Lumbar plexus imaging

1. ANATOMY OF
LUMBOSACRAL
PLEXUS
DR. JOAN LUCIYA JOSEPH
DNB RESIDENT

2. • At the level of the L2–L5 transverse processes, the ventral rami of the L1–L4
spinal nerve roots coalesce in or posterior to the psoas major muscle to form
the lumbar plexus.
• The lumbar plexus gives rise to the iliohypogastric, ilioinguinal,
genitofemoral, femoral, and lateral femoral cutaneous nerves, which exit
lateral to the psoas major muscle and the obturator nerve and lumbrosacral
trunk ,which course medial to the psoas major muscle.
• The ventral rami of the L4–S3 spinal nerve roots coalesce anterior to the
piriformis muscle to form the sacral plexus, which gives rise to the sciatic,
pudendal, superior gluteal, and inferior gluteal nerves.
• The lumbar and sacral plexuses connect in the lumbrosacral trunk to form the
lumbrosacral plexus.

3. Illustration shows the anatomy of the
lumbar plexus from T12 to L5.
Peripheral nerve branches (*) to the
psoas and iliacus muscles are also
illustrated.

4. Illustration shows the anatomy of
the sacral plexus from L4 to S5,
with “L” denoting lumbar and “S”
denoting sacral nerve roots. + =
peripheral nerve branches to the
piriformis, * = quadratus femoris
and inferior gemellus, ** = obturator
internus and superior gemellus, x =
levator ani, coccygeous, and
sphincter ani externus

5. ILIOHYPOGASTRIC AND
ILIOINGUINAL NERVES
• The iliohypogastric and ilioinguinal
nerves are formed by the anterior
division of the L1 root.

6. • The iliohypogastric nerve courses anterior to the psoas major muscle and runs
inferolaterally over the anterior aspect of the quadrates lumborum muscle and
posterior to the kidney.
• It then pierces the transversus abdominis muscle and runs superior to the iliac
crest in the lateral abdominal wall.
• Its terminal branch courses parallel to the inguinal ligament and exits the
aponeurosis of the external oblique muscle.

7. • The ilioinguinal nerve also courses over the quadratus lumborum muscle, along
the inferior aspect of the iliohypogastric nerve, sending branches to the
iliohypogastric nerve.
• Subsequently, the ilioinguinal nerve pierces the lateral abdominal wall and runs
medially to the level of the inguinal ligament.
• Because of their small size, depiction of the ilioinguinal and iliohypogastric are
inconsistent unless they are abnormally enlarged; however, they are often seen
at STIR imaging at the site where they exit the psoas muscles.

8. Coronal STIR image shows the
iliohypogastric (arrowheads), left
ilioinguinal (black arrow), and lateral
femoral cutaneous (white arrows) nerves
where they exit the psoas muscles

9. GENITOFEMORAL NERVE
• The genitofemoral nerve arises from
the anterior divisions of the L1 and
L2 roots and pierces the psoas major
muscle inferior to the iliohypogastric
and ilioinguinal nerves at,
approximately, the level of L3–L4.

10. • Subsequently, the genitofemoral nerve divides into two branches—medial
genital and lateral femoral—that run downward over the anterior aspect of the
psoas major muscle.
• The medial genital branch enters the inguinal canal and courses parallel to the
spermatic cord in men and the round ligament of the uterus in women.
• The lateral femoral branch travels lateral to the femoral artery, posterior to the
inguinal ligament, and enters the proximal thigh, where it pierces the sartorius
muscle distal to the inguinal ligament.

11. MR image shows the
genitofemoral nerves (arrows)
at the anterior margin of the
psoas muscle. The right sciatic
nerve (arrowhead) is also seen
to split.

12. OBTURATOR NERVE
• The obturator nerve is formed by
the anterior rami of the L2–L4
roots.

13. • It emerges from the medial border of the psoas major muscle beneath the
common iliac vessels, immediately lateral to the sacrum, and courses along the
lateral wall of the lesser pelvis to enter the obturator foramen.
• Just before it enters the thigh, it is separated into anterior and posterior
branches by the adductor brevis muscle.
• The anterior branch of the obturator nerve courses superficial to the adductor
brevis muscle and terminates at the distal aspect of the adductor longus muscle,
where it forms a subsartorial plexus that communicates with the anterior
cutaneous branches of the femoral and saphenous nerves and innervates the
distal medial thigh.
• The obturator nerve is consistently depicted on axial and coronal MR
neurographic images because it courses longitudinally along the pelvic sidewall.

14. • The accessory obturator nerve, which is present in about onethird of people,
arises from the ventral rami of the L3 and L4 roots, descends along the medial
border of the psoas muscle, crosses the superior pubic ramus, and courses
under the pectineus muscle, where it divides into numerous branches.
• One of these branches supplies the pectineus muscle, another is distributed to
the hip joint, and a third one communicates with the anterior branch of the
obturator nerve

15. • Coronal T1 image showing normal
obturator nerve.

16. FEMORAL NERVE
• The femoral nerve arises from the posterior divisions of the L2–L4 roots,
descends through the psoas major muscle, and emerges from the lower part of
the lateral muscle border.
• It courses inferiorly between the psoas major and iliacus muscles, posterior to
the iliac fascia, and runs beneath the inguinal ligament and into the thigh, where
it splits into anterior and posterior divisions; the saphenous nerve derives from
the posterior division.
• The femoral nerves are consistently seen at MR neurography, which depicts
areas that are symmetric in terms of hyperintensity and size at the level of the
iliopsoas crotch and isointense (including their anterior and posterior divisions)
at the site of the inguinal ligament.

17. Coronal STIR image shows
normal femoral (arrows) and
ilioinguinal (arrowheads) nerves

18. LATERAL FEMORAL CUTANEOUS
NERVE
• The lateral femoral cutaneous nerve is a sensory branch that derives from the
posterior divisions of the L2 and L3 roots.
• It pierces the lateral side of the psoas major muscle, runs obliquely over the
iliacus muscle, and lies immediately medial to the anterior superior iliac spine,
leaving the pelvis inferior to the lateral aspect of the inguinal ligament and over
the sartorius muscle.
• In the thigh, it briefly courses under the fascia lata and, before breaching the
fascia, divides into anterior and lateral branches that often communicate with
the cutaneous branches of the femoral and saphenous nerves to form the
patellar plexus.
• On axial T2-weighted SPAIR MR neurographic images, the lateral femoral
cutaneous nerve is consistently isointense along the anterior surface of the
iliopsoas muscle.

19. Normal lateral femoral cutaneous nerves.
Axial T2-weighted SPAIR MR image
obtained at the level of the pelvis shows
both lateral femoral cutaneous nerves
medial to the anterior superior iliac spine

20. SCIATIC PLEXUS
• The sciatic plexus is formed by the ventral rami of the L4–S3 nerve roots, which
join to form the tibial, common peroneal, and posterior femoral cutaneous
nerves.
• After the sciatic plexus exits the pelvis through the greater sciatic foramen
(which descends anterior, above, or through the piriformis muscle), the tibial and
common peroneal components become enclosed in a common nerve sheath,
forming the sciatic nerve.
• The sciatic nerve then enters the gluteal region and courses inferiorly between
the adductor magnus and gluteus maximus muscles to the distal onethird of the
thigh, where it divides into the tibial and common peroneal trunks.

21. • The posterior femoral cutaneous nerve is a sensory branch of the sacral plexus
that arises from the posterior divisions of the S1 and S2 roots and the anterior
divisions of the S2 and S3 roots and travels immediately posterolateral to the
sciatic nerve.
• The terminal segments of the posterior femoral cutaneous nerve communicate
with the sural nerve .
• The sciatic plexus and sciatic nerves are consistently seen at MR neurography,
normally with symmetric morphologic characteristics and signal intensity,

22. Coronal STIR image shows
normal sciatic nerves (arrows)

23. PUDENDAL PLEXUS
• The pudendal plexus is formed along the posterior wall of the pelvis by the
anterior divisions of the S2–S4 and C1 nerve roots.
• It branches into the perforating cutaneous, pudendal, anococcygeal, visceral,
and muscular nerves, the most important of which is the pudendal nerve
(formed by the S2–S4 nerve roots), which passes between the piriformis and
coccygeus muscles and exits the pelvis through the lower part of the greater
sciatic foramen.
• The pudendal nerve then crosses the ischial spine between the sacrotuberous
and sacrospinous ligaments and re-enters the pelvis through the lesser sciatic
foramen.

24. • The pudendal nerve then courses through the pudendal (Alcock) canal with the
internal pudendal vessels. The pudendal canal is located along the lateral wall
of the ischiorectal fossa and medial to the obturator internus muscle, with its
outer border formed by the obturator fascia.
• Finally, the pudendal nerve divides into the perineal nerve (the larger branch)
and the dorsal nerve (the smaller branch) of the penis or clitoris.
• On axial T1- weighted and fat-suppressed T2-weighted MR neurographic
images, the pudendal nerve is easily seen between the sacrotuberous and
sacrospinous ligaments at the level of the ischial spine and in the pudendal
canal.

25. SUPERIOR GLUTEAL NERVE
• The superior gluteal nerve originates from the L4–S1 nerve roots and exits the
pelvis through the greater sciatic foramen, above the piriformis muscle, with the
superior gluteal artery and vein.
• It then branches into the superior and inferior nerves: The inferior gluteal nerve
originates from the L5–S2 nerve roots and exits the pelvis through the greater
sciatic foramen, inferior to the piriformis muscle.
• Because of their small size, the gluteal nerves are not visualized at MR
neurography unless they are abnormally enlarged.

27. MRI TECHNIQUE
• High field strength (ie, 3 Tesla MRI preferable to 1.5 Tesla) is preferable for its
higher signal to noise ratio.
• An optimal field of view will be large enough to assess the region of interest
while small enough to decrease imaging time and potential artifacts.
• Images should extend superiorly through L1 as this is the most cephalad
position of the lumbar plexus ventral rami, and should be obtained in both the
axial and coronal planes.
• Coronal imaging should be centered at L3-4, which is the fulcrum for the normal
lumbar lordotic curvature.
• Oblique imaging centered on the sacrum can be obtained to better evaluate the
sciatic and pudendal nerves.

28. • Full evaluation of the lumbosacral plexus should include both T1-weighted
images and a fluid sensitive fat-suppressed sequence such as STIR.
• A suggested protocol would therefore include pre-contrast and postcontrast T1-
weighted sequences in addition to a T2 fat saturated sequence in axial and
coronal planes.
• Intravenous gadolinium can be employed as a contrast agent and will
demonstrate areas of breakdown of the blood–nerve barrier such as in tumor-
related plexopathies, inflammatory conditions, and posttraumatic neuromas.

29. THANK YOU