Term
TR determines the amount of ___ weighting
Flip Angle controls the amount of ___ weighting
TE controls the amount of __ weighting |
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Definition
TR = T1 & PD
Flip Angle = T1 & PD
TE = T2 |
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Term
| The four main considerations determining image quality are: |
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Definition
1. SNR (signal to noise ratio)
2. CNR (contrast to noise ratio)
3. Spatial Resolution
4. Scan Time |
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Term
| The signal to noise ratio is___ |
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Definition
| The ratio of amplitude of the signal received to the average amplitude of the noise |
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Term
| The ___ is the voltage induced in the receiver coil by the precession of the NMV in the transverse plane |
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Definition
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Term
| The __ represents frequencies that exist randomly in space and time |
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Definition
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Term
| Name 3 factors that contribute to noise |
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Definition
1. Build of the patient
2. the area under examination
3. inherent noise of the system |
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Term
| Increase the signal will ____ the SNR |
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Definition
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Term
| List a few factors that affect the SNR |
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Definition
1. magnetic field strength of the system (B0)
2. PD of the area under examination
3. Voxel volume
4. TR, TE, & Flip Angle
5. NEX
6. Receive Bandwidth
7. Coil Type |
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Term
| Increase the magnetic field strength = ____ SNR |
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Definition
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Term
| Areas with high PD (increse PD) = ___SNR |
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Definition
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Term
| The ___ of the pixel represents the strength of the MRI signal generated by a unit of volume of patient tissue (Voxel) |
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Definition
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Term
| Pixel Area = ____ / _____ |
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Definition
| Pixel area = FOV / Matrix size |
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Term
| a ___ matrix is one with a low number of frequency and/or phase encodings and results in a low number of pixels in the FOV |
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Definition
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Term
| a ___ matrix is one with a high number of frequency and/or phase encodings and results in a large number of pixels in the FOV |
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Definition
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Term
Fine Matrix = __pixels and voxels
Coarse matrix = __ pixels and voxels
Which has a higher SNR |
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Definition
Small ; Large
Large voxels (coarse matrix) have a higher SNR |
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Term
| Any selection that decreases the size of the voxel in turn decreases the SNR and vise versa, this is acheived in 3 ways |
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Definition
1. Changing the slice thickness
2. Changing the image matrix
3. Changing the FOV |
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Term
Thicker slice has __ SNR
Thinner slice has __ SNR |
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Definition
Thicker = more SNR
Thinner = less SNR |
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Term
Large voxel volumne (coarse matrix) = __SNR
Small Voxel volumne (fine matrix) = ____SNR |
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Definition
Large voxel volumne = more SNR
Small voxel volumne = less SNR |
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Term
Large FOV = __SNR
Small FOV = ___SNR |
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Definition
Large FOV = more SNR
Small FOV = less SNR |
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Term
FYI:
TR, TE, & flip angle influence image contrast, but also influence the SNR |
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Definition
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Term
| Spin Echo Pulse Sequences generally have ___ SNR than Gradient Echo Pulse Sequence |
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Definition
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Term
| The flip angle controls that amount of ___. The maximum signal amplitude is created with a flip angle of __° |
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Definition
| Transverse Magnetization ; 90° |
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Term
| The TR controls the amount of ___ magnetization that is allowed to recover before the next excitation pulse is applied. |
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Definition
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Term
A __ TR allows Full recovery of the longitudinal mag so that more is available to be flipped in the next repetition. a __ TR does NOT allow full recovery of longitudinal mag, so less is available to be flipped.
Long (increased) TR = __SNR
Short (decreased) TR = __SNR |
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Definition
Long ; Short
Increase TR = More SNR
Decrease TR = Less SNR |
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Term
The TE controls the amount of __ magnetization that is allowed to decay before an echo is collected.
Long (increased) TE = __ SNR
Short (decreased) TE = __SNR |
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Definition
Transverse
Increase TE = Less SNR
Decreased TE = More SNR |
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Term
Summary:
A Long TR increases SNR and a short TR reduces SNR
A Long TE reduces SNR and a short TE increases SNR
The lower the flip angle, the lower the SNR (90° is optimal) |
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Definition
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Term
| ___ is random and is in a different position each time data is stored, __ is not random and always occurs in the same place when it is collected |
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Definition
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Term
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Definition
Increase
However, it is not the best way to increase SNR because it doubles the scan time while only increasing the SNR by 1.4 times. In order to double the SNR with NEX, we would need to increase the NEX by 4x which would greatly lengthen the scan time and allow for more pt movement |
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Term
| Receive Bandwidth is the range of frequencies that are sampled during the readout gradient. Decreasing the receive bandwidth results in less ___ being sampled relative to signal. |
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Definition
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Term
| Halving the bandwidth increases the SNR by __%. But is also increase the Minimum TE available. |
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Definition
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Term
| To induce maximum signal, the coil must be positioned in the ___ plane perpendicular to B0 |
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Definition
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Term
Summary:
To acheive the highest possible SNR,
-use spin echo pulse sequences where possible
-try NOT to use a very short TR & very long TE
-Use the correct coil and ensure it is well positioned
-use a coarse matrix
-use a Large FOV
-select thick slices
-use as many NEX as possible |
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Definition
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Term
| The ___ is defined as the difference in the SNR between two adjacent areas. |
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Definition
| Contrast to Noise Ratio (CNR) |
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Term
| CNR is controlled by the same factors that affect SNR. The ___ is the most critical factor affecting image quality. |
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Definition
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Term
| CNR is controlled by the same factors that affect SNR. The ___ is the most critical factor affecting image quality. |
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Definition
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Term
| CNR can be increased by... |
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Definition
1.Using a T2 weighted Image
2.Using Contrast agents
3.Using chemical pre-saturation techniques
4.Using Magnetization transfer contrast (MTC) |
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Term
| The ___ is the ability to distinguish between two points as separate and distinct and is controlled by the Voxel Size |
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Definition
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Term
| List three things that affect the voxel sie |
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Definition
1. Slice Thickness
2. Matrix
3. FOV |
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Term
| ___ voxels give better spatial resolution |
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Definition
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Term
| In large voxels, individual signal intensities are averaged together and not represented as distinct within the voxel. This results in ____ |
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Definition
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Term
If you want to increase spatial resolution you need to
___ slice thickness
___Matrix
___FOV |
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Definition
Thinner slice thickness
Fine Matrix
Small FOV |
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Term
| Square pixels always provide better ___ than rectangular pixels |
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Definition
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Term
| ___ is the time to complete data acquisition or the time to fill kspace |
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Definition
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Term
| The ___ of a gradient is the time required for it to acheive the correct slope. |
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Definition
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Term
| Steep Gradient slopes have a ___ rise time than Shallow Gradient slopes |
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Definition
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Term
| For good spatial resolution you need thin slices, fine matrix, and small FOV, what kind of gradient slope does each of these require? |
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Definition
Thin Slice = Steep slice select gradient
Fine Matrix = High proportion of Steep Phase Encoding Gradient
Small FOV = Steep Frequency encoding gradient |
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Term
To acheive the shortest scan time possible:
use the __TR
select the __matrix
__the NEX |
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Definition
Shortest TR
Coarsest matrix
Minimum NEX |
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Term
| There are good tables on Pg. 135-137 |
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Definition
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Term
| ____ is adventgeous in that very small lesions can be demonstrated, because there is no slice gap and the slice thickness can be drastically reduced |
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Definition
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Term
FYI:
The other main advantage of volume imaging is that as data are collected from a slab, the slab can be manipulated to look at the anatomy in any plane at any angle of obliquity |
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Definition
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Term
| The disadvantage of volume imaging is what? |
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Definition
| The scan times are relatively long |
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Term
| Scan time = __x___x___x___ |
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Definition
| scan time = TR x NEX x #phase encodings x #slice encodings |
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Term
| In volume imaging, you need to obtain equal resolution in every plane and at every angle of obliquity, each voxel should be symmetrical (___) |
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Definition
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