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THRALL - Thom > 5. Principles of CT and MRI > Flashcards

5. Principles of CT and MRI Flashcards

1
Q

What are the 2 primary advantages of CT / MRI over other modalities? 1 disadvantage?

A

Adv:

1) Tomographic nature
2) Increased contrast resolution (e.g.smaller differences in x-ray attenuation detectable in CT due to reduced scatter and more sensitive detectors -> fluid from ST!; MR even greater contrast resolution -> use of combined sequences)

Dis:

1) POORER SPATIAL RESOLUTION

=>CT 0.3mm and MR 1mm respectively

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2
Q

What is generally considered to be the limiting factor to resolution in CT / MR?

A
  • SLICE THICKNESS -> tends to be largest voxel dimension
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3
Q

What are the advantages of helical scanning? When / why would you use sequential scanning?

A

=> produces data volume rather than single slices . Interpolation of data to reonstruct

  • Reduced motion artefact
  • Increased speed
  • Less prone to step artefact
  • May use sequential when e.g. scanning area with limited motion such as head. Less strain on tube
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4
Q

What are CT detectors made from?

A
  • Ceramic solid state detectors

=> scintillation crystal -> reacts with Xray, and through amplifaction emits light, converted to digital

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5
Q

What does multi-detector refer to?

A
  • DEPTH OF ACQUISITION -> multiple detectors in plane of gantry allow multiple contiguous slices to be acquired at a time

=> QUICKER ACQUISITION

e.g. 2.5mm thick slices of 30mm thorax: 64 slice <2 secs; 1 slice 60-120 secs

reduced interscan delays

=> MORE EFFICIENT USE OF RADIATION

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6
Q

What does temporal resolution refer to?

A
  • Resolution improvements due to minimising motion artefacts and tissue misregistration. See with MDR scanning and helical aquisition
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7
Q

What is Linear Attenuation Coefficient?

A

Attenuation mainly depends on ELECTRON DENSITY OF A MEDIUM

LAC: Measures fraction of radiation removed in passing through a given thickness of a specific material

=> absorption probability described by LAC (µ)

Nt = N0 e-µx

Where N0 = Number of initial photons at tube exit, Nt = number of transmitted electrons meaured by detector, e = base of natural log (2.718), x = thickness of absorber, and µ is LAC present along xray path

Rearranged, LAC can be derived as N values measured in system, and other values are known.

THIS CALCULATION is performed multiple times by the machine! approx 800 transmission calculations, 1000 diffferent projection angles per image! => 800,000 measurements total

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8
Q

What is filtered backprojection?

A
  • Once µ is calculated for a single ray -> mathmateical process (think sudoku) where voxel values are assigned

Values are transformed into HU / CT numbers

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9
Q

What is the formula for calculation of HU for different tissues?

A

HU of tissue = [(µtissue - µw) / µw] x1000

Where pure water µw = 0

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10
Q

In basic terms, how to reconstruction filters affect image?

A
  • Can determine level of edge reinforcement in raw data:

Low pass / ST: emphasis ST, smoother but more blurry e.g. brain

Bone: more spatial resolution -> sharper but grainier (noisy)

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11
Q

What is the typical range of HU measureable by CT?

A
  • 1000 - +3095
  • 4096 shades of grey (12 Bits)
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12
Q

What do window width and window level refer to?

A
  • ww = number of shades of grey displayed (max to min)
  • wl = HU at centre of window

=> adjusted for specific tissues

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13
Q

List 3 uses of CT contrast

A
  • Evaluation of perfusion characterisitcs of tissues
  • Angiography -> assessment of vascular phases
  • Excretory urography (superior to XR) -> estimate GFR and assess collecting system
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14
Q

What are the advantages / disadvantages of cone beam CT?

A

Cone shaped beam -> reduced patient radiation

Adv:

  • High spatial resolution

Dis:

  • Increased scatter
  • Lower contrast resolution (Lower contrast to noise ratio)
  • Lower temporal resolution of cesium iodide detectors -> inc motion artefact
  • Longer reconstruction times
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15
Q

What is Faraday’s law of induction?

A
  • When a current is fun through a coiled wire, a magnetic field is produced in a direction that is perpendicular to the flow of current, and that is proportional in strength to the current.
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16
Q

What is B0 and what is net magnetization? What features contribute to / affect net mag?

A
  • B0 refers to the axis of the externally administered magnetic field
  • Net mag: Spins align with B0, SLIGHTLY more parallel with field > antiparallel with field
  • > Net mag increass proportionally to STRENGTH OF FIELD
  • > Dependent on PROTON DENSITY of tissues
17
Q

What is precession? And what is larmour freq?

A
  • Precession describes the wobbling behviour of spins under the influence of B0
  • Motion controlled by RF pulse
  • Larmour (/resonance) freq = frequency of precession of protons in B0, and is proportional to magnet field strength

w0 = gammaB0

Where gamma = constant, or gyromagnetic ratio, for each type of nuclei

Hydrogen protons -> GM ratio = 42.6MHz/Tesla

18
Q

Define flip angle, resonance, excitation and relaxation

A
  • RF pulse application to protons at larmour freq -> energey transfer (RESONANCE) causes energetic EXCITATION of protons, with more protons adopting antiparallel orientation

=> Results in net macroscopic magnetization shift away from z axis, towards xy axis = angle of change in direction of mag = FLIP ANGLE

  • Once RF removed, return of spins to normal equlibrium = RELAXATION
19
Q

Define the processes of T1 and T2 relaxation

A
  • Realignment of spins with B0 on end of RF pulse -> Relaxation in longitudinal axia, with loss of energy into lattice = T1 (longitudinal)
  • Spins precess in coherent fashion as approaching XY axis. Loss of phase coherence after RF removes releases energy into lattice = T2 (transverse)

TWO DISTINCT PROCESSES OCCURING SIMULTANEOUSLY

=> tissue specific

20
Q

What is T2* relaxation?

A
  • Occurs due to inhomogeneity in the magnetic field e.g. metal, blood, calcium, air, or local variation in magnet strength
  • In stead of normal dephasing of spins once RF removed, very rapid dephasing occurs -> T2* relaxation
  • Tissues dont relax at specific time associated with tissue type, but much faster
21
Q

Define TR and TE

A

SPIN ECHO SEQUENCES - designed to control for T2* relaxation

  • TE: time of echo

Time from iniital 90deg pulse to echo. Echo results after second (usually 180deg) rephasing pulse -> this allows for ‘slow’ and ‘fast’ processing spins to become in synch, with resultant larrge transverse signal (echo)

  • TR: Time to repetition

Time from 90 deg pulse to 90 deg pulse (or repeat of whole sequence)

NB: Basic sequences, 1 row of image data each TR, so: 256x256 matrix, 500msec TR => 128,000 m/sec (128 sec) to acquire

22
Q

How do recieving coils receive signal?

A
  • Aligned loops of wire perpendicular to transverse axis -> when spins in transverse plane, induce current in coils (proportional to transverse field strength)
23
Q

Explain how TE / TR relate to T1w, T2w and PD images

A

In each instance, need to seperate tissues based on slow or fast relaxation (and thus differential signal intensity)

  • T2w:

Reliant on TE

LONG TR, LONG TE

Allow short T2 tissues to decay, while highlighting signal from low T2 tissues (high signal at time of echo)

  • T1w

Reliant on TR

SHORT TR, SHORT TE

Allow short T1 tissues to decay while highlighting signal from long T1 tissues

** OPTIMIZING ONE INHIBITS THE OTHER!**

  • PD images

Both T1 and T2 effects inhibited

LONG TR and SHORT TE

Varying intensity levels for same tissue

24
Q

Describe the different gradients used to localise components of the image

A
  • Slice selection gradient:

Selects slice along B0 -> Gradient creates differential magnetif field strength, and as such allows differential slices to be flipped due to predictable variation in larmour frequency (fucntion of magnet strengthY)

  • Phase encoding gradient:

Soon after 90deg pulse

Gradient across slice -> Different PHASE

  • Freq encoding gradient:

During echo

gradient across row -> Differental PRECESSIONAL FREQ

=> allows individual voxel signal to be mapped

25
Q

What are fast spin / turbo spin sequences?

A
  • Spin echo sequences where multiple 180 rephasing pulse sequences are applied for a single TR
  • More signals localised, speeds up acquisition
26
Q

How are inversion recovery sequences produced? What is Time of inversion (TI)?

A
  • Initial 180 pulse -> reverses proton alignment (-z)
  • Relaxation towards +z, all tissues eventually cross z=0 (nulled) -> predictable
  • If normal Spin echo seq started AFTER 180 pulse, can place read signal when desired tissue is nulled

=> DEPENDENT ON T1 RELAXATION TIME (LONGITUDINAL)

TI = Time from 180 inversion pulse and 90 deg RF pulse

FAT = SHORT TI as short T1 relaxation (STIR IMAGES USE THIS)

FLAIR similar, but can be T2 or T1w

27
Q

How do gradient echo sequences differ from spin-echo?

A
  • Technique:

Use smaller flip angles

No 180 deg refocusing pulse

Gradients used to dephase and rephase transverse mag -> generate echoes

Shorter TR

No compensation for T2* relaxation / inhomogeneity -> if long TE = T2* weighted

  • Time:

Short TR and smaller flip -> Shorter aquisition

Reduced motion

  • Utility:

Angiography (less motion)

Magnetic susceptility => haemorrhage

28
Q

Describe the different types of magnetic susceptibility properties

A
  • Paramagnetic:

Small +ve susceptilbility

E.g. magnesium, molbdenum, lithium

  • Ferromagnetic

Strong +ve susceptibility

E.g. iron, nickel, cobalt

  • Diamagnetic

Weak -ve susceptibility

E.g. gold, silver

29
Q

What is the predominant action of gadolinium?

A
  • Shortened T1w relaxation -> generate greater signal on T1w seq

BIt confusing, but correct. Either accept or investigate further….

30
Q

How does chemical fat saturation work?

A
  • Only in high field
  • > Exploit difference in precessional freq of fat and water = 220Hz at 1.5T, becomes smaller at lower strengths
  • Freq specific preparation pulse -> selectively excite lipid protons, followed by spoiling gradient that dephases fat signal

=> Generated signal only arises from non-fatty tissues

31
Q

Briefly, what are DWI and PWI?

A
  • DWI = diffusion weighted. Sensitive to BROWNIAN motion of water molecules -> THUS CYTOTOXIC OEDEMA, seen in restricted diffusion with stroke
  • PWI = T2*w following bolus contrast -> semiquantifiy susceptibility-induced signal loss over time, and thus blood flow.
32
Q

How are SNR and Spatial resolution related?

A
  • Spatial resolution: improved by decreasing voxel size e.g. smaller slices, smaller FOV, larger matrix
  • SNR: requires volume of tissue per voxel, so decreases with factors improving resolution. Can average signal over multiple acquisitions (NSA), but at cost of increased scan time.
  • > also, some coils help. Parallel imaging = use of multichannel phased array coil. Signals from each element combined to imprve SNR without longer aquisition
33
Q

List some benefits / disadvantages of low field MR (<1T)

A
  • Adv:
    Less susceptibility

Open magnet design, can help with patient access

Accidents more preventable

Dis:

Reduced SNR / spatial resolution

THRALL - Thom (40 decks)

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