- Concerning nuclear spin (I), which of the following is true?
- Spin is due to rotation of the nucleus about its axis.
- Protons have spin, but neutrons do not.
- Spin can only have integer or half-integer values.
- Another name for spin is "precession".
Spin is a fundamental quantum property of subatomic particles and does not result from their physical rotation. Many subatomic particles besides the proton have spin, including the neutrons and electrons. Spin is quantized and can take on a limited number of discrete values, so c) is true. When placed in an external magnetic field, nuclear spin results in precession, but spin and precession are not the same. Link to Q&A discussion
- Which of the following spins (I) could a nucleus not possess?
Quantum mechanics restricts nuclear spin to only integer or half-integer (1/2, 3/2, 5/2, etc) values, so I = 3/4 is not permitted. Link to Q&A discussion
- Concerning nuclear spin (I), which of the following statements is false?
- A longer but equivalent name for "spin" is "spin angular momentum".
- For hydrogen (¹H) MRI it is common and acceptable to use the terms "nucleus", "spin", and "proton" interchangeably.
- Routine clinical MRI measures signal from hydrogen (¹H) nuclei only.
- The hydrogen (¹H) nucleus contains one proton and one electron.
The hydrogen (¹H) nucleus contains only a single proton surrounded by an electron cloud, so d) is false. The other statements are all true. Link to Q&A discussion
- Concerning nuclear spin (I), which of the following statements is false?
- Protons and neutrons each have spin = ½.
- To determine net nuclear spin (I), you simply add up the number of protons and neutrons and divide by 2.
- Different isotopes of the same element commonly have different nuclear spins.
- Every element has at least one isotope with non-zero spin.
Net nuclear spin (I) does depend on the total number of protons and neutrons, but no simple formula for I exists as interactions between more elementary components (quarks and gluons) must be considered. The other statements — a), c) and d) — are all true. Link to Q&A discussion
- Concerning nuclear spin (I) and NMR, which of the following statements is false?
- All nuclei can undergo NMR except those containing even numbers of both protons and neutrons.
- Every element in the periodic table has at least one isotope that can undergo NMR.
- Across the periodic table nuclear spins (I) with values ranging from 0 to 8 can be found.
- Nuclei with I = 0 readily undergo NMR.
Only nuclei with non-zero spins can undergo NMR, so d) is false. The other statements are all true. Link to Q&A discussion
- Which of the following statements concerning the magnetic dipole moment is false?
- It is a representation of the nucleus modeled as a tiny bar magnet with north and south poles.
- An alternative representation is a vector (μ) arising from a small current loop.
- Like a compass needle, a dipole moment will tend to align with an externally applied magnetic field to assume its lowest energy state.
- The dipole moment will precess when placed in an external magnetic field.
The magnetic dipole moment (μ) will experience a torque causing it to align with an externally applied magnetic field, but will not precess around the field. Link to Q&A discussion
- Which of the following statements about a magnetic dipole placed in an external magnetic field is false?
- The dipole is at its lowest energy state when pointing in a direction opposite the field.
- The torque (twisting force) experienced by the dipole is directly proportional to the strength of the external field.
- The torque (twisting force) experienced by the dipole depends on the angle between the dipole and the external field.
- The energy of the dipole-external field system depends on the angle between the dipole and the external field.
The dipole (μ) experiences a torque (τ) given by the vector cross-product τ = μ x B0. In terms of scalar magnitudes: ||τ|| = ||μ|| ||B0|| sin θ, where θ is the angle between them. The energy (E) is defined by the dot product, E = −μ • B0 = −||μ|| ||B0|| cos θ. The energy is at its maximum (not a minimum), when the dipole points opposite the field (θ = 180°), so answer a) is false, . Link to Q&A discussion
- The dipole magnetic moment (μ) is directly proportional to nuclear spin (I), connected by a constant called the
- Gyromagnetic ratio (γ)
- Planck's constant (h)
- Nuclear susceptibility (χ)
- Chemical shift (δ)
The two are connected by the gyromagnetic ratio (γ), described through the relationship: μ = γI Link to Q&A discussion
- Which of the following statements about the gyromagnetic ratio (γ) is false?
- It can be expressed in units of MHz/Tesla.
- It may have a negative value.
- It is different for each element.
- It is the same for all isotopes of a given element.
The gyromagnetic ratio (γ) is a constant of proportionality between the dipole magnetic moment (μ) and nuclear spin (I). It is commonly expressed in units of MHz/T. Although usually a positive number, it may be negative. A negative value for γ means that the magnetic moment and spin point in opposite directions. The gyromagnetic ratio (γ) is different for every isotope of every element, so answer d) is false. Link to Q&A discussion
- The ¹H nucleus has a spin (I) = ½. When placed in an external magnetic field, its number of measurable spin states (eigenstates) will be
The number of observable spin states for a nucleus with spin I equals 2I +1. So the ¹H nucleus has 2(½) + 1 = 2 possible spin states. Link to Q&A discussion
- Besides ¹H, two nuclei commonly studied by NMR spectroscopy are ³¹P and ¹³C. The fact that all three isotopes have identical nuclear spins (I) = ½ means that when placed in an external magnetic field
- They will have exactly the same resonant frequencies.
- They will each exhibit two discrete measurable energy states.
- The difference in energy states will be the same for each isotope.
- They will have the same gyromagnetic ratio (γ).
All spin-½ particles will exhibit two discrete energy states when placed in an external magnetic field, so answer b) is true. The isotopes will have different gyromagnetic ratios (γ), different resonant frequencies, and different energy levels, however. Link to Q&A discussion
- ²³Na is another NMR active isotope used for imaging research. It has a nuclear spin (I) = 3/2. Compared to the ¹H nucleus with I = 1/2, in an external magnetic field
- ²³Na will have a higher gyromagnetic ratio (γ) than ¹H.
- ²³Na will have a higher resonance frequency than ¹H.
- ²³Na will manifest 3 discrete energy states.
- ²³Na will manifest 4 discrete energy states.
The number of observable energy states for a nucleus with spin I equals 2I +1. The ²³Na nucleus therefore has 2(3/2) + 1 = 4 possible spin states, so answer d) is correct. The gyromagnetic ratio (γ) and hence resonance frequency do not depend on I. Link to Q&A discussion
- The famous experiment demonstrating how spin-½ particles can be physically separated into two groups by a magnetic field was performed in 1922 by
- Stern and Gerlach
- Planck and Dirac
This is a description of the Stern-Gerlach experiment, often provided as tangible proof for the quantization of nuclear angular momentum visible in the macroscopic world. Link to Q&A discussion
- Which of the following names refers to the highest energy spin state of an ¹H nucleus in a magnetic field?
The highest energy state occurs when the spin has a direction opposite that of the main magnetic field and is denoted spin-down (correct answer), anti-parallel, or |−½>. Link to Q&A discussion
- Why don't all the nuclear spins simply fall to their lowest energy states to minimize total system energy?
- They actually do. The above statement is false.
- Such a highly skewed distribution of energy states is prohibited by quantum mechanics.
- Thermal molecular collisions tend to equalize the distribution of nuclei between lower- and higher-energy states.
- The distribution of energy states cannot be known due to the Heisenberg Uncertainty Principle.
The natural tendency for spins to fall to lower energy states is offset by thermal collisions that tend to equalize the two energy levels. The result is a compromise predicted by the Boltzmann distribution. Link to Q&A discussion
- According to quantum mechanical (QM) theory and experiment, which statement is true?
- A spin-½ particle like the proton must exist exclusively in either the spin-up or spin-down state.
- Conclusive evidence for the QM restriction described in part a) is provided by the Stern-Gerlach experiment.
- Scientists have been able to trap and manipulate spins in a superposition between their up- and down-states.
- There is nearly universal agreement among scientists that the Copenhagen Interpretation of QM is correct.
Quantum mechanics does not require a proton to exist exclusively in one of its two primary (eigen)states, only a linear combination of the two. Thus answer a) is false. The Stern Gerlach experiment only shows that when a measurement is made a group of spin-½ particles that one of two results becomes physically manifest. This does not speak to the underlying quantum reality prior to measurement, however, so b) is also false. Haroche and Weinland, working independently, were awarded the Nobel Prize for Physics in 2012 for their experiments trapping and manipulating spins in superposition states, so c) is true. The Copenhagen Interpretation of QM, while still the most popular, is far from being universally accepted, so d) is false. Recent polls of quantum physicists found the Many Worlds Interpretation gaining significant ground. Of course, neither may be correct! Link to Q&A discussion
- Nuclear precession can be considered the result of a "twisting force" or torque (τ) on a spin's angular momentum as it interacts with an externally applied magnetic field. The orientation of this torque is
- Collinear with the spin.
- Collinear with the spin but in the opposite direction.
- Collinear with the magnetic field.
- Perpendicular to both the spin and magnetic field.
The torque (τ) is perpendicular to both the spin angular momentum and the magnetic field. This creates a "twisting force" that can be thought of as "pushing the spin from the side" resulting in nuclear precession rather than alignment with the field. Link to Q&A discussion
- Precession may be expressed in either angular (ω0) or cyclic (f0) frequency. The two are are related by the equation
- ω0 = 2 π f0
- f0 = 2 π ω0
- ω0 = π f0
- f0 = π ω0
The correct equation is a). Angular frequency (ω0) is measured in radians per second, where 2 π radians/sec = 360°/sec = 1 cycle (or revolution) per second = 1 Hertz (Hz). Thus the cyclic frequency (f0) must be multiplied by 2 π to obtain angular frequency (ω0). Link to Q&A discussion
- The conventional units for angular frequency (ω0) are
- Cycles per second (cps)
- Hertz (Hz)
- Revolutions per minute (rpm)
The correct answer is c), radians per second. The other options are used to express cyclic frequency (f0). Link to Q&A discussion
- Which of the following is the correct form of the Larmor equation?
- f0 = γ B0
- f0 = γ / B0
- f0 = B0 / γ
- None of these equations is correct.
The correct answer is a), f0 = γ B0. Link to Q&A discussion
- What is the approximate gyromagnetic ratio (γ) of the ¹H nucleus?
- 10.7 MHz/Tesla
- 42.6 MHz/Tesla
- 64.0 MHz/Tesla
- 128 MHz/Tesla
The correct answer is b), 42.6 MHz/Tesla. I don't think you need to memorize this number, but it can easily be retrieved as I do believe everyone involved with MRI should know that a 1.5T scanner has a resonance frequency of about 64 MHz. Thus γ ≈ 64 MHz ÷ 1.5 Tesla, giving a γ of about 42.6 MHz/T. Link to Q&A discussion
- If the ¹H resonance frequency in a 1.5T scanner is about 64 MHz, what is the approximate ¹H resonance frequency at 7T?
- 128 MHz
- 256 MHz
- 298 MHz
- 426 MHz
Resonance frequency is directly proportional to field strength. So the ¹H resonant frequency at 7.0T can be calculated as 64 MHz x (7.0/1.5) ≈ 298 MHz. Link to Q&A discussion
- The gyromagnetic ratio (γ) of the ¹³C nucleus is about 10.7 MHz/T. What is the ¹³C
resonance frequency at 3.0T?
- 10.7 MHz
- 21.4 MHz
- 32.1 MHz
- 64.2 MHz
By the Larmor equation: f0 = γ B0 = (10.7 MHz/T) x 3.0T = 32.1 MHz. Link to Q&A discussion
- Sir Joseph Larmor developed his famous equation to allow calculation of NMR resonance frequencies in different magnetic fields.
False. Larmor developed his equation in the late 19th Century before NMR was even discovered. He was investigating the splitting of optical spectra by magnetic fields, and his equation described the precession of orbital electrons. In the 20th Century the Larmor equation was found to apply to any particle with spin angular momentum and was hence fully applicable to NMR. Link to Q&A discussion
- Which of the following statements about nuclear precession is true?
- Nuclear precession will not begin until a radiofrequency pulse is applied.
- Sustaining nuclear precession requires the continual input of energy from the environment.
- Protons in every drop of water in the ocean and in every snowflake at the north pole are precessing right now.
- It is impossible to obtain MR images using the earth's magnetic field because it is so small.
Nuclear precession occurs spontaneously when protons are placed in any magnetic field. No energy input is required to start or sustain precession. Precession occurs even in the tiniest magnetic fields, including that of the earth, so answer c) is true. In fact, crude MR images using the earth's magnetic field alone have been obtained. Link to Q&A discussion
- The slight difference in resonant frequencies noted between ¹H-nuclei in different molecular environments is due to
- Different gyromagnetic ratios
- Different local magnetic fields
- Different relaxation times
- Different spin quantum numbers
This question concerns the origin of chemical shift. Variable shielding and deshielding of ¹H-nuclei by molecular electron clouds results in slightly different local magnetic fields that each nucleus experiences. These different local fields alter resonance frequencies slightly. Link to Q&A discussion
- Chemical shifts (δ) are typically reported in units of
- Gauss (G)
- Millitesla per meter (mT/m)
- Parts per million (ppm)
- Percent (%)
Chemical shifts are typically reported in ppm, which is independent of field strength. Note that ppm, like %, is dimensionless. Link to Q&A discussion
- The abbreviation "ppm" stands for
- Proton paramagnetic moment
- Proton-proton magnetization
- Precession per minute
- Parts per million
The correct answer is d), "parts per million", the commonly used field-independent method to report chemical shifts. Link to Q&A discussion
- The chemical shift (δ) between water and fat protons measured at 1.5T is approximately 3.5 ppm. What would their chemical shift be at 3.0T?
- 1.75 ppm
- 3.5 ppm
- 7.0 ppm
- 10.5 ppm
The chemical shift (δ) in ppm is independent of field strength, so b) is correct. Link to Q&A discussion
- The methyl protons of two brain metabolites, N-acetyl aspartate (NAA) and Creatine (Cr), have a chemical shift difference of 1.0 ppm. At a field strength of 1.5 T (where the water Larmor frequency is 64 MHz), their difference in frequency would be about
- 64 MHz
- 1.0 MHz
- 64 kHz
- 64 Hz
The frequency difference is just the Larmor frequency x the chemical shift = 64 MHz x 1 ppm = (64 x 106 Hz) x (1.0 x 10−6) = 64 Hz. Link to Q&A discussion
- 110 Hz
- 220 Hz
- 440 Hz
- 660 Hz
Field strength is doubled, so Larmor frequency is doubled, and frequency difference due to chemical shift is also doubled. The correct answer is therefore c). Link to Q&A discussion
- M can be considered the vector sum or average millions of individual nuclear spins.
- Using M allows the NMR phenomenon to be analyzed using classical (rather than quantum) physics.
- At equilibrium, M is aligned with the external magnetic field.
- At equilibrium, M precesses around the direction of the external magnetic field.
All statements are true except d). At equilibrium, M is stationary and aligned with the external magnetic field. Application of an RF-pulse is required to tip M out of alignment with B0 at which time it will begin to precess. Link to Q&A discussion
- Is effectively zero in all directions.
- Has a significant non-zero longitudinal component.
- Has significant non-zero transverse components.
- Spontaneously precesses.
In the absence of a strong external magnetic field, individual proton spins are randomly oriented in space and their vector sum is essentially zero in all directions. In reality, however, some small external magnetic field is always present, if only from the earth itself. Thus M is only effectively (but never completely zero) in all directions. Link to Q&A discussion
- This premise of the question is false; M develops transverse components from the very beginning.
- The transverse components are not seen at first because they take several seconds to develop.
- The transverse components are neutralized by the precession of M.
- The individual spins contributing to M have randomly dispersed transverse components lacking phase coherence.
Answer d) is correct because the spins have random transverse components of angular momentum. An RF-pulse or energy input near the Larmor frequency will be necessary to generate some phase coherence and transverse components. Link to Q&A discussion
- Spin density (ρ)
The initial growth of longitudinal magnetization (Mz) is a simple exponential with time constant T1. Link to Q&A discussion
- Net magnetization (M) develops when an unmagnetized sample of tissue is placed in an external magnetic field.
- Initially M grows in the longitudinal direction as the individual spins seek to align with B0.
- When tipped out of alignment with B0, M will precess at the same resonance frequency as the individual nuclei comprising it.
- M will continue to precess even when completely inverted and pointing in the −z direction (i.e. opposite to B0).
Only d) is false. Even though the individual spins comprising M are always precessing, M itself does not precess unless tipped out of alignment with B0 allowing it to develop nonzero transverse components. Once M is completely inverted 180° with respect to B0, it no longer has transverse components and is no longer precessing. If left alone after such an inversion, M will simply regrow along the z-axis to return to its initial orientation and magnitude aligned with B0. Link to Q&A discussion
- Tipping the net magnetization (M) out of initial alignment with B0 requires absorption of energy by the spin system.
- In MRI, the source of energy required to initiated NMR is typically provided by a rotating/oscillating radiofrequency field named B1.
- This tipping of (M) is a manifestation of the NMR phenomenon.
- Nuclear precession and resonance are essentially the same.
All statements are true except d). Nuclear precession is experienced by all non-zero spin particles when placed in an external magnetic field and requires no input of energy. NMR is a special condition of a spin system requiring the absorption and release of energy over a narrow range of frequencies. Link to Q&A discussion
- Which of scientist first experimentally demonstrated the NMR phenomenon in the 1930's and gave it its name?
- Felix Bloch
- Isidor Rabi
- Edward Purcell
- Peter Mansfield
Isidor Rabi is credited with naming NMR and being the first to demonstrate its existence experimentally in a molecular beam of LiCl. Link to Q&A discussion
- Which of the following statements about the NMR discoveries of Felix Bloch and Edward Purcell is true?
- The two worked together in adjacent labs at Harvard.
- Their experimental setups were nearly identical.
- Their initial reports were published simultaneously in 1946.
- Only Bloch received the Nobel Prize for his research.
Bloch performed his research at Stanford at the same time Purcell was working independently at MIT. Their experimental setups were quite different, Bloch detecting an induction signal from water and Purcell measuring energy absorption in solid paraffin wax. Their initial reports were published simultaneously in the January, 1946 issue of the journal Physical Review, so answer c) is true. Bloch and Purcell jointly received the Nobel Prize for Physics in 1952. Link to Q&A discussion
- The radiofrequency (RF) field used to inject energy into a spin system to induce nuclear resonance is called
The correct answer is B1. B0 is the main magnetic field and Mxy are the transverse components of net magnetization induced by B1. There is no field called B2. Link to Q&A discussion
- For resonance to occur, B1 must be applied exactly at the Larmor frequency.
The B1 must only be applied reasonably close to the Larmor frequency, not exactly. Energy transfer is optimized when B1 is precisely on resonance, but appreciable tipping of M will still occur even if the frequency is not perfect. Link to Q&A discussion
- For resonance to occur, B1 must be applied exactly perpendicularly to B0.
The only requirement is that B1 not be collinear with B0 and have have some perpendicular components. Energy transfer is optimized when B1 is perfectly orthogonal to B0, however. Link to Q&A discussion
- Which of the following statements about flip angle using conventional RF-pulses is false?
- Flip angle depends on the strength of the RF-pulse.
- Flip angle depends on the duration of the RF-pulse.
- Flip angle is measured relative to the direction of B1.
- More energy is injected into the system by a 180°- than a 90°-pulse.
Flip angle is measured relative to the direction of B0, not B1. The other statements are all true. Link to Q&A discussion
- (Advanced) Which of the following statements concerning the spin-system immediately after a 90°-pulse is true?
- If the z-component of angular momentum were measured for all protons, an equal number of spin-up and spin-down states would be observed.
- The 90°-pulse causes the spins to precess around B1.
- The spins all become locked into phase coherence with one another.
- The spin angular momentum for each proton is turned so that it points horizontally in the direction of B1.
This is a pretty tricky question that really probes one's understanding of the NMR phenomenon. Only a) is true. If the longitudinal angular momentum of all spins were measured (by passing them through all Stern-Gerlach device, for example), an equal distribution of spin-up and spin-down states would be observed. Remember that spins do not exist in pure eigenstates, so it is not correct to say there are an equal number of spin-up and spin-down protons (though that statement is commonly found in textbooks). But the idea is much the same. Option b) is false, but only barely so. Precession after a 90°-pulse occurs around the direction of B0, just as it did prior to the RF-pulse. During the application of the RF-pulse, however, procession occurs around both B0 and B1, described in more detail in the Q&A's about the rotating frame. Option c) is false, as no "magical" locking together of spins occurs. What we call "phase coherence" is simply the same initially skewed longitudinal distribution of spin energies that have been rotated into the transverse plane by the RF-pulse. Finally, option d) is false. By the Heisenberg uncertainty principle we cannot know the direction a spin is "pointing", so the statement is meaningless. But I'm pretty sure the spins would not all be "pointing" horizontally if we were able to know! Link to Q&A discussion
- When stimulated to higher energy levels by RF-excitation, protons spontaneously release this energy and fall back to lower levels.
Spontaneous emission of absorbed energy is extremely improbable at the (MHz) frequencies where NMR occurs. Virtually all energy release by an NMR spin system must be induced by direct interaction with its external environment. This is the principle underlying T1 relaxation. Link to Q&A discussion
- The complex motion of the net magnetization vector (M) when acted upon by both B0 and B1 can be simplified by considering the system in the
- Laboratory frame of reference.
- Rotating frame of reference.
- Earth's frame of reference.
- Adiabatic frame of reference.
The rotating frame of reference allows the motion of (M) to be simplified by removing the simple Larmor precession around B0. It's like riding on a merry-go-round instead of watching it from stationary ground nearby. Link to Q&A discussion
- (Advanced) What happens if the B1 field is not applied exactly at the Larmor frequency?
- Spins will not be affected and will continue to precess only around B0.
- Spins will stop precessing around B0 and begin to precess around B1.
- Spins will precess around an effective field Beff in the rotating frame that takes into account of off-resonance B1 and B0 effects.
- The spin system will experience random and unpredictable fluctuations in energy levels.
The correct answer is c). It describes the "off-resonance" condition which is commonly encountered in MR imaging where a span of RF-frequencies is transmitted and gradients affect Larmor frequencies across the object being imaged. Link to Q&A discussion
- (Advanced) Which of the following statements about adiabatic excitation is false?
- Unlike "conventional" RF-pulses that are purely amplitude-modulated, adiabatic RF-pulses are also frequency-modulated.
- The fat-suppression technique SPAIR uses adiabatic inversion.
- Adiabatic pulses are relatively insensitive to B1 field inhomogeneities.
- Doubling the duration of a 90°-adiabatic pulse creates a 180°-adiabatic pulse.
All statements are true except d). Unlike "conventional" RF-pulses, the flip angle of an adiabatic pulse is NOT proportional to its magnitude and duration. An adiabatic pulse cannot be easily scaled or stretched to change its effect. Link to Q&A discussion
- Which of the following is not a synonym for T1 relaxation?
- Spin-spin relaxation
- Spin-lattice relaxation
- Longitudinal relaxation
- Thermal relaxation
Spin-spin relaxation is a synonym for T2 relaxation. Link to Q&A discussion
- Which of the following is/are synonyms for T2 relaxation?
- Spin-spin relaxation
- Transverse relaxation
- Thermal relaxation
- Both a) and b)
Thermal relaxation is a synonym for T1. Link to Q&A discussion
- Which of the following statements about T1 relaxation is false?
- T1 is the time constant for regrowth of longitudinal magnetization (Mz).
- T1 relaxation requires an energy transfer between spins and their environment ("lattice").
- T1 relaxation results in a net energy loss from the spin system.
- This energy loss occurs by spontaneous emission of photons from the protons.
All are true except d). T1 relaxation requires release of absorbed energy from the spin system to the external environment, but this such energy transfer must be stimulated by interaction with a fluctuating field near the Larmor frequency arising from a nearby proton or molecule. Link to Q&A discussion
- When an unmagnetized sample is placed in a magnetic field, an internal magnetization (M) will develop and grow to a maximum value in the longitudinal direction (M0). The first order exponential time constant for this growth is defined as
This is the definition of T1 as described by Felix Bloch. Link to Q&A discussion
- T1 is the time required for the longitudinal magnetization Mz to grow from zero to about ____ of its maximum value (M0)
The equation for exponential regrowth is Mz = M0 (1 − e−t/T1). After one time constant (i.e., at t = T1), Mz will have reached (1 − e−1) or about 63% of its maximum value (M0). Link to Q&A discussion
- T2 is the time required for the transverse components of magnetization M0 to decay approximately ____ from their maximum initial value after a 90°-pulse.
The equation for exponential decay of transverse magnetization immediately after a 90°-pulse where the initial magnetization (M0) has been tipped into the transverse plane is given by: Mxy = M0 e−t/T2. After one time constant (i.e., at t = T2), Mxy will have decayed by (e−1) or to about 37% of its initial value. Link to Q&A discussion
- Which of the following statements concerning T2 relaxation is false?
- Any process causing T2 relaxation also results in T1 relaxation.
- A major cause of T2 relaxation is dephasing of spins by static local field inhomogeneities.
- Another major cause of T2 relaxation is spin-spin "flip-flop" interactions.
- T2 relaxation may occur with or without energy transfer/loss from the spin system.
Only a) is false, but its converse is true, "Any process causing T1 relaxation also results in T2 relaxation". The other options are correct. Link to Q&A discussion
- If the T1 relaxation time for brain tissue is 1000 ms, what is its relaxation rate (R1)?
- 1000 msec
- 1 sec
- 1 sec−1
- 1 msec−1
Relaxation rate is merely the inverse of relaxation time. So R1 = 1/T1 = 1/1000 msec = 1/(1 sec) = 1 sec−1 Link to Q&A discussion
- Which of the following statements concerning T1 and T2 relaxation times in tissues at 1.5T are correct?
- Liquids (like CSF and urine) have the longest T1 and T2 values.
- For most solid organs (like the brain and liver) T1 values are about 10x longer than T2 values.
- Dense fibrous tissues (like tendons and cartilage) have very short T2 values.
- Fat has a relatively short T1 value compared to most other tissues.
- All are true.
These basic facts and relationships about tissue T1 and T2 values should be known by anyone involved with MR imaging. Link to Q&A discussion
- Which of the following relaxation time pairs for tissue is impossible?
- T1 = 4000 ms, T2= 2000 ms
- T1 = 1000 ms, T2 = 100 ms
- T1 = 500 ms, T2 = 20 ms
- T1 = 500 ms. T2 = 600 ms
Because every process that causes T1 relaxation also causes T2 relaxation, but T2 relaxation can occur without T1 relaxation, T2 is always less than or equal to T1. Thus the combination in choice d) is impossible. Link to Q&A discussion
- Which of the following biological materials would be expected to have the shortest T2 values?
- Achilles tendon
- Quadriceps muscle
T2 relaxation occurs most quickly when local static magnetic fields are present that dephase the spins. The more "solid" and "drier" the tissue, the shorter is its T2 value. Of the choices above, tendon has the shortest T2. Link to Q&A discussion
- Which of the following biological materials would be expected to have the shortest T1 values?
- Scalp fat
- Red bone marrow
Because of their size and shape, triglycerides (the main component of adipose tissue as in scalp fat) have a relatively large fraction of molecular motions near the Larmor frequency, making them efficient at T1 relaxation. So the correct answer is a). Yellow bone marrow has relatively short T1 due to fat deposition, but red marrow has much less fat and is filled with water-containing hematopoietic cells. Link to Q&A discussion
- Which of the following biological materials would be expected to have the longest T2 values?
- Colloid cyst in the thyroid
- Urine in the bladder
"Free" or "unbound" water molecules tumble most rapidly averaging out static dipolar interactions and hence have the longest T2 values. So pure liquids like spinal fluid and urine have the longest T2 values. Link to Q&A discussion
- Which of the following statements about T2* is false?
- T2* is always longer than T2.
- T2 is always longer than T2*.
- T2* includes not only "true" T2 effects but also effects of magnetic field inhomogeneities.
- T2* includes not only "true" T2 effects but also effects of magnetic field inhomogeneities.
T2* includes not only "true" T2 effects but also effects of magnetic field inhomogeneities. Link to Q&A discussion
- Which of the following molecular mechanisms is the most important for causing T1 and T2 relaxation in ¹H NMR?
- Dipole-dipole interactions
- Chemical shift anisotropy
- Chemical shift anisotropy
- Molecular translation/diffusion
All these mechanisms make contributions, but the dipole-dipole interaction is the most important. Dipole-dipole interactions are "through space" magnetic field disturbances typically occurring between nuclear or electron spins on the same or closely apposed molecules. Link to Q&A discussion
- Because they each have spin = ½, electrons and protons are equally effective at producing dipole-dipole interactions.
Due to its small size, the electron has a much greater charge to mass ratio and hence a much larger gyromagnetic ratio (γ). The electron is thus many thousand times more powerful than the proton at dipole-dipole interactions. Link to Q&A discussion
- Intramolecular dipole-dipole interactions are more powerful than intermolecular ones.
The strength of the dipolar interaction is inversely proportional to the sixth power of the distance between spins (1/r6). Thus dipole-dipole interactions between spins on the same molecule (intramolecular) are more powerful than more distantly spaced spins on different molecules (intermolecular). Link to Q&A discussion
- In dipole-dipole interactions, T1 relaxation is most efficient (and T1 values are shortest) for
- Small, rapidly tumbling molecules
- Molecules tumbling near the Larmor frequency
- Large, slowly moving molecules
- Macromolecules bound to a rigid biologic scaffold or matrix
Since T1 relaxation requires stimulated energy exchange at the Larmor frequency, spins on molecules tumbling at near the Larmor frequency are most efficient at T1 relaxation. Link to Q&A discussion
- In dipole-dipole interactions, T2 relaxation is most efficient (and T2 values are shortest) for
- Small, rapidly tumbling molecules.
- Molecules tumbling near the Larmor frequency.
- Large, slowly moving molecules.
- Macromolecules bound to a rigid biologic scaffold or matrix
Stationary local magnetic fields are most effective at inducing T2 relaxation, so nearly immobile macromolecules (especially those bound to bones, cell membrane, or collagen) have the shortest relaxation times of all. Small, rapidly tumbling molecules have very long T2 values. Link to Q&A discussion
- Considering the effects of macromolecules on relaxation times, which of the following is false
- Macromolecules themselves often have extremely short T2 values and their signal may decay too quickly to be detected using conventional MRI techniques.
- A "bound" or "hydration" layer of water molecules with restricted motion is generally found adjacent to macromolecules.
- These "bound" water protons interact exclusively with macromolecules and not with the "free water" pool.
- Bound water molecules tumble more slowly, producing shortening of T1 and T2. This partially explains the reduced T1 and T2 values of tissues compared to free water.
Only c) is false. The bound water molecules interact and exchange magnetization both with the macromolecules and the free water pool. Link to Q&A discussion
- Which of the following ¹H-containing molecules account for nearly 100% of the signal recorded within the brain parenchyma using routine MRI sequences?
- N-acetyl aspartate (NAA)
Triglcerides are present in scalp fat, but not in the brain parenchyma itself. Myelin is present in the brain, but as an immobile macromolecule has an extremely short T2 and whose signal thus decays too rapidly to be detected using conventional MRI sequences. NAA is the brain metabolite with the largest peak seen on MR spectroscopy of the brain, but its concentration is many thousand times lower than that of water. Link to Q&A discussion
- By irradiating tissue with an off-resonance RF-pulse it is possible to affect image contrast by transferring energy between macromolecular and free-water pools. This process is known as
- T1 exchange
- Magnetization transfer
- Chemical shift
- Energy swap
This is a brief description of MT. Link to Q&A discussion
- As field strength increases from 0.5T to 3.0T, the T1 of most tissues
- Remains about the same
- Decreases then increases
For most biological tissues, empirical measurements suggest that T1 increases approximately as B0⅓. Therefore, measured T1 values of most tissues will approximately double as field strength is raised from 0.3 T to 3.0 T. Link to Q&A discussion
- As field strength increase from 0.5T to 3.0T, the T2 of most tissues
- Remains about the same
- Decreases then increases
The effect varies depending on tissue type, but generally there is relatively little change in T2 values over the 0.5T to 3.0T range. T2 definitely shortens for fields > 3.0T, however. Link to Q&A discussion
- NMR relaxation theory from basic science nearly completely explains T1 and T2 values observed in tissues.
Basic science relaxation theory adequately explains the T1 and T2 behavior of simple homogenous materials and solutions, but is far from explaining the observed relaxation times observed in biological tissues. Multi-compartment models may give some insights, but no fully comprehensive description yet exists. Link to Q&A discussion