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741
Pictorial
MR Imaging Walther
von Sinner,1
Hydatid of the
in Hydatid
cysts
presence
Lambertus
may develop of a viable
Disease
te Strake,1
anywhere parasite
David
in the body
(Echinococcus
Clark,2
as a result
Echinococcus granulosus is a zoonosis of worldwide tribution. Humans may become infested after ingesting
dispara-
sitic eggs directly, from contact with dogs, or indirectly, from contaminated water or food. Hatched parasitic embryos migrate through the intestinal mucosa and enter venules and lymphatics. The liver filters 60-70% of the embryos and the lungs, 1 5-25%; 1 0-i 5% will reach other organs via the systemic circulation. Embryos that are not destroyed transform into small cysts that will grow 2-3 cm each year. The wall of the parasitic cyst consists of a germinal layer (endocyst) and a laminated proteinaceous membrane (ectocyst). The host reacts against the cyst by forming a dense fibrous
(pericyst),
nutrients
which contains
to the parasite.
The
cyst shows
that the disease
manifestations
depending
blood vessels
natural
history
is a dynamic on the
that provide of the hydatid
entity with different
degree
and
severity
to W. von Sinner. 2 Department of Radiology, AJR 157:741-745,
October
Aiyadh 199i
after revision April 24, 1991. King Faisal Specialist Hospital
Military
Hospital,
0361 -803X/91/1
Riyadh,
574-0741
Hassan
Stages
Sharif2
of Development
of Hydatid Cysts
Saudi
Simple
Viable
Hydatid
Cyst
The wall of the intact viable hydatid cyst consists of layers: (1 ) parasitic membranes consisting of gelatin proteinaceous material and (2) an outer layer consisting dense fibrous capsule from reactive host tissue. In the intact cyst, these layers cannot be separated on images. On Ti -weighted spin-echo (SE) images, the cyst is isointense
relative
to the
fluid
in the
cyst
and
appears
two and of MR wall on
T2-weighted images as a low-intensity rim surrounding the homogeneous high-signal cyst contents (Fig. 1). In the liver, the low-intensity rim is a nonspecific finding and has been
described tions
Hydatid
in a number
of other benign
and malignant
condi-
[1].
Cyst with Daughter
Cysts
On Ti-weighted images, the viable daughter cysts may be isointense or slightly hypointense compared with the contents (“hydatid sand”) of the mother cyst (Fig. 2). Daughter and mother cysts are isointense on T2-weighted images. The development of daughter cysts is an early sign of degeneration of the mother cyst.
of de-
generative changes of the parasite. Effective new drugs are now available to treat hydatid disease medically. We demonstrate the use of MR in diagnosing simple and complicated hydatid cysts and in monitoring the results of chemotherapy.
Received January 22, 1991 ; accepted 1 Department of Radiology, MBC #28,
and
granulosus).
Depending on the condition of the parasite, the host reaction, and therapy, the hydatid cyst will degenerate and may eventually collapse, leaving an area of calcification in the host tissue. The purpose of this pictorial essay is to demonstrate the use of MR imaging in the diagnosis of the various stages of hydatid disease and in monitoring response to therapy.
capsule
Essay
& Research
Hydatid The severe
Centre,
Cyst
Roentgen
Parasitic
Membranes
presence of detached membranes is a sign of more degeneration and rupture of the parasitic membranes
P.O. Box 3354.
Arabia.
© American
with Detached
Ray Society
Aiyadh
1 121 1 . Saudi Arabia.
Address
reprint
requests
VON
742
SINNER
ET AL.
AJR:157,
Fig. 1.-Simple A, Ti-weighted wall is isointense B, T2-weighted
October
1991
viable hydatid cyst of liver. MR image (SE 733/20). Cyst relative to cyst contents. MR image (SE 2000/100).
Cyst
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wall, consisting of both intact parasitic membranes and pericyst, is of low signal intensity compared with high signal
Fig. 2.-Hydatid
of cyst contents.
liver cyst containing daughter cysts.
A, Ti-weighted MR image (SE 640/20) shows multiple daughter cysts containing low-intensity fluid compared with intermediate of mother cyst. B, T2-weighted MR image (SE 2000/100). Contents of mother cyst and daughter cyst have same high signal intensity. C, Coronal Ti-weighted MR image (SE 640/20) in patient with large subdiaphragmatic hydatid cyst containing multiple daughter
[2].
These
membranes
are
floating
seen
appear dark on both Ti - and T2-weighted The pericyst is also dark on T2-weighted to be isointense images.
relative
to normal
within the cyst and images (Fig. 3).
images,
liver tissue
but tends
on Ti -weighted
External
Rupture
Rupture
of a Hydatid
Cyst
[2-4].
In a hepatic
cavity,
leading
cyst, contents
to extensive Hydatid
Disease
will spill into the peritoneal
intraperitoneal
in the process
ofdegeneration
ofthe
hydatid
cyst is the rupture of the pericyst. The cyst contents are no longer contained by the pericyst and spill into the surrounding parenchyma
[2-4].
During
seeding
(Fig. 5).
of the Lung
Cyst
In the lung, the cyst may rupture The next stage
cysts.
External rupture of a cyst occurs when both the wall of the cyst and the capsule of the organ harboring the cyst rupture
Complicated Intrahepatic
of Hydatid
signal of hydatid sand
this process,
bile ducts
and veins
may be eroded, releasing cyst contents into the biliary system or the blood stream. On MR images, rupture of the cyst is easily recognized as an area of discontinuity of the lowintensity cyst wall (Fig. 4).
into the bronchial
tree.
The patient will cough up cyst contents that may be aspirated, causing extensive reactive pneumonitis. MR may show communication between the bronchial tree and the cyst (Fig. 6). End-Stage During
Calcified the
and calcification
Cyst
course
of the
disease,
spontaneous
of the cyst may occur,
leaving
collapse
an area of
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AJR:157,
October
MR
1991
Fig. 3.-Hydatid parasitic membranes
age (SE 2000/100). membranes (arrow)
liver cyst with on T2-weighted
detached MR im-
Both detached and
pericyst
parasitic are
of low
IN HYDATID
DISEASE
743
Fig. 4.-Intrahepatic rupture of liver cyst on MR image (SE 2100/50). Low-intensity pericyst is disrupted, and fluid contents spill into liver parenchyma
Fig. 5.-External rupture of liver cyst. T2weighted MR image (SE 2000/100) shows disruption of low-intensity rim of pericyst (black arrow) with spillage of fluid into perihepatic space (white arrows). (Reprinted with permission from von Sinner
(arrows).
intensity.
[2].)
Fig. 6.-Complicated hydatid cyst of lower lobe of right lung. MR image (SE 950/20) shows lower lobe bronchi in close contact with cyst wall (arrows and arrowheads). Spread of cyst contents into lower lobe caused infiltratesand reactive pneumonitis.
calcification
in the host tissue.
Partial calcifications
wall
are degenerative changes that living and dead parasites. Calcifications
on CT, although of signal dropout
they may be suggested (Fig. 7).
Response to Treatment to Hydatid Disease
of Hepatic
of the cyst
may be seen with both are better appreciated
on MR by an area
Parenchymal
Reaction
Albendazole, in combination with praziquantel, is a new, powerful drug for the treatment of hydatid disease. The regression in size of hydatid cysts and the changes in the host tissue during by MR (Fig. 8).
chemotherapy
Hydatid
Outside
Disease
can be adequately
Spread
of Hydatid
Disease
Across
the Diaphragm
The spread of hydatid disease across the diaphragm is a known complication that is well shown on sagittal MR scans (Fig. 9).
Musculoskeletal
Involvement
of Hydatid
MR imaging is a useful technique of bone involvement and soft-tissue appearance
of hydatid
may mimic
tumors
disease
or other
Disease
in determining the extent extension (Fig. 1 0). The
in bone
inflammatory
is not
specific
and
conditions.
assessed Cardiac
the Lungs and Liver
Hydatid disease may occur anywhere in the body. Some typical examples of cysts at locations other than the liver or the lungs are illustrated (Figs. 9-12).
Cardiac
Hydatid
Disease
hydatid
disease
may be due to hematogenous
spread or rupture of a lung cyst. ECG-gated MR imaging allows accurate assessment of the location of a cardiac cyst and its relation to valves, ventricular septum, and myocardium (Fig. 11).
744
VON
SINNER
ET AL.
AJR:157,
October
1991
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Fig. 7.-End-stage collapsed calcified liver cyst. A, CT shows two calcifications in liver. B, Ti-weighted MR image (SE 733/120) shows only larger calcified cyst as an area of signal dropout (arrows).
Fig. 8.-Monitoring
of response
to albendazole
treatment. A, MR image (SE 2100/100) obtained before treatment with albendazole. Excellent contrast resolution of MR allows delineation of extensive reactive edematous changes in liver surrounding hydatid cyst (arrows). B, MR image (SE 2000/100)obtained after treatment shows decrease of size of hydatid cysts and of surrounding reactive changes (arrows).
Fig. 9.-Subphrenic and supradiaphragmatic spread of hydatid cysts. T2-weighted sagittal MR image (SE 2000/100). Multiplanar imaging capability and excellent contrast resolution of MR allow
differentiation
among
supradiaphragmatic
cyst with surrounding changes, left hemidiaphragm phrenic cyst.
lung
reactive pulmonary (arrow), and sub-
Fig. 10.-Hydatid cyst of bone. Coronal Tiweighted MR image (SE 533/20) shows extensive involvement of lower lumbar spine, sacrum, and right iliacbone with soft-tissue extension.
Hydatid
Disease
of the Brain
Hydatid cysts of the brain are rare, and they may reach a large size before the patient becomes symptomatic. On MR, the diagnosis is easily suggested by the presence of a cyst with a low-intensity rim (Fig. 12).
Discussion When treating hydatid disease, it is important to realize that the hydatid cyst represents a living organism. Like other organisms, it ages, undergoes degeneration, and eventually may die. The host attempts to contain the cyst by forming a
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AJR:157,
October
MR
1991
IN HYDATID
DISEASE
745
B
Fig. ii.-Cardiac
hydatid
disease.
A, CT scan shows calcified intracardiac cyst. B, MR images (SE i800/50) show low-intensity areas (arrowheads) probably corresponding to calcifications seen on CT (A). Precise location of cyst within ventricular septum and its extension into myocardium (arrows) are well shown on MR. C, Ti-weighted MR image (SE 750/30) in a different patient shows a low-intensity mass in right atrium in close contact with tricuspid valve (arrow). D, MR image (SE 1000/50) shows a high-signalintensity mass, suggesting cystic nature of lesion shown in C.
dense fibrous capsule around the parasitic membrane. Surgical removal of the hydatid cyst was until recently the only treatment. However, imidazole compounds combined with praziquantel taken orally have proved to be effective in destroying the germinal layer of the parasite, thus inducing or accelerating the natural process of degeneration and death of the cyst. MR is capable of adequately demonstrating all features of hydatid disease, with the exception of calcifications. MR also is useful in monitoring the response to treatment.
REFERENCES
Fig. 12.-Simple
hydatid cyst of brain. T2-weighted
40) shows low-intensity rim of cyst wall surrounded reactive changes in compressed brain consistent
MR image (SE 2000/
by high-signal-intensity with gliosis (arrows).
1 . Hahn PF, Stark DO, Saini S. et al. The differential diagnosis of ringed hepatic lesions in MR imaging. AJR 1990;154:287-290 2. von Sinner WN. Ultrasound, CT, and MAI of ruptured and disseminated hydatid cysts. Eur J Radiol i990;1 1 :31-37 3. Lewall DB, McCorkell SJ. Rupture of echinococcal cysts: diagnosis, classification, and clinical implications. AJR i986;1 46:391-394 4. Lupetin AR, Dash N. Intrahepatic rupture of hydatid cyst: MR findings. AJR 1988;151 :491 -492