Gradient Recalled Echo(GRE)

GRADIENT ECHO PULSE SEQUENCE ,TYPES AND APPLICATIONS

1. MRI GRADIENT ECHO PULSE SEQUENCE,TYPES AND APPLICATIONS Presenter : Sujan Karki B.Sc. MIT 3 rd year National Academy of Medical Sciences(NAMS) Bir Hospital

2. Topics Included Introduction Spatial encoding (briefly) Coherent gradient echo Incoherent or Spoiled gradient echo Steady state free precession Balanced steady state Fast gradient echo (EPI) Reference

3. INTRODUCTION Gradients are the coils of wire that when current is passed through them ,they alter the magnetic field strength in a controlled and predictable way They either add or subtract from the existing field in a linear fashion Isocentre is the point where field strength remain unchanged even when the gradient is switched on Today, most MRI scanner manufacturers employ distributed windings in a “fingerprint” pattern.

4. Gradient Axes Here are three gradient coils situated within the bore of the magnet They are named according to the axis along which they act when they are switched on. Shows these directions in a typical superconducting magnet,some manufacturers may use a different system. Fig:Gradient labeling .

5. Why is it called RECALLED ECHO ?? In GRE we will intentionally dephase the FID and rephase (or recall) it at a more convenient time, namely, at TE. In the figure we have bilobed gradient in x direction which first dephases the spins and then rephases them in the readable form. The area under the negative lobe is equal to half the area under the positive lobe (??) . The refocusing occurs at the midpoint of the positive lobe. In GRE we first diphase the FID and then Rephase it in time TE so it is called Gradient Recalled Echo.

6. WHAT IS THE REASON FOR NOT USING A 180 degree RF pulse IN GRE?? If we use 180 degree RF pulse then the longitudinal magnetization will be flipped to south and we need longer TR ,which is not desirable in GRE sequence.

7. HISTORY OF GRE NOMENCLEATURE Bloch et al and Purcell et al demonstrated the phenomenon of nuclear induction in the year 1946 but in the year 1950 Hahn recorded the transient MR signal after the RF which is now called as FID. In the same year Hahn reported the discovery of a remarkable new type of MR signal the SE which could be generated by application of 2 successive RF pulse In collaboration with other scientists Hahn discovered that train of three or more RF pulse could produce third type of MR signal STIMULATED ECHO. Allen D etal 1993

8. Why gradient echo sequences ???? For fast imaging applications because we use short TR and TE in GRE and it is applicable in real time imaging ,breath hold techniques and 3D imaging etc. 2.RF energy deposition is less in comparison to SPIN echo sequences( SAR) because it uses the low flip angle and is applicable when the heating risk are concern 3. Image contrast is flexible because we can adjust TR,TE and Flip Angle to characterize a tissue. 4 .To obtain bright blood signal during the cardiovascular and angiographic examinations the inflowing blood in the vessel is in motion and it might not see the various RF pulses.

9. Some Drawbacks of gradient echo 1 )T2* weighting rather then T2 weighting because GRE do not use 180 degree refocusing pulse 2) It is more sensitive to off-resonance (field inhomogeneity , susceptibility) because it do not have 180 refocusing pulse. 3) Peripheral nerve stimulation ( tingling sensation ) and acostic noise

10. Spatial encoding

11. Slice Selection Gradient When the gradient is turned on it alters the magnetic field strength in the linear fashion at the selected slice and changes the precessional frequency of that particular slice to resonate with the applied RF. In the given figure if we want excite the slice A then we should apply the RF of 63.76MHz and if slice b is to be excited then we should apply the RF of 63.86MHz Slice gap is the space between two slices Too small slice gap leads to the cross excitstion artefact It is switched on during the delivery of RF pulse for about 3.2ms ( Where W is Lamour frequency ,Bo is main magnetic field strength , Gz is gradient on z axis and z is distance of slice from isocentre ) Magnetic field gradients figure isocenter Magnetic field gradients

12. Slice thickness Slice thickness is determined by range of frequency called bandwidth more specifically transient bandwidth because RF is transmitted at the instant. thin slice = steep gradient and narrow bandwidth = spatial resolution also increases thick slice = shallow gradient and broad bandwidth = spatial resolution will decrease.

13. Phase Encoding Gradient Once the gradient is turned off the spins will stop precessing , but they will retain the phase they had due to the phase-encoding gradient It is turned on for about 4 ms and amplitude and polarity is altered for each phase encoding gradient. We use phase encoding gradient after the slice excitation and before the frequency or readout gradient We use phase encoding in 1 direction for 2D imaging and we use it in 2 direction for 3D imaging . Phase shift is the change in the position after the gradient is switched off but the processional frequency will be same as earlier.

14. Frequency Encoding Gradient Frequencies within the signal are read by the system during its application so it is also called readout gradient or measurement gradient , it is turned on for about 8 ms. Frequency change or frequency shift caused by the gradient is used to locate the signal. The slope of frequency encoding gradient determines the size of the FOV and therefore the image resolution. A large current produces a higher amplitude(steep gradient ) and this creates small FOV A small current produces a lower amplitude(shallow gradient ) and creates larger FOV

15. Basic Gradient Echo Sequence FID decay due to T2 decay and spin dephasing Gradient accelerates spin dephasing gradient can also rephase the spins and produce an echo

16. Hydrogen protons before the application of RF pulse Protons after the application of RF pulse move to the transverse plane Here after the gradients causes the change in magnetic field and the proton starts dephasing The rephasing gradient rephases the protons and at echo is collected Now again dephasing of protons by the second half of bilobed gradients

17. Steady state Condition where TR is shorter than both T1and T2 relaxation time of tissues Transverse magnetization has not completely decayed before the application of next TR So residual transverse magnetization accumulation over the successive TR affects the contrast. The only process that has time to occur is T2* To maintain the steady state short TR(22-50) and medium flip angle( 30-45 ) is required. Most of gradient echo sequence utilizes steady for fastest image acquisition .

18. Coherent gradient echo( FFE,GRASS,FISP) It uses the steady state by using very short TR and medium flip angle so there is a residual transverse magnetization We use rewinder gradient or rephasing in the opposite polarity of phase encoding direction with same amplitude and it results in the rephasing of the residual transverse magnetization and keep it in phase therefore it is preserved when the next excitation is applied As the residual transverse coherence is retained the tissues with long T2 times are hyperintense like blood , csf and fluid. Conventionally they produces the T2* weighted images with long TE but by manipulating parameter we can obtain T1 and PD weighting images also.

19. Typical parameters(For T2* weighting ) TR: 20-50ms TE:10-15ms flip angle:30-45 degree FOR T1 weighting TR: 400ms TE:5ms flip angle:90 degree For PD weighting TR: 400ms TE:5ms flip angle:20 degree Applications We use coherent gradient echo to produce T2* weighting in a very short scan time and as water is hyperintense they are used in angiographic, myelography and arthographic examinations. They also are used to determine whether an area contains fluid. Coherent axial slice of abdomen coherent Sagittal slice of knee

20. Incoherent gradient echo(SPGR,FLASH,T1-FFE It uses the steady state by using very short TR and medium flip angle Uses a gradient rephasing instead of 180 degree RF pulse Eliminates the residual transverse magnetization so that tissue with long T2 times are not allowed to dominate image contrast but T1/PD contrast can be obtained TE should be as short as possible to minimize the T2* effects Incoherent sagittal image of ankle IncoherentCoronal image Post contrast

21. Why spoiling is important ???? It eliminates the transverse magnetization after each TR which prevents the errors related to transverse magnetization. It shortens the TR, if spoilers are not used then we have to wait for 5xT2* With the help of spoilers we can enhance T1 contrast

22. RF SPOILING It is considered as the powerful technique of removing the signal contribution from the residual transverse magnetization Here in each TR magnetization is tipped in different axis. In quadratic phase cycling, the phase for the nth RF pulse is given by phi=n(n+1)phi and constant increment, phi. From spoiling we get T1 contrast because of proper phase increment If we suppress the residual transverse magnetization then the signal will be independent of T2,resulting in T1 signal . Karla L Miller etal 2011

23. Gradient Spoiling It is one of the easiest method of spoiling where the gradient pulse is used to create a range of phase angles across the voxel. The main purpose here is avoid rephasing in each TR. We should apply the gradient of different amplitude in each TR to avoid the rephasing However achieving a broad range of variable areas require either strong gradient or long TR ,making this impractical in most circumstances Karla L Miller etal 2011

24. Long TR Spoiling For long TR spoiling we must have to wait for about 5 times the T2* decay, ie we must let the complete dephasing We must wait for about 99% of the recovery of longitudinal magnetization . It will increase the scan time Qa in mri

25. Typical parameters(For T1 weighting ) TR: 20-50ms TE:5-10ms flip angle:30-45 degree In addition: Average scan time –  several seconds for single slice , minutes for volumes. Applications 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 . Incoherent or spoiled gradient-echo sequences also demonstrate good T1 anatomy and pathology after gadolinium contrast enhancement

26. T2 VS T2* T2 is defined as a time constant for the decay of transverse magnetization arising from natural interactions at the atomic or molecular levels transverse magnetization decays much faster than would be predicted by natural atomic and molecular mechanisms; this rate is denoted  T2* ("T2-star") . T2* is always less than or equal to T2. T2* results principally from inhomogeneities in the main magnetic field.  T2* weighting are generally used for the detection of small hemorrhages and calcifications

27. To produce same contrast there are these differences in parameters in spin echo and gradient echo Via ucla radiography

28. Ernest Angle It is the flip angle which gives the maximum MRI signal at the given TR and T1

29. Spoiled GRE &Ernest Angle

30. Balanced Steady-State Free Precession ( bSSFP,True FISP,FIESTA) "Balanced" means that the net gradient-induced dephasing over a TR interval is zero. The signal intensity is seen to be the ratio   T2/T1 . To ob tain balanced steady state we sample both FID and ECHO In balanced gradient echo gradients are applied in slice and frequency axis . Higher flip angle and shorter TR is used than in coherent echo so higher SNR and shorter scan time highlight fluids such as CSF and blood, making them ideal for cardiovascular MRI, MR cisternography/myelography, MR urography, and MR enterography .

31. banding artefact The refocusing mechanism fails if intravoxel dephasing exceeds ±180º manifest by band-like artifacts. They are also more problematic in 3D acquisitions where  TR  values may exceed 10-15 msec. banding artifacts appear as a result of off-resonance effects, improved shimming can also mitigate the appearance of these artifacts. To ameliorate the banding artifacts, the  TR  should be minmized ,

32. Typical parameters( T2/T1 contrast ) • 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. Advantages Disadvantages Shorter scan time Reduced SNR in 2D acquisitions Reduced artifacts from flow Loud gradient noise Good SNR and anatomical detail in 3D imaging Susceptible to artefacts Image demonstrate good contrast Requires high performance gradients Advantages and disadvantages of balanced gradient echo Axial balanced gradient-echo image of the lumbar spine.

33. Steady State Free Precession (SSFP,T2FFE,PSIF) It gives T2 weighted image RF contain various amplitude and some are capable of rephasing the FID. Each RF pulse therefore not only produces its own FID, but also rephases the FID produced from the previous excitation. The echo from the first excitation pulse occurs at the same time as the third excitation pulse. RF cannot be transmitted and received at the same time. To prevent this, a rewinder gradient is used to speed up the rephasing process after the RF rephasing has begun. Via mri at glance

34. Ssfp cont. In SSFP here are two TE: Actual TE (time between echo and next RF pulse) Effective TE (time from the echo to the excitation pulse that created its FID) rephasing is done by RF so that to reduce the off-resonance effects i.e more T2 than T2*. Via mri at glance Acronyms of SSFP

35. T2 weighting image produced by SSFP SPSS is generally used in acquiring T2 weighting contrast which is useful in brain and joints for the 2D and 3D acquisitions. They are replaced by TSE as TSE produce the T2 weighting image in shorter scan time. Image via MRI in practice

36. Dual Echo Steady State(DESS) DESS generates the FID-like and Echo-like signals from the steady-state free precession individually. Phase-encoding and slice-select gradients are balanced to maintain the transverse steady state.  The contrast of DESS is unique as it combines features from both the FID-signal of FISP with the Echo-signal of PSIF. Fluid is extremely bright and Bone is relatively dark due to T2* dephasing from trabeculae .

37. MERGE MERGE  ("Multiple Echo Recombined Gradient Echo") is a spoiled T2*-weighted sequence for spinal and musculoskeletal imaging By reversing the frequency-encoding gradient rapidly, several individual gradient echoes can be generated at different  TEs . The number of echoes is limited by T2*-decay, but typically between 3-5 echoes are recorded.

38. GRASE(Gradient and Spin Echo) It is a hybrid technique that generates and records a series of alternately acquired gradient echoes and spin echoes from a train of RF-pulses.  In its original implementation, a 90°-RF pulse was followed by series of eight 180°-refocusing pulses, generating eight spin echoes.  Centered about (and overlapping) each spin echo, three gradient echoes were produced by rapidly switching the readout gradient polarity.  The final data set consisted of 24 separate MR signals having properties intermediate between spin-echo and gradient echo contrast.  The advantage of GRASE is that the gradient echo component would provide increased sensitivity for detecting calcifications and hemorrhages (often difficult to see on SE) while not having too much susceptibility artifact at normal anatomic interfaces. Additionally the energy deposition ( SAR ) is lower than a comparable fast spin-echo sequence because there are fewer RF-pulses. T2- vs T2*-weighting can be obtained

39. FIESTA_C/CISS modification of the basic FIESTA/ TrueFISP sequence composed of a pair of TrueFISP acquisitions run back-to-back preceded by an automatic shimming procedure.  The first uses phase alternation of the RF-pulses (+α, −α, +α, −α, ...) while the second does not (+α, +α, +α, etc ). When the paired data sets are combined in maximum intensity projection, the phase errors cancel, resulting in an image largely free of dispersion banding combination of paired signals is performed automatically after data collection FIESTA-C/CISS is currently the sequence of choice for CSF-cisternography for visualizing cranial nerves at the skull base.

40. FAST GRADIENT IMAGING AND EPI (single shot imaging) First we apply RF pulse the we switch the slice select gradient In EPI the we wont isolate the dephasing and the rephasing lobe but bring them closer Phase encoding gradient is applied at the zero oscillating the gradient magnetic field If we have very strong magnetic field then we can turn on and off for very short time As we move on time there will be progressively more and more phase encoding gradient so filling in k space will be from center to outwards Frequency is inverting in each time so filling in k space will be in rectilinear fashion We will get T2* weighting image We may fill the entire k space in single RF so it may also be called as single shot imaging VIA EINSTEIN COLLEGE OF RADIOLOGY ( youtube )

41. Bibliography MAGNETIC RESONANCE IMAGING Physical and Biological PrinciplesStewart Carlyle Bushong , ScD, FAAPM, FACR Professor of Radiologic Science Baylor College of Medicine Houston, Texas Geoffrey Clarke, PhD Professor and Chief of Graduate Education Department of Radiology The University of Texas Health Science Center at San Antonio San Antonio, Texas MRI Basic Principles and Applications Brian M. Dale, PhD MBA Mark A. Brown, PhD Richard C. Semelka , MD FIFTH EDITION MRI at a Glance Catherine Westbrook Steady-state MRI: methods for neuroim agingKarla L Miller†1, Rob HN Tijssen1 , Nikola Stikov2 & Thomas W Okell12011 CT AND MRI OF THE WHOLE BODY Sixth Edition John R. Haaga , MD, FACR, FSIR, FSCBT, FSRS MRI From Picture to Proton MRI in Practice 4 th and 5 th Edition Catherine Westbrook MSc, FHEA, PgC (HE), DCRR, CTC Rapid Gradient-Echo Imaging Brian A. Hargreaves, PhD*2012 FLASH Imaging. Rapid NMR Imaging Using Low Flip-Angle Pulses A. HAASE, J. FRAHM, D. MATTHAEI, W. H,&NICKE, AND K.-D. MERBOLDT www.ucla radiography Questions and answers in mri Steady-State MR Imaging Sequences: Physics, Classification, and Clinical Applications1 Gradient echo MR imaging techniques and acronyms Einstein institute of medicine