Introduction to MRI

Introduction to MRI Welcome to the exciting world of MRI! This is a tutorial that will cover the basic aspects of MRI. Copyright © 1994-1998 Ray Ballinger, All Rights Reserved. Send questions and comments to Ray Ballinger at mridoc@ ufl.edu URL: http://ballingerr.xray.ufl.edu/mritutor/index.html PDF Version Conversion done by Markus Busold of "The International NMR Resource site" URL: http://www.nmr.de Last Modified: September 22, 1998 Basics of MRI Instrumentation Image Characteristics Artifacts Pulse sequences Safety Contrast Agents MR Angiography MR Spectroscopy Special Topics Glossary References Basics of MRI The following topics will be discussed: Magnetism Resonance and RF Relaxation References Magnetism Magnetism is a property of matter that is a result of the orbiting electrons in atoms. The orbiting electrons cause the atoms to have a magnetic moment associated with an intrinsic angular momentum called 'spin'. Magnetic field strengths are measured in units of gauss (G) and Tesla (T). One Tesla is equal to 10,000 gauss. The earth's magnetic field is about 0.5 gauss. The strength of electromagnets used to pick up cars in junk yards is about the field strength of MRI machines (1.5-2.0T). You will run across four terms describing the magnetic properties of materials, such as contrast agents, used in MRI. These terms are ferromagnetism, paramagnetism, superparamagnetism, and diamagnetism Ferromagnetism Paramagnetism Superparamagnetism Diamagnetism Ferromagnetism Ferromagnetic materials generally contain iron, nickel, or cobalt. These materials include magnets, and various objects one might find in a patient, such as aneurysm clips, parts of pacemakers, shrapnel, etc. These materials have a large positive magnetic susceptibility, i.e., when placed in a magnet field, the field strength is much stronger inside the material than outside. Ferromagnetic materials are also characterized by being made up of clusters of 10^17 to 10^21 atoms called magnetic domains, that all have their magnetic moments pointing in the same direction. The moments of the domains is random in unmagnetized materials, and point in the same direction in magnetized materials. The figure to the above illustrates the effect of a ferromagnetic material (grey circle) on the magnetic field flux lines (blue). The ability to remain magnetized when an external magnetic field is removed is a distinguishing factor compared to paramagnetic, superparamagnetic, and diamagnetic materials. On MR images, these materials cause susceptibility artifacts characterized by loss of signal and spatial distortion. This can occur with even fragments to small to be seen on plain x-ray. This is a common finding in a cervical spine MRI post anterior fusion. Paramagnetism Paramagnetic materials include oxygen and ions of various metals like Fe, Mg, and Gd. These ions have unpaired electrons, resulting in a positive magnetic susceptibility. The magnitude of this susceptibility is less than one one-thousands of that of ferromagnetic materials. The effect on MRI is increase in the T1 and T2 relaxation rates (decrease in the T1 and T2 times). The figure above illustrates the effect of a paramagnetic material (grey circle) on the magnetic field flux lines (blue). Gd is used as a in MR contrast agents. At the proper concentration, Gd contrast agents cause preferential T1 relaxation enhancement, causing increase in signal on T1-weighted images. At high concentrations, as is sometimes seen in the urinary bladder, loss of signal is seen instead, a result of the T2 relaxation effects dominating. Superparamagnetism Superparamagnetic materials consist of individual domains of elements that have ferromagnetic properties in bulk. Their magnetic susceptibility is between that of ferromagnetic and paramagnetic materials. The figure to the left illustrates the effect of a superparamagnetic material (grey circle) on the magnetic field flux lines (blue). Examples of a superparamagnetic materials include iron containing contrast agents for bowel, liver, and lymph node imaging. Diamagnetism Diamagnetic materials have no intrinsic atomic magnetic moment, but when placed in a magnetic field weakly repel the field, resulting in a small negative magnetic susceptibility. Materials like water, copper, nitrogen, barium sulfate, and most tissues are diamagnetic.The figure above illustrates the effect of a diamagnetic material (grey circle) on the magnetic field flux lines (blue). The weak negative magnetic susceptibility contributes to the loss of signal seen in bowel on MRI after administration of barium sulfate suspensions. Resonance and RF Protons in a magnetic field have a microscopic magnetization and act like tiny toy tops that wobble as they spin.The rate of the wobbling or precession is the resonance or Larmor frequency. In the magnetic field of an MRI scanner at room temperature, there is approximately the same number of proton nuclei aligned with the main magnetic field Bo as counter aligned. The aligned position is slightly favored, as the nucleus is at a lower energy in this position. For every one-million nuclei, there is about one extra aligned with the Bo field as opposed to the field. This results in a net or macroscopic magnetization pointing in the direction of the main magnetic field. Exposure of individual nuclei to RF radiation (B1 field) at the Larmor frequency causes nuclei in the lower energy state to jump into the higher energy state. On a macroscopic level, exposure of an object or person to RF radiation at the Larmor frequency, causes the net magnetization to spiral away from the Bo field. In the rotating frame of reference, the net magnetization vector rotate from a longitudinal position a distance proportional to the time length of the RF pulse. After a certain length of time, the net magnetization vector rotates 90 degrees and lies in the transverse or x-y plane. It is in this position that the net magnetization can be detected on MRI. The angle that the net magnetization vector rotates is commonly called the 'flip' or 'tip' angle. At angles greater than or less than 90 degrees there will still be a small component of the magnetization that will be in the x-y plane, and therefore be detected. Relaxation T1 Relaxation The return of excited nuclei from the high energy state to the low energy or ground state is associated with loss of energy to the surrounding nuclei. Nuclear magnetic resonance was originally use to examine solids in the form of lattices, hence the name "spin-lattice" relaxation. Macroscopically, T1 relaxation is characterized by the longitudinal return of the net magnetization to its ground state of maximum length in the direction of the main magnetic field. The rate of return is an exponential process as is shown in the following figure. The T1 relaxation time is the time for the magnetization to return to 63% of its original length. After two T1 times, the magnetization is at 86% of its original length. Three T1 times gives 95%. Spins are considered completely relaxed after 3-5 T1 times. Another term that you may hear is the T1 relaxation rate. This is merely the reciprocal of the T1 time( 1/T1). T1 relaxation is fastest when the motion of the nucleus (rotations and translations or "tumbling rate") matches that of the Larmor frequency. As a result, T1 relaxation is dependent on the main magnetic field strength that specifies the Larmor frequency. Higher magnetic fields are associated with longer T1 times. T2 Relaxation Microscopically, T2 relaxation or spin-spin relaxation occurs when spins in the high and low energy state exchange energy but do not loose energy to the surrounding lattice. This results macroscopically in loss of the transverse magnetization. In pure water, The T2 and T1 times are approximately the same, 2-3 seconds. In biological materials, the T2 time is considerably shorter than the T1 time. For CSF, T1=1.9 seconds and T2=0.25 seconds. For brain white matter, T1=0.5 seconds and T2=0.07 seconds (70 msec). T2 relaxation occurs exponentially like T1 relaxation with 63% of the transverse magnetization gone after one T2 period as shown in the graph. T2* Relaxation T2* relaxation is the loss of signal seen with dephasing of individual magnetizations. It is characterized macroscopically by loss of transverse magnetization at a rate greater than T2. It is caused by magnetic field inhomogeneity an occurs in all magnets. The relationship between T2 and T2* can be illustrated by the multiecho spin echo sequence shown in the diagram below. The 180 degree RF pulses used to generate the echo are rephasing the spins that have undergone T2* decay. The gradual decline in signal from subsequent echos reflects T2 decay (See Figure). Unlike spin echo sequences, gradient echo sequences do not refocus T2* decay. Therefore, gradient echo sequences are more susceptible to ferromagnetic foreign bodies that distort the main magnetic field homogeneity. Instrumentation This section will cover the following topics: Magnets RF and Gradient Coils Electronics and Data Processing References RF and Gradient Coils I have grouped these two topics together, although they serve different purposes. RF Coils RF coils are the "antenna" of the MRI system that broadcasts the RF signal to the patient and/or receives the return signal. RF coils can be receive-only, in which case the body coil is used as a transmitter; or transmit and receive (transceiver). Surface coils are the simplest design of coil. They are simply a loop of wire, either circular or rectangular, that is placed over the region of interest. The depth of the image of a surface coil is generally limited to about one radius. Surface coils are commonly used for spines, shoulders, TMJ's, and other relatively small body parts. Paired saddle coils are commonly used for imaging of the knee. These coils provide better homogeneity of the RF in the area of interest and are used as volume coils, unlike surface coils.

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