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Fluid-attenuated inversion recovery

Fluid-attenuated inversion recovery (FLAIR) is a magnetic resonance imaging (MRI) pulse sequence that utilizes an inversion recovery technique with a long inversion time to suppress the high signal intensity from cerebrospinal fluid (CSF) and other free fluids, producing images with T2-weighted contrast where brain parenchyma is highlighted while fluids appear dark.^[1] This suppression is achieved by selecting an inversion time (TI) that corresponds to the null point of the longitudinal magnetization recovery for fluids, typically around 2,000–2,600 milliseconds at 1.5 Tesla field strength, allowing for enhanced detection of pathological changes adjacent to CSF spaces that might otherwise be obscured on conventional T2-weighted images.^[2] Developed in the early 1990s by Graeme Bydder, Joseph Hajnal, and Ian Young, FLAIR was initially introduced as an advancement in neuroimaging to address limitations of standard spin-echo sequences, with early implementations at low field strengths like 1.0 T demonstrating its utility in visualizing brain structures without fluid signal interference.^[1] These technical adaptations have enabled FLAIR's integration into routine clinical protocols across various MRI systems. FLAIR's primary clinical value lies in its sensitivity for detecting subtle abnormalities in the central nervous system, particularly white matter lesions, cortical infarcts, and periventricular pathologies, making it indispensable for diagnosing conditions like multiple sclerosis, acute ischemic stroke, encephalitis, and neoplastic diseases.^[3] In multiple sclerosis, for instance, FLAIR excels at revealing leptomeningeal enhancement and juxtacortical lesions that are less conspicuous on T2-weighted imaging alone.^[4] For acute stroke evaluation, a FLAIR-negative diffusion-weighted imaging mismatch can indicate early-stage ischemia within hours of onset, aiding in thrombolysis decisions.^[5] Post-contrast FLAIR further enhances detection of blood-brain barrier disruptions, such as in meningitis or metastases, by showing lower thresholds for gadolinium enhancement compared to T1-weighted sequences.^[3] Recent advancements as of 2025 include AI-based reconstruction for accelerated imaging and enhanced applications in brain tumor detection.^[6]^[7] Despite its advantages, FLAIR is susceptible to artifacts like CSF pulsation or incomplete fluid suppression at higher fields, which can mimic pathology and necessitate complementary sequences for accurate interpretation.^[8] Overall, FLAIR remains a cornerstone of modern neuroimaging, continually refined for applications in both research and clinical practice.

Introduction and History

Definition and Overview

Fluid-attenuated inversion recovery (FLAIR) is a magnetic resonance imaging (MRI) pulse sequence that utilizes inversion recovery to suppress signals from fluids, such as cerebrospinal fluid (CSF), thereby producing T2-weighted images in which CSF appears dark rather than bright.^[3] This suppression is achieved through a specific inversion time that nullifies the fluid signal, minimizing interference from high-signal fluids in the imaging field.^[9] The term FLAIR stands for Fluid-Attenuated Inversion Recovery, and it serves as a variant of conventional T2-weighted MRI tailored for enhanced tissue contrast by attenuating fluid contributions.^[10] In standard T2-weighted imaging, the bright signal of CSF can obscure pathological changes near fluid-filled structures, but FLAIR addresses this by providing strong T2 weighting with fluid suppression, resulting in improved lesion-to-background contrast adjacent to CSF spaces.^[11] This core purpose enables better visualization of abnormalities in proximity to fluids, such as those bordering brain ventricles, without the partial volume effects common in non-suppressed sequences.^[3] FLAIR is routinely incorporated into clinical MRI protocols, particularly for neuroimaging, where it complements other sequences to offer a clearer depiction of tissue pathologies against suppressed fluid backgrounds.^[12] As a foundational adaptation of inversion recovery principles, it prioritizes diagnostic utility in fluid-rich environments while maintaining the sensitivity of T2-weighted contrast for edematous or inflammatory changes.^[3]

Development and Key Milestones

The fluid-attenuated inversion recovery (FLAIR) sequence emerged as an advancement in magnetic resonance imaging (MRI) techniques during the early 1990s, building on foundational inversion recovery methods that had been developed in the 1970s and 1980s to enhance tissue contrast by selectively nulling signals based on T1 relaxation times.^[13] These earlier inversion recovery sequences, initially explored for their ability to suppress unwanted signals in basic MRI scans, laid the groundwork for more specialized applications in neuroimaging.^[14] FLAIR was invented by Graeme Bydder, Joseph V. Hajnal, and Ian R. Young at Hammersmith Hospital in London, with the primary goal of suppressing cerebrospinal fluid signals to improve visualization of brain pathologies adjacent to fluid-filled spaces.^[15] The technique was first demonstrated in 1992 through a seminal study by Hajnal and colleagues, who applied FLAIR to brain imaging and highlighted its potential for detecting abnormalities in white matter by nulling fluid signals while preserving T2-weighted contrast.^[16] By the mid-1990s, FLAIR began to be integrated into commercial MRI scanners, enabling broader accessibility beyond research settings and facilitating its evaluation in clinical trials for various neurological conditions.^[17] In the late 1990s, the introduction of fast spin-echo variants significantly reduced scan times, making FLAIR more practical for routine use and establishing it as a standard protocol in brain imaging.^[17] The development of 3D FLAIR sequences around 2000 further expanded its capabilities, allowing for isotropic voxel imaging and multiplanar reconstructions that improved lesion detection without the limitations of slab boundaries in 2D acquisitions.^[18] By the early 2000s, FLAIR had achieved widespread clinical adoption, particularly in the diagnosis of multiple sclerosis, where its sensitivity to periventricular and juxtacortical lesions surpassed conventional T2-weighted imaging, influencing diagnostic criteria and patient management protocols.^[11]

Principles and Physics

Inversion Recovery Fundamentals

In nuclear magnetic resonance (NMR), relaxation processes govern the return of magnetized spins to equilibrium after perturbation by radiofrequency pulses. Longitudinal relaxation, characterized by the time constant T1, describes the recovery of the net magnetization vector along the direction of the static magnetic field, typically taking hundreds of milliseconds to seconds in biological tissues due to energy exchange between spins and their lattice environment.^[19] Transverse relaxation, governed by T2, refers to the dephasing and decay of magnetization in the plane perpendicular to the field, occurring on shorter timescales (tens to hundreds of milliseconds) from spin-spin interactions and local field inhomogeneities.^[19] These T1 and T2 values vary across tissues—such as shorter T1 in fat (~250 ms at 1.5 T) compared to longer T1 in cerebrospinal fluid (~2000–4000 ms)—providing inherent contrast in magnetic resonance imaging (MRI). The inversion recovery mechanism begins with a 180° radiofrequency pulse that inverts the longitudinal magnetization from its equilibrium state (M0) to -M0, initiating T1 recovery toward positive equilibrium.^[20] This is followed by an inversion time (TI) during which magnetization recovers exponentially, with the recovered value depending on the tissue's T1. At the end of TI, a 90° pulse (or spin-echo sequence) tips the longitudinal magnetization into the transverse plane for signal readout, enabling selective manipulation of signals from tissues with differing T1 values.^[20] The signal intensity in an ideal inversion recovery sequence, neglecting transverse relaxation, is given by:

M(TI) = M_0 \left(1 - 2e^{-TI/T1}\right)

assuming full recovery before the next repetition (long TR).^[20] The null point, where signal is zero, occurs when the magnetization crosses the transverse plane, at TI = T1 \ln(2) \approx 0.693 T1; for example, this TI nulls fat signal at approximately 170 ms (for T1 = 250 ms).^[20] By tuning TI to the T1 of specific tissues, inversion recovery generates contrast through selective suppression or enhancement: short TI emphasizes tissues with short T1 (e.g., fat appears bright), while TI matched to longer T1 values (e.g., water in edema) can null them, highlighting adjacent structures.^[20] This T1-based contrast mechanism allows differentiation of pathologies like lesions from normal tissue based on relaxation differences.^[20] Inversion recovery techniques originated in NMR spectroscopy in 1949 for T1 measurement and were adapted for MRI in the 1970s to produce basic T1-weighted contrast in early imaging experiments.^[19]

FLAIR-Specific Adaptations

Fluid-attenuated inversion recovery (FLAIR) adapts the standard inversion recovery sequence by integrating T2-weighting through a prolonged echo time (TE) applied after the 90° excitation pulse, which emphasizes differences in T2 relaxation times among tissues while the inversion preparation suppresses fluid signals. This modification enables high-contrast imaging of parenchymal abnormalities by combining the nulling effect of inversion with the sensitivity of T2-weighted contrast to pathology. The long TE, typically exceeding 100 ms, ensures that short-T2 components like fat decay rapidly, while longer-T2 tissues and lesions retain signal, enhancing differentiation in the post-inversion recovery period.^[3] The core mechanism for cerebrospinal fluid (CSF) suppression in FLAIR involves selecting an inversion time (TI) that nulls the CSF signal, generally set to approximately 2000–2500 ms at 1.5 T magnetic field strength. This TI value targets the long longitudinal relaxation time (T1) of CSF, which is around 4000 ms, allowing the magnetization of CSF to reach its null point during recovery, while its transverse relaxation time (T2) of about 2000 ms contributes to the overall fluid attenuation when combined with the long TE. As a result, CSF appears hypointense, providing a dark background that improves the visibility of adjacent structures.^[17]^[21] In FLAIR imaging, pathological lesions characterized by extended T1 and T2 relaxation times—such as those associated with edema, inflammation, or demyelination—manifest as hyperintense regions against the suppressed CSF background, facilitating their detection near fluid-filled spaces. This contrast behavior stems from the incomplete recovery of lesion magnetization at the chosen TI, coupled with their slower T2 decay compared to normal tissue. Unlike conventional T2-weighted sequences, where the hyperintense CSF signal can mask or mimic periventricular or sulcal pathology, FLAIR's fluid attenuation eliminates this interference, thereby enhancing diagnostic specificity for subtle lesions.^[3]^[22] Field strength influences FLAIR adaptations, particularly the TI selection, as T1 relaxation times increase with higher magnetic fields; at 3 T, CSF T1 extends to roughly 4500–5000 ms, necessitating a longer TI of about 2400–2800 ms to achieve effective nulling, compared to the shorter values at 1.5 T. This adjustment accounts for the field-dependent prolongation of T1 across tissues, ensuring optimal CSF suppression without over- or under-nulling, while maintaining the T2-weighted contrast through consistent long TE application.^[23]^[21]

Technical Implementation

Pulse Sequence Details

The fluid-attenuated inversion recovery (FLAIR) pulse sequence is structured as a T2-weighted inversion recovery technique designed to suppress cerebrospinal fluid (CSF) signal while highlighting parenchymal abnormalities. It commences with a non-selective 180° radiofrequency (RF) inversion pulse that inverts the longitudinal magnetization across the imaging volume. This is followed by an inversion time (TI) period, during which magnetization recovers toward equilibrium; the TI is specifically selected to null the CSF signal based on its long T1 relaxation time. At the end of TI, a slice-selective 90° RF excitation pulse is applied, initiating transverse magnetization. Subsequent 180° refocusing RF pulses—typically one in conventional spin-echo FLAIR or multiple in fast variants—form spin echoes, with the signal acquired at a long echo time (TE) to emphasize T2 contrast between tissues.^[1]^[24] In a typical timing diagram for standard 2D FLAIR, the repetition time (TR) exceeds 8000 ms to allow sufficient longitudinal recovery, TI ranges from 2000 to 2600 ms for CSF nulling at 1.5 T or 3 T field strengths, and TE is set between 100 and 150 ms to enhance T2 weighting. These parameters ensure that at the excitation point, CSF magnetization crosses the zero point, yielding minimal signal, while brain parenchyma with shorter T1 recovers sufficiently to produce positive signal during readout. For example, at higher fields like 7 T, TI may extend to approximately 3100 ms and TR to over 20,000 ms to account for prolonged relaxation times and specific absorption rate (SAR) limits, often using an adiabatic inversion pulse for uniform B1 field distribution.^[2]^[1] Variants of the FLAIR sequence adapt the core structure for efficiency and resolution. In 2D FLAIR, acquisition occurs in axial, sagittal, or coronal planes with anisotropic voxels, suitable for routine clinical scanning. The 3D FLAIR variant employs volumetric excitation and encoding, yielding isotropic voxels (e.g., 1 mm³) that enable multiplanar reformatting without partial volume effects, though it requires longer scan times unless accelerated. Fast or turbo FLAIR incorporates an echo train via multiple 180° refocusing pulses (turbo factor of 16–32) after the 90° excitation, acquiring several k-space lines per TR to reduce overall acquisition time from minutes to seconds, while maintaining T2 weighting through effective TE selection.^[2] Gradient pulses play a critical role in spatial localization and artifact mitigation throughout the sequence. Slice-selection gradients are superimposed on the RF pulses to excite specific planes, phase-encoding gradients vary stepwise to fill k-space in the phase direction, and frequency-encoding (readout) gradients linearize the field during signal acquisition for position decoding. Spoiler gradients, applied immediately after readout and before the next inversion pulse, dephase any residual transverse magnetization, preventing steady-state effects or ghosting artifacts from prior excitations.^[1]^[25] In post-contrast FLAIR, intravenous gadolinium administration shortens the T1 of enhancing lesions or tissues, causing them to recover magnetization faster than CSF during TI, resulting in bright signal on the otherwise dark CSF background without interference from fluid signal. This adaptation leverages the sequence's inherent CSF suppression to improve conspicuity of subtle enhancements, such as in leptomeningeal disease, using the same core timing but performed after contrast injection.^[26]

Imaging Parameters and Optimization

In clinical practice, the key imaging parameters for fluid-attenuated inversion recovery (FLAIR) sequences in brain MRI are tailored to balance contrast, resolution, and scan efficiency. Typical field of view (FOV) is set to 22-24 cm to encompass the entire brain, with a matrix size of 256×256 or 256×192 to achieve adequate spatial resolution without excessive scan prolongation.^[27]^[28] Slice thickness is commonly 3-5 mm for 2D acquisitions, enabling whole-brain coverage while minimizing partial volume effects; thinner slices (e.g., 3 mm) are preferred at higher field strengths for improved detail.^[29] The number of excitations (NEX) is often 2 to enhance signal-to-noise ratio (SNR) without significantly extending acquisition time.^[27] Optimization of FLAIR involves adjusting the inversion time (TI) based on magnetic field strength to effectively null cerebrospinal fluid (CSF) signal. At 1.5 T, a TI of approximately 2100-2200 ms is standard, while at 3 T, it is increased to 2250-2400 ms to account for longer T1 relaxation times of CSF.^[30]^[31] Parallel imaging techniques, such as SENSE or GRAPPA, are routinely employed to reduce scan time by factors of 2-3 while maintaining image quality, particularly beneficial for motion-prone patients.^[27] Scan times for standard 2D brain FLAIR typically range from 4-6 minutes, influenced by repetition time (TR, often 8000-11000 ms), echo time (TE, 100-140 ms), and the use of fast spin-echo readout.^[27]^[28] Strategies to minimize motion artifacts include shortening TR where possible or incorporating faster variants like turbo FLAIR, ensuring diagnostic utility in uncooperative subjects.^[32] Hardware considerations emphasize the use of a dedicated multi-channel head coil as the standard receiver to maximize SNR in neuro imaging.^[33] Additional fat suppression pulses are occasionally applied in non-neurological FLAIR applications to mitigate orbital or calvarial fat signals, though this is rarely needed for routine brain protocols.^[31] Quality control in FLAIR imaging relies on quantitative metrics such as SNR (target >20 for robust lesion detection) and contrast-to-noise ratio (CNR) between gray matter and CSF (ideally >15 post-nulling).^[27] These are validated through phantom testing or region-of-interest analysis to ensure consistent suppression of CSF hyperintensity and optimal tissue differentiation across scanners.^[28]

Clinical Applications

Neurological Imaging

Fluid-attenuated inversion recovery (FLAIR) imaging plays a pivotal role in the diagnosis and evaluation of multiple sclerosis (MS), particularly for visualizing periventricular and juxtacortical plaques, where cerebrospinal fluid (CSF) suppression enhances lesion conspicuity against surrounding white matter.^[34] In MS patients, FLAIR sequences reveal hyperintense lesions that correspond to areas of demyelination and inflammation, often appearing as ovoid or finger-like extensions perpendicular to the ventricles, aiding in fulfilling diagnostic criteria such as those from the McDonald criteria.^[34] This CSF nulling effect, achieved through inversion recovery, distinguishes FLAIR-detected plaques from adjacent CSF signal voids, improving detection sensitivity compared to conventional T2-weighted imaging in supratentorial locations.^[35] In acute stroke, FLAIR identifies acute to subacute infarcts as hyperintense regions within the affected brain parenchyma, typically becoming visible several hours after onset and facilitating differentiation from chronic lesions.^[36] Hyperintense vessels on FLAIR, indicative of slow flow in occluded vessels, correlate with large-vessel occlusion and poor collateral circulation, providing prognostic insights in the hyperacute phase.^[37] The sequence's ability to highlight ischemic changes supports its integration into multimodal protocols, such as DWI-FLAIR mismatch assessment, to identify patients eligible for thrombolysis within hours of onset.^[36] FLAIR imaging enhances the detection of brain tumors and associated inflammation, particularly through post-contrast leptomeningeal enhancement that outlines subtle meningeal spread in conditions like leptomeningeal metastases or carcinomatosis.^[38] In encephalitis and meningitis, FLAIR reveals hyperintense signals in the subarachnoid space and cortical regions, reflecting exudates, edema, or inflammatory changes, with high sensitivity for identifying meningeal involvement even without contrast.^[39] For instance, in viral encephalitis, FLAIR hyperintensities in the temporal lobes or basal ganglia aid in pathogen-specific localization, while in bacterial meningitis, it detects non-enhancing exudates effectively.^[40] In epilepsy, FLAIR is instrumental in identifying hippocampal sclerosis, characterized by T2/FLAIR hyperintensity and atrophy in the hippocampus, which is a common substrate in temporal lobe epilepsy.^[41] This visualization supports preoperative evaluation by delineating sclerotic changes that may not be as apparent on standard T2 sequences. For traumatic brain injury, FLAIR sensitively detects diffuse axonal injury (DAI) through non-hemorrhagic white matter hyperintensities at gray-white junctions, corpus callosum, and brainstem, correlating with shearing forces and aiding in grading injury severity.^[42] Spinal cord applications of FLAIR are more limited but valuable in cervical imaging for demyelinating diseases like MS, where it helps visualize short-segment lesions by suppressing surrounding CSF signal, though T2-weighted sequences remain primary.^[43] In spinal MS, FLAIR-detected hyperintensities indicate active plaques, particularly in the cervical region, contributing to overall diagnostic confidence when combined with brain findings.^[44]

Non-Neurological Uses

In musculoskeletal imaging, FLAIR sequences suppress signals from synovial and joint fluids, enhancing the delineation of inflammatory changes, cartilage defects, and soft tissue lesions. For instance, T2-weighted spectral presaturation with inversion recovery FLAIR (T2W SPIR-FLAIR) effectively nulls intra-articular fluid in the hip while preserving high signal from thickened synovium in patients with spondyloarthritis-related synovitis, achieving 100% sensitivity, 94.8% specificity, and 96.7% accuracy compared to contrast-enhanced T1-weighted imaging, with a contrast-to-noise ratio of 55.20.^[45] This non-contrast approach reduces risks associated with gadolinium, such as allergic reactions or nephrogenic systemic fibrosis, and demonstrates excellent agreement (Kappa = 0.929) with enhanced sequences. Accelerated deep learning-reconstructed FLAIR with fat saturation further supports knee synovitis evaluation, yielding synovitis scores (mean 10.69 ± 8.83) equivalent to standard contrast-enhanced T1-weighted fat-suppressed imaging (mean 10.74 ± 10.32; P = 0.521) in under 2 minutes, with high inter- and intra-reader reproducibility (Cohen's κ = 0.82–0.96).^[46] At 7T field strength, fat-suppressed FLAIR provides superior conspicuity of synovial inflammation in psoriatic and rheumatoid arthritis, scoring mild to severe synovitis in affected knees, though it underestimates volumes by 18.6 ± 9.5% relative to contrast-enhanced T1-weighted sequences (P < 0.01), positioning it as a promising research tool for detailed joint assessment.^[47] Post-contrast FLAIR applications extend to body imaging by improving lesion conspicuity near fluid interfaces, though integration remains adjunctive to standard T1-weighted sequences. In pediatric imaging, for orbital applications, contrast-enhanced fat-suppressed FLAIR excels in assessing pediatric uveitis, identifying uveal tract inflammation severity and extent with greater diagnostic confidence than non-contrast sequences, guiding targeted therapy in inflammatory eye conditions.^[48] Research extensions of FLAIR include exploratory uses in cardiac and breast imaging to exploit fluid-tissue contrast, though not yet routine. In cardiac studies, FLAIR detects edema in myocardial or pericardial regions post-event, correlating with white matter changes in associated brain imaging, but requires optimization for motion. In breast research, synthetic FLAIR-derived sequences from multi-dynamic multi-echo acquisitions show potential for lesion characterization without additional scans, improving efficiency in dense breast evaluation.

Advantages and Limitations

Key Benefits

Fluid-attenuated inversion recovery (FLAIR) imaging provides superior lesion conspicuity compared to conventional T2-weighted sequences by nulling the cerebrospinal fluid (CSF) signal, creating a dark background that enhances the visibility of hyperintense pathologies adjacent to ventricles, sulci, or the cortical gray-white matter junction.^[11] This suppression is particularly beneficial for detecting multiple sclerosis (MS) plaques, where FLAIR has demonstrated improved detection rates, for example, 3D-FLAIR identified 29% more periventricular lesions than 2D-FLAIR.^[49] In the posterior fossa, optimized FLAIR sequences detect significantly more infratentorial MS lesions, with mean counts of 7.5 versus 4.1, 4.8, and 5.8 for conventional sequences including T2-weighted imaging.^[50] Post-contrast FLAIR further enhances diagnostic accuracy by improving visualization of subtle blood-brain barrier disruptions and leptomeningeal enhancement without the confounding glare from bright CSF seen in standard T1-weighted post-contrast images.^[51] This inherent T1-weighting in FLAIR allows for sensitive depiction of enhancement patterns in superficial parenchymal lesions and metastases, often outperforming routine contrast-enhanced T1 sequences for cortical and meningeal abnormalities.^[38] The multiplanar capability of 3D FLAIR enables isotropic voxel acquisition and reformatting in any orientation, facilitating precise lesion volumetry and quantification in neurological conditions like MS.^[52] This volumetric approach supports thinner slices (typically 1 mm or less), reducing partial volume effects and improving detection of small lesions that might be obscured in thicker 2D acquisitions.^[32] FLAIR integrates effectively as a complementary sequence in multi-parametric MRI protocols, enhancing the assessment of diffusion-weighted imaging (DWI) findings in acute stroke or combining with T1-weighted sequences for comprehensive evaluation of lesion characteristics and enhancement.

Artifacts and Challenges

Motion artifacts in FLAIR imaging often manifest as blurring or ghosting, particularly due to patient head movement during the extended acquisition times required by the sequence's long repetition time (TR).^[53] These artifacts can simulate hyperintense pathologic changes in the subarachnoid space or brain parenchyma, complicating interpretation.^[54] Periodic motions, such as those from breathing or swallowing, exacerbate ghosting, with intensity increasing alongside the amplitude of movement and signal from affected tissues.^[55] Mitigation strategies include patient sedation for uncooperative subjects or implementation of faster acquisition techniques to minimize scan duration.^[56] Flow-related artifacts in FLAIR primarily arise from cerebrospinal fluid (CSF) pulsation and vascular flow, leading to hyperintense ghosting or incomplete CSF suppression within the ventricles and subarachnoid spaces.^[53] These pulsation effects produce discrete ghosts spaced according to the periodicity of cardiac or respiratory cycles, often mimicking intraventricular pathology such as hemorrhage or tumors.^[57] Such artifacts are more conspicuous in regions with turbulent flow, like the aqueduct or basal cisterns, and can degrade meningeal and cortical conspicuity.^[58] They become particularly prominent at higher field strengths like 3T, where enhanced T2 contrast amplifies residual non-nullified CSF signals.^[59] B1 inhomogeneity poses a significant challenge in high-field FLAIR imaging, causing signal voids or inhomogeneous suppression, especially at 3T and above.^[60] This transmit field nonuniformity leads to bright or dark banding artifacts near the skull base and temporal lobes, reducing diagnostic confidence in deep brain structures.^[61] At ultra-high fields like 7T, dielectric effects and increased radiofrequency wavelength shortening intensify these inhomogeneities, often resulting in pronounced signal loss in peripheral brain regions.^[62] Corrections involve B0 shimming to improve field homogeneity or adiabatic inversion pulses to achieve more uniform flip angles across the imaging volume.^[63] FLAIR sequences typically require scan times of 4-10 minutes, prolonging patient discomfort and elevating risks of claustrophobia or involuntary motion in susceptible individuals. Recent deep learning-based reconstructions have accelerated FLAIR acquisition, potentially reducing scan times and mitigating motion artifacts while preserving diagnostic quality (as of 2024).^[64] Additionally, FLAIR exhibits reduced sensitivity for detecting acute hemorrhage, where blood products may appear isointense against suppressed CSF, and calcifications, which lack the susceptibility weighting needed for clear visualization.^[65] Parameter adjustments, such as optimizing inversion time, can partially address these issues but do not fully overcome inherent sequence limitations. At higher field strengths like 3T and beyond, FLAIR suffers from amplified susceptibility artifacts, resulting in signal distortions and voids near air-tissue interfaces or metallic implants.^[60] These effects, driven by shortened T2* relaxation times, are more severe than at 1.5T, potentially obscuring lesions in susceptibility-prone areas like the brainstem or orbitofrontal regions.^[66] Fat suppression in FLAIR is also less reliable at ultra-high fields due to B1 variations, leading to incomplete nulling in adipose tissues and confounding soft-tissue evaluation.^[62]

References

[1]
Use of fluid attenuated inversion recovery (FLAIR) pulse sequences ...
Fluid attenuated inversion recovery pulse sequences with a long echo time (TE) have been used to image the brain in one volunteer and four patients.
[2]
Fluid attenuated inversion recovery (FLAIR) MRI at 7.0 Tesla - NIH
The aim of the present study is to assess fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) at three different field strengths (the ...
[3]
Fluid-attenuated inversion recovery (FLAIR): clinical prospectus of ...
Fluid-attenuated inversion recovery (FLAIR) is a magnetic resonance imaging (MRI) sequence that produces strong T2 weighting, suppresses the CSF signal, and ...
[4]
Fluid attenuated inversion recovery | Radiology Reference Article
Feb 16, 2013 · Fluid attenuated inversion recovery (FLAIR) is a special inversion recovery sequence with a long inversion time. This removes signal from ...
[5]
Fluid-Attenuated Inversion Recovery (FLAIR) for Assessment of ...
Background and Purpose Our aim was to evaluate fluid-attenuated inversion recovery (FLAIR) sequence in the diagnosis of cerebral infarction with MRI.
[6]
Physics for clinicians: Fluid-attenuated inversion recovery (FLAIR ...
A pedagogical review of fluid-attenuated inversion recovery (FLAIR) and double inversion recovery (DIR) imaging is conducted in this article.
[7]
https://www.nature.com/articles/s41598-025-94267-9
[8]
Fluid attenuated inversion recovery | Radiology Reference Article
Feb 16, 2013 · FLAIR is a special inversion recovery sequence with a long inversion time. This removes signal from the cerebrospinal fluid in the resulting images.
[9]
Fluid-Attenuated Inversion Recovery Magnetic Resonance Imaging ...
Hajnal JVBryant DJKasuboski L Use of fluid attenuated inversion recovery (FLAIR) pulse sequences in MRI of the brain. J Comput Assist Tomogr.1992;16:841-844 ...
[10]
An Introduction to Neuroimaging Methods for the Nurse Scientist - NIH
Structural MRI technique which suppresses CSF signal to provide image and is highly sensitive to pathological abnormalities. ... FLAIR MRI is an excellent ...Table 2 · Structural Mri · Functional Mri<|control11|><|separator|>
[11]
Inversion Recovery (IR) Sequence - MRI Questions
Historically, IR techniques were widely used in the "early days" of MRI (c. 1980-1985). They produced excellent image contrast, especially for T1-weighted ...Missing: 1970s | Show results with:1970s
[12]
[PDF] Inversion Recovery and Early Contrast Studies in the Brain - ISMRM
Inversion Recovery and Early Contrast Studies in the Brain: A Brief History ... When MRI of human subjects began, there was no data available about the NMR ...
[13]
Ian Robert Young OBE and the development of MRI - RAD Magazine
The Fluid Attenuated IR (FLAIR) sequence was developed by Jo Hagnal who used an inversion pulse to null CSF and doubled the TE of T2-weighted SE sequences to ...
[14]
MR of the brain using fluid-attenuated inversion recovery (FLAIR ...
FLAIR sequences provide high sensitivity to a wide range of disease. The basic sequence is easy to implement but is relatively time consuming.
[15]
T2-FLAIR - Questions and Answers in MRI
Originally just called "FLAIR", this technique was developed in the early 1990's by the Hammersmith research team led by Graeme Bydder, Joseph Hajnal, and Ian ...Missing: inventors | Show results with:inventors
[16]
Determination of Efficiency of 3D Fluid-Attenuated Inversion ... - NIH
Nov 1, 2023 · In the year 2000, a single-slab 3D-FLAIR sequence was introduced, effectively eliminating the magnetization transfer phenomena and slab boundary ...
[17]
Magnetic Resonance Imaging - Mathematics and Physics of ... - NCBI
Equation 4.4 shows that the time variation of the spatial frequency vector is determined by the integral over the gradients; the time sequence of the RF pulses ...
[18]
The Basics of MRI
S = k ρ (1-2exp(-TI/T1)+exp(-TR/T1)) Inversion Recovery (180-90-180) S = k ρ (1-2exp(-TI/T1)+exp(-TR/T1)) exp(-TE/T2) Gradient Recalled Echo.Missing: fundamentals | Show results with:fundamentals
[19]
Cerebrospinal fluid T1 value phantom reproduction at scan room ...
Jun 9, 2019 · The value at that time was 3532.81–4704.57 ms at 1.5 T and it ranged from 4052.41 to 5701.61 ms at 3.0 T. The highest correlation with the ...
[20]
Optimal combination of FLAIR and T2-weighted MRI for improved ...
Suppressing CSF in the fluid-attenuated inversion recovery (FLAIR) pulse sequence enhances lesion visualization. Both T2w and FLAIR are routinely acquired in ...Missing: adaptations | Show results with:adaptations
[21]
Impact of Inversion Time for FLAIR Acquisition on the T2-FLAIR ...
Jan 14, 2021 · For example, in stroke imaging, TI of 2,600 ms in 3T or 2,200 ms in 1.5T has been used to detect the DWI-FLAIR mismatch sign (33). Our research ...
[22]
Inversion Recovery, STIR and FLAIR - MRI sequences - IMAIOS
The aim of a FLAIR sequence is to suppress liquid signals by inversion-recovery at an adapted TI. Water has a long T1. Nulling of the water signal is seen at ...
[23]
[PDF] Overview of MRI Pulse Sequences and Image Acquisition
Different tissues relax at different rates (R1 = 1/T1). How to recognize IR pulse sequence in diagram? Initial. 180o RF pulse, then a waiting period.
[24]
Contrast‐Enhanced Fluid‐Attenuated Inversion Recovery in ...
Feb 15, 2022 · This review provides the fundamental mechanism of contrast-enhanced FLAIR, systematically describes its current and potential clinical application,
[25]
Standard 3.0 T MR Imaging - Radiology Key
Sep 16, 2017 · Sequence TR (ms) TE (ms) Other parameters (TI, FA, ETL) Slice thickness (mm) No. of slices FOV Matrix NEX Examination time (min:s) 500 Min ...
[26]
[PDF] Optimizing MR Imaging Procedures - AAPM
Jul 27, 2005 · Scan time depends on # TR. Conventional SE: one k-space line per echo ... FOV & matrix size. FOV & matrix size. Contrast. Signal-to-Noise.
[27]
Consensus recommendations for a standardized Brain Tumor ...
Similar to T2-weighted MRI, the recommended slice thickness for 3 T scans is 3 mm with no interslice gap, and 1.5 T scanners should acquire images up to 4 mm ...
[28]
Impact of Inversion Time for FLAIR Acquisition on the T2 ... - Frontiers
Jan 13, 2021 · For example, in stroke imaging, TI of 2,600 ms in 3T or 2,200 ms in 1.5T has been used to detect the DWI-FLAIR mismatch sign (33). Our research ...
[29]
TI MRI | Inversion time for FLAIR,STIR and PSIR
A typical range of TI values at 3T might be between 700 ms to 1200 ms, with around 900 ms often being optimal for nullifying the signal from CSF and enhancing ...
[30]
Iterative denoising accelerated 3D SPACE FLAIR sequence for ...
Imaging parameters for both 3D-FLAIR sequences are presented in Table 1. Table 1. Imaging parameters for 3D-FLAIR MRI sequences. Empty Cell, Conv-FLAIR, Acc ...<|separator|>
[31]
[PDF] ACR–ASNR–SPR Practice Parameter for the Performance and ...
The most commonly accepted basic imaging protocols for MRI of the brain currently include a T1-weighted sequence in the sagittal plane (or a T1-weighted ...
[32]
Current and New Directions in MRI in Multiple Sclerosis - PMC
White matter lesions characteristic of MS are best seen on fluid-attenuated inversion recovery (FLAIR) sequences that suppress signal from the CSF and allow for ...
[33]
FLAIR2: A Combination of FLAIR and T2 for Improved MS Lesion ...
FLAIR and double inversion recovery are important MR imaging scans for MS. The suppression of signal from CSF in FLAIR and the additional suppression of WM ...
[34]
Fluid-attenuated Inversion Recovery Hyperintense Ischemic Stroke ...
Early development of stroke lesion on FLAIR within 4.5 hours of onset is associated with reduced likelihood of favorable 90-day outcome after IV thrombolysis.
[35]
Significance of hyperintense vessels on FLAIR MRI in acute stroke
Jul 25, 2000 · HVS on FLAIR MRI is an indicator of slow flow and early ischemia as a result of large vessel occlusion or stenosis and inadequacy of collateral circulation.<|control11|><|separator|>
[36]
Importance of Contrast-Enhanced Fluid-Attenuated Inversion ...
This article describes the diagnostic importance of CE-FLAIR imaging for various intracranial pathologic conditions, as well as normally enhancing structures ...Missing: review | Show results with:review
[37]
Contrast-enhanced weighted-T1 and FLAIR sequences in MRI of ...
T1-W images without contrast were able to diagnose 78 cases of meningitis (92.8% of them), and FLAIR sequences could diagnose 82 patients (97.6% of them).
[38]
Magnetic Resonance Imaging Findings in Viral Encephalitis
MRI findings include cerebellitis, myelitis with hyperintensities seen on T2W/FLAIR sequences. ... It can manifest in several forms – encephalitis, meningitis ...
[39]
Hippocampal Sclerosis: Correlation of MR Imaging Findings with ...
These findings, atrophy and T2 hyperintensity of the hippocampus, are often referred to as the two primary MR imaging findings of hippocampal sclerosis.
[40]
Diffuse axonal injury: a case report and MRI findings - PubMed Central
Nov 1, 2021 · FLAIR and DWI have been proven to be sensitive in detecting non-hemorrhagic lesions seen with DAI [6,7].
[41]
Demyelinating Diseases of the CNS (Brain and Spine) - NCBI - NIH
Feb 11, 2024 · MRI is the most sensitive imaging technique for detecting MS plaques throughout the brain and spinal cord. Proton density (PD) or T2-weighted MR ...
[42]
Neuroradiological evaluation of demyelinating disease - PMC
Magnetic resonance imaging (MRI) has become a critically important tool in diagnosis and differentiation of different demyelinating disorders.
[43]
The application of T2W SPIR-FLAIR in the diagnosis of hip synovitis ...
T2W SPIR-FLAIR can effectively suppress the intra-articular fluid signals, while retaining the signals of thickened synovial membranes and can be used for the ...
[44]
Inflammatory Knee Synovitis: Evaluation of an Accelerated FLAIR ...
Aug 1, 2024 · Inflammatory Knee Synovitis: Evaluation of an Accelerated FLAIR Sequence Compared With Standard Contrast-Enhanced Imaging ... Musculoskeletal ...
[45]
Comprehensive assessment of knee joint synovitis at 7 T MRI using ...
Feb 21, 2020 · Seven T FLAIR-FS ultra-high field MRI is a potential non ... One of the applications of 7 T systems is musculoskeletal disease and particularly ...
[46]
The Diagnostic Utility of Contrast-Enhanced FLAIR Imaging in ... - NIH
Jun 28, 2024 · CE-FLAIR-FS plays a significant role in the diagnosis of pediatric uveitis, identifying the involvement and severity of the uveal inflammation and guiding the ...
[47]
Diagnostic value of 3D-FLAIR magnetic resonance sequence in ...
Jul 17, 2020 · 3D-FLAIR sequence is of greater sensitivity than standard 2D-FLAIR and T2 sequences in MS brain lesions depiction, and it is recommended to be included in MR ...Missing: history adoption
[48]
Improving Detection of Multiple Sclerosis Lesions in the Posterior ...
Jul 1, 2019 · The optimized FLAIR sequence detected significantly more posterior fossa lesions than any other sequence: 7.5 versus 5.8, 4.8, and 4.1 (P values ...
[49]
Contrast-Enhanced Fluid-Attenuated Inversion Recovery Imaging for ...
Apr 1, 2003 · CONCLUSION: Contrast-enhanced FLAIR imaging seems to improve detection of leptomeningeal disease when compared with routine contrast-enhanced T1 ...Abstract · Mr Imaging Methods And... · Results<|control11|><|separator|>
[50]
The Technical and Clinical Features of 3D-FLAIR in Neuroimaging
3D-FLAIR allows thinner slices with multi-planar reformation capability, a higher flow sensitivity, high sensitivity to subtle T 1 changes in fluid.Missing: introduction date<|control11|><|separator|>
[51]
FLAIR sequence in MRI – what is it? - United-Imaging
FLAIR (Fluid-Attenuated Inversion Recovery) – this is a special type of T2-weighted sequence that suppresses the cerebrospinal fluid signal, which allows ...Missing: definition | Show results with:definition
[52]
Artifacts in Magnetic Resonance Imaging - PMC - NIH
An artificially hyperintense signal on FLAIR images can result from magnetic susceptibility artifacts, CSF/vascular pulsation, motion, but can also be found in ...
[53]
Hyperintensity in the Subarachnoid Space on FLAIR MRI | AJR
At many institutions, the FLAIR pulse sequence has become a routine part of MRI studies of the brain. First described by Hajnal et al. [1] in 1992, FLAIR ...
[54]
Common and Uncommon Artifacts in T1 FLAIR SAG Sequences of ...
The intensity of these ghost artifacts increases with the amplitude of the periodic motion, as well as with the signal intensity of the moving tissue.
[55]
Reducing motion artifacts - Questions and Answers in MRI
Adjust imaging sequences and parameters. a. Increasing number of signals averaged (NSA, NEX) will reduce artifacts and increase signal-to-noise but at expense ...
[56]
Normal MRI Appearance and Motion-Related Phenomena of CSF
Whereas random movement leads to blurring, periodic motion, such as with CSF pulsation, cardiac motion, and respiratory motion, leads to ghosting artifact. This ...
[57]
Reduction of CSF and Blood Flow Artifacts on FLAIR Images of the ...
May 1, 2001 · The KRISP FLAIR sequence can suppress CSF and blood flow artifacts and improve the conspicuity of the meninges, cortex, brain stem, and cerebellum.
[58]
Intraventricular cerebrospinal fluid pulsation artifacts on low-field ...
CSF constantly pulsates within the ventricles; therefore, any residual nonnulled CSF signal also causes pulsation artifacts that may result in ghosting effects ...
[59]
Imaging Artifacts at 3.0T - Wiley
Sep 6, 2006 · The main advantage of 3T MRI over the standard clinical field strength of 1.5T is that it approximately doubles the signal-to-noise ratio (SNR).
[60]
B1-inhomogeneity artifacts on a 7 T MR FLAIR images of a 42-year ...
This review highlights the historical development of VWI at 7T, tracing its evolution from early attempts to key technical advancements, development of pulse ...
[61]
Emerging Use of Ultra-High-Field 7T MRI in the Study of Intracranial ...
Jan 1, 2020 · However, 7T imaging is limited by artifacts caused by increased transmit field (B1) inhomogeneity. The superiority over lower-field MR ...Clinical Applications · Intracranial Tumors · Neurovascular Pathology
[62]
Dielectric effect - Questions and Answers in MRI
Abnormal bright and dark areas due to B1 field inhomogeneity are frequently noted at very high fields (3T and above). ... artifacts are progressively important as ...<|separator|>
[63]
Reduction of CSF Artifacts on FLAIR Images by Using Adiabatic ...
Feb 1, 2001 · The purpose of this study was to investigate the possibility that some artifactual high signals produced in CSF with fluid-attenuated inversion-recovery MR ...
[64]
Differentiating Artifacts From True Pathology on MRI | AJR
This article outlines artifactual findings commonly encountered in neuroradiologic MRI studies and offers clues to differentiate them from true pathology.Artifact Recognition · Common Artifacts · Motion Artifact<|separator|>
[65]
The Holy Grail in diagnostic neuroradiology: 3T or 3D? - PMC
Dec 23, 2010 · These factors make imaging at higher field strengths more challenging, although most problems (SAR, susceptibility) can be overcome by ...