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27 Cards in this Set
- Front
- Back
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a system that uses coordinates to establish position is known as?
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Coordinate system
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What is NMV (net magnetization vector)?
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NMV reflects the relative balance between spin-up (low energy) nuclei and spin-down (high energy) nuclei and reflects the net magnetic moment of protons in a magnetic field.
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Symbolized by M ?
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NMV
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What is NMV (net magnetization vector)?
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Symbolized by M
Present the sum of the mgnetization of all H protons May have two components: transverse and longitudinal Longitudinal – Parallel with Bo Transverse – perpendicular to Bo |
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(NMV) Longitudinal component is also named –
and is symbolized by - Transverse component is named – and is symbolized by - |
longitudinal magnetization
(symbolized Mz) transverse magnetization (symbolized Mxy) |
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Describe "RF pulse"
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A second smaller magnetic field in the form of radio – frequency (RF)
Symbolized B1 RF pulse is introduced to the H protons in order to cause them to move from low energy state to the high energy state |
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Bo vs. B1
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Bo – a magnetic field that is constant; main magnetic field;
B1 – a magnetic field that is rapidly rotating (oscillating) and always oriented perpendicular to Bo |
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Describe the MR signal.
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Once equilibrium has been established, a second, small magnetic field in the form of an RF pulse (B1) is introduced to NMV (symbolized M) at the Larmor frequency
Introduction of B1 causes M to shift in size and direction (and causes protons to move in phase) away from Bo This will result in transverse component to increaseAs the NMV is flipped into the transverse plane as a result of resonance, it passes across the receiver coil inducing a voltage in it. This voltage is the MR SIGNAL. |
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Introduction of B1 causes what to happen?
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Introduction of B1 - -causes M to shift in size and direction (and causes protons to move in phase) away from Bo
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The frequency of the signal is the same as the Larmor frequency – the magnitude of the signal depends on the amount of magnetization present in the transverse plane
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the MR signal.
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Describe "Relaxation".
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Begins at the time the B1 is turned off
Hydrogen nuclei give up absorbed RF energy M returns to Bo The magnetic moments of hydrogen lose coherency |
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The very first MR signal captured by coils -
It is not a real MR signal. |
Free induction decay (FID) signal
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free induction decay (FID):
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‘free’ because of the absence of the RF pulse; and
• ‘induction decay’ because of the decay of the induced signal in the receiver coil. |
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The withdrawal of the RF produces several effects:
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Nuclei emit energy absorbed from the RF pulse
Nuclei lose precessional coherence or dephase |
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What are two consequences of the application of an RF pulse at the Larmour frequency?
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1. NMV moves out of alignment with external magnetic field (B0) by the flip angle.
2. Magnetic moments of nuclei move into phase with each other. |
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What is the free induction decay signal?
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When an external RF pulse is switched off, the NMV of the protons try to align with B0. As this relaxation process occurs, the magnitude of the transverse magnetization decreases. This causes a decrease in the voltage induced in the receiver coil. The induction of reduced signal in the receiver coil is called the free induction decay signal.
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Does spin-lattice relaxation refer to T1 recovery or T2 decay?
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T1 recovery which is caused by nuclei giving up their energy to the surrounding environment or lattice.
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What does T1 relaxation time measure?
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the time it takes for 63% f the longitudinal magnetization to recover in the tissue.
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What does T2 relaxation time measure?
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The time it takes for 63% of the transverse magnetization to be lost?
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Does spin-spin relaxation refer to T1 recovery or T2 decay?
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T2 decay which is caused by nuclei exchanging energy with neighboring nuclei.
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What is TR?
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The time from the application of one RF pulse to the application of the next RF pulse. It controls the amount of T1 relaxation that has occured.
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What is TE?
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the echo time (TE) refers to the time from the application of the RF pulse and the peak of the signal induced in the coil. It controls the amount of T2 relaxation that has occurred when the signal is read.
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What is Faraday's Law?
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A magnet moving (rotating) in the vicinity of a conductor will induce a current to flow in the conductor
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What properties of a RF excitation pulse allow it to tip the NMV of a proton utilizing relatively low energies? (3)
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It is applied in the transverse plane
It is applied at the resonance frequency the resonance frequency is in the radiofrequency range |
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What is flip angle?
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When an external RF energy is applied to a proton at its Larmour frequency it causes the net magnetization vector (NMV) to move out of alignment with the external field (B0). The angle to which NMV moves out of alignment is called the flip angle.
The magnitude of the flip angle depends on the amplitude and duration of the RF pulse. |
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What determines flip angle? (2)
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The magnitude and duration of the RF pulse applied.
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What is the Larmour frequency of hydrogen in a 1 T magnet? 1.5 T magnet?
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The gyro-magnetic ratio for hydrogen is 42.57 MHz/T
Larmour frequency for hydrogen: 42.58 MHz at 1 T 63.86 MHz at 1.5 T |
