The Insane Engineering of MRI Machines — Transcript

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1. 0:00You are probably familiar with magnetic resonance imaging machines, either through the claustrophobic
2. 0:05sensation of being inside one or from seeing the incredibly high detail cross sections
3. 0:10of the human body they provide.
4. 0:12.
5. 0:13MRIs have completely changed how we view and understand our bodies.
6. 0:17It has never been easier to visualize organs with such high details.
7. 0:21We can safely locate and identify tumors in the kidneys, brain, stomach, and pancreas.
8. 0:27We can inject paramagnetic contrast agents into the bloodstream to locate blockages in
9. 0:31the heart, allowing doctors to accurately implant life-saving stents to open blood vessels.
10. 0:37A tiny keyhole surgery that patients can recover from quickly, where once dangerous open heart
11. 0:42surgery was the only option.
12. 0:45Magnetic resonance imaging has truly changed the nature of medical diagnosis and treatment.
13. 0:50A technology that seems straight out of a sci-fi novel, how they work is a complete
14. 0:55mystery to most.
15. 0:57To unlock the power of this machine physicists and engineers first had to discover and master
16. 1:02the principles of quantum mechanics, superconducting magnets, computer science, and mathematics.
17. 1:13The clean futuristic white facade of an MRI machine hides a world of complicated and marvelous
18. 1:18engineering that you may not have ever considered.
19. 1:21Prior to their introduction to medicine, we peered into our bodies using harmful ionizing
20. 1:26x-rays or low-detailed ultrasounds.
21. 1:29While incredibly useful, even to this day, these two imaging techniques pale in performance
22. 1:34to the safe, millimeter resolution of MRIs.
23. 1:37Capable of creating a 3D reconstruction of the body rather than a flat 2D image.
24. 1:43Achieving all of that without any moving parts.
25. 1:47Imaging in medicine relies on collecting signals from the body based on the innate physical
26. 1:52properties of tissues.
27. 1:54Ultrasound imaging relies on how sound waves bounce off tissues of different densities.
28. 1:59X-rays form images based on the absorption of high-energy radiofrequency waves.
29. 2:04Magnetic resonance imaging, however, relies on something far less intuitive, the quantum
30. 2:09properties of the hydrogen atom.
31. 2:11The human body is teeming with hydrogen atoms
32. 2:14located in water, carbohydrates, and proteins.
33. 2:17To image the body, MRI machines exploit a quantum property of these hydrogen atoms.
34. 2:23A property called spin.
35. 2:25How it does this is a marvel of modern physics and engineering.
36. 2:29Spin is an innate property of particles just like mass and charge.
37. 2:34Spin makes particles behave like tiny bar magnets.
38. 2:37The proton inside the hydrogen nucleus behaves like a magnet.
39. 2:41The orientation of its magnetic north is described probabilistically.
40. 2:46Under normal circumstances, this probability is evenly distributed.
41. 2:50This means that the combined magnetic field of many hydrogen atoms cancels out.
42. 2:55But, this changes when the hydrogens are inserted into a large external magnetic field just
43. 3:01like the ones MRIs create.
44. 3:04This changes the distribution of the tiny magnets, augmenting the number of atoms aligned
45. 3:09with the external magnetic field.
46. 3:12This imbalance is the source of the MRI signal, as we can manipulate these tiny magnets to
47. 3:18produce signals that can be processed into images.[1]
48. 3:21Once the atoms are aligned in the machine's incredibly strong magnetic field the machines
49. 3:27give them a tiny nudge using a magnetic radiofrequency pulse.
50. 3:31This pulse comes from this set of coils inside the machine that send pulses by simply applying
51. 3:37an alternating current through its coils at a very specific frequency.
52. 3:42This nudge misaligns these hydrogen bar magnets to shift their magnetic field perpendicular
53. 3:48to the large magnetic field the machine is creating.
54. 3:51Naturally, the spins want to align their orientation back to their original position, aligned with
55. 3:57the machine's magnetic field, but they don’t fall back immeditely, they decay in a spiraling
56. 4:03motion.
57. 4:04This decay causes a changing magnetic field, and by placing a coil of wire nearby, this
58. 4:10changing magnetic field can induce currents that can be read as a clear signal.
59. 4:15MRI machines can use the same coils to send the “nudge pulses” and to read the signal
60. 4:20from the body.
61. 4:21These coils are placed here, as close to the patient as possible but still Inside the MRI
62. 4:27tube.
63. 4:28For higher contrast and resolution, some machines use separate coils to transmit pulses and
64. 4:33receive the signal.
65. 4:35This allows the receiver coils like these to be placed much closer to the body, maximizing
66. 4:40the strength of the signal.
67. 4:41[2]
68. 4:42We can increase the number of hydrogens, aligning with the external magnetic field by increasing
69. 4:47the strength of the external magnetic field.
70. 4:50In turn, the hydrogen magnet can induce a larger current as it spirals back.
71. 4:56So, by simply increasing the strength of the MRIs field, we increase the strength of the
72. 5:01signal collected and therefore improve image quality.
73. 5:06Common MRI field strengths are 1.5 to 3 tesla, around 300,000 times stronger than the earth's
74. 5:12magnetic field and 30,000 times stronger than your common fridge magnet. [3] You absolutely
75. 5:18do not want anything ferrous around these machines during operation, a field this strong
76. 5:24can lift nearby wheelchairs straight off the ground.
77. 5:27For research purposes, MRIS can produce even higher magnetic fields, up to 20 teslas.
78. 5:33Achieving this intense magnetic field comes as no easy feat.
79. 5:37Early MRIs used permanent magnets as their source of the main magnetic field but only
80. 5:42reached strengths of 0.5T limiting the resolution of the machine [4].
81. 5:48Electromagnets can be used to reach stronger magnetic fields, but standard electromagnets
82. 5:53can’t produce a 1.5 tesla field.
83. 5:56Higher magnetic fields require higher electric currents that would melt ordinary wires.
84. 6:02To achieve larger currents in the wires, engineers required superconducting coils. [4]
85. 6:08Superconductors are another sci-fi technology.
86. 6:11Temperature affects all metallic conductors.
87. 6:14With resistance gradually lowering with temperature.
88. 6:17But superconducting materials are special, in that their resistance drops to zero at
89. 6:22temperatures close to minus 273 degrees Celsius, or absolute zero.
90. 6:28In theory, when this happens, an electric current could travel in a loop of superconducting
91. 6:34material indefinitely, never needing a power source.
92. 6:37In reality, this means that the main superconducting coil in MRIs does not consume any power directly.
93. 6:44Rather, the main consumption of energy is just to keep the coil cooled down so the current
94. 6:50will travel endlessly, leaving the MRI magnet permanently on.
95. 6:54The energy needed to run an MRI for a full year is equivalent to 25 four-person households,
96. 7:01around 130,000 to 140,000 kWh per year.
97. 7:06[5]
98. 7:07The most common superconducting material used in MRIs is Niobium–titanium.
99. 7:10[6].
100. 7:11The demand for High-resolution images is so large that 80% of all the Nb-Ti we extract
101. 7:18from the earth goes into an MRI machine [7].
102. 7:21To achieve the incredibly low temperatures needed for superconductivity, we need a very
103. 7:27cold refrigerant.
104. 7:29Early MRI machines used to submerge their superconducting wires in a bath of liquid
105. 7:34Helium.
106. 7:35Pouring one thousand (1000) liters of liquid helium, at minus 269 degrees celsius, into
107. 7:40the machine to cool the superconducting coil as close to absolute zero as possible.
108. 7:46This evaporated the helium, allowing it to escape the machine as a gas.
109. 7:50Meaning, early MRI machines required regular refillings of liquid helium.[8]
110. 7:56Even though helium is an incredibly common gas in the universe, it is so light that it
111. 8:00can escape our atmosphere into space.
112. 8:03We extract helium from underground gas caverns, where it accumulated as a byproduct of uranium
113. 8:08and thorium radioactive decay.
114. 8:11But once we allow that helium to escape into the atmosphere, it is gone for good.
115. 8:16Floating to the top of our atmosphere and gradually being blown into space by solar
116. 8:20winds.
117. 8:21We will eventually run out of natural helium.
118. 8:24This method of cooling was costly and unsustainable, costing up to $26,000 per year in helium refills.[9]
119. 8:31To avoid this refilling problem, modern MRI machines use a vacuum-sealed chamber that
120. 8:37holds the liquid helium without letting it evaporate.
121. 8:40This eliminates the need for refilling and minimizes the cost of operation.
122. 8:45These so-called “Zero Boil off” machines are now the norm in MRI technology.
123. 8:50They use an electric refrigerant cycle like the one in your fridge but on steroids.
124. 8:55This cycle keeps the helium in its liquid phase and keeps the magnets cool enough to
125. 9:00maintain them in their superconducting state [8].
126. 9:03Now that we know why and how the hydrogen atoms are aligned, we need a way to turn that
127. 9:08information into an image, and to do that we need to know the physical location of the
128. 9:13hydrogen atoms.
129. 9:14Recall that MRIs detect signals from the spiral decay pattern of hydrogen atoms after they
130. 9:19have been “nudged”.
131. 9:21These spirals decay with a unique rotational frequency (⍵),. interestingly, this is the
132. 9:26only frequency that can “nudge” the hydrogen atoms too.
133. 9:30Luckily, rotational frequency is dependent on something we control, the magnetic field
134. 9:34strength.
135. 9:35For example, hydrogen resonates at 64 MHz at 1.5 Tesla and at 128 MHz at 3 tesla[10].This
136. 9:44means the atoms in a weaker magnetic field will rotate slower while atoms in a stronger
137. 9:49field will rotate faster.
138. 9:51To nudge these atoms we need a radiofrequency wave at 64 MHz or 128 MHz.
139. 9:59We can use this to our advantage to image individual slices.
140. 10:04If we can apply a gradient to the magnetic field strength, we can selectively nudge atoms
141. 10:09along the gradient by applying the corresponding frequency.
142. 10:13This gradient is applied precisely using a separate set of regular electromagnets, aptly
143. 10:18named gradient coils.
144. 10:20To image a slice near the weaker end of the tube, the machine sends a “nudging pulse”
145. 10:25centered at a lower rotational frequency say 63.998 MHz and to image a slice on the other
146. 10:33side of the tube the pulse is centered at 64.002MHz.
147. 10:38This still feels like magic.
148. 10:40It’s not terribly obvious how detecting the decay from these spiraling hydrogen atoms
149. 10:45can create images.
150. 10:47The receiving coil can only measure the sum of all these spiraling decays.
151. 10:52We need a way of processing the signal to create an image.
152. 10:56Let's start by understanding how to get contrast between tissues before delving deeper into
153. 11:01the techniques used to actually form images.
154. 11:05We need a way of identifying different kinds of tissues to form an image, to do that we
155. 11:11need to contrast the tissues using two different signal types.
156. 11:15The first one deals with how quickly atoms re-align themselves with the large magnetic
157. 11:20field after a nudging pulse.
158. 11:23This is called T1 relaxation.
159. 11:25The second measure comes from the physical reality of interaction between hydrogens.
160. 11:30The atoms do not realign with the magnetic field uniformly.
161. 11:34In tissue, hydrogens interact with each other and with their surroundings.
162. 11:39Right after the nudging pulse is sent, the small interactions cause the spins to fall
163. 11:44out of uniformity.
164. 11:46Since the coil can only measure the sum of all these spiral decays, then the failing
165. 11:51out of order would create a decaying signal.
166. 11:54This is called T2 decay.
167. 11:57Importantly, the two rates are not equal, and even more important they are dependent
168. 12:02on the tissue.
169. 12:04Hydrogens in fat have different intrinsic characteristics and interactions than hydrogens
170. 12:08in water.
171. 12:09This difference is what lets technicians contrast the tissues.[10]
172. 12:13We can emphasize the T1 signal by sending pulses rapidly and listening to the signal
173. 12:18immediately, as the dephasing effects of T2 do not have enough time to take place.
174. 12:24We can emphasize T2 by sending pulses slowly and listening for longer, allowing the dephasing
175. 12:29to occur.
176. 12:30[11]
177. 12:31Just like a photographer can play with camera settings to take pictures of either the bright
178. 12:35sky or a dim environment.
179. 12:37MRI technicians can also play with two settings to take images of contrasting tissues, the
180. 12:43time between pulse repetition, and how long to wait to listen for a signal.
181. 12:48These two parameters are chosen by the technician each time an MRI is performed and they choose
182. 12:53them depending on what the doctor would like to image.[10]
183. 12:56For example, T1 is used to image fatty tissues while suppressing the signal from water.
184. 13:02But maybe the doctor wants to assess the cerebral spinal fluid in the spine or the brain, and
185. 13:07for that T2 signals are emphasized to enhance the signal from water-based fluids.[12]
186. 13:13Remember, the signal is collected as a sum from a singular slice, The machine needs to
187. 13:18form a 2D image of each slice.
188. 13:22There is one extra layer of complexity since the receiver coil can only measure the sum
189. 13:27of the signals from all the decaying hydrogens in the slice.
190. 13:31This is yet another story where old mathematics discoveries guide modern technology.
191. 13:37In 1822 Joseph Fourier created a mathematical framework that deconstructs complex waves
192. 13:44into simple additions of simpler waves.
193. 13:46This framework can be extended to 2D grayscale geometries.
194. 13:51Just like a musical melody can be simplified into additions of simpler notes.
195. 13:55Any image can be deconstructed into a weighted average of simpler black and white stripes.
196. 14:01This is what MRIS uses to create images.
197. 14:04Instead of sampling individual pixels, MRIs sample different striped patterns.
198. 14:09In reality, what does this mean for the spinning hydrogens?
199. 14:13What is the physical representation of these patterns?
200. 14:16It is easier if we imagine the slice as a grid of rotating hydrogens where we color
201. 14:21the phase of the atoms in grey scale.
202. 14:23When rotating in unison the grid is all white.
203. 14:27Atoms rotating 180 degrees out of phase are colored black.
204. 14:31MRIs exploit this to physically create the striped patterns needed for the Fourier analysis.
205. 14:38Using another set of gradient coils MRIs precisely change the phase of rotating hydrogens to
206. 14:44create striped patterns.
207. 14:46By controlling how long the Y and the X gradients are turned on, the MRI can create patterns
208. 14:51in all directions and frequencies.
209. 14:54Slowly the machines start to imprint all the patterns with different frequencies and orientations
210. 14:59to sample their relative strengths in physical space.
211. 15:02And slowly by adding more and more patterns the image starts to emerge.
212. 15:08This is what forms the image for each 2D slice.
213. 15:11Then the machine goes on to the next slice of the body and starts the process all over
214. 15:16again.
215. 15:17Turning on and off coils inside a magnetic field might sound familiar if you’ve ever
216. 15:22played with a speaker.
217. 15:23As basically they are the same things.
218. 15:26Gradient coils rapidly turning on and off are the source of the loud clunking sounds
219. 15:30in MRIs.
220. 15:32MRIs are constantly evolving and the core technology is starting to branch off.
221. 15:37Researchers want the highest resolution and strive to increase the strength of the magnet,
222. 15:42while some hospitals and companies are making them smaller and cheaper, as more practical
223. 15:46and cheap MRIS are incredibly useful in the field.
224. 15:50[13]
225. 15:51It's hard to fathom the complexity of all these systems, from superconducting wires
226. 15:54to vacuum-sealed helium reserves to rapidly changing magnetic gradients, and even more
227. 16:00incredible to think of the first people who figured out how to combine these technologies
228. 16:05together to peer into our bodies.
229. 16:08This intricate dance of quantum physics and carefully manipulated gradients has allowed
230. 16:13MRIs to change the world of medicine.
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