Magnets & Scanners Self Assessment

Magnets & Scanners Quiz

Basic Electromagnetism

  1. When the current flowing through a wire reverses direction, the magnetic field around the wire
    1. Does not change
    2. Increases
    3. Disappears
    4. Reverses direction

    Ampère’s Law states that a current (i) in a wire induces a magnetic field (B) around the wire. if the direction of current flow reverses, the direction of the field does also, so d) is correct. The magnitude of the field depends on the magnitude of the current, so b) and c) are false. Link to Q&A discussion

  2. The bulk magnetic properties of matter derive primarily from
    1. Protons
    2. Neutrons
    3. Electrons
    4. Whole nuclei

    The combination of intrinsic electron spin and electron orbital angular momentum is primarily responsible for the bulk magnetic properties of matter. Protons, neutrons, and whole nuclei possess spin but the size of the magnetic effect is relatively small and limited to juxta-nuclear region of the atom only. Link to Q&A discussion

  3. If the current in a wire doubles, the induced magnetic field
    1. Doubles
    2. Quadruples
    3. Remains the same
    4. Is reduced by half

    Ampère’s Law states that a current (i) in a wire induces a magnetic field (B) around the wire proportional to that current. If the current doubles, the magnitude of B also doubles. Link to Q&A discussion

  4. The direction of magnetic field lines surrounding a wire can be determined using
    1. The right-hand rule
    2. The left-hand rule
    3. Faraday's Law
    4. Lenz' Law

    Fleming developed the right hand rule in which if you grasp a wire carrying current with the right hand and point your thumb in the direction of the current, your fingers will curl around the wire in the direction of the induced magnetic field. Link to Q&A discussion

  5. The voltage induced across a stationary conductor in an external static magnetic field
    1. Depends on the angle of the conductor with the magnetic field
    2. Increases with time
    3. Is zero
    4. Depends on the strength of the magnetic field

    This is an example of the Faraday-Lenz Law, where the induced voltage is directly proportional to the rate of change of the magnetic field (dB/dt). In this case both the conductor and magnetic field are static, so dB/dt = 0 an the induced voltage is zero. Link to Q&A discussion

  6. Concerning the relationship between electricity and magnetism, which of the following statements is false?
    1. A constant current in a wire induces a constant magnetic field around the wire.
    2. A changing current in a wire induces a changing magnetic field around the wire.
    3. A constant magnetic field induces voltage in a nearby stationary wire.
    4. A changing magnetic field induces voltage in a nearby wire.

    Ampère’s Law states that a current (i) in a wire induces a magnetic field (B) around the wire. If the current is constant, the magnetic field is constant, but if the current fluctuates, so will the magnetic field. Thus (a) and (b) are true. The Faraday-Lenz Law states that a changing magnetic field can induce an electric potential (voltage) in a nearby wire. The magnitude of this voltage (V) is proportional to the rate of change of the magnetic field (dB/dt). Thus (d) is true. A constant magnetic field does not change with time (dB/dt = 0) and thus will not induce a voltage in a nearby wire. Hence (c) is false. Link to Q&A discussion

  7. Which question about the Tesla (T) is correct?
    1. It is the official unit for magnetic induction field strength in the cgs system.
    2. 1 Tesla = 1,000 Gauss (G)
    3. 1 G = 1 mT
    4. It is one of the coolest cars on the road

    The Tesla is the unit for magnetic induction field strength in the International System of Units (SI), formerly known as the mks (meter-kilogram-second) system. Gauss is the equivalent unit in the cgs (centimeter-gram-second) system, so a) is false. 1 Tesla = 10,000 G, and 1 G = 0.1 mT, so both b) and c) are false. This leaves option d) as the correct answer, which everyone knows anyway! Link to Q&A discussion

  8. Concerning magnetic field strengths, which statement is true?
    1. The earth's magnetic field is about 0.5 G.
    2. A junkyard electromagnet that picks up cars is much stronger than the main field of most MR scanners.
    3. Research MR scanners for humans now exist with field strengths exceeding 20 T.
    4. Higher field strength scanners have wider bores than lower field strength scanners to accommodate the extra flux lines

    The earth’s magnetic field at the equator is about 0.5 G, so a) is the correct answer. Junkyard electromagnets generally have field strengths of about 1T, limited by the flux density of steel, so they are much weaker than most MR scanners, and thus b) is false. The largest current human scanners are 11.7T, so c) is false. Higher field strength scanners have smaller bores, not larger ones, so d) is false. Link to Q&A discussion

  9. Which of the following materials is paramagnetic?
    1. Water
    2. Fat
    3. Bone
    4. Air

    Most biological tissues (including water, fat, and bone) are weakly diamagnetic. Molecular O2 is paramagnetic, overwhelming the weak diamagnetism of the other components of air N2 and CO2. Link to Q&A discussion

  10. A material that is weakly repulsed by a magnetic field is known as
    1. Paramagnetic
    2. Diamagnetic
    3. Superparamagnetic
    4. Ferromagnetic

    Diamagnetic materials generate an internal polarization (J) that opposes the externally applied field, so b) is correct. The polarization of the other classes of materials is in the direction of the field and are attracted by the field. Link to Q&A discussion

  11. Susceptibility (χ) is negative for materials that are
    1. Paramagnetic
    2. Superparamagnetic
    3. Diamagnetic
    4. Ferromagnetic

    Susceptibility (χ) is negative when the internal polarization (J) points opposite to the main magnetic field (B). By definition, only diamagnetic materials have negative susceptibilities. Link to Q&A discussion

  12. Ferromagnetic materials form magnetic ________ when arrays of electron spins become linked via quantum exchange interaction.
    1. Flux lines
    2. Poles
    3. Vectors
    4. Domains

    Exchange interaction is a quantum effect in which unpaired electrons link together to form individual magnetic domains which behave as individual small "magnets". Link to Q&A discussion

  13. Comparing superparamagnetic and ferromagnetic materials, which statement is false?
    1. Ferromagnetism is usually more powerful than superparamagnetism.
    2. Ferromagnetism persists when the magnetizing field is removed.
    3. Superparamagnetism persists once the external field is removed.
    4. Superparamagnetism can be thought of as a single-domain particle.

    Ferromagnetic materials retain memory (remenance) of prior magnetization, while superparamagnetic particles are single domain but whose magnetization disappears once the the magnetizing field is removed. Link to Q&A discussion

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MR Magnets

  1. The most common design configuration for clinical MR scanners is
    1. Open bore superconducting
    2. Closed bore superconducting
    3. Open bore permanent
    4. Dipolar electromagnet

    Over 90% of MR scanners sold today are of the closed bore (cylindrical) superconducting type. The second most common is open bore permanent. Link to Q&A discussion

  2. The highest field strength permitted for adults in routine clinical practice by the United States Food and Drug Administration (FDA) is
    1. 3.0 Tesla
    2. 7.0 Tesla
    3. 8.0 Tesla
    4. 11.7 Tesla

    The current FDA limit for clinical MR scanners is 8.0 T for adults and children older than 1 month; it is 4.0 T for children younger than 1 month. The highest field strength clinical scanner commercially manufactured are 7.0 T, although human-sized research scanners of up to 11.7 T exist. Link to Q&A discussion

  3. The first company to produce a clinical whole-body MRI scanner for commercial use was
    1. GE
    2. Fonar
    3. Siemens
    4. Technicare

    Raymond Damadian formed the first company FONAR to manufacture MR scanners for clinical use in 1982. Link to Q&A discussion

  4. The direction of the main magnetic field (Bo) in a cylindrical closed bore scanner is
    1. Longitudinal (along the main axis) of the cylinder
    2. Horizontal (cross-wise to the cylinder and parallel to the floor)
    3. Vertical (cross-wise to the cylinder and perpendicular to the floor)
    4. Can be at any angle depending on which gradients are turned on

    The main magnetic field of cylindrical scanners is along the long axis of the cylinder (solenoid). Answer d) is patently false as gradients do not alter the direction of the (Bo) field. Link to Q&A discussion

  5. Which of the following is not an advantage of low- and intermediate-field (< 1.0 T) MR scanners?
    1. Lower price
    2. Lower fringe field
    3. Improved detection of gadolinium enhancement
    4. Lower energy deposition in tissues

    Gadolinium enhancement relative to tissues is reduced at lower fields, a disadvantage. Link to Q&A discussion

  6. Which of the following is not an advantage of high-field (≥ 1.0 T) MR scanners?
    1. Higher signal-to-noise
    2. Better detection of calcifications and hemorrhage
    3. Smaller artifacts around metallic implants
    4. Better magnetic field homogeneity

    Susceptibility artifacts around metal objects are worse at high fields than at lower fields. Link to Q&A discussion

  7. Quoted specifications for four different magnets are given below. Which one has the best homogeneity?
    1. <1 ppm over a 40 cm DSV
    2. <1 ppm over a 20 cm DSV
    3. >1 ppm over a 40 cm DSV
    4. >1 ppm over a 20 cm DSV

    Homogeneity specifications are typically quoted as magnetic field deviation in parts per million (ppm) over a defined spherical volume (DSV). So a magnet with the smallest ppm over the largest DSV would be the most homogeneous — answer a). Link to Q&A discussion

  8. Poor magnetic field homogeneity may affect image quality in the following ways
    1. Shading artifacts
    2. Spatial distortion
    3. Poor fat suppression
    4. Only a) and b)
    5. All of the above

    Poor magnet homogeneity may affect multiple aspects of image quality, including all of the above plus blurring, intensity loss, curved slice profiles, and banding artifacts. Link to Q&A discussion

  9. Which of the following statements about passive shimming is true?
    1. Its primary purpose is to correct for field distortions produced by a patient's body.
    2. Ferromagnetic materials cannot be used for passive shimming.
    3. Passive shimming is affected by room temperature.
    4. Once the field is calibrated and magnetic homogeneity achieved, the passive shim materials can be removed.

    Passive shimming involves the placement of ferromagnetic elements (typically plates or rods to correct for static field inhomogeneities in an empty magnet, not one containing a patient. Thus a) and b) are both false. The shim elements are made of metal and hence the quality of shimming depends upon scanner and room temperature, so answer c) is true. The shim elements are not removed, but remain in the magnet after shimming is completed, so d) is false. Link to Q&A discussion

  10. Which of the following statements about superconductivity is correct?
    1. All elements can become superconducting if the temperature is low enough.
    2. Only metals can become superconductors.
    3. The magnetic field is zero inside the center of a superconducting wire.
    4. The resistance of a wire linearly decreases toward zero as the temperature falls below the transition temperature (TC).

    Only about half of the known elements can become superconductive at low temperatures, so a) is incorrect. Most of these are metals or metal alloys, but some non-metal ceramics may also possess superconductivity. The onset of superconductivity is immediate, with resistance dropping to zero once the transition temperature is crossed; thus answer d) is false. The correct answer is c), which describes the Meissner effect, the complete expulsion of all magnetic fields from the interior of the superconductor with creation of a state of perfect diamagnetism. Link to Q&A discussion

  11. The superconducting component in the main windings of nearly all clinical MR scanners is an alloy of
    1. Niobium (Nb) and Titanium (Ti)
    2. Niobium (Nb) and Copper (Cu)
    3. Nickel (Ni) and Titanium (Ti)
    4. Nickel (Ni) and Copper (Cu)

    The main windings of nearly all clinical superconducting scanners use Niobium (Nb) - Titanium (Ti) alloy microfilaments embedded in a copper core. The copper core is not superconducting but serves to support the microfilaments and serve as a current shunt in the event of a quench. Link to Q&A discussion

  12. The cryostat of a typical superconducting MR scanner contains all of the following except
    1. Liquid helium
    2. Liquid nitrogen
    3. Main magnet windings
    4. superconducting shim coils

    Although common in the past, modern refrigeration systems have obviated the need for liquid nitrogen, so only liquid helium is present within the cryostat of today's MR scanners. Link to Q&A discussion

  13. The temperature of liquid helium is approximately
    1. 4 °K
    2. 0 °K
    3. −4 °K
    4. −400 °C

    The temperature of liquid helium used to cool the magnet main windings is just a few degrees above absolute zero, about 4 °K. This is several degrees less than the transition temperature needed to maintain superconductivity in the NbTi alloy wires. No temperature equal or below absolute zero (0 °K or −273.15 °C) is possible, so answers b), c) and d) are all false. Link to Q&A discussion

  14. MRI facilities often display a sign on the door that says: "Warning! The magnet is always on." This sign would not strictly apply to a
    1. Resistive magnet scanner
    2. Permanent magnet scanner
    3. Superconducting magnet scanner
    4. The sign is applicable to all types of scanners, always.

    Only FONAR still make still makes a resistive (electromagnet-type) MR scanner for human use, which can be turned off when not in use. All permanent magnet and superconductive scanners remain at full field strength at all times, however, so the sign applies to nearly all MR facilities. Link to Q&A discussion

  15. Pushing the "big red button" near the door to a room housing an MRI scanner
    1. Immediately opens the door even if scanning is in progress
    2. Initiates a controlled quench of the magnetic field
    3. Turns off all electric power to the scanner and room
    4. Calls emergency providers (911) and sounds an alarm

    The "big red button" is often labeled "Magnet Stop" or "Emergency Run Down" and when pressed, initiates a controlled quench of the magnetic field. It should be used only in a life-threatening emergency, such as a patient pinned in the scanner by a metal object. Link to Q&A discussion

  16. During a magnetic quench, why should patients and employees be evacuated from the scan room?
    1. Even in small quantities gaseous helium causes burning and irritation to the eyes.
    2. Asphyxiation may occur.
    3. Severe frostbite would be likely.
    4. The released helium may catch fire or explode.

    Helium is odorless and colorless, but can potentially fill the scanner room resulting in asphyxiation. Fortunately, quench pipes typically vent nearly all released helium out of the room so death from a quench has never occurred in a medical setting to my knowledge. Direct contact with liquid helium would cause burns, but a mist of helium gas would not likely cause eye injury or frostbite (although this could occur in theory with high concentrations of gas in the room). Link to Q&A discussion

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Gradients

  1. Magnetic field gradients for imaging are typically measured in units of
    1. Millitesla per meter (mT/m)
    2. Gauss per second (G/s)
    3. Tesla (T)
    4. Tesla per meter per second (T/m-s)

    A magnetic field gradient is defined as the change in magnetic field measured over a certain distance, so the only appropriate unit is answer a), millitesla per meter= [magnetic field strength]/distance. Gradient strength is also reported in Gauss/cm (G/cm). Link to Q&A discussion

  2. For a supine patient, enabling the z-gradient alone to alter the magnetic field within a patient during slice selection would create a(n)
    1. Axial slice
    2. Coronal slice
    3. Sagittal slice
    4. Oblique slice

    Applying a single gradient causes a frequency variation of protons as a function of position along the direction of the gradient. This change in frequency can be used for spatial encoding. If the gradient is played out during slice selection and again during signal readout, a slice can be selected perpendicular to the gradient directio Link to Q&A discussion

  3. What is the effect of applying the x- and z-gradients simultaneously during slice selection?
    1. The image will be distorted.
    2. Significant interslice cross-talk will occur.
    3. An oblique slice will be created.
    4. The scanner will display a warning that such a combination is not allowed.

    Applying a single gradient during slice selection creates a slice perpendicular to the axis of the gradient. Applying two (or three) gradients simultaneously produces single (or double) oblique slices respectively. Link to Q&A discussion

  4. When the y-gradient is turned on, what happens to the direction of the main (Bo) field?
    1. The Bo field now points slightly to the right.
    2. The Bo field now points slightly toward the ceiling.
    3. The Bo field is reversed.
    4. The Bo field remains pointing in its original (z)-direction.

    Gradients do NOT generate transverse components that rotate/tip Bo from side-to-side or up-and-down. They act only to create in-plane "skewing" of the z-components of Bo. Link to Q&A discussion

  5. The basic coil configuration used to generate the z-gradient in a cylindrical MR scanner is known as
    1. Maxwell pair.
    2. Double saddle.
    3. Golay.
    4. Fingerprint.

    Z-gradients are typically produced using two looped coils carrying current in opposite directions, a configuration known as the Maxwell pair. The other coil types listed are used primarily for x- and y-gradients. Link to Q&A discussion

  6. Which of the following statements about eddy currents is false?
    1. They create a wide range of image artifacts, including ghosts and blurring.
    2. They are a manifestation of Faraday's Law of induction.
    3. They especially affect traditional spin-echo sequences with long TE's.
    4. They create tissue heating.

    Eddy currents are produced rapidly changing magnetic fields (especially from the switching of imaging gradients) and are thus a manifestation of Faraday's Law. They are typically seen with rapid gradient echo or echo-planar sequences using short TE's; hence answer c) is false. They are uniformly undesirable, creating various artifacts and tissue heating problems. Link to Q&A discussion

  7. Concerning actively shielded gradients, which statement is true?
    1. The are the most effective way to reduce eddy currents in superconducting systems.
    2. The are the most effective way to reduce eddy currents in superconducting systems.
    3. Current in these coils runs in a direction opposite to their associated imaging gradients.
    4. All of the above are true.

    Active shielded gradients are nearly universally used in superconducting scanners because of their high effectiveness in reducing eddy currents in the magnet itself. They do not affect eddy currents generated in tissues, however. Link to Q&A discussion

  8. Which of the following methods is not used to reduce eddy currents:
    1. Actively shielded gradients.
    2. Self-shielded gradients.
    3. Active shimming coils.
    4. Pre-compensation.

    The terms “Actively shielded” and “self-shielded” gradients are synonymous, and are the most effective way to reduce eddy currents. Pre-compensation distortion of waveforms is also helpful. Active shimming coils are used to make the main magnetic field more homogeneous and have nothing to do with eddy currents or their correction. Link to Q&A discussion

  9. Typical values for peak gradient strength in a modern 1.5 T scanner are in the range of
    1. 1-10 mT/m.
    2. 20-50 mT/m.
    3. 200-400 mT/m.
    4. 500-1000 mT/m.

    Lower field strength permanent magnets have peak gradients in the range of 15-25 mT/m. Link to Q&A discussion

  10. The time for a gradient to ramp from zero to its maximum value is known as its
    1. Rise time.
    2. Gradient time.
    3. Slew rate.
    4. Duty cycle.

    This is the definition of rise time. Link to Q&A discussion

  11. The definition of gradient slew rate is
    1. Peak gradient strength ÷ main field strength (Bo).
    2. Peak gradient strength ÷ total time the gradient is on.
    3. Peak gradient strength ÷ Rise time.
    4. The number of times a gradient is turned on and off per second.

    Slew rate is defined to be Peak gradient strength ÷ Rise time. Link to Q&A discussion

  12. The units for slew rate are given in
    1. Millitesla per meter (mT/m).
    2. Tesla per meter per second (T/m/s).
    3. Tesla per second (T/s).
    4. Milliseconds (ms).

    Peak gradient strength is typically measured in mT/m and rise time is measured in ms. Their ratio, slew rate, therefore has units T/m/s. Link to Q&A discussion

  13. Typical maximum slew rate values quoted for modern 1.5 T scanners are in the range of
    1. 1−2 T/m/s.
    2. 10−20 T/m/s.
    3. 100−200 T/m/s.
    4. 1000−2000 T/m/s.

    Answer c) is correct. Slew rates much higher than this can be obtained in research scanners but are limited due to the risk of peripheral nerve stimulation by the rapidly switching gradients. Link to Q&A discussion

  14. A gradient that ramps from 0 to a peak amplitude of 30 mT/m in 0.25 ms has a slew rate of
    1. 30 T/m/s.
    2. 60 T/m/s.
    3. 90 T/m/s.
    4. 120 T/m/s.

    Slew rate = [Peak gradient strength] ÷ [Rise time] = [30 mT/m] ÷ [0.25 ms] = 120 T/m/s. Link to Q&A discussion

  15. Which gradient specification is generally the most important when assessing how well an MR system is capable of performing rapid, high-resolution imaging?
    1. Peak gradient strength.
    2. Slew rate.
    3. Rise time.
    4. Magnetic field strength.

    The slew rate incorporates both peak gradient strength and rise time into its definition and is the best single gradient specification to consider. Static magnetic field strength is an important consideration for high-resolution imaging but is not a gradient specification. Link to Q&A discussion

  16. How many sets of paired physical gradients are present in an MR scanner?
    1. 1
    2. 2
    3. 3
    4. 6

    Three sets of paired primary gradients are present in all MR scanners, producing field distortions along the three cardinal directions (x, y, and z). Link to Q&A discussion

  17. Which of the following statements about gradient duty cycle is false
    1. It is commonly measured in percent (%).
    2. It represents the fraction of time that the gradient works at maximum amplitude.
    3. Its value depends on the pulse sequence timing parameters and number of slices.
    4. Its value is independent of the type of pulse sequence (SE, IR, etc).

    The duty cycle is defined as the percent time a gradient works at maximum amplitude during a pulse sequence. Its value will therefore vary with the pulse sequence and timing parameters, so d) is false. Link to Q&A discussion

  18. Which of the following statements about the gradient subsystem is true
    1. The gradient coils are located within the cryostat.
    2. Gradient coils generate considerable heat during operation.
    3. The gradient coils are cooled by liquid helium.
    4. Increasing power supplied to a gradient decreases the slope of the gradient.

    Gradient systems are not superconducting and are not located within the cryostat. They do generate considerable heat and typically require water and air cooling (though not cooling by liquid helium). Increasing power to a gradient increases (rather than decreases) its strength/slope. Link to Q&A discussion

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RF and Coils

  1. Which coils are located closest to the patient in an MR scanner?
    1. Gradient coils.
    2. RF-receiver coils.
    3. Shim coils.
    4. Body RF-transmit coils.

    RF-receiver coils are placed closest to the patient so they may detect the MR signal with highest sensitivity. Link to Q&A discussion

  2. In the construction of a superconducting MR magnet, which is the correct order of coils from outermost to innermost?
    1. Main magnet windings, gradient coils, RF coils, shielding coils.
    2. Gradient coils, shield coils, main magnet windings, RF coils.
    3. Shielding coils, main magnet windings, gradient coils, RF coils.
    4. RF coils, shield coils, main magnet windings, gradient coils.

    Shielding coils are the outermost coils. Their function is to reduce the external fringe fields from the main magnet windings, the next coil encountered. Next come resistive shim coils used to improve magnet homogeneity. Then the gradients, then the RF transmit coils, and finally the RF receiver coils (closest to the patient). Link to Q&A discussion

  3. Although most local RF coils are "receive only", some specially designed to operate in "transmit-receive (T/R)" mode. T/R coils commonly offered by MR vendors include all of the following except
    1. Head coils.
    2. Knee coils.
    3. Spectroscopy coils.
    4. Spine coils.

    Standard spine and body phased surface coil arrays are not offered for transmit/receive dual operation. Link to Q&A discussion

  4. A 1.5 T MR scanner has a base operating frequency of approximately 64 MHz. In the electromagnetic spectrum, this is considered to be in the range of
    1. Infrared frequencies.
    2. Radio frequencies.
    3. X-ray frequencies.
    4. Microwave frequencies.

    MRI operates in the same frequency range as those used for radio and TV transmission. Link to Q&A discussion

  5. Use of a single element surface coil placed directly on the patient offers which advantages?
    1. High signal-to-noise.
    2. Increased depth of penetration.
    3. Capability for larger fields-of-view.
    4. All of the above.

    A single element surface coil offers high signal-to-noise for receiving signals from voxels beneath the coil. The depth of penetration is limited, however, usually to less than 75% of the diameter of the coil. Larger FOV's are not possible, limited again by the size of the coil. Link to Q&A discussion

  6. Comparing 10 cm and 20 cm diameter surface coils, which of the following is false?
    1. The sensitive volume of the 20 cm coil is larger.
    2. The penetration depth of the 20 cm coil is greater.
    3. The 20 cm coil has higher signal-to-noise for voxels immediately under the coil.
    4. The 20 cm coil offers a larger field of view.

    For voxels immediately under the coil, the 10 cm coil would have superior SNR than for the 20 cm one. The other statements are all true. Link to Q&A discussion

  7. Comparing linear and quadrature coils
    1. Quadrature coils offer twice the signal-to-noise.
    2. Quadrature coils offer four times the signal-to-noise.
    3. Quadrature coils offer about 40% greater signal-to-noise.
    4. Quadrature coils are about 40% larger.

    Quadrature coils can be the same size as linear coils, but offer a signal-to-noise ratio of approximately √2 (≈ 1.41), or about 40% greater than that of linear coils. Link to Q&A discussion

  8. A sinusoidal wave can be described by the equation S(t) = A sin (ωt − φ). The constant A represents
    1. Angular frequency.
    2. Difference in height between positive and negative peaks.
    3. Half the difference in height between positive and negative peaks.
    4. Phase shift.

    "A" is called the "amplitude" of the sinusoidal wave measured from zero to its maximum positive value. The full excursion between positive and negative peaks is 2A, so "A" is ½ this difference. Link to Q&A discussion

  9. An MR scanner employs three different magnetic fields— the main field (B0), gradient fields (G), and radiofrequency field (B1). In terms of relative strength from weakest to strongest, the proper ranking is
    1. B1 < G < B0
    2. G < B0 < B1
    3. G < B1 < B0
    4. B1 < B0 < G

    The main magnetic field (B0) is by far the strongest, typically larger than 1 Tesla. Gradient fields are of intermediate strength with values in the mT range. Radiofrequency fields are the smallest, with B1 values on the order of 10−50 μT. Link to Q&A discussion

  10. Which of the following is not an advantage of parallel (multi-)transmit RF?
    1. Decreased RF-energy deposition in tissues.
    2. Reduced shading artifacts.
    3. Increased standing waves due to dielectric effect.
    4. More uniform excitation.

    Parallel transmit RF excitation offers many advantages, particularly as field strength increases, including improved homogeneous excitation and artifact reduction. The incorrect answer a) standing waves due to dielectric effect are decreased, not increased with parallel transmission. Link to Q&A discussion

  11. Comparing phased array and parallel array coils, which of the following is true?
    1. Both types of coils offer improved signal-to-noise and increased field-of-view.
    2. Overlap of coil elements is avoided in both types.
    3. Phased array coils are also known as switchable arrays.
    4. Both can be used equally well with parallel imaging acquisition methods.

    Only answer a) is true. Overlap of coil elements is common design feature of phased array coil systems, but must be avoided in parallel arrays. Although some phased array coils are compatible with parallel imaging applications, most are not. Finally, phased and switchable arrays are not the same, the latter largely being abandoned 20+ years ago. Link to Q&A discussion

  12. Parallel imaging systems are composed of coil elements, segments, and channels. The proper transmission hierarchy beginning with the patient and proceeding upward through the processing chain is
    1. Elements → channels → segments
    2. Elements → segments → channels
    3. Elements → channels → segments
    4. Channels → elements → channels

    Proper order is given above. Link to Q&A discussion

  13. Advantages of parallel receiver coil arrays include all of the following except
    1. Increased signal-to-noise.
    2. Increased field-of-view.
    3. Ease of design.
    4. Reduced imaging time.

    Parallel coil configurations offer all the advantages above, with the exception of ease of design. In fact, it is rather difficult to pack many small coils closely together in these arrays without significant magnetic interactions. Link to Q&A discussion

  14. The effective depth of penetration for signal reception from a 20 cm diameter single loop surface coil is approximately
    1. 10−15 cm
    2. 20−25 cm
    3. 30−40 cm
    4. 40−50 cm

    The penetration depth for reception from a simple loop RF surface coil is typically only about 50-75% of the coil's diameter. Link to Q&A discussion

  15. Concerning the main transmit RF-body coil, which statement is false?
    1. It is commonly used to receive the MR signal
    2. It is built into the scanner gantry housing and cannot be seen by the patient
    3. It is considered a transceiver coil, capable of both RF transmission and reception.
    4. Its transmission field (B1) is perpendicular to the main magnetic field (B0).

    The body coil is hidden within the scanner gantry housing. Its primary purpose is to transmit the B1 field which is perpendicular to B0. It is a transceiver coil capable of both RF transmission and reception. However, it is seldom used for reception because better signal-to-noise is obtained using surface coils for this purpose. Hence, answer a) is false. Link to Q&A discussion

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SITE PLANNING

  1. Which of the following components of an MR system is typically not located in an adjoining equipment room?
    1. RF-power amplifiers
    2. Gradient amplifiers
    3. Helium pump
    4. Gradient coils

    Gradient coils are an integral part of the MR scanner itself and are not located in a separate equipment room. Link to Q&A discussion

  2. Where is the master computer located that controls the MR scanner and processes data into images?
    1. In the MR scanner room
    2. In the MR scanner control room
    3. In the nearby MR equipment room
    4. At least 25 meters distant from the main scanner to avoid interference

    The master computer is located in the scanner console in the control room immediately adjacent to the magnet room. Because of scanner shielding there is not need for it to be in a remote location (answer d is false). Link to Q&A discussion

  3. The function of the array processor is to
    1. Generate triggers for the array of RF-pulses and gradient waveforms used for imaging
    2. Reconstruct the raw NMR data into images
    3. Calculate RF frequency offsets and gradient strengths for desired slice selection and field-of-view
    4. Activate and/or disable various coil elements in an array

    The array processor is a special board within the main computer that operates the MR scanner. It is responsible for performing Fast Fourier Transformation (FFT) of the raw data and constructing the data into images. Link to Q&A discussion

  4. Which scanner is the heaviest (and would thus require the most floor support)?
    1. 0.35 T Permanent magnet system
    2. 0.6 T Resistive magnet system
    3. 1.5 T Superconductive system
    4. 3.0 T Superconductive system

    Permanent magnet systems may weigh over 35,000 pounds (16,000 kg), over 3 times more than a superconductive scanner. Link to Q&A discussion

  5. Which scanner is would have the lowest overall siting and operational costs?
    1. 0.35 T Permanent magnet system
    2. 0.6 T Resistive magnet system
    3. 1.5 T Superconductive system
    4. 3.0 T Superconductive system

    Notwithstanding additional siting costs regarding their weight as described in the previous question, permanent magnet scanners do not require cryogens nor a sophisticated chiller system, so their operational costs are extremely low. Their fringe fields are typically very small as well, allowing them to have much smaller room requirements. Resistive electromagnet scanners, by comparison, have high operational costs due to use of electricity and increased environmental cooling requirements. Superconducting scanners are the most expensive to site due to their size, fringe fields, and cooling requirements. Link to Q&A discussion

  6. Which component of a superconducting MR scanner does not require specialized cooling to maintain function?
    1. Main coil windings
    2. Gradient coils
    3. Gradient amplifiers
    4. Radiofrequency coils
    5. Radiofrequency amplifiers

    The main coil windings, of course maintained at superconducting temperatures by liquid helium. Both gradient coils and amplifiers get very hot and must be cooled by circulating water/antifreeze exchanged through chiller circuitry. Radiofrequency amplifiers are usually in the same cabinet as gradients and also require air and/or water cooling. The RF transmit coils themselves do get warm but require no separate cooling. The RF receive coils close to the patient do not heat up at all. Link to Q&A discussion

  7. The B0 field of an MR scanner is most homogeneous at
    1. At the opening (gantry) of the magnet
    2. At bore level about 1 meter directly in front of the magnet
    3. In the middle of the bore at isocenter
    4. On the outside of the magnet immediately against its wall

    The B0 field is most homogenous at magnet isocenter. Link to Q&A discussion

  8. Which scanner would have the largest fringe field?
    1. 0.35 T Permanent magnet system
    2. 0.6 T Resistive magnet system
    3. 1.5 T Superconductive system
    4. 3.0 T Superconductive system

    Fringe fields are generally directly related to field strength, so the higher the main field, the greater the fringe. Thus the correct answer is d). Magnet configuration also is important. Specifically, C-shaped magnets (the typical configuration for permanent scanners) have relatively low fringe fields. Link to Q&A discussion

  9. If one moves from 1 meter to 2 meters away from a magnet, the fringe field will be reduced by a factor of approximately
    1. √2
    2. 2
    3. 4
    4. 8

    In theory the strength of a magnetic fringe field is inversely related to the third power of the distance (1/r³) from the magnet isocenter. Thus moving twice as far away from the magnet, the fringe field should fall by a factor of approximately 1/2³ = 1/8. Link to Q&A discussion

  10. The fringe fields of cylindrical superconducting magnet are highest
    1. In the x-direction (transverse and horizontal to the axis bore)
    2. In the y-direction (transverse and vertical to the axis bore)
    3. In the z-direction (along the axis bore)
    4. They are equal in all directions

    Fringe fields are significantly higher along the z-axis (the direction of B0). Link to Q&A discussion

  11. The primary purpose for passive magnetic shielding is
    1. To reduce fringe magnetic fields outside the scanner room.
    2. To keep extraneous radiofrequency noise from entering the scanner room.
    3. To constrain the NMR signal to remain within the bore of the magnet for better reception.
    4. To reduce the effects of moving equipment (such as cars and elevators) from distorting the magnetic field.

    Passive shielding typically involves placing iron posts or sheets of steel in selected places around scanner floor or wall to minimize fringe field extension outside the scanner room. Passive shielding is generally not necessary with modern self-shielded scanners unless they are closely space or near other sensitive equipment. Link to Q&A discussion

  12. Concerning passive shielding, which statement is true?
    1. It is performed by placing heavy copper plates along the walls of the scanner room.
    2. It is a method to reduce extraneous radiofrequency interference with the MR signal.
    3. It is more commonly required for 7.0T than for 1.5 T installations.
    4. Active shielding technology found in modern scanner design has not changed the need for it.

    Passive shielding is a method to reduce fringe magnetic fields, so a) copper lining of the walls to reduce b) RF-interference are incorrect. It is more needed for higher field strength installations, so c) is true. Active shielding technology in modern scanners has reduced the need for passive methods, so d) is false. Link to Q&A discussion

  13. Passive magnetic shielding of the scanner room is typically achieved using sheets or rods made of
    1. Copper
    2. Iron
    3. Aluminum
    4. Lead

    A ferromagnetic substance such as iron or steel is required to constrain the fringe field lines. Link to Q&A discussion

  14. The fringe magnetic field arising from an MR scanner
    1. Can be eliminated by active shielding.
    2. Can be eliminated by passive shielding.
    3. Can be reduced by radiofrequency shielding.
    4. None of the above.

    Active and passive shielding can reduce, but not eliminate fringe fields. Radiofrequency shielding reduces noise but does not affect fringe fields Link to Q&A discussion

  15. What is the “5 Gauss Line”?
    1. A place inside the scanner where x- and y-gradients differ in strength by less than 5 Gauss (5 mT).
    2. The boundary in an MRI center inside of which one’s credit cards will be erased.
    3. A fringe field line that may pose danger to patients with certain pacemakers
    4. A fringe field line in the scanner room safe for patients but which MR technologists should avoid crossing.

    The 5-Gauss Line was established by the US Food and Drug Administration (FDA) as a boundary to which the unsuspecting public should not be exposed. The value was based on the fact that the reed switch in older pacemakers could be flipped by exposure to this level of stray magnetic field, potentially converting a patient’s demand pacemaker into asynchrony mode. It should be recognized that it is not just a line, but a surface that extends outward from the scanner in 3 dimensions. Thus it can extend into the floors above and below the scanner as well as to the sides. Link to Q&A discussion

  16. Which statement about ACR Safety Zones 1 and 2 is correct?
    1. Both Zones 1 and 2 lie outside the 5 Gauss line.
    2. Patient safety screening is required before entering Zone 2
    3. The general public should not be admitted to Zone 1; it is only for MR patients and their families.
    4. Patients with pacemakers can at risk if allowed to enter Zone 2.

    Zone 1 is for the general public. Entry is generally restricted beginning in Zone 2, as this is where safety screening takes place. Both lie outside the 5 Gauss line and are safe for everyone. Link to Q&A discussion

  17. Which statement about ACR Safety Zone 3 is false?
    1. Patients should not be admitted to Zone 3 unless they have undergone safety screening.
    2. Ferromagnetic objects may not be brought into this area.
    3. The MR operator's console is located in this area.
    4. Medical personnel should not be admitted to this area unless they have undergone MR safety training.

    Zone 3 includes areas within the 5-Gauss line, so all patients and family members need to be screened prior to entry. Zone 3 includes the area where the MR operator's console is located. The fringe fields in Zone 3 are sufficiently small that there is no risk for flying ferromagnetic objects to be propelled into the scanner. Nevertheless, there is generally easy direct access from Zone 3 into the scanner room (Zone 4) where dangerous flying objects can occur. Ferromagnetic objects in Zone 3 are discouraged but not forbidden; they certainly should not be brought near the door of the scanner room. For these reasons all medical personnel must be trained/educated in MR safety before being allowed into Zone 3. Link to Q&A discussion

  18. Which statement about ACR Safety Zone 4 is true?
    1. Accompanying family members should never be allowed access to Zone 4.
    2. Zone 4 is synonymous with the room containing the MR scanner.
    3. Zone 4 is includes the scanner, the operator's console, and equipment room (where gradient amplifiers are located).
    4. A locked door requiring badge, key, or combination access must be present and remain closed between Zone 3 and Zone 4 except when moving patients.

    Zone 4 is the scanner room itself, so b) is true and c) is false. Family members may be allowed in the scanner room provided they have been appropriately screened, so a) is false. The door to the scanner room is not locked and is frequently left open when scanning is not in progress (though we recommend having a strap across it to prevent inadvertent entry). Ferromagnetic materials should not be brought into Zone 4 as the risk of them being propelled into the scanner is high. Link to Q&A discussion

  19. Why might large trucks on a road 20 meters away from an MR scanner be of potential concern for siting?
    1. Their CB radios operate at the same frequencies as the MR signal.
    2. The scanner magnetic field can be affected by the dense iron in their chassis as they pass by.
    3. The physical vibrations they produce can affect image quality.
    4. At this distance heavy truck traffic should be of no concern at all.

    Environmental vibrations can significantly affect scanner performance, and sites should undergo vibration testing prior to installation of a scanner. The frequent passage of heavy trucks on a nearby road would be one possible cause. Other vibration sources include nearby air conditioning equipment, motors, and building elevators. RF-interference from CB radios should not be a special problem, as these frequencies would normally be filtered out by standard RF-shielding. At a distance of 20 meters, moving metal should not cause a static field disturbance; however, this could be of concern if the trucks passed as close as 10 meters by. Link to Q&A discussion

  20. The loud noise produced by an MR system during a scan is primarily due to
    1. Vibrations of the gradient coils
    2. Vibrations of the radiofrequency coils
    3. Vibrations of the main magnet windings
    4. Vibrations from the chiller and helium pump

    The noise produced during a scan is primarily due to electromechanical vibrations generated by gradients as they are rapidly turned on and off during a pulse sequence. This is transmitted to other structures in the magnet housing that may also vibrate secondarily and amplify the noise. Link to Q&A discussion

  21. Which of the following sequences would likely generate the loudest noise during scanning?
    1. T2-weighted Turbo spin-echo (TSE) imaging of the spine
    2. Dixon fat-water imaging of the liver
    3. Echo-planar diffusion tensor imaging of the brain
    4. MR spectroscopy of the prostate

    The loudest sequences are those where gradients are switched on and off most rapidly, such as in echo-planar imaging and short TE gradient echo imaging. Link to Q&A discussion

  22. Which of the following statements about MR scanner noise is false
    1. Sound levels can reach up to 120 dB for some sequences.
    2. Although potentially uncomfortable for the patient, there is no real risk to hearing.
    3. Ear protection is mandatory for all patients undergoing MR imaging.
    4. New quiet pulse sequences can reduce noise levels to within 10 dB of background.

    Sound levels indeed can reach 120 dB for some sequences, especially echo-planar ones. This can result in damage to the inner ear and produce hearing loss, so option b) is false. Thus ear protection is mandatory for all patients. Link to Q&A discussion

  23. Which of the following methods can reduce scanner noise?
    1. Avoidance of echo-planar sequences
    2. Use of "soft" gradient pulses with longer rise times
    3. Use of 3D ultrashort TE sequences
    4. All of the above

    These strategies will all produce a reduction of noise levels during scanning. Link to Q&A discussion

  24. Newer "quiet" MR sequences with longer gradient ramp times are now available. Which of the following statements about these sequences is true
    1. They can reduce noise levels to within 10 dB of background.
    2. This strategy can be applied to all pulse sequences.
    3. They can be employed with no signal-to-noise penalty.
    4. They do not affect number of slices for a given TR.

    Newer quiet sequences can reduce noise levels to less than 10 dB of background, so answer a) is true. They can be used on many (but not all) pulse sequences. Because of the increasing rise and fall times there is a shorter sampling window and SNR is reduced. A penalty in the maximum number of slices may also occur at constant bandwidth due to the increased time spent in ramping gradients. Link to Q&A discussion

  25. Radiofrequency shielding of the scanner room is most commonly achieved by lining the walls with thin sheets of
    1. Iron
    2. Aluminum
    3. Copper
    4. Lead

    A thin layer of copper around the entire room is most commonly used in scanner installations. It acts as a Faraday cage and is effective at reducing penetration of extraneous radiofrequencies. However, virtually any conductive metal could be used for this purpose, and both steel and aluminum cages are occasionally used. Link to Q&A discussion

  26. The primary purpose for radiofrequency shielding is
    1. To confine fringe fields to the scanner room itself.
    2. To constrain the NMR signal to remain within the bore of the magnet for better reception
    3. To keep extraneous radiofrequency noise from entering the scanner room
    4. To reduce the effects of moving equipment (such as cars and elevators) from distorting the magnetic field.

    RF-shielding primarily prevents extraneous radiofrequency noises from outside the scanner room from entering and contaminating the MR signal. Link to Q&A discussion

  27. In most clinical MR sites, the principal source RF-interference that needs to be excluded comes from cell phones, TV, and radio transmissions.
    1. True
    2. False

    The most common form of RF interference comes from noise generated from nearby electrical equipment (transformers, motors, pumps) or electronic devices (computers, pulse oximeters, cardiac monitors). Link to Q&A discussion

  28. A device that allows a plastic oxygen hose to be passed through the wall of an MR scanner room without disrupting the integrity of the RF-shielding is called a
    1. Penetration panel
    2. Bandstop filter
    3. Waveguide
    4. Faraday cage

    The correct answer is waveguide (c). This looks like a pipe mounted within the wall and has a design that it blocks/traps radiofrequencies in the Larmor frequency range from passing through. This device is commonly part of the penetration panel, which also includes bandstop filters for wires. The Faraday cage is the entire enclosure around a scanner room to produce RF shielding. Link to Q&A discussion

  29. A common location for RF-leakage into the scanner room is
    1. Around the door
    2. Along seals of the scanner window
    3. At the penetration panel
    4. Along the junction of copper plates in the scanner room’s walls

    Because of repetitive opening and closing, RF-seals around the door are frequently damaged and a common source of RF-leakage into the room. Link to Q&A discussion

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