Magnets and Scanner

Question 1

What imparts magnetic characterics to all materials?

Answer 1

Angular momenta of electrons (and to a lesser extent, nuclei) impart some magnetic characteristics to all materials.

Question 2

What generates a magnetic field?

Answer 2

Any current, moving charge, or changing electrical potential also generates a magnetic field.

Question 3

What is the today’s understanding of the magnetism? Today we understand that the static magnetic fields associated with lodestones and permanent magnets derive principally from the ____ within those materials.

Answer 3

Today we understand that the static magnetic fields associated with lodestones and permanent magnets derive principally from the total angular momentum of electrons within those materials.

Question 4

What is spin?

Answer 4

Possessed by lone electrons; quantized fundamental property of nature denoted by the letter S.

Question 5

In addition to spin, what does electrons orbiting a nucleus possess?

Answer 5

In addition to S, electrons orbiting a nucleus also possess orbital angular momentum (L).

Question 6

Total Angular Momentum

Answer 6

Spin + Orbital Angular Momentum

Question 7

Who first demonstrated the relationship between current and strength of the resultant magnetic field?

Answer 7

Andre Marie Ampere

Question 8

Who demonstrated principle of magnetic induction?

Answer 8

Michael Faraday

Question 9

How did Faraday demonstrate principle of magnetic induction?

Answer 9

Not only do electrical currents produce magnetic fields, but changing magnetic fields induce electrical currents. By measruing the voltage produced in a coil by a moving magnet.

Question 10

Three years after Faraday demonstrated the principle of magnetic induction, who showed that the current induced was so directed as to oppose the change in magnetic flux?

Answer 10

Heinrich Lenz

Question 11

emf = -N(delta magnetic flux / delta time)

Answer 11

Lenz’s Law represents the rate change of the magnetic field. The negative sign reflects Lenz’s principle that the induced current creates a “counter field” in a direction opposite to magnetic field(B).

Question 12

Maxwell’s Four Equations in words

Answer 12

Maxwell’s four equations in order mean: 1) wherever a charge exists, an electric field E diverges from it or into it; 2) magnetic field lines (B) can only exist in closed loops; 3) an electric field (E) is produced by a changing magnetic field (dB/dt) and 4) either a constant current or changing electric field creates a circulating magnetic field.

Question 13

Maxwell’s four equations in order mean: 1) ; 2) magnetic field lines (B) can only exist in closed loops; 3) an electric field (E) is produced by a changing magnetic field (dB/dt) and 4) either a constant current or changing electric field creates a circulating magnetic field.

Answer 13

Maxwell’s four equations in order mean: 1) wherever a charge exists, an electric field E diverges from it or into it; 2) magnetic field lines (B) can only exist in closed loops; 3) an electric field (E) is produced by a changing magnetic field (dB/dt) and 4) either a constant current or changing electric field creates a circulating magnetic field.

Question 14

Maxwell’s four equations in order mean: 1) wherever a charge exists, an electric field E diverges from it or into it; 2) ; 3) an electric field (E) is produced by a changing magnetic field (dB/dt) and 4) either a constant current or changing electric field creates a circulating magnetic field.

Answer 14

Maxwell’s four equations in order mean: 1) wherever a charge exists, an electric field E diverges from it or into it; 2) magnetic field lines (B) can only exist in closed loops; 3) an electric field (E) is produced by a changing magnetic field (dB/dt) and 4) either a constant current or changing electric field creates a circulating magnetic field.

Question 15

Maxwell’s four equations in order mean: 1) wherever a charge exists, an electric field E diverges from it or into it; 2) magnetic field lines (B) can only exist in closed loops; 3) 4) either a constant current or changing electric field creates a circulating magnetic field.

Answer 15

Maxwell’s four equations in order mean: 1) wherever a charge exists, an electric field E diverges from it or into it; 2) magnetic field lines (B) can only exist in closed loops; 3) an electric field (E) is produced by a changing magnetic field (dB/dt) and 4) either a constant current or changing electric field creates a circulating magnetic field.

Question 16

Maxwell’s four equations in order mean: 1) wherever a charge exists, an electric field E diverges from it or into it; 2) magnetic field lines (B) can only exist in closed loops; 3) an electric field (E) is produced by a changing magnetic field (dB/dt) and 4)

Answer 16

Maxwell’s four equations in order mean: 1) wherever a charge exists, an electric field E diverges from it or into it; 2) magnetic field lines (B) can only exist in closed loops; 3) an electric field (E) is produced by a changing magnetic field (dB/dt) and 4) either a constant current or changing electric field creates a circulating magnetic field.

Question 17

What is Tesla?

Answer 17

SI unit of magnetic field strength

Question 18

How is Tesla Defined?

Answer 18

Force per unit length exerted on a current-carrying wire

Question 19

1 Tesla =

Answer 19

1 Newton / Ampere-Meter

Question 20

When a current is passed along the wire, a magnetically generated (Lorentz) force

Answer 20

When a current is passed along this wire, a magnetically generated (Lorentz) force deflects the wire upward.

Question 21

Lorentz Force is proportional to the current(I), The length of the wire(L), Strength of the magnetic field(B)

Answer 21

F = i * L * B

Question 22

In general, however, the structure of the magnetic field is more complex, and B may have –

Answer 22

In general, however, the structure of the magnetic field is more complex, and B may have different values and directions of action at different points in space.

Question 23

A magnetic field is therefore formally defined to be an __ (denoted by the boldfaced letter B) whose magnitude B and direction at each point in space define how the field will act on a charge moving at that location.

Answer 23

A magnetic field is therefore formally defined to be an array of vectors (denoted by the boldfaced letter B) whose magnitude B and direction at each point in space define how the field will act on a charge moving at that location.

Question 24

Vector Form of Lorentz Force Law

Answer 24

F = I x B

Question 25

Relationship between Gauss and Tesla

Answer 25

1T = 10,000 Gauss

Question 26

Relative Magnetic Field Strength
Earth, Refrigerator Magnet

Answer 26

Earth : 50 uT, Refrigerator Magnet: 5 mT

Question 27

How is Gradient Defined?

Answer 27

Gradient(G) is defined as change in field over a change in distance

Question 28

When does a magnetic gradient exist?

Answer 28

Whenever a magnetic field differs in magnitude or direction between

Question 29

The main magnetic field, the gradient (G) is a vector, possessing both magnitude and direction. By convention, the direction of the main magnetic field is designated to be —-. For completeness, therefore, we should say the gradient (G) is a vector of magnitude 2 mT/meter in the z-direction.

Answer 29

Note we have calculated only the magnitude of the gradient. But like the main magnetic field, the gradient (G) is a vector, possessing both magnitude and direction. By convention, the direction of the main magnetic field is designated to be the z-axis. For completeness, therefore, we should say the gradient (G) is a vector of magnitude 2 mT/meter in the z-direction.

Question 30

The gradient of B is denoted ∇B, where ∇ is known as the “—” operator. Because B is a vector, ∇B is a Jacobian or 2nd order tensor, a matrix of 9 partial derivatives of the 3 principal components of B (Bx, By, and Bz) with respect to the 3 cardinal directions (x, y, and z).

Answer 30

The gradient of B is denoted ∇B, where ∇ is known as the “del” operator. Because B is a vector, ∇B is a Jacobian or 2nd order tensor, a matrix of 9 partial derivatives of the 3 principal components of B (Bx, By, and Bz) with respect to the 3 cardinal directions (x, y, and z).

Question 31

Fortunately, in MR imaging, we can often assume relatively simplified cases like the one above. For example, the x-, y- and z-gradients used for imaging are generally considered to create variations of B only with respect to the —- component. Thus as a good first order approximation, we need only consider the partial derivatives in the third column of the Jacobian, assuming all —-.

Answer 31

Fortunately, in MR imaging, we can often assume relatively simplified cases like the one above. For example, the x-, y- and z-gradients used for imaging are generally considered to create variations of B only with respect to the Bz component. Thus as a good first order approximation, we need only consider the partial derivatives in the third column of the Jacobian, assuming all derivatives in the first two columns are zero.

Question 32

Isn’t a gradient some type of coil?

Answer 32

Gradient Coils produce the gradient fields. gradient describes how a magnetic field changes in space. An ideal, uniform magnetic field in a vacuum contains no gradients

Question 33

gradient describes how a magnetic field changes in space. An ideal, uniform magnetic field in a vacuum contains no gradients. However, all real-life MR magnets have gradients due to field imperfections (—). Additionally, any object in the scanner (e.g., the human body) interacts with and distorts the field, an effect called —–.

Answer 33

gradient describes how a magnetic field changes in space. An ideal, uniform magnetic field in a vacuum contains no gradients. However, all real-life MR magnets have gradients due to field imperfections (inhomogeneities). Additionally, any object in the scanner (e.g., the human body) interacts with and distorts the field, an effect called susceptibility.

Question 34

gradient is used synonymously with gradient coils, a set of electromagnets embedded in the body of the MR magnet assembly. When an electrical current is passed through these coils, the main magnetic field is focally distorted in certain places, creating magnetic gradients of the first kind. Gradient coils alter the main magnetic field in a — manner in the x-, y-, and z-directions and are used to —- the MR signal

Answer 34

gradient is used synonymously with gradient coils, a set of electromagnets embedded in the body of the MR magnet assembly. When an electrical current is passed through these coils, the main magnetic field is focally distorted in certain places, creating magnetic gradients of the first kind. Gradient coils alter the main magnetic field in a predictable manner in the x-, y-, and z-directions and are used to spatially encode the MR signal

Question 35

What is Susceptibility?

Answer 35

Susceptibility is a measure of the extent to which a substance becomes magnetized when it is placed in an external magnetic field. A synonym for susceptibility is “magnetizability”.

Question 36

When matter interacts with the magnetic field, an — is created that either opposes or augments the external field.

Answer 36

When matter interacts with the magnetic field, an internal magnetization or polarization (J) is created that either opposes or augments the external field.

Question 37

What is diamagnetism?

Answer 37

If the polarization opposes the applied field, the effective field within the object is reduced, the lines are dispersed

Question 38

How to calculate magnetic susceptibility?

Answer 38

Magnitude of Internal Polarization(J) / Strength of an external magnetic field(Bo)

Question 39

What is the dimension of magnetic susceptibility?

Answer 39

Since it is the ratio of two magnetic fields, susceptibility is a dimensionless number.
Internal Magnetization / Polarization(J)

Question 40

What kinds of substances have negative susceptibilities?

Answer 40

Diamagnetic substances have negative susceptibilities (χ < 0); paramagnetic, superparamagnetic, and ferromagnetic substances have positive susceptibilities (χ > 0).

Question 41

What kinds of substances have positive susceptibilities?

Answer 41

Diamagnetic substances have negative susceptibilities (χ < 0); paramagnetic, superparamagnetic, and ferromagnetic substances have positive susceptibilities (χ > 0).

Question 42

What kinds of magnetism does biological tissues have?

Answer 42

Nearly all biological tissues are weakly diamagnetic.

Question 43

Nearly all biological tissues are weakly diamagnetic. However, some can be paramagnetic. Why?

Answer 43

Some tissues contain focal accumulations of metals such as iron, gadolinium, copper, or manganese that concentrate the magnetic field and are therefore paramagnetic.

Question 44

Nearly all biological tissues are weakly diamagnetic. However, some can be Superparamagnetic. Why?

Answer 44

A few tissues also contain chunky iron-based protein conglomerates (ferritin and hemosiderin) that are superparmagnetic

Question 45

Are there ferromagnetic substances in our body?

Answer 45

Other than trace amounts of magnetite that do not contribute to bulk susceptibility, there are no endogenous ferromagnetic substances in the human body.

Question 46

If there are no ferromagnetic susbtances in our body, why do we have to be careful in MRI?

Answer 46

many extrinsic metallic foreign bodies and surgical implants are ferromagnetic, and these are commonly encountered in MR imaging

Question 47

What causes susceptibility?

Answer 47

Susceptibility is caused by interactions of electrons and nuclei with the externally applied magnetic field.

Question 48

Please explain paramagnetism in terms of spins

Answer 48

When nuclear and electron spins in a material orient in the same direction as the magnetic field, their individual magnetic moments locally augment the field. This augmentation of the external field is called paramagnetism.

Question 49

What do you call magnetic effects opposing the applied field?

Answer 49

Diamagnetism.

Question 50

What determines the overall magnetic susceptibility of a material?

Answer 50

Electrons are much smaller than nuclei, so we can think of their spins being as being “concentrated” into smaller volumes. As a consequence of this spin/size discrepancy, electron-field interactions are thousands of times stronger than nuclear ones. Electrons, not nuclei, primarily determine overall magnetic susceptibility of a material.

Question 51

Expalin why orbitals are important in magnetism.

Answer 51

In addition to having spin, electrons “orbit” the nucleus and possess a second quantum property - orbital angular momentum (L). How electrons are distributed among orbitals is critical in determining magnetic susceptibility. Orbitals containing paired electrons contribute to diamagnetism; orbitals with unpaired electrons contribute to paramagnetism.

Question 52

In most substances there are several competing diamagnetic and paramagnetic effects whose net sum determines bulk susceptibility. The major mechanisms contributing to susceptibility are briefly described below. Give me a list.

Answer 52

Langevin (Larmor) diamagnetism.
Van Vleck paramagnetism.
Nuclear paramagnetism.
Curie Paramagnetism.
Mechanisms in simple metals (Pauli Paramagnetism and Landau Diamagnetism).
Exchange Coupling and Magnetic Domains.

Question 53

What causes Langevin(Larmor) Diamagnetism?

Answer 53

Results from Angular momentum (L) of electrons in filled orbitals. Through a quantum process similar to Lenz’ Law, the orbital motion of paired electrons generates an internal field that opposes the externally applied one. The effect is relatively weak, but because all molecules contain at least some filled orbitals, Langevin diamagnetism is present in all materials. Langevin diamagnetism dictates the overall susceptibility of a material unless it is overridden by a more powerful mechanism.
Water, most biological materials, nonmetallic atoms, stable salts, and covalently bonded molecules have no unpaired electrons, so their overall susceptibilities are dominated by Langevin diamagnetism. Their susceptibilities (χ) are weak and negative, typically with values close to −0.00001.

Question 54

Is Langevin Diamagnetism present in all materials?

Answer 54

Yes. Results from Angular momentum (L) of electrons in filled orbitals. Through a quantum process similar to Lenz’ Law, the orbital motion of paired electrons generates an internal field that opposes the externally applied one. The effect is relatively weak, but because all molecules contain at least some filled orbitals, Langevin diamagnetism is present in all materials. Langevin diamagnetism dictates the overall susceptibility of a material unless it is overridden by a more powerful mechanism.

Question 55

Which materials are dominated by Langevin Diamagnetism?

Answer 55

Water, most biological materials, nonmetallic atoms, stable salts, and covalently bonded molecules have no unpaired electrons, so their overall susceptibilities are dominated by Langevin diamagnetism. Their susceptibilities (χ) are weak and negative, typically with values close to −0.00001.

Question 56

Explain what Van Vleck Paramagnetism is.

Answer 56

Van Vleck paramagnetism. This can be considered a minor counter-effect to Langevin diamagnetism, becoming important only for a few polyatomic molecules and certain atoms/ions with shells exactly one electron short of being half-filled. In water, for example, Van Vleck paramagnetism is only 10% the strength of Langevin diamagnetism.

Question 57

What effect can be considered a minor counter effect to Langevin Diamagnetism?

Answer 57

Van Vleck Paramagnetism. Important only for a few polyatomic molecules and certain atoms or ions with shells exactly one electron short of being half filled. In water, Van Vleck paramagnetism is only 10% the strength of Langevin Diamagnetism.