150 Questions
What type of relaxation is responsible for T1 recovery time?
Spin-lattice energy transfer
What is the name of the process by which hydrogen loses energy given to them by the RF excitation pulse?
Relaxation
What type of artifact is caused by the lines of magnetic flux becoming attracted to a ferromagnetic item?
Magnetic susceptibility
What is the purpose of using a short TR in gradient-echo sequences?
To minimize the scan time
What determines the size and location of pixels in an image?
The amplitude of the frequency-encoding gradient
What is Zone III?
The region in which free access by unscreened non-MR personnel or ferromagnetic objects or equipment can result in serious injury or death
What is the Nyquist theorem used for?
To accurately digitize a modulation of several frequencies
What type of contrast is derived from differences in the T2 decay times of the tissues?
T2 contrast
What is the purpose of the sampling window?
To store the data points in a line of k-space
What is the main purpose of gradient coils?
To generate images of the body
What is the effect of doubling the TE?
An increase in SNR
What is the purpose of the 180° RF rephasing pulse in spin-echo pulse sequences?
To rephase the magnetic moments of hydrogen nuclei
What type of flow is difficult to image using MRI?
Turbulent flow
What is the purpose of the RF inverting pulse in EPI sequences?
To produce T1 weighting
What is the purpose of STIR and FLAIR sequences?
To improve the clarity of pathology adjacent to CSF
What is the purpose of the frequency-encoding gradient?
To determine the FOV in the frequency direction
What is the difference between GE-EPI and SE-EPI?
GE-EPI begins with 90° and 180° RF pulses, while SE-EPI begins with a variable RF excitation pulse
What is the purpose of a cryostat in an MRI system?
To reduce heat transfer from adjacent components
What does high signal indicate?
High intrinsic contrast and hyperintensity
What is the primary source of noise in MRI?
The patient's body
What is the duty cycle of the gradient field?
The ratio of the maximum amplitude to the total acquisition time
What is the precessional frequency of fat nuclei?
Slightly different from water nuclei
What is the difference between frequency encoding and phase encoding?
Frequency encoding relies on what happened during the short time-frame of the sampling window, whereas phase encoding relies on what happened during the entire scan.
What is the effect of gradient spoiling on magnetic moments?
It alters the magnetic field strength experienced by the coherent magnetization
What is the turbo factor or echo train length (ETL) in spin-echo pulse sequences?
The number of 180° RF rephasing pulses performed every TR
What is the remedy for magnetic susceptibility artifact?
Using spin-echo sequences instead of gradient-echo
What type of contrast is produced by gradient-echo sequences?
T1 and T2 contrast
What is meant by the term “MR safe”?
An item that has been demonstrated to pose no known hazards in all MR imaging environments
What is the precessional path determined by?
The Larmor frequency
What is the number of mobile hydrogen protons per unit volume of a tissue called?
Proton density
What is the purpose of the RF excitation pulse?
To flip the longitudinal components of magnetization into the transverse plane
What is the purpose of presaturation?
To saturate fat signal
What is the purpose of shimming the magnet in an MRI system?
To compensate for inhomogeneity
What is the effect of a steep frequency-encoding gradient?
A small FOV in the frequency direction
What is the most important factor in gradient performance?
Gradient amplitude
What is the effect of chemical shift artifact in the renal area?
All of the above
What is the purpose of gradient amplitude in an MRI system?
To spatially locate or encode signal
What is the purpose of contrast agents?
To increase the conspicuity of tissues
What type of contrast does the DRIVE pulse sequence typically generate?
T1 contrast
What determines how far up and down a line of k-space is filled?
The amplitude of the phase-encoding gradient
What is the purpose of gradient-echo MRI?
To create a gradient in the magnetic field
What happens when the phase-encoding gradient is applied?
The phase of the magnetic moments of hydrogen nuclei changes according to their spatial location.
What type of personnel are Level 1 personnel?
Office staff and patient aides who have passed minimal safety education to ensure their own safety as they work within Zone III
What is the name of the signal that is produced when the RF excitation pulse is switched off?
The FID signal
What is the difference between coherent and rewound gradient-echo sequences?
Coherent sequences use gradient rewinding to produce a gradient-echo while rewound sequences use gradient rephasing to produce a gradient-echo
How can aliasing be reduced?
Minimizing the FOV
What is the relationship between the Nyquist frequency and the digital sampling frequency?
The digital sampling frequency is twice the Nyquist frequency.
What is the rate of T2 decay?
Exponential
What type of coil is typically used in receive-only MRI systems?
Array of loops
What are the molecules of gadolinium bonded to?
Metal ions
What type of nuclei do not have a net spin?
Nuclei with an even number of protons and neutrons
What is the range of flux density typically used in an MRI system?
1.0 to 8.0 T
What is the purpose of the 90° RF excitation pulse in the IR pulse sequence?
To induce maximum signal in the receiver coil
What is the effect of short TRs in gradient-echo MRI?
It decreases the impact of magnetization of tissues
What is the purpose of rewinding in gradient-echo sequences?
To maintain the coherency of residual transverse magnetization
What is the difference between presaturation and SPIR?
Presaturation applies a 180° RF pulse
What is the purpose of the slice-select gradient?
To spatially locate or encode signal
What type of personnel are Level 2 personnel?
MRI technologists, radiologists, and nursing staff who have been extensively trained in MRI safety
What determines whether a line in the top or bottom half of k-space is filled?
The polarity of the phase-encoding gradient
What is the effect of a shallow frequency-encoding gradient?
A large FOV in the frequency direction
What type of artifact is caused by uneven excitation of nuclei within the patient due to RF excitation pulses applied at flip angles other than 90° and 180°?
Shading
What is the name of the magnetic field associated with the RF excitation pulse?
The B1 field
What is the purpose of the 180° RF inverting pulse in the DRIVE pulse sequence?
To flip the NMV through 180°
What is the difference between T1 and T2 relaxation?
T1 relaxation depends on the molecular tumbling rate, while T2 relaxation depends on the Larmor frequency of magnetic fields
What is the purpose of the three gradient coils situated within the bore of the magnet?
To spatially locate or encode signal
What is the purpose of the FFT?
To calculate the exact position of spins located along the phase encoding axis
What is the advantage of using an open system for MRI?
Easy side access to the patient by clinicians
What determines how far to the left and right k-space is traversed?
The amplitude of the frequency-encoding gradient
What is the purpose of the phase-encoding gradient?
To achieve different pixel sizes in the phase direction
What is the effect of water molecules on the relaxivity of gadolinium?
They decrease the T1 recovery times of nearby hydrogen nuclei
What can cause vertigo when ultra-high-field systems are used?
The main magnetic field B0
What is the purpose of out-of-phase imaging?
To null signal from voxels containing fat and water
What is the main RF transceiver in closed-bore MRI systems called?
Body coil
What is the purpose of RF spoiling?
To dephase the residual transverse magnetization
What is the purpose of the trailing edge of the fan?
To slow down magnetic moments
What determines the time interval between each data point?
The digital sampling frequency
What is the name of the phenomenon that occurs in nuclei with an odd number of both protons and neutrons resulting in an even mass number?
Whole integral spin value
What is the effect of increasing the TR for an image?
The signal from fat is higher than that of water
What type of image is hyperintense on?
T1-weighted image
What is the effect of shallow phase-encoding slopes on spin-echoes?
They have a low amplitude
What is the purpose of the leading edge of the fan?
To speed up magnetic moments
What type of contrast is produced by incoherent or spoiled gradient-echo pulse sequences?
Proton density contrast
What is the time interval between the RF excitation pulse and when the center of k-space is filled?
The effective TE
What type of image quality factor is resolution?
Frequency resolution
What is used to encode the frequency of the signal being received?
A gradient
What determines the pixel size in the phase axis of the image?
The steepest phase-encoding gradient
What is the remedy for shading artifact?
Ensuring the coil is loaded correctly
What is the shape of the magnet bore in a closed-bore system?
Tunnel-shaped
What is the most important measure of gradient performance?
Gradient power duty cycle
What is the effect of patients moving to the isocenter of the magnet?
A sensation of moving along a curved trajectory
What is the purpose of changing the phase angle of the transverse magnetization every TR?
To dephase the residual transverse magnetization
In the event of a large projectile incident, what may be required to remove the projectile?
Quenching of the superconducting magnet
What determines the scan time?
The TR
What is the main difference between T1 and T2 contrast?
T1 contrast is likely to occur if vectors do not fully recover their longitudinal magnetization between each RF excitation pulse, while T2 contrast is likely to occur if the TR is shorter than the total recovery times of the tissues
What is the definition of a steady state?
A condition in which the energy “in” is equal to the energy “out”
What is the result of sampling at the same frequency as the frequency we are trying to digitize?
A representation of a straight line
What type of nuclei have two possible energy states?
Hydrogen nuclei
What is the most efficient way to reduce scan time?
Decreasing the flip angle
What is the purpose of the phase-encoding gradient?
To cause a change of phase of the magnetic moments of hydrogen nuclei
What is the main purpose of MRI?
To generate images of the body
What type of agents are typically superparamagnetic macromolecules of iron?
T2 agents
What is the difference between shallow and steep gradient slopes?
Steep gradient slopes alter the precessional frequency of magnetic moments of hydrogen nuclei between two points more than shallow gradient slopes
What is the purpose of the quench pipe in a cryostat?
To fill the helium and exhaust the atmosphere
What type of image contrast is produced by the SS-TSE pulse sequence?
In-phase transverse magnetization
What type of artifact is caused by the anatomy in the FOV being folded onto the top of anatomy outside the FOV?
Aliasing
What type of images can be produced using gradient-echo sequences?
All of the above
What is the result of sampling at less than once per cycle?
An artifact called aliasing
What is the purpose of the gradient in an MRI system?
To spatially locate or encode signal
Which of the following can affect SNR?
All of the above
What is the purpose of respiratory triggering and gating?
To limit the number of scans
What is the TI used for STIR?
The time it takes the fat vector to recover from full inversion to the transverse plane
What is the remedy for aliasing?
Minimizing the FOV
What is the effect of magnetic field inhomogeneities on spin-echo pulse sequences?
They cause T2* dephasing
What type of hazard can RF radiation cause?
Cancer
What is the purpose of the refrigeration unit in a cryostat?
To reduce heat transfer from adjacent components
What is the effect of pathology on diffusion?
It becomes restricted
What is the purpose of the RF excitation pulse?
To flip the longitudinal components of magnetization into the transverse plane
What is the name of the two time parameters used to measure the amount of T1 and T2 relaxation?
TR and TE
What is the purpose of the frequency-encoding gradient?
To cause a change of frequency of the magnetic moments of hydrogen nuclei
What is the most important factor in quickly applying the magnetic field?
Slew rate
What is the purpose of reducing the TE in gradient-echo pulse sequences?
To acquire images in a single breath-hold
What type of hazard can RF radiation cause?
Genetic damage
At which magnetic field strength is SNR highest?
1.5 T
What type of design is typically used in the main RF transceiver of closed-bore MRI systems?
Birdcage design
What is the result of sampling twice per cycle?
An absent frequency in the data
What is the purpose of the outer shell of the cryostat?
To reduce heat transfer from adjacent components
What is the effect of the 180° RF rephasing pulse on T2 decay?
It is unaffected
What is the TI used for FLAIR?
The time it takes the CSF vector to recover from full inversion to the transverse plane
What is the purpose of the TR period in EPI sequences?
To achieve images in one TR period
What is the effect of increasing the magnetic field strength?
An increase in the precessional frequency of fat nuclei
What is the effect of altering the relative proportion of central to outer lines filled in k-space?
Image quality is significantly affected
What is the purpose of paramagnetic contrast agents?
To increase the conspicuity of tissues
What type of nuclei have a half-integral spin value?
Nuclei with an odd number of both protons and neutrons
What is the effect of increasing the TR for an image?
The signal from fat is higher than that of water
What type of artifact is caused by signal from fat in the image being misplaced?
Chemical shift
What is the purpose of single-shot imaging?
To fill all k-space at once
What type of contrast is determined by the ADC?
Diffusion contrast
What is the digital sampling frequency not directly selected in the scan protocol?
The digital sampling frequency
What is the purpose of gradient-echo scans?
To image the inside of the body
What is the purpose of the inner circumference of the cryostat?
To contain the patient bore and MRI system components
What is used to achieve a very short TE?
Partial or fractional echo
What is the main difference between T1 and T2 contrast?
T1 contrast is likely to occur if vectors do not fully recover their longitudinal magnetization between each RF excitation pulse, while T2 contrast is likely to occur if the TR is shorter than the total recovery times of the tissues
What is the remedy for chemical shift artifact?
Increasing the receive bandwidth
What is the difference between volume coils and surface coils?
Volume coils produce a higher signal-to-noise ratio
What is the purpose of the RF excitation pulse in SPIR?
To produce no transverse magnetization in fat
What is FLAIR especially useful for?
Seeing periventricular and cord lesions more clearly
What is the purpose of the 180° RF inverting pulse in the IR pulse sequence?
To flip the NMV through 180°
What type of hazard can RF radiation cause?
The antenna effect
What is the relationship between the amplitude of the phase-encoding gradient and the spatial frequency of the waveform derived from it?
The amplitude of the phase-encoding gradient is directly proportional to the spatial frequency of the waveform.
What type of nuclei align their magnetic moments antiparallel or spin-down to the main B0 field in the classical description?
High-energy nuclei
Study Notes
- MRI relies on the spinning motion of specific nuclei present in biological tissues.
- There are a limited number of spin values depending on the atomic and mass numbers.
- A nucleus has no spin if it has an even atomic and mass number, e.g. six protons and six neutrons, mass number 12.
- In nuclei that have an even mass number caused by an even number of protons and neutrons, half of the nucleons spin in one direction and half in the other.
- The forces of rotation cancel out, and the nucleus itself has no net spin.
- However, in nuclei with an odd number of protons, an odd number of neutrons, or an odd number of both protons and neutrons, the spin directions are not equal and opposite, so the nucleus itself has a net spin or angular momentum.
- Typically, these are nuclei that have an odd number of protons (or odd atomic number) and therefore an odd mass number.
- This means that their spin has a half-integral value, e.g. inline.
- However, this phenomenon also occurs in nuclei with an odd number of both protons and neutrons resulting in an even mass number. This means that it has a whole integral spin value, e.g. 1, 2, 3.
- For hydrogen nuclei, there are only two possible energy states: low-energy and high-energy.
- Low-energy nuclei do not have enough energy to oppose the main B0 field (shown as a white arrow on Figure 1.4).
- These are nuclei that align their magnetic moments parallel or spin-up to the main B0 field in the classical description (shown in blue in Figure 1.4).
- High-energy nuclei do have enough energy to oppose the main B0 field.
- These are nuclei that align their magnetic moments antiparallel or spin-down to the main B0 field in the classical description.
- The precession of the magnetic moments around B0 produces the magnetic field inside the MRI scanner.
- The precessional path is determined by the precessional frequency, which is often called the Larmor frequency.
- The magnetic moments of the hydrogen nuclei are out of phase with each other, and therefore the NMV does not process. Resonance is achieved by transmitting an RF pulse called an RF excitation pulse. This is produced by a transmit coil (see Chapter 9).
- The RF excitation pulse is derived from the magnetic component only (the electric field produces heat), and unlike the B0 field, which is stationary, the RF excitation pulse produces an oscillating magnetic field, termed B1.
- The B1 field is applied at 90° to B0 at a narrow range or bandwidth of frequencies centered around a central frequency (termed the transmit bandwidth; see Chapters 5 and 6).
- The magnetic field associated with the RF excitation pulse B1 is very weak compared with that of the main external field B0.
- When the RF excitation pulse is switched off, the NMV is influenced only by B0, and it tries to realign with it. To do so, the hydrogen nuclei lose energy given to them by the RF excitation pulse. The process by which hydrogen loses this energy is called relaxation.
- As relaxation occurs, the NMV returns to realign with B0 because some of the high-energy nuclei return to the low-energy population and therefore align their magnetic moments in the spin-up direction. At the same time, but independently, the magnetic moments of hydrogen lose coherency due to dephasing. This occurs because of inhomogeneities in the B0 field and due to interactions between spins in the patient’s tissue (see Chapter 2).
- As the magnitude of transverse coherent magnetization decreases, so does the magnitude of the voltage induced in the receiver coil. The induction of decaying voltage is called the free induction decay (FID) signal. This is because spins freely precess influenced only by B0, signal decays with time, and magnetic moments of the spins induce a current in the receiver coil.
- The TR and TE are two time parameters that are used to measure the amount of T1 and T2 relaxation that have occurred when reading a signal.
- TR determines how long it takes for the RF excitation pulse to decay, while TE determines how long it takes for the peak of the induced signal to occur.
- TR and TE affect the amount of T1 and T2 relaxation that has occurred, respectively.
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