32 MAGNETIC RESONANCE IMAGING

Transcription

1 32 MGNETIC RESONNCE IMGING R T R M J. P I E R C E C H E R Y L D U O S E OUTLINE Principles of magnetic resonance imaging, 330 Comparison of magnetic resonance imaging and conventional radiography, 330 Historical development, 330 Physical principles, 331 Equipment, 333 Safety of magnetic resonance imaging, 336 Examination protocols, 339 Clinical applications, 345 Spectroscopy, 353 Functional magnetic resonance imaging, 354 Conclusion, 355 Definition of terms, 355 F G W V W G

2 Magnetic Resonance Imaging Principles of Magnetic Resonance Imaging Magnetic resonance imaging* (MRI) is a noninvasive examination technique that provides anatomic and physiologic information. Similar to computed tomography (CT) (see Chapter 31), MRI is a computerbased cross-sectional imaging modality. The physical principles of MRI are totally different from those of CT and conventional radiography, however, in that no ionizing radiation is used to generate the MR image. Instead, MRI creates images of structures through the interactions of magnetic fields and radio waves with tissues. MRI was originally called nuclear magnetic resonance (NMR) imaging, with the word nuclear indicating that the nonradioactive atomic nucleus played an important role in the technique; however, this term was dropped because of public apprehension about nuclear energy and nuclear weapons neither of which is associated with MRI in any way. *lmost all italicized words on the succeeding pages are defined at the end of this chapter. Comparison of Magnetic Resonance Imaging and Conventional Radiography ecause MRI provides cross-sectional images, it serves as a useful addition to conventional x-ray techniques. On a radiograph, all body structures exposed to the x-ray beam are superimposed into one flat image. In many instances, multiple projections or contrast agents are required to distinguish one anatomic structure or organ clearly from another. Cross-sectional imaging techniques such as ultrasonography, CT, and MRI more easily separate the various organs because there is no superimposition of structures. Multiple slices (cross sections) are typically required to cover a single area of the body. In addition to problems with overlapping structures, conventional radiography is limited in its ability to distinguish types of tissue. In radiographic techniques, contrast (the ability to discriminate between two different substances) depends on differences in x-ray attenuation within the object and the ability of the recording medium (e.g., film or digital detectors) to detect these differences. Radiographs cannot detect small attenuation changes. Generally, conventional radiographs can distinguish only air, fat, soft tissue, bone, and metal because of the considerable differences in attenuation between each group. Most organs, such as the liver and kidneys, cannot be separated by differences in x-ray attenuation alone, unless the differences are magnified through the use of contrast agents. CT is much more sensitive to small changes in x-ray attenuation than is plain film or digital radiography. CT can distinguish the liver from the kidneys on the basis of their different x-ray attenuation and by position. Similar to CT, MRI can resolve relatively small contrast differences among tissues. These tissue differences are unlike the differences in x-ray attenuation and the exiting radiation that produces the image, however. Contrast in MRI depends on the interaction of matter with electromagnetic forces other than x-rays. Historical Development The basic principle of MRI (discussed more fully in the next section) is that protons in certain atomic nuclei, if placed in a magnetic field, can be stimulated by (absorb energy from) radio waves of the correct frequency. fter this stimulation, the protons relax (release energy), while an electrical signal (the MRI signal) is induced into a receiver antenna, which is digitized into a viewable image. Relaxation times represent the rates of signal decay and the return of protons to equilibrium. Separate research groups headed by loch and Purcell first discovered the properties of magnetic resonance in the 1940s. Their work led to the use of spectroscopy for the analysis of complex molecular structures and dynamic chemical processes. In 1952, loch and Purcell were jointly awarded the Nobel Prize in Physics, and spectroscopy is still in use today. This finding suggested that images of the body might be obtained by producing maps of relaxation rates. Nearly 20 years after the properties of magnetic resonance were discovered, Damadian showed that the relaxation times (T1/T2) of tumors differed from the relaxation times of normal tissue. This finding suggested that images of the body might be obtained by producing maps of relaxation rates. In 1973, Lauterbur published the first crosssectional images of objects obtained with MRI techniques. These first images were crude, and only large objects could be distinguished. Mansfield further showed how the signals could be mathematically analyzed, which made it possible to develop useful imaging techniques. Mansfield also showed how extremely fast imaging could be achieved. Since that time, MRI technology has advanced rapidly. Very small structures are commonly imaged quickly and with increased resolution and contrast. In 2003, the Nobel Prize in Physiology or Medicine was jointly awarded to Lauterbur and Mansfield for their discoveries in MRI. 330

3 Physical Principles SIGNL PRODUCTION The structure of an atom is often compared with the structure of the solar system, with the sun representing the central atomic nucleus. The planets orbiting the sun represent the electrons circling around the nucleus. MRI depends on the properties of the nucleus. Most MRI scanners image the element hydrogen, which is the most abundant element in the body, found mostly in fat and water. Hydrogen with a single proton in its nucleus is the strongest nuclear magnet on a per-nucleus basis creating the strongest MRI signal. Strong signals are necessary to produce diagnostic images. Many atomic nuclei have magnetic properties, which means they act like tiny bar magnets (Fig. 32-1). Normally, protons point in random directions in the human body, as shown in Fig If the body is placed in a strong, uniform magnetic field, the protons attempt to align in the direction of the magnetic field, similar to how iron filings line up with the field of a toy magnet. The word attempt is appropriate because the protons do not line up precisely with the external field but at an angle to the field, and they rotate around the direction of the magnetic field in a manner similar to the wobbling of a spinning top. This wobbling motion, depicted in Fig. 32-3, is called precession and occurs at a specific frequency (rate) for a given atom s nucleus in a magnetic field of a specific strength. These precessing protons can absorb energy if they are exposed to radiofrequency (RF) pulses, which are very fast bursts of radio waves, provided that the radio waves and nuclear precession are of the same frequency. This absorption of energy by the precessing protons is referred to as resonance. The resonant frequency varies depending on the field strength of the MRI scanner. t a field strength of 1.5 tesla, the frequency is approximately 63 MHz; at 1 tesla, the frequency is approximately 42 MHz; at 0.5 tesla, the frequency is approximately 21 MHz; and at 0.2 tesla, the frequency is approximately 8 MHz. efore exposure to the RF pulse, a slight majority of the hydrogen protons are oriented with the direction of the magnetic field. This causes the tissues to be magnetized in the longitudinal direction, which is also parallel to the magnetic field. When the RF pulse is applied, and the protons absorb the energy, the result is a reorientation of the bulk of the tissue magnetization into a plane perpendicular to the main field. This is known as the transverse plane. The magnetization in the transverse plane also precesses at the same resonant frequency. The precessing transverse magnetization in the tissues creates an electrical current in the receiving coil or antenna. This follows Faraday s law of induction, in which a moving magnetic field induces electrical current in a coil of wire. The electrical current in this application is measured as the MRI signal, which is similar to the broadcasting radio waves that induce current in a car radio antenna. The MRI signal is picked up by this sensitive antenna or coil, amplified, and processed by a computer to produce a sectional image of the body. This image, similar to the image produced by a CT scanner, is a digital image that is viewed on a computer monitor. ecause this is a digital image, it can be manipulated, or postprocessed, to produce the most acceptable image. dditional processing can be performed on a three-dimensional workstation if applicable, and hard copies can be produced if necessary. Many other nuclei in the body are being used in MRI. Nuclei from elements such as phosphorus and sodium may provide useful or differing diagnostic information than hydrogen nuclei, particularly in efforts to understand the metabolism of normal and abnormal tissues. Metabolic changes may prove to be more sensitive and specific in detecting abnormalities than the more physical and structural changes recognized by hydrogen-imaging MRI. Nonhydrogen nuclei may also be used for combined imaging and spectroscopy, in which small volumes of tissue may be analyzed for chemical content. Physical Principles S N S N N N Fig proton with magnetic properties can be compared with a tiny bar magnet. Curved arrow indicates that a proton spins on its own axis; this motion is different from that of precession. S Fig In the absence of a strong magnetic field, the protons (arrows) point in random directions and cannot be used for imaging. S N S 0 One nucleus Toy top Fig Precession. The protons (arrow) and the toy top spin on their own axes. oth also rotate (curved arrows) around the direction of an external force in a wobbling motion called precession. Precessing protons can absorb energy through resonance. 0 represents the external magnetic field acting on the nucleus. The toy top precesses under the influence of gravity. 331

4 Magnetic Resonance Imaging SIGNIFICNCE OF THE SIGNL Conventional radiographic techniques, including CT, produce images based on a single property of tissue: x-ray attenuation or density. MR images are more complex because they contain information about differing properties of tissue proton density, relaxation rates, and flow phenomena. Each property contributes to the overall strength of the MRI signal. Computer processing converts signal strength to shades of gray on the image. Strong signals are represented by white in the image, and weak signals are represented by black. One determinant of signal strength is the number of precessing protons in a given volume of tissue. This signal produced by the excited protons is proportional to the number of protons present. Signal strength depends on the concentration of protons, or proton density. Most soft tissues, including fat, have a similar number of protons per unit volume; the use of proton density characteristics alone poorly separates most tissues. Some tissues have few hydrogen nuclei per unit of volume; examples include the cortex of bone and air in the lungs. These tissues have a weak signal as a result of low proton density and can be easily distinguished from other tissues. MRI signal intensity also depends on the relaxation times of the nuclei. Relaxation is the release of energy by the excited protons, which occurs at different rates in different tissues. Excited nuclei relax through two processes. The process of nuclei releasing their excess energy to the general environment or lattice (the arrangement of atoms in a substance) is called spin-lattice relaxation. The rate of this relaxation process is measured in milliseconds and is labeled as T1. Spin-spin relaxation is the release of energy by excited nuclei through interaction among themselves. The rate of this process is also measured in milliseconds but is labeled as T2. The rates of relaxation (T1 and T2) of a hydrogen nucleus depend on the chemical environment in which the nucleus is located. Chemical environment differs among tissues. The chemical environment of a hydrogen nucleus in the spleen differs from that of a hydrogen nucleus in the liver. The relaxation rates of these nuclei differ, and the MRI signals created by these nuclei differ. The different relaxation rates in the liver and spleen result in different signal intensities and appearances on the image, enabling the viewer to discriminate between the two organs. Similarly, fat can be separated from muscle, and many tissues can be distinguished from others, based on the relaxation rates of their nuclei. The most important factor in tissue discrimination is the relaxation time. The signals produced by MRI techniques contain a combination of proton density and T1 and T2 information. It is possible, however, to obtain images weighted toward any one of these three parameters by stimulating the nuclei with certain specific radiowave pulse sequences. In most imaging sequences, a short T1 (fast spin-lattice relaxation rate) produces a high MRI signal on T1-weighted images. Conversely, a long T2 (slow spin-spin relaxation rate) generates a high signal on T2-weighted images. The final property that influences image appearance is flow. For complex physical reasons, moving substances usually have weak MRI signals. (With some specialized pulse sequences, the reverse may be true; see the discussion of magnetic resonance angiography [MR] later in the chapter.) With standard pulse sequences, flowing blood in vessels produces a low signal and is easily discriminated from surrounding stationary tissues without the need for the contrast agents required by regular radiographic techniques. Stagnant blood, such as an acute blood clot, typically has a high MRI signal in most imaging schemes as a result of its short T1 and long T2. The flow sequences of MRI may facilitate the assessment of vessel patency or the determination of the rate of blood flow through vessels (Fig. 32-4). G L IVC S K Fig T2-weighted image of an abdomen showing the flow void produced by flowing blood., aorta; G, gallbladder; IVC, inferior vena cava; K, kidney; L, liver; S, spleen. 332

5 Equipment MRI requires a patient area (magnet room), an equipment room, and an operator s console. separate diagnostic workstation is optional. CONSOLE The operator s console is used to control the imaging process (Fig. 32-5). Sitting at the console allows the operator to interact with the system s computers and electronics to manipulate all necessary examination parameters and perform the appropriate examination. Images are viewed on a computer monitor to ensure that the examination is of appropriate diagnostic quality. Images can be manipulated here, and hard copy can be produced if necessary. n independent or three-dimensional workstation may be used to perform additional imaging manipulation or postprocessing when required. EQUIPMENT ROOM The computer room houses the electronics necessary for transmitting the radio-wave pulse sequences and for receiving and analyzing the MRI signal. The raw data and the computer-constructed images can be stored on a computer disc temporarily but are usually transferred to a magnetic tape or an optical disc for permanent storage and retrieval. The equipment room houses all the electronics and computers necessary to complete the imaging process. The RF cabinet controls the transmission of the radio-wave pulse sequences. The gradient cabinet controls the additional timevarying magnetic fields necessary to localize the MRI signal. The array processors and computers receive and process the large amount of data received from the patient and construct the images the operator sees on the operator s console. Equipment Fig Operator s console. This device controls the imaging process and allows visualization of images. (Courtesy General Electric Healthcare.) 333

6 Magnetic Resonance Imaging MGNET ROOM The magnet is the major component of the MRI system in the scanning room. This magnet must be large enough to surround the patient and any antennas that are required for radio-wave transmission and reception. ntennas are typically wound in the shape of a positioning device for a particular body part. These are commonly referred to as coils. The patient is usually placed within the coil or surface coils are placed directly on the patient and are used in the imaging of superficial structures. The patient and the coil still must be within the magnet, however, to be exposed to the proper magnetic field for imaging. The patient lies on the table and is advanced into the imaging magnetic field (Fig. 32-6). Various magnet types may be used to provide the strong uniform magnetic field required for imaging, as follows: Resistive magnets are simple but large electromagnets consisting of coils of wire. magnetic field is produced by passing an electrical current through the wire coils. High magnetic fields are produced by passing a large amount of Fig Patient prepared for MRI. (Courtesy General Electric Healthcare.) current through numerous coils. The electrical resistance of the wire produces heat and limits the maximum magnetic field strength of resistive magnets. The heat produced is conducted away from the magnet by a cooling system. Superconductive (cryogenic) magnets are also electromagnets. Their wire loops are cooled to very low temperatures with liquid helium to reduce electrical resistance. This permits higher magnetic field strengths than produced by resistive magnets. Permanent magnets are a third source for producing the magnetic field. permanent magnet has a constant field that does not require additional electricity or cooling. The early permanent magnets were extremely heavy even compared with the massive superconductive and resistive units. ecause of their weight, these magnets were difficult to place for clinical use. With improvements in technology, permanent magnets have become more competitive with the other magnet types. The magnetic field of permanent magnets does not extend as far away from the magnet (fringe field) as do the magnetic fields of other types of magnets. Fringe fields are a problem because of their effect on nearby electronic equipment. Various MRI systems operate at different magnetic field strengths. The choice of optimal field strength for imaging is controversial. Magnetic field strength is measured in tesla (T) or gauss (G). Most MRI examinations are performed with field strengths ranging from 0.2 to 3.0 tesla. Resistive systems generally do not exceed 0.6 tesla, and permanent magnet systems do not exceed 0.3 tesla. Higher field strengths require superconductive technology, with popular field strengths of 1.5 tesla and 3 tesla. Most research has concluded that field strengths used for diagnostic clinical imaging do not produce any substantial harmful effects. Regardless of magnet type, MRI units are difficult to install in hospitals. Current units are quite heavy up to 10 tons for resistive and superconductive magnets and approximately 100 tons for some permanent magnets. Some institutional structures cannot support these weights without reinforcement. In addition, choosing a location for the MRI unit can be difficult because of fringe fields. With resistive and superconductive magnets, the fringe field extends in all directions and may interfere with nearby electronic or computer equipment, such as television monitors and computer tapes. In addition, metal objects moving near the magnetic fringe field, such as automobiles or elevators, may cause ripples in the field, similar to the ripples caused by a pebble thrown into a pond. These ripples can be carried into the center of the magnet, where they distort the field and ruin the images. MRI sites must be located far enough away from moving metal objects. Efforts continue to be made to find more ways to shield the magnetic fringe field to prevent its extension beyond the patient area. 334

7 Stray radio waves present another difficulty in the placement of MRI units. The radio waves used in MRI may be the same as the radio waves used for other nearby radio applications. Stray radio waves can be picked up by the MRI antenna coils and interfere with normal image production. MRI facilities require specially constructed rooms to shield the receiving antennas from outside radio interference, adding to the cost of the installation. In recent years, specialty units have become available for limited applications. One example is an extremity MRI scanner (Fig. 32-7). This unit is designed so that the patient can sit comfortably in a chair while having an extremity or musculoskeletal joint imaged. These units are lightweight (approximately 1500 lb) and take up less space than conventional MRI scanners, and they produce good image quality (Fig. 32-8). Fig Extremity MRI scanner, 1.0 tesla. (Courtesy ONI Medical Systems, Inc, Wilmington, M.) Equipment Fig Coronal MRI of the knee obtained with extremity MRI scanner. (Courtesy ONI Medical Systems, Inc, Wilmington, M.) 335

8 Magnetic Resonance Imaging Safety of Magnetic Resonance Imaging MRI is generally considered safe. It is often preferred over CT for imaging of children because it does not use ionizing radiation, which has known potential adverse health effects. growing child s body is thought to be more susceptible to the effects of ionizing radiation. Nevertheless, many potential safety issues concerning MRI must be raised some related to potential direct effects on the patient from the imaging environment and others related to indirect hazards. Opinions differ about the safety of the varying magnetic and RF fields to which the patient is directly exposed. Many studies in which experimental animal and cell culture systems were exposed to these fields over long periods have reported no adverse effects, whereas others have reported changes in cell cultures and embryos. Some energy is deposited in the patient during imaging and is dissipated in the body as heat. The resulting changes seem to be less than the levels considered clinically significant, even in areas of the body with poor heat dissipation, such as the lens of the eye. The significance of direct short-term exposure (i.e., exposure of a patient) and long-term exposure (i.e., exposure of an employee who works with MRI) is unclear. No clear association of MRI with adverse effects in humans has been proven, but research is continuing. Numerous hazards related to MRI have been well documented. Objects containing magnetic metals (e.g., iron, nickel, cobalt) in various combinations may be attracted to the imaging magnet with sufficient force to injure patients or personnel who may be interposed between them. Scissors, oxygen tanks, and patient gurneys are among the many items that have been drawn into the magnetic field at MRI sites. Metallic implants within patients or personnel can become dislodged within the body and cause injury if they are in delicate locations. Examples include intracranial aneurysm clips, auditory implants, and metallic foreign bodies in the eye. Long-standing, firmly bound surgical clips, such as from a cholecystectomy, do not pose problems. Electronic equipment can malfunction when exposed to strong magnetic fields. The most critical items in this category are cardiac pacemakers and the similar automatic implantable cardiac defibrillators. Patients, visitors, and personnel should be screened to ensure that they do not have metallic objects on or in their bodies that could be adversely affected by exposure to strong magnetic fields. Patients have received local burns from wires, such as electrocardiogram (ECG) leads, and other monitoring devices touching their skin during MRI examinations. These injuries have resulted from electrical burns caused by currents induced in the wires or thermal burns caused by heating of the wires. Such burns can be prevented by checking wires for frayed insulation, ensuring that no wire loops are within the magnetic field, and placing ad- 336

9 ditional insulation between the patient and any wires exiting the MRI system. The varying magnetic forces in an MRI unit act on the machine itself, causing knocking or banging sounds. These noises can be loud enough to produce temporary or permanent hearing damage. The use of earplugs or nonmagnetic headphones can be helpful in preventing auditory complications and are highly recommended to be used by each patient being scanned. Claustrophobia can be a significant impediment to MRI in 10% of patients (Fig. 32-9). Patient education is perhaps most important in preventing this problem, but tranquilizers, appropriate lighting and air movement within the magnet bore, and mirrors or prisms that enable a patient to look out of the imager may be helpful. Claustrophobia can also be prevented by having a family member or friend accompany the patient and be present in the room during the scan. In superconductive magnet systems, rapid venting (quench) of the supercooled liquid gases (helium) from the magnet or its storage containers into the surrounding room space is a rare but potential hazard because the relative concentration of oxygen in the air could be reduced to unsafe levels. Unconsciousness or asphyxiation could result. Oxygen monitoring devices in the magnet or cryogen storage room can signal personnel when the oxygen concentration becomes too low. Personnel may then evacuate the area and activate ventilation systems to exchange the escaped gas for fresh air. Safety of Magnetic Resonance Imaging Fig Patient inside a superconducting 1.5-tesla magnet. Some patients cannot be scanned because of claustrophobia. (Courtesy General Electric Healthcare.) 337

10 C CC V CL Magnetic Resonance Imaging H P H Fig Two images (different patients) from a 3.0-tesla superconductive MRI scanner, showing excellent resolution of images., This image shows remarkable anatomic detail in a midsagittal image of the head., air in sinuses;, brainstem; C, cerebrum; CC, corpus callosum; CL, cerebellum; V, ventricle., This coronal image of the pelvis shows anatomic relationships of the prostate (P), which is enlarged and elevating the bladder (). Hips (H) and acetabula () are also shown. loop of the sigmoid (S) colon is on top of the bladder. This degree of resolution in coronal or sagittal images would be difficult to obtain by reformatting a series of transverse CT slices. 338

11 Examination Protocols IMGING PRMETERS The availability of many adjustable parameters makes MRI a complex imaging technique. Knowledge of the patient s clinical condition or probable disease is important in choosing the proper technique and in imaging the correct area of the body. The operator may choose to obtain MR images in sagittal, coronal, transverse, or oblique planes. These are independently directly acquired images with equal resolution in any plane (Fig ). In contrast, data can be obtained only in the transverse plane with CT. Sagittal and coronal CT images are be generated by reformatting the data. nother MRI technique, especially when numerous thin slices or multiple imaging planes are desired, is three-dimensional imaging. In this technique, MRI data are collected simultaneously from a three-dimensional block of tissue rather than from a series of slices. Special data collection techniques and subsequent computer analysis allow the images from the single imaging sequence to be displayed in any plane (Fig ). Slice thickness is important in the visualization of pathology. More MRI signal is available from a thicker slice than a thinner slice, so thicker slices may provide images that are less grainy. Small pathologic lesions may be hidden, however, by the surrounding tissues in the thicker slices. Slice thickness may need to Examination Protocols Fig Single slice from three-dimensional acquisition of the knee on a 3.0-tesla MRI unit. Data from an entire volume within the imaging coil are obtained concurrently. The data may be reconstructed into thin slices in any plane, such as the sagittal image shown here. This imaging sequence shows hyaline cartilage (black arrow) as a fairly high signal intensity rim overlying the bone. Meniscal fibrocartilage (white arrow) has low signal intensity. High signal intensity from joint fluid in a tear (curved arrow) within the anterior horn of the meniscus is visualized. 339

12 Magnetic Resonance Imaging be adjusted based on the type of lesion under investigation. nother important MRI parameter is overall imaging time. s imaging time (per slice) is lengthened, more MRI signal is available for analysis. Image quality improves with increased signal. Fewer patients can be imaged, however, when extended data acquisitions are performed. In addition, patient motion increases with prolonged imaging times, which reduces image quality. The imaging sequence is a crucial parameter in MRI. Depending on the choice of pulse sequence, the resulting images W F may be more strongly weighted toward proton density, T1, or T2 information. Depending on the relative emphasis given to these factors, normal anatomy (Fig ) or a pathologic lesion (Fig ) may be easily recognized or difficult to see. It is not unusual for a lesion to stand out dramatically when one pulse sequence is used yet be nearly invisible (same MRI signal as surrounding normal tissue) with a different pulse sequence. Research continues to determine the optimal pulse sequences for scanning various patient problems. G G W F lthough varying the timing parameters of an individual pulse sequence can alter the relative weighting of information received, certain classes of pulse sequences tend to emphasize information about proton density, T1, T2, and flow. Spin echo sequences are the classic imaging sequences usually used with appropriate timing parameters (TE and TR) to yield T1-weighted images, but they can also provide proton density weighted images and T2-weighted images. Inversion recovery is a sequence that accentuates T1 information but can also provide a special result in that the timing parameters (inver- V V Fig xial 3.0-tesla images through a normal brain., T1-weighted image shows relatively low differentiation of gray matter (G) and white matter (W) within the brain., Heavily T2-weighted image shows improved differentiation between gray and white matter. Cerebrospinal fluid within the ventricles (V) also changes in appearance with change in pulse sequence (low signal on T1-weighted image); fat (F) normally shows high signal intensity, whereas on the T2-weighted image, the signal intensity of fat is less than cerebrospinal fluid. 340

13 T sion time) can be chosen to minimize signal intensity in a particular tissue. Fat is usually the tissue chosen to have its intensity minimized, and so-called fatsuppressed images can be useful when the high signal from extensive fat overwhelms small signal intensity differences in the tissues of interest. dditional techniques to suppress the signal from fat or fluid or both have been developed for pulse sequences other than inversion recovery. Standard imaging sequences such as spin echo and inversion recovery are timeconsuming and slow patient throughput, or productivity. MRI engineers and physicists have developed faster pulse sequences to speed up examinations. The oldest and most common type of faster imaging sequence is the gradient echo pulse sequence. In the early 1990s, a fast gradient echo pulse sequence, known as rapid acquisition recalled echo, was created. In recent years, an even faster sequence, called echo planar imaging, has been implemented. Some imaging sequences are short enough that imaging can be accomplished during a breath hold. Many fast pulse sequences are sensitive to flow and may be used to provide images of blood vessels. (See the discussion of MR later in this chapter.) POSITIONING Patient positioning for MRI is usually straightforward. Generally, the patient lies supine on a table that is subsequently advanced into the magnetic field. s previously discussed, it is important to ensure that the patient has no contraindications to MRI, such as a cardiac pacemaker or intracranial aneurysm clips. Claustrophobia may be a problem for some patients as previously noted because the imaging area is tunnel-shaped in most MRI system configurations (see Fig. 32-9). T Examination Protocols C T Fig xial MRI showing the use of different pulse sequences and their effect on the visualization of the cerebellopontine angle tumor., T1-weighted image shows that limited contrast exists between the tumor (T) and normal brain., Lesion becomes dramatically more obvious using the pulse sequence of the T2-weighted image. C, Lesion is still visible on FLIR pulse sequence but not as well as the T2 weighted image. Choice of pulse sequence is critical. These images also show how the lack of bone artifact makes MRI superior to CT for imaging of posterior fossa lesions. 341

14 Magnetic Resonance Imaging COILS The coils used for MRI are necessary for transmitting the RF pulse and receiving the MRI signal (as described earlier in the section on signal production). Some coils can transmit and receive (transceiver coils), whereas others only may receive the signal (receiver coils). The body part to be examined determines the shape of the antenna coil that is used for imaging (Fig ). Most coils are round or oval, and the body part to be examined is inserted into the coil s open center. Some coils, rather than encircling the body part, are placed directly on the patient over the area of interest. These surface coils are best for the imaging of thin body parts, such as the limbs, or superficial portions of a larger body structure, such as the orbit within the head or the spine within the torso. nother form of receiver coil is the endocavitary coil, which is designed to fit within a body cavity such as the rectum. This enables a receiver coil to be placed close to some internal organs that may be distant from surface coils applied to the exterior body. Endocavitary coils also may be used to image the wall of the cavity itself (Fig ). PTIENT MONITORING lthough most MRI sites are constructed so that the operator can see the patient during imaging, the visibility is often limited, and the patient is relatively isolated within the MRI room (see Fig. 32-9). t most sites, intercoms are used for verbal communication with the patient, and all units have panic buttons with which the patient may summon assistance. These devices may be insufficient, however, to monitor the health status of a sedated, anesthetized, or unresponsive patient. MRI-compatible devices are available to monitor multiple physiologic parameters such as heart rate, respiratory rate, blood pressure, and oxygen concentration in the blood. Local policy and patient condition dictate which physiologic parameters are monitored. Fig Examples of coils used for MRI. Upper row, left to right, Foot/ankle coil, breast coil, and knee coil. Lower row, left to right, Shoulder coil, functional head coil, and wrist coil. (Courtesy Invivo Corporation.) Fig xial image of prostate obtained with an endorectal coil. The increased resolution allowed by the endorectal coil makes it possible to perform MRS (PROSE). The spectroscopy map shows an elevated citrate level (arrow) consistent with tumor. (Courtesy GE Healthcare.) 342

15 CONTRST MEDI Contrast agents widen the signal differences in MR images between various normal and abnormal structures. good orally administered agent for identifying bowel loops in MRI scans has not yet been identified. In CT scanning, the use of high-attenuation, orally administered contrast medium allows clear differentiation of the bowel from surrounding lower attenuation structures. In MRI scans, the bowel may lie adjacent to normal or pathologic structures of low, medium, and high signal intensity, and these intensities may change as images of varying T1 and T2 weighting are obtained. It is difficult to develop an agent that provides good contrast between the bowel and all other structures under these circumstances. ir, water, fatty liquids (e.g., mineral oil), dilute iron solutions (e.g., Geritol), gadolinium compounds designed for intravenous (IV) use, barium sulfate, kaolin (a clay), and various miscellaneous agents all have been used none with complete success. t this time, the MRI contrast agents most commonly used in the United States for routine clinical use in the whole body are gadolinium-containing compounds. Gadolinium is a metal with paramagnetic effects. Pharmacologically, an intravenously administered gadolinium compound acts similar to radiographic iodinated IV agents: It distributes through the vascular system, its major route of excretion is the urine, and it respects the bloodbrain barrier (i.e., it does not leak out from the blood vessels into the brain substance unless the barrier has been damaged by a pathologic process). Gadolinium compounds have lower toxicity and fewer side effects than IV iodinated contrast media used in radiography and CT. New contrast agents used for MR examinations are known as blood pool agents and contain a gadolinium base. y binding to albumin found in the body, these blood pool agents produce shorter T1 relaxation times than regular gadolinium compounds, resulting in a brighter signal on the final image. These agents also prolong retention in the bloodstream, allowing for longer imaging times. Imaging using these agents may allow estimates of tissue perfusion and ischemia. Enhancement of heart muscle could assist in differentiating healthy, ischemic, or infarcted myocardial tissue. Gadolinium compounds are used most commonly in evaluation of the central nervous system. The most important clinical action of gadolinium compounds is the shortening of T1. In T1-weighted images, this provides a high-signal, high-contrast focus in areas where gadolinium has accumulated by leaking through the broken blood-brain barrier into the brain substance (Fig ). In gadolinium-enhanced T1- weighted images, brain tumors or metastases are better distinguished from their surrounding edema than in routine T2- weighted images. Gadolinium improves the visualization of small tumors or tumors that have a signal intensity similar to that of a normal brain, such as meningiomas. IV injections of gadolinium also have been used in dynamic imaging studies of body organs such as the liver and kidneys, similar to techniques using standard radiographic iodinated agents in CT. Iron oxide mixtures are known as superparamagnetic contrast agents. These agents are referred to as T2 contrast agents because they shorten the T2 relaxation times of normal tissue. These contrast agents are approved for use in the detection and diagnosis of liver lesions. Numerous novel contrast agents for MRI are under development, but many are not yet approved for routine clinical use. Selective enhancement of lymph nodes may allow tumor involvement to be detected directly, obviating the need to rely on crude size criteria for abnormality. The production of contrast agents with an affinity for specific tumors may also be possible. Radioactive-labeled antibodies against tumors are available for use in nuclear medicine, and appropriately labeled antibodies could carry paramagnetic compounds to tumor sites. Examination Protocols C Fig Use of IV gadolinium contrast medium for lesion enhancement in axial images of the brain., T1-weighted sequence. single brain lesion (arrowhead) is seen as a focal area of low signal intensity in a large area of edema. The borders of the lesion are difficult to delineate., FLIR image. High signal areas (arrows) represent tumor and surrounding edema. C, T1-weighted image obtained using similar parameters after IV administration of gadolinium. Lesion borders and size (arrow) are much more conspicuous. 343

16 Magnetic Resonance Imaging GTING Gated imaging is another technique for improving image quality in areas of the body in which involuntary patient motion is a problem. patient can hold the head still for prolonged data acquisition, but heartbeat and breathing cannot be suspended for the several minutes required for standard MRI studies. Even fast pulse sequences are susceptible to motion artifact from the beating heart; this is a problem when images of the chest or upper abdomen are desired. If special techniques are not used, part of the MRI signal may be obtained when the heart is contracted (systole) and part when the heart is relaxed (diastole). When information is combined into one image, the heart appears blurred. This problem is analogous to photographing a moving subject with a long shutter speed. Similar problems in MRI occur with the different phases of respiration. Gating techniques are used to organize the signal so that only the signal received during a specific part of the cardiac or respiratory cycle is used for image production (Fig ). Gated images may be obtained in one of two ways. In one technique of cardiac gating, the imaging pulse sequence is initiated by the heartbeat (usually monitored by an ECG). The data collection phase of the pulse sequence occurs at the same point in the cardiac cycle. nother method is to obtain data throughout the cardiac cycle but record the point in the cycle at which each group of data was obtained. fter enough data are collected, the data are reorganized so that all data recorded within a certain portion of the cardiac cycle are collated together: data collected during the first eighth of the cycle, second eighth of the cycle, and so on. Each grouping of data can be combined into a single image, producing multiple images at different times in the cycle. LV LVW RV LV LVW P Fig ECG gated images of the heart., Left ventricular outflow tract (LVOT)., Short-axis images., aorta; LV, left ventricle; LVW, left ventricular wall; P, papillary muscles; RV, right ventricle. 344

17 OTHER CONSIDERTIONS When MRI was introduced, long imaging times were required to obtain enough information to reconstruct the sectional images, and this remains the standard for most routine imaging. With advances in technology, it has become possible to obtain enough data quickly (within seconds) to reconstruct an image by using special fast-imaging pulse sequences. These fastimaging pulse sequences are becoming more popular for specialized applications, such as obtaining a dynamic series of images after IV administration of contrast agents. In many such sequences, fluid has a high signal intensity. This high signal intensity can produce a myelogramlike effect in studies of the spine or an arthrogramlike effect in evaluation of joint fluid (see Fig ). Quality assurance is important in a complex technology such as MRI. Calibration of the unit is generally performed by service personnel. Routine scanning of phantoms by the technologist can be useful for detecting any problems that may develop. Clinical pplications CENTRL NERVOUS SYSTEM MRI has become the modality of choice for imaging of the central nervous system. It is routinely used in almost all examinations of the brain with the exception of acute trauma. MRI is superior in the brain because of its inherent ability to differentiate the natural contrast among tissues such as gray and white matter (see Fig ). This ability allows MRI to be more sensitive than CT in detecting changes in white matter disease such as multiple sclerosis. The development of specialized pulse sequences such as fluid attenuation inversion recovery (FLIR) helps visualize lesions in the paraventricular area that were previously difficult to detect. MRI is also better at imaging the posterior fossa (cerebellum and brainstem) because cortical bone does not produce any signal in MRI (see Fig ). This area is often obscured on CT because of the beam hardening artifact. lmost all brain lesions such as primary and metastatic tumors, pituitary tumors, acoustic neuromas (tumors of the eighth cranial nerve), and meningiomas are better shown by MRI. The additional use of IV gadolinium based contrast materials has allowed better differentiation and increased sensitivity in detecting these lesions (Fig ). Cerebral infarction is identified sooner using diffusion-weighted imaging compared with CT. Diffusionweighted imaging also gives MRI the ability to determine the age of lesions or differentiate acute from chronic ischemic changes. Clinical pplications T Fig xial MRI of the brain in a patient with acoustic nerve tumor arising from the seventh and eighth cranial nerve complex., Precontrast T1-weighted image shows inhomogeneous area of abnormality (white arrows), with mass effect expanding the area of the nerve complex., Image obtained at the same level after gadolinium enhancement. ctive tumor (T) shows high signal intensity. 345

18 Magnetic Resonance Imaging MRI is also routinely used to image the spinal canal and its contents. The ability of MRI to image directly in the sagittal plane allows for the screening of a large area in a single examination. T2-weighted pulse sequences permit the separation of cerebrospinal fluid and the spinal cord similar to myelography without the use of contrast media (Fig ). ecause of its inherent ability to differentiate slight changes in soft tissue contrast, MRI is exquisitely sensitive at detecting spinal cord tumor and cystic changes within the cord. The visualization of bony marrow is useful in the detection and diagnosis of metastatic disease and pathologic and nonpathologic vertebral fractures and diskitis (infection). The most prolific use of MRI in the spine is the imaging of disk disease. Direct visualization of the posterior longitudinal ligament in the sagittal plane and vertebral disks in the oblique plane shows the severity of herniated disks (Fig ). The use of IV gadolinium contrast material helps differentiate between disk herniations and postoperative scar tissue, which is a crucial clinical distinction. CHEST MRI is extremely sensitive to physiologic motion, so imaging within the chest is difficult. dvances in multislice/helical CT and the technical challenges of imaging moving anatomy have limited the use of MRI for examining the chest. Cardiac gating (imaging only during a certain part of the cardiac cycle), respiratory gating or triggering, breath-hold scans, and ultrafast imaging sequences have enabled MRI to excel at cardiac imaging, however. MRI is able to show anatomy and produce functional (ejection fractions, chamber volume) data similar to nuclear medicine and echocardiography. The study of congenital heart disease, imaging of masses, and evaluation of heart muscle viability are now routine (Fig ). MRI may also be used to image the chest wall, thoracic outlet, and brachial plexus region. Since its approval in 1991 by the U.S. Food and Drug dministration (FD) as a supplemental imaging tool, MR mammography or breast MRI has become an essential part of breast imaging. The FD lists breast MRI in their screening criteria. reast imaging is routinely used to screen high-risk patients. Surgeons also use it preoperatively to define the extent of the lesion, look for additional lesions, and image the contralateral breast. In addition, it is being used to monitor adjuvant therapy (chemotherapy and radiation therapy) (Fig ). F CSF S D HNP N Fig Sagittal T2-weighted MRI through thoracic spine. High signal from CSF (F) outlines the normal spinal cord (S), giving a myelogramlike effect without the use of contrast agents. Fig Sagittal T2-weighted image of lumbar spine. Spinal canal is filled with high signal intensity cerebrospinal fluid (F) except for low signal intensity linear nerve roots running within the spinal canal. Normal vertebral disks have a high signal intensity nucleus pulposus (N). Desiccated disks (D) show low signal intensity. t L4-5, note the herniated nucleus pulposus (HNP) protruding into the spinal canal and compressing the nerve roots. 346

19 M D Fig Cardiac MRI: four-chamber view from two different patients., T2-weighted image showing normal myocardium in the wall of the left ventricle (M) before the administration of contrast medium., Delayed enhancement image (inversion recovery) showing bright signal in the wall of the left ventricle represented infarcted or dead myocardium (DE). Clinical pplications Fig MRI breast image postprocessed showing contrast washin and washout. The patient is a 68-year-old woman with an enhancing mass in the left breast at 1 o clock position (arrow). 347

20 Magnetic Resonance Imaging DOMEN lthough abdominal imaging is also affected by respiratory motion, the use of ultrafast scanning techniques with the ability to acquire two-dimensional and threedimensional volumes in a breath-hold has made MRI extremely useful in the abdomen as a problem-solving tool. Typically not used as a primary diagnostic tool, MRI is used to follow-up questionable results from other modalities such as CT and ultrasound. One exception is liver imaging, in which MRI may be more sensitive in detecting primary and metastatic tumors. The use of liver-specific IV contrast agents has improved the differentiation of liver lesions. MRI has the ability to predict the histologic diagnosis of certain abnormalities such as hepatic hemangiomas, which have a distinctive appearance. The use of in-phase and out-of-phase images can distinguish between benign and malignant adrenal tumors (Figs and 32-24). PELVIS Respiratory motion has little effect on the structures in the pelvis. s a result, these structures can be better visualized than structures in the upper abdomen. The ability of MRI to image in the coronal and sagittal planes is helpful in examining the curved surfaces in the pelvis. ladder tumors are shown well, including tumors at the dome and base of the bladder that can be difficult to evaluate in the transverse dimension. In the prostate (see Fig ), MRI is useful in detecting neoplasm and its spread. In the female pelvis, MRI can be used to image benign and malignant conditions (Fig ). MUSCULOSKELETL SYSTEM MRI produces excellent images of the limbs because involuntary motion is not a problem, and MRI contrast among the soft tissues is excellent. The lack of bone artifact on MRI permits excellent visualization of the bone marrow. On plain film radiography and occasionally on CT, dense cortical bone is often hidden in the marrow space. s previously stated, calcium within tumors is better visualized with CT, however, because of the lower MRI signal from calcium. Overall, the ability to image in multiple planes, along with excellent visualization of soft tissues and bone marrow, has rapidly expanded the role of MRI in musculoskeletal imaging. MRI is particularly valuable for the study of joints, and it is replacing arthrography and, to a lesser extent, arthroscopy in the evaluation of injured knees (see Fig ), ankles, and shoulders. Small joints also are well evaluated with MRI. Local staging of soft tissue and bone tumors is best accomplished with MRI. Early detection of ischemic necrosis of bone is another strength of MRI. S P H K H K H P K Fig Multiple images through the liver of a patient with hemangioma (H)., xial T1-weighted image., xial T2- weighted image. C, xial T1-weighted postcontrast image. This image shows the classic fill-in of contrast material from the periphery of the lesion toward the center. MRI also shows the other abdominal organs and their relationship quite well: kidneys (K), pancreas (P), stomach (S), and aorta (). C 348