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ABMP MRI Physics practice questions

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Question 1Basic MR Physics and Nuclear Magnetism

Which particle property is directly responsible for the magnetic moment of the hydrogen nucleus used in MRI?

  1. Its orbital electron cloud
  2. Its intrinsic spin angular momentum Correct answer.
  3. Its large atomic mass
  4. Its electric quadrupole moment

Explanation

A nucleus with nonzero spin has an associated magnetic dipole moment, which is what allows alignment with an external field. The hydrogen nucleus has spin 1/2, giving it a strong magnetic moment. Electron cloud and mass do not create the nuclear magnetic moment.

Question 2Basic MR Physics and Nuclear Magnetism

The Larmor frequency of a nucleus is determined by which relationship?

  1. f = γB₀/(2π) Correct answer.
  2. f = B₀/(γ2π)
  3. f = 2πγB₀
  4. f = γ/(2πB₀)

Explanation

The Larmor frequency equals the gyromagnetic ratio times the field divided by 2π when expressing frequency in hertz. This linear relationship means frequency scales directly with field strength. The other arrangements invert or misplace the constants.

Question 3Basic MR Physics and Nuclear Magnetism

At 1.5 T, the approximate precessional frequency of protons is closest to which value?

  1. 128 MHz
  2. 42 MHz
  3. 64 MHz Correct answer.
  4. 21 MHz

Explanation

Proton gyromagnetic ratio is about 42.58 MHz/T, so at 1.5 T the frequency is roughly 64 MHz. This is the standard operating frequency for clinical 1.5 T systems. Doubling the field would give about 128 MHz.

Question 4Basic MR Physics and Nuclear Magnetism

In thermal equilibrium, the net magnetization vector of tissue in a static field points in which direction?

  1. Parallel to B₀ along the longitudinal axis Correct answer.
  2. Anti-parallel to B₀ only
  3. At 45 degrees to B₀
  4. Perpendicular to B₀ in the transverse plane

Explanation

Slightly more spins occupy the lower-energy parallel state, producing a small net magnetization aligned with B₀. This longitudinal magnetization is the source of signal when tipped into the transverse plane. The transverse component is near zero at equilibrium.

Question 5Basic MR Physics and Nuclear Magnetism

The gyromagnetic ratio of a nucleus represents which quantity?

  1. Spin quantum number per unit mass
  2. Energy per unit field
  3. Field strength per unit frequency
  4. Magnetic moment per unit angular momentum Correct answer.

Explanation

The gyromagnetic ratio is the ratio of magnetic dipole moment to angular momentum. It links precessional frequency to applied field. Its units are typically MHz/T or rad/s/T.

Question 6Basic MR Physics and Nuclear Magnetism

Which statement best describes the Bloch equation's role in MRI physics?

  1. It models time evolution of magnetization under fields and relaxation Correct answer.
  2. It predicts the atomic number of tissue nuclei
  3. It calculates only the static field homogeneity
  4. It determines the RF coil's impedance

Explanation

The Bloch equation describes how magnetization precesses under applied fields while relaxing via T1 and T2. It is the foundation for understanding pulse sequence behavior. It does not address coil impedance or static field mapping alone.

Question 7Basic MR Physics and Nuclear Magnetism

After a 90° RF pulse, the net magnetization immediately lies in which orientation?

  1. In the transverse plane Correct answer.
  2. At the magic angle
  3. Opposite to B₀
  4. Along the longitudinal axis

Explanation

A 90° pulse rotates longitudinal magnetization entirely into the transverse plane. This transverse magnetization precesses and induces signal in the receive coil. Longitudinal magnetization is momentarily zero right after the pulse.

Question 8Basic MR Physics and Nuclear Magnetism

A 180° RF pulse applied to equilibrium magnetization produces which immediate effect?

  1. No change in magnetization
  2. Inversion of longitudinal magnetization Correct answer.
  3. Creation of transverse magnetization
  4. Complete destruction of all spins

Explanation

A 180° pulse flips longitudinal magnetization to the opposite direction, producing inversion. This is used in inversion recovery and spin-echo refocusing. No transverse component is created by an ideal 180° pulse alone.

Question 9Basic MR Physics and Nuclear Magnetism

T1 relaxation is best characterized by which process?

  1. Recovery of longitudinal magnetization toward equilibrium Correct answer.
  2. Conversion of spin to charge
  3. Precession frequency drift
  4. Decay of transverse magnetization from dephasing

Explanation

T1 describes the return of longitudinal magnetization to its equilibrium value via energy exchange with the lattice. It is also called spin-lattice relaxation. Transverse decay is governed by T2.

Question 10Basic MR Physics and Nuclear Magnetism

Which mechanism primarily causes T2 decay in tissue?

  1. RF coil heating
  2. Longitudinal field drift
  3. Energy transfer to the lattice only
  4. Spin-spin interactions causing loss of phase coherence Correct answer.

Explanation

T2 reflects dephasing of transverse spins through local spin-spin interactions. These interactions vary precessional frequencies and reduce coherence. T1 involves lattice energy exchange instead.

Question 11Basic MR Physics and Nuclear Magnetism

T2* differs from T2 because T2* additionally includes which contribution?

  1. Chemical shift only
  2. Lattice energy exchange
  3. RF pulse duration
  4. Static field inhomogeneity dephasing Correct answer.

Explanation

T2* combines true T2 decay with reversible dephasing from static field inhomogeneities. A spin echo can refocus the inhomogeneity part but not the intrinsic T2 part. Thus T2* is always shorter than or equal to T2.

Question 12Basic MR Physics and Nuclear Magnetism

The equilibrium Boltzmann distribution of proton spin states at room temperature results in what population difference?

  1. Complete alignment in the parallel state
  2. An equal split between states
  3. A very small excess in the parallel state Correct answer.
  4. A large majority in the anti-parallel state

Explanation

Thermal energy greatly exceeds the Zeeman splitting at room temperature, so only a tiny fraction more spins occupy the lower-energy state. This small excess produces the detectable net magnetization. Complete or majority alignment does not occur at physiologic temperatures.

Question 13Basic MR Physics and Nuclear Magnetism

Which quantum number determines the number of possible nuclear spin orientations in a magnetic field?

  1. The spin quantum number I Correct answer.
  2. The atomic mass number A
  3. The magnetic susceptibility χ
  4. The principal quantum number n

Explanation

A nucleus with spin I has 2I+1 allowed orientations (m values) in a field. For protons with I = 1/2, there are two states. Atomic mass and susceptibility do not set the number of orientations.

Question 14Basic MR Physics and Nuclear Magnetism

A nucleus with an even number of protons and an even number of neutrons typically has which spin property?

  1. Spin of 3/2
  2. Spin of 1
  3. Spin of 1/2
  4. Zero net spin Correct answer.

Explanation

Even-even nuclei pair their nucleons so that spins cancel, giving zero net spin. Such nuclei are not directly observable in conventional MRI. Odd nucleon counts are required for nonzero spin.

Question 15Basic MR Physics and Nuclear Magnetism

In the rotating frame at the Larmor frequency, the effective magnetic field experienced by on-resonance spins is which?

  1. Reversed in direction
  2. Unchanged in magnitude and direction
  3. Reduced to near zero along the transverse axes Correct answer.
  4. Doubled relative to the laboratory frame

Explanation

Transforming to a frame rotating at the Larmor frequency removes the main B₀ precession, so on-resonance spins see essentially no effective field. Off-resonance spins experience a residual field proportional to their frequency offset. This simplifies description of RF excitation.

Question 16Basic MR Physics and Nuclear Magnetism

The energy difference between proton spin states in a magnetic field is given by which expression?

  1. ΔE = f/h
  2. ΔE = hf²
  3. ΔE = h/f
  4. ΔE = hf Correct answer.

Explanation

The Zeeman energy gap equals Planck's constant times the Larmor frequency. Resonant absorption of a photon at that frequency flips the spin. The other forms have incorrect dimensional relationships.

Question 17Basic MR Physics and Nuclear Magnetism

Which factor directly determines the magnitude of the net equilibrium magnetization in a voxel?

  1. Proton density and field strength Correct answer.
  2. Gradient slew rate
  3. RF coil Q factor
  4. Receiver bandwidth

Explanation

Net magnetization scales with the number of protons and the applied field strength through the Boltzmann distribution. More protons and higher fields yield larger magnetization. Coil and gradient parameters affect acquisition, not the equilibrium magnetization itself.

Question 18Basic MR Physics and Nuclear Magnetism

Free induction decay occurs because transverse magnetization does which?

  1. Rotates into the longitudinal axis instantly
  2. Dephases while relaxing after excitation Correct answer.
  3. Grows steadily over time
  4. Remains perfectly coherent indefinitely

Explanation

After a 90° pulse, transverse spins dephase from T2 and field inhomogeneity while also relaxing, so the induced signal decays. This decaying signal is the free induction decay. It does not persist indefinitely or grow.

Question 19Basic MR Physics and Nuclear Magnetism

Applying a 90° pulse to magnetization that is already inverted (180°) produces which result?

  1. Transverse magnetization from a 270° net rotation Correct answer.
  2. Full longitudinal recovery
  3. A second inversion
  4. Zero magnetization

Explanation

A 90° pulse adds to the existing 180° orientation, giving a net 270° rotation that places magnetization in the transverse plane. The resulting transverse component has opposite phase to a simple 90° excitation. It does not return magnetization to equilibrium.

Question 20Basic MR Physics and Nuclear Magnetism

The precessional angular frequency of a spin in a field is expressed as which relationship?

  1. ω = γB₀ Correct answer.
  2. ω = γ/B₀
  3. ω = 2πγB₀
  4. ω = B₀/γ

Explanation

Angular frequency equals the gyromagnetic ratio times the field, with γ in rad/s/T. Dividing by 2π converts this to frequency in hertz. The other expressions have incorrect proportionality.

Question 21Basic MR Physics and Nuclear Magnetism

Why does the net magnetization of tissue remain small even in a strong MRI field?

  1. Spins cancel because all are anti-parallel
  2. Thermal energy greatly exceeds the Zeeman splitting Correct answer.
  3. Protons have no magnetic moment
  4. The field is too weak to align any spins

Explanation

At body temperature, kT is far larger than the small Zeeman energy gap, so the population difference is tiny. This yields a small but measurable net magnetization. The field is strong enough to create a difference, just a small one.

Question 22Basic MR Physics and Nuclear Magnetism

Which statement correctly describes the relationship between T1 and T2 in most soft tissues?

  1. T1 is generally longer than T2 Correct answer.
  2. T2 is generally longer than T1
  3. T1 is always shorter than 10 ms
  4. T1 and T2 are always equal

Explanation

In most biologic tissues, longitudinal recovery takes longer than transverse decay, so T1 exceeds T2. Typical values at 1.5 T are hundreds of milliseconds for T1 and tens of milliseconds for T2. Equality or reversal is not typical.

Question 23Basic MR Physics and Nuclear Magnetism

A spin echo refocuses which component of transverse dephasing?

  1. Reversible dephasing from static field inhomogeneity Correct answer.
  2. RF pulse phase errors only
  3. Irreversible T2 decay from spin-spin interactions
  4. Longitudinal T1 recovery

Explanation

The 180° refocusing pulse reverses phase accumulated from static field differences, recovering signal lost to inhomogeneity. True T2 decay from random spin-spin interactions cannot be refocused. T1 recovery is a separate longitudinal process.

Question 24Basic MR Physics and Nuclear Magnetism

Which nucleus is most commonly imaged in clinical MRI because of its abundance and gyromagnetic ratio?

  1. Carbon-13
  2. Sodium-23
  3. Hydrogen-1 Correct answer.
  4. Phosphorus-31

Explanation

Hydrogen-1 is abundant in water and fat and has a high gyromagnetic ratio, giving strong signal. Other nuclei are far less abundant or have lower sensitivity. This makes protons the primary imaging nucleus.

Question 25Basic MR Physics and Nuclear Magnetism

Which condition is required for a nucleus to be observable with conventional MRI?

  1. A large electric quadrupole moment
  2. A nonzero nuclear spin Correct answer.
  3. An even number of protons
  4. A mass number greater than 20

Explanation

Only nuclei with nonzero spin possess a magnetic moment and can generate an MR signal. Even-even nuclei with zero spin are not observable. Mass and quadrupole moment do not determine basic observability.

Question 26Basic MR Physics and Nuclear Magnetism

Which property of the hydrogen nucleus makes it the dominant signal source in clinical MRI?

  1. It possesses a single unpaired proton with a large natural abundance in tissue Correct answer.
  2. It has the highest gyromagnetic ratio of any nucleus
  3. It carries an electric quadrupole moment that enhances signal
  4. It has the longest T2 relaxation time of all nuclei

Explanation

Observability requires a nonzero nuclear spin, and hydrogen (a single proton) has spin 1/2 with no quadrupole moment, giving a clean signal. Combined with its very high concentration in water and fat, this makes it the dominant source. Abundance and gyromagnetic ratio together, not either alone, drive its dominance.

Question 27Basic MR Physics and Nuclear Magnetism

A sample is placed in a stronger static magnetic field while temperature is held constant. What happens to the fractional excess of protons in the lower energy spin state?

  1. It decreases because relaxation accelerates
  2. It stays essentially unchanged because thermal energy dominates
  3. It becomes negative above a threshold field
  4. It increases in proportion to the field strength Correct answer.

Explanation

The energy gap between spin states grows linearly with field strength, so the Boltzmann factor favors the lower state more strongly at higher field. Because the gap remains tiny compared with thermal energy, the excess is small but still grows in direct proportion to B0. This field dependence underlies the SNR gain obtained at higher field.

Question 28Basic MR Physics and Nuclear Magnetism

A spin is off-resonance by a small amount in the rotating frame. How does its behavior differ from an on-resonance spin during a long, low-amplitude RF pulse?

  1. It immediately flips by exactly 180 degrees regardless of pulse duration
  2. It precesses faster about the applied RF field than an on-resonance spin
  3. It experiences a reduced effective field and precesses about a tilted effective field direction Correct answer.
  4. It experiences no RF field at all and simply relaxes to equilibrium

Explanation

In the rotating frame, an off-resonance spin sees a residual field along z in addition to the RF field along the transverse plane. The vector sum is a tilted effective field smaller in magnitude than the RF field alone, and the spin precesses about that tilted axis. This is why slice-selective pulses and off-resonance effects produce imperfect tip angles.

Question 29Basic MR Physics and Nuclear Magnetism

Two tissues have the same proton density but different relaxation times. Immediately after a 90° pulse, which factor determines the initial amplitude of the transverse magnetization?

  1. The proton density of each tissue Correct answer.
  2. The T1 of each tissue
  3. The T2 of each tissue
  4. The field strength alone

Explanation

A 90° pulse tips longitudinal magnetization fully into the transverse plane, so the initial transverse amplitude equals the equilibrium magnetization that existed before the pulse. At that instant relaxation times have not yet acted, so tissues with equal proton density give equal initial signal. T1 and T2 shape the subsequent time evolution rather than the starting amplitude.

Question 30Basic MR Physics and Nuclear Magnetism

In a rotating frame turning at exactly the Larmor frequency, which description of the effective magnetic field for an on-resonance spin is physically correct?

  1. The static field doubles while the RF field is removed
  2. The static field vanishes and only the RF field remains along the transverse axis Correct answer.
  3. The effective field points along the longitudinal axis with magnitude equal to the RF amplitude
  4. The effective field is zero because the spin is stationary in this frame

Explanation

Transforming into a frame rotating at the Larmor frequency removes the resonance offset, so the large static field no longer contributes an effective component along z. What remains is the applied RF field, which appears static in this frame and lies in the transverse plane. Spins then nutate about that transverse axis at a rate set by the RF amplitude.

Question 31Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

On a spin-echo sequence, which two time intervals define T1 and T2 weighting respectively?

  1. TE controls T1 weighting and TR controls T2 weighting
  2. TR controls T1 weighting and TE controls T2 weighting Correct answer.
  3. TR controls both T1 and T2 weighting equally
  4. TE controls both T1 and T2 weighting equally

Explanation

Repetition time governs how much longitudinal magnetization recovers between excitations, so it sets T1 contrast. Echo time governs how much transverse dephasing occurs before signal readout, so it sets T2 contrast.

Question 32Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

A tissue has T1 of 1000 ms. After a 90-degree pulse, approximately what fraction of longitudinal magnetization has recovered at 1000 ms?

  1. About 37%
  2. About 50%
  3. About 86%
  4. About 63% Correct answer.

Explanation

Longitudinal recovery follows 1 - e^(-t/T1). When the elapsed time equals one T1, recovery is 1 - e^-1, which is roughly 0.63.

Question 33Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

Which statement correctly describes T2* relative to T2?

  1. T2* is unrelated to T2 and reflects only longitudinal recovery
  2. T2* equals T2 in all tissues regardless of field homogeneity
  3. T2* is longer than T2 because inhomogeneity slows dephasing
  4. T2* is shorter than T2 because static field inhomogeneity adds to spin-spin dephasing Correct answer.

Explanation

T2* includes both irreversible spin-spin dephasing and reversible dephasing from static field inhomogeneity. Because additional dephasing mechanisms shorten the observed decay, T2* is always less than or equal to T2.

Question 34Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

What is the primary purpose of the 180-degree refocusing pulse in a spin-echo sequence?

  1. To spoil residual transverse magnetization after readout
  2. To reverse static dephasing and form an echo at TE Correct answer.
  3. To shorten the tissue T1 relaxation time
  4. To saturate fat signal before excitation

Explanation

The refocusing pulse flips spins so that those dephased by static field differences rephase, producing an echo at TE while irreversible T2 decay continues.

Question 35Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

In a gradient-echo sequence with a short TR and a flip angle less than 90 degrees, what is the main contrast effect?

  1. Contrast becomes independent of TR and flip angle
  2. T2 weighting dominates because TE is long
  3. T1 weighting is preserved while scan time is reduced Correct answer.
  4. Proton density weighting dominates at all flip angles

Explanation

A reduced flip angle leaves more longitudinal magnetization available, allowing short TR without complete saturation. This maintains strong T1 weighting while shortening acquisition time.

Question 36Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

Which sequence is most appropriate for producing heavy T2 weighting in fluid-sensitive imaging?

  1. Long TR with short TE
  2. Short TR with long TE
  3. Short TR with short TE
  4. Long TR with long TE Correct answer.

Explanation

T2 weighting requires a long TE to allow transverse decay differences to develop, and a long TR to reduce T1 effects. Together they emphasize tissues with long T2 such as fluid.

Question 37Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

Which intrinsic tissue property determines the rate of transverse magnetization decay after a 90-degree pulse?

  1. T1 relaxation time
  2. Chemical shift
  3. Proton density
  4. T2 relaxation time Correct answer.

Explanation

Transverse decay is governed by spin-spin interactions characterized by T2. T1 describes longitudinal recovery instead.

Question 38Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

What does the inversion time (TI) control in an inversion-recovery sequence?

  1. The time from excitation to echo formation
  2. The delay between the inversion pulse and the excitation pulse Correct answer.
  3. The interval between successive excitation pulses
  4. The duration of the readout gradient

Explanation

TI is the time allowed after the 180-degree inversion pulse before the 90-degree excitation, and it determines how much longitudinal magnetization has recovered, thus setting the contrast.

Question 39Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

A tissue with a long T1 is imaged with a short TR. What signal behavior is expected compared with a tissue of short T1?

  1. Higher signal because long T1 recovers faster
  2. Higher signal because long T1 reduces transverse decay
  3. Equal signal because TR affects both tissues identically
  4. Lower signal because longitudinal recovery is incomplete Correct answer.

Explanation

With a short TR, tissues with long T1 have recovered less longitudinal magnetization before the next excitation, so they yield lower signal than short-T1 tissues.

Question 40Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

Two tissues have identical T1 but different T2. Which parameter change best separates them on a spin-echo image?

  1. Increasing TR
  2. Decreasing TE
  3. Decreasing TR
  4. Increasing TE Correct answer.

Explanation

Differences in T2 manifest as differences in transverse decay, which grow as TE lengthens. TR influences longitudinal recovery and therefore T1 contrast, not T2 contrast.

Question 41Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

Why does a very long TR reduce T1 contrast on a spin-echo image?

  1. Transverse magnetization decays completely before readout
  2. T2 decay becomes negligible at long TR
  3. Flip angle is automatically reduced at long TR
  4. Most tissues fully recover longitudinal magnetization, removing recovery differences Correct answer.

Explanation

When TR is much longer than tissue T1 values, nearly all tissues recover most of their longitudinal magnetization, so differences in recovery rate no longer produce contrast.

Question 42Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

In a spoiled gradient-echo sequence, what is the role of RF or gradient spoiling?

  1. To convert T1 contrast into T2 contrast
  2. To increase the flip angle beyond 90 degrees
  3. To destroy residual transverse coherence before the next excitation Correct answer.
  4. To lengthen the tissue T2 relaxation time

Explanation

Spoiling removes leftover transverse magnetization so that each TR interval starts from a consistent longitudinal state, which is necessary for stable T1-weighted steady-state contrast.

Question 43Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

A steady-state free precession sequence uses very short TR and maintains residual transverse coherence. What contrast does this typically provide?

  1. High signal with contrast driven by the T2/T1 ratio Correct answer.
  2. Proton density weighting only
  3. Pure T1 weighting identical to spin echo
  4. Complete suppression of fluid signal

Explanation

When transverse coherence is preserved across very short TRs, the steady-state signal depends on the ratio of T2 to T1, giving bright signal from fluids and high SNR.

Question 44Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

At what inversion time does a tissue's longitudinal magnetization cross zero during inversion recovery?

  1. TI equal to the tissue T1
  2. TI equal to twice the tissue T1
  3. TI equal to the tissue T2
  4. TI equal to about 0.69 times the tissue T1 Correct answer.

Explanation

Starting from full inversion, longitudinal magnetization follows M0(1 - 2e^(-TI/T1)). Setting this to zero gives TI = T1 ln2, approximately 0.69 T1.

Question 45Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

STIR achieves fat suppression by exploiting which property of fat?

  1. Its large chemical shift relative to water
  2. Its short T1, allowing a short inversion time to null it Correct answer.
  3. Its low proton density relative to water
  4. Its long T2, allowing a long TE to null it

Explanation

STIR applies an inversion pulse and selects a TI that nulls fat based on its short T1, so fat signal is suppressed regardless of field homogeneity.

Question 46Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

Why does spectral fat saturation depend on magnetic field homogeneity?

  1. It relies on the short T1 of fat to null its signal
  2. It relies on differences in T2 between fat and water
  3. It relies on a narrow-frequency presaturation pulse centered on the fat peak Correct answer.
  4. It relies on spoiling gradients applied during readout

Explanation

Spectral saturation excites a narrow frequency band at the fat resonance. If local field inhomogeneity broadens or shifts the fat peak, the presaturation pulse misses it and suppression fails.

Question 47Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

A spin-echo sequence has TR of 500 ms and TE of 15 ms. Which weighting is most likely?

  1. Proton density weighted
  2. T2 weighted
  3. T1 weighted Correct answer.
  4. T2* weighted

Explanation

A short TR limits longitudinal recovery and a short TE minimizes T2 decay effects, so differences in T1 dominate the image contrast.

Question 48Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

Which change would most increase T2 weighting while keeping TR fixed?

  1. Shortening TE
  2. Lengthening TE Correct answer.
  3. Lengthening TR
  4. Reducing flip angle

Explanation

T2 contrast arises from differences in transverse decay, which accumulate over the echo time. Lengthening TE therefore increases T2 weighting independent of TR.

Question 49Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

Why does a long echo train in fast spin echo tend to blur images?

  1. Later echoes have lower amplitude and greater T2 decay, reducing effective resolution Correct answer.
  2. TR becomes progressively shorter during acquisition
  3. Slice thickness increases with echo number
  4. Flip angle increases progressively along the echo train

Explanation

In fast spin echo, later echoes are acquired at longer effective TE values, so they carry more T2 decay and lower SNR. Their contribution degrades the point spread function and blurs the image.

Question 50Relaxation, Contrast Mechanisms, and Pulse Sequence Timing

What is the effect of a 180-degree inversion pulse applied before a 90-degree excitation on the dynamic range of T1 contrast?

  1. It converts the sequence to pure T2 weighting
  2. It reduces signal-to-noise by halving proton density
  3. It eliminates T1 contrast by equalizing recovery rates
  4. It increases T1 contrast by starting magnetization from negative values Correct answer.

Explanation

Inverting longitudinal magnetization gives tissues a larger range over which T1 recovery differences can evolve before excitation, which amplifies T1 contrast compared with saturation recovery.