Week 6 Flashcards
(103 cards)
1
Q
What are the four key factors for MRI parameters?
A
- Signal to noise ratio (SNR)
- Contrast resolution
- Spatial resolution
- Acquisition time
2
Q
What is SNR?
A
The ratio between the signal created by the transmit coil and the amount of noise produced by the patient
3
Q
If SNR is increased, how does contrast respond?
A
Increases
4
Q
What word is used to describe an image’s quality when SNR is too high?
A
Grainy
5
Q
SNR relies on proper placement of what two things?
A
The coils and the specific body part
6
Q
The ability to differentiate between the intensities of adjacent tissues on an MRI image is known as what?
A
Contrast resolution
7
Q
Contrast occurs due to different relaxation times of each tissue. True or false?
A
True
8
Q
What are the two main factors that control contrast?
A
- TR
- TE
*Sometimes TI
9
Q
What is spacial resolution?
A
The degree of sharpness on an image 
10
Q
How is spatial resolution adjusted?
A
By adjusting pixel or voxel size within an image
11
Q
What does acquisition time refer to?
A
Either the scan time for a particular sequence or the total exam time, depends on the context.
12
Q
There are many factors that _________ affect scan time for both _______ and ________ imaging.
A
Directly; 2D and 3D
13
Q
What are the five parameters that directly affect acquisition time?
A
- TR
- NEX
- Number of phase encoding steps
- ETL
- Slice thickness (for 3D imaging)
14
Q
What is the 2D scan time formula?
A
[(TR * NEX * Encodes) / ETL] / 60
15
Q
When calculating scan time, the TR must be in milliseconds. True or false?
A
False; TR must be in seconds
16
Q
[(TR * NEX * Encodes * Slices) / ETL] / 60
What does this formula calculate?
A
3-D scan time
17
Q
If scan time formula leaves you with 7.68 minutes, how does one convert that to minutes and seconds?
A
0.68 * 60 = 40.8 seconds
7 minutes and 41 seconds
18
Q
What does TR stand for?
A
Repetition time
19
Q
What is TR measured in?
A
Milliseconds
20
Q
What is repetition time?
A
The time between each initial RF pulse, regardless of whether it’s a 90° or 180°
21
Q
If TR increases, What two factors will also increase?
A
- Contrast (more T1 information is gathered)
- SNR
22
Q
As mentioned in week five, echo time refers to the time from the initial RF pulse to the peak echo within the same sequence. If the TE increases, what two factors are affected and how?
A
- Decrease in contrast (less T2 information gathered)
- Decrease in SNR
23
Q
What are the three parameter requirements to achieve T1 weighted images?
A
- A short TR, 400-700 ms
- A short TE, 10-14 ms
- A short ETL, 4-6
24
Q
What two things are T1 weighted images ideal for?
A
- Tumor evaluation
- Exams utilizing IV contrast
25
What are the three parameter requirements to achieve T2 weighted images?
1. A long TR, 4000–6800 ms
2. A long TE, 105-115 ms
3. A long ETL, 10-16
26
What two things are T2 weighted images ideal for?
1. Brain
2. Spine
27
What are the three parameter requirements to achieve Proton Density (PD) weighted images?
1. A long TR, 4000-6800 ms
2. A short TE, 10-14 ms
3. A long ETL, 10-16
28
What is PD weighted images ideal for?
Evaluating cartilage
29
As discussed in week five, inversion time refers to the time from the initial pre-180° pulse to the initial 90° pulse in an inversion recovery sequence. As TI Increases, how does it affect contrast and SNR?
1. Decreases contrast (Less T1 information is gathered due to fat suppression)
2. Decreases SNR.
30
The numerical value of the area of interest being scanned in the slice direction is known as what?
Field of view (FOV)
31
FOV is measured by what?
Centimeters
32
If field of view is doubled, how much does SNR also increase?
SNR is quadrupled
33
If FOV is decreased, what factors are affected and how?
1. SNR is decreased
2. Resolution is increased
34
FOV is constructed by the ______.
Matrix
35
What is the matrix?
The grid of pixels or voxels that relate to image resolution.
36
If a matrix is comprised of large pixels, it’s known as what?
Coarse
37
A fine matrix is composed of pixels of what size?
Small
38
Assuming field of view remains unchanged, what factors are affected if the matrix is increased and how?
1. Decrease in SNR
2. Increase in resolution.
3. Increase an acquisition time
39
The rows and columns that make up the matrix grid are not interchangeable and are fixed values. True or false?
False; the two directions are interchangeable, but do retain specific characteristics.
40
What are the two directions of the matrix grid known as?
1. Phase encoding
2. Frequency encoding
41
Motion artifacts on an MRI image will always be seen in which matrix direction?
Phase encoding
42
If the phase encoding direction is increased, what factors are affected and how?
1. SNR decreases.
2. Resolution increases.
3. Acquisition time increases.
43
When viewing matrix values, which direction will always represent the larger value?
Frequency encoding
44
If the frequency encoding direction increases, what factors are affected and how?
SNR barely decreases. Very little impact.
45
What is sliced thickness measured in?
Millimeters
46
If the image slices become thicker, what factors are affected and how?
1. SNR increases.
2. Resolution decreases.
3. Acquisition time decreases
47
What does gapping, spacing, or skipping refer to in MRI imaging?
The adjustable spacing between consecutive slices in the slice direction
48
What risk does the technologist run when utilizing gapping?
Missing anatomy
49
How is specific absorption rate (SAR) affected when increasing the gap between slices?
SAR is decreased
50
As mentioned in week three, the flip angle refers to how far magnetization is knocked from the longitudinal to the transverse direction. If the flip angle is increased, what factors are affected and how?
1. SNR increases.
2. Contrast decreases.
51
Decreasing a flip angle will increase the specific absorption rate. True or false?
False; decreasing the angle will also decrease SAR
52
The flip angle that is the most optimal per TR and results in the best SNR is known as what?
Ernst angle
53
What does NEX stand for?
Number of excitations
54
Define NEX.
The number of times each line of k-space is filled during one TR cycle
55
If NEX is decreased, what factors are affected and how?
1. SNR decreases
2. Acquisition time decreases.
56
The number of echoes collected per single TR period is known as what?
Echo train length (ETL)
57
What does bandwidth refer to an MRI?
The range of frequencies used to transmit and receive image signals.
58
What is frequency measured in?
Hertz (Hz)
59
If bandwidth is widened, what factors are affected and how?
1. SNR decreases.
2. Acquisition time decreases.
60
What are the two types of bandwidth and when are they used?
1. Transmit bandwidth: used during RF excitation.
2. Receiver bandwidth: used during signal sampling
61
During 2-D pulse sequence image acquisition, _______ slices are selected via ____________ and excited with RF pulses ______ at a time.
Individual; linear variation; one
62
Increasing what will allow for steeper variation and thinner slices?
Gradient amplitude
63
Pixels are used in _____ imaging and voxels are used _____.
2D; 3D
64
Pixels and voxels are usually measured in what?
Millimeters
65
Assuming the field of view remains the same, what factors are affected by increasing pixel/voxel size and how?
1. Resolution decreases.
2. SNR increases.
3. Acquisition time decreases.
66
What is the formula for calculating pixel size?
(FOV/phase) * (FOV/frequency) = pixel size
Remember, the matrix is made of two values, phase and frequency, with frequency always being the larger value!
FOV is displayed in cm, ensure it is converted to mm!
67
Which image acquisition always has a higher SNR, 2-D or 3-D?
3D
68
3-D image acquisition adds another direction, depth, to the matrix - what is it?
A second phase direction
69
Voxel depth is determined by ______.
Slice thickness
70
What is the formula for calculating voxel size?
(FOV/phase)(slice thickness) * (FOV/frequency)(slice thickness) = voxel size
Ensure all units of measurement are converted to mm!
71
Slice order is an imaging option which dictates the order in which slices are selected and sampled during image acquisition. What are the two options for slice order?
1. Sequential: 1, 2, 3…
2. Interleaved: 1, 3, 5, then 2, 4, 6
72
What are saturation pulses and bands?
An RF pulse or physical band that is applied at a specific location to minimize undesired signals.
73
What are two reasons that a saturation pulse or band may be used?
1. To reduce any vascular flow artifacts (think ToF and PCA MRAs)
2. To reduce breathing motion
74
What is another term for flow compensation?
Gradient moment nulling
75
What imaging option refers to a flow motion reduction technique that makes adjustments during signal read out to reduce artifacts and correcting?
Flow compensation
76
What are the three types of fat and water suppression techniques?
1. STIR
2. T2 fat sat. (aka CHESS)
3. Dixon technique.
77
As discussed in week five, short time inversion recovery is a fat suppression technique with _____ inversion times.
Short
78
T2 fat sat yields similar images as STIR. What makes this technique unique?
It uses an RF and spoiler pulse, tuned to the same frequency as fat, to achieve suppression
79
Which of the three fat and water suppression techniques is the newest?
The Dixon technique
80
How does the Dixon technique allow for fat-only and/or water-only selections in one sequence?
By collecting 2 to 3 separate echoes at different TEs in a pulse sequence
81
There are two options that allow for the syncing of data acquisition with motion cycles to reduce artifacts. Which option is retrospective and how does it work?
Triggering; images are collected throughout the scan and then motion intervals are subtracted with post processing.
82
There are two options that allow for the syncing of data acquisition with motion cycles to reduce artifacts. Which option is prospective and how does it work?
Gating; by collecting images between the motion cycles with the use of respiratory belts, pulse oximeters, sensors, and biomarkers
83
In and out of phase imaging is commonly used in the imaging of what anatomy?
Abdominal
84
How is in-phase/out-of-phase imaging similar to pre/post-contrast imaging?
Just like how identical imaging is acquired before and after contrast injection, in- and out-of-phase imaging is acquired before and after dephasing
85
How is in-phase/out-of-phase imaging acquired?
With a gradient echo that has the same TR, but different TE
86
What imaging option is an under sampling method that shortens the phase direction pixels, resulting in less phase and coding steps?
Hint: Remember surface coils!
Rectangular field of view (RFoV)
87
If a rectangular field of view is utilized, what factors are affected and how?
1. Acquisition time increases.
2. SNR increases.
88
What risk is associated with utilizing RFoV?
Any anatomy outside of the rectangular field of view will become distorted and wrapped. Ensure the entire anatomy of interest is covered in the phase direction.
89
What does an aliasing effect do to an image?
It causes data outside of the selected FoV to become mismapped on the opposite side of the image (in the phase direction)
90
What’s another name for the aliasing effect?
Phase wrap effect
91
When using a RFoV, the technologist runs the risk of an aliasing effect. What 2 imaging options can also be utilized to help prevent this?
1. Anti-aliasing
2. No phase wrap
92
Anti-aliasing and no phase wrap are both options of preventing aliasing effect artifacts. What’s the difference between them?
Anti-aliasing increases data collection within the FoV and No-Phase Wrap omits data collection outside the field of view.
93
Also known as ARC and Grappa, THIS imaging method accelerates data acquisition by sampling less k-space with the help of more receiver coils.
Parallel imaging
94
When parallel imaging is utilized, what factors are affected and how?
1. Acquisition time decreases.
2. SNR decreases.
95
As the name suggests, parallel imaging places receiver coils parallel with one another. Why?
So the coils can sample half of the area around them, and still complete an image - thus creating the images faster
96
Parallel imaging is utilized with phase array coils, which are designed to be ideal for imaging the chest. Why?
Parallel imaging reduces acquisition time. This subsequently means it also reduces how long the patient must hold their breath and, in turn, also reduces motion artifacts.
97
What is the primary cause for image corruption, resulting in repeats and longer exams?
Motion
98
Why is motion always found in the phase encoding direction?
Phase encoding takes three times longer than frequency encoding. The longer it takes, the more likely motion is to occur
99
Propeller (GE) and Blade (Siemens) are proprietary terms for the same motion reduction technique. How does it work?
It uses radial (semicircle) k-space filling and post processing algorithms to reduce involuntary motion
100
Similar to Instagram, MRI post-processing can add filters. How does the process of adding a filter work?
Filtering adds artificial value to data appearance before or after acquisition by hiding any noise or drop off
101
What are the two types of filters?
1. PURE
2. SCIC
102
How does the PURE filter work?
Adds a calibration scan to reduce any variables in the coil intensity
103
How does the SCIC filter work?
Automatically corrects any outlier low intensities, reduces noise, and improves contrast.
