MRI102 Week2 Ch1 Assignment

What is the Net Magnetic Vector (NMV), and why is it important in MRI? 

The net magnetization vector (NMV) represents the overall magnetic orientation of excess nuclei that haven't cancelled each other out. These are the nuclei we utilize for MRI scans.

How does tissue-specific NMV affect image contrast in MRI? 

Each tissue's NMV reacts differently and emits a slightly different signal. These differences allow us to distinguish between tissues when generating an image.

What happens to the magnetic moments of hydrogen nuclei in a strong magnetic field? 

In a strong magnetic field (B0), hydrogen nuclei precess. Precession only occurs when nuclei are exposed to B0, such as when a patient enters the MRI scanner room

What is precession, and when does it occur in MRI? 

Precession is an additional motion where nuclei wobble in the direction of B0, or around the axis of B0.

Define precessional frequency and identify its units. 

Precessional frequency is the rate at which nuclei precess or wobble.

The units for precessional frequency are megahertz (MHz), indicating millions of rotations per second.

What is the Larmor equation, and what relationship does it describe? 

The Larmor equation defines the relationship between precessional frequency, the gyromagnetic ratio of hydrogen, and B0:

PrecessionalFrequency=GyromagneticRatio∗B0

This equation shows that precessional frequency has a direct relationship with B0, given that the gyromagnetic ratio is constant.

What is the gyromagnetic ratio for hydrogen, and why is it significant in MRI? 

The gyromagnetic ratio of hydrogen is a constant value of 42.5 MHz/Tesla.

It represents the relationship between B0 and the angular momentum of the nuclei.

(Angular momentum results from combining all three motions of an atom.)

The gyromagnetic ratio is significant because it is part of the Larmor equation, which is used to calculate precessional frequency.

Calculate the Larmor frequency at 1.5T. 

63.87 MHz.

What is the difference between being “in phase” and “out of phase” in the context of hydrogen nuclei? 

• All nuclei in the body are initially out-of-phase, meaning they are oriented in random directions.

• When exposed to B0B0​, nuclei precess (wobble), but remain out of sync with each other

• RF (radio frequency) pulse:

  • Causes nuclei to transition from out-of-phase to in-phase.

  • Nuclei begin wobbling together as a single unit.

Define Frequency. 

Frequency is the rate at which something repeats itself

Explain the concept of resonance in MRI. 

Definition:

Resonance is the process of matching the RF pulse's frequency with the precessional frequency of the nuclei.

• Purpose:

  • Allows for efficient transfer of energy into the nuclei.

  • When energy is transferred in, the same amount of energy is echoed back out.

What must be true about the RF energy in order for resonance to occur? 

• The RF energy must be transferable, meaning the RF pulse must be able to deposit its energy into the nuclei.

• Transferability is achieved when the frequency of the RF pulse matches the precessional frequency of the nuclei.

What happens to the NMV after excitation? 

• The net magnetization vector (NMV) is initially in the longitudinal plane (vertical).

• After excitation with an RF pulse, the NMV tilts into the transverse plane (horizontal).

• This occurs because the NMV gains enough energy to overcome the pull of B0 and move out of alignment.

What is the Flip Angle, and what does a 90-degree flip angle represent? 

The flip angle is the degree to which the NMV (Net Magnetization Vector) is rotated from the longitudinal plane into the transverse plane by the RF pulse.

A 90-degree flip angle is often used, positioning the NMV entirely in the transverse plane.

Explain the longitudinal and transverse planes in MRI.  

The longitudinal plane is the direction of the B0 field (main magnetic field).

The transverse plane is perpendicular to the B0 field

Describe how an MR signal is produced following resonance. 

A strong signal is produced when nuclei are in phase in the transverse plane.

What is the function of the receiver coil in the MRI system? 

Here's how the MRI signal is produced:

1. Energy is transmitted into the patient via the RF pulse.

2. Resonance is achieved; nuclei absorb energy.

3. Nuclei tilt into the transverse plane and become in phase.

4. The coil detects the echo (RF pulse) from the nuclei in the transverse plan.

5. The echo induces a voltage within the coil.

   It is crucial that the nuclei are in phase in the transverse plane for the coil to detect a signal.

Using Faraday’s Law, explain how the MR signal is detected. 

According to Faraday's Law, a moving electromagnetic field induces a current in a conductive medium (the coil).

1. The NMV emits an echo, which is a radio frequency pulse.

2. This moving electromagnetic energy induces a current inside the coil (copper wire).

3. A voltage is then induced in the coil.

What is Free Induction Decay (FID)? 

FID occurs immediately after excitation and is a random burst of energy. It is caused by the decay of the nuclei space combined with the inhomogeneity of the B0 field.

Describe the two processes that occur during relaxation after the RF pulse ends.

Nuclear magnetic relaxation involves two simultaneous processes:

The energy absorbed by the nuclei is released. NMV returns to the longitudinal plane

Nuclei lose their phase coherence and become incoherent.