3D FLAIR vs. 2D Dual echo T2/DP in Multiple Sclerosis
Multiple Sclerosis
Multiple sclerosis (MS) is considered as an autoimmune chronic demyelinating disease of the central nervous system (CNS). It is one of the most common causes of disability in young adults. The cause of MS remains unknown despite many years of research. Its development, however, is influenced by a combination of genetic, immunological, and environmental factors. MS affects both the brain and the spinal cord, resulting in a wide variety of neurological symptoms that vary in type and severity [1, 2]. MS lesions, also known as scars, form when the immune system attacks the myelin sheets. Depending on the location and type of lesions that take place, MS symptoms and relapses will occur, and thereby, normal functioning will be impaired. Even though scarring heals, and normal functioning is restored in certain situations, persistent scarring can cause lasting impairment to motor and sensory processes [3].
MRI has become a critical tool for the establishment of a definitive MS diagnosis. However, identifying typical MS lesions on MRI images remains a challenge for neurologist and neuro-radiologist following-up on their MS patients. Indeed, new lesions occurrence is a major contributor to switching treatment. The location, shape, and size of the lesions give the greatest perspective of tissue damage, lesion activity, and disease accumulation to date. Furthermore, MS lesion follow-up acts as a tool for tracking their dissemination in space and time. When using a gadolinium-based contrast agent during the MRI scan, a better understanding of the pathogenesis of blood-brain barrier (BBB) permeability could even be evaluated. It is therefore why effective MRI sequences for better sensitivity in lesion detection are highly needed. However, due to changes in pulse sequences (e.g., FLAIR vs PD/T2), the use of contiguous and thinner slices (3 mm vs 5 mm), the acquisition and the interpretation of MRI images may be complex. Thus, the development and application of a common standardized MRI protocol is critical for the diagnosis and follow-up of MS patients.
MS is an inflammatory demyelinating disease characterized by the occurrence of white matter lesions in the brain and spinal cord.
Lesions profile on MRI sequences
2D Dual echo T2/PD sequence
MS lesions are identified on T2-weighted images as high signal foci against a low signal white matter background. However, periventricular lesions are frequently indistinguishable from the nearby high T2-weighted CSF signal of the ventricles. Luckily, T2 and proton density (PD)-weighted images can be acquired concurrently in a single dual-echo, spin-echo sequence, thereby providing complementary data to improve lesion detection in the periventricular regions. Indeed, the lesions contrast/signal can be increased using the PD image that in turn decreases the CSF signal of the ventricles (Table 1).
2D Fluid attenuated inversion recovery (FLAIR) sequence
Using a fluid attenuated inversion recovery (FLAIR) sequence can overcome the issue of periventricular lesions detection and identification. The FLAIR sequence addresses this issue by suppressing the CSF signal of the ventricles, all while preserving a substantial T2 weighting. The FLAIR sequence has also been shown to be better than other sequences in detecting cortical and juxtacortical lesions. FLAIR is therefore a regularly acquired MRI sequence that has great value especially when an MS diagnosis is suspected. The only disadvantage is its poor lesion detection quality in the posterior fossa and spinal cord, where the dual-echo T2/PD-weighted sequence is favored (Table 1).
3D Gadolinium-enhanced and unenhanced T1-weighted sequences
Lesions with a high T2-weighted signal will sometimes also appear on T1-weighted images as hypointense areas compared to a normal white matter background. If a pitch-black signal is observed, they are sometimes referred to as “black holes”. These newly appearing lesions usually follow a reversible phase of oedema, inflammation, or demyelination, and will eventually disappear with time. However, when accompanied with permanent axonal loss, they will remain and turn into chronic black holes.
Following the administration of a gadolinium-based contrast agent intravenously five minutes prior the acquisition of a T1-weighted image, BBB breakdown in the presence of active inflammation can be rendered detectable. These new lesions can typically remain enhanced for a month on average, thereby providing helpful information for the monitoring of disease progression (Table 1).
MS lesions appear as high signal foci (hyperintense) against a low signal white matter background on T2-weighted and FLAIR images and as low signal on T1-weighted images.
Table 1: MRI sequences parameters in a multiple sclerosis protocol.
Lesion load measurement: 2D vs 3D sequences
As previously stated, sensitivity in lesion detection with MRI is highly required. Lesion detectability could be improved in several ways. For instance, increasing the field strength (1.5T vs 3T), will result in an improved signal-to-noise ratio (SNR), and thereby an anticipated improvement in lesion detection. Furthermore, the sequences’ selection is obviously critical (T2 vs FLAIR). While FLAIR excel in detecting cortical and juxta-cortical lesions as well as periventricular lesions, dual-echo T2/PD-weighted images have better detection rate in the posterior fossa and spinal cord.
Not so long ago, 3D sequences have become available. These provide great spatial resolution, excellent SNR, and multiplanar reconstruction. Compared to 2D sequences, 3D sequences have longer acquisition time (Table 1). However, this issue has nowadays been resolved with the emergence of new sequences that rely on single-slab techniques instead of multi-slab ones, which in turn drastically reduce acquisition time. Nonetheless, even with the time improvements applied, 3D FLAIR for instance, is acquired in around 6-7 minutes while 2D FLAIR in around 2-3 minutes. In clinical practice, whenever 2D FLAIR is privileged over 3D FLAIR, it is always acquired with a 2D T2 sequence to improve the detection of infratentorial lesions [4].
The advantage of 3D sequences in this case rely in the multiplanar reconstruction that is done in no time, whereas 2D sequences would require additional acquisitions in different planes [5]. Thus, when compared to 2D sequences used in MS, 3D FLAIR images with their multiplanar reconstruction capabilities demonstrated high sensitivity in lesion detection and location [6-8]. Missing new appearing lesions due to high slice thicknesses (3-5 mm) with 2D images is no longer an issue (Figure 1). In fact, the acquisition of thinner slices without intersectional gaps with 3D sequences (1 mm) has greatly and positively impacted lesion detection in MS along with the improvement of spatial resolution and the reduction of partial volume effects [9] (Figure 2). Furthermore, despite the higher spatial resolution, thus the smaller voxels, 3D sequences have the advantage of better and higher signal and contrast to noise ratio [6, 7]. Indeed, even though 2D FLAIR suppresses the CSF signal for better visibility of lesions, it also led to a reduced SNR and decreased contrast between gray matter and white matter especially in elderly patients [10].
Combined to high-field strength (3T), 3 D sequences are superior to standard 2D sequences and are currently replacing them in MS imaging protocols.
Figure 1. Ability of 3D FLAIR sequences to better detect lesions compared to 2D axial T2 sequences due to its higher resolution and better signal to noise ratio allowing a better contrast between lesions and tissue.
Figure 2. Multiplanar reconstruction of a high-resolution 3D FLAIR sequence compared to a 2D axial T2 sequence acquired with a 3 mm slice thickness.
Infratentorial lesions’ detection: 3D FLAIR vs. 2D T2/PD
Where 2D FLAIR failed to accurately detect lesions in the posterior fossa and infratentorial regions of the brain of MS patients (due to CSF flow artifacts and lower contrast between lesions and white matter), 2D T2 excelled. It is therefore why both 2D FLAIR and T2 sequences go hand in hand in an MS MRI protocol. However, with the emergence of the 3D FLAIR sequence, lesion detection in the infratentorial regions has been greatly improved. Indeed, a study has found an average of 32% more lesions with 3D FLAIR than with 2D T2/PD images [8]. This finding was attributed to 3D FLAIR higher spatial resolution that favored the detection of distinct smaller lesions (increased number of slices and decreased slice thickness), some of which appeared to be one or confluent lesions on 2D T2/PD [11, 12].
Furthermore, the lack of CSF and blood flow artifacts (clearly reduced with 3D FLAIR), along with the thinner slice thicknesses and no intersectional gaps (less partial volume effects), increased the detection rate of infratentorial lesions [13]. To this end, 2D T2/PD is nowadays being considered optional in some MRI protocols published following international consensus consortium meetings for MS due to the higher sensitivity of 3D FLAIR [14-16]. Indeed, the clinical importance of precise lesion detection in infratentorial regions, added to the significant superiority of 3D FLAIR compared to 2D T2/PD, are sufficient arguments for the replacement of the conventional 2D T2 and FLAIR sequences by the new 3D FLAIR in MS MRI protocols.
3D FLAIR has proven to be superior to both 2D FLAIR as well as 2D T2/PD in detecting infratentorial lesions but also whole brain lesions, due to its increased resolution and signal to noise ratio.
Rad tips
When expecting a regular MS patient in the department, it is always important to keep in mind that reproducibility is key for their clinical follow-up. MRI sequences should be optimized to improve lesions detection. To do so, it is always preferable to use 3D instead of 2D sequences. This specially applies for T2-weighted images. Instead of acquiring both 2D T2 and 2D FLAIR sequences, it would be better to just acquire 3D FLAIR that has been shown to have higher detectability rate than the 2D sequences. 3D sequences have higher resolution and thinner slice thicknesses, thereby limiting the partial volume effects and increase lesions detection. However, if 3D sequences are not available, it is always better to acquire the images with the smallest slice thickness possible (usually 3 mm for 2D). In case of MS, 2D T2/PD and 2D FLAIR should always be acquired together since one is more sensitive to infratentorial lesions while the other is best for supratentorial. You can always use the parameters listed in Table 1 as reference for optimized MS protocol sequences.
References
- Confavreux, C. and S. Vukusic, The clinical course of multiple sclerosis. Handb Clin Neurol, 2014. 122: p. 343-69.
- Filippi, M., et al., Multiple sclerosis. Nat Rev Dis Primers, 2018. 4(1): p. 43.
- 3. Confavreux, C. and S. Vukusic, *Natural history of multiple sclerosis: a unifying concept.*Brain, 2006. **129**(Pt 3): p. 606-16.
- Wattjes, M.P., et al., Evidence-based guidelines: MAGNIMS consensus guidelines on the use of MRI in multiple sclerosis–establishing disease prognosis and monitoring patients. Nat Rev Neurol, 2015. 11(10): p. 597-606.
- Patzig, M., et al., Comparison of 3D cube FLAIR with 2D FLAIR for multiple sclerosis imaging at 3 Tesla. Rofo, 2014. 186(5): p. 484-8.
- Bink, A., et al., Detection of lesions in multiple sclerosis by 2D FLAIR and single-slab 3D FLAIR sequences at 3.0 T: initial results. Eur Radiol, 2006. 16(5): p. 1104-10.
- Moraal, B., et al., Multi-contrast, isotropic, single-slab 3D MR imaging in multiple sclerosis. Eur Radiol, 2008. 18(10): p. 2311-20.
- Hannoun, S., et al., Diagnostic value of 3DFLAIR in clinical practice for the detection of infratentorial lesions in multiple sclerosis in regard to dual echo T2 sequences. Eur J Radiol, 2018. 102: p. 146-151.
- Dolezal, O., et al., Detection of cortical lesions is dependent on choice of slice thickness in patients with multiple sclerosis. Int Rev Neurobiol, 2007. 79: p. 475-89.
- Okuda, T., et al., *Brain lesions: when should fluid-attenuated inversion-recovery* *sequences be used in MR evaluation?* Radiology, 1999. **212**(3): p. 793-8.
- Tawfik, A.I. and W.H. Kamr, Diagnostic value of 3D-FLAIR magnetic resonance sequence in detection of white matter brain lesions in multiple sclerosis. Egyptian Journal of Radiology and Nuclear Medicine, 2020. 51(1): p. 127.
- Toledano-Massiah, S., et al., Accuracy of the Compressed Sensing Accelerated 3D-FLAIR Sequence for the Detection of MS Plaques at 3T. AJNR Am J Neuroradiol, 2018. 39(3): p. 454-458.
- Gramsch, C., et al., Diagnostic value of 3D fluid attenuated inversion recovery sequence in multiple sclerosis. Acta Radiol, 2015. 56(5): p. 622-7.
- Cotton, F., et al., OFSEP, a nationwide cohort of people with multiple sclerosis: Consensus minimal MRI protocol. J Neuroradiol, 2015. 42(3): p. 133-40.
- Brisset, J.C., et al., New OFSEP recommendations for MRI assessment of multiple sclerosis patients: Special consideration for gadolinium deposition and frequent acquisitions. J Neuroradiol, 2020. 47(4): p. 250-258.
- Traboulsee, A., et al., Revised Recommendations of the Consortium of MS Centers Task Force for a Standardized MRI Protocol and Clinical Guidelines for the Diagnosis and Follow-Up of Multiple Sclerosis. AJNR Am J Neuroradiol, 2016. 37(3): p. 394-401.
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