MRI Physical and physiological basis, precession ,spin , T1 relaxation, T2 relaxation.

1. Physical And Physiological Basis Of Magnetic Relaxation,
Image Contrast And Noise
By AALIA ABDULLAH
ASSISTANT PROFESSOR RADIOLOGY

2. • The phenomenon of magnetic relaxation is fundamental to
the functioning of MRI. It involves the relaxation of excited
nuclear spins back to their equilibrium state and forms the
basis for contrast in MRI images. There are two types of
relaxation processes:
• longitudinal (T1) relaxation spin-lattice relaxation and
• transverse (T2) relaxation. spin-spin relaxation

3. • Magnetic Relaxation:
• Spin-Up and Spin-Down: In a magnetic field, hydrogen
protons align either parallel (spin-up) or antiparallel (spin-
down) to the direction of the magnetic
field.

4. • Precession: When protons are placed in a magnetic field, they
precess around the axis of the field at a frequency known as the
Larmor frequency.

5. Longitudinal (T1) Relaxation:
• Longitudinal relaxation refers to the process by which the
magnetization vector of the nuclear spins in a tissue sample returns to
its equilibrium along the direction of the external magnetic field (z-axis)
after being perturbed by an external radiofrequency (RF) pulse.
• The time constant associated with this process is called T1, or the
longitudinal relaxation time.
• T1 relaxation is primarily influenced by the interactions between
neighboring nuclear spins and their environment.

6. • T1 Relaxation: Involves the return of the longitudinal
magnetization component (parallel to the main magnetic field)
to its equilibrium state. T1 relaxation times are influenced by the
type of tissue and provide contrast between different tissues in
MRI images.
• T2 Relaxation: Involves the decay of the transverse
magnetization component (perpendicular to the main magnetic
field) due to interactions among neighboring protons. T2
relaxation times also contribute to tissue contrast in MRI
images.

7. • 2. Image Contrast:
• Tissue Properties: Different tissues in the body have varying
relaxation times (T1 and T2) due to their molecular composition,
density, and water content.
• Contrast Mechanisms: By manipulating the timing between
RF pulses and measuring the signals emitted during T1 and T2
relaxation processes, MRI scanners can create images with
different contrasts, highlighting specific tissue properties.

8. • The physical and physiological factors influencing T1
relaxation include:
• Tissue Type: Different tissues have different T1 relaxation
times due to variations in molecular composition and structure.
• Proton Density: T1 relaxation is influenced by the density of
hydrogen nuclei (protons) within a tissue.
• Motion: Tissues with mobile protons (e.g., fluids) exhibit shorter
T1 relaxation times compared to tissues with more restricted
motion.
• Proton Exchange: Tissues with fast proton exchange
processes (e.g., water exchange between compartments) tend
to have shorter T1 relaxation times.

9. • The physical and physiological factors influencing T2
relaxation include:
• Tissue Type: Tissues with shorter T2 relaxation times appear
darker in MRI images due to faster signal decay.
• Inhomogeneities: magnetic field inhomogeneities in tissues
can lead to faster T2 relaxation.
• Spin-Spin Interactions: Interactions between nuclear spins
within a tissue contribute to T2 relaxation. These interactions
are influenced by the local environment.
• Motion and Diffusion: Tissues with restricted motion and
diffusion (e.g., solid tissues) tend to have longer T2 relaxation
times, while tissues with more fluid-like motion have shorter T2
relaxation times.

10. • 3.Noise: Noise in MRI can be attributed to both physical and physiological factors:
• Physical Sources of Noise: Thermal noise, radiofrequency interference (RFI),
gradient coil vibrations, and electronic noise contribute to the overall noise level in
MRI images.
• Physiological Sources of Noise: Subject motion, physiological motion (e.g.,
breathing, heartbeat), and natural variations in tissue properties introduce
variability into the acquired signals.
• Reducing noise involves improving hardware components, optimizing imaging
sequences, and utilizing post-processing techniques to enhance signal-to-noise
ratio (SNR) while preserving image details.

11. • Physiological Factors:
• Blood Flow: Blood flow affects tissue relaxation times and,
consequently, image contrast. Dynamic contrast-enhanced MRI is used
to visualize blood perfusion in tissues.
• Blood Oxygenation: Blood oxygenation influences T2* relaxation,
leading to functional MRI (fMRI) techniques that can depict brain
activity.

12. • Magnetic relaxation is at the core of MRI principles, allowing the
creation of detailed images by manipulating the interactions between
magnetic moments of hydrogen protons. Contrast in MRI images is
achieved by exploiting variations in relaxation times among different
tissues. However, noise is an inherent challenge in MRI due to thermal
and other sources. Understanding the physical and physiological basis
of these factors is essential for optimizing image quality, acquiring
accurate diagnostic information, and advancing MRI technology.