The Basic Principles of the Magnetic Resonance Imaging Image Production - Case Study Example

The Basic Principles of the Magnetic Resonance Imaging (MRI) Image Production Magnetic Resonance Imaging (MRI) is a new, revolutionary and exciting imaging technique used in generating images of the internal structural anatomy of the human body to help in the diagnosis of diseases. It mainly relies on the principles of magnetism and quantum mechanics. Necessarily, the main component of an MRI is a magnetic field which is used to stabilize the hydrogen protons and prepare them for manipulation or excitation through the introduction of electromagnetic pulses. The excitation processes results in the reception of signals from body tissues by the MRI’s scanner and the generation of their images. MRI is considered comparatively safe for both patient and operator because no chemical bonds are broken and there is no harmful radiation (Consiglio 5). A The Magnetic Properties of Matter MRI is essentially dependent on the principles of magnetism or the ability of matter to be attracted to other matter. Matter exhibits magnetism in one of the three ways: diamagnetism; paramagnetism, and; ferromagnetism. Diamagnetic substances react to a magnetic field by slightly repelling it; paramagnetic substances by slightly being attracted to it, and; ferromagnetic substances aligns and strongly reacts to when they come in contact with a magnetic field (See Fig. 1). Both diamagnetic and paramagnetic substances do not manifest their magnetic inclinations outside of the magnetic field, whereas ferromagnetic substances like iron retain their magnetic nature even outside of the magnetic field (Westbrook 302-304; Faulkner 4-5; Caldemeyer et al 770). The MRI process uses magnetization to manipulate the human body so that certain microscopic chemical and physical information can be obtained from it. This is possible only however, if the body has natural chemical components that has magnetic qualities. The role of ferromagnetic substances, like iron, cobalt and nickel, in MRI is to act as permanent magnet Homogenous Magnetic Field Diamagnetic Substance Slight repelled reaction to a magnetic field Paramagnetic substance Slight attracted reaction to a magnetic field Ferromagnetic substance Strong attracted reaction to a magnetic field Fig. 1 The Different Magnetic Reactions of Matter in the same way as earth’s natural magnetic quality. The application of the principle of magnetism in MRI also implies the application of knowledge in quantum mechanics. Atoms, the basic unit of life, exhibit certain magnetic qualities because all atoms contain protons which have mass, a positive charge, and constant spin ( ). B The Hydrogen Proton The magnetic field in the MRI targets the hydrogen protons in the body. This is because hydrogen is abundant in the human body as a component of H2O which in turn constitutes 70% of it. Other magnetically active nuclei, also found in the human body, are the elements of carbon, fluorine, phosphorus and sodium. MRI makes use of the signal that hydrogen protons manifest when manipulated or excited to generate images of structures in the internal anatomy (Faulkner 7). The nucleus of a hydrogen atom consists of one proton which has a positive charge and one neutron which has a neutral charge. An electron, with a negative charge, orbits around the nucleus. As the charges cancel each other out, the hydrogen atom is neutral under normal condition. Like all protons, the hydrogen proton spins randomly and constantly but always at the same speed. The mass of the hydrogen proton and its constant spinning give the hydrogen atom a slight magnetic quality making it susceptible to external magnetic fields (Weishaupt et al 2). C Effect of a Magnetic Field on a Hydrogen Proton When a hydrogen proton is exposed to a magnetic field, two things happen to the spinning proton: first, it wobbles and aligns its spinning to the direction of the magnetic field, and; second, it loses energy causing its spinning to decelerate. “The rate at which spins wobble when placed under a magnetic field” is called the Larmor frequency and is represented by the equation ωo = γo . Bo , where ωo stands for the Larmor frequency expressed in Megahertz, γo stands for the gyromagnetic ratio which is a constant and Bo is the strength of the magnetic field (van Geuns et al 149). In short, exposure of hydrogen protons to a magnetic field ultimately caused them to align, decelerate and move uniformly along the magnetic field of the Bo (Weishaupt et al 4). At the point when the hydrogen protons become stabilized along the Bo (or the Z axis), the MRI begins to initiate the process of excitation of the protons to measure, read and differentiate the signals of the different body tissues through the introduction of a radiofrequency energy (RF). D The Excitation Process The excitation of the hydrogen protons is important because the disturbance of the proton spins creates a signal that is captured by the MRI machine. As earlier discussed, the exposure of the hydrogen protons to a magnetic field causes what were once randomly and constantly spinning protons to precess, align and stabilize along the magnetic field line (the Bo or the Z axis) (see Fig. 2). In drawing number 3 of the Fig. 2, an electromagnet pulse is applied to the stabilized hydrogen protons and the result is their transverse magnetization or the flipping over of the spins from the Z-axis to the XY axis (Weishaupt et al 5). It is during the transverse magnetization, when the spinning takes place along the XY axis that the spinning protons give off signals recordable by the antenna coals located within the MRI scanner as they induce electrical or radio signals (Sands & Levitin 67). However, the excitation of the hydrogen protons eventually dissipates as they return to their previous relaxed and stabilized state spinning along the Z axis. The return to such previous state is called relaxation, of which there are two types: longitudinal relaxation (T1), and transverse relaxation (T2). The significance of this relaxation phase is the variance in time by which different tissues of the body take to relax and the resulting differences in brightness and intensity of images they create in the MRI scanner. The differences between the two types of relaxation is that in T1, the hydrogen protons relax back to their previous state as they give off the energy artificially induced on them by the RF to their surroundings while the T2 relaxation occurs when the hydrogen protons eventually lose coherence brought about by the individual differences in angular spins along the XY axis resulting in the loss of transverse magnetism without an accompanying loss of energy Fig. 2 Stabilization, transverse magnetism & relaxation (Weishaupt et al 9). Water and fat, for example, have different transverse and longitudinal relaxation times. Water has a longer T1 and T2 than fat because it has difficulty in transferring energy to its surroundings (or lattice) than fat and because water molecules move rapidly around making it easier for hydrogen protons to continue moving even with different angular spins without easily colliding with one another and developing incoherence in the process, respectively. In addition, there are variations in the relaxation periods of healthy tissues from unhealthy ones as pathologic tissues are observed to have more water content than normal. The implication of all this is that an MRI technician can employ specially designed electromagnetic impulses for the purpose of emphasizing certain tissues and obtain a detailed image of them in the scanner (Sands 67). The intensity and brightness of an MR image is therefore dependent on one of the three: proton mass in which case the image is called proton-density weighted (Fig 3) which can be made by minimizing T1 and T2; T1 time which is the time between the recovery/relaxation and the next excitation, in which case the image is called T1 weighted-image (Fig 4), and; T2 time which is the time the MR fades from the point of excitation, in which case the image is called T2-weighted image (Fig 5) (Weishaupt 11). Fig 3, Proton density- Fig 4, T1 density- Fig 5. T2 density- weighted weighted weighted In the images shown above, Fig. 2 is a proton density-image of a knee with a tear in the posterior horn. The PD MRI sequence is commonly used in imaging of that part of the body. Fig. 4, on the other hand, shows a T1 density-weighted image of the lumbar spine with the fat bright, marrow brighter than the disk because of its fat content, hydrated disks between vertebrae darker than the bone marrow, and a low signal return of the cerebrospinal fluid (CSF). Finally Fig. 5 shows a T2 weighted image of the lumbar spine with high signal return of the CSF because of high water content. E Parts of the MRI Magnet Fig 6 As can be seen in Fig. 3, the MRI scanner uses magnet – permanent, resistive or superconductive - to stabilize hydrogen protons which is measured in units of Tesla. The MRI uses magnet stronger than the natural magnetic strength of the earth which is 0.5 Gauss (1 Tesla is equal to 10,000 Gauss) (Faulkner 2). For orthopedic MRI scanning, for example, the optimal strength is deemed at 1.5T (McKie 14). The 1.5 T scanners remain the benchmark of MRI scanning but there is a move to introduce the 3T scanners to clinical applications (Lee 37). Some types of MRI scanners are the standard tunnel type like Fig. 3, the open magnet, the short bore magnets and the standing or sitting magnets. The gradient coils of the scanner are the electrical conductors which can be turned on and off to generate magnetic fields in three orthogonal axes: Z, X or Y directions (Carmi et al 134). The radio frequency coil, which must completely surround the part of the body being examined, transmits and receives MR signals. This is the part of the MRI that sends the electric pulses to the H-protons for excitation (Peart 766-767). Presently, MRI is in the forefront of medical and scientific studies and diagnoses. It is employed in the detection of blood oxygen level dependent contrast (BOLD) in the brain (Norris 794); neurovascular imaging through diffusion-weighted imaging (Luypaert 19); the evaluation of epileptic cases because of its ability to differentiate and trace the origins of seizures (Connor 787) and various other areas in medicine. MRI has therefore made possible the proper and accurate diagnosis of many diseases that used to be difficult to accurately identify through more detailed and enhanced imaging of internal anatomical structures of the human body. In addition, MRI has the added distinction of being a non-invasive, comparatively safe imaging modality. F Conclusion The MRI is one of the most important discoveries and invention in the late 20th century yet at its heart lays basic principles of physics. Guided by the fact that the human body is made up of three-quarters of water, the MRI technique focuses on the hydrogen atom, one of the two elements of water. More importantly, hydrogen is capable of magnetic moment due to its positively charged and constantly spinning proton. This implies that the hydrogen proton can be manipulated in the laboratory by exposing it to a magnetic field and exciting it with electromagnetic frequency or RF. The manipulation of the H-protons allows the MRI to detect signals from them that are varied by their tissue location (indicated by differences in intensity and brightness) as tissues differ in their water content and therefore their hydrogen density. These signals received by the MRI generate detailed images of the internal structure of the human anatomy which enables accurate diagnoses of pathological conditions. Bibliography Caldemeyer, Karen & Kenneth A. Buckwalter, 1999. “Radiologic Images in Dermatology: The Basic Principles of Computed Tomography and Magnetic Resonance Imaging” Indianapolis, Indiana J Am Academy Dermatology. Carmia, Eyal Carmia, & Siuyan Liub & Noga Alona, & Amos Fiata & Daniel Fiatc. “Resolution Enhancement in MRI” Science Direct. Elsever. Connor, S. E. J. & J. M. JAROSZ. 2001. Magnetic Resonance Imaging of Patients with Epilepsy. The Royal College of Radiologists Consiglio, Ni-orn. 2006. “MRI and Patient Safety” The Canadian Journal of Medical Radiation Technology. Faulkner, Wm. 2006. Basic Principles of MRI. http://www.e-radiography.net/mrict/Basic_MR.pdf Lee, Jonathan & Steven P. Shannon. 2007. “Tesla Magnetic Resonance Imaging (MRI)—Is it Ready for Prime Time Clinical Applications?” The Canadian Journal of Medical Radiation Technology. Luypaert, Luypaert & S. Boujraf & S. Sourbron & M. Osteaux. 2001. Diffusion and perfusion MRI: Basic Physics. Elsevier Science Ireland Ltd. Norris, David. G. Norris, 2006. “Principles of Magnetic Resonance Assessment of Brain Function,” Journal of Magnetic Resonance Imaging, 23:794–807 (2006) McKiea, S. & J. Brittendenb. 2005. “Basic Science: Magnetic Resonance Imaging” Current Orthopaedics 19, 13–19 Elsevier Ltd. Peart, Olive. 2005. “Mammography and Breast Imaging: Just the Facts” Just the Facts Series McGraw-Hill Just the Facts. Edition, illustrated. McGraw-Hill Professional, 2005 Sands, Mark & Abraham Levitin. 2004. “Basics of Magnetic Resonance Imaging,” Seminars in Vascular Surgery, Vol 17, No 2 (June), 2004: pp 66-82 Van Geuns, Robert-Jan & Piotr A. Wielopolski & Hein G. de Bruin & Benno J. Rensing & Peter M.A. van Ooijen & Marc Hulshoff, & Matthijs Oudkerk, & Pim J. de Feyter. 1999. “Basic Principles of Magnetic Resonance Imaging,” Progress in Cardiovascular Diseases, Vol. 42, No. 2 (September/October), 1999: pp 149-156 Weishaupt, Dominik & Victor D. Kochli & Borut Marincek. 2006. How Does MRI Work? An Introduction to the Physics and Function of Magnetic Resonance Imaging, 2nd ed, Springer Verlag Berlin Heidelberg New York. Westbrook, Catherine & Carolyn Kaut-Roth & John Talbot. 2005. MRI in Practice, edition3, illustrated, revised. Wiley-Blackwell. Read More