Introductory NMR & MRI: Video 01: Precession and Resonance

Summary

In this video, Paul Kahan introduces the basic principles of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). He uses a mechanical device to demonstrate how atomic nuclei behave in a magnetic field, comparing them to a spinning wheel. Key concepts like angular momentum, precession, and resonance are explained through the wheel's movement. These principles are then linked to the behavior of hydrogen nuclei in MRI, highlighting how resonance is central to reorienting nuclei in a magnetic field. The video illustrates how understanding these phenomena is crucial to mastering NMR and MRI.

Takeaways

• 🧲 The video discusses the fundamental principles of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).
• 🌐 Atomic nuclei behave like tiny compass needles when placed in a magnetic field, aligning with the field.
• 🔄 The concept of angular momentum is crucial in understanding nuclear magnetism, demonstrated by the spinning wheel analogy.
• 💫 Precession is a key phenomenon observed when spinning objects like the wheel are suspended, and it's central to MRI technology.
• 📏 Precession frequency remains consistent regardless of the axle's orientation, except when the axle is aligned with the gravitational field.
• 🌀 The hydrogen nucleus has two natural states in a magnetic field, which are significant for NMR and MRI.
• 🔄 To observe precession, the wheel's axle must be tilted from the vertical, analogous to the alignment of atomic nuclei.
• 🤚 Applying a torque to the wheel, similar to how NMR is performed, can reorient it and make the precession visible.
• 🎶 The concept of resonance is central to NMR, where an external force must match the natural precession frequency of the nuclei.
• 🧲 In NMR, a secondary oscillating magnetic field is used to apply the necessary torque to the atomic nuclei.
• ⏳ Over time, the wheel (and by analogy, atomic nuclei) will return to equilibrium, reducing the visibility of precession.

Q & A

What is the purpose of the video series presented by Paul Kahan?

The purpose of the video series is to explore the basic concepts behind nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) using a simple apparatus to demonstrate fundamental principles.

What does the apparatus in the video demonstrate?

The apparatus demonstrates the basic principles of nuclear magnetic resonance, such as the behavior of atomic nuclei in a magnetic field and the concept of precession.

How do atomic nuclei behave in a magnetic field?

Atomic nuclei have magnetism, and when placed in a magnetic field, they align like small compass needles with the magnetic field.

What is the purpose of using a mechanical device like the spinning wheel?

The spinning wheel is used to demonstrate key principles of nuclear magnetism, such as angular momentum and precession, which are important for understanding NMR.

What is precession, and how is it demonstrated with the spinning wheel?

Precession is the wobbling motion that occurs when an object with angular momentum, like the spinning wheel, is suspended and subjected to a force. It is demonstrated by suspending the spinning wheel and observing how it moves in circles.

How does the orientation of the wheel's axis affect the precession?

The precession frequency remains constant regardless of the axis orientation, but the precession becomes harder to observe when the axis is either perfectly vertical or perfectly horizontal.

What are the two natural states of hydrogen nuclei in a magnetic field?

Hydrogen nuclei have two natural states in a magnetic field: one where the axis is pointing up and another where it is pointing down.

How can the precession be made visible when the wheel is in a low-energy vertical position?

To make precession visible, a torque must be applied in the horizontal plane to tilt the axis away from the vertical position, allowing the wheel to begin precessing.

What is resonance in the context of nuclear magnetic resonance?

Resonance occurs when the torque applied to reorient the atomic nuclei matches the precession frequency of the nuclei. In NMR, this is achieved by applying an oscillating magnetic field orthogonal to the main magnetic field.

What happens to the spinning wheel's precession over time?

Over time, the precession of the spinning wheel dies away as it gradually returns to its equilibrium state, similar to how atomic nuclei return to equilibrium after being disturbed in NMR.

Outlines

00:00

🧑‍🔬 Introduction to Nuclear Magnetic Resonance and Magnetic Resonance Imaging

Paul Kahan introduces the video series, which explores nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). He presents a basic NMR apparatus and explains the key principles underlying these technologies. The focus will be on understanding the behavior of atomic nuclei in magnetic fields. He begins by illustrating how atomic nuclei act like tiny compass needles, aligning with magnetic fields, which he demonstrates using a wheel in Earth’s gravitational field. The first key idea is that atomic nuclei have magnetism, and the second is that they possess angular momentum.

05:00

🌀 Demonstrating Precession and Angular Momentum with a Spinning Wheel

Paul demonstrates angular momentum by spinning the wheel and suspending it from a string. This action shows a phenomenon called precession, which is key to understanding magnetic resonance. The wheel precesses at a steady frequency regardless of its orientation, except when in a natural low-energy position, where precession is hard to see. This is analogous to the behavior of atomic nuclei, specifically the hydrogen nucleus, which has two natural states in a magnetic field: pointing up or down. Precession is only visible when the axle is tilted away from the vertical.

⚛️ Resonance: Matching Frequencies for Magnetic and Nuclear Precession

Paul explains how to disturb the wheel's low-energy position by applying torque, which reorients the wheel and makes precession visible. This concept mirrors how resonance works in NMR, where a secondary magnetic field is applied orthogonally to the primary one, matching the frequency of nuclear precession. This process allows the reorientation of atomic nuclei in magnetic fields, similar to how the wheel's axle is moved. Resonance involves applying torque in synchrony with precession frequency, a principle central to NMR.

🔄 Return to Equilibrium: The Damping of Precession

In the final demonstration, Paul shows how, after the wheel is moved into a position that highlights precession, it gradually reorients itself back to equilibrium, and precession becomes less visible over time. This process mirrors the behavior of atomic nuclei in NMR as they return to their equilibrium state after being disturbed. This natural return to equilibrium is another key phenomenon in understanding nuclear magnetic resonance.

Keywords

💡Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR) refers to the phenomenon where atomic nuclei resonate in a magnetic field when exposed to an oscillating secondary magnetic field. This concept is central to the video, as it underpins the technology of magnetic resonance imaging (MRI). The speaker demonstrates how atomic nuclei, like the hydrogen nucleus, behave in magnetic fields, linking the idea of resonance with the mechanical demonstration of the spinning wheel.

💡Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI) is a medical imaging technique that uses NMR principles to visualize structures inside the human body. The video uses basic physics demonstrations to explain how atomic nuclei behave in magnetic fields, laying the groundwork for understanding how MRI machines generate detailed internal images by manipulating nuclear resonance.

💡Atomic Nuclei

Atomic nuclei are the small, dense centers of atoms made up of protons and neutrons. In the video, they are discussed in the context of their magnetic properties and angular momentum, which cause them to align with magnetic fields and exhibit behaviors such as precession when disturbed. The hydrogen nucleus, particularly, is highlighted for its importance in NMR and MRI.

💡Magnetic Field

A magnetic field is a region of space where magnetic forces are exerted, which influences objects with magnetic properties, such as atomic nuclei. In the video, the speaker demonstrates how atomic nuclei align with an applied magnetic field, analogous to a compass needle aligning with the Earth's magnetic field. The magnetic field is a crucial element in both NMR and MRI.

💡Precession

Precession is the phenomenon where the axis of a spinning object, like a top or atomic nucleus, slowly rotates around another axis. In the video, precession is demonstrated with a spinning wheel, showing how its axle moves in a circular pattern. This mirrors how atomic nuclei precess when placed in a magnetic field, a key concept for understanding NMR.

💡Angular Momentum

Angular momentum is the rotational equivalent of linear momentum, a property of rotating objects, including atomic nuclei. In the video, angular momentum is demonstrated using a spinning wheel, which helps to explain the behavior of atomic nuclei in a magnetic field. The nuclei’s angular momentum interacts with the magnetic field, leading to precession.

💡Torque

Torque is a twisting force that causes objects to rotate. The video uses the concept of torque to explain how forces acting on atomic nuclei (or the spinning wheel) can cause changes in orientation. The torque applied by the magnetic field reorients the atomic nuclei, just as the Earth's gravitational field exerts torque on the spinning wheel.

💡Resonance

Resonance occurs when a system oscillates at a specific frequency in response to an external force. In the context of the video, the speaker shows how applying a force at the same frequency as the spinning wheel’s natural precession causes resonance. This principle is directly related to how NMR works, as a secondary magnetic field is applied at the resonance frequency of atomic nuclei.

💡Equilibrium

Equilibrium refers to a state of balance where forces are equal and opposing. In the video, the concept of equilibrium is demonstrated when the spinning wheel, or atomic nuclei, return to a stable orientation after being disturbed. In NMR, atomic nuclei have natural equilibrium states in a magnetic field, and understanding how they return to these states is important for analyzing the signals produced during resonance.

💡Hydrogen Nucleus

The hydrogen nucleus, which consists of a single proton, plays a key role in NMR and MRI because of its abundance in the human body and its strong magnetic properties. The video highlights how the hydrogen nucleus can occupy two states in a magnetic field, aligning with or against the field. This behavior forms the basis for NMR’s ability to detect changes in nuclear orientation.

Highlights

Introduction to nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) concepts.

Demonstration of basic principles of NMR using a simple apparatus.

Explanation of atomic nuclei behavior in a magnetic field.

Analogy using a wheel and Earth's gravitational field to explain how nuclei align in a magnetic field.

Introduction to angular momentum and its importance in nuclear magnetism.

Demonstration of angular momentum with a spinning wheel and its relation to nuclear spins.

Explanation of precession and its significance in magnetic resonance.

Observation that the precession frequency remains the same, regardless of the axle’s orientation.

Illustration of how atomic nuclei, like hydrogen, have two natural states in a magnetic field: aligned up or down.

Importance of tilting the axis to observe precession in nuclei and the wheel analogy.

Introduction to the concept of resonance and how applying torque at the correct frequency affects precession.

Demonstration of mechanical resonance using the wheel to explain nuclear resonance.

Explanation of how resonance in NMR involves applying an oscillating magnetic field orthogonal to the main field.

Illustration of how resonance reorients atomic nuclei from their natural equilibrium states.

Discussion of relaxation and the return of nuclei to equilibrium after resonance, similar to how the spinning wheel's precession dies away.

Transcripts

my name is Paul Kahan and in this series

of videos we're going to be looking at

some of the basic ideas behind nuclear

magnetic resonance and magnetic

resonance

imaging and to help us look at those

ideas we've got this apparatus here

behind me it's a very simple nuclear

magnetic resonance apparatus which can

demonstrate most of the basic principles

we need to understand in order to

appreciate what magnetic resonance

imaging and nuclear magnetic resonance

can

do in thinking about magnetic resonance

we'll be thinking about Atomic nuclei

and their behavior in a magnetic

field and I want to start by

demonstrating some of the basic physics

behind the behavior of atomic nuclei in

a magnetic field and to help me do that

I've got a mechanical device and it's

this wheel over

here the first principle of

understanding Atomic nuclei in a

magnetic field is to realize that those

nuclei have magnetism and so like little

Compass needles when they're placed in a

magnetic field they tend to line up with

the

field and I can demonstrate that idea

with this wheel there's no magnetic

field that's playing a role here but we

have the Earth's gravitational field and

the effect of that gravitational field

is that if I try to suspend this wheel

on a string like this it twists and

finds a natural orientation pointing

along the Earth's gravitational field

why is that well the combination of the

weight force of the wheel and the string

causes a torque to act that torque or

twisting force reorients the wheel so

that it hangs

vertically the second idea that's really

important in understanding nuclear

magnetism is the idea that the nuclei

themselves have angular moment

let me demonstrate angular momentum with

this wheel I can do that simply by

spinning

it now let's try that same trick of

suspending the wheel like I did before

on the string now that the wheel is

spinning something really remarkable

happens that effect is called precession

and that procession idea is very very

important in understanding the way

magnetic resonance

Works let's look at that procession a

little more closely and see how it

depends on the orientation of the axle

of the wheel I'm going to start with the

wheel with its axis

horizontal takes about a second to go

around what happens if we Orient the

axle slightly differently like this for

example takes about a second to go

around in fact it doesn't matter what

orientation we have this axle the

precession fre frequency is pretty much

the

same however if we do suspend the wheel

in that natural low energy

position the Precision is very very hard

to see if we tilt it a bit it

reappears so we have to have the axle of

the wheel tilted from the vertical in

order to see the

procession by the way there's another

position where the procession is very

hard to see as well and that's where the

axle pointing up like this but anywhere

in

between pointing up and pointing down

the procession is very

visible by the way these two states of

pointing down pointing up turn out to be

very relevant in the case of the atomic

nuclei that we'll be looking at in these

series of videos and that's the hydrogen

nucleus it turns out that the hydrogen

nucleus has two very natural States in a

magnetic field one pointing One

Direction and One pointing in the other

direction so what have we learned here

we've seen that in order to observe the

procession we need to have the axle

tilted at some angle away from the

vertical and that begs the question if

we did start in that low energy position

with the axle pointing vertically

downwards how could we disturb it in

such a way that we could make the

procession visible and I want to

demonstrate that to

you just watch this

trick

I did it I used my finger to apply

torque to the wheel that brought the

axle away from the vertical position

into the

horizontal what was the basic idea

behind the trick the first thing is that

I had to apply the torque in the

horizontal plane I had to apply it in a

direction normal to or at right angles

to the torque that the Earth's

gravitational field is

applying the second thing is that I had

to move my finger around

in synchron with the wheel keeping the

reorientation of that Torque from my

finger at the same frequency as the

procession frequency let me just

demonstrate that

again so this movement of the torque

following exactly the procession

frequency of the wheel is known as

resonance the frequency is exactly the

same in both cases and the idea of

resonance is really Central to what

we'll be looking at when we reorient

atomic nuclei from their natural

equilibrium States in a magnetic

field so that's wheel mechanical

resonance when we perform nuclear

magnetic resonance we have to do the

same trick we have to apply a torque

that's orthogonal to the torque of the

magnetic field on the spins and we have

to have that torque varying with time

time or oscillating an exact frequency

match with the natural procession

frequency of the nucleus spins and how

do we apply that torque we do it with

another magnetic field a magnetic field

that's orthogonal to the main magnetic

field that tries to orient the spins and

a magnetic field that's oscillating in

time in exact synchrony with the

procession frequency of the atomic

nuclei and that is nuclear magnetic

resonance there's one more thing I want

to show you let's go back to the

wheel suppose we do this wheel

mechanical resonance and end up with the

axle pointing in a direction that really

shows the procession very

nicely the horizontal orientation as

shown here let's look what happens with

time gradually the wheel reorients and

the visibility of the procession starts

to die

away

this returning to some sort of

equilibrium is something we'll also see

happening with atomic

nuclei