Stereotactic anatomy of the posterior cranial fossa. A study of the transcerebellar approach to the brainstem

Validation of a magnetic resonance imaging guided stereotactic access to the ovine brainstem

Background

Anatomical differences between humans and domestic mammals preclude the use of reported stereotactic approaches to the brainstem in animals. In animals, brainstem biopsies are required both for histopathological diagnosis of neurological disorders and for research purposes. Sheep are used as a translational model for various types of brain disease and therefore a species-specific approach needs to be developed. The aim of the present study was to establish a minimally invasive, accurate and reproducible stereotactic approach to the brainstem of sheep, using the magnetic resonance imaging guided Brainsight^TM frameless stereotactic system.

Results

A transoccipital transcerebellar approach with an entry point in the occipital bone above the vermis between the transverse sinus and the external occipital protuberance was chosen. This approach provided access to the target site in all heads. The overall mean needle placement error was 1.85 ± 1.22 mm.

Conclusions

The developed transoccipital transcerebellar route is short, provides accurate access to the ovine caudal cranial fossa and is a promising approach to be further assessed in live animals.

The human locus coeruleus 3-D stereotactic anatomy

PURPOSE

The main goal of this work was to study the stereotactic anatomy of the human locus coeruleus (LC), important relay of adrenergic and dopaminergic human brainstem (HB) circuitry, to allow its easy localization on MRI and in microsurgical procedures.

METHODS

Forty LC were studied from 20 adult HB of both sexes. The melanin pigmentation of its cells was used to identify and localize them and so to define the 2-D and 3-D LC contours. These HB were cut on a cryomicrotome with 3-D referenciation. The slices were coloured with haematoxyline-eosin. On the slices, digitized images of the cells were referenced to the midline, the fourth ventricle floor plane and the pontomedullary junction plane with an appropriate computer program.

RESULTS

The LC revealed to be a symmetric, thin and elongated nucleus, divergent caudally except in its superior part, with a sub-ependymal location on the superior dorsal lateral pons. The main LC dimensions are: length 12.0-17.0 mm (m 14.5); width 2.5 mm; height 2.0 mm. The 3-D references of the LC center are: 3.2 ± 0.3 mm to the midline; 1.1 ± 0.2 mm to the IV ventricle floor and 18.5 ± 1.5 mm to the ponto-medullary junction.

CONCLUSIONS

The human LC is a nucleus thinner and longer than previously described (in average 14.5 mm long and 2-2.5 mm thick), localized 1 mm under the IV ventricle, 3 mm apart from the midline and centered 14-21 mm above de ponto-medullary junction. No correlation was found between LC and pons dimensions, the gender or the age.

Stereotactic biopsies of focal brainstem lesions

BACKGROUND

Stereotactic biopsies of the brainstem (SBB) are a selected group of stereotactic operations owing to the lower incidence of brainstem lesions requiring biopsy, the greater complexity, and the higher risks of these procedures. Usually, the lower the lesion in the brainstem, the greater the risks involved. The approach of the different target locations by distinct routes, transcerebral and transcerebellar, is still a matter of debate. Moreover, pure medullary lesions are seldom biopsied, whereas diffuse brainstem lesions, typical of children's brainstem gliomas and rather frequent in most published series, depend less and less on the histopathological confirmation for treatment. In this study, the authors present their own experience on SBB aiming to discuss mainly their indications, approach routes choice, and procedure techniques.

METHODS

The authors reviewed a series of 30 SBB (27 adults and 3 children) out of 450 stereotactic biopsies of the central nervous system performed for the last 10 years. All cases but one presented as focal brainstem masses, 19 mainly in the pons and 10 in the midbrain. Eleven lesions (10 in the midbrain and one midbrain-pontine) were approached by a transfrontal route. All the others were approached by a suboccipital transcerebellar route.

RESULTS

Twenty-six (87%) out of the 28 cases (93%) where a positive histopathological result was obtained had a specific diagnosis: 18 tumors (14 astrocytomas, 2 primary brain lymphomas, 1 oligodendroglioma, and 1 ganglioglioma), 2 toxoplasmosis, 2 sarcoidosis, 1 aspergillus abscess, 1 vasculitis, 1 acute inflammatory demyelinating disease, and 1 progressive multifocal leucoencephalopathy. Four nontumoral lesions arose in patients with AIDS. Morbidity was restricted to 2 cases consisting of transient cranial nerve deficits.

CONCLUSIONS

Stereotactic biopsies are specially useful for the diagnosis of focal brainstem lesions. Midbrain and midline lesions should be approached through the cerebrum, whereas most of the pontine lesions through the cerebellum. Overall, this procedure should be performed with "functional-like" precision and require some technical refinements to reach maximal accuracy with minimal morbidity.

Anatomy based research in neurosurgery

The Anatomy Based Research in Neurosurgery is an important part of Applied Neuroanatomy that mainly concerns neurosurgical procedures and related problems, namely neuroimaging, brain mapping and surgical techniques. This includes a wide variety of research fields such as 2-D and 3-D structure visualization and referencing (stereotaxy), intraoperative imaging cartography and targeting (neuronavigation), as well as microsurgical, functional and endoscopic anatomy. The methods involved in this investigation regard predominantly microsurgical proceedings and morphological variability studies, both on living or post-mortem material (the strict neuroimaging techniques remaining a main tool for radiologists). The present paper provides an overview of the main techniques of neuroanatomical research applied to neurosurgery. Their major indications and requirements are described and discussed. Special attention is paid to some anatomical techniques such as microdissection and serial sectioning as well as 2-D and 3-D image procedures that are explained in detail.

Stereotactic Transcerebellar Approach to Pontine Lesions through the Middle Cerebellar Peduncle

SUMMARY

A stereotactic approach to the pons through the middle cerebellar peduncle based on MR studies was used to biopsy 18 patients. The stereotactic coordinates and angles were defined with reference to three orthogonal planes (midsagittal, IVth ventricular floor and pontomedullary junction). The pathological diagnoses were in keeping with clinical outcome and comprised five highgrade astrocytomas, three low-grade astrocytomas, two glioblastomas multiforme, two oligodendrogliomas, two primitive neuroectodermic tumours, two lymphomas, one medulloblastoma, and one tuberculosis. This approach provides a high yield of positive histological diagnoses with little morbidity (transient neurological deficits in two cases) and thereby avoids inappropriate therapy.

A brainstem stereotactic atlas in a three-dimensional magnetic resonance imaging navigation system: first experiences with atlas-to-patient registration

OBJECT

The authors describe a computer-resident digital representation of a stereotactic atlas of the human brainstem, its semiautomated registration to sagittal fast low-angle shot three-dimensional (3-D) magnetic resonance (MR) imaging data sets in 27 healthy volunteers and 24 neurosurgical patients, and an analysis of the subsequent transforms needed to refine the initial registration.

METHODS

Contour drawings from the atlas, which offer the 70th percentile of variation of anatomical structures, were interpolated into an isotropic 3-D representation. Initial atlas-to-patient registration was based on the fastigium/ventricular floor plane reference system. The quality of the fit was evaluated using superimposition of the atlas and MR images. If necessary, the atlas was tailored to the individual anatomy by using additional transforms. On average, the atlas had to be stretched by 2 to 6% in the three directions of space. Scale factors varied over a broad range from -8 to +19% and the benefit of visual interactive control of the atlas-to-patient registration was evident. Analysis of distances within the pons measured in the midsagittal MR imaging slices and the required scale factors revealed significant correlations that may be used to reduce the amount of user interaction in the coregistration substantially. In 70.6% of the cases, the atlas had to be shifted in a cranial direction along the brainstem axis (in 25.5% of cases 3-4 mm, in 45.1% of cases 1-2 mm). This was due to a more caudal position of the fastigium cerebelli on the MR images compared with the atlas.

CONCLUSIONS

This observation, in conjunction with the variability of the height of the fourth ventricle in our MR imaging data (range 6.1-15.2 mm, mean 10.1 mm, standard deviation 1.8 mm) calls into question the role of the fastigium cerebelli as an anatomical landmark for localization within the brainstem.