Sonografi Melalui Fossa Post

8/18/2019 Sonografi Melalui Fossa Post

1/5

F aye C .

Laing ,

M D

{149}Ma ry

C . F ra tes, M D

{149}

ou g las L . B row n , M D

{149} aro l B .

B en son , M D

D on a ld N . D i S a lv o , M D

{149}

e ter M . D ou b ilet , M D , P h D

S on og ra p h y o fth e F eta l P o ster io r F ossa :

F a lse A p p ea ra n ce of M ega -C istern a M a gn a

and

D an d y -W alk er V ar ian t’

24 7

O b ste tr ic a l U ltra so u n d

PURPOSE :

T o esta b lish th at in cor -

rec t scan n in g of th e fe ta l p oster ior

fossa m ay fa lse ly crea te th e ap p ear-

an ce o f a m ega -c istern a m agn a

(M C M ) or D an d y -W alk er var ian t

(DWV ) .

M ATER IA LS AND

METHODS :

Afte r

rou t ine

sonography ,

10 0

consecutive

fe tuses (gesta tio na l age

range ,

25-40

w eek s) w ithou t

ab n orm ality u n d er-

w ent ad d ition a l p o ster ior fossa scan -

n in g in an attem p t to create th e ap -

p earan ce o f an M CM (an terop o ster ior

d iam e te r large r th an 10 mm ) or DW V .

So n ogra m s w ere ob ta in ed in a p lan e

inferior

to or a n gled m ore co ro n ally

th a n th e rou tin e ax ia l p lan e .

RESULTS: T h e p o ster ior fossa w as

d ep ic ted in 93 fe tuses . P seu d o -M C M

occu rred in 35

(38 ) o f 9 3 fe tu ses: 12

(30 ) o f 40 fe tu ses ag ed 25 .0 -29 .9

w eek s, 11 (50 ) o f 22 fetu ses aged

30 .0 -34 .9 w eek s, and 1 2 (3 9% ) o f 31

fe tuses ag ed 35 .0 -40 .0 w eek s. P seu do-

DW V

occu rred in 40 (43 ) o f 93 fe -

tu se s : 20 (50% ) of 40 fe tuses aged

25 .0 -29 .9

w eek s, 1 2 (5 5 ) o f 22 fe-

tuses

a ged 3 0 .0 -3 4 .9 w eek s, an d e igh t

(26 ) o f 3 1 fetu ses aged 35 .0 -40 .0

weeks .

ON LUS I ON S on o grap h y of th e

p oster io r fossa in an an g led sem i-

co ron al p la n e sh ou ld b e a vo id ed b e-

ca u se it m a y create an a p p ea ra n ce

th a t m im ics an ab n orm ality .

Index

te rm s: B rain , ab norm alities,

10 .184 ,

1 5.1 45 2, 1 5.1 45 9

#{149}

e tu s, a bn or ma li ti es ,

85 6.8 744 3, 856 .87 49 #{149}

etus , cen tra l ne rv ous

sys tem , 85 6.8 7443 , 856 .87 49 #{149} e tus, US ,

856 .12981 #{149}U ltraso und (U S), ar tifac t

R ad io logy 19 94 ; 192 :2 47-25 1

I

From th e D epartm en t o f R ad io lo gy ,

Brigham

and W om en’s H osp ita l, 75 F ranc is S t, B oston ,

MA 02115 . F rom the 1993 R SNA sc ien tific as sem -

b ly . R ece ived Janu ary 17 , 19 94 ; rev is ion re -

qu es ted M arch 3 ; rev ision rece ived M arch 14 ;

accep ted M arch 18 . A ddress rep rin t re que sts to

F .C .L .

0

RSNA ,

1994

S ONOGRAPH IC eva lua tion of fe tal

in trac ran ia l con ten ts has p r im ar-

ily em ph as ized the app ea ran ce o f su -

p ra ten to ria l ana tom ic s truc tu re s; the

poste r io r fos sa h as rece ived re la tive ly

le ss atten tion . S evera l recen t a rtic les

d iscuss th e so nog raph ic app ea ran ce

o f fetu se s w ith m an ifes tatio ns o f th e

Dandy-W alker com p lex , w hich is con -

side red a sp ec trum of ab norm a litie s

tha t encom passes the D andy -W alke r

m a lfo rm a tion , th e D andy -W alke r

va rian t (DW V), and m eg a-c is te rna

m agna (M CM ) (1 -4 ). Th ese a rtic le s

p re sen t th e so nog raph ic fin d ing s as-

soc ia ted w ith the .D andy-W alker com -

p lex b u t do n o t em phasize th e p rop er

im ag ing p lan e fo r scann in g the poste -

rio r fos sa , no r d o th ey d iscu ss the p0-

ten tial p itfa lls in d iag nosis o f the

D andy-W alk er com p lex .

W e h av e fou nd th at it is re la tive ly

easy to c rea te th e appea rance o f DW V

and M CM w ith im prop er scann in g

techn iques. Th e pu rpo se o f th is rep ort

is to a le rt so nograp hers and so no lo -

g ists to th is po ten tial sou rce of er ro r.

MATER IALS

AND

METHOD S

The stu dy group co nsisted o f 10 0 con-

secu tiv ely scan ned fetuse s w hose ge sta -

tiona l ag es w ere d ete rm ined to be b e-

tw een 25 an d 40 w eek s on the b as is o f

biom e tric d ata o r prev io us u ltraso un d ex -

aminat ion . No abno rm ality w as de tected

in any o f th ese fetu se s. Th e ex am ina tion

pe rio d w as F eb rua ry 11 th roug h M arch 31 ,

1993 . Ind ication s for sono graph y inc lud ed

size -da te d iscrepancy , ana tom ic su rvey

ex am ina tion , p ossib le in trau te r ine g row th

re ta rda tion , and vag in al b leed in g . Th e ex-

am ina tions w ere pe rfo rm ed by m eans o f

high- reso lu t ion

r ea l- tim e s on og ra ph ic

eq u ipmen t with 3 .5 - o r 5 -M Hz ph ased- o r

curv i l inea r -a r ray t ransducers with select-

ab le focu sing (A cu son , M oun ta in V iew ,

Cal i f ) .

After initial

scann ing by a son ographe r ,

one o f si x fu l l - t ime sta ff sono lo g ists re -

v iew ed the im ages and re scan ned the p a-

tien t. In each case , a tran sab dom ina l scan -

rnng

approach w as u sed an d a specia l

e ffo r t w as m ade to d ep ict th e poste r io r

fos sa an d reco rd a m easu rem en t o f th e

an terop oste rio r (A P ) d im en sion of the c is-

terna m agna (F ig 1 ). Th is m easu rem en t

w as m ade on ax ia l im ages tha t show ed th e

ve rm is ce rebe lli in th e m id lin e,

symme t r i -

ca l w ith resp ect to th e o cc ip ita l ca lvana

an d cerebe lla r h em ispheres (5 ). T he m ea-

su rem en t w as ob ta in ed in th e m id line ,

ex ten d ing from th e inne r m arg in o f the

occip ital ca lva ria to the pos ter io r m arg in

of the verm is cereb effi. T he leve l o f the

brain s tem , basila r ar te ry , fo u rth ven tr icle ,

o r po ste rio r a rachno id re flectio ns w as con -

side red ap propria te fo r t hi s m ea su re m en t.

In ad d ition , an attem pt w as m ade to ob -

tam and reco rd an ax ial im age of the

fou rth ven tric le

(F ig 2 ). S can s w ere then

ob ta ined

an d reco rd ed in a sem ico ron al

p lane th at w as in fe r io r and an g led sligh tly

m ore co ron a l to the rou tin e ax ia l p lane in

an a ftem pt to create the appea ran ce o f an

M CM (A ? d iam e te r, 1 0 .0 m m) (5 ), a

DWV ,

o r bo th . W hen ev er a flu id -filled ,

s litlike space loca ted be tw een and in fe rio r

to th e ce reb ella r h em isph ere s w as fou nd

that superficially

app eared to comm un i-

cate

with

th e fou rth v en trid e bu t w as

p ro ved to b e sep ara te from th e fou rth

ven tr icle with appropriate scanning tech -

n ique, it w as ca lled a pseud o-DW V (Fig 3).

An M CM dep ic ted on an an g led v iew w as

co nside red a p seu do-M CM (F ig 4) .

R E S U L T S

Fo r p re sen ta tion of th e da ta , th e

10 0 fe tuse s w ere subd iv ided in to

th ree g roup s acco rd in g to g esta tion al

ag e: g roup 1 , 25 .0 -29 .9 w eeks

n =

40);

g ro up 2 , 3 0 .0 -3 4 .9 w eeks

n =

24 ); and

g ro up 3 , 3 5 .0 -4 0 .0 w eeks n = 36). T he

re su lts o f eva lua tion of th e po ster io r

fos sa in th ese fe tuse s a re lis ted in the

Tab le.

D ep ic tion of th e p oste rio r fossa w as

techn ica lly su ccessfu l in 93 o f 100 fe -

tu ses. F or each o f the th ree grou ps,

th e resp ectiv e success rate s w ere 40 of

40 fetu se s (10 0% ), 2 2 of 24 fe tu ses

(92% ), an d 31 of 36 fe tuse s (8 6% ). T he

Abb rev iat ion s: # {1 92}Y a nte rop osterio r, CN S =

cen tra l ne rvou s sys tem , DW V = Dandy-Walker

varian t, M CM = m ega-cis terna m agna .


2/5

_._4 { 149}

k

L

Figure 1 . A xia l sonogram show s m easure -

ment o f the AP diam eter (be tw een cursors )

of th e norm a l c istern a m agn a. N ote th at the

verm is ce rebe lli is m id line and t he c er eb el la r

hem isp he res a re sym m etric in app earanc e. In

th is exam p le, the AP diam e ter is 7 .6 mm .

7

..

‘

k

L -H 1 ;

t

- I

F igure 2 A xial son ogram show s th a t the

no rm a l fou rth 4 th ) ven tric le (ar row ) is lo -

ca ted ju st an terio r to the verm is

V) . The up-

per portion o f th e c is te rna m agna

CM )

is

s ee n p os te ri or ly .

}JU

spac e (arrow ) be tw een the ce reb ella r hem i-

sph eres th at sup erf ic ia lly m im ics a DWV .

(b ) S onogram ob ta ined w ith app ropria te

scan angula tion show s a no rm a l p osterio r

fossa in the sam e fetus as in a .

PIF1bL LI

.

H HL

.

‘ ..

. .

.

.1

.

_

--‘

=

11 . i {248}

L B /AJ

-

.

-1

{149}--

8

LB /AJ

cistern a m agna 6 .1 mm in d iam ete r (b etw een cursors ) in th e sam efe tus as in a.

24 8 {149}

ad io log y Ju ly 1 994

in itia l m ean A P d iam ete r o f th e c is-

te rna m agna , m easured in the co n-

ven tion al m ann er , w as 6 .1 mm in

g ro up 1 (rang e, 1 .5 -1 0 .3 mm ), 6 .2 m m

for g ro up

2 (ran ge , 2 .4 -8 .3

mm ), and

8 .6 mm for group 3 (range , 3 .0 -1 1 .0

mm ). T here w as a s ing le fe tus each in

group s 1 and 3 w ith c iste rna m agn a

d iam ete rs la rger than 10 .0 mm . Du r-

in g subsequen t ex am in ation s o f th ese

tw o fetuses, m easurem en ts less than

1 0 .0 mm w ere recorded , and the d i-

am ete rs m easu red a t pos tna ta l c ran ia l

exam ina tion w ere co nside red w ith in

norm al lim its .

Am ong the 93 fetuses in w hom the

pos ter io r fossa w as adequ ate ly im -

aged , the fo urth ven tric le w as v is ib le

in 7 0 fe tuses (75% ). T he su ccess ra te

fo r dep ic tion of th e fou rth v en tric le

w as no t dependen t on the ag e of the

fe tu s , becau se it w as seen in 2 9 of 40

fe tuses in g rou p 1 (7 3% ), 17 of 22 fe-

tu se s in g ro up 2 (7 7% ), an d 24 o f 31

fe tuses in g rou p 3 (7 7% ).

O n th e sem ico rona l an g led v iew s,

the m ean AP d im en sion of the c is-

te rna m agna increased in each gro up

to the fo llow ing : 9 .3 mm in gro up 1

(range , 5 .0 -1 5 .0 mm ), 10 .7 mm in

gro up 2 (rang e, 5 .8 -20 .6 mm ), and

10 .6 mm in g roup 3 (ran ge , 4 .0 -19 .0

mm ). A pseudo-M CM (ie , th e c iste rna

m agn a w as large r than 1 0 mm on the

ang led v iew ) w as v isib le in a to ta l o f

35 (38% ) of 93 fe tuses in w hom the

po ste rio r fo ssa w as ev a lu ab le . In

g roup 1 , 12 (30% ) of 40 fe tuses dem -

onstrated th is fin d ing , w h ile in g roup s

2 an d 3 th e respec tive n um bers w ere

11 (5 0% ) o f 22 fe tuse s and 12 (39% ) o f

3 1 fe tu se s.

The sem ico ro na l an g led v iew cre -

ated a pseud o-DW V app earance in 40

of 93 fetuses (4 3% ). F or each o f the

th ree grou ps, the respec tiv e ra tes fo r

c rea ting the varian t w ere 20 of 4 0 fe -

tuses (50% ), 12 of 22 fetuses (55% ),

an d eig h t o f 31 fe tuse s (2 6% ). In tw o

fe tu ses , the sono lo g ist w as un ab le to

dem on stra te c lear ly tha t th e fo u rth

ven tr icle w as sep a ra te from th e flu id -

filled slit be tw een the cerebe lla r h em i-

sph ere s, and the q uestion of a true

DW V w as ra ised (F ig 5 ) . F ind in gs a t

sub sequen t exam ina tio ns and pos tn a-

ta l sonog raph ic eva lua tion in each of

th ese cases w ere in te rp reted as w ith in

norm a l lim its .

D I S C U S S I O N

Deve lopm en ta l poste r io r fos sa ab -

norm alitie s th at can be iden tified w ith

son ograp hy typ ica lly a ffec t the size

and appea rance o f the c iste rna

m agna . W hen the c iste rn a m agna is

too sm all, th e fetu s is a t r isk fo r a C h i-

an II m alfo rm a tion , w h ich is a lm o st

a lw ays assoc ia ted w ith sp ina b if id a

and m ening oce le o r m en ing om yelo -

ce le (6 ). W henever th e c is te rn a m agna

is too la rg e , the fe tus is at risk fo r th e

D andy -W alk er com p lex , w hich en-

com passes the D andy-W alker m alfo r-

m ation , th e DW V , and M CM (7 ).

The ciste rna m agna , the re fo re , is

cruc ial in eva lua tion of the pos te rio r

fos sa , an d its app earanc e can pro v id e

c lu es abou t the s ta tus o f the rh om ben-

ceph a lon and sp in e. M ahony e t a l (5 ) ,

who in itially desc ribed th e tech n ique

fo r scann ing th e feta l c is te rn a m agna ,

suggest th at ax ia l scans be ob ta in ed


3/5

Results of So nog rap h ic Ev alu atio n of the Pos ter io r Fossa in 1 00 F etu se s

Vo lum e 192

{149}

um ber 1

Rad io logy {149}49

Ax ia l

Four th

Sem ico r o n a l

Po ste rio r F ossa

A P D ia me te r

Ventr ic le

AP D iam eter Pseud o-M CM

Pseudo -DWV

Gestat ional

D epic ted of

C istern a M agn a

Vis ib le

of

C isterna M agn a

Vis ib le t

V is ib le

A ge (w k)

Y es N o

(m m )* Y es N o (m m )*

Y es N o

Y es N o

G ro up 1 ,2 5.0 -2 9.9

n = 40 ) 40 (10 0) 0 (0)

6 .1 /1 .5 -10 .3 29 (7 3) 1 1 (27)

9 .3 /5 .0 -1 5 .0 12 (3 0) 28 (7 0) 2 0 (5 0) 2 0 (50)

G ro up 2 ,3 0.0 -3 4.9

n

= 24) 22 (92 ) 2 (8 ) 6 .2 /2 .4 -8 .3

1 7 (77 ) 5 (23) 10 .7 /5 .8 -20 .6

1 1 ( 5 0) 11 (50 ) 12 ( 5 5) 1 0 (4 5)

G ro up 3 , 35 .0-4 0 .0

n

= 36 )

3 1 (86 ) 5 (1 4) 8 .6 /3 .0 -11 .0

24 (77 ) 7 (23) 10 .6 /4 .0 -19 .0 1 2 (39 ) 19 (61)

8 (26 ) 2 3 (74)

A ll fetuses 93 (93) 7 7)

NA 70 (75 ) 2 3 (25)

N A 35 (38 ) 5 8 (62 ) 40 (43 ) 5 3 57)

N ote-N um be rs in pa ren the ses are p ercen tages .

*

M ean/ran ge. NA =

n o t a p pl ic a bl e .

t

P s e u do -MCM is de fin ed as a cisterna m agn a larger th an 10 mm in diam e ter o n the angled

v ie w.

F igu re 5 . Sonog ram of the p os terio r fos sa of

th is fe tus is sug ge stiv e of a DWV . The fo urth

v en tric le w as never cle arly dep ic ted as a

sepa rate ana tom ic struc ture .

w ith “10#{176}o 15 #{17 6}f in clina tion from

the can thom ea ta l line a t th e in ferio r

por tion of the th ird cereb ra l y en -

tric le .” M cL eary e t a l (8 ) su gges t tha t

pos te rio r fossa con ten ts co u ld be op ti-

m ally exam ined by “ro ta ting the

tran sdu cer to a p lan e app rox im ate ly

30#{176}rom R eid ’s base line .” U nfortu -

na tely , gu ides th a t p resc ribe spec ific

degrees o f ang u la tion re la tive to the

can thom ea tal line or R e id b ase lin e

m ay be usefu l fo r rad iograp h ic sku ll

exam in a tion bu t a re o f lim ited usefu l-

ness fo r sonograp hers. A na tom ic

landm arks th at have been sugges ted

fo r defin itio n of a s tandard ized leve l

fo r m easu rem en t o f th e c iste rn a

magn a inc lud e the aq ueduc t o f S y l-

v ius , the th ird v en tr ic le , and the cho-

ro id p lex us w ith in th e tem poral horn

of the late ra l v en tr ic le (5 ), s truc tu res

tha t a re each e ith er very sm all o r no t

c lea rly d ef in ed .

D esp ite these d iff icu ltie s and the

lack of a sing le key an atom ic lan d-

m ark to gu ide transducer pos itio n ,

recogn ition of the no rm a l app ea ran ce

of the c is te rna m agna , ce reb ella r

hem isph eres , and v erm is ce rebe lli is

no t d ifficu lt (F ig 1). In our study , th e

pos ter io r fos sa w as su ccessfu lly im -

aged in 93% o f the fe tu ses. T h is is

som ewhat b e tte r than the in itia l expe-

rience of M ahony e t a l (5 ), w ho re -

por ted a 73% success ra te . M ahony e t

a ! (5 ) and F illy e t a l (9 ) conc luded tha t

the ab ility to im age the pos ter io r fos sa

decreases w ith advanc ing ges ta tiona l

age , and in ag reem en t w ith the ir f ind -

ings , w e observ ed a fa ilu re ra te o f 8%

and 14% in fetu se s tha t w ere 30 .0 -

34 .9 and 35 .0 -4 0 .0 w eeks g esta tio na l

a ge , r es pe ct iv el y.

A n enlarged c is te rn a m agna (a c is -

terna m agna w ith an A P diam ete r 10

mm

or la rger) adverse ly a ffec ts feta l

ou tcom e , and lik e la te ra l ven tr icu lo -

m egaly , th is find ing and sh ou ld ale rt

son ograph ers to cr itica lly ana lyze the

scan fo r changes co nsis ten t w ith the

D andy-W alker com plex , as w ell as to

sea rch care fu lly fo r o ther fe ta l abnor-

m alitie s. Iden tifica tion of DW V or

MCM is pa rticu la rly im portan t, b e-

cause

desp ite the fac t th a t the so no-

g raph ic chang es a re typ ica lly less se -

vere than th ose seen in fetu ses w ith

D andy-W alker m alfo rm ation , fe tuses

w ith DW V or M CM are a t a rela tive ly

inc reased risk fo r an eu p lo idy (2 -4 ).

S onograph ic reco gn itio n of classic

Dandy-W alke r m alfo rm a tion is usu -

a lly no t d ifficu lt, because th e f in d ings

a re no t sub tle and con sist o f (a ) ap la -

si a

o r severe hy pop las ia o f the verm is

ce rebe l l i , b) a la rge pos te rio r fo ssa

cys t tha t is d ue to a sev ere ly d ila ted

fourth v en tr ic le , and

c)

inc reased

p oste rio r fossa v o lum e. H ydroceph a-

lus and v ario us o ther ex tra -cen tra l

n ervo us sy stem (CN S ) ab norm a litie s

a re also freq uen tly p re sen t (1 ) . O f

seven cases desc ribed by N yberg e t a l

(1 ), fo u r had m ultip le ex tra -CN S ab-

no rm alitie s (tw o of w hom had abnor-

m al k ary oty pe s) .

In com parison w ith D andy -W alker

m alfo rm ation , ch aracte ristic fea tu res

o f DW V (w hich h as a freq uency ap-

p rox im ate ly tw ice tha t o f D andy-

W alker m alfo rm atio n , acco un ting fo r

33% of pos te rio r fo ssa

malformations)

(10 ) cons ist o f (a ) re la tiv ely m ild hy-

p op lasia o f the verm is ce rebe lli (m os t

o ften invo lv ing its in fe rio r asp ec t),

b) le ss sev ere d ilata tion o f the fou rth

v en tr ic le , and c) a rela tive ly no rm al

po ste rio r fo ssa vo lum e (11). E stro f f e t

a l (4 ) desc ribed th e prena ta l so no-

g raph ic fea tu res o f 1 7 fetuses w ith

DW V and no ted tha t la te ra l v en tr icu -

lom ega ly w as p resen t on ly 24% o f the

tim e , w hile 47% of fe tu ses had ex tra -

CN S abnorm ali ties an d 29% had ab -

no rm al k aryo types .

M CM is th e m o st com m on o f the

th ree co nd itio ns desc rib ed in th e

spectrum of D andy-W alker com plex

and accoun ts fo r 54% of cys tic p oste -

no r fossa m alfo rm atio ns (11 ). Th ese

fe tu ses typ ica lly have the few es t pos-

terio r fo ssa abnorm alitie s, u sua lly lim -

ited to iso la ted en largem en t o f the

c is te rna m agna . T he verm is ce rebe lli

is in tac t, and there is a variab ly en -

larg ed pos te rio r fossa v o lum e (11 ).

In itial sono grap h ic repo rts co nside red

MCM a clin ica lly ben ig n co nd ition (5 ) ,

a ltho ugh a m ore recen t rep ort has

no ted a re la tive ly h igh frequency of

an eu p lo idy (espec ia lly trisom y 18 ) (3 ).

L a te ra l ven tricu la r d ila tation is typ i-

ca lly absen t in fe tu ses w ith an abnor-

m al kary o type (2 ).

I f sonograp hy is inco rrectly per-

fo rm ed , it is re la tive ly easy to crea te

th e app ea ran ce o f an M CM or a DW V

(F igs 3a , 4a ). M ahony e t a l (5 ) w ere

th e f irs t to no te th is po ten tial so u rce

o f error w hen they ob served tha t “the

ap pa ren t size o f the cis terna m agn a

co u ld be spuriou sly en la rged by in -

creas in g the ang les o f transducer

in clina tion .” In our s tu dy , w e m ade a

de libe ra te e ffo rt to c rea te p seudo -

M CM s and pseudo-DW V s and to de-

te rm ine h ow often it w as po ssib le to

crea te a fa lse -po sitiv e resu lt. T he tech-

n iqu e invo lved in ten tiona l scann ing


4/5

; .

-

a. b.

F igure 6 . M R im ag es o bta ined in a 13-day -o ld neona te w ho w as b orn at 37 w eek s ge sta tion al

age . T h e pos te rio r fo ssa is norm al. M R im ag in g w as p erfo rm ed by m eans of a g rad ien t echo

w ith g rad ien t spo i ler p u lse s and the fo llow in g param ete rs: repe tition tim e, 45 m sec; echo

time,

7

m sec; fl ip ang le , 3 5#{176} .a ) S em ico ron al M R im ag e thro ugh the p osterio r fos sa sh ow s cere -

b rosp in al flu id in th e v alle cu la ce reb ell i tha t m im ics th e app earanc e of a DW V . (b ) A n appro -

pria te ly ang led M R im ag e con firm s norm a l p osterio r fos sa w ith a n orm al ap pearan ce . (Im ag es

cour tesy of A . Jam es B a rko vich , M D , D epar tm ent o f R adio log y , U niv ersi ty o f C alifo rn ia San

F ran cisco M edical C enter, San Franc isco .)

show s tha t th e fe ta l v allecu lac ereb ell i can b e

ra the r larg e. Th e an te rio r exp ans ion of the

va llecu la ce rebe lli can m im ic com mun ica tion

b etw een cerebro sp inal flu id and the fo urth

ven tric le (a rrow ).

a. b. C.

25 0

{149}

ad io logy J u ly 1 994

in

a sem icorona l p lane tha t w as in fe-

no r

an d ang led in a m o re corona l

fash ion than is norm ally recom -

m end ed . Th e re su lts (T ab le) in d icate

th at fo r each ag e group , th e m ean A P

diam ete r o f the c iste rna m agna in -

c reased on the an g led scans . In itia lly

norm al m easurem en ts becam e c lea rly

pa tho log ic o n the ang led scans o b-

tam ed in fe tuses in group s 2 and 3 .

In 35% o f cases , a pseudo-M CM

w as created . T h is w as m ost ap paren t

in the 3 0 .0 -34 .9 -w eek age g roup , in

w hich it w as o bserv ed in 50% o f fe -

tuses. A pseudo-DW V w as crea ted in

4 0% of fe tuses . It, too , w as m ost com -

m on in the 3 0 .0 -34 .9 -w eek age group ,

in w h ich it w as seen in 5 5% of fe tuses.

A n M CM and a DW V w ere no t p ro -

duced in any fe tus . T hese varied ap-

pearances m ay b e due to ana tom ic

va ria tio ns o f the poste r io r fo ssa , the

precise ang le an d scann ing p lan e

used to im age the p oste rio r fossa , o r

bo th .

R ev iew o f th e “ab norm al” im ages

revea led th at the s ize , shape , and un-

du latio n o f the poste r io r fo ssa f lu id

co llection s d id resem ble th ose seen in

fe tuses w ith true M CM and DW V .

Com pariso n of the abn orm a l im ages

w ith m agn etic resonan ce (M R ) im ages

ob ta ined w ith a sim ila rly ang led

p lane of sec tio n in neona tes su ggests

tha t th e f lu id -filled , s litlike space lo -

ca ted be tw een the m o st in fe rio r a s-

pec t o f the cerebe lla r hem isph eres is

due to f lu id w ith in the va llecu la ce r-

ebe lli. T h is ana tom ic stru ctu re tends

to ex pand sligh tly in its an te rio r as -

pec t, the reby resem bling the p atho -

lo g ic com m un ica tion be tw een the

c iste rna m agn a and the four th yen-

tric le in fe tuse s w ith a tru e ab norm a l-

ity (F ig 6 ).

S evera l im por tan t po in ts shou ld b e

em phasized to enab le reco gn itio n of

th ese fa lse -p ositiv e find ings and

the reby avo id m isd iagn osis . F irst,

w hen fe tuses a re in co rrec tly scanned ,

th e im age is o rien ted in a sem ico ron al

p lane . T h is o ften resu lts in concurren t

im ag ing of th e p oste rio r fossa to -

g eth e r w ith portion s o f th e up pe r cer-

v ica l sp ine , th ickened soft tissues o f

th e po ste rio r n eck due to a pa rtia l vo l-

um e e ffect, and a rela tive ly ro und

co nfigu ra tion to th e shap e of the head

(F igs 3a , 4 a ). E very effo rt sho u ld b e

m ade to o b ta in scans w ith less trans-

Figure

8 . P os tna tal so nog ram s of the fetus in F ig ure 5 . (a) C oro nal so nog ram thro ugh the an terio r fo n tane lle do es no t cle arly sh ow the cis -

te rna m agna or fou rth v en tric le . (b ) M id lin e son og ram thro ugh the an terio r fon tane lle sh ow s a gro ssly no rm a l p osterio r fossa . Su btle abn or-

m ali ties can b e ov erlook ed on th is v iew . (c) S onogram obta ined w ith a tran scran ial ax ial ap pro ach show s a fo urth ven tric le w ith a n orm al ap -

p earanc e. T he p oste rio r fossa w as cons ide red n orm al o n th e bas is o f sono gram s obta ined w ith th is approach .


5/5Vo lum e 192 {149}umbe r 1 Radio logy #{149}51

duce r angu latio n to dem onstra te no r-

m a l ana tom ic featu re s (F igs 3b , 4 b ).

B ecause o f shad ow ing from the o ssi-

f led occ ip ita l bon e (a p articu la r p ro b -

lem la te in p regn an cy), an ino ppo r-

tu ne p osition of th e feta l sku ll, o r

bo th , how ev er , acq u isitio n o f op tim a l

im ages is som etim es im possib le . R e -

p ea t scans o b ta ined on a d iffe ren t day

m ay som e tim es be m ore succes sfu l.

S econd , eve ry effo rt sho u ld be

m ad e to im age the fou rth v en tric le as

a sep ara te ana tom ic s tru ctu re . I t

sh ou ld b e v isib le on scans o b ta ined in

a m ore su pe rio r ax ial p lane and

sh ou ld no t comm unica te w ith poste -

n o r fossa flu id (F igs 2 , 3b ) . A s o ur re -

su its in d ica te, h ow ev e r, c lea r d ep ic-

tio n of th e fo u rth ven tr ic le is n o t

a lw ay s po ssib le (in th is s tudy , it w as

seen in 70% of fe tuse s in w hom the

p oste rio r fossa w as v isib le) , and the

n orm a l an ter io r exp an sion of the v al-

lecu la ce rebe lli c an m im ic com m un i-

ca tion b e tw een ce reb rosp ina l f lu id

and the fo u rth ven tr icle (F ig 7) .

If th e d iag nosis rem a ins in q uestion

(F ig 5 ), p ostna ta l im ag ing shou ld be

p erfo rm ed . T h e q uestion of w he ther

postna ta l im ag ing sho u ld be pe r-

fo rm ed w ith so nog raph y , M R im ag -

in g , o r com pu ted tom og raph y has n o t

been d efin itiv ely answ ered . M any

au thoritie s w ou ld fav or M R im aging

to op tim ize sub tle ana tom ic ch an ges

in the po ste rio r fos sa. if cran ia l so nog -

rap hy is pe rfo rm ed , im ag ing th ro ugh

the an te rio r fon tan elle m ay cause a

rela tive ly sub tle abn orm a lity to be

ov erloo ked (4 ) (F ig 8a , 8b ). S cans

sh ou ld also be o b ta ined th roug h the

fo ram en m agnum , as w e ll a s w ith a

tran sc ran ia l ax ia l app roach , to op ti-

m a lly d ep ict th e fou rth v en tric le and

the co n ten ts o f th e po ster io r fossa

(1 2 ,13 ) (F ig 8c ) . U

References

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DR , M ack LA , F itzsim -

m onsJ, H ic kok D , M ahony BS . D andy-

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1 98 8; 7 :6 5- 71 .

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A JR 1 98 9;

153:1289-1300 .

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ov ich A J . D etec tion of fe ta l cen tra l n er-

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1989 ; 172:403-408 .

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four th ven tric le an d ad jacen t m enin gea l

structu res.

J N euro rad io l 1982 ; 9 :103 -133 .

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Po ste rio r fossa m alfo rm atio ns. A JN R 1992;

13:691 724

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featu res

of

D andy -W alk er varian t w ith oc-

c ip ita l cep haloce le . JC U 1992 ; 20 :13 9-1 41 .

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R ifk in MD .

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