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MRI (Taryn) > MRI Final Review > Flashcards

MRI Final Review Flashcards

1
Q
  1. What are the two theories used to describe the basic principles of MRI?
A

Classical Theory and Quantum Theory.
* The north/south axis of each nucleus is represented by a magnetic moment and is used in the principles of classic theory of MRI.
*Quantum Theory describes discrete quantities of energy - spin up and spin down.
(the nucleus of Hydrogen has one positively charged proton and spins or moves. It has a North and South pole of equal strength.)

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2
Q
  1. What is the Lamor equation and describe each parameter of this equation?
A

It is used to measure the precessional frequency using the strength of the magnetic field.

  • The Lamor equation governs the precessional frequency, it states: ῳ = Bo x γ
  • ῳ = Precessional frequency
  • Bo = Magnetic field strength of the magnet
  • γ = Gyro-magnetic ratio (defined in another card)
  • Precessional frequency is therefore often called the Larmor frequency
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3
Q

When using MRI, cables from RF coils and ECG leads should be:

A

Placed straight, without touching the patient or the magnetic bore, through the imager.

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4
Q

Three types of magnetism?

A

Diamagnatism -
* have no net magnetic moment without external magnetic field. Therefore slightly repelled by magnetic field.
(“dead” not really affected by magnet)
Paramagnetism -
*small magnetic moment due to unpaired electrons.
*Magnetic moments random and cancel each other.
* Align with the direction of the external magnet.
Ferromagnetism -
* show strong attraction and align to external magnet.
* Can become dangerous projectiles
* retain their magnetism even when removed from magnetic field.
* South to North

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5
Q

What does this sign mean:

yellow triangle with MR

A

MR conditional:
An item that has been demonstrated to pose no known hazards in a specified MRI environment with specified conditions of use.

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6
Q

Family members and additional personnel accompanying the patient into the scan room:

A

Should be screened as if they are going through MRI procedure themselves.

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7
Q

What would you do to reduce peristalses in an abdominal MRI?

A

Use antispasmodic agents eg. Buscopan tablets, IV buscopan, IV glugagen or intramuscular can be given before the scan.

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8
Q

In preparation of the MRI exam patients are encouraged to?

A

Either arrive at MRI centre wearing a tracksuit without any metal zips or studs or change into a hospital gown.

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9
Q

Aliasing (wrap around) occurs because the tissue outside the FOV is:

A

Undersampled.

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10
Q

In a spin echo sequence the time between the 90 degree pulse and the peak of the signal is the:

A

TE

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11
Q

MAGNETIC FIELD SHIELDING

can be achieved either actively or passively. In order to achieve passive shielding, the room can be lined with:

A

Steel

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12
Q

Complete this sentence:

Signal or voltage is only induced in the receiver coil if there is (A) magnetism in the (B) plane that is (C) phase.

A

Signal or voltage is only induced in the receiver coil if there is
(A - coherent) magnetism in the
(B - transverse) plane that is
(C - in) phase.

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13
Q

The MRI signal is changed from the FID into an understandable format by a series of mathematical equations known as?

A

Fourier transform

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14
Q

Why is the use of pre-saturation bands important to use when imaging the C-spine?

A

To mask motion from: swallowing and pulsation of the carotid arteries. Phase mismapping

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15
Q

What is the Magic angle artefact?

A

It is an artefact seen in certain tendons as high signal intensity that may mimic pathology and is caused when certain structures with tightly bound fibres lie at an angle of 55 degrees to the main magnetic field altering it’s signal intensity.

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16
Q

To eliminate chemical shift artefact use:

A

Chemical saturation

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17
Q

Name two categories that diagnostic imaging contrast usually depends on:

A 
Intrinsic and Extrinsic parametres.
Intrinsic:
*T1 recovery
*T2 decay
*Proton density
*flow
* ADC (apparent dffusion coefficient)
Extrinsic (can change):
*TR repetition time
*TE echo time
*FA flip angle
*TI Inversion time
Echo train length/turbo factor
*b-value
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18
Q

It is the ability to distinguish between two points as separate and distinct and is controlled by voxel size:

A

Spacial resolution.

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19
Q

Which sequence is often used in multiple sclerosis imaging to demonstrate plaques:

A

FLAIR

20
Q

The liquid cryogen is commonly used to maintain the magnet at superconducting temperatures is:

A

Helium

21
Q

What is the artefact called that is seen on the outer edges of an abdominal MRI (looks like psychedelic flowers):

A

Moires artefact

22
Q

What is phase mismapping artefact also known as?

A

Ghosting or blurring artefact.

23
Q

What are three types of magnets?

A

*Permanent magnets: ferromagnetic, no stray force lines, South to North, doesn’t require power source so low operating costs.
*Electromagnets:
Coil with charge through, two types (solenoid and resistive).
Resistive - disadvantage = stray magnetic field lines. Can turn off, 0.3T max due to expense. Runs Head to Foot.
*Super conducting (what we use) . Uses Helium which reduces temperature = less resistance, therefore less power needed.
Runs Head to Foot.

24
Q

Things that effect resistance of a magnet:

A

Temperature.
Cross sectional area.
Length of the wire.
Type of material used.

25
Q

What does FID stand for?

A

Free Induction Decay:

  • As T2 decay occurs, there is a decrease in the signal produced and this is the FID (induction of reduced signal).
  • When we switch off the RF pulse the NMV is again influenced by Bo and tries to realign with it. In order to do so the hydrogen nuclei must lose the energy given to them by the RF pulse.
  • This process in which hydrogen loses this energy is call relaxation.
  • Some of the high-energy nuclei returns to the low-energy population and align their magnetic moments to the magnetic field.
  • The magnetization in the transverse plane decreases as well as the voltage induced in the receiver coil.
  • this induction of reduced signal is called FID.
26
Q

Name two advantages of a Spin Echo Sequence.

A

Good image quality, versatile, true T2 weighting, available on all systems, gold standard for image contrast and weighting.

27
Q

The term used to describe RF absorption is:

A

Specific Absorption Rate

28
Q

Explain the difference between cross talk and cross excitation?

A
  • Cross excitation: energy given to nuclei in adjacent slices by the RF pulse.
  • Cross talk: energy given to adjacent slices due to spin lattice relaxation. There is no remedy for cross-talk as it is caused by the natural dissipation of energy by the nuclei.
29
Q

RF SHIELDING:

can be achieved by lining the MRI room with:

A

Copper

30
Q

It is good practice for all MRI patients that undergo MRI to be monitored:

A

Visually and/or verbally.

31
Q

The inversion recovery spin echo sequence begins with a:

A

180 degree RF pulse.

32
Q

Which two cranial nerves are seen when imaging the IAMs?

A

Vestibulocochlear and facial nerve (VII and VIII)

33
Q

A quench is used to:

A

Rapidly removed the superconductivity and the magnetic field.

34
Q

Name any 2 factors T1 and T2 relaxations depends on?

A
  • The inherent energy of the tissue.
  • How closely packed the molecules are to another.
  • How well the molecules tumbling rate matches the Lamor Frequency.
35
Q

Hydrogen nuclei have a magnetic moment because they possess a property called?

A

Spin

36
Q

T2 relaxation time is defined as when:

A

63% of the transverse magnetization has decayed.

37
Q

What does y stand for in the Lamor frequency?

A

Y = gyro-magnetic ratio: is a constant and expresses the relationship between the angular momentum and the magnetic moment of each MR active nucleus at 1T.
The unit is MHz/T
Y of hydrogen = 42.57/T

38
Q

What are a pixel and a voxel?

A

Pixel: picture element
Voxel: Volume element

39
Q

The following items are usually allowed to enter the MRI scan room in high magnetic systems:

A

Copper tools

40
Q

The purpose of shimming the magnet is to:

A

Make the B0 field as homogenous as possible.

41
Q

In order to evaluate the range of motion in TMJs the MRI studies using surface coils, doing the MRI images acquired in the Sag plane are:

A

The patient’s mouth open and closed.

42
Q

SAR:

Factors important when monitoring SAR:

A

Specific Absorption Rate.
Factors:
weight of the patient and the pulse sequence.

43
Q

Recovery =

A

T1 Recovery - when the amount of magnetization in the longitudinal plane gradually increases.

44
Q

Decay =

A

T2 Decay - when (at the same time as recovery but independently) the amount of magnetization in the transverse plane gradually decreases.

45
Q

TR =

A

time to repetition.

Time between pulses to show tissue types separately.

46
Q

TE =

A

Echo time.
Time of the pulse to maximum signal before it begins decaying.
How long it stays in the transverse plane.

MRI (Taryn) (5 decks)

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