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1 Fundamentals
MRI Fundamentals
Question | Answer |
---|---|
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 |