Non-invasive MR imaging of human brain lymphatic networks with connections to cervical lymph nodes

Abstract

Meningeal lymphatic vessels have been described in animal studies, but limited comparable data is available in human studies. Here we show dural lymphatic structures along the dural venous sinuses in dorsal regions and along cranial nerves in the ventral regions in the human brain. 3D T2-Fluid Attenuated Inversion Recovery magnetic resonance imaging relies on internal signals of protein rich lymphatic fluid rather than contrast media and is used in the present study to visualize the major human dural lymphatic structures. Moreover we detect direct connections between lymphatic fluid channels along the cranial nerves and vascular structures and the cervical lymph nodes. We also identify age-related cervical lymph node atrophy and thickening of lymphatics channels in both dorsal and ventral regions, findings which reflect the reduced lymphatic output of the aged brain.

Fig. 1. Phantom studies of head and neck structures using 3D-T2 FLAIR.

Echo time (TE) related changes in 3D-T2 FLAIR signal can be seen in six phantom images (a), with the strongest signals observed at TE 601 in solutions of higher protein concentrations. The ratio of albumin signal intensity (SI) to agar SI with respect to albumin concentration can be seen in (b). Increasing TE values resulted in increasing signal-noise-ratio (SNR) across the protein samples analyzed, with the highest SNR observed at TE of 601 ms. The rectangular phantom images depict inversion time (TI) related signal changes related to albumin concentration (c). Signal suppression of water was more prominent at TI 1400 ms, and signal was significantly greater at all protein concentrations at TI 1600 ms. The ratio of SI albumin to SI agar with respect to albumin concentration can be seen in (d) for IR 1400 ms and 1600 ms. The greater TI value of 1600 ms yielded a greater SNR across the protein samples analyzed. Simultaneous MR imaging was performed of a rectangular phantom and a healthy adult male subject (e–g). Parasagittal dural lymphatic signal intensity corresponds to that of the higher albumin concentrations between 2000 and 4000 mg/dl (e). SNR shows a positive relationship with albumin concentration up to 15 mg/dl, then a negative relationship from 15 to 60 mg/dl, followed by a marked increase in SNR between 60 and 4000 mg/dl (f). An example image identifying key structures in the head and neck region from which signal intensity measurements were obtained, including the cervical lymph nodes and parasagittal dural lymphatics, is provided in (h). These findings demonstrate that this T2-FLAIR sequence’s parameters are sensitive to lymphatic fluid and lymphatic tissue without the need for contrast agents. TR relaxation time.

Fig. 2. Study profile describing the patient selection process.

In this figure, application of our inclusion and exclusion criteria to the study population is represented in detail. SD, standard deviation.

Fig. 3. Dorsal dural lymphatic system MR images and metrics from multiple subjects.

a Sagittal FLAIR image shows linear signal increase representing the dural-parasagittal lymphatics around the walls of the superior sagittal sinus (arrowheads), straight sinus (arrows) and confluent sinus (double arrow). Coronal (b) and axial (c) FLAIR images depict thick dural-parasagittal lymphatic signals along the sagittal sinus (arrowheads) and the cortical veins. Similar but less prominent lymphatic signals are also seen along the wall, the transverse sinus (d, arrowheads), sigmoid sinus, and jugular vein (e, arrowheads). f Sagittal image depicts the dural lymphatic structures in the posterior aspect of the foramen magnum (arrowheads). g Illustration of distribution and relative volume of dorsal dural lymphatic structures (green) and venous system (blue) derived from T2-FLAIR h. 3D representation of dural lymphatics as defined from T2-FLAIR shown from dorsal view. The dural lymphatics (bright parasagittal irregular structures) are defined from T2-FLAIR and co-registered with rough segmentation of the brain and the dural-parasagittal lymphatics (green).

Fig. 4. Ventral dural lymphatic system MR images and metrics from multiple subjects.

a–c FLAIR images show the ventral dural lymphatic elements in the anterior cranial fossa (arrowheads), olfactory bulb (double arrow), optic grove (arrows) and diaphragma sella (double arrowhead; c). d, e Ventral dural lymphatic elements can be seen at the level of the orifices of neural foramina of the trigeminal nerves (d, arrowheads) and edge of Meckel’s cave dura (arrows; e). Ventral dural lymphatic elements can also be seen at the level of the orifices of the internal auditory canals on axial (f, arrowheads) and coronal (g, arrowheads) views. These structures are visible as well at the level of the jugular foramina around the IX–XI complex (arrowheads) on axial (h) and coronal (i) and views (arrowheads). j 3D representation of ventral dural lymphatics as defined from T2-FLAIR shown from ventral view. The ventral dural lymphatics (bright ventral irregular structures around the ventral dural surfaces and neural foramina) as defined from T2-FLAIR and co-registered with rough segmentation of the brain and the dural lymphatics (green).

Fig. 5. Depictions of lymphatic drainage at the levels of the skull bases and neck.

Axial (a) and sagittal (b) FLAIR images show prominent lymphatic fluid signals along the wall of the jugular vein (JV) in the upper neck region (green arrowheads). a Lymphatic signal connections with internal carotid artery (A), JV and cranial nerves (CN) can be seen (green arrows). c Axial magnified image reveals lymphatic signal communications along the cranial nerve IX–XI complex (CN), internal carotid artery (A) and jugular vein (JV). Corresponding color overlay depicts veins (blue), meningeal lymphatic tissue/flow (green; arrows), ICAs (red), and CNs (yellow). Sagittal (d), axial (e, g, h), and coronal (f) FLAIR images depict lymphatic connections from the cranial nerve IX–XI complex to the deep cervical lymph nodes (DCLN; green arrows and green arrowheads) and retropharyngeal lymph nodes (RPLN; green arrows and green arrowheads). Similar types of connections can also be seen between CNs and the ICAs as well as between the ICAs and lymph nodes (green arrows and green arrowheads).

Fig. 6. Age effects on dorsal and ventral dural lymphatic structures.

a Dorsal parasagittal dural lymphatic system in young and aged female subjects. Sagittal FLAIR images and coronal magnified FLAIR images reveal a significant increase in thickness and extension of the parasagittal dural lymphatics structures along the sagittal sinus (arrowheads) in the aged subject. b Comparisons of the 3D representation of dorsal parasagittal dural lymphatics as defined from T2-FLAIR. Images depict the brain for the lateral (upper image) and dorsal (lower image) views. c Comparison of visibility of dorsal lymphatics between older and younger age groups and across decades of age. d–k Comparison of thickness and signal intensity values of dorsal and ventral lymphatic systems between older and younger age groups and across the age groups. Student t test was used to compare older and younger age groups. All tests are two-sided. l Comparisons of retropharyngeal and vascular lymph node SI and short diameter by age groups. Student t test was used to compare older and younger age groups. All tests are two-sided. Data are presented as mean values ± SEM. [Age: 15–30; N = 35], [Age: 30–45; N = 10], [Age: 45–60; N = 16], [Age: 60–75; N = 15], [Age: 75–90; N = 5] and [Age: <50; N = 52] and [Age: >50; N = 29].

Fig. 7. Comparisons of the dorsal and ventral dural lymphatic system by sex.

a Representative sagittal FLAIR images and coronal magnified FLAIR images reveal increased thickness, extension, and SI of the parasagittal; dural lymphatics structures along the sagittal sinus (arrowheads) in the male subject compared to the female subject (both the male and female are young adults of the same age). b Comparisons of the 3D representation of dorsal dural lymphatics as defined from T2-FLAIR shown from dorsal view. c–j Comparison of thickness and signal intensity values of dorsal and ventral lymphatic systems between male and female subjects. Student t test was used to compare older and younger age groups. All tests are two-sided. k Comparisons of retropharyngeal and vascular lymph node SI and short diameter by sex. Student t test was used to compare older and younger age groups. All tests are two-sided. l Comparison of visibility of dorsal lymphatics by sex. Data are presented as mean values ± SEM. [Female; N = 45], [Male; N = 36].

Fig. 8. A schematic model of cranial nerve, dura, and dural lymphatic flow at cranial foramina.

a Illustration of hypothetical lymphatic flow from dural lymphatics to the epineurium of a generic extra-cranial nerve. Dural lymphatic flow in this model becomes prominent at the level of the foramen before entering the epineural space. This lymphatic flow follows the cranial nerve epineurium and later routes to cervical lymph nodes. b Axial MR image reflecting this model showing dural lymphatic signal extending into the neural foramen and along the cranial nerve outside of the skull (arrows indicate direction of flow).

Fig. 9. Schematic model of the brain lymphatic system and its drainage pathways.

The overall system is divided into dorsal and ventral systems. The dorsal system includes ISF-lymphatic fluid present along dural venous sinuses that drains along the jugular veins and foramen magnum. The thickest of these fluid spaces occurs at the midsagittal level. Decreased caliber of this space present at the level of the posterior sagittal sinus is most likely due to effects of gravity while in the supine position during scanning. The ventral system includes ISF-lymphatic fluid along the olfactory nerves in the anterior cranial fossa, optic groove, dura at the orifice of the Meckel’s caves, dura at the orifice of the internal auditory canals, and dura at the level of the orifices of jugular foramen. The green signal at the orifices of neural foramina represents lymphatic fluid. These pathways channel ISF-lymphatic fluid outside of the intracranial along the space epidural surfaces of the cranial nerves. ISF-Lymphatic signals along the cranial nerves, jugular veins and petrous and cervical ICAs in the neck show connections between each other and cervical lymph nodes. The schematic depicts direct connections between meningeal ISF-Lymphatic fluid to deep neck nodes via multiple neural foramina and along the skull base structures in humans.