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MRI pulse sequence

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This table does not include uncommon and experimental sequences.

Group Sequence Abbr. Physics Main clinical distinctions Example
Spin echo T1 weighted T1 Measuring spin–lattice relaxation by using a short repetition time (TR) and echo time (TE).

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[1]
  • Low signal for fat in standard Spine Echo (SE),[1] though not with Fast Spin Echo/Turbo Spin Echo (FSE/TSE). FSE/TSE is the standard of care in modern medicine because it is faster. With FSE/TSE, fat has high signal due to disruption of hyperfine J-coupling between adjacent fat protons.[3]
  • Low signal for paramagnetic substances[2]

Standard foundation and comparison for other sequences

Proton density weighted PD Long TR (to reduce T1) and short TE (to minimize T2).[4] Joint disease and injury.[5]
Gradient echo (GRE) Steady-state free precession SSFP Maintenance of a steady, residual transverse magnetisation over successive cycles.[7] Creation of cardiac MRI videos (pictured).[7]
Effective T2
or "T2-star"		 T2* Spoiled gradient recalled echo (GRE) with a long echo time and small flip angle[8] Low signal from hemosiderin deposits (pictured) and hemorrhages.[8]
Susceptibility-weighted SWI Spoiled gradient recalled echo (GRE), fully flow compensated, long echo time, combines phase image with magnitude image[9] Detecting small amounts of hemorrhage (diffuse axonal injury pictured) or calcium.[9]
Inversion recovery Short tau inversion recovery STIR Fat suppression by setting an inversion time where the signal of fat is zero.[10] High signal in edema, such as in more severe stress fracture.[11] 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).[12]
Double inversion recovery DIR Simultaneous suppression of cerebrospinal fluid and white matter by two inversion times.[13] High signal of multiple sclerosis plaques (pictured).[13]
Diffusion weighted (DWI) Conventional DWI Measure of Brownian motion of water molecules.[14] High signal within minutes of cerebral infarction (pictured).[15]
Apparent diffusion coefficient ADC Reduced T2 weighting by taking multiple conventional DWI images with different DWI weighting, and the change corresponds to diffusion.[16] Low signal minutes after cerebral infarction (pictured).[17]
Diffusion tensor DTI Mainly tractography (pictured) by an overall greater Brownian motion of water molecules in the directions of nerve fibers.[18]
Perfusion weighted (PWI) Dynamic susceptibility contrast DSC Measures changes over time in susceptibility-induced signal loss due to gadolinium contrast injection.[20]
  • Provides measurements of blood flow
  • In cerebral infarction, the infarcted core and the penumbra have decreased perfusion and delayed contrast arrival (pictured).[21]
Arterial spin labelling ASL Magnetic labeling of arterial blood below the imaging slab, which subsequently enters the region of interest.[22] It does not need gadolinium contrast.[23]
Dynamic contrast enhanced DCE Measures changes over time in the shortening of the spin–lattice relaxation (T1) induced by a gadolinium contrast bolus.[24] Faster Gd contrast uptake along with other features is suggestive of malignancy (pictured).[25]
Functional MRI (fMRI) Blood-oxygen-level dependent imaging BOLD Changes in oxygen saturation-dependent magnetism of hemoglobin reflects tissue activity.[26] Localizing brain activity from performing an assigned task (e.g. talking, moving fingers) before surgery, also used in research of cognition.[27]
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 aneurysm, stenosis, or dissection[28]
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.[29] Detection of aneurysm, stenosis, or dissection (pictured).[28]
(VIPR)

Diffusion MRI

Main article: Diffusion MRI
DTI image

Diffusion MRI measures the diffusion of water molecules in biological tissues.[30] Clinically, diffusion MRI is useful for the diagnoses of conditions (e.g., stroke) or neurological disorders (e.g., multiple sclerosis), and helps better understand the connectivity of white matter axons in the central nervous system.[31] In an isotropic medium (inside a glass of water for example), water molecules naturally move randomly according to turbulence and Brownian motion. In biological tissues however, where the Reynolds number is low enough for laminar flow, the diffusion may be anisotropic. For example, a molecule inside the axon of a neuron has a low probability of crossing the myelin membrane. Therefore, the molecule moves principally along the axis of the neural fiber. If it is known that molecules in a particular voxel diffuse principally in one direction, the assumption can be made that the majority of the fibers in this area are parallel to that direction.

The recent development of diffusion tensor imaging (DTI)[32] enables diffusion to be measured in multiple directions, and the fractional anisotropy in each direction to be calculated for each voxel. This enables researchers to make brain maps of fiber directions to examine the connectivity of different regions in the brain (using tractography) or to examine areas of neural degeneration and demyelination in diseases like multiple sclerosis.

Another application of diffusion MRI is diffusion-weighted imaging (DWI). Following an ischemic stroke, DWI is highly sensitive to the changes occurring in the lesion.[33] It is speculated that increases in restriction (barriers) to water diffusion, as a result of cytotoxic edema (cellular swelling), is responsible for the increase in signal on a DWI scan. The DWI enhancement appears within 5–10 minutes of the onset of stroke symptoms (as compared to computed tomography, which often does not detect changes of acute infarct for up to 4–6 hours) and remains for up to two weeks. Coupled with imaging of cerebral perfusion, researchers can highlight regions of "perfusion/diffusion mismatch" that may indicate regions capable of salvage by reperfusion therapy.

Like many other specialized applications, this technique is usually coupled with a fast image acquisition sequence, such as echo planar imaging sequence.

Steady-state free precession imaging

Main article: Steady-state free precession imaging

Steady-state free precession imaging (SSFP MRI) is an MRI technique which uses steady states of magnetizations. In general, SSFP MRI sequences are based on a (low flip angle) gradient-echo MRI sequence with a short repetition time which in its generic form has been described as the FLASH MRI technique. While spoiled gradient-echo sequences refer to a steady state of the longitudinal magnetization only, SSFP gradient-echo sequences include transverse coherences (magnetizations) from overlapping multi-order spin echoes and stimulated echoes. This is usually accomplished by refocusing the phase-encoding gradient in each repetition interval in order to keep the phase integral (or gradient moment) constant. Fully balanced SSFP MRI sequences achieve a phase of zero by refocusing all imaging gradients.

New methods and variants of existing methods are often published when they are able to produce better results in specific fields. Examples of these recent improvements are T*
2-weighted turbo spin-echo (T2 TSE MRI), double inversion recovery MRI (DIR-MRI) or phase-sensitive inversion recovery MRI (PSIR-MRI), all of them able to improve imaging of brain lesions.[34][35] Another example is MP-RAGE (magnetization-prepared rapid acquisition with gradient echo),[36] which improves images of multiple sclerosis cortical lesions.[37]

Magnetization transfer MRI

Main article: Magnetization transfer

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.[38]

The most common use of this technique is for suppression of background signal in time of flight MR angiography.[39] There are also applications in neuroimaging particularly in the characterization of white matter lesions in multiple sclerosis.[40]

T1rho MRI

T1ρ (T1rho): 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.[41]

Proton density weighted

Proton density (PD) weighted images are created by having a long repetition time (TR) and a short echo time (TE).[42] On images of the brain, this sequence has a more pronounced distinction between gray matter (bright) and white matter (darker gray), but with little contrast between brain and CSF.[42]

Fluid attenuated inversion recovery (FLAIR)

Main article: Fluid attenuated inversion recovery

Fluid attenuated inversion recovery (FLAIR)[43] is an inversion-recovery pulse sequence used to nullify the signal from fluids. For example, it can be used in brain imaging to suppress cerebrospinal fluid (CSF) so as to bring out periventricular hyperintense lesions, such as multiple sclerosis (MS) plaques. By carefully choosing the inversion time TI (the time between the inversion and excitation pulses), the signal from any particular tissue can be suppressed.

Susceptibility weighted imaging (SWI)

Main article: 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, hemorrhage and iron storage. It is used to enhance the detection and diagnosis of tumors, vascular and neurovascular diseases (stroke and hemorrhage), multiple sclerosis,[44] Alzheimer's, and also detects traumatic brain injuries that may not be diagnosed using other methods.[45]

Fast spin

Fast spin echo (FSE), also called turbo spin echo (TSE) is a sequence that results in fast scan times. In this sequence, several 180 refocusing radio-frequency pulses are delivered during each echo time (TR) interval, and the phase-encoding gradient is briefly switched on between echoes.[46]

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) inflammatory bowel disease, but also obesity, chemotherapy and celiac disease.[47] Techniques to suppress fat on MRI mainly include:[48]

  • Identifying fat by the chemical shift of its atoms, causing different time-dependent phase shifts compared to water.
  • Frequency-selective saturation of the spektral 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 paramagnetic properties of neuromelanin and can be used to visualize the substantia nigra and the locus coeruleus. It is used to detect the atrophy of these nuclei in Parkinson's disease and other parkinsonisms, and also detects signal intensity changes in major depressive disorder and schizophrenia.[49]

References

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