9/22/22 Flashcards
- At 1 Tesela the frequency difference between that of fat and water is approximately
147 Hz
- How well a material attracts the imaginary lines of the magnetic field is defined as:
Magnetic Permeability
- _______ occurs after initial RF application, causing phase differences to appear between precessing spins, resulting in decay of transverse magnetization.
Dephasing
- The difference in chemical shift is approximately ___ parts per-million
3.5
- Aside from the center frequency, what other indicator in Image 82 signifies it was acquired at 1.5 Tesla?
The chemical shift between fat and water is 220 Hz
-The net magnetization vector (NMV), when aligned with the magnetic field direction, is aligned ________.
Along the longitudinal axis
- When thermal equilibrium is reached:
There are more hydrogen protons in the low energy state
- At 1.5 Tesla, the chemical shift between fat and water is:
220 Hz
Image 60 represents the ( w0 = y x B0):
Larmor equation
-Hydrogen protons in the anti-parallel state are referred to as:
Both A and C spin down and/or high energy spins
-Immediately following the application of the 90degree RF pulse, the precessing protons:
Both A and B begin to precess in phase and tip into the transverse plane
-Immediately following the application of the 90degree RF pulse, the transverse magnetization is:
Non-zero
-In one T2 relaxation time:
63% of the transverse magnetization has decayed
-In one T1 relaxation time:
63% of the longitudinal magnetization has recovered
______ magnets can be “shut off” the quickest in case of an emergency
Resistive
A magnetic vector possesses attributes of both _____ and _____
Magnitude, direction
Calculate the Larmor frequency for H at 0.5 Tesla.
21.28 Mhz
All the following are TRUE except:
The RF field oscillates at the Faraday frequency
What is defined as the excess number of hydrogen protons aligned with the static magnetic field direction (B0)?
Longitudinal magnetization
The energy used to form MRI images is _____ the patient’s tissues.
Emitted from
The extent to which a material or tissue becomes magnetized in an external magnetic field is called:
Magnetic susceptibility
If the RF energy transmitted is ___ Mhz, the slice through the patient would be in a magnetic field of .995T.
42.36
The precessional frequency of Hydrogen according to the Armor equation at 3T is:
127.71 Mhz
The protons that align themselves with the direction of the static magnetic field (B0) exist in a ____ energy state that those anti-parallel to the magnetic field.
Lower
The first documented human MR image was obtained on a 15 Mhz RF system, what field strength did it operate at?
0.352 Tesla
What would the scan time be in a 3D volume acquisition as follows: TR 32ms, TE 4 ms, Flip Angle 15degrees, FOV 18cm, 2mm slice thickness, 96 slices, 192x256 matrix, 1 NEX?
9 min 50 sec
3D scan time formula: TR x Phase Matrix x NEX x #slices
Calculate the voxel volume for the following pulse sequence parameters: TR 450, TE 20, FOV 24cm, 320x320 matrix, 3mm slice thickness.
1.69mm3
To calculate voxel volume, first determine pixel area. FOV/matrix xFOV/matrix = pixel area. Pixel area x slice thickness = voxel volume.240/320 x 240/320 = 0.5625 mm2. 0.5625 x 3 = 1.69mm3
Calculate the voxel volume for the following pulse sequence: TR 2000, TE 85, FOV 28cm, matrix 272 x 320, 5mm slice thickness, 3 NSA.
4.55 mm3
Calculate the pixel area for the following sequence: TR 450, TE 12, FOV 18cm, 224x256 matrix, 4 NEX.
0.56mm2
FOV/matrix x FOV/matrix = pixel area
______ is when only half the views of k-space are filled in the frequency axis.
Partial or fractional echo
A method of reducing MRI scan time by not filling as many lines of k-space in the phase direction, while maintaining spatial resolution, is known as:
Rectangular FOV
The overall signal-to-noise ratio will increase in 3D volume imaging when there is an increase in:
A and/or D increase in either FOV and/or increase in the number of slices or partitions
Which of the following pulse sequence parameter combinations would yield an image with the most blurring?
6000TR, 80TE, 256x256, 5mm slice thickness, 16 ETL, 4NSA
The longer the ETL the increased chance of blurring in the Image
Reducing the number of lines filled in k-space will produce an image:
With a decrease in spatial resolution
If the NEX is increased from 2 to 6, the signal-to-noise ratio increase would be
1.7 - When increasing NEX/NSA, SNR increases by the square root of the % increase. For example an increase of 2 to 6 NEX (3x greater) would have an increase in SNR of square root of 3 (1.73 times) more SNR than the original sequence.
All of the following would aid in reducing scan time EXCEPT:
Reducing TE
Each line of K-space is defined by the:
Phase encoding gradient
High amplitude signals are stored in the:
Center portion of k-space
Increasing NSA will:
Improve SNR
A reduction in FOV will result in:
Decrease in SNR - increase the spatial resolution by lowering pixel size
Reducing the slice thickness will result in:
A decrease in SNR - decrease partial volume averaging
The timing of the RF pulses in an MRI pulse sequence controls:
Image contrast
The smallest object that can be resolved in an image acquired with a 20cm FOV and a 224x224 matrix is _____mm.
0.89
200mm/224 = 0.892mm
With a recFOV that is twice as tall as it is wide, and the phase direction oriented along the smaller “width” dimension of the FO, which of the following relationships between the frequency and phase steps yields square pixels?
Phase steps must be half the number of frequency steps
The pixel size of the images which result from a 3D acquisition using a 240mm FOV, a 272x272 matrix, 32 slices and a volume of 64mm is:
0.88 x 0.88 mm
An increase in TR will produce all the following EXCEPT:
Decrease the number of available slices
A set of specifically timed instructions to the magnet is known as:
A pulse sequence
All the following affect acquisition time EXCEPT:
TE
All the following yield a higher signal-to-noise ratio EXCEPT:
Increase TE
An increase in TE will yield a decrease in SNR
As the number of acquisitions (NEX) is increased from 1 to 3, which of the following does NOT occur?
The pixel size is increased
Reducing the FOV only will result in:
Decreased signal-to-noise
A reduction in FOV will yield a decrease in SNR, but an increase in spatial resolution
The scan time of a fast spin echo sequence utilizing a TR of 3500ms, a TE of 90ms, a 256x256 matrix, 1NSA, a 220mm FOV and an echo train length of 5 is ____ minutes, approx.
3
TR (3500) x Phase matrix (256) x NSA (1)/ETL (5) = 179200 ms, then divide by 1000 to get 179.2 seconds, or nearly 3 minutes.
Assuming all other parameters are the same, a _____ ms TR provides the best SNR ratio.
800
The longer the TR, the better the SNR
The smallest object that can be resolved in an image acquired with a 20cm FOV and a 224x224 matrix is ____mm
0.89
200mm/224 = 0.892mm
What formula is used to calculate the size of a pixel?
FOV/matrix
Which parameter will provide the better spatial resolution?
4mm slice thickness, 256x256 matrix
If a sequence with a matrix of 256x512 is adjusted to 512x512,
the SNR will decrease and the scan time will increase
In a fast spin echo sequence with a 28cm FOV, 256x512 matrix, TR 2200, 90 TE, 5mm slice thickness, 8 ETL and 3 NEX, the scan time would be:
3 mins 31 seconds
Decreasing the TR will:
Both A and B
With regards to k-space, the data containing high resolution are located along:
The outer lines
With regards to k-space, the data containing signal and contrast information are located along:
the central lines
Assuming all other parameters are the same, an echo time of ___ms provides the best SNR.
15
Fat has a ____ T1 relaxation time and a ____ T2 relaxation time
Short, short
Water has a ____ T1 relaxation time and a ____ T2 relaxation time
Long, long
If the repetition time is doubled, the scan time will:
Double
Which of the following is NOT affected by changing the slice thickness?
The T1 contrast
TR is the controlling factor of T1 weighting/contrast in an image; slice thickness does nothing to alter the T1 information
Which of the following would NOT compensate for the change in SNR as a result of decreasing the FOV?
Reducing the number of excitations
A reduction in excitations (NEX/NSA) would reduce SNR,which has the same effect as decreasing the FOV
Reducing the phase matrix from 256 to 192 will:
All of the above
SNR Increases with all the following adjustments EXCEPT:
Decreasing pixel size
Smaller pixel size (higher phase matrix) leads to improved resolution, but decrease SNR
Reducing the TR:
B and D only
decreases the SNR and increases saturation effects
-The option to reconstruct multiple planes from a single data set is advantages in a 3D pulse sequence due to the thin slices used in 3D sequences and the lack of interstice gap
A and C only
Presaturation pulses are often used to:
Reduce flow artifacts
If the radiologist requires 3mm slices axially acquired through the IAC, with a slice gap of 0.5mm, and requires 4.2cm total coverage, how many slices must be selected?
12
SNR would increase in a 3D volume acquisition with an increase in:
Slices, FOV
The time during which the frequency encoding gradient is on:
Increases with a reduction in receiver bandwidth
Creating additional images in various planes from a 3D data set is accomplished by a technique known as:
Multi-planar reconstruction (MPR)
Which of the following is NOT a result of reducing the FOV?
Anatomical structures are displayed smaller in the image
If 24 4mm slices are planned with a gap of 1mm, the total anatomic cover is ____cm
12
Reducing the TR:
Decreases the SNR of the image
Decreasing the receiver bandwidth
Inverts the SNR of the image
All of the following are true mechanisms of saturation EXCEPT:
Partial saturation yields T1 weighting; occurs when NMV is 180degrees
