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46 Section I: Diagnostics and Planning

cranial ultrasound is diagnostically accurate when compared with
MRI and useful for determining initial clinical management
[125,126].
The size of the craniotomy determines the size of the transducer
used. Subcortical lesions can be insonated at a frequency of 7.5 to 15
MHz or more, which provides a high‐resolution image [127]. For
superficial lesions, small‐footprint linear transducers can be used
which have little near‐field artifact. Deeper lesions may require a
lower‐frequency transducer because attenuation is less with lower‐
frequency sound. The selected probe is draped in a sterile cover
filled with sterile jelly and all air bubbles should be eliminated.
Sterile irrigation with saline is employed during the procedure to
ensure optimal coupling. The probe frequency is adjusted (with a
variable frequency probe) to suitably insonate both superficial and
deep structures as required [128].
Several investigators have described the benefits of using intra-
operative ultrasonography [129–138]. In a study of 186 patients,
Rubin and Dohrmann [139] found intraoperative ultrasonography
to be more useful for small subcortical lesions. The literature
describes a number of novel uses of intraoperative ultrasonography,
ranging from the more traditional localization of subcortical lesions
to the localization of contusions in trauma and monitoring of ven-
tricular catheter placement [140,141]. The efficacy of ultrasound in
Figure 4.23 Transverse T2‐weighted MRI of an aged dog with cortical atro- localizing the lesion, especially for metastases and high‐grade
phy due to senile changes. Note the sulcal widening and mild ventriculo- tumors, is good [132,133,142]. Even with low‐grade diffuse glio-
megaly. Increased intraventricular pressure in obstructive hydrocephalus mas, ultrasound has been shown to be better able to demarcate the
may result in reduced or absent CSF signal within the sulci. hyperechoic tumor, which may not be discernible on CT [143,144]
and difficult to localize with the naked eye at surgery. However,
minimized with real‐time information obtained by real‐time imag- there remain concerns regarding the ability of ultrasound to resolve
ing techniques [122]. Intraoperative image guidance may also pro- differences between peritumoral edema, infiltrative margin, and
vide critical information during resection of tumors with a normal parenchyma [145]. Interestingly, in a study where histologi-
consistency similar to normal brain tissue by delineating T2‐ cal correlation was attempted, ultrasound showed a good positive
weighted imaging margins. This information, coupled with intra- predictive value for tumor‐infiltrated margin [146]. However, it was
operative stimulation mapping data for lesions involving less reliable in cases followed after treatment, where diffuse changes
functionally eloquent cortical and subcortical areas, enables the related to the treatment effect could not be differentiated from
surgeon to maximize resection without causing additional neuro- recurrence of tumor [147]. It is also unable to provide histological
logical morbidity. In addition, intraoperative anatomical informa- characterization of lesions [131,132] but is an excellent tool for dif-
tion is helpful by visualizing any brain displacement that may occur ferentiating solid and cystic lesions. Attempts have even been made
intraoperatively as a result of resection cavity, brain retraction, or to perform volumetric studies using intraoperative ultrasound.
CSF leakage, causing significant discrepancy between preoperative However, its efficacy vis‐á‐vis MRI remains to be proven [135,147–
imaging data and the surgical field [124]. 149]. With advances in image resolution and use of contrast ultra-
Intraoperative imaging is more powerful than direct visualiza- sound, there could be better scope in the future [150–152]. Another
tion in detecting diseased tissue from normal brain parenchyma recent application of intraoperative ultrasound in cranial neurosur-
because it permits one to see beyond the exposed surgical field. gery has been to correct for the effect of brain shift after craniotomy
Following resection, intraoperative imaging techniques may be or stereotactic localization of lesions [153].
used to determine the extent of resection and to check for any resid- The use of intraoperative micro‐Doppler sonography has become
ual disease. Overall, intraoperative image guidance techniques help invaluable to vascular neurosurgeons in human medicine. With
the neurosurgeon to plan surgery, approach and resect the tumor, current technology, vessels less than 1 mm in diameter can be dis-
and evaluate the extent of resection. cretely insolated to assess for patency. Although crude compared
with transcranial Doppler or duplex sonography, micro‐Doppler
Intraoperative Ultrasonography can determine vessel patency, direction of flow, and the presence of
Intraoperative ultrasonography provides a method for obtaining laminar versus turbulent flow. With this technology, anastomotic
real‐time imaging of the intracranial contents during surgical pro- sites can easily be evaluated in bypass surgery, and the patency of
cedures for precise localization of a lesion within the surgical field. parent vessels and their branches can be assessed during aneurysm
The craniotomy used for the surgery provides the acoustic window clipping.
necessary to image the CNS tissue. The portability, low cost, safety, Recently, a report described 25 dogs which underwent craniot-
and real‐time evaluation capability of intraoperative ultrasound omy or craniectomy procedures for removal/debulking of an intrac-
make this technology an important adjunct in the treatment of neu- ranial mass using intraoperative ultrasound‐guided visualization of
rosurgical disease [123]. It has been shown to be useful in the man- the mass. Mass removal was accomplished using an ultrasonic aspi-
agement of intracranial tumors (Figure 4.24), cysts, abscesses, rator. Of the 25 patients, 24 survived surgery and 17 were eventually
vascular malformations, and hematomas. Recent studies reveal that discharged to the owners’ care. In all patients, the intracranial mass