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MRI > Physics > Flashcards

Physics Flashcards

1
Q

What is the TR range for T1 images?

A

400-750 ms (1.5T)
600 - 850 ms (3T)

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2
Q

Describe Faradays Law

A

Moving magnetic field passing through a coil of wire will induce current to flow in that wire.

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3
Q

What does TR control?

A

T1

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4
Q

What does TE control?

A

T2

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5
Q

⬇️ TR and ⬇️ TE =

A

T1 weighted image

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6
Q

⬆️ TR and ⬇️ TE =

A

PD weighted image

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7
Q

⬆️ TR and ⬆️ TE =

A

T2 weighted image

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8
Q

Describe TE (time of echo) Also known as 2 Tau

A

Time of echo - from 90 degree RF to center of echo

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9
Q

Why do small coils have higher signal-to-noise ratio?

A

Smaller coils see less NOISE. Signal is NOT increased but SNR goes up with less noise.

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10
Q

What are the two types of shims?

A
  1. Hard/passive shim
  2. Electronic/active shim
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11
Q

Which shim is permanently placed during install process to shim the magnetic field?

A

The hard/passive shim

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12
Q

What shim uses specialized electromagnets to correct the magnetic field?

A

Electronic/active shim (must be checked with each p.m.)

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13
Q

Describe (FID)
Free Induction Decay
(aka T2*)

A

Dephasing of protons in the
x/y plane. This is the first step of tissue relaxing.

Protons are FREE from the INDUCING energy of the RF, so they DECAY.

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14
Q

The T1 time of a tissue is the time it takes for ____ of the NMV to return to Bo.

A

63%

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15
Q

The T2 time of a tissue is the time it takes for the NMV to decay down to _____ of its original value.

A

37%

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16
Q

What is a Faraday cage?

A

RF shielding: Copper sheets that line all MRI rooms to keep RF in and outside RF out.

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17
Q

What is the Larmor Frequency?

A

How fast the Hydrogen atoms wobble (“precess”). Wobble is always 42.57 MHz!

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18
Q

What is the Larmor Equation?

A

Wo = yBo

Wobble/frequency = (gyromagnetic ratio)(magnet)

Ex:
Wo= (42.57) (3T) = 127.71 MHz

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19
Q

What is the difference between T1 and T2 relaxation?

A

T2 is a decay process - protons spread out into X/Y.

T1 is a re-growth process - protons return to Bo.

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20
Q

What are 2 types of T2 relaxation?

A

Spin/Spin (True T2) - recover at the next 90°

T2* - recover at 180° or gradient reversal.

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21
Q

Name 2 types of resolution.

A
  1. Spatial resolution - ability to see Small Structures.
  2. Temporal resolution- imaging quickly over Time
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22
Q

What is T1 aka?

A

T1 Recovery or Spin Lattice

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23
Q

What is T2 aka?

A

Spin Spin Relaxation, True T2, spin spin interaction

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24
Q

Who created the electromagnet?

A

Serbian inventor, Nikola Tesla

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25
Q

Define gauss

A

The measurement of the magnetic field strength in the periphery of the magnet scan room.

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26
Q

What are the primary contributors to noise in an image?

A

The patient and system electronics

27
Q

What is the FDA SAR restriction?

A

Whole body limit of
4W/kg for 15 min of scanning per sequence.

28
Q

Define Magnetophosphenes

A

Visual light flashes induced by gradient field strength.

29
Q

What is the function of the gradient magnetic field?

A

To create image orientation and spatial resolution (detail)

30
Q

What are the 3 gradients used in MRI?

A

Slice select gradient
Frequency encoding gradient
Phase encoding gradient

31
Q

What is the function of the Slice Select gradient?

A

Determines:
Slice thickness
Scan plane
Orientation of pulse sequence

32
Q

What is the function of the Frequency/Readout Encoding Gradient?

A

Encodes the long axis of anatomy.
Does not contribute to scan time but is on during sampling of echo.

33
Q

What is the function of the Phase Encoding Gradient?

A

Shapes images detail
Scan time
Switches on/off to fill Kspace

34
Q

Formula: Spin Echo Time

A

TR x Phase x NEX = ms
(Convert to minutes: Divide ms by 60,000 then take seconds and multiply by 60)

35
Q

Two gradients applied at the same time during slice selection are used to _____.

A

Encode oblique slice planes

36
Q

An isotopic voxel refers to a ____ shaped voxel.

A

Cubic

37
Q

A gradient is defined as:

A

A small change in the magnetic field along a particular axis.

38
Q

Where are the collected echos stored prior to being processed into an image via Fourier Transform?

A

Raw data file on the computer.

Raw Data ➡️ Fourier ➡️ Kspace

39
Q

What effect does a narrow receiving bandwidth have on susceptibility (metal) imagine?

A

Increases susceptibility artifact. Never use narrow bandwidth on metal implants.

40
Q

Define Ernst angle

A

The optimal flip angle that yields the maximum signal for a particular spin in the least amount of time.

41
Q

As flip angle increases, what happens to SNR?

A

SNR increases to a point called the Ernst angle.

42
Q

The general public is limited to magnetic field of ____ gauss.

A

5

43
Q

Heart valves are considered acceptable to scan at ____.

A

1.5T

44
Q

Sedated patients must always be monitored with ______

A

A pulse oximeter

45
Q

Normal resting heart rate for adults:

A

60 to 90 beats per min

46
Q

Tachycardia

A

Heart rate exceeding 100 beats/min

47
Q

Bradycardia

A

Heart rate below 60 beats/min

48
Q

Normal pulse rate:

A

60-100 beats/min

49
Q

Pathogen

A

Any disease producing agent, esp a virus, bacterium or microorganism.

50
Q

Etiology

A

The origin of a disease

51
Q

Enteral

A

Oral, sublingual or rectal delivery

52
Q

Parenteral

A

Any route other than mouth (PO) or rectal

53
Q

Nosocomial infection

A

Infection acquired during hospital stay or receiving medical treatment

54
Q

Fomite

A

Object that carries microorganisms between people via indirect contact

55
Q

Vector borne infections

A

Microorganisms transferred via insect. Ex malaria

56
Q

The source of infection where pathogens thrive in numbers sufficient to cause a threat is known as ______.

A

Reservoir

57
Q

What is the TE range for T2?

A

70-120 ms

58
Q

What is TR range for T2?

A

2000-6000 ms

59
Q

What is TR range for PD?

A

1500-3000 ms

60
Q

What is TE range for PD?

A

10-30 ms (same as T1!)

61
Q

FOV % Matrix =

A

FOV % Matrix = pixel size

62
Q

Where is the storage location of MR signal data?

A

K-space

63
Q

Where are high amplitude signals stored in Kspace?

A

Center

64
Q

Describe Half Fourier / Zero Fill

A

Kspace: Acquiring more than half of PHASE dimension kspace samples, then interpolating the data with 0s for the other half.

MRI (2 decks)

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