The British Journal of Radiology, 82 (2009), 662–665
The ‘‘dot-in-circle’’ sign — a characteristic MRI finding in
mycetoma foot: a report of three cases
1
R S CHERIAN, DMRD, DNB, 1M BETTY, MBBS, 2M T MANIPADAM, MD, 3V M CHERIAN,
3
P M POONNOOSE, MS, 3A T OOMMEN, MS and 1R A CHERIAN, DMRD, DNB, FRCR
MS,
Departments of 1Radiology, 2General Pathology and 3Orthopaedics, Christian Medical College, Vellore-632004, TamilNadu, India
ABSTRACT. Three patients referred for MRI of the foot were found to have imaging
features characteristic of mycetoma. Two patients presented with recurrent soft tissue
masses, which were operated on several times and not suspected to be of infective
aetiology. The third patient had typical clinical features with a history of blackish
granule discharge. In all three patients, MRI showed conglomerate areas of small
round discrete T2 weighted hyperintense lesions, representing granulation tissue
surrounded by a low-signal-intensity rim representing intervening fibrous septa.
Within many of these hyperintense lesions, there was a central low-signal-intensity
dot, which gives rise to the ‘‘dot-in-circle’’ sign that has been very rarely described in
the literature. This sign is an easily recognisable and unique appearance that is highly
suggestive of mycetoma.
Mycetoma is a localised chronic suppurative infection
characterised by exuberant granulation tissue formation
involving the subcutaneous tissues, which can extend to
involve the deeper tissues [1]. It is caused by different
species of fungi or by filamentous bacteria known as
actinomycetes. Mycetomas caused by fungi are known as
eumycetoma, whereas those caused by aerobic actinomycetes are called actinomycetomas. Clinically, the disease
follows an indolent but progressive course after initially
presenting as a firm, painless nodule. Eventually, the
lesions communicate via sinuses onto the skin surface or
involve the adjacent bone to cause a form of osteomyelitis
[2]. Early diagnosis before the sinuses and fungal grain
discharge occurs is sometimes difficult. If untreated,
however, mycetomas can lead to significant destruction
and deformity. Biopsy or microbiological culture can
provide the diagnosis, but this may not always be
possible, especially if the organism is fastidious [3].
The purpose of this study is to describe the characteristic MRI appearances of mycetoma foot. With the
increasing availability of MRI, especially in areas of the
dry tropics where organisms causing mycetomas are
endemic, this would aid in the early diagnosis of this
condition and facilitate appropriate management.
Methods and materials
Three patients were referred for evaluation of soft
tissue masses involving the foot during the period 2006
to 2007. Plain radiographs were available for all patients.
MRI was performed for the first two patients using a 3T
MRI scanner (Intera Achieva, Philips Medical Systems,
Address correspondence to: Rekha S. Cherian, Radiology
Department, Christian Medical College, Vellore-632004, TamilNadu, India. E-mail: vijitkc@hotmail.com
662
Received 9 April 2008
Revised 29 May 2008
Accepted 7 July 2008
DOI: 10.1259/bjr/62386689
’ 2009 The British Institute of
Radiology
The Netherlands) and for the third patient using a 1.5T
MRI scanner (Siemens Avanto, Erlangen, Germany). The
sequences were obtained using a phased-array surface
coil and included fat suppressed T2 weighted, proton
density (PD) and T1 weighted images. Intravenous
gadolinium contrast was not administered. The studies
were read by radiologists with MRI experience of 3–7
years. All three lesions underwent histopathological
examination and were proven to be eumycetoma.
Results
Patient 1
A 30-year-old male soldier presented with a recurrent
soft tissue mass in the left foot, which had been operated
on elsewhere four times. Reports of previous surgery
and histopathology were not available. Examination
revealed a swelling in the dorsum and medial aspect of
the foot. The tibialis anterior tendon was non-functioning. Plain radiograph revealed a soft tissue mass in the
dorsum of the foot with no calcification.
MRI showed a soft tissue mass in the subcutaneous
plane of the dorsum of the foot, anterior to the tibialis
anterior tendon (Figure 1a–d). The mass showed conglomerate areas of multiple, discrete, small 2–5 mm
round hyperintense lesions, which were separated by a
low-signal-intensity rim in the subcutaneous plane of the
dorsum of the foot. Within many of these hyperintense
lesions, there was a central low-signal-intensity dot. A
separate focus of involvement was seen in the medial
aspect of the foot. At this time, fungal aetiology was not
suspected and the patient underwent wide excision of
the lesion, with skin grafting. The tibialis anterior tendon
was sacrificed. The tissue was sent for histopathology,
which showed eumycetoma.
The British Journal of Radiology, August 2009
The ‘‘dot-in-circle’’ sign – a characteristic MRI finding in mycetoma foot
Figure 1. A 28-year-old man (patient
1) with a recurrent soft-tissue mass.
(a) MRI T2 weighted fat suppressed
sagittal image (repetition time (TR)
2407, echo time (TE) 70) showing a
lesion in the subcutaneous plane of
the foot (arrow). The lesion has the
appearance of conglomerates of
small discrete round 2–5 mm hyperintense lesions, with a peripheral
low-signal-intensity rim and fibrous
network extending between the
hyperintense lesions. (b) T1 weighted
sagittal image (TR 524, TE 12) showing the corresponding isointense or
hypointense signal of the granulation tissue surrounded by low-signalintensity rim (fibrous septa). (c,d)
Proton density coronal images (TR
2420, TE 42): (c) magnified view
showing the ‘‘dot-in-circle’’ sign
(arrows) and (d) showing a small
separate focus of involvement in the
medial aspect of the foot (arrow).
Patient 2
A 57-year-old male industrial manager from North
East India presented with swelling of the left foot of 5
years’ duration, which had been operated on twice with
recurrence. No history of pain or discharging sinuses
was present. The patient underwent MRI (Figure 2a,b)
which showed a lesion in the subcutaneous plane of the
medial aspect of the foot, which also infiltrated the
adjacent muscles. In view of the similarity of the MRI
appearance to the previous case, mycetoma was con-
sidered. The patient underwent an excision biopsy.
Histopathology showed eumycetoma (Figure 2c,d).
Patient 3
A 38-year-old housewife from West Bengal presented
with swelling of the left foot for the past 10–15 years,
with watery and blackish granule discharge. This had
been operated on twice, with a previous biopsy
performed elsewhere revealing fungal aetiology. On
Figure 2. A 57-year-old man (patient
2) with a recurrent soft-tissue swelling
of the left foot. (a) MRI T2 weighted
fat suppressed (repetition time (TR)
5177, echo time (TE) 70) and (b)
proton density coronal (TR 858, TE
42) images. Both show conglomerates
of discrete small round hyperintense
lesions with peripheral low-signalintensity rim and central hypointense
dot in the medial aspect of the foot
(arrow), also infiltrating the adjacent
muscles. (c) Haematoxylin and eosin
(H&E) 6 50. Fungal organisms surrounded by granulation tissue. (d)
H&E 6 200. Colonies of fungal organisms rimmed by neutrophils and giant
cells.
The British Journal of Radiology, August 2009
663
R S Cherian, M Betty, M T Manipadam et al
(a)
(b)
(c)
Figure 3. A 38-year-old woman with swelling of the left foot with two discharging sinuses. (a) Plain radiograph showing
changes of chronic osteomyelitis in the navicular bone, calcaneus and third metatarsal (arrows). (b) MRI proton density sagittal
image (repetition time (TR) 3000, echo time (TE) 28) showing the ‘‘dot-in-circle’’ sign within the navicular bone (arrow) and
calcaneus (curved arrow). (c) T2 weighted fat suppressed sagittal image (TR 5900, TE 28) showing the ‘‘dot-in-circle’’ sign within
the navicular bone.
examination, there was a diffuse swelling on the plantar
and dorsal aspects of the left foot, with two discharging
sinuses. Plain radiograph (Figure 3a) showed chronic
osteomyelitis in the calcaneus, navicular bone and the
third metatarsal. MRI (Figure 3b,c) showed extensive
altered signal in the navicular bone, calcaneus and talus,
with the ‘dot-in-circle’ sign seen within the navicular
bone, calcaneus and several soft tissue foci. A sinus tract
was seen within the calcaneus. The patient underwent a
biopsy from a soft tissue lesion, and histopathology
demonstrated eumycetoma.
All three patients were started on long-term itraconazole and followed up. The first patient returned after 1
year with a small nodule at the post-operative site. MRI
revealed this to be scar tissue with no soft tissue lesion
present to suggest recurrence.
Discussion
Mycetomas are mainly, but not exclusively, found in
the dry topics where there is low annual rainfall. They
were first described in the Madura district of Southern
India [4]. The disease usually involves an extremity
(most commonly the foot, lower leg or hand) and
occasionally the head or back. Infection occurs by direct
implantation of organisms that are normal inhabitants of
the soil secondary to a penetrating injury, such as a thorn
prick [2].
Mycetoma is characterised by the formation of aggregates of the organism, known as ‘‘grains’’, which are
found within abscesses surrounded by abundant granulation tissue [2]. This could account for the appearance we
observed on MRI of conglomerates of small (2–5 mm)
round hyperintense lesions, representing the granulation
tissue, surrounded by low-signal-intensity rim, representing intervening fibrous septa. The central low-signalintensity dot is the result of susceptibility effect caused by
the presence of fungal grains. This is a unique appearance,
easily recognisable once known, that appears to be highly
suggestive of mycetoma. This has been very rarely
described in the literature. It was first described by
Sarris et al [3] in two cases of mycetoma involving the soft
tissues of the foot; they called the appearance the ‘‘dot-incircle’’ sign. They also postulated that this appearance
reflects the unique pathological features of mycetoma.
664
Kumar et al [5] described a similar appearance in a case of
mycetoma of the thigh. Histologically, our cases also
showed fungal grains surrounded by acute and chronic
inflammatory cells including a few giant cells.
Czechowski et al [4] had described findings of small
lesions of T1 and T2 low signal intensity caused by the
susceptibility effect of the fungal grains. The appearances
we observed of conglomerate hyperintense areas separated by fibrous septa, together with a central dot, were
not described in their article.
In the absence of typical clinical features of discharging sinuses, mycetoma foot may clinically mimic a
neoplasm or a chronic bacterial or tuberculous infection
[2]. MRI findings that are specific to the diagnosis of this
condition would aid in the early diagnosis of this chronic
infection and differentiate it from such conditions. MRI
findings of tuberculous infection include synovial thickening around the tendons and synovial fluid collections
that may contain small low-signal-intensity foci in the
tendon sheath or bursa (rice bodies) caused by caseous
material or debris [6, 7]. These features help to
differentiate tuberculous infection from other infections.
In mycetoma involving the soft tissues of the foot,
antimicrobial therapy is sometimes curative. When there
is bone involvement, non-surgical cure is unlikely and
partial resection or amputation may be required [1]. Early
diagnosis of this disease and bone involvement is therefore essential for appropriate management. In India, the
disease is still endemic and early recognition is important.
The first two patients, despite repeated surgery elsewhere,
were not diagnosed as having fungal infection. The
variability in the quality of laboratory facilities could also
have contributed to the delay in diagnosis.
A limitation of this study is the small number of cases.
Nevertheless, awareness of this characteristic sign is
important in view of the scarcity of reports that describe
it in the literature and with increasing use of MRI in the
evaluation of soft tissue tumours. The dot-in-circle
appearance, first described by Sarris et al [3], appears
to represent a characteristic MRI feature in the diagnosis
of mycetoma.
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