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MRI > Physics Of MRI > Flashcards

Physics Of MRI Flashcards

1
Q

The first documented human MR image was obtained using what MHz RF/Tesla system?

A

15 MHz/0.3 Tesla

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2
Q

What is the Gyromagnetic Ratio used to calculate the Larmor Equation?

A

42.57(42.6) MHz per Tesla = The value of Hydrogen.

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3
Q

What is the definition of precessional frequency?

A

The rate at which the protons precess(spin) around the net mag. direction. AKA-resonant frequency. Equal to the product of the magnetic field and the gyromagnetic ratio. Termed the Larmor Frequency.

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4
Q

What is the Larmor frequency for Hydrogen (H) at 0.5 Tesla?

A

21.28 MHz

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5
Q

What is the resonance frequency in a 1.5/3 Tesla magnet?

A

63.86 MHz (1.5) / 127.71 MHz(3)

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6
Q

1 Newton per ampere-meter = (?)
1 Coulomb = (?)
1 Coulomb/second = (?)

A

1 Tesla (T)
6.24 X 10 18e-
1 ampere

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7
Q

What is the difference in chemical shift?

A

3.5 ppm (parts per million)

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8
Q

The frequency difference between fat and water at 1.5T and 3T respectively is?

A

147 Hz
1.5T - 220 Hz and 3T - 440 Hz

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9
Q

Magnetic Field inhomogeneity is expressed as what unit of measurement?

A

PPM (Parts-Per-Million)

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10
Q

10,000 Gauss is equal to how many Tesla?

A

1 Tesla
EX.= 30,00 Gauss system is a 3T MR machine.

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11
Q

Faraday’s Law of induction states-

A

That a changing magnetic field will induce voltage.

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12
Q

The most common systems used are what?

A

Superconducting magnet systems due to the high field strength and imaging capabilities. The magnetic field is parallel to the long axis of B0.

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13
Q

Resistive magnets -
Permanent magnets -

A

-Can be shut off the quickest in case of emergency
-Blocks of ferromagnetic plates used to generate a mag. field and are heavy.

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14
Q

What are the 3 types of magnetic states of matter?

A

Ferromagnetic which are iron or iron-like substances that can generate a relatively strong magnetic field. Usually Dangerous materials for MR. EX= Stainless Steels, iron, nickels, etc.
Paramagnetic which have a very weak magnetic field. Gadolinium is used a contrast material because it is paramagnetic.
Diamagnetic which has no magnetic field.

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15
Q

The force that an object can be attracted to the magnetic field is reliant upon what three things?

A
  1. The SPECFIC ferromagnetic nature/properties of the object
  2. the MASS of the object.
  3. The FIELD STRENGTH (TESLA) of the MR machine - (1.5/3)
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16
Q

What is magnetic susceptibility?

A

The extent to which a material or tissue becomes magnetized in an external mag. field.

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17
Q

What is magnetic permeability?

A

How well a material attracts the imaginary lines of the magnetic field.

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18
Q

Iron is permeable and sometimes placed inside the walls of an MR room for what reason?

A

It attracts the imaginary lines of the mag. field. The iron shielding does not allow the mag. field to extend out into the adjacent safe areas where patients are.

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19
Q

Protons that align themselves with static mag. field (B0) are in what energy state?

A

They are low energy protons are parallel to the external field.

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20
Q

Protons that are anti-parallel are what?

A

High energy, referred to as “spin down”. They are going against the net magnetization which gives them more energy.

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21
Q

Longitudinal magnetization is what?

A

AKA Net Magnetization Vector (NMV) and is the excess number of hydrogen protons aligned w/ the static mag. field. The Mag. Vector possesses attributes of BOTH magnitude, (which is strength) and direction. When the NMV is aligned w/ the mag. field direction, it is aligned along the LONGITUDINAL AXIS.

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22
Q

Free Induction Decay (FID) is produced from what?

A

An RF pulse

23
Q

RF Excitation is what?

A

Process of moving the net magnetization away from the Z axis.

24
Q

What is the Magnetic Moment?

A

The specific magnetic property or activity individual protons endure at a microscopic level in regards to normal or static state.

25
Q

What is the Larmor Equation?

A

insert picture

26
Q

What are the different frequencies of Hydrogen per MHz/T?

A

insert picture

27
Q

What are Longitudinal and Transverse directions?

A

Longitudinal- The net magnetization direction, B0 or the direction in which aligned protons are pointing in the presence of static magnetic filed.
Transverse- The opposing 90* perpendicular direction of the net mag. in which the static protons are pointing in the presence of a static magnetic field.
Transverse direction is the alignment direction protons point after interaction w/ a RF excitation pulse.

28
Q

What is Relaxation Time and the difference between T1 and T2 Relaxation Time?

A

Relaxation Time - the time in which the neg mag. of a proton returns back to longitudinal direction from the transverse direction. T1 is the amount of time it takes for two types of tissue (fat/water) to return back to longi. plane from trans. plan. T2 is the amount of time it takes for two types of tissue to become dephased.

29
Q

What are the amplitudes of T1 and T2 relaxation?

A

T1 = 63%
T2 = 37%

30
Q

What is Dephasing?

A

the progressive drop-off of coherence of precessing protons as they relax back to the longi. mag.

31
Q

The RF excitation pulse will ONLY flip net mag. from longi. to trans. direction when?

A

When the radio frequency pulse and precessional frequency have the same value. 42.6 MHz/T

32
Q

The Flip Angle is?

A

How fare the net mag. tilts from longi. to trans. direction.

33
Q

What two factors does the Flip Angle rely on?

A

The amplitude of RF pulse
Duration of RF pulse

34
Q

According to Lenz’s Law, what is another name for an induced magnetic field that opposes a current produced by electromagetic induction

A

Eddy Current

35
Q

What unit is used to represent magnetic field homogeneity?

A

ppm (parts per million)

36
Q

What type of MRI magnet remains powered indefinitely when cooled below 4 kelvin?

A

Superconductive electromagnets

37
Q

MRI superconductive electromagnets contain conductive coils composed of:

A

Niobium-Titanium (Nb-Ti)

38
Q

Which type of MRI magnet is capable of producing the strongest magnetic field?

A

Superconductive electromagnets

39
Q

The homogeneity of magnetic fields produced by superconductive electromagnets is typically:

A

< 1ppm

40
Q

In order to remove the magnetic field produced by MRI superconductive electromagnets, what must occur?

A

A QUENCH

41
Q

Which type of MRI magnet is the most expensive to purchase?

A

Superconductive electromagnet

42
Q

The cryogen typically used in modern day superconductive electromagnets is:

A

Liquid Helium

43
Q

The steel tank that functions as the house for both the magnetic coils and cryogens is known as the:

A

Cyrostat

44
Q

What is another term for cryostat?

A

Dewar

45
Q

What type of MRI magnet requires a constant power supply to maintain its magnetic field?

A

resistive electromagnets

46
Q

What is the source of power of a resistive electromagnet MRI system?

A

Electricity

47
Q

What type of MRI magnet is inexpensive to purchase, but very expensive to operate:

A

Resistive electormagnets

48
Q

A magnetic field strength of 10 kilogauss is equal to:

A

1 T(tesla) or 10,000 G(Gauss)

49
Q

The ferromagnetic substance that commonly composes MRI permanent magnets is:

A

ALNICO (Aluminum, Nickle, Cobalt)

50
Q

What term describes the strength of magnetic forces that act on substances exposed to external magnetic fields?

A

Magnetic Susceptibility

51
Q

Which of the following are classifications of magnetic susceptibility?

A

Dia-mag, para-mag, super-para-mag, ferromag, non-mag.

52
Q

Diamagnetic substances display no magnetic moment because they have paired orbital electrons. True or False

A

True

53
Q

All of the following are diamagnetic substances EXCEPT: (Lead, Water, Iron, Copper)

A

Iron

MRI (3 decks)

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