Saturday 11 August 2012

Cranial Nerves MRI Planning Protocol

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MRI is the imaging modality of choice when any cranial nerve pathology is suspected.
The routine MRI Brain sequences augmented by a 3 D Gradient Echo Steady- State sequences such as FIESTA or FISP are sufficient to demonstrate most of the pathologies of cranial nerves. Intravenous gadolinium-DTPA may be required occasionally and provides additional diagnostic information.
The protocol that we follow in our institute on 1.5 Tesla GE Signa Excite for MRI evaluation of Cranial Nerves is “MRI BRAIN FOR CRANIAL NERVES” includes routine MRI Brain sequences followed by FIESTA.

Routine Brain includes: Axial T1 SPGR, FLAIR, T2w, Dwi,T2*GRE, Coronal T2, Sagittal T2.
For Cranial Nerves axial 3 D FIESTA.
In selected cases Contrast enhanced T1 SPGR axial sos coronal and sagittal.

How to plan MRI FIESTA for Trigeminal Nerve and 7th-8th Cranial Nerve complexes ? 
 Slab Oriented at right angle to vertical axis of brain stem covering whole brain stem.
What is FIESTA ?
FIESTA stands for 'Fast Imaging Employing Steady State Acquisition', provides images of fluid filled structures with very short acquisition times, uses the T2 steady state contrast mechanism to provide high SNR (Signal / Noise Ratio) images with strong signal from fluid tissues while suppressing background tissue for contrast and anatomic detail of small structures. In addition, the ultra short TR and TE enable extremely short acquisition times – shorter than FSE – and the images can be post processed using MIP, volume rendering, or 3D navigator techniques.

What is Steady State Free Precession Imaging (SSFP) ? 
Steady state free precession (SSFP) imaging is a Gradient Echo MRI technique which uses steady states of magnetizations achieved by a series of radiofrequency (RF) irradiation and natural relaxation behaviors of spins. So the influencing factors include: the flip angles of RF pulses, repetition time (TR) of pulse repeats, the relaxation time constants including longitudinal (T1) and transverse (T2) ones, plus if gradient moments (i.e. the integral of gradients with time) in one TR are zero, etc.
In general, SSFP MRI sequences are based on a (low flip angle) gradient-echo MRI sequence with a short repetition time which in its generic form has been described as the FLASH MRI technique.

Commercial names for Steady-State Free Precession (SSFP)
GE : FIESTA  (Fast Imaging Employing Steady-stateAcquisition)
Siemens : FISP (Fast Imaging with Steady-statePrecession)
Philips: FFE (Fast Field Echo), b-FFE (Balanced Fast Field Echo)

Usefulness of FIESTA:
This protocol and the FIESTA sequence is useful for evaluation of Trigeminal Nerve in Trigeminal Neuralgia, Facial and Vestibulo choclear Nerves in IAC for any space occupying lesion or Vascular loop, Abducens nerves for Lateral rectus palsy and in addition Hypoglossal and Glassopharyngeal Nerves.

Related Article : Cranial nerves normal MRI Anatomy

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