Page 52 - Zoo Animal Learning and Training

44 Section I: Diagnostics and Planning

A B

Figure 4.19 Transverse T2‐weighted MRI (A) and CT with lung window and level (B) of a dog with pneumocephalus. Note the presence of intraventricular
gas (arrow in A, B) that in this case was secondary to surgery for a frontal meningioma. Air appears as a signal void on MRI and in this case the air bubble
is seen surrounded by CSF. Differentiation of signal voids can be difficult on MRI but is easy in CT where gas is hypoattenuating and easily differentiated
from bone, ligaments, or metal.

Intraoperative Imaging
Image‐guided procedures have recently been introduced into the
neurosurgical armamentarium and have provided major advances
in neurosurgery. In essence, these promising technologies have
emerged from the need to acquire data that is more accurate than
that obtained from routine preoperative imaging. Classic surgical
planning relies on preoperative imaging and indirect localization
methods. Although current neuroimaging techniques can elegantly
define anatomy and pathology in most clinical settings, preopera-
tive images have significant limitations, for example distortion
between the image space and the physical space, which may result
in less‐than‐optimal localization of the lesion [122]. In addition,
intraoperative alterations in the anatomical characteristics of the
surgical field may be visualized only with the use of intraoperative
imaging modalities. These techniques may also be beneficial in
determining the extent of surgical resection during brain tumor
surgery. This section presents several techniques that may provide
intraoperative image guidance during brain surgery.

Rationale for Intraoperative Image Guidance
Problems related to the accuracy of localization in neurosurgery are
mainly caused by anatomical and physiological properties of the
Figure 4.20 Transverse T1‐weighted postcontrast MRI of a cat that pre- brain that prevent wide surgical exposures and, in most cases, direct
sented with progressive obtundation following a bite to the head 2 weeks visualization of surrounding structures; however, a safe neurosurgi-
previously. The MRI shows a defect within the frontal bone, meningitis,
cellulitis, and subdural empyema (arrow). cal approach to a mass lesion requires precise spatial knowledge of
the relevant pathology as it relates to surrounding bone and vascu-
lar structures and the localization of the lesion with respect to nor-
mal tissue. Although conventional neurosurgery training and
disease. It is often seen in older animals and is not associated with subsequent experience enable the surgeon to navigate safely within
thinning of the calvaria (Figure 4.23) [119,120]. the brain parenchyma, additional intraoperative anatomical infor-
mation is still valuable, especially in situations in which individual
Inflammatory Diseases anatomical variations or prior treatment complicate the anatomy
Inflammatory CNS disease may be associated with a normal MRI. [123]. Mass lesions, together with their surrounding edema, often
In one study, 6 of 25 dogs with inflammatory CSF had a normal distort normal anatomical relationships, thus posing a significant
MRI examination [121]. Investigation of suspected inflammatory/ challenge to the neurosurgeon trying to navigate using conven-
infectious disease therefore requires CSF analysis. tional landmarks. The effect of such anatomical alterations may be