11 Magnetic Resonance

Question 1

Which nuclei have no nuclear

magnetization?

Answer 1

Nuclei with an even number of protons and an even number of neutrons.
–Even numbers of protons pair up with their magnetization aligned in opposite directions and cancel each other (as do even numbers of neutrons).

Question 2

Which nuclei have the largest nuclear magnetization?

Answer 2

Hydrogen nuclei.

Question 3

Why H nuclei the basis of most clinical magnetic resonance (MR) imaging?

Answer 3

The abundance of hydrogen in the body, together with the large nuclear magnetization,

Question 4

Give me one reason that MR signals are weak?

Answer 4

So few nuclei contribute to the MR signal.

Question 5

Define the Larmor frequency (fL)?

Answer 5

–The precession frequency (MHz) of nuclei in a magnetic
field (Bo).
–The Larmor frequency is directly proportional to the magnetic field strength.
–Larmor frequency (fL) for protons is 42 MHz at 1 T.

Question 6

Radiofrequency (RF) electromagnetic fields are generated using what?

Answer 6

volume or surface coil.

Question 7

Define Resonance?

Answer 7

This occurs when an applied RF field interacts with the net nuclear magnetization.

Question 8

Define the longitudinal magnetization?

Answer 8

–The component of the net magnetization vector parallel to the main magnetic field.
–By convention, the longitudinal magnetization is taken to point in the z-axis.
–Grows exponentially from the initial value of zero to the equilibrium value of Mz with a time constant T1

Question 9

Define the transverse magnetization?

Answer 9

–The component perpendicular to the main magnetic field.

–By convention, the transverse magnetization is taken to be in the x-y plane.

Question 10

Define the free induction decay (FID) signal?

Answer 10

–The detected voltage.
–The FID signal is an oscillating voltage at the Larmor frequency (fL).
–The induced FID is obtained in a receiver coil placed around the sample.
–FID signals are detected, digitized, and used to produce MR images.

Question 11

T1 relaxation

Answer 11

–At a time equal to T1, 63% of the magnetization has formed.
–Full magnetization is normally taken to occur after a time interval of approximately 4 × T1.
–Longitudinal magnetization decays as Mz× e−t/T1 where t is the elapsed time.
–T1 relaxation is called longitudinal relaxation and spin-lattice relaxation.

Question 12

T1 and tissue

Answer 12

–T1 is long in liquid materials & in solids (hair).
–T1 is short in medium-viscosity materials and in fat.
–Contrast agents such as gadolinium-DTPA cause T1 to be shortened.
–For tissues, T1 increases with increasing magnetic field strength.
–Doubling the magnetic field strength increases tissue T1 by approximately 2^0.5.

Question 13

Define repetition time TR?

Answer 13

–Generating a N2 matrix MR image requires the acquisition of N sequential signal acquisitions that are obtained with a repetition time TR.
–Short TR times are less than ∼300 ms at 1.5 T and less than ∼450 ms at 3T.

Question 14

transverse relaxation(T2 relaxation), also spin-spin relaxation

Answer 14

–After a 90-degree pulse, the magnetization vector rotates at the Larmor frequency in the transverse (x-y).
–The induced FID signal decays as e−t/T2 where t is the time.
–At a time equal to T2, the signal has decayed to 37% of its original value.
–After a time ∼4 T2, the transverse magnetization signal is negligible.
–T2 decreases with increasing viscosity and decreasing molecular mobility.
–Tissue T2 values are approximately independent of magnetic field strength.

Question 15

TE (time to echo)

Answer 15

–MR signals are most often obtained in the form of echoes from transverse magnetization.
–Short TE values will result in little loss of transverse magnetization (i.e., little T2 decay).
–Short TE values therefore produce no differences (contrast) between tissues that have different T2 values.
–T2-weighted images are obtained with a long TE.
–Long TE values are typically greater than 60 ms.

Question 16

(T2inhomogeneity)*

Answer 16

–Given by 1/T2∗ = 1/T2 + 1/T2inhomogeneity

–For tissues, T2∗ ≤ T2 ≤ T1.

Question 17

Magnetic shimming

Answer 17

Used to make small corrective changes to the main field to improve the magnetic field uniformity.

Question 18

Magnetic shimming with passive techniques

Answer 18

Pieces of iron at specific locations.

Question 19

Magnetic shimming with active techniques

Answer 19

Electrically energized coils.

Question 20

Permanent magnets

Answer 20

–Have low operating costs and small fringe fields.

–Limitations: heavy and generate fields only up to ∼0.35 T.

Question 21

Resistive magnets

Answer 21

–It can generate magnetic fields up to ∼0.5 T.
–Resistive magnets can be turned on and off, but consume a large amount of power and need cooling because of the heat generated.

Question 22

Superconducting magnets

Answer 22

–Field strengths higher than those of resistive and permanent magnets.
–Use a wire-wrapped cylinder (i.e., a solenoid) to generate the uniform magnetic field.
–Superconducting magnets must be kept very cold using liquid helium (4◦ K) as a refrigerant.
–A perpetually circulating electric current of hundreds of amps creates the magnetic field.
–The magnetic field is always on.

Question 23

Superconductivity

Answer 23

–It is the ability of certain materials to conduct electrical current without any resistance.

Question 24

Magnet quench

Answer 24

–If the wire temperature rises, the system loses its superconducting properties and the energy stored in the magnetic field is converted to heat resulting in a magnet quench.

Question 25

What will happen when MR field strength increases?

Answer 25

–T1 relaxation time, SNR, and RF energy deposition in the patient.
–Some image artifacts may also increase with increasing magnetic field strength.

Question 26

Magnetic gradients

Answer 26

– Used to code the spatial location of the MR signal.

Question 27

Axial gradients (z)

Answer 27

–Are produced using Helmholtz coils.

–Used to produce a gradient along the z-axis.

Question 28

Linear gradient

Answer 28

–Gradients that change the main field as a function of x or y distance are normally produced by saddle coils.
–When activated, these gradients superimpose a linear gradient on the main magnetic field.

Question 29

Eddy currents

Answer 29

Generated when gradient field change with the time.

Question 30

Radiofrequency

Answer 30

–RF is electromagnetic radiation with frequencies in the range of approximately 1 MHz to 10 GHz.

Question 31

Volume coils

Answer 31

Are designed to transmit and receive uniform RF signal throughout a volume, e.g., the head coil or body coil.

Question 32

Linear volume coil

Answer 32

Coils receive the signal from only one of the x- or y-axes of the rotating transverse magnetization.

Question 33

Quadrature volume coil

Answer 33

Coils receive the signal in both the x- and y-axes, therefore increasing the overall SNR and reducing image artifacts.

Question 34

Surface coils

Answer 34

Have increased sensitivity close to the coil, but the signal drops off with increasing distance from the coil.

Question 35

Phased array coils

Answer 35

–are a combination of many surface coils around the body part being examined.
–Phased arrays try to obtain uniform signals from the enclosed volume with an improved signal detection of individual surface coils.

Question 36

Parallel imaging

Answer 36

–Parallel imaging uses separate signals from phased array coils.
–Many individual surface coils in a phased array coil detect the same signal from the same place in the body.

Question 37

Magnetic shielding

Answer 37

–Magnetic shielding usually consists of thick iron plates or layers of special steel sheet metal embedded in the MR magnet room walls.

Question 38

RF shielding uses

Answer 38

–Prevent RF signals (radio broadcasts) getting into the coils and increasing the background noise.
–Prevents the powerful RF pulses from escaping and interfering with outside electronic equipment.

Question 39

RF shielding

Answer 39

–The RF shielding is a Faraday cage, which consists of conductive sheet metal lining the MR magnet room.
–Copper is the best material, with copper screen also used over windows.

Question 40

How the image appears with short T1?

Answer 40

–Tissues with short T1 values appear bright on T1-weighted images.

Question 41

How the image appears with long T2?

Answer 41

–Tissues with long T2 values appear bright on T2-weighted images.

Question 42

Pixel size equals what?

Answer 42

–Pixel size equals the field of view divided by the data acquisition matrix size.

Question 43

The signal-to-noise ratio (SNR) is increase by what?

Answer 43

–Increasing slice thickness.
–Decreasing matrix size.
–Reducing RF bandwidth during signal detection.
–High static magnetic field strength.
–Doubling the field strength will generally double SNR.
–SNR increases as the square root of the number of image acquisitions.
–Four acquisitions (repeats) will double the SNR at the expense of a quadrupling of the total image acquisition time.
–Use of smaller surface coils improves SNR.
–Quadrature detection provides an increase of √2 in SNR.

Question 44

Chemical shift artifacts

Answer 44

• Are caused by the slight difference in resonance frequency of protons in water and in fat.
• Can produce light and dark bands at the edges of the kidney or the margins of vertebral bodies.

Question 45

Truncation artifacts

Answer 45

–Truncation artifacts in the spinal cord may simulate a syrinx.
–Truncation artifacts are sometimes referred to as Gibbs ringing.

Question 46

ghost images

Answer 46

–Patient motion results in ghost images that appear in the phase-encode direction.

Question 47

Wraparound artifact

Answer 47

–Wraparound artifact occurs when the FOV is smaller than the structure and imaged objects outside the FOV are mapped to the opposite side of the image.
–Wraparound is caused by undersampling (aliasing).

Question 48

Acquisition time to generate an image with N pixels in the frequency encode direction and M pixels in the phase encode direction

Answer 48

M TR.

Question 49

Reconstructing an N × M MR image requires a total of Fourier transforms

Answer 49

N + M 1D Fourier transforms.

Question 50

Spin echo (SE) pulse sequences

Answer 50

–The SE sequence of 90-degree and 180-degree RF pulses is repeated after a repetition time (TR).

Question 51

Proton density-weighted images are obtained with

Answer 51

Long TR (>2,000 ms) to minimize T1 differences and a short TE (<20 ms) to minimize T2 differences.

Question 52

Gradient recalled echo (GRE)

Answer 52

Techniques make use of low flip angles.

GRE images are T2∗ weighted (not T2).

Question 53

Inversion recovery (IR)

Answer 53

–180-degree pulse to invert the longitudinal magnetization.
–Longitudinal magnetization recovers with a time constant T1.
–Complete recovery of longitudinal magnetization takes 4T1.
–The 90-degree pulse is known as the readout pulse.
–Readout pulses flip any longitudinal magnetization into the transverse plane.
–A refocusing 180-degree pulse at time TE/2 produces an echo at time TE (echo).
–The size of the signal obtained with the readout pulse is strongly dependent on the values of T1 and TI.

Question 54

Short time inversion recovery

Answer 54

(STIR) sequences for fat suppression.

–STIR has a TI value that is selected to null the signal from fat.

Question 55

Fluid attenuated inversion recovery (FLAIR)

Answer 55

–The signal from fluids is suppressed.

–FLAIR has a TI value that is set to eliminate a CSF signal.

Question 56

Three-dimensional imaging times

Answer 56

–N1 × N2 × TR, where N1 is the number of phase-encoding steps in one plane and N2 is the number in the orthogonal plane.

Question 57

A typical resolution in clinical MR

Answer 57

0.3 lp/mm, and about half the resolution in clinical CT.

Question 58

The FDA guideline for limiting RF absorption in any gram of tissue

Answer 58

8 W per kg in the extremities.

Question 59

The size (single domain) of a superparamagnetic particle such as SPIO

Answer 59

0.04 μm.