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Chapter 4: Advanced Imaging: Intracranial Surgery 33

In humans, CT is usually the first study of choice for evaluation hyperintense (bright) on a T2‐weighted image [47]. A routine brain
of a patient with suspected acute intracranial pathology because of protocol in veterinary medicine includes transverse precontrast and
its availability, ease of use, short acquisition time, and high sensitiv- postcontrast T1‐weighted, T2‐weighted, T2W‐FLAIR (fluid attenu-
ity for detection of acute hemorrhage and fractures. It can provide a ated inversion recovery), and sagittal and dorsal T2‐weighted
wealth of information about the brain, including ventricular size, images. In some cases T1‐weighted (postcontrast) images may be
presence of brain edema, mass effect, presence and location of hem- useful in the sagittal and dorsal planes (e.g., assessment of the pitui-
orrhage or masses, midline shift, evolving ischemic injuries, fractures, tary gland and extraaxial masses). The T2W‐FLAIR sequence sup-
benign and malignant osseous pathology, and the paranasal sinuses presses the signal from CSF and allows assessment of periventricular
[29–32]. Its availability and short acquisition time also allow changes difficult to see on T2‐weighted images. This sequence has
frequent repeat scanning of the brain that can contribute to the high sensitivity for inflammatory CNS disease. FLAIR images are
management and follow‐up of patients in the acute, subacute, and particularly useful if there is hydrocephalus or an intracranial cyst.
chronic phases in both inpatient and outpatient settings. In veterinary T2* gradient‐echo (GRE) images are very useful for evaluation of
patients, CT of the brain is often less useful than in human patients hemorrhage or bony changes. In some cases, diffusion‐weighted
due to differences in anatomy and image quality. imaging (DWI) may be helpful in aging infarcts and in the identifi-
In neurosurgery, CT of the head is used for preoperative and cation of small infarcts in the peracute stage [47]. Images acquired
postoperative evaluation of patients for hemorrhage, infarction, with fat suppression (short‐tau inversion recovery, or postcontrast
inflammation [33], hydrocephalus, mass effect, fracture, and post- T1‐weighted with fat saturation) are useful for showing pathology
surgical assessment [30–32,34–45]. CT was historically the study of in the extracranial soft tissues, particularly orbital disease and
choice when evaluating for acute hemorrhage because it had higher pathology affecting the bone marrow. In unstable patients, the
sensitivity and specificity than MRI. With developments in MRI sequences most likely to give a diagnosis and assess secondary
pulse sequences, MRI currently has similar or greater accuracy for effects of increased ICP should be obtained first (transverse and
the detection of intracranial hemorrhage than CT (Figure 4.6) [46], sagittal T2‐weighted images), in case the MRI study needs to be
but usually requires anesthesia in veterinary patients. Intracranial aborted.
hemorrhage is typically described in terms of its location within the Like CT, MRI provides cross‐sectional images, but unlike CT
head, such as epidural, subdural, subarachnoid, intraventricular, these can be acquired in any anatomical plane without repositioning
and parenchymal, with each type of hemorrhage having sufficiently the patient. This means that multiplanar reformatting of images
distinct appearance and location. Epidural hemorrhage has a bicon- is not necessary; reformatted CT images can be of very high
vex contour of its borders in relation to the cranial vault and adja- quality but primary image acquisitions have the best detail. The
cent brain parenchyma and is usually the result of acute trauma effects of radiofrequency currents and magnetic fields on protons
associated with an acute fracture across branches of meningeal generate magnetic resonance images. The strength of magnets
arteries that hemorrhage into the epidural space. available for clinical use in veterinary medicine range from 0.2
to 7 Tesla (T) [48–51]. Low‐field magnets are less expensive but
Magnetic Resonance Imaging image quality is affected by lower signal to noise and scan times
The physics of magnetic resonance image generation is highly com- can be prolonged [52].
plex and specific details are beyond the scope of this chapter. The As with CT, patterns of contrast enhancement are an important
combination of magnetic fields and radiofrequency pulses used to feature of MRI. However, the mechanism of action of contrast
create a magnetic resonance image is called a pulse sequence media in CT compared with MRI is quite different. CT contrast
Spin‐echo pulse sequences are designed to emphasize various media are iodinated and alter X‐ray attenuation within the patient,
types of proton relaxation. Images generated using spin‐echo pulse with the iodine atoms directly visualized on the image. MRI con-
sequences are either T1‐weighted or T2‐weighted depending on trast media are paramagnetic and function by changing the relaxa-
whether T1 or T2 relaxation controls tissue contrast [47]. tion rate of protons. In MRI the contrast media is not seen directly
T1 relaxation relates to the spins of protons in the patient that are on the image, as the mechanism of enhancement is indirect. In
perturbed by a radiofrequency pulse realigning into their normal MRI, increased concentration of contrast medium changes the
position parallel with the main magnetic field. T2 relaxation relates relaxation time of protons in the immediate area leading to a signal
to the rate of dephasing of the protons immediately after being per- change in the image. In both CT and MRI, contrast media accumu-
turbed by the radiofrequency pulse. Most important is that the dif- late in regions of hypervascularity or altered vascular permeability.
ference in these relaxation times can be used to influence tissue Most MRI contrast studies are based on changes in T1 relaxation,
contrast [47]. For example, fluid has very long T1 and T2 relaxation where increased relaxation leads to increased signal coming from
times and will create low signal (dark gray/black) in a T1‐weighted regions of increased contrast medium.
image but will create high signal (white) in a T2‐weighted image. Specialized imaging sequences other than spin‐echo pulse
Thus, because most CNS lesions have increased fluid associated sequences are often helpful for CNS imaging. It is possible to sup-
with edema, they will be conspicuous as regions of increased signal press the signal from fat; this is beneficial due to the high signal
in a T2‐weighted image. However, some substances, such as pro- emitted from fat in many pulse sequences. This high signal from fat
teinaceous exudates and methemoglobin, have short T1 relaxation can hide smaller, adjacent lesions or may also be misinterpreted as
times and will have high signal in a T1‐weighted image. Thus, the disease [53]. The signal from fat can be suppressed in a spin‐echo
signal characteristics of a lesion in T1‐ versus T2‐weighted images pulse sequence by the addition of a radiofrequency saturation pulse
can be used to estimate its composition. that nulls the signal from fat within the image. The high signal of fat
As a general rule, T2‐weighted images are often the most useful may also be reduced using a type of pulse sequence called short‐tau
for neuroimaging as they provide the best soft tissue contrast. On a inversion recovery (STIR) sequence.
T2‐weighted image, fluid and fat are hyperintense. As most pathol- Certain types of hemorrhage have paramagnetic effects that can
ogy results in an increase in water content, pathology is often be imaged with MRI [54]. A very effective way of detecting these