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CIHR Strategic Training Program in Vascular Research VASCPROG 560 Vascular Imaging Techniques Module 6 - Magnetic Resonance Imaging (MRI) CIHR Strategic Training Program In Vascular Research Navigation through this Module This module was generated using Microsoft PowerPoint and then converted to Adobe Acrobat. You will need Adobe Acrobat Reader to view the content. Different web browsers may display WebCT content differently. Please contact Jackie Williams at the email address below if you experience difficulties viewing any module. Instead of a course textbook, all the modules contain links to excellent information that can be found on the internet. It is important that you visit these links to get more background on the topics. These also may be printed out to read in more detail later, or to be saved for future reference. If you have any difficulty in accessing any of the links within these modules please send an email to [email protected]. Sometimes the sources of the links change and adjustments will be made to correct this. When you have finished the module, please go to the Module 6 Quiz under the Quizzes icon on the Course Home Page. 1 Credits Information in this module comes from several sources, including lecture notes given by Robarts Scientists as part of the coursework in the Department of Biomedical Engineering at the University of Western Ontario, and the Radiology Residents’ Course entitled “Physics of Diagnostic Imaging”, given in partnership with the Department of Diagnostic Radiology and Nuclear Medicine, London Health Sciences Centre, Imaging Research Laboratories, Robarts Research Institute, St. Joseph’s Health Centre, and The Lawson Research Institute, all in London, Ontario. Some of the figures in the module were used with permission from “Figures from the Essential Physics of Medical Imaging, Second Edition. Jerrold T. Bushberg, J. Anthony Seibert, Edwin M. Leidholdt Jr. & John M. Boone. These are denoted by the publisher’s mark, ©2003 Lippincott, Williams & Wilkins. Other sources of information were: W. Huda & R. Slone. Review of Radiologic Physics, 2nd edition. Lippincott Williams & Wilkins. 2003. Chapter 12 – Magnetic Resonance I would like to thank Dr. Grace Parraga, and Dr. Yi-Fen Yen for permission to reproduce some of their lecture material. Jackie Williams also wrote some of the content and organized the module in its present format for WebCT. 2 Introduction Magnetic Resonance Imaging (MRI) is another rather complex modality, and it is difficult to write a concise module that will help students understand the basics, without going into too much detail. Where possible, links to more detailed websites have been included to assist students who need more information, but students whose current research does not involve MRI can ignore them. This module will give a basic overview of the principles of MRI and how it is used. For students who would like more in-depth information, or for those who prefer an interactive approach, IMAIOS offers a free e-course on MRI physics that is excellent. You will be required to register, but the registration is free, The link is: MRI step-by-step, interactive course on magnetic resonance imaging The module starts with a brief description of MRI and what makes it such an outstanding imaging technique. Then the basic physics of how it works are explained, followed by a description of the hardware. MR angiography is described in Module 8 – Angiographic Techniques. Images from Functional MRI 3 What is Magnetic Resonance Imaging (MRI)? Of all imaging techniques used in medicine, MRI produces the clearest and most detailed soft tissue contrast on stationary parts of the body. MRI can be used on moving body parts, such as the heart, but this requires advanced techniques that will be explained later in the module. MRI is based on the principles of nuclear magnetic resonance (NMR), which is one of the spectroscopic techniques used to study the properties of molecules. When the technology became available for clinical use in hospitals in the 1970s, the word nuclear was dropped from the name, as it was thought this might frighten patients, and the term magnetic resonance imaging was adopted. Both terms are used in this module, but they are synonymous. Medical MRI is based on the fact that the composition of the human body is more than 70% water, which means that a patient can become magnetized by magnetizing their water molecules. During the magnetization process, the water molecules absorb and then emit radio waves. In contrast to x-ray imaging that uses energy in the x-ray frequency of the electromagnetic spectrum, magnetic resonance imaging requires the absorption and emission of energy in the radio frequency range. This makes it a safe, nonionizing form of imaging. The whole process of how the final MR image is obtained is very complicated, and involves advanced mathematics, of which only a brief overview is given in this module. For any students who would like another, more advanced online source of MRI information, there is an excellent website written by Dr. Joseph P. Hornak from the Rochester Institute of Technology at: The Basics of MRI – Once at the site, click on the image to enter the website. 4 Why Use MRI? Compared to x ray and CT, MRI is noninvasive unless contrast agents are used. It is well tolerated and has an excellent safety record. As the MR signal is generated from the water content in tissues, solid tissues such as bone are transparent, while soft tissue delineation is excellent because of the differences in water content in different tissues. It is the best imaging modality for looking at abnormalities in the brain where differences in white and gray matter can be clearly seen, and also gives excellent images in other organs. However, just because MR provides clearer images doesn’t necessarily mean it should be chosen over other imaging modalities in every situation. MRI is very expensive and other modalities may be just as effective at correctly diagnosing a patient, even if the images are not as clear. In some cases (high resolution skeletal imaging for example) CT remains the preferred modality. MR is also excellent at depicting the vasculature and the heart, internal organs, breast, pelvis, and the musculoskeletal system. As well as showing anatomy, MR can be used to show the functional aspects of tissues. It is most often used to look at brain function by measuring the blood oxygenation in response to different stimuli. Then it is referred to as fMRI (functional MRI). 5 Potential Risks and Contraindications Although multiple studies have been performed, no significant permanent biological hazards have been demonstrated as a result of exposure to patients from the magnetic fields or radiofrequency electromagnetic pulses used in magnetic resonance imaging. However, there can be adverse effects on various medical devices implanted into patients and therefore all patients must be carefully screened to determine if MR scanning can be safely performed. Patients who cannot undergo an MRI should be able to be scanned using CT. Potential risks and contraindications include the following: 1. Cardiac pacemakers: Absolute contraindication. These patients cannot be scanned. 2. Cerebral aneurysm clip: Unless there is documented proof that a non-ferromagnetic clip was used, these patients cannot be scanned. 3. Metal fragments in body (bullet, BB, shrapnel, etc.): Safe, unless in contact with vital organ, such as heart, spinal cord, eye. 4. Surgical clips: Safe. 5. Pregnancy. While there are no known hazards, MRI is not proven to be safe during pregnancy. If a pregnant woman must undergo an MRI, she may be asked to sign a special consent form. 6. Claustrophobic patients. Patients with claustrophobia will usually find it impossible to go inside an MRI machine. 6 Basic Steps in Acquiring MR Images The diagram below shows the steps in acquiring MR images in the necessary sequence. Magnetization of Patient Repeat Hundreds of Times Patient Absorbs Radio Waves Patients Emits Radio Waves Formation of Image Acquisition of Different Types of Images Viewing, Filming, and Archiving of Images 7 The Concept of Slices MRI uses computers to transform digital data into three dimensions, one slice at a time... Just as in CT. The slices are as thin as a few millimeters and can be generated from any part of the body in any direction, giving an advantage over any other imaging modality. Neither the machine nor the patient needs to move to produce images from different directions – this is all done by manipulating the gradient magnets. 8 Where does the MR Signal Come From? The original term, nuclear magnetic resonance, was a very good one in explaining the basis of the technique. It is the ability of the nucleus of the atom to resonate in the presence of a magnetic field which makes MRI possible. The basic stages in NMR are: 1. A person is placed in a constant magnetic field. 2. Then, another magnetic field that is oscillating in the radiofrequency (RF) range of the electromagnetic spectrum is applied for a certain length of time, which makes the body’s nuclei resonate. 3. After the RF radiation is switched off, the nuclei continue to resonate and actually start to emit RF radiation which can be detected as an NMR signal. There is a fundamental difference between all the previous imaging modalities that have been described and MRI. In the other modalities there is an interaction where the body attenuates or reflects ultrasound waves or x rays, whereas in MRI, the signal is not coming from attenuation or reflection of the RF radiation, but from the tissues themselves that have been stimulated by the RF radiation. Only nuclei that have the property of spin and which have an odd number of protons and neutrons can be made to resonate and are able to be used in MRI. These nuclei possess intrinsic magnetism (or magnetic moment) so that each is a magnetic dipole and acts like a tiny bar magnet. 9 Where does the MR Signal Come From? MRI depends on the fact that hydrogen atoms can be magnetized. In fact, any nucleus that has an odd number of protons and neutrons has a net magnetic moment, so other possible candidates for MRI would be 19F, 23Na, and 31P, but as 1H is the most abundant isotope of hydrogen, it is the one that is used. The spinning proton is like a gyroscope. With a gyroscope, the earth’s gravity tries to pull it down, but its fast spin keeps it upright. Within a spinning nucleus, the interaction of the magnetic moment of the nucleus, and the applied magnetic field, causes the nucleus behave in much the same manner. Nuclei with even numbers of protons and neutrons have no net magnetic moment. However, nuclei with unmatched protons and neutrons (odd numbered nuclei), such as 1H, have a net magnetic moment, which produces an overall nuclear spin and with it a slight positive charge. This induces a weak magnetic field because the nuclear spin acts as a circular current. This magnetization of water molecules is what allows the patient to become magnetized. 10 Application of a Magnetic Field Normally, nuclei spin randomly, pointing in all directions. In the presence of a magnetic field (Bo), however, they align themselves both parallel (spin up) and anti-parallel (spin down) to the magnetic field, with a preference for the parallel direction, as this requires less energy. Spin up and spin down protons cancel each other, and it is only the very few excess protons in the spin up direction that produce a tiny net magnetization. Parallel direction = Low Energy State Anti-parallel direction = High Energy State Protons in their natural state (No external magnetic field) Protons in the presence of a large external magnetic field (Bo). More protons are aligned in the low energy state than the high energy state. So biological tissues in a large magnetic field have a small net magnetization of unpaired hydrogen protons pointing in the same direction as the magnet field. 11 The Boltzmann Distribution So, to recap, the number of spins in the lower energy level is slightly greater than the number in the higher energy level at room temperature. The entire nuclear magnetic resonance (NMR) signal is generated by this tiny energy difference between the spins in the lower energy state and the spins in the higher energy state. This is called the BOLTZMANN distribution. The Boltzmann distribution is dependent on the temperature and the different chemical components in the environments of the organs (e.g. protons in the leg will be in a different chemical environment than the brain). Most importantly, it depends on the field strength of the magnet – the stronger the magnet the greater the energy difference between the protons in the high versus low energy states. To give an idea of how tiny this energy difference is, the difference in numbers between the two populations is approximately 1 in 10,000,000! These exceedingly weak MR signals must be maximized to produce a signal-to-noise ratio (more on this later). The units of measurement of a magnetic field are Tesla and Gauss. 1T (Tesla) = 10,000 Gauss The earth’s magnetic field is 0.5 Gauss (or 0.00005 T) A fridge magnet is between 5-100 Gauss MRI machines are named according to their field strength in Teslas e.g. the most common MRI machines in routine clinical use are 1.5T magnets. Machines at 3 and 4T are considered high field strength, although there are 11T machines available at the present time. 12 Precession and the Larmor Frequency Precession Besides the slight difference in the population numbers, the magnetic field causes the dipole moments to become aligned at an angle to the magnetic field (Bo). This is called precession and it is a wobbling motion that occurs when spinning objects are subjected to an external force. This is the same motion as when a toy spinning top slow downs and starts to wobble (precess). Spin frequency is defined as: Larmor frequency (Hz) = magnetic field strength x gyromagnetic constant The rate of precession is very important in MRI as the frequency increases as the magnetic field strength increases. This is referred to as the Larmor frequency. For any given magnetic field strength, 1H will precess with a certain Larmor frequency (fo) and it gives rise to a cone-shaped movement in the xyz plane pointed in the direction of the magnetic field (see diagram on following page). B0 = the external magnetic field f0 B0 2 Where: F0 = the Larmor frequency 2 = the gyromagnetic constant The gyromagnetic constant is a specific number for each different nuclear species. This means that hydrogen nuclei under a specified magnetic field will spin at a predictable frequency. If the magnetic field changes, the spin frequency changes. 13 Precession and the Larmor Frequency This diagram shows the formula for calculating the Larmor frequency. To recap, the Larmor frequency is the frequency of precession of the nucleus about the vertical axis. The gyromagnetic ratio is a constant for a given nucleus, and is 42 MHz/Tesla for hydrogen, so 1H has a Larmor frequency of 21 MHz in a 0.5 Tesla magnet, and 63 MHz at 1.5 Tesla. 14 Resonance (Excitation) Resonance in magnetic resonance imaging refers to the same phenomenon that allows a guitar string to make a note when plucked. The plucking makes the guitar string vibrate, which then finds a frequency called its resonant frequency. Different resonant frequencies lead to different notes. In MRI, a magnetic force is used to stimulate the vibration of nuclei. So once in the main magnet, although the dipole moments are precessing at the same frequency, they are not necessarily in phase (i.e. aligned) – they are all pointing in different directions. To be of any use on MR imaging they must be brought in phase. This is achieved by applying a radio frequency (RF) electromagnetic wave whose frequency is equal to the Larmor frequency. It is usually applied for fairly short duration and is referred to as an RF pulse. Phase of Precessing Nuclei Before the RF pulse all of the magnet moments are precessing out of phase. The net magnetic moment is static and vertical. net magnetization After the RF Pulse All of the magnet moments are precessing in phase. The net magnetic moment is now rotating and has a component in both the x-y plane and in the z-direction 15 Excitation (Resonance) To recap, when atomic nuclei are exposed to a RF wave, they absorb energy from it and become excited. This phenomenon is referred to as resonance. The RF pulse is applied perpendicular to the external magnetic field (z-axis), this forces the magnetization vector out of alignment and to rotate towards the x-y plane. Immediately after the RF pulse all the nuclei are pointing in the same direction and have the same precession angle (i.e. they are in phase). An RF pulse is named after the size of the precession angle it produces – the larger the pulse the larger the angle. The two most common pulses are the 90o and the 180o pulse. When the RF wave stops, the atoms relax back to the equilibrium state and release absorbed energy to the environment as RF wave emissions. The emitted RF waves can then be detected by sensors. The component of the net magnetization vector that runs parallel with the magnetic field is called the longitudinal magnetization. The component that runs perpendicular to the magnetic field is called the transverse magnetization. ***It is this component in the horizontal plane that produces the NMR signal. 16 Adding an RF Pulse Protons are aligned in the direction of the external magnetic field RF pulse at the Larmor frequency keeps up with the precessing magnetization, which eventually forces it down to 90 degrees to the external magnetic field. The tip angle is dependent on the amplitude and duration of the RF pulse 17 Generation of the MR Signal from Patient 18 After the RF Pulse – T1 and T2 Relaxation net magnetization The following pages will describe both T1 and T2 relaxation. These are important concepts in MRI as they help provide the processes determining the contrast in MR images. 19 T1 Relaxation (aka Longitudinal or Spin-Lattice Relaxation) T1 is the time constant for the z-component of the magnetization generated by the excited nuclei to return to equilibrium. Remember that the application of the RF pulse forces the nuclei to precess in phase and drags them down to the transverse plane – at this point the NMR signal can be detected. When the pulse stops, the T1 relaxation phase starts as the nuclei start to recover the longitudinal component (see diagram on previous page). T1 is exponential and is defined as the time taken for the z-component of the magnetization to return to 63% of its equilibrium value (see diagram below). T1 is a time constant and not the time taken for full longitudinal recovery. During this time the NMR signal being emitted by the patient is losing intensity. 20 T1 Relaxation (aka Longitudinal or Spin-Lattice Relaxation) To recap, when the RF pulse stops, the excited nuclei return from the high energy state to the low energy state, losing that energy to the surrounding nuclei. The interaction between these nuclei and the surrounding molecular lattice structure and T1 was originally called “spin-lattice relaxation”. T1 relaxation is also known as longitudinal relaxation, as it is a return to the longitudinal axis. The nuclei bump into the other nuclei in the surrounding lattice, so the size and speed of the surrounding nuclei affect the rate of T1 (i.e. length of T1 time). Different body tissues contain different sizes of molecules and it is these differences that cause differences in T1 times and hence create contrast between tissues. Very small molecules rotate too quickly, leading to a small number of potential resonant frequencies, and large macromolecules rotate so slowly that no frequencies comparable to the Larmor frequency exist. Medium-sized molecules provide the shortest T1 times, so fat produces a short T1 time, while water and most proteins produce long T1 times. T1 times range from 0.1 to 1 second in soft tissues and from 1 to 4 seconds in aqueous tissues and water. T1 times increase with magnetic field strength. The RF pulse must be applied many times with relaxation and signal measurement occurring after each application. The number of repetitions is in multiples of 64, usually 128 or 256. A minimum of 128-256 signal samples are needed to form an image. Contrast in the MR images occurs because a high signal will show as a white area in the image, whereas no signal will appear as black. Intermediate amounts of signal appear in the image as different shades of gray. 21 Free Induction Decay (FID) and T2 Immediately after a 90o RF pulse stops, the now transverse magnetization vector rotates at the Larmor frequency in the x-y (horizontal) plane perpendicular to the external magnetic field. It sends out an NMR signal called the free induction decay (FID) signal. Free refers to the fact that it is no longer under the influence of the RF pulse. The FID signals are detected by a receiver coil, then digitized, stored in the computer and then transformed into MR images using a Fourier transform analysis. After the RF pulse is switched off, the protons start to lose phase gradually (decay) until they return to their original state (dephasing). It is this decay rate that is referred to as T2. T2 decreases with increasing molecular size and decreased molecular mobility. Large molecules in the body and solids have short T2 times whereas liquids have long T2 times. 22 FID Signals from the Patient TE and TR are explained in the next pages. 23 Pulse Sequences The NMR signal is composed of four separate components – The amplitude (size of the signal) – The frequency - The phase - The duration of resonance Of course, a single NMR signal from one proton doesn’t provide much information, but a whole sequence of RF pulses can be used to manipulate the amplitude of the NMR signal, which eventually becomes translated into an intensity value on the final image. The larger the amplitude, the brighter the intensity from that part of the body. It is the difference in amplitude of the NMR signals in different body regions that produce the contrast in the final image, amplitude is often referred to as intensity. There are many different pulse sequences that are preset on MRI machines that can manipulate the quality of the final image. Different sequences are used for different body parts to enhance the features that will help with diagnosis. The different pulse sequences will not be described here, but there is a good description of the most common ones at: MR Pulse Sequences There are certain parameters that are manipulated in pulse sequences, which will be explained in the next few pages. For example, the simplest (but not the most commonly used) pulse sequence is called saturation recovery. It involves repeated 90o RF pulses with the NMR signal being measured after every pulse. The repetition time (time between pulses) is called TR. 24 TR If TR is long (about 3-4 the length of T1) then the nuclei have time to return to equilibrium before the next RF pulse. The amplitude of the NMR signal after each pulse, therefore only contains proton density information. If TR is shorter (say equal to T1) then the nuclei do not have time to return to equilibrium (to their full vertical position) before the next RF pulse. 25 TE (Time to Echo) The next parameter to understand is TE (time to echo), which is the time between the 90o RF pulse and when the echo signal is sampled. The larger the TE, the larger the T2 contribution in the echo. 26 MRI Interactive Program on the Web All that has been presented in the module so far is shown in an interactive form in a demonstration program found at the following website: http://www.simplyphysics.com/MRIntro.html (This takes you to the introductory page – click on the link called Go to MRI Introduction) They say a picture is worth a thousand words and to actually see the movements made by the protons is probably worth more than any number of words. This program gives an excellent and clear description of the physics that should help cement the information. I highly recommend students to visit this site. 27 Instrumentation There are three basic types of magnets used in MRI systems: Resistive Magnets have many coils of wire wrapped around a cylinder through which an electric current is passed (a solenoid). When connected to an electricity supply this generates a magnetic field. These magnets are lower in cost to construct than a superconducting magnet, but require huge amounts of electricity (up to 50 kilowatts) to operate because of the natural resistance in the wire. These magnets are generally too expensive to run in any but very low field MR machines (below 0.35T). Permanent Magnets produce a magnetic field that is always running at full strength, so it costs nothing to maintain the field. Unfortunately, these magnets are extremely heavy and are not feasible for any but very low field MR machines (less than 0.4T). Superconducting magnets are by far the most commonly used. They are somewhat similar to a resistive magnet in that they have coils of wire through which a constant current of electricity is passed create the magnetic field. The important difference is that the wire is continually bathed in liquid helium at 452.4 degrees below zero, which is kept in a well insulated vacuum tube. The extremely low temperature causes the resistance in the wire to drop to zero, reducing the electrical requirement for the system dramatically and making it much more economical to operate. Superconductive systems are still very expensive, but they can easily generate 0.5-tesla to 4.0-tesla fields, allowing for much higher-quality imaging. 28 Magnet Assembly Most magnets are long, narrow cylinders with other concentric cylinders which perform different tasks inside them. The main magnet has miles of wire wound around it and it is the electricity in this wire that creates the magnetic field. Another type of magnet found in every MRI system is called a gradient (coil) magnet. There are usually three gradient magnets inside the MRI machine. Magnet Bore Shim Coils Gradient Coils RF Coils RF coils create the B1 field which rotates the net magnetization in a pulse sequence. Three types; RF pulse transmitter and receiver coils, receiver only coils, and transmitter only coils. MRI machines have many different coils designed for different parts of the body according to size. The gradient coils produce the gradients in the Bo magnetic field. These are also magnets, but with very, very low strength compared to the main magnetic field, ranging from 180 gauss to 270 gauss, or 18 to 27 millitesla. They produce a variable field. Shim Coils superimpose small corrective field differences on the main field to improve the magnetic field uniformity. A more realistic diagram of the Magnet assembly is shown on the next page. 29 Superconducting Magnetic Resonance System Diagram from W. Huda & R. Slone. Review of Radiologic Physics, 2nd edition. Lippincott 30 Williams & Wilkins. 2003. Signal-to-Noise Ratio If you hang around people working in the MRI field long enough (usually less than half an hour!) you will hear the term signal-to-noise ratio (often written as S/N or SNR). SNR is a measure of signal strength relative to background noise. Noise is simply unwanted electrical or electromagnetic energy that degrades the quality of signals and data and it occurs in both analog and digital signals. The ratio is usually measured in decibels (dB). Although SNR is an issue in other imaging modalities, it is a much more important concept in MRI as the signal is small to begin with, and extraneous noise has a more serious effect on the quality of the final image. Obviously, a high signal-to-noise ratio in which the signal predominates is the aim. Many of the factors that affect the SNR can be manipulated by the design of the MR equipment and the methods used to acquire images. Adjusting the SNR is a tradeoff between the contrast (which is the intensity difference between two adjacent regions) and spatial resolution. Images with low signal to noise (i.e. a lot of noise) diminish the ability to see low contrast structures. There are many factors that affect SNR and it is not important that you know them all, or their action. Some of the main factors that affect the SNR are: The strength of the main magnet The coil selection The receiver bandwidth The voxel size The pulse sequence parameters The relationship between these is shown in the formula for SNR: SNR K voxelsize measurements bandwidth 31 RF Coils RF pulses are usually applied through one of the different transmitter coils designed for different parts of the body that are supplied with MRI machines. These coils usually conform to the contour of the body part being imaged, or are located as close as possible to it during the exam. The same coils can be used as receivers, but sometimes the receiver is a separate coil. Receiver coils detect the FID signals coming from the patient. The proper selection of RF coils can greatly increase the SNR. – Small RF coils maximize the SNR of weak FID signals, but this is at the expense of a smaller region that can be imaged – Quadrature coils can also improve the SNR, as can Phased Array coils. Quadrature coils are actually made up of two linear coils that are orthogonal to each other. Each coil transmits a pulse, but being orthogonal, they are 90o out of phase with each other. Quadrature coils usually have a birdcage design. Phased Array coils use several surface coils that overlap, but the overlap must be at the correct geometry to be effective. It may be of interest to students that Robarts scientists spun off a state-of-theart MR coil company, XLResonance, that designs different MR coils for clinical and research purposes. 32 Gradient Coils Gradient coils are magnets used to produce a known variation in the constant magnetic field of the main magnet. It is the gradient coils that perform the 'magic' which generates information about position from NMR. They work by changing the magnetic field so that at some points it is higher than at others. This provides a way of knowing where in the sample the signal is coming from. These coils are firmly attached to the inside of the scanner, but the huge magnetic forces of the main magnet make the coils bang against their housing. It is this banging of the gradient coils that makes the loud rhythmic noise when the MR is scanning. MR systems have three gradient coils in the x, y, and z orientations, so that they can provide a gradient field in any direction. The following images here and over the page show what these gradients look like. The x-gradient (gradient slopes in the x plane) 33 Gradient Magnetic Fields Gradient magnetic fields (usually just referred to as “gradients”) introduce a spatial variation in the location. The larger gradient the more rapidly the slope of the magnetic field varies. The gradient magnetic fields all point in the same direction as the main magnetic field (Bo), but the variation or slope can be in any direction. The convention is to apply gradient magnetic fields along the x, y or z planes, but in principle they can be applied in any direction. The Z-Gradient The Y-Gradient 34 Spatial Encoding Spatial encoding is a crucial component of MRI. It is the process in which the spatial position of spins is encoded by applying a temporary gradient field. The methods used for spatial encoding are slice selection, frequency encoding and phase encoding. The "slice select" gradient is used to locate the level of each plane. The "frequency" and "phase encoding" gradients are used to locate points of intensity within each plane. These steps lead us to how the final image is reconstructed from the raw data. This uses a mathematical procedure called the inverse Fourier Transform. The next step is to understand the concept of K-space and how it transforms the raw data into the familiar MR image with all its shades of gray. For more information visit the following website from the e-MRI, Magnetic Resonance Imaging physics and technique course on the web. You will need to register to view the pages. Spatial Encoding 35 K-Space The concept of k-space is one that even MR professionals have some difficulty grasping. K-space is a map of all the waves of the actual image by a code that is spatial and numerical. The k-space map corresponds to numbers that contain the inherent information on the direction, strength (amplitude) and frequency of every wave in each slice that makes up the total image. The K-space map is divided into 4 quadrants with each quadrant being symmetrically the same. The numbers represent the raw data of the wave patterns that are present in any particular slice. The "raw data" sampled during the echo tells the direction, strength (amplitude) and frequency of the waves. The frequency of a wave can be known by where it appears in one of four quadrants, so the distance of the point from the centre of k-space gives the frequency. This means that waves closer to the centre of k-space have a higher frequency. The brightness of a point (the larger the number, the brighter the point) represents its strength (amplitude). Any point that is joined from the centre of K-space to any angle will show the direction of the wave. The centre 20% of the k-space map provides 90% of the contrast in the image. This site is by Dave Higgins from the Department of Medical Physics at University of Leeds, UK and has several multimedia elements that demonstrate how k-space works. MRI Tutorials - K-Space 36 Functional MRI (fMRI) Functional MRI is a relatively new imaging modality that is used for brain mapping. The technique allows visualization of the metabolic activity within the brain, rather than the anatomy, making it similar to Positron Emission Tomography (PET). FMRI is performed on a regular MR machine, but there are two tissue contrast mechanisms in MR imaging that show functional activity. These are increases in blood flow and microvascular oxygenation. The technique will not be described in any more detail in this module, but if any students are interested in learning more there is an excellent website from the FMRIB Group at the Oxford Centre for Functional Magnetic Resonance Imaging of the Brain, which is based at the Radcliffe Infirmary, Oxford University, UK. at: FMRIB - Brief Introduction to fMRI For students who are really interested, one the best written sources is the book by scientists at this centre called “Functional MRI: An Introduction to Methods”, Peter Jezzard, Paul M. Mathews, & Stephen M. Smith (Eds). 38