核磁共振造影(MRI)提供良好的軟組織訊號對比。臨床上，磁振造影對比劑用於增加組織之間的對比度差異。在核磁共振造影中，微氣泡藉由產生磁化率效應而成為血管內核磁共振磁化率對比劑。然而，和其他核磁共振對比劑相比，微氣泡的磁化率效應仍是相對較弱的。因此，本篇研究將以假體實驗和活體內動態腦部核磁共振造影來探討帶氧微氣泡的磁化率增強效應。我們將測量不同濃度的帶氧微氣泡和不帶氧微氣泡的橫向弛豫率；同時，利用分析橫向弛豫率圖以研究微氣泡向上浮動的現象，確保量測的橫向弛豫率的準確性。結果顯示，在靜態和動態假體實驗中，帶氧微氣泡可以誘導比沒帶氧微氣泡更強的磁化率效應；而微氣泡濃度和橫向弛豫率的關係表明，橫向弛豫率可以做為另一個評估磁化率效應的參數。在大鼠腦中，上矢狀竇出現明顯的微氣泡磁化率效應，但是在其他腦區只出現些微的訊號衰減情形。在本篇研究中，大鼠腦中的訊號變化難以說明帶氧微氣泡能增強磁化率效應，然而，我們在長T2的區域中發現T2變化，這可能為評估微氣泡的磁化率提供更多的資訊。

Magnetic resonance imaging (MRI) provides great soft tissue contrast. In clinical, MRI contrast agents are used to increase the contrast difference between tissues. Gas-filled microbubbles in MRI become an intravascular MR susceptibility contrast agent due to their magnetic susceptibility effect. However, microbubble susceptibility effect is relatively weak compared with other MR contrast agents. Therefore, enhancement of susceptibility effect by oxygen-carrying microbubbles is investigated by phantom experiments and in vivo dynamic brain MRI in this study. Transverse relaxation rates (R2 and R2*) of microbubbles with (O2/PFP-MBs) and without (PFP-MBs) oxygen in different concentrations are measured. At the same time, relaxation rate mappings are analyzed to investigate the microbubble upward migration to ensure the accuracy of R2 and R2* measurements. Results show that O2/PFP-MBs can induce stronger susceptibility effect than PFP-MBs in static and dynamic phantom. The relationship between microbubble concentration and R2 also indicates that R2 can be another parameter for quantitative assessments of susceptibility effect. In rat brain, microbubble susceptibility effect appeared obviously in the superior sagittal sinus, while other brain regions have only a little signal attenuation. In our study, signal changes in rat brain can hardly tell the enhancement of susceptibility effect of O2/PFP-MBs, while T2 alteration is found in long T2 regions, which may provide more information for the evaluation of microbubble susceptibility.

1. V. Baliyan, C. J. Das, R. Sharma, A. K. Gupta, Diffusion weighted imaging: Technique and applications. World J Radiol 8, 785-798 (2016).
2. K. Murase, in High-Resolution Neuroimaging - Basic Physical Principles and Clinical Applications. (2018), chap. Chapter 9.
3. B. Wu et al., An overview of CEST MRI for non-MR physicists. EJNMMI Phys 3, 19 (2016).
4. B. E. McGehee, J. M. Pollock, J. A. Maldjian, Brain perfusion imaging: How does it work and what should I use? J Magn Reson Imaging 36, 1257-1272 (2012).
5. G. H. Jahng, K. L. Li, L. Ostergaard, F. Calamante, Perfusion magnetic resonance imaging: a comprehensive update on principles and techniques. Korean J Radiol 15, 554-577 (2014).
6. P. Young, W. Brinjikji, Update on state of the art magnetic resonance angiography techniques. Journal of Vascular Diagnostics, (2015).
7. J. F. Schenck, The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds. Med Phys 23, 815-850 (1996).
8. D. R. Lide, Magnetic susceptibility of the elements and inorganic compounds. (2005), pp. 134-139.
9. J. Zhang et al., Clinical Applications of Contrast-Enhanced Perfusion MRI Techniques in Gliomas: Recent Advances and Current Challenges. Contrast Media Mol Imaging 2017, 7064120 (2017).
10. E. Lapointe, D. K. B. Li, A. L. Traboulsee, A. Rauscher, What Have We Learned from Perfusion MRI in Multiple Sclerosis? American Journal of Neuroradiology 39, 994-1000 (2018).
11. I. S. Oliveira, S. S. Hedgire, W. Li, S. Ganguli, A. M. Prabhakar, Blood pool contrast agents for venous magnetic resonance imaging. Cardiovascular Diagnosis and Therapy 6, 508-518 (2016).
12. K. K. Wong et al., In vivo study of microbubbles as an MR susceptibility contrast agent. Magnetic resonance in medicine 52, 445-452 (2004).
13. J. Ophir, A. Gobuty, R. E. McWhirt, N. F. Maklad, Ultrasonic backscatter from contrast producing collagen microspheres. Ultrasonic Imaging 2, 67-77 (1980).
14. V. Sboros, C. M. Moran, T. Anderson, W. N. McDicken, An in vitro comparison of ultrasonic contrast agents in solutions with varying air levels. Ultrasound in Medicine and Biology 26, 807-818 (2000).
15. J. Hancock, H. Dittrich, D. E. Jewitt, M. J. Monaghan, Evaluation of myocardial, hepatic, and renal perfusion in a variety of clinical conditions using an intravenous ultrasound contrast agent (Optison) and second harmonic imaging. Heart (British Cardiac Society) 81, 636-641 (1999).
16. A. L. Klibanov, Ultrasound contrast materials in cardiovascular medicine: from perfusion assessment to molecular imaging. Journal of cardiovascular translational research 6, 729-739 (2013).
17. T. H. Bryant et al., Improved Characterization of Liver Lesions with Liver-Phase Uptake of Liver-specific Microbubbles: Prospective Multicenter Study. Radiology 232, 799-809 (2004).
18. C. J. Harvey et al., Pulse-inversion mode imaging of liver specific microbubbles: improved detection of subcentimetre metastases. The Lancet 355, 807-808 (2000).
19. N. C. Nanda et al., Multicenter Evaluation of SonoVue for Improved Endocardial Border Delineation. Echocardiography 19, 27-36 (2002).
20. J. Cwajg et al., Detection of angiographically significant coronary artery disease with accelerated intermittent imaging after intravenous administration of ultrasound contrast material. American Heart Journal 139, 675-683 (2000).
21. J. R. Lindner, Microbubbles in medical imaging: current applications and future directions. Nature Reviews Drug Discovery 3, 527 (2004).
22. T. R. Porter, F. Xie, Myocardial Perfusion Imaging With Contrast Ultrasound. JACC: Cardiovascular Imaging 3, 176-187 (2010).
23. J. M. Tsutsui, F. Xie, R. T. Porter, The use of microbubbles to target drug delivery. Cardiovascular ultrasound 2, 23-23 (2004).
24. C. R. Mayer, N. A. Geis, H. A. Katus, R. Bekeredjian, Ultrasound targeted microbubble destruction for drug and gene delivery. Expert Opinion on Drug Delivery 5, 1121-1138 (2008).
25. Z. Fan, R. E. Kumon, C. X. Deng, Mechanisms of microbubble-facilitated sonoporation for drug and gene delivery. Therapeutic delivery 5, 467-486 (2014).
26. S. Meairs, A. Alonso, G. Hennerici Michael, Progress in Sonothrombolysis for the Treatment of Stroke. Stroke 43, 1706-1710 (2012).
27. J. J. Pacella et al., Treatment of microvascular micro-embolization using microbubbles and long-tone-burst ultrasound: an in vivo study. Ultrasound in medicine & biology 41, 456-464 (2015).
28. J. J. Choi et al., Microbubble-size dependence of focused ultrasound-induced blood-brain barrier opening in mice in vivo. IEEE transactions on bio-medical engineering 57, 145-154 (2010).
29. K.-H. Song, B. K. Harvey, M. A. Borden, State-of-the-art of microbubble-assisted blood-brain barrier disruption. Theranostics 8, 4393-4408 (2018).
30. M. A. O'Reilly, Y. Huang, K. Hynynen, The impact of standing wave effects on transcranial focused ultrasound disruption of the blood–brain barrier in a rat model. Physics in Medicine and Biology 55, 5251-5267 (2010).
31. C.-H. Fan, C.-K. Yeh, Microbubble-enhanced Focused Ultrasound-induced Blood–brain Barrier Opening for Local and Transient Drug Delivery in Central Nervous System Disease. Journal of Medical Ultrasound 22, 183-193 (2014).
32. J. D. Martin, D. Fukumura, D. G. Duda, Y. Boucher, R. K. Jain, Reengineering the Tumor Microenvironment to Alleviate Hypoxia and Overcome Cancer Heterogeneity. Cold Spring Harb Perspect Med 6, a027094 (2016).
33. S. M. Fix et al., Oxygen microbubbles improve radiotherapy tumor control in a rat fibrosarcoma model - A preliminary study. PLoS One 13, e0195667 (2018).
34. J. R. Eisenbrey et al., Sensitization of Hypoxic Tumors to Radiation Therapy Using Ultrasound-Sensitive Oxygen Microbubbles. Int J Radiat Oncol Biol Phys 101, 88-96 (2018).
35. A. Upadhyay, S. V. Dalvi, Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications. Ultrasound Med Biol 45, 301-343 (2019).
36. M. S. Khan et al., Oxygen-Carrying Micro/Nanobubbles: Composition, Synthesis Techniques and Potential Prospects in Photo-Triggered Theranostics. Molecules 23, (2018).
37. H. L. Liu, C. H. Fan, C. Y. Ting, C. K. Yeh, Combining microbubbles and ultrasound for drug delivery to brain tumors: current progress and overview. Theranostics 4, 432-444 (2014).
38. V. Paefgen, D. Doleschel, F. Kiessling, Evolution of contrast agents for ultrasound imaging and ultrasound-mediated drug delivery. Front Pharmacol 6, 197 (2015).
39. S. Sirsi, M. Borden, Microbubble Compositions, Properties and Biomedical Applications. Bubble Sci Eng Technol 1, 3-17 (2009).
40. K. H. Martin, P. A. Dayton, Current status and prospects for microbubbles in ultrasound theranostics. Wiley Interdiscip Rev Nanomed Nanobiotechnol 5, 329-345 (2013).
41. A. L. Alexander, T. T. McCreery, T. R. Barrette, A. F. Gmitro, E. C. Unger, Microbubbles as novel pressure-sensitive MR contrast agents. Magnetic resonance in medicine 35, 801-806 (1996).
42. R. Dharmakumar, D. B. Plewes, G. A. Wright, On the parameters affecting the sensitivity of MR measures of pressure with microbubbles. Magnetic resonance in medicine 47, 264-273 (2002).
43. J. S. Cheung, A. M. Chow, H. Guo, E. X. Wu, Microbubbles as a novel contrast agent for brain MRI. Neuroimage 46, 658-664 (2009).
44. S. L. Peng et al., Using microbubbles as an MRI contrast agent for the measurement of cerebral blood volume. NMR Biomed 26, 1540-1546 (2013).
45. R. H. Abou-Saleh et al., The influence of intercalating perfluorohexane into lipid shells on nano and microbubble stability. Soft Matter 12, 7223-7230 (2016).
46. S. Unnikrishnan, A. L. Klibanov, Microbubbles as ultrasound contrast agents for molecular imaging: preparation and application. AJR Am J Roentgenol 199, 292-299 (2012).
47. J. N. Meegoda, S. Aluthgun Hewage, J. H. Batagoda, Stability of Nanobubbles. Environmental Engineering Science 35, 1216-1227 (2018).
48. C. A. McKenzie, Z. Chen, D. J. Drost, F. S. Prato, Fast acquisition of quantitative T2 maps. Magnetic resonance in medicine 41, 208-212 (1999).
49. T. Ueguchi et al., Air microbubbles as MR susceptibility contrast agent at 1.5 Tesla. Magnetic resonance in medical sciences : MRMS : an official journal of Japan Society of Magnetic Resonance in Medicine 5, 147-150 (2006).
50. K. P. Whittall, A. L. MacKay, Quantitative interpretation of NMR relaxation data. Journal of Magnetic Resonance (1969) 84, 134-152 (1989).
51. L. Li, Q. Wei, H. B. Li, S. Wen, G. J. Teng, Evaluation of microbubbles as contrast agents for ultrasonography and magnetic resonance imaging. PLoS One 7, e34644 (2012).
52. 朱書葦, 國立清華大學, 新竹市 (2018).
53. M. Kaya, T. S. t. Gregory, P. A. Dayton, Changes in lipid-encapsulated microbubble population during continuous infusion and methods to maintain consistency. Ultrasound Med Biol 35, 1748-1755 (2009).
54. M. Chaplin, Nanobubbles (ultrafine bubbles).
55. C. Leuze et al., The separate effects of lipids and proteins on brain MRI contrast revealed through tissue clearing. Neuroimage 156, 412-422 (2017).
56. K. R. Thulborn, J. C. Waterton, P. M. Matthews, G. K. Radda, Oxygenation dependence of the transverse relaxation time of water protons in whole blood at high field. Biochimica et biophysica acta 714, 265-270 (1982).
57. Z. Zhou et al., Artificial local magnetic field inhomogeneity enhances T2 relaxivity. Nat Commun 8, 15468 (2017).
58. V. Hubert et al., Clinical Imaging of Choroid Plexus in Health and in Brain Disorders: A Mini-Review. Front Mol Neurosci 12, 34 (2019).