Clover Image Production: Data Acquisition, Processing and Storage

Vocabulary flashcards focusing on MR image production, data acquisition principles, pulse sequences (SE, FSE, GRE, IR, EPI), gradient mechanisms, and imaging parameters.

Slice Selection Gradient (SSG)

A gradient ($G_s$) applied simultaneously with RF excitation or refocusing pulses to select the specific anatomical slice being imaged.

Phase Encoding Gradient (PEG)

A gradient ($G_p$) applied after slice selection that varies the phase of hydrogen nuclei across the body to spatially encode data and fill different lines of k-space.

Frequency Encoding Gradient (FEG)

Also known as the readout gradient ($G_f$), this gradient encodes spatial information based on precessional frequency during MR signal acquisition.

Free Induction Decay (FID)

The initial MR signal decay that occurs immediately following an RF excitation pulse due to transverse dephasing.

Echo Signal

The MR signal created when dephased spins are rephased, representing the point at which the MRI system captures data to reconstruct an image.

Repetition Time (TR)

The time interval between consecutive RF excitation pulses applied to the same slice.

Echo Time (TE)

The time interval measured from the application of the initial RF excitation pulse to the peak of the generated echo signal.

Spin Echo (SE) Pulse Sequence

A pulse sequence that uses a $90^\text{o}$ RF excitation pulse followed by a $180^\text{o}$ RF rephasing pulse to correct for field inhomogeneities and generate an echo.

Spin Echo Advantages

Key advantages of conventional spin echo sequences including predictable image quality, high image quality, reduced magnetic susceptibility artifacts, and body-wide versatility.

Spin Echo Disadvantages

Primary limitations of conventional spin echo sequences, specifically longer scan times and high sensitivity to motion artifacts.

Tau (\tau)

The time period between the initial $90^\text{o}$ RF pulse and the $180^\text{o}$ rephasing pulse in a spin echo sequence, equal to half of the echo time ($\frac{1}{2}\text{TE}$).

Conventional Spin Echo (CSE)

A baseline pulse sequence that utilizes one $90^\text{o}$ pulse and one $180^\text{o}$ pulse per TR period to produce a single echo and fill one line of k-space.

Fast Spin Echo (FSE)

Also called Turbo Spin Echo (TSE) or RARE, a sequence applying multiple $180^\text{o}$ refocusing pulses after a single excitation pulse to acquire an echo train and fill multiple lines of k-space per TR.

Echo Train Length (ETL)

Also referred to as the turbo factor, it is the total number of rephased echoes collected following a single RF excitation pulse in an FSE sequence.

Effective TE

In Fast Spin Echo imaging, the TE selected to fill the central lines of k-space, which predominantly determines image contrast and weighting.

Driven Equilibrium (DRIVE)

A pulse sequence modification that uses a reverse $-90^\text{o}$ pulse at the end of an echo train to drive transverse magnetization back into the longitudinal plane, enhancing water and CSF signal.

Inversion Recovery (IR)

A pulse sequence starting with a $180^\text{o}$ inversion pulse followed by a $90^\text{o}$ excitation pulse after an inversion time (TI) to produce heavy T1 weighting or suppress specific tissues.

Inversion Time (TI)

The time interval between the initial $180^\text{o}$ inversion pulse and the subsequent $90^\text{o}$ RF excitation pulse in an inversion recovery sequence.

Null Point

The precise point during longitudinal relaxation when a tissue's net magnetization vector crosses zero, resulting in zero signal intensity if excited at that moment.

STIR (Short Tau Inversion Recovery)

An inversion recovery pulse sequence utilizing a short TI ($150\,\text{ms} - 175\,\text{ms}$) to suppress signal from fat tissue.

FLAIR (Fluid Attenuated Inversion Recovery)

An inversion recovery pulse sequence utilizing a long TI ($1700\,\text{ms} - 2200\,\text{ms}$) to suppress signal from fluid such as cerebrospinal fluid (CSF).

Gradient Echo (GRE) Sequence

A pulse sequence characterized by variable flip angles ($<90^\text{o}$) and gradient reversal (bipolar gradients) instead of $180^\text{o}$ RF pulses to rephase transverse magnetization.

Spoiler Gradient

A gradient pulse used to dephase and eliminate residual transverse magnetization between TR cycles.

Rewinder Gradient

A gradient pulse used to reverse phase shifts and maintain phase coherence of residual transverse magnetization.

Steady State

A condition in rapid GRE imaging where TR is shorter than tissue T1 and T2 relaxation times (typically TR $20\,\text{ms} - 50\,\text{ms}$), establishing stable longitudinal and transverse magnetization.

Residual Transverse Magnetization (RTM)

Transverse magnetization remaining from previous RF excitation pulses when the TR is shorter than the T2 relaxation time of the tissue.

Coherent GRE

Also called rewound GRE, a pulse sequence that preserves residual transverse magnetization using rewinder gradients to yield $T2^*$ weighting.

Spoiled GRE

Also called incoherent GRE, a sequence that uses RF or gradient spoiling to eliminate residual transverse magnetization, generating T1 weighting.

Balanced GRE

A coherent GRE sequence with fully balanced gradients along all axes to eliminate phase errors from flowing blood/CSF, producing signal based on the $T1/T2$ ratio.

Echo Planar Imaging (EPI)

An ultra-fast MRI acquisition technique that uses a continuous train of oscillating gradient echoes to collect multiple or all lines of k-space during a single TR period.

Entry Slice Phenomenon (ESP)

An inflow effect in MRA where fresh, unsaturated blood entering a slice produces high signal intensity compared to saturated stationary tissues.

Single-Shot Fast Spin Echo (SSFSE)

An FSE pulse sequence technique that collects all required lines of k-space in a single TR period using a very long echo train length.

Multiplanar Reconstruction (MPR)

A post-processing technique applied to 3D volumetric MRI datasets to generate reformatted images in any desired anatomical plane.