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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

MR imaging in hydatid disease.

Hydatid cysts may develop anywhere in the body as a result of the presence of a viable parasite (Echinococcus granulosus). Depending on the condition ...
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