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