Use of Landmarks in Skull of Dog (Canis familiaris) for Neuronavigation - Article Example

www.academia-research.com Sumanta Sanyal d: 14/11/06 Use of Landmarks in Skull of Dog (Canis familiaris) for Neuronavigation Introduction Data provided by MRI and MEG (Magnetoencephalography) can be functionally used to conduct successful intraoperative neuronavigation that can enable planning a precise surgical approach to targeted lesions within the defined surrounding neurovascular structures. The incorporated data can enable the surgeon to avoid eloquent areas of the brain during surgery. Additionally, intraoperative MRI or some other scanning method can enable radical resection of lesions, possible immediate control of tumour remnants and continuing updates to assist in the neuronavigation process with intraoperative images providing fresh images of brain shifts (Ganslandt, O., et al, 2002). The technique is quite common with human patients undergoing intricate neurosurgery but this paper investigates how it may be utilised in canid ones, specifically dogs (Canis familiaris). The Imaging Process Image-guided neuronavigation functions on the basis of stereotaxy. The brain is conceived as a geometric entity that can be trisected by three spatial planes orthogonal to each other. These planes are the horizontal, frontal and sagittal ones based on the Cartesian co-ordinate system (Ganslandt, O., et al, 2002). Precise surgical guidance is available by referencing objects within this three-dimensional system in the brain with three-dimensional images aligned along parallel co-ordinate axes displayed on the console of a computer-workstation providing the surgeon with point-to-point imagery of the actual locations in the brain. This allows maximum accuracy of operation (Ganslandt, O., et al, 2002). This spatial accuracy afforded by functional imaging systems such as, primarily, magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), when incorporated within the neuronavigation process, allows precise surgery near eloquent areas of the brain with minimum morbidity (Ganslandt, O., et al, 2002). Characteristics: Canis familiaris Attempts to apply congruency in both morphological and behavioural characteristics of the domestic dog (Canis familiaris) immediately runs in several problems because there are innumerable breeds all over the world and the variety is so diverse that it is bewildering. Nevertheless, it is generally agreed that the domestic breed diverged from gray wolves (Canis lupus) 3,25,000 to 1 million years ago. The domestic dog is a very close relative of the wild gray wolf with only about 0.2% divergence in mitochrondrial DNA sequence (Jordana, J., et al, 1999). It is also acknowledged that the gray wolf is a breed that is found both in the New World and the Old and dogs probably originated from the Old World breed and crossed over with humans to the New World to form the special breeds found there today (Olori, J., 2005). This is even though the fond belief of dog-breeders in the New World is that their breeds are exclusively diverged from gray wolves there. At least, the early dog breeds in the New World were so while more recent breeds may have been bred down from the northern gray wolves that are large-bodied (Jordana, J., et al, 1999). There is also evidence to suggest that there are four distinct lineages that descended independently from gray wolves. Each lineage is called a clade (Olori, J., 2005). The comparative study conducted on 25 different breeds from different parts of the world on the morphological, though Canis familiaris is probably the most morphologically diverse species on earth, and behavioural traits of the domestic dog revealed that morphological data was more congruent than behavioural ones (Jordana, J., et al, 1999). In the context of the purpose of the paper it was found that the cranial profile varied from medium to large to small dogs in the following manner. Medium dogs - Subconvex cranium Large dogs - Convex cranium Small dogs - Rectilinear cranium (Jordana, J., et al, 1999, Table 1) The second morphological feature that is germane to this paper is the shape of the occipital crest. This is as follows as per the same comparative study. Medium dogs - Pronounced Large dogs - Slightly pronounced Small dogs - Null (Jordana, J., et al, 1999, Table 1) This is true of all clades and based on this morphological fact the paper shall now proceed with identifying some landmark features in a dog's cranium that may assist in neuronavigation. Cranial Development The breed identified here to illustrate landmark features and subsequent planar demonstration for neuronavigation is the small Mexican hairless xoloitzcunitle, a very close relative of the Chihuahua (Olori, J., 2005). This breed has not interbred with European stock but it has been crossed with subspecies like the Pekingnese and the Pug, both brachycephalic varieties. Its principal characteristic is its dwarfism and, being a small subspecies, its cranial shape is rectilinear. The Mexican hairless has a short muzzle and relatively flat face with a fontanelle where other larger canids have the sagittal crest (Olori, J., 2005). This flatter face with short muzzle does give its cranium a rectilinear shape. This is because while in other dogs, especially the larger ones, as juveniles grow towards adulthood the skull bones ossify and harden, giving the cranium an elongated shape. Thus, cranial shape, in dogs, relies mostly upon postnatal growth. In the Mexican hairless, which is a very small dog, juveniles grow towards adulthood without the skull bones completely ossifying and a gap is left where there is a suture in other dogs and this gives the face a round, puppy-like shape (Olori, J., 2005). Thus, medium and larger dogs, growing into adulthood and closely approaching the ancestral shape of larger wolves, have elongated craniums that may be either subconvex, for medium breeds, and convex ones for the larger ones with the most elongated cranium shapes. Cranial Imaging While the diversity in cranial shape among different breeds of dog depending upon their size may seem germane to neuronavigative purposes the principal requirement will be identifying unique cranial and facial features of the particular dog undergoing operation. The need for neuronavigative landmarks is essentially because, in the process of the imaging the patient pre-operatively it is found essential to shift the patient to the intraoperative stage. Anatomical landmarks and/or fiducial markers, about 6 to 12 of them, are rigidly registered during the final preoperative stage. After the patient is changed to the intraoperative stage the same landmarks are now registered again and conformity between the pre- and intraoperative stages is compared. The best-fit version on computer algorithm generated data is maximum shift 1.6mm to 3mm (Ganslandt, O., et al, 2002). This is the maximum permissible shift in axial co-ordinates for the human patient. Since dogs, most breeds, have slightly smaller heads and there is no literature advising on the maximum permissible shift it is taken for the purpose of this paper that the maximum shift should be kept within this margin and especially favoured towards the lower limit - 1.6mm - in the case of small breeds like the Mexican hairless, Chihuahuas and others. The particular position of the lesions determines the earmarking of landmarks. Surface landmarks may not earmark tumours deeper into the cranial crevices while landmarks that are superficial or near the surface may earmark those nearer the surface. Osseous processes like neural genu, arterial and venal canals and other canals and processes like the orbital ones may be registered for the purpose of landmarking though the particularity depends primarily upon the position of the lesions, as already mentioned earlier. Non-bony features like large veins and arteries, the various brain surfaces and continuity of the skin may also be registered for purposes of landmarking (Ganslandt, O., et al, 2002). Conclusion In conclusion it must be noted that there is very little literature available on neuronavigation in a favoured domestic species like Canis familiaris. It is strange that while humans have made themselves safer by the means of this advanced surgical technique they have not paid similar attention to their best friend in the animal world - the domestic dog. Pictorial depiction, of the dog cranium, though easily available, has been avoided in the absence of exact information on the tumour position. Reference Jordana, J., et al, Comparative analysis of morphological and behavioural characters in the domestic dog and their importance in the reconstruction of phylogenetic relationships in canids, Genet. Mol. Biol. Vol. 22, No.1, Mar. 1999. Extracted on 12th November, 2006, from: http://www.scielo.br/scielo.phpscript=sci_arttext&pid=S1415-47571999000100011&lng=en&nrm=iso&tlng=en Olori, Jennifer, 2005, Canis familiaris, University of Texas. Extracted on 11th November, 2006, from: http://www.digimorph.org/specimens/Canis_familiaris/ Ganslandt, O., et al, Neuronavigation: concepts, techniques and applications, Neurology India, Vol. 50, Issue 3, PP. 244-255, 2002. Extracted on 11th November, 2006, from: http://www.neurologyindia.com/article.aspissn=0028-3886;year=2002;volume=50;issue=3;spage=244;epage=55;aulast=Ganslandt Read More