Physics CH 3 and 4
A technique used to visualize the morphology of the heart and great vessels is:
Double IR
What type of sequence has the following advantages and disadvantages? 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. Disadvantages -Chemical shift artifact is common. -Peripheral nerve stimulation due to fast switching of gradients. -Susceptible to artifacts.
EPI Sequences.
What is the name of the signal detected in the receiver coil at time TE after application of 180 degree RF rephasing pulse?
Echo
A balanced gradient-echo sequence does not correct the phase shift errors created by flowing CSF or blood.
False
In a fast or turbo gradient-echo sequence, very short TEs are achieve by two methods : ramped sampling and use of broad receive bandwidth.
False
In STIR sequences, which tissue is being suppressed?
Fat
Which statement is not true about FSE pulse sequences?
Fat is brighter than on SE pulse sequences
At 1 Tesla field strength, in a steady state, which of the following brain tissue will demonstrate high signal intensity?
Fat with options listed but water is the highest.
STIR sequences can suppress the signal from all of the following EXCEPT:
Fluid (CSF)
What do we mean when we say that the first echoes are "free" in conventional spin echo sequences?
Free echoes refer to those that are acquired while the system is waiting for the last echo to occur, e.g. dual echo spin echo. The PD weighted image acquired from the first echo does not cost anything in terms of scan time because the system waits for the second echo to occur.
The gradient that is on during the production of the echo is the:
Frequency
The point in a tissue's longitudinal recovery where there is no component of magnetization and therefore no signal created on an IR pulse sequence is called:
Null point
Fluid attenuated inversion recovery (FLAIR) sequences are typically used for the evaluation of:
Periventricular white matter disease
Imaging parameters in GRE T2*-weighted sequences should have:
Small flip angle, long, TR, long TE
A pulse sequence with a 90⁰ RF excitation pulse followed by a 180⁰ RF rephasing pulse is known as a(an):
Spin echo
Define steady state?
Steady state is defined as a stable condition that does not change over time. In this state, the energy given to the hydrogen nuclei equal the energy lost by them.
In a steady state which tissue will demonstrate a high signal intensity?
Tissue where T1 recovery and T2 decay times are similar.
Why do conventional IR sequences produce heavily T1 weighted images?
Tissues begin their recovery from full inversion as opposed to from the transverse plane as in conventional spin echo. This allows more time for differences in the T1 recovery times between tissues to become apparent.
What is the purpose of a pulse sequence?
To rephase the magnetic moments of hydrogen nuclei at a point in time when the signal from these nuclei can be read.
EPI often run in conjunction with single-shot imaging and the TR is set at infinity.
True
In a gradient-echo sequence, Flip angle and TR determine whether vectors are saturated or not and, therefore, the degree of T1 contrast in the image.
True
In a gradient-echo sequence, the gradient echoes are created by a bipolar gradient. True or False?
True
In a steady state, irrespective of the magnetic field strength, the residual transverse magnetization is rephased by subsequent RF pulses to form stimulated echoes. True or False?
True
In a steady-state, the "energy in" is determined by the flip angle and the "energy out" is determined by TR. True or False?
True
In an EPI sequence, PD weighting is achieved by selecting short TE.
True
In gradient-echo sequence, TE controls the amount of T2* decay and therefore T2 contrast. True or False?
True
In the steady state, image contrast is not due to differences in the T1 recovery and T2 decay times of tissues but rather due to the ratio of T1 recovery time to T2 decay time. True or False?
True
The steady state is equal amounts of longitudinal and transverse magnetization.
True
What is the extrinsic contrast parameter unique to TSE/FSE?
Turbo factor or echo train length.
In FLAIR sequences, which tissue is being suppressed?
CSF
IR pulse sequences are characterized by:
A 180⁰ inversion RF pulse followed by a 90⁰ excitation RF pulse
What is the general definition of the steady state?
A condition that does not change over time
Define echo planar imaging?
A rapid technique that begins with a series of one or more RF pulses and is followed by a series of gradient-echoes. These echoes are generated by oscillation of the readout gradient.
What is the definition of a pulse sequence?
A series of RF pulses, gradient applications, and intervening time periods.
The pulse sequence consisting of a variable flip angle RF excitation pulse followed by a pulse of the magnetic field gradients is called:
Gradient echo
What does GRASE stand for?
Gradient echo and spin echo.
What type of echoes are generated in EPI?
Gradient echoes.
Gradient echo T1-weighted sequences use:
Gradient or RF spoiling
Spin echo sequences:
Have a 90-degree excitation pulse followed by 180-degree rephasing pulse
What are the advantages of the driven equilibrium modification of TSE/FSE?
Images where there is a high signal in water is permitted in short TRs and, therefore, scan times. This is possible because the transverse magnetization is driven back up into the longitudinal plane by a restoration pulse. It is not, therefore, necessary to wait for full T1 recovery.
Gradient echo sequences acquired for high signal from fluid are known as all of the following EXCEPT:
Incoherent gradient echoes
The time between 180⁰ inversion pulse and the 90⁰ excitation pulse is known as:
Inversion time
How does balance GE differ from coherent or rewind GE?
It uses a balance gradient scheme to correct for phase errors in flowing blood and CSF and an alternating RF excitation scheme to enhance steady-state effects.
How does balanced GE differ from coherent or rewind GE?
It uses a balanced gradient scheme to correct for phase errors in flowing blood and CSF and an alternating RF excitation scheme to enhance steady-state effects.
Imaging parameters in GRE T1-weighted sequences should have:
Large flip angle, short TR, short TE
What parameters are suitable in a FLAIR sequence and why?
Long TI to suppress CSF, long TE to enhance T2 weighting, and long TR to allow full recovery of spins from saturation.
How are very short TEs achieved in fast or turbo gradient echo?
Ramped sampling, partial echo, and broad receive bandwidth.
What are two consequences of generating the steady state in GE sequences?
Residual transverse magnetization that is rephased to produce a stimulated echo. Images where contrast is dependent on T1/T2 rather than absolute values of T1 recovery time and T2 decay time.
What are the consequences of increasing this parameter?
Shorter scan time, more T2 weighting, less magnetic susceptibility, more J coupling, more MTC, fewer slices for a given TR, and possibly some blurring if the turbo factor is very long.
What are the consequences of increasing ETL time?
Shorter scan time, more T2 weighting, less magnetic susceptibility, more j coupling, more mtc, fewer slices for a given TR, and possibly some blurring if the turbo factor is very long.
A technique that allows k-space to fill from a single echo train is known as:
Single shot imaging
Gradient echo sequences acquired for the evaluation of hemorrhagic lesions rely on:
Susceptibility
How is the The required to null the signal be calculated?
T1 required to null the signal from a tissue is always 0.69 times its T1 relaxation time.
What weighting are Incoherent GRE (spoiled) sequences Typically?
T1W
What weighting are Coherent GRE sequences Typically?
T2* W.
Coherent GRE sequences are typically:
T2*-weighted
Short tau inversion recovery images are:
T2-weighted, fat saturated images
What weighting are Reverse GRE sequences Typically?
T2W
Which combination of TR and TE create a T2-weighted image?
TE 90ms, TR 4000ms
Which of the following statements is incorrect regarding image weighting mechanism in a gradient-echo sequence?
TR and flip angle do not control saturation.
Which TR is considered appropriate to maintain a steady state?
TR below 50ms
the number of TR periods needed to reach the steady state depends on which factors?
TR length, flip angle, and relaxation times of tissues.
What signals are sampled in coherent or rewind GE sequences?
The FID and the stimulated echo.
Why should STIR sequences not be used after administering gadolinium?
The T1 recovery times of enhancing structures are shortened by gadolinium so that they approach the T1 recovery time of fat. In a STIR sequence, therefore, enhancing tissue may also be nulled.
How is the TI required to null the signal be calculated?
The TI required to null the signal from a tissue is always 0.69 times its T1 relaxation time.
What is the Ernst angle?
The flip angle that produces maximum signal intensity in a tissue with a given T1 recovery time in a given TR.
What is the advantage of using hybrid sequences for EPI?
The hybrid sequence uses the benefits of both types of rephasing methods; i.e. the speed of gradient rephasing and the ability of the RF pulse to compensate for T2* effects.
Why is the stimulated echo not sampled in incoherent or spoiled gradient echo?
There is a phase lock between the transmit and receive coils so that only the most recently created transverse magnetization is detected by the receive coil.
How do the flip angle and TR affect weighting in GE sequences?
They determine whether vectors are saturated or not and, therefore, the degree of T1 contrast in the image.
What are the consequences of generating a steady state in a gradient-echo sequence? a. Residual transverse magnetization that is rephased to produce a stimulated echo. b. Images where contrast is dependent on T1/T2 rather than absolute values of T1 recovery time and T2 decay time. c. All tissues signal will be nulled. d. All tissue will exhibit the same signal intensity
a and b are both correct
Black blood acquisitions produce images where blood flow is dark and are based on:
spin echo sequences
