MRI EQUIPMENTS.pptx

MRI EQUIPMENTS ARE DESCRIBED IN DETAILIN THIS PPT. CONTENT TAKEN FROM MUTIPLE BOOKS AND GENERALS.

1. Magnetic Resonance Imaging (MRI) MR. ROHIT BANSAL ASSISTANT PROFESSOR (RADIOPHYSICS) MAMC, AGROHA

2. Magnetic Resonance Imaging (MRI) is a spectroscopic imaging technique that used in medical setting to produce the images of internal structure of the body. MRI is based on the principle of Nuclear Magnetic Resonance (NMR), which is a spectroscopic technique used to obtain microscopic chemical and physical data about molecules. It is a non-invasive test. An MRI scanner uses powerful magnetic fields, radio frequency waves and a computer program to produce high quality MR images of internal structures. MRI scanner produces cross sectional images of soft tissue similar to CT scanner without use of ionizing radiation. MRI was originally called NMRI (Nuclear Magnetic Resonance Imaging) and is a form of NMR, though the use of ‘nuclear’ in the acronym was dropped to avoid negative associations with the word. Certain atomic nuclei are able to absorb and emit radiofrequency energy when placed in an external magnetic field. Introduction

3. Over the years Magnetic Resonance Imaging hereafter referred as MRI, has become a popular and widely available means of a cross sectional imaging modality. That is not coincidental; MRI has gone through a fast passed round of development since its discovery. Now every self-respecting hospital or clinic has one or more MRI scanners to battle the conquest for more precise and accurate imaging and diagnosis of pathology. Even as we speak the development is still in full swing. As MRI becomes more and more accepted the need for more qualified staff also increasing. Through the years operation of MRI scanners has become easier with each new software release, but this doesn’t eliminate the need for proper understanding of how MRI works. MRI works with a host of parameters, such as TR, TE, Flip Angle, Gradients etc. A thorough understanding of these parameters is vitally important in order to produce a successful MR image.

4. The story of MRI starts in about 1946 when Felix Bloch proposed in a Nobel Prize winning paper some rather new properties for the atomic nucleus. He stated that the nucleus behaves like a magnet. He realized that a charged particle, such as proton, spinning around its own axis has a magnetic momentum. He wrote down his finding in what we know as the Bloch Equations. It would take until the early 1950s before his theories could be verified experimentally. In 1960 Nuclear Magnetic Resonance Spectrometers were introduced for analytical purposes. During the 1960s and 1970s NMR spectrometers were widely used in academic and industrial research. Spectrometry is used to analyze the molecular configuration of material based in its NMR spectrum. In the late 1960s Raymond Damadian discovered the malignant tissue had different NMR parameters than normal tissue. He mused that, based on these differences, it should be possible to do tissue characterization. Based on this discovery he produced the first ever NMR image of rat tumor in 1974. History of MRI

5. In 1977 Damadian and his team constructed the first super conducting NMR scanner (known as The Indomitable) and produced the first image of the human body, which took almost 5 hours to scan. At the same time Paul Lauterbur was pioneering in the same field. One could discuss who was responsible for bringing MRI to us, although, in all fairness, one could accept that both gentlemen had their contribution. The name Nuclear Magnetic Resonance (NMR) was changed into Magnetic Resonance Imaging (MRI) because it was believed that the word nuclear would not find wide acceptance amongst the public. The rest is, as they say, history. In the early 1980s just about every major medical imaging equipment manufacturer researched and produced MRI scanners. Since then, a lot has happened in terms of development. The hardware and software became faster, more intelligent and easier to use. Because of the development of advanced MRI pulse sequences more applications for MRI opened up, such as MR Angiography, Functional Imaging and Perfusion / Diffusion scanning. And yet, the end is not in sight. The development of MR is still in full swing and only time will tell what the future has in store for us.

6. Several processes must be completed to produce magnetic resonance images, including image acquisition and image formation. To complete these processes a number of system components are required, including hardware (instrumentation and equipment) and software programs (pulse sequences and image formation programs). The process includes nuclear alignment, radiofrequency excitation, spatial encoding and image formation, and the hardware required to complete such processes includes: – A strong magnet to generate the static magnetic field (B0). – A gradient system consisting of three coils to produce linear field distortions in the x-, y-, and z-directions and the corresponding amplifiers. A radiofrequency (RF) transmitter with a transmit coil built into the scanner. Components of MRI

7. A highly sensitive RF receiver to pick up and amplify the MR signal. Alternatively, imagers may use a single RF coil switched between the transmit and receive modes. Additional coils either receive coils or transmit/receive coils. Various computers for controlling the scanner and the gradients (control computer), for creation of the MR images (array processor), and for coordinating all processes (main or host computer, to which are connected the operator’s console and image archives). Other peripheral devices such as a control for the patient table, electrocardiography (ECG) equipment and respiration monitors to trigger specialized MR sequences, a cooling system for the magnet, a second operator’s console (e.g., for image processing), a device for film exposure, or a PACS (picture archiving and communication system).

8. The magnetic susceptibility of a substance is the ability of external magnetic fields to affect the nuclei of a particular atom, and is related to the electron configurations of that atom. Para magnetism Paramagnetic substances contain unpaired electrons within the atom that induce a small magnetic field about themselves known as the magnetic moment. An example of a paramagnetic substance is Titanium , Aluminium . Diamagnetism When an external magnetic field is applied, diamagnetic substances show a small magnetic moment that opposes the applied field. Substances of this type are therefore slightly repelled by the magnetic field and have negative magnetic susceptibilities. Examples of diamagnetic substances include Copper , Zinc, Bismuth, Silver, Gold etc . Ferromagnetism When a ferromagnetic substance comes into contact with a magnetic field, the results are strong attraction and alignment. Example are Iron, Cobalt, Nickel etc. Magnetic Susceptibility

9. Things to remember – Magnetism. Paramagnetic substances add to (increase) the applied magnetic field. Super-paramagnetic substances have a magnetic susceptibility that is greater than that of paramagnetic substances but less than that of ferromagnetic materials. Diamagnetic substances slightly oppose (decrease) the applied magnetic field. Diamagnetic effects appear in all substances. However, in materials that possess both diamagnetic and paramagnetic properties, the positive paramagnetic effect is greater than the negative diamagnetic effect, and so the substance appears paramagnetic. Ferromagnetic substances are strongly attracted to, and align with the applied magnetic field. They are permanently magnetized even when the applied field is removed. Moving a conductor through a magnetic field induces an electrical charge in it. Moving electrical charge in a conductor induces a magnetic field around it.

10. In the field of MRI contrast agents, gadolinium is always considered to be paramagnetic, but in its refined state, gadolinium is a silver metal that has unpaired electrons and magnetic domains, which are the characteristics of ferromagnetism. Ferromagnetic elements have what is known as a “Curie temperature,” above which they cease to exhibit ferromagnetic properties. In the case of gadolinium, this is 20 °C. Below this temperature, gadolinium exhibits a magnetic moment of its own, which does not rely on the presence of an external magnetic field. At human body temperature (or any temperature greater than 20 °C), gadolinium is paramagnetic, hence its description as such when used as a contrast agent. Where a compound contains atoms of different magnetic susceptibility, the net magnetic susceptibility is dictated by the number of each type of atom in the compound and their electron configuration. For example, in water, oxygen is paramagnetic; however, water has a surfeit of hydrogen atoms and therefore exhibits a net diamagnetic effect. This is one of the factors that cause an adverse effect on field homogeneity when a patient is placed into an MRI scanner. How can gadolinium be ferromagnetic, yet be classed as a paramagnetic contrast medium?

11. Closed-bore systems Closed-bore systems are the most popular type of MRI scanner worldwide. They feature the familiar tunnel-shaped magnet bore and resemble, in shape, a larger version of a computed tomography (CT) scanner. Longitudinal table movement allows the patient to be positioned with the region of interest lying at the centre of the magnet bore. This encloses the patient to the front, back, and sides, but still allows limited access. Patients having lower extremity scans may be positioned feet first, which permits most of the body to remain outside of the bore. Closed bore scanners generate the main magnetic field using toroidal superconducting solenoid electromagnets positioned in circumference to the cylindrical bore. This type of scanner can generate very high magnetic field strengths, typically between 1 and 3 T for clinical use and up to 8 T (and above) for research studies. The highest field currently generated by this type of magnet for live-animal research is 21 T. Scanner Configurations

12. Open systems Open systems have a different design, whereby the patient is positioned on a wider imaging table that is manoeuvred between two magnetic poles that are located above and below the imaging volume. This only encloses the patient above and below, leaving a relatively unobstructed view from all sides. This is advantageous when scanning large animals, humans having a large habitus (broad or obese), and nervous/claustrophobic patients (such as small children), who may find the open access more tolerable. The design also facilitates easy side access to the patient by clinicians when undertaking interventions such as biopsies. Importantly, these scanners also permit a degree of sideways table movement. This is very useful when imaging lateral body structures such as the shoulder or elbow, as it allows the region of interest to be positioned closer to the isocentre of the magnet rather than at the edge of the imaging volume where there may be poorer field homogeneity. Flexion and extension views of the spine are also possible as patients have the space to adopt positions that are not possible in the confines of a closed-bore scanner. At least one manufacturer offers upright open MRI systems that permit weight-bearing examinations. Open scanners use large permanent magnets or superconducting solenoids to generate the main magnetic field. The maximum currently available field strength for an open superconducting MRI system is 1.2 T.

13. Extremity systems Extremity scanners, as the name suggests, are designed to scan limbs and are smaller in size than their whole-body counterparts. The typical design is approximately the size and shape of a domestic washing machine having a narrow aperture in the centre that is large enough to accommodate an arm or leg. Slightly larger models are the size of a fluoroscopy unit and may be angled to allow weight-bearing views of the spine, hips, and knees. The magnetic field is typically generated by permanent magnets and is therefore restricted to below 1 T. This has certain negative trade-offs in terms of scan time and image quality but offers advantages . The small physical size of the scanner and reduced magnetic fringe field means that they can be located in small rooms and offices. They are also cheaper to purchase, and running costs may be lower, as permanent magnets do not require electrical power or liquid helium fills to maintain the magnetic field.

14. The magnet is the major part of the MRI machine. The magnet allows the MRI machine to produce high quality images. The function of an MRI magnet is to generate a strong, stable, spatially uniform magnetic field. The strength of magnetic field is measured in Tesla or Gauss (1Tesla = 10000 Gauss). The strength of Earth’s magnetic field is approximately 0.6G. Most of the MRI systems use a magnetic field of 0.5 to 3.0 Tesla. There are also imaging systems used clinically known as ultra-low magnetic fields (0.1T) and ultra-high (10T) magnetic fields, but they are uncommon. About 85% of clinical scanners used worldwide are 1.5T . As of July 2004, the Food and Drug Administration Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices (FDA CDRH) increased to a limit of 4T for infants up to one month and up to 8T for any age above this. Three types of magnets can be used in MRI, such as Permanent magnet, Resistive magnet and Superconducting magnet. Magnets

15. Permanent Magnet Permanent magnet MRI scanners do not employ electromagnets. Instead, they are equipped with large discs of a ferromagnetic alloy such as neodymium, boron, and iron, or aluminium, nickel, and cobalt (alnico ) . Neodymium magnets are also known as rare-earth magnets (despite neodymium being neither “rare” nor “earth”) and are some of the most powerful permanent magnets. The ferromagnetic discs are known as pole shoes and are typically mounted on a yoke that positions them directly above and below the imaging volume . The magnetic field is created by the inherent ferromagnetism of the alloy, namely the combined forces of unpaired electrons in the atoms of the metal that create a macroscopic magnetic field. The advantages of this type of magnet are that it does not require electrical power or cryogenic cooling. These advantages are somewhat offset by the fact that these magnets are unable to generate a flux density of more than 0.5 T, are typically very heavy (17 US tons), and cannot be switched off in an emergency.

16. Advantage: Low power consumption. Low operating cost. Small fringe field. No cryogen Disadvantage: Limited field strength (less than 0.5T) Very heavy No quench possible

17. Resistive Magnet: The magnetic field is generated by current, which runs through loops of wire. Resistive MRI scanners employ copper-wound solenoids that operate just below normal room temperature. The principal advantage of this type of system is that the field strength can be adjusted and the magnet switched off safely after use. Industrial resistive magnets can achieve ultra-high field strength; however, they typically feature very narrow magnet bores. To attain a maximum flux density of around 0.4 T, in a solenoid of a size required for human scanning, a current of over 10 kilowatts (kW) is required. Like an electric bar fire, the resistivity in the windings produces significant heat, and water cooling is required to prevent damage to the system (which would otherwise become incandescently hot). This is achieved by sitting the solenoid magnets inside a water-filled vessel through which chilled water is constantly circulated. Superconducting magnets were introduced to avoid resistivity issues. These devices use cryogens (coolants) to reduce the temperature of the windings to within 4° of absolute zero (4  Kelvin's (K)). This enables a substantially higher flux density using a solenoid large enough to fit a patient inside.

18. Advantages: Low capital cost. Light weight. Can be shut off. Disadvantages: High power consumption. Limited field strength (up to 0.5T). Water cooling required. Large fringe field.

19. Superconducting Magnets: Superconducting electromagnets create a magnetic field in the same way as a resistive magnet; however, the windings of the solenoid are spun from a type of metal alloy that is superconductive ( typically niobium/titanium ). This means that the resistivity of the metal decreases to zero when the metal is cooled below a certain critical temperature (known as the transition temperature). To understand how this affects the design of the MRI scanner, it is first necessary to describe how the cooling system works. Cryostat The term cryostat is derived from the Greek words meaning “cold” and “stable.” The cryostat is a somewhat larger version of a thermal vacuum flask that you might use to keep your wine chilled. The cryostat contains the cryogen liquid helium and liquid nitrogen , which has a boiling point of just 4.2 K (−268.9 °C). The primary function of the cryostat is to prevent heat transfer from the adjacent system components (particularly the gradient coils) to the cryogen. This thermal insulation reduces the rate at which the liquid helium boils off to the atmosphere. The outer structure consists of a hollow cylindrical steel tank. This is almost entirely seal-welded except for an aperture through which the helium is filled and through which also passes an atmospheric exhaust vent (quench pipe). The entire outer tank is evacuated of air, which largely reduces heat transfer by thermal convection.

20. On top of the outer shell of the cryostat is a refrigeration unit that chills the metal superstructure of the cryostat, helping to prevent heat transfer by conduction. The area inside the cylinder of the cryostat is known as the warm bore. This contains not only the patient bore but also the components of the MRI system that operate at room temperature. Inside this outer tank is a similarly shaped secondary cryogen chamber constructed from aluminium. In Figure, the cryogen chamber is shown half removed from the steel cryostat for clarity. The outside wall of the cryogen chamber is swathed in layers of aluminized polyester sheeting with insulating spacers. This highly reflective insulating material is familiar to anyone who has seen a space-blanket used to protect patients (or marathon runners) from hypothermia. The term “space-blanket” refers to the fact that the material was originally developed by NASA (National Aeronautics Space Administration ) to provide an insulation layer in space-suits. This was necessary to protect the astronauts from extremes in temperature during the moon landings. Its highly reflective surface forms a very efficient heat shield that prevents heat transfer by thermal radiation. This combination of features considerably reduces heat transfer, thereby the helium boil-off rate. Many modern scanners also feature a helium recondensing or recycling system that further reduces helium loss to a negligible amount. Such scanners are unlikely to require a helium refill during their operational lifetime.

21. MRI systems use superconducting magnets to produce powerful magnetic fields. These magnets are bathed in liquid helium to maintain superconductivity. If the temperature of the coil goes above the threshold thus the resistance will develop in the coils. The resistance will create heat, which turns the liquid helium into gas . This results in the helium from the cryogen escaping rapidly. So , a quench refers to the sudden loss of superconductivity. If for some reason the gas should escape into the room instead of being vented outside the building, there is a risk of asphyxiation or frostbite. Quench

22. When sitting an MRI scanner, it is desirable that the main magnetic field is not permitted to extend into areas other than the magnet room. This is because a powerful magnetic field can adversely affect nearby equipment (in imaging departments) and creates a potential safety hazard if members of the public gain access to a field strength of 5 G or greater. Shielding is achieved in one of two ways: passive shielding, which requires the scanner to be surrounded by large steel plates, or active shielding, which uses additional solenoid magnets. Magnet Shielding

23. Passive shielding To maintain safety, it was necessary to site unshielded scanners in buildings with a large footprint, away from other hospital departments, thereby preventing the fringe field from extending into adjacent public areas. The fringe field was further reduced by passive shielding, which requires large steel plates incorporated either around the scanner or in the walls of the magnet room. Passive shielding reroutes the fringe field away from the outside environment and back toward the scanner. Passive shielding has several major disadvantages. The iron cladding can weigh over 20 tons, it is very expensive, and the proximity of ferromagnetic metal can adversely affect the homogeneity of the scanner that it is intended to shield. For these reasons, passive shielding has now been replaced by active shielding in most clinical scanners and in some ultra-high-field research systems.

24. Active shielding In addition to the main magnet solenoids, two larger diameter solenoids positioned at each end of the bobbin. These are colloquially known as bucking coils in that they oppose (buck) the effect of the main magnet windings. Their function is to actively shield the local environment by constraining the 5 G footprint of the fringe field to within a short distance from the scanner. To do this, the bucking coils carry a current flowing in the opposite direction to the main magnet windings, reversing the flux. This design was formerly suited only to magnets with a flux density up to 3 T, because in ultra-high-field research magnets. In modern ultra-high-field-strength systems, active shielding is permitted, even at 8 T. Remember that although active shielding allows convenient sitting of an MRI scanner, it does not remove the safety risk from projectiles. If anything, we should be more cautious about projectile safety because active shielding causes a very steep static field gradient .

25. The function of the shim system is to ensure homogeneity of the magnetic field within the imaging volume. One of the first tasks after the installation of a new MRI scanner is to assess the homogeneity of B0 with the scanner in situ. A homogenous magnetic field is desirable for two main reasons. Firstly, any distortions in the magnetic field lead to geometric distortion of the images . Secondly, excitation of the hydrogen nuclei is frequency-dependent . Passive shimming Passive shimming uses shims to adjust for large changes in magnetic field homogeneity. The inner circumference of the warm bore of the cryostat is lined with several long plastic trays that fit along the full length of the bore. These shim trays have around 14–16 spaced compartments. Shim System

26. Active shimming Active shimming uses electromagnets instead of ferromagnetic shims and is used in addition to passive shimming. There have been various designs over the years using both resistive (room temperature) coils and superconducting solenoids. Resistive shimming coils were often positioned close to the gradient coils. The advantage of using resistive shim coils is that the shim can be manipulated at any time by adjusting the current flowing through the windings. In modern scanners, active shimming is usually performed by additional superconducting solenoids inside the cryostat. Their advantage is that no additional electrical power is required Gradient offset (dynamic) shimming The final method for shimming uses the gradient set, which is another electromagnet that is designed to manipulate the magnetic field during image acquisition. In terms of shimming it is possible to apply a current to the gradient coils that offsets any minor inhomogeneity in the main magnetic field.

27. Gradients are coils of wire that, when a current is passed through them, alter the magnetic field strength of the magnet in a controlled and predictable way. They add or subtract from the existing field in a linear fashion so that the magnetic field strength at any point along the gradient is known. When a gradient is applied the following occur. • At isocentre the field strength remains unchanged even when the gradient is switched on. • At a certain distance away from isocentre the field strength either increases or decreases. The magnitude of the change depends on the distance from isocentre and the strength of the gradient. • The slope of the gradient signifies the rate of change of the magnetic field strength along its length. The strength or amplitude of the gradient is determined by how much current is applied to the gradient coil. Larger currents create steeper gradients so that the change in field strength over distance is greater. The reverse is true of smaller currents. Gradients

28. • The polarity of the gradient determines which end of the gradient produces a higher field strength than isocentre (positive) and which a lower field strength than isocentre (negative). The polarity of the gradient is determined by the direction of the current flowing through the coil. As the coils are circular, current either flows clockwise or anticlockwise. • The maximum amplitude of the gradient determines the maximum achievable resolution. • The speed with which gradients can be switched on and off are called the rise time and slew rate. Both of these factors determine the maximum scan speeds of a system . The Z gradient alters the magnetic field strength along the Z axis. The Y gradient alters the magnetic field strength along the Y axis. The X gradient alters the magnetic field strength along the X axis

29. Slice Selection Gradient As a gradient alters the magnetic field strength of the magnet linearly, the magnetic moments of nuclei within a specific slice location along the gradient have a unique precessional frequency when the gradient is on. A slice can therefore be selectively excited by transmitting RF at that unique precessional frequency. Example: a 1.5 T field strength magnet with a gradient imposed that has changed the field strength by 10 gauss between slice A and B. • The gradient has changed the field strength by 10 G. • The precessional frequency of magnetic moments has changed by 100 Hz. • To excite nuclei in slice A an RF pulse of 67.76 MHz must be applied. • Slice B and all other slices are not excited because their precessional frequencies are different due to the influence of the gradient. • To excite slice B, another RF pulse with a frequency of 63.86 MHz must be applied. Nuclei in slice A do not resonate after the application of this pulse because they are spinning at a different frequency. The scan plane selected determines which gradient performs slice selection.

30. • The Z gradient selects axial slices , so that nuclei in the patient’s head spin at a different frequency from those in the feet. • The Y gradient selects coronal slices , so that nuclei at the back of the patient spin at a different frequency from those at the front. • The X gradient selects saggital slices , so that nuclei on the right-hand side of the patient spin at a different frequency from those on the left. • A combination of any two gradients selects oblique slices . Slice Thickness The slice thickness is determined by the slope of the slice select gradient and the transmit bandwidth. It affects in-plane spatial resolution. Thin slices require a steep slope and narrow transmit bandwidth and improve spatial resolution. Thick slices require a shallow slope and broad transmit bandwidth and decrease spatial resolution.

31. Phase Encoding Gradient After a slice has been selected, the magnetic field strength experienced by nuclei within the excited slice equals the field strength of the system. The precessional frequency of spins within the slice is equal to the Larmor frequency. The frequency of the signal from the slice also equals the Larmor frequency, regardless of the location of each signal. The system has to use gradients to gain two-dimensional information representing the spatial location of the spins within the slice. When a gradient is switched on, the precessional frequency of a nucleus is determined by its physical location on the gradient. The gradient also changes the phase of the magnetic moment of each nucleus. The phase of a magnetic moment is its place on the ‘cone’ or precessional path at any moment in time. It can be compared to the position of the little hand on a clock.

32. Imagine a watch telling the time of 12 o’clock. The hour and minute hand are both located over the number 12. Assume that the position of the hour hand at this point is equivalent to the phase of a magnetic moment of a nucleus experiencing B 0 . When the phase-encoding gradient is switched on, the magnetic field strength, precessional frequency, and phase of the magnetic moments of nuclei change according to their position along the phase-encoding gradient. Magnetic moments of nuclei experiencing a higher magnetic field strength gain phase, i.e. move further around the watch to say 4 o’clock, because they travel faster while the gradient is switched on. Magnetic moments of nuclei experiencing a lower field strength lose phase, i.e. move back around the watch to say 8 o’clock, because they travel slower while the gradient is switched on. Magnetic moments of nuclei at magnetic isocenter do not experience a changed field strength, and their phase remains unchanged, i.e. 12 o’clock. There is now a phase difference or shift between magnetic moments of nuclei positioned along the axis of the phase-encoding gradient. When the phase-encoding gradient is switched off, the magnetic field strength experienced by nuclei returns to the main field strength B 0 and therefore the precessional frequency of all the magnetic moments of hydrogen nuclei returns to the Larmor frequency . However, the phase difference between the nuclei remains. The nuclei travel at the same speed (frequency) around their precessional paths, but their phases or positions on the watch are different because the phase-encoding gradient was applied. Another way of saying this is that the phase difference between the magnetic moments of nuclei located along the gradient is remembered when the phase-encoding gradient is switched off. This difference in phase is used to determine the position of nuclei along the phase-encoding gradient.

33. Frequency Encoding Gradient The frequency encoding gradient is switched on during the echo. It is often called the readout gradient because, during its application, frequencies within the signal are read by the system. The gradient is normally switched on for 8 ms and the echo is usually centred to the middle of the gradient application. The readout gradient is switched on in the positive direction. • The slope of the frequency encoding gradient determines the size of the FOV and therefore the image resolution .

34. The radiofrequency (RF) system comprises a powerful RF generator (the Larmor frequency at 1.5 T is 63.8 MHz, which is in the range of FM transmitters) and a highly sensitive receiver. The stability of these two components is crucial: as both the frequency and the phase of the signal are needed for spatial encoding, any distortions, e.g., by phase rotation introduced by the receiver, would result in a blurred image. Moreover , to adequately detect the weak MR signal, effective RF shielding of the scanner room is necessary to prevent interference from external sources. This can be achieved by housing the magnet in a closed conductive structure known as a Faraday cage. The RF subsystem also includes the transmit and receive coils. These may be combined coils acting as both transmitters and receivers such as the body coil which is integrated into the scanner. It is not visible from the outside and consists of a “cage” of copper windings encircling the patient. The RF transmitter serves to deliver pulses that correspond to the resonant frequency of hydrogen atoms. RF coils consist of loops of wire which, when a current is passed through them, produce a magnetic field at 90° to B0. Radiofrequency System

35. Transmit coils Energy is transmitted at the resonant frequency of hydrogen in the form of a short intense burst of radio frequency known as a radio-frequency pulse. The main coils that transmit RF in most systems are: • A body coil usually located within the bore of the magnet itself; • A head coils. The body coil is the main RF transmitter and transmits RF for most examinations excluding head imaging (when the head coil is used). The body and head coil are also capable of receiving RF, i.e., acting as receiver coils. Receiver coils RF coils placed in the transverse plane generate a voltage within them when a moving magnetic field cuts across the loops of wire. This voltage is the MR signal that is sampled to form an image. In order to induce an MR signal, the transverse magnetization must occur perpendicular to the receiver coils.

36. RF coil types The configuration of the RF transmitter and receiver probes or coils directly affects the quality of the MR signal. There are several types of coils currently used in MR imaging: transmit/receive coils; surface coils and phased array coils. Transmit/receive coils A coil both transmits RF and receives the MR signal and is often called a transceiver. It encompasses the entire anatomy and can be used for either head or total body imaging. Head and body coils of a type known as the bird-cage configuration are used to image relatively large areas and yield uniform SNR over the entire imaging volume. However, even though the volume coils are responsible for uniform excitation over a large area, because of their large size they generally produce images with lower SNR than other types of coils. The signal quality produced by these coils has been significantly increased by a process known as quadrature excitation and detection.

37. Surface coils Coils of this type are used to improve the SNR when imaging structures near the surface of the patient. Generally, the nearer the coil is situated to the structure under examination, the greater the SNR. This is because the coil is closer to the signal-emitting anatomy, and only noise in the vicinity of the coil is received rather than over the entire body. Surface coils are usually small and especially shaped so that they can be easily placed near the anatomy to be imaged with little or no discomfort to the patient. However, signal (and noise) is received only from the sensitive volume of the coil that corresponds to the area located around the coil. The size of this area extends to the circumference of the coil and at a depth into the patient equal to the radius of the coil. There is therefore a fall off of signal as the distance from the coil is increased in any direction. Intra-cavity coils (such as rectal coils) or local coils, They can be used to receive signal deep within the patient. As the SNR is enhanced when using local coils, greater spatial resolution of small structures can often be achieved. When using local coils, the body coil is used to transmit RF and the local coil is used to receive the MR signal.

38. Phased array coils These consist of multiple coils and receivers whose individual signals are combined to create one image with improved SNR and increased coverage. Therefore, the advantages of small surface coils (increased SNR and resolution), are combined with a large FOV for increased anatomy coverage. Usually up to four coils and receivers are grouped together either to increase longitudinal coverage or to improve uniformity across a whole volume. During data acquisition each individual coil receives signal from its own small usable FOV. The signal output from each coil is separately received and processed but then combined to form one single, larger FOV. As each coil has its own receiver the amount of noise received is limited to its small FOV, and all the data are acquired in a single sequence rather than four individual ones.

39. The patient transport system is typically a large non-ferromagnetic patient couch (table) that is raised and lowered to facilitate patient access and driven horizontally into the magnet bore. This movement is controlled from a panel situated on the front cone of the scanner cover. From a geometric perspective, the aim of the couch movement is to position the centre of the region of interest at the true isocenter of the imaging volume where the magnetic homogeneity is greatest. The front cone of the scanner is equipped with laser positioning lights. The region to be scanned is cantered to a crosshair marker formed by the lasers, and the position is recorded by pushing a button on the front panel. The table is then activated to automatically move the anatomical region into the isocenter of the bore. In open MRI systems, the couch top may also be adjusted sideways, which is of great benefit when imaging lateral structures such as the shoulders or elbows. The patient couch may also include posterior receive coil elements and sockets into which surface coils can be mounted. There is typically a light source inside the magnet bore, and a patient cooling fan may also be included. Manufacturers also offer patients a prismatic or reflective headset that allows them to watch videos, or pleasant immersive scenery during the procedure. This may alleviate nervousness and provide information to the patient about the duration of each acquisition. There is also a call-button and a patient microphone to allow two-way communication between the patient and staff at the imaging console. Patient Transport System

40. The entire process of an MRI acquisition is orchestrated by the host computer. The graphical user interface (GUI) of the system is used to identify the patient, usually via a network link to the radiology information system. The age, weight, gender, and physical orientation of the patient may also be recorded. This ensures that the system is prepared to calculate the amplitude of the RF pulses and label the physical orientation of anatomy shown on the images. The interface also usually contains pages or tabs relating to the various parameters that are required for the scan. These may be grouped by function, for example, a geometry page that allows adjustments to FOV, image matrix, and slice thickness. The parameters required for an MRI scan are numerous, and it is undesirable (from a patient-throughput perspective) for us to manually enter these factors in every acquisition. Computer System And Graphical User Interface

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