3: Fundamentals of MRI Physics Flashcards
Humans are made up of ___ % water
95 %
Hydrogen is in what state normally?
Spinning, non-coherent, disarrayed
Net Magnetization
the direction in which MOST protons point, when in the presence of a magnetic field
Static Magnetization Field or B0
The constant external magnetic field present in Zone 4 (MRI Room)
Precessing
Specific “spinning” activity protons have when in the presence of a magnetic field or when randomly misaligned
Precessional Frequency
The rate at which protons precess or spin around the net magnetization direction. How many times is the Hydrogen atom spinning around the magnetic field direction?
2 different names for Precessional Frequency
- Resonant frequency
- Larmor frequency
Larmor equation definition
the formula used to calculate the precessional frequency of hydrogen.
Gyromagnetic Ratio
the numerical constant used to calculate the Larmor Equation, the value for hydrogen is 42.6 MHz/T
Magnetic moment
INDIVIDUAL. the specific magnetic property or activity individual protons endure at a microscopic level in regards to normal or static state
Magnetization Moment
OVERALL. the overall amount of individual protons aligned parallel with the static magnetic field
Net Magnetization Vector or moment
When most of the protons magnetic poles align together in the direction of the magnetic field or B0, Coherent state
Paralell
Low energy protons become parallel with the B0 because it is weaker compared to the magnetic field.
Anti-parallel
High energy protons become anti-parallel with B0 because it is stronger compared to the magnetic field
Spin Up protons
Parallel, low energy, weaker compared to B0
Spin Down Protons
Anti-parallel, High energy, stronger compared to B0
Spinning properties of Hydrogen
- Net magnetization of H+ becomes static (aligned with B0)
- Hydrogen will precess around the magnetic field direction, depending on the strength of the magnet
Higher the magnetic field, the (more or less) parallel protons, (lower or higher) quality of imaging
The higher the magnetic strength, the more parallel protons, higher quality of imaging.
The higher the magnetic strength, the (the higher or lower) the precessional frequency
Higher
Larmor Equation (formula)
pf = (Y/2pie)B
precessional frequency = (gyromagnetic ratio) magnetic field strength
Gyromagnetic ratio (constant = 42.6)
Frequency of Hydrogen (MHz/T) on 0.5 Tesla
42.6 x 0.5 T = 21.3 MHz
Frequency of Hydrogen (MHz/T) on 1.0 Tesla
42.6 x 1.0 T = 42.6 MHz
Frequency of Hydrogen (MHz/T) on 1.5 Tesla
42.6 x 1.5 T = 63.9 MHz
Frequency of Hydrogen (MHz/T) on 3 Tesla
42.6 x 3 T = 127.8 MHz
What would the net magnetization’s precessional frequency of a hydrogen atom be in an MRI machine with a magnet field strength of 1.5?
- pf = 63.9 MHz *
pf = (Y/2pie)B
pf = (42.6)(1.5)
pf = 63.9 MHz
If the precessional frequency is 127.8 MHz, what would the magnetic strength of the magnet be?
- 3 Tesla *
pf = (Y/2pie)B
127.8 = (42.6)B
127.8 / 42.6 = 3 Tesla
Longitudinal direction
also known as the net magnetization direction, B0, or the direction in which aligned protons are pointing in the presence of a static magnetic field
Transverse direction
is the opposing 90 degree perpendicular direction of the net magnetization in which the static protons are pointing in the presence of a static magnetic field. The alignment direction protons point after interaction with RF excitation pulse
Relaxation time and how is it measured?
the time in which the net magnetization of protons returns back to longitudinal direction from the transverse direction. Measured in milliseconds (ms)
T1 Relaxation
what is it? what amplitude % is needed?
Measured in milliseconds (ms) the amount of time it takes for two types of tissue (fat vs. water)to return back to the longitudinal plane from the transverse plane * amplitude of 63% *
T2 Relaxation, what amplitude is needed? how is it measured?
Measured in milliseconds (ms) the amount of time it takes for two types of tissue (fat vs. water) to become dephased * amplitude of 37% *
Dephasing
the progressive drop-off of coherence of precessing protons as they relax back to the longitudinal magnetization
We differentiate tissue by _____
measuring their different relaxation times
Excitation and Relaxation Process
- all protons become aligned with the met magnetization or B0
- RF excitation pulse is applied to the aligned protons
- Aligned proton become excited by RF pulse and flips into the transverse plane
- RF pulse ends and the met magnetization begins to relax and returns to the longitudinal plane.
* The amount of time it takes for it to go back to the longitudinal direction is the relaxation time*
RF Excitation Pulse will only flip net magnetization from longitudinal to transverse direction when…
BOTH the RF pulse and the Precessional frequency have the same value - 42.6 MHz/T ** Changes depending on what magnet you’re using*
Flip Angle
The degree of how far the net magnetization tilts from the longitudinal to transverse direction.
Flip angle relies on 2 major factors:
- Amplitude of the RF pulse
- Duration of the RF pulse
T1 and T2 relaxation occurs (separately or simultaneously)
simultaneously
Spin-Lattice Relaxation
T1 Relaxation
Spin-Spin Relaxation
T2 Relaxation
Amplitude needed for T1
63 %
Amplitude needed for T2
37%
Which relaxes much faster resulting in a shorter T1 relaxation time? fat or water
fat
In T2 As tissue begins to dephase, they need to reach a separation of _____ % to reach a significant amplitude.
37%
Which de-phases faster resulting in a darker appearance than the other? fat or water?
Fat de-phases much faster than water
Spatial localization
the ability to excite a specific location in a patient’s body.
Gradients
The physical components consisting of copper wire with passing electrical current that produces magnetic field variations in a patient’s body. Varies the magnetic field to allow a specific slice to be excited and flipped into the transverse plane. Cause positive and negative variations.
Measurement used for gradient field applications
mT/M milliTesla / meter
X Gradient
Sagittal
Y Gradient
Coronal
Z Gradient
Axial
Gradients work in a ______ fashion
linear
Gradients shift magnetic strength by a value measured in …
milliTesla / meter Mt/M
Gradient field variation
in order to excite a particular slice, gradients need to create a linear positive to negative variation in the magnetic field. 4mT/M pulse, than the field will vary +4, -4.
Gradient variation formula
B (+) = B0 + ( G x D / 1,000) (stronger)
B (-) = B0 - ( G x D / 1,000) (weaker)
Magnetic strength B0
Gradient strength G
distance from center of magnet D
What is the strongest magnetic field that can be measured in a 1.5 Tesla magnet, 2 meters from the center of the magnet when at 6mT/M gradient pulse is applied?
B (+) = B0 + ( G x D / 1,000)
B+ = 1.5 + (6 x 2 / 1,000)
B+ = 1.5 + .012
B+ = 1.512
if weaker, you would B (-) = B0 - ( G x D / 1,000)
A RF pulse would be considered…
High or low energy?
ionizing or non ionizing?
low energy, non ionizing
