Download - Rad T 265 MRI Lecture
![Page 1: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/1.jpg)
Rad T 265MRI Lecture
![Page 2: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/2.jpg)
No Magnetic Field
= No NetMagnetization
Protons align with a magnetic field…
![Page 3: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/3.jpg)
In a magnetic field, protons can take either high- or low-energy
states
![Page 4: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/4.jpg)
The difference between the numbers of protons in the high-energy and low-energy states results in a net magnetization (M) and gives rise to the
Larmor Equation.
002 Bv
![Page 5: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/5.jpg)
MRI Equipment
• Main Magnet
• Shim Magnet
• Gradient Coil
• RF system
![Page 6: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/6.jpg)
Main Magnetic Field
▪Typically oriented to the long axis of the patient
▪Bo
Increase Bo
Homogeneity Precessional frequency Chemical shift
![Page 7: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/7.jpg)
Type of Magnets - Resistive
▪Require constant electrical current
▪Max field is less than 0.3T
![Page 8: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/8.jpg)
Type of Magnets - Permanent
▪Can be built in a variety of shapes and configurations
▪Tend to be heavy
▪Lower field strengths
▪Made of aluminum, nickel, and cobalt - alnico
![Page 9: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/9.jpg)
Type of Magnets - Superconductor
▪Most common
▪Lowest electrical costs
▪Highest field strength
▪Use cryogens
Helium-450 F, -269 C, 4.2 K
Nitrogen-320 F, -160 C, 77.3 K
![Page 10: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/10.jpg)
Shim CoilsIncrease homogeneity
▪Passive▸Steel plates attached to the magnet
▪Active▸Electromagnets with an opposite polarity
![Page 11: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/11.jpg)
Gradient Fields
▪Measured in mT/m
▪Rise time▸1 ms for 0 to 10mT/m is good
![Page 12: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/12.jpg)
Gradient Coils ▪Produce noise
▪They rattle in their mountings
▪Greatest stress is caused
by obliques
![Page 13: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/13.jpg)
![Page 14: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/14.jpg)
Faraday’s LawUsed for most MRI activities
▪Ramping the magnet
▪Shimming
▪RF field
▪Gradients
▪MR signal
![Page 15: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/15.jpg)
RF Receiver Coil
▪Designed to detect transverse magnetization
▪Based on Faraday’s Law▸Variable magnetic fields produce an electric
current in a loop of wire
![Page 16: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/16.jpg)
RF Shielding
▪Copper is preferred▸Expensive
▪Aluminum can be used
▪Problems with the RF shield produce zipper artifacts
![Page 17: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/17.jpg)
Gauss
▪10,000 gauss = 1 tesla
▪Earth’s magnetic field is 0.5 g
![Page 18: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/18.jpg)
Larmor Equation
▪Precession frequency is based on Bo
▪For a 1 T magnetic the precessional frequency is 42.6
Mhz
![Page 19: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/19.jpg)
RF
▪Needs to be perpendicular to Bo
▪Needs to be at the precession frequencySpins are only in phase during RF pulsesWhen the pulse ends dephasing begins immediately
![Page 20: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/20.jpg)
Inherent Tissue Characteristics
▪T1, T2, PD, flow, motion
▪We can only demonstrate these not change them
▪Pulse sequences are used to maximize differences in tissue characteristics
![Page 21: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/21.jpg)
Weighting
• T 1
T 1 weighted images have a short TE and TR
Provide more anatomical info – better spatial resolution
• T 2T 2 weighted images have a long TE and TR
More pathologic info
![Page 22: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/22.jpg)
MRI Contrast Agents
▪Types; Paramagnetic, Ferromagnetic
▪Administration
▪Reactions
▪Contraindications
![Page 23: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/23.jpg)
Paramagnetics
▪Gadolinium▸Positive contrast▸Shortens T1 relaxation▸Appears brighter on the image▸Elimination half life 1 - 2 hrs
![Page 24: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/24.jpg)
![Page 25: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/25.jpg)
Ferromagnetics
▪Ferumoxides▸ Negative contrast▸ Shorten T2 relaxation▸ Appears darker on the image
![Page 26: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/26.jpg)
SAR Dependent on
▪Duration of RF▸Flip angle and strength
▪Frequency▸Pulse sequence and strength
▪Patient Mass▸Weight
![Page 27: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/27.jpg)
SAR LimitsIncrease core temp 1 C
▪Whole body▸ 0.4 W/kg
▪Head▸3.2 W/kg
▪Small Volume▸8.0 W/kg
![Page 28: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/28.jpg)
Static Field Exposure
▪Whole body▸3T
▸ Extremities▸5T
![Page 29: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/29.jpg)
High Field ExposurePossible effects
▪Magnetophosphenes
▪Nausea
▪Vertigo
▪Metallic taste
![Page 30: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/30.jpg)
Fringe Field
▪Public is limited to 0.5 mT▸0.5 mT = 5 gauss
▪No pacemakers beyond this line
![Page 31: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/31.jpg)
Noise Limitations
▪Earplugs are necessary above 100 db
▪Remember noise is related to gradient activity
▪Gradients are rattling in their supports
![Page 32: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/32.jpg)
Quench
▪Uncontrolled release of cryogens
▪Helium and nitrogen replace oxygen
▪Asphyxiation
![Page 33: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/33.jpg)
Non Compatible DevicesAbsolute contraindications
▪Cardiac pacemakers
▪Internal defibrillators
▪Biostimulators
▪Implanted infusion pumps
▪Cochlear implants
▪Metallic orbital FB
![Page 34: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/34.jpg)
Non Compatible Devices Continued
Safe to image
▪Surgical hemostasis clips
▪Orthopedic prostheses
▪Dental work▸Except magnetic dentures
▪IUDs
▪Intra vascular coils
![Page 35: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/35.jpg)
Wires
▪Important to remember that coiled wires will generate a current and that currents produce heat.
▪Faraday’s Law
![Page 36: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/36.jpg)
MR Data AcquisitionFourier Transformation
▪Process that takes a complex signal and breaks it down into its component parts
![Page 37: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/37.jpg)
Types of Pulse Sequences
▪SE, IR, STIR, GE
▪RARE, FLARE, FLAIR, FSE
▪EPI,
![Page 38: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/38.jpg)
Spin Echo
▪Uses a 90 RF followed by a 180 RF
▪Traditionally the most popular sequence
▪Can provide T1 or T2 information
![Page 39: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/39.jpg)
IR, STIR
▪Uses a 180 RF followed by a 90 RF and then a 180 RF
▪Provides heavy T1 weighting▪Can be used to minimize signal by varying the TI
time
![Page 40: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/40.jpg)
Gradient Echo
▪Uses an initial RF pulse, usually less than 90
▪Rephases the spins by using a gradient instead of other RF pulses
![Page 41: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/41.jpg)
FSE
▪Uses ETL▪ETL - obtain more than 1 echo per TR▪Different from regular ME because second echo and beyond
is used to fill the same k- space, not a new one
![Page 42: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/42.jpg)
EPI
▪Similar to FSE
▪Difference is all the phase encoding steps are acquired during one TR
![Page 43: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/43.jpg)
Length of sequence
▪SE, IR, traditional sequences
▪TR x NSA x #PE
![Page 44: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/44.jpg)
TR Controls
▪T1 relaxation
▪Spin lattice
▪Longitudinal
![Page 45: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/45.jpg)
TE Controls
▪T2
▪Spin spin
▪Transverse relaxation - dephasing
![Page 46: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/46.jpg)
Affecting SNR
▪Slice gap
Increase slice gap, increase SNR, less
cross-talk
▪Slice thickness
Increase slice thickness, increase SNR,
more anatomy per slice = more signal
Also, increase partial volume and decrease
resolution
![Page 47: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/47.jpg)
Affecting SNR
▪FoVIncrease FoV, increase SNR (more anatomy)
Decrease resolution
This is the same effect we discussed in CT
▪Increase matrixIncrease resolutionDecrease SNR, smaller pixels
![Page 48: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/48.jpg)
Affecting SNR
▪Increasing TR increases SNR
Provides more relaxation
▪Decreasing TE increases SNRLess dephasing occurs
![Page 49: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/49.jpg)
Types of Suppression
▪STIR; short tau inversion, suppresses fat
▪FLAIR; suppresses fluids, long T1 values
▪Heavy T2; long TE and TR, maximizes T2 values
▪Spectral fat suppression; based on freq difference between fat and water
![Page 50: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/50.jpg)
GatingUsed to eliminate or minimize physiologic motion
▪Peripheral pulse
▪Respiratory
▪Cardiac
▪NOTE ALL INCREASE TR▸Or decrease slices
![Page 51: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/51.jpg)
MR AngiographyTwo major techniques
▪Time of flight
▪Phase contrast
![Page 52: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/52.jpg)
Time of Flight (ToF)Needs MIP
▪Uses slice entry phenomena
▪Can also be called▸Paradoxical enhancement▸Flow related enhancement
▪Evaluates vessel morphology
![Page 53: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/53.jpg)
A. Safety issues surrounding static field.
![Page 54: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/54.jpg)
• RF pulse determines “flip angle” Rotation determines amount
of magnetization measured
• Field strength determines resolution Increased magnetization
leads to increased signal
Pulse and Field Effects
cosMM Z
Images adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm
sinMM XY
Muscle
Tissue
Difference
B0= 0.2 T
B0 = 1.5 T
![Page 55: Rad T 265 MRI Lecture](https://reader036.vdocuments.net/reader036/viewer/2022062516/56812c3b550346895d90c23c/html5/thumbnails/55.jpg)
Short TR
Short TE
Long TE
Long TR
T1
T2
PD