Chapter 9

Identify the requirements of an MRI scanner to ensure homogenous, powerful magnetic field and a system that can both transmit and receive pulses of electromagnetic radiation.
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Identify the requirements of an MRI scanner to ensure homogenous, powerful magnetic field and a system that can both transmit and receive pulses of electromagnetic radiation.
a. . Powerful magnet to create magnetic fields over a 40-50 cm spherical volume. b. . A shim system to improve the homogeneity of the magnetic field.
c. . A gradient system to create linear slopes in field strength in any direction.
d. . An RF transmission system to generate and transmit pulses of electromagnetic radiation.
e. . A set of RF receiver coils to detect signal from the patient.
f. . A computer system to allow input of parameters and displaying images.
g. . A computer subsystem capable of coordinating the application of RF pulses and gradients and reconstructing the acquired data into images and storing them.
Use the diagram below to identify key parts of the MRI scanner.
A. . Shim Coil
B. . Main magnet
C. . Active shielding D. . Quench
E. . Gradient amplifier F. . RF transmitter
G. . RF amplifer
Image: Use the diagram below to identify key parts of the MRI scanner.
Attraction to an external magnetic field (B0):
Diamagnetic
Paramagnetic
Ferromagnetic
Weak
Weak
Large
Magnetic Susceptibility:
Diamagnetic
Paramagnetic
Ferromagnetic
Small
Small Positive
Large Positive
Electrons:
Diamagnetic
Paramagnetic
Ferromagnetic
Evenly Paired
Unpaired
Unpaired
What happens when exposed to the B0:
Diamagnetic
Paramagnetic
Ferromagnetic
Magnetic fields cancel out

Slight increase in magnetic field

High attraction to the static magnetic field
What happens when removed from the B0:
Diamagnetic
Paramagnetic
Ferromagnetic
Doesn't retain any magnetism

Electrons path loses alignment, no magnetism once removed

Retain magnetic moments
List examples
Diamagnetic
Paramagnetic
Ferromagnetic
Hydrogen, and helium

Calcium, Oxygen, and many metals, aluminum, titanium, and platinum

Iron, nickel, cobalt, gadolinium
Gadolinium contrast is highly ferromagnetic? (T/F)
False
Closed system
encloses the body front, back, and sides. Generates the main magnetic field using toroidal superconducting solenoid electromagnets
Open SystemOpen on the sides and only encloses patient above and below. To generate a magnetic field the scanners, use a large permanent magnet to allow for a 1.2 T superconducting MRI system.Extremity SystemAbout the size of a washing machine. The field is typically generated by a permanent magnet and is restricted to below 1T.Identify the six (6) main requirements required for high-quality anatomical imaging.a. . the field strength (flux density) must be high typically between 1 and 8 T b. . Fringe field having a strength of (5 G) must not extend outside of safety zones III and IV c. . Field must be spatially homogenous to a very high degree. d. . Homogeneity must extend over a large spherical imaging volume (40 cm) to accommodate the required anatomical FOVs. e. . Field must be temporally stable. This means that the flux density must not vary over time. f. . Weight and bulk of the magnet must be kept at a level that does not pose any problems with installation in a normal imaging department.Permanent Magnets-Large disks of ferromagnetic allow such as boron, iron, nickel, and cobalt. 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.Resistive Magnets -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.Superconducting Magnets -The same way as a resistive magnet; however, the windings of the solenoid are spun from a type of metal alloy that is superconductive [5] (typically niobium/titanium).In the Right hand grip rule, the thumb indicates: a. The magnetic field direction b. The current direction c. The number of windings in the solenoid d. None are correct.a. The magnetic field directionThis type of magnet uses cryogens to reduce resistance. a. Permanent magnets b. Resistive magnets c. Superconducting magnets d. Paramagnetic magnetsb. Resistive magnetsThe difference between Resistive and Superconducting magnets is that the resistive magnets use cryogens to reduce resistance. T/FFalseIdentify the physical construction of a cryostata. . Outer structure consists of a hollow cylindrical steel tank b. . Area inside of the chamber is termed the warm bore. c. . Quench pipe d. . RefrigerationIdentify the properties of cryogen.a. . Extremely low temperature b. . Readily available c. . Be able to shield heat transferBriefly explain why solenoid coils of the MRI scanner are broken into segments vs all one unit.A single cylindrical solenoid of a suitable length for an MRI scanner is not capable of creating a large enough homogenous imaging volume to achieve the required FOV. Splitting into evenly spaced solenoids ensure that homogenous imaging volume if large enough to cover the requisite 40 cm sphereSolenoid windings are made of: a. Niobium b. Titanium c. Copper d. Gold e. A-C f. B-De. A-CBriefly discuss the steps for ramping up the magnet.When the parallel winding with a heating coil is heated, the terminals of the solenoid are connected to an external power supply to power up the magnet When the required field strength is achieved, the heater is turned off, and the persistent switch becomes superconductive. This creates a closed loop of superconductive wire and effectively bypasses the external power supply because the flowing electrons show a preference for the non resistive circuit.Identify the measurement and symbol used for magnetic field strength.Amperes(A) per meter and is given the symbol HIdentify the measurement and symbol used for flux density.Tesla and is given the B symbolAs the field strength increases, the signal to noise ration increases. T/FTAs the field strength increases, chemical shift increases. T/FFHow should the receive bandwidth be adjusted to decrease chemical shift artifact?Increase bandwidthAs the field strength increases, the energy deposition in the patient's body decreases. T/FFPassive• Reroutes the fringe field away from the outside environment and back towards the scanner • Causes lines of flux to pass though steel cladding in preference to air • Disadvantages-iron cladding can weigh over 20 tons, very expensive, proximity of ferromagnetic metal can adversely affect the homogeneity of the scanner that is intended to shield • Replaced by active shielding in most clinical scanners and some ultra-high-field research systemsActive• 2 larger diameter solenoids positioned at each end of the bobbin-known as bucking coils (oppose the effect of the main magnet windings) • Function is to actively shield the local environment by constraining the 5G footprint of the fringe field within a short distance from the scanner • Bucking coils carry a current flowing in the opposite direction to the main magnet windings, reversing the flux • We should be more cautious about projectile safety because active shielding causes a very steep static field gradientWhat happens if a current-carrying conductor is placed perpendicular to an external magnetic field?The Lorentz force acts on the moving charges in the wire and causes physical defection of the conductorWhat is the purpose of the Shim System?Ensure homogeneity of the magnetic field within the imaging volumeIdentify the unit of measurement for magnetic field homogeneity.ppm over the 40cm (diameter) spherical imaging volumeDescribe the following: Passive Shimmingo Uses shims to adjust for large changes in magnetic homogeneity o Inner circumference of the warm bore of the cryostat is lined with long plastic trays that fit along the full length of the bore- designed to accommodate the small ferromagnetic shims o Used to balance out any inhomogeneity present in the bare field by manipulating the lines of fluxDescribe the following: Active Shimmingo Uses electromagnets of ferromagnetic shims and is used in addition to passive shims o Resistive shimming coils-positioned close to the gradient coils, the shim can be manipulated at any time by adjusting the current flowing through the windings o Active shimming is usually performed by additional superconducting solenoids inside the cryostat o Advantage is there is no electrical power requiredDescribe the following: Gradient offset (dynamic shimming)o It is possible to apply a current to the gradient coils that offsets any minor inhomogeneity in the main magnetic field o By introducing a large patient into the bore, the homogeneity can be changed by up to 9pom by diameter repulsion-dynamic shimming can help correct this o The primary purpose of the gradient system is to create linear slopes along the main magnetic fieldIdentify the imaging plane that will be a result from the active gradient. : a. Z-gradient b. X-gradient c. Y-gradienta. Z-gradientElements of the gradient coil are supplied by three separate power sources called ___. a. Gradient amplifiers b. Gradient dampeners c. Gradient coils d. Gradient residuala. Gradient amplifiersDegradation of image quality can occur if the gradient fields are allowed to extend into the main magnet solenoids. This is the result of: a. Rapid currents b. Ghosting c. Riptides d. Eddy currentsd. Eddy currentsAll of the following are true about gradient coils, except: a. When activated, the gradient coils change the main magnetic field in a linear slope. b. The polarity of a gradient may be changed by reversing the current flow in the gradient coil elements. c. Adding current in the same direction of the windings of the main magnet solenoid simulates the increasing of windings in the coil thus increasing the magnetic strength. d. Adding current to the coil makes the field strength throughout the bore of the MRI scanner the same.d. Adding current to the coil makes the field strength throughout the bore of the MRI scanner the same.When a gradient amplifier is applied, the Larmor frequencies of magnetic moments vary depending on where they lie on the linear slope. (T/F)Tdentify the measurement unit for Gradient Amplitude.mT/m or G/cmIdentify the measurement unit for Gradient Rise Time.microsecondsIdentify the measurement unit for Gradient Slew Rate.T/m/sIdentify the safety considerations for Gradient power duty cycle.a. In terms of MRI hardware, rapid gradient switching can cause overheating of the gradient coils b. Some implanted devices may be adversely affected, and manufacturers place conditions of scanning related to gradient slew rate c. The physiological limitation to gradient slew relates to the fact that a time-changing magnetic field causes an induced voltage in a nearby conductor. This includes a patient's nervous system.What technical factors determine the geometry of the MRI image due to the gradient amplitude?a. . Slice thickness b. . Phase resolution c. . FOVPulse sequences that require rapid gradient switching tend to cause the highest levels of acoustic noise. (T/F)TA gradient that has a positive lobe equal to the negative lobe is known as a _______ gradient. a. Balanced b. Sloped c. Unbalanced d. None are correct.a. BalancedThe gradient system determines the scan time, slice location, spatial encoding and indirectly the image contrast. (T/F)TThe ______________ of a gradient is determined by the amount of current passing through the coil.amplitudeThe ____________ of a gradient is determined by the direction of the current flowing through the coil.polarityThe amplitude of a gradient determines the ___________ ______________.spatial resolutionThe _____________ determines how fast data can be acquired.slewWhat is the purpose of the transmitted RF system?To target a volume slice of tissue within the region to be examined and increase the energy level of a proportion of the hydrogen nuclei within that slice.What is the gyromagnetic ratio of Hydrogen at 1 T?42.58 MHz/TIdentify the material used and the name of the protective shielding used to prevent RF frequencies from entering or leaving the MRI suite.The protective shielding is the Faraday Cage. Copper was the original material of choice. Aluminum panels are now favored.Define transmit bandwidth.The transmitted RE is an electromagnetic wave, which contains a narrow range of frequencies centered around the Larmer frequency of the magnetic moments of hydrogen nuclei at the external field strength of the system, The narrow range of frequencies is the transmit bandwidth.Identify the roles of the RF Transmit System: Waveform generator:Generates a sinc pulseIdentify the roles of the RF Transmit System: Frequency Synthesizer:Ensures that the frequencies in the pulse are centered around the Larmor frequency of the magnetic moments of the spins.Identify the roles of the RF Transmit System: RF Amplifier:Provides enough energy to couple with the nuclei in the region of interest.Describe the purpose of an RF Transmitter coil.Generates radio waves at the desired frequency, which are transmitted into the anatomical area.RF homogeneity improves as coil sizes increase. The trade-off is that the filling factor can be compromised. This affects the SNR and image quality. To improve filling factor, what considerations should be used when using an RF transmit coil?a. . Positioned close to patient b. . Enclose the entire region of interest c. . Must be evenly spacedWhat technical factor determines when the RF receive system should receive the signal?Time TEAmplification, filtering, and digitization of analog signals are done inside the receiver coil assembly or body of the MRI scanner. T/FTCompare and Contrast Surface RF receive coils and Volume RF receive coils.Surface coils are not typically used as RF transmitters because of their poor RF homogeneity. They are described as receive-only coils. Volume coils they offer a comparatively homogenous RF field over a large imaging volume unlike surface coils. Volume coils are typically as transmit coils like surface coils are.Choose all that apply] Identify the true statements about receiver coils. a. Receiver coils are a critical part of image optimization. The selection of the appropriate receiver coil for the area under examination is very important b. Large coils provide large coverage but a relatively poor SNR. This is because the anatomical area may not fill the entire sensitive area of the coil. The filling factor is low. c. Small coils provide small coverage but relatively good SNR. This is because the sensitive volume of the coil will be filled with tissue. The filling factor is high. However, aliasing is more likely d. Phased array coils of the linear and volume type are the best option, as they combine the benefits of using small coils with those of using large onesa. Receiver coils are a critical part of image optimization. The selection of the appropriate receiver coil for the area under examination is very important b. Large coils provide large coverage but a relatively poor SNR. This is because the anatomical area may not fill the entire sensitive area of the coil. The filling factor is low. c. Small coils provide small coverage but relatively good SNR. This is because the sensitive volume of the coil will be filled with tissue. The filling factor is high. However, aliasing is more likely d. Phased array coils of the linear and volume type are the best option, as they combine the benefits of using small coils with those of using large onesWhere in the MRI scanner is the magnetic homogeneity the greatest?True isocenter of the imaging volumeWhat is the role of the Pulse Generator Module (PGM) in the MRI process?The PGM is an independent computer that is responsible for sending instructions relating to the timing, amplitude, and shape of the transmitted RF pulses and the timing and duration of the sampling window. It also is connected to the gradient amplifiers and generates pulses to determine the shape and duration of the gradients used during the scan.What is the format used for storing patient images in a PACS system?DICOM