Understanding the posttreatment imaging appearance of the internal auditory canal and cerebellopontine angle

Understanding the Posttreatment Imaging Appearance of the Internal Auditory Canal and Cerebellopontine Angle Timothy L. Larson The intent of this article is to become familiar with the post-treatment appearance of the cerebellopontine angle (CPA) and internal auditory canal (IAC), This includes a review of the pertinent pathology, surgical approaches, and post-treatment imaging appearance. A post-treatment imaging algorithm is suggested. © 2003 Elsevier Inc. All rights reserved.

coustic schwannomas (AS) are by far the most frequently encountered IAC/CPA tumors. Meningiomas are second in frequency and may also have an IAC component. Both lesions along with epidermoids account for over 90% of CPA tumors. Over 60 other lesions account for the remaining 10% and include arachnoid cysts, choroid plexus papillomas, hemangioblastomas, lipomas and solitary metastasis. 1 Differentiation of all these lesions is beyond the scope of this article. Lesions which may present in a purely intracanalicular fashion include meningiomas, facial nerve neuromas, lipomas and hemangiomas. In the differential diagnosis it is important to consider non-surgical tumor mimics such as neuritis (CNs VII or VIII), scar/granulation tissue or metastasis (Fig 1). Aneurysms and intraxial lesions extending to the CPA deserve special mention. It is critical to recognize these preoperatively, as the treatment is radically different than for the more commonly encountered lesions. Most AS arise from the vestibular nerves. 2 In reality however the common nerve bundle (combined cochlear and the superior/inferior vestibular nerves) is often involved with tumor or adhesions. 3 This may create difficulty identifying the nerve of origin. Involvement of the common bundle contributes to the difficulty of "hearing preservation surgery. ''4'5'6 While hearing preservation may be desirable (particularly in a only or better hearing ear), it is subservient to the goal of complete tumor removal and facial nerve (CN VII) preservation. Involvement of critical structures by the tumor (aggressive histology or fibrosis) or unusual clinical situations (for example, relief of brainstem compression in a young child with normal hearing) results in incomplete resection of tumors 1-2 percent of the time. 7 There are three primary options available to the clinician once a tumor is diagnosed - observation, radiotherapy and surgery (see the chapter in this

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issue by Huang and Vermuellen). AS will serve as the model tumor for surgical and radiation therapy discussion purposes, being far and away the most common IAC/CPA tumor. SURGICAL APPROACHES TO THE IAC/CPA There are three primary approaches to the IAC/ CPA regions with variations of each described in the literature. The approach used depends primarily on the expertise and training of the operating surgeons more than on the size or location of the tumor. Radiologists should be familiar with local preferences. Technical details regarding the removal of specific tumor types are beyond the scope of this article.

Retrosigmoid (RS) Approach The RS technique is the surgical approach most commonly used for both CPA and IAC lesions. This is a team approach, the neurosurgeon performing the suboccipital craniotomy and cerebellar retraction, the neurootologist removing the tumor. Advantages include no tumor size limitation and good exposure to the lower cranial nerves/CPA. Once access is achieved and the tumor is identified, it's relationship to the brainstem and seventh CN VII is determined. CN VII monitoring greatly mitigates the risk of injury to this nerve? The combination of presentation, imaging appearance and use of intraoperative facial nerve monitoring distinguishes CN VII lesions from other tumors. If the tumor extends to the fundus of the IAC, a modified RS approach is used, the posterior wall of the canal

From the Swedish Hospital and Medical Center, Seattle, WA Address reprint requests to Dr. Timothy L. Larson, Seattle Radiologists, Swedish Hospital and Medical Center, 1229 Madison #900, Seattle, WA 98104. © 2003 Elsevier Inc. All rights reserved, 0887-2171/03/2403-0189530.00/0 doi: l O.lO53/sult.2003.S0887-2171(03)00036-2

Seminars in Ultrasound, CT, and MRI, Vol 24, No 3 (June), 2003: pp 133-146

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Fig 1. Post-operative axial T1 weighted image of the IACs after gadolinium administration demonstrates intense enhancement within the IAC (arrow) extending from the fundus to the CPA. Note also dural enhancement posterior to the fundus. The patient initially presented with right-sided hearing loss and a pre-operative MRI exam (not shown) consistent with an intracanalicular AS. At surgery there was no evidence of tumor; CN VIII appearing thickened and edematous. The post-operative diagnosis was neuritis of CN VIii.

drilled away (Fig 2). If the labyrinth is preserved during drilling, the superior vestibular quadrant remains unexposed, violation of the labyrinth required for complete exposure of this quadrant. 9 If the labyrinth is violated, the posterior semicircular canal (PSCC) is most vulnerable. Reported disad-

Fig 2. Post-operative axial CT image at the level of the IACs in a patient after resection of an AS via the RS approach. Note the craniotomy defect (angled arrow). The posterior wall of the IAC has been drilled away (straight arrow) with partial destruction of the labyrinth, small bone fragments present at the fundus. Labyrinthitis ossificans has nearly obliterated the horizontal semicircular canal, barely visible on this scan (curved arrow).

TIMOTHY L. LARSON

Fig 3. Axial T1 weighted MR image after gadolinium administration at the level of the IACs. The patient has un, dergone resection of an AS on the left via a translabyrinthine approach. Note the wide exposure achievable with this approach, extending nearly to the carotid anteriorly and the sinodural plate posteriorly (straight arrows). This allows visualization virtually to the midline. Fat and fascia fills the surgical defect. The patient was diagnosed as having neurofibromatosis Type 2, a second AN on the right (angled arrow).

vantages of the RS approach include higher incidence of CSF leak/headaches, higher risk of recurrence and bone removal occurring after opening of the dura (potential meningeal irritation). ~°

Translabyrinthine (TL) Approach The TL approach is commonly used if there is very poor hearing on the side of the tumor with no concern for hearing preservation. Advantages reported include the early identification of the facial nerve lateral to the tumor and low incidence of tumor recurrence or CSF leak/headache, a° Following a mastoidectomy, the labyrinth is drilled away and partially or completely sacrificed enroute to the IAC/CPA. The cochlea may be spared or sacrificed (transcochlear approach). If cochlear channel implantation is a future option, attempts are made to spare the cochlear nerve also. This approach is useful for tumors of all sizes, exposure facilitated by extensive drilling of bone lateral to the tumor. This approach achieves wide access to the CPA nearly to the midline, extending as far anteriorly as the carotid, posteriorly to the sinodural plate (Fig 3). After tumor removal, the bone defect is covered with fascia and the defect filled with fat. There is complete hearing loss postoperatively. This approach is usually performed by a

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the transverse crest and facial nerve both obstructing the surgeons view of the inferior one-half. This necessitates either blind dissection or use of a mirror/endoscope to assure complete tumor removal. 1~ Due to limited access to the posterior fossa, the MFA is often reserved for tumors less than 2 cm. in size. The MFA may be combined with the RS approach for large tumors and is also useful for lesions involving the cephalad portion of the petrous apex or geniculum of CN VII. COMPLICATIONS OF SURGERY

Fig 4. Axial CT images of the left temporal bone following resection of an acoustic tumor via a middle cranial fossa approach, in the top image the craniotomy defect is evident (angled arrow). The superior semicircular canal was violated along its anterior margin (straight arrow) resulting in complete hearing loss. Just anterior to this defect a portion of the petrous ridge was resected, this defect filled with low-attenuation fat, In the lower image, the labyrinthine segment of CN VII is visualized (thick arrow). As one can see from these images, the surgeon will encounter CN VII before reaching the tumor. The patient developed a CSF leak through a surgical defect in the posterior wall of the IAC (lower image). The leak was conrolled by obliteration of the mastoid via a TL approach (star).

neurotologist without neurosurgical assistance. Since this approach is less familiar to neurosurgeons, potential problems could arise if neurosurgical assistance is required intraoperatively.

Middle Cranial Fossa (MFA) Approach This is a team approach, the neurosurgeon performing the craniotomy and elevating the middle cranial fossa dura and retracting the temporal lobe. Tumor removal is performed by the neurotologist. As opposed to the RS, this is primarily an extradural approach initially. Advantages reported include better hearing preservation, low incidence of CSF leak/headache, bone removal prior to opening the dura and good exposure to the lateral portion of the IAC (Fig 4). Disadvantages include a higher incidence of facial nerve injury (as the nerve lies directly in the line of approach to the tumor) and occasional memory disturbance or aphasia, l° The fundus of the IAC may be incompletely visualized,

Complications include hemorrhage, stroke, arterial/venous injury, infection/meningitis, brainstem/CN injuries, edema, CSF leak, and death. 7 The frequency of cranial nerve injury has decreased significantly with the availability of real time monitoring. Cranial nerves V and VII are most commonly monitored. Both nerves may be difficult to identify at the time of surgery, either from displacement/effacement, adhesions or involvement by tumor. The desire for nerve preservation may lead to subtotal resection of tumor. Delayed onset paresis/paralysis most likely relates to edema and may be relieved by steroids. Death usually relates to complications from comorbid conditions related to anesthesia/surgery rather than from the surgery itself. Arterial injury most commonly involves the anterior inferior cerebellar artery (AICA). This may lead to the AICA syndrome with cerebellar peduncular infarction (hearing loss, ataxia, vertigo, dysmetria, facial weakness, dysphagia/dysarthria). 12 Venous complications include bleeding, air embolism, dural sinus thrombosis or cerebral edema post-occlusion of a dominant dural sinus system or cerebral vein (Fig 5). Pneumocephalus (with small volumes of air) is common in the immediate post-operative period, tension pneumocephalus rare 13'14 CSF leaks potentially occur with all three approaches. There is an increased incidence with larger tumors, particularly those requiring combined approaches. 15 They are minimized by careful attention to closure of all potential sites of leak. Bone wax or fascia are used to isolate CSF from air cells. 16 The CSF may tracks to the middle ear and then drain to the nose via the eustachian tube (ET). Some advocate occlusion of the ET to prevent this (TL approach only). Leaks often resolve with conservative measures but may require corrective sur-

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Fig 5. Following resection of a right-sided CPA meningioma, the patient became obtunded. A phase contrast MR-venogram demonstrated absence of flow in the superior sagital and right transverse sinuses.

gery. On rare occasions, leaks may reoccur if the fat/fascia used to obliterate the mastoid is resorbed (Fig. 6). With a persistent CSF leak, delayed onset meningitis is the primary concern. Meningeal symptoms in the immediate post-operative period are common and may relate to "local" meningeal irritation. 17 STEREOTACTIC RADIOSURGERY

The goals are arrest of tumor growth with no or limited morbidity. I° The method of radiation ther-

apy has changed over the years. Current methods, particularly sterotactic radiotherapy (SR), allow for concentration of the highest dose in the center of the lesion with relative sparing of the periphery (the portion which may be intertwined with adjacent cranial nerves). These newer methods minimize damage to adjacent normal structures. Longterm results of older methods are moot given the improvement in technique and the new techniques are too new to have produced long-term results. For instance, although there is long-term data

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Fig 6. (A) Axial T1 weighted image of the IACs following contrast administration in a patient after resection of an AS on the left, RS approach. Note the widened appearance to the IAC after resection of the posterior "lip" of the porous acousticus. There is linear dural and labyrinthine enhancement (arrow). The patient subsequently underwent a TL procedure with obliteration of the cavity with fat (large star) because of a persistent CSF leak. (B) Six months later the CSF leak reoccurred. An axial T1 weighted image of the IACs after gadolinium administration demonstrates that the mastoid fat graft has resorbed and has been replaced with fluid (small star).

available for SR per se, newer techniques include delivering ever-lower doses to the periphery of lesions (to minimize injury to adjacent structures). Given the inherent slow growth pattern of AS and the ever changing SR protocols, it may be some time before an accurate comparison of surgery versus SR is meaningful. SR alone is used most commonly with ASs. In the IAC/CPA region, it is this tumor's response to radiation that has been most closely studied. After radiation, the center of the lesion is felt to undergo cell death and fibrosis. The evolution of tissue change in the periphery remains uncertain. 1° Radiographically there is diminished contrast en-

hancement centrally in the first 6-12 months (Fig 7). Areas of "cystic necrosis" may develop. In the first 1-2 years the apparent tumor size may increase, this likely related to edema. Measuring tumor volume (easily obtained by tracing the tumor margins) is a more sensitive method for determining change in tumor size than are bidimensional measurements. If edema causes tumor enlargement, intervention ranging from steroids to ventricular shunting is preferable to early surgical intervention due to the "friability" of tumor in this stage. 2° In light of the above, the following general treatment guidelines have been suggested. SR is

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Fig 7. Axial T1 weighted images of the IAC after contrast administration. The patient presented with hearing loss on the left. (A) Image at the time of diagnosis demonstrated a intensely enhancing lesion of the IAC/CPA consistent with AN (arrow). There is early brainstem compression. (B) 12 months after SR, the lesion has diminished slightly in size. The lesion has developed areas of low T1 signal within from either cyst formation or necrosis (arrow). (C) 24 months after SR the lesion has diminished further in size. There is now CSF apparent between the AN and the brainstem (arrow). The lesion enhances more uniformly than in image B suggesting that scar tissue is replacing the previously cystic/necrotic areas.

preferable for elderly patients and those at an excessive risk for surgery. For tumors greater than 3cm (especially with brainstem compression that would be exacerbated by edema), surgery is indicated. For tumors with CPA component of 2-3cm without brainstem compression, SR has better CN preservation rates than surgery. For intracanalicular lesions, there is lower CN morbidity with surgery and comparable hearing preservation rates. SR in young patients remains controversial because of the concern for radiation-induced tumors.l° Complications relate closely to dose, particularly to the peripheral zone. After SR for AS, facial weakness has been reported in up to 53% of patients. Trigeminal symptoms are less frequent, seen most often in those patients with larger tumors (significant CPA component). Neuropathies of CN V and VII are more common with intracanalicular lesions, is Progressive hearing loss has a delayed onset, typically beginning at least 12 months after treatment, Hirato et a119 reporting hearing preservation rates of 85% at 3 months and 50% at 24 months. Hydrocephalus may be noncommunicating (related to tumor compression on

the brain stem) or communicating. Less common complications in the "modem" era include brainstem/cerebellar, vascular and lower cranial nerve injuries (Fig 8). Occasionally tumors will continue to enlarge in spite of SR and require surgery. It remains controversiai whether prior SR makes surgery more difficult or not. Slattery and Brackman a° report that in patients requiring surgery after SR that morbidity is increased, brainstem implant results are poor and that viable tumor remains. Pollock et al, 21 on the other hand, note that less than 2% of patients will need surgery after SR for AS. They found no clear relationship between prior SR and the difficulty of subsequent surgery. The long-term risk from radiation induced tumors remains a concern, particularly when radiating younger patients. Though the incidence is estimated to be low, the tumors induced are aggressive and usually fatal. 22 It remains to be seen whether improvements in sparing peritumoral tissues from radiation injury will reduce the incidence of secondary tumors. In any event, a high index of suspicion needs to be maintained when following these patients long-term.

POST-TREATMENT APPEARANCE OF IAC/CPA

Fig 8. MRI images obtained in an elderly woman after SR to a CPA meningioma. She developed progressive ataxia and dizziness after treatment. (A) Axial T1 weighted image of the posterior fossa after gadolinium administration. There is a uniformly enhancing meningioma with dural attachment immediately posterior to the IAC (star). There is a curvilinear zone of enhancement (arrows) involving the lateral margin of the cerebellar peduncle. This was interpreted as being radiation injury/necrosis. (B) Axial T2 weighted image at the same level as that in image (A) redemonstrates the meningioma (star). There is a prominent zone of edema surrounding the area of radiation injury that involves the cerebellar peduncle and extends into the central portion of the left cerebellar hemisphere {area between curved arrows). Images 2 years later {not shown) demonstrated resolution of the imaging abnormalities without encephalomalacia. The patient's symptoms however did not resolve.

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At our institution, the "baseline" 6 month examination consists of axial 2-ram T 1 weighted images without and with gadolinium through the posterior fossa. This assumes that the lesion and surgical field were confined to the posterior fossa. An exception would be after epidermoid surgery where diffusion imaging is included. A full examination of the brain is included only if warranted clinically. The pre-contrast images are important for detection of fat or hemorrhage (which may spuriously suggest enhancement if only post-gadolinium images were obtained) (Fig 9). Fat-saturation imaging post-gadolinium has been suggested as an alternative. Unfortunately uneven fat saturation can cause difficulty obtaining optimal images, especially near bone/soft tissue interfaces in and

POST-OPERATIVE IMAGING OF THE IAC

The timing and frequency of post-treatment imaging is variable. It is dictated by the surgical pathology and completeness of resection. For benign disease after complete resection, typically a baseline MRI is performed 6 months postoperatively. If the exam is normal and the patient remains stable clinically, a subsequent MRI is obtained at 18 months. If the scan findings at 18 months are unchanged or improved, then a 3 to 5-year exam post-surgery is obtained. If there are questionable changes on the 18-too. exam, then interval 12-month examinations are obtained until the significance of the finding is clarified. More frequent imaging is necessary if the lesion exhibited aggressive histology/clinical features or if surgery resulted in incomplete resection. Some have advocated post-operative imaging only if the patient becomes symptomatic. If the ear is "dead", these symptoms may indicate brain stem compression, a less than optimal situation.

Fig 9. Images obtained through the IACs following gadolinium administration in a patient after resection of an AS via a middle fossa approach. (A) This coronal image demonstrates the fat graft that was used to close the defect in the roof of the right IAC. The fat extends into the IAC, making differentiation of fat. dural enhancement or residual tumor impossible (curved arrow). There is minor signal distortion from metal along the cephalad margin of the porous acousticus. (B) This axial image demonstrates similar imaging characteristics as in (A). It was obtained before contrast administration. (C) Following image B, this axial T1 weighted image was obtained with the addition of contrast and fat saturation. It is now possible to differentiate minor dural enhancement in the lateral aspect of the IAC (angled arrow) from the fat graft. There was no evidence of residual tumor.

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around the IAC. On subsequent examinations (in uncomplicated cases) only post-gadolinium images are necessary. This postoperative protocol is sufficient for the vast majority of patients. It saves a considerable amount of money during the longterm follow-up of these patients (as opposed to the cost of a full head/IAC examination without and with contrast). It is rare that the tumor histology would favor CT over MRI. The one exception would be with jugulotympanic glomus tumors where both CT and MRI are necessary. With glomus tumors the earliest sign of residual disease may be subtle progressive bone destruction, MR being relatively insensitive to this. Post-operative evaluation of the IAC/CPA by CT alone is less preferable, performed only if there are absolute contraindications to MR scanning. After uncomplicated translabyrinthine surgery, CT or MR imaging reveals a large triangular defect in the mastoid and inner ear, the apex directed towards the fundus of the IAC (Figs 3, 4, 6). The extent of labyrinthine resection is variable with portions of the far anterior or posterior labyrinth often preserved. With MR imaging, the fat graft filling the bulk of the surgical defect is the predominant feature with the medial fascial component (covering the fundus) often not apparent. Early in the post-operative course fluid and blood will be present. If a portion of the labyrinth was preserved, there may be T1 shortening within from blood products. Enhancement following gadolinium administration results from "labyrinthitis" or formation of granulation tissue (Fig 6). The labyrinth may be transgressed with the RS/tVICF techniques also, least commonly with the MCF approach (Figs 2, 4). Post-operative changes from the MFA may be minimal on MR and missed if not made aware of the surgical history. Coronal imaging facilitates visualization of the surgical approach and post surgical changes involving the temporal lobe and the fundus of the IAC. The temporal craniotomy should be apparent. There is often a small volume of fat/fascia covering the portion of bone drilled away for exposure of the tumor/IAC (primarily the "roof" of the IAC). This fat may also extend into the IAC (Fig 9). If the original tumor was confined to the fundus, post-surgical distortion will be minimal. CT will show these bony changes to better advantage (Fig 4) but is insensitive for early de-

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Fig 10. Axial T1 weighted images after gadolinium administration in three different patients following surgery via a RS approach. (A) Normal post-operative appearance, the only evidence of surgery being the surgical defect related to the RS approach (not shown). (B) Satisfactory post-operative appearance following AS removal. The posteriomedial margin of the IAC has been resected (arrow). The porous acousticus appears widened as a result. There is minor dural enhancement along the posterior margin of the fundus with no evidence of tumor. (C) Questionable residual disease following resection of a CPA meningioma that partially extended into the IAC. There is linear dural enhancement in the IAC with more focal enhancement (angled arrow) along the posterior margin of the porous acousticus. Pre-gadolinium images did not demonstrate fat here. The focal enhancement has remained stable for 18 months and continues to be monitored.

tection of tumor recurrence. The IAC/CPA may appear normal. More commonly there is variable thin, linear enhancement related to either dural hyperemia or granulation tissue along the course of the surgical approach. The CPA may appear widened, particularly if the CPA component of the tumor was large. The craniotomy defect after a retrosigmoid approach may not be well seen if only a limited MRI study is performed. The cerebellum often shows some degree of encephalomalacia from infarction/ retraction though it may appear perfectly normal. 23 The CPA will be widened if the original tumor had a significant CPA component (from chronic brainstem compression). If the tumor was small, the CPA will look normal (Fig 10). Rarely the CPA on the side of surgery will be "narrowed", this seen

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Fig 11. Axial T1 weighted image after gadolinium administration. The patient had transection of CN VII during resection of a large AS. Post-surgical changes include the craniotomy defect (curved arrow) and labyrinthine enhancement (fat arrow). The ipsilateral CPA cistern is narrowed (boxed arrow) following CN VII grafting, the brainstem drawn towards the side of surgery.

with nerve reanastamosis, typically the facial nerve (Fig 11). If the posterior wall of the IAC was drilled away, this results in a very wide appearance to the porous acousticus (Figs 2, 10, 11). Tumor extending to the fundus of the IAC requires more extensive drilling, which may involve partial destruction of the posterior semicircular canal. T1 shortening in the labyrinth results from hemorrhage, enhancement following gadolinium administration from labyrinthitis or granulation tissue. With all three procedures, scar, fascia and dura will enhance in a linear or whorled fashion and is easily recognized as post-operative change. 24 The extent of this depends on the size of the original lesion and surgical approach used (Figs 6, 9, 10).

This may extend from the brain stem to the fundus of the IAC or be found anywhere about the periphery of the CPA. The prominence of this enhancing tissue will decrease over time, typically 12-24 months, then remain stable (personal observation). Round, ovoid or globular areas of enhancement need to be viewed with suspicion for residual or recurrent disease (Fig 12). If muscle was included with the fascia, nodular enhancement may result, mimicking residual diseaseY Long-term follow-up imaging will easily clarify the significance of these findings. Uncommon imaging findings include signal distortion from metallic fragments (most commonly fragments from drill bits used in and about the

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Fig 12. Axial T1 weighted image after gadolinium administration. The patient had resection of a CPA hemangioblastoma. CN VII grafting results in a narrowed CPA cistern (angled arrow), the brainstem deviated towards the side of surgery. There is focal enhancement at the fundus of the IAC that has remained stable (fat arrow). The larger focus of enhancement demonstrated progressive enlargement on serial exams and represents residual disease (thinner arrow).

TIMOTHY L. LARSON

temporal bone if irrigation was inadequate) (Fig 13). Air and blood will be present if imaging is obtained acutely (Fig 14). On rare occasion, gauze or pledgetts are present, this most commonly done to "protect" the brainstem from ectatic vessels (essentially a Jenetta procedure) (Fig 15). On occasion tumors are only debulked or subtotally resected. In one report, 17 patients (44%) who underwent incomplete resection demonstrated progression of residual disease. 26 10 of these patients required additional treatment. Incomplete resection or debulking may be performed intentionally in exceptional clinical circumstances. (Fig 16) A common scenario is when complete resection risks injury to the facial nerve or other critical structures. In these "near-total resection" scenarios, tumor (or just the tumor capsule) may be left behind (Fig 17). The residual disease may be partly devitalized by cautery. Finally, there are rare pa-

Fig 13. Axial T1 weighted image after gadolinium administration. There is distortion of signal (area surrounded by arrows), which obscures the left IAC, making early detection of residual disease impossible. The signal distortion was not apparent on the preoperative exam.

POST-TREATMENT APPEARANCE OF IAC/CPA

Fig 14. Axial CT image of the posterior fossa. The patient demonstrated a diminished mental status following removal of a CPA tumor. CT demonstrated hemorrhage (long thin arrow) with mild brainstem compression (surgically confirmed). There is air centrally in the hematoma (shorter arrow) with smaller volumes of air in the basilar cisterns.

tients that have progressive disease after both surgery and radiation, with either recurrent tumor or associated peritumoral cysts. Treatment in these patients is highly individualized (Fig 18). When residual or recurrent disease is discov-

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Fig 15. (A) Axial TI weighted image after gadolinium administration demonstrates an intensely enhancing AS which fills the left IAC (arrow). (B) Axial T1 weighted image without gadolinium. Following surgery, there is a widened appearance to the left CPA cistern and porous acousticus. T1 shortening with increased signal in the cochlea results from hemorrhage. Interposed between the ectatic basilar artery (angled arrow) and the brainstem is a structure (arrows) that demonstrates irregular T1 signal. This is related to a pledget placed here at the time of surgery.

Fig 16. Axial T1 weighted images after gadolinium administration at the level of the IACs, The patient was a normal hearing asymptomatic 12-year-old adopted male being evaluated for absence spells. (A) A large AN fills and expands the right IAC (star). The CPA component broadly effaces and compresses the brainstem. Not shown is a smaller intracanalicular AN on the left consistent with NF Type 2. (B) Because the child was asymptomatic, it was elected to only debulk the lesion to relieve brainstem compression and preserve CN VII. Following partial resection there was resolution of brain stem compression. Tumor remained in the IAC and lateral portion of the right CPA cistern (arrow). Hearing was lost and facial nerve function remained normal.

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Fig 17. Axial T1 weighted image after gadolinium administration through the IACs following resection of an AS with melanotic features. This demonstrates a thick linear area of intense enhancement which parallels the brainstem from the anterior margin of the CPA to the fiocculus. At the time of surgery, tumor was adherent to the brainstem and CN Vii and the entire tumor could not be safely removed.

ered, it is helpful to obtain tumor volumes (obtained by tracing the tumor and calculating the volume). This is most useful in benign disease when a period of observation is desired. On sequential exams the growth rate can then be estimated/calculated. These measurements are more accurate than bidimensional measurements. This is useful for anticipating subsequent surgery or adjuvant therapy. POSTOPERATIVE IMAGING OF THE CPA

Postoperative imaging in the CPA depends on the histology of the original tumor. For AS tumors extending to the CPA, the observations described in that section suffice. For benign CPA lesions

(meningioma, dermoid, hemangioblastoma, etc), exams limited to the posterior fossa (as described for IAC lesions) are adequate. They may be extended slightly to encompass the entire posterior fossa. For epidermoids, the addition of diffusion imaging is key. Malignant lesions may require imaging of the entire neural axis without and with gadolinium. The imaging of these is beyond the scope of this article. Bony changes are dependent on the approach used and usually pose no difficulty in interpretation. For large lesions or for correction of CSF leak after surgery, a combination of approaches may have been used (Fig 4). Correlation with the surgeon or operative report (if performed at an "out-

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side"

i n s t i t u t i o n ) is u s e f u l

in t h e s e

situations.

A t r o p h y or e n c e p h a l o m a l a c i a o f a c e r e b e l l a r h e m i s p h e r e is c o m m o n . T h i s r e l a t e s e i t h e r to i n f a r c t i o n or r e s e c t i o n ( t r a n s c e r e b e l l a r a p p r o a c h ) . F i n e l i n e a r areas o f e n h a n c e m e n t are o f t e n s e e n a l o n g t h e a p p r o a c h r o u t e a n d site o f t h e r e s e c t e d lesion. A s w i t h A S i m a g i n g , f o c a l g l o b u l a r , r o u n d or o v o i d areas o f e n h a n c e m e n t n e e d to b e v i e w e d w i t h suspicion. T h e i m a g i n g i n t e r v a l is m o r e v a r i a b l e than with AS tumors and depends on the tumor histology, completeness of resection and imaging n e e d s r e l a t e d to a d j u v a n t t h e r a p y . SUMMARY Fig 18. Axial T1 weighted images after gadolinium administration through the posterior fossa demonstrates a large cystic structure (cross) that surrounds an enhancing tumor along the anterior margin of the widened porous (star). In 1983 the patient had undergone resection of an AS. Tumor recurrence became apparent 15 years later. The patient underwent SR with no further growth of the tumor on serial exams. In 2002 the patient experienced vague dizziness and a MRI examination done at that time demonstrated brainstem compression from a peritumoral cyst. The tumor itself was unchanged. Symptoms resolved following drainage of the cyst, this performed via a RS approach.

W h e n i n t e r p r e t i n g p o s t o p e r a t i v e cases i n v o l v i n g s u r g e r y in or a r o u n d the I A C / C P A , it is h e l p f u l to k n o w a n d u n d e r s t a n d t h e s u r g i c a l a p p r o a c h e s to these regions. Each surgical approach has a typical p o s t - o p e r a t i v e a p p e a r a n c e . U n d e r s t a n d i n g this facilitates the d i f f e r e n t i a t i o n o f a n o r m a l p o s t - o p e r a t i v e i m a g i n g e x a m i n a t i o n vs. o n e d e m o n s t r a t i n g r e s i d u a l disease. F o r i n d e t e r m i n a t e e x a m i n a t i o n s , a f o l l o w u p i m a g i n g a l g o r i t h m is s u g g e s t e d .

REFERENCES

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