MRI in Practice chapter 4 Gradient echo pulse sequences.

Advantages of 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

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. -There are two spoiling methods: RF Spoiling and Gradient spoiling.

Scan tip: coherent or rewound GE pulse sequences

-As both the FID and stimulated echo are sampled in this sequence, it is possible to obtain PD-, T1-, and T2*-weighted images. -It is also normally possible to use this sequence out of the steady state with a TR similar to that used in T1-weighted spin-echo (e.g. 400 ms). -This enables acquisition of multiple slices in the TR period and reduces some artifacts. -It is worth noting that if any of the other gradient-echo sequences yield poor image quality it might be worth trying coherent gradient-echo. -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°

Things to remember- balanced gradient echo.

-Balanced gradient-echo is a steady state sequence in which longitudinal magnetization is maintained during the acquisition, there by 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

Balanced gradient-echo 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 _______ ______ _______, these gradients are applied in the slice and frequency axes. -In addition, higher flip angles and shorter TRs are used than in coherent gradient-echo producing a higher SNR and shorter scan times. -Normally, this combination of flip angle and TR results in saturation and therefore enhanced T1 contrast. -However, saturation is avoided by changing the phase of the RF excitation pulse every TR. -This is achieved by selecting a flip angle of 90°, for example, but in the first TR period only applying half of this, i.e. 45°. -In successive TRs, the full flip angle is applied but with alternating phase angles so that the resultant transverse magnetization is created at a different phase every TR. -Consequently, saturation is avoided, and fat and water, which have T1/T2 relaxation times approaching parity, return a higher signal than tissues that do not. -The resultant images display a high SNR; good CNR between fat, water, and surrounding tissues; and fewer flow voids. -Also, these images are obtained in a very short scan time.

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. -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). -GE-EPI begins with an RF excitation pulse of any flip angle and is followed by EPI readout of gradient-echoes -SE-EPI begins with a 90° RF excitation pulse followed by a 180° RF rephasing pulse followed by an EPI readout of gradient-echoes. -Application of the rephasing pulse helps to "clean up" some of the artifacts caused by magnetic field inhomogeneities and chemical shift. -SE-EPI has longer scan times but generally a better image quality than GE-EPI, but the extra RF pulses increase RF deposition to the patient. --EPI sequences can begin with any type of RF pulse. -An example is EPI-FLAIR (180°/90°/180° followed by EPI readout) where CSF is nulled, but the sequence is significantly faster than in conventional FLAIR sequencing -EPI sequences are often run in conjunction with single-shot imaging. --In all single-shot techniques, all k-space is filled at once so the recovery rates of vectors in individual tissues are not critical. -For this reason, the TR is said to equal infinity (because it is infinitely long). -Either PD or T2* weighting is achieved by selecting either a short or long effective TE, which corresponds to the time interval between the RF excitation pulse and when the center of k-space is filled. -T1 weighting is produced by applying an RF inverting pulse before the RF excitation pulse to produce saturation.

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

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

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

extrinsic contrast parameters (Weighting mechanism 1)

- the RF excitation pulse flip angle is 90° (as in spin-echo pulse sequences). Under these circumstances, TE controls T2 contrast, and T2 contrast increases as the TE increases. -The same is true in gradient-echo pulse sequences except that T2 is termed T2* to reflect the fact that magnetic field inhomogeneities are not compensated for by gradient rephasing.In spin- echo pulse sequences, TR controls T1 contrast, and T1 contrast increases as the TR decreases. --This is because short TRs do not permit complete recovery of the vectors, and, therefore, subsequent 90° RF excitation pulses cause saturation. -In gradient-echo pulse sequences, the TR and the flip angle control the amount of T1 relaxation and saturation that occurs. -Apart from the added variable of the flip angle, weighting rules in gradient-echo are the same as in spin-echo. -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.

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 and a leading edge consisting of nuclei with faster precessing magnetic moments. -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. -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. --A maximum signal is induced in the receiver coil, and this signal is called a gradient-echo. --Gradients that rephase in this way are called rewinders. --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.

(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 ______ _____ 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.

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. --Therefore in gradient-echo pulse sequences, a gradient is used to rephase transverse magnetization instead.

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

bipolar gradient

-Gradient-echoes are created by a _______ ________. This means that it consists of two lobes, one negative and one positive. -The frequency-encoding gradient is used for this purpose. -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.

Gradient spoiling

-Gradients are used to dephase and rephase residual magnetization. -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.

Learning tip: 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 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. -They are also heavily reliant on T2* relaxation processes. -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.

residual transverse magnetization (Weighting mechanism 3)

-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 ________ _________ _________, and it 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 extrinsic contrast parameters (TR, TE, and flip angle) are selected to generate the steady state and to enhance T2* contrast. -However, the influence of the other two weighting mechanisms is also evident. -The effect of residual transverse magnetization is seen from the high signal from water in the stomach. Water is also hyperintense in this image because water has a good parity between its T1 recovery and T2 decay times. -Fat is also bright on this image for the same reason. Muscle is hypointense because it does not have a parity between its T1 recovery and T2 decay times.

Reverse-echo gradient-echo (SS FP)

-In this sequence, only the stimulated echo is read. -_________ _______ ______ ______ 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. -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. -As previously described, every RF pulse regardless of its net magnitude contains energies that have sufficient magnitude to rephase spins and produce a stimulated echo. -To do this, the stimulated echo is repositioned so that it does not occur at the same time as the subsequent RF excitation pulse. -A rewinder gradient accelerates rephasing so that the stimulated echo occurs sooner than it normally does. -Rewinding is achieved by applying the positive lobe of the frequency-encoding gradient. -The resultant gradient-echo is dominated by the stimulated echo and therefore demonstrates better T2 weighting than conventional gradient-echo sequences. -This is because the TE (called the effective TE) is longer than the TR. -In this sequence, there are usually two TEs: Actual and Effective

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

Uses for incoherent or spoiled gradients

-Only the FID is sampled so that T1 weighting predominates -As the stimulated echo contains mainly T2* and T2 information, and this is spoiled, these pulse sequences produce T1- or PD-weighted images. -This is because image contrast is mainly influenced by the FID that contributes T1 and proton density contrast. -However, flowing water (blood and CSF) may have a rather high signal due to gradient rephasing. -These sequences are used for 2D and volume acquisitions, and, as the TR is short, 2D acquisitions are used to obtain T1-weighted breath-hold images. --also demonstrate good T1 anatomy and pathology after gadolinium contrast enhancement

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. --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. -enables only gradient-echoes produced from the most recently created transverse magnetization to affect image contrast.

disadvantages of 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 GE pulse sequences

-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* contrastIn addition: -Average scan time - seconds for slice-by-slice acquisitions to several minutes for volumes.

random things to know

-SSFP- steady state free procession -coherent samples everybody and everything -Spoilers- get rid of magnetization -Rewinders- put everything back into phase. -The effective TE is longer than the TR- you get more T2 weighting on a gradient echo.

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

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*

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

Ernst angle

-The _____ _____ is the flip angle that provides optimum signal intensity for a tissue with a given T1 recovery time scanned using a given TR. --The optimum signal intensity in all three tissues is about 12°, but to obtain good contrast between them, larger flip angles between 30° and 45° are required.

stimulated echo

-The ______ ______ 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 (bright) when the stimulated echo is used to create the gradient-echo.

effective TE

-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.

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 FSE are a good choice, but there are several disadvantages with these sequences. -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.

uses and limitations of EPI

-The rapid scan times of the EPI pulse sequence decreases physiological motion in MR images, which is advantageous when imaging the heart and coronary vessels and when performing interventional techniques. -Rapid imaging also enables visualization of physiology such as perfusion and blood oxygenation. -As this pulse sequence requires rapid switching of gradient polarity, particularly in the frequency encoding axis, nerve stimulation sometimes occurs. I -n addition, gradient noise is high so acoustic insulation and ear protection are essential -In addition, many artifacts are seen in EPI including distortion and chemical shift. -As each gradient-echo is acquired rapidly, there is relatively little chemical shift in the frequency direction. -However, there is a larger chemical shift along the phase axis. -This phase directional chemical shift artifact does not appear in conventional spin or gradient-echo acquisitions since echoes with different phase-encoding gradient applications are acquired at the same time after RF excitation. -In single-shot imaging, however, the length of time required to perform a train of phase-encoding gradient applications means that phase data are encoded at different times after RF excitation. - This results in chemical shift that is larger than in spin-echo imaging. -Other artifacts seen in single-shot imaging include blurring and ghosting.

learning tip: echo formation in the steady state

-The steady state involves repeatedly applying RF excitation pulses using very short TRs. -As the TR is shorter than either the T1 or T2 relaxation times of the tissues, there is a build-up of residual transverse magnetization over successive TR periods. -This is because there is not enough time in between RF excitation pulses for the transverse magnetization to either dephase or realign with B0. -In the steady state, this residual transverse magnetization is rephased by subsequent RF excitation pulses in the sequence and produces echoes. -This might seem odd as, so far, we have assumed that RF excitation pulses excite, and RF rephasing pulses rephase. -RF excitation pulses usually have a magnitude of 90° and rephasing pulses a magnitude of 180°. In fact, any pair of RF pulses may produce an echo. -Their magnitude is irrelevant - they can all excite and rephase. -RF pulses are pulses of electromagnetic radiation and consist of an oscillating magnetic field. -If the frequency of the oscillating magnetic field matches the precessional frequency of the magnetic moments of hydrogen nuclei (Larmor frequency), resonance occurs. -The magnitude of the magnetic field (B1) determines the energy of the pulse and therefore the flip angle. -An RF excitation pulse resulting in a 90° flip angle is common in spin-echo pulse sequences. -However, we have already learned that RF pulses with flip angles other than 90° are still excitation pulses. -The same is true of RF rephasing pulses - they still have the capability to rephase magnetic moments even if they do not have a magnitude of 180°

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.

learning tip: how to differentiate common steady state sequences

-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. • 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- orT2*-weighted images depending on theTE. • 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

WeIghtIng In gradient-echo pulse sequences

-There are essentially three different processes that affect weighting in gradient-echo pulse sequences, and sometimes all three overlay each other in the image. These are as follows: • Extrinsic parameters (TR, TE, and flip angle) • The steady state • Residual transverse magnetization.

Using extrinsic contrast parameters in gradient-echo - (PD weighting)

-To obtain a PD-weighted image, both T1 and T2* processes are minimized so that the differences in proton density of the tissues are demonstrated. -To minimize T2* decay, the TE is short so that neither the fat nor the water vectors have had time to decay. To minimize T1 recovery, the flip angle is small and the TR long enough to permit full recovery of longitudinal magnetization before the next RF excitation pulse is applied.

Using extrinsic contrast parameters in gradient-echo - (T1 weighting)

-To obtain a T1-weighted image, differences in the T1 recovery times of the tissues are maximized, and differences in the T2* decay times of the tissues are minimized. -To maximize differences in T1 recovery times, neither fat nor water vectors are given time to recover full longitudinal magnetization before the next RF excitation pulse is applied. --To avoid full recovery of their longitudinal magnetization, the flip angle is large and the TR short so that the fat and water vectors are still in the process of recovering when the next RF excitation pulse is applied. -To minimize differences in T2* decay times, the TE is short so that neither fat nor water has time to decay

Using extrinsic contrast parameters in gradient-echo - (T2* weighting)

-To obtain a T2*-weighted image, differences in the T2* decay times of the tissues are maximized, and differences in the T1 recovery times are minimized. -To maximize differences in T2* decay times, the TE is long so that fat and water vectors have had time to dephase. -To 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. -In practice, small flip angles produce such little transverse magnetization that full longitudinal recovery occurs even if the TR is short

Advantages of coherent GE pulse sequences

-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. -In addition, a technique called ramped sampling is used. -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. -Many fast sequences use extra pulses, applied before the pulse sequence begins, to pre- magnetize the tissue. -This premagnetization is achieved by applying a 180° RF inverting pulse before the rest of the pulse sequence begins. -This inverts the NMV into full saturation, and at a specified delay time, the pulse sequence itself begins. -This enhances T1 contrast and may also null signal from certain organs and tissues as in inversion recovery pulse sequences. -Fast gradient systems permit multi slice gradient-echo sequences with very short TEs. -Multiple images are therefore acquired in a single breath-hold and are free from respiratory motion artifacts. -In addition,______ ______ _______ acquisitions are useful when temporal resolution is required. -This is especially important after the administration of contrast agents when the selection of fast gradient-echo permits dynamic imaging of an enhancing lesion.

scan tip: parameter selection in gradient-echo - what's going on behind the scenes?

-When we alter extrinsic contrast parameters in gradient-echo pulse sequences, behind the scenes, we determine image weighting. -When we select the TR and the flip angle in the scan protocol, we control how much T1 recovery is permitted between each RF excitation pulse and how far the vectors are moved by the RF excitation pulse. -We therefore control the extent to which T1 contrast influences image weighting. -We also determine the SNR, the scan time, and the slice number, but these factors are not usually as important as weighting. -In most gradient-echo pulse sequences, the TR and the flip angle are selected to maintain the steady state rather than to control T1 contrast. When we select the TE in the scan protocol, we control how muchT2* decay is permitted between the RF excitation pulse and the peak of the gradient-echo. -We therefore control the extent to which T2* contrast influences image weighting. -We also determine the SNR, but this is not usually as important as weighting.

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 trailing edge of the fan consists of nuclei whose magnetic moments slow down because they are situated on the gradient axis that has a lower magnetic field strength relative to isocenter. -The leading edge of the fan consists of nuclei whose magnetic moments speed up because they are situated on the gradient axis that has a higher magnetic field strength relative to isocenter. -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. --Gradients that dephase in this way are called spoilers, and the process of dephasing magnetic moments with gradients is called gradient spoiling.

Coherent or rewound gradient-echo- pulse sequences

-as both the FID and stimulated echo are sampled in this sequence, it is possible to obtain PD-, T1-, and T2*-weighted images. -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. --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. -generally used to create T2*-weighted images in a very short scan time -In addition: use gradient moment rephasing to accentuate T2* and reduce flow artifact -average scan time - seconds for single slice, minutes for volumes

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. -The main purpose of these two mechanisms is to enable shorter TRs and therefore scan times than are common with spin-echo pulse sequences.

Things to remember: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

Uses of GE sequences

-reduction in scan time because of greatly reduced scan time. -for dynamic contrast enhancement -For angiographic studies -To acquire T2* (no 180 pulse) T1, and PD weighting. -Used for single slice or volume breath- hold acts in the abdomen.

Steady state (Weighting mechanism 2)

-stable condition that does not change over time. -energy in equals energy out. -The RF excitation pulse gives energy to hydrogen nuclei, and the amount of energy applied is determined by the flip angle. -Energy is lost by hydrogen nuclei through spin-lattice energy transfer, and the amount of lost energy is determined by the TR. -Therefore, by selecting a certain combination of TR and flip angle, the overall energy of the system remains constant, as the energy "in" as determined by the flip angle equals the energy "out" as determined by the TR. -As RF has a low frequency and hence low energy, for most values of flip angle very short TRs are required to achieve the steady state. -In fact, the required TRs are shorter than the T1 and T2 relaxation times of tissues. -Therefore, unlike spin-echo, where even with short TRs some transverse magnetization decays, in gradient-echo there is no time for transverse magnetization to decay before the pulse sequence is repeated. -This magnetization influences weighting as the receiver coil is positioned in the transverse plane. -The number of TR periods needed to reach the steady state depends on the TR, flip angle, and the relaxation times of tissues. -However, in gradient-echo sequences, a short TR is deliberately used to minimize the scan time. -As the TR is so short, magnetization in tissues does not have time to reach its T1 recovery or T2 decay times before the next RF excitation pulse is applied. --Therefore, 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. -In tissues where T1 recovery and T2 decay times are similar, signal intensity is high, and where they are dissimilar, signal intensity is low. -In the human body, fat and water have this parity (fat, very short T1 recovery and T2 decay times; water, very long T1 recovery and T2 decay times); therefore these tissues return high signal intensity in steady state sequences. -Tissues such as muscle do not have this parity (very short T2 decay time and very long T1 recovery time), so they return a low signal in steady state sequences. --Typically, TRs less than 50 ms are considered appropriate to maintain the steady state. -The optimum flip angle is determined by the Ernst angle equation.

Uses: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. I -t is also sometimes used in joint and abdominal imaging.

Uses: Reverse-echo gradient-echo pulses sequences

-were used to acquire images that demonstrate true T2 weighting. -They were especially useful in the brain and joints with both 2D and 3D volumetric acquisitions. -FSE has now largely replaced this sequence, as it produces better T2 weighting in short scan times. -However, the process of shifting the stimulated echo is used in sequences where rapid data acquisition and long TEs are required. An example of this is in perfusion imaging

a train of RF pulses generates multiple echoes

. Two "signals" are created at each RF pulse: • A FID • An echo (called a spin-echo.These echoes are also termed Hahn or stimulated echoes). -Any two 90° RF pulses produce a Hahn echo. -Any two RF pulses with varying amplitude, i.e. with flip angles other than 90° produce stimulated echoes. -This latter type of echo is used in steady state gradient-echo sequences as variable flip angles are common. -In practice, echo production is so rapid that the tails of FID signals merge with stimulated echoes resulting in a continuous signal of varying amplitude.

ghosting

In EPI acquisitions, half FOV ghosts occur as a result of small errors in the timing and shape of readout gradients. -This causes differences between echoes acquired with positive and negative readout gradients. -These errors cause a ghost of the real image that appears shifted in the phase direction by one half of the FOV. -Since it is difficult to eliminate these errors, a correction is usually performed during image reconstruction using information acquired during a reference scan.

SS FP (reverse echo gradient echo)

Suggested Parameters for ______________: To maintain 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

coherent or rewound

suggested parameters for ________ or ________ pulse sequences: To maintain the steady state: • Flip angle- 30°-45° • TR- 20-50 ms. To maximize T2*: • Long TE- 10-15 ms.

negative

To dephase the FID faster you make the first lobe _________

T2*

You cannot get true T2 weighting on Gradient echos You get _______ weighting.

Double Echo Steady State (DESS)

_____ ______ ______ _______ is a steady state sequence that generates two echoes. -One is a coherent gradient and the other a reverse-echo gradient-echo. -Both are combined in the final image. -The coherent gradient-echo provides resolution and the reverse-echo gradient-echo T2 contrast.

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.

Spoiling gradients

________ ________ are often applied at the end of each TR to eliminate any transverse magnetization should there be any present. -In addition, crusher pulses are sometimes used around the 90° and 180° RF pulses that destroy the unwanted effects of each pulse. -Therefore, in spin-echo pulse sequences, not only is there nothing in the transverse plane at the end of the TR period to rephase, but the RF excitation pulses only excite, and RF rephasing pulses only rephase. -Crusher pulses make sure that they only have one function. -In gradient-echo pulse sequences in the steady state, these strategies are not used. -There is therefore residual transverse magnetization present at the end of each TR period. -In addition, every RF pulse is an excitation and rephasing pulse, and rephases the residual transverse magnetization to produce an echo.

Blurring

_________ occurs as a result of T2* decay during the acquisition. -If the train of gradient-echoes takes a similar time to decay, signal from the end of the acquisition is reduced, resulting in a loss of resolution and blurring.

rewinders

gradients that rephase

balanced gradient echo (parameters)

suggested parameters for _______ _______ ______: • 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.

incoherent or spoiled gradient echo

suggested parameters for _________ or __________ ________ _______: To maintain the steady state: • Flip angle- 30°-45° • TR- 20-50 ms. To maximize T1: • Short TE- 5-10 ms. In addition: • Average scan time - several seconds for single slice, minutes for volumes.

gradient spoiling

the use of gradients to dephase magnetic moments - the opposite of rewinding

180's

these decrease magnetic susceptibility.

FID

• The _____ 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 (dark) when the FID is used to create the gradient-echo.

actual TE

• 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.

stimulated echo and FID

• When both the _________ ______ and the ______ are used to create the gradient-echo, T1, proton density, and T2* weighting are achievable.