Term
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Definition
| Number of excitations - repeating identical signal acquisitions for the purpose of signal averaging. |
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Term
| How is the acquisition time determined? |
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Definition
| Time = TR x #PEG steps x NEX |
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Term
| If the K-space is not square, which direction is usually the smallest and why? |
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Definition
| The phase encode gradient is typically smaller and this is used to reduce acquistion time. |
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Term
| What is multislice data acquisition? |
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Definition
| Several slices are selectively excited in a sequential timing scheme during the TR interval. |
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Term
| Describe 3 other methods of filling the K-space. |
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Definition
Centric filling - filliing central rows first
Keyhole filling - outer rows are filled first, then the center rows are filled when an event like contrast occurs.
Spiral filling - starts at the center of K-space and spirals outward. |
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Term
| What is propeller imaging? |
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Definition
| Propeller - Periodically rotated overlapping parallel lines with enchanced reconstruction. A wide rectangle at the center is filled first, then it is rotated about the center. Repeat filling and rotating. This causes overlapping information at the center of K-space. |
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Term
| What is parallel imaging? |
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Definition
| A technique that fills k-space by using the response of multiple receive RF coils that are coupled together with independent channels. |
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Term
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Definition
| SENSitivity Encoding - using a measured sensitivity profile for each coil used in parallel imaging to determine where the signal is coming from. |
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Term
| What are common image matrix sizes in MRI? |
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Definition
| 512x256, 512x512, and 1024x512 |
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Term
| What are common pixel sizes in MRI? |
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Definition
0.5 to 1 mm for large FOV
0.1 to 0.2 for limited FOV |
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Term
| Typical slice thickness in MRI. |
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Definition
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Term
| What effects do TR, TE, and flip angle have on SNR? |
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Definition
Longer TR increases SNR.
Longer TE reduces SNR.
Smaller flip angle reduces SNR. |
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Term
| For a given pulse sequence, what is SNR proportional to? |
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Definition
SNR ≈ Voxelx,y,z x √NEX / √BW
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Term
| What is the voxel volume equal to? |
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Definition
| Voxel Volume = (FOVx / No. Pixelsx) x (FOVy / no. Pixelsy) x (slice thickness, z) |
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Term
| SNR is _____ to voxel volume. Doubling the matrix size over the same FOV will _______ the SNR. Doubling the FOV with the same matrix size will _____ the SNR. |
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Definition
SNR proportional to voxel volume.
Double the matrix size - reduce SNR by half.
Double the FOV - double the SNR. |
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Term
| SNR is proportional to the _____ of NEX. Doubling the NEX will ______ the SNR. |
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Definition
square root of the NEX.
Double NEX - increase SNR by √2 -> 40%. |
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Term
| SNR is proportional to the _______ of the receiver bandwidth. A narrow BW results in a ______ SNR. |
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Definition
inverse square root
higher |
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Term
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Definition
Time to sample the signal, ΔT
ΔT = 1 / (RF BW) |
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Term
| Effect of magnetic field strength on SNR. |
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Definition
SNR ≈ B1 to B1.5
Double the field strength results in 2-3 times the SNR. |
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Term
| Fringe fields below _____ are considered safe. |
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Definition
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Term
Field uniformity is expressed in _____.
Typical values range from ____ to _____. |
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Definition
parts per million (ppm)
< 1 ppm to 10 ppm |
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Term
The most common bioeffect of MR is ________.
This is caused by _______. |
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Definition
tissue heating.
Caused by RF energy deposition and/or by rapid switching of high strength gradients. |
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Term
What does SAR stand for?
Give a value. |
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Definition
Specific Absorption Rate
< 4 W/kg for the whole body average |
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Term
| How many MRI incidents are in TJC Sentinel Event Database? How many resulted in death? |
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Definition
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Term
| What are the main categories of injuries found in MRI? At what percentage? |
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Definition
1. Burns (most common, > 70%)
2. Projectiles (10%)
3. Electronic Device Malfunctions |
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Term
| Electronic devices that can malfunction due to MRI fields include |
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Definition
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Term
| MRI related burns come from |
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Definition
| wires, leads, implants, staples, clamps |
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Term
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Definition
| ferromagnetic flying objects like IV poles and O2 tanks as well as clips moving in the brain and fragments moving in the eye. |
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Term
| What 4 actions are taken to ensure MRI safety: |
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Definition
1. controlling access
2. posting warning signs
3. removing ferromagnetic objects
4. screening patients |
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Term
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Definition
Zone 1 - general public
Zone 2 - reception and changing areas
Zone 3 - restricted area (control room) - trained MR personnel and screened patients only
Zone 4 - the magnet room |
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Term
| MRI warning signs include: |
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Definition
1. Magnetic is always ON
2. Projectile hazard
3. Danger to those with implants (pacemakers) |
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Term
| Screening MRI patients involves: |
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Definition
| Questionairre regarding previous surgeries, implants, and foreign metal objects |
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Term
| What is important for safety in MRI patient positioning? |
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Definition
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Term
| How does Fast Spin Echo Imaging work? |
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Definition
| A series of 180 degree refocusing RF pulses follows the 90 degree excitation pulse. |
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Term
| What is ETL? What does it do to the acquisition time? |
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Definition
Echo Train Length - Used in Fast Spin Echo
ETL is the number 180 degree refocusing RF pulses used per TR interval. It reduces the acquisition time by a factor of ETL. |
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Term
| What happens to the echo size during Fast Spin Echo. |
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Definition
| Each echo in the Echo Train gets smaller due to T2 relaxation. |
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Term
| What is Echo Planar Imaging? |
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Definition
| A single RF excitation pulse is used. The FEG is continuously cycled to acquire echos. Usually a small matrix is used. This is very fast. Mainly used in diffusion imaging. |
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Term
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Definition
1. increases with the square of the magnetic field
2. increases with the square of the RF flip angle
3. increases linearly with shorter TR
4. increases with the number of RF pulses per cycle
5. increases with patient size |
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Term
| Gradient field bioeffects include |
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Definition
peripheral nerve stimulation (PNS)
sound pressure (noise) levels |
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