Technical issues for MRI examination of the posterior fossa
Introduction
Imaging of the posterior fossa by magnetic resonance imaging (MRI) is typically performed as part of a standard brain examination. Hence, it offers the advantages, as well as challenges and limitations, of current brain MRI. The normal anatomy of the posterior fossa is well visualized on MRI. The pons, medulla, 4th ventricle, cerebellum, cisterns, and vascular structures are best seen on T1-weighted images. Lesions deforming the basic normal anatomy may be obvious just by morphology on these images. Identification of hyperintensity on T2-weighted images is most useful for the detection of parenchymal lesions. Imaging may be compromised by the presence of artifact as with all MRI. In the posterior fossa, motion artifact seen as ghosting from the sigmoid and transverse sinuses may be especially apparent. Conventional MRI imaging of the brain is discussed extensively in textbooks and recent reviews [1], [2]. Newer and more advanced techniques including quantitation of conventional MRI data, magnetic resonance spectroscopy, and quantitative imaging including magnetization transfer may enhance future analysis of posterior fossa abnormalities. Structures of posterior fossa, in particular the brain stem and cerebellum, figure prominently in clinical disability scales, and thus targeted assessment of damage in these regions might be expected to correlate better with clinical disability when compared to whole brain measures.
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Resolution, contrast, and signal
The posterior fossa is typically imaged as part of a routine brain study with approximately 1 mm resolution in plane and 3 mm slice thickness, for an overall volume element (voxel) measurement of 3 mm3. Timing parameters for fast spin echo acquisition include 4–6 s repetition times (TR) and effective echo time (TE) of 90 ms for T2-weighting. Fluid attenuated inversion recovery (FLAIR) imaging may be performed with TR of 11 s and TE 140 ms and inversion time of 2200 ms. Application of FLAIR in
Motion artifact in the posterior fossa
The presence of the sigmoid and transverse sinuses in the posterior fossa gives rise to potentially troublesome ghost artifacts due to flowing blood. They are ubiquitous in axial slices at the level of the sinuses and manifest as ‘copies’ of the vessel superimposed on the image. Certain aspects of this artifact are under the control of the imaging technologist and should be understood for optimal imaging results. Specifically, any structures that periodically change position or velocity with
Investigation of cerebellar ataxia
Conventional and non-conventional MR techniques have been applied in the posterior fossa, including magnetic resonance spectroscopy (MRS), offering the possibility of insight into neuronal loss. This takes on increased importance in light of recent findings in multiple sclerosis (MS) that describe greater than expected axonal loss in that disease [6]. In a 1995 study, Davie et al. applied several conventional and non-conventional techniques to a study of this region. They examined 11 MS
Posterior fossa atrophy
Of interest in the study of dementias is the presence of atrophy in the posterior fossa, particularly in the brain stem and cerebellum. This was explored in two recent studies by Filippi et al. and by Edwards et al., who found that both brain stem and cerebellar volumes were reduced in MS. Significant correlations were found between measures of atrophy in these regions and clinical measures including the Scripps neurological rating scale and the infratentorial functional score [9], [10].
Conclusions
MRI of the posterior fossa, with optimal technique for that region, offers the potential for complementary information useful in the diagnosis and prognosis of MS. There may be substantial differences between lesions of the posterior fossa and supratentorial lesions, and non-conventional techniques may provide new insight into their investigation.
Acknowledgements
Supported in part by NIH grant NS34353.
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