6: Parameters and Options Flashcards
Signal to Noise Ratio SNR:
the ratio between the peak frequency signal generated of the transmit coil to the amount of noise from the patient.
signal from coil: noise from patient
Signal to Noise Ratio SNR is reliant upon….
Coil placement in relation to the body part being examined
Poor coil placement will result in…..
an increase in noise, which is also termed “Grainy”
Contrast Resolution
the ability to differentiate between intensities of adjacent tissue on an MR image
How can contrast be adjusted?
TR and TE values
Contrast between tissues occur due to…..
the different relaxation times of each type of tissue
SNR and Contrast (direct or indirect) relationship
Direct: +SNR = +Contrast
Spatial Resolution
the degree of sharpness of an image
What controls Spatial Resolution
Size of pixels or voxels, matrix size
How to determine Voxel size?
FOV / Matrix X Slice thickness
Matrix and Resolution relationship (direct or indicrect)
Direct: +Matrix = +Resolution
Acquisition time:
the length of scan time for a particular sequence or total exam time
What affects scan time? (5)
TR
NEX/NSA (# of signal averages)
Phase encoding steps (#)
ETL
Slice thickness (in 3D imaging)
*Adjusting these values will result directly on acquisition time.
Formula to calculate 2D Scan time
TR x NEX x Phase Encodes
———————————————– / 60
ETL
* TR convert to seconds
Formula to calculate 3D scan time
TR x NEX x Phase Encodes x # of slices
———————————————– / 60
ETL
- TR convert to seconds
When calculating scan time, if there is no ETL….
divide by 60
What would the scan time be in a 2D FSE sequence utilizing TR = 600 ms, an effective TE = 14 ms, 2 signal averages, and a matrix of 512 x 256?
TR x NEX x Phase / ETL / 60
- Convert to Seconds ! *
.6 (s) x 2 (NEX) x 256 (Phase)
.6 x 2 x 256 = 307.2
307.2 / 60 = 5.12
5 mins
.12 x 60 = 7.2 seconds
Answer = 5 mins 7 seconds
What would the scan time be in a 2D FSE sequence utilizing TR = 3000 ms, 2 signal averages, 384 phase encoding lines of matrix with and echo train of 16?
2 mins 24 seconds
What would the scan time be in a Brain 3D FSE sequence utilizing TR = 3000ms, 2 signal averages, 192 phase encoding lines of matrix with an increase in echo train to 24 with 10 slices?
8 minutes
Repetition time (TR)? and how is it measured?
the time from the initial RF pulse to the initial RF pulse of the next pulse. milliseconds
TR value is (directly or indirectly) proportional to the amount of longitudinal relaxation allowed (63%)
directly
+TR = +long relaxation time = +T1 information = increase overall fat signal
Echo Time (TE)? how is it measured?
the time from the initial RF pulse to the peak resultant echo of the same pulse, measured in milliseconds
1/2 TE would be considered the time from the initial 90 degree pulse to refocusing 180 degree pulse
TE value is (directly or indirectly) proportional to the amount of transverse relaxation allowed (37%)
directly
+TE = -SNR = - T2 contrast = increase in chance of Susceptibility artifact
An increase in TE would have (decrease or increase) in SNR
decrease
TR and TE and used to control…
image contrast, “weighted” images
What is needed to control contrast in Inversion Recovery images?
TI (inversion time) TR (repetition time) and TE (echo time)
T1 Weighted images require:
TR
TE
ETL
Ideal for imaging…
- Short TR and Short TE
- TR: 400 - 700 ms
- TE: 10 - 14 ms
- ETL: 4-6
Ideal for Mass or Contrast imaging (pre and post)
What weighted image is best for Mass or Contrast imaging?
T1 Weighted
T2 Weighted images require
TR:
TE:
ETL:
Ideal for imaging…
- Long TR and Long TE
- TR: 4000 - 6800 ms
- TE: 105 - 115 ms
- ETL: 10 - 16
Ideal for Brain and Spine imaging
What weighted image is best for Brain and Spine imaging?
T2 Weighted
Proton Density Weighted images require
TR
TE
ETL
Ideal for imaging…
- Long TR and Short TE
- TR: 4000 - 6800 ms
- TE: 10 - 14 ms
- ETL: 10 - 16
Ideal for Cartilage imaging
What weighted images is best for Cartilage imaging?
Proton Density Weighted
Inversion Time (TI)
the time from the initial 180 degree FR pulse to the 90 degree pulse in an inversion recovery pulse sequence
Relation between TI, Fat Suppression, and T1 contrast:
+ TI = + Fat suppression = - T1 contrast
Field of View (FOV)? how is it measured?
the numerical value area of interest being scanned in the slice direction, measured in cm
FOV is constructed by interchangeable, adjustable columns and rows. These columns and rows together are considered the…
Frequency and phase directions of the imaging matrix.
Increasing FOV by a multiple of 2 will result in ______ value of signal to noise (SNR)
quadrupled
Relationship between FOV, SNR, and Resolution
+ FOV = + SNR = - Resolution
Slice Thickness? how is it measured?
the thickness of the slice in the slice direction, measured in millimeters
A thicker slice will contain (less or more) protons compared to a thinner slice, thus allowing a (higher or lower) signal to noise ratio (SNR)
more protons, higher SNR
Relationship between slice thickness, SNR, and spatial resolution:
+ Slice thickness = + SNR = - Spatial resolution
Gap
also termed “skip”, an adjustable spacing between consecutive slices in the slice direction
- Can be manually entered for each sequence
Relationship between skip, coverage, SAR, risk of missing pathology
+ Skip = + Coverage = - SAR = + Risk of missing pathology
Crosstalk
an artifact of overlapping slice, creates void on intersecting
Matrix
an adjustable grid of columns and rows within a FOV that directly relates to image sharpness or resolution on an MRI image
The value of each matrix is reliant upon…
how many pixels/voxels are in each column and row
Matrix is (directly or indirectly) related to pixel size
directly
Fine (higher) matrices have (larger or smaller) pixel size
smaller
Coarse (lower) matrices have (larger or smaller) pixel size
larger
Relationship between Matrix, SNR, Pixel size, and Resolution:
+ Matrix (Fine) = - SNR = - Pixel size = + Resolution
Flip angle? how is it measured?
how far the net magnetization has been flipped from the longitudinal transverse plane, measured in degrees
Ernst Angle
the most optimal flip angle per TR and results in the best SNR
In regards to SAR, doubling your flip angle will result in an (decrease or increase) of body absorption by a factor of ____.
increase, 4
Relationship between Flip angle, SNR, Contrast, and SAR
+ Flip angle = + SNR = - Contrast = + SAR (times two)
Number of Excitations (NEX / NSA)
the number of times each like of k-space is sampled during one TR cycle
*Comparing MRI to taking a picture, NEX would be similar to take double or triple exposing an exact image
Increasing NEX by double will (decrease or increase) SNR by the square root of ___.
increase, 2
Relationship between NEX, SNR, and scan time
+ NEX = + SNR = + Scan time
Echo Train Length (ETL)
the amount of echoes collected in one TR period
As ETL increases, the resultant echoes become (stronger or weaker) as they become further away from the initial 90 degree pulse
weaker
Relationship between ETL, SNR, and Scan Time
+ ETL = - SNR = - Scan Time
Bandwidth? and how is it measured?
the range of high and low frequencies used to transmit and receive signal, measured in Hertz (Hz)
Transmit bandwidth
the range of frequencies used during RF excitation
Receiver bandwidth rBW
the range of frequencies used by the receiver to sample signal
Phase Encoding affects… (3)
one direction of the imaging matrix which contributes directly to …
- resolution or image sharpness
- motion
- scan
Relationship between Bandwidth, SNR, range, scan time
+ Bandwidth = - SNR = wide range = - Scan time
Relationship between Phase, SNR, Slight resolution, and scan time:
+ Phase = - SNR = + slight resolution = + scan time
Phase encoding is an important step in matrix filling as it directly corelates to ____
motion
It is important to remember that while phase and frequency directions can be switched, ___ will always be ween in the ____ direction
motion, phase direction
Frequency Encoding is opposite of… and contributes to…
opposite direction of phase encoding direction, ONLY contributes slight increase in spatial resolution
Relationship between Frequency, SNR, resolution, time
+ Frequency = - SNR = slight increase on resolution = slight decrease in time
+ TR
SNR, Contrast, Resolution, Scan time
+ TR = + SNR, + Contrast T1, + Scan Time
no affect on resolution
+ TE
SNR, Contrast, Resolution, Scan time
+ TE = - SNR, - Contrast T2, + Scan Time
no affect on resolution
+ TI
SNR, Contrast, Resolution, Scan time
+ TI = - Contrast T1, + Scan time
no affect on SNR or resolution
+ FOV
SNR, Contrast, Resolution, Scan time
+ FOV = + SNR, - resolution
no affect on Contrast or Scan time
+ Slice Thickness
SNR, Contrast, Resolution, Scan time
+ Slice thickness = + SNR, - Resolution, - Scan time (less slices)
No affect on Contrast
+ Gap
SNR, Contrast, Resolution, Scan time
+ Gap = - Scan time (less slices)
no affect on SNR, Contrast, or Resolution
+ Matrix
SNR, Contrast, Resolution, Scan time
+ Matrix = - SNR, + Resolution, + Scan time
no affect on Contrast
+ Flip Angle
SNR, Contrast, Resolution, Scan time
+ Flip Angle = + SNR, - Contrast T1
no affect on Resolution or Scan time
+ NEX / NSA
SNR, Contrast, Resolution, Scan time
+ NEX = + SNR, + Scan time
no affect on Contrast or Resolution
+ ETL
SNR, Contrast, Resolution, Scan time
+ ETL = - SNR, - Scan time
no affect on Contrast or Resolution
+ Bandwidth
SNR, Contrast, Resolution, Scan time
+ Bandwidth = - SNR, - Scan time
no affect on Contrast or Resolution
+ Phase
SNR, Contrast, Resolution, Scan time
+ Phase = - SNR, + Resolution, + Scan time
no affect on Contrast
+ Frequency
SNR, Contrast, Resolution, Scan time
+ Frequency = - SNR, + Resolution
no affect on Contrast or Scan Time
2D Imaging
An acquisition method in which each slice is isolated and excited by creating a linear variation in combination with RF excitation in a progressive format
What is the difference between 2D and 3D imaging?
the process in which data is collected in the slice direction
In 2D imaging, increasing _____ will allow steeper gradient variation and thus the ability to sample thinner slices
Amplitude
In 2D imaging, increasing amplitude will….
allow steeper gradient variation and thus the ability to sample thinner slices.
3D imaging:
an acquisition method in which an entire region of interest is excited during each data acquisition step
For 3D scanning, you need to excite…
an entire region of interest, the addition of an extra phase encoding step allows depth excitation, relaxation, and data acquisition
What is a major benefit of 3D imaging compared to 2D?
the result of overall higher SNR due to a repetitive, volumetric excitation and acquisition method
Pixel size? How is it measured?
the size (width and height) of one single cube within a 2-dimensional matrix, measured in millimeters
Adjusting pixel size will directly effect…
spatial resolution
Increasing pixel size will result in …
a decreased matrix value thus decreasing imaging sharpness and scan time
How do you calculate pixel size?
Pixel size = FOV / Matrix
Relationship between Pixel Size, Spatial Resolution, SNR, and Time:
+ Pixel size = - Spatial Resolution = + SNR = - Time (with FOV remaining constant)
What is the pixel size in an MRI brain image with a FOV of 24 cm and a matrix of 256x 192?
240 mm / 192 mm = 1.25
240 mm / 256 mm = 0.94
1.25 x 0.94 = 1.2
1.2 mm
Voxel size? How is it measured?
the size (width, height, and depth) of one single cube in a 3-dimensional matrix, measured in millimeters
Voxel depth is adjusted through…
slice thickness
How do you calculate Voxel Size?
Voxel size = FOV / Matrix * Slice Thickness
What is the Voxel size in a 3d MRI Brain image with a FOV of 24 cm, a matrix of 256 x 192, and a slice thickness of 2 millimeters?
240 mm / 192 mm x 2 mm = 2.5
240 mm / 256 mm x 2 mm = 1.88
2.5 x 1.8 = 4.7 mm
4.7 mm
Slice Order:
the order in which slices are selected and sampled during image acquisition
Sequential slice order:
Within the image matrix, each pixel is acquired in the traditional 1, 2, 3, 4, 5… fashion
SLICE 1
SLICE 2
SLICE 3
SLICE 4
Interleaved slice order:
within the image matrix, each pixel is acquired in a 1, 3, 5, 7, 9… followed by a 2, 4, 6, 8, 10…
*When scanning under “interleaved method”, 2 acquisitions must be used allowing 1 acquisition for odd and another acquisition for even slices
SLICE 3
———–
SLICE 5
———–
SLICE 7
GO BACK
SLICE 4
———–
SLICE 6
————
SLICE 8
————
Interleaved slices helps reduce…
crosstalk
also to go back and help when patient is moving
Saturation Pulse (Saturation Band) (Sat-band)
an RF pulse applied in a specific location before initial 90 RF pulse to dephase spins and minimize their signal
Saturation bands are utilized for two reasons:
- reduction of breathing motion (l spine), perpendicular to the phase direction of the matrix
- reduce vascular flow artifact: place outside the FOV
Flow compensation or Gradient Moment Nulling:
a flow motion reducing technique that utilizes extra adjustments to correct for flow related dephasing, adds adjustments during signal readout to limit dephasing.
How to use Flow Compensation or Gradient Moment Nulling?
Scanning Z axis:
Scanning X or Y axis:
- Turn on Flow Compensation option
- If scanning in a Z-direction, choose “Slice” as flow compensation direction
- If scanning in an X or Y direction, choose “Frequency” as Flow Compensation direction.
STIR: Short Tau Inversion Recovery:
a fat suppression technique that utilizes a short inversion time (TI) that essentially nulls signal from fat, thus creating significant contrast between fat and water.
T2 Fat Saturation “Fat Sat” CHESS
creates a similar result to STIR, but demonstrates fat suppression by tuning an RF pulse and spoiler at the same frequency as fat with resultant nulling of fat
Dixon Technique
newest fat suppression; collects 2-3 separate echoes at different echo times in a pulse sequence, this results in the ability to select “fat only” and “water only” or both in one sequence.
Gaiting and Triggering:
the ability to reduce motion artifact caused by breathing, cardiac movement, or blood flow by synchronizing data acquisitions with cardiac or respiratory cycles with the use of respiratory belts, pulse oximeters, or biomarkers.
Gating:
a PROSPECTIVE way of acquiring image data between cycles
Triggering:
RETROSPECTIVE way of acquiring data consistently through the cycle and subtracting areas of increase activity
In Phase/Out of Phase
normally used in abdominal imaging, is typically a gradient echo with the same TR values but different TE values. Similar to pre and post contrast imaging, it takes the same image after intervention, fat vs. water content in suspected pathological tissue.
Aliasing; Phase Wrap:
an effect that causes data acquired outside of a field of view in the phase direction to become unsorted and mismapped onto the opposing side of the FOV in the phase direction
Anti-Aliasing; Oversampling:
increase in data acquisition when RFOV is on to prevent phase wrap
No Phase Wrap:
imaging option which compensates for any tissue from being sampled outside of the FOV in the phase direction from being in the acquired
Parallel imaging: ARC: Grappa:
imaging method used to accelerate data acquisition by sampling less k-space data with the help of more receiver coils
Receiver coils placed parallel with another will sample half the area in close proximity to complete an image within a reduced time
Parallel imaging is optimal for:
shortening breath holding sequences resulting in less motion artifact
Relationship between Parallel imaging, Time, SNR, and Resolution
Parallel imaging on = - Time = - SNR = no effect on resolution.
Motion is always found along… why?
the phase direction because phase encoding takes approximately 3x longer than frequency encoding dedicating more time to overall k-space filling in the phase direction. The chances of motion in the phase direction is higher
Phase encoding time: seconds to minutes
Frequency encoding time: milliseconds
Propeller or Blade:
motion reduction techniques that utilizes radial k-space filling with an additional post processing algorithms that reduce involuntary motion during acquisition
Filtering
the process of adding value to data before or after acquisition, does not inherently improve image quality, just adds value to its appearance by masking any presence of noise or contrast drop off
Filtering: Pure
used to reduce variable coil intensities through the use of a calibration scan
Filtering: SCIC
a filter that automatically corrects for low intensities, reduces noise and improves contrast.
FDA limit for static magnetic field for clinical MR examination:
8.0 Tesla
TI relaxation is defined once:
63% of the longitudinal magnetization has RECOVERED
Transverse direction can be defined as:
- Net magnetization vector protons perpendicular to B0 or b(null)
- Perpendicular direction to the longitudinal direction
