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MRI Flashcards

1
Q

What is Spin Echo

A
  1. 90 degree pulse applied
  2. 180 degree pulse applied at TE/2
  3. Echo taken at TE

180 degree pulse is applied to remove magnetic field inhomogeneities

In Spin Echo - T1 weighted, T2 weighted and Proton density image can all be produced (by altering TR adn TE)

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2
Q

What is an ECHO train?

A
  • Type of spin echo
  • 90 degree followed by successive 180 degree each with an increasing TE
  • There is a limit to the echo train length (as signal is decaying via T2*)
  • PD Image initially produced on first TE followed by T2 weighted images which get stronger
    *
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3
Q

What is a turbo spin echo sequence?

A
  • Spin echo sequence
  • Echo train is first created (90 followed by succesive 180 degree pulses)

​After each echo a phase encoding gradient is applied (reduces time massively)

  • Better spatial resolution
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4
Q

Which sequence is quicker?

A

Gradient echo is quicker than spin echo

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5
Q

Why does Spin Echo take a long time?

A
  • TR is the most time consuming part of the pulse sequences
  • A 180 degree pulse also takes time to deliver
  • If we shorten TR
    • there wont be sufficient time between 90 degree pulses
    • there will be very little longitudinal magnetisation and as a result very little transverse magnetisation

The solution is to:

  1. Find a way of refocusing the dephasing protons without a slow 180 degree pulse
  2. Find a way to preserve longitudinal magnetisation before flipping (therefore maintining signal)
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6
Q

What happens in Gradient echo?

A

Initially the same steps as a spin echo apply

  1. RF pulse (of any flip angle compared to 90 degrees in spin)
  2. Slice select gradient
  3. Phase encoding gradient
  4. Frequency encoding gradient

A magnetic field gradient is applied (instead of a 180 degree pulse)

  1. After the frequency encoding gradient
    1. Negative magetic field gradient applied causing protons to dephase and quicker loss of Mxy signal
    2. Positive magetic field gradient applied causing partial rephasing creating a signal
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7
Q

Gradient Echo - flip angle used?

A

Can use any flip angle

Using a smaller flip angle means quicker recovery and another RF pulse can then be applied generating 2 echoes

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8
Q

What type of images do Gradient echoes produce?

A

T2*

Cannot produce a true T2 weighted image as no 180 degree pulse to discount field inhomogeneities

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9
Q

In Gradient echo how do we overcome small Mxy signal caused by short TR?

A

Use a small flip angle (10-35 degrees)

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10
Q

Why are gradient echo sequences faster?

3 resons

A
  1. Shorter TR
  2. Small flip angles
  3. No time consming 180 degree pulse
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11
Q

What does a large flip angle in gradient echo produce?

A

T1 weighted images

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12
Q

Advantages and disadvantages of permanent magnets?

A

Max strength 0.2 - 0.3

Usually made from rare earth metals

  • neodymium-iron-boron
  • samarium-cobalt

Fringe field is fully contained within the permanent magnets

Advantages:

  • always on
  • dont require an energy input

Disadvantages

  • thermal instability
  • limited magnetic field strength
  • very heavy
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13
Q

What are resistive magnets?

A

Otherwise known as electromagnets where electric current is passed through a wire

Max strength is 0.6T field strength

Made out of copper or aluminium wrapped around an iron core

Disadvantages

  • Are only magnetic when a current is flowing
  • generates alot of heat so impracticale at high field strengths
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14
Q

What are superconducting magnets?

A

Most common magnet used in MRI scanners

Electric current is passed through a superconducting material Has zero electrical resistance

Coils of a supermagnet are made from Niobium titanium inside a copper matrix

Once an electrical current is sent through the material, it flows permanently, creating a magnetic field

Considerations

  • need to be cooled to 4 degrees kelvin (-269 degrees celsius) - liquid helium or nitrogen used to do this
  • liquid helium or nitrogen otherwise known as cryogens
  • Magentic field only turned off by quenching
    • superconducting material warms up, electrical resistance increases and magnetic field disappears

ADVANTAGES

  • High magnetic field strengths (30,000 times higher than earths field)
  • Excellent magnetic field homogeneity

very expensive however

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15
Q

What coils are used in MRI?

3 types

A
  1. Gradient coils
  2. RF coils
  3. Shim coils
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16
Q

RF Coils

A
  • Send and receive the RF pulse to excite the protons (cannot do both simultaneously however)
  • Should be as close to the region of interest as possible
  • Size of coils do not affect spatial resolution ​
  • Produce magentic field at right angles to main field
  1. Body coil
    • usually a permanent part of the scanner
    • can transmit RF pulse and receive MR signal
  2. Head coil
    • transmits and receives
    • used for brain scanning
  3. Surface coils
    • only receive
    • improve
      • ​SNR
      • Resolution
      • Smaller FOV
    • ​are not used in ‘Whole Body Imaging’
    • result in LESS UNIFORMITY

Can cause heat damage !!

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17
Q

Gradient Coils

A

Gradient coils change field strength in a LINEAR fashion

Three sets of gradient coils

  1. Slice selecting gradient coil
  2. Frequency encoding gradient coil
  3. Phase encoding gradient coils

MRI scanner noise is due to switching on/off of GRADIENT COILS

Can induce currents in patients!!

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18
Q

Shim Coils

A

Used to make magnetic field as homogenous as possible

Has nothing to do with fringe fields

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19
Q

Gadolinium

A
  • Shortens T1 and T2 (time taken for recovery of both)
  • Gadolinium cheleated with DTPA as it is toxic on its own
  • Does not pass blood brain barrier

Post contrast images obtained are T1 weighted (shortening of T1 causes higher signal)

TR can also be reduced when using contrast (resulting in faster scan times)

Can result in nephrogenic systemic fibrosis

  • starts as fibrosis of the skin and connective tissue of extremeties
  • no known cases in people with normal kidney function
  • most cases assocaited with Gadiodiamide

Gad can be given even in severe renal impairment but with consideration

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20
Q

MR Angiography

What do spin echo sequences demonstrate?

A
  1. Flow void
  • seen in fast flowing blood​
  • protons in blood get flipped by 90 degrees
  • since this blood then flows onwards, it is replaced by blood that is not in transverse plane. A 90 degree pulse hits the new blood but no signal as BOTH 90 AND 180 DEGREE PULSES REQUIRED TO GIVE A SIGNAL IN SPIN ECHO
  1. Flow enhancement (seen in both spin echo and gradient echo)
  • seen in veins (slow flowing blood)
  • protons in SLICE get flipped to 90 degrees
  • this slice slowly gains longitudinal magnetisation
  • unexcited blood from outside slice moves in and posseses full longitudunal magnetisation
  • after further RF pulse there is high transverse signal in blood vs tissue surrounding it

Spin echo is better at assessing vessel wall

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21
Q

Gradient Echo in MR Angio (Time of flight)

A

Gradient echo used

  • Tissue band subjected to multiple TRs which saturate tissue giving no signal (small flip angle followed by gradient echo early on leaving very low longitudinal magnetisation)
  • Unsaturated blood flowing inwards then generates strong signal
  • Slow flowing blood near to the slice can become saturated leading to loss of signal

Pulsatile flow seen as ghosting artefacts

BLOOD AND CSF BRIGHT ON GRADIENT ECHO

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22
Q

Diffusion weighed imaging

A
  • T2 weighted imaging
  • Uses spin echo planar imaging

Two large gradient pulses are applied either side of the 180 degree pulse in a spin echo planar imaging sequence

  • for moving protons (normal) - a gradient will cause a reduction in signal
  • for stationery protons (in restricted diffusion) - a gradient will create a high signal (with low ADC signal)

HIGH SIGNAL FOR TISSUES THAT ARE DIFFUSION RESTRICTING

ADC

  • Measures the degree of diffusion weighting
    • high B value = heavily diffusion weighted

Removes the effect of T2

  • Restricted diffusion = low ADC signal
  • Normal Diffusion = high ADC
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23
Q

What is Aliasing?

A

Causes wrap around artefact (2 noses sign)

Due to too small FOV

Does not occur if the whole body part is in FOV

Most commonly in phase encoding gradient (but can occur in both)

Reduced by:

  • increasing FOV
  • using surface coil
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24
Q

Motion Artefact

A

Due to patient movement

Occurs in phase encoding direction

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25
Q

Chemical Shift Artefact

A

Larmour frequency for hydrogen within fat and water is slightly different (results in Chemical Shift artefact)

  • Chemical shift artefact produces a black or white band at the fat tissue interface. (Signal void usually).
  • Occurs in the FREQUENCY ENCODING direction and is a result of spatial misregistration of water and fat molecules
  • Chemical shift is more pronounced at higher magnetic field strength. Chemical shift artefact can be combated using STIR sequences as this suppresses fat.

Reduced by:

  • HIGHER BANDWIDTH FOR PIXEL AND WIDER RECEIVER BANDWIDTH
  • using steeper phase encoding gradient and reducing the field of view
  • Chemical shift is exploited in Dixon sequences where fat can be nulled
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26
Q

Truncation Artefact

Known as:

  • Ringing artefact
  • Gibbs artefact
A

Parallel stripes at high contrast interfaces

Seen in:

  • CSF and spinal cord
  • brain edges

Due to undersampling in phase encoding direction

  • corrected by increasing number of phase encodings
  • reducing FOV
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27
Q

MRI Safety - Gradient Coil FIelds

A

Electrical fields produced perpendicular to gradient fields. Cause Eddy currents

​

  • Nerve stimulation
  • Arrythmias
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28
Q

Pregnancy and MRI

When should it be avoided?

A

Should be avoided during first trimester of pregnancy

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29
Q

What noise do MRI scanners produce in Db?

A

Most dont exceed 120Db

Hearing protection required if >90Db

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30
Q

Radiofrequency Fields

What do they cause?

A

Burns due to microwave heating

Specific absorption ratio

  • this is the RF energy deposited per unit mass of tissue
  • unit is W/kg
  • SAR is greater for larger body parts and for 180 degree pulses
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31
Q

Relaxation times T1 and T2

A

T1 is always longer than T2

T2 relaxation time is always longer than T2*

32
Q

T1 times

TE

TR

A

Short TE (15ms)

Short TR (300-800ms)

33
Q

T2 times

TE

TR

A

Long TE (90-140ms)

Long TR (1000-2000ms)

34
Q

TE cannot be longer than TR

A
35
Q

Do protons that are subject to a strong magnetic field precess in phase?

A

No. They align with magentic field but precess in different phases.

Phase syncronisation happens after 90 degee pulse

36
Q

Can free induction decay occur at any flip angle?

A

Yes all apart from a 180 degree pulse.

Any angle that results in tranverse magnetisation can result in FID.

37
Q

Does external magnetic field strength alter T1 time?

A

Yes…..it lengthens the T1 relaxation time

38
Q

What signal does fat return on turbo spin echo sequences?

A

Hyperintense signal

Results from multiple refocusing pulses that suppress J coupling and lengthen T2 giving larger signal

39
Q

Cortical bone T1 and T2 times?

A

Long T1 and very short T2

Appears BLACK

40
Q

Substances with low T2 signal?

A
  1. Melanin
  2. Calcification
  3. Fibrous tissue
  4. High protein content
  5. Flow void
41
Q

In what dimension is phase encoding applied?

A

Usually applied along smallest dimension

e.g. laterally in brain imaging or AP in abdominal imaging

This allows shorter phase encoding steps and cuts down on scan time

42
Q

K Space

A

Centre = contrast and low spatial frequencies

Periphery = spatial resolution and high spatial frequencies

43
Q

Can half of K space be acquired to produce an image?

A

Yes

Acquiring half of K space is sufficient to obtain an image.

  • known as zero filling
  • may result in a blurry image
  • ‘mirroring’ can also take place where acquired half can be mirrored to missing half. Known as half or fractional NEX
44
Q

Single Shot FSE (Fast/turbo spin echo)

Also known as Echo planar imaging

HASTE (Half fourier single short turbo spin echo)

A

Type of turbo spin echo where whole of K space is filled with one 90 degree pulse and echo train of 180 degree pulses.

90 degree followed by repeated 180 degree pulses and phase encoding gradients (reversal gradients applied in frequency endoing direction in between phase encoding gradient and next 180 degree pulse)

Produces highly T2 weighted images. (initial images are PD then as echo train lengthens, images at the end are more T2 weighted).

Used in MRCP

Echo planar imaging can produce:

  • T1
  • T2
  • T2*

Only half of K space is filled in HASTE (much quicker than turbo spin echo)

45
Q

What is Ernst Angle?

A

The excitation flip angle that provides maximal MR signal for T1

(NOT CONTRAST BETWEEN TISSUES)

46
Q

What is Spoiling?

A

Is a type of Gradient echo sequence

Is an Incoherent sequence

Spoiling elimintates residual transvere magnetisation (T2 signal)

Can be achieved by random variations in the phases of excitation pulse

47
Q

What do Coherent or Rewound gradient echo sequences do?

A

They preserve signal by preserving residual magnetisation

  • use very short TR
  • water is very bright on these sequences
  • used for
    • ​myelography
    • angiography

Are very sensitive to motion and flow

  • this destroys the signal
48
Q

What is opposed phase imaging?

(Chemical shift imaging)

A

Are paired gradient echo sequences:

  • in phase
  • out of phase to differntiate water and fat e.g dixon sequences

Main purpose is to identify microscopic fat content of tissues by showing signal drop out on out of phase imaging

  • It is a gradient echo sequence
  • Difference in Larmour freq between fat and water is 220Hz at 1.5T

Uses different TE but same TR

  • TE should be at point where fat and water have a 180 degree phase difference

India Ink appearance in out of phase imaging

Using a higher magnetic field strength results in increased suppression of fat in out of phase imaging

49
Q

FLAIR

A

Suppression of fluid. Fat will appear bright

Combines inversion with a T2 weighted spin echo sequence

50
Q

Inversion Recovery

A

Not susceptible to magnetic field inhomogenetities as they work on T1 relaxation times

Lowers overall SNR in tissues

51
Q

Are STIR sequences sensitive??

A

NO

Any tissue with a T1 close to fat will also be nulled e.g.

  • haemorrhage
  • protein rich fluid
  • gad
52
Q

What is SNR in MRI?

A

SNR is inversely proportional to the square root of bandwidth

SNR is proportional to the SQUARE ROOT of the number of sequence repetitions

Increasing bandwidth increases noise

Decreasing bandwidth increases SNR

  • Spin echo returns higher signal than gradient echo

Using smaller coils improves SNR but NOT spatial resolution

53
Q

What are superparamagnetic iron oxide particles used for?

A

Used in hepatic imaging to diagnose hepatic tumours

54
Q

Perfusion Brain Imaging

A

Based on the T2* shortening upon first passage of an exogenous contrast agent

  • Performed using T2* imaging
55
Q

What are phased array coils?

A

Are smaller coils that can be used as surface coils

  • Allow for larger FOV and increased SNR
56
Q

Can patients with intracranial clips be images in MRI?

A

Yes if they are titanium.

Metallic heart valves are also safe

57
Q

MRI Fields Controlled Areas

A

MR Controlled area is where stray fields are greater than 0.5mT

Inner MR controlled area 3mT

58
Q

T1 and T2

FAT

WATER

A

Fat

T1: 250ms

T2: 80ms

CSF

T1: 2000ms

T2: 150ms

Water

T1: 3000

T2: 3000

59
Q

What controls T1 weighting in spin and gradient echo?

A

TR

60
Q

Gradient echo

What flip angle used in T1?

What flip angle used in PD weighting?

A

T1 - large flip angle

PD - small flip angle

61
Q

What determines slice thickness?

A

Transmit bandwidth

  • Increasing the transmit bandwidth increases the slice thickness

Increased slice thickness = reduced resolution

62
Q

When is Slice selection gradient switched on?

A

During initial RF pulse and during 180 degree pulse (in spin echo)

63
Q

How does phase encoding gradient affect spatial resolution?

A

Steeper phase encoding gradients results in increased spatial resolution

64
Q

Does coil size affect spatial resolution?

A

NO

Dedicated surface coils increase SNR though

65
Q

Summary of gradients

A
66
Q

Effect of TE and TR on SNR

A

Increasing TE results in more LOSS of transverse magnetisation = lower SNR

Increasing TR results in more RECOVERY of longitudinal magnetisation = higher SNR

67
Q

What determines scan time?

A

TR x number of phase encoding steps/NEX

68
Q

FOV

A

The steeper the phase/freq encoding gradient = smaller FOV

Higher gradient strengths are needed for smaller FOV

69
Q

Can gadolinium alone be excreted by the body?

Manganese

A

No

Gad has 7 unpaired electrons and is paramagnetic

High T1 signal

70
Q

Iron Oxide contrast

A

Normal tissues are low signal

Excreted by liver

Liver lesions will appear as high signal on T2

Known as a T2 enhancing agent

71
Q

Faraday cage

A

Protects from external RF fields (not fringe field)

Surround the scanner to protect from external electromagnetic (RF) fields

Usually copper shielding

72
Q

What nuclei can be used in MRI?

A

Only nuclei with an odd number of PROTONS or NEUTRONS

73
Q

Ferrous artefacts cause….

A

Signal drop out

Causes rapid dephasing of Mxy signal

Affects Gradient echo more than spin echo

74
Q

Can T1 and T2 images be produced simultaneously?

A

NO

They would cancel eachother out

75
Q

What causes Noise in MR?

A

Due to induction of the RF coils

76
Q

What is magnetic susceptibilty?

A

Where tissues magnetise to different degrees and so have different precessional frequency

Most noticed between boundaries with a large difference

e.g metal implants

Physics (10 decks)

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