5.4  Case study – Acoustic neuroma

Acoustic neuroma may be also referred to as vestibular schwannoma.  These are benign tumours that arise from Schwann cells, especially near Scarpa’s ganglion of the superior vestibular nerve.  The tumours frequently arise in the IAMs and grow medially into the cistern.  Expansion of the bony canal is a typical feature of this tumour.  Acoustic neuromas are slow growing and present as a cerebello pontine (CPA) mass at the time of diagnosis.  Bilateral neuromas are a common feature in neurofibromatosis.

MRI has replaced CT and CT gas cysterography previously utilised in the imaging of intracranial Schwannomas.  MRI combined with contrast is the most sensitive modality in the identification and assessment of acoustic neuromas.  MRI with long TR/TE values is also very sensitive with this respect.  Acoustic neuromas typically enhance intensely after the administration of Gd-DTPA on T1-weighted images. 

A minority of acoustic neuromas at the CPA cistern present with a cystic component within or surrounding the tumour.  The shape of the tumour on MR images is totally dependent on its location and is best appreciated with contrast enhancement on coronal and axial sections.  Intra-canalicular tumours are small and round and may follow the entire length of the canal.  When the tumour extends medially beyond the porus acousticus towards the CPA cistern it has a cone shape lying ion the side.  The surrounding structures as the brain stem and the cisternal space may be also distorted if the tumour is large.  A small percentage of these tumours may be associated with cystic changes and the presence of an arachnoid cyst with the CPA.

When acoustic neuromas affect the cochlea, on CT and MRI it is seen as a soft tissue mass.  On CT of the temporal bone, bone erosion and widening of the basal turn of the cochlea are identified.  On MRI the mass enhances homogeneously after contrast injection. 

Therefore, MRI is effective fast and safe in the imaging of acoustic neuromas.  However, CT should be reserved for cases in which visualisation of osseous structures is crucial, such as congenital anomalies, trauma, osteodystrophies, inflammation and tumours that affect the temporal bone (Rao et al, 2001).

5.4.1        Patient

This 38-year old gentleman presented with hearing loss in the left ear.  This was accompanied by tinnitus and pain in the left ear.  This patient also complained of headaches and presented with some problem with balance. 

5.4.2        Material and methods

Refer to section 3.5.2

5.4.3        Protocols and pulse sequence parameters

The sequences taken for this examination started with a PD/T2 FSE sequence of the whole brain. The sequences after were as follows:

Table 5 sequences used in MRI of the IAMs

Pulse sequence	TR	TE	NEX	Matrix	Slice thickness	Slice spacing	FOV
Coronal T1 SE	440	14	2	256 x 224	3.00	0	18 x 18
Axial T1 SE	440	10	2	256 x 224	3.00	0	18 x 18
Coronal T1 FSE + Gd-DTPA	460	22	2	256 x 224	3.00	0	18 x 18
Axial T1 FSE + Gd-DTPA	500	22	2	256 x 224	3.00	0	18 x 18

The protocol for acoustic neuroma of Massachusetts General Hospital includes sagittal T1, axial FLAIR, axial T2, and axial DWI through the whole brain.  They include axial and coronal T1 3mm through IAMs, MRS and post contrast 3mm axial and coronal T1 with fat saturation.  This protocol is very similar to our local protocol.  Our system does not support DWI and MRS. It is becoming   current in our department to scan patients with a suspected lesion on the IAMs using 3D in coherent GRE with and without contrast administration.  These sequences yield images with very high spatial resolution which is very important for the small structures in the IAMs.  Also, after the examination, reformations in the plane of the tumour enable the radiologist to depict the exact origin of the tumour.  A 3D T2 FSE sequence also being performed before the administration of contrast.  This produces images with not only a high spatial. 

Figure 16 a and b:  3mm coronal T1-weighted images precontrast and post contrast respectively; 17 c and d are axial 3mm T1-weighted precontrast and post contrast

      

      

 As can be observed in these images the lesion enhanced homogenously after contrast administration

resolution but also with a high SNR.  3D sequences use contiguous slices and will not suffer of the cross-talk artefacts.  This protocol is also recommended by Brown (1999) to MRI users since these sequences define the cochlea and semi-circular canals and outlines the cranial nerves.  Another patient has been scanned also having an acoustic neuroma using the 3D techniques as seen in figure 17.

5.4.4        Results

After scanning the patient with the above protocol, a left-sided well-defined lesion was identified measuring 4cm maximally, situated in the left CPA.  This lesion is arising from the IAMs and is likely to represent an acoustic neuroma according to the radiologist.  It enhances brightly after Gd-DTPA administration, which supports this

There is also, pontine and cerebellar compression with some pressure on the 4^th ventricle.

5.4.5        Evaluation of the examination

The examination in this case was successful.  The patient was cooperative and the images achieved were acceptable.  The lesion can be clearly seen in the region of the IAMs.  It became more conspicuous after injection of Gd-DTPA were it enhanced homogenously.   However, the post-contrast images appear grainy and noisy.  A possible explanation to this is the use of FSE in post contrast sequences due to the increased ETL.  Although FSE sequences reduce scan times they also increase blurring and reduce SNR.

5.4.6        Treatment and prognosis (adapted from CancerBACUP, 2001)

The treatment for an acoustic neuroma depends on a number of factors including your general health and the size and position of the tumour. The results of your tests will enable your doctor to discuss the best type of treatment for you.

For most people, surgery is the best form of treatment for acoustic neuroma. In many cases the tumour can be completely removed and no further treatment is necessary. Before you have surgery your doctor will discuss the possible risks associated with this type of operation.

Figure 17    a:3D IR FSPGR; b: 3D IR FSPGR + Gd DTPA; c: 3D T2 reformation;

d:3D IRFSPGR reformation; e: 3D T2