Presentation1, radiological imaging of popliteal fossa masses.

Radiological imaging of Popliteal fossa masses.
Dr/ ABD ALLAH NAZEER. MD.

Axial PD MRI of the popliteal fossa.

Coronal PD MRI of the popliteal fossa.

SYNOVIAL
Pigmented villonodular synovitis (PVNS):
PVNS: Knee (80%) > ankle > hip > shoulder > elbow
Unknown etiology - possibly a reactive inflammatory process
Abnormal synovium prone to hemorrhage
Hemosiderin deposits in synovium and with nodule
formation due to repeated hemorrhagic effusions
Managed as low-grade, locally aggressive neoplasm.
Imaging features:
MRI diagnostic in 95% of cases:
Gradient echo imaging shows "blooming" phenomenon
of hemosiderin-laden nodules.
Large effusion +/- erosions.
Intense inhomogeneous enhancement.

MRI PD FS sagittal (a) and T1 axial (b) demonstrating a large posterior intra-articular
lobulated mass with villonodular proliferation of hemorrhagic synovium demonstrated
areas of low signal intensity representing hemosiderin (red arrows). Typical imaging
appearances of PVNS which was histologically confirmed.

Typically appearances of PVNS. Radiograph of the knee (a) demonstrates
increased soft tissue density in the posterior aspect of the knee (red
arrow). MR confirmed a large intra-articular lobulated lesion with areas
of nodular low signal intensity regions (hemosiderin) seen on sagittal PD
(b) which 'blooms' on axial T2* gradient echo (c).

MRI PD sagittal (a) demonstrating a well-defined low signal lesion
abutting the posterior cruciate ligament which demonstrates intense
inhomogeneous enhancement post contrast on sagittal (b) and axial
(c) T1FS which is typical for PVNS. Contrast can make small lesions
more conspicuous but usually not required. Pre T1FS not shown.

Pigmented villonodular synovitis. PD-weighted MRI sequences, both in the sagittal
plane, showing nodular synovial thickening interspersed with hypointense foci,
especially in the suprapatellar and posterior recesses of the knee.

Synovial chondromatosis:
Knee (50-65%) > hip > shoulder > elbow
Benign neoplastic synovial proliferation and formation
of cartilaginous (chondromatosis) or osseous bodies
(osteochondromatosis)
Malignant transformation is extremely rare and no
reliable distinguishing feature.
Imaging features:
Multiple, round, similar-sized calcified bodies seen on
radiograph.
80% have erosions detectable by MR
Bodies are of variable MR signal, depending on proportion
of calcium, chondroid, and mature ossific tissue.

Lateral radiograph of the knee (a) and sagittal gradient echo T2* (b)
demonstrates innumerable tiny ossified bodies both in the anterior
and posterior joint space. Numerous bodies can become conglomerate,
appearing as a "mass" within the joint as in this case. This was a case
of biopsy proven synovial osteochondromatosis.

Synovial chondromatosis. Sagittal proton density (PD)-weighted MRI scans with fat
suppression. Multiple intra-articular loose bodies located in the suprapatellar recess
(arrow) and posterior recess (asterisk), where a popliteal cyst (Baker's cyst) can be seen.

Primary synovial
Chondromatosis.

BURSAL:
Baker's cyst:
Fluid-filled sac with neck arising from interspace between
gastrocunemeous muscle and semimembranosus tendon.
Contralateral subclinical Baker cysts common.
Imaging features:
Must see neck arising from medial gastrocunemeous-
semimembranosus tendon interspace to make diagnosis of
Baker cyst.
Characteristic "talk-bubble" configuration on transverse
scans
Free fluid tracking adjacent to cyst indicates recent leakage.
Acute pain suggestive of rupture or hemorrhage into cyst

Axial PD MRI of the popliteal fossa demonstrating the characteristic appearance of a
Baker's cyst. A cystic structure with a 'talk bubble' configuration is seen arising between
the tendons of the medial head of the gastrocunemeous and semimembranosus muscles.

Transverse ultrasound (a) demonstrates a large echogenic solid lesion in
the popliteal fossa with no increased Doppler signal. T1 axial (b) and
sagittal (c) demonstrates a large T1 hyperintense collection arising
between the medial head of the gastrocunemeous (blue arrow) and
semimembranosus tendon (red arrow) in keeping with a large hemorrhagic
Baker's cyst. Signal loss of the knee joint due to total knee arthroplasty.

Baker's cyst. A 48-year-old male presented with swelling in the
popliteal fossa: sagittal (A) and axial (B) T2W gradient-echo images
show a distended gastrocnemius--semimembranosus bursa (arrows).

SOFT TISSUE:
Ganglion:
Myxoid tissue encased in a fibroblastic collagenous
wall (pseudocapsule) with no synovial lining
Arises from joint or tendon but intraosseous (Muir
et al) or intramuscular (Parks et al) locations have
been reported
In the knee, commonly arises from cruciate
ligaments or joint capsule.
Imaging features:
T2: Uniformly hyperintense soft tissue cystic mass

MR STIR axial (a), coronal (b) and sagittal (c) of the knee
demonstrating a uniformly T2 hyperintense multilobulated
soft tissue cystic mass arising from the posterior knee joint
capsule which was histologically proven as a ganglion.

Standard non-contrast PD sequenced MRI scans revealing ganglion cyst arising
from ACL, enveloping ACL fibres and predominantly extending posteriorly.

2 cm cyst in the posterior intercondylar notch, mainly
surrounding the PCL and Humphrey's ligament.

Lipoma:
Benign lipomatous tumour composed of mature
adipose tissue representing almost 50% of soft
tissue masses.
Well-defined, oblong-shaped encapsulated mass with
fine linear striations parallel to long axis of tumour
Characterised by location as:
1-superficial to the deep fascia
2-deep to the deep fascia
3-intramuscular .
Imaging features:
Should be similar to subcutaneous fat on any imaging
modality.

AP and lateral radiographs of the knee demonstrating increased fat density within the
soft tissues of the posteromedial aspect of the knee (blue arrow). Note how this is similar
to the normal density of the suprapatellar pouch that contains fat (red arrow) suggesting
this mass is fatty in nature. Lipomas follow fat density/signal on all modalities.

Axial (a) and coronal (b) T1 and corresponding axial (c) and coronal (d) fat suppressed MRI
sequences demonstrating the fatty lesion seen in the radiograph of Figure 10. The lesion is
high signal on T1 in keeping with fat and demonstrates complete suppression in keeping
with a lipoma. Due to its size biopsy was performed confirming a simple lipoma.

Lipoma arborescens. PD-weighted MRI sequences, both in the sagittal plane, with fat suppression (A)
and without (B), showing intra-articular anomalous tissue, with a frond-like aspect, presenting a
hyperintense signal in the PD-weighted sequences without fat suppression and showing a reduction in
the signal strength when fat suppression is applied. Moderate joint effusion can also be seen.

Fibromatosis:
Spectrum of benign fibroblastic disorders with tendency to
infiltrate adjacent tissues
Tendency for local recurrence (65%) without metastasis.
Imaging features:
Intense enhancement post contrast
Extends along fascial planes
Locally infiltrative
US: Nonspecific soft tissue mass, variable echogenicity,
smooth well-defined margins.
non-contrast CT: Soft tissue mass hyperdense to muscle
MRI: T1 - Hypointense to muscle; T2 - Hyperintense with
bands of hypointense signal regions (fibrous components).

Axial T1 (a), axial STIR (b)
and corresponding T1 and
STIR coronal MRI images
(c) and (d) respectively,
demonstrate a soft tissue
mass hypointense to
skeletal muscle (blue
arrow) that was low signal
on all sequences. There
curvilinear areas of low
signal represent areas of
mature collagen.

Fasciitis:
Benign mass-forming fibrous proliferation
Extension of mass along fascia suggests diagnosis
Three types:
Subcutaneous = well-circumscribed round nodule
attached to fascia extending into superficial fat
Fascial = poorly circumscribed fascial mass with
satellite growth pattern
Intramuscular = ovoid intramuscular mass attached to
fascia.
Imaging features:
Similar intensity to skeletal muscle on T1, intermediate to
high signal intensity on T2
Enhancement depends on myxoid and fibrous content.

MRI depicting intra-articular nodular fasciitis. Axial T2-weighted (A),
sagittal T2-weighted (B), and sagittal T1-weighted (C) images.

Sagittal T1 (a) demonstrates a soft tissue lesion anterior to the popliteal vessels
which is similar intensity to skeletal muscle on T1 and high signal intensity on
axial STIR (b). Pre (c) and post (d) contrast T1FS demonstrates predominantly
peripheral enhancement. Biopsy of the lesion confirmed proliferative fasciitis.

Epidermal inclusion cyst:
benign lesion formed by the inclusion of dermal
squamous epithelium with retention of keratinous
debris and cholesterol or sebaceous materials
Well-circumscribed subcutaneous mass.
Imaging features:
T1 isointense to muscle with mild heterogeneous
signal ranging from low to high signal
T2 Hyperintense signal with low or high signal
debris.
Absent to low-level central contrast enhancement
with enhancing capsule.

Transverse ultrasound image of a superficial subepidermal lesion
with internal speckles and posterior acoustic enhancement. This
lesion was histologically confirmed as an epidermal inclusion cyst
- though the classic 'pseudotestis' appearance is not seen.

Axial MR images of the knee. Dermal soft tissue session seen in
figure 15 is low on T1 (a) and high on T2 (b) with internal low signal
debris. Post contrast T1FS (c) demonstrates only subtle peripheral
enhancement. Biopsy proven epidermal inclusion cyst.

Heterotopic ossification/Myositis ossificans:
Benign Heterotopic formation of bone and cartilage in soft
tissue.
Stem cells for osteoid production exist within affected soft tissues
Stimulus is usually traumatic though it may be in apparent or
forgotten by patient.
Imaging features:
Evolving radiographic stages with stages of bone
development (indistinct soft tissue through to mature
bone formation)
Peripheral bone formation with central immature bone
(versus parosteal osteosarcoma which has organized bone
centrally, less mature bone peripherally).

AP and lateral radiographs of
the knee demonstrates
ossification of the soft tissues at
the posterior aspect of the knee.
Importantly, no connection to
the femur is seen and periosteal
reaction. Axial CT (b) and MR
STIR (d) demonstrates bone
density within the soft tissues
posterior to the medial femoral
condyle with no aggressive
features. Differentials include
heterotopic ossification but a
parosteal osteosarcoma cannot
be entirely excluded although
less likely. Biopsy confirmed
heterotopic ossification.

Heterotopic myositis ossificans.

Soft tissue sarcoma:
Malignant tumour that arises from connective (mesenchymal) tissue other than
bone
Classified according to tissue type rather than anatomical origin.
Grow centrifugally from single focus
Suspicious features include a soft tissue mass that is increasing in size, has a size
>5cm or is deep to the deep fascia, whether or not it is painful
Imaging features:
Features depend on the type of tumour and may have, internal cystic component,
myxoid tissue, fat, calcification or necrosis
Large heterogeneous hypoechoic mass
Usually located in deep (subfascial) tissues
Well-encapsulated
Usually hypervascular with disorganized vascular pattern on color Doppler
imaging
± Myxoid tissue: Well-defined, intra-tumoral anechoic or hypoechoic areas.
± Necrosis: Poorly-defined hypoechoic areas.
± Calcification: Discrete intra-tumoral echogenic foci with acoustic shadowing.
± Hemorrhage: Ill-defined intra-tumoral echogenic areas without acoustic
shadowing

Axial STIR (a) demonstrates a complex soft tissue lesion superficial to the deep
fascia. Pre (b) and post (c) contrast T1Fs demonstrates heterogenous
enhancement. Soft tissue sarcomas are tumors without specific imaging
findings beyond solid enhancing, soft tissue mass and needs biopsy for
histological diagnosis. Biopsy confirmed undifferentiated pleomorphic sarcoma.

NDIFFERENTIATED PLEOMORPHIC SARCOMA
(formerly malignant fibrous histiocytoma):
5% are extensively hemorrhagic, presenting as
fluctuant mass often misdiagnosed as hematoma
Underlying malignancy should be excluded in
patients thought to have spontaneous
musculoskeletal hemorrhage.
Imaging features:
Calcification present peripherally in 5-20%
Similar signal intensity to muscle on T1 MRI
Heterogeneously hyperintense to muscle on
fluid-sensitive sequences.

Axial T2FS (a) and sagittal T1 (b) MRI demonstrates a high T2/low T1 signal mass
in the posterior aspect of the knee with internal areas of intermediate and low
signal which bloom on sagittal gradient echo T2* (c) in keeping with calcification.
This lesion was histologically proven as undifferentiated pleomorphic sarcoma.

LIPOSARCOMA:
Liposarcomas account for approximately 10% of all
soft tissue sarcomas
Liposarcomas arise from fat cells and can occur
anywhere in the body.
There are 6 different sub-types; the most common are:
1-well-differentiated liposarcoma,
2-pleomorphic liposarcoma
3-myxoid liposarcoma
Imaging features:
Non-specific soft tissue mass with myxoid and fat
tissue ~< 25% fat.

Transverse ultrasound of the knee demonstrates a fatty lesion with increased
Doppler signal (a). Panoramic ultrasound view (b) of the lesion demonstrates
a large fatty lesion. Axial STIR MR (c) and T2 sagittal (d) demonstrates mixed
signal intensity mass with heterogeneous enhancement (e) post contrast (pre
contrast T1FS not shown). Biopsy confirmed a myxoid liposarcoma.

SYNOVIAL SARCOMA:
Contrary to name, synovial sarcoma is not does not
arise intra-articular or from synovium
Malignant soft tissue tumour with predilection for
juxta-articular regions of young patients especially
popliteal fossa of knee or tendon sheath.
Imaging features:
Enhancing soft tissue mass near a joint
Calcification, hemosiderin, cystic change, fluid-fluid
levels relatively common.

Transverse ultrasound (a) demonstrates a soft tissue lesion
with increased Doppler signal. Axial T1 MR (b) and sagittal
T2FS (c) demonstrates a juxtaarticular low T1/high T2 soft
tissue mass which enhanced post contrast (images not
shown). Lesion proved to be a synovial sarcoma.

Synovial sarcoma. Fat-suppressed T1-weighted MRI sequences in the axial (A) and sagittal
(B) planes, after intravenous injection of paramagnetic contrast. Voluminous multiloculated
expansile lesion, with the "bowl of grapes" aspect, showing intense enhancement after
contrast administration. The femoral cortical bone (arrow) is poorly defined.

a) Lateral radiograph showing scattered calcification in synovial
chondromatosis (white arrow), and b) T2-weighted sagittal MR scan of
the knee showing heterogenous malignant synovial chondromatosis (white

VASCULAR:
Popliteal artery aneurysm:
Popliteal artery is most common lower extremity location (70% of all peripheral
aneurysms).
Imaging features:
Fusiform or saccular arterial enlargement, often with laminar thrombus within.
Longitudinal panoramic view of the popliteal artery
demonstrating a focal fusiform aneurysm with mural thrombus.

CT axial (a) and coronal reformat (b) demonstrating
a right popliteal aneurysm (yellow arrow) which is
completely occluded with thrombus.

MR axial (a) and coronal T1 (b), coronal STIR (c) demonstrating the occluded right
popliteal artery which contains hyperintense T1 mural thrombus with central low
signal material in keeping with chronic blood products. Image (d) is a zoomed up STIR
coronal image demonstrate layers of differing blood products within the aneurysm.

Popliteal artery pseudoaneurysm. Sagittal PD-weighted MRI sequence (A) and sagittal fat-suppressed T1-weighted
MRI sequence obtained after intravenous injection of paramagnetic contrast (B). Small bone projection from the
tibial metaphysis (arrow) into the popliteal fossa, where there is an oval image with heterogeneous content, which
shows partial enhancement by contrast, representing a partially thrombosed pseudoaneurysm.

Vascular malformations and hemangiomas:
Hemangiomas are vascular tumours that are rarely apparent at
birth, grow rapidly during the first 6 months of life, involutes with
time and do not necessarily infiltrate but can sometimes be
destructive
Vascular malformation/arteriovenous malformation (AVM) is a
diffuse mass of dilated tortuous serpiginous vessels involving both
arteries and veins. In contrast to hemangiomas, they are present at
birth, slow growing, infiltrative, and destructive
Almost all AVMs and nearly 40% of hemangiomas eventually
require intervention.
Imaging features:
Flow voids in high flow AVM
High signal intensity on fat-suppressed T2 : Bright "bag of worms"
Low T2 signal intensity foci may reflect phleboliths, scars or
hemosiderin deposition best seen on gradient echo sequences.

MR Axial T1 (a), T2FS (b) and post contrast T1FS (c)
demonstrating a low T1/High T2 soft tissue mass with
intense enhancement. Intralesional low signal areas are
suggestive of flow voids. Biopsy confirmed a hemangioma.

Hemangiopericytoma (HPC):
Entity arising from pericytes with 70% being benign
Characterised by its branching vascular pattern that (although can be seen in numerous
benign and malignant tumours).
Solitary fibrous tumour and hemangiopericytoma are histologically similar and currently
used almost interchangeably and are thought of as being part of the
same histopathologic spectrum.
Lower extremity > retroperitoneum > head/neck > trunk > upper extremity
Longitudinal ultrasound image (a) demonstrating a vascular soft tissue mass. Axial MR T2 (b)
and sagittal T1 (c) demonstrates a predominantly hyperintense lesion with internal branching
flow-voids (red arrows) in keeping with vessels. Biopsy proved a hemangiopericytoma.

Myopericytoma:
Benign neoplasm thought to represent a subset of hemangiopericytoma (HPCs)
WHO defines myopericytoma as a distinct entity encompassing myopericytoma
and glomangiopericytoma.
Arises from perivascular myoid cells.
Imaging features:
Typically well-defined subcutaneous mass typically occurring in the lower extremities.
Isointense to muscle on T1
Hyperintense on T2 weighted sequences
Intense enhancement post contrast
MR axial STIR (a) and pre (b) and post (c) axial T1FS demonstrates an intermuscular
mass more conspicuous post contrast. Biopsy confirmed a myopericytoma.

NEURAL:
Peripheral nerve sheet tumour (PNST):
Benign tumour arising from nerve; histologically
classified as either schwannoma or neurofibroma
Any peripheral nerve can be affected
Intermuscular > subcutaneous > intramuscular.
Imaging features:
Typically fusiform or oblong-shaped and oriented
along long axis of nerve.
Nerve entering or exiting from tumor is seen in
majority (90%)
Posterior enhancement is common (70%).

Coronal (a) and axial (b) MR T2FS
demonstrating the classical fusiform 'target'
lesion with the nerve seen entering and
exiting from the lesion in keeping with a
peripheral nerve sheath tumour (PNST).
Axial STIR MR of a neurofibroma which
demonstrates the typical 'target' appearance
of a nerve sheath tumour. Central (red
arrow) fibrocollagenous component (low T2)
surrounded by peripheral (blue arrow)
myxoid component (high T2).

Peripheral nerve sheath schwannoma.

Synovial sarcoma. A 23-
year-old male with
palpable mass. (A) Sagittal
T1W image demonstrates
hypointense oblong mass in
semimembranosus muscle
(arrow), which has solid
internal enhancement
(arrow) on post-contrast
T1W fat-saturated image
(B). (C) Photomicrograph (H
and E, ×200) shows a highly
cellular spindle cell
neoplasm composed of
densely packed cells with
scant cytoplasm. The tumor
is characterized by its lack
of nuclear pleomorphism
and a mitotic rate that is
lower than expected for
such a cellular tumor,
consistent with a synovial
sarcoma.

Malignant peripheral nerve sheet tumour:
Malignant spindle cell sarcoma of neural origin
25-70% associated with Neurofibromatosis 1
10-20% radiation-induced
5-10% of all soft tissue sarcomas
Imaging features:
Infiltrative, ill-defined, fusiform mass > 5 cm
MR sagittal T1 (a) and axial STIR (b) demonstrates a large fusiform T2 hyperintense mass in the region
of the common peroneal nerve. Biopsy confirmed a malignant peripheral nerve sheath tumour.

LYMPHOID:
Non Hodgkin's Lymphoma:
Extra nodal lymphoma soft tissue lymphoma is reported as
<0.01%
Must indicate to pathology if lymphoma is suspected to allow
appropriate immunohistochemical analysis to be performed.
Staging and management of lymphoma is drastically different
compared to sarcomas
Imaging findings:
Infiltrative mass - encases neurovascular bundle
Isointense to muscle on T1
Hyperinternse of T2
Contrast imaging does not add further to the tissue
characterization: both homogenous and heterogenous
enhancement post contrast has been reported.

Axial MR T1 (a) and STIR (b) demonstrates an ill-defined infiltrative
soft tissue mass encasing the popliteal artery - typical of lymphoma.
Axial CT angiogram (c) of the lower leg demonstrated a patent
popliteal artery. Biopsy confirmed Non-Hodgkin's lymphoma.

MUSCULAR/TENDON:
Giant cell tumour of tendon sheath (GCTTS):
Benign synovial proliferation within tendon sheath,
Same pathologic entity as pigmented villonodular synovitis (PVNS) but in a different location
Lobulated soft tissue mass immediately adjacent to tendon
Hand > wrist > ankle/foot > knee > elbow > hip
Imaging features:
Low T1 and T2 signal intensity soft tissue lobulated mass with intense inhomogeneous
enhancement
Low signal hemosiderin rim "blooms" on gradient echo imaging
Sagittal T1 MR (a) and axial STIR (b) demonstrates a soft tissue mass with peripheral low signal rim on
both sequences suggestive of hemosiderin. Biopsy confirmed a giant cell tumour of the tendon sheath.

Leiomyosarcoma:
Vascular malignant neoplasm arising from smooth muscle
Arises in skin, soft tissue, blood vessel
Imaging features:
Nonspecific soft tissue mass +/- calcification, necrosis, hemorrhage, or cystic
change giving rise to fluid-fluid levels
Aggressive lesion invading adjacent bone.
AP radiograph (a) of the knee demonstrates abnormal increased soft tissue density in the medial
aspect of the keen (blue arrow). Axial (b) and sagittal (c) STIR MR demonstrates a large soft
tissue mass with multiple fluid-fluid levels (red arrows). Biopsy confirmed a leiomyosarcoma.

Parosteal osteosarcoma (differential diagnosis
- heterotopic ossification/myositis ossificans):
Low-grade osteosarcoma arising on surface of bone
65% of all lesions occur in the posterior cortex of the distal
femoral metadiaphysis
Fusiform mass along length of bone which tends to "wrap
around" as it enlarges
Initially associated with the surface and can present as an
area of cortical thickening, and therefore be misinterpreted
as a healing stress fracture
As the mass grows, parts of it may be adjacent to cortex
without being contiguous to it, giving appearance of "cleft"
Bone is mature at site of origin, less mature at periphery
[opposite ossification pattern of myositis ossificans

Chondrosarcoma:
malignant cartilaginous tumours that account for 20-27% of all
primary malignant bone tumours.
Predominantly in the long bones
Primary or secondary (osteochondroma, enchondroma)
Multiple subtypes:
conventional intramedullary chondrosarcoma (or central
chondrosarcoma):
juxtacortical chondrosarcoma:
myxoid chondrosarcoma:
extraskeletal chondrosarcoma
Imaging features:
1-Destructive lesion with intralesional calcification
(chondroid matrix)
2- High T2 signal of chondroid matrix
3- Heterogenous contrast enhancement

Lateral radiograph of the knee demonstrating areas of calcification
within the soft tissue posteriorly (blue arrow). MR Sagittal T1 (b),
sagittal (c) and axial (d) T2FS demonstrating high T2 signal mass.
Biopsy confirmed an extraskeletal chondrosarcoma.

Presentation1, radiological imaging of popliteal fossa masses.

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Presentation1, radiological imaging of popliteal fossa masses.