Chapter 3 Questions

Front 1

What condition must be met in order to maintain virtually zero resistance in a superconducting magnet?

Back 1

The superconducting windings must be maintained at a temperature approaching absolute zero. (or 4 degree K)

*This can be achieved by bathing it in liquid helium.

Hint 1

At this temperature the wire achieves near zero electrical resistance which allows huge currents to flow through it thereby generating the magnetic field.

Front 2

If a voltage source was connected to a solenoid, so that the current flowed as in the diagram below, how would a magnetic field be generated?

Back 2

To the right.

Hint 2

Right hand rule
*Rule states that within the fingers of the right hand curled so that the finger tips point in the direction of the flow of current through a coil of wire then the extended thumb points in the direction of the resulting magnetic field.

Front 3

The purpose of the shims is to:

Back 3

Make the magnetic field more uniform.

*Provides homogeneity of the main magnetic field thereby removing most of the field inhomogeneities.

Front 4

The purpose of magnetic shielding is to:

Back 4

Keep the fringe magnetic field closer to the magnet.

*Note: Magnetic shielding is not designed to control RF energy and does not act as a Faraday shield.

Front 5

What keeps the RF energy used in scanning from interfering with the external environment and keeps the stray radio signals in the environment from creating artifacts in the MR images?

Back 5

RF or Faraday shielding

Front 6

The correct location of the z-gradient coils in a superconductive MRI system is wrapped ______________________ a cylinder within the bore of the magnet.

Back 6

Completely around each end of

*The X gradient is generated by coils wrapped on the top and bottom of the cylinder.
***There is no gradient coil which is wrapped in a spiral fashion. Although tha main magnetic field can be generated in this way.

Front 7

Which of the following is affected by the transmitter bandwidth?

Back 7

The slice thickness

Front 8

Which of the following MRI system components is responsible for the timing and correct performance of each component once the sequence has been started?

Back 8

Pulse sequence controller

Front 9

What performs calculations on MR echo and image data?

Back 9

The array processor

Front 10

Which of the following describes the operational mechanism of an electromagnet?

Back 10

A power supply is connected to the ends of a coil of long wire, generating a strong magnetic field.

Front 11

Gradient rise time is defined as which of the following?

Back 11

The time required for a gradient to reach its specified amplitude beginning from a resting state.

Front 12

Which of the following describes the percentage of time that a gradient is applied at a specific amplitude?

Back 12

Duty cycle

*If a gradient is at its resting state 60% of the time during a measurment, it is said to have a 40% duty cycle.

Front 13

The slew rate is:

Back 13

The rate of change in gradient strength during a transition from one amplitude to another.

Front 14

The MRI system's host computer is responsible for:

Back 14

Communicating the sequence parameters to the rest of the system components.

Front 15

The actual timing of the gradients is achieved by:

Back 15

The pulse system controller

Front 16

The MR signal is digitized by the:

Back 16

Analog to digital converter or ADC

Front 17

The Fourier Transform is performed by which MRI system component?

Back 17

Array processor

* Image reconstruction is possible with the application of a Fourier Transform calculation. Since such calculations are very sophisticated and many thousands of them are required in order to reconstruct a single image, specialized hardware is required which is dedicated to achieving such calculations at high speed.

Front 18

Which of the following functions is a responsibility of the RF coils?

Back 18

Detection of the MR signal

Front 19

What is responsible for the amplification of the MR signal?

Back 19

Preamplifiers

*It is considered to be a separate component from the RF coils.

Front 20

Which of the following components is NOT located within the bore of the magnet?

Back 20

Passive magnetic shielding

Front 21

The MRI system's array processor is responsible for:

Back 21

The calculation of the MR signals into the resulting MR image.

Front 22

The echoes emitted from the patient's body are passed through a pre-amplifier to:

Back 22

Greatly increase the strength of their tiny signals.
*Uses to ADC

Front 23

A pulse sequence to select a 3 mm sagittal slice requires 40 amps of electrical current through the gradient coil. If the current is increased to 80 amps, a ________ will result.

Back 23

Thinner slice

Front 24

___________ magnets cannot lose their magnetic field without being disassembled.

Back 24

Permanent

Front 25

Which of the following is an example of Faraday's law of induction?

Back 25

Magnetic variations as a result of relaxation mechanisms causing the induction of current into a RF coil.

Hint 25

Faraday's law of induction states that "any magnetic change which occures near a neighboring conductive material will cause a voltage to be induced across that conductive material."

Front 26

________ Shielding is another name for Faraday shielding.

Back 26

RF

Front 27

What types of sheilds control the vaious types of magnetic fields?

Back 27

Magnetic, gradient, and active sheilding

*They do not function as Faraday shields and do not affect RF transmissions or signals.

Front 28

Which of the following statements is FALSE regarding the best results obtainable with a receiver coil?

Back 28

The largest possible receiver coil should always be selected.

*The coil should NOT be much larger that the ROI.

Front 29

When a different RF coil is used for transmitting than the RF coil used for transmitting than the RF used for receiving, they must be decoupled. What is decoupling?

Back 29

Electrically isolating the two coils so that the RF energy is only applied to the transmitter coil.

Front 30

_____________ magnets are routinely used in producing a magnetic field of 1.5T.

Back 30

Superconducting

Front 31

_____________ magnets require a constant connection to a power source in order to maintain their magnetic field.

Back 31

Resistive

Front 32

Which of the following statements is TRUE?

A. Permanent magnets use more cryogen than superconducting magnets.
B. Cryogens may be refilled while a patient is in the magnet.
C. Leaving the room door open in a room whose walls comprise the Faraday shield may adversely affect the image quality.
D. The use of magnetic shielding allows people with pacemakers to have an MRI exam.

Back 32

Leaving the room door open in a room whose walls comprise the Faraday shield may adversely affect the image quality.

Front 33

Which of the following types of magnets can have their magnets can have their magnetic reduced to zero in the shortest amount of time?

Back 33

Resistive (Can be reduced to zero in a few seconds)
*Superconducting requires a few minutes

Front 34

A sequence is run to select a transverse slice 5 mm from isocenter, towards the patient's feet, along the z-direction. If the direction of current in the x-gradient coils is reversed:

Back 34

A mirror image of the same slice would be generated. Reversing the current in the x- gradient will result in left and right being reversed in the image.

Front 35

___________ magnets are used on most MRI systems produced today.

Back 35

Superconductive

Front 36

If gradients takes 1.5 msec to reach an amplitude of 8mT/m, the slew rate is _______ mT/m/sec.

Back 36

5333

(8 mT/m) / (0.0015 sec)

Front 37

The fringe field of a 1.5 T superconductive magnet extends along which of the following directions?

Back 37

all directions

Front 38

How will an extra steel I-beam in the floor, about 150 cm from the magnet's isocenter, affect the MR system?

Back 38

It will distort the homogeneity of the main magnetic field.

Front 39

Which of the following statements is True?
A. Superconducting magnets can produce magnetic fields up to 10 T.
B. Resistive magnets may only have horizontal magnetic fields.
C. Superconducting magnets are never used is MRI systems.
D. Permanent magnets are ideal for use in mobile systems.

Back 39

A. Superconducting magnets can produce magnetic fields up to 10 T.

Front 40

A pulse sequence is run to select a transverse slice located 5 mm from isocenter toward the patient's feet, along the z-direction. What would be the result if the direction of current flow in the z-gradient coils were reversed?

Back 40

A transverse image from tissues located 5 mm from isocenter towards the patient's head, along the z-direction would be generated.

Front 41

Some manufactures have incorporated a Faraday shield into the design of their magnets, thus eliminating the need to enclose the entire magnet room with RF shielding. Which of the following is TRUE regarding this self shielded type of MR system?

Back 41

Installation costs will be decreased since expensive metal in the magnet room walls can be avoided.

*Faraday shielding does NOT necessisarily incorporate magnetic shielding, therefore Faraday shielding does NOT allow the trailer to be parked any closer to heavy moving objects.

Front 42

The actual size of the Faraday room if properly designed should not affect the signal to noise of the images. T/F?

Back 42

True. It should NOT

Front 43

A given MRI system has a maximum gradient amplitude of 10 mT/m. For a specific pulse sequence and set of parameters, the prescription of a 3 mm sagittal slice requires an x-gradient amplitude of 8 mT/m, at the narrowest bandwidth. If all parameters remain constant except for slice thickness, a _____ mm sagittal slice is NOT possible on this system.

Back 43

2

*Compared to the 3 mm slice, a 2 mm slice requires an amplitude of (3mm/ 2 mm) x 8 mT/m =12 mT/m

Front 44

A region of interest extends about 5 cm below the surface of the skin. If you could only have four circular coils to chose from, a ___ cm diameter coil will have the best sensitivity to signal from the region of interest and at the same time provide the highest signal to noise ratio.

Back 44

10

Front 45

Which of the following would result if a 5 cm diameter circular coil was used to image the shoulder of a professional weight lifter?

Back 45

The portion of the shoulder closest to the coil would have plenty of signal intensity, but the signal intensity from deeper structures would be greatly decreased.

*Images created by circular coils always show the brightest signal adjacent to the coil and a gradual loss of signal in tissues as distant increases from the coil.

Front 46

Upon installing a new MRI system, the service engineer mistakenly connects the x-gradient amplifier to the y gradient coil. The y gradient coil and the z gradient amplifier to the x gradient coil. What type of image would result when the technologist runs the first sagittal slice?

Back 46

A coronal image (the machine mistakes the x for a y which is Really a coronal image)

Front 47

Around which of the following magnets is it necessary to restrict ferromagnetic objects?

Back 47

All types of magnets