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Back in 1882, the 26-year-old Serbian American inventor and engineer Nikola Tesla discovered about rotating magnetic fields (Hunt, 2018). Little did he know that just 130 years later, his discovery would be the foundation for a powerful medical diagnostic tool that has saved millions of lives called Magnetic Resonance Imaging (MRI). Today, more than 3 million scans are made every year just in England which comprises less than 1% of the world's population (NHS, 2018).
In 1930, Isidor Isaac Rabi started his research on molecular beams where he succeeded in precisely measuring the nuclear motion of molecules and atoms including hydrogen, lithium and deuterium.
His work which was issued in 1937 won him a Nobel Prize in Physics in 1944 (Editors of Encyclopedia Britannica, 2018). In his experiment, he used a 0.2T electromagnet and a hairpin to produce a field. He passed a beam of lithium chloride across a vacuum then into the apparatus. His team recorded the resonance peaks for lithium and chlorine as expected. He called this "nuclear magnetic resonance" which is now widely known as NMR.
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When World War II ended on August 14, 1945, the MIT Radiation Laboratory, which was more commonly called as Rad Lab, was preparing to close down. However, a few of the researchers were asked to stay on. One of them was Edward Mills Purcell. During fall in 1945, Purcell suggested to his colleagues Robert Vivian Pound and Henry C. Torrey that they determine the absorption of radiofrequency energy caused by nuclear magnetic moments in a solid material that contains protons, which in this case is paraffin.
(Pound, 2000) They made a resonant cavity using a coaxial cable that can oscillate at 30 MHz. They filled the cavity with 850 cc of solid paraffin. The magnetic field strength was then increased slowly by placing the apparatus in between the gap of a large magnet at Harvard's Research Laboratory of Physics. A sudden increase by 20 times was observed at roughly 7100 oersteds or 0.71T. This verified that the resonance of a proton (hydrogen nuclei) is at that nuclear magnetic resonance peak. (Purcell, Torrey & Pound, 1945)
1079511366500Just one month later at Stanford University, Felix Bloch and two of his collaborators William Webster Hansen and Martin Packard did experiments to study nuclear induction on water. The experiment was carried out using 1 gram of water put in glass bulbs with a volume of 1cc which were then wrapped with receiver coil and a.c. coils to give the field modulation. Lastly, the apparatus was placed in the magnet gap as it was for Purcell's experiment. This time however, resonance was detected by observing the proton's induction signal on the oscillograph screen. (Block, Hansen & Packard, 1946)
Bloch and Purcell were awarded together the Nobel Prize in Physics in 1952 in recognition for their works about Magnetic Resonance phenomena. Up to 1970s, the NMR was only used for physical and chemical analysis. It wasn't until an American physician named Raymond Vahan Damadian did a study on detecting tumours using NMR in 1971. His study suggested that results can differentiate between malignant and normal tumours as cancerous tissue has lower water content compared to normal ones. This study opened the possibility for NMR being used to discriminate benign and malign surgical specimens. (Damadian, 1971)
At the same time, American chemist Paul Lauterbur tried forming images of pure water and clams. He called this technique "zeugmatography" which come from the Greek word "zeugma" which means "that which is used for joining". It was performed using two glass capillaries with a diameter of 1 mm, both of which are attached to the inside wall of a 4.2-mm diameter glass tube of D2O. Magnetic field gradients were applied at four different dimensions by rotating it 45°. A two-dimensional tomographic image was then formed by back-projection (Lauterbur, 1973). In 1975, Richard Robert Ernst found a new way to form two-dimensional images by applying a sequence of pulsed field gradients at right angles to each other. The gradients are timely switched with each other using Fourier transform with the aid of phase and frequency encoding. This was called NMR Fourier Zeugmatography (Kumar, Welti & Ernst, 1975).
In 1977, English physicist Peter Mansfield and his grad student Andrew A Maudsley developed a line-scan method of imaging by relying on the properties of NMR spin echoes. This shortened the time needed to do a scan from hours to just minutes. He tried out this method on a liquid mineral oil sample shaped into cylindrical annulus. The samples were exposed to radiation selectively and the response can be seen in Fig. 3b which shows discrete projection profiles. (Mansfield & Maudsley, 1977a) It took 40.5 min for the first image of the mineral oil to be produced. The first human body part to be scanned using NMR was Maudsley's finger. The two images took 15 minutes and 23 minutes with a delay of 0.3s and 0.5s respectively. (Mansfield & Maudslet, 1977b)
During this time, Damadian was finishing the first ever human scan which he called the Indomitable which is a superconducting magnet made up of niobium-titanium wire. Damadian volunteered himself to be the first guinea pig of his creation after hours of no result; they figured that his body fat percentage is too high for the machine. Seven weeks after the experiment, on July 3, 1977, a grad student Larry Minkoff noted that there had been no side effects on Damadian thus he volunteered himself to be the next guinea pig. Mere 5 hours after the test started, the first ever human scan was produced. (Wakefield, 2000) However, his methods were deemed too slow thus Lauterbur and Mansfield's methods were used which resulted to them being awarded the Nobel Prize in Medicine back in 2003.
Over the past 40 years, many scientists have advanced the technology of MRI. This includes German pharmaceutical company Schering's patenting in 1981 of Gd-DTPA dimeglumine which is a MRI contrast agent which improves visibility of the images produced by MRI (Gadoxate Disodium, 2004). The improvement of superconductors as well as faster computing also made the process faster.
The nuclei of some atoms have a property called spin. They are usually represented as tiny spinning tops. However, it is important to remember that the particle is actually not spinning. Since the nuclei are charged, a spinning proton or nucleus of a hydrogen atom produces two spin states or magnetic moments where they precess with or against the direction of an applied strong and uniform magnetic field. (Khan Academy, 2018)
The angular frequency of the precession is known as the Larmor frequency. This depends on the particular type of nucleus and is proportional to the strength of the flux density of the applied magnetic field.
In living tissues, there are large amounts of waters, and hence hydrogen atoms. It is these atoms that are used to produce the images in MRI scans. Nuclear magnetic resonance relies on encouraging more nuclei to flip into the higher energy state by superimposing on top of the strong permanent magnetic field which oscillates at the same frequency as Larmor frequency, which is in the radiofrequency (RF) region. (Rigden, 1986)
Once the RF field is switched off, the nuclei drops back into its lower energy state and the net magnetization goes back to its initial maximum value once again as it emits a RF pulse and exchanges energy with the external environment. This emitted pulse is then detected and processed by the highly calibrated non-uniform magnetic field superimposed on a uniform magnetic field of large magnitude. The time it takes for the atom to relax which is between the end of the RF pulse and the emitting of the RF is called the relaxation time which is dependent on the type of tissue in which the atom occurs. For example, tissues with higher water content take longer time whereas tissues with higher fat percentage take shorter times. (Bloch, 1946)
The non-uniform field superimposed on large uniform field determines the position of the resonating nuclei. By changing the field, the position of the detection slice can also be changed.
The patient lies on a bed surrounded by large coils that produce the uniform field. Gradient coils also surround the patient that produces non-uniform fields. The resonant frequency is different for each small part of the body but this can be identified by the computer. The RF field is applied in pulses and the patient is moved slowly through the coils providing a set of information at each position taken which is then transmitted to a computer that builds up the image.
The advantage of MRI is that it has no ionizing radiation which can kill or mutate cells thus there is no side effects for patients. Furthermore, the image produced shows good contrast between different soft tissues. 3D images can also be produced using the MRI.
However, patients with pacemakers or any metallic implanted devices inside their bodies are not able to use the MRI with the exception of some recent technologies due to the risk of the metal heating up and potentially burning the tissues near it (Are MRI Scans Safe, 2018). Also, the patient must lie very still during the procedure which could last between 15 to 90 minutes thus anesthesia may have to be used for pediatric patients to keep them from moving. This is an added risk. Its small area also proves it hard and uncomfortable for claustrophobic
The reason why MRI scans take such a long time is due to the low signal-to-noise ratio (SNR). The SNR can be improved by using better contrast agents and higher-field scanners. It can also be improved by using stronger magnets. As of now, MRIs widely available to the public have magnetic field strengths of 1.5T and 3T. There are some 7T MRI machines however patients rarely ever get to use them due their expensiveness. MRIs with stronger magnets can cost up to a million US dollars which is enough to strain a hospital's budget. Another way to improve the SNR is by using high-density MRI coils array. However, they are inflexible and heavy thus causing discomfort to the patients.
3450590000In 2016, researchers have discovered a new method to increase the quality of the MRI images which is by developing low-cost flexible MRI radiofrequency coils. These coils can be made using a screen-printer to perfectly fit each individual patient. As the coils are closer to the patient's body, sensitivity is increased and image produced are clearer. (Corea et al., 2016)
The researchers of the University of York has also found a way to increase the signals produced of the analytes such as glucose and pyruvate by hyperpolarizing them without changing their identity through the Signal Amplification By Reversible Exchange (SABRE) where ammonia is first hyperpolarized using parahydrogen and then used as a carrier to exchange the NH protons with the analyte. Since the analytes have already been polarized, there is no need to use a superconducting magnet which is currently used in MRIs. Cheaper magnets can be used thus hospitals in developing countries that does not have the infrastructure to operate current scanners will be able to utilize this new invention and carry out MRI scanning. Furthermore, since the technique is non-toxic, doctors will be able to repeat scans with the same patient and thus doctor is able to observe clearly any changes and thus treatment is highly likely to be successful. (Iali, Rayner & Duckett, 2018)
History of the Magnetic Resonance Imaging. (2019, Nov 18). Retrieved from https://studymoose.com/history-of-the-magnetic-resonance-imaging-essay
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