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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 .
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_._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
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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
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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 .
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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
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