MR Basics Module 1 PDF

Module 1 Transcript For educational and institutional use. This transcript is licensed for noncommercial, educational in- house or online educational course use only in educational and corporate institutions. Any broadcast, duplic...

Module 1 Transcript For educational and institutional use. This transcript is licensed for noncommercial, educational in- house or online educational course use only in educational and corporate institutions. Any broadcast, duplication, circulation, public viewing, conference viewing or Internet posting of this product is strictly prohibited. Purchase of the product constitutes an agreement to these terms. In return for the licensed use, the Licensee hereby releases, and waives any and all claims and/or liabilities that may arise against ASRT as a result of the product and its licensing.  MR Basics Module 1 Fundamentals 1. ASRT Animation 2. MR Basics – Fundamentals Welcome to Module 1 of MR Basics – Fundamentals. This module was written by Cathy Dressen, M.H.A., R.T.(R)(MR). 3. (License Agreement) 4. Module Objectives After completing this module, you will be able to:  Compare the different types of magnets used in magnetic resonance (MR) imaging.  Describe the principles of electromagnetism.  Identify the fundamentals of nuclear magnetism.  Explain how an image is acquired using MR.  Describe how MRI signal is produced and detected.  Discuss tissue characteristics associated with relaxation. 5. Introduction Magnetic resonance (MR) imaging is a fascinating, highly technical and diagnostically useful modality. The images produced by MR display virtually all anatomy and pathology of the human body with excellent detail. MR imaging equipment has evolved over several decades, and scientists and manufacturers continue to modify the technology to improve the speed and image quality of MR examinations. MR imaging depends heavily on technologist skill and input, and so the MR technologist’s understanding of MR concepts is crucial to acquiring diagnostic-quality images for the health care team. The knowledge base for MR education and training includes human anatomy, computer systems, atomic structure, electromagnets and magnetic fields. 6. Advantages of MR Even though MR is a relatively new modality – having only been in clinical use since the mid- 1980s — it has become the imaging method of choice for many indications. The MR scanner is an expensive piece of equipment, costing between $1 and $3 million. The price can rise even higher when imaging departments add special hardware and software options. MR offers advantages for its high price, however. X-ray examinations expose patients to ionizing radiation, and long-term radiation exposure has been linked to a higher risk of cancer and other health problems. MR uses no ionizing radiation, thus making this diagnostic examination ideal for patients who require frequent exams or who are otherwise more susceptible to the effects of ionizing radiation. MR images display excellent image contrast between the body’s various soft tissues, which makes it especially useful in imaging the brain, muscles, heart and cancers compared with other medical imaging modalities, such as computed tomography (CT) or radiography. 7. MR vs CT Comparisons often are made between MR and CT, which can produce cross-sectional images of the body. MR images are different from those generated by CT, however, and provide some unique and significant advantages over CT, such as:  Superior soft-tissue contrast resolution.  Ability to scan in all 3 planes. 1   No artifacts from bone or air. Technologist flexibility to manipulate the degree of brightness for various soft-tissue types by adjusting MR imaging parameters. 8. MR vs CT Each of these images is a sagittal view of the skull or brain. Notice how each modality produces different contrast which highlights different details of the skull or brain. 9. MR vs CT There are many types of MR scanners used for various purposes: Long-bore scanners are the traditional, closed-bore MR scanners. Short-bore scanners are 50% shorter and 5% wider than conventional long-bore scanners. They are designed to make patients feel less confined. Open-bore scanners originally were designed to accommodate larger patients and patients with claustrophobia. Ultra-low field scanners contain magnets with field strengths of 0.2 to 0.01 tesla (T). They typically are used for orthopedic applications in which the bore encloses only the body part of interest. Low-field open scanners operate at field strengths from 0.2 to 0.4 T but are being redesigned to operate at higher strengths to improve image quality. High-field scanners include 1.5-T and 3.0-T strengths and higher. Magnets operating at 3.0 T or higher are considered ultra-high field MR scanners. Ultra-high field scanners once were used solely for research, but many hospitals now have MR equipment with 3.0-T technology. Clinical scanners with superconducting magnets are commercially available with field strengths up to 3.0 T, and whole-body systems at research facilities have fields as high as 9.4 T. Upright scanners are low-field strength magnets that chiropractors use to display spinal alignment through flexion and extension. 10. Magnet Types Magnets can be permanent, resistive or superconducting. They also can have different magnetic field strengths and directions. 11. Permanent Magnet Permanent magnets use open designs for larger patients. They do not require use of electrical currents to maintain their magnetic fields. Permanent magnets have vertical magnetic fields. 12. Resistive Magnet Resistive magnets depend on electrical currents to generate their magnetic fields (electromagnetism). They typically have limited, or low, field strengths of 0.6 T. 13. Superconducting Magnet Superconducting magnets use cryogens (i.e., liquid gases, such as helium) to control their temperatures. Electrical current flows without any resistance through superconducting wires immersed in the cryogens. Superconducting magnets are the most common type used in MR scanners, even though they are the most expensive to operate. 2 14. Knowledge Check 15. Magnetic Field Strength Magnetic field strength is measured in tesla and gauss, although tesla is the preferred international unit. One T is equal to 10,000 gauss. The differences in the 3 types of magnets in the previous slides also can be described in terms of their tesla power. Superconductive magnets have field strengths greater than 0.5 T and up to 7.0 T. The field strength of permanent magnets generally is less than 0.3 T, and resistive magnets have field strengths of less than 0.2 T. 16. Magnetic Field Strength This table compares magnetic field strengths in tesla and gauss. The strength of a 1.5 T magnet is equivalent to 15,000 gauss, and a 3.0 T magnet is equal to 30,000 gauss, and so on. The magnetic strength for clinical imaging can be 15,000 to 30,000, or more, times the strength of the earth’s magnetic field, which is 0.5 gauss. This is a very powerful force. 17. MR Safety Operating the MR system is an important responsibility. The systems are multimillion dollar pieces of equipment that require special training, along with specific safety procedures. Ensuring that safety policies and procedures are in place can maximize the safety of all physicians, faculty, staff and patients. Safety must be the MR technologist’s number one concern at all times while operating MR equipment. As we mentioned earlier, an MR magnet is surrounded by a very strong magnetic field, which has the potential to attract ferrous metal. The magnetic field also can interfere with the normal operation of electronic devices. For these reasons, technologists must obtain a detailed medical history for every person who enters the MR room. This includes not only patients, but also all hospital or medical staff and volunteers. The repercussions of a technologist’s failure to appropriately screen a patient, volunteer or staff member could be serious injury or death. Because of how MR functions, metal in or on a person’s body can affect the acquired images or injure the patient and staff members. Patients and medical staff need to inform the MR technologist when a patient has certain medical devices, such as a cardiac pacemaker or artificial heart valve, metal plate, pin or other metal implant, insulin or other infusion pump, aneurysm clip, stent, IUD or cochlear implant. 18. MR Safety A magnetic field surrounds the MR magnet. Anything that enters the force of the magnetic field potentially experiences effects from the force. The main risk of a high-field magnet is its magnetic torque, which means the ability of the magnet to attract a magnetically sensitive material. The 5-gauss line is the point at which the magnetic field becomes very dangerous. Metallic objects can become flying projectiles at the 5-gauss line. In MR physics, magnetic susceptibility is the degree to which a substance becomes magnetized when placed in a magnetic field; the effect of an externally applied magnetic field and resulting magnetization is called magnetic induction. MR safety will be covered in-depth in later modules in this series. 19. Planes MR imaging has an advantage over other modalities because it can acquire images in any desired plane. It displays the conventional 3 planes used in imaging: coronal, sagittal and axial. Coronal is a cross-sectional plane, for example, across the shoulders, dividing the body into front and back halves. Sagittal is a cross-sectional plane that divides the body down the middle into left and right halves. Axial orientation divides the body perpendicular to the long axis of the body, creating upper and lower body cross-sections. With MR, images also can be acquired at any oblique angle to the traditional planes. There is tremendous benefit to acquiring the “slices” or “cuts” at any angle or plane; this allows the 3  anatomy to be displayed from different points of view. The health care team can see structures and pathology or disease from multiple perspectives. MR imaging is extremely operator-dependent. It is important for MR technologists to have a sound understanding of cross-sectional anatomy. In addition, knowledge about pathology is important in choosing the proper technique and imaging parameters. Only then can the technologist provide the best possible diagnostic image quality to assist clinicians in making accurate diagnoses. 20. MR Defined — Evolution MR technologists should be familiar with the history and evolution of MR technology to understand the technology’s basic concepts. Founded on the principles of nuclear magnetic resonance, the technology we now know as MR was first termed “nuclear magnetic resonance,” or NMR. NMR then became “nuclear magnetic resonance imaging,” or NMRI, and subsequently magnetic resonance imaging, or MRI, in the late 1970s because of the negative connotations associated with the word “nuclear.” 21. History of MR Imaging In many ways the MR imaging historical timeline began when Nikola Tesla discovered the rotating magnetic field in 1882. The first basic NMR device was developed by Isidor Rabi in 1938. Interestingly, Rabi, a physicist at Columbia University in New York City, noted the NMR effect of radio waves in a magnetic field in the late 1930s, but considered the phenomenon to be an artifact of his apparatus and disregarded its importance. Rabi was awarded the Nobel Prize in Physics in 1944 for his invention of the atomic and molecular beam method of observing atomic spectra that enabled more precise measurements of nuclear magnetic moments than had been previously possible. This method could provide data related to the magnetic properties of certain substances. Rabi succeeded in detecting and measuring single states of rotation of atoms and molecules, and in determining the mechanical and magnetic moments of the nuclei. 22. History of MR Imaging In 1946 Felix Bloch of Stanford University and Edward Purcell of Harvard University developed instruments that could measure the magnetic resonance in bulk material such as liquids and solids. They found that when certain nuclei were placed in a magnetic field they absorbed energy in the radiofrequency (RF) range of the electromagnetic spectrum, and re-emitted this energy when the nuclei returned to their original state. Bloch and Purcell won the 1952 Nobel Prize in Physics for their work. 23. History of MR Imaging Sir Joseph Larmor, an Irish physicist who lived from 1857 to 1942, demonstrated earlier that the strength of the magnetic field and the RF pulse matched one another. This relationship now is known as the Larmor frequency — that is, the angular frequency of precession of the nuclear spins is proportional to the strength of the magnetic field. The phenomenon was called nuclear magnetic resonance based on the following:  Nuclear — only the nuclei of certain atoms reacted in this way.  Magnetic —a magnetic field is required.  Resonance — the direct frequency dependence of the magnetic and RF fields. 24. MR History — Clinical Application Research on NMR continued to advance, but it was not until the 1970s that technological developments could be applied to acquiring diagnostic images of the human body. At that time, Sir Peter Mansfield, a British physicist and professor, demonstrated how to mathematically analyze MR signals to convert them to images. Mansfield also developed echo planar imaging. Peter Lauterbur, an American chemist and professor at the University of Illinois, discovered how to create 2-D images by introducing gradients to the magnetic field. 4  Physician Raymond Damadian was said to have built the first NMR body scanner in 1977 after acquiring the patent in 1972. His machine, named “Indomitable,” is now located in the Smithsonian Institution. 25. MR Innovation The pioneers of MR have received many prestigious awards. In 1991 Richard Ernst of Switzerland received the Nobel Prize in Chemistry for his work on high-resolution pulsed Fourier transform spectroscopy, and in 2002 Kurt Wüthrich received the Chemistry Prize for developing MR spectroscopy that demonstrated 3-D molecular structures in solution. Although Mansfield and Lauterbur received the Nobel Prize in 2003, Damadian was overlooked, which caused some controversy in the MR community. 26. What Is an MR Examination? An MR examination is a diagnostic procedure in which strong magnetic fields, radio waves and a computer are used to acquire images of the body’s interior. The images are multidimensional and are not produced using ionizing radiation. Any area of the body may be scanned. It’s important to remember when working around a superconducting magnet that the magnetic field is always on. Many people in the health care environment incorrectly assume that MR technologists turn the magnetic field on and off, so technologists must educate others that under normal conditions, the field never is turned off. 27. MR Examination To perform an MR examination, the technologist places the body part to be examined in the center of the MR scanner’s bore (isocenter). When the patient’s entire body or body part under study is exposed to the strong magnetic field, the hydrogen nuclei in the body align with the magnetic field. RF energy then is used to flip the hydrogen atoms out of alignment with the magnetic field. After the RF energy is removed, the nuclei realign with the magnetic field. As they realign, the nuclei in the patient’s body produce a signal. Special coils receive and transmit the signal to a computer, which converts the data to an image. During the exam, the MR scanner makes a loud knocking or thumping sound that changes in frequency or pattern. This is the sound of the gradients switching on and off multiple times per second. Ear plugs or headphones protect the patient’s hearing and provide comfort. 28. MR Requirements Three components are required to produce MR images. First, the anatomy of interest must be placed in a magnetic field. Second, the MR scanner must transmit RF pulses that are absorbed by the patient’s protons and then emitted as a signal. Finally, an RF coil must receive the signal and transmit it to the computer. 29. MR Physics To understand MR imaging, it’s important to understand the basic principles of magnetic resonance. This knowledge base is commonly referred to as MR physics, and many MR concepts are built on this foundation. The human body is made up of trillions of atoms. The central part of each atom is the nucleus, and electrons orbit around the nucleus. The nucleus contains 2 kinds of particles: protons and neutrons. Protons are positively charged. Neutrons have no charge and are therefore neutral. 30. MR Physics MR is concerned with the characteristics of hydrogen protons, so an MR technologist should have a basic understanding of protons’ physical properties. Protons:  Are positively charged.  Are located in nuclei. 5   Spin on their axes. In 1946, while working at Stanford University, physicist Felix Bloch discovered that protons inside the nucleus of any atom behave like tiny magnets. The Bloch equation explains that tiny charged particles in the nucleus behave as though they spin on an imaginary axis like a top. When the protons spin, they create a small magnetic field called a “magnetic moment.” 31. The Faraday Law Both a magnetic field and a charged particle are necessary for MR imaging. The concept of electromagnetism is used in many parts of the MR system and MR imaging. Electromagnetic induction is the process of using magnetic fields to produce voltage in a complete circuit. Michael Faraday discovered the concept of electromagnetic induction in 1831 while using different combinations of wires and magnetic strengths and currents. He found that it wasn’t until he moved the wires that he produced voltage. Faraday discovered that changes in a magnetic field could induce an electromotive force and current in a nearby circuit. Any change in the magnetic environment of a coil of wire induces a voltage in the coil. The voltage is generated no matter how the change is produced — by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil in or out of the magnetic field or rotating the coil relative to the magnet. 32. Electromagnetism and MR An essential concept used in MR imaging is the connection between electricity and magnetism; in other words, a magnetic field is created whenever the charged particle moves. The Faraday law of induction states that a changing magnetic field produces an electric current in a closed-loop circuit. In addition, an electric current flowing in a circuit produces a magnetic field around the wire of that circuit. MR systems use many different types of coils that are customized to the body part being imaged. The coil emits a measurable RF wave. 33. Electromagnetic Induction This slide demonstrates how a changing magnetic field, such as a magnet moving through a conducting coil, generates an electrical field and therefore tends to drive a current through the coil. The induced electromotive force in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit. As the magnet moves back and forth, a current is induced in the wire. 34. Vectors Every magnet has a north and south pole and is surrounded by a magnetic field. Drawing lines from the poles to create vectors demonstrates the direction and strength of the magnetic field. The vector represented by the red arrow in the image corresponds to the main magnetic field, which usually is written as B0. The direction of the vector shows the orientation of the magnetic field; its length is proportional to the field strength, or magnitude. 35. Hydrogen Atoms MR imaging takes advantage of one of the most common particles in the human body — the hydrogen proton. MR uses hydrogen because it contains an odd number of protons and therefore generates a measureable MR signal. If the number of protons were even, there would be an equal amount of spins, and the spins would cancel each other out, resulting in no signal. The terms “hydrogen proton” and “hydrogen nucleus” sometimes are used interchangeably in the literature. 6  Water molecules are composed of hydrogen and oxygen atoms. Water constitutes about two- thirds of the human body’s weight, and this high amount of water explains why MR imaging is so successful. The water content of tissues and organs varies, and the pathologic processes of many diseases cause changes in measurable water content, which are reflected in the MR images. In addition, the body’s soft tissues, such as fat, muscle, blood, cartilage and ligaments, demonstrate easily-delineated contrast on MR images. 36. No External Field Applied All hydrogen protons in the human body spin under normal circumstances. Patients don’t notice these magnetic forces because there is no external magnetic field and the protons rotate randomly. Look at this animation. It demonstrates the random behavior of hydrogen protons under ordinary conditions — that is, when no magnetization is applied. If you attempt to measure the magnetization in the body, the measurement would be 0, because equal amounts of tiny magnetic fields would cancel each other out. 37. External Field Applied Applying an external magnetic field causes the hydrogen protons to align with or against the magnetic field. You’ll remember that the main magnetic field is referred to as B0. Look at this animation. The hydrogen protons with low-energy states align parallel to the main magnetic field. Hydrogen protons with higher-energy states oppose the main magnetic field in an antiparallel position. 38. Increasing Field Strength Increasing the static, or main, magnetic field strength causes more hydrogen protons to align with the field. 39. External Field Applied When an external magnetic field is applied, the hydrogen protons align either in the same direction as the magnetic field or in the opposite direction of the exposure field. The 2 alignment directions are: Low-energy, or parallel, orientation, referred to as spin-up nucleus. High-energy, or antiparallel, orientation, which is referred to as a spin-down nucleus. Parallel protons have lower-energy states than antiparallel protons. It takes slightly less energy to line up with the magnetic field than against it. Therefore, there always are slightly more protons parallel to the magnetic field than opposed to the field. 40. Net Magnetization Vector Remember that when the patient is placed in the bore of the MR scanner, some of the hydrogen protons align parallel and some antiparallel to the magnetic field. This animation illustrates that concept. Net magnetization is the amount of magnetized protons that remain after parallel and nonparallel energy states cancel out. The resulting measurable amount of magnetization generates the MR signal. Net magnetization reflects the balance between spin-up and spin-down nuclei. The exact energy difference depends on the MR scanner’s field strength, that is, whether it is a 0.2 T, 1.5 T or 3.0 T magnet. The energy difference between spin-up and spin-down nuclei increases as the field strength increases. At high field strengths, the magnitude of the net magnetization vector is larger than at low field strengths, resulting in a better signal. An improved signal leads to better image quality. The net magnetization vector measurement plays an important role in the MR imaging process. It represents the sum of many small magnetic moments generated by the slight majority of hydrogen protons or nuclei oriented parallel to the external magnetic field. The strength and 7  direction of the net magnetization vector are determined by the behavior of these nuclei, and any change in the vector reflects a change in their status. 41. Net Magnetization Vector The net magnetization vector is formed when pairs of parallel and antiparallel nuclei cancel each other out. The magnetic moments of the unpaired nuclei create a sum called the net magnetization vector. Only the unpaired nuclei form the MR signal. This animation illustrates the concept of net magnetization vector. 42. Net Magnetization Vector Net magnetization vector is represented by the vector symbol M. The animation on this slide demonstrates the net magnetization vector (M) of a sample of protons aligned with an external magnetic field, B0. 43. Net Magnetization Vector Let’s review. In the presence of an external magnetic field B0: The nuclei align in 1 of 2 positions depending on their energy state. Low-energy nuclei align with the field in parallel position. High-energy nuclei align against the field in antiparallel position. 44. Knowledge Check 45. Knowledge Check 46. Magnetic Fields When operating an MR system, the MR technologist uses 3 different magnetic fields:  Static magnetic field — the main field created by the MR magnet, or B0.  Radiofrequency (RF) magnetic field — created by the RF transmitters B1.  Gradient magnetic field — created by the gradient coils Bg.  It’s important to understand how these magnetic fields affect the MR environment. 47. Magnetic Fields B0 is referred to as the external static magnetic field. This is the strong, main magnetic field of an MR system. A magnetic field having the same strength across the entire field is called homogeneous. The strength of the magnetic field can vary across the field, which is referred to as inhomogeneity. Lack of homogeneity can lead to artifacts (unwanted signal intensities on the MR images) or signal loss on the images. The American College of Radiology (ACR) MR quality control standards recommend annual checks of MR equipment to ensure that the magnetic field is stable. The uniformity of the main magnetic field is measured in parts per million, or ppm. In MR the homogeneity of the static magnetic field is an important criterion for image quality. Magnetic field homogeneity is required for high-resolution imaging. 48. Precession The body is made up mostly of water and fat, both of which contain a high number of hydrogen atoms. The hydrogen nucleus has an unpaired proton. Protons play a major role in generating the MR signal because they each have a magnetic field. Each proton spins and the direction of the proton’s spin is randomly distributed in nature. If the human body is placed inside a large magnet with a magnetic field strength much greater than the earth’s natural magnetic field, the tiny spins of lower-energy particles align with the strong magnetic field. 8  The influence of B0 produces an additional spin, or “wobble,” which is called precession. Precession is the motion of net magnetization as it wobbles around the main magnetic field of the MR scanner. MR measures the signal from the wobbling protons, and this phenomenon happens thousands of times during an MR examination. 49. Precessional Frequency The speed at which hydrogen precesses depends on the strength of B0 and is called the “precessional frequency.” The precessional frequency of hydrogen in a 1.5 T magnetic field is 63.84 MHz. The precessional paths of the individual hydrogen nuclei are random. Precessional frequency is the speed at which the magnetic direction of a hydrogen proton rotates around the direction of the applied magnetic field. The Larmor equation is used to calculate precessional frequency. 50. In-phase Precession Two terms describe the phase, or direction, in which the hydrogen nuclei rotate when precessing: in-phase or out-of-phase precession. Play this animation. In-phase precession, or phase coherence, refers to hydrogen nuclei precessing or rotating in unison. This refers to magnetic moments that are in the same place on the precessional path around B0 at any given time. 51. Out-of-phase Precession Out-of-phase precession refers to the hydrogen nuclei precessing randomly. Play this animation. When an RF energy source or RF excitation pulse is transmitted, there is phase coherence. Once the RF pulse or energy is removed, or shut off, the hydrogen nuclei begin to precess out-of- phase. These are magnetic moments that are not on the same precessional path at any given time around B0. 52. Larmor Equation The rate or speed that the magnetization precesses is described by a relationship called the Larmor equation. This scientific equation describes the influence of the magnetic field strength on the precessional frequency of the protons’ net magnetization. The f in the Larmor equation stands for the frequency of the wobbling of the net magnetization; it is measured in hertz (Hz), or cycles per second. The k is the frequency of hydrogen. The B stands for the magnetic field strength, which is measured in tesla. The stronger the magnetic field, the higher the precessional frequency. 53. Larmor Equation The Larmor equation is used to calculate the rate of precession of a given element. It describes the resonant precessional frequency of a nuclear magnetic moment in an applied magnetic field. Precessional frequency depends on the type of element and the strength of the external magnetic field. The equation states that the precessional frequency (ω) is equal to the gyromagnetic ratio of an element (γ) times the magnetic field strength (B0). The gyromagnetic ratio is the precessional frequency of an element at 1.0 T. The ratio is a constant and is different for each atom. For example, the gyromagnetic ratio for hydrogen is 42.58 MHz for a 1.0-T magnet. 54. Knowledge Check 55. Larmor Frequency and Magnetic Field Strength 9  There is a linear relationship between the precessional, or Larmor, frequency and the applied magnetic field strength. The graphic on this slide shows that if B0 increases, the Larmor frequency increases and vice versa. 56. Radiofrequency As the hydrogen nuclei precess in-phase in the B1 plane, a changing magnetic field is created. Placing a receiver coil (antenna) in the path of the changing magnetic field induces a current. The coil emits a measureable RF signal. MR systems use many different types of coils customized to the body part being imaged. To receive a signal from the protons in the body part being imaged, MR technologists apply RF energy to the patient. This process uses a principle called resonance. The RF energy “disturbs” the alignment of the net magnetization of hydrogen protons. For resonance to occur, the frequency of the transmitted RF energy must match the precessional frequency of the receiving object. 57. Resonance Resonance helps transfer energy efficiently. This is true, for example, when pushing a child on a swing. The child swings back and forth at a particular rate. If an adult pushes the swing at the right time, he or she transfers energy efficiently to the swing and child. If the adult consistently pushes at the right time, he or she is in resonance with the swing, and the efficient transfer of energy allows the child to swing higher. When resonance occurs, all magnetic moments move to the same position, with the same precessional path around B0. 58. Generating MR Signal RF energy must be applied at exactly the Larmor frequency of hydrogen for resonance of hydrogen to occur. As the magnetic field strength increases, the difference between high energy and low energy increases; higher RF energy, or frequency, is required to produce resonance. The goal of MR is to acquire images that help radiologists differentiate various body tissues to determine if tissue is normal or demonstrates pathology. The process of resonance and relaxation make these differences apparent. To differentiate tissues, the MR technologist must generate a signal by changing the alignment of the hydrogen protons’ net magnetization with the static magnetic field. MR changes the tissue in 2 directions: longitudinal and transverse. 59. Longitudinal Direction Longitudinal magnetization is defined as the component of net magnetization that is parallel to the magnetic field B0 at equilibrium. 60. Transverse Magnetization As the hydrogen nuclei respond to the RF pulse, or excitation, the net magnetization vector rotates away from its longitudinal position. By definition, transverse magnetization is the component of net magnetization that is oriented perpendicular to the longitudinal axis. The angle or distance at which the net magnetization vector moves away from the longitudinal position is referred to as the flip angle. The net magnetization and directions are relative to B0. 61. RF Excitation Pulse The amount of RF energy used to tilt the magnetization from the longitudinal to transverse direction is called the RF excitation pulse. The stronger the RF energy or the longer the RF pulse is applied, the farther the net magnetization vector will be tilted away from longitudinal direction. For example, an RF pulse that has enough energy to tilt the net magnetization from the longitudinal direction a full 90° into the transverse (x-y) plane is called a 90° RF pulse. A 180° RF pulse produces a 180° flip angle. The technologist applies this RF pulse twice as long as a 90° 10  RF pulse. When flip angles of less than 90° are used, only a portion of net magnetization vector tilts to the transverse plane. Therefore, a smaller number of low-energy spins become high- energy spins. If flip angles of greater than 90° are used, more high-energy spins occur than low- energy spins. The RF angle can vary depending on the imaging technique the MR technologist uses. The technologist can control the MR signal by controlling the transmitted RF pulse. 62. Resonance Let’s review some of the basic material we’ve covered this far. If the human body is placed in a magnetic field and an RF excitation pulse is transmitted at the Larmor frequency to hydrogen nuclei in the body, these nuclei absorb energy and resonate. Two physical changes occur as the hydrogen nuclei absorb the RF energy: The nuclei enter a high-energy state and align themselves antiparallel to the external magnetic field as they precess in-phase. The changes in the nuclei are represented by the net magnetization vector. They either rotate, or precess, away from the longitudinal axis or rotate in a new position. After precession, either a state of longitudinal relaxation or transverse magnetization occurs. A coordinate system helps to identify the x, y and z planes. 63. RF Energy Absorption Before an RF pulse is applied, the net magnetization (shown by the small red arrow in the left figure) aligns parallel to the static magnetic field and the z-axis. After an RF pulse at the Larmor frequency is applied, the protons absorb the energy, causing the net magnetization to rotate away from the z-axis, as seen in the central and right figures. 64. Superconducting Magnet This slide demonstrates the coordinate system in a scanner. Superconductive MR scanners that use cryogen gases have horizontal fields. Open MR scanners have vertical fields; this distinction is important for MR technologists to remember. 65. Coordinate System The coordinate system is an important concept for MR imaging. The direction parallel to the static magnetic field is the longitudinal direction, which also may be called the z-axis, or direction. For typical 1.5-T superconducting magnets with cylindrical bores, the z direction is horizontal and corresponds to the head-to-foot (or foot-to-head) direction. The plane perpendicular to this direction is called the transverse plane, or the x-y plane. When a patient is positioned head first and supine in a superconducting magnet, the x direction often is selected as left-right across the patient and the y direction often is the anterior-to-posterior direction. Interestingly, the transverse plane matches the axial plane for typical 1.5-T magnets. 66. Relaxation The amount of RF angle can vary, depending on the imaging technique used. The amount of RF energy or power in the RF pulse depends on the strength and duration of the RF energy the MR technologist selects. After a 90° RF pulse is applied, the net magnetization begins to precess from the longitudinal orientation to the transverse plane in a spiral motion. Net magnetization spirals downward in a rotating vector. Before the technologist applies the RF pulse, the longitudinal magnetization is equal to the full magnitude of net magnetization, and transverse magnetization is 0. After the RF pulse is transmitted, however, and net magnetization rotates 90°, the transverse magnetization equals the full magnitude of net magnetization and longitudinal magnetization equals 0. 11  Turning the RF energy off causes the net magnetization vector to return to a state of equilibrium, or rest. The protons within each tissue relax at different rates after the RF energy is turned off, which allows MR technologists to acquire images that display differences among the tissues. 67. Types of Relaxation There are 2 types of relaxation in MR imaging: longitudinal, or T1, relaxation, which also is referred to as spin-lattice relaxation, and transverse, or T2, relaxation, also referred to as spin- spin relaxation. Both of these processes occur at the same time but are completely independent of one another. To recap this information: The return of resonating hydrogen nuclei to equilibrium influences the type of signal obtained for imaging; 2 events occur as excited hydrogen nuclei return to their original states; and energy is released as the system becomes more random. This series of events is called relaxation. 68. Definition of T1 When the MR technologist stops the RF energy, the net magnetization vector begins to respond to the pull of the main magnetic field. The recovery of net magnetization back into the longitudinal field is called T1 relaxation. The longitudinal component of net magnetization becomes steadily greater in size, or magnitude, after the RF energy is turned off. T1 relaxation is a time constant, relative to the time it takes for the longitudinal magnetization to reach 63% of the original magnetization. In other words, T1 relaxation is the time the hydrogen nuclei need to recover to magnetization in the longitudinal plane. The graphic on the slide demonstrates the process whereby energy absorbed by the excited protons, or spins, is released back into the longitudinal plane. 69. Longitudinal Relaxation Let’s apply the fundamental concepts presented earlier to more complicated MR situations. Net magnetization that is aligned with the longitudinal direction is called longitudinal magnetization. After a 90° RF pulse flips the longitudinal magnetization into the transverse plane, the magnetization is called transverse magnetization and the resulting longitudinal magnetization is 0. The magnetization then begins to return in the longitudinal direction, that is, T1 relaxation. The rate that longitudinal magnetization recovers differs for various tissues and it is the fundamental source of contrast in T1-weighted images. T1 is a parameter that helps to characterize specific tissue and depends on the static magnetic field strength. Net magnetization does not rotate back up, but always increases parallel to the static magnetic field, which is the longitudinal direction. 70. Transverse Relaxation T2, or transverse, relaxation begins when the net magnetization aligns with the z direction and then a 90° RF pulse rotates the net magnetization into the transverse plane. The figure on this slide demonstrates this process. Net magnetization is made up of many precessing protons. During the RF pulse, the protons begin to precess together, or become in-phase. Immediately after the 90° RF pulse stops, the protons begin to dephase because of several effects, such as the spin-spin relaxation process. 71. T2 Relaxation Immediately following the application of an RF pulse, the energized protons begin to tilt the net magnetization vector into the transverse plane. Over time, the amount of magnetization in the transverse plane begins to decrease, or decay. This process of decaying magnetization in the transverse plane is called T2 relaxation. It is the time it takes 63% of transverse magnetization to be lost. In other words, T2 relaxation is the time in which transverse magnetization has decayed to 37% of its value because of spin-spin interactions. T2 relaxation takes place in the x-y plane and occurs more rapidly than T1 relaxation. 12 72. Relaxation Process In physics terms, T1 and T2 relaxation are time concepts that characterize rates of magnetic relaxation. The T1 and T2 relaxation processes occur simultaneously. After the application of a 90° RF pulse, the transverse magnetization (T2 decay) dephases, while the longitudinal magnetization rephases parallel to the static magnetic field. After a few seconds, most of the transverse magnetization is dephased and most of the longitudinal magnetization has rephased. During the relaxation process, the spins shed the excess energy acquired from the 90° RF pulse in the form of RF waves. To produce an MR image, the RF waves are captured. The RF waves can be gathered using a receiver coil. The receiver coil can stand alone or be combined with a transmitter coil or a transmit/receive coil. Many different types of coils are available for imaging various body types. 73. Free Induction Decay (FID) The signal received at the beginning of T2 relaxation is strong and then becomes weaker. This transient signal effect is called free induction decay (FID). Free induction decay is signal that would be received in the absence of any magnetic field. The figure on this slide demonstrates the effect that takes place in the z plane. A free induction decay curve is generated as excited nuclei relax. The amplitude of the free induction decay signal decreases over time as net magnetization returns to equilibrium. If, for example, a 90° pulse produces transverse magnetization of the spins, there is a transient MR signal that decays toward 0 with a characteristic time constant of T2 (or T2*). This decaying signal is the free induction decay. 74. T1-weighted Contrast The graph on this slide shows the longitudinal magnetization curves for white matter, gray matter and cerebrospinal fluid (CSF); the MR image demonstrates a brain with T1-weighted contrast. Different tissues have different rates of T1 relaxation. If an image is obtained when the T1 relaxation curves are widely separated, T1-weighted contrast is maximized. 75. T2-weighted Contrast Likewise, the graph shown on this slide displays the transverse magnetization curves for white matter, gray matter and CSF; the image on the right shows the brain with T2-weighted contrast. Tissues have varying rates of T2 relaxation. If an image is obtained when the T2 relaxation curves are widely separated, T2-weighted contrast is maximized. 76. Excitation When the transverse magnetization rotates rapidly, it creates RF excitation that induces a voltage signal, or puts energy into the spin system. Using various excitation pulse sequences causes the signal to reflect various responses in MR images. 77. T1 and T2 Relaxation In T1 and T2 relaxation, the spins align with the external magnetic field, and the time it takes them to return to 63% of final maximum value is the relaxation time. After T1 and T2 relaxation occur, MR technologists must collect a signal from the patient to take advantage of the relaxation process. The technologist applies a second burst of RF energy that causes the body to emit a measurable signal following the transmitted pulse. This signal is called an echo. The size of the echo depends on both the T1 and T2 relaxation processes and on the number of protons in the patient’s body part or tissue. The number of protons that generate the MR echo in any given body tissue is called proton density. 78. Computers in MR Imaging 13  MR imaging is a digital modality that uses thousands of complex calculations. The MR computer performs 3 basic tasks: General scanner operation, Image processing and Data collection or measurement control. Some MR systems use multiple computers or consoles. Patient images can be displayed or recorded on film or other media, such as CD-ROMs or DVDs. MR technologists postprocess images at computer workstations, and radiologists can choose window and display controls at their interpretation workstations. Technologists also can transfer images electronically using the digital imaging and communications in medicine, or DICOM, standard, which allows equipment such as scanners, digital cameras, printers or viewing stations, to accept and process the patients’ images and data accurately. 79. Advances in MR MR imaging has undergone many changes in the past 10 years. The trend in clinical settings has been toward using magnets with a field strength of 3.0 T. Compact scanner design allows the new generation 3.0-T systems to occupy old 1.5-T scanner sites. For neurological examinations, increased field strength translates directly to increased sensitivity. The 3.0-T neurological equipment can be used to obtain higher resolution images or substituted for increased speed and, thus, shorter examination times. Currently, however, other clinical applications at 3.0 T are more variable in their reproducibility (e.g., cardiac imaging). MR imaging also has moved from a principally diagnostic role to use as a tool in treatment. Interventional MR refers to minimally invasive procedures such as MR-guided biopsies, thermal ablations and intravascular stent placements. Intraoperative MR is used in neurosurgery to guide the precise resection of tumors. 80. Integrated Modalities Integrated, or fused, imaging with MR is a new development that will affect patient assessment, particularly in oncology. Positron emission tomography (PET) often is used to diagnose and manage cancer treatment; however, the modality provides no structural data. Although PET and CT systems have been integrated to provide combined functional and anatomical detail, the fused images still lack adequate soft-tissue contrast. Patients, therefore, may have to undergo a separate MR exam, so that physicians can gather critical soft-tissue information. Integrating PET with MR in a single piece of equipment has therefore been an important clinical and technological goal. In 2011 the FDA approved the first fused equipment that performs both PET and MR scans. In a single system, the Biograph mMR from Siemens Medical Solutions uses PET to show the function of organs, soft tissues and other internal structures, while simultaneously providing MR images of the organs. MR-PET imaging delivers lower radiation doses than PET-CT, along with improved soft tissue contrast. Developers envision many innovative uses for this technology in the areas of oncology, neurology and cardiology. 81. MR-safe Pacemakers MR has developed rapidly since its introduction as a clinical tool in the early 1980s. In addition to increasing the field strength of magnets and developing fusion imaging, future developments in coil technology and new MR-specific contrast agents will continue to provide novel tools for clinical diagnosis. Finally, manufacturers are addressing other challenges in MR imaging. For example, MR-safe pacemakers are being manufactured and have been approved by the FDA. Under certain conditions patients with these pacemakers can now be scanned using MR. 14 82. Conclusion This concludes Module 1 of MR Basics – Fundamentals. you should now be able to:  Compare the different types of magnets used in magnetic resonance (MR) imaging.  Describe the principles of electromagnetism.  Identify the fundamentals of nuclear magnetism.  Explain how an image is acquired using MR.  Describe how MRI signal is produced and detected.  Discuss tissue characteristics associated with relaxation. 83. References American College of Radiology. (n.d.). Retrieved October 2011, from MRI accreditation: http://www.acr.org/accreditation/mri.aspx Philip W Ballinger, E. F. (2003). Merrill"s Atlas of Radiographic Positions & Radiologic Procedures. St Louis: Mosby. Cohen MS. A selection of slides on MRI basics. University of California Los Angeles website. www.ccn.ucla.edu/bmcweb/sharedcode/slides/SlideFiles.html. Accessed November 20, 2010. Mitchell D, Cohen M. MRI Principles. 2nd ed. Philadelphia, PA: Saunders; 2003. MRI basic physics Q & A. ReviseMRI.com website. www.revisemri.com/questions/basicphysics/. Accessed June 20, 2011. Westbrook C, Kaut Roth C, Talbot J. MRI in Practice. 3rd ed. Malden, MA: Blackwell Publishing Ltd; 2005. Woodward P. MRI for Technologists. 2nd ed. Mill Valley, CA: McGraw-Hill; 2000. 15