s1042368008000132.pdf

Neurosurg Clin N Am 19 (2008) 175–205

Imaging of Acoustic Neuromas

Hugh D. Curtin, MD a , * , William L. Hirsch, Jr, MD b

a Massachusetts Eye and Ear Inﬁrmary, Harvard Medical School, Boston, MA, USA

b Radiologic Physicians Associates, Fairmont, WV, USA

Imaging has become a sensitive method of

evaluating patients with possible acoustic neuromas

(ANs). Magnetic resonance imaging (MRI) con-

tinues to evolve but is already considered the pre-

ferred imaging study for the evaluation of a patient

with suspected AN [1–13] . A negative gadolinium-

enhancedMRI examination is considered a reliable

indicator that the patient does not have an AN.

Other imaging techniques remain as useful tools in

certain clinical settings, but many techniques, con-

sidered state of the art less than a decade ago,

have faded into almost complete obsolescence [3] .

Imaging of the eighth cranial nerve sheath

tumors has progressed from plain radiography

to today’s MRI [14] . In this evolution, the evalu-

ation has progressed from attempts to show subtle

ﬁndings that suggested the possibility of a lesion

to actual visualization of the smallest of tumors

deep within the internal auditory canal (IAC) it-

self. Because of the increasing acceptance of

MRI as the imaging procedure of choice, much

of the following discussion deals with MRI. Other

modalities, such as computed tomography (CT),

are discussed where appropriate. Imaging strate-

gies in various clinical situations are discussed.

It should be remembered that either gadoli-

nium-enhanced MRI or contrast-enhanced CT

can demonstrate almost any AN [2] . The usual

clinical situation, however, is that the clinician is

trying to ensure that the patient does not have

an AN, and so the most desirable test is the one

that is the most sensitive. CT may not visualize

the intracanalicular region well, and it is here

that MRI establishes its advantage.

Anatomy and imaging

The bony anatomy is demonstrated in excellent

detail by high-resolution CT when performed with

a bone algorithm. The cortical edges of the IAC

are sharply deﬁned, and intricate internal anat-

omy of the labyrinth is routinely visualized. The

bone algorithm allows limited visualization of the

soft tissues, however, and the contents of the IAC

are not seen.

The soft tissue or standard algorithm gives an

improved visualization of the soft tissues, but the

contrast of the nerves versus the cerebrospinal ﬂuid

(CSF) is still insucient to allow demonstration of

the ﬁne soft tissue elements within the IAC. In

addition, there is still a problem of artifact streak-

ing obscuring the region of the porus acusticus

and the cerebello-pontine angle (CPA) cistern.

MRI, on the other hand, gives excellent soft

tissue visualization but does not show the bony

detail nearly as well as CT. Cortical bone gives

a lack of signal or signal void on MRI. Air also is

seen as a lack of signal on the MRI scan. In

a normal situation, therefore, the observer is not

able to diﬀerentiate the otic capsule from the air-

ﬁlled middle ear. Both appear black. The petrous

apex often contains fat, which is seen as bright

signal on the T 1 -weighted (short TR/TE) image.

Fluid does give some signal on the T 1 -weighted

image. Fluid is not nearly as bright as fat but still

can be seen quite easily, especially when con-

trasted against the signal void of the dense bone

of the inner ear. Thus the perilymph/endolymph

in the labyrinth and the CSF in the IAC can be

seen on the image (see Fig. 1 A).

The article of originally appeared in

Otolaryngologic Clinics of NA: Volume 25, issue 3,

June 1992; p. 553–608.

* Corresponding author. Hugh D. Curtin, MD, 230

Lothrop Street, Pittsburgh, PA 15213.

doi:10.1016/j.nec.2008.02.012

neurosurgery.theclinics.com

176

CURTIN & HIRSCH

Fig. 1. (A)T 1 -weighted (TR 600/TE 20) magnetic resonance image of the temporal bones. There is enough signal from

the soft tissue of the facial nerve (VII) and the ﬂuid in the internal auditory canal (IAC) as well as in the horizontal semi-

circular canal (arrow) to allow visualization contrasted against the mastoid and otic capsule. Note that the ﬂuid and soft

tissue signal is not nearly as bright as the fat (large arrowhead) in the petrous apex. Small arrowheads indicate neural

elements. (B)T 2 -weighted image (TR 2500/TE 90) turns the cerebrospinal ﬂuid and the ﬂuid in the vestibule (V) bright

white. The intermediate signal from the cochlea (C) does not indicate that there is tissue within but rather that there is

partial volume eﬀect. The bright ﬂuid in the cochlea is averaged with the contiguous bone to give an intermediate signal

intensity. Nerve elements in the IAC (arrowheads) are extending from the root exit zone at the pontomedullary junction

(arrow). (C) Extreme T 2 -weighted image (TR 4000/TE 120) shows the bright signal in the cochlea, the vestibules, and

IAC. On the right side a nerve can be seen from the pontomedullary junction to the fundus of the IAC.

On the T 1 -weighted image, the soft tissue and

the CSF have diﬀerent signal intensities. Soft tis-

sues such as the nerves (or brain) have somewhat

greater signal than the CSF. Often the nerves can

be seen crossing the CPA cistern and occasionally

can be followed into the IAC (see Fig. 1 A) [28,48] .

The diﬀerences in signal intensity are not great

enough on a T 1 -weighted image to see the actual

nerves consistently within the canal and to follow

the nerves to their exit points at the lateral aspect

of the canal [15] .

The T 2 -weighted (long TR/TE) image can be

used to increase the signal diﬀerence between the

nerves and the surrounding ﬂuid ( Fig. 1 B, C). A

IMAGING OF ACOUSTIC NEUROMAS

177

heavily T 2 -weighted image makes ﬂuid bright rel-

ative to the soft tissues. Thus the nerves can be

seen more frequently if thin enough slices are ob-

tained. The relative signal intensity from the laby-

rinth also increases, and so the inner ear structure

may also be more conspicuous.

Acoustic neuroma

The appearance of an AN depends on the

internal architecture of the tumor, its site of origin

along the neural pathway, and the size of the

lesion as well as the speciﬁcs of the imaging

procedure being performed. The tumor can have

a somewhat variable internal architecture. The

tumor is made up of various concentrations of

Antoni A and Antoni B type histologic patterns.

Both types can often be identiﬁed within the same

tumor. This variability of histology along with the

presence of cystic areas and even small hemor-

rhages is thought to account for the wide spec-

trum of appearance of the AN.

An AN is a soft tissue tumor. Although the

lesion can certainly come into contact with the

brain, the role of imaging is usually to try to

contrast the lesion against the CSF. It may be

dicult to appreciate an AN on CT done without

contrast administration because the density dif-

ferences are insucient for consistent visualiza-

tion. On MRI without contrast administration,

the tumor has a diﬀerent appearance than CSF

(Figs. 2 and 3 ). The tumor is brighter than CSF on

aT 1 -weighted image. The appearance on a

T 2 -weighted image is variable. The small intraca-

nalicular tumors have, in our experience, been

consistently darker than CSF on heavily T 2 -

weighted images ( Fig. 4 ).

On MRI without contrast enhancement, small

or even fairly large cystic areas can often be

appreciated within the tumor. These cysts

may be dark or bright on a T 1 -weighted image.

Bright signal may represent small areas of hemor-

rhage, [16] . but an elevated protein content within

a cyst could result in a similar phenomenon

( Fig. 5 ).

Virtually all ANs have an altered blood–

brain barrier [17] . Thus on either MRI or CT,

the tumors enhance after intravenous injection

of a contrast agent that crosses such an

abnormal blood–brain barrier. This phenome-

non is responsible for the now classic appear-

ance of the AN on both CT and MR [4,18]

(Figs. 6–9 ).

Fig. 2. T 1 -weighted magnetic resonance image without

contrast, acoustic neuroma left side. The tumor ﬂattens

against the cerebellar peduncle (arrowheads). A small

area of lower signal intensity represents a cystic area

(arrow). The lesion can be followed into the internal

auditory canal.

The tumor shows as a bright white lesion

protruding from the IAC on CT (see Figs. 6 A

and 7 ). On the MR image, the bright white of

the high signal from the enhancing tumor is seen

on the T 1 -weighted image (see Figs. 8 and 9 ).

There have been scattered reports of nonenhanc-

ing ANs, but if they exist, they are exceedingly

rare. Some authors have recommended that, at

least with CT, the contrast agent be injected 10

to 15 minutes before imaging to allow the contrast

agent time to cross the blood–brain barrier into

the lesion [19] . This has not seemed to be a prob-

lem on MRI.

Frequently the entire tumor enhances uni-

formly. If there are cystic regions within the

lesion, they may not enhance, and the ﬂuid

contained within the cystic areas will have the

same appearance after contrast administration as

before ( Fig. 10 ). These are avascular collections,

so the contrast agent cannot reach the material

within the cavity.

Calciﬁcations are occasionally mentioned as

rarely being present in ANs. They are extremely

uncommon. In fact, if anything more than

minimal calciﬁcation is present, an alternative

diagnosis, such as meningioma, should be con-

sidered. Calciﬁcations, when present, are expected

to be tiny, and they are unlikely to be seen on

178

CURTIN & HIRSCH

Fig. 3. (A)T 1 -weighted magnetic resonance image (unenhanced); large acoustic neuroma is seen ﬁlling the internal au-

ditory canal (IAC) and cerebellopontine angle cistern. The pons and the cerebellar peduncle as well as the fourth ven-

tricle are pushed toward the contralateral side. There is a collection of cerebrospinal ﬂuid (CSF) (arrowhead)

posterolateral to the acoustic neuroma. (B)T 2 -weighted image shows the acoustic neuroma in the cistern protruding

into the IAC. A small amount of CSF is trapped in the lateral aspect of the IAC (arrow), thus the tumor is not completely

ﬁlling the canal. The ﬂuid collection posterolateral to the neuroma (arrowhead) shows the same brightening as the CSF.

MRI, where small calciﬁcations are averaged

together with contiguous soft tissue and

become virtually invisible. CT is much more

likely to show small ﬂecks of calcium if they are

present.

The shape of an AN is determined by its point

of origin and its size. These lesions are thought to

arise at or near the glial–Schwann cell junction.

This junction point is usually found just inside the

IAC, but the actual site is variable enough that

Fig. 4. (A)T 1 -weighted magnetic resonance image (without gadolinium). There is a slight increase in signal on the side of

the acoustic neuroma (arrow). This would be dicult to call positive with certainty. (B)OnaT 2 -weighted image the

cerebrospinal ﬂuid (CSF) is bright white. The tumor now shows as an area of low signal (arrow) contrasted against

the CSF. Compare with the CSF-ﬁlled internal auditory canal on the normal side.

IMAGING OF ACOUSTIC NEUROMAS

179

Fig. 5. Hemorrhage into an acoustic neuroma. This patient had an acoustic neuroma diagnosed previously. The patient

had an acute hearing loss and underwent magnetic resonance imaging. At surgery the patient had a large hemorrhage

into the acoustic neuroma. (A)T 1 -weighted (TR 600/TE 20) image shows an acoustic neuroma on the right side with

apparent ﬂuid/ﬂuid level (arrow). (B)T 1 -weighted image slightly superior to that found in A, again shows ﬂuid level rep-

resenting layering of hemorrhage. Also note the bright signal (arrowhead) in the fat of the petrous apex. (C)T 2 -weighted

image showing even greater contrast between the ﬂuid components of the hemorrhage.

the AN can develop completely within, com-

pletely outside, or partly inside, partly outside

the canal. Most commonly, the lesion arises

just inside the meatus and then grows out into

the CPA cistern. Such a lesion is said to have both

an intracanalicular and an extracanalicular

component.

Intracanalicular tumor

As a tumor enlarges, several things happen

that allow detection. First, the CSF, which usually

is found within the canal, is replaced with soft

tissue. This eﬀect is dicult to see on CT but can

be detected on MRI in many cases. Before the

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