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Spin, Precession, Resonance and Flip Angle | MRI Physics Course | Radiology Physics Course #3
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Spin, Precession, Resonance and Flip Angle | MRI Physics Course | Radiology Physics Course #3

Introduction to Nuclear Magnetic Resonance

In this section, we will discuss the phenomenon of nuclear magnetic resonance (NMR) and the concept of spin in quantum physics.

Understanding Spin in Quantum Physics

  • The classical model describes a charged particle rotating around its own axis with angular momentum, which induces a magnetic field.
  • However, this classical model is not an accurate representation of what happens within particles in the body.
  • Quantum physics breaks down properties into discrete measurable values, including spin.
  • Spin describes how a subatomic particle reacts to an external magnetic field.

Spin Values and Magnetic Moments

  • Protons are made up of quarks with their own spin values connected by gluons.
  • The net spin value of a proton is 1/2.
  • Neutrons also have a spin value of 1/2 but can have opposite directions, resulting in cancelation when two neutrons are within the same nucleus.
  • Atoms with a net spin value of zero are unaffected by an external magnetic field.

Importance of Hydrogen in NMR

  • Many different atoms can undergo nuclear magnetic resonance, but hydrogen is commonly used due to its abundance in the body and its net spin value.

Classical Model vs. Quantum Mechanical Model

In this section, we will compare the classical model and the quantum mechanical model to understand nuclear magnetic resonance better.

Classical Model

  • The classical model describes a charged particle rotating around its own axis with angular momentum, inducing a magnetic field represented by the magnetic moment.
  • This model helps us visualize what happens but does not accurately represent particles' behavior within the body.

Quantum Mechanical Model

  • Quantum physics provides a more accurate understanding of particles' behavior through discrete measurable values for properties like charge, mass, color, and spin.
  • Spin describes how a particle reacts to an external magnetic field.
  • Quantum properties can be broken down into distinct measurable values.

Quantum Mechanical Model and Magnetic Moments

In this section, we will delve deeper into the quantum mechanical model and its relationship with magnetic moments.

Quantum Properties

  • Quantum properties like charge, mass, color, and spin have distinct measurable values.
  • These properties describe how subatomic particles react to external forces or fields.

Magnetic Moments

  • Spin value represents the spin angular momentum of a particle in response to an external magnetic field.
  • Protons have their own spin values connected by gluons within the nucleus.
  • The net spin value of a proton is 1/2.
  • Neutrons also have a spin value of 1/2 but can cancel each other out when two neutrons are within the same nucleus.

Importance of Quantum Physics in Describing Magnetic Moments

In this section, we explore why quantum physics is necessary to accurately describe magnetic moments within particles.

Limitations of Classical Model

  • The classical model helps visualize what happens but falls short in explaining certain phenomena observed in particles within the body.
  • For example, a neutron has no charge yet possesses a magnetic moment. It wouldn't have a magnetic moment if it were simply rotating on its axis.

Quantum Physics Explanation

  • Quantum physics provides a more accurate description by breaking down properties into discrete measurable values.
  • Spin describes how particles react to an external magnetic field.
  • Understanding quantum physics is essential for describing the magnetic moments within particles accurately.

Describing Magnetic Moment Within Neutrons

In this section, we discuss how quantum physics helps describe the magnetic moment within neutrons.

Quantum Physics and Neutrons

  • Quantum physics is necessary to describe the magnetic moment within a neutron accurately.
  • Neutrons have a spin value of 1/2, even though they have no charge.
  • The classical model's idea of rotation does not explain the observed magnetic moment in uncharged particles like neutrons.

Quantum Properties and Discrete Measurable Values

In this section, we explore how quantum properties can be broken down into discrete measurable values.

Quantum Properties

  • Quantum properties include charge, mass, color, and spin.
  • These properties can be broken down into distinct measurable values.
  • Charge describes how subatomic particles react to other charged particles or external forces.

Understanding Charge as a Quantum Property

In this section, we delve deeper into understanding charge as a quantum property.

Charge as a Quantum Property

  • Describing charge itself is challenging, but it explains how subatomic particles react to other charged particles or external forces.
  • Electrons with negative charges repel other negatively charged subatomic particles and attract opposite charges.

Spin as a Quantum Property

In this section, we focus on spin as a quantum property and its role in describing particle behavior.

Spin as a Quantum Property

  • Spin describes how subatomic particles or particles react to an external magnetic field.
  • It represents the type of reaction or behavior exhibited by the particle when subjected to an external force.
  • Protons have their own spin values connected by gluons within the nucleus.

Spin Angular Momentum and Subatomic Particles

In this section, we discuss spin angular momentum and its relationship with subatomic particles.

Spin Angular Momentum

  • Spin angular momentum describes how a particle reacts to an external magnetic field.
  • Protons, for example, have their own spin values connected by gluons within the nucleus.
  • The net spin value of a proton is 1/2.

Net Spin Value of Nucleus

In this section, we explore the net spin value of a nucleus and its implications.

Net Spin Value

  • A nucleus with an even number of protons and neutrons will have a net spin value of zero.
  • Neutron pairs and proton pairs cancel each other out in terms of their magnetic moments.
  • Atoms with a net spin value of zero are unaffected by an external magnetic field.

Cancelation of Magnetic Moments in Nuclei

In this section, we discuss how magnetic moments can cancel each other out within nuclei.

Magnetic Moment Cancelation

  • When two neutrons are within the same nucleus, they can have opposite directions for their magnetic moments.
  • This results in cancelation, where the two magnetic moments counteract each other.

Atom's Spin Value and External Magnetic Field

In this section, we explore how an atom's spin value affects its interaction with an external magnetic field.

Atom's Spin Value

  • Atoms with a net spin value of zero are unaffected by an external magnetic field.
  • Oxygen 16 and carbon 12 isotopes have even numbers of protons and neutrons, resulting in canceled spin values.
  • Hydrogen has a single proton with a spin value of 1/2, making it influenced by an external magnetic field.

Importance of Hydrogen in NMR

In this section, we discuss why hydrogen is commonly used in nuclear magnetic resonance (NMR).

Reasons for Using Hydrogen

  • Hydrogen is the most abundant isotope in the body.
  • Hydrogen has a net spin value, making it suitable for undergoing nuclear magnetic resonance.
  • Other atoms can also undergo NMR, but hydrogen's abundance and properties make it a preferred choice.

Magnetic Moment and Spin of Hydrogen Protons

This section discusses the magnetic moment and spin of hydrogen protons, as well as their relationship.

Magnetic Moment and Spin

  • Hydrogen protons have a magnetic moment, which describes the magnetic field around them.
  • The terms "free hydrogens," "protons," and "spins" are used interchangeably to refer to hydrogen atoms.
  • In the quantum world, hydrogen protons can exist in both spin up and spin down states simultaneously.
  • The net magnetic moment of a group of hydrogens is obtained by summing up their individual magnetic moments.
  • The magnitude of the magnetic moment is linked to the spin of the proton through the gyromagnetic ratio.

Gyromagnetic Ratio and Larmor Frequency

This section explains the gyromagnetic ratio and its importance in determining the Larmor frequency.

Gyromagnetic Ratio

  • The gyromagnetic ratio links a specific atom's spin to its magnetic moment.
  • For hydrogen atoms, the gyromagnetic ratio is 42.5 megahertz per Tesla (MHz/T).
  • Multiplying the gyromagnetic ratio by the spin gives us the magnitude of the magnetic moment.

Larmor Frequency

  • The Larmor frequency is calculated by multiplying the gyromagnetic ratio with the strength of an external magnetic field.
  • It represents the precessional frequency at which hydrogen protons align with a magnetic field.
  • Knowing the Larmor frequency is crucial for matching radiofrequency pulses in MRI imaging.

Calculation of Larmor Frequency

This section explains how to calculate the Larmor frequency using the gyromagnetic ratio and magnetic field strength.

Calculating Larmor Frequency

  • The Larmor frequency can be calculated by multiplying the gyromagnetic ratio of the atom with the strength of the magnetic field.
  • The resulting frequency is measured in megahertz (MHz).
  • This calculation is essential for determining the appropriate radiofrequency pulse in MRI imaging.

Net Magnetization Vector and Transverse Magnetization

This section discusses the net magnetization vector and its relationship with transverse magnetization.

Net Magnetization Vector

  • As hydrogen protons align with a magnetic field, a net magnetization vector is formed.
  • The net magnetization vector lies parallel to the external magnetic field along the longitudinal or Z-axis.
  • Processing protons have different phases, causing their transverse magnetization values to cancel each other out.

Transverse Magnetization

  • Transverse magnetization refers to the component of magnetization perpendicular to the main magnetic field.
  • In MRI imaging, inducing transverse magnetization becomes important for generating useful signals.

Precessional Frequency and Radiofrequency Pulse

This section explains precessional frequency and its relation to radiofrequency pulses.

Precessional Frequency

  • The precessional frequency represents the specific frequency at which atoms process or spin in a magnetic field.
  • It depends on the type of atom present in the magnetic field.

Radiofrequency Pulse

  • A radiofrequency pulse (B1) can be applied at a specific precessional frequency to induce transverse magnetization.
  • The radiofrequency pulse is perpendicular to the main magnetic field and helps generate useful signals in MRI imaging.

Nuclear Magnetic Resonance

This section explains the concept of nuclear magnetic resonance and how it is used to measure signals from hydrogen atoms.

Introduction to Nuclear Magnetic Resonance

  • Nuclear magnetic resonance occurs when the net magnetization vector of hydrogen spins fans out due to a radio frequency pulse.
  • When the radio frequency pulse matches the precessional frequency of hydrogen spins, they start spinning in phase with each other, resulting in transverse magnetization.
  • Transverse magnetization can be measured by placing a coil transverse to the main magnetic field and reading out the signal strength. The amplitude of the signal is proportional to the transverse magnetization.

Flip Angle and Signal Strength

  • The flip angle, determined by the duration of the radio frequency pulse, affects the signal strength. A larger flip angle results in a higher signal amplitude.
  • By applying a radio frequency pulse for a longer period of time, we can increase the flip angle until it reaches 90 degrees, which gives us maximum signal strength.

Resonance and Frequency Selection

  • Resonance allows us to induce transverse magnetization and measure a signal. It also enables us to select specific groups of hydrogen atoms based on their Larmor frequencies.
  • The Larmor frequency depends on both the gyromagnetic ratio of hydrogen protons and changes in magnetic field strength along an applied gradient.
  • To achieve resonance, we need to match our radio frequency pulse with the precessional frequency of hydrogen protons. If they don't match, energy transfer will not occur.

Flipping Magnetization Vector

  • It takes time for processing hydrogen atoms to gain transverse magnetization after a radio frequency pulse. The duration of the pulse determines the flip angle of the net magnetization vector.
  • By applying the radio frequency pulse for double the time it took to reach 90 degrees, we can flip the net magnetization vector a full 180 degrees. This results in all transverse magnetization being lost and the vector aligning anti-parallel to the main magnetic field.

Conclusion

  • Nuclear magnetic resonance is based on spin angular momentum within protons, which align with an external magnetic field and process at a frequency dependent on field strength and atom type. Resonance allows us to measure signals and select specific groups of hydrogen atoms based on their Larmor frequencies.
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Video description

*Pass your radiology physics exam first time. Complete radiology physics past paper question bank* 👇 https://learnradiologyphysics.com/ ========================= Let's discuss nuclear magnetic resonance. First we will look at what nuclear spin is and how it determines the magnetic moment. Then we'll see how non zero spin atomic particles behave in an external magnetic field. We'll discuss the role of the gyromagnetic ratio and magnetic field strength on precession frequency, using the Lamour equation to calculate this relationship. Lastly, we discuss resonance secondary to the application of a radiofrequency pulse. Quantum physics videos for those interested: SPIN: https://www.youtube.com/watch?v=DCrvanB2UWA HEISENBERG UNCERTAINTY PRINCIPLE: https://www.youtube.com/watch?v=TQKELOE9eY4 ENTANGLEMENT: https://www.youtube.com/watch?v=v1_-LsQLwkA ========================= *Not sure these radiology physics question banks are for you?* If you’re preparing for a radiology physics exam and feeling overwhelmed by formulas, theory, or endless reading, you’re not alone. Most candidates don’t fail because they didn’t study enough, but because they didn’t practise the right way. The fastest way to build confidence in radiology physics is simple: 👉 Do high-quality past-paper style questions. Instead of passively reading notes, you’ll practise the way the exams actually test you. With carefully written questions that reflect real exam structure, difficulty, and marking logic. *Why question banks work for radiology physics exams* Radiology physics isn’t about memorising every fact. It’s about recognising patterns, understanding how concepts are tested, and applying physics under exam pressure. These question banks help you: Identify high-yield examinable topics Learn how questions are phrased across different exams and modalities Recognise common exam traps and misconceptions Reinforce understanding through repetition and explanation Build the confidence that comes from knowing you’ve seen this before Every question is written with exam relevance in mind, aligned to major international curricula, and structured to mirror real-world past papers, not generic AI generated physics quizzes. *Who these radiology physics question banks are for* These are ideal if you: Are short on time and want maximum exam return Feel confident reading theory but struggle with exam questions Want a structured way to revise X-ray, CT, MRI, ultrasound, and nuclear medicine physics Are preparing for FRCR, RANZCR AIT, ARRT, ABR Core, MICR Part 1, or FC Rad Diag (SA) or similar exams in the Radiography and Veterinary fields. If you’ve ever thought “I understand this topic… but I’m not sure I could answer it in an exam”, this is exactly the gap these question banks are built to close. Radiology physics exams reward practice, familiarity, and confidence. And confidence comes from doing focused, exam specific practice over and over again. Happy studying, Michael #radiology #radres #FOAMrad #FOAMed 0:00 NUCLEAR MAGNETIC RESONANCE 0:26 SPIN 6:40 MAGNETIC MOMENT (µ) 7:39 GYROMAGNETIC RATIO (Y) 8:46 LARMOR FREQUENCY 10:25 PRECESSION 15:50 FLIP ANGLE