MRI Signal Production (First Part - until receiver bandwidth)

Transmitter bandwidth

- Is the bandwidth used during the transmission of RF pulses - Used to excite a discrete slice thickness or volume

inversely

- Receiver bandwidth and sampling time are ________ proportional. Increasing the bandwidth shortens the sampling time. It however reduces the signal amplitude and increases the noise level in pixels resulting in a significant reduction in the signal-to-noise (SNR) ratio of the image

Slice selection gradient

- has gradually increasing magnetic field strength from one end to another - It determines the slice position.

Free Induction decay (FID) Gradient echo (GRE) Spin echo (SE) Stimulated echo

4 types of MR Signal

magnetic dipole moment

All nuclei have a spin; those with an odd number of protons and/or neutrons will have a property called a ___________

cycles per second or Hertz

Bandwidth is measured in ___________

Low SAR

Best to choose when scanning neonates and sedated patients due to their inability to feedback the operator - When scanning pregnant patients to avoid any potential RF harm to the fetus

SNR

Decreasing the bandwidth increases the ________. This method can be used to boost the SNR in high resolution scans with very small FOV.

bandwidth

Defined as the amount of frequencies or wavelengths that can be transmitted or received in a limited amount of time

MR Spectroscopy

Free induction decay does not contribute for MR image but is used for ___________

chemical shift artifacts

Higher receiver bandwidth reduces __________

metallic susceptibility artifacts

Higher receiver bandwidth sequences are also effective in reducing ___________.

blurry images

Increasing the receiver bandwidth can be a method to produce less ______ as it reduces echo spacing. In TSE sequences, higher bandwidth increases the turbo factor due to short echo spacing. This increase in turbo factor results in higher fat signal in TSE scans.

claustrophobic

Increasing the receiver bandwidth shortens the scan time by allowing shorter TR and TE values which is helpful for ___________ and moving patients.

Net Magnetization (M)

Is split into two, perpendicular components

Receiver Bandwidth

Is the range of frequencies used during the reception of RF pulses

nuclei, external magnetic field

Joseph Larmor (1857- 1942) developed an equation that shows the relationship between a ______ and an _______

Joseph Larmor

Larmor Frequency is named after the Irish physicist ________ (1857- 1942)

small parts

Lower bandwidths will reduce the minimal field of view which can be used to scan _________

Fourier Transformation

MR signals are transformed into MR image by computers using mathematical methods such as __________________

Transmitter bandwidth Receiver bandwidth

MRI is designed with two types of bandwidths:

Free Induction decay (FID) - 1 RF Pulse Gradient echo (GRE) -1 RF Pulse + gradient reversal Spin echo (SE) - 2 RF Pulse Stimulated echo - 3 or more RF Pulses

Method of formations in Free Induction decay (FID) Gradient echo (GRE) Spin echo (SE) Stimulated echo

Free Induction Decay

One of the four basic types of MR signal produced in different ways

transverse magnetization

Only __________ is received by coil antennas 90deg RF pulse turns the longitudinal magnetization into transverse magnetization Stronger transverse magnetization creates a stronger MR signal

Fast

Reduce scan times in claustrophobic and moving patients - Will reduce the TR and TE values significantly allowing the user to reduce the TR and TE values manually, reducing scan time. - Most appropriate option to select when scanning areas with metal implants because it can significantly reduce susceptibility and distortion artifacts

Free Induction Decay

Signal that appears immediately after a 90° pulse - Is a damped oscillation at the resonance frequency recorded when the net magnetization is tipped into the transverse plane

RF pulses

Slice thickness is determined by the bandwidth of ________.

RF transmitting RF receiving

The RF pulse consists of the following RF coils:

higher

The ________ the transmitter bandwidth, the higher the lowest possible slice thickness

TM vector

The __________ starts reducing in its magnitude immediately after its formation because of dephasing of protons.

flip angle

The angle of the pulse is called the ___________

gradient coils

The gradient fields are produced by coils called as __________.

directly

The lowest possible slice thickness of a pulse sequence is ________ proportional to the transmitter bandwidth of the RF pulse type

MR signal

The receiver RF coil receives this current as _____________.

transverse magnetization

The strength of the signal is proportional to the magnitude of the ___________

• Slice selection gradient • Phase encoding gradient • Frequency encoding (read out) gradient

The three gradients are:

"gradient fields"

These magnetic fields have different strength in varying location hence these fields are called _______ or simply "gradients"

X, Y, and Z

Three more magnetic fields are superimposed on the main magnetic field along _______ axes to localize from where in the body signals are coming.

Low specific absorption rate (SAR) Normal Fast

Three types of RF pulses (based on Siemens)

precession frequency

Transverse magnetization vector has a __________ It constantly rotates at Larmor frequency in the transverse plane and induces electric current

1. Excitation 2. Relaxation

What are the two parts of MR measurement

Reducing

________ the bandwidth will increase the TR, TE and echo spacing >> increase in scan time, chances of chemical shift, distortion and susceptibility artifacts

Magnetic moment

_________ is characterized by its alignment with an external magnetic field analogous to a small bar magnet. These particular nuclei are also called dipoles because they have two poles like the north and south pole of a bar magnet.

Bandwidth

_________ is the range of frequencies. The wider the bandwidth, thicker is the slice

Transverse Magnetization (Mxy)

along the x-y axis of the magnetic field The net magnetization is the source of MR signal

Longitudinal Magnetization (Mz)

along the z-axis of the magnetic field

RF receiving

antenna for receiving MR signal

RF transmitting

antenna for sending RF pulses

Free induction Decay (FID)

describes how transverse magnetization decays quickly over time

Transmitter bandwidth

describes the properties of transmitted radiofrequency pulses

Receiver bandwidth

describes the quality of MR signal

Excitation

energy in

Relaxation

energy out

90° pulses

flips magnetization in the transverse direction (xy-plane)

180° pulses

flips the magnetization in the longitudinal direction (z-plane)

Fast

higher bandwidth pulse

Low specific absorption rate (SAR)

low bandwidth pulse

Normal

medium bandwidth pulse

90° pulses 180° pulses

two types of standard RF pulses