MRI Basics 2018 cchs

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Magnetism
the extent to which a material becomes magnetized when placed within a magnetic field
Diamagnetic
Magnetic susceptibility <0.
Slightly repelled by magnets.
Diamagnetic materials
Mercury, silver, copper, carbon, hydrogen have this type of magnetism
Paramagnetic
Magnetic susceptibility >0.
Weakly influenced by external magnetic field.
No measurable self-magnetism.
Paramagnetic materials
Platinum, oxygen, tungsten, manganese, aluminum, gadolinium contrast agents are this type of magnetic materials
Ferromagnetic
Magnetic susceptibility >1.
Easily magnetized
Ferromagnetic materials
Iron, cobalt, nickel, dysprosium are this type of magnetic materials
Permanent magnets
type of magnet with no power needed and no cryo cooling
Electromagnets
type of magnets that uses electrical current
B0=KI
Copper wire wrapped in loops to form a coil
Resistive magnets
type of magnets with horizontal flux lines
Must have power supply = high operational cost
.3T max strength
Hydrogen in the body
62% of all atoms found in the human body and 10% of total mass.
Single charged spinning nucleon.
No net charge.
Superconducting electromagnets
type of magnets that must be cryo cooled with liquid helium
High field strengths (1.5T, 3T, 7T, etc.)
High capital cost, low ops cost
Hybrid magnets
type of magnets that are a combination of superconducting and permanent
Niche magnets
type of magnets that are low strength with a specific use (extremities, etc)
Fringe field
the magnetic field outside the bore. It is stronger the closer you get.
Electromagnetism
A moving electric charge, produces a magnetic field.
Strength of the magnetic field is proportional to amplitude and velocity of the moving charge.
MRI Zones I-IV
I - general public
II - screening are
III - area around magnet, control room
IV - magnet
Gradient coil strength depends on:
: number of loops in coil, current in loops, and diameter and spacing of loops
Gradient amplitude
gradient strength, measured in mT/m or G/cm
Gradient rise time
:time to reach highest amplitude
Slew rate:
:gradient strength over distance
70 mT/m/s average, 120 mT/m/s is high speed
Duty cycle
:% of time during one TR that gradient can be at max amplitude
RF coils
:can be a transmitter, receiver, or both
Must be transmitted at resonant frequency of hydrogen for resonance to occur (wobble or jiggle)
Types of transmit/receive coils
head
extremity
some breast coils
Types of receiver coils
detect/encode signal
volume or birdcage coils
surface coils
Hemholtz pair
phased array
What are volume coils?
type of coil that transmit RF
receives MR signal
encompasses entire anatomy
What are surface coils?
type of RF coil that has better signal to noise
small, placed close to anatomy
can have signal drop off
What are array coils
type of RF coil that includes multiple coils and receivers, data from each coil is combined
-spine, pelvic, breast, cardiac, TMJ
What is the minicomputer?
part of the operator interface
-fast, high capacity
-must be able to multi-task
What does the image processor do?
part of the operator interface that converts data to image
What is the hard disc drive?
part of the operator interface
-image, parameter, and raw data storage
B0
With the application of a magnetic field, magnetic moments align with OR against the external magnetic field
Precess
a rotational motion about an axis of a vector whose origin is fixed at the origin of the coordinate system
Precession
wobble associated with atom being exposed to the magnetic field
directly proportional
frequency of the precession is _______ _________ to the strength of the magnetic field.
Larmor frequency
The resonant frequency of a spin within a magnetic field of a given strength. It defines the frequency of electromagnetic radiation needed during excitation to make spins change to a high-energy state, as well as the frequency emitted by spins when they return to the low-energy state.
(Rate of precession)
Larmor Equation
-W0 also known as larmor frequency or resonance frequency -λ is the gyromagnetic ratio
--Unique to each atom
RF Frequency Pulse (RF pulse)
A short burst of electromagnetic energy delivered to the patient by the RF transmitter.
Resonance
Absorption or emission of RF by a nucleus in a magnetic field following RF excitation at the Larmor frequency of that nucleus.
Must apply an electromagnetic RF pulse at this type of frequency of hydrogen for the RF to be absorbed by hydrogen.
What happens when once the RF transmitter is turned off?
-Absorbed RF energy is retransmitted, producing a signal.
-Excited spins return to original orientation.
-Spins become "out of phase" as they return to their original orientation.
Gradient
Will cause a change in the magnetic field strength within the magnet.
-As spins are out of phase, this can be applied to rephase them.
Relaxation
process of losing energy.
Recovery
when the amount of magnetization in the longitudinal plane increases
Decay
when the amount of magnetization in the transverse plane decreases
Types of Imaging Pulse Sequences
-gradient echo (GRE)
-spin echo (SE)
-Inversion recovery (IR)
Image weighting parameters
TR--repetition time
TE--time to echo
TI--inversion time
FA--flip angle
Intrinsic contrast paramters
controlled by:
T1 recovery time
T2 decay time
proton density
flow
apparent diffusion coefficient (ADC)
Extrinsic contrast parameters
controlled by:
TR
TE
Flip angle
TI
Turbo factor, echo train length
b value
proton density contrast
-difference in signal intensity between tissues that are a consequence of their number of protons per unit volume
Fat relaxation times at 1.5 Tesla
240-250 ms T1 time
60-80 ms T2 time
at 1.5 Tesla
Water or CSF relaxation times at 1.5 Tesla
2200-2400 ms T1 time
500-1400 ms T2 time
at 1.5 Tesla
Spin echo using one echo
-used for T1, T2, or PD
-90 degree excitation pulse followed by...
-180 degree pulse (generation of spin echo)
Spin echo using two echoes
-used for PD/T2 scans
-short TE, long TE
-long TR
Spin echo time parameters
short TE: 10-25 ms
long TE: >60 ms

short TR: 300-700 ms
long TR: 2000ms

Spin Echo: 90 degrees
Spin echo pulse sequence
1) 90 degree excitation pulse
2) NMV in transverse plane
3) FID signal
--T2* dephasing
4) 180 degree pulse
--Maximum signal
Weighting (High, Low, T1, T2, PD)
T1: Short TE, Short TR
T2: Long TE, Long TR
PD: Short TE, Long TR
Gradient Echo Pulse Sequence
1) Variable RF excitation pulse
--variable flip angle
2)Transverse component of magnetization occurs
3) Discontiue RF pulse
--dephase
4) Rephase with gradients
--gradient echo signal
Gradient Echo sequence parameters: T1
short TE, short TR, large flip angle
Gradient Echo sequence parameter: T2*
long TE, long TR, small flip angle
Gradient Echo sequence parameter: PD
short TE, long TR, small flip angle
Gradient echo sequence parameters
short TE: 5-10 ms
long TE: 15-25 ms

short TR: <50 ms
long TR: >100 ms

short flip angle: 5-20 degrees
long flip angle: 70-110 degrees
repetition time or TR
Time from the application of one RF pulse to the application of the next RF pulse for each slice, measured in milliseconds
Repetition time or TR determines the amount of ______________ that is allowed to occur between the end of one RF pulse and the application of the next
longitudinal relaxation
T1 relaxation
TR determines the amount of ____________ that has occurred when the signal is read
echo time or TE
The time from the application of the RF pulse to the peak of the signal induced in the coil, measured in milliseconds
decay
The echo time or TE determines how much ________ of transverse magnetization is allowed to occur
T2 relaxation
TE controls the amount of ______________ that has occurred when the signal is read
When is the slice selection gradient is applied?
Slice selection gradient is applied during excitation
When is the phase encoding gradient applied?
Phase encoding gradient is applied after excitation (before application of 180 refocusing pulse)
Steepness of the slope of phase encoding gradient determines...
determines the degree of phase shift/phase matrix
When is the frequency gradient applied?
Frequency encoding gradient is applied during signal collection (during rephasing and dephasing of the echo (called readout gradient), typically 8 ms (4 ms rephasing, 4 ms dephasing)
Steepness of slope of frequency encoding determines...
...determines size of anatomy covered (FOV)
A thin slice can be obtained by applying what type of gradient and/or bandwidth?
steep gradient and/or narrow bandwidth
A thick slice can be obtained by applying what type of gradient and/or bandwidth?
broad bandwidth and/or shallow gradient
Sampling frequency/rate
:rate at which frequencies are sampled or digitized during readout
Sampling time (acquisition window)
:duration of readout gradient (frequency encoding gradient)
Receive bandwidth
: the range of bandwidths of signal that will be sampled during readout
Sampling frequency is _________ to receive bandwidth and __________ to sampling time.
Proportional, inversely proportional
Gradient Axes table: Slice selection
Sagittal: X
Coronal: Y
Axial (body): Z
Axial (head): Z
Gradient Axes table: Phase Encoding
Sagittal: Y
Coronal: X
Axial (body): Y
Axial (head): X
Gradient Axes table: Frequency Encoding
Sagittal: Z
Coronal: Z
Axial (body): X
Axial (head): Y
Father of MRI
Felix Bloch
Who won the Nobel Prize in 2003 for the invention of MRI
Paul Lauterbur and Peter Mansfield
Who discovered the "Magnetic Moment"?
Otto Stern
Who measured the "Magnetic Moment"?
Isador Rabi
What is Wolfgang Puli best known for?
Pauli Exclusion Principle
Introduced concept of the spinning electron, and angular momentum
George Uhlenbeck
Who discovery of gamma rays (who and when)
Pierre and Marie Curie in 1896
Who is "Crazy Ray"?
Raymond Damadian
What are "Crazy Ray's" contributions to MRI
1971-- FONAR
1974-- Demonstrated malignant tissue--awarded first MRI patent
1976-- Completed Indomitable
What is an alternative name for MRI
"zeugmatography"
Paul Lauterbur
Defined "zeugmatography"
chemical shift
artifact caused by the frequency difference between fat and water
To maximize SNR...
INCREASE:
NEX, slice thickness, FOV, TR

DECREASE: matrix, receive bandwidth, TE
To maximize resolution (assuming a square FOV)...
INCREASE:
matrix

DECREASE: slice thickness, FOV
To minimize scan time...
INCREASE: nothing

DECREASE:
TR, phase matrix,
NEX, slice number in volume imaging
Advantages of increasing TR...
Increase SNR
Increase number of slices
Disadvantages of increasing TR...
Increased scan time
Decreased T1 weighting
Advantages of decreasing TR...
Decreased scan time
Increased T1 weighting
Disadvantages of decreasing TR...
Decreased SNR
Decreased number of slices
Advantage of increasing TE
Increased T2 weighting
Disadvantage of increasing TE...
Decreased SNR
Advantage of decreased TE
Increased SNR
Disadvantage of decreased TE...
Decreased T2 weighting
Advantages of increased NEX...
Increased SNR of all tissues
Reduced flow artifact due to increased signal averaging
Disadvantage of increased NEX
Directly proportional increase in scan time
NEX
number of excitations, the number of times an echo is encoded with the same slope of phase encoding gradient.
Advantage to decreased NEX
Decreased scan time
Disadvantage to decreased NEX
Decreased SNR
decreased signal averaging
Advantages of increasing slice thickness...
Increased SNR in all tissues
Increased coverage of anatomy
Disadvantages of increasing slice thickness..
Decreased spatial resolution
increased partial voluming in slice select direction
Advantages of decreasing slice thickness...
Increased spatial resolution and reduced partial voluming in slice select direction
Disadvantage of decreasing slice thickness....
Decreased SNR in all tissues
Decreased coverage of anatomy
Advantages of increased FOV...
Increased SNR
Increased coverage of anatomy
Decreased likelihood of aliasing
Disadvantage if increased FOV...
Decreased spatial resolution
Advantages of decreased FOV...
Increased spatial resolution
Disadvantages of decreased FOV...
Decreased SNR
Decreased coverage
Increased aliasing (pFOV)
Advantage of increased (p)Matrix...
Increased spatial resolution
Disadvantages of increased (p)Matrix...
Increased scan time
Decreased SNR if pixel small
Advantages of decreased (p)Matrix
Decreased scan time
Increased SNR if pixel large
Disadvantage of decreased (p)Matrix...
Decreased spatial resolution
Advantages of increased receive bandwidth
Decreased chemical shift
Decreased minimum TE
Disadvantage of increased received bandwidth
Decreased SNR
Advantage of decreased received bandwidth
Increased SNR
Disadvantages of decreased received bandwidth
Increased chemical shift
Increased minimum TE
Advantage of large coil
Increased area of received signal
Disadvantage of large coil
Decreased SNR
Sensitive to artifacts
aliasing with small FOV
Advantages of small coil
Increased SNR
less sensitive to artifacts
less prone to aliasing with small FOV
Disadvantage to small coil
Decreased area of received signal
steady state
...
gyromagnetic frequency
...