| MRI of the IAMs |
|
5.2
Clinical indications for MRI of the IAMs The
common indication for an MRI of the IAMs include the following: ·
Symptoms to exclude acoustic neuromas ·
Facial palsy/numbness ·
Diagnosis of posterior fossa lesions ·
Hemifacial spasm ·
Trigeminal nerve neuralgia (Westbrook,
1999) 5.3
Image interpretation Refer
to section 3.3 (MRI of the Brain). 5.4
Image optimisation of MRI of the IAMs 5.4.1
Equipment Refer
to section 3.4.1 5.4.2
Image quality Since
the IAMs are very small structures the aim of the examination is to
achieve the highest spatial resolution possible with good SNR. The inherent SNR is usually good due to the high proton
density of the brain if used in association with a good head coil.
However, in the region of the IAM the low proton density of the
petrous bones and the mastoids reduces the SNR.
In this respects fine thickness slices with minimum spacing are
required to achieve good spatial resolution.
Fine matrices and smaller FOVs also help in maintaining resolution
but also reduce the SNR. Nevertheless, this requires higher NEX values to maintain SNR,
which increase the scan time. A
high-resolution T2 FSE sequence is usually highly indicated in the
assessment of the IAMs. A
frequency encoding matrix of 512 in conjunction with FSE sequences provide
extremely high spatial and contrast resolution.
The latter is achieved since the CSF provides a high signal in
comparison with the low signal from the nerve.
Westbrook (1999) states that at time if the contrast on this
sequence is adequate contrast enhancement may not be required.
The T2 FSE sequences is usually very useful
in the coronal plane when looking at the posterior fossa. 3D
SPGR sequences before and after contrast enhancement are being used in the
assessment of the IAMs even at St Luke’s Hospital (See fig 17) This
method eliminates slice gap and very thin sections.
Moreover, since reformation is possible in any plane better
assessment of IAM structure involvement is possible.
However, Westbrook, (1999) states that T2 FSE is still superior to
this method. 5.4.3
Artefacts Flow
motion from the venous sinuses is often troublesome in the posterior fossa.
GMN is helpful in this respect, but is reserves for T2-weighted
images since GMN increases signal in vessels and the TE.
Spatial pre-saturation is also helpful, where slabs are placed
superior and inferior to the FOV. When
the flow artefacts are severe, Pe gating may be used but this increases
exam time drastically (Westbrook, 1999). 5.5
Protocols and pulse sequences In
the evaluation of the IAMs, 3mm thick contiguous axial and coronal images
through the temporal bones are obtained.
Rao et al (2001) recommend SE sequence with a TR of 600 to 800ms
and a short TE of 25ms, a matrix of at least 256 x 256 and an NEX of 2. The authors also recommend the injection of Gd-DTPA in IAMs
(0.1mmmol/kg). Acoustic
neuromas, CPA meningiomas and other benign tumours enhance after contrast
injection. T1-weighted GRE
images are excellent for the definition of neural and vascular structures
in the jugular foramen. These
usually involve a TR of 100, a TE of 15 a 90-degree flip angle and flow
compensation. In
T1-weighted images cranial nerves are almost isointense with the brain.
On T2-weighted images the nerves appear hypointense in contrast to
the CSF. Rao et al (2001)
also recommends the use of 3D in visualising the IAMs since the spatial
resolution is very high in these images. Rao et al (2001) suggests the use of Gd-DTPA in combination
with suppression in the evaluation of the jugular foramen and petrous
apex. Long TR and TE
sequences, which are heavily T2-weighted images, are indicated for
demonstrating small tumours without contrast.
|