magnetic resonance imaging 2
問題一覧
1
- Parameters that are inherent to subject being scammed and uncontrolled by the technologist.
• Proton density
• T1 sequence
• T2 sequence
intrinsic parameters
2
- Parameters that are controlled by the techndo-gist depending on the capabilities of the mRl scanner.
• Repetition time
• Echo time
• Inverse time
• Flip angle
extrinsic parameters
3
- AKA Spin Density, is the quantity of resonating spins (precessing protons) in agiven evolume tissue:
• The signal strength produced by precessing protons is equal to the number of protons present.
• Achieved by long relaxation times (TR)
between pulses.
• Produces low contrast.
- Proton density alone poorly separates tissues.
proton density
4
true or false
• Most soft tissues and fat have similar proton sensity thus poorly distinguished.
• Bone and hair air in the lungs have low proton density and produces weak signals, thus easily distinguished From other tiesues.
true
5
proton density AKA
spin density
6
- the release of energy by the excited protons, which occurs at difference rates in different tissues.
relaxation
7
• Relaxation Time: the most important Factor in tissue discrimination.
• Tissues can be distinguished From one another based on relaxation rates of their nuclei.
• Types : T, and Ta sequences.
relaxation time
8
- The process nuclei undergo as they release their excess energy to the general environment
• Usually labelled as spin lattice relaxation.
• Measured in milliseconda.
• Inuelves a types of relaxation: spin lattice longitudinal, end thermal celaxation time.
T1 sequence
9
3 types relaxation of T1 sequence
spin lattice,longitudinal,thermal relaxation time
10
the time required For some of the magnetic moment to regain original orientation after
RF excitation.
spin lattice relaxation time
11
the time required for precessing spins to align with the constant external magnetic Field to 63% of maximum strength.
longitudinal relaxation time
12
the time required For the thermally excited system to return to its equilibrium state.
• Ti relaxation of water in 3 seconds.
• Demonstrates significant different in dehydrated tissues.
thermal relaxation time
13
- The process nuclei undergo as they release their excess energy through interaction amongst themselves.
• Usually labelled as spin-spin relaxation
• Measured also in milliseconds
• Involves a types of relaxation: spin-spin , transverse relaxation time.
T2 sequence
14
2 types relaxation of T2 sequence
spin spin,transverse relaxation time
15
- the time requred after :
• Precessing spins hove aligned with the external magnetic Field due to the RE pulse
• hosing 63% oF coherence due to interactions between spins.
spin spin relaxation time
16
the time required for transverse magnetization to reduce strength by 37% of its maximum.
transverse relaxation time
17
true or false
Use of signals in MRI
- The signals produced in mRI techniques contain a combination of proton density and relaxation (Ti and Tz)
• Rates of Relaxation (Ti, Tz) depends on the environment in which the nucleus is located.
• Chemical environment among tissues difFer.
• Relaxation rates of nuclei in diFFeren+
tissues differ = mRl signals in different tissues
also diFFer = difFerent appearance on image.
true
18
Extrinsic Parameters
- the longest time interval between pulse sequences, usually between 300 - 3000 ms.
repetition time (TR)
19
Extrinsic parameters
- the shortest time between a 90° pulse and its echo during a spin echo pulse, usually 10 - 100 ms
echo time (TE)
20
Extrinsic parameters
angle of inclination Fronen longitudinal axis.
flip angle (FA)
21
Extrinsic parameters
the time between a 180 degrees pulse and 90 degrees pulse in the inversion recovery usually 200-2000 ms
inverse time (TI)
22
- used to stimulate the nuclei to obtain images that are weighted with any of the three intrinsic parameter.
• short T1 = Fast spin lattice relaxation rates incresed mRl signal on T1 weighted images.
• long T2 = slow spin-spin relaxation rate increased MRI signal on T2 weighted images.
pulse sequences
23
most common MEI pulses
90-180 degrees
24
true or false
Common MRl pulses
- Most common mRl pulses: ao and 180 degrees
• B1 = determines the distance that magnetic moment (mo) processes from 2 axis.
• A 90 degrees pulse tips 90° From 2 axis, the y exis
• The spinning mo induces an Em Fore that creates detectable voltage in the
coil oF wire.
true
25
TRUE OR FALSE
creation of an MRI signal
- Frequency of induced voltage matches the larmour frequency making it possible to identify the element of emitting the signal.
• The voltage detected is the MRI signal.
• Signal emitted by protons decay
exponentially because of dephasing.
• Free Induction Decay: the gradual loss of MRI signal due to dephasing.
true
26
- AKA Saturation Recovery, uses a series of 90° RF pulses to create repeated Free induction decays.
• Contrast: depends on pD and T12 •
• Sequence: short TR T1 partial saturation.
• Use: demonstrating the presence oF cysts, subdural Fluids, and blood Flow.
partial saturation
27
patial saturation AKA
saturation recovery
28
- The most popular and commonly used, introduced by Herman Carr and Edward purcell
- Uses 20° and 180° RFpulses (refocusing pulse) 50 TE
• sequences: short TR and TE For Ti weighting, long FR and TE For Te weighting both are usually obtained.
• Use: in almost all (general) applications.
spin echo recovery
29
fastest imaging MRI technique
echo planar imaging EPI
30
uses multiple rephrasing RF pulses 180 degrees
fast spin echo technique (FSE)
31
uses GE with a reverse polarity rephrasing gradient.
gradient refocused echo GRE
32
uses GE with short TR, enabling mz and Mxy be equal.
steady state free precession (SSFP)
33
uses GE low FA and TR to deplase Mxy.
fast low angle shot (FLASH)
34
- The uses gradient pulses other tham REpulses, usvally gradient pulses of ao degrees.
• Contrast: controlled by FA, TI and pD.
• Sequence: short TR and 90° FA.
• VEe: For neurologic and orthopedic applications.
gradient echo recovery
35
uses GRE with the techniques of SSEP
FISP
36
Uses 180° gradient pulses and GRE.
GRASS
37
- Uses a three REpulse sequence that consist of 180° REpulse (inversion pulse) then Followed by a spin echo sequence.
• Use: For tissue suppression and modiFying inersion time settings.
inversion recovery
38
used as basic fat suppression sequence
STIR
39
uses long T1 techniques for fluid suppression
FLAIR
40
TRUE OR FALSE
- Magnetic Field lines will be present in all forms of magnet :
• Bar permanent magnet
• Cylindrical coil
• Iron core electromagnet
• Straight current carrying wire
• Circular current carrying wire
true
41
(Bo) : the primary Field produced by the main magnet of the scanner.
static magnetic field
42
(BI): the secondary field.
radiofrequency field
43
: causes Field distortions produced by the gradient coils
time varying magnetic field TVMF
44
- The main magnetic field of the scanner that is used to magnetize the patient's tissue.
- Provides the static external magnetic Field, available in different Field strengths and types.
• From low to high magnetic Fields.
• Permanents and electromagnets
• Longitudinal and transverse directions
primary magnet
45
Ultra low field strength
<0.2 tesla
46
Low field strength
0.2-0.4 tesla
47
Mid field strength
0.5-1.4 tesla
48
Ultra high field strength
4.0-8.0 tesla
49
an Ferrous material made magnetic through exposure to an external magnetic Field.
permanent magnet
50
• has high resistance and requires a large amount of electricity.
resistive magnet
51
True or false
Permanent Magnet
- An artificially induced magnet in nature.
- Created by means of exposing a Ferromagnetic material to an external magnetic Field to induce magnetism.
• Bo: up to 0.3 Tesla
• Bo: Direction: can be transverse or longitudinal
• Very heavy, inexpensive maintenance and materials, temperature sensitive.
• Low Fringe field.
- Made up of Ferrous material (e. magnetized ceramics, Ferrous plates, iron, cobalt, nickel)
- magnetic Field is always active.
- No power is required to activate magmetic Field
- Can only produced low Field strengthe
- Usually oF vertical Field direction,
- Produces a weak Fringe Fiold.
true
52
TRUE OR FALSE
Advantages of Using a Permanent Magnet
- much less expensive (to buy and maintain) than the other types of MRI magnets.
- Costs virtually nothing to operate.
- Scaner table moves in two directions.
- Weak Fringe Fields and less SAR to the patient
• Specific Absorption Rate (SAR) : the amount of power (head) deposited by a radio frequency Field in a certain mass of tissue, expressed in W/kg.
true
53
: the amount of power (head) deposited by a radio frequency Field in a certain mass of tissue, expressed in W/kg.
specific absorption rate SAR
54
TRUE OR FALSE
Disadvantages of Using a Permanent Magnet
- how magnetic Field strength
- Inhomegenous magnetic Field (produces relatively
no uniporm Fields
- magnets used are very heavy (approx 20 tons)
- Low signal to noise ratio
• Signal to Noise Ratio (SNR): satio of the average signal intensity oven the unavaidable background noise.
• V SNR = L image quality
true
55
• ratio of the average signal intensity over the unavaidable background noise.
• mababa SNR = mababa image quality
signal to noise ratio SNR
56
TRUE OR FALSE
Field Strength of an Electromagnet
- Number of loops in the coil
- Electric current passing through the loop
- Diameter of the lasp
- Spacing between loops
true
57
TRUE OR FALSE
- Number of loops in the coil: the more loops the electromaghet has, the stronger the magnetic Field.
true
58
TRUE OR FALSE
- Current passing through the loop: the higher the electric current, the stronger the magnetic Field.
true
59
- Diameter of the loop: the smaller the diameter of the loops, the stronger the magnetic Field.
true
60
- Spacing between the loops: the smaller the spacing between the loops, the stronger the magnetic Field.
true
61
Permanent magnet (Bo)
0.3 tesla,transverse,longitudinal
62
Resistive Magnet
- An electromagnet in nature limited by cooling requirements.
- Resistance is due to the amount of solenoid that produce huge amount of heat and requives large amount of electricity.
• Bo: 0.02 - 0.4 Tesla
• Bo: Direction : longitudinal
• High clectrical cost, inexpensive material solenoia, and produces uniform magnetic Fielo.
• hongitudinal Fringe Field problem.
resistive magnet
63
Resistive magnet
0.02-0.4 tesla,longitudinal
64
TRUE OR FALSE
characteristics of a Resistive magnet
- Operates at room temperature.
- Uses standard conducting materials (e.g. copper) shaped into a solenoid.
- Requires high electric current to produce magnetic Field.
- Produces a uniform magnetic Fied.
- Limited to low Field applications.
true
65
TRUE OR FALSE
Disadvantages of Using a Resistive Magnet
- High electric power consumption: a great deal Of power is required to achieved high magnetic Field strength.
- Active current is used to induced a magnetic Field.
- Heat produced by the magnet needs to be cooled by water (using a water chiller)
- High operating cost: large power requirements oF the magnetic coils and the associated cooling system.
true
66
TRUE OR FALSE
true
67
- Connected to the mR scanner system to help maintain the bore cool and avoid high tempe ratures.
• High temperatures causes damage to the system and loss oF eFficiency in producing magnetic Field.
• Chiller system has a close circuit where glycol (cooling liquid) circulates through.
• Glycol carries heat as it passes through the mRI system, then cooled down as it passes through the chiller.
MRI chiller system
68
Superconducting magnet
4 tesla,transverse
69
TRUE OR FALSE
Characteristics of a superconducting Magnet
- Made oF (superconductive) coils of Niobium Titanium alley filament wrapped around a large loop of copper.
• Super conductive materials when exposed to low temperantures has no resistance which eliminates problems regarding those of resistive magnets.
• Ramping: method of bringing magnets to a superconductive state via cryogenic cooling (use of special cooling agents).
true
70
• method of bringing magnets to a superconductive state via cryogenic cooling (use of special cooling agents).
ramping
71
: an insulating chamber that contains thy cryogens and electromagnet
dewar (cryostat)
72
• the cooling liquid used by the cryostat (c.g. liquid Helium a15k, liquid Nitrogen >7 K).
• The magnet is housed insulated container (Dewar) that Functions like a thermos battle.
• 1200 liters: typical volume of liquid Helium Used.
• Quenching: the rapid heating of electromagnets that vaporize the cryogens.
cryogen
73
• typical volume of liquid Helium Used.
1700 liters
74
• the rapid heating of electromagnets that vaporize the cryogens.
quenching
75
TRUE OR FALSE
Use of Cryogen in Super conductive mkI scanners
- Current posses through the magnet to induce a magnetic Field and rumps it to strength, then super cooled by cryogens to eliminate resistance.
• Liquid helium and nitrogen are subject to evaporation and must be replaced periodically
• Replacement is a signiFicant cost, since cryogens are very expensive and handling requires special
training.
• Superconducting magnets do not need continous supply of electric current to Function.
true
76
• used to recondense some helium vapor back into the liquid helium bath.
cryocooler (cold head)
77
TRUE OR FALSE
Advantages of using a superconducting magnet
- induces high intensity magnetic Field
- High Field homogeneity
- Low power consuption
- High signal noise rotio = Faster scanning time
- Scamers are built horizontally
true
78
TRUE OR FALSE
Disadvantages of using a Superconducting Magnet
- High capital cost requirement
- High specific absorption rate (SAR)
- Production oP chemical shiFt artifact
- Increased production of motion artifact
- Louder noises produced by the scanner.
- InsufFicient research.
true
computed tomography
computed tomography
Kenneth Manlapaz · 46問 · 1年前computed tomography
computed tomography
46問 • 1年前interventional radiology 2
interventional radiology 2
Kenneth Manlapaz · 58問 · 1年前interventional radiology 2
interventional radiology 2
58問 • 1年前Radiationtherapy
Radiationtherapy
Kenneth Manlapaz · 79問 · 1年前Radiationtherapy
Radiationtherapy
79問 • 1年前computed tomography 2
computed tomography 2
Kenneth Manlapaz · 11問 · 1年前computed tomography 2
computed tomography 2
11問 • 1年前QAQC 2
QAQC 2
Kenneth Manlapaz · 33問 · 1年前QAQC 2
QAQC 2
33問 • 1年前radiotherapy
radiotherapy
Kenneth Manlapaz · 19問 · 1年前radiotherapy
radiotherapy
19問 • 1年前POSI 2
POSI 2
Kenneth Manlapaz · 30問 · 1年前POSI 2
POSI 2
30問 • 1年前問題一覧
1
- Parameters that are inherent to subject being scammed and uncontrolled by the technologist.
• Proton density
• T1 sequence
• T2 sequence
intrinsic parameters
2
- Parameters that are controlled by the techndo-gist depending on the capabilities of the mRl scanner.
• Repetition time
• Echo time
• Inverse time
• Flip angle
extrinsic parameters
3
- AKA Spin Density, is the quantity of resonating spins (precessing protons) in agiven evolume tissue:
• The signal strength produced by precessing protons is equal to the number of protons present.
• Achieved by long relaxation times (TR)
between pulses.
• Produces low contrast.
- Proton density alone poorly separates tissues.
proton density
4
true or false
• Most soft tissues and fat have similar proton sensity thus poorly distinguished.
• Bone and hair air in the lungs have low proton density and produces weak signals, thus easily distinguished From other tiesues.
true
5
proton density AKA
spin density
6
- the release of energy by the excited protons, which occurs at difference rates in different tissues.
relaxation
7
• Relaxation Time: the most important Factor in tissue discrimination.
• Tissues can be distinguished From one another based on relaxation rates of their nuclei.
• Types : T, and Ta sequences.
relaxation time
8
- The process nuclei undergo as they release their excess energy to the general environment
• Usually labelled as spin lattice relaxation.
• Measured in milliseconda.
• Inuelves a types of relaxation: spin lattice longitudinal, end thermal celaxation time.
T1 sequence
9
3 types relaxation of T1 sequence
spin lattice,longitudinal,thermal relaxation time
10
the time required For some of the magnetic moment to regain original orientation after
RF excitation.
spin lattice relaxation time
11
the time required for precessing spins to align with the constant external magnetic Field to 63% of maximum strength.
longitudinal relaxation time
12
the time required For the thermally excited system to return to its equilibrium state.
• Ti relaxation of water in 3 seconds.
• Demonstrates significant different in dehydrated tissues.
thermal relaxation time
13
- The process nuclei undergo as they release their excess energy through interaction amongst themselves.
• Usually labelled as spin-spin relaxation
• Measured also in milliseconds
• Involves a types of relaxation: spin-spin , transverse relaxation time.
T2 sequence
14
2 types relaxation of T2 sequence
spin spin,transverse relaxation time
15
- the time requred after :
• Precessing spins hove aligned with the external magnetic Field due to the RE pulse
• hosing 63% oF coherence due to interactions between spins.
spin spin relaxation time
16
the time required for transverse magnetization to reduce strength by 37% of its maximum.
transverse relaxation time
17
true or false
Use of signals in MRI
- The signals produced in mRI techniques contain a combination of proton density and relaxation (Ti and Tz)
• Rates of Relaxation (Ti, Tz) depends on the environment in which the nucleus is located.
• Chemical environment among tissues difFer.
• Relaxation rates of nuclei in diFFeren+
tissues differ = mRl signals in different tissues
also diFFer = difFerent appearance on image.
true
18
Extrinsic Parameters
- the longest time interval between pulse sequences, usually between 300 - 3000 ms.
repetition time (TR)
19
Extrinsic parameters
- the shortest time between a 90° pulse and its echo during a spin echo pulse, usually 10 - 100 ms
echo time (TE)
20
Extrinsic parameters
angle of inclination Fronen longitudinal axis.
flip angle (FA)
21
Extrinsic parameters
the time between a 180 degrees pulse and 90 degrees pulse in the inversion recovery usually 200-2000 ms
inverse time (TI)
22
- used to stimulate the nuclei to obtain images that are weighted with any of the three intrinsic parameter.
• short T1 = Fast spin lattice relaxation rates incresed mRl signal on T1 weighted images.
• long T2 = slow spin-spin relaxation rate increased MRI signal on T2 weighted images.
pulse sequences
23
most common MEI pulses
90-180 degrees
24
true or false
Common MRl pulses
- Most common mRl pulses: ao and 180 degrees
• B1 = determines the distance that magnetic moment (mo) processes from 2 axis.
• A 90 degrees pulse tips 90° From 2 axis, the y exis
• The spinning mo induces an Em Fore that creates detectable voltage in the
coil oF wire.
true
25
TRUE OR FALSE
creation of an MRI signal
- Frequency of induced voltage matches the larmour frequency making it possible to identify the element of emitting the signal.
• The voltage detected is the MRI signal.
• Signal emitted by protons decay
exponentially because of dephasing.
• Free Induction Decay: the gradual loss of MRI signal due to dephasing.
true
26
- AKA Saturation Recovery, uses a series of 90° RF pulses to create repeated Free induction decays.
• Contrast: depends on pD and T12 •
• Sequence: short TR T1 partial saturation.
• Use: demonstrating the presence oF cysts, subdural Fluids, and blood Flow.
partial saturation
27
patial saturation AKA
saturation recovery
28
- The most popular and commonly used, introduced by Herman Carr and Edward purcell
- Uses 20° and 180° RFpulses (refocusing pulse) 50 TE
• sequences: short TR and TE For Ti weighting, long FR and TE For Te weighting both are usually obtained.
• Use: in almost all (general) applications.
spin echo recovery
29
fastest imaging MRI technique
echo planar imaging EPI
30
uses multiple rephrasing RF pulses 180 degrees
fast spin echo technique (FSE)
31
uses GE with a reverse polarity rephrasing gradient.
gradient refocused echo GRE
32
uses GE with short TR, enabling mz and Mxy be equal.
steady state free precession (SSFP)
33
uses GE low FA and TR to deplase Mxy.
fast low angle shot (FLASH)
34
- The uses gradient pulses other tham REpulses, usvally gradient pulses of ao degrees.
• Contrast: controlled by FA, TI and pD.
• Sequence: short TR and 90° FA.
• VEe: For neurologic and orthopedic applications.
gradient echo recovery
35
uses GRE with the techniques of SSEP
FISP
36
Uses 180° gradient pulses and GRE.
GRASS
37
- Uses a three REpulse sequence that consist of 180° REpulse (inversion pulse) then Followed by a spin echo sequence.
• Use: For tissue suppression and modiFying inersion time settings.
inversion recovery
38
used as basic fat suppression sequence
STIR
39
uses long T1 techniques for fluid suppression
FLAIR
40
TRUE OR FALSE
- Magnetic Field lines will be present in all forms of magnet :
• Bar permanent magnet
• Cylindrical coil
• Iron core electromagnet
• Straight current carrying wire
• Circular current carrying wire
true
41
(Bo) : the primary Field produced by the main magnet of the scanner.
static magnetic field
42
(BI): the secondary field.
radiofrequency field
43
: causes Field distortions produced by the gradient coils
time varying magnetic field TVMF
44
- The main magnetic field of the scanner that is used to magnetize the patient's tissue.
- Provides the static external magnetic Field, available in different Field strengths and types.
• From low to high magnetic Fields.
• Permanents and electromagnets
• Longitudinal and transverse directions
primary magnet
45
Ultra low field strength
<0.2 tesla
46
Low field strength
0.2-0.4 tesla
47
Mid field strength
0.5-1.4 tesla
48
Ultra high field strength
4.0-8.0 tesla
49
an Ferrous material made magnetic through exposure to an external magnetic Field.
permanent magnet
50
• has high resistance and requires a large amount of electricity.
resistive magnet
51
True or false
Permanent Magnet
- An artificially induced magnet in nature.
- Created by means of exposing a Ferromagnetic material to an external magnetic Field to induce magnetism.
• Bo: up to 0.3 Tesla
• Bo: Direction: can be transverse or longitudinal
• Very heavy, inexpensive maintenance and materials, temperature sensitive.
• Low Fringe field.
- Made up of Ferrous material (e. magnetized ceramics, Ferrous plates, iron, cobalt, nickel)
- magnetic Field is always active.
- No power is required to activate magmetic Field
- Can only produced low Field strengthe
- Usually oF vertical Field direction,
- Produces a weak Fringe Fiold.
true
52
TRUE OR FALSE
Advantages of Using a Permanent Magnet
- much less expensive (to buy and maintain) than the other types of MRI magnets.
- Costs virtually nothing to operate.
- Scaner table moves in two directions.
- Weak Fringe Fields and less SAR to the patient
• Specific Absorption Rate (SAR) : the amount of power (head) deposited by a radio frequency Field in a certain mass of tissue, expressed in W/kg.
true
53
: the amount of power (head) deposited by a radio frequency Field in a certain mass of tissue, expressed in W/kg.
specific absorption rate SAR
54
TRUE OR FALSE
Disadvantages of Using a Permanent Magnet
- how magnetic Field strength
- Inhomegenous magnetic Field (produces relatively
no uniporm Fields
- magnets used are very heavy (approx 20 tons)
- Low signal to noise ratio
• Signal to Noise Ratio (SNR): satio of the average signal intensity oven the unavaidable background noise.
• V SNR = L image quality
true
55
• ratio of the average signal intensity over the unavaidable background noise.
• mababa SNR = mababa image quality
signal to noise ratio SNR
56
TRUE OR FALSE
Field Strength of an Electromagnet
- Number of loops in the coil
- Electric current passing through the loop
- Diameter of the lasp
- Spacing between loops
true
57
TRUE OR FALSE
- Number of loops in the coil: the more loops the electromaghet has, the stronger the magnetic Field.
true
58
TRUE OR FALSE
- Current passing through the loop: the higher the electric current, the stronger the magnetic Field.
true
59
- Diameter of the loop: the smaller the diameter of the loops, the stronger the magnetic Field.
true
60
- Spacing between the loops: the smaller the spacing between the loops, the stronger the magnetic Field.
true
61
Permanent magnet (Bo)
0.3 tesla,transverse,longitudinal
62
Resistive Magnet
- An electromagnet in nature limited by cooling requirements.
- Resistance is due to the amount of solenoid that produce huge amount of heat and requives large amount of electricity.
• Bo: 0.02 - 0.4 Tesla
• Bo: Direction : longitudinal
• High clectrical cost, inexpensive material solenoia, and produces uniform magnetic Fielo.
• hongitudinal Fringe Field problem.
resistive magnet
63
Resistive magnet
0.02-0.4 tesla,longitudinal
64
TRUE OR FALSE
characteristics of a Resistive magnet
- Operates at room temperature.
- Uses standard conducting materials (e.g. copper) shaped into a solenoid.
- Requires high electric current to produce magnetic Field.
- Produces a uniform magnetic Fied.
- Limited to low Field applications.
true
65
TRUE OR FALSE
Disadvantages of Using a Resistive Magnet
- High electric power consumption: a great deal Of power is required to achieved high magnetic Field strength.
- Active current is used to induced a magnetic Field.
- Heat produced by the magnet needs to be cooled by water (using a water chiller)
- High operating cost: large power requirements oF the magnetic coils and the associated cooling system.
true
66
TRUE OR FALSE
true
67
- Connected to the mR scanner system to help maintain the bore cool and avoid high tempe ratures.
• High temperatures causes damage to the system and loss oF eFficiency in producing magnetic Field.
• Chiller system has a close circuit where glycol (cooling liquid) circulates through.
• Glycol carries heat as it passes through the mRI system, then cooled down as it passes through the chiller.
MRI chiller system
68
Superconducting magnet
4 tesla,transverse
69
TRUE OR FALSE
Characteristics of a superconducting Magnet
- Made oF (superconductive) coils of Niobium Titanium alley filament wrapped around a large loop of copper.
• Super conductive materials when exposed to low temperantures has no resistance which eliminates problems regarding those of resistive magnets.
• Ramping: method of bringing magnets to a superconductive state via cryogenic cooling (use of special cooling agents).
true
70
• method of bringing magnets to a superconductive state via cryogenic cooling (use of special cooling agents).
ramping
71
: an insulating chamber that contains thy cryogens and electromagnet
dewar (cryostat)
72
• the cooling liquid used by the cryostat (c.g. liquid Helium a15k, liquid Nitrogen >7 K).
• The magnet is housed insulated container (Dewar) that Functions like a thermos battle.
• 1200 liters: typical volume of liquid Helium Used.
• Quenching: the rapid heating of electromagnets that vaporize the cryogens.
cryogen
73
• typical volume of liquid Helium Used.
1700 liters
74
• the rapid heating of electromagnets that vaporize the cryogens.
quenching
75
TRUE OR FALSE
Use of Cryogen in Super conductive mkI scanners
- Current posses through the magnet to induce a magnetic Field and rumps it to strength, then super cooled by cryogens to eliminate resistance.
• Liquid helium and nitrogen are subject to evaporation and must be replaced periodically
• Replacement is a signiFicant cost, since cryogens are very expensive and handling requires special
training.
• Superconducting magnets do not need continous supply of electric current to Function.
true
76
• used to recondense some helium vapor back into the liquid helium bath.
cryocooler (cold head)
77
TRUE OR FALSE
Advantages of using a superconducting magnet
- induces high intensity magnetic Field
- High Field homogeneity
- Low power consuption
- High signal noise rotio = Faster scanning time
- Scamers are built horizontally
true
78
TRUE OR FALSE
Disadvantages of using a Superconducting Magnet
- High capital cost requirement
- High specific absorption rate (SAR)
- Production oP chemical shiFt artifact
- Increased production of motion artifact
- Louder noises produced by the scanner.
- InsufFicient research.
true