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411-Exam 2
411- Exam 2
| Question | Answer |
|---|---|
| tuning controls | adjust the resonance frequency |
| pulse sequence | protocol dependent ; varies the image contrast displayed |
| RF flip angle | gradient imaging can choose the flip angle used |
| matrix size | overall columns and rows which influence pixel size |
| field of view | changing the size of the image |
| T/F: FOV plays a role in size of pixel | true |
| number of signal averages (NSA) or (NEX) | the number of times a signal is averaged |
| receiver bandwidth | range of frequencies to sample |
| window width/level | sets the contrast scale of gray, black, white on image |
| primary magnet is called what | B0 (static field) |
| three types of primary magnets? | permanent, resistive, superconducting electromagnet |
| permanent magnet | once magnetized it stays on |
| resistive magnet and superconducting magnet | has to apply electricity to power magnet, if electricity is off so is the magnet |
| Permanent magnet field strengths | 0.2, 0.3, 0.5, 0.6 Tesla |
| permanent mri's also known as | open MRI |
| how much does a permanent magnet machine weigh? | 100 tons |
| T/F: you can quench a permanent magnet? | False |
| most common type of MRI scanners today | superconductive |
| faraday's law of magnetic inductions says? | moving a magnet or changing a magnetic field over time in the presence of a conductor will induce a voltage in the conductor. (basic prinicple by which MR signals are detected) |
| diamagnetic magnetism | slight negative repelling effect (weak), unaffected by a magnetic field, gold, silver, copper, mercury, carbon, water |
| paramagnetic magnetism | slight increase when in magnetic field, no magnetic properties of their own, very weakly magnetized, gadolinium, aluminium, platinum, manganese, tungsten |
| superparamagnetism | property occuring principally in small, single-domain magnetic particles without magnetic memory. it is more closely related to ferromagnetism. (iron oxide) |
| ferromagnetic | become magnetized wehn placed in magnetic field, easily magnetized, iron, steel, nickel, cobalt |
| nuclear magnetism | certain nuclei have properties that cause them to display magnetism |
| what is the most abundant magnetically active nuclei in the body | hydrogen protons specifically in water and fat |
| what is the purpose of the static magnetic field | magnetizes tissues |
| f= | resonant or larmor frequency |
| y/2pi= | gyromagnetic ratio |
| B= | magnetic field in Tesla |
| hydrogren protons precess at what in a 1.5 T | 63.9 mHz/ 1T |
| hydrogen protons precess at what in a 1T | 42.6 mHz |
| hydrogen protons precess at what in 3T | 128 mHz |
| T/F: at equilibrium, net magnetizaton is always aligned with the magnetic field | true |
| flip angle | when thee net magnetization has been roatated at an angle from the z-axis from a RF pulse |
| when can a signal be created? | when NM is somewhere in the XY plane |
| when is the strength of a signal the greatest? | when the flip angle is 90 degrees |
| Free induction decay is? | a fading electrical signal produced by atomic nuclei as they spin back to their resting state after a RF pulse |
| what symbol is used to describe a flip angle less than 90 degrees? | a |
| what is the primary MRI signal? | Free Induction Decay (FID) |
| what happens when the NMV is in the Z plane (equilibrium)? | no signal is transmitted |
| shim coils | used to make the static field uniform (homogenous) |
| Radiofrequency System | used to transmit RF into the patient and receive the induced signal from the patient |
| Gradient System (coils) | secondary magnet that is superimposed on the static field that can vary the magnetic field according to which gradient coil is used |
| Active shimming | current passing through the coils of wire generating a perpendicular magnetic field |
| passing a specific amount of current in a given direction through each shim coil will do what | will either add or subtract to the main magnetic field |
| Passive shimming | placing plates of metal on surface of the cylinder will compensate for variation in magnetic field |
| B1 is used to provide what? | Provides the necessary energy to cause net magnetization of the tissues to tip and rotate through the transverse plane where the receiving coils are located sends out (transmits) RF and recieves (detects) MR signals from tissue |
| transmit only coils | only transmits RF into patient; less heat |
| receive only coils | receive signal back to patient but does not transmit RF; cause more heat |
| volume coils | go all the way around a volume of tissue |
| examples of volume coils | body coil (saddle coil), Birdcage (head coil), Solenoid ( tube shaped coil) |
| local surface coils | placed over/around region of interest, most commonly recevie only (can be any type), reduced area of coverage, must be placed perpendicular to the XY plane for maximum signal intensity |
| multiple coils example | helmholtz pair/ maxwell coil |
| helmholtz pair | two coils placed on one patient for better signal |
| maxwell coil | three coils placed on one patient |
| phased-array | consists of multiple coils and multiple receivers; larger anatomy coverage; improved SNR |
| Linear array | spines |
| volume array | pelvic/torso; breast, cardiac |
| Gradient Coils | spatially encode MRI signals |
| Gradient amplifiers | power supplier to gradient coils |
| what are band of gradient coils made of and what are they hooked up to? | Made of copper, hooked up to gradient amplifiers |
| Gradient Amplitude | severity of slope of the magnetic field |
| Rise Time | time required to ramp up to the desired gradient amplitude (time needed to turn on gradients) |
| Fall time | time required for field to return to resting value |
| Slew rate | time required for acceleration of the gradient to its maximum amplitude before it's switched off |
| Duty cycle | percenatge of time a gradient is applied at a specific amplitude |
| The loud banging noise in MRI is caused by what | the switching of the gradient coils (rapid alteration of current) |
| Z-gradient coil (top) | head to foot direction |
| X-gradient coil (middle) | right to left direction |
| Y-gradient coil (bottom) | A to P direction |
| Z gradient coil will give you what kind of slice | axial (transverse) slice |
| X-gradient coil will give you what kind of slice | sagittal slice |
| Y-gradient coil will give you what kind of slice | coronal slice |
| slice select gradient does what | locating a slice within the scan plane selected |
| transmit bandwidth | the band of RF frequencies sent out into the patient to match the frequencies of a particular set of protons in a particular slice |
| narrow bandwidth will give you what | thin slices |
| broad bandwidth will give you what | thick slices |
| frequency encoding gradient | reading the signal from the patient located along the long axis of the anatomy |
| phase encoding gradient | reads the signal from the patient that is located along the short axis of the anatomy |
| k-space | storage location of the collected data from the signals; raw data; symmetrical data |
| Pulse Sequence | the grouping of two or more RF pulses. A pulse sequence is a series of RF and gradient pulses used to disturb and manipulate the orientation of the net magnetization of protons and to spatially encode the information contained within the MR signal (echo |
| Types of Pulse sequences | Spin echo, inversion recovery, Gradient recall echo, echo planar imaging |
| Spin Echo | conventional and fast spin echo |
| inversion recovery | stir and flair |
| RF pulse diagram | visual representation of information about what is occuring during a pulse sequence |
| Phase Coherence | state in which rotating objects move in phase or in unison |
| what are the dephasing mechanisms | magnetic field inhomogenealities, susceptibility, chemical shift |
| spin echo is characterized by what | initial 90 pulse and then a 180 degree pulse. The 180 degree pulse is applied to refocus the spins that have dephased due to an nonuniform field. |
Created by:
jayciebest