single band RF pulse
A Z-Gradient Array for Spatially Oscillating Magnetic Fields in Multi-Slice Excitation Koray Ertan1, 2, Soheil Taraghinia1, 2, Alireza Sadeghi1, 2, Ergin Atalar1, 2 1National
Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey 2Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey
Introduction
Methods
• Design of multi-slice excitation, refocusing and inversion RF pulses are limited by SAR, peak RF voltage, duration of the RF pulse [1]
Theory • Traditionally, different spatial positions are encoded to different frequencies bijectively which makes the RF pulse more demanding. • Aim is to create such a magnetic field to assign same magnetic field to M slice locations with N gradient channel as in Figure 1a. 1Point Per Slice (1PPS ) Method (Figure 1b) At the center point of each slice location • Magnetic field should be equal • Absolute value of the derivative with respect to z direction should be equal for same slice thickness • Second derivative of the magnetic field wrt z direction should be zero for intra-slice homogeneity since laplacian of a static magnetic fields are zero. If z derivative is zero due to radial symmetry other derivatives also vanishes • 3M-1 channel is required 2 Point Per Slice (2PPS ) Method (Figure 1c) At two off-center points in each slice location • Magnetic field should be equal • Absolute value of the derivative with respect to z direction Figure 1: Illustrative Design Method should be equal for same slice thickness for M slice selection with N channel • Previous constraint automatically satisfies second derivative array (a) Slice locations (b) 1PPS constraint in 1PPS and provides better intra-slice homogeneity method and equations (c) 2PPS method • 4M-1 channel is required and equations REFERENCES
• Figure 4 shows that proposed technique is possible for the off-centered slice locations by applying different current combinations to each channel. • Slice thickness performances (constant slice thickness in the whole slice plane) is similar for the off-centered slice locations. • Maximum available gradient performances are almost same for the 1PPS method and slightly changes for the 2PPS method; however, 2PPS method is advantageous • In the experiments field map of each channel is measured with the scanner and currents are optimized according to required slice locations • Experimental result in Figure 5 shows that even with the 1PPS method 3 slice selection with a single band RF pulse is possible
Figure 2: Experimental Setup (a) 9 channel gradient array. Inside the gradient coil, there is Tx/Rx birdcage coil and its RF shield. (b) Custom made gradient amplifiers, power distribution units and FPGA [8]
• In order to overcome limitations, there are techniques to design multi-band RF pulses such as phase optimization [2], time shifting [3], PINS [4], MultiPINS [5], parallel transmission [6], root flipping [7]. • Purpose of this study is excite multiple slice locations with a single band RF pulse. This is possible if different slice locations are mapped to same frequency so that single band RF pulse can affect multiple locations. In order to dynamically create such a magnetic field distribution for each different slice location set, nine channel z-gradient array is implemented and proof of concept has been demonstrated with both simulations and experiments.
Presentation number: 81 Session Title: Novel hardware concepts Session Time: 29.09.2016, 16:00 - 17:30
Gradient Coil • Diameter = 25cm • Each turn has 36 turns • Wire thickness is 0.8mm
Amplifiers and Control Experiments and Simulations • Simulations in MATLAB • User interface (C#) • H-bridge (50V, 30A) • Experiments in 3T (Tim Trio, Siemens) • PWM signals with FPGA
Results • Figure 3 shows the simulation results for 1PPS and 2PPS methods • 9 channel is enough for exciting 3 slices with 1PPS and 2 slices with 2PPS • Slice seperation is 9cm for 1PPS and 13.6cm for 2PPS to cover all volume of 27.5cm with slice thickness of 5mm
Figure 4: Simulated selected slices for 1PPS (3slice) and 2PPS (2 Slice) Same parameters with Figure 3 is used only slice locations are changed at each step to cover the whole volume. Each plot shows the attainable gradient strength with the current hardware.
Figure 5: Experimental coronal phantom image shows that 3 slice selection in z direction with a single band RF pulse is possible. Slice seperation is 9 cm and slice thickness is 6.6mm
Discussion and Conclusion
• Under maximum voltage and • Proposed technique can be also used for refocusing and inversion pulses current limitations of the amplifiers • Proposed technique can be combined with the existing methods [1-7] to multiply the obtained gradients are 19mT/m and number of slices without no increase in RF demand especially for the 2PPS method since 26mT/m for 1PPS and 2PPS it creates almost linear gradients in 2 different region. methods • Available gradient strength strongly depends on the slice distance, length of each channel and the coil diameter which we are currently working on. • 2PPS manages to excite slices with • Although 1PPS method produces slightly curved slices, depending on the diameter of rectangular profile while 1PPS VOI, curvature of the slice profiles of 1PPS method can be considered as negligible and excites curved slices (Figure 3e,f) we are currently working on obtaining transverse images to observe the effect. Figure 3: Simulations for 1PPS and 2PPS methods. (a, b) Slice profiles (Red boxes are the imaging radius • Conlusion: Multi-slice excitation with a single band RF pulse without increasing the of the RF coil) (c, d) Oscillatory magnetic field at the center line (e, f) percentage error of slice thickness duration, SAR and peak power of the RF pulse is possible with the proposed gradient as a function of radius. array system
[1] Barth, Markus, et al. "Simultaneous multislice (SMS) imaging techniques."Magnetic resonance in medicine 75.1 (2016): 63-81. [2] Sbrizzi A, Poser BA, Tse DH, Hoogduin H, Luijten PR, Berg CA. RF peak power reduction in CAIPIRINHA excitation by interslice phase optimization. NMR in Biomedicine 2015;28:1393-1401. [3] Auerbach EJ, Xu J, Yacoub E, Moeller S, Uğurbil K. Multiband accelerated spin‐echo echo planar imaging with reduced peak RF power using time‐shifted RF pulses. Magnetic resonance in medicine 2013;69:1261-1267. [4] Norris DG, Koopmans PJ, Boyacioğlu R, Barth M. Power independent of number of slices (PINS) radiofrequency pulses for low‐power simultaneous multislice excitation. Magnetic resonance in medicine 2011;66:1234-1240. [5] Eichner C, Wald LL, Setsompop K. A low power radiofrequency pulse for simultaneous multislice excitation and refocusing. Magnetic resonance in medicine 2014;72:949-958. [6] Guérin B, Setsompop K, Ye H, Poser BA, Stenger AV, Wald LL. Design of parallel transmission pulses for simultaneous multislice with explicit control for peak power and local specific absorption rate. Magnetic Resonance in Medicine 2015;73:1946-1953. [7] Sharma, Anuj, Michael Lustig, and William A. Grissom. "Root‐flipped multiband refocusing pulses." Magnetic resonance in medicine 75.1 (2016): 227-237. [8] Feasibility of Z gradient Array for Variable Volume of Interest”ESMRMB 2015, Soheil Taraghinia, Koray Ertan and Ergin Atalar
Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey 2Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey
Introduction
Methods
• Design of multi-slice excitation, refocusing and inversion RF pulses are limited by SAR, peak RF voltage, duration of the RF pulse [1]
Theory • Traditionally, different spatial positions are encoded to different frequencies bijectively which makes the RF pulse more demanding. • Aim is to create such a magnetic field to assign same magnetic field to M slice locations with N gradient channel as in Figure 1a. 1Point Per Slice (1PPS ) Method (Figure 1b) At the center point of each slice location • Magnetic field should be equal • Absolute value of the derivative with respect to z direction should be equal for same slice thickness • Second derivative of the magnetic field wrt z direction should be zero for intra-slice homogeneity since laplacian of a static magnetic fields are zero. If z derivative is zero due to radial symmetry other derivatives also vanishes • 3M-1 channel is required 2 Point Per Slice (2PPS ) Method (Figure 1c) At two off-center points in each slice location • Magnetic field should be equal • Absolute value of the derivative with respect to z direction Figure 1: Illustrative Design Method should be equal for same slice thickness for M slice selection with N channel • Previous constraint automatically satisfies second derivative array (a) Slice locations (b) 1PPS constraint in 1PPS and provides better intra-slice homogeneity method and equations (c) 2PPS method • 4M-1 channel is required and equations REFERENCES
• Figure 4 shows that proposed technique is possible for the off-centered slice locations by applying different current combinations to each channel. • Slice thickness performances (constant slice thickness in the whole slice plane) is similar for the off-centered slice locations. • Maximum available gradient performances are almost same for the 1PPS method and slightly changes for the 2PPS method; however, 2PPS method is advantageous • In the experiments field map of each channel is measured with the scanner and currents are optimized according to required slice locations • Experimental result in Figure 5 shows that even with the 1PPS method 3 slice selection with a single band RF pulse is possible
Figure 2: Experimental Setup (a) 9 channel gradient array. Inside the gradient coil, there is Tx/Rx birdcage coil and its RF shield. (b) Custom made gradient amplifiers, power distribution units and FPGA [8]
• In order to overcome limitations, there are techniques to design multi-band RF pulses such as phase optimization [2], time shifting [3], PINS [4], MultiPINS [5], parallel transmission [6], root flipping [7]. • Purpose of this study is excite multiple slice locations with a single band RF pulse. This is possible if different slice locations are mapped to same frequency so that single band RF pulse can affect multiple locations. In order to dynamically create such a magnetic field distribution for each different slice location set, nine channel z-gradient array is implemented and proof of concept has been demonstrated with both simulations and experiments.
Presentation number: 81 Session Title: Novel hardware concepts Session Time: 29.09.2016, 16:00 - 17:30
Gradient Coil • Diameter = 25cm • Each turn has 36 turns • Wire thickness is 0.8mm
Amplifiers and Control Experiments and Simulations • Simulations in MATLAB • User interface (C#) • H-bridge (50V, 30A) • Experiments in 3T (Tim Trio, Siemens) • PWM signals with FPGA
Results • Figure 3 shows the simulation results for 1PPS and 2PPS methods • 9 channel is enough for exciting 3 slices with 1PPS and 2 slices with 2PPS • Slice seperation is 9cm for 1PPS and 13.6cm for 2PPS to cover all volume of 27.5cm with slice thickness of 5mm
Figure 4: Simulated selected slices for 1PPS (3slice) and 2PPS (2 Slice) Same parameters with Figure 3 is used only slice locations are changed at each step to cover the whole volume. Each plot shows the attainable gradient strength with the current hardware.
Figure 5: Experimental coronal phantom image shows that 3 slice selection in z direction with a single band RF pulse is possible. Slice seperation is 9 cm and slice thickness is 6.6mm
Discussion and Conclusion
• Under maximum voltage and • Proposed technique can be also used for refocusing and inversion pulses current limitations of the amplifiers • Proposed technique can be combined with the existing methods [1-7] to multiply the obtained gradients are 19mT/m and number of slices without no increase in RF demand especially for the 2PPS method since 26mT/m for 1PPS and 2PPS it creates almost linear gradients in 2 different region. methods • Available gradient strength strongly depends on the slice distance, length of each channel and the coil diameter which we are currently working on. • 2PPS manages to excite slices with • Although 1PPS method produces slightly curved slices, depending on the diameter of rectangular profile while 1PPS VOI, curvature of the slice profiles of 1PPS method can be considered as negligible and excites curved slices (Figure 3e,f) we are currently working on obtaining transverse images to observe the effect. Figure 3: Simulations for 1PPS and 2PPS methods. (a, b) Slice profiles (Red boxes are the imaging radius • Conlusion: Multi-slice excitation with a single band RF pulse without increasing the of the RF coil) (c, d) Oscillatory magnetic field at the center line (e, f) percentage error of slice thickness duration, SAR and peak power of the RF pulse is possible with the proposed gradient as a function of radius. array system
[1] Barth, Markus, et al. "Simultaneous multislice (SMS) imaging techniques."Magnetic resonance in medicine 75.1 (2016): 63-81. [2] Sbrizzi A, Poser BA, Tse DH, Hoogduin H, Luijten PR, Berg CA. RF peak power reduction in CAIPIRINHA excitation by interslice phase optimization. NMR in Biomedicine 2015;28:1393-1401. [3] Auerbach EJ, Xu J, Yacoub E, Moeller S, Uğurbil K. Multiband accelerated spin‐echo echo planar imaging with reduced peak RF power using time‐shifted RF pulses. Magnetic resonance in medicine 2013;69:1261-1267. [4] Norris DG, Koopmans PJ, Boyacioğlu R, Barth M. Power independent of number of slices (PINS) radiofrequency pulses for low‐power simultaneous multislice excitation. Magnetic resonance in medicine 2011;66:1234-1240. [5] Eichner C, Wald LL, Setsompop K. A low power radiofrequency pulse for simultaneous multislice excitation and refocusing. Magnetic resonance in medicine 2014;72:949-958. [6] Guérin B, Setsompop K, Ye H, Poser BA, Stenger AV, Wald LL. Design of parallel transmission pulses for simultaneous multislice with explicit control for peak power and local specific absorption rate. Magnetic Resonance in Medicine 2015;73:1946-1953. [7] Sharma, Anuj, Michael Lustig, and William A. Grissom. "Root‐flipped multiband refocusing pulses." Magnetic resonance in medicine 75.1 (2016): 227-237. [8] Feasibility of Z gradient Array for Variable Volume of Interest”ESMRMB 2015, Soheil Taraghinia, Koray Ertan and Ergin Atalar
