Physics (MRI)

MRI requires what 3 things
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Terms in this set (119)
Why are resistive magnets called resistive?Because of the large amount of resistance produced in the wiresWhat is the field strength limit for resistive magnets?Limited to under 0.7 TWhat does increased resistance in a resistive magnet produce?HeatHow does a superconducting magnet differ from an electromagnet?Uses special materials and conditions to reduce electrical resistance to near zeroWith superconducting magnets, reducing resistance allows for?Allows for greater current flow resulting in a higher strength magnetic fieldResistance is dependent onThe material of the wire and the temperature of the wireWhat is the most common field strength of a magnet for medical imaging?1.5 TWithin the bore, used for imagingMain magnetic fieldStray magnetic field outside the boreFringe fieldWhat is used to confine the fringe field?Magnetic shielding (active and passive)The placement of heavy ferromagnetic plating in the walls or floor of the magnet room or even the magnet itself.Passive Magnetic ShieldingEmploys an extra set of superconducting windings in the scanner to oppose a portion of the main magnetic field.Active ShieldingCoils that transmit and receiveRF subsystem________ produces RF energy that excites hydrogen protons.TransmitterIn order for the transmitter to produce RF energy to excite hydrogen protons, they must be at a resonant frequency ofHydrogen_____ detects and collects the MR signal, which is used to create the image with RF subsystemReceiverSome coils of a RF subsystem have dual function, meaning they canTransmit and receiveRF shielding is used to ?Block incoming RF signals which can interfere with the signal coming from the ptWhat material is used for RF shieldingCopper or stainless steelWhat is shielded in RF shieldingWalls, doors, and windowsUsed to increase or decrease main magnetic field strength linearly, over distanceGradient CoilsThick bands of conductive material wrapped around a cylinderGradient coilsGradient coils allow forSpatial localizationWhich gradient coil goes from right to leftX coilWhich gradient coil goes from A to PY coilWhich gradient coil goes from head to footZ coilWhat produces noise during the scanGradient coilsWhy are there 3 gradient coils?Because we have to locate the body part in 3 areasMRI requires what 3 things1. Strong magnetic field 2. Radio frequency source 3. MRI active nucleusMRI active nuclei have magnetic characteristics, meaning they haveAn odd number of protons and neutronsA tiny magnetic field generated by a spinning MR active nuclei (spin, proton)Magnetic momentThe magnetic moment spin is described as _________, since it contains two magnetic polesa magnetic dipoleThe most abundant atom in the human bodyHydrogenHas one proton in its nucleus and a north and south pole represented by a magnetic momentHydrogenWithout an external magnetic field, protonsAre spinning randomlyWhen placed in a strong magnetic field (Bo), the magnetic fields align in one of two waysParallel or anti-parallelWhen the magnetic moment aligns parallel it lines what direction?The main magnetic field (low-energy nuclei)When the magnetic moment aligns anti-parallel it lines in what direction?Opposite to the external magnetic field (high-energy nuclei)Slightly more than half of the magnetic moments align in which direction?Parallel to the main magnetic fieldWhat happens with net magnetizationmagnetic moments of the parallel nuclei cancel out the smaller number of anti-parallel nucleiParallel nuclei are added together resulting inNet magnetization vector (NMV)The net magnetization vector is produced in the direction ofThe main (external) magnetic field (Bo) (longitudinal magnetization-parallel to magnetic field)NMV is parallel to Bo (magnetic field strength) and cannot be measured, so it must beTipped into a transverse planeRf pulse is used toDisturb protons and move them from longitudinal to transverse plane (transverse magnetization)When protons move from longitudinal to transverse planeTransverse magnetizationProtons spin onTheir own axisMain magnetic field produces an additional spin calledPrecessionRotation that can be described as a wobblePrecessionPrecession spin follows what kind of pathA circular path (known as precessional path) at a certain speedThe speed at which spins precess around Bo (magnetic field strength)Larmor FrequencyAlso known as precessional or resonant frequencyLarmor FrequencyWo=Precessional frequency (MHz)Bo=Magnetic field strength (T)Represents precessional frequency of MR active nuclei at 1TGyro-magnetic ratio (MHz/T)What is the gyro-magnetic ratio of hydrogen?42.6 MHz/TTransfer of energy between two objectsResonanceRF pulse is used toGive energy to hydrogen protonsFor RF pulse to give energy to hydrogen protons, the RF pulse must haveThe same frequency as hydrogen (Larmor Frequency)Can you change the Larmor Frequency?NoWhat is the result of Resonance?NMV moves away from the longitudinal to transverse planeThe result of resonance is a change in?The direction of magnetic fieldWhen do protons lose energy?Once the RF pulse is removedTransverse magnetization decreases asThe protons spiral back toward the main magnetic fieldThere are two types of relaxation that occur at the same time, but they are _______ of each otherIndependentRecovery of the NMV back into the longitudinal directionT1 relaxationT1 relaxation is defined as the time it take ___% of the longitudinal magnetization to recover63Each tissue has a uniqueT1 relaxation timeKnown as spin-spin relaxationT2 relaxationT2 Relaxation is the decaying of magnetization in the _______ planeTransverseT2 relaxation is defined as time it takes 63% of the _________ to be lostTransverse magnetizationThis type of relaxation shows as dark fat and bright fluidT2This type of relaxation shows fluid as dark and fat as brightT1This type of relaxation is good for showing anatomyT1What indicated which tissues are present and allow for contrast between different tissues?Differences in relaxation timesWater has a long or short recovery time?LongFat has a long or short recovery time?ShortThe process of MR signal collection that generates an imagePulse SequenceHow are the parameters for pulse sequence selected?Selected by the operatorWhat are the two most common pulse sequences?1.Spin echo sequence 2. Gradient echo sequence_________ determines image contrast.Image parametersEcho Time and Repetition Time are?Imaging parametersThe time from RF pulse to the collection of the echoEcho time (TE)The time from one RF pulse to the next RF pulseRepetition Time (TR)The ability to distinguish two structures as separate and distinctSpatial ResolutionSpatial Resolution is controlled byPixel/ voxel size (matrix size, FOV, slice thickness)Signal-to-noise ratio is controlled by ______ and affected by ?Voxel volume; Affected by slice thickness, FOV, matrix size, mag field strength, type of RF coil, MRI parameters, system electronicsWhat increases the chance of motionIncreased scan timeFactors that affect scan times are-TR -Number of phase encoding steps -NSA (number of times the data is collected=number of signal averages) like double exposing in xrayWhat type of contrast is used most often in MRIGadolinium-based contrastChemical substance that alters relaxation times of tissuesGadoliniumHow is gadolinium introducedIntravenously as a chelateWhat is the half-life of gadolinium1.5 hoursGadolinium is excreted via _____.KidneysWhat are the side effects of MR contrast?Transitory headache, nausea, vomiting, lightheadedness, low blood pressureWhat is gadolinium's worst side effect?Development of nephrogenic systemic fibrosis (NSF)Who should avoid MR contrast agents when possiblePatients with kidney disease or impaired renal functionAll pts and personnel must be ______ before entering the MRI scan roomScreened_________ objects become airborne in the presence of a magnetic fieldFerromagneticSmall objects have a terminal velocity of 40 mph at ___ T.1.5Magnetic field strength varies over time, becauseGradients are switched on and offGradient magnetic field may cause_______ or _______Significant heating (burns/fire) or peripheral nerve stimulation (tingling)Acoustic noise is created as current is passes throughGradient coilsAcoustic noise can cause some reversible and irreversible effects includingTransient hearing loss and patient annoyanceRF field can causeHeatingA measurement of RF absorption and the patients' ability to dissipate heatSpecific Absorption Rate (SAR)Specific absorption rate is expressed inWatts/kg and limited by FDAProcess where there is a sudden loss of absolute zero temperature in the magnetic coilsQuenchingWith quenching, venting system sends ____ to the outside environmentHeliumWhat are the possible effects of quenchingPossible asphyxia, frostbite, or eardrum rupture