Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
Physics I week 4
| Question | Answer |
|---|---|
| Gradients | Are used to rephrase or dephaase magnetic moments of hydrogen nuclei |
| GRE echo pulse sequence | use variable RF excitation pulse flip angles less than 90 degrees. Use to rephase the transverse magnetic moments of hydrogen nuclei to form an echo instead of RF pulse. |
| Variable Flip angle | Gradient-echo pulse sequence use a RF excitation pulse that is a variable and flips the NMV through any angle less than 90° |
| Gradient rewinding | result in a large component of magnetization remaining in the longitudinal plane after the RF excitation pulse is switched off. In gradient-echo pulse sequences, a gradient is used to rephase transverse magnetization |
| Rewinders | all the magnetic moments are in the same place at the same time and are therefore rephased by the gradient. A maximum signal is induced in the receiver coil |
| Spoilers | The magnetic moments of nuclei are therefore no longer in the same place at the same time, and so magnetization is dephased by the gradient |
| polarity | The gradient field adds or subtracts from the main magnetic field (B0) is dependent on the direction of the current that passes through the gradient coils |
| Gradient spoiling | With no gradient applied, all the magnetic moments of hydrogen nuclei precess at the same frequency at the same field strength. |
| bipolar gradient | The frequency encoding gradient (readout) is used to dephase and then to rephase the magnetic moments, the result will be a gradient echo. Means that it consists of two lobes, one negative and one positive |
| frequency-encoding gradient | It is initially applied negatively, which increases dephasing and eliminates the FID. Its polarity is then reversed, which rephases only those magnetic moments that were dephased by the negative lobe. |
| positive lobe | These nuclei are then rephased and create the gradient-echo at the time TE. |
| negative lobe | gradient is half that of the area under the positive lobe |
| Low flip angles | less longitudinal magnetization is converted to transverse magnetization during the excitation phase of the sequence, less time is required for relaxation (short TR) |
| T2* | reflect the fact that magnetic field inhomogeneities are not compensated for by gradient rephasing. |
| TR and the flip angle | In gradient-echo pulse sequences, controls the amount of T1 relaxation and saturation that occurs |
| Extrinsic parameters (TR, TE, and flip angle), The steady state, Residual transverse magnetization. | There are essentially three different processes that affect weighting in gradient-echo pulse sequences, and sometimes all three overlay each other in the image |
| TE | Controls the amount of T2* decay and therefore T2 contrast |
| T2 contrast | increases as the TE increases |
| TR | Controls the amount of T1 recovery and therefore T1 contrast. |
| T1 contrast | increases as the TR decreases |
| T1 contrast is maximized | If the combination of flip angle and TR causes saturation of the vectors (i.e. they never fully recover their longitudinal magnetization during the TR period) |
| T1 contrast is minimized | If the combination of flip angle and TR does not cause saturation of the vectors (i.e. they recover most, or all, of their longitudinal magnetization during the TR period) |
| T2*-weighted image | differences in the T2* decay times of the tissues are maximized, and differences in the T1 recovery times are minimized |
| maximize differences in T2* decay times | the TE is long so that fat and water vectors have had time to dephase |
| minimize differences in T1 recovery times | the flip angle is small and the TR long enough to permit full recovery of the fat and water vectors before the next RF excitation pulse is applied |
| PD-weighted image | both T1 and T2* processes are minimized so that the differences in the tissues are demonstrated. |
| minimize T2* decay | the TE is short so that neither the fat nor the water vectors have had time to decay. |
| Steady state | TR is less than T1 & T2 relaxation times of tissues. Also defined as generally as a stable condition that does not change over time |
| T2 weighted images | Tissues that have a long T2 decay times (water CSF) are the main components of transverse magnetization and contribute to the contrast |
| Coherent or rewound gradient-echo | Uses a variable F/A followed by gradient rephasing. TR is shorter than T1 and T2 relaxation times of tissue. Reverses the slope of the phase encoding gradient after the readout. Gradient echo has information from the FID and the stimulated echo. |
| Coherent/rewound uses | determine whether a vessel is patent or whether an area contains fluid. They may be acquired slice by slice or in a 3D volume acquisition. As the TR is short, slices can be acquired in a single breath-hold. |
| coherent/rewound parameters | To maintain steady state: F/A 30-40 degrees TR 20-50 ms To maximize T2*: Long TE 10-15 ms |
| coherent/rewound advantages | - Very fast scan times - sensitive to flow/useful in angiography - can be acquired in volume acquisitions |
| coherent/rewound disadvantages | - Reduced SNR in 2D - Magnetic susceptibility increases - Loud gradient noises |
| Incoherent or spoiled gradient-echo | These sequences dephase / spoil this magnetization so the effect on image contrast is minimal. Only transverse magnetization from excitation is used. FID, enabling T1 and PD contrast to dominate |
| RF spoiling | RF excitation pulses are transmitted not only at a certain frequency to excite each slice but also at a specific phase. Every TR, the phase angle of the transverse magnetization is changed. |
| phase-locked circuit | that the receiver coil discriminates between transverse magnetization that has just been created by the most recent RF excitation pulse and residual transverse magnetization created by previous RF excitation pulses. |
| Gradient spoiling | uses gradients to dephase magnetic moments; it is the opposite of rewinding. Slice select, phase encoding and frequency encoding are used to dephase residual magnetization. T2* effects are reduced. |
| Incoherent/spoiled uses | T1 or PD. 2D and 3D. Breath holds sequences. Good T1 anatomy and post contrast imaging |
| Incoherent/spoiled advantages | - Short scan times - used after gadolinium injection - acquired in a volume acquisition - Good SNR and anatomic detail in 3D |
| Incoherent/spoiled disadvantages | - Reduced SNR in 2D acquisitions - Magnetic susceptibility increases - Loud gradient noise |
| Incoherent/spoiled parameters | maintain steady state: - F/A 30°–45° - TR 20–50 ms. To maximize T1: - Short TE 5–10 ms. |
| SE | rephased by an RF pulse with a large flip angle; The 180 RF pulse rephases the transverse magnetization to create a spin echo. |
| No gradient | means that the magnetic moments of hydrogen nuclei precess at the same frequency as the magnetic field strength (except for the inhomogeneity’s which are small) |
| TSE/FSE | is used for 2D or 3D mode; an example would be for the Brain/IAC. In 2 D acquisitions multiple slices are interleaved because the TR is longer than the sequence length (time). |
| F/A + TR | causes saturation of vectors - T1 contrast is maximized; meaning longitudinal magnetization during TR period, doesn’t fully recover |
| T1 weighted gradient echo | F/A is large and TR is short, with a long TE to maximize T2* |
| T2* weighted gradient echo | F/A is small and TR is long to prevent saturation TE is short to minimize T2* |
| PD weighted gradient echo | F/A and TR prevents saturation F/A is small TR is long. TE is short to minimize T2* |
| Ernst Angle | the flip angle the highest signal intensity in a tissue with a given T1 recovery time in a given TR. Be able to recognize and label this graph. |
| Residual transverse magnetization | is what is left over from the previous RF pulses in the steady state conditions. It is rephased by subsequent RF pulses to form stimulated echoes. |
| Actual TE | Time between the peak of the GRE echo and next RF pulse, it is the TE that is selected in scan protocol but not the TE that controls T2 contrast. |
| Effective TE | Time from peak of GRE-echo to a previous RF pulse (The RF pulse that created the FID) this is the TE that determines T2 contrast. |
| Balance gradient-echo | coherent gradient echo it corrects for errors in flowing blood and CSF it uses an alternating RF excitation scheme to enhance steady state. These gradients are applied in the slice and frequency axis. |
| Balance gradient-echo advantages | Fast, shorter scan times, reduced artifact from flow, Good SNR & image contrast. |
| Balance gradient-echo disadvantages | decreased SNR in 2D, very loud noises, Artifacts needs high performance gradients. |
| Balance gradient-echo uses | Developed for heart and great vessels it is also used in spinal imaging; cervical spine, IAC (internal auditory meatus as CSF flow is reduced and occasionally in joint and abdominal imaging). |
| EPI Echo | is a method of filling k-space in a single or multiple shot by oscillating the frequency-encoding gradient and reading the resultant gradient-echoes |
| Hybrid sequences | combine gradient and spin echo sequences uses benefits of both |
| Flip angle | Controls the amount of saturation and therefore T1 contrast. |
| reverse-echo gradient echo | characterized by a rewinder gradient that repositions the stimulated echo so that it is read by the system. True T2 makes a significant contribution to image contrast because the residual transverse magnetization contains mainly T2 contrast data. |
| reverse-echo gradient echo uses | used to acquire images that demonstrate true T2 weighting. brain and joints, 2D and 3D |
| reverse-echo gradient echo advantages | Fast shorter scan times, Truer T2 than in conventional gradient–echo, Can be acquired in a volume acquisition, Good SNR and anatomical detail in 3D |
| reverse-echo gradient echo disadvantages | Reduced SNR in 2D acquisitions, Loud gradient noise, Susceptible to artifacts, Image quality can be poor |
| reverse-echo gradient echo parameters | To maintain the steady state: Flip angle: 30°–45° TR: 20–50 ms actual TE should be as short as possible to enhance T2 contrast. |
| Balance gradient-echo parameters | Flip angle variable (larger flip angles increase signal) • Short TR less than 10 ms (reduces scan time and flow artifact) • Long TE 5–10 ms. |
| EPI advantages | Very fast shorter scan times, Reduced artifact from respiratory and cardiac motion, All three types of weighting can be achieved, Functional information acquired, Scan time savings can be used to improve phase resolution |
| EPI disadvantages | Chemical shift artifact is common, Peripheral nerve stimulation due to fast switching of gradients, Susceptible to artifacts |
Created by:
MRIsipin