Radio Frequency Interactions with Medical Devices

Size: px

Start display at page:

Download "Radio Frequency Interactions with Medical Devices"

Janice Richards
5 years ago
Views:

1 Radio Frequency Interactions with Medical Devices Justin Peterson University of Western Ontario MBP 3970Z April 1, 2011

2 Introduction Magnetic Resonance Imaging (MRI) systems are an extremely important technique when considering the internal structure of an object, in this case a human. If a person experiences an accident and suffers a severely broken joint a device is used to stabilize the joint to ensure proper healing. If the accident is severe enough an MRI is needed to check for damage in other areas that could otherwise go unnoticed by a physician. Human tissue naturally undergoes heating near the surface of the skin during an MRI but an electrically conductive, elongated device could concentrate the Radio Frequencies (RF) and cause damaging effects [1]. To prevent damaging the tissue of the patient, tests must be performed on each device to ensure the degree of heating is acceptable. The factors affecting the rate of heating in the device include the strength of the magnet, the geometry and composition of the device, and the position in the bore. The testing should focus on the worst case scenario that the device could experience during an MRI scan. The first objective of this study was to determine the relationship between radial position in the bore and rate of temperature change of the device; the second objective was to determine the ideal worst case scenario for future testing. Theory The three main components of an MRI system are the main magnet, the gradient coils and the RF coils. The main magnet produces a uniform magnetic field along the length of the bore; this field aligns the hydrogen nuclei in what is to be imaged. The gradient coils produce a magnetic field that varies in strength along a plane (in any orientation) of the operator s choice, which make the strength of the magnetic field dependent on position in the bore. The RF coils produce a pulse, which causes the hydrogen nuclei to precess about an axis; the nuclei will precess at different speeds according to the position on the plane produced by the gradient coils. The nuclei release energy as they stop precessing; the RF coils detect this energy and send information to a computer, which builds the image [1]. The MRI was a 3T Trio model manufactured by Siemens. A phantom is a general representation of the body on the MRI bed; it is large enough for the device to be placed representative of where it would be in a real scenario. The phantom is made of an electrical insulator and is non-magnetic; it must be filled with a material that has a conductivity similar to that of human tissue. The gel that filled the phantom had a conductivity of 4.80 ms/cm. The device is comprised of two stainless steel pins connected by a carbon fiber core surrounded in a fiberglass sheath. The stainless steel pins have threads in one end, which are to anchor the device into bone. The carbon fiber/fiberglass rod connects the pins outside the body at some distance from the skin surface. 2

3 Methods Before the tests were performed the phantom gel was made using distilled water, Sodium Chloride (NaCl), and Poly-acrylic acid, partial sodium salt (PAA). Pour 8 L of distilled water into a container and measure the conductivity (should be 0 ms/cm). Next, 1.32 g/l NaCl was added to the distilled and mixed thoroughly and the conductivity was measured to be 2.40 ms/cm. Next, 10.0 g/l of PAA was mixed into the water/nacl solution. The entire solution was mixed for a total of 10 minutes (until the mixture was completely smooth ); this conductivity was measured to be 4.80 ms/cm. The part and batch numbers of the PAA were kg and MKAA3066, respectively. The part and batch numbers of the NaCl were and This process was repeated five times until a total of 40 L of gel was made, 30 L of the gel was poured into the phantom to a depth of 10 cm. The probes used to measure the change in temperature during the tests were optical sensors accurate to 0.2 C and a program named Neoptix collected the data. There were eight temperature sensors placed throughout the phantom gel and the device. Figure 1: Temperature Probe Placement 1: Tip of the furthest pin in the gel 2: Tip of the closest pin in the gel 3: Mid-way up the furthest pin out of the gel 4: Mid-way up the closest pin out of the gel 5: Along the left side of the phantom 6: Outside of the phantom (Ambient) 7: Along the right side of the phantom 8: Outside of the phantom (Ambient) Figure 2: The device was placed in five different radial positions: -240mm, -210mm, - 160mm, 0mm, and +230mm with 0 being the center of the MRI bore. In all five tests the device was anchored with a space of 550 mm from the bottom of the bar to the phantom gel. 3

4 Results All the numbers and figures in this study were calculated and plotted using Microsoft Excel Temperature ( C) Temperature Change -160 mm From Centre of Bore Sensor 1 Sensor 2 Sensor 3 Sensor 4 Sensor 5 Sensor 6 Sensor Sensor 8 Time (s) Figure 3: The temperature change at eight probes placed on the device, in the gel and in the surrounding environment. The start and end of RF exposure are at 60 and 240 seconds, respectively. Temperature( C) Temperature Change +230 mm from centre of Bore Time (s) Sensor 1 Sensor 2 Sensor 3 Sensor 4 Sensor 5 Sensor 6 Sensor 7 Sensor 8 Figure 4: The temperature change at eight probes placed on the device, in the gel and in the surrounding environment. The start and end of RF exposure are at 60 and 140 seconds, respectively. 4

5 Varying Radial Position of Device on Temperature Temperature ( C) mm mm mm 0 mm mm Time (s) Figure 5: Sensor 1 data from five tests at different radial positions along the bore of the MRI. Data was collected during the first 20 seconds of each test. dt/dt ( C/s) Rate of Temperature Change Dependency on Radial Position in MR Bore Radial Position in Bore (mm) Figure 6: Rate of temperature change at five different positions along the radius of the bore calculated from Sensor 1 data during the first 20 seconds of data collection. 5

6 In Figures 3 and 4, there was 60 seconds of steady temperature data before the RF exposure was turned on. In Figure 3 the device pin temperature increased at a rate of C/s while in Figure 4 the device pin increased at a rate of C/s. Almost instantly after the exposure stopped, the rate of temperature change began decreasing. Figure 5 depicts the temperature change with time for five radial positions along the MRI bore, using the first 20 seconds of this data the rate of temperature change for each position was calculated. Figure 6 shows spatially how the temperature rate of change varies with radial position along the bore. Discussion In this study, the effect of radial position on rate of temperature change due to radio frequency was tested. Between all eight of the optical temperature probes, Sensor 1 and 2 showed a rise in temperature large enough to look at (Figure 3). For simplicity, only Sensor 1 was analyzed because Sensor 2 had a smaller rate of temperature change. This is consistent with a study by the American Society for Testing and Materials (2009) [1] ; the ends of an elongated device will experience the greatest amount of heating. Figure 1 shows the position of Sensors 1 and 2 as the ends of the device, although Sensors 3 and 4 were also near the ends of the device but they did not undergo heating comparable with the first two sensors. This could be due to the induced electric field from the gradient coils, which cause an induced current around the loop of the device. This induced current travels around the device from one conductive pin to the bar to the other conductive pin but when the electrons attempt to migrate to the other pin. The gel acts as a resistor in the circuit and as a result the pin and the gel in the vicinity experiences a greater amount of heating. If the circuit were flowing in a clockwise direction around the device, it would explain why Sensor 1 was hotter than Sensor 2. Sensor 1 was chosen as a common probe among all tests because it experienced the highest rate of temperature change among all probes. As seen in Figure 6, the highest rates of temperature change that were experienced by the device occur along the edge of the MRI bore (furthest away from the center). As the device position moved from the edge of the bore to the center the rate of heating decreased from a maximum of C/s to a minimum of C/s. A possible explanation for this is the induced electric field associated with the gradient coils of the MRI. The electric field produced by the gradient coils is related to the strength and direction of the rate of change of the magnetic field [2]. This electric field is generated along the length of the bore, which is why the device experiences the highest rate of heating in the configuration tested, with the bar parallel with the field. The electric field is strongest near the coils that produce them or furthest from the center of the bore. 6

7 There is a strong trend that suggests the worst case scenario for device testing along the radial position would be along the edge of the MRI bore. This was not significant because a statistical analysis could not be performed with a small sample size of one for each group of testing. If the sample size were increased, there would be a parabolic trend between radial position and rate of temperature change much stronger than that seen in Figure 6. An interesting area of study would be testing the device in different positions along the length of the MRI bore. Since the device experienced a greater amount of heating on the furthest pin, a test of the device farthest away from the torso might result in a greater amount of heating on both pins. These results are accurate for a Siemens 3T Trio device only, other MRI devices with differing magnetic field strength and manufacturing specifications will produce different results [1]. Conclusion The two objectives of this study were to determine the relationship between radial position in an MRI bore and rate of temperature change of a passive medical implant and to determine the ideal worst case scenario for future testing. As radial distance increased from the center of the bore, the rate of temperature change increased as a parabolic trend. A larger sample size could have produced significant results and a better parabolic trend. The worst case scenario for radial position is along the edge of the MR bore (farthest from the center). An interesting area of research would be to test the device along the length of the bore to determine where exactly the ideal worst case scenario would be for testing of similar devices. These results can be used to refine and update the current standards when testing passive medical devices in the future. 7

8 References [1] Miller, Stephen. Standard Test Method for Measurement of Radio Frequency Induced Heating Near Passive Implants During Magnetic Resonance Imaging. Tech. no. F a. Pennsylvania, Print. [2] Glover, P. M., and R. Bowtell. "Measurement of Electric Fields Induced in a Human Subject Due to Natural Movements in Static Magnetic Fields or Exposure to Alternating Magnetic Field Gradients." Physics in Medicine and Biology 53.2 (2008): Web. 1 Apr

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

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