MRI in practice 5th edition Chapter 4 Gradient-echo pulse sequence WCUI

The steady state produces two signals:

A FID made up of transverse magnetization that has just been created by switching off an RF excitation pulse A stimulated echo made up of the residual transverse magnetization component that builds up over time.

How gradients rephase

A gradient is applied to incoherent (out of phase) magnetization to rephase it. The magnetic moments initially fan out due to T2* decay, and the fan has a trailing edge consisting of nuclei with slowly precessing magnetic moments (shown in purple) and a leading edge consisting of nuclei with faster precessing magnetic moments (shown in red). A gradient is then applied so that the magnetic field strength is altered in a linear fashion along the axis of the gradient. The direction of this altered field strength is such that the slowly precessing magnetic moments in the trailing edge of the fan experience an increased magnetic field strength and speed up. In Figure 4.2, these are the purple spins that experience the red "high end" of the gradient. At the same time, the faster precessing magnetic moments in the leading edge of the fan experience a decreased magnetic field strength and slow down. In Figure 4.2, these are the red magnetic moments that experience the purple "low end" of the gradient. After a short period of time, the slow magnetic moments speed up sufficiently to meet the faster ones that are slowing down. At this point, all the magnetic moments are in the same place at the same time and are therefore rephased by the gradient.

VARIABLE FLIP ANGLE

A gradient-echo pulse sequence uses an RF excitation pulse that is variable and therefore flips the NMV through any angle (not just 90°). Typically, a flip angle of less than 90° is used. This means that the NMV is flipped through a lower angle than it is in spin-echo sequences when a larger 90° flip angle is usually applied. As the NMV is moved through a smaller angle in the excitation phase of the pulse sequence, it does not take as long for the NMV to achieve full relaxation once the RF excitation pulse is removed. Therefore, full T1 recovery is achieved in a much shorter TR than in spin-echo pulse sequences. As the TR is a scan time parameter, this leads to shorter scan times.

The _________ is the time between the peak of the gradient-echo and the next RF excitation pulse. This is the TE selected in the scan protocol in these sequences, but it is not the TE that determines T2 contrast.

Actual TE

GRADIENT REPHASING

After the RF excitation pulse is withdrawn, the FID immediately occurs due to inhomogeneities in the magnetic field and T2* decay. In spin-echo pulse sequences, the magnetic moments of hydrogen nuclei are rephased by an RF pulse. As a relatively large flip angle is used in spin-echo pulse sequences, most of the magnetization is still in the transverse plane when the 180° RF rephasing pulse is applied. Consequently, this pulse rephases this transverse magnetization to create a spin-echo. In gradient-echo pulse sequences, an RF pulse cannot rephase transverse magnetization to create an echo. The low flip angles used in gradient-echo pulse sequences result in a large component of magnetization remaining in the longitudinal plane after the RF excitation pulse is switched off. The 180° RF pulse would therefore largely invert this magnetization into the − z direction (the direction that is opposite to B0) rather than rephase the transverse magnetization [1]. Therefore in gradient-echo pulse sequences, a gradient is used to rephase

Things to remember - balanced gradient-echo.

Balanced gradient-echo is a steady state sequence in which longitudinal magnetization is maintained during the acquisition, thereby preventing saturation This is achieved by altering the phase angle of each RF excitation pulse every TR A balanced gradient scheme is used to correct for flow artifacts

disadvantages of EPI.

Chemical shift artifact is common Peripheral nerve stimulation due to fast switching of gradients Susceptible to artifacts

Things to remember - coherent or rewound gradient-echo.

Coherent gradient-echo is a steady state sequence that utilizes a short TR and medium flip angle. A reversal of the phase-encoding gradient rewinds all transverse magnetization so that its coherency is maintained. Both the FID and the stimulated echo are sampled so that T1, T2*, and PD weighting are possible. This sequence is usually used with T2* weighting with a long TE to image water.

Coherent gradient-echo, incoherent gradient-echo, and reverse-echo gradient-echo pulse sequences are differentiated according to whether they use one or both of these signals.

Coherent gradient-echo samples both the FID and the stimulated echo to produce either T1-/PD- or T2*-weighted images depending on the TE Incoherent gradient-echo samples the FID only to produce images that are mainly T1/PD weighted Reverse-echo gradient-echo samples the stimulated echo only to produce images that are T2 weighted

Gradient-echo sequences are classified according to which of these signals they use. They are generically referred to as follows:

Coherent or rewound gradient-echo Incoherent or spoiled gradient-echo Reverse-echo gradient-echo Balanced gradient-echo Fast gradient-echo.

The __________ is the time from the peak of the gradient-echo to a previous RF excitation pulse (i.e. the RF pulse that created its FID). This is the TE that determines T2 contrast, as this is the time allowed for T2 decay in the gradient-echo.

Effective TE

Ernst = cos-1 [e(-TR T1)]

Ernst is the Ernst angle in degrees TR is the repetition time (ms) T1 is the T1 relaxation time of a tissue (ms) This equation determines the maximum signal intensity for a tissue with a certain T1 relaxation time at different TR values. When the flip angle is larger than the Ernst angle, saturation and therefore T1 contrast increase. When the flip angle is less than the Ernst angle, contrast relies more on PD

WEIGHTING IN GRADIENT-ECHO PULSE SEQUENCES

Extrinsic parameters (TR, TE, and flip angle) The steady state Residual transverse magnetization.

Things to remember - fast imaging techniques.

Fast or turbo versions of the traditional gradient-echo sequences use strategies such as ramped sampling and fractional echo to reduce scan times. EPI 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. Ultrafast sequences are commonly used to acquire functional rather than anatomical information

Advantages of balanced gradient-echo.

Fast shorter scan times Reduced artifact from flow Good SNR and anatomical detail in 3D Images demonstrate good contrast

Advantages reverse-echo gradient-echo

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

Suggested parameters To maintain the steady state:

Flip angle 30°-45° TR 20-50 ms.

To maintain the steady state:

Flip angle 30°-45° TR 20-50 ms.

Suggested parameters To maintain the steady state

Flip angle: 30°-45° TR: 20-50 ms The actual TE affects the effective TE. The longer the actual TE, the shorter the effective TE. The actual TE should therefore be as short as possible to enhance T2 contrast.

Things to remember - gradient-echo pulse sequences

Gradient-echo sequences use gradients to rephase the magnetic moments of hydrogen nuclei and usually flip angles less than 90°. Both of these strategies permit a shorter TE and TR than in spin-echo pulse sequences. Low flip angles mean that, as less longitudinal magnetization is converted to transverse magnetization during the excitation phase of the sequence, less time is required for relaxation. This is why a short TR can be used. The speed of rephasing is increased using a gradient. A bipolar application of the frequency-encoding gradient enables magnetic moments of the hydrogen nuclei to rephase faster than when using an RF rephasing pulse. This permits a short TE, which means that a shorter TR can be used for a given number of slices than in spin-echo. Although faster than RF rephasing, inhomogeneities are not compensated for in this type of sequence. Magnetic susceptibility artifacts therefore increase

Gradient spoiling

Gradients are used to dephase and rephase residual magnetization. Gradient spoiling is the opposite of rewinding. In gradient spoiling, the slice-select, phase encoding, and frequency-encoding gradients are used to dephase residual magnetization so that it is incoherent at the beginning of the next TR period. T2* effects are therefore reduced. The uses and parameters involved in these pulse sequences are similar to those used in RF spoiling. However, most manufacturers use RF spoiling in incoherent or spoiled gradient-echo pulse sequences.

The good and bad of gradient-echo pulse sequences

Gradients rephase the magnetic moments of hydrogen nuclei much faster than RF pulses, and therefore echoes are generated faster than in spin-echo pulse sequences. The TEs are therefore shorter than in spin-echo. The TE is not part of the scan time equation, but, as we saw in Chapter 3, the TE determines how long we sit at each slice waiting for an echo. When the TE is short, a given number of slices are acquired in a short TR, and, therefore, the scan time is shorter than that of the spin-echo pulse sequences. However, in gradient-echo sequences, there is no compensation for magnetic field inhomogeneities. Gradient rephasing does not remove the contribution made by T2* decay processes. This is because the rephasing lobe of the bipolar gradient only affects the magnetic moments that are dephased by the dephasing lobe of the gradient. Magnetic moments dephased due to magnetic field inhomogeneities are not affected. Gradient-echo sequences are therefore very susceptible to certain artifacts that rely on magnetic field inhomogeneities such as magnetic susceptibility (see Chapter 8). They are also heavily reliant on T2* relaxation processes (see Chapter 2). As a result, in gradient-echo pulse sequences, T2 weighting is termed T2* weighting, and T2 decay is termed T2* decay to reflect the contribution made by magnetic field inhomogeneities to image contrast.

The trick is to imagine how far the vectors are flipped by the RF excitation pulse (flip angle) and then how long they are given to recover their longitudinal magnetization (TR).

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), then T1 contrast is maximized. 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), then T1 contrast is minimized. These rules, along with those of how the TE controls T2* contrast, are used to weight images in gradient-echo pulse sequences.

Things to remember - incoherent or spoiled gradient-echo.

Incoherent gradient-echo is a steady state sequence that utilizes a short TR and medium flip angle RF spoiling ensures that residual transverse magnetization is not sampled. This is achieved by altering the phase angle of each RF excitation pulse every TR and locking this to the receiver coil Only the FID is sampled so that T1 weighting predominates

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 [6]. A phase-locked circuit is used, which means 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. This is possible because the phase angle of the residual transverse magnetization is different from that of the newly created transverse magnetization.

There are two spoiling methods

RF spoiling Gradient spoiling

disadvantages reverse-echo gradient-echo

Reduced SNR in 2D acquisitions Loud gradient noise Susceptible to artifacts Image quality can be poor

disadvantages of balanced gradient-echo.

Reduced SNR in 2D acquisitions Loud gradient noise Susceptible to artifacts Requires high performance gradients

disadvantages of coherent or rewound gradient-echo.

Reduced SNR in 2D acquisitions Magnetic susceptibility increases Loud gradient noise

disadvantages of incoherent or spoiled gradient-echo

Reduced SNR in 2D acquisitions Magnetic susceptibility increases Loud gradient noise

Things to remember - reverse-echo gradient-echo

Reverse-echo gradient-echo is a steady state sequence that utilizes a short TR and medium flip angle Rephasing of the stimulated echo is initiated with an RF pulse but the echo is repositioned by a rephasing gradient Only the stimulated echo is sampled, and due to its repositioning, the TE of this echo is long enough to include T2 rather than T2* contrast

SI = PD e-TE/T2* (1-e-TR/T1) [sin θ /(1 - cos θ e-TR/T1)]

SI is the signal intensity in a tissue PD is the proton density TE is the echo time (ms) T2* is the T2* relaxation time of the tissue (ms) TR is the repetition time (ms) T1 is the T1 relaxation time in the tissue (ms) θ is the flip angle [sin θ/(1 − cos θ e−TR/T1)] is the flip angle function This equation shows why the signal intensity from a tissue depends on intrinsic and extrinsic contrast parameters. Compare this equation with Equation (2.4). The flip angle function is added, and T2 becomes T2*. The flip angle function shows how the flip angle, TR, and T1 relaxation time all determine whether a tissue is saturated. If α = 0° or 90°, then sin α = 1 and cos α = 0. This equation is then identical to Equation

ramped sampling is used in Fast gradient-echo

Sampling begins before the frequency-encoding gradient reaches its maximum amplitude. These measures ensure that the TE is kept to a minimum so that the TR and therefore the scan time are reduced.

Suggested parameters To maximize T1:

Short TE 5-10 ms

Advantages of incoherent or spoiled gradient-echo.

Shorter scan times Can be used after gadolinium injection Can be acquired in a volume acquisition Good SNR and anatomical detail in 3D

Comparison of extrinsic parameters - spin-echo and gradient-echo

Spin-echo Long 2000 m s+ Long 70 m s+ 90° Short 300-700 m s+ Short 10-30 m s+ 90° Gradient-echo Long 100 m s+ Long 15-25 ms Small 5°-20° Short less than 50 ms Short less than 5 ms Medium 30°-45° Large 70 °+

T1 and T2 relaxation times and signal intensity of brain tissue in the steady state at 1 T.

T1 T2 T1/T2 Signal Intensity Water 2500 2500 1 ↑ Fat 200 100 0.5 ↑ Cerebral spinal fluid 2000 300 0.15 ↓ White matter 500 200 0.2 ↓

The following parameters are a good place to start:

T1 weighting TR 400 ms/TE 5 ms/flip angle 90° PD weighting TR 400 ms/TE 5 ms/flip angle 20° T2* weighting TR 400 ms/TE 15 ms/flip angle 20°

Blurring occurs as a result of

T2* decay during the acquisition

Coherent or rewound gradient-echo pulse sequences are generally used to create

T2*-weighted images in a very short scan time

TEeff =2 × TR- TEact

TEeff is the effective TE in ms TEact is the TE set at the console in ms TR is the repetition time in ms This equation shows that the effective TE is longer than the TR so T2 weighting increases. It also shows that shorter actual TEs increase T2 weighting

Gradient-echo pulse sequences - summary of what's going on behind the scenes.

TR Controls the amount of T1 recovery and therefore T1 contrast. In practice, selected to maintain the steady state TE Controls the amount of T2* decay and therefore T2 contrast Flip angle Controls the amount of saturation and therefore T1 contrast. In practice, selected to maintain the steady state b value Determines how much phase shift there is across an area of tissue per s in DWI

Things to remember - weighting mechanism gradient-echo pulse sequence.

TR and flip angle control whether the NMV is saturated. Saturation is required for T1 weighting only TE controls T2* weighting For a T1-weighted gradient-echo, the flip angle and TR combination ensures that saturation occurs. The flip angle is large and the TR short to achieve this. In addition, the TE is short to minimize T2* For T2*-weighted gradient-echo, the flip angle and TR combination prevents saturation. The flip angle is small and the TR long to achieve this. In addition, the TE is long to maximize T2* For PD-weighted gradient-echo, the flip angle and TR combination prevents saturation. The flip angle is small and the TR long to achieve this. In addition, the TE is short to minimize T2*

The number of TR periods needed to reach the steady state depends on the

TR, flip angle, and the relaxation times of tissues

Gradient-echo pulse sequences differ from spin-echo pulse sequences in two ways

They use variable RF excitation pulse flip angles as opposed to 90° RF excitation pulse flip angles that are common in spin-echo pulse sequences. They use gradients rather than RF pulses to rephase the magnetic moments of hydrogen nuclei to form an echo.

Scan tip: T2* vs true T2

The difference between the terms T2 and T2* is well demonstrated in imaging of the cervical spine. If, for example, the suspected pathology is a herniated disk causing cervical myelopathy using a T2* gradient-echo sequence such as coherent or rewound, gradient-echo is a good choice. The disk is demonstrated as a low-signal-intensity disk bulge herniating into a high-signal-intensity CSF-filled thecal sac. If, however, the pathology is subtle, for example, a small multiple sclerosis plaque within the cord, then this might be missed in gradient-echo sequences. As the TE is not long enough to measure the differences in actual T2 decay times of the tissues, subtle pathologies that do not produce any changes around them become less visible. To see these pathologies, it is important to use pulse sequences that use long TEs. They are likely to produce images where the differences in the T2 decay times of the tissues are observable because there is enough time for these processes to occur before the echo is generated. Conventional spin-echo and TSE are a good choice, but there are several disadvantages with these sequences (see Chapter 3). It is difficult to use a long TE in gradient-echo sequences because they are designed to be used with a short TR to achieve short scan times. Reverse-echo gradient-echo pulse sequences allow the combination of a short TR and a long TE so that true T2 weighting is achieved at the same time as a short scan time.

Things to remember - the steady state.

The steady state is created when the TR is shorter than the T1 and T2 relaxation times of tissues. Residual transverse magnetization therefore builds up over time The residual transverse magnetization is rephased by subsequent RF pulses to form stimulated echoes The resultant image contrast is therefore determined by the ratio of T1 and T2 in a tissue and whether the FID or the stimulated echo is sampled

The weighting of various steady state gradient-echo sequences depends on whether the stimulated echo, or the FID, or both are used to generate the gradient-echo. Their contrast is determined by which of these are used to create the gradient-echo. This is the echo that is read by the MR system to produce an image

The stimulated echo contains mainly T2*/T2 weighted information because it is generated from the residual transverse magnetization. As water has the longest T2 decay time, water is a large component of the residual transverse magnetization and therefore the stimulated echo. Water is likely to be hyperintense when the stimulated echo is used to create the gradient-echo. The FID tends to create contrast that relies on T1 and proton density effects. This is because it does not contain residual transverse magnetization. Water is likely to be hypointense when the FID is used to create the gradient-echo. When both the stimulated echo and the FID are used to create the gradient-echo, T1, proton density, and T2* weighting are achievable.

Define Bipolar gradient

This means that it consists of two lobes, one negative and one positive. The frequency-encoding gradient is used for this purpose (see Chapter 5). 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. It is only these nuclei (those whose magnetic moments are dephased by the negative lobe of the gradient and are then rephased by the positive lobe) that create the gradient-echo at time TE. The area under the negative lobe of the gradient is half that of the area under the positive lobe

Advantages of coherent or rewound gradient-echo.

Very fast scans Very sensitive to flow so useful for angiography Can be acquired in a volume acquisition

Advantages of EPI.

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

FAST GRADIENT-ECHO

Very fast versions of some gradient-echo pulse sequences acquire a volume in a single breath-hold. These usually employ coherent or incoherent gradient-echo sequences, but the TE is significantly reduced. This is achieved by applying only a portion of the RF excitation pulse so that it takes much less time to apply and switch off. Only a proportion of the echo is read (partial echo) and the receive bandwidth is widened.

How gradients dephase

With no gradient applied, all the magnetic moments of hydrogen nuclei precess at the same frequency, as they experience the same field strength (in reality they do not because of magnetic field inhomogeneities, but these changes are relatively small compared with those imposed by a gradient). A gradient is applied to coherent (in phase) magnetization (all the magnetic moments are in the same place at the same time). The gradient alters the magnetic field strength experienced by the coherent magnetization. Some of the magnetic moments speed up, and some slow down, depending on their position along the gradient axis. Thus, the magnetic moments fan out or dephase because their frequencies are changed by the gradient

The steady state is generically defined as

a stable condition that does not change over time

residual transverse magnetization

affects image contrast, as it induces a voltage in the receiver coil. Tissues with long T2 decay times (i.e. water) are the main component of this residual transverse magnetization and enhance T2 contrast.

The ____________ pulse sequence is a modification of the coherent gradient-echo sequence. 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. As the area of the gradient under the line equals that above the line, moving spins accumulate a zero-phase change as they pass along the gradients. As a result, the magnetic moments of flowing spins are coherent and have a high signal intensity. This gradient scheme is the same as flow compensation or gradient moment rephasing. In balanced gradient-echo, these gradients are applied in the slice and frequency axes

balanced gradient-echo

Gradient-echoes are created by a

bipolar gradient

Hybrid sequences

combine gradient- and spin-echoes, such as GRASE (gradient- and spin-echo). Typically, a series of gradient rephasing is followed by an RF rephasing pulse. 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.

A maximum signal is induced in the receiver coil, and this signal is called a

gradient-echo

Balanced gradient-echo was developed initially for

imaging the heart and great vessels, but is also used in spinal imaging, especially the cervical spine and internal auditory meatus, as CSF flow is reduced. It is also sometimes used in joint and abdominal imaging.

Echo planar imaging (EPI)

is a rapid acquisition technique that begins with a sequence of one or more RF pulses and is followed by a series of gradient-echoes. These gradient-echoes are typically generated by oscillation of the readout gradient

half FOV ghosts

occur as a result of small errors in the timing and shape of readout gradients

Whether a gradient field adds or subtracts from the main magnetic field depends on the direction of current that passes through the gradient coils. This is called the

polarity of the gradient

Incoherent or spoiled gradient-echo

pulse sequences begin with a variable flip angle RF excitation pulse and use gradient rephasing to produce a gradient-echo. The steady state is maintained so that residual transverse magnetization is left over from previous TR periods. These sequences dephase or spoil this magnetization so that its effect on image contrast is minimal. Only transverse magnetization from the previous excitation is used, i.e. the FID, enabling T1 and proton density contrast to dominate

Coherent or rewound gradient-echo

pulse sequences use a variable flip angle RF excitation pulse followed by gradient rephasing to produce a gradient-echo. The steady state is maintained by selecting a TR shorter than the T1 and T2 relaxation times of tissues. There is therefore residual transverse magnetization left over when the next RF excitation pulse is applied. These sequences maintain the coherency of this residual magnetization by rewinding. This is achieved by reversing the slope of the phase-encoding gradient after readout [5] (Figure 4.14). Rewinding rephases all transverse magnetization regardless of when it is created so that it is in phase or coherent at the beginning of the next TR period. Therefore, the resultant gradient-echo contains information from the FID and the stimulated echo. These sequences may therefore be used to achieve T1-, PD-, or T2*-weighted images, although traditionally they are used in conjunction with a long TE to produce T2* weighting.

In the steady state, there is coexistence of both longitudinal and transverse magnetization. The transverse component of magnetization does not have time to decay and builds up over successive TRs. This transverse magnetization is produced because of previous RF excitation pulses but remains over several TR periods in the transverse plane. It is called

residual transverse

In gradient-echo sequences, the TE is not long enough to measure the T2 decay time of tissues, as a TE of at least 70 ms is required for this. In addition, gradient rephasing is inefficient so that gradient-echoes are dominated by T2* effects. True T2 weighting is difficult to achieve. The ___________ overcomes this problem in obtaining images that have a sufficiently long TE and less T2* than in other steady state sequences.

reverse-echo gradient-echo

Gradients that rephase in this way are called

rewinders.

Apart from the added variable of the flip angle, weighting rules in gradient-echo are the same as in

spin-echo

A different image contrast is achieved by beginning the sequence either with a variable RF excitation pulse termed gradient-echo EPI (GE-EPI) or with 90° and 180° RF pulses termed

spin-echo EPI (SE-EPI)

Reverse-echo gradient-echo pulses sequences were used to acquire images that demonstrate

true T2 weighting

Suggested parameters: balanced gradient-echo.

• 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.