Gradients
Gradient sequences uses a gradient to rephrase spins and usually flip angles less than ___
90 degrees
Functional imaging techniques are used to image the function or Physcial off of a system rather than its __
Anatomy
A balanced gradient scheme is used in all three gradients to correct for flow __
Artefacts
Functional mri relies on a process called __ to produce a signal in areas of the brains where there is increased activity after performing a function (such as finger tapping)
BOLD
Three disadvantages of EPI
Chemical shift artefact is common Peripheral nerve stimulation due to fast switching of gradients Susceptible to artefacts
EPI is best for
Diffusion Perfusion Real time Cardiac Breath hold techniques
The residual transverse magnetization is rephrased by subsequent RF pulses to form stimulated __
Echoes
Both the __ and the stimulated echo are sampled so that T1, T2* and PD weighting are possible
FID
The resultant image contrast is therefore determined by the ratio of T1 and T2 in a particular tissue and whether the __ or the stimulated echo is sampled
FID
Although faster than RF rephasing, ___ are not compensated for Im this type of sequence.
Inhomogeneities
Two ultrafast sequences
Fast or turbo Echo planar
Four advantages of an incoherent gradient echo
Fast scan times Can be used after gadolinium injection Can be acquired in a volume acquisition Good SNR and anatomical detail in 3D
Four advantages of a balanced gradient echo
Fast scan times Reduced artefacts from flow Good SNR and anatomical detail in 3D Images demonstrate good contrast
Four advantages of a steady state free precession
Fast scan times Truer T2 than in conventional gradient echo Can be acquired in a volume acquisition Good SNR and anatomical details in 3D
TR and __ __ control whether the NMV is saturated
Flip angle
Steady state free precession parameters
Flip angle 30-45 TR <50 ms TE 7ms
Balanced gradient echo parameters
Flip angle 90 TE 15ms TR <10ms
Ultrafast sequences are commonly used to acquire __ rather than anatomical echoes
Functional
Balanced gradient echo is best for
IAC internal auditory canal CSF
Contrast enhanced images use an
Incoherent gradient
Magnetic susceptibility artefacts therefore __
Increase
The speed of rephasing is ___ using a gradient
Increased
For a T1 weighted gradient echo, the flip angle and TR combination must ensure that saturation occurs. The flip angle must be __ and the TR must be short to achieve this. In addition the TE must be short to minimize T2*
Large
Low flip angles mean that as less longitudinal is converted to transverse during the excitation phase of the sequence, ___ time is required for relaxation. This is why a short TR can be used in gradients
Less
For T2* weighted gradient echo, the flip angle and TR combination must prevent saturation. The flip angle must be small and the TR __ to achieve this. The TE must be long to maximize T2*
Long
Only the stimulated echo is sampled and due to its repositioning the TE is this echo is __ enough to include T2 rather than T2* contrast
Long
Balanced gradient echo is a steady state sequence in which ___ magnetization is maintained during the acquisition thereby preventing saturation
Longitudinal
Coherent gradient is best for
MRA angiography
Fast or turbo versions of the traditional echo sequences use strategies such as __ sampling and fractional echo to reduce scan times
Ramped
Four disadvantages of a steady state free precession
Reduced SNR in 2D acquisitions Loud gradient noise Susceptible to artefacts Image quality can be poor
Three disadvantages of an incoherent gradient echo
Reduced SNR in 2D acquisitions Magnetic susceptibility increases Loud gradient noise
Four disadvantages of a balanced gradient echo
Reduced SNR in 2D acquisitions Loud gradient noise Susceptible to artefacts Requires high performance gradients
Three disadvantages of a coherent gradient echo
Reduced SNR in 2D acquisitions Magnetic susceptibility increases Loud gradient noise
Rephasing of the stimulated echo is initiated with an RF pulse but the echo is repositioned by a __ gradient
Rewinder
Spatial localization can be achieved with spectroscopy by using __
STEAM stimulated echo acquisition mode
A bipolar application of the frequency encoding gradient enables spins to be rewound into phase in the middle of the sampling window when the system is reading the echo. This permits a ___ TE, which means that a shorter TR can be used for a given number of slices than in spin echo
Short
Coherent gradient echo is a steady state sequence that utilizes a __ TR and medium flip angle
Short
Gradient rephasing is faster than RF rephasing and therefore these sequences use a shorter TR and TR than a spin echo, result in __ scan times
Short
Incoherent gradient echo is a steady state sequence that utilizes a __ TR and medium flip angle
Short
Both of these strategies permit a ___ TE and TR than in a spin echo
Shorter
The steady State is created when the TR is __ than the T1 and T2 relaxation times of the tissues. Residual transverse magnetization therefore builds up over time
Shorter
EPI is a method of filling k space in a __ or __ shot by oscillating the frequency encoding gradient and reading the resultant gradient echos
Single or multiple
For PD weighting gradient echo, the flip angle and TR combination must prevent saturation. The flip angle must be __ and the TR long. The TE must be short to minimize T2*
Small
__ is a technique that enables evaluation of tissue metabolism by looking at its molecular and chemical structures
Spectroscopy
Only the FID is sampled so that __ weighting predominates
T1
Saturation is required for __ weighting only
T1
Coherent gradient echo are usually used with __ weighting with a long TE to image water
T2*
TE controls __ weighting
T2*
This is achieved by altering the phase angle of each RF excursion pulse every __
TR
T1 weighting gradient parameters
TR <50ms short Flip angle: 60-120 large TE: <5 ms short
Coherent gradient parameters for T2*
TR short 35ms Flip angle medium 30 TE long 15ms
Incoherent gradient parameters for T1 weighting
TR short 35ms Flip angle medium 35 TE short 5ms
Proton density gradient parameters
TR: >50ms long Flip angle: 5-20 small TE: 5ms short
T2* weighting gradient parameters
TR: >50ms long Flip angle: 5-20 small TE: 15ms relatively long
A reversal of the phase encoding gradient rewinds all __ magnetization so that it's coherency is maintained
Transverse
RF spoiling ensures that residual __ magnetization is not sampled
Transverse
Five advantages of EPI
Very fast scan times Reduced artefact from respiratory and cardiac imaging All three types of weighting can be achieved Functional information required Scan time savings can be used to improve phase resolution
Three advantages of a coherent gradient echo
Very fast scans Very sensitive to flow so useful for angiography Can be acquired in a volume acquisition