MRI Pulse Sequences and Parameters

A pulse sequence is a structured set of radiofrequency (RF) pulses and magnetic field gradients used to generate an MR image.

Pulse sequences are broadly categorized into spin echo and gradient echo sequences.

A spin echo sequence involves two primary RF pulses: a 90-degree pulse to tip the magnetization and a 180-degree pulse to rephase the protons.

The time between two 90-degree pulses is called the repetition time (TR), and the time between the 90-degree pulse and the echo signal is called the echo time (TE).

Inversion Recovery (IR) is a technique applied to spin-echo sequences where a 180-degree pulse is used to invert the magnetization before the 90-degree pulse. This helps to suppress signal from certain tissues.

Echo planar imaging (EPI) is a technique used with gradient echo sequences to rapidly acquire data by applying a series of gradient pulses to encode spatial information.

Gradient echo sequences use gradient pulses to dephase and rephase the magnetization instead of a 180-degree pulse like spin-echo.

The spin echo sequence, with its two key pulses and defined timing parameters, forms the foundation for understanding other more complex pulse sequences.

In conventional SE sequence, one line of K-Space is filled per TR. This is modified to have more than one echo (line of K-Space) per TR, sending multiple 180-degree pulses after the 90-degree pulse. Each 180-degree pulse obtains one echo.

This sequence involves sending two 180-degree pulses after each 90-degree pulse, acquiring two echoes per TR. The PD + T2 double echo sequence is an example.

This sequence sends multiple 180-degree pulses after each 90-degree pulse, acquiring multiple echoes per TR. Each echo fills a single K-Space line, resulting in faster scanning.

This sequence acquires all echoes needed to form an image in a single TR, using many 180-degree pulses.

The number of 180-degree pulses sent after each 90-degree pulse in a Fast Spin-Echo sequence. It determines the speed and weighting of the image.

The TE at which the center of K-Space is filled in a Fast Spin-Echo sequence. It determines the weighting of T1 and T2.

This technique uses a long TR and short TE to primarily visualize proton density. It's one of the most widely used techniques for generating images.

This technique uses a long TR and long TE to primarily visualize water content. It helps identify areas of inflammation and disease.

A type of MRI sequence where the signal is produced by a gradient echo, meaning that the signal is rephrased by gradients instead of a 180 degree pulse. This allows for faster scanning times.

In a GRE sequence, the dephasing caused by magnetic field inhomogeneity is not eliminated because there is no 180-degree pulse. This results in a faster T2 decay, often referred to as T2*.

A type of GRE sequence where the residual transverse magnetization is destroyed after each TR, preventing it from interfering with the next TR. This results in a more controlled signal and can help create stronger image contrast.

A type of GRE sequence where the residual transverse magnetization is not destroyed, but rather refocused. This allows for a steady state of magnetization, resulting in consistent signal intensity over time.

The time it takes for the longitudinal magnetization to recover to 63% of its original value after an RF pulse. It is a crucial parameter in determining the scan time and image contrast.

The time it takes for the transverse magnetization to decay to 37% of its original value. It is influenced by factors such as magnetic field inhomogeneity and is another key parameter that affects the image contrast

A type of pulse sequence that only acquires a portion of the k-space data, the other half being reconstructed mathematically using a half-Fourier transform. This results in significantly faster scan times.

A type of MRI sequence that acquires all of the K-space data in a single excitation, significantly reducing the scan time by half.

A type of pulse sequence that uses a variable flip angle excitation pulse and frequency encoding gradient rephasing to produce a gradient echo. It also 'spoils' or dephased the remaining transverse magnetization to minimize its effect on image contrast, resulting in increased T1 weighting.

This sequence uses a 180-degree inverting pulse before the usual spin-echo or gradient echo sequence, typically with spin echoes. The inverting pulse flips the longitudinal magnetization (LM) from positive to negative, causing saturation of all tissues. Then, as tissues recover their LM at different rates depending on their T1 values, the 90-degree pulse is applied, leading to increased T1 contrast in the images.

The time interval between the inversion 180-degree and the excitatory 90-degree pulses.

A type of magnetic resonance imaging (MRI) sequence that uses a series of short repetition times (TR) and echo times (TE) to allow for rapid image acquisition. It emphasizes steady-state saturation, resulting in bright signal from tissues with long T2 values.

A specific type of steady-state sequence that uses a coherent gradient echo pulse. It enhances T2 weighting.

The time interval between the application of a 90-degree excitation pulse and the receipt of an echo signal.

A type of MRI imaging that uses either GRE or EPI sequences to highlight areas of restricted diffusion of extracellular water, often seen in conditions like infarction. The affected area shows up as bright on the images. It's particularly helpful in differentiating between recoverable and irreparable brain tissue after a stroke.

A dynamic MRI technique that captures images of the brain while a person is performing a task or at rest. It allows researchers to study brain activity in real-time by observing changes in blood flow and oxygen levels.

A technique that reduces background signal to improve the visibility of blood vessels and specific disease processes. It makes features like vessels stand out more clearly.

A specialized MRI technique designed to visualize blood vessels by detecting the difference in signal intensity between flowing blood and stationary tissue. This creates a clearer picture of blood vessels compared to standard MRI.

A technique that uses coherent GRE pulse sequences to saturate background tissue while allowing flowing blood to show a strong signal. It's commonly used to visualize arteries and veins in the head, neck, and limbs.

A technique utilizing coherent GRE sequences to suppress background signal and provide detailed images of blood vessels. Although it offers excellent background suppression, it typically involves longer scan times compared to other MRA techniques.

Short Inversion Recovery (STIR) is an IR sequence variation that utilizes a short TI value to suppress the signal from fat. By selecting a TI corresponding to the time it takes for fat to recover from inversion to the transverse plane, the fat signal is nullified, leaving only other tissues in the image.

Fluid Attenuated Inversion Recovery (FLAIR) is an IR sequence variation that utilizes a longer TI value (around 2000ms) to suppress the signal from free water. This makes it ideal for visualizing lesions or structures that have a high water content, as the background water signal is reduced.

Single Shot EPI (SS-EPI) is a variation of EPI where all the lines of k-space required to form an image are acquired in a single TR.

Spin Echo EPI (SE-EPI) is a type of EPI where multiple 180-degree pulses are used to generate echoes, contributing to the image formation.

Gradient Echo EPI (GE-EPI) is a variation of EPI where gradient pulses are used for rephasing instead of a 180-degree pulse. This allows for quicker acquisition times compared to SE-EPI.

Perfusion Weighted Imaging (PWI) is a dynamic MR imaging technique that utilizes gradient echo (GRE) or EPI sequences with contrast agents to assess the uptake of the contrast agent in lesions. It is used in studying vascular abnormalities in various organs such as brain, pancreas, liver, and prostate.