MRI in Practice
Terms in this set (301)
Where does most of the mass of an atom come from? What can this category be divided into?
The nucleons. Protons and neutrons.
What is the difference between atomic number and mass number?
Atomic number: sum of protons in the nucleus. Mass number: sum of protons and neutrons in the nucleus
What are isotopes?
Same number of protons, different number of neutrons.
What are the three types of motion in an atom?
Electrons spinning about their own axis, electrons orbiting the nucleus, the nucleus itself spinning about its own axis.
What is angular momentum? What are they also referred to as? Why are these useful in the MRI environment?
In nuclei with odd mass numbers the nucleus of an atom itself will have a spin. Referred to as MRI active nuclei, useful because they tend to align their axis of rotation to an applied magnetic field.
What is the law of electromagnetic induction?
Refers to three forces: motion, magnetism, and charge. If two of these are present, the third is automatically induced.
What hydrogen isotope is the MR active nucleus?
What is the difference between "spin up" and "spin down" nuclei?
Spin up: low energy nuclei, align their spins parallel to the external field (parallel) because they do not contain enough energy to oppose the magnetic field. Spin down: high energy nuclei, align anti-parallel to the external field. Always more aligned parallel to the external field.
What is the net magnetization vector?
Relative balance between spin-up and spin-down nuclei.
What is the gyromagnetic ratio of hydrogen?
What is the Larmor equation?
wo = Bo x lambda
What is the difference between in phase (coherent) and out of phase (incoherent) magnetic moments?
Coherent: magnetic moments are in the same place along the precessional path around Bo at any given time. Incoherent: magnetic moments are not in the same place on the precessional path.
How is MR signal produced? What determines the frequency and amplitude of this signal?
Coherent magnetization cuts across a coil, producing magnetic field fluctuations in the coil that induce electrical voltage. Frequency is the same as the Larmor frequency, magnitude depends on the amount of magnetization present in the transverse plane.
What is the free induction decay?
Induction of reduced signal that occurs as the transverse magnetization decreases.
What is repetition time?
Amount of time between the application of one RF pulse to another RF pulse.
What is echo time?
Time from the application of the RF pulse to the peak of the signal induced in the coil, measures how much decay of transverse magnetization is allowed to occur (amount of T2 relaxation allowed to occur)
Intrinsic contrast parameters include:
T1 time, T2 time, proton density, flow, and apparent diffusion coefficient.
Extrinsic contrast parameters include:
TR, TE, flip angle,TI, ETL, b value
If a tissue has a large transverse component of coherent magnetization at TE, what does that mean for the contrast of that tissue?
Contrast will be high.
What are the three factors that determine T1 and T2 relaxation times?
Inherent energy of the tissues, how closely packed the molecules are, how well tumbling rate matches the larmor frequency of hydrogen.
How does inherent energy of the tissues affect T1 and T2?
Tissues with low inherent energy are more able to easily absorb energy from hydrogen nuclei. Especially important in T1 relaxation.
How does how closely packed the molecules are affect T1 and T2?
When molecules are more closely spaced there is more efficient interaction between the magnetic fields of the neighboring hydrogen nuclei. Especially important in T2 decay.
How does how well the molecular tumbling rate matches the Larmor frequency of hydrogen affect T1 and T2?
If there is a good match between the Larmor frequency and the molecular tumbling rate, then the energy exchange between the hydrogen nuclei and the molecular lattice is efficient.
What factors contribute to the T1 recovery of fat?
Low inherent energy that can easily absorb energy into its lattice, molecular tumbling rate matches the Larmor frequency which allows for efficient energy exchange. T1 of fat is short.
What factors contribute to the T1 recovery of water?
High inherent energy so cannot easily absorb energy into its lattice, less efficient exchange of energy from hydrogen nuclei to the surrounding lattice due to high tumbling rate. T1 of water is long.
What factors contribute to the T2 decay of fat?
Hydrogen and fat molecules are closely packed together, so spin spin interactions are more likely to occur. T2 time of fat is short.
What factors contribute to the T2 decay of water?
Molecules are far apart, so spin-spin interactions are less likely to occur. T2 time of water is long.
What is the basic MRI contrast?
Proton density weighting.
What does partial saturation refer to? Full saturation?
Partial saturation: NMV is pushed beyond 90 degrees. Full saturation: NMV is pushed to a full 180 degrees.
What is the TAU time? What is it equal to?
Time it takes spins to rephase after the 180 degree RF pulse is removed. Equal to the time it takes for spins to dephase after the application of the 90 degree RF pulse.
What is the polarity of a gradient?
Whether a gradient field adds or subtracts from the main magnetic field; depends on the direction of current passing through the gradient coils.
What are spoilers?
Gradients that dephase magnetic moments.
What are rewinders?
Gradients that rephase magnetic moments.
How is T1 weighting generated with gradient echo imaging?
Flip angle is large, and TR is short so fat and water vectors are still relaxing when the next RF pulse is applied.
How is T2* weighting generated with gradient echo imaging?
TE is long so fat and water vectors have sufficient time to decay, flip angle is small so the TR is long enough to permit full recovery of the fat and water vectors.
How is proton density weighting generated with gradient echo imaging?
TE is short to minimize T2* decay, flip angle is small and TR is long enough to permit full recovery of the longitudinal magnetization to reduce T1 recovery.
Define spatial encoding.
Magnetic field is altered in a linear fashion so the precessional frequency experienced by the nuclei can be predicted (spatial encoding).
How does slice selection work?
When a gradient coil is switched on the precessional frequency of located along its axis is altered in a linear fashion. Slices can be selectively excited by transmitting RF with a band of frequencies corresponding to the Larmor frequencies of the spins in a particular slice.
How is slice thickness encoded by the machine? How are thin slices obtained? How are thick slices obtained?
1) Slope of the slice select gradient: steep slopes result in greater difference in precessional frequency between two points on the gradients, opposite is true of shallow gradients. 2) Transmit Bandwidth: must contain the range of frequencies to match the difference in precessional frequency between two points. Thin: steep slice select gradient, narrow transmit bandwidth. Thick: shallow slice select gradient, broad transmit bandwidth.
When is the slice select gradient turned on?
Spin echo: during 90 degree excitation pulse and 180 degree refocussing pulse. Gradient echo: during excitation pulse only.
How does frequency encoding work? What is another name for the frequency encoding gradient?
When the frequency encoding gradient is turned on the magnetic field strength and precessional frequency along the axis of the gradient is altered linearly, which allows it to be located along the axis of the gradient. Readout gradient.
When is the frequency encoding gradient turned on? What is the duration of this gradient referred to?
When the signal is received. Duration is referred to as the sampling time or acquisition window.
How does phase encoding work?
When the gradient is turned on, the magnetic field strength and precessional frequency of nuclei are altered. As the speed of precession changes, so does the accumulated phase of the magnetic moments.
When is the phase encoding gradient turned on?
After the application of the excitation pulse.
What is the sampling rate, or sampling frequency?
Rate at which frequencies are sampled or digitized during the acquisition window per second.
What determines the number of data points collected during the acquisition window? What determines the size of that?
The frequency matrix. The frequency matrix obtainable is determined by the sampling frequency.
What is the sampling interval equal to?
What is the Nyquist Theorem?
Tells us how fast to sample a frequency or frequencies in order to digitize accurately. When digitizing a signal with a range of frequencies, the highest frequency must be sampled at least twice as fast to accurately digitize or represent it.
What is the Nyquist frequency?
Maximum frequency that can be sampled.
How do you calculate sampling frequency?
Sampling Frequency = 2 x Nyquist frequency
What is the receive bandwidth? How can we calculate it?
Range of frequencies we wish to sample or digitize during readout. Receive bandwidth = 2 x the highest frequency (Nyquist frequency)
Where are the frequency and phase axes of K-space located?
Frequency: horizontal. Phase: vertical.
What is the unit of K-space?
Describe how K-space fills.
Phase gradient is altered every TR so that the different gradients will fill the different lines of K-space. Steeper gradients will fill the most outer lines, whereas shallow gradients will fill the central lines. Frequency encoding gradient is turned on to lay out individual data points in the line.
What mathematical process is applied to generate an image from the data collected in K-space?
Fast Fourier Transform.
What is a pseudo-frequency?
A frequency that depends on the degree of phase shift produced by the gradient,
What frequency information is contained in each line of K-space? Will it be the same or different?
Same pseudofrequency, different frequency.
What frequency information is contained in each column of K-space? Will it be the same or different?
Same frequency data, different pseudofrequency data.
What is conjugate symmetry?
The symmetry of data in K-space. The points are mirror images across the origin of K-space, with identical amplitudes but opposite phases.
What do the central lines of K-space contribute to in the final image? What do the outer lines of K-space contribute to in the final image?
Central lines: signal and contrast. Outer lines: resolution.
Describe the signal amplitude and resolution contained in the central portion of K-space. Describe the signal amplitude and resolution contained in the periphery of K-space.
Central: high signal amplitude, low resolution. Outer: low signal amplitude, high resolution.
How long is the scan time?
The same amount of time it takes to fill K-space. Equal to TRxNEXx#PE/ETL
What does the amplitude of the frequency encoding gradient determine about K-space?
How far right and left K-space is traversed, determines the size of the FOV in the frequency direction.
What does the amplitude of the phase encoding gradient determine about K-space?
How far up and down a line of K-space is filled.
What is partial echo imaging?
Part of the signal or echo is read during application of the frequency encoding gradient. The system can extrapolate the rest of the data given that K-space is symmetrical.
How is partial echo imaging different from partial Fourier?
Partial echo imaging only reads part of the echo during acquisition, whereas with partial Fourier only part of the lines of K-space have been filled.
When can partial averaging be used?
Where a reduction in scan time is necessary and the resultant signal loss is not important.
What are the three methods of acquiring data?
1) Sequential, 2) Two-Dimensional Volumetric, 3) Three-Dimensional Volumetric.
What are sequential acquisitions?
Acquire all data from slice one before moving on to acquiring data from slice two. Slices are displayed as they are acquired.
What are 2D volumetric acquisitions?
Fill one line of K space for slice one, one line of K-space for slice two, etc, until this line of K-space has been filled for all of the slices, then the next line will be filled for all the slices, and so on.
What is the most common way of acquiring data?
Two Dimensional Volumetric Acquisitions.
What are 3D volumetric acquisitions?
Acquires data from the entire volume of tissue rather than in slices. Excitation pulse is not slice selective, so it excites the entire imaging volume. At the end of the acquisition, the volume is divided into discrete locations by the slice select gradient, which separates the slices according to their phase value along the gradient. Slices will be contiguous - no slice gap!
What are the four main considerations in image quality?
SNR, CNR, Spatial Resolution, Scan Time
What does noise refer to in the MRI environment?
Frequencies that exist randomly in space and time.
How does magnetic field strength affect SNR?
As the field strength increases fewer nuclei have enough energy to align their spins in opposition to Bo. The number of spin-up nuclei increases, and the NMV increases in size.
Why does doubling NEX only increase SNR by a factor of root 2?
Signal stays the same, but noise will change as it is random and in a different position each time the data is stored.
What other than SNR will increasing NEX do?
Decreased motion artifact.
What is the contrast to noise ratio?
The difference in the SNR between two adjacent areas, determines the eyes ability to distinguish areas of high signal from areas of low signal.
What is magnetization transfer contrast and how does it work?
Transfer of magnetization between bound and free protons, causing a change in the T1 of the free protons. If you selectively saturate bound protons by destroying the transverse magnetization of the bound protons, then you can reduce the intensity of the signal from the free protons.
What does "rise time" refer to in the context of gradients?
The time required for the gradient to achieve the correct slope.
What are the main advantages of volume imaging? The main disadvangates?
Advantages: no slice gap so small lesions can be clearly visualized, superior SNR, fewer NEX required, slab can be manipulated to examine from every angle. Disadvantages: long scan times.
What does the "effective TE" in TSE imaging refer to? Why is it important?
The TE at which the operator wishes to weight the resulting image. Important because the echoes in TSE imaging are generated at different TE times and will all have different weightings, so the machine will ensure that the shallow gradients that produce the most signal will be centered around the effective TE. These images will have more effect on image contrast.
What are the two main contrast differences between spin echo and fast spin echo? What are these differences caused by?
1) fat remains bright on T2 weighted images. 2) magnetization transfer effects are increased, so muscle can appear darker. Both of these are due to the effects of the multiple 180 degree refocusing pulses.
What is J-coupling?
Spin-spin interactions in fat.
What is an effect of repeated 180 degree refocusing pulses? Why might this be undesirable?
Reduces magnetic susceptibility artifact. Can be detrimental when looking for small hemorrhages.
Why does image blurring occur with scans that have a long ETL? Where will you see this effect? How do you reduce this effect?
Late echoes with low signal amplitude contribute to resolution in the image, but if the echoes are negligible then information is lost and blurring occurs. Will see this effect at the edges of tissues with different T2 decay values. Decrease echo spacing, or reducing the ETL.
What is an undesirable effect of increasing ETL, other than potential blurring?
As the ETL increases the number of slices available per TR decreases.
What is a single-shot fast spin echo sequence?
The lines of K-space are all acquired in one TR. Combines partial Fourier technique with fast spin echo - half the lines of K-space are acquired in one shot, and the other half are transposed.
What are some of the problems with using single-shot fast spin echo sequences?
SNR is reduced, SAR is increased.
What is the driven equilibrium Fourier transform? Why is it used? What kind of sequence uses it?
Applies an additional reverse flip angle excitation pulse to drive any transverse magnetization into the longitudinal plane to be available for excitation at the beginning of the next TR. That way you don't have to wait long periods for T1 relaxation to occur. Increases signal from CSF when using shorter TRs. RESTORE, DRIVE, FR-FSE.
What is pathology weighting? What sequence will you see this weighting using and how do you acquire it?
Produces an image that is primarily T1 weighted but where pathological processes appear bright. IR sequences, lengthen the TE time.
What controls contrast in an inversion recovery sequence?
Parameters for a PD IR sequence:
TR: 3000+ ms. TE: 10-20 ms. TI: 1800 ms.
Parameters for a Pathology Weighted IR sequence:
TR: 3000+ ms. TE: 70+ ms. TI: 400-800 ms.
Describe a fast inversion recovery sequence. What are some common ones?
180 degree inversion pulse, 90 degree excitation pulse, multiple 180 degree refocusing pulses. STIR and FLAIR sequences.
What does tau refer to in the context of a STIR sequence?
The short TI.
What does the null point refer to in the context of a STIR sequence?
The point that it takes fat to recover from full inversion so there is no longitudinal magnetization corresponding to fat.
What is TI time in general for any tissue in the body?
0.69 times its T1 relaxation time.
Why does post-gad enhancement occur in FLAIR images?
Long echo trains in FLAIR cause fat to remain bright, and gad reduces T1 relaxation time so that it is similar to that of fat.
What is double IR imaging? What is it used for?
Two 180 degree pulses applied - one that is non-slice selective and inverts all spins in an imaging volume, and one that is slice selective and re-inverts spins within a slice. Used for black blood imaging to look at the heart and great vessels.
What is triple IR imaging? What is it used for?
Adds a third 180 degree refocusing pulse at the TI of fat to null fat and blood together. Used to determine fatty infiltration of the heart walls.
Why does TR not always affect the image contrast of GRE imaging? What does normally control this?
Once a certain value of TR is exceeded the NMV will recover fully regardless of the flip angle selected. The flip angle and TE generally control this.
What TR and flip angle combination would you need to maintain a steady state?
TR: less than 50 ms. FA: 30-45 degrees.
What does steady state refer to in the MRI environment?
Coexistence of both longitudinal magnetization and transverse magnetization that has built up as the result of previous excitations, and remains over several TR periods. Residual transverse magnetization will be rephased by RF pulses to form a spin echo.
How will images with residual transverse magnetization appear on the MRI?
Tissues with long T2 times will appear bright on the image. Contrast will be due to the ratio of T1 and T2.
How do you classify sequences with residual transverse magnetization?
In phase (coherent) or out of phase (incoherent).
What is a Hahn echo?
An echo formed by any two 90 degree RF pulses.
What is a stimulated echo?
An echo produced by any two RF pulses of varying amplitudes. Used in steady state gradient echo sequences
How to you ensure the steady state is maintained in coherent gradient echo imaging? How is the residual magnetization kept coherent?
Selecting a TR that is shorter than the T1 and T2 times of the tissues so residual transverse magnetization is left over when the new excitation pulse is delivered. Residual magnetization is kept coherent by "rewinding" - reversing the slope of the phase encoding gradient after readout.
What would you use coherent gradient echo pulse sequences for?
T2* weighted images
What parameters should you select to maintain the steady state?
FA: 30-45 degrees. TR: 20-50 ms, TE: 15-25 ms
How are incoherent gradient echo (spoiled) pulse sequences acquired? What kind of weighting are they used for?
Steady state is maintained so residual transverse magnetization is left over from previous repetitions, then this magnetization is dephased so its effect on image contrast is minimal. Only transverse magnetization from the previous excitation is used. T1 contrast.
How do you achieve spoiling in incoherent gradient echo imaging? Describe these methods.
RF spoiling: RF is transmitted at a particular frequency to excite a slice at a particular phase, and then the next TR period transverse magnetization is generated at a different phase. Gradient spoiling: gradients are used to dephase and rephase residual magnetization
What parameters can be used for incoherent gradient echo?
FA: 30-45 degrees, TR: 20-50 ms, TE: 5-10 ms.
What are steady state free precession sequences used for? How does it do this?
Have less T2* weighting than other gradient echo sequences. Will need to digitize frequencies from the stimulated echo and not from the FID. Therefore the stimulated echo can't occur at the same time as the subsequent excitation pulse; a rewinder gradient is applied to speed up rephasing.
What parameters are characteristic of steady state free precession sequences?
TE is longer than the TR, actual TE is the time between the echo and the next excitation pulse, and the effective TE is the time from the echo to the next excitation pulse that created its FID. Rephasing is mostly initiated by an RF pulse rather than by a gradient, the echo is just repositioned by the gradient.
How do you calculate effective TE?
effective TE = (2xTR) - TE
What parameters are typically used for steady state free precession sequences?
FA: 30-45 degrees. TR: 20-50 ms. TE: should be as short as possible.
What is balanced gradient echo imaging?
A modification of coherent gradient echo imaging that uses a balanced gradient to correct for phase errors in flowing blood and CSF, both the FID and spin echo are collected in a single readout
What are the benefits of balanced gradient echo compared to coherent gradient echo?
Fat and water produce higher signal, greater SNR, fewer flow artifacts, shorter scan times.
How does balanced gradient echo produce higher SNR in shorter scan times than coherent gradient echo?
Higher flip angles and shorter TRs. Saturation is avoided by changing the phase of the excitation pulse every TR.
What are the parameters used for balanced gradient echo imaging?
Flip angle: 90 degrees. TR: 10 ms. TE: 15 ms.
How are fast gradient echo sequences acquired?
Only a portion of RF excitation pulse is applied and only a portion of the echo is read. May use extra pulses applied before the sequence begins to affect the contrast obtained: 180 degree pulses can be used to enhance T1 contrast or null signal from certain tissues, and a 90-180-90 degree pulse combination produces T2 contrast by producing transverse magnetization, rephasing it, and then driving the coherent magnetization into the longitudinal plane
What kinds of k-space filling are used for fast gradient echo sequences? Why would you have to use a method different from normal acquisitions?
Centric, keyhole. Used to enhance signal and contrast and achieve rapid scan times.
Describe centric k-space filling.
Fills k-space line by line, but fills the central lines first instead of working upwards and downwards. Achieved by applying all the shallowest phase encoding gradients first and leaving the steep ones until the end of the sequence. This way, central lines are filled with echoes have their highest amplitude.
Describe keyhole k-space filling.
Central lines are filled only during certain parts of the sequence.
What kind of k-space imaging is commonly used for angiography?
Keyhole K-Space filling.
What is echo planar imaging?
Single shot imaging that consists of a train of gradient echoes.
What is blipping?
Starts at maximum positive polarity, then phase shifts one line down, then that line is filled, and so on and so forth until maximum negative polarity is achieved.
What is spiral k-space filling?
K-space filling begins at the center, and then spirals out to fill the remainder of k-space. Ensures enhanced filling of the central lines.
What are some common variations on spiral k-space filling?
What is elliptical k-space filling?
Central ellipse portion of k-space is acquired as a volume acquisition during a contrast enhanced angiogram. Will show arterial phase only.
What is propeller k-space filling?
Lines are acquired as a block, but that block is rotated about the central axis of k-space
What are the two methods of acquiring multi-shot EPI sequences?
K-space segmentation by acquisition fills a section of k-space at a time. K-space segmentation by echo uses an ETL that is repeated several times.
What kinds of gradient power supply modifications can be used to increase power to the gradients for single shot sequences?
Resonant power supplies allow the readout and phase gradients to oscillate at the same frequency, reducing gradient requirements. Non-resonant power supplies produce any gradient waveform so that both EPI and conventional sequences can be run off of the same supply.
Describe a gradient echo EPI sequence.
Excitation pulse of any flip angle is followed by an EPI readout of gradient echoes.
Describe a spin-echo EPI sequence.
Excitation pulse of 90 degrees followed by a 180 degree refocussing pulse followed by an EPI readout of gradient echoes..
What are GRASE sequences? What are the benefits of using these sequences?
Hybrid gradient and spin echo sequences. Has the speed of a gradient echo sequence and the ability of the RF pulse to compensate for T2* effects.
What are some safety concerns of EPI and GRASE sequences?
Rapid switching of gradients causes nerve stimulation and gradient noise is quite severe.
Where will chemical shift be observed on an EPI sequence? Why?
Phase axis. The phase encodes are applied at different times after excitation.
Describe parallel imaging.
Multiple lines of K-space are filled every TR by assigning them to certain coils that are coupled together to enable them to acquire data simultaneously.
What is the reduction or acceleration factor?
Equal to the number of coils used in parallel imaging; the factor by which scan time will be decreased.
What are the three primary types of flow phenomena?
Time of flight, entry slice phenomenon, intra-voxel dephasing.
What are the four principle types of flow?
Laminar flow, spiral flow, vortex flow, turbulent flow.
Describe laminar flow.
Flow is at different but consistent velocities across a vessel. Flow is faster in the center of the lumen than at the vessel wall, where resistance slows down the flow.
Describe spiral flow.
Direction of flow is spiral.
Describe vortex flow.
Flow is initially laminar but passes through a stricture or stenosis in the vessel. High velocity in the lumen of the vessel, but at the vessel wall the flow spirals.
Describe turbulent flow.
Flow is at different velocities that fluctuate randomly. The difference across the vessel changes erratically.
What are the different flow mechanisms? Which ones can be compensated for by the machine?
First order motion laminar flow (constant velocity), second order motion acceleration, third order motion jerk. First order flow can be compensated for because the system can only correct for flow that is at a constant velocity and direction during data acquisition.
What is time of flight phenomenon?
Flowing nuclei may not receive both the excitation and rephasing pulse.
How does time of flight phenomenon appear on spin echo pulse sequences?
Nuclei flowing perpendicular to the slice may receive either the 90 degree excitation pulse or the 180 degree rephasing pulse but not both. In both instances, no signal will be returned and the vessel will appear dark.
How does speed of flow affect the time of flight phenomena in spin echo sequences?
As flow velocity increases, signal loss also increases (high velocity signal loss) and as the velocity of flow decreases signal loss also decreases. This is referred to as flow related enhancement.
How does TE affect the time of flight phenomena in spin echo sequences?
At higher TE a higher proportion of flowing nuclei have exited the slice between the 90 degree and 180 degree pulses, so signal loss will be greater.
How does slice thickness affect the time of flight phenomena in spin echo sequences?
Nuclei take longer to travel through a thick slice compared to a thin slice, and so are more likely to receive both a 90 degree and 180 degree pulse in this circumstance. As the slices get thinner, nuclei are more likely to receive only one pulse and signal void increases.
How does time of flight phenomenon appear on gradient echo pulse sequences?
The rephasing gradient is not slice selective, so a flowing nucleus that receives an excitation pulse at any point will be rephased regardless of its slice position and produce a signal. Often said to be flow sensitive sequences.
What happens to nuclei that receive repeated RF pulses during an acquisition with a short TR?
They will become saturated because their magnetic moments are more likely to be oriented in the spin-down direction because their TR is not long enough for longitudinal recovery.
What does "fresh" nuclei refer to?
Nuclei that have not received repeated RF pulses, whose magnetic moments are mostly oriented in the spin-up direction.
What is the entry slice phenomenon?
At short TR stationary nuclei will become saturated after repeated RF pulses, however nuclei flowing perpendicular to the slice enter fresh and produce a different signal.
What affects the magnitude of the entry slice phenomenon?
TR, slice thickness, velocity and direction of flow.
How does TR affect entry slice phenomenon?
Reduces the magnitude of the entry slice phenomenon.
How does slice thickness affect entry slice phenomenon?
Nuclei traveling through thick slices receive more RF pulses than nuclei traveling through thin slices, so entry slice phenomenon is decreased in thick slices as opposed to thin.
How does velocity of flow affect entry slice phenomenon?
Fast flowing nuclei are more likely to have traveled to the next slice when RF is delivered, so entry slice phenomenon is decreased as the velocity of flow increases.
How does direction of flow affect entry slice phenomenon?
Co-current flow: flow travels in the same direction as slice selection. Flowing nuclei are more likely to receive repeated RF excitations as they move from one slice to the next so spins will become saturated quickly. Counter-current flow: flow travels in the opposite direction to slice excitation, so flowing nuclei will stay fresh as they have not likely received the previous excitation pulses.
What is intra-voxel dephasing?
Nuclei flowing along a gradient rapidly accelerate or decelerate. If a flowing nucleus is adjacent to a stationary nucleus in a voxel there is a phase difference between the two nuclei as the flowing nucleus has either lost or gained phase as it moves along the gradient. Nuclei in the same voxel being out of phase with each other results in a loss of total signal amplitude from the voxel.
What is intra-voxel dephasing dependent on? When can intra-voxel dephasing be compensated for?
Depends on the degree of turbulent flow in a vessel. When flow is laminar then intra-voxel dephasing can be compensated for.
What are the methods of reducing flow phenomena?
Even echo rephasing, gradient moment nulling, spatial presaturation.
What is even echo rephasing?
If two or more echoes are produced, intra-voxel dephasing can be reduced by acquiring the second and then every even echo at a multiple of the first TE. At the first TE the spins will be out of phase, but they will be in phase at two times the TE.
What is gradient moment rephasing (nulling)? What gradient performs this function?
Gradients are used to correct the altered phases that occur in intra-voxel dephasing back to their original values. The slice select/readout gradient performs this task.
What is reduced as the phase shifts in flowing nuclei are corrected for?
Flow motion artifacts.
Why is gradient moment nulling referred to as a method of "first order motion compensation"?
It is most effective on slow laminar flow as it assumes a constant velocity and direction across the gradients.
Why is minimum TE increased and maximum number of slices per TR decreased with gradient moment nulling?
If the system needs to perform extra gradient tasks more time must elapse before it is ready to read an echo.
How do saturation bands work?
90 degree RF pulse is delivered to the volume of tissue before the excitation pulse, so that with the excitation pulse the spins in the volume are brought to 180 degrees and saturated.
What does chemical shift refer to? What is its typical value?
The frequency difference between fat and water. 220 MHz at 1.5T
How is fat/water saturation obtained?
Machine sends 90 degree RF pulse at the precessional frequency of fat or water into the imaging volume before the excitation pulse.
What is sat TR?
The interval of difference between the presaturation pulses. Equal to the scans TR divided by the number of slices
What is a SPIR sequence and how is it acquired?
An RF pulse at the precessional frequency of fat is applied to the imaging volume with a magnitude of 180 degrees, and after a time TI which corresponds to the null point of fat the 90 degree excitation pulse is applied. Combines fat sat and STIR methods.
What is the advantage of a SPIR sequence over a fat sat sequence?
Does not rely on a homogenous field to find the precessional frequency of fat.
What is the advantage of a SPIR sequence over a STIR sequence?
Does not affect the appearance of post gad images.
What are the reasons that phase mismapping occurs in the phase direction only? (2 reasons)
Phase encoding gradient has a different amplitude every TR, and the time delay between phase encoding and readout.
How does respiratory compensation/respiratory ordered phase encoding work?
System reads the signal from the transducer and fills the central lines of K-space when chest/abdominal wall motion is at a minimum, so most of the data that provides the image is acquired when movement is at a minimum.
How is respiratory gating/triggering different from respiratory compensation/respiratory ordered phase encoding?
Gating times the excitation with a certain phase of the respiratory cycle, compensation reorders the acquisition.
What is a drawback of respiratory compensation?
May be fewer slices available per TR.
What is a drawback of respiratory gating/triggering?
TR is determined by how rapid the patient is breathing which may cause contrast changes, and the scan time may be lengthened.
How does a respiratory navigator work to compensate for motion artifact? What is the drawback of using it?
System monitors signal intensity within a ROI and throws out data acquired outside the prescribed boundaries. Scan time may increase, or SNR decreases.
How is aliasing compensated for in the frequency direction?
Frequency filter filters out frequencies that occur outside the selected FOV.
What else can chemical misregistration be referred to as?
Out of phase artifact.
What causes the truncation artifact? What fixes it?
Undersampling data so interfaces of high and low signal are incorrectly represented in the image. Increasing the number of phase encoding steps fixes this.
What is the difference between cross-excitation and cross-talk?
Cross excitation: adjacent slices receive energy from the RF excitation pulse that is meant to excite their neighbors, which pushes the NMV of the nuclei to the transverse plane so that they may become saturated when they are excited. Cross talk: due to spin lattice relaxation energy may dissipate into nuclei of neighboring slices.
What is the shading artifact? How will it look? What causes it?
Appears as a difference in signal intensity across the imaging volume. Main cause is the uneven excitation of nuclei within the patient due to RF pulses applied at flip angles other than 90 or 180 degrees, can be caused by abnormal loading on the coil, or inhomogeneities in the magnetic field.
How do you fix the shading artifact?
Ensure the coil is loaded correctly, that the patient is not touching the coil at any point, ensure proper prescan parameters have been obtained before the scan.
Why does Moire artifact appear as a banding pattern?
Pixels are wrapped on top of each other each other due to anatomy existing outside the FOV producing signal, inhomogeneities cause this wrap to be in and out of phase.
What kinds of sequences can be used to acquire black blood imaging?
Spin echo, TSE, or inversion recovery.
What kinds of sequences can be used to acquire bright blood imaging?
What are balanced gradient echo sequences usually used for? What kind of contrast do these sequences have?
Cardiac imaging, MRCP, MR myelography, evaluation of the IACs. Have contrast that is weighted to T2/T1.
What kind of imaging is used for MRAs? Why?
GRE sequences. Signal from moving spins is enhanced due to the inflow effect/entry slice phenomenon, and spins from stationary tissue are suppressed.
What methods can be used to suppress stationary spins in MRA imaging?
1) Two acquisitions can be performed that treat stationary spins identically, but differentiate between moving spins and subtract them. 2) Short TR is used to saturate spins within an imaging volume is combined with the inflow effect, so a high degree of vascular contrast can be achieved.
What are the four different common MRA techniques?
1) digital subtraction MRA, 2) time of flight MRA, 3) phase contrast MRA, 4) contrast enhanced MRA.
Describe the technique of digital subtraction MRA.
An image is acquired during systolic phase (when signal loss is present in the arteries due to intra-luminal dephasing) and during diastole (in which high signal will be obtained from both the arteries and veins). Subtracting these two images will allow you to acquire the signal from either arteries or veins and simultaneously subtract out background materials.
Describe the technique of time of flight MRA.
Enhancement is related to the flow of blood that is perpendicular to the slice plane. Uses incoherent GRE sequences and gradient moment nulling to enhance flow. Flip angle and TR are selected so that stationary spins are saturated, and the inflow effect from magnetized flowing fresh spins produces high vascular signal.
What kinds of parameters would you use for 2D MRA?
FA of 45-60 degrees in conjunction with a TR of 40-50 ms.
Why would you use a 2D TOF MRA? What are the trade-offs?
Slow flow, wider area of coverage. Motion can produce artifacts and signal voids within the vessels.
Why would you use a 3D TOF MRA? What are the trade-offs?
High resolution images of small vessels with fast flow. Spins will take longer to flow through the slab resulting in them receiving more RF and saturating faster, meaning that it is inappropriate for slow flow and that the maximum thickness of the slab is limited.
How can you minimize unwanted background signal on a TOF MRA?
1) Choose a TE that keeps the signals of fat and water out of phase with each other so that they cancel each other out. Should be a short TE to minimize intra-voxel dephasing, phase ghosting, and signal loss. 2) magnetization transfer contrast - off resonance RF pulses are applied to suppress the signals from macromolecules and allow visualization of bright vasculature.
What is the MOTSA technique? Why would you use it? What are some of the restrictions?
Multiple overlapping thin section angiography. Can be used to increase coverage on the 3D TOF imaging. 3D slabs must overlap to prevent the Venetian blind artifact - the differences in saturation between spins leaving slab A and fresh spins entering slab B results in an obvious boundary between slices.
Describe the technique of phase contrast MRA.
Related to the change in flowing blood. Phase shift is related to the flow of blood velocity, flow direction, and the type of scan acquired. Phase contrast MRA can provide information about vascular anatomy, flow velocity, multi-directional flow, and flow direction. Uses gradient echo pulse sequences with small flip angles and bipolar gradients.
What are bipolar gradients and what do they do?
Two lobes that are equal in strength, one in the positive direction and one in the negative direction. They allow for the distinction between stationary tissues and spins within flowing blood.
What is the VENC? What does it do?
A bipolar gradient that produces a greater phase shift in moving spins than in stationary spins. Relies on the stationary spins momentarily becoming phase advanced when the first lobe is applied, but restored to their original position by the end of the second, whereas moving spins will have changed position between the applications of the two lobes and will not experience an equal and opposite second lobe.
How is the VENC selected? What units does the VENC use? What is a commonly used value of VENC to evaluate both arterial and venous flow?
Based on the blood flow velocity that is to be imaged. cm/s. 50 cm/s.
When arterial flow is to be imaged, should high or low VENC be used? Does this correspond to high or low amplitude gradient pulses?
High VENC, low amplitude gradient pulses.
Why does high VENC correspond to low amplitude gradient pulses?
In fast flowing blood, spins travel rapidly along the gradient, so the gradient only needs to be shallow to achieve the degree of shift needed.
What can happen if the VENC selected is lower than the velocity of blood flow within the vessel? Why does this occur?
Aliasing. In laminar flow the viscosity of blood results in drag so the center of the vessel has the highest velocity, so the signal will be mismapped out of the vessel lumen.
What are the flow encoding axes?
The X, Y, and Z gradients.
What is a major advantage of phase contrast MRA over TOF MRA?
Phase contrast MRA can evaluate blood flow in any direction, whereas TOF MRA can only evaluate blood flow perpendicular to the slice plane.
What is the most inferior chamber of the heart? The most superior?
Left ventricle. Right ventricle.
What does the P wave of an ECG trace represent? The QRS complex? The T wave?
Atrial systole. Ventricular systole. Ventricular diastole.
What is prospective gating? Retrospective gating?
Scan is timed to and triggered from the beats of the heart. Acquires data and times to the cardiac cycle during reconstruction.
Why is the TR used for cardiac imaging termed the effective TR?
Because it is totally dependent on the cardiac cycle, which is not perfectly constant.
How does the MRI compensate for the fact that the patients heart rate fluctuates throughout the scan?
Waiting periods built into the pulse sequences
What is the trigger window? What percentage usually compensates for variations in heart rate in normal patients? Patients with arrhythmias?
The waiting period before each R wave. 10-20% normal. 25% for arrhythmias.
What is the trigger delay?
The waiting period after each R wave.
What is the available imaging time of a scan for cardiac imaging?
the R-R interval - (trigger window + trigger delay)
What is pseudo-gating?
Patients heart rate is measured prior to commencing imaging, and a TR is selected that matches the R-R interval.
What is multiphase imaging? Single phase imaging?
Images acquired at a given location during each phase of the cardiac cycle. Used to assess physiology or function. Takes images during the same phase of the cardiac cycle.
Diamagnetic materials have paired electrons. With no applied external field the substance will show no magnetic moments. When an external magnetic field is applied, the substance will show a small magnetic moment that opposes the applied field.
Paramagnetic materials have unpaired electrons. With no applied external field the substance will have many small magnetic moments, but these moments will occur in a random pattern and cancel each other out. When an external magnetic field is applied, the substances magnetic moments will align with the direction of the field and add together.
Ferromagnetic materials have half filled electron shells. When an external magnetic field is applied it results in a strong attraction and alignment. Ferromagnetic materials retain their magnetization when the external magnetic field has been removed, becoming permanent magnets.
What is the most common substance used to produce a permanent magnet?
An alloy of aluminum, nickel, and cobalt - alnico
What is a solenoid electromagnet? What determines the strength of the magnetic field?
Wires are wrapped to form a loop, looks like a spring. Strength of the magnet is determined by the amount of current passed through the wire, the number of loops in the spring, and the distance between the loops. A type of resistive magnet
What is the typical configuration of the MRI?
Two solenoids are used to generate the magnetic field, one at either end of the magnet bore. Further windings are located along the length of the bore to improve homogeneity - referred to as bucking coils.
What is the bobbin?
The two solenoids generating the primary magnetic field and the windings located along the length of the bore.
What are the four gradient characteristics?
Gradient strength/amplitude, gradient speed/rise time, slew rate, duty cycle.
What is gradient strength/gradient amplitude? What is it measured in? What is a typical gradient amplitude?
How strong or steep a particular gradient is. Measured in mT/m. 10-40 mT/m
What is gradient speed/gradient rise time? What is it measured in? What is a typical gradient rise time?
Time it takes each gradient to reach maximum amplitude. Measured in microseconds. 120 microseconds
What is slew rate? What is it measured in? What are typical slew rates?
Time it takes for a given gradient to reach maximum amplitude and what that amplitude is. Measured in mT/m/s. 70 mT/m/s
What is duty cycle? What is it measured in? What is a typical duty cycle for spin echo imaging? For echo planar imaging?
The percentage of the time the gradient is permitted to work. Expressed as a percentage. 10% for spin echo. 50% for EPI.
What is a bipolar or balanced gradient system?
Each gradient pulse is balanced by an equal but opposite gradient pulse.
What is ramp sampling? Why would you use it? What are the drawbacks?
Data points are collected when the rise time is almost complete. Sampling occurs while the gradient is still reaching maximum amplitude, while it is at maximum amplitude, and as it begins to decline. Reduces time. Requires reconstruction programs to reduce artifacts, and resolution may be lost.
What is Faradays law of induction?
dB/dt = dv. dB: changing magnetic field. dt: the changing time. dv: changing voltage
What is a Helmholtz pair?
Two coils combined with B1 fields in the same direction.
What is a Maxwell pair?
Two coils combined with B1 fields in the opposite direction.
What is MR conditional 1?
Object is acceptable for use in the MR environment despite showing positive magnetic field interactions during testing. Weakly ferromagnetic.
What is MR conditional 2?
Coils, filters, stents, clips, occluders, and other implants that become firmly implanted in the tissue six weeks following placement.
What is MR conditional 3?
Transdermal patches with metal foil which have been reported to heat during scanning.
What is MR conditional 4?
Halo vest or cervical fixation device that may contain ferromagnetic components, however magnetic field interactions have not been determined. No report of injury in association with this device.
What is MR conditional 5?
Object can be scanned in MR environment only if specific guidelines and recommendations are followed.
What is MR conditional 6?
Determined to be conditional according to terminology specified by the ASTM.
What is MR conditional 7?
Device is not intended for use during the operation of an MR system for an MR procedure.
What is MR conditional 8?
Pertains to a device that has MR labeling at 1.5T and 3T.
What limits for whole body heating constitute normal mode limit?
0.5 degrees celcius or 2 W/kg
What limits for whole body heating constitute first level controlled mode?
1 degree celcius or 4 W/kg
What limits for whole body heating constitute second level controlled mode?
Greater than 1 degree celcius or 4 W/kg
What are the IEC/FDA limits for localized heating?
Head normal mode: 38 degrees celcius or 3.2 W/kg averaged over the head mass; Torso normal mode: 39 degrees celcius or 10 W/kg over any 10g; Extremities normal mode: 40 degrees celcius or 10 W/kg over any 10 g.
What factors affect SAR?
RF pulse characteristics and patient characteristics.
What is the recommended SAR level for imaging in the USA?
4 W/kg in the whole body averaged over 15 minutes, 3.2 W/kg in the head averaged over 10 minutes, 8 W/kg in the head or torso, per gram of tissue over 5 min, and 12 W/kg in the extremities, per gram of tissue over 5 min.
What are the two forces ferromagnetic materials are exposed to in the MR environment?
Translational force and rotational force.
Describe rotational force at magnetic isocenter.
Rotational force is greatest at magnetic isocenter.
Describe translational force at magnetic isocenter.
Increases as the object approaches isocenter.
What are level 1 MR personnel?
Have had minimal safety education, can work within Zone 3 areas.
What are level 2 MR personnel?
Individuals who have been more extensively trained and educated in broader aspects of MR safety issues.
What characteristics are implanted devices tested for in the MRI environment?
Torque and heating, functionality of the device, device interference with image quality, artefacts, safety associated with particular devices.
What are the two main considerations in imaging patients with bullets/shrapnel?
What is the material made of , and where is it in the body?
What is the current recommendation by the FDA for imaging pregnant patients?
If other non-ionizing imaging is suboptimal, or the information to be gained by MR imaging would have required more invasive testing MRI is acceptable.
What are some of the rules regarding pregnant employees in the MR environment?
Can enter the scan room, but should stay out while RF and gradient fields are employed.
How should patients that are unable to be contacted visually and verbally be monitored?
What effect will gadolinium have on T1 weighted images? T2 weighted images?
T1: T1 relaxation will be shortened and lesions will appear bright. T2: T2 relaxation will be shortened and lesions will appear dark.
How do you change intrinsic parameters? How do these changes influence these intrinsic parameters?
Cannot be changed, but can be influenced by alterations in static field strength and temperature. As patient temperature increases T1 and T2 relaxation changes. As Bo is increased T1 increases and T2 decreases.
What influences the tumbling rate of molecules in a gadolinium chelate based solution?
Viscosity and temperature of the solution.
How do dipole-dipole interactions contribute to gadolinium contrast based enhancement?
If a tumbling rate with a large magnetic moment is placed in the presence of water spins, local magnetic field fluctuations occur. For gadolinium, these fluctuations are near the Larmor frequency, so T1 relaxation times of nearby spins are reduced.
Why would you utilize gadolinium as a T2 contrast agent?
Perfusion imaging - dynamic imaging to investigate blood supply into a volume of tissue.
What is relaxivity?
The effect of a substance on relaxation rate.
What is the LD50 for gadolinium?
What does the ACR recommend in terms of gadolinium injections?
Technologists can inject as long as a doctor is present during an injection of any contrast agent.
What other agents are commonly used as T1 contrast agents in the body, and what are they commonly used to image?
Manganese - images the liver. Hyperpolarized helium gas - inhalation imaging of the lungs.
What are the naturally enhancing structures in the brain?
Falx, choroid plexus, pineal gland, pituitary gland/infundibulum, vessels, temporal bone.
What will be demonstrated on the first pass of gadolinium in the liver?
Arterially-fed cancerous lesions.
How does the MRI acquire cardiac tagging sequences? What is an example of this type of sequence? Why are tagged sequences useful?
Thin saturation pulses are applied immediately after the R wave, which suppresses specific parts of the myocardium by nulling the magnetization. Tagged tissue will produce decreased signal intensity, and the position of this tag can be traced. SPAMM sequences (spatial modulation of magnetization). Provide 2D or 3D information about the shape and dimensions of the heart cavity and its walls at different timepoints throughout the cardiac cycle.
What is SPAMM imaging?
Spatial modulation of magnetization. Two perpendicular sets of parallel stripes form a grid on the image that can be tracked through the cardiac cycle.
Other that SPAMM modulation, what other kinds of tagging techniques can be utilized in cardiac imaging?
Tags can be obtained in any geometric pattern.
What does the apparent diffusion coefficient show?
Net displacement of molecules diffusing across an area of tissue per second.
How are DWI images sensitized to motion?
Application of two gradients on either side of the 180 degree RF pulse. Stationary spins will acquire no net phase change after the gradients have been applied. Moving spins will acquire this phase change and result in signal loss.
Why is restricted diffusion bright on a diffusion weighted image?
Spins in restricted tissue are refocused as they stay in place during the application of both gradients. However, in normal tissue where diffusion is random refocusing is not complete and signals cancel.
How are ADC maps acquired? Why will restricted diffusion appear dark on an ADC map?
Post processing - the ADC for each voxel of tissue is calculated and a signal intensity is allocated according to its value. Restricted tissue has a low ADC, so it will be darker.
Why can DWI imaging be used to image early stage stroke patients? How can this be useful?
After the onset of ischemia but before permanent tissue damage cells will swell and absorb water from the extracellular space, so diffusion will be restricted. Differentiates between areas of irreversible and reversible ischemia.
What do perfusion weighted images examine? How are these images acquired?
Regional blood flow in tissues - the volume of blood that flows into one gram of tissue. The water in arterial blood will be tagged, and ultra fast imaging methods are used.
What does the time intensity curve refer to in regards to perfusion imaging?
Applied after data acquisition, used to ascertain blood volume, transient time, and measurement of perfusion. Time intensity curves for multiple images acquired during and after injection are combined to generate a cerebral blood volume map.
What is an alternative to contrast tagging in perfusion imaging? How are these images acquired?
Continuous arterial spin labeling (CASL). Arterial spins are attenuated by inversion or saturation pulses outside the FOV. An untagged reference image is also acquired and subtracted from the reference image.
What are susceptibility weighted images?
Use the susceptibility differences between tissues to generate contrast. Long TE is used and signal from tissues with different magnetic susceptibilities become out of phase.
Explain how blood oxygen dependent imaging creates contrast?
When oxygen is bound the magnetic properties of iron are suppressed (diamagnetic), when not bound the molecule becomes more magnetic (paramagnetic). BOLD utilizes these differences in magnetic susceptibility to examine cerebral blood flow and oxygen consumption
What are single voxel MR spectroscopy techniques? How are they acquired? What is the limitation of single voxel images?
STEAM and PRESS. Three intersecting slices to locate a single voxel from which to measure the spectrum. Suffer from SNR and chemical shift artefacts, can suffer from motion artefacts.
What spectroscopy measurements can be an indication for tumors?
NAA drop indicates cell invasion, choline elevation indicates tumor growth, lactate changes indicate anaerobic status, lipid elevation indicates tumor necrosis.
What is MR microscopy?
Uses extremely fine resolution to image structures with the same resolution as pathology sections.
What weighting would be used for a digital subtraction MRA?
What pulse sequence would you use for the TOF MRA?
T1 incoherent GRE
What pulse sequence would you use for a CE MRA?
T1 3D GRE
What are some drawbacks associated with presaturation pulses?
Increased RF heating, decreased number of slices available per TR.
What are some drawbacks associated with gradient moment nulling?
Increases minimum TE due to the extra gradients used, fewer slices available per TR.
What happens if the VENC selected is too high? Too low?
Vessel appears smaller than actual diameter. Signal void in the middle of the vessel from aliasing.
What is an important safety consideration when hooking a patient up to monitoring equipment for their MRI scan?
Always check the cables for damage, avoid looping the cables, make sure the cables do not touch either the patient or the bore of the magnet (run cables down the middle of the patient and place pads between patient and cables).
What weighting/pulse sequence would you use for cine sequences of the heart?
T2* coherent gradient echo
How do you calculate the time for 3D acquisitions?
TR x #PE x NEX x # slice encodings
What does iPAT stand for?
Integrated parallel acquisition techniques
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