MRI

What is longitudinal magnetization?

The component of the net magnetization vector parallel to the main magnetic field. Mz = M_0*(1-exp(-t/T1))

What are some parallel imaging artifacts:

They can have slightly different versions of other artifacts. Aliasing may occur. SENSE ghosts is aliasing that has more distortions in the center of the image. Inhomogeneity and noise increase with acceleration.

Explain RF artifacts and potential solutions:

Zipper artifacts can be caused by EM energy leaking in. Check electronics, room shielding, lights for potential causes. Slice crosstalk artifact from adjacent slices can be solved with slice interleaving, 2 groups of slices where they interleave like a zipper when combined, each set has a larger gab between images. Doubles image time unless you solve it with rectangular RF pulses.

MRI acceptance tests?

5 gauss line measurement and marking. Vibration measurement: Vibrations can cause coils to become un coupled and can cause artifacts RF shielding test Physics testing for coils and image quality Physical mechanical checks: table motion, accuracy, docking, ventilation, lighting Software and networking check Homogeneity check Gain calibration Gradient accuracy check post homogeneity

Describe inversion recovery pulse and how it works. Give two examples.

A 180 degree pulse flips the magnetization into the -z direction before excitation pulses come. The timing of the excitation pulse after the inversion time can be used to eliminate or reduce signals from certain tissues. STIR and FLAIR are common examples. STIR=short tau inversion recover suppresses fat signals in favor of water. FLAIR = Fluid Attenuation Inversion Recovery to suppress CSF.

Explain magnetic resonance spectroscopy:

A way to measure tissue chemistry by recording signals from metabolites through frequency shifts in PPM. Can do in-vivo exams but in-vitro allows for extra scan time and better SNR. Chemical shifts from electron cloud shielding of nuclei, which creates difference frequencies. Use chemical saturation techniques like CHESS (Chemical shift selective) or STIR to remove water and fat signals. Can have single voxel option for initial diagnosis (high SNR), then multivoxel to assess distribution of specific metabolite.

Describe aliasing and when does it occur:

Aliasing is also called phase wraparound. It's usually caused by having anatomy outside of the FOV. This can be on the sides of 2D image or at the ends of 3D images.

Describe the MR safety conditionality assessment for questionable objects:

All sights should have a minimum 1000 G handheld magnet or ferromagnetic detection device to test objects with. All portable objects should be screened and in writing. Exceptions that are deemed necessary should only be brought in under direct supervision and should be physically restricted at all times. If it is not clearly indicated, never assume an objects safety compatibility. Document the results, date, time, and tester. All metallic objects should be properly labeled as MR safe, unsafe, or conditional.

Describe Gibbs truncation artifacts:

Also called ringing artifact, it occurs at boundaries where intense and abrupt changes occur. Creates parallel lines adjacent to high contrast surfaces. It's caused by Fourier harmonics having overshoot at the corner instead of resulting in a perfect rectangle. Can be minimalized by increasing the number of k-space lines in the dimension that it's happening in. They can't be completely eliminated. Post processing can help.

Dielectric Artifact?

Are caused RF wavelengths in tissues as a function of field strength. Stronger field strengths create smaller wavelengths that affect anatomy that is the same or smaller size Causes abnormally bright and dark areas from B1 inhomogeneity. Particularly prominent in 3T body imaging and obese patients

Safety risks of cryogens?

Asphyxiation f gases replace oxygen in air. Frostbite if temperatures get exceedingly low or people come into contact with the liquids. Fire hazards in the event of a quench where gases escape into the magnet room. Pressure considerations are rare, but are possible.

Explain k-space errors and how they can be fixe:

Bad datapoints in k-space such as with spikes results in dark streaks across the image. Usually caused by breakdown in connection within the coil. Coils may need to be replaced. Reprocessing of data in certain cases can help. RF data clipping means the gain is set too high and the unit must be recalibrated.

Why is a refocusing gradient needed during slice selection?

Brings the transverse direction phase dispersal back into line to prevent signal loss.

MRI siting concerns?

Can't have ferromagnetic materials. Walls should be concrete. Magnet should have venting ducts. The room should have a loading area for liquid helium and space to install the magnet. If multiple MRs are near each other, each magnet should be shielded. Shimming and MRI orientation is important here. Shielding to decrease fringe fields. Should have barriers to prevent accidental introduction of ferromagnetic objects to the room with labeled warnings.

Alternate methods to fill k-space:

Centric filling: fill center frequency lines first before the outer 2/3rds Keywhole: fill outer 2/3rds then center Sprial: Fills center of k-space efficiently first Propeller (Periodic Rotatied Overlapping Parallel Lines with Enhanced Reconstruction): Redundant info at the center of k-space improves SNR.

What are the RF attenuation limits for the magnet room?

Commonly 100 dB attenuation at 100MHz for 1.5 T scanners. (150-170 MHz for 3T). dB = 10*log(P2/P1); where P is power Faraday cage shield should also be electronically isolated from the building ground.

What is transverse magnetization?

Component of synchronized precession. If the magnetic field is in the Z direction, the transverse magnetization would rotate about the x-y axis at the angular frequency equal to the Larmor frequency. Mxy = M_0*(exp(-t/T2))

Coil layout from out to bore center?

Cryosystem: Active shield coil, active shim, main magnet coil Gradient shielding Gradient coils Active shimming resistive coils Passive shimming RF body coils Bore

Describe object and device screening for MRI safety:

Devices that are not explicitly documented as MRI safe should be screened and documented as MR safe, conditional, or unsafe. It should not be allowed unrestricted access through zone 3 otherwise. Use the handheld >=1000 G magnet to check for detectable magnetic forces. If none are felt, it is labeled Conditional. Only non-metal objects can be labeled Safe. Unsafe devices can still be permitted to the scanning room in specific situations. Should be under direct supervision of designated level 1 or 2 personnel and be physically restrained.

Define diamagnetic, paramagnetic, and ferromagnetic.

Diamagnetic: materials are slightly negative susceptibility and oppose applied magnetic fields Paramagnetic: materials are slightly positive susceptibility and enhance magnetic fields locally Ferromagnetic: materials augment magnetic fields. Sometimes called superparamagnetic.

Describe the sources of chemical shifts? What is the fat-water shift? How does chemical shift affect imaging and spectroscopy?

Different materials have slightly different resonant frequencies. Fat-water has a shift of ~3.5 ppm, which will result in different frequency changes vs magnetic field strength. Spectroscopy will measure signals at different frequencies depending on material composition.

Image phase stability artifacts (ghosting) causes and potential solutions?

EPI can cause read direction artifacts from deviations in linear course of gradients. Caused by eddy currents or poor shimming. This results in phase mismatching that creates ghosting.

Explain EPI:

Echo planar imaging moves back and forth across the frequency direction to image multiple lines of k-space with a single TR.

How do materials react to magnetic fields?

Electrons possess spins as they orbit the nucleus as well as angular momentum. Paired electrons cancels the magnetic field. Magnetic fields exist in dipoles. North is the origin of field lines, south is the return.

What are the Bloch Equations?

Equations describing the motion M in components X, Y, and Z: Mx(t) = M0*exp(-t/T2)*sin(wt) My(t)=M0*exp(-t/T2)*cos(wt) Mz(t)=M0*(1-exp(-t/T1))

Explain MRI transfer contrast and its use:

Excite a separate body of protons that transfers its excitation to an adjacent group of unexcited protons to create image with reduced signal while decreasing the signal in the bulk water. Used to image the heart, eye, knee cartilage. Used along with MRA to suppress unwanted signal and improve contrast of flow enhanced signal.

How do k-space and image space relate?

FOV_x = 1/dv FOV_v = 1/dx dv is the k-space step size, x is image space pixel size

General MRI unit considerations for support electronics?

Fringe fields above a certain amount can effect electronics with slow moving electron beams, such as monitors and color TVs. Commercial products typically won't have limits or recommendations. Magnetic storage drives may be erased if not careful. PMTs from CT scanners and cameras can also be affected. (AAPM rpt 20 has a table or recommendations). Unplug and remove all unused electronic devices from the MR room prior to scan. All coils should be checked by the scanning technologist. Electrically conductive materials may heat up, which is a risk. Avoid loops.

Risks associated with contrast agents in MRI?

Gadolinium naturally is toxic and must be bound and prevented from accumulating in the body. Reactions range from mild (nausea and vomiting) to severe (allergic-like reactions). Premedication can help prevent reactions. Nephrogenic systemic fibrosis was linked to use of certain gadolinium contrast agents and occurs in patients with chronic kidney diseases. It is defined as an estimated glomerular filtration rate of <30 mL/min/1.73 m2. (Normal is 90 mL/min or higher). Contrast-induced nephropathy is impaired kidney function 48-72 hours after contrast injection. Can take up to 2 weeks to recover.

ACR Recommended action criteria for the large phantom?

Geometric accuracy within +/- 2 mm from true values. Failure at >=3 mm difference. Gradients poorly calibrated and field inhomogeneity can cause this. High contrast spatial resolution must see <=1 mm resolution in both directions. Too much image filter, eddy currents, and ghosting can cause failure. Slice thickness ramps must be 5mm +/- 0.7 mm. Can be caused by distorted RF pulse shapes. Slice thickness = 0.2(top ramp*bottom ramp)/(top+bottom) Slice position accuracy bar difference must be <=5mm. Most commonly caused by slice prescription error and poor positioning. Image uniformity should have a PIU of >=87.5%. PIU = 100*(1-(high-low)/(low+high)) Commonly caused by poor phantom positioning within the coil and image ghosting Percent signal ghosting should be <=2.5%. Ghosting ratio = (top+bottom-(left+right))/(2*large ROI) Ghosting can be caused by vibration or instability within the measured signal or pulse cycle. Low Contrast Detectability must see >=30 spokes for 1.5T> <3T T1, >=25 for T2, and >=37 spokes for >=3T. Most commonly caused by mispositioned slices, phantom tilt, ghosting, or incorrect slice size.

Two different basic gradient coil designs?

Gradient coils are closed loops of wire where the magnetic field is slightly changed versus distance on the axis going through the center of the loop (like through the hole of a donut). Use the right hand rule for direction of increasing gradient field. Helmholtz coils have current facing the same direction to increase magnetic field in one direction. Maxwell coil configuration has current in opposite directions to raise the field in one direction and lower it in the other.

Explain a basic gradient echo pulse and spin echo pulse:

Gradient echo starts with a 90 degree excitation RF then allows signal to echo then decay. Flip angle does a lot to determine tissue contrasts. Longer TR means similar to spin echo, but with T2*. Spin echo starts with an initial 90 degree excitation RF pulse then a 180 degree refocusing pulse at time TE/2. The second pulse changes Mz to Mxy and removes extrinsic inhomogeneities and their dephasing effects. (Does not need to be and rarely is actually 90 and 180 degrees)

Gradient-field inhomogeneity:

Gradient fields can become distorted as distance from the isocenter increases. Can cause spatial distortion from incorrect distances if not corrected for. Usually software corrects for this.

Static field inhomogeneity artifacts are caused by what and how are they solved?

Inhomogeneity can cause geometric distortions, shading, blurring, decreases in intensity, improper fat sat. It's usually fixed by calling service engineers. Shim currents may need to be adjusted.

How does magnetic field strength change with distance for MRI?

It follows the inverse cube law, decreasing by a factor of d^3. Typical monopole follows a 1/d^2, but dipole fields follow a 1/d^3 relationship.

What's the difference between a linear and quadrature coil?

Linear polarization is a 2d wave from 2 coils. Circular polarization is from 4 coils and adds a dimension to the wave. Most common quadrature coils are birdcage coils and transverse electromagnetic coils.

MRI static field bioeffects:

Lorentz forces can affect the ionic currents in the ear and has been noted to cause nausea and dizziness at 7T and above. Motion induced currents from Faraday-Lenz Law can cause visual light flashes during eye motion, metallic taste during tongue motion, and can cause implants to shift if not careful. Susceptibility-induced forces can occur when objects are polarized within a magnetic field.

MRI Instrumentation:

MRI main unit will be composed of: Main magnet, gradient coils, RF coils to excite frequencies, passive shim sheets in the bore, patient within the bore, computer to control it all. High field MRIs may have a closed bore model. Resistive coils can be made of permanently magnetized bars, standard resistive coils with current running through, or superconducting. First two are "open systems". Resistive coils can be turned off if need be. Cryostat is the large volume of liquid helium. Cold head is the cryopump. Also serves to re-condense evaporated helium back to liquid. Chillers heat with water/ethylene glycol to cool coils and amplifiers. Passive RF shielding around the room and active shielding around the main coil windings but within the cryostat.

Equation for bulk magnetization (thermal equilibrium)?

M_0=N(gamma*h_bar)^2*B_0/(4kT); gamma is the gyromagnetic ratio = 42.57 MHz/T h_bar =6.626*10^-34/(2pi) J*sec k is the Boltzman Constant = 1.38*10^-23 J/K T is the absolute temperature in Kelvin B_0 is the magnetic field strength in T

What properties affect net magnetization?

Magnetic field strength, proton density, and temperature. At equilibrium, a slight majority of atoms will exist in the low-energy parallel direction to the magnetic field. N- = N+*(exp(-E/(kT))); N- and N+ are the higher and lower energy level aligned protons, respectively E is the energy difference between the spin states k is the Boltzmann's constant (1.3805*10^-23 J/K) T is the temperature in K

Explain metal susceptibility artifacts and how they may be mitigated?

Magnetic fields altered by objects with different susceptibilities. Ferromagnetic objects are bright. Borders between bone/tissue, air/tissue can cause this. Reduce this with spin echo or lower TEs. Shim fields and larger acquisition matrices for better spatial resolution at borders can help. Metal effects can be reduced with larger BW_receive or removed with saturation bands.

Give an overview of magnetic resonance angiography (MRA) and artifacts:

Magnetic resonance imaging of the heart and blood vessels for evaluation of pathology. There are three main types of angiography, with contrast, without contrast, and and susceptibility weighted imaging (BOLD). Phase contrast angiography applies gradients to change phase linearly with velocity. Time of flight can be used to excite blood before entering the volume of interest. BOLD is sensitive to deoxyhemoglobin, which forms when oxygen is released from blood. Uses a long TE and susceptibility effects in the blood to visualize veins. Flow artifacts: stationary spins accumulate phase by T, moving spins by T^2, which will change during heart beats. Three lobed gradient can be used to undo this phase shift (Gradient Moment Nulling). If blood is not needed, can use pre-saturation bands to remove the flowing signal. Susceptibility artifacts are common around stents, clips, and endovascular coils. Can be mistaken for stenosis when mild and loss of flow when severe. High velocity signal loss is when protons flow out of the slice during echo reformation. Can also be caused by turbulant flow dephasing protons.

Conversion of Hz to PPM:

Meant to measure the Hz or PPM difference between materials. Fat/water difference is ~3.5 ppm, for example, so the difference in frequency is: df = G [T] x 42.58 [Mhz/T] x 3.5 [PPM]

Equations for T1 and T2 recovery?

Mz = M_0*(1-exp(-t/T1)) Mxy = M_0*exp(-t/T2)

What is the connection between the number of protons and neutrons and an atom's magnetic moment?

Nuclei with both even protons and neutrons have no magnetic moment (0 spin). Odd-odd creates whole spin, odd-even create fraction spins (1/2, 3/2, 5/2...)

Passive vs active shielding materials:

Passive shielding can be iron or steel beams within the walls, steel or copper plates. Active shielding is composed of at least one extra set of superconductive windings that opposes the magnetic field of the main windings. This shielding has field points in opposite directions to the main coils to do this.

What are shims in MR?

Passive shims are sheets/pellets of ferromagnetic metal placed inside the bore. Active shimming is done with coils placed within the scanner to homogenize the magnetic field. Can be superconducting coils within the cryosystem or resistive coils outside. Resistive coils are dynamic while the superconducting ones are set at installation.

Motion errors and how to solve them?

Patient motion changes the phase. Appears as ghosting and blurring in the phase direction. Use breath holds if able. Respiratory gating on longer acquisitions may work. Phase reordering can help compensate. Real-time navigator echo gating find the diaphragm position to help keep the images still. Electrocardiographic gating for heartbeats can be used. Small children may need to be sedated. Patients can be gently strapped down in certain cases.

SAR limits in MRI? (Specific Absorption Rate)

Per the FDA, SAR should be limited to produce less than a 1° C body-core temperature increase. SAR averaged over the patient mass over a 6-min period < 4 W/kg level 1(2 for normal). Partial body 4-10 W/kg for ratio between exposed mass and total patient mass over 6 minutes. Head is 3.2 W/kg averaged over 6 min. Extremity 40 W/kg over 6 min (20 for normal)

Describe perfusion MRI and functional MRI:

Perfusion uses tracers such as H2, He3, O17, F19 for tissues. Gadolinium for blood pool agent since it produces shorter T2*. Functional MRI looks at blood flow to specific areas of the brain. Uses Blood Oxygen Level Dependant acquisition techniques (BOLD) to get T2 weighted

How does the flip angle, RF pulse amplitude, shape, and duration affect the image signal?

RF pulse amplitude, shape, and duration affect the excitation flip angle. Flip angle = w * t w is the angular frequency; t is the pulse duration. (flip angle = RF amplitude * flip angle) A flip angle of 90 degrees gives the largest displacement and detectable MR signal. Different flip angles can change tissue contrasts.

Explain coherent gradient echo:

RF pulse is timed with dephasing and rephasing to generate an echo at a selected time TE. A rewinder phase gradient of same strength but opposite polarity after the echo to realign the phase after the target echo to prevent continueing echos (short TR will create multple echos). Little tissue contrast (reliant on T2/T1 contrast and tissues with long T2 often have long T1). Used for blood contrast techniques (angiography). Called Gradient Recalled Acquisition in the Steady State (GRASS, Fast Imaging with Stead-State Precession (FISP), and/or Fourier Acquitred Stead State (FAST).

How do RF coils excite select spins?

RF pulses resonate at the same frequency as desired nuclear spins (Larmor frequency) using tuning and matching. Better tuning and matching, better signal. This process is now done automatically during prescans. RF pulse inputs energy into atoms at the resonant frequency and excites them into the transverse plane.

Why are there RF filters on electrical lines? Why are waveguides on cables built with a 4:1 length:width ratio?

RF signal can interfere and create noise in signals traveling through power lines. Waveguides are needed to block RF signal. The 4:1 cylinder ratio will be more effective at blocking RF signal at the desired ranges.

Factors that effect SAR:

SAR = (sigma*A^2*w^2*B_1^2*D)/(2*m); sigma is the conductance = 1/Resistance A is tissue area w is the frequency B_1 is the RF field strength = B1*cos(wt) D is the duty cycle m is the mass

SNR vs receiver bandwidth, voxel size, flip angle, proton density, TR, TE, and number of excitations?

SNR increases with TR, voxel volume, proton density SNR decreases with TE, more excitations (EPI length), receiver bandwidth (more noise from unneeded frequencies) SNR can increase with flip angle. Ernst angle is the maximum SNR for a given TR and T1.

SNR vs MRI field strength?

SNR varies linearly with B_0

Explain parallel imaging:

Sample few lines in k space using phase array coils and reconstructing based on sensitivity maps to get complete images more quickly. SENSE (Phillips) is most common. Many vendors have some version of it. It has better SNR as the acceleration increases. Doesn't do as well in heterogeneous regions (lungs). Doesn't do well with patient motion, but software can correct for this. SENSE stands for Sensitivity Encoding. GRAPPA (Siemens) is the next most common. GRAPPA takes slightly longer. Has better tolerance with small FOVs. Works better with single shot EPIs. GRAPPA stands for Generalized Auto-calibrating Partial Parallel Acquisition. SENSE takes the image, reconstructs, then unwraps. GRAPPA takes the image, estimates missing lines in k-space iteratively using harmonics, then reconstructs to the complete image.

Explain steady-state free precession in MRI:

Short TR doesn't allow TE enough time to recover fully. SSFP sequence emphasices acquisition of only the stimulated echo (1 echo) and not other echos from previous RF pulses for short TR. RF pulsed are timed at 2 x TR where a rewinder gradient is used to speed up rephasing by initial RF pulse so echo reforms and ecayse just before next RF pulse.

Pregnancy considerations for MRI?

Should consider 3 questions: Will the reason for the exam affect the baby? Can it wait until after birth? Can the information be acquired through other non-ionizing means. Avoid contrast if possible. Some Gd will pass through the fetus and to the kidneys and liver. No recorded deleterious effects from MR or Gd on pregnant patients, but risk is not 0.

Explain the MRI signal receiver steps:

Signal is detected through a coil and goes through coil decoupling to separate MR output signal from the input signals. This signal is very weak so it goes through a preamplification using a low-noise preamplifier. The analog-to-digital converter receives this signal from shielded wires and can sample it to create digital signals (newer systems can sample to 16 bit data).

What is the slew rate? The rise time?

Slew rate is the rate of change in the gradient field strength. Rise time is the time it takes to ramp from 0 to maximum for a gradient.

What are eddy currents?

Small currents induced in the core material by the alternating flux Generate heat and waste energy. Opposes changes in gradient magnetic fields and can cause inaccuracies in gradient fields because of it. Also can cause peripheral nerve stimulation. Can use pre-emphasis to counter and reduce eddy currents with distorted input gradient waveforms. Other ways are gradient coil design and active shielding of gradient coils.

Define spoiling for MRI: How is it sometimes used:

Spoiling is meant to make sure there are no transverse signals remaining during steady state magnetization. Done by adding phase shift to successive RF pulses so it does not generate the eddy currents from gradient spoiling (the superior method), using long TR times, or apply the SS gradient and maybe the readout gradient at varying strengths (either linearly or semi-randomly). Very short TR steady-state acquisitions can't give T1 weighting because of small differences in longitudinal magnetization with small flip angles or the dominance of T2* effects for larger flip angles from residual transverse magnetization. Spoiling can reduce the T2* effect. Spoiled transverse magnetization gradient recalled echo (SPGR) is used for 3D acquisitions.

Quench procedures?

Staff should be knowledgeable of the hazards regarding magnetic field, cryogen hazards, oxygen sensors, and emergency response procedures. Procedures are facility specific.

Describe diffusion imaging:

Stationary water molecules are in phase and have stronger signal than moving water. Pulse sequence that is more sensitive to moving water must be used. Average displaced distance is 10 um over ~50 ms. Pulse sequence with T2 weighting has a diffusion sensitive gradient on either side of a 180 degree pulse, so phase is refocused to undo the phase change from the initial gradient for stationary molecules. Combine with fast imaging (EPI) to get an image. Diffusing water is detected as a signal loss. If movement is random, must choose a b-value, which is a math factor to indicate diffusion sensitization. 3D diffusion tensor imaging can be used in brains after a stroke.

What causes peripheral nerve stimulation, acoustic noise?

Strong gradients changing rapidly can cause peripheral nerve stimulation. Limits are in place for these changes in units of dB. Acoustic noise can be caused by certain sequences. Sound pressures can get as high as 110-130 dB. Also caused by switching gradient currents. Scans lasting for <15 min should be <115 dB and 60 min should have <105 dB. At no point should acoustic noise be >=140 dB.

Different types of RF coils:

Surface coils Used to achieve high SNR and resolution for anatomy near the surface of patient. Usually receive only. Volume coils: Uniform SNR over the entire image, but are larger and thus have lower SNR. Phased array coils: multiple coils and receivers created multiple overlapping loops. Each coil is small and gives good SNR. Coils come together to create a composite image. Multi-channel encoding coils: Have several (as many as 32) elements to perform parallel imaging.

Describe T1-weighting, T2-weighting, and proton:

T1: Short TE, Short TR T2: Long TE, Long TR Proton: Short TE, Long TR T1: bone is dark, white matter is bright, csf is dark, fat is bright T2: bone is dark, csf is bright, fat is bright, white matter is dark, gray matter is bright proton: csf is bright, fat is less bright, gray matter is light gray, white matter is dark gray Short TR: 5-10ms Short TE: 2-5 ms Long TR: 100-800 ms Long TE: 10-50 ms

How do T1, T2, and T2* relate to one another? How does B0 affect these values? How does field homogeneity and susceptibility affect these values?

T1>T2>T2* T2* accounts for the inhomogeneities that shorten the relaxation time. T1 is longer for larger B0 while T2 is unaffected. Magnetic field inhomogeneity shortens T2 to T2*. T1 values are unaffected. Susceptibility can decrease homogeneity, decreasing T2*. It will also change the B0 field within the vicinity, which will change the T1.

TE vs Effective TE:

TE is time from the last RF to the next Echo. Effective TE is used during steady state free precession (SSFP). Effective TE is the time from the echo to the RF pulse that created it (Short TR may result in different values so effective TE may be longer than TR). Effective TE = 2 x TR - actual TE

Describe superconductivity:

Temperature is dropped to T_c (critical temperature) to eliminate electrical resistance almost completely, letting electrons flow freely without additional energy. (liquid helium boils at 4.2 K=-269 deg C) NbTi alloy is common in MRI. It's embedded in an insulating copper matrix.

What is the Ernst Angle?

The flip angle that produces maximum signal intensity in a tissue with a given T1 recovery time in a given TR. Ernst Angle = arccos(exp(-TR/T1))

Which component, longitudinal or transverse, is directly measurable during MR? Which must be indirectly measured?

The transverse signal can be directly measured. The longitudinal must be indirectly measured.

What is the relationship between the receiver bandwidth and the frequency FOV? Dwell time?

They are one and the same. Dwell Time = 1/BW_R; it's the time between ADC samples

Tradeoffs between bandwidth, FOV, gradient levels?

Transmit bandwidth helps determine the size of the excited slice. Receiver bandwidth determines the range of frequencies read to create the image. Larger receiver bandwidths means more frequencies. Stronger gradients means larger change in frequency per unit length for the imaging object. FOV is determined by the receiver bandwidth and the frequencies per pixel.

Explain how slices are selected during MRI acquisition: What happens if this is done imperfectly?

Transmit bandwidth selects the target frequencies within the gradient to excite (gradient from negative to positive value with "null gradient" in the middle). Excitation RF pulse excites those frequencies to select the slice. Narrow transmit BW and low gradient strength gives better slice resolution, but can lower SNR. If done incorrectly, slices are incorrectly selected, can cause slice crosstalk artifact (cross-excitation) if slices are closely spaced, or zipper artifacts in the frequency encode direction.

Transmit vs receive vs transmit/receive coils?

Transmit only are in the scanner body coil. (Transmit a homogenous B1) Receive only coils are as close to the ROI as possible. (maximum SNR). Resonates and stores energy at Larmor frequency. Transmit receive coils are special coils where the receive coils detune during transmit mode to protect them. (Low SAR) Often separated to improve SNR.

Describe a birdcage coil:

Two RF-transmit rings connected to conductive elements to make a cage. Can serve as both RF transmitters and receivers.

What is the Larmor equation?

W_0 = B_0 x lambda; B_0 is the magnetic field strength lambda is the Larmor frequency (42.58 MHz/T for H2O)

What is Faraday's Law?

When a wire is moved across a magnetic field, a small EMF is produced in the wire. Summarizes the way voltage can be generated. Depends on: -the strength of the magnetic field -velocity of the magnetic field as it moves past the conductor -angle of the conductor to the magnetic field -number of turns in the conductor

Define the MRI safety zones:

Zone 1: All areas freely accessible to the general public without supervision. Magnetic fields are < 0.5 mT (5 G) Zone 2: Still a public area, but it is between zones 1 and 3. Safety screening typically happens here. It has an increased degree of restriction. Zone 3: Area near the magnet room. Fringe fields are strong enough to potentially be a physical hazard to unscreened patients and personnel. Should have physical restrictions and be under personnel supervision and the MR medical director or level 2-designated physician. Zones should be clearly marked, even in unoccupied areas. Zone 4: The magnet room. Has the strongest field and all ferromagnetic objects must be excluded. This will be within zone 3 and be marked with as potentially hazardous. Must provide direct visual observation by level 2 personnel for access pathways. Should have lighted signs for the magnet being on.

What are actively shielded gradient coils for?

Counteracts the gradient field outside of the shield coil to reduce eddy currents. Surrounds the gradient coil with current running in an opposite direction to imaging gradient.

Rapid MRI techniques?

EPI, FSE, parallel imaging (SENSE, mSENSE, SMASH, GRAPPA)

Explain FSE:

Fast spin echo repeats the echo with multiple 180 degree pulses. Multiple lines of k space are scanned with each TR. May create too much tissue heating and will be less sensitive to susceptibility.

Describe the two kinds of chemical shift artifacts:

First kind is a fat-water shift. This shift is from the frequency difference between fat and water. If the system is mapped to water, it may spatially misread fat signals to be from a lower section of the gradient field. It looks like dark bands. Second kind is also called India Ink and phase cancellation artifact. Signals at tissue interfaces destructively interfere when they have both water and fat within a voxel. For 1.5T, it happens every 4.4 msec starting at 2.2 msec TE. Can be reduced by swapping the phase and frequency directions. Changing the receiver bandwidth can reduce the artifact as well. Fat suppression techniques work well.

MR contrast agents and indications?

Gadolinium or air. Gadolinium decreases T1 values in the surrounding tissue. Air creates pockets of no signal. Used to view organ structure or function. Can see blood flow as well. Brightens T1 images.

What is GRASE?

Gradient And Spin Echo hybrid sequence - Uses the speed of gradient and ability of RF to compensate for T2* effects (contains more 90 & 180 pulses) Start with a spin echo then follow up with gradient echos in an EPI acquisition.

Explain data synthesis:

Imaging can be sped up by taking advantage of the redundant nature of k-space. A full image with reduced SNR can be taken with 1/2+1 line of k-space in either the frequency or phase direction. Phase to reduce scan time, frequency to reduce motion artifacts.

What is the Quality (Q) factor?

Its a ratio of the stored and dissipated energy in a coil. It mathematically describes the efficiency to detect MR signal of a coil and is a dimensionless indicator. The SNR of the coil element is proportional to sqrt(Q) Q = average energy stored/average energy dissipated per cycle = w*L/R w is the Larmor frequency, L is the inductance of the coil, and R is the coil resistance. Q ratio = Q unloaded / Q loaded = (R coil + R sample)/R coil

Define MR safe, unsafe, and conditional:

MR safe: Device has no known hazards in all MR imaging environments. MR Conditional: No known hazards with specific conditions of use, such as field strength limits, magnetic field time rate of change (dB/dt), RF fields, and SAR. MR unsafe: Known to be hazardous in MR environments. Such as ferromagnetic objects.