Head Peter
M. Som,
Gary
D.
MD
#{149} William
Fullerton,
P. Dillon, Lidov,
#{149} Mika
PhD
Hypointense Differential and Relation
terms:
Magnetic
parative studies tissue characterization,
nuses,
23.25,
es, fungus, ies,
23.205
1990;
the
sinuses, Paranasal
si-
diseases, sinus-
#{149}
#{149} Paranasal
Sinusitis,
Radiology From
Paranasal 23.492
(MR),
#{149} Paranasal
#{149}
#{149}
(MR), corn-
resonance
23.1214
23.3811,
23.1214
)
Magnetic
CT, 23.121 1
23.205,
resonance
#{149}
sinuses,
MR
San
Francisco
of Radiology
(W.P.D.);
Department School
of Radiology, University of Pittsburgh of Medicine, Pittsburgh (H.D.C.); and ment of Radiology, University of Texas Science Center, San Antonio (G.D.F.). March 2, 1990; revision requested May sion
dress ,c
received reprint
RSNA,
May 15; accepted requests to P.M.S.
1990
M
MD
Radiology
#{149}
that and
AGNETIC resonance (MR) imaging can provide information reflects the actual composition structure of the tissues being
studied, features that can in some cases allow distinction to be made between substances of similar computed tomographic (CT) attentuation, as with some sinonasal tumors and inflammatory secretions (1). It might seem logical to assume that the plethora of information made available on
MR imaging confident be made
May
DepartHealth
Received 8; revi-
16. Ad-
studies
imaging between
by distinct
lesions.
would
allow
differentiation most pathological-
However,
this
a to
is
clearly not the case in all instances. For example, the fairly common MR appearance of an intrasinus region of low signal intensity on signal void surrounded by tissue that has bow signal intensity on Ti-weighted sequences, intermediate signal intensity on proton-density-weighted sequences, and high signal intensity on T2-weighted sequences is one that is shared by a variety of lesions. The purpose of this article is to examine the entities with this common MR appearance, review the current explanations of the causes of these aneas of low signal void, discuss the
intensity diagnostic
on signal refine-
ments and problems that are by MR imaging, and examine role of CT in these cases.
PATIENTS
(P.M.S., ML.) and Otolaryngology (P.M.S.), Mount Sinai Medical Center, City University of New York, 1 Gustave Levy P1, New York, NY 10029; Department of Radiology, University of California,
D. Curtin,
created the
stud-
23.25
176:777-781
Departments
#{149} Hugh
Neck
Paranasal Sinus Foci: Diagnosis with MR Imaging to CT Findings’
Despite the plethora of information provided by magnetic resonance (MR) imaging that allows differentiatlon of some substances that are indistinguishable at computed tomography (CT), there are diagnostic problems. In particular, there are several quite disparate substances that all appear as either low signal intensity or signal void on Tiweighted images and even lower signal intensity or signal void on T2-weighted images. These substances include air, desiccated secretions, mycetomas, acute hemorrhage, calcium, bone, and enamel. When they are surrounded by material that has long Ti and T2 relaxation times, a not uncommon MR appearance in the sinonasal cavities, they may be impossible to differentiate from one another. The current explanations for the low signal intensities are presented, the similarities in the MR appearance are illustrated, and the use of CT to resolve diagnostic problems is discussed. CT appears to be the best modality for initially examining patients with suspected routine inflammatory disease or fungal infection. Index
MD MD
and
AND
METHODS
The MR file material from three hospitals was reviewed to retrospectively bocate studies of patients who had surgicalby proved lesions with the MR appearance already described. Forty cases were found, all of which also had CT scans. The MR images were obtained on a Gyrex S5000 (Elscint, Boston) 0.5-T unit, a Signa (GE Medical Systems, Milwaukee) 1.5-T unit, or a Gyroscan (Philips Medical Systems North America, Shelton, Conn) 0.5-T unit. The images were all obtained as multisection, spin-echo proton-density-weighted, T2weighted, or Ti-weighted sequences. The
Ti-weighted images were obtained with repetition times (TRs) of 200-600 msec and echo times (TEa) of 20-50 msec. The proton-density-weighted images and T2weighted images had TRs of 1,500-2,500 msec, with first-echo TEs of 20-50 msec and second-echo TEa of 80-120 msec. MR images were 5-mm-thick sections with a i-mm intersection gap. The acquisition matrix varied from 256 X 128 to 220 X 220 to 256 X 256. Axial and coronal images were obtained in all patients, and sagittal images were obtained in most patients. In randomly selected cases, gadopentetate dimeglumine (Magnevist; Benlex Laboratories, Wayne, NJ) was used. The CT scans were obtained on GE 9800 units (GE Medical Systems) as axial and coronal studies with 5-mm contiguous scans. Studies obtained both before and after administration of contrast material were available in most cases.
RESULTS The diagnoses in the 40 cases in which the MR images had the appeanance of an intnasinus central area of low signal intensity or signal void surrounded by a zone that was characterized by prolonged Ti or T2 relaxation times were as follows: acute or subacute sinusitis with some remaining sinus aeration (n 12), aspengibbosis with an inflammatory nesponse in the mucosa surrounding a mycetoma (n 9), desiccated secretions on chronic polyps with an inflammatory reaction in the surrounding sinus mucosa (n 12), acute (less than 24 hours) intrasinus hemornhage with mucosal inflammation (n 3), and dentigenous cysts (n 4). In seven cases of chronic secretions, three mycetomas, and two cases of acute hemorrhage, central regions of low signal intensity on Ti-weighted images became progressively lower in signal intensity on proton-density- and T2-weighted images, often appearing as areas of signal void on Abbreviations: tion
TE
echo time,
TR
repeti-
time.
777
T2-weighted studies. In the remaining cases, including those with some residual sinus aeration and those with dentigerous cysts, there were localized areas of signal void on all imaging sequences. The CT and MR images presented in Figures 1-6 are representative studies of all these cases.
Patients with intnasinus hemornhage of oven 48 hours duration had a central area of high to intermediate signal intensity on all imaging sequences surrounded by a zone of matenial characterized by prolonged Ti and T2 relaxation times, which represented mucosal inflammation. Gadopentetate dimeglumine was given to four patients with chronic secretions and two patients with aspergilbosis. The inflamed mucosa showed enhancement in all cases; however, the secretions and mycetomas did not. Gadopentetate dimeglumine administration did not add information that was useful in arriving at a diagnosis. The CT scans of the patients with aspergillomas, desiccated secretions, chronic polyps, and intrasinus hemorrhage all showed moderately high attenuation (60-180 HU) in the intra-
sinus area, with a thin surrounding zone of lower attenuation (iO-20 HU) separating the more central region from the bone. In the cases of sinusitis, the remaining sinus air was cleanly identified as an area of zero relative attenuation, while the patients with dentigerous cysts clearly had a displaced tooth of high attenuation, eccentrically located in the cyst wall with some remodeling of the sinus walls. These findings are summanized in the Table.
ous
A focus of hypomntensity or signal void within an inflamed pananasal sinus is a common MR occurrence.
cyst); and (e) air. A review of our current understanding of the causes of the MR signab hypointensity or signal void reveals that for chronic inspissated secretions and dried polyps, either low signal intensity or signal void is observed once the mucous protein concentration is greaten than 35%-40% (2,3). Materials that appear as low signal intensity on Ti-weighted sequences show progressively lower signal intensities on proton-densityand T2-weighted images, often being depicted as signal voids on T2weighted images. In such cases in the present study the secretions were all of a thick, pastelike consistency. Matenials that appeared as signal voids
Most often one might assume that such an area represents a pocket of air, as it usually does on CT scans. However, the MR differential diagnosis of these lesions can involve the intrasinus presence of five types of materials, each of which is surrounded by either an inflammatory mucosal response on cyst fluid. These matenials are (a) chronic secretions (desiccated secretions and polyps); (b) mycetomas; (c) acute hemorrhage; (d) calcium, bone, on enamel (dentiger-
on Ti-weighted studies were signal voids on all imaging sequences, and in this study they were all found at surgery to be dried concretions. The 35%-40% protein concentration represents the range in which all of the free water and some of the bound water have been eliminated from the secretions, allowing direct binding to occur between the macnomolecular proteins (4). This event is reflected grossly by a sudden increase in secretion viscosity. Below
DISCUSSION
1. (a) Axial T2-weighted MR image shows an apparently expanded ethmoid complex, which contains some relatively bow-signal-intensity secretions in the postenor ethmoid cells. The remaining ethmoid cells bilaterally and the sphenoid sinuses all appear to be aerated with mild infiammatory mucosal thickening. (b) Axial non-conFigure
trast-enhanced
CT
scan
taken
the
same
day
as a shows the ethmoid cells and sphenoid sinuses to be totally opacified. There is clear expansion of the ethmoid cells. Each sinus is filled with an area of increased attenuation, which for the most part is separated from the sinus wall by a zone of lower attenuation. These changes represent chronic dried secretions and polyps.
a.
a.
b.
Figure
2.
Ti-weighted
(a),
proton-density-weighted
b.
c. (b),
and
T2-weighted
dimeglumine. Mucosa in sphenoid and ethmoid sinuses shows enhancement the middle and posterior right ethmoid cells and the right sphenoid sinus. signal void in C. Diagnosis at surgery was chronic secretions and sinusitis. 778
#{149} Radiology
(c) MR
images
obtained
after
administration
on all images. In a, bow-signal-intensity This material has a lower signal intensity
of gadopentetate
material in b and
is seen in appears as a
September
1990
protein mixtures have T2 relaxation times that are on the order of a few milliseconds and Ti relaxation times that are only slightly longer. Thus, at the present time with most commencially available MR units with spin-
this protein concentration the secretions are strictly liquid in nature. However, above this protein concentration the secretions rapidly progress from a thick paste to a desiccated solid. These semisolid and solid
echo TEs of about 20 msec, secretions with very short T2 values and short Ti values appear as either voids (desiccated concretions) on all sequences or as areas of low signal intensity on Ti-weighted sequences and prognessively lower signal intensity on proton-densityand T2-weighted studies (thick, pastelike samples). Mycetomas either have a thick, cheesy consistency or are solid “stones.”
Structurally,
contain primarily some glycoproteins, lecubar
a. primarily pears
filling
3.
(a) Axial T2-weighted
in the ethmoid uninvolved (arrow). the right sphenoid
MR image
shows
high-signal-intensity
inflamed
mucosa
sinuses and the left sphenoid sinus. The right sphenoid sinus (b) CT scan taken within several days of a shows a soft-tissue sinus (arrow). This was a mycetoma with aspergilbosis.
mycelia
proteins,
and
iron
and
man-
ganese (5,6). The Ti and T2 relaxation times in these cheesy (primarily hypointense signal intensities) on solid (signal void) mycetomas are on the order of a few milliseconds, with the T2 being shorter than the Ti. These relaxation times primarily reflect the lack of hydration and the physical state of the mycetomas. The presence of paramagnetic elements may only further shorten the relaxation times. Thus, for analogous rea-
b.
Figure
the
carbohydrates with other macnomo-
apmass
sons,
the
signal
intensities
are
like
those of the chronic secretions already described. In the case of acute intrasinus hemorrhage, at beast two major factors account for the low signal intensity on Ti-weighted sequences and the even lower signal intensity on T2-weighted sequences. The first is the susceptibility
effect
of deoxyhemogbobin,
which produces local field heterogeneity and thus T2 shortening (7). On Ti-weighted sequences with TEs of about 20 msec, these ultrashort T2 values cause a decrease in signal intensity;
b.
a.
Figure
4.
(a) Axial proton-density-weighted
ty thickened
and
redundant
mucosa
in the
MR image left
nasal
fossa
shows and
intermediate-signal-intensimaxillary
sinus.
There
thus,
the
Ti-weighted
images
have some T2 modulation. The second factor results from the formation of a fibnin clot, which effectively squeezes the serum (water) from the remaining protein complex (8). Thus, the clot represents a poorly hydrated, semisolid, macnomoleculan protein mixture. As with the chronic protein secretions, the very short T2 results
ap-
pears to be residual air in the left antrum (arrow). On T2-weighted images, this mucosa displayed high signal intensity. (b) Coronal CT scan taken 1 day after a shows high-attenuation material filling the left nasal fossa and left maxillary sinus with lower-attenuation material more peripherally in the sinuses. No air is seen in these areas. Diagnosis was aspergilbosis with mycetomas.
in low
signal
intensity
on
Ti-weight-
ed sequences and even lower signal intensity on T2-weighted sequences. In the case of bone, calcium, and enamel, their rigidly fixed and solid
I
Figure
5.
(a) Axial
shows some inflamed mucosa with residual by in the
right
ethmoid
T2-weighted
MR
image
high-signal-intensity sinus aeration pnimanicomplex.
The
sphe-
noid sinuses appear normal. (b) Axial CT scan shows opacification of the entire right ethmoid complex and the right sphenoid sinus. There are higher-attenuation masses with tions.
a.
Volume
b.
176
#{149} Number
3
surrounding Diagnosis
lower-attenuation was aspergiblosis
with
secremy-
cetomas.
Radiology
#{149} 779
b.
a.
c.
nature is reflected in T2 relaxation times that are on the order of a few microseconds. As a result, their signals are not detected on MR images. In the case of air, there simply are too few protons to give a detectable signal; thus, a signal void is observed. The obvious problem pointed out by this study is that materials as disparate as air and tooth enamel, which on CT scans are easily distinguished, on MR images appear similar not only to each other but also to chronic secretions, mycetomas, and acute hemorrhage in some cases. Especially
in the
case
of chronic
se-
cretions and mycetomas, in which signal voids can be observed on all MR sequences, confusion with air is a very real problem. Such a totally opacified sinus can be misinterpreted on MR images as having some residuab sinus aeration with only a small amount of inflammatory mucosal thickening (3). This situation leads to misdiagnosis. Because fresh hemorrhage usually has a low-intensity signab
on
Ti-weighted
sequences,
con-
fusion with air is somewhat less likeby, but confusion with some chronic secretions and mycetomas can occur. In the near future there will probably
be clinical
MR
units
that
can
short
T2 relaxation
times
should
give more detectable signals, allowing them to be more easily distinguished from air, bone, and enamel. However, until that time, the diagnostic dilemma gist, particularly
ages, may
where appear
exists for the on T2-weighted
all of these as signal
radioboim-
substances voids.
Fortunately, chronic secretions, mycetomas, and hemorrhage all display increased attenuation on CT scans and can be clearly differentiat780
#{149} Radiology
e. 6.
ances.
Proton-density-weighted
MR
(a) Intermediate-signal-intensity
images
mucosa
of
five
patients
in left maxillary
with
similar
sinus
and
MR
nasal
appear-
cavity.
Mu-
cosa displayed high signal intensity on T2-weighted sequence. Diagnosis was sinusitis with some residual sinus aeration. Note similarity to Figure 4a. (b) Mucosal thickening in the left antrum and minimal mucosal thickening in the right antrum. Mucosa displays intermediate signal intensity, which became high signal intensity on T2-weighted images. There is resid-
ual air in each sinus (arrows). Note similarity to c. (c) Mucosal thickening in the right antrum displays intermediate signal intensity, which became high signal intensity on T2weighted images. Within the sinus is an area of low signal intensity (arrow), which became lower in intensity on the T2-weighted images. Diagnosis was aspergillosis with mycetoma. (d) Slightly expanded right maxillary sinus. The sinus is filled with intermediate-signal-intensity material, which displayed high signal intensity on the T2-weighted images. Penipherally there is an area of signal void (arrow), which remained a signal void on the T2weighted studies. This was a tooth in a dentigerous cyst. (e) Low-signal-intensity material in the left antrum. This material appeared as even lower signal intensity on the T2-weighted studies. The periphery of both sinuses and the “fluid” within the right sinus had high signal intensity on both sequences. Diagnosis was old mucosal hemorrhage bilaterally with fresh intrasinus hemorrhage in the left antrum.
pro-
vide spin-echo TEs on the order of a few milliseconds. At that time, substances like dried secretions, mycetomas, and acute hemorrhage that have very
d. Figure
ed from air and dentigerous cysts. All the cases of dentigerous cyst that involved the maxillary sinus had associated expansion of the sinus cavity secondary to remodeling of the sinus walls by pressure from the cyst. The tooth was also eccentric, lying in the cyst
wall.
tensity
Since or signal
the
bow
void
MR
signal
of chronic
inse-
cretions, mycetomas, air, on acute hemorrhage is more centrally placed within the sinus, confusion with a dentigerous cyst should occur infrequently.
Overall, for
the
it appears low
signal
that intensity
the
reasons on
Ti-
weighted intensity
sequences, on
on the presence are seen with cetomas, and hemorrhage They reflect of hydration
the
T2-weighted
bower
signal
sequences,
of signal voids that chronic secretions, myeven acute, clotted are somewhat similar. varying degrees of lack and the semisolid on sol-
id physical state of the gardless of the mechanism
material, that
nepro-
duces the loss of free water on the precise order of events that produces the firm physical structure. These are, in a real sense, nonspecific protein
and
Although
hydration
no case
effects.
of intrasinus September
fi1990
of CT and
Summary
MR Findings
In 40 Cases MR Signal
No. of Cases
Lesion Chronic
Pastelike
secretions consistency
7
.
.
. .,,
.
.
Proton-Density-
Ti-weighted
CT Appearance of increased
Central area attenuation
weighted
Low
separated
from
Lower
bone
by zone of lower-attenuation Desiccated,
rocklike
5
Intensity
.
material Central area
of increased
attenuation
separated
by zone material
Signal from
void
than
T2.weighted
on
Lower
than
on
Ti-weighted
density-weighted
sequence
sequence
Signal
void
Lower
than
proton-
or signal
Signal
void
Lower
than
void
bone
of lower-attenuation
Mycetomas
Cheesy
consistency
3
Central
area
Low
of increased
attenuation separated from bone by zone of lower-attenuation Desiccated,
6
rocklike
on
Ti-weighted sequence
material Central area
of increased attenuation separated from bone by zone of bower-attenuation
Signal
void
Signal
void
Lower
than
on
proton-
density-weighted sequence or signal Signal
void
Lower
than
void
material
Hemorrhage
Acute
3
(24 h) Air remaining in inflamed sinus
12
attenuation
Tooth
in dentigerous
4
cyst
with
ily
identified
and
distinguished
with
CT. MR imaging may play a secondary role in the case of a sinus hemorrhage of greater than 48 hours duration; while the CT appearance will be the same as that of chronic secre-
176
#{149} Number
3
High Signal
void
High Signal
void
Signal
void
Signal
void
Signal
void
material
tions,
polyps,
and
a mycetoma,
5.
the
tensity
or signal
void
for
the
other
Undoubtedly, shorten TEs
6.
le-
with the approach of the continued devel-
and
opment of software, the role of MR imaging will expand in these patients. However, at present, CT appears to be a more reliable first examination, with MR imaging being used to help
differentiate
book-alikes.
some
of the
7.
940-942.
Fungal
sinusitis:
Lawson matory Som
MD,
1988;
PM,
merman
RA,
WP,
Dillon
WP,
Rajagopalan
4.
on PM,
WG,
sinonasal
C, 10.
In:
Ti and T2 172:515-520.
shorten-
1988;
GD.
in chronically
Stark
resonance
11.
Z. secre-
Physiologic
Magnetic
Mosby,
Zim-
sinonasal
secretions.
relaxation.
eds.
GD,
Fullerton
MR signal
1990; 174:73-78. Fullerton GD. netic
inflamMR.
B, Marom
1989;
Som
pointense
Sasaki
and with
Fullerton
obstructed
tions: observations ing. Radiology
sated
HF,
167:803-808.
Dillon
Chronically
3.
Biller
W. Sinonasal tumors tissues: differentiation
Radiology 2.
RI.
12.
Hy-
inspis-
DD,
of mag-
with
CT
and
Scientific Society
Chicago,
November
Hayman
LA,
of clot
AM,
and soft
of the
images
Martel
of blood: W,
M,
masses:
Efon
an
Braunstein
in vitro EM,
of primary
tumors.
Soft-tissue
retraction
10:1155-1158.
CT evaluation
756. Sundaram
1, 1989.
JJ, et al.
Ford
and
ap-
America,
26-December KH,
1989;
tissue
MR
Exhibit-
Assembly
formation MR
the
of North
Taber
AJNR
Aisen
of
hemorrhage.
AJR
McGuire
1986;
MH, histologic
et al.
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F. for
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Som
PM,
Shugar
M,
Stollman and
evaluation with 1 987; 162:499-503. JMA,
AL.
Use
computed
Troy
KM.
Sacher
of magnetic
reso-
tomography
management
of a patient
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with
MR
in the intrasinus
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Bradley
imaging.
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J, et al.
Malat
modeling
75th
nance
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basis
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Shapiro
DW,
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Kennedy
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MR imaging. Radiology 1988; 169:439444. Janick PA, Hackney DB, Asakura T, Gross-
fect
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U
Amos
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HP,
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man
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Willett
Zinsser
Zinreich
MRI 1.
WK,
eds.
Norwalk,
(11,12).
sions
Joklik
CM,
Ti- and T2-weighted MR sequences will exhibit high signal intensity for the hemorrhage and bow signal in-
Louis:
Volume
void
soft-tissue-
attenuation
brosis or calcified sinolith was in our case material, these entities could be part of the differential diagnosis discussed in this article. Sinobiths are rare and can occur with little adjacent mucosab reaction. In theory, a sinolith could be completely overlooked on an MR study. A localized area of intrasinus fibrosis is uncommon and occurs primarily in the postoperative patient. Such scars have been reported to have bow signal intensity on all imaging sequences (9,10). Because of the problems we have mentioned, one could make a good argument for the use of CT as the mitial diagnostic study on patients who are clinically suspected of having routine chronic inflammatory disease or a fungal infection. With this approach, a more accurate assessment of the degree of sinus involvement can be made. In addition, a dentigerous cyst and residual sinus air can be eas-
High Signal
2) or signal
=
1)
=
(mucosa)
Area of no attenuation (tooth) along margin of expanded sinus filled
(,z
(n
void
Same as acute hemorrhage Area of no attenuation (air) surrounded by area of soft-tissue
on proton-
Ti-weighted
St
36-55.
Radiology
#{149} 781