ABMP MRI Physics flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Larmor equation

The precession frequency of a spin is f0 = (γ/2π)B0. For hydrogen-1 at 1.5 T, f0 is approximately 63.9 MHz; at 3 T, approximately 127.7 MHz.

Gyromagnetic ratio

The constant γ relating magnetic moment to angular momentum and Larmor frequency to field. For 1H, γ/2π is about 42.58 MHz/T, determining resonance frequency at a given B0.

Net magnetization M0

The equilibrium vector sum of nuclear magnetic moments aligned with B0. It arises from a small Boltzmann population excess and is proportional to proton density and B0.

Bloch equation

An equation of motion describing precession and relaxation of macroscopic magnetization: dM/dt = γM × B − (Mx i + My j)/T2 − (Mz − M0)k/T1.

Flip angle

The angle by which an RF pulse rotates magnetization, given by θ = ∫γB1(t)dt. A 90° pulse places M0 into the transverse plane; a 180° pulse inverts Mz.

T1 relaxation

Longitudinal (spin-lattice) recovery of Mz toward equilibrium, described by Mz(t) = M0(1 − e^(−t/T1)). T1 is always longer than or equal to T2 in tissue.

T2 relaxation

Transverse (spin-spin) decay of Mxy described by Mxy(t) = Mxy(0)e^(−t/T2). It reflects irreversible dephasing from microscopic field fluctuations.

T2 star

Effective transverse decay including reversible dephasing from static field inhomogeneity, with 1/T2* = 1/T2 + 1/T2′. It is always shorter than true T2.

Spin-echo signal equation

Signal is proportional to ρ(1 − e^(−TR/T1))e^(−TE/T2). Long TR with short TE gives proton-density weighting; long TR with long TE gives T2 weighting.

Inversion time null point

In inversion recovery, Mz(t) = M0(1 − 2e^(−TI/T1)); a tissue is nulled when TI = T1·ln2. STIR nulls fat and FLAIR nulls CSF using this relation.

Ernst angle

The gradient-echo flip angle that maximizes signal for a given TR and T1: α = arccos(e^(−TR/T1)). It balances signal against saturation at short TR.

Slice thickness formula

Slice thickness equals RF bandwidth divided by the product of γ and slice-select gradient: Δz = Δf/(γGss). Thinner slices require a larger gradient or narrower RF bandwidth.

Phase encoding

A brief gradient applied before readout whose area sets the ky location in k-space. The number of phase-encoding steps determines the image matrix size in that direction.

Frequency encoding

A readout gradient applied during signal acquisition that maps spatial position linearly to Larmor frequency, allowing position along that axis to be decoded by Fourier transform.

k-space FOV relation

Field of view equals 1/Δk, where Δk is the k-space sampling interval. Spatial resolution equals 1/(k_max), the highest sampled spatial frequency.

Slice profile

The through-plane excitation profile, equal to the Fourier transform of the RF pulse envelope. A sinc-shaped RF pulse produces a roughly rectangular slice profile.

Quadrature coil advantage

A quadrature (circularly polarized) coil detects two orthogonal signal components, improving SNR by a factor of about √2 relative to a linear coil and reducing transmit power needs.

Surface coil behavior

A small coil placed near tissue provides high SNR close to the coil but nonuniform sensitivity and limited field of view, because sensitivity falls with distance from the coil plane.

Receive bandwidth effect

Wider receive bandwidth admits more noise, lowering SNR, but reduces chemical-shift artifact. Narrower bandwidth improves SNR but increases shift and susceptibility-related artifacts.

Specific absorption rate

RF power deposited per unit mass, expressed in W/kg. It scales approximately with B0 squared, flip angle squared, and sequence duty cycle, and is subject to regulatory limits.

Aliasing artifact

Wraparound of signal from outside the field of view into the image, occurring when the object exceeds the FOV. Larger FOV, oversampling, or saturation bands prevent it.

Gibbs artifact

Truncation ringing near sharp edges caused by finite k-space sampling. It appears as oscillating bands parallel to high-contrast boundaries and is reduced, not resolved, by zero-filling.

Motion ghosting

Replicated or smeared signal along the phase-encode direction caused by inconsistent motion between phase-encode lines. Swapping phase and frequency directions moves the artifact.

Chemical shift artifact

Spatial displacement of fat relative to water along the frequency-encode direction due to their ~3.5 ppm resonance difference. It increases with field strength and decreases with wider bandwidth.

Echo-planar imaging

A fast technique filling k-space after a single excitation using rapid gradient switching. It is prone to geometric distortion and T2* blurring, especially near susceptibility boundaries.

Parallel imaging SNR

Coil-sensitivity encoding skips phase-encode lines, reducing scan time. SNR decreases roughly with the square root of the acceleration factor, and unfolding artifacts can occur.

Balanced SSFP

A steady-state sequence with fully balanced gradients giving high SNR and T2/T1-weighted contrast. It is sensitive to off-resonance, producing dark banding artifacts.

Compressed sensing

A reconstruction approach exploiting image sparsity and incoherent undersampling to recover images from fewer samples than Nyquist requires, enabling faster acquisitions.

Daily QA phantom checks

Routine quality assurance measures SNR, geometric accuracy, slice thickness, and image uniformity using a phantom, comparing results against established baseline values.

Coil element failure sign

A failed or detuned array element produces regional signal dropout or nonuniformity in the image. Comparing individual element images localizes the faulty coil component.

Apparent diffusion coefficient

A quantitative measure of water mobility derived from diffusion-weighted images. Signal follows S = S0·e^(−b·ADC); highly cellular tissue restricts diffusion and lowers ADC.

Quantitative susceptibility mapping

A method estimating tissue magnetic susceptibility from gradient-echo phase data after removing background fields and performing dipole inversion, useful for iron and hemorrhage.