W10 Flashcards

Question

free induction decay (FID)

Answer 1

**free** because of the absence of the RF pulse and **induction decay** because of the decay of the induced signal in the receiver coil

Answer 2

areas of high signal as well as areas of low signal

Answer 3

for ex: *fat, cerebro-spinal fluid (CSF), muscle* **high signal** (***white***) = large transverse component of magnetization **intermediate signal** (***grey***) = medium transverse component of magnetization **low signal (*black*)** =small transverse component of magnetization

Answer 4

**Repetition time (TR)** - time from the application of one RF pulse to the application of the next **Echo time (TE)** - time between RF excitation pulse and signal collection **Flip angle** - angle through which the NMV is moved due to RF excitation. **Turbo-factor (TF)** or **echo train length (ETL)** - number of 180° RF pulses and resultant echoes. **Time from inversion (TI)**: used on inversion recovery sequences. **b value**: used in diffusion weighted imaging.

Answer 5

* T1 recovery (relaxation process, where nuclei emit E absorbed from the RF puse through spin lattice energy transfer). * T2 decay (cumulative dephasing of spin‐spin interactions and inhomogeneities). * Proton density (PD). * Flow. * Apparent diffusion coefficient (ADC)

Answer 6

* relaxation processes * field inhomogeneities * susceptibility effects.

Answer 7

**recovery** of the **longitudinal** magnetization

Answer 8

**decay** of **transverse** magnetization

Answer 9

the magnetic moments of just the excess number of parallel spins

Answer 10

both oscillate at the same frequency

Answer 11

how fast the magnetic moments of spins are precessing and is measured in MHz

Answer 12

the position of a magnetic moment of a spin on its precessional path at any moment in time

Answer 13

At rest the magnetic moments of the spins are **out of phase with each other**.

Answer 14

a net absorption of energy (***excitation***), and also changes the balance between the number of spins in the low‐ and high‐energy populations => orientation of the NMV to B0 depends on this balance

Answer 15

- caused by the exchange of energy from nuclei to their surrounding environment or lattice (= ***spin lattice energy transfer***) => nuclei's magnetic moments relax or return to B0 (= regain their longitudinal magnetization) rate at which this occurs is an exponential process and it occurs at different rates in different tissues => T1 time of a tissue is an intrinsic contrast parameter that is inherent to the tissue being imaged T1 recovery = e time it takes for 63% of the longitudinal magnetization to recover, period during which this occurs is the time between one excitation pulse and the next or the ***TR*** => TR determines how much T1 recovery occurs in a tissue T1 relaxation - result of nuclei exchanging the energy given to them by the RF pulse to their surrounding environment and **efficiency of this process determines the T1 time of the tissue in which they are situated** for ex: - *since fat can absorb energy quickly, the T1 time of fat is very short* => nuclei dispose of their energy to the surrounding fat tissue and return to B0 in a short time. - *Water is very inefficient at receiving energy from nuclei* => T1 time of water is quite long => nuclei take a lot longer to dispose of their energy to the surrounding water tissue and return to B0. repetition time (TR) controls how much of the NMV in fat or water has recovered before the application of the next RF pulse: - short TR does not permit full longitudinal recovery in most tissues => contrast difference between fat and water due to their different T1 recovery times; - long TR allows full recovery of the longitudinal components in most tissues => there is no difference in the longitudinal components magnitude => no contrast difference

Answer 16

caused by the exchange of energy from one nucleus to another (= spin-spin E transfer), occurs as a result of the magnetic fields of the nuclei interacting with each other => loss of phase coherence (= dephasing) => decay of the NMV in the transverse plane, also an exponential process and occurs at different rates in different tissues => T2 decay time of a tissue is a contrast parameter and is characteristic to the tissue being imaged T2 = time it takes for 63% of the transverse magnetization to be lost due to dephasing => transverse magnetization is reduced to 63% of its original value => 37% remains; it occurs over time between the excitation pulse and the MR signal = the ***echo time, TE*** => TE determines how much T2 decay occurs in a tissue T2 relaxation = result of the spins of adjacent nuclei interacting with each other and exchanging energy, efficiency of this process depends on how closely packed the molecules are to each other, for ex: - *In fat, the molecules are closely packed together* => spin‐spin is efficient => T2 time of fat is therefore very short - *In water, the molecules are spaced apart* => spin‐spin is not efficient => T2 time of water is therefore very long TE controls how much transverse magnetization has been allowed to decay in fat and water when the signal is read => short TE does not permit full dephasing in either fat or water => There is little contrast difference between fat and water due to differences in T2 decay times using a short TE. A long TE allows dephasing of the transverse components in fat and water => There is a contrast difference between fat and water due to differences in T2 decay times when using a long TE.

Answer 17

tissues with a low proton density are always dark on an MR image

Answer 18

dark or bright depending on their velocity and the pulse sequence used

Answer 19

differences in the T1 relaxation times of tissues must be demonstrated => TR is selected that is short enough => image with **contrast that is predominantly due to the differences in T1 recovery times of tissues**, differences between the T1 times of tissues is exaggerated **to diminish T2 effects, TE must also be short** => to sum up, T1 weighted img: short T1 (400 ms), short TE (10 ms) => we get an img, where tissues with short T1 relaxation times such as fat, are bright (high signal), tissues with long T1 relaxation times such as water, are dark (low signal) => best demonstration of anatomy, but also pathology if used after contrast enhancement (gadolinium)

Answer 20

differences in the T2 relaxation times of tissues must be demonstrated => TE is selected that is long enough => image with contrast that is predominantly due to the differences in the T2 decay times of tissues, differences between the T2 times of tissues is exaggerated **to diminish T1 effects, long TR is selected** => T2 weighted image: long T2 (4000 ms), long TR (100 ms) => Tissues with a long T2 decay time such as water are bright (high signal), tissues with short T2 decay times such as air, are dark (low signal) => T2 weighted images best demonstrate **pathology**

Answer 21

differences in the proton densities must be demonstrated => both T1 and T2 effects are diminished, by selecting selecting a long TR (4000 ms) and short TE (20 ms) respectively => image with contrast that is predominantly due to differences in the proton density of the tissues => tissues with a low proton density are dark (low signal), tissues with a high proton density are bright (high signal) => **anatomy + some pathology**

Answer 22

are always dark, regardless of the weighting

Answer 23

D. GM is darker than WM on T1 and GM is brighter than WM on T2.

Answer 24

mechanisms that permit refocusing of spins so that images can be acquired with different types of contrast

Answer 25

1. the inherent energy of the tissue 2. how well the rate of molecular tumbling matches Larmor

Answer 26

magnetic field strength: field strength **increases**, tissues **take longer to relax**

Answer 27

TR: for good T1 contrast, the TR must be short

Answer 28

magnetic field strength: field strength **increases**, tissues **take longer to dephase**

Answer 29

TE: for good T2 contrast, the TE must be long

Answer 30

how closely the molecules are packed together

Answer 31

**True**. Spin angular momentum is an intrinsic property of an atom; those with equal numbers of protons and neutrons have no netspin and so no magnetic moment. With only a single proton, hydrogen has a large magnetic moment.

Answer 32

**True**. This is known as the Larmor frequency and is unique for each element. The frequency varies with magnetic field strength and very slightly depending on what substance the proton is contained within.

Answer 33

**False**. Although all the hydrogen atoms will be affected, nearly as many will run anti-parallel, or in the opposite alignment to the field.

Answer 34

**False**. It is the population difference between ‘spin up’ and ‘spin down’ states when subject to a strong magnetic field that produces the NMR signal. At room temperature in a 1 T magnet, the population differs by two for every million nuclei involved.

Answer 35

**True**. However, as this vector is in alignment with the magnetic field, it cannot be detected. By applying RF energy to the protons, their net magnetism can be altered into the x,y plane lying perpendicular to the static magnetic field (mxy); this provides the signal for MRI.

Answer 36

**True**. The protons comprising hydrogen nuclei are most useful for medical imaging because they are so prevalent in the human body.

Answer 37

**True**. C-13, O-17, F-19 and P-31 can all be used for MRI. At present imaging using anything other than protons is mainly for research purposes.

Answer 38

**False**. Approximately half the protons will align ‘spin up’ and half will be ‘spin down’. At 1 T approximately two extra protons out of every million will align spin up, and this difference accounts for the ‘detectable’ protons that allow the formation of the MR image.

Answer 39

**False**. In the resting state the mxy will be the sum of all the individual out of phase vectors pointing in random directions and thus will be equal to 0.

Answer 40

**False**. A 1 T field is approximately 20 000 times greater than the earth’s field strength of 50 mT.

Answer 41

**False**. The z-axis is in line with the field. To enable creation of an MR signal, an RF pulse is applied which tips the net magnetic vector

Answer 42

**False**. Signal increases with static field strength.

Answer 43

**False**. Increased PD (number of protons per unit volume) will give an increased signal.

Answer 44

**False**. A coil can act in both transmit and receive modes, but not simultaneously.

Answer 45

**True**. Water protonsform the basis of most MRI, but protons are also very prevalent in fat

Answer 46

**True**. The signal is only detectable following the application of RF energy

Answer 47

**True**. The application of RF photons at the Larmor frequency will energize the hydrogen protons, causing them to precess in time with one another; this creates a signal which is detectable.

Answer 48

**False**. A 180-degree pulse has enough energy to reverse the net magnetic vector. A 90-degree pulse is halfas energetic (note this is the total energy of the pulse).

Answer 49

**False**. Following an RF pulse, those protons in phase and creating a detectable MR signal will immediately begin to dephase due to T2 relaxation, and field inhomogeneities and the signal will rapidly diminish.

Answer 50

**False**. It also depends on the - field strength - RF pulse flip angle and – to an extent depending on the pulse sequence timings used – - on T1 and T2 relaxation processes - magnetic field inhomogeneities. With a higher field strength, more protons are initially spin up.

Answer 51

**True**. For hydrogen nuclei at 1 T it is approximately 42.6 MHz. It is proportional to field strength, and is also slightly affected by the substance it is in. The Larmor frequency for hydrogen within fat and water is very slightly different, which is the *cause for chemical shift artefact*.

Answer 52

**False**. Photons with the same frequency as the Larmor frequency have the correct energy to cause the magnetic vector of a single proton to flip from spin up to spin down. *At 1 T this is approximately 0.2 meV*. ***This frequency lies within the radio-wave end of the electromagnetic spectrum, which is why there is no ionizing radiation in MRI.***

Answer 53

**False**. A 180-degree pulse has approximately twice the number of photons, but the photons still all must match the Larmor frequency and thus have the same energy.

Answer 54

**True**. An initial 90-degree RF pulse flips the net magnetic vector such that it lies in the transverse xy plane, reducing the net mz to 0. It also causes the individual protons to precess in phase and thus produces a detectable net mxy rotating at the Larmor frequency.

Answer 55

**True**. The signal used to generate the image is caused by the rotating magnetic vector inducing a current to flow in the RF coils in the same way that a dynamo works. This current is used to form the image.

Answer 56

**False**. T1 recovery is in the *longitudinal direction* and is also known as spin–lattice relaxation. T1 is the ***time for 63% maximum recovery***.

Answer 57

**True**. Stronger magnetic fields hasten the precession, this lengthens T1.

Answer 58

**True**. This occurs as they both have a short T1

Answer 59

**True**. T1-weighted imagesrequire a short TR and a short TE. Tissues with a short T1 will appear bright with these settings

Answer 60

**True**. Protons dephase faster than they realign with the static magnet.

Answer 61

**False**. It is transverse relaxation

Answer 62

**True**. Tissues with a long T2 relaxation time give a high MR signal with T2 weighting. TE should ideally be of the order of the T2 relaxation time of the tissues of interest.

Answer 63

**False**. *CSF, urine, amniotic fluid and water* all have ***long T2*** and are therefore high signal. *Blood* flowing into the imaged slice after the initial 90-degree RF pulses ***does not produce signal***

Answer 64

**True**. The difference is minimal, approximately 10 ms

Answer 65

**True**. This has most effect in solids which have a very short T2.

Answer 66

**False**. Spin–spin relaxation leads to T2 decay through the transfer of energy between adjacent nuclei.

Answer 67

**False**. This is true of T1. T2 is unaffected by magnet strength

Answer 68

**True**. If only a single 90-degree pulse is applied, the MR signal undergoes FID; ***true spin–spin relaxation is enhanced by magnetic field inhomogeneities***. This situation is referred to as T2' and is generally countered using a 180-degree rephasing pulse.

Answer 69

**True**. T2 is the time when 37% of the transverse magnetization remains. T1 is the time at which 63% of the longitudinal signal has recovered.

Answer 70

**False**. T1 recovery is also known as spin–lattice relaxation and describes the recovery of the longitudinal magnetization of its equilibrium value along z, with associated loss of energy to the surrounding lattice. T2 decay is spin–spin relaxation and describes the dephasing of the mxy vector due to the small fields caused by surrounding dipoles. An FID signal, however, has amplitude which depends on T2' relaxation. T2' is more rapid than T2 and includes the dephasing effects of magnetic field inhomogeneities.

Answer 71

**True**. T2' is the decay caused by field inhomogeneities plus tissue relaxation. A further 180-degree RF pulse in SE sequences helpsto reduce the effect of field inhomogeneities by rephasing the mxy and thus tissues show a true T2 signal.

Answer 72

**True**. Conversely, T2 is the time for the mxy to fall to 37% of its value.

Answer 73

**True**. Fat and large molecules such as proteins in fluid are effective at removing energy in spin–lattice relaxation; this shortens T1. However, in solids, where water is more tightly bound, T1 relaxation becomes less efficient and T1 lengthens.

Answer 74

**False**. In water, where molecules are moving around very rapidly, dipole–dipole interactions are very brief, making T2 relaxation less efficient, leading to a long T2.

Answer 75

**False**. It takes several hours for the current in the coil to build up. Once the unit is superconducting, the supply is shorted and the current continues to flow within the magnet without drawing any further external current.

Answer 76

**True**. If the cooling fails, the system will rapidly heat up due to a loss of superconductivity causing resistive heat losses in the magnet windings. This results in rapid boil-off of liquid helium.

Answer 77

**False**. The Faraday cage is a wire mesh that serves to screen the scanner from external RF interference.

Answer 78

**False**. This is inverse square law which applies to radiation exposure. Modern MRI scanner static fields generally fall-off more rapidly due to active shielding.

Answer 79

**False**. Surface coils give better SNR and/or resolution for near-surface tissues but less uniformity. The signal from near to the surface coil is much brighter than on the opposite side of the FOV.

Answer 80

**False**. The strength of a 0.5 T magnet is 0.5 T. It does not matter what type of magnet is producing it. However, only superconducting magnets are capable of producing fields of *above 0.5 T*. Resistive magnets are limited to approximately 0.5 T by heating effects, and permanent magnets are only made in field strengths up to about 0.3 T.

Answer 81

**False**. The term paramagnetic is used to describe how some molecules behave within a magnetic field; gadolinium is an example of a paramagnetic substance.

Answer 82

**False**. A permanent magnet cannot be shut down at all. A resistive magnet may be shut down rapidly by switching off the power. A superconducting magnet may be shut down but only very slowly if a quench is to be avoided. In a true emergency, the helium is released raising the temperature of the magnet and thus removing the superconducting properties. This has the effect of rapidly reducing the field strength, but even this method is not as fast as switching off a resistive magnet and can prove very expensive and damaging to the equipment.

Answer 83

**True**. This reduces field inhomogeneities and thus allows better images to be obtained.

Answer 84

**True**. This is most commonly seen in spinal MRI with surface coils along the back giving very detailed images of the spine.

W10 Flashcards

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