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Chapter 4: Advanced Imaging: Intracranial Surgery 35

susceptibility weighted imaging (SWI) using phase and magnitude nuclei have the highest sensitivity and thus yield the largest signal,
information from the magnetic resonance signal can be used to cre- but other factors including temperature and, most significantly,
ate images with exquisite sensitivity for hemorrhage. SWI has been magnetic field strength, have an impact on the signal produced.
shown to be significantly more sensitive than conventional T2* Numerous studies have used MRS for the diagnosis of brain tumors,
GRE images in showing cerebral hemorrhage [56]. The MRI effects differentiating types of brain tumor, grading tumors, determining
of hemorrhage are quite complex, being dependent on the age of the brain tumor margins, and monitoring response to treatment in
hemorrhage and the pulse sequence used. humans [60,62–81]. In a study by Fountas et al. [66], MRS was suc-
cessful in establishing a correct diagnosis of tumor type in 85.6% of
Magnetic Resonance Spectroscopy the population, whereas standard MRI was correct in only 78% of
Nuclear magnetic resonance spectroscopy (MRS) is an analytical the cases. A study using MRS imaging in conjunction with perfu-
technique that employs the magnetic resonance hardware used in sion MRI enabled the differentiation between metastatic lesions,
chemistry for many years and more recently with in vitro research lymphoma, and glioblastoma multiforme, the most aggressive glial
and clinically to determine the structure of compounds and compo- cell tumor in humans, with high specificity [63]. Differentiation of
sition of mixtures of compounds. In vivo this technique allows tumor types, without the use of biopsy, can allow diagnosis, treat-
examination of the biochemical status of living tissue in a noninva- ment, and follow‐up of brain tumors without the use of invasive
sive manner. Most research with, and clinical applications of, MRS biopsy techniques. This would be especially useful in the veterinary
have involved the human brain. Examples include research into community, where brain biopsy is performed less commonly.
human brain tumors and brain abscesses, which typically require Numerous studies using MRS have shown that brain tumors
invasive surgical biopsies or ultrasound‐guided aspirates for diag- exhibit decreased levels of N‐acetylaspartate (NAA) and increased
noses. Research over the past few decades has elucidated that the levels of choline (Cho), causing a decrease in the NAA/Cho ratio
use of MRS in these disease processes may aid in diagnosis and [69,82]. These findings are thought to be associated with the
monitoring of treatment, without the use of invasive techniques. absence of normal neuronal tissue and increased membrane turno-
Until recently, MRS in animals has typically been in the context of ver [59]. Other changes such as increased lactate and lipid may be
models for human neurological diseases but the utility of MRS in evident, with alterations in metabolism (anaerobic glycolysis) or
clinical veterinary medicine is now being investigated (Figure 4.7) necrosis or ischemia associated with tissue damage and areas of
[57,58]. Hydrogen is the most frequently used nucleus in MRS, decreased blood supply [82]. Occasionally, increased levels of myo-
referred to as proton MRS or H‐MRS. Hydrogen has a high sensitiv- inositol are detected when a short echo time is used. This is thought
ity because of its high gyro‐magnetic ratio, high natural abundance to be associated with increased number of glial cells, which contain
and favorable relaxation time and because it can be analyzed using myoinositol, especially in tumors such as high‐grade gliomas
conventional MRI hardware and radiofrequency coils [59,60]. The [70,83]. High‐grade brain tumors in children have a 60% lower
latter two reasons for selecting hydrogen for MRS are because the NAA/Cho ratio, a 50% higher Cho to creatine (Cr) ratio (Cho/Cr),
magnetic resonance signal of hydrogen nuclei within fat and water and a 43% lower NAA/Cr ratio than nonneoplastic lesions; low‐
are obtained to produce images. Using specialized equipment, other grade tumors have a 50% higher Cho/Cr ratio than nonneoplastic
nuclei can be used for in vivo MRS. These include carbon‐13, nitro- lesions [70]. Other studies have demonstrated similar findings sug-
gen‐15, and phosphorus‐31. These nuclei have lower sensitivity and gesting high‐grade tumors will have higher Cho levels and therefore
lower natural abundance. Therefore, their use results in lower reso- lower NAA/Cho levels than low‐grade tumors [70,73]. In addition,
lution, longer scan times, and increased cost due to the different high‐grade tumors have higher lactate and lipid levels than
equipment needed for analysis [59,61]. As mentioned, hydrogen low‐grade tumors [73,80]. Another study using preoperative, high

A B

Figure 4.7 Transverse T2‐weighted MRI (A) and MRS spectra (TE = 144 ms) (B) of an English Springer Spaniel with confirmed fucosidosis. The T2‐
weighted image shows symmetrical T2 hyperintensity within the white matter, which is a nonspecific finding. On MRS there is an abnormal lactate peak
(inverted doublet, arrow) and elevated NAA peak (asterisk), which in dogs appear to be characteristic of fucosidosis.