MRI pulse sequence

An MRI pulse sequence in magnetic resonance imaging (MRI) is a particular setting of pulse sequences and pulsed field gradients, resulting in a particular image appearance.^[1]

A multiparametric MRI is a combination of two or more sequences, and/or including other specialized MRI configurations such as spectroscopy.^[2][3]

Overview table

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Group	Sequence	Abbr.	Physics	Main clinical distinctions	Example
Spin echo	T1 weighted	T1	Measuring echo time (TE).	Lower signal for more water content, hemorrhage.[5] High signal for fat[4][5] High signal for paramagnetic substances, such as MRI contrast agents[5] Standard foundation and comparison for other sequences
	T2 weighted	T2	Measuring spin–spin relaxation by using long TR and TE times	Higher signal for more water content[4] Low signal for fat[4] − Note that this only applies to standard Spin Echo (SE) sequences and not the more modern Fast Spin Echo (FSE) sequence (also referred to as Turbo Spin Echo, TSE), which is the most commonly used technique today. In FSE/TSE, fat will have a high signal.[6] Low signal for paramagnetic substances[5] Standard foundation and comparison for other sequences
	Proton density weighted	PD	Long TE (to minimize T2).[7]	Joint disease and injury.[8] High signal from meniscus tears.[9] (pictured)
Gradient echo (GRE)	Steady-state free precession	SSFP	Maintenance of a steady, residual transverse magnetisation over successive cycles.[10]	Creation of cardiac MRI videos (pictured).[10]
	Effective T2 or "T2-star"	T2*	Spoiled gradient recalled echo (GRE) with a long echo time and small flip angle[11]	Low signal from hemosiderin deposits (pictured) and hemorrhages.[11]
	Susceptibility-weighted	SWI	Spoiled gradient recalled echo (GRE), fully flow compensated, long echo time, combines phase image with magnitude image[12]	Detecting small amounts of hemorrhage (diffuse axonal injury pictured) or calcium.[12]
Inversion recovery	Short tau inversion recovery	STIR	Fat suppression by setting an inversion time where the signal of fat is zero.[13]	High signal in edema, such as in more severe stress fracture.[14] Shin splints pictured:
	Fluid-attenuated inversion recovery	FLAIR	Fluid suppression by setting an inversion time that nulls fluids	High signal in lacunar infarction, multiple sclerosis (MS) plaques, subarachnoid haemorrhage and meningitis (pictured).[15]
	Double inversion recovery	DIR	Simultaneous suppression of cerebrospinal fluid and white matter by two inversion times.[16]	High signal of multiple sclerosis plaques (pictured).[16]
Diffusion weighted (DWI)	Conventional	DWI	Measure of Brownian motion of water molecules.[17]	High signal within minutes of cerebral infarction (pictured).[18]
	Apparent diffusion coefficient	ADC	Reduced T2 weighting by taking multiple conventional DWI images with different DWI weighting, and the change corresponds to diffusion.[19]	Low signal minutes after cerebral infarction (pictured).[20]
	Diffusion tensor	DTI	Mainly tractography (pictured) by an overall greater Brownian motion of water molecules in the directions of nerve fibers.[21]	Evaluating white matter deformation by tumors[21] Reduced fractional anisotropy may indicate dementia.[22]
Perfusion weighted (PWI)	Dynamic susceptibility contrast	DSC	Measures changes over time in susceptibility-induced signal loss due to gadolinium contrast injection.[23]	Provides measurements of blood flow In cerebral infarction, the infarcted core and the penumbra have decreased perfusion and delayed contrast arrival (pictured).[24]
	Arterial spin labelling	ASL	Magnetic labeling of arterial blood below the imaging slab, which subsequently enters the region of interest.[25] It does not need gadolinium contrast.[26]
	Dynamic contrast enhanced	DCE	Measures changes over time in the shortening of the gadolinium contrast bolus.[27]	Faster Gd contrast uptake along with other features is suggestive of malignancy (pictured).[28]
Functional MRI (fMRI)	Blood-oxygen-level dependent imaging	BOLD	Changes in oxygen saturation-dependent magnetism of hemoglobin reflects tissue activity.[29]	Localizing brain activity from performing an assigned task (e.g. talking, moving fingers) before surgery, also used in research of cognition.[30]
Magnetic resonance angiography (MRA) and venography	Time-of-flight	TOF	Blood entering the imaged area is not yet magnetically saturated, giving it a much higher signal when using short echo time and flow compensation.	Detection of dissection[31]
	Phase-contrast magnetic resonance imaging	PC-MRA	Two gradients with equal magnitude, but opposite direction, are used to encode a phase shift, which is proportional to the velocity of spins.[32]	Detection of dissection (pictured).[31]	VIPR )

Spin echo

T1 and T2

Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T1 (

Proton density (PD)- weighted images are created by having a long repetition time (TR) and a short echo time (TE).^[36] On images of the brain, this sequence has a more pronounced distinction between grey matter (bright) and white matter (darker grey), but with little contrast between brain and CSF.^[36] It is very useful for the detection of arthropathy and injury.^[37]

Gradient echo

A gradient echo sequence does not use a 180 degrees RF pulse to make the spins of particles coherent. Instead, it uses magnetic gradients to manipulate the spins, allowing the spins to dephase and rephase when required. After an excitation pulse, the spins are dephased, no signal is produced because the spins are not coherent. When the spins are rephased, they become coherent, and thus signal (or "echo") is generated to form images. Unlike spin echo, gradient echo does not need to wait for transverse magnetisation to decay completely before initiating another sequence, thus it requires very short repetition times (TR), and therefore to acquire images in a short time. After echo is formed, some transverse magnetisations remains. Manipulating gradients during this time will produce images with different contrast. There are three main methods of manipulating contrast at this stage, namely steady-state free-precession (SSFP) that does not spoil the remaining transverse magnetisation, but attempts to recover them (thus producing T2-weighted images); the sequence with spoiler gradient that averages the transverse magnetisations (thus producing mixed T1 and T2-weighted images), and RF spoiler that vary the phases of RF pulse to eliminates the transverse magnetisation, thus producing pure T1-weighted images.^[39]

For comparison purposes, the repetition time of a gradient echo sequence is of the order of 3 milliseconds, versus about 30 ms of a spin echo sequence.^{[citation needed]}

Inversion recovery

Inversion recovery is an MRI sequence that provides high contrast between tissue and lesion. It can be used to provide high T1 weighted image, high T2 weighted image, and to suppress the signals from fat, blood, or cerebrospinal fluid (CSF).^[40]

Diffusion weighted

Perfusion-weighted imaging (PWI) is performed by 3 main techniques:

• Dynamic susceptibility contrast (DSC):
  T2 weighted) quantifies susceptibility-induced signal loss.^[45]
• Dynamic contrast enhanced (DCE): Measuring shortening of the
  gadolinium contrast bolus.^[46]
• Arterial spin labelling (ASL): Magnetic labeling of arterial blood below the imaging slab, without the need of gadolinium contrast.[47]

The acquired data is then postprocessed to obtain perfusion maps with different parameters, such as BV (blood volume), BF (blood flow), MTT (mean transit time) and TTP (time to peak).

In

Phase contrast

Phase contrast MRI (PC-MRI) is used to measure flow velocities in the body. It is used mainly to measure blood flow in the heart and throughout the body. PC-MRI may be considered a method of magnetic resonance velocimetry. Since modern PC-MRI typically is time-resolved, it also may be referred to as 4-D imaging (three spatial dimensions plus time).^[51]

Susceptibility weighted imaging

Susceptibility-weighted imaging (SWI) is a new type of contrast in MRI different from spin density, T₁, or T₂ imaging. This method exploits the susceptibility differences between tissues and uses a fully velocity-compensated, three-dimensional, RF-spoiled, high-resolution, 3D-gradient echo scan. This special data acquisition and image processing produces an enhanced contrast magnitude image very sensitive to venous blood,

Magnetization transfer (MT) is a technique to enhance image contrast in certain applications of MRI.

Bound protons are associated with proteins and as they have a very short T2 decay they do not normally contribute to image contrast. However, because these protons have a broad resonance peak they can be excited by a radiofrequency pulse that has no effect on free protons. Their excitation increases image contrast by transfer of saturated spins from the bound pool into the free pool, thereby reducing the signal of free water. This homonuclear magnetization transfer provides an indirect measurement of macromolecular content in tissue. Implementation of homonuclear magnetization transfer involves choosing suitable frequency offsets and pulse shapes to saturate the bound spins sufficiently strongly, within the safety limits of specific absorption rate for MRI.^[54]

The most common use of this technique is for suppression of background signal in time of flight MR angiography.^[55] There are also applications in neuroimaging particularly in the characterization of white matter lesions in multiple sclerosis.^[56]

Fat suppression

Fat suppression is useful for example to distinguish active inflammation in the intestines from fat deposition such as can be caused by long-standing (but possibly inactive)

Techniques to suppress fat on MRI mainly include:[59]

• Identifying fat by the chemical shift of its atoms, causing different time-dependent phase shifts compared to water.
• Frequency-selective saturation of the spectral peak of fat by a "fat sat" pulse before imaging.
• Short tau inversion recovery (STIR), a T1-dependent method
• Spectral presaturation with inversion recovery (SPIR)

Neuromelanin imaging

This method exploits the

T1 rho (T1ρ) is an experimental MRI sequence that may be used in musculoskeletal imaging. It does not yet have widespread use.[61]

Molecules have a kinetic energy that is a function of the temperature and is expressed as translational and rotational motions, and by collisions between molecules. The moving dipoles disturb the magnetic field but are often extremely rapid so that the average effect over a long time-scale may be zero. However, depending on the time-scale, the interactions between the dipoles do not always average away. At the slowest extreme the interaction time is effectively infinite and occurs where there are large, stationary field disturbances (e.g., a metallic implant). In this case the loss of coherence is described as a "static dephasing". T2* is a measure of the loss of coherence in an ensemble of spins that includes all interactions (including static dephasing). T2 is a measure of the loss of coherence that excludes static dephasing, using an RF pulse to reverse the slowest types of dipolar interaction. There is in fact a continuum of interaction time-scales in a given biological sample, and the properties of the refocusing RF pulse can be tuned to refocus more than just static dephasing. In general, the rate of decay of an ensemble of spins is a function of the interaction times and also the power of the RF pulse. This type of decay, occurring under the influence of RF, is known as T1ρ. It is similar to T2 decay but with some slower dipolar interactions refocused, as well as static interactions, hence T1ρ≥T2.^[62]

Others

• Saturation recovery sequences are rarely used, but can measure
  spin-lattice relaxation time (T1) more quickly than an inversion recovery pulse sequence.^[63]
• Double-oscillating-diffusion-encoding (DODE) and double diffusion encoding (DDE) imaging are specific forms of MRI diffusion imaging, which can be used to measure diameters and lengths of axon pores.^[64]

References