MRI Physics - Basic Principles
Terms
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- atomic number
- total protons in an atom
- mass number
- total protons and neutrons in an atom
- is the mass number of an isotope of an atom usually an even or odd number
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odd number
most atoms have balanced numbers of neutrons and atoms (i.e. even mass number) isotopes of atoms usually have slightly more or fewer neutrons than protons (i.e. odd mass number) - what are the three types of motion present within an atomm
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electrons spinning on their own axis
electrons orbiting the nucleus
nucleus spinning on its own axis - do atomic nuclei usually have a net spin
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no, most nuclei do not usually have a net spin (i.e. no angular momentum)
however, in nuclei with odd mass numbers, spin directions of neutrons and protons are not balanced and therefore there is a net spin or angular momentum. Such nuclei are known as "MRI active nuclei." - What is the law of electromagnetic induction?
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It refers to the three types of individual forces: motion, magnetism, and charge.
If two of these are present, then the third is automatically induced.
MR active nuclei that have a net charge and are spinning (motion), automatically acquire a magnetic moment and align with an external magnetic field. - name some MR active nuclei and their mass numbers (7)
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1 hydrogen
13 carbon
15 nitrogen
17 oxygen
19 fluorine
23 sodium
31 phosphorous - what does the term low energy and high energy nuclei refer to?
- When hydrogen nuclei (a single proton) are placed in an external magnetic field, some align parallel to the field (low energy nuclei, spin-up) and a smaller population align anti-parallel to the magnetic field (high energy nuclei, spin-down).
- What is NMV (net magnetization vector)? Is it larger or smaller with higher or lower external field strengths?
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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.
As the strength of the external magnetic field (B0) is increased the energy difference between spin-up and spin-down nuclei increase. As a result fewer nuclei have enough energy to be aligned anti-parallel to the external field and NMV increases. This increase in NMV at higher field strengths results in improved signal to noise seen at higher field strengths. - What is the Larmour equation?
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This equation describes the precessional frequency of a nucleus in an external magnetic field.
It tells us two important facts:
1) All MR active nuclei have their own gyro-magnetic constants. For this reason when they are exposed to the same field strength they precess at different frequencies.
2) As the external field strength increases the Larmour frequency increases. - What are two important concepts that are implied by the Larmour equation?
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It tells us two important facts:
1) All MR active nuclei have their own gyro-magnetic constants. For this reason when they are exposed to the same field strength they precess at different frequencies.
2) As the external field strength increases the Larmour frequency increases. - What is resonance?
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It is the phenomena that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.
When a nucleus is exposed to an external perturbation that has an oscillation similar to its own natural frequency, the nucleus gains energy from the external force. - 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:
42.57 MHz at 1 T
63.86 MHz at 1.5 T - 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. - What determines flip angle? (2)
- The magnitude and duration of the RF pulse applied.
- What are two consequences of the application of an RF pulse at the Larmour frequency?
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NMV moves out of alignment with external magnetic field (B0) by the flip angle.
Magnetic moments of nuclei move into phase with each other. - What is the free induction decay signal?
- 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.
- Does spin-lattice relaxation refer to T1 recovery or T2 decay?
- T1 recovery which is caused by nuclei giving up their energy to the surrounding environment or lattice.
- What does T1 relaxation time measure?
- the time it takes for 63% f the longitudinal magnetization to recover in the tissue.
- What does T2 relaxation time measure?
- The time it takes for 63% of the transverse magnetization to be lost?
- Does spin-spin relaxation refer to T1 recovery or T2 decay?
- T2 decay which is caused by nuclei exchanging energy with neighboring nuclei.
- What is TR?
- 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.
- What is TE?
- 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.
- What is the T1 of myocardium at 1.5T? T2?
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T1 is approx 900 ms
T2 is appros 70 ms - What is the T1 and T2 of fat at 1.5T?
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T1 260 ms
T2 110 ms - What is Faraday's Law?
- A magnet moving (rotating) in the vicinity of a conductor will induce a current to flow in the conductor
- 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 - What are Helmholtz coils?
- Two circular loops with current in the same direction that produce fields that combine to form a uniform field
- What are Maxwell Coils?
- Two circular loops with current in the opposite direction produce fields that combine to form a uniform field gradient
- Is T1 relaxation a field-dependent or field-independent process?
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Yes, T1 relaxation requires a net transfer of energy between the lattice and the spin systems
Due to the energy requirement, the T1 relaxation process is field dependent, i.e. the higher the field, the longer the T1 since the body (lattice) continues to transfer energy at the same rate, and more energy is required at higher fields - Is T2 relaxation a field-dependent or field-independent process?
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It is essentially field independent
T2 describes the loss of coherence between spins and involves an energy exchange between pairs of spins.