MRI Flashcards
Which planes do gradient coils allow spatial encoding in and what plane of images do they produce?
x axis : runs along horizontal axis - sagittal images
y axis: runs along vertical axis - coronal images
z axis: runs along long axis - axial images
Identify the pulse sequence
Conventional Spin echo
Identify the pulse sequence
Fast Spin echo
Identify the pulse sequence
Inversion Recovery (Spin Echo)
Identify the pulse sequence
Dual echo spin echo sequence
Identify the pulse sequence
FLAIR
Identify the pulse sequence
FLAIR
What is an MRI pulse sequence?
An MRI pulse sequence is a programmed set of changing magnetic gradients.
How are MRI sequences grouped?
MRI sequences can be grouped according to the type of sequence (e.g. spin echo) or by general image weighting (e.g. T1 or T2) and additional features (e.g. fat suppressed or gadolinium enhanced).
What is the purpose of multiple sequences in MRI?
Multiple sequences are needed to adequately evaluate a tissue, and the combination of sequences is referred to as an MRI protocol.
What is Time to Echo (TE) in MRI pulse sequences?
The echo time (TE) refers to the time between the application of the radiofrequency excitation pulse and the peak of the signal induced in the coil. It is measured in milliseconds. The amount of T2 relaxation is controlled by the TE.
What is Time to Repetition (TR) in MRI pulse sequences?
The repetition time (TR) is the time from the application of an excitation pulse to the application of the next pulse. It determines how much longitudinal magnetisation recovers between each pulse. It is measured in milliseconds.
What are Spin-echo pulse sequences?
Spin-echo pulse sequences are one of the earliest developed and still widely used of all MRI pulse sequences. They can be adjusted to give T1-weighted, proton density, and T2-weighted images.
What is Flip Angle in MRI pulse sequences?
The flip angle is an MRI phenomenon by which the axis of the hydrogen proton shifts from its longitudinal plane to its transverse plane by excitation with radiofrequency pulses.
What are Inversion Recovery Pulse Sequences?
Inversion recovery pulse sequences are used to selectively null the signal for certain tissues and can generate heavily T1-weighted images. They involve a preparatory 180° RF pulse to invert longitudinal magnetisation.
What is Short tau inversion recovery (STIR)?
Short tau inversion recovery (STIR) is a fat suppression technique with an inversion time that nulls the signal of fat. It cannot be used with gadolinium injection due to T1 shortening effects on tissues.
What is Fluid attenuated inversion recovery (FLAIR)?
Fluid attenuated inversion recovery (FLAIR) is a special inversion recovery sequence with a long inversion time to remove signal from cerebrospinal fluid in the resulting images.
What does the FLAIR pulse sequence do?
removes signal from the cerebrospinal fluid in the resulting images
Brain tissue on FLAIR images appears similar to T2 weighted images with grey matter brighter than white matter but CSF is dark instead of bright.
How is the signal from fluid nulled in the FLAIR pulse sequence?
The inversion time (TI) of the FLAIR pulse sequence is adjusted such that at equilibrium there is no net transverse magnetisation of fluid.
What is the purpose of the FLAIR sequence in brain imaging?
It is particularly useful in the detection of subtle changes at the periphery of the hemispheres and in the periventricular region close to CSF.
(Infarction, Multiple Sclerosis, Subarachnoid haemorrhage, head injuries)
What is Phase-sensitive inversion recovery (PSIR) in MRI?
refers to an inversion recovery MRI pulse sequence that accounts for the positive and negative polarities and preserves the information of tissue magnetisation during the recovery from the initial 180° inversion pulse and factually doubles the dynamic range.
What is Diffusion-weighted imaging (DWI) based on?
measuring the random Brownian motion of water molecules within a voxel of tissue.
What is the fundamental idea behind diffusion-weighted imaging?
The attenuation of T2* signal based on how easily water molecules are able to diffuse in that region.
What are the key parameters for T1 MRI Pulse Sequence?
TR: <800
TE: <30
flip angle: 90°
What are the key parameters for T2 MRI Pulse Sequence?
TR: >2000
TE: >80
flip angle: 90°
What are the key parameters for FLAIR MRI Pulse Sequence?
TR: >3000
TE: >80
flip angle: variable
inversion time: 1700–2200
What properties do nuclei need to have for MRI?
Spin
charge
What are good MR nuclei?
1H, 13C, 19F, 23Na, 31P
What are biological tissues predominantly made of?
12C, 16O
What are nuclei made of in MRI?
Nuclei are made of protons and neutrons
Both have spin ½. Protons have charge.
Which atoms have spin in MRI?
Only atoms with an odd number of protons or neutrons have spin
Good MR nuclei are 1H, 13C, 19F, 23Na, 31P.
What is the most abundant atom in the body for MRI?
Hydrogen is the most abundant atom in the body
The majority of hydrogen is in water (H2O). Essentially all MRI is hydrogen (proton) imaging.
What does a moving charged particle generate in MRI?
A moving (spinning) charged particle generates its own little magnetic field
Spinning particles with mass have angular momentum. Angular momentum resists attempts to change the spin orientation.
What is the purpose of RF electromagnetic fields in MRI?
RF electromagnetic fields are used to manipulate the magnetization of specific types of atoms
Some atomic nuclei are sensitive to magnetic fields and their magnetic properties are tuned to particular RF frequencies.
What is T1 relaxation in MRI?
T1 relaxation – Flipped nuclei realign with the magnetic field
What is T2 relaxation in MRI?
T2 relaxation – Flipped nuclei start off all spinning together, but quickly become incoherent (out of phase)
What is T2* relaxation in MRI?
T2* relaxation – Disturbances in magnetic field (magnetic susceptibility) increase the rate of spin coherence T2 relaxation
What are the relaxation times used for image contrast in MRI?
Different tissues have different relaxation times. These relaxation time differences can be used to generate image contrast
T1 - Gray/White matter. T2 - Tissue/CSF. T2* - Susceptibility (functional MRI). Relaxation time in milliseconds.
What are some safety concerns in MRI?
Safety in Magnetic Resonance Imaging. MRI may pose significant hazards to patients and staff due to: 1. Electromagnetic fields. 2. The auditory effects of noise. 3. Inert gas quench. 4. Claustrophobia. 5. I.V. contrast agent.
What are the significant hazards to patients and staff during MRI?
- Electromagnetic fields
- The auditory effects of noise
- Inert gas quench
- Claustrophobia
- I.V. contrast agent
These are potential hazards that may arise during Magnetic Resonance Imaging.
What should be ensured before performing MRI on a patient?
Ensure a detailed history of the patient is carried out to check for any ferromagnetic implants such as pins in broken bones or cardiac pacemakers.
This precaution is necessary to avoid any complications during the MRI procedure.
What happens to radiofrequency above 10MHz during an MRI examination?
It will be converted to heat, which will be distributed by connective heat transfer through blood flow.
This explains the process of converting radiofrequency to heat during an MRI.
What guidelines are created when using MRI to limit temperature rise?
Guidelines are created to limit the rise in temperature of the skin, body core, and local tissues to levels lower than those that may cause tissue damage (thermal).
These guidelines help ensure patient safety during MRI procedures.
Why is there noise during an MRI scan?
The noise is due to vibration in the gradient coils and other parts of the scanner caused by varying magnetic fields.
This explains the source of noise during MRI scans.
What is the noise level that may be achieved during an MRI scan?
Up to 95dB.
This noise level is due to the operation of the MRI scanner.
How is the MRI unit super cooled?
By using liquid helium and nitrogen.
This cooling method is essential for the operation of the MRI unit.
What should be done to prevent severe frost bites in case of a breakdown in the MRI unit?
Install a well-ventilated room with an oxygen monitor to raise an alarm.
This precaution helps prevent severe frost bites in case of a breakdown.
What precaution should be taken when entering the control area of the MRI suite?
Remove all ferromagnetic objects before entering.
This precaution is necessary to avoid interference with the MRI equipment.
