shot Turbo Spin Echo(HASTE)
(HASTE).
Miscellaneous imaging sequences
Miscellaneous imaging sequences refer to a variety of
specialized techniques used in medical imaging, particularly in
magnetic resonance imaging (MRI) and computed tomography
(CT) imaging. These sequences are designed to provide specific
information about tissues or structures in the body.
Figure 4: Miscellaneous imaging sequences
Advantages of Miscellaneous imaging sequences
 Enhanced Diagnostic Information
 Tissue Specificity
 Multi-Modal Imaging
 Functional Information
 Quantitative Analysis
 Early Disease Detection
 Minimally Invasive Planning
 Research and Discovery
 Patient Comfort and Safety
Miscellaneous Type of imaging
sequences
 Enhanced Fast Acquired Steady State Technique (CE FAST)
 Contrast Enhanced Gradient Recalled Acquisition in the
Steady State (CE GRASS)
 Double Echo Steady State (DESS)
 Turbo Gradient Spin Echo (TGSE)
 Gradient and Spin Echo (GRASE)
 Magnetization Prepared Rapid Gradient Echo (MP RAGE)
Questions:
1. What is Gradient Echo Sequence?
2. What are the Gradient Echo Sequence family?
3. What is Miscellaneous imaging sequences?
4. What are the advantages of Miscellaneous imaging sequences?
5. What are the Miscellaneous imaging sequences types?
References.
1. R. Khandpur-Biomedical Instrumentation_ Technology and ApplicationsMcGraw-Hill Professional (2004)
2. https://radiologykey.com/miscellaneous-magnetic-resonanceimaging/What are the Gradient Echo Sequence family?
3. https://www.researchgate.net/figure/Gradient-echo-images-aresuperior-to-FLAIR-in-the-detection-of-hemosiderinThis_fig3_8036960What are the advantages of Miscellaneous imaging
sequences?
4. https://mriquestions.com/gradient-echo.html