1. mechanical frame for assemble and stability 2. confines the fringe field 3. intensifies the Bo

Part IV Flashcards by Queenie Francisco

heart of MRI system

primary magnets

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function of Primary Magnets

provide a sustained
homogenous Bo during examination

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Criteria for selection for primary magnets

desired field strength - magnetic flux density (↑strength, ↑ price)
siting limitation - location of installation

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types of primary magnets

permanent magnets
electromagnets - resistive and superconductive

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first permanent magnet

lodestone

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-occurs naturally
-can be synthesized
-inexpensive & widely used

permanent magnet

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ealiest commercial magnets

ferrite magnets made up of Fe

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1930 magnets developed, higher mf intensity than ferrite magnets

AlNiCo

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recently introduced magnets with higher mf intensity

rare earth magnets
ex. neodymium

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pupose of open mri system

parents can stay with kid, claustro and anxious pt

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produces mf in open mri system

individual brick size ferromagnetic ceramic material, oriented into an array, up to 1m on a side (2-5 layers)

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part of OMS, positioned in each magnet for shimming - above magnet

pole face

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[oms] refine the homogeneity of the mf after installation of MRI system

adjusting screws

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iron yolk purposes

mechanical frame for assemble and stability
confines the fringe field
intensifies the Bo

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permanent magnet advantages (SLA)

signif. fringe field: 0.5mT
low electric power consumption
absence of cooling system

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permanent magnet disadv.

limited Bo intensity
poor mf homogeneity
excessive wt. 90000 kg/ approx. 100 tons)

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z-axis of permanent magnet

vertical instead of horizontal

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pm is mostly used for [..]

low mf imaging - extremity imaging

0.3 T - whole body

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[pm] mf Bo

up to 0.3T (usually less)

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[pm] mf homogeneity

10-50 ppm (↑ppm, ↑ inhomegeneity)

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[pm] weight

90 000 kg

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[pm] cooling system

none

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[pm] power consumption

20kW

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[pm] dist to 0.5 mT fringe field

<1m

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magnets used in older products, Bo produced by a classical electromagnet

Resistive EM

no. of coils of resistive em that balances Bo

4 separate coils- intensity, uniform Bo

Resistive EM produces [orientation] Bo

vertical coupled to permanent magnets

material that Solenoid resistive em uses

Aluminum Strips

adv and disadv of aluminum strips

widely available, light mat. lower conductivity rate

aluminum weight

1/3 of that of Cu but 60% conductivity of Cu

Resistive EM field strength

0.3 T

Resistive EM Adv [LePTSEL]

-less expensive - lower f. strength -r. less precision -easily turned off - desired mf strength -shimming less difficult -easier siting -lighter than perm magnet (4,000 kg)

Resistive EM mf homogeneity

10-50 ppm

Resistive EM Disadv

-consumes large current (0.2=60-80 kW) -req for cooling: Water-cooler with secondary single pass system thru heat exchanger

used for analytical spectroscopy and high energy physics (reach up 14 T)

Superconducting EM

requires broader rf bandwidth

Superconducting EM

[Superconducting EM] Mf bo

0.5-4T

[Superconducting EM] mf homogeneity

0.1-5 ppm

[Superconducting EM] weight

10 000 kg

[Superconducting EM] cooling system

cryogenic

[Superconducting EM]power consumption

20kW

[Superconducting EM] distance to 0.5 ml fringe field

10 m

[Superconducting EM] Adv

1. high mf intensity (↑MRI signal=↑SNR) 2.↑ SNR = ↑SR and CR 3. field could be ho0monized/shimmed

[Superconducting EM] Disadv

1. intense fringe field (compromises site selection) 10m 2. usage of cryogen ( expensive, ↑work load)S

superconducting em

nobium-titanium(NbTi) alloys

critical temp:

9K - conductor becomes superconductor

liquified gasses that keeps the conductor cold

cryogen

2 liquid gasses

lHe vaporizes @ 4 K lN vaporizes at 27 K

container housing the superconducting wire and cryogen (DEWAR)

cryostat

separation to maintain temp

helium vessel

cause of heat up and vaporization of liquid Helium

thermal radiation

solution for thermal rad :

surround helium compartment with concentric insulating compartments (lN)

replenishing device that replaces nitrogen compartment

cryocoolers

purpose of cryocoolers:

cool radiation shields recondense the helium in main mag system

recondenses helium vapors back into liquid helium bath

cool head

the superconductor becomes resistive that it starts to heat up

quench

cause of quench

lack of lHe or mechanical trauma to crystat

location of GC

secondary cylinder

GC materials

broad thick copper conducting bands (10mm wide and 4 mm thick) placed in a strong epoxy resin

GC casing purpose

prevents the coils that causes thump/noise

GC material

broad thick Cu conducting bands (10mm wide & 4 mm thick) placed in a strong epoxy resin casing

Z GC / Gz, Gss / Maxwell Coils Location:

wound on cylinder at opposite ends

Z GC / Gz, Gss / Maxwell Coils Purpose

Bz: changes mf into the z-axis Bss: transverse selection (top-down / AXIAL/ separates superior from inferior)

X GC / Gx, GR Location

2 positioned saddle shaped coil in set of 2 positioned on either side of the cylinder

X GC / Gx, GR Purpose

1. produce mf lat; side to side 2. slice selection (sagittal) 3. Br - readout gradient mf 4. phase &frequent encoding

Y GC / Gy, Go Location

2 positioned saddle shaped coil in set of 2 -vertical axis

Y GC / Gy, Go Purpose

1. produce mf vertically; A-P 2. slice selection (coronal) 3. phase and frequency encoding

gc energized simultaneously (3 gradient)

combined gradients

Combined Gradients Purpose

1. produce a single selection 2. used for slice selection (oblique images)

made up of coils of wire that can produce a radiosignal at the larmour frequency behaves as transmitter/receiver

RF probe

source of RF, master of freq source

Frequency Synthesizer

2 types RF Probe

Homogenous volume coils (T&R) Inhomogenous coils (R coils only)

Quadratus Coil Purpose

1. improves SNR by detecting MR signal from multi. detectors 2. replaces saddle design for all homo application 3. birdcage resonator

widely used coil T & R low signal sensitivity

saddle coils

robust; homogenous volume coil

body coil

body coil (location)

inside the gradient coil - close to gc

body coil purpose

can image the whole body; T & R

head/extremity coil pupose

cranial ana and LE (w/ high reso) typical quadratus birdcage head coil-neck coil and thoracic/lumbar surface, spine coil -total spine

surface coil location:

placed nead the poi

surface coil is encased with a [..]

rubberized/plastic matrix

surface coil purpose

1. obtain high SNR images of the specific anatomy 2. used as R but used in head/body coils to transmit RF 3. better CR and SR

surface coils disadv

1. small fov 2. positioning coil = more time

phased array and matrix coil purposes

-to overcome limited FOV -can image spine, br, and pelvis -uniform img - large imging vol -use eithe linear R/quaddrature R can be used -allows parallel imaging

contains 32,64 more indiv coils; used for gen imaging and parallel imaging

matrix coils

Superconducting EMs operators @:

0.5T, 1.0T, 1.5T

Superconducting EMs specialty system:

3T and 4T

Part IV Flashcards

(86 cards)