Physics CH.1-5 Midterm MRI
What process must occur in Spin Echo image to rephase that portion of the net magnetization that has been dephased over time after the application of the inital 90 degree RF pulse?
180 degree RF pulse
In a conventional double SE pulse sequence using a long TR, what is the type of contrast usually acquired?
1st echo - PD weighted 2nd echo - T2 weighted
If fractional averaging of partial (half) Fourier imaging is used in 2D sequence, approximately what percentage of SNR is reduced compared with a conventional 1 NEX 2D sequence?
30
Which method of image acquisition data from an entire volume of tissue, then uses a method called slice encoding to separate the images?
3D
The thickness of an MRI image can be changed by which of the following methods?
Altering the gradient slope and RF bandwidth
After being placed in an external magnetic field, high energy hydrogen nuclei point in which direction?
Anti-parallel
During slice selection, the Z gradient is used to select slices in which plane?
Axial
Of the following parameters which would give the highest SNR? a. 512x128, 3mm STH, 12cm FOV, 4NEX b. 128x128, 2mm STH, 16cm FOV, 2NEX c. 256x256, 2mm STH, 20cm FOV, 2 NEX
B
Pixel size is equal to which of the following ? a. matrix size/FOV b.FOV/Matrix Size c. Frequency encoding/matrix d. phase encoding/matrix
B
When placed in an external magnetic field, hydrogen nuclei ___?
Become algined
Image data along both the phase and frequency axis with the highest signal amplitude is stored in which area of K space?
Central Lines
The ability to distinguish one structure from another on an image is known as ?
Contrast to noise
Which Parameter(s) effect total scan time? a. TR b. Number phase encoding steps c. Number of excitations d. All of the above
D
Which of the following is a parameter that directly affects SNR? a. Voxel size b. NEX c. TR d. All of the above
D
With the paramters given, which protocol has the longest aquisiton time? a. Long TR/ Low NEX / Low Matrix b. Short TR/ High NEX/ Low Matrix c. Short TR/ Low NEX / High Matrix d. Long TR/ High NEX/ High Matrix
D
In FSE pulse sequence what parameter is most responisble for image contrast?
ETL
The Negatively charged component of an atom is call the?
Electron
The signal created after applying the 90 degree RF pulse is known as a ?
FID Signal
The amplitude of the phase and frequency encoding gradients determines the dimesion of what parameter?
FOV
The process of filling K space by sampling only half of the echo and interpolating the rest is known as ?
Fractional/Partial echo
The gradient that is turned on just before the 180 degree rephasing pulse is known as the ___?
Frequency Encoding Gradient
Gradient strength is measured in what form of unit?
Gauss/cm or Millitesla/m
Magnetic Susceptibility effects are seen most prominent in which pulse sequence ?
Gradient
In a T1 weighted image, tissues with short T1 relaxation times produce what type of signal ?
High
The stronger the magnetic field the __ the precessional frequency?
Higher
Increasing the number of frequency encoding steps will?
Increase Spatial Resolution and Decrease Frequency Aliasing
The precessional frequency of hydrogen nuclei that experience increased magnet field strenth due to gradient coils __?
Increases
Dephasing of the net vector in the transverse plane is caused by two factors, imperfections in the external magnetic field and ?
Interactions between surrounding nuclei
The center of the magnet where the magnetic field strength remains unchated even during the application of gradient magnetic fields is know as the ?
Isocenter
The equation that is used to determine the precessional frequency of magnetic nuclei is known as?
Larmor Equation
In a T1 weighted image, CSF has a ___ T1 relaxation time and therefore appears ____?
Long/Dark
When the phase encoding gradient is activated, shallow slopes produce what type of signal amplitude?
Low
In a T2 weighted image, a long TE is used to?
Maximize T2 effects
In a T2 weighted image, a long TR is used to?
Minimize T1 effects
In a T1 weighted image, a short TE is used to ?
Minimize T2 effects
Magnetization in the transverse plane can also be known as ?
Mxy
Magnetization in the longitudinal plane can also be known as ?
Mz
What describes the number of times data is collected with the same amplitude of phase encoding gradient called?
NEX
The sum of all of the unmatched parallel protons in an external magnetic field makes up what is called the ?
Net Magnetic Vector
The neutral component of an atom is called the?
Neutron
With the exception of hydrogen, all matter is composed of three components. They are ?
Neutrons, Electrons, Protons
The theorem that states that a frequency must be sampled at least twice in order to reproduce it reliably is known as the __?
Nuquist Theorem
Image data with high spatial resolution is stored in which area of K space?
Outer Lines
The are of K space filled with the shallowest phase encoding slopes is known as ?
Outer Lines
After being placed in an external magnetic field, low energy hydrogen nuclei point in which direction?
Parallel
When placed in an external magnetic field, the hydrogen nuclei that are of clinical interest are __?
Parallel unpaired
The process of filing only a percentage of K space with acquired data and filling the rest with zeros is known as ?
Partial averaging
When only part of the signal is read by the fequency encoding gradient, it is called?
Partial or Fractional Echo
The spacial location of signal according to its precessional phase is known as ___?
Phase Encoding
The type of rotation that is displayed by hydrogen nuclei in an external magnetic field is known as ?
Precession
The speed at which protons rotate in an external magnetic field is known as ?
Precessional Frequency
The positive charged component of an atom is called the?
Proton
the nucleus of the hydrogen atom is made up of one __?
Proton
MRI images that are based on differences in the amount of hydrogen nuclei in tissues are know as?
Proton Density Images
A spin echo image with a long TR and a short TE is known as what type of image?
Proton Density/Spin Density
A 180 degree RF pulse is used to ___ the dephasing net vector in the transverse plane.
Refocus
The RF pulse used to move nuclei into a higher energy state is at a frequency known as ?
Resonance Frequency
What relationship does SNR have to voxel volume?
SNR is proportional to voxel volume
The rate at which signal samples are taken during frequency encoding is known as the ___?
Sampling Rate
During the sampling of signal, the sampling time is inversley proportional to ___?
Sampling Rate/Receive Bandwidth
In a T1 weighted image, fat has a ___ T1 relaxation time and therefore appears ___?
Short/Bright
In a T2 weighted image, fat has a ___ T2 relaxation time and therefore appears ___?
Short/Dark
The three primary functions that gradients perform during MR scanning are ___?
Slice Selction, Phase Encoding, Frequency Encoding
Three characteristics commonly used to define the quality of an MRI image are ?
Spatial Resolution, Contrast, SNR
The signal produced after the 180 degree RF pulse is applied is known as a ?
Spin Echo Signal
The presence of random noise causes the doubling of the NEX to increase SNR by?
Square root of 2
In MRI, thin slices are achieved by applying a ___ gradient slope or a ___ bandwidth?
Steep/Narrow
MRI images that are based on the differences in longitudinal relaxation characteristics of tissues are known as ??
T1 Weighted Images
A spin echo with a short TR and short TE is known as what type of image?
T1 weighted
MRI images that are based on the differences in the transverse relaxation characteristics of tissues are known as ?
T2 Weighted Images
The time it takes for transverse magnetization to decay to 37% of its original value is known as ?
T2 relaxation time
A spin echo image with a long TR and long TE is known as what type of image?
T2 weighted
What controls the amount of transverse magnetization that is allowed to decay before an echo is collected?
TE
The image parameter that primarilyh affects T1 Weighting is ?
TR
Hydrogen nuclei begin to precess in phase when what occurss?
The proper RF is turned on
Within a perfect magnetic field all protons rotate at?
The same frequency
With the paramters given, which protocol gives the higest Spacial Res?
Thin slice, sml FOV, High matrix
During a conventional spin echo pulse sequence, each slice is selected, phase encoded, and frequency encoded once per TR. TURE or FALSE
True
What type of Flip Angle do GRE pulse sequences use?
Variable
MRI signals can only be detected in which of the following planes?
X,Y, & Transverse
In a 3DFT pulse sequence, increasing the number of slices from 32 to 64 causes what result?
increase SNR and scan time
