MRI (physics)

What is needed to create a magnetic field?
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What does a magnet produce and what is its function?A static magnetic field - Function: to magnetize tissueWhat is Bo?Main magnetic fieldWhat is the static magnetic field measured in?TeslaWhat are 3 different types of magnets that can be used in an MR scanner?- Permanent magnet - Resistive magnet - Superconducting magnetWhat is a permanent magnet?Magnetic field produced by using permanently magnetized bricksWhat is a permanent magnet made of?Aluminum, nickel, cobalt alloy "alnico"When do we use permanent magnets?In open magnet scannersWhat is the magnetic field strength in a permanent magnet?Up to .35 TWhat are the issues with permanent magnets?- Heavy - Not able to produce a strong magnetic field which is important to image qualityRelationship between magnetic field strength and image quality?Increase magnetic field strength = increase in image qualityWhat do resistive magnets utilize?Principles of electromagnetism - EM are used to produce a static magnetic fieldWhat is the relationship between number of coils and field strength in MRI?MRI machines require multiple coils - Increasing # of coils, increases field strengthHow is current passed in resistive magnets?Current is passed through loops of wire which generates a static magnetic fieldWhy are resistive magnets called "resistive"?Because of the large amount of resistance produced in wiresWhat does increased resistance produce (resistive magnets)?Because there is an increase in current needed, there is more heat production, which leads to higher resistanceWhat is the field strength of a resistive magnet?Limited to under .7TWhat are 2 advantages to resistive magnets?- They can be turned off - They have a small fringe fieldWhat is the main issue with resistive magnets?There is a lot of resistance producedWhat are 3 disadvantages to resistive magnets?- They have high power consumption which limits the field strength - They require water to cool the system - There is a high operational cost because of high power consumptionWhat is higher in resistive magnets than in permanent magnets?field strengthWhat are superconducting magnets?Electromagnets but they use special materials and conditions to reduce electrical resistance to near zeroWhat do superconducting magnets allow for?Greater current flow resulting in a higher strength magnetic fieldWhat does resistance depend on?The material the wire is made of and the temperature of the wireWhat is the meaning of superconductivity?is a property of a material to conduct electricity without resistanceWhat does a superconducting magnet do to resistance and temperature?Eliminates resistance and decreases temperature so we can increase current to have a stronger magnet - Temperature is 0 kelvinHow is temperature maintained?- By the use of cyragons (helium or liquid nitrogen) - Allows coils to be kept at a low temperatureWhen is power needed for a superconducting magnet?when magnet is first installed the power is only needed to ramp up the magnet (only need power to the coil to start it once) - as long as there is 0 kelvin temp, additional power is not neededWhat materials are used for superconducting magnets?Niobium and titaniumWhat is the magnetic field strength in superconducting magnets?- 3 tesla in clinical - 7 tesla is approved for some use4 Advantages of a superconducting magnet?- Can be turned off - Low power consumption - Capable of high field strengths - Stable, homogeneous magnetic field3 Disadvantages to superconducting magnets?- Must maintain cryogens - Expensive - Larger fringe fieldWhat is the main magnetic field?- Within the bore, used for imaging - Where the pt is located, we want this as high as possibleWhat is the fringe field?Stray magnetic field outside the bore of the magnet - We want this reducedWhat is magnetic shielding used for?To confine the fringe fieldWhat are the 2 types of magnetic shielding?Active and passive shieldingWhat is the RF subsystem?Coils that transmit and receive RF signal - Used for image productionWhat are the 2 types of RF coils?- Transmitter - ReceiverWhat is the transmitter ?- Produces RF energy that excites hydrogen photons - Must be at a resonant frequency of hydrogen - Transmits signalWhat is the receiver?- Detects and collects the MR signal - Signal is used to create the imageWhat is a dual function coil?Can transmit and receive signalwhat are 4 examples of coils?1. chimney foot coil 2. flex abdomen coil 3. inherent coil 4. array coilwhat are chimney foot coils used for?can be used to scan the foot or the head (transmit/receiving coil)what are inherent coils used for?used for large body parts when a external coil is not needed - they are built into the machinewhat is an array coil used for?used to scan spine, or abdomenWhat is RF shielding used to block?blocks incoming RF signals which can interfere with the signal coming from the patientWhat is the material of RF shields and why?Copper or stainless steel is used to keep RF frequencies out of the roomWhere are the RF shields located?Walls, doors, and windowsWhat would happen if there was an RF leak coming into the MR room?we would see artifacts on the image - Can also happen if the door is not closed all the wayWhat is a gradient coil used for?To increase or decrease main magnetic field strength linearly over distance in small incrementsWhat are gradient coils made of?Thick bands of conductive material wrapped around a cylinder placed around the bore of the magnetWhat do gradient coils allow for?Spatial localizationWhat is spatial localization?Knowing where the signal is coming fromWhat are the 3 gradient coils?X coil, Y coil, Z coil - they produce loud noises during scanningwhere is the x coil positioned?Right to leftWhere is the y coil positioned?Anterior to posteriorWhere is the z coil positioned?Head to footWhat 3 things are required to produce a MRI image?1. Strong magnetic field 2. Radio frequency source 3. MRI active nucleusWhat are the strong magnetic field and radio frequency source a part of?Instrumentation - Magnetic field can come from a permanent, resistant or superconducting magnetWhat is the MRI active nucleus from?The patient is the nucleus that we are scanningWhat characteristics do MRI active nuclei have?Magnetic characteristicsHow can you crease a net magnetic charge?By the nucleus having an odd number of protons and neutronsWhat is a magnetic moment?a tiny magnetic field generated by a spinning MR active nuclei (spin, proton)How is the spin described in a magnetic moment?The spin is described as a magnetic dipole, since it contains two magnetic polesWhy does MRI use hydrogen atoms?Because hydrogen only has 1 proton in its nucleusWhat is the most abundant atom in the human body?HydrogenWhat are other MRI active nuclides other than hydrogen?Fluoride, carbon, nitrogen, oxygen, phosphor, hydrogenHow many protons are in hydrogens nucleus?1 proton which gives it a large magnetic momentWhat do all magnetic moments have?A north and a southHow will protons move if they do not have an external magnetic field?They will spin randomlyWhat happens when protons are exposed to a strong magnetic field?They will align with the external magnetic field lines - We need to align hydrogen atoms in the patient's body so that we can image themWhen a MRI active nuclei is place in a strong magnetic field (Bo) what 2 ways can the magnetic moments align?Parallel or antiparallelWhat does it mean if the magnetic moments align parallel and which nuclei will align parallel?- They align in the direction of the main magnetic field - Low energy nuclei because they do not have enough energy to oppose the magnetic forceWhat does it mean if the magnetic moments are anti-parallel and what nuclei will be antiparallel?- Opposite to the external magnetic field - High energy nuclei - Have enough energy to oppose the magnetic fieldWhen all the dipoles are random what is the net magnetic field?0Will there be more parallel or antiparallel?Parallel because majority of the hydrogen are at a lower energy stateHow do you determine net magnetization?- Magnetic moments of the parallel nuclei cancel out the smaller number of anti-parallel nuclei - Net magnetization vector is determined by adding parallel nuclei togetherHow is longitudinal magnetization produced?When the NMV is produced in the direction as the main (external) magnetic field (Bo)NMV example7 point up, 5 point down the NMV is the 2 arrows added togetherIf the NMV is parallel to the Bo what happens?- It cannot be measure - It has to be tipped into a transverse planWhat is RF pulse used for?- To distribute protons and move them from the longitudinal plane to the transverse - We give energy to the low energy protons to move them to transverseIf the protons are moved from longitudinal to transverse this is called...Transverse magnetizationWhat 2 things produces voltage in a coil?A change in magnetic field strength or directionWhich coil in MRI needs to have an induced voltage to produce an image?The receive coilWhat is precession?- When the main magnetic field produces an additional spin - The additional rotation is described as a "wobble"How do hydrogen nuclei move?They will follow a circular path (known as the precessional path) at a certain speedWhat does precession speed allow us to solve?- The frequency. - Stronger the magnetic field strength, the faster the hydrogen nuclei will precessWhat is the meaning of precissional or resonant frequency (lamor frequency)?The speed at which spins precess around BoWhat is the Larmor frequency equation?ω0 = B0 λWhat do the symbols in the lamor frequency equation stand for?- ω0= precessional frequency (MHz) - B0= magnetic field strength (T) - λ= gyro-magnetic ratio (MHz/T)Example of how to calculate larmor frequency:Bo= 2 λ= 42.6 2 x 42.6= 85.2 precessional frequencyDefine gyro-magnetic ratio:- The precessional frequency of MR active nuclei at 1T - MHz/T - gyro-magnetic ratio is a constant (always is the same number)What is the gyro-magnetic ratio of hydrogen?42.6MHz/TWhat is resonance?Transfer of energy between two objectsWhat is the RF pulse used for?To give energy to hydrogen protonsWhat is needed for the RF pulse to transfer energy (resonance) to the hydrogen?- RF pulse must have the same frequency as the precessional frequency of hydrogen - RF frequency = precessional frequency of H - Lamor frequencyWhat is the result of resonance or transfer of energy?- NMV moves away from the longitudinal to the transverse plane - Magnetic field changes directionWhat does faradays law state about changing magnetic field?If we change magnetic field (direction of strength) we will induce voltage in a coil of wireWhat happens in the receiving coil when the magnetic field direction is changing?- Receiving coil will pick up the change in direction and produce voltage - Voltage is out MRI signal to produce the imageHow does the RF pulse determine if it is transverse or longitudinal?- When RF pulse is on, energy is given to H and it is transverse - When RF pulse is off, energy is lost and H will move to longitudinal (the main magnetic field) - NVM will be constantly going back and forthWhat are the 2 different types of relaxation occurring?- One type describes transverse, the other type describes longitudinal - These 2 different types are independent of each otherWhat is T1 relaxation?Recovery of the NMV back into the longitudinal directionWhat is T1 relaxation defined as?The time it takes for 63% of the longitudinal magnetization to recoverWhat does each tissue having a different T1 time mean?It allows us to see different tissues with different contrastWhat is T2 relaxation?Decaying of magnetization in the transverse planeWhat is another name fore T2 relaxation?Spin-spin relaxationWhat is T2 relaxation defined as?The time it takes 63% of the transverse magnetization to be lostWhat do different relaxation times of tissues determine?- All of the tissues that we image relax at different rates - These times indicate which tissues are present on an image allowing for different contrasts by using different sequences in the machineWhat are the 4 different sequences?T1, T2, spin echo, gradient echoWhat are 2 extremes of sequences with water and fat?- Water has a long T1 recovery - Fat has a short T1 recovery *This will allow for different contrast displays*What is a pulse sequence?The process of MR signal collection that generates an imageHow does a tech set up a pulse sequence?By selecting parameters - Different pulse sequence will produce different contrastWhat are the 2 most common pulse sequences?- Spin echo sequence - Gradient echo sequenceWhat are imaging parameters and what do they determine?Parameters set on the control panel that determine image contrastWhat are 2 examples of imaging parameters?- Echo time (TE) - Repetition time (TR)What is echo time (TE)?The time from RF pulse to the collection of the echoWhat is repetition time (TR)?The time from one RF pulse to the next RF pulseWhat is spatial resolution?The ability to distinguish 2 structures as separate and distinctWhat is spatial resolution controlled by?Pixel and voxel sizeWhat is pixel and voxel size affected by?Matrix size, FOV, and slice thickness (3D)What is signal to noise ratio?Ratio of the signal to ratio of noiseIf SNR is high, what happens to image quality?Image quality increasesWhat is SNR controlled by?The voxel volumeWhat 3 things affect voxel volume?- Slice thickness - FOV - Matrix sizeWhat 4 things is SNR affected by?- Magnetic field strength - Type of RF coil - System electronics - MRI parametersWhat happens to spatial resolution if SNR is increases?Spatial resolution is low - InverseFor what body structures do we want a high SNR and a low SNR?- High SNR: large structures (ex whole abdomen) - Smaller SNR: small structures for an increase in spatial resolutionWhat do longer repetition times (TR) allow for?They give the patient more time to move which decreases image qualityWhat are 3 factors that affect scan times?- TR - Number of phase encoding steps - NSAWhat is NSA?The number of times the data is collected - We can collect data more than once which increases image qualityWhat 3 factors can be manipulated to decrease scan times?- Decreasing TR - Decrease # of phase encoding steps - Decreasing NSA *But, decreasing these also decrease image quality*What contrast agents are mostly used?- Gadolinum-based contrast agents (Heavy metals) - These agents allow us to see vasculature similar to iodine in xrayWhat do contrast agents affect in MRI?Contrast agents are a chemical substance that alter relaxation times of tissuesHow are contrast agents given to the patient?They are introduced intravenously as a chelateWhat is the half life of gadolinium contrast agents?1.5 hoursHow are contrast agents filtered out of the body?filtered out in the urine and Excreted via kidneysWhat are the 5 side effects of contrast?transitory headache, nausea, vomiting, lightheadedness, low blood pressure - Side effects usually only last several minutesWhat is gadoliniums worst side effect (not listed above)?the development of nephrogenic systemic fibrosis (NSF)Who does nephrogenic systemic fibrosis (NSF) affect?- Patients with kidney disease or impaired renal function - Patients with these diseases should avoid MR contrast agents when possibleWhat is the first thing that must be done before an MR exam?- All patients and personnel must be screened on documentation and verbally by the tech before entering the MRI scan room - Pts must also be scanned to ensure they are allowed to have contrast agentswho may be affected by the MR magnet and what can happen if they are exposed to a strong magnet?those with surgical implants or metal within the body. - This metal can move due to being exposed to a strong magnetWhat happens if a ferromagnetic object comes into the presence of a strong magnetic field?They may become airborne (AKA projectiles)What terminal velocity do small objects have?In the presence of a 1.5T magnet, they may move at 40 mphWhat specific objects should not be brought into the scan room?Scissors, hemostats, pins, oxygen tanks, and sandbags - There are MRI safe objects that you can bring in the room to replace these itemsWhat one of the MRI magnets is at a higher risk of causing accidents?Superconducting magnetsWhat is varied over time and why?Magnetic field strength because gradients are switched on and offWhat can happen from gradient coils that is damaging to the machine?Significant heating (can cause burns or fire)What are 2 things that can happen from gradient coils that the patient can physically feel?Peripheral nerve stimulation (tingling) and cause uncomfortWhat causes acoustic noise in the machine?As the current is passed through the gradient coils, noise occursWhat can be caused from acoustic noise?some reversible and irreversible effects, including transient hearing loss and patient annoyanceWhat should be given to the patient to protect from noise?Some form of ear protection such as earplugs or headphonesWhat can the RF field cause in the pt and machine?Heating in material occurring in the bore and the patientWhat is the specific absorption rate (SAR)?Measurement of RF absorption and the patients ability to dissipate heatWhat is specific absorption rate (SAR) expressed in and who is it limited by?Expressed in Watts/Kg and limited by the FDA - There are warnings on control panel once absorption rates have been exceededHow can we prevent burns to the patient?Use padding to avoid skin to skin contact with the machine and patientWhat are factors that influence the specific absorption rate (SAR)?Room temperature, air flow in the bore, pts medical history, patients age, etc.Why should we screen patients for metallic implants?They can create serious damaging effects - Implants and prostheses must be identified prior to entering the magnetic fieldWhat are the 3 classifications of device implants?- Safe - Unsafe (cannot scan) - Conditional (have to make sure certain conditions are fulfilled)What is an example of conditional classification of an implant?If a pt gets an implant, they say to wait 6 weeks to get an MRI after the device is implanted so that scar tissue is formed - This is so that tissue doesn't get torqued in the pts bodyWhat can the magnetic field negatively cause?Torque, heating, or artifactsHow can it be determined if the prosthesis or implant is MRI compatible?Based on the make and model numberWhat is quenching?Process where there is a sudden loss of absolute zero temperature in the magnetic coilsHow is quenching initiated?By pushing the quenching buttonWhat happens once you push the quenching button?The venting system sends helium to the outside environment releasing it from the machineWhat happens if the venting system fails?The helium will replace the oxygen in the room and cause an increase in pressure - no one should be in the room at the time of quenchingWhat can happen if the pressure increases from the helium?Possible asphyxia, frostbite, or eardrum ruptureWhat type of magnets is quenching used for?Superconducting magnetsWhy would the quenching button be pushed?- if there is something stuck inside the scanner that people are not strong enough to remove themselves - Once pressed, chryagens are released into the atmosphere, the magnetic field would be gone, and you would be able to remove whatever was stuck to the bore.