MRI Image Basics
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When does T1 relaxation occur?
When nuclei give up their energy to the surrounding tissue
What is T1 relaxation also known as?
- Spin lattice relaxation
- longitudinal relaxation
- Z axis regrowth
What is T1 relaxation time?
Recovery of 63% longitudinal magnetization of a specific tissues original magnetization
T2 relaxation time is defined as ______
Time it takes for 63% of the transverse magnetization to decay
Fat has _____T1 relaxation time
Fat has a _____T2 relaxation time
Water has a ____T1 relaxation time
Water has a _____ T2 relaxation time
The Larmor frequency of hydrogen in water is ______ than hydrogen in fat
To obtain ____signal , there must be a small component of magnetization in the transverse plane in the coil
To obtain ____ signal, there has to be a large component of magnetization in the transverse plane received in the coil
On T1 sequences fat has a ____ signal
On T1 sequences water has ___ signal
A T1 weighted image utilizes what kind of TR and TE?
In T2 sequences Fat has ____ singal
In T2 sequences water has _____signal
T2 weighted images utilize what TR and TE?
Long TR does what to T1 information
Long TE does what to T2 information
What controls T1 weighting/contrast in an image?
What controls T2 weighting/contrast in an image?
What Factors will Increasing the TR affect?
- Improve SNR
- Increase scan time
- Reduce T1 weighting
- Increase the available # of slices
What factors will Decreasing TR affect?
- Lower SNR
- Reduce scan time
- Increase T1 weighting
- Reduce the available # of slices
The _____ the TE the better the overall SNR
What happens to scan time if the TR is doubled?
Scan time will double
What factors will reduce scan time?
- Reduce TR
- Reduce NEX/NSA
- Increase parallel imaging factor
- Utilize RecFOV
- Utilize Half-Fourier/halfscan
- Coarse Matrix
What factors does slice thickness directly affect?
- SNR of the image
- Image resolution
- Anatomical coverage
If NEX/NSA is reduced what happens to SNR?
SNR is reduced
What does NEX not have an effect on?
What happens to pixel size when a higher phase matrix is used?
- improved resolution
- decreased SNR
What factors will increase SNR?
- Increase NEX/NSA
- Increasing FOV
- Decreasing phase matrix
- Increasing TR
- Increasing slice thickness
- Reducing bandwidth
- Reducing ETL
If you use longer ETL there is an increased chance of ______
blurring in the image
What formula is used to calculate the size of a pixel?
FOV / matrix
Reducing the FOV by a factor of 2 will reduce voxel volume by a factor of ____
Reducing FOV in half will reduce the ____ and ____ dimensions each by ___
phase and frequency dimensions by 2
Formula for scan time in spin echo
TR x phase matrix x NEX
Formula for Scan time in fast spin echo
TR x phase matrix x NEX/ETL
Formula fo scan time in gradient echo
TR x Phase matrix x NEX
Formula to determine pixel size
Pixel size= FOV / Matrix
Voxel size formula
Voxel size=FOV/matrix + Slice thickness
what is an Isotropic voxel
equal measurements in all 3 dimensions
Pixel area formula
Phase x frequency
Voxel volume formula
Pixel area x slice thickness
What is Kspace
Storage location of MR signal data
Each line of k space is defined by ______-
phase encoding gradient
What is located in the central lines of K space?
Signal and contrast information
What is located in the outer lines of K space?
High resolution data
Reducing the number of lines filled in K space will produce what in an image?
Decreased spatial resolution
_____slope creates more detail
What is the approximate ETL for T1 weighting?
What is the ETL for PD weighting?
What is the ETL for T2 weighting?
When you increase the ETL what happens to slices?
It reduces the number of slices
Echo Spacing is defined as _________
The time between successive 180* echoes in a FSE pulse sequence
What is a typical echo spacing for a FSE?
The echo of the ______ will have the most signal
What type of contrast will there be more of with a longer ETL?
How many rows of space are filled per TR period in a spin echo sequence?
How many rows of kspace are filled per TR in a FSE sequence?
same as the ETL
What is elliptic centric kspace filling?
collecting the low frequency(high amplitude signal) data points in space at the start of the scan in a spiral fashion
During dynamic enhanced imaging for vasculature, contrast is administered and space is filled with _______
centric kspace filling
Where is the maximum signal stored in kspace?
What is half fourier aka?
What is half fourier?
acquiring a bit more than half the phase dimension kspace samples then interpolating the data with zeroes for the remaining half
What is partial or fractional echo?
- Only half the views of space are filled in the frequency axis
- An acceleration technique used to shorten TR and TE
When increasing NEX/NSA, SNR increases by _______
square root of the % increase
What happens when FOV is decreased?
- Decreased SNR
- Increased spatial resolution
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