Nuclear Magnetic Resonance (NMR)

Question 1

What is NUCLEAR MAGNETIC RESONANCE (NMR)?

Answer 1

A physical phenomenon in which nuclei in a strong magnetic field are unsettled by a weak oscillating magnetic field and respond by producing an electromagnetic signal.

Question 2

What are the 4 STEPS in NMR?

Answer 2

1) Apply External Magnetic Field
2) Emit RF Pulse
3) Detect Sinusoidal Induced Current Caused by the Magnetic Moment
4) Relaxation

Question 3

What happens in the ‘APPLY EXTERNAL MAGNETIC FIELD’ stage of NMR?

Answer 3

• Protons, electrons, neutrons and therefore nuclei have spin that is the vector of its components (theoretical spin, not physical)
• The nuclei have a charge distribution and therefore an associated magnetic moment with spin
• Nuclei with an odd number of neutrons have spin -
• Clinical NMR MRI is at 1.5-3T

Question 4

What happens in the ‘EMIT RF PULSE’ stage of NMR?

Answer 4

• Hydrogen atoms have 2 spin states; ‘spin up’ (j=-1/2) and ‘spin down’ (j=1/2)
• Spin up is a lower energy state and is therefore the preferred state however, quantum states that not all nuclei can have the same state
• A radiofrequency pulse with a frequency that matches the Larmor frequency is fired perpendicularly to the magnetic field to produce resonance.
• The resonance causes the precessing nuclei to align in phase creating a net vector that is no longer vertical
• The resonance causes more nuclei to become anti-parallel so there is an equal number of parallel and anti parallel

Question 5

How does the MAGNETIC FIELD STRENGTH AFFECT THE ALIGNMENT of the nuclei in NMR?

Answer 5

The stronger the magnetic field the greater the difference between the number of parallel and anti-parallel nuclei, creating a greater net magnetic moment

Question 6

What is the ZEEMAN EFFECT?

Answer 6

Quantized energy states exist in the presence of an external magnetic field; E = -mγ h/2π B0
m = possible spin value
γ = gyromagnetic ratio
h = Planck’s Constant
B0 = magnetic field strength

Question 6

What is the LARMOR FEQUENCY?

Answer 6

AKA the processional frequency
The rate of precession of the net magnetic moment of nuclei around the external magnetic field
f = (γB)/(2π)

Question 7

What is RESONANCE?

Answer 7

When something experiences a wave with a frequency that matches their natural frequency the wave is absorbed and amplifies the objects frequency

Question 8

What happens in the ‘DETECT SINUSOIDIAL INDUCED CURRENT CAUSED BY THE MAGNETIC VECTOR’ stage of NMR?

Answer 8

• As the net vector is horizontal (equal number of parallel and antiparallel nuclei) but continues to process around the magnetic field
• The angle the net vector changes once the RF wave is introduced is known as the knock-angle (max of 90 degrees), it depends on the strength and how long the RF wave is pulsed.
• The rotating motion of the nuclei can be transformed into a sinusoidal wave

Question 9

What are the 2 KEY FLIP ANGLES used in MRI?

Answer 9

π/2 (90 degrees) - net vector transverse
π (180 degrees) - net vector aligns negative on the z axis (antiparallel)

Question 10

What happens in the ‘RELAXATION’ stage of MRI?

Answer 10

• Once the FR wave is shut off the vector returns back to its original, lower energy state
• As the vector has gone from a high energy state to a lower state the energy must be released, they are released in the form of radio waves
• T1 relaxation (longitudinal relaxation) - energy is lost to the surrounding
• T2 relaxation (Transverse relaxation) - spin-spin relaxation

Question 11

What is FREE INDUCTION DECAY (FID)?

Answer 11

With net transverse magnetization, from the perspective from the y-axis a sinusoidal wave is observed causing an induced sinusoidal current in the RF coil.
Through Fourier transforms the frequencies can be resolved
As the rate of precession is unique for different tissues the tissue can be identified based of this response.

Question 12

What is T1 RELAXATION?

Answer 12

• Longitudinal relaxation, quantified by the time constant T1 (63% of signal)
• The antiparallel return to, lower energy, parallel
• Caused by the interaction of spin with surrounding molecules - collisions/attraction/repulsion.
• Fat relaxes quickly (brighter)
• Water relaxes slower (darker)

Question 13

What is T2 RELAXATION?

Answer 13

• Transverse relaxation, quantified by time constant T2
• Spin-spin relaxation
• The time taken for the vectors to go out of phase in the transverse plane
• Bone/fat relaxes quicker (dark)
• Water relaxes slower (bright)

Question 14

What is APPARENT T2 (T2*)?

Answer 14

The experimentally measured FID
The FID signal decays quicker in practice due to the fluctuating magnetic field - harder to keep a constant field strength at high powers

Question 15

What are GRADIENT COILS?

Answer 15

• Separate coils that generate a weaker magnetic field that superimpose the primary field
• Distorts the primary field in a predictable pattern
• By applying a gradient in current over the length of the target we can use the distorted signal to locate the area being scanned.
• Creates a link between frequency and spatial location

Question 16

What is PHASE ENCODING?

Answer 16

The sampling of the signal in a sequence of defined phases
The combination of Phase and Frequency encoding datasets are known as K-space - used as the reconstruction dataset for MRI

Question 17

What EFFECTS THE RESOLUTION OF EACH AXIS in NMR?

Answer 17

X - frequency encoding
Y - phase encoding
Z - slice thickness

Question 18

How does the SIGNAL TO NOISE RATIO (SNR) EFFECT THE RESOLUTION?

Answer 18

High resolution - low SNR (increase in noise)
Low resolution - high SNR (less noise but can’t see small objects)

Question 19

What is IMAGE WEIGHTING?

Answer 19

The outcome of a pulse sequence designed to preferentially highlight one parameter over others
By setting certain parameters specific tissues can be highlighted
Weighting doesn’t suppress other parameters

Question 20

What are the PARAMETERS USED IN PROTON WEIGHTED MRI?

Answer 20

Long repetition time
Short echo time

Question 21

What are the PARAMETERS USED IN T1 WEIGHTED MRI?

Answer 21

Short repetition time
Short echo time

Question 22

What are the PARAMETERS USED IN T2 WEIGHTED MRI?

Answer 22

Long repetition time
Long echo time

Question 23

What is REPETITION TIME in MRI?

Answer 23

The time between corresponding consecutive points in a pulse sequence
Comparable to time pulse repetition frequency in ultrasound

Question 24

What is ECHO TIME?

Answer 24

The time between the center of the RF pulse and the signal response
Comparable to the time of flight in ultrasound

Question 25

What does PROTON DENSITY WEIGHTING DO?

Answer 25

• Seeks to minimize T1 and T2*
• Looks at density of hydrogen atoms only
• Marrow/fats are bright
• Bone are dark

Question 26

What does T1 WEIGHTING DO?

Answer 26

• Emphasizes fat and tissues with fast T1 relaxation
• Large flip angle
• Echo time must be short otherwise image will be too uniform
• Fluid is dark
• Fat is bright

Question 27

What does T2 WEIGHTING DO?

Answer 27

• Small flip angle
• Fluid is bright
• Fat is dark

Question 28

What is INVERSION RECOVERY PULSE SEQUENCE (IR)?
What is the INVERSION TIME?

Answer 28

• An inversion pulse followed by 90 degree pulse then another inversion pulse
• Allows for the T1 of different tissues to become separate
• Used to discriminate in favor of specific tissues
• STIR / FLAIR
• As this requires longer scan times there’s an increase in flow related artefacts
• Inversion time is the time between the initial inversion and the 90 degree pulse

Question 29

What is STIR IMAGING?

Answer 29

• Short T1 IR
• Suppresses fatty tissue response

Question 30

What is FLAIR IMAGING?

Answer 30

• Long T2
• Suppresses cerebrospinal fluid
• Fluid attenuated IR

Question 31

What AFFECTS THE CONTRAST of MRI images?

Answer 31

• Tissue dependent parameters (T1, T2, spin and proton density)
• Technical parameters (echo time, repetition time)

Question 32

What is FUNCTIONAL MRI (fMRI)?

Answer 32

Used to observe blood flow/brain activity
Captured using specific MRI techniques such as diffusion and perfusion
Has poor sensitivity requiring longer activation times

Question 33

What are the 3 CONFIGURATIONS OF MRI?

Answer 33

• Open bore
• Closed bore
• Extremity MRI