Posterior Fossa- IAC

Common Indications IAC:

• Asoustic neuroma

• facial palsy/numbness

• hemifacial spasm

IAC: Patient lies supine on the examination table with their head within the head coil.

The head is adjusted so that the interpupillary line is parallel to the table and the head is straight.

IAC: Longitudinal alignment light lies in the midline and the horizontal alignment light passes through

the nasion

Slice coverage for saggital IAC

R —> L entire brain

Sequences sag. IAC

• T1: SE, FSE, incoherent GRE

FOV sag. IAC

220-240 × 100%

Sag IAC phase

A —>P

Matrix sag. & coronal IAC

256-320 × 75-100%

Slice/gap sagittal IAC

5/1, 4/1

Slice angulation for sagittial IAC: coronal and axial

parallel to the midline

Axial coverage for IAC

S —> I from foramen magnum to superior border of petrous ridge

Axial IAC sequenes

• T1: SE, FSE, 3D incoherent GRE*

• T2: SE, FSE

Axial and coronal IAC FOV:

180-200 × 100%

Axial phase IAC:

R —> L

Axial matrix IAC:

256-512* x 75-100%*

Axial slice/gap IAC:

3/1, 0.5/0* (512×100% matrix)

Slice angulation for axial IAC: Sagittal and coronal

Parallel to IAC/cochlear nerves

Coronal slice coverage for IAC

A —> P cerebellum to clivus

Coronal sequences IAC

• T1: SE, FSE

• T2: SE, FSE

coronal phase IAC

R —> L

Coronal IAC slice/gap

3/1

Slice angulation for coronal IAC: Sagittal and axial

Sagittal is perpendicular to the ACPC line. Axial is parallel to the CN VIII nerves

Technical issues for IAC:

• IAMs are very small structures and this examination is usually carried out to exclude a small acoustic neuroma situated within the canal.

  • Therefore, it is important to achieve the highest spatial resolution possible in keeping with good SNR.

• To optimize spatial resolution even further, the FOV is reduced compared with standard brain imaging.

  • As a result of all these measures, the NEX/NSA may have to be increased to maintain SNR.

• IAC is mostly T2 sequences. T1-weighted sequences yield low inherent contrast

Artifact for IAC:

• Flow motion from the venous sinuses is often troublesome in the posterior fossa.

  • GMN minimizes this artifact but it not only increases the signal in vessels but also the minimum TE available and is therefore reserved for T2-weighted sequences. 

  • Spatial presaturation pulses placed Superior and Inferior to the FOV are also beneficial.

Contrast usage for IAC:

As T1-weighted sequences yield low inherent contrast between the petrous ridge and the IAM and acoustic neuromas demonstrate good enchantment, contrast is usually necessary.