1. This document discusses various types of soft tissue and bone infections, including cellulitis, necrotizing fasciitis, gas gangrene, abscesses, septic arthritis, and osteomyelitis. 2. Imaging findings are described for each condition, with MRI typically being the most sensitive for detecting early soft tissue and bone infection. CT and ultrasound can also be useful in certain settings. 3. Proper diagnosis of soft tissue and bone infections is important as many require urgent surgical treatment and have high mortality if not treated promptly.

1. Ahmed Elsammak, MD

2. Risk factors :
1. Vascular insufficiency
2. Soft-tissue ulcer (often secondary to diabetes)
3. Immunosuppression
4. Foreign bodies
5. DM

3. Definition:
Acute infectious process limited to the skin and
subcutaneous tissues.
Radiographic findings
1. Nonspecific soft-tissue swelling
2. Obliteration of fat planes

5. Cross-sectional imaging:
1. Thickening of the skin and the septae in the subcutaneous
tissue.
2. Occasionally, small fluid collections may be present in the
subdermal areas or superficial to the fascia.
In practice:
Diagnosed when Doppler is performed to rule out DVT

7. Cellulitis: refers to infection superficial to the superficial
fascia.
septic fasciitis refers to infection that has extended to involve
the fibrous fascia itself.
Necrotizing fasciitis
Fulminant form of septic fasciitis associated with rapid
spread of infection and prominent tissue necrosis.
Surgical emergency, so rapid and accurate diagnosis is
important

8. Diagnosis:
Gas within soft tissues

9. Fournier gangrene is necrotizing fasciitis of the perineum. It is
a true urological emergency due to the high mortality rate but
fortunately the condition is rare.
Epidemiology
Fournier gangrene is typically seen in diabetic men aged 50-
70 but is rarely seen in women.
Clinical presentation
perineal/scrotal pain, swelling, redness
crepitus from soft tissue gas (up to 65%)
fever and leukocytosis

10. Pathology
The source of infection can usually be identified, most
commonly anorectal (such as from a fistula or perianal
abscess)
Imaging: CT
1. soft tissue gas
2. a cause of infection may be apparent (e.g. perianal
abscess, fistula)
Treatment Immediate surgical interference
Prognosis 33% mortality

12. Radiographs
Nonspecific soft-tissue mass
Gas bubbles or gas-liquid levels are visualized.
CT
Heterogenous fluid collection with thick irregular margins
that enhance after the administration of intravenous contrast.
Inflammatory changes in the soft tissues adjacent to the
abscess may lead to overestimation of its size.
CT is particularly useful for detecting gas present within the
abscess cavity.
Both radiographs and CT can demonstrate retained foreign
bodies that may be the cause of abscess.

13. MRI
Soft-tissue abscesses have variable signal characteristics on MRI.
The typical abscess shows central areas of low signal intensity on
T1-weighted (T1W) images and high signal intensity on T2-weighted
(T2W) images.
Though the proteinaceous granulation tissue on the inner margin
may show intermediate signal on T1W images.
The thick, irregular wall enhances after administration of
intravenous gadolinium.
Adjacent soft tissue and skin inflammation are generally present.

15. Site:
The buttock and thigh are the most commonly affected
locations
Appearance:
The muscle is typically edematous, partially necrotic, and can
contain multiple abscesses.
Myonecrosis:
Muscle infection with extensive gas formation due to
myonecrosis, also referred to as “gas gangrene,”

18. Definition:
infection of normally existing synovial-lined bursal
cavities.
Causes:
Deep bursal infection is typically hematogenous
Superficial bursae, such as those overlying the
olecranon or patellar tendon, are often infected
secondary to penetrating trauma.

19. Imaging:
Conventional radiographs typically demonstrate
nonspecific soft-tissue fullness; gas formation within
the bursa is uncommon
CT and MRI
typically show cystic structure with wall
enhancement in the expected position of the
bursa
Surrounding inflammation is minimal.
Can cause adjacent joint effusions, which are
typically reactive (infection don’t extend to joints)

22. Predisposing factors:
As before
Rheumatoid arthritis
Foreign bodies such as joint prostheses.
Pregnancy is an often overlooked risk factor. The
increased laxity of the sacroiliac joints during
pregnancy makes them prone to fluid accumulation
and seeding during episodes of transient bacteremia

23. Children Vs adults
In children, septic arthritis involves the large joints
of the extremities and is typically hematogenous in
etiology.
 In adults:
The etiology is usually direct inoculation from
penetrating trauma to hands and feet.
Hematogenous spread in adults is less common
than direct inoculation, but when it does occur it
typically involves the 5 “S” joints: sacroiliac joint,
symphysis pubis, sternoclavicular joint, spine, and
acromioclavicular joint (shoulder).

24. Major complications:
Premature physeal closure
Premature osteoarthritis
Avascular necrosis
Osteomyelitis.

25. Radiographic findings:
Very early: particularly in young children whose
joints are relatively lax, the joint space may be
widened by a reactive effusion

27. Radiographic findings:
Early :
Joint effusion
Periarticular soft-tissue swelling
Aggressive periarticular osteopenia.

29. Radiographic findings:
Later:
Joint space narrowing secondary to cartilage
destruction and osseous erosions are identified.
Joint-space loss is uniform and is not accompanied
by sclerosis or productive bony changes, features
which help differentiate infection from degenerative
arthropathy.

30. Any monoarticular destructive arthritis should be regarded
as infectious until proven otherwise
MRI features
Synovitis
Periarticular soft-tissue inflammation
Periarticular bone edema.
 Differentiating between secondary osteomyelitis and reactive
bone marrow edema with MRI can be extremely difficult. Cortical
erosions, asymmetric edema on one side of the joint, and
extensive marrow involvement suggest osteomyelitis rather than
reactive marrow edema

31. In practice, the most requested modality to diagnose septic
arthritis is ultrasound because it detects effusion in clinically
suspected cases before radiography.
Most cases seen in practice are neonates with septic hip
after admission in incubators
Comparison with the asymptomatic side is helpful
Scanning of the entire hip is important however fluid is best
seen anterior to femoraal neck

38. Phemister’s triad
Ill-defined erosions
Profound osteoporosis
Relative preservation
of joint space till late

39. Rice bodies
Thickened areas of
synovial proliferation,
typically detached from
the underlying synovium
Characteristic
appearance on MRI,
appearing as 5 mm to10
mm elongated
fragments shaped like
rice kernels, with
intermediate T1W signal
and low signal on T2W
images

40. Osteomyelitis refers to infection of the bone marrow
Infective osteitis
Infection of the bone cortex.
From overlying soft-tissue infection.
Organism:
S. aureus is the most common pathogen.
Patients with diaphyseal infarction due to sickle cell disease
have an increased predilection for infection
with Salmonella organisms, though S. aureus still remains the
most common causative organism in sickle cell osteomyelitis.

41. Age:
All age groups, but is most common in early childhood.
Children
•Via the hematogenous route
•Metaphysis is the initial site of infection due to its slow, looping
capillary blood supply.
•Blood vessels between the metaphysis and epiphysis do not close
until after the first year of life, therefore, osteomyelitis in the infant
can readily spread into the epiphysis and adjacent articulation.
Adults
Direct spread from overlying soft-tissue infection, trauma
(penetrating or surgical).

43. Imaging characteristics:
Radiographs
Radiographic findings of acute osteomyelitis are typically not
present for 7 to 14 days following the onset of infection.
The early radiographic manifestations of osteomyelitis consist of:
- Permeative osteolysis
- Endosteal erosions
- Intracortical fissuring
- Periostitis.
Differential diagnosis:
- Ewing sarcoma or aggressive neoplasm
- Stress fracture

45. Late radiographic findings: destruction
CT
insensitive for early marrow infection
More sensitive than radiographs for detection of early periostitis
and trabecular or cortical erosion.
Nuclear medicine
Technetium-99m methylene diphosphonate, either alone or in
conjunction with Gallium-67 or Indium-111 labeled leukocytes.
Scintigraphy is more sensitive for early osteomyelitis

46. Nuclear medicine
Findings:
Abnormal increased uptake on all 3 phases, with increasing activity
on the delayed images.
Gallium and Indium scanning are adjunct techniques that are
particularly useful when there is an underlying disorder (e.g.
trauma or surgery) that produces bone remodeling.
False negative
Uncommonly, marrow pressure may be sufficiently increased to
produce hypoperfusion, resulting in a false-negative bone scan.

49. MRI
very sensitive for early osteomyelitis
Currently the imaging examination of choice for detection of
marrow infection.
Findings:
T1WI loss of the normal fatty signal of marrow on T1W images.
This finding is less apparent in the infant and very young child, in
whom the marrow is largely hematopoietic and little fatty marrow
exists.
T2W and STIR: in increased signal intensity of the infected marrow
space.

50. Gadolinium enhancement is present, except in rare cases where the
bone is poorly perfused due to vascular compromise from high
marrow pressure or underlying vascular disease leading to bone
infarction.
Subperiosteal abscess might be present
MRI tends to overestimate the extent of infection due to difficulty
distinguishing adjacent reactive edema from frank marrow
infection.

51. c

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53. When infection becomes chronic, the classic radiographic
appearance is thickened cortex and variable mixtures of lucency
and density.
Chronic osteomyelitis may remain clinically silent for years, then
reactivate. Such reactivation generally implies the presence of a
necrotic, infected bone as the nidus of reactivation.
Radiologist should identify a sequestrum because it has to be
surgically removed

54. Sequestrum is necrotic bone, isolated from living bone by
granulation tissue. It appears relatively dense because it has no
blood supply,
Involucrum is a shell of bone that surrounds a sequestrum.
Cloaca is a cortical and periosteal defect through which pus drains
from an infected medullary cavity
Sinus Chronic osteomyelitis of the tibia or femur is often
associated with a chronically draining sinus tract.
If the drainage occurs over many years (usually decades), the tract
may develop a squamous cell carcinoma.

59. Brodie’s abscess
Subacute or chronic osteomyelitis in a child
Usually found in the metaphysis and may present in
epiphysis
Geographic lytic lesion witha well defined,often broad
sclerotic margin
Oval, with the long axis parallel to the long axis of the bone
Borders the growth plate.
Radiographically nonaggressive, unlikeacute osteomyelitis.
Clinically, patients with Brodie’s abscess may not have
associated fever or elevated erythrocyte sedimentation rate.