Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
Cíle letního kurzu 2
Nedokončeno, nemělo by to ovlivňovat žádné zákony o ochraně osobních údajů ani z
| Question | Answer |
|---|---|
| What are some disadvantages to MRI? | MR image intensity is relative and not an absolute quantity Expensive to purchase and operate Not safe for certain patient populations (pacemakers, metal) |
| What are some advantages to MRI? | True 3D image Excellent soft tissue contrast differentiation |
| Who discovered Nuclear Magnetic Resonance (NMR) | Bloch and Purcell in 1952 |
| If the angle is zero, what is the cross product? | The cross product is 0 -> No matter what the magnitude of the original vectors are (sin0* = 0) |
| The only imaging parameter that changes is matrix size, it increases. What happens to CNR of the image? | can be either higher or lower depending on what signals impact new voxels |
| Describe to someone with no knowledge of MRI, how an MRI test works. | Patient goes inside strong magnetic field, use multiple RF pulses and additional magnetic field pulses to make an image |
| How much is one Tesla worth in Gauss? | 1T = 10000 Gauss |
| What is the earth's magnetic field? | 0.5 Gauss |
| Health Canada and the FDA put a limit on MRIs for clinical use. What was the limit for them? | 3T was the maximum for clinical use |
| A long TE gradient echo is used with a short TR. What sort of sequence is this? | Inherently a T2* weighted sequence |
| How do you add T2-weighting to a pulse sequence? | A long repetition time (TR) and a long echo time (TE) are used |
| What is T2* weighting sensitive to? | T2* weighting is sensitive to anything that makes the magnetic field non-uniform (poor shim) |
| Describe how signal dephasing can be used to introduce T2 weighting How is T2 weighting caused? What does it result in? | Caused from Spin-Spin relaxation, results in dephasing of magnetic moments of the spins. Caused by one spin transferring energy to another spin rather than into lattice. |
| When comparing contrast, what does echo time (TE) primary control? | Echo time primary controls amount of T2-weighting |
| When comparing contrast, what does repetition time (TR) primary control? | TR primarily controls the amount of T1-weighting |
| What TR/TE would you use for a T2 weighted imaging sequence? | TR & TE should be set to long values. This results in T2 relaxation times of tissues to impact image contrast. Water appears bright, where fatty tissues like white matter are dark |
| If I use a short TR combined with a short TE, how will this effect my T1? T2? | Using this combination will maximize T1, BUT will keep T2 minimized |
| What are some uses of T1-weighted images? | T1 weighted images are used for anatomy, pathology postcontrast enhancement |
| A long TE and TR are used. How does CSF appear on an image? | It would appear bright as it is mostly water, and water has a long T2. |
| A long TE and TR are used. How does white matter appear on an image? | It would appear dark as fatty tissues have a short T2 |
| What instances is T1 good for? | Imaging detailed anatomy, detection of contrast enhancement |
| What instances is T2 good for? | Imaging edema and other fluids |
| What are the TR/TE for a basic T1 weighted imaging sequence? | Utilize a short TE and a short TR |
| What does a short TR do? FIX THIS LATER | minimizes signal differences due to proton density differences between tissues FIX THIS LATER |
| What does a short TE do? | Minimizes signal differences due to T2 differences between tissues |
| A short TE and a short TR are used. How does water appear on an image? | Water appears dark as it as a long T1 |
| A short TE and a short TR are used. How does fat appear on an image? | Fat would appear bright on an image, short T1 |
| Describe the concept of saturation. | Saturation refers to the process where all longitudinal magnetization (Mz) of spins is flipped into transverse plane, typically by applying a 90* RF pulse. Basically removes Mz, suppresses signal from certain tissue |
| How can saturation be used to introduce T1 weighting? | Used to introduce T1 weighting by using an RF pulse to tip Mz away from equilibrium. Using different time delay between saturation pulse and signal acquisition, get different T1 |
| Describe the role of an inversion recovery pulse in creating T1 weighting | Inversion recovery pulse is able to null certain tissues (with the most common being free fluid). Works if set to T1 when one tissue is exactly Mz = 0 (longitudinal magnetization) Uses a 180 pulse to flip longitudinal magnetization to 0 before the puls |
| Describe in detail how inversion recovery pulses work in creating T1 weighting | Add 180* RF pulse with delay time, before we do pulse sequence. Delay time is inversion time (TI). Have short TI to get T1 weighting |
| Define Signal to Noise Ratio (SNR) | ratio of relative signal to noise |
| Examine the role of SNR in evaluating MRI image quality | Strong signals result in bright voxels/pixels Weak signals result in dark voxels/pixels Contrast in an image is the difference in signal strengths between 2 types of tissue |
| Define Contrast to Noise Ratio (CNR) | Difference in SNR between two points |
| Examine the role of CNR in evaluating MRI image quality | If the contrast difference is high between 2 signals but noise is also high, then the CNW is low. If there's a small difference in 2 signals, if the noise is even lower, the CNR will be higher |
| Introduce the concept of a voxel | 3D location in the patient. (Volumetric pixels) |
| How does acquisition matrix size relate to spatial resolution? | A large acquisition matrix size (more pixels) results in better spatial resolution (finer details in the image) |
| Determine the role played by NEX in the SNR/CNR and scan time in the quality of images | NEX directly impacts SNR/CNR. Increasing NEX improves SNR, but also increases scan time and improved CNR |
| Determine the role played by FOV/resolution in the SNR/CNR quality of images | Lowering FOV or increasing resolution (smaller voxel size) generally leads to a lower SNR and CNR |
| How does spatial resolution relate to image quality factors such as SNR and CNR | Increasing spatial resolution (smaller voxels) can improve CNR, but lowers SNR |
| Describe the interaction in a pulse sequence between SNR, CNR, resolution and scan time | Increasing SNR needs a longer scan time or sacrificing resolution, while increased resolution impacts SNR or longer scan times. Higher SNR and appropriate contrast can improve image quality. |
| Define the common imaging parameters used in MRI (TR, TE, flip angle, etc) | TR - Repetition Time, TE - Echo Time, Flip Angle - Angle of the NMV to B0 |
| Describe the effect of frequency encoding bandwidth on spatial resolution (how does it affect spatial resolution, SNR) | increasing bandwidth will lower spatial resolution but increase SNR |
| Examine the role played by these acquisition parameters in image quality: Bandwidth, spatial resolution, SNR | If bandwidth increases, spatial resolution will decrease. SNR also increases and decreases motion artifacts |
| Introduce the concept of partial k-space acquisitions | There is symmetry relation between - and + values in k-space. Known as conjugate symmetry. To save time in a scan, only collect half phase encode data. |
| Discuss how to acquire a proton density MR image | Long TR, short TE. The larger the proton density of a tissue, the brighter it appears in a PD weighted image |
| Provide an overview of how/why one would introduce contrast in MRI | See different things with different contrasts. T1 weighted contrast shows detailed anatomy, detection of contrast enhancement. T2 is good for imaging edema and other fluids |
| Demonstrate the introduction of T2* weighting to an MRI sequence | To only lengthen the TE allows for contrast in an image (T2* Weighted) Short TR, long TE |
| How does lowering the FOV at a fixed matrix size affect resolution | Increase and have smaller voxel size |
| What happens if I only increase FOV without changing any other parameters? (Hint: specifically what happens to resolution and SNR) | Resolution will decrease and SNR will increase |
| The matrix size is smaller at a fixed FOV. How does this impact resolution and voxel size. | Lower resolution, but larger voxel size |
| The matrix size is decreased while FOV isn't changed. What happens to resolution and SNR? | Resolution will decrease, SNR will increase |
| As the matrix size increases, the resolution and scan time ________. | Increase |
| As the matrix size increases, the SNR will ________ | Decrease |
| If slice thickness increases, what happens to the resolution and SNR? | As slice thickness increases, the resolution decreases and SNR increases |
| If the FOV increases, what happens to the resolution and SNR? | As FOV increases, resolution decreases but SNR increases |
| What is repetition time (TR)? | Defined as the time between the first RF pulse and the application of the next RF pulse for that same slice |
| What happens if a flip angle less than 90* is used? | Less transverse magnetization is generated |
| What is "partial volume" | A single voxel would contain a mixture of multiple tissue values, and we lose contrast |
| Why do we not use partial k-space data acquisitions all the time? | One half of k-space has the same noise character as the other, and our SNR drops. Therefore we trade-off SNR for scan time |
| What is the TR/TE like during Proton Density (PD) Imaging? | A Long TR is used to minimize signal differences due to T1 differences between tissues A short TE is used to minimize signal differences due to T2 differences between tissues |
| The echo time of a gradient echo sequence is longer than the repetition time. What sort of weighted image does this produce? | This produces a T2* weighted image (short TR, long TE) |
| Describe the CROSS PRODUCT | Math equation with vectors. Cross product of two vectors results in a vector which is perpendicular to both of the original vectors |
| Describe the DOT PRODUCT | The DOT product of two vectors result in a value that only has a magnitude |
| Discuss the concept of a differential equation, and summarize qualitatively what a differential equation represents | A differential equation involves an unknown function of one or more of its derivatives (change of function with respect to a variable) |
| Examine the principles underlying the Fourier Transform | A mathematical operation that takes a signal measured as a function of time and converts it to a plot of signal as a function of frequency |
| Discuss the underlying physical meaning of the Fourier Transform | The Fourier Transform shows us what frequencies we measured and what the amplitude was of each frequency |
| How does the Fourier Transform relate its application to the processing of images | The Fourier Transform an image into its frequencies, and reverse the process by doing an Inverse Fourier Transform on the frequencies to get our image back |
| Identify the fundamental physical processes that underlie the phenomenon of magnetic resonance | Nuclear magnetic resonance, nuclear spin. All atomic particles behave as they are spinning. Each has angular momentum, and can only have certain angular momentum values. Means things are quantized |
| How does nuclear spin relate to the generation of nuclear magnetization | Can only have certain angular momentum values. It is said to be quantized. If the spinning is quantized, the spinning is what creates the magnetic field, the magnetization is also quantized |
| Give a real world example for how a quantum mechanical concept can be understood as a classical picture | A compass has a magnetic needle that turns to point along the direction of earths magnetic field. Needle turns as if its quantized (only certain values make it turn to where it needs to go) If other direction than magnetic field, keeps turning |
| What does Resonance mean? | When there is a particular natural frequency that something wants to absorb/emit energy |
| Examine how the interactions between nuclear magnetization and an applied magnetic field lead to precession | A hydrogen nucleus in a magnetic field wobbles in a circular path. Every single hydrogen nucleus wobbles (precession) at a very specific frequency. Precession path is a circle that is about the magnetic field that was applied |
| Derive the Larmor Equation and examine its role in MRI | Larmor Frequency is the precessional rotation per second of the nuclear magnetic moment about the external magnetic field (B0) |
| Introduce the concept of radio frequency pulses and assess how these pulses would interact with the energy levels of the nuclear spins | The RF pulse results in a resonant absorption of energy that causes the spins to change energy levels Excite the spins we disturb from equilibrium, detect a signal |
| Identify the role of the "rotating frame of reference" in understanding MRI pulse sequences | Always assume B0 is pointed in the z-direction A coordinate system which is rotating about the z-direction An RF pulse results in a rotation of the NMV away from z-direction, into the x-y plane |
| Discuss the process by which applied magnetic fields and the net magnetic vector interact | If B1 is orientated in the x-direction and the equilibrium of M is in the z-direction, then the effect of the pulse is a rotation away from z and into the x-y plane |
| What is the flip angle? What is it controlled by? | The angle by which the NMV rotates due to an RF pulse is known as the flip angle. The flip angle is controlled by the amplitude and length of the RF pulse |
| Summarize the concept of MRI signal phase | With an RF pulse in affect on the NMV is a magnetic field that spins (oscillating) around. Changing magnetic field induces voltage in anything that conducts. If a coil of wire is around something, voltage is picked up to measure and digitize |
| Describe the overall concept of magnetic resonance relaxation | Relaxation is the process by which the energy that we put into the nuclear spins returns to how they were at equilibrium |
| Explain the definition of a Free Induction Decay | Free Induction Decay is the loss of signal due to relaxation |
| Demonstrate the role played by intrinsic and extrinsic magnetic field inhomogeneities (not uniform) | Intrinsic could be tissue characteristics, while extrinsic could be an issue with main magnetic field or hardware limitations |
| The effect of the interactions that cause the system to return to equilibrium is know as: reversible dephasing of MRI signals OR irreversible dephasing of MRI signals | Irreversible dephasing of MRI signals is from the effect of the interactions that cause the system to return to equilibrium |
| How is a Spin Echo pulse sequence formed? What is the result of it | A spin echo can be formed by using a combination of a 90* and 180* RF pulse, which results in a refocusing of the magnetization |
| Differentiate between the process of T1 and T2 relaxation | T2 was caused from a loss of phase coherence T1 is characterized by the actual loss of energy in the spin system |
| Identify the physical processes that lead to T1 relaxation | T1 causes an actual net loss of energy to be transferred out of the spins (which happens because the stimulated transition results in a photon being emitted). T1 only occurs at Larmor Frequency |
| Explain the key mechanisms that drive differences in T1 | T1 is the change in magnetization in the longitudinal direction (parallel to B0). It measures the rate of return of the NMV to its equilibrium. It is caused from coupling between spins and environment |
| Describe a pulse sequence for measuring T1 | Inversion Recovery -> two RF pulses, 180* RF pulse first, wait some time, then 90* RF pulse. Repeated many times, wait for things back to equilibrium, vary time, and have info to measure T1 |
| Discuss the role played by molecular motion in determining the MR relaxation times | The rate of molecular wobbling relative to the Larmor Frequency determines the energy exchange and spin dephasing. |
| Discuss the reasons why different tissues in the body have different relaxation times | Different molecular structures, different states they are in such as solid/liquid/gas, some tissues flow while others are stationary |
| The contrast difference is high between 2 signals. Noise is also high. How is CNR? | Even though there's high contrast difference, with noise appearing high, the CNR will be low |
| There is a small difference between two signals, with low noise. How is CNR? | CNR will be higher, noise will be lower |
| What are the parts that make up the MRI magnet | The magnet, active shielding, patient bed, shim coils, gradient coils, RF coil, spectrometer |
| Examine how shim coils are both constructed and used | Shim coils are used to help make the magnetic field more uniform. they are similar to shims from carpentry. |
| Describe how gradient coils are constructed and analyze the magnetic field patterns that they create | Gradient coils are inside shim coils; 3 more coils of wire that when current goes through them, they create magnetic field gradients. Gradients are turned on and off rapidly to form part of pulse sequence |
| Examine the role of eddy currents and how they can be corrected | Unwanted flowing currents are eddy currents. Corrected with actively shielded gradient and eddy current compensation |
| Describe how gradient coils are constructed | Shim coils and gradient coils are built together, each shim coil shape and each gradient coil direction being wound on concentric cylinders |
| Discuss the magnetic field patterns created by gradient coils | Turned on and off quickly; slew rate. Gradient pulses then form part of the pulse sequence along with the RF that makes up creating an MR image |
| Summarize the spectrometer electronics | Heart of electronics: instructions, applies them in order to make image. Sends each instruction to any pulse/signal to whatever relevant amplifier (RF, gradient, shim) |
| Discuss how amplifiers interact between the spectrometers and their respective coils | Spectrometer creates pulses with very precise amplitude, frequency and phase. Helps make stronger signals to shims, RF, gradient amplifiers |
| Illustrate the basis of an LCR "resonant" circuit | Inductance (L), Capacitance (C), Resistance (R) Just a coil of wire, has inductance, and resistance. Attach solenoid to capacitors at right value, it will resonant. If put pulse of energy, slosh back and forth at solenoid |
| How can a LCR circuit produce an oscillating magnetic field | Just a coil of wire, has inductance, and resistance. Attach solenoid to capacitors at right value, it will resonant. If put pulse of energy, slosh back and forth at solenoid |
| Describe the construction of a simple RF coil | Solenoid (simple RF coil). has a wire, has inductance, and resistance. Attach solenoid to capacitors at right value, then will resonant at the Larmor frequency. |
| Identify the role played by anatomy specific RF coils | RF coils have anatomy "fit" into them. The higher the filling factor, the greater the SNR |
| Discuss the basis for phase array coils and how they improve image quality | Multiple RF coils integrated into one, each coil is independent. Each coil is very sensitive to signals from tissues around it, and less sensitive as you move away from it. Closer the tissue is to the coil, the better |
| Discuss what is meant by a magnetic field gradient in MRI | A gradient is a linear change in magnetic field strength |
| Explain how a gradient alters the field within a patient | A gradient can alter the field within a patient by spatially modulating the amplitude of the magnetic field (which is always in the Z direction) in a linearly varying fashion |
| Explain how a gradient affects the MR frequency and phase | Gradients can introduce a spatially varying MR frequency and phase |
| Explain the relationship between the shape/length of an RF pulse and its frequency profile | Module D - Topic 12 -> basic soft RF pulse is a shape called sinc "box car". The shorter the sinc is in time, the wider the boxcar is in frequency. The longer sinc is in time, the narrower the boxcar in frequency |
| Explain how a "soft" RF pulse and magnetic field gradients work together to perform slice selection. (Hint: think of what spins are excited) | Applying a soft (shaped) RF pulse while a gradient is on, the only the spins that will be excited are those within the slice of that patient that has those frequencies (slice selection) |
| Explain the key acquisition parameters (pulse width, center frequency, gradient strength) that define slice selection | Center frequency is choosing what the center of the RF pulse bandwidth is, pulse width is changing the length of the RF pulse and gradient strength impacts the greater range of Larmor Frequency across the patient |
| Explain how a signal recorded while a gradient is on relates to the Fourier Transform of an object | Fourier Transforming a signal recorded with a gradient being on, acquired a 1 dimensional image of the projection of that object |
| Explain an example of one-dimensional profile through an understanding of how frequency encodes space | Fourier Transforming a signal with a gradient on gives us a 1D image of the projection of that object. Projection meaning that we collapse an object down to 1D by summing up how much there is of an object at each point along the direction of projection |
| Describe a basic 1D frequency encoding pulse | Spins sit at Larmor frequency. RF pulse applied, detect signal from object. Gradient turned on, spins in different locations along gradient direction will experience different fields. Signal recorded. Fourier Transform = 1D image of projection |
| Identify how a signal recorded while a gradient is on relates to the Fourier Transform of the object | The Fourier Transform of the MR signal recorded with a gradient on is actually a plot of the signal amplitude vs position... (example: an image) |
| Explain what a soft RF pulse is | RF pulses that modulate amplitude during the pulse contain a narrow band of frequencies |
| Explain what a hard RF pulse is | RF pulses that simply switch on and off. Contain a very broad range of frequencies |
| How are gradient area and dephasing related? | The amount of dephasing caused by the frequency encoding gradient depends on the gradient area (how strong it is, how long it has been applied) |
| Discuss what is meant by k-space | K-Space is the Fourier Transform of our image It is the "space" in which we record data that we acquire in MR |
| Explain how gradient area and k-space trajectory are related (Module D) | The location within k-space is proportional to the gradient area! |
| How can we use k-space to better understand how to combine frequency and phase encoding | Frequency encoding has every time we digitize a new time point we move to a new location of k-space Phase encode use a gradient amplitude that changes with every repitition, moves us to new location in k-space |
| Explain a basic 2D frequency and phase encoded pulse sequence | A combination of frequency & phase encoding can fill k-space. Apply a phase encode gradient, followed by frequency encode gradient, we can collect the data needed to fill k-space, then Fourier Transform! |
| Explain the concept of "complex" k-space data | Complex numbers are made with real and imaginary numbers If we know the real (Mx) and imaginary (my) components, then we know the magnitude and phase data All MRI K-space data is "complex" |
| Explain a slice selective 2D dimensional gradient echo pulse sequence | Slice selection to resolve space in one direction Phase encoding to resolve space in another direction Frequency encode to resolve space in the third direction |
| Explain how a slice selective 2D gradient moves through k-space and generates a 2D Fourier representation of the object | Use slice selection to resolve space in one direction, phase encoding to resolve space in another direction, frequency encode to resolve space in the third direction. |
| Give a short definition of a Steady State Sequence | Condition where the TR is less than T1 and T2 relaxation times of tissues. Also defined generically as a stable condition that does not change over time |
| Explain spoiled imaging sequences, like FLASH | Fast Low Angle SHot = use a very short TR, a low flip angle, and spoiling the transverse magnetization before you do the next RF pulse to acquire another k-space line |
| Explain balanced imaging sequences, such as FISP (hint: NOT what it suppresses, how it actually works) | Uses a phase encode "re-winder gradient". This gradient is always equal and opposite to whatever phase encode gradient was applied. Gets around the issue of un-phased magnetization "re-appearing" during subsequent TR |
| Explain how FISP and FLASH can be extended to 3D | Excite the entire volume, collect true 3D k-space of object; phase encode in 2 directions, then Fourier Transform the data |
| Explain the concept of a spin echo train, and how it can be used to accelerate T2 weighted sequences | Repeat 180* pulse separated by a time TE, can measure a train of corresponding echoes (again separated by TE) More than one spin echo could allow for more than one line of k-space per 90* RF excitation pulse |
| Explain the FSE sequence and examine how image contrast can be manipulated through k-space ordering | FSE is a pulse sequence that uses multiple applications of the phase encoding gradient to encode multiline of k-space in a TR. |
| Explain the FLAIR method for additional contrast weighting | Making TI (inversion time) a different specific (long) value, we can eliminate signals from free fluids like CSF. This is called FLAIR (Fluid Attenuated Inversion Recovery) |
| Explain the STIR method for additional contrast weighting | By selecting TI (inversion time), we can decide which tissue we want to eliminate. If TI is a specific (short) value, we can eliminate signals from fat. |
| Provide an overview of single shot techniques | Single shot techniques are fast spin echo sequences where all the lines of k-space are acquired during a single TR period |
| Explain briefly what the Echo Planar Imaging sequence is | Use bipolar read-out gradients that alternate back and forth. In between each read-out, apply a very brief and small phase encode gradient to shift us one line up (or down) in k-space. Known as phase encode "blip" |
| Construct and discuss the Spiral Imaging pulse sequence | K-space data is acquired in a spiral formation sequence rather than in rows on the standard cartesian plane. Data collection starts at the center of the axis, and proceeds to spiral outward |
| Explain the concept of multi receiver arrays and how they can be used to acquire parallel information | Multiple independent coils are able to collect under sampled data, tissue is close to coils. Images could be collected, but then "un-alias" the images. |
| Explain how the sensitivity profiles of the receiver coils can be used to acquire independent data | Each independent coil is most sensitive only to tissues close to that coil. Can collect under-sampled data and get a non-aliased image IF simultaneously collecting data using multiple coils |
| Describe SENSE for parallel imaging in a very basic way (Hint: involves un-alias images) | Include spatial sensitivity coils in the image reconstruction and instead combine the information from the image, can "un-alias" the images. Under-sample and speed up data acquisition with parallel imaging. |
| Explain the overview of SMASH for parallel imaging | Similar to SENSE: rather than combining images, it combines the data in k-space |
| Discuss the benefits of parallel imaging | Under-sample and speed up acquisition time The more individual phased array coils we have, the more we can under-sample |
| Where is information about image contrast primarily located? | near the middle of k-space |
| If two vectors are pointing in the same direction, what is their cross product? Are they added together, subtraction, or it equals 0? | It equals 0 |
| In order to have good CNR, you can have one of two things. What are they? | High SNR High contrast between tissue |
| A coronal slice is produced using slice selection. What sort of parameters were used to create the slice? (slice select, frequency and phase encoding) | Slice select y-gradient Frequency encode z Phase encode x |
| A sagittal slice is acquired using slice selection. What sort of parameters were used to create the image? | An slice select x-gradient Frequency encode z Phase encode y |
| When frequency encoding, what should the minimum sampling rate be? | 2x highest frequency in signal obtained |
| Discuss the fundamental physical processes that occur during magnetic resonance | Nuclear magnetic resonance, nuclear spin. All atomic particles behave as they are spinning. Each has angular momentum, and can only have certain angular momentum values. Means things are quantized |
| Explain how nuclear spin relates to the generation of nuclear magnetization | Can only have certain angular momentum values. It is said to be quantized. If the spinning is quantized, the spinning is what creates the magnetic field, the magnetization is also quantized |
| What causes time course of a gradient amplitude to go faster or slow down than expected? | Eddy currents |
| In a general sense, what is resonance | when there is a particular natural frequency that something wants to absorb/emit energy |
| Describe what it is to be "in resonance" using a real life example | Push a kid on a swing. Based on rope length, it will swing back and forth at a specific length. Push it more or less often they wont swing as high (absorb energy). If at right frequency, they go higher |
| Explain the interactions between nuclear magnetization and an applied magnetic field lead to precession (hint: think hydrogen example) | A hydrogen nucleus in a magnetic field wobbles in a circular path. Every single hydrogen nucleus wobbles (precession) at a very specific frequency. Precession path is a circle that is about the magnetic field that was applied |
| Discuss the Larmor Equation. What is its role in MRI? | Larmor Frequency is the precessional rotation per second of the nuclear magnetic moment about the external magnetic field (B0) |
| What does an RF pulse do to energy levels | The RF pulse results in a resonant absorption of energy that causes the spins to change energy levels |
| How do RF pulses interact with the energy levels of nuclear spins? | The RF pulse results in a resonant absorption of energy that causes the spins to change energy levels |
| Discuss the role of the "rotating frame of reference" in understanding MRI pulse sequences | Always assume B0 is pointed in the z-direction A coordinate system which is rotating about the z-direction An RF pulse results in a rotation of the NMV away from z-direction, into the x-y plane |
| Discuss the process by which applied magnetic fields and the net magnetic vector interact | If B1 is orientated in the x-direction and the equilibrium of M is in the z-direction, then the effect of the pulse is a rotation away from z and into the x-y plane |
| Is there a connection between how an RF pulse works and the concept of an RF flip angle? | The angle by which the NMV rotates due to an RF pulse is known as the flip angle. The flip angle is controlled by the amplitude and length of the RF pulse |
| Summarize the concept of MRI signal phase | With an RF pulse in affect on the NMV is a magnetic field that spins (oscillating) around. Changing magnetic field induces voltage in anything that conducts. If a coil of wire is around something, voltage is picked up to measure and digitize |
| Discuss the overall concept of magnetic resonance relaxation | Relaxation is the process by which the energy that we put into the nuclear spins returns to how they were at equilibrium |
| What is Free Induction Decay (FID)? | Loss of signal due to relaxation |
| What is the role played by intrinsic magnetic field inhomogeneities | Intrinsic inhomogeneities could come from the characteristics of the tissue being examined |
| What is the role played by extrinsic magnetic field inhomogeneities | Extrinsic inhomogeneities could come from imperfections with the main magnetic field or from external sources like hardware limitations or presence of ferromagnetic materials near scanner |
| Irreversible dephasing of MRI signals is from the effect of interactions that ______ ___ ___ __ _____ __ _________. | Irreversible dephasing of MRI signals is from the effect of the interactions that cause the system to return to equilibrium |
| Discuss the physical principles underlying reversible vs irreversible dephasing | |
| Discuss the Spin Echo pulse sequence | A spin echo can be formed by using a combination of a 90* and 180* RF pulse, which results in a refocusing of the magnetization |
| Why does a spin echo form? | Forms when two successive RF pulses of any flip angle are employed |
| How do you characterize T1 relaxation? (Hint: think loss of ______ ______) | T1 is characterized by the actual loss of energy in the spin system |
| How is T2 relaxation caused? (Hint: loss of ______ _____) | T2 was caused from a loss of phase coherence |
| What are the physical processes that lead to T1? | T1 causes an actual net loss of energy to be transferred out of the spins (which happens because the stimulated transition results in a photon being emitted). T1 only occurs at Larmor Frequency |
| Describe the key mechanisms that drive differences in T1 | T1 is the change in magnetization in the longitudinal direction (parallel to B0). It measures the rate of return of the NMV to its equilibrium. It is caused from coupling between spins and environment |
| Explain a pulse sequence for measuring T1 (hint: inversion recovery) | Inversion Recovery -> two RF pulses, 180* RF pulse first, wait some time, then 90* RF pulse. Repeated many times, wait for things back to equilibrium, vary time, and have info to measure T1 |
| What is the mechanism of T1? The direction is it in after the RF pulse? | The mechanism of T1 depends on their being motion of magnetic dipoles at the Larmor frequency. T1 is the rate of recovery of magnetization in the longitudinal direction after RF pulse |
| Why do different tissues in the body have different relaxation times | Different molecular structures, different states they are in such as solid/liquid/gas, some tissues flow while others are stationary |
| What does STIR stand for? What does it do? | Short Tau Inversion Recovery It can use a TI with a very specific (short) value to eliminate signals from fat |
| What does FLAIR stand for? What does it do? | Fluid Attenuation Inversion Recovery It is able to utilize a specific (long) value to eliminate signals from CSF |
| What parts make up the MRI magnet? | Magnetic, patient bed, active shielding, shim coils, gradient coils, rf coil over patient |
| Explain how shim coils are made and used | Shim coils are used to help make the magnetic field more uniform. they are similar to shims from carpentry. |
| Explain how gradient coils are made | Gradient coils are inside shim coils; 3 more coils of wire that when current goes through them, they create magnetic field gradients. |
| Explain how gradient coils analyze the magnetic field patterns that they create | Gradients are turned on and off rapidly to form part of pulse sequence |
| What are the two key points of the Steady State? | Magnetization is less on subsequent RF pulses (Magnetization Saturation) Repeat RF pulses with short TR, same magnetization value, but lower then equilibrium. Known as steady state magnetization value |
| What is the Ernst Angle? | the flip angle that generates the highest signal intensity in a tissue with a given T1 recovery time and in a given TR |
| What is Spoiling in MRI? How does it occur? | The act of removing transverse magnetization. This occurs when large gradients are added to the end of a pulse sequence |
| What are some clinical applications of FLASH imaging? | MR angiography, single breath hold abdominal imaging, cardiac imaging, 3D imaging |
| What is FISP? | Fast Imaging with Steady state Precession |
| How does FISP work? (think TR and rephasing) | Utilizes very short TRs and involves rephasing of the effects of gradients applied during the sequence |
| What are two clinical applications for FISP imaging? | Good to use in urinary/biliary tract imaging |
| During EPI, are k-space lines acquired in multiple acquisitions, or a single acquisition? | All k-space lines are filled in a single acquisition in echo planar imaging |
| The k-space lines are filled in a ________ pattern during Echo Planar Imaging | Raster pattern |
| The k-space lines are filled in a ______ pattern during Spiral Imaging | Spiral pattern |
| What are the advantages of Echo Planar Imaging? | Imaging speed. Ability to image an entire slice in 50-100ms |
| What are the disadvantages of Echo Planar Imaging? | Lower SNR compared to FSE or FLASH because the bandwidth is so much larger (which lets in more noise) More sensitive to magnetic field inhomogeneities which leads to image distortions |
| What does SENSE stand for in MRI? | SENSitivity Encoding |
| What is Aliasing? | "Wrapping" of the anatomy -> if an object may not fit into the FOV anymore, it wraps over a side of the image |
| What does SMASH stand for in MRI? | Simultaneous Acquisition of Spatial Harmonics |
| What is Parallel Imaging? | Technique that fills k-space more efficiently. Fills many lines of k-space per TR, acquired by assigned to certain coils coupled together in an array. Enables them to acquire data simultaneously |
| Explain the SMASH Parallel Technique in detail | Acquire (coil sensitivity, partial k-space) Reconstruct (missing lines of k-space using coil sensitivity maps) Combine (rather than combining images, combines data from k-space) |
| What is a hard RF pulse, how big/small is the range of frequencies? | Switch on, switch off. Contain a broad range of frequencies |
| What is a soft RF pulse, how big/small is the range of frequencies? | Continuously varying amplitude during the phase. Contain a narrow band of frequencies |
| concepts such as angular frequency is an equation (change in phase divided the change of time) | This flash card is just to remind Carter that the Larmor Frequency Equation exists |
| How can Saturation be used to introduce T1 weighting | Shorten TR, less signal from things that have a long T1. Some variation due to partial recovery. T1 contrast is introduced. Got T1 contrast by going faster at the cost of loss of control |
| What does Saturation refer to? How is it achieved? | Saturation refers to a state where the net magnetization of nuclear spins is lowered to 0. Achieved by applying an RF pulse that flips the magnetization, giving no net signal. |
| What is Inversion Recovery? What does it do? What is a disadvantage of it? | Inversion time, things regrew, get T1 contrast but able to control it more. Choose which one is black (back to flair/stir concept) Disadvantage is slower |
| When frequency encoding and the read-out gradient is twice that of the dephasing gradient area, where do we end up in k-space? | Acquired a complete line of k-space |
| Give a real life example of what it is like for something to be in Resonance | Push a kid on a swing. Based on rope length, it will swing back and forth at a specific length. Push it more or less often they wont swing as high (absorb energy). If at right frequency, they go higher |
| TRUE OR FALSE: The resonance frequency of a nucleus is directly proportional to a magnetic field it is in | TRUE |
| How is the flip angle controlled? | By the length and amplitude of an RF pulse |
| TRUE OR FALSE: The RF pulse does not lock every individual magnetic moment into phase | FALSE: The RF pulse locks ALL the individual magnetic moments into phase |
| TRUE OR FALSE: The Free Induction Decay rate is gradual | FALSE: The FID decay rate is exponential |
| What does T2* stand for? | It stands for the decay time constant |
| What sequence is also known as Spin-Spin Relaxation? | T2 -> It involves interactions between spins |
| The amplitude of the spin echo is described by _____ (T1 or T2), which is the irreversible dephasing of the component | T2 |
| Which of these is NOT typically used to describe a magnetic field gradient? (Slew rate, phase, direction, amplitude) | Phase |
| TRUE OR FALSE: As TE gets longer, SNR usually decreases | FALSE |
| What sequence is also known as Spin=Lattice Relaxation? | T1 -> the actual net loss of energy to be transferred out of the spins |
| What is STIR? What is it meant to do? | Short Tau Inversion Recovery -> A pulse sequences that suppresses signal from fat |
| What is FSE / TSE? | Pulse sequence that uses multiple applications of the phase-encoding gradient to encode many lines of k-space in a TR |
| What is the Zeeman Effect? | coupling between hydrogen nuclei and the external magnetic field B0 |
| What is shimming? | Process where the evenness of the magnetic field is optimized |
| What is Single Shot FSE? (SS-FSE) | Fast spin-echo sequence where all the lines of k-space are acquired during a single TR period |
| What is the Slew Rate? | The strength of the gradient over distance |
| What is Partial Volume? | The loss of spatial resolution when large voxels are used |
| What is Parallel Imaging? | Technique that uses multiple coils to fill segments of k-space |
| What does NEX mean? | Number of excitations. Also know as NSA -> number of signal averages. Number of times a line of k-space is filled with data |
| What is K-space? | An area in the array processor where data on spatial frequencies is stored |
| T2* is defined as the time it takes for magnetization to decay to ____ of its original amplitude | 37% |
| What is the real reason that dephasing caused by T2 mechanisms is random? | Caused by random magnetic fields of other spins that are moving Therefore, the magnetic field and hence precession frequency are not the same before and after a 180* pulse |
| What is the Fringe Field? | the magnetic field outside the bore of the magnet |
| The spin echo only reverses the portion of dephasing that was due to ______ | inhomogeneity of the magnetic field and not that due to other spins |
| What is Echo Planar Imaging? (EPI) | Single shot or multi-shot acquisition that fills k-space with data from gradient echoes |
| T1 is specifically the time required for magnetization to return to ____ of its equilibrium value | 63% of its equilibrium value |
| When does T1 relaxation occur? | When there is motion at the Larmor Frequency |
| When does the best T2 relaxation occur? | When there is slow (or even no motion) |
| How are Eddy Currents caused? Are they useful? | No they are not useful. They are caused by pulsing a gradient to quickly and there is a gradient field in the other metal objects in the MRI. |
| How can we cancel out Eddy Currents? | Activate a second gradient coil winding that is in the opposite direction and larger diameter (actively shielded gradient) Eddy current compensation |
| If I want to use slice selection to create an axial image, what are my parameters for slice select, frequency and phase encoding? | Use the z-gradient, y frequency encode, and x phase encode |
| What does a Radio Frequency (RF) coil do? | both transmits and receives RF energy. Acts as an antenna |
| What are the requirements for an RF coil? (3) | RF field needs to be as uniform as possible over anatomy Magnetic field component needs to be orthogonal to the direction of B0 Shape of coil needs to conform anatomy as well as possible! |
| If current passes through a solenoid in a certain direction, it creates a uniform magnetic field inside the solenoid. The more current (energy) flowing through it, the ______ ______ ____ _____ | stronger the magnetic field |
| What are the names of the three acquisition parameters that define slice selection? | Pulse width, center frequency, gradient strength |
| In MRI, we don't actually acquire an image. We acquire the ______ _______ of the image | We acquire the Fourier Transform of the image (Module D - Topic 13) |
| What is pulse width? | Changing the length of a pulse |
| What is center frequency? | Choosing the center of the RF pulse bandwidth will effectively change the center frequency. This allows for selection of the slice I want |
| What is gradient strength when referring to slice selection? | The stronger the gradient, the greater range of the Larmor Frequency across the patient |
| Explain the role played by the flip angle in determining contrast for steady state sequences (Hint: think of saturation) | Using a TR much less than T1 results in saturation, and a low flip angle decreases the saturation, with the Ernst angle having the highest SNR for a given TR and T1 |
| Examine how FSE image contrast can be manipulated through k-space ordering (hint: think TE eff) | The EFFECTIVE ECHO TIME (the echo time that determines T2 contrast) is equal to the true TE x the number of echoes acquired to reach the middle of k-space (where image contrast is determined) |
| Do Fat molecules have a very efficient T1 or T2? | Very efficient T1 because of their spectral density |
| Does gray matter have a shorter T1 than white matter? | NO: White matter has a shorter T1 than gray matter because it is "white" from all the lipids! |
| Why is the T2 in the liver short? | Due to all the fat having all the iron (which is magnetic and therefore causes dephasing) |
| Why is T2 long in CSF? | T2 is long in CSF due to it being basically water, which tumbles quickly |
| Which is almost always longer, T1 or T2? | T1 is almost always longer than T2, with typical T1s of 100s of milliseconds and typical T2s of 10s of milliseconds in the human body |
| What is FISP? What sort of imaging is it used for? | Fast Imaging with Steady State Precession Good for urinary/biliary tract imaging |
| What is SS-FSE? Basic on how it works? | Single-Shot Fast Spin Echo Fast spin echo, all lines of k-space are acquired during a single TR period |
| What is SMASH? Give the basis on how it works | Simultaneous Acquisition of Spatial Harmonics Acquire (coil sensitivity, partial k-space) Reconstruct (missing lines of k-space, using coil sensitivity maps) Combine (Combines data in k-space) |
| What is SENSE? Give the basis on how it works | SENSitivity Encoding Acquire (coil sensitivity, partial k-space) Reconstruct (images from each coil) Combine (Combines images from all coils using coil sensitive maps) |
| What is the basis for Parallel Imaging? | Under-sample using phased array coil, then use knowledge of coil sensitivity maps to get the image |
| What are the drawbacks of Parallel Imaging? | Low SNR as we are under-sampling and speeding up our acquisition time |
| An image was created using a Z-gradient, y frequency encoding and x as phase encoding. What sort of image was produced? | Axial |
| An image was created using a X-gradient, Z frequency encoding and y phase encoding. What sort of image was produced? | Sagittal |
| An image was created using a Y-gradient, Z frequency encoding and x phase encoding. What sort of image was produced? | Coronal |
| How much dephasing occurs when the gradients are at the exact point when the area of the readout gradient is equal but opposite area of the dephasing gradient? | Zero; spins are stationary |
| The longer the RF pulse, the ______ the bandwidth | narrower |
| The shorter the RF pulse, the _____ the bandwidth | broader |
| A gradient is on a specific amplitude where it moves in the positive y direction. The gradient changes so it slowly goes back towards zero. What happens next? | Movement in the Ky direction slows |
| How is the maximum cross product magnitude reached? | When the original vectors themselves are perpendicular (sin90* = 1) |
| What is the Fourier Transform in MRI | Complex math operation that converts a signal measured as a function of time ---> CONVERTS IT to a plot of signal as a function of frequency |
| What does the Fourier Transform show us? (hint: frequency) | What frequencies we measured and what the amplitude was of each frequency |
| If high spatial frequencies are thrown away, and we only have frequencies that are closer to zero, how will the image appear? | It will appear blurry due |
| TRUE OR FALSE: The resonance frequency of a nucleus is DIRECTLY proportional to a magnetic field it is in | TRUE |
| What does B1 refer to? | Refers to another magnetic field that is weaker than the external magnetic field |
| What happens to MRI signal once an RF excitation pulse is turned off? | Spins begin to dephase Causes the signal meant to be measured to decrease over time, because the net magnetization will slowly shrink as the individual magnetic moments become out of phase (as they were at equilibrium |
| The B1 pulse is only effective if it is ________ to the main magnetic field | Orthogonal |
| What is something that we need to be aware of that could be dangerous to human tissue when creating a B1 pulse? | SAR (Specific Adsorption Rate) Create electrons to jiggle, causes them to heat up Health Canada put a limit on SAR (limits how strong an RF pulse is) |
| A patient is getting an MRI scan of their brain, and the requisition states "potential subarachnoid cyst". What protocol / TR, TE will you choose? Why? | A long TR and long TE will be used to find the cyst as water has a long T2, and the cyst contains water |
| An axial image of the abdomen shows the liver appearing dark. What TR/TE or what sequence was used? | A T2 sequence was used if the liver is appearing dark. The liver has lots of iron (which has a short T2), causing it to appear dark. |
| What is the name of the sequence that uses a very short TR, a low flip angle, and spoiling the transverse magnetization before the next RF pulse to acquire another line of K-space? | This sequence is know at FLASH -> Fast Low Angle SHot |
| What sort of images does the FLASH sequence produce? | Very commonly used to get routine T1-weighted anatomical images |
| What is the SPGR sequence? | SPoiled GRadient Echo. It is another term for FLASH sequence |
| What kind of image contrast is produced with FISP imaging? | Complex mix of T1 and T2* |
| Does FISP imaging produce a low SNR or a high SNR? | High SNR |
Created by:
192846
Popular Czech sets