Non-invasive measurement of oxygen saturation in the spinal vein using SWI: Quantitative evaluation under conditions of physiological and caffeine load
Research Highlights
►Non-invasive measurement of oxygen saturation in the spinal vein was feasible by using SWI. ►Absolute value of oxygen saturation can be calculated by using SWI. ►SWI can be a useful tool for the clinical evaluations of spinal venous oxygenation. ►Regulatory function exists in the spinal arteries as well as in the brain.
Introduction
Susceptibility-weighted imaging (SWI) is extremely sensitive to differences in local magnetic susceptibility, which can be induced by hemorrhage, iron deposit, calcification, and deoxygenated hemoglobin (deoxy-Hb) (Haacke et al., 2004). SWI has been reported to be useful for the evaluation of the intracranial veins (Koopmans et al., 2008), because SWI can clearly depict normal venous blood, utilizing the phase shift induced by paramagnetic effect of deoxy-Hb in the vein. Specifically, oxygen saturation in the vein can be measured using SWI, from the susceptibility phase difference (∆φ) between blood in the vein and surrounding tissue with an infinite cylinder approximation (Shen et al., 2007).
SWI has been reported to be useful for clinical diagnosis and for the assessment of some diseases, mainly those affecting the brain; nonetheless, SWI can also be beneficial in the assessment of conditions of the spinal cord. For example, patients with spinal arteriovenous malformations (AVM) exhibit increased oxygen saturation in the draining vein. Normalized oxygen saturation in the draining vein can be detected by SWI (Fujima et al., 2010). In the case of patients with multiple sclerosis (MS), decreased oxygen uptake leading to decreased venous vasculature in the brain was assessed by using SWI (Ge et al., 2009), and this technique can be applied to spinal MS as well. Evaluation of tumor tissue vascularization and visualization of the dilated draining vein by using SWI in patients with spinal tumors could help in the grading of gliomas (Park et al., 2009).
In healthy individuals, an auto-regulatory function maintains a constant cerebral blood flow (CBF) despite changes in blood pressure (Lassen, 1959). CBF is additionally regulated by changes in the partial arterial pressure of oxygen or carbon dioxide (Ainslie and Duffin, 2009). For example, hyperventilation leads to decreased carbon dioxide and increased oxygen levels in an artery, which promotes vasoconstriction of the cerebral artery and decreased CBF. Conversely, breath holding increases carbon dioxide and decreases oxygen in an artery, leading to increased CBF. SWI was previously used to evaluate these changes in CBF, by measuring the phase shift of a vein, which reflects venous deoxy-Hb (Fushimi et al., 2010, Sedlacik et al., 2008b). CBF was shown to be modulated by the activity of drugs such as caffeine (Cameron et al., 1990) or acetazolamide (Grossmann and Koeberle, 2000). SWI was also used to evaluate vasoconstriction in the small arteries, and decreased CBF caused by caffeine (Sedlacik et al., 2008a). However, although these studies reported a change in blood flow, they did not quantify oxygen saturation. Furthermore, all these studies were performed in the brain. To our knowledge, no such study has been performed in the spinal cord.
Since the spinal cord is an important part of the central nervous system, we elected to ask if the same mechanisms governed the regulation of oxygen saturation in the spinal cord and the brain. The main goals of our study were to use SWI to 1) quantify oxygen saturation in the spinal cord and 2) to evaluate changes in oxygen saturation and spinal blood flow after a physiological or caffeine load.
Section snippets
Subjects
Five healthy volunteers (4 males and 1 female; age range, 28–30 years; average, 29 years) were recruited for this study. None of the study participants had a past history of spinal cord diseases or symptoms. For evaluation, the subjects were asked to fill in a questionnaire that enquired about their caffeine-related habits. None of them seemed to consume large amounts of caffeine; their daily caffeine intake was 1–2 cups of coffee. Therefore, all subjects were treated as one uniform group and
Results
MR examinations were successfully performed on all study participants with no adverse events recorded before or after the examination. During breath holding, on an average, they were observed to breathe (small breath) once in 20–30 s, and this breath lasted for about 2–4 s. The ASV could be detected in all participants (Fig. 2, Fig. 3). For the physiological load measurements, the average Y values for Rc, Bh, Ox and Hv were 0.80, 0.84, 0.81, and 0.75, respectively (Fig. 4). ANOVA analysis showed
Discussion
In this study, we used SWI to non-invasively measure changes in oxygen saturation of the ASV after a physiological or a caffeine load. We showed that a physiological or caffeine load resulted in alterations in blood flow, which in turn induced changes in venous oxygenation in the spinal cord as well as in the brain. We also demonstrated the feasibility of using SWI as a non-invasive tool to detect and evaluate these changes by measuring oxygen saturation in the spinal vein.
Our results on oxygen
Acknowledgment
The authors thank the entire staff at Hokkaido University Hospital for their assistance in data collection.
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