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1 Fundamentals

MRI Fundamentals

QuestionAnswer
Requirements for a magnet - Magnetic field - External Rf energy source - Odd number of Nuclear Protons.
The Hydrogen Atom consists of ? A stable nucleus with an odd number of protons. consists of a single proton and is the simplest most abundant atom in the human body.
The partical of interest in clinical MRI The proton within the hydrogen atom
A positively charged particle Proton
An excited, hight energy state of proton alignment Antiparallel, North to north aligned.
A more relaxed, low-energy state of proton alignment Parallel, north to south aligned. with main mag field.
Parallel orientation of protons, is also called? Ground state
The sum of the magnetic effects of all the protons exposed to the main magnetic field. The net magnetic moment
The overall net magnetization occurs in the parallel direction, This net mag is also called ? Equilibrium magnetization
The frequency is established by the rate of precession of the net magnetization
The precessional frequency of the MR signal is often called The resonant frequency
The resonant frequency is determined by the strength of the magnetic field
The relationship between resonant frequency and the magnetic field strength is described by The Larmor Equation
This mathematical constant states, As the magnetic field strength increases, the resonant frequency of the net magnetization increases The Gyromagnetic Ratio.
The gyromagnetic ratio for hydrogen protons is 42.6 MHz/T
Larmor Equation = f = (y/ 2pie) B
the resonant frequency depends on the magnetic field strength
T or F Their is a linear relationship between the resonant frequency or Larmor frequency of the protons magnetization and the magnetic field strength to which the protons are exposed ? T
resonant (Larmor) frequency of 3T 127.8 MHz
resonant (Larmor) frequency of 1.5T 63.9 MHz
resonant (Larmor) frequency of 1T 42.6 MHz
resonant (Larmor) frequency of 0.5T 21.3 MHz
Throught what process can tissues be distinguished The relaxation process
What happens at the precessional frequency of tissues RF energy is transmitted into patients tissue, and the patients protons will generate the MR signal.
In order to differentiate tissues in MRI, we must ? Generate a signal from the body by disturbing the alignment of protons net magnetization within the main magnetic field.
Once we stop the disturbance, the magnetization will Move back, or relax, toward its equilibrium position through two distinct relaxation processes
The phenomenon which permits the efficient transfer of energy from one object or system to another Resonance
The direction of the main magnetic field is referred to as ? The longitudinal direction
The angle that the net mag is tilted away from the main mag field is known a s? RF flip angle
RF flip angle depends on what two things ? - RF amplitude - RF pulse duration
The process by which energy is lost to the environment is called relaxation Relaxation
The two mechanism of proton relaxation - T1 spin-lattice. - T2 spin-spin.
Refers to the longitudinal (spin-lattice) relaxation T1
Refers to the transverse (spin-spin) relaxation T2
T/F Both T1 & T2 relaxation processes occur simultaneously TRUE
Responsible for the regrowth of longitudinal net mag T1
The time required for the net mag to regrow to 63% of its final amplitude, is what type of relaxation? T1
White matter has a very ____ T1 relaxation time SHORT / brightest
Gray matter has a _______ T1 relaxation time somewhat shorter
CSF has a very ____ T1 relation time LONG / darkest
T1 white matter exponential curve Highest amplitude
T1 CSF exponential curve lowest amplitude / darkest
T1 weighted image white matter Brightest / short
T1 weighted image CSF darkest
A loss in traverse magnetization is called dephasing
Reasons why Dephasing occurs - Chemical shift. - Inhomogeneities. - mag susceptibility. -Spin spin interactions
The signal decay, as a result of the dephasing of traverse mag is called ? FID free induction decay
Represents the amplitude of the precessing MR signal in the transverse plane during the time course of dephasing FID
The phenomenon in which energy is transferred from one proton to another is called A spin-spin interaction
The reduction of traverse magnetization due to spin-spin interactions is known as? T2 relaxation
The time at which the traverse magnetization has decayed to 37% of its full value due entirely to spin spin interactions T2 relaxation
T/F Tissues with longer T2 relaxation times exhibit a longer lasting traverse magnetization ? TRUE (i.e. dephase more slowly)
CSF has a _____ T2 relaxation time LONG / brightest
gray matter has an ______ T2 relaxation time intermediate
White matter has a _____ T2 relaxation time SHORT / darkest
T2 CSF exponential curve Highest amplitude / longest relaxation time / brightest
T2 white matter exponential curve Lowest amplitude / shortest relaxation time / darkest
TE = The time from the center of the 90 RF pulse to the center of the echo is called echo time, or TE.
Can be varied in order to control the contrast between tissues in the images TR & TE
The parameter which controls how much contrast due to T1 relaxation is seen between tissues TR. as the TR is shortened, the T1 contrast is improved.
The perimeter which controls how much contrast due to T2 relaxation is seen between the tissues TE. as the TE is lengthened, the T2 contrast is improved.
The amount of time required for the longitudinal magnetization to recover 63% of its full magnitude T1 relaxation
As ___ is shortened, contrast based on the recovered longitudinal magnetization of each tissue becomes greater. TR
The TR controls the degree to which the longitudinal magnetization recovers after an RF excitation pulse according to the _____ time of each tissue . T1
An RF pulse of what flip angle will maximize the component of the net magnetization projected into the transverse plane ? 90
Before any RF energy is transmitted to the protons, the net mag is initially aligned with _____? The main magnetic field
What is the time from the 180 degree RF pulse to the echo ? TE
The ____ pulse is place exactly halfway between the time of the 90 degree pulse and the desired echo time 180 degree pulse
The time from the 90 pulse to the next 90 pulse is the ___? TR
The introduction of a contrast agent which further shortens the ____ time of blood will make perfused tissues appear like _____ _____ tissues. T1. Shorter T1 tissues
A vascular lesion would appear______ on a T1-weighted contrast-enhanced image as compared to tits appearance on a T1-weighted unenhanced image. Brighter
The protons ______ contributes to its magnetic properties. 1. spinning motion 2. positive charge 3. mass not mass
What is the time from the initial Rf excitation pulse to the echo called? TE
The _____ is the time between successive RF excitation pulses TR
The ___ is the time between the initial 180 degree RF pulse and the 90 degree excitation pulse in an inversion recovery sequence. TI
The time course of the loss of transverse magnetization describes T2 relaxation time
Is the result of spin spin interaction T2 relaxation
Substances with a LONG T2 relaxation time exhibit a lower rate of spin-spin interactions, and this results in a slower loss of _______ magnetization. Traverse
edema has a ____ T2 time longer
Longer T2 times will generate a ____ signal in the traverse plane and results in a ______ appearance on the image. Longer signal. Brighter appearance.
On a T2-weighted image, CSF appears bright because it has a _____ relaxation time. LONG T2
Because CSF has long T1 time, A _____ TR must be used in a T2-weighted image in order to eliminate T1-weighted contrast effects. Long TR
What perimeter controls the amount of contrast seen in an image due to T2 relaxation TE
T2 relaxation refers to _____ th degree to which spin-spin interactions have occurred
The result of using 45 degree flip angle type of a Reduced flip angle, used in fast imaging sequences, such as gradient echo pulse sequences. allowing the sequence to be repeated much more quickly
Referes to the longitudinal recovery of magnetization T1 relaxation
Refers to the transverse decay of magnetization T2 relaxation
Describes the time course of spin lattice interactions which cause the recovery of the longitudinal magnetization T1 relaxation
Describes the time course of spin spin interactions which caused the decay of traverse magnetization T2 relaxation
Who has the shorter relaxation time, T1 or T2 T2
Used to determine the processional frequency of the protons net magnetization Larmor equation
The longitudinal magnetization gradually recovers according to the____time of the populations protons T1
T1 relaxation is also called Spin lattice relaxation/longitudinal relaxation
T2 relaxation is also called spin spin relaxation
Generally tissues with the shortest T1 will appear________ on the reconstructed image Brighter
Projects the longitudinal magnetization into the traverse plane The initial RF excitation pulse
The_____ is the time between the initial RF inversion pulse and the RF excitation pulse TI
_____ Controls the amount of time that T2 relaxation is allowed to occur TE
Longer echo times allow_______ T2 relaxation to occur Greater
______ is used to maximize the rephasing of the transverse magnetization during the echo 180 degree RF pulse
Created by: tomvilla21
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