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