Authors: Carol Yan, Corin So, Deng Yuqi, Stephen Tai Adviser: Professor Ed X. Wu

Diagnosis of Stroke Risk By Diffusion MRI

Diffusion MRI as a diagnostic tool

Nuclear spin of water molecules in

the body

Magnetic signals loss because of

the displacements

Amount of signal loss measured as

diffusion coefficient

Artificial magnetic

field

Magnetic signals are generated by water molecules

Diffusion of water

molecules

Small displacement

of water molecules

Interaction

Figure 7. Flow chart of the process of diffusion MRI

Traditional MRI can only detect stroke after bleeding happens (Figure 3), but diffusion MRI can sense the changes in the brain before the blood vessel breaks [2] (Figure 4). This is because the high tissue pressure in the risk area causes decrease in diffusion activity, which is reflected on the diffusion weighted image.

Figure 3. Diffusion weighted image before stroke. The white area is at high risk of stroke. [3]

Figure 4. Diffusion weighted image after stroke. The white area shows hemorrhage. [4]

Figure 8. 3D ellipsoid constructed [8]

Combining ellipsoids at different locations gives an overall view of diffusion in the brain [6] (Figure 9).

Figure 9. Virtual image showing intensity and preferred direction of diffusion. [9]

b. The concept of nuclear spin: Atomic nuclei such as the nuclei of hydrogen atoms rotate at certain speed, this phenomenon is called nuclear spin [6]. When a nucleus rotates, it will produce a magnetic field, which is a detectable signal (Figure 6).

Figure 6. Rotating nucleus, analogous to a bar magnet. [7]

Figure 5. Diffusion of a drop of ink in the water has no preferred direction. [5]

a. The concept of diffusion: Diffusion is the random motion of molecules driven by internal thermal energy [2]. For example, the motion of a drop of ink in the water is diffusion (Figure 5).

The combination. Diffusion of water molecules causes a loss of the signals produced by nuclear spin, and the magnitude loss is measured. The process is shown in the flow chart (Figure 7).

2. The principles: a combination of two concepts

1. Overall view: the early detection of stroke risk

In this stage, diffusion coefficients measured are mathematically computed to quantitatively describe the diffusion process. At a certain location, a three dimensional ellipsoid (Figure 8) is drawn with axes representing the diffusion magnitudes in different directions.

3. The final step: image construction

What is stroke: Stroke occurs when an artery leading to/within the brain either bursts or is blocked by a clot [1]. Figure 1. Ischemic stroke [1] Types of stroke: 1. Ischemic stroke (Figure 1) - Caused by blockage within blood vessel a. Atherosclerosis Reduced blood flow b. Rupture of plaque Blood clots Blood vessel blocked Permanent damage [1] 2. Hemorrhagic stroke (Figure 2) - Caused by rupture of a weakened blood vessel

Introduction to stroke

a. Aneurysm: Ballooning of blood vessel b. Arteriovenous malformation (AVM): Cluster of abnormally formed blood vessels [1] Effects of stroke: - Partial loss of body function. • Paralysis on the left/right side of the body • Vision or speech problems • Memory loss [1]

Figure 2. Hemorrhagic stroke [1]

1. Metallic Implants (Figure 13) Diffusion MRI is not suitable for patients with metallic implants. - Metal does not contain hydrogen Causes signal loss Distorts magnetic field of surrounding areas [13] - Ferromagnetic surgical clip Experiences a torque and twists Hemorrhage [14] 2. Chemical Shift (Figure 14) It is present at interfaces of tissues which have different molecular organization, e.g. interfaces of fat and other tissues, as the frequency of nuclear spin in fat is lower than in other soft tissues. Fat will be displaced slightly in the image [13].

Limitations & Potential Hazards

Future Development Study of multiple sclerosis (MS): MS is the hardening of tissue in the brain or spinal cord. Diffusion MRI has the ability to assess in vivo the presence of tissue damage occurring in the normal-appearing white and gray matter. It is sensitive to MS damage and capable of detecting its evolution over relatively short periods of time. [15]

Biomarker of response to cancer therapy: Diffusion-weighted MRI can be used for the characterization of malignancy, including determination of lesion aggressiveness and monitoring response to therapy. [16]

Acknowledgements

References

We would like to thank Professor Ed X. Wu from the Department of Electrical and Electronic Engineering, Faculty of Engineering, the University of Hong Kong, who provided some information and advices for these posters. Also we would like to thank Miss Miranda Legg from the Centre for Applied English Studies, The University of Hong Kong.

[1] American Stroke Association. Available at: http://www.strokeassociation.org/. Accessed April 9, 2012. [2] de Graaf RA. In Vivo NMR Spectroscopy Principles and Techniques. 2nd ed. West Sussex, England: John Wiley & Sons; 2007:141-179. [3] Stroke Prognosis. Available at: http://www.strokeprognosis.info/. Accessed April 14, 2012. [4] BrighamRAD. Available at: http://brighamrad.harvard.edu/Cases/bwh/hcache/93/step-2.html. Accessed April 9, 2012. [5] PlaguePhotography on deviantART. Available at: http://plaguephotography.deviantart.com/art/Ink-Diffusion-158876076. Accessed April 14, 2012. [6] Brown MA, Semelka RC. MRI Basic Principles and Applications. 4th ed. Hoboken, NJ: John Wiley & Sons; 2010:161-175. [7] Maher K. Basic Physics of Nuclear Medicine/MRI & Nuclear Medicine. Wikibooks. Available at: http://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/MRI_%26_Nuclear_Medicine. Accessed April 14, 2012. [8] Dougherty G. Image analysis in medical imaging: recent advances in selected examples. Biomed Imaging Interv J. 2010;6(3):e32. Available at: http://www.biij.org/2010/3/e32/. Accessed April 14, 2012. [9] Tuch DS, Wedeen VJ, Dale AM, et al. Conductivity tensor mapping of the human brain using diffusion tensor MRI. PNAS. 2001;98(20):11697-11701. Available at: http://www.pnas.org/content/98/20/11697/F3.expansion.html. Accessed April 14, 2012. [10] The National Institute of Biomedical Imaging and Bioengineering. Available at: http://www.nibib.nih.gov/Research/Highlights/Connectome. Accessed April 9, 2012. [11] Dr. Wes. Available at: http://drwes.blogspot.com/2010/04/heart-strings.html. Accessed April 9, 2012. [12] Indiana University. New Map IDs The Core Of The Human Brain. ScienceDaily. June 30, 2008. Available at: http://www.sciencedaily.com/releases/2008/06/080630200947.htm. Accessed April 14, 2012. [13] Bushong SC. Magnetic Resonance Artifacts. Magnetic Resonance Imaging: Physical and Biological Principles. 3rd ed. Philadelphia, PA: Mosby; 2003:374-392. [14] Bushong SC. Biological Effects of Magnetic Resonance Imaging. Magnetic Resonance Imaging: Physical and Biological Principles. 3rd ed. Philadelphia, PA: Mosby; 2003:396-402, 405-408. [15] Rovaris M, Gass A, Bammer R, et al. Diffusion MRI in multiple sclerosis. Neurology. 2005;65(10):1526-1532. [16] Patterson DM, Padhani AR, Collins DJ. Technology Insight: water diffusion MRI—a potential new biomarker of response to cancer therapy. Nature Reviews Clinical Oncology. 2008;5:220-233.

Figure 13. A small band of metal in this patient’s ponytail produced the “cone-head” artifact. [13]

Figure 14. The dark rim of signal along the right bladder wall occurs because of a chemical shift at the interface between pelvic fat and bladder urine and wall. [13]

3. Motion (Figure 15) It is caused by voluntary, involuntary and microscopic physiological motion, producing “ghosting” or irregular wavelike lines of increased and decreased signal [13]. 4. Biological Effects of MRI a. Static magnetic field: Polarization - No scientific evidence shows that intense static magnetic fields produce harmful effects in mammals. b. Transient gradient magnetic fields: Induced current - It can induce neurological signals in nerve cells and muscle fibers. - Time of applying magnetic field is too short to produce such effects. c. Radiofrequency (RF) field: Thermal heating - It consists of an electric and magnetic field. - Electric field is well shielded by using guard rings. - Magnetic field induces eddy currents within conductive tissues, therefore producing heat. [14]

Figure 15. A. An image of abdomen obtained during breath-holding. B. A similar image obtained in equal duration with quiet respiration, resulting in “ghosting”. [13]

A

B

Advantages of diffusion MRI

Figure 10. Diffusion MRI image of the human brain [10]

Figure 11. Diffusion MRI image of muscle fibers of the heart [11]

Imaging tissue architecture : Tissue architectures such as cell membranes can influence the process of diffusion. So diffusion weighted image allows us to see the micro-architecture of the tissue imaged [2] (Figure 10).

Examining fibrous tissues: Diffusion along fibers can be shown on the diffusion MRI image. So fibrous tissues such as white matter of brains and fibers of muscles (Figure 11) can be imaged for diagnosis [6].

Mapping nerve connections: According to the diffusion intensity, the nerve fibers can be tracked and mapped [6]. Thus, models can be built to reflect the nerve connections (Figure 12).

Figure 12. Model of nerve connection built according to diffusion MRI image [12]