By: Brett Empringham. Supervisor: Dr. Blaine A. Chronik. Medical Biophysics 3970Z: Six Week Project. University of Western Ontario

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1 Relationship Between Insertion Depth and MRI Induced Heating of External Stabilizing Brace By: Brett Empringham Supervisor: Dr. Blaine A. Chronik Medical Biophysics 3970Z: Six Week Project University of Western Ontario April 1, 2011

2 Acknowledgements I would like to thank the following individuals for their support and help in the completion of my report. Dr. Blaine A. Chronik for generously donating his time and effort to educating and helping me through the experiments. Baraa Al-Khazraji and Nicole Novielli for their patient help and support throughout the entire year. Dr. Ian Macdonald for his continuous support and demonstrations throughout the duration of the course.

3 Introduction MRI is an increasingly popular imaging technique in medicine. Its use has exploded in the last thirty years since its introduction to medicine due to its effectiveness in imaging soft tissue such as muscle, tendon, brain and spinal tissue. It works by placing the body in a large magnetic field and exposing it to radio frequency pulses. Normally, this is a very safe process. The body is not exposed to any radiation and the contrast agents that are sometimes used are quite safe. However, oftentimes a patient needs an MRI scan because they are involved in an accident. This accident may require medical hardware to support bones and help with healing. These devices may not be suitable to be placed within the MRI scanner. Magnetic force, torque, and induced heating must be considered. The results of experimental research on these topics are highly variable, suggesting that it is a complex issue (Mattei, E, et al. 2008). Therefore, MRI compatibility must be determined experimentally for each device. This paper focuses on the induced heating of an external leg stabilizing brace with two pins, supplied by ExtraOrtho TM. It keeps all other variables as consistent as possible to isolate the effect of altering the insertion depth of the device into the phantom (substitute for human tissue) on its induced heating. The hypothesis is that as the device is inserted deeper into the phantom the induced heating decreases. Theory An MRI machine works using magnetic fields and radio frequency pulses to create a usable image for doctors. It contains three distinct components that are utilized in a coordinated manner to obtain and create an image. The three main components are the primary magnet, the gradient coils and the radio frequency coils. The primary magnet is used to create a powerful and largely homogeneous magnetic field within the bore. Creating this magnetic field was a difficult challenge in the early development, but is now created using strategically placed coils of wire (Chronik, B. A. 2010). These coils contain high currents which induce a mostly uniform magnetic field along the bore. This current creates the magnetic field according to the Biot-Savart Law which states B= (u o /4π)*(I*dL*x /r^2) (Chronik, B. A. 2010). In this equation, B is the magnetic field, (u o /4π) is a constant, I is the current through the wire, dl is the length of the differential element of the wire, is the unit vector in the direction from the element of wire to the place where the magnetic field is being calculated, and r is the distance between these two points. This suggests that the induced magnetic field is proportional to the current through the wire. Primary magnets have a large range of strengths, and can be as weak as 0.2 T and as strong as 11 T (Chronik, B. A. 2010). In a clinical setting, the maximum strength of the primary magnet is 3 T, and the majority of scans are done using a 1.5 T primary magnet. The field strength of the MRI machine used in these experiments is 3T in strength. When exposed to a

high magnetic field, the magnetic fields of hydrogen nuclei have a tendency to align with the primary field. This gives them a net magnetization. (Coyne, K.

4 high magnetic field, the magnetic fields of hydrogen nuclei have a tendency to align with the primary field. This gives them a net magnetization. (Coyne, K. 2011) The next component of a Magnetic resonance imaging machine is the set of gradient coils. These are also coils of wire which contain electrical current (Chronik, B. A. 2010). They induce a relatively low strength magnetic field in the bore. The strength of this field is dependent on the position within the machine. The principle of superposition states that the net field strength at any point in space is equal to the sum of the individual field strengths. Due to this principle, the total magnetic field caused by the primary magnet and the gradient coils is also dependent on position. (Gelman, N. 2010) Next, radio frequency coils are used to rotate the magnetization of the nuclei around the axis defined by the primary field. The precession frequency of the nuclei is dependent on magnetic field strength and is called the larmor frequency (Gelman, N. 2010). This is the frequency that the radio frequency coils must emit (Mackeiwich, B. 1995). As the nuclei relax back to their original state they release a radio frequency signal which is sensed by the radio frequency coils. The signal that they emit is dependent upon the local net magnetic field, and thus on the location within the bore. A computer that uses a mathematical transform called Fourier transform can be used to transfer the radio frequency signals into a usable image of the tissue. (Hornak, J. P. 2010)

5 Figure 1: This figure provides a visual of a Magnetic Resonance Imaging scanner. (Coyne, K. 2011) The strong magnetic fields created by MRI machines create numerous dangers for patients with devices or implants. The most obvious is physical force and torque. There is also a danger that the device may experience induced heating (Woods, T. O. 2006). Induced heating occurs in tissue and the device when electrical currents flow through them. This heating is called Joule heating and is proportional to the resistance multiplied by the square of the current. This current is a result of voltage according to Ohm s law. Ohm s law states that I=V/R, where I is the current, V is the voltage and R is the resistance (Chronik, B. A. 2010). Voltage is caused by two things. The first is concentration of charges, and the other is changing magnetic flux. Changing magnetic flux causes voltage according to Faraday s law which states that the voltage induced in a circuit is proportional to the time rate of change of the magnetic flux through the circuit. Concentration of charge causes voltage as well and therefore causes current. (Chronik, B. A. 2010) Various factors affect the amount of induced heating during radio frequency exposure. Implantable devices can be made of various materials which have different conductive properties. Therefore they will respond differently to induced voltage. The device used in this experiment contains a bar made of carbon fiber and fiberglass, and pins made of implant grade stainless steel. Location within the bore is another factor that plays a large role in the heating of the device. Heating tends to be greatest near the outer edge of the bore, however the reasoning for this is unclear. Heating is also dependent upon the wavelength of the radio frequency pulses, and thus on the strength of the primary magnet (Mattei, et al, 2008). The relationship between the length of the device and the radio frequency wavelength affects the rate of the temperature increase. However, the relationship is not easy to predict, therefore tests for a device must be done on both 1.5 and 3.0 T MRI systems. To research MRI compatibility of medical devices, a phantom must be used that accurately models human tissue. A phantom is the material that the device is placed in when it is scanned during tests. This phantom must have certain properties that are similar to those of human tissue. In the testing of induced heating, the electrical conductivity, thermal diffusivity, heat capacity and viscosity of the phantom must be regulated (Miller, S. 2009). Refer to the appendix 1 for the specific values of these parameters. An appropriate phantom can be made by mixing poly-acrylic acid, table salt and distilled water together. The phantom must also have a similar shape and size of a typical human torso. Refer to the appendix 1 for the specific dimensions of the phantom container used in this experiment.

6 Methods First, an appropriate phantom must be made. It is convenient to make the phantom in eight litre batches, therefore the quantities of ingredients described here are such that eight litres are made. Eight litres of distilled water is measured into a large container. The conductivity of the distilled water is measured to ensure that it is zero. Next, g of NaCl is measured into the container using an electronic balance. The conductivity is again measured to determine if it is close to 240 ms/m g of Poly-acrylic acid is then added to the solution. The solution is then mixed for ten minutes, using a hand held blender. Since many small blenders may overheat if used for ten minutes, multiple blenders may be used for shorter durations of time. When the solution is adequately mixed, it should be fairly consistent and not granular. The conductivity of the final mixture is measured and should be ms/m (Miller, S. 2009). At the time of the experiment, about 40 litres are poured into an acrylic container shaped approximately like a human torso. Once the phantom is prepared, the device is clamped into place using the phantom suspension apparatus. It is oriented 2 cm from the left side of the phantom when looking at the front of the MRI machine. It is suspended such that the bottom of the supporting bar is about 80 mm from the surface of the phantom. Next, the temperature probe apparatus and computer software is set up. The probes are placed in the phantom and on the device using tape. Probes one and two are attached to the tips of the pins, with probe one being on the pin nearest to the head of the container. Probes three and four are attached halfway up the pins such that probe three is on the same pin as probe one. Probe five is placed in the phantom on the left side, probe six is placed in the phantom near the middle, and probe seven is placed in the phantom on the right side. Probe eight is placed outside of the gel in order to sense the temperature of the room. Figure 2 provides a visual representation of the locations of the probes. With the probes in place, the phantom is inserted into the MRI machine. The command is given to the computer to begin recording temperature. After approximately two minutes, or when the temperature probes are reporting steady state temperatures, radio frequency exposure is initiated. Exposure continues for up to seven minutes, or when the hottest probe reaches sixty degrees Celsius. At this point, radio frequency exposure is discontinued and the temperatures are recorded during the cooling process. Once the device is cooled, the device is inserted 10 mm further into the phantom and scanning is repeated. The experiment is repeated four times.

7 Figure 2- This figure provides a visual representation of the locations of the temperature probes. Probes 1 and 2 are on the tips of the pins, while probes 3 and 4 are halfway down the pins. Probes 5, 6 and 7 are in the phantom and probe 8 is outside of the phantom.

8 Results Change in Temperature ( o C) Temperature Increase at Various Points in Phantom with Bottom of Bar 78 mm from Surface of Phantom Time (s) probe 1 probe 2 probe 3 probe 4 probe 5 probe 6 probe 7 probe 8 Figure 3: This figure shows the temperature increase at all eight probes during the experiment where the device protrudes furthest from the phantom. The x-axis displays the time of the experiment. It is important to note that the time does not begin at zero, but begins at about 35 seconds before radio frequency. The y-axis represents the change in temperature from the original temperatures of the probes. The dashed line closest to the left indicates the start of radio frequency exposure and the one to the right indicates the end of exposure.

9 Temperature Increase at Various Points in Phantom with Bottom of Bar 38 mm from Surface of Phantom 35 Change in Temperature ( o C) Time (s) probe 1 probe 2 probe 3 probe 4 probe 5 probe 6 probe 7 probe 8 Figure 4: This figure shows the temperature increase at all eight probes during the experiment where the device is driven the furthest into the phantom. Along the x-axis is the time of the experiment. Along the y-axis is the change in temperature from the original temperature of the probes. This experiment is noteworthy because it is the only one where probes three and four, which are the ones halfway down the pins, are submersed into the phantom. The dashed line closest to the left of the figure represents the start of radio frequency exposure and the line to the right represents the end of exposure.

10 30 Temperature Increases of Probe 1 at Different Depths Temperature Change ( o C) Time of Radio Frequency Exposure (s) 78mm from phantom surface 68 mm from phantom surface 58 mm from phantom surface 48 mm from phantom surface 38 mm from phantom surface Figure 5: This figure displays the change in temperature of probe one in each of the experiments. That is, the temperature at the hottest probe when the device is at different depths in the phantom. On the x-axis is the time since the commencement of radio frequency exposure in seconds. Along the y-axis is the change in temperature from the original probe temperature in degrees Celsius. This figure only shows the first twenty seconds of radio frequency exposure to highlight the initial rate of heating. The results indicate that only probes that are immersed into the phantom sense significant change in temperature. This can be seen by observing figures 3 and 4. In figure 3, probes one and two are the only probes that are in the phantom, and as a consequence are the only probes that report a significant rise in temperature. Figure 4 shows similar results, but with probes one, two, three and four heating up. Figure 3 illustrates how probe one, which is the one nearest to the back of the MRI machine, heats up faster than probe two. At the end of radio frequency exposure, probe one senses a 38.0% greater increase in temperature than probe two. See calculation 1 in Appendix 2 for details of the calculation. From figure 4, information about the heating at the tips of the pins relative to the middle of the pins can be obtained when the bar is 38 mm from the phantom surface. By comparing the probes that are on the pin closest to the back of the machine, we see that the probe at the tip of the pin increased in temperature 743 % more than the probe in the middle. See calculation 2 in Appendix 2 for details of the calculation. A similar result is obtained from the other pin.

11 Figure 5 provides information about the differing rates of induced heating at differing insertion depths. Specifically, when the bar is 78 mm from the surface of the phantom, the heating is 225% greater than it is at 38 mm from the surface after 20 seconds. Again, see calculation 3 in Appendix 2 for details of the calculation. Discussion As discussed in the Theory section, the induced heating is caused by currents flowing through the material. The electrical currents created inside an MRI are caused by both the gradient coils and the radio frequency coils. Both of these components induce a voltage in the loop created by the device and the phantom surface. The changing currents and orientation of currents within the machine create a changing magnetic field according to Biot-Savart law. This changing field results in a changing flux through the circuit and thus a voltage, according to Faraday s law. Note that this effect requires a complete circuit in order for current to flow. The other cause of current is the concentration of charge in the gradient coils. As charge concentration changes, an induced voltage occurs within the MRI machine. This effect does not require a complete circuit and is strongest along the outer edge of the bore. The reasoning for this is unclear. In experiments one through four, it can be seen that probes one and two are the only probes that sense heating. In experiment five, probes one, two, three and four all reported an increase of temperature, with the one and two experiencing a greater increase than three and four. Since these probes are the only ones that are in the phantom, it appears that the device only heats up during radio frequency exposure at the points where it is submersed in the gel. This is an interesting finding and may reflect the high resistance in the circuit at the junction of gel and the pin. As discussed earlier, Joule heating is proportional to resistance, thus there would be a great deal of heating at this junction The next interesting finding that can be observed from the results is how probe one consistently heats up faster than probe two. This suggests that the induced heating is greater near the back of the MRI machine. The reasoning for this is unclear and likely reflects the design of the system. Every machine works using the same premise, however they all have slight differences which may cause this uneven heating. The results suggest that there is a strong negative relationship between the insertion depth of the device and the induced heating. This is caused by a number of effects. When the device is inserted deeper into the tissue, it creates a smaller circuit loop than it does when it is not inserted as deep. Thus, according to Faraday s law, the induced voltage created along the loop by changing magnetic flux is reduced. The lower induced voltage results in a lower current according Ohm s law, thus the Joule heating is reduced. The next reason is associated with the

12 amount of phantom that the heating pins touch. As the pins are driven deeper into the phantom, more gel is exposed to heating. Thus, the heat is distributed over more matter and the localized heating at the temperature probe is decreased. The last possible reason is based on the geometric positioning of the device. At the greatest protrusion from the phantom, the device is located in the upper left portion of the bore, as seen when looking at the front of the MRI machine. Therefore, when it is pushed down deeper into the gel, it becomes further away from the outer edge of the bore. As discussed earlier, the induced heating is greatest along the outer edge. Thus, when the device is moved down it moves to a region of lower heating and the device does not heat up as much. It is clearly beneficial for medical devices to be MRI compatible. However, testing must be done to be sure that the devices are safe in every circumstance. Testing should test devices in the worst case scenario. Therefore, patients can be sure that they are safe in a clinical setting. The information obtained from the results of this experiment can be used to help determine this worst case scenario. The results show that the induced heating is greatest near the back of the MRI machine when looking at it from the front. Therefore the device should be positioned near the back during a test of MRI compatibility. Secondly, the device heats up significantly less when it is inserted deeper into the phantom. This has interesting applications in health care. When devices are implanted in individuals who will need to have an MRI scan done, heating can be minimized by implanting the device as deep as possible. Shortening the pins to the minimum necessary length may reduce heating even further. In these ways, health care can be improved for individuals who need to have an MRI scan and have a medical device implanted in their body. It is important to note that testing must also be done on MRI systems of different magnetic field strengths to determine if the same findings are applicable for all clinical MRI machines. Conclusion This experiment was designed to investigate the factors that affect the induced heating of a medical device caused by Magnetic resonance imaging. More specifically, the insertion depth of the pins of the device was changed to determine the effect on the heating. The results indicate that as the device is inserted deeper, induced heating decreases. The heating was greatest on the pin closest to the back of the MRI machine. Additionally, the heating was greatest on the tips of the pins, and significant heating occurred only on the parts on the pins that were submersed in the phantom. The finding that induced heating decreases as insertion depth increases agrees with the hypothesis, however the other results were surprising.

13 References Chronik, B. A. (2010). Medical Biophysics 2128a: Fundamental Concepts of Medical Imaging. University of Western Ontario, Lecture Notes. Coyne, K. (2011). MRI: A Guided Tour. Florida State University. Gelman, N. (2010). Medical Biophysics 3505F: Mathematical Transform Applications in Medical Biophysics. University of Western Ontario, Lecture Notes. Hornak, J. P. (2010). The basics of MRI. Rochester Institute of Technology. Mackeiwich, B. (1995). Intracranial Boundary Detection and Radio Frequency Correction in Magnetic Resonance Images, Simon Fraser University. Mattei, E., Triventi, M., Calcagnini, G., Censi, F., Kainz, W., Mendoza, G., Bassen, H. I., Bartolini, P. (2008). Complexity of MRI induced heating on metallic leads: Experimental measurements of 374 configurations. BioMedical Engineering Online, 7:11. Miller, S. (2009). Standard Test Method for Measurement of Radio Frequency Induced Heating Near Passive Implants During Magnetic Resonance Imaging. American Society for testing and Materials. Woods, T. O. (2006). Standards for Safety of Medical Devices in MRI. U.S. Food and Drug Administration: Center for Devices and Radiological Health.

14 Appendix 1 Phantom container dimensions: Parameter Value Height 7.0 Width 17.0 (+/- 1/32 ) Length 24.0 (+/- 1/32 ) Wall thickness ¼ all sides Bottom thickness ½ Material= acrylic Regulated Parameters of the Phantom: Parameter Approximate Value Conductivity 240 ms/m Thermal Diffusivity 1.3x10-7 m 2 /s Specific Heat Capacity 4184 J/kg o C Viscosity Such that no bulk transport occurs (convection)

15 Appendix 2 Calculation 1 Percent increase=( larger value - smaller value)/(smaller value )*100 =(38.66 o C o C)/( o C)*100 =38.0% Therefore probe 1 experiences a 38.0% greater temperature increase than probe 2. Calculation 2 Percent increase=(32.54 o C o C)/(3.86 o C)*100 =743% Therefore probe 1 experiences a 743% larger increase in temperature than probe 3. Calculation 3 Percent increase=(25.79 o C o C)/(7.93 o C)*100 =225% Therefore probe 1 experienced a 225% greater temperature increase when the bar was 78 mm from the phantom than when it was 38 mm from the surface.

Radio Frequency Interactions with Medical Devices

Radio Frequency Interactions with Medical Devices Justin Peterson University of Western Ontario MBP 3970Z April 1, 2011 Introduction Magnetic Resonance Imaging (MRI) systems are an extremely important