midterm 2

Question 1

are loops of wire or thin conductive sheets on a cylindrical shell lying just inside the bore of an MR scanner. When current is passed through these coils a secondary magnetic field is created

Answer 1

Gradients

Question 2

slightly distorts the main magnetic field in a predictable pattern, causing the resonance frequency of protons to vary in as a function of position

Answer 2

gradient field

Question 3

primary purpose of the gradient system in MRI is to create ___ in the magnetic field. This enables the MRI scanner to localize signals from specific locations within the body and create detailed images.

Answer 3

spatial variations

Question 4

Types of gradients:

Answer 4

• X-Gradient (Slice selection)
• Y-Gradient (Frequency encoding)
• Z-Gradient (Phase encoding)

Question 5

critical component of the MRI system that plays a key role in generating the RF pulses used to excite the protons in the body.

Answer 5

Radio frequency (RF) transmitter

Question 6

• a specific type of receive RF coil.
• typically placed directly on or near the region of the body being imaged to capture the RF signals emitted by the protons.
• highly sensitive to the tissue immediately beneath them, allowing for high signal-to-noise ratio (SNR) and better image quality in localized areas.

Answer 6

Surface coil

Question 7

process of reducing magnetic field inhomogeneities.

Answer 7

Magnetic shimming

Question 8

2 main types of shimming:

Answer 8

passive and active (sometimes both may be used)

Question 9

uses Shim coils, which have electric currents directed through them to generate their own magnetic field cancelling the remaining field gradients in the main magnetic field

Answer 9

active shimming

Question 10

uses ferromagnetic pellets or pieces of sheet metal attached to the MRI scanner bore

Answer 10

passive shimming

Question 11

used to cool the superconducting magnets that generate the strong magnetic fields necessary for MRI imaging

Answer 11

cryogens

Question 12

Main Cryogens Used in MRI:

Answer 12

Liquid Helium (He-4):
Liquid Nitrogen (N₂):

Question 13

• most commonly used cryogen in MRI systems, particularly for cooling the superconducting magnets.
• Boiling point of ____, which is low enough to keep superconducting magnets at the temperatures required for them to operate in a superconducting state.

Answer 13

Liquid Helium (He-4)
4.2 K (-452.0°F / -268.9°C)

Question 14

• used as a secondary cooling agent, typically for pre-cooling the magnet before the liquid helium is introduced
• boiling point of ____\, which is significantly higher than that of liquid helium but still cold enough to help cool the superconducting magnet.

Answer 14

Liquid Nitrogen (N₂)
77 K (-195.8°F / -196.2°C)

Question 15

• Used to control the computer.
• computer initiates the appropriate radio-wave transmissions and then receives and analyzes the data.
• independent diagnostic workstation may be used to perform the same function as those of the operator’s console, depending on system configuration. However, usually only the operator’s console can control the actual imaging process.

Answer 15

Console

Question 16

houses the electronics necessary for transmitting the radio-wave pulse sequences and for receiving and analyzing the MRl signal. The raw data and the computer-constructed images can be stored on a computer di k temporarily but are usually transferred to a magnetic tape or an optical disk for permanent storage and retrieval.

Answer 16

Computer Room

Question 17

influences the quality and clarity of the resulting images. It refers to the ratio of the desired signal (the true information, such as tissue details) to the background noise (unwanted random variations that reduce image clarity)

Answer 17

Signal-to-noise Ratio

Question 18

represents the strength of the information coming from the tissues being imaged. It is directly related to factors like tissue properties, magnetic field strength, and the pulse sequence used.

Answer 18

Signal (S)

Question 19

represents random variations in the signal that are not related to the tissue properties but come from various sources, including thermal noise in the MRI equipment, electronic noise, or even patient movement.

Answer 19

Noise (N)

Question 20

Pulse Sequences

Answer 20

• Spin Echo (SE) Sequence
• Gradient Echo (GRE) Sequence
• Inversion Recovery (IR) Sequence
• Echo Planar Imaging (EPI) Sequence
• Diffusion-Weighted Imaging (DWI) Sequence

Question 21

Single RF pulse generates a free induction decay (FID), but two successive RF pulses produce a ____.

Answer 21

spin echo (SE)

Question 22

The time between the middle of the first RF pulse and the peak of the spin echo is called the ____.

Answer 22

echo time (TE)

Question 23

pulse sequence consists of 90°-pulse, a 180°-pulse, and then an echo.

Answer 23

Spin Echo

Question 24

The time between the middle of the first RF pulse and the peak of the spin echo is called the echo time (TE). The sequence then repeats at time _____

Answer 24

TR time repetition

Question 25

It has a sequence of 180° — {90°−180°−echo}

Answer 25

- Inversion Recovery Pulse Sequence - Think of it as Spin Echo na 90-180-echo (or pulse, pulse, echo) pero dinagdagan pa ng isang 180 degree pulse sa una. So, pulse, pulse, pulse, echo siya.

Question 26

The time between the 180° inverting pulse and the 90°-pulse is called the ___. The repetition time (TR) and echo time (TE) are defined as they are for spin echo

Answer 26

inversion time (TI)

Question 27

a variant of inversion recovery sequence where cerebrospinal fluid (CSF) is suppressed to better visualize lesions in the brain.

Answer 27

- FLAIR (Fluid Attenuated Inversion Recovery) - often T2-weighted, as they highlight areas with increased water content while suppressing CSF

Question 28

- manipulation of the FID using a bipolar gradient - First gradient lobe dephases the FID while second lobe rephases it a later time, generating the

Answer 28

Gradient Echo (GRE)

Question 29

fastest acquisition method in MRI (100 ms / slice), but with limited spatial resolution.

Answer 29

EPI (echo planar imaging)

Question 30

* Measures the diffusion of water molecules in tissue. * Particularly useful in detecting early changes in stroke or cancer - image contrast is determined by the random microscopic motion water protons. It measures the motion of spins (specifically in water). The signal is dependent on the diffusion coefficient within the material i.e. how freely the water can diffuse. The more a particle can move in a given amount of time, the higher the diffusion coefficient.

Answer 30

Diffusion-Weighted Imaging (DWI) Sequence