Methods
offer valuable insights, but are insufficient to reconstruct and recognize
convoluted and smaller nerves of the PNS, such as the nerves of the pelvic lum-
bosacral plexus, which are clinically significant in conditions like endometriosis.
A
NERVE FIBER QUERIES
L5_left = crossing(VertebralCanalL5Left)
then anterior of(PiriformisMuscleLeft)
then left of(LevatorAniMuscles)
then not posterior of(Sacrum)
…
L5_right = crossing(VertebralCanalL5Right)
then anterior of(PiriformisMuscleRight)
then right of(LevatorAniMuscles)
then not posterior of(Sacrum)
...
S1_left = crossing(SacralHoleS1Left
then anterior of(PiriformisMuscleLeft)
then left of(LevatorAniMuscles)
then not posterior of(Sacrum)
...
S1_right = crossing(SacralHoleS1Right)
then anterior of(PiriformisMuscleRight)
then right of(LevatorAniMuscles)
then not posterior of(Sacrum)
...
S2_left = crossing(SacralHoleS2Left)
then anterior of(PiriformisMuscleLeft)
then left of(LevatorAniMuscles)
then not posterior of(Sacrum)
...
S2_right = crossing(SacralHoleS2Right)
then anterior of(PiriformisMuscleRight)
then right of(LevatorAniMuscles)
then not posterior of(Sacrum)
...
S3_left = crossing(SacralHoleS3Left)
then anterior of(PiriformisMuscleLeft)
then left of(LevatorAniMuscles)
then not posterior of(Sacrum)
...
S3_right = crossing(SacralHoleS3Right)
then anterior of(PiriformisMuscleRight)
then right of(LevatorAniMuscles)
then not posterior of(Sacrum)
...
B1 B2
B1
Fig. 1.The Visionerves framework (blue) from image acquisition on the left to the final
3D model merging anatomical structures and nerve fibers on the right. The method is
composed of: phase A (green) - Anatomical Structure Segmentation - where a U-Net-
based algorithm takes as input a morphological (e.g. T2-w) MR image and outputs
an anatomical segmentation; phase B (red) - Peripheral Nerves Reconstruction (B1)
and Recognition (B2) - where a tractography algorithm takes as input a DW image
(together with output segmentation of phase A), and outputs a fibers reconstruction,
which in turn is passed as input (together with nerve fibers queries and result of phase
A) to our symbolic AI filtering method for recognizing the targeted nerves.
4 G. La Barbera et al.
3 The Visionerves Method
The proposed Visionerves method consists of two primary phases (Figure 1): (A)
anatomical structure segmentation from a morphological (e.g. T2-w) MR image
(see Section 3.1); and (B) peripheral nerves reconstruction and recognition from
multi-gradient DW image, leveraging the result of phase A (see Section 3.2). As
described before, a final 3D model can be created, merging the extracted nerve
information with the anatomical segmentation.
3.1 Phase A - Anatomical Structure Segmentation
AU-Net-basedalgorithminspiredbythewell-knownnnU-Net[14]anditsderiva-
tives (nnDetection [1] for localization and nnInteractive [15] for error correction)
is employed for rapid and fully automated modeling of anatomical structures
from morphological MRI images, such as T2-w or T1-w. A preprocessing step
(to have a standard dataset representation as in [14]) includes a reorientation
of the images in the RAS coordinate system (left to right, posterior to ante-
rior, inferior to superior), a correction of field heterogeneity, and a resampling
to a common voxel size. The segmentation pipeline includes three custom U-Net
subsystems: one for localization (bounding box determination), one for semantic
segmentation, and one for error correction via user interaction.
This phase A of the pipeline enables fast and user-friendly 3D anatomical
model generation, avoiding manual segmentation, however it is not central to our
methodanditcanbeeasilyreplacedwithalternativestate-of-the-artapproaches.
For these reasons we will not elaborate on it further. Nevertheless it is important
to mention that in addition to enabling the functioning of phase B, this part also
allows for the visualization of the nerves in their anatomical context, leading to a
better understanding of their relationships to the different anatomical structures.
3.2 Phase B - Peripheral Nerves Reconstruction and Recognition
For the identification of the PNS, in contrast to previous works [20,24], we pro-
pose to represent and formalize anatomical knowledge, usually given in natural
language, in first order logic, with the associated syntactic reasoning abilities.
Taking inspiration from [6], we also propose to associate it with fuzzy semantics.
Spatial relations between nerves and anatomical structures play an important
role in the description of nerves. They are represented as predicates in the logic,
for which a degree of satisfaction is computed using mathematical morphology
and fuzzy sets [3]. These relations include distances, directions and connectiv-
ity with respect to segmented structures from phase A. This hybrid approach
combines formal descriptions (syntactic part) with concrete representations in
the spatial domain and as degrees of satisfaction taking values in[0,1](semantic
part). Fuzzy representations hence inherently solve the semantic gap, establish-
ing links between abstract clinical concepts and image information.
Visionerves 5
Phase B1 - Automatic T ractography for Reconstruction.The first part
of phase B consists of fiber reconstruction via a tractography algorithm [22] ap-
plied to the multi-gradient DWI image. A preprocessing (as in [22]) includes
denoising, Gibbs artifact removal, eddy currents and motion corrections, and
correction of field heterogeneity. Following this, a tractography algorithm is ap-
plied (algorithm choice left to the user depending on DWI parameters and the
studied body region). In order to narrow the reconstruction space as well as make
this process automatic and reproducible, ROIs for seeding and inclusion zones
(which are obligatory to be crossed in the specified order) are produced using
either directly the segmented anatomical structures or by using spatial relations
to define regions where they are satisfied (e.g. region “anterior of structure A”
AND “to the right of structure B”). In this part, spatial relations are binarized, in
order to be used in tractography algorithms. In addition, the created regions are
large enough to not limit the reconstruction space too much, given the potential
deviations from normal anatomy. It is important to note that, in order to have
a good voxel matching between anatomical structures (produced from the MRI)
and DWI, a registration might be necessary.
Phase B2 - Symbolic AI for Recognition.Once the tractogram is recon-
structed, we perform a filtering to recognize only the nerve fibers, differentiat-
ing them from muscle and tissue fibers as well as potential noise. The medical
knowledge encoded in the logic is translated into queries used by the recognition
algorithm. For each nerve bundle, a query representing its anatomical path is
created using the spatial relations, with the possibility of combining the relations
with AND/OR operators, creating exclusion zones with NOT operators, and or-
dering the nerve segments with THEN (i.e. “sequential" AND) operators. Once
the query is built, its degree of satisfaction is assessed at each point along the
fiber by mapping every point to the corresponding defined fuzzy regions. A fiber
is considered to satisfy a query if it sequentially validates all specified spatial
relations, where to validate means that the average of the non-zero fuzzy values
along the fiber is higher than a specified threshold. All fibers that fulfill these
criteria are then aggregated to form a bundle representing the targeted nerve.
More details on the fuzzy logic modeling and on phase B2 can be found in [2].
4 Application on the pelvic region for endometriosis
We applied our Visionerves method on the lumbosacral plexus in the 10 en-
dometriosis cases, for whom a nerve fiber analysis could be a strong aid in the
understanding of this disease as explained in Section 1. Furthermore, since this
is a highly innervated region with different muscles and organs, it represents an
ideal subject of application for our method.
4.1 Database
For the segmentation system of phase A, we used 168 T2-w MRI images of 131
patients (ranging from 2 months old to 20 years old) belonging to a proprietary
6 G. La Barbera et al.
database licensed by the Hôpital Necker-Enfants malades of Paris), with refer-
ence images manually annotated by expert surgeons and radiologists over the
course of several years using 3DSlicer software [8]. Pelvic bones (L5 vertebra, hip
bones and sacrum), muscles (piriformis, obturator and levator ani), visceral or-
gans (bladder, colon and rectum) and reproductive organs (ovaries, uterus and
vagina) were labeled, in addition to other regions of interest (sacral foramina
from S1 to S3, sacral canal, intervertebral foramina of L5) facilitating nerve
detection.
We then applied the complete Visionerves method (phases A and B) on
10 different adult female patients (5 diagnosed and 5 suspected endometriosis,
both groups in an age range from 20 to 50 years old), gathered at Hôpital
européen Georges-Pompidou of Paris. For each patient, a couple of T2-w MRI
(reconstruction voxel size 0.5×0.47×0.47mm3) and multi-gradient DWI image
(acquisition voxel size 3.3×2.3×3.6mm3, NEX 1, 50 directions, b-value 600) was
acquired in a 3T GE Signa Architect machine during a preliminary research (for
a future prospective study) and used retrospectively after anonymization. Nerve
Reference
reconstructions were created using the method of phase B1 plus the
use of a ROI mask, in order to select the fibers passing through, exclusively and
completely, within it. This further constrains the search area to the region we
consider to represent the true pathway of fiber passage. Such a ROI mask was
produced under the supervision of expert surgeons and radiologists via manual
segmentation of tubes enclosing each bundle of nerve fibers (given the difficulty
of accurately segmenting these structures). All the manual annotations detailed
in this paragraph were performed using 3DSlicer software [8].
Finally the nerve queries were written with the help of clinical experts, lever-
aging anatomy books [11], literature [18] and knowledge, and aiming to make
them generalizable across different cases. For example a query for recognizing
the left S2 nerve is (the parameters defining the relations and threshold values
are not mentioned here for the sake of readability):
S2_left=crossing(SacralHoleS2Left) then anterior_of(PiriformisMuscleLeft)
then left_of(LevatorAniMuscles) then not posterior_of(Sacrum)
then not (crossing(SacralHoleS1Left) or crossing(SacralHoleS3Left))
then not left_of(PiriformisMuscleLeft)
then not anterior_of(ObturatorMuscleLeft)
then not between(ObturatorMuscleLeft, ObturatorMuscleRight)
4.2 Results and Discussion
The networks in phase A were implemented from scratch using Tensorflow 2.16.
We used 89 T2-w images for training, 16 as validation set and 63 as test set.
All images were preprocessed as described in Section 3.1 with a common voxel
size of 0.88×0.88×0.88mm3. The method showed high quality segmentation of
the pelvic structures described in Section 4.1 with Dice indices exceeding 85%
for dense structures and Average Surface Distance less than 2 mm for elon-
gated or small structures. The pelvic region was consistently localized by the
Visionerves 7
first network and the U-Net-based error correction proved effective for minor
pelvic structures, providing satisfying results for clinicians in around 2 minutes
in worst case scenarios. Although the error correction phase reduces the level of
automation of phase A, it serves to decouple potential segmentation errors from
directly propagating into the results of phase B.
T able 1.Quantitative results of phase A of the Visionerves method in average (and
standard deviation) for 10 endometriosis cases on different anatomical structures using
Dice and Average Symmetric Surface Distance (ASSD). Results are shown before the
use of the U-Net-based error correction. Bones are L5 vertebra, hip bones and sacrum;
muscles are piriformis, obturator and levator ani; visceral organs are bladder, colon and
rectum; reproductive organs are ovaries, uterus and vagina; specific ROIs are sacral
foramina from S1 to S3, sacral canal and intervertebral foramina of L5.
Structure Bones Muscles Visceral organsReproductive organsSpecific ROIs
Dice [%] 95.7 91.3 89.1 86.4 86.5
(0.14) (0.54) (0.92) (0.78) (1.26)
ASSD [mm] 0.22 0.36 0.97 0.93 0.43
(0.09) (0.42) (1.18) (0.62) (0.51)
The results of the complete Visionerves method on the 10 subjects with both
T2-w and DWI acquisitions are shown in Table 1 for phase A (segmentation).
These results are reported for completeness, even though segmentation is not
the central focus of our method; they are shown before error correction, which
was rarely necessary. Phase B results (reconstruction and recognition) are shown
in Table 2 and were obtained using the 10 nerve reference reconstructions avail-
able from our patient cohort. After preprocessing, nerve bundles were extracted
using raw tractography. The sacral or intervertebral foramen corresponding to
each nerve (for each side) served as the seed labelmap, and fiber selection was
constrained to those containing at least one point within the region traversed
by the sciatic nerve (where all the four analyzed fiber bundles are confluent, see
Figure 2 for a better understanding). This region was constructed using the bi-
narized spatial relations that we defined in Section 3.2. This phase was executed
using MRTrix3 software [22] and we used a FOD-based algorithm with deter-
ministic tracking, called “SD STREAM" [21], with minimum FOD amplitude
for seeds of 0.15, FOD cut-off of 0.10, maximum angle of 45 degrees and step
size of 3mm. This raw tractogram is referred as just “Tractography” in Table
2, and represents the current state of the art in PNS nerve recognition for tra-
ditional methods. Learning-based approaches could not be evaluated, primarily
due to the challenges associated with detailed manual nerve segmentation in the
T2-w images, and, more critically, the infeasibility of training a neural network
given the only 10 cases with DWI and the lack of publicly available pre-trained
models. We then applied our filtering method based on symbolic AI using the
nerve queries defined as in Section 4.1 (see also Figure 1) to the raw tractograms
produced. These results are referred as “+ Filtering” in Table 2. Since the Dice
8 G. La Barbera et al.
score is not well adapted to the thin and tubular structure of the nerves, we
also used the precision score (recall was not considered, as it cannot exceed the
performance achieved by tractography) and multiple distance metrics that are
the Average Symmetric Surface Distance (ASSD), the Average Symmetric Cen-
terline Distance (ASCD) and the Absolute Length Difference (ALD). In order
to make these measurements, each fiber bundle was transformed into a single
labelmap.
T able 2.Quantitative results of the Visionerves method (pre- and post-filtering) in
average (and standard deviation) for 10 endometriosis cases on 4 different lumbosacral
nerves (divided by sides) using Dice, precision, Average Symmetric Surface Distance
(ASSD), Average Symmetric Centerline Distance (ASCD) and Absolute (Euclidean)
Length Difference (ALD).
Nerve
Bundle
Visionerves Dice [%] Precision
[%]
ASSD
[mm]
ASCD
[mm]
ALD [mm]
L5
left
Tractography 49.56
(19.22)
35.09
(18.69)
9.66
(4.42)
31.67
(19.33)
30.64
(33.73)
+ Filtering 70.70
(16.26)
63.65
(17.35)
5.02
(4.38)
14.27
(16.11)
26.49
(30.52)
L5
right
Tractography 55.81
(14.29)
40.21
(14.21)
6.30
(3.63)
18.28
(12.53)
22.25
(27.78)
+ Filtering 64.86
(9.85)
58.74
(14.31)
3.49
(2.38)
10.01
(8.89)
24.98
(30.65)
S1
left
Tractography 56.37
(21.52)
42.37
(21.95)
6.02
(3.48)
14.14
(8.34)
36.12
(22.75)
+ Filtering 74.31
(11.95)
70.02
(16.47)
2.37
(1.80)
7.20
(7.00)
21.37
(23.47)
S1
right
Tractography 60.40
(24.97)
47.79
(27.36)
6.59
(4.39)
13.44
(11.44)
14.98
(18.03)
+ Filtering 74.46
(19.94)
69.65
(25.02)
2.12
(2.54)
5.81
(5.32)
15.03
(18.34)
S2
left
Tractography 50.96
(18.43)
36.57
(20.23)
7.27
(3.11)
19.30
(15.20)
35.76
(35.40)
+ Filtering 69.32
(12.48)
59.45
(17.39)
3.52
(2.27)
11.95
(18.01)
32.40
(27.03)
S2
right
Tractography 56.78
(17.69)
41.83
(17.67)
6.37
(4.11)
13.54
(8.82)
25.30
(17.36)
+ Filtering 70.71
(10.05)
61.19
(12.66)
3.01
(2.61)
6.37
(5.75)
17.77
(17.68)
S3
left
Tractography 44.54
(18.73)
30.55
(16.40)
7.48
(3.32)
17.47
(7.07)
37.68
(32.23)
+ Filtering 54.78
(15.95)
56.90
(21.35)
3.19
(1.83)
7.14
(2.82)
30.61
(21.72)
S3
right
Tractography 39.49
(18.56)
26.32
(15.11)
9.66
(4.40)
21.69
(12.07)
33.70
(27.81)
+ Filtering 54.06
(18.77)
55.26
(23.43)
4.28
(3.54)
6.88
(2.67)
28.55
(22.80)
Visionerves 9
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