Page 40 - Zoo Animal Learning and Training

32 Section I: Diagnostics and Planning

A B

Figure 4.5 Sagittal B‐mode (A) and color flow Doppler (B) ultrasound images of the brain of a 5‐month‐old Shih Tzu presented with seizures and poor
growth (due to congenital dwarfism). Because of an open fontanelle, ultrasonography allowed assessment of the brain and in this case shows the presence
of a small quadrigeminal cistern cyst (arrow). Color flow Doppler shows the direction and some assessment of blood flow velocity. The color flow Doppler
study shows blood flow within the rostral cerebral and callosal arteries. To obtain resistive index measurements, pulsed wave Doppler is used to allow meas-
urement of diastolic and systolic velocities within a blood vessel.

Recently, work has been published on coregistering ultrasound with lateral ventricles and bedside evaluation of critically ill patients
MRI to aid tissue identification and this may have a future in veteri- where CT and MRI techniques are unavailable.
nary neurosurgery [12].
Ultrasonography can be useful in animals with thin calvaria or Computed Tomography
open fontanelles that allow an acoustic window. The ultrasonographic CT can be performed in various planes depending on patient posi-
anatomy of the brain has been described, and its utility for identifying tion and the CT gantry angle within its limited arc. Historically,
hydrocephalus and other congenital malformations, such as arach- direct coronal‐plane CT imaging of the paranasal sinuses or brain
noid cysts, Dandy–Walker syndrome, and cerebellar herniation asso- was performed with the patient in a supine “hanging‐head” posi-
ciated with Chiari‐like malformation, has been reported [5,13–15]. tion with the head of the patient literally hanging over the edge of
Doppler ultrasonography is also useful for evaluating blood flow the CT scanner table or with the patient in the prone position and
in the CNS [16–18]. Transcranial Doppler ultrasound can be used the neck hyperextended. CT imaging of the brain and spine is now
to evaluate the blood flow in the basilar artery via the foramen mag- usually performed in the axial plane with the patient in a prone or
num in most patients, and the cerebral arteries can be evaluated in supine position on the scanner table and the head and neck in a
animals with a persistent fontanelle (Figure 4.5). The resistance neutral position. The need for a direct coronal patient position has
index, a measure of resistance to blood flow, can be calculated for been made redundant since the advent of high‐resolution multipla-
the basilar artery and has been shown to be related to intracranial nar reconstruction capabilities on newer multislice CT scanners.
pressure (ICP) and neurological status in dogs [19,20]. The current These reconstruction capabilities can generate axial images in 0.5–
literature describes Doppler sonographic findings in inflammatory 0.6 mm increments, which can then be reformatted into the sagittal,
disease, abscessation, ischemia, hemorrhage, and neoplasia of the coronal, and oblique planes with image quality nearly identical to
brain in dogs [21–26]. that obtained from direct scanning [28].
Ultrasonography was adapted for imaging the brain in experi- A typical routine brain CT scan consists of 3–4 mm axial
mental dogs in one study [27]. A transducer originally designed images through the entire brain from the skull base to the vertex
for transesophageal echocardiography was adapted for real‐time without the intravenous injection of iodinated contrast material.
volumetric endoscopic imaging of the brain. The purpose of the With multislice CT scanners the brain is usually imaged in helical
study was to evaluate the clinical feasibility of real‐time three‐ mode with at least 50% slice overlap. This is usually followed by
dimensional intracranial ultrasound. A transcalvarial acoustic another set of axial images through the brain after the intrave-
window was created under general anesthesia by placing a 10‐mm nous administration of a contrast agent, typically iodinated con-
burr hole in the parietal calvaria of a 50‐kg dog. The burr‐hole trast material (600 mg iodine/kg) injected through an 18‐ or
was placed in a left parasagittal location to avoid the sagittal sinus, 20‐gauge intravenous catheter. Scanning intervals can be and are
and the transducer was placed against the intact dura mater for adjusted for clinical need and indications such as patient age and
ultrasound imaging. Images of the lateral ventricles were pro- size, need for higher‐resolution images of specific anatomy such
duced, including real‐time three‐dimensional guidance of a nee- as the orbits, temporal bone, and skull base, or for CT angiogra-
dle puncture of one ventricle. In a second canine subject, phy. With the multidetector CT scanners, images can be recon-
contrast‐enhanced three‐dimensional Doppler color flow images structed into submillimeter axial images that can be used to
were made of the cerebral vessels, including the complete circle of generate two‐dimensional and three‐dimensional reformatted
Willis. Clinical applications may include real‐time three‐dimen- sagittal and coronal images and thus better delineate parenchy-
sional guidance of cerebrospinal fluid (CSF) extraction from the mal, vascular, and osseous anatomy.