2016
Alon, Leeor; Deniz, Cem M; Carluccio, Giuseppe; Brown, Ryan; Sodickson, Daniel K; Collins, Christopher M
Effects of anatomical differences on electromagnetic fields, SAR, and temperature change Journal Article
In: Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering, vol. 46, no. 1, pp. 8–18, 2016, ISSN: 15525031.
@article{Alon2016,
title = {Effects of anatomical differences on electromagnetic fields, SAR, and temperature change},
author = {Leeor Alon and Cem M Deniz and Giuseppe Carluccio and Ryan Brown and Daniel K Sodickson and Christopher M Collins},
url = {http://doi.wiley.com/10.1002/cmr.b.21317},
doi = {10.1002/cmr.b.21317},
issn = {15525031},
year = {2016},
date = {2016-02-01},
journal = {Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering},
volume = {46},
number = {1},
pages = {8--18},
abstract = {Electromagnetic field simulations are increasingly used to assure RF safety of patients during MRI exams. In practice, however, tissue property distribution of the patient being imaged is not known, but may be represented with a pre-existing model. Repeatedly, agreement in transmit magnetic (B1 ) field distributions between two geome- tries has been used to suggest agreement in heating distributions. Here we examine rel- ative effects of anatomical differences on B1 distribution, specific absorption rate (SAR), and temperature change (DT). Numerical simulations were performed for a single surface coil positioned adjacent a homogeneous phantom and bovine phantom, each with slight geometric variations, and adjacent two different human body models. Experimental dem- onstration was performed on a bovine phantom using MR thermometry and B1 mapping. Simulations and experiments demonstrate that B1 distributions in different samples can be well correlated, while notable difference in maximum SAR and DT occur. This work illustrates challenges associated with utilizing simulations or experiments for RF safety assurance purposes. Reliance on B1 distributions alone for validation of simulations and/or experiments with a sample or subject for assurance of safety in another should be performed with caution.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Electromagnetic field simulations are increasingly used to assure RF safety of patients during MRI exams. In practice, however, tissue property distribution of the patient being imaged is not known, but may be represented with a pre-existing model. Repeatedly, agreement in transmit magnetic (B1 ) field distributions between two geome- tries has been used to suggest agreement in heating distributions. Here we examine rel- ative effects of anatomical differences on B1 distribution, specific absorption rate (SAR), and temperature change (DT). Numerical simulations were performed for a single surface coil positioned adjacent a homogeneous phantom and bovine phantom, each with slight geometric variations, and adjacent two different human body models. Experimental dem- onstration was performed on a bovine phantom using MR thermometry and B1 mapping. Simulations and experiments demonstrate that B1 distributions in different samples can be well correlated, while notable difference in maximum SAR and DT occur. This work illustrates challenges associated with utilizing simulations or experiments for RF safety assurance purposes. Reliance on B1 distributions alone for validation of simulations and/or experiments with a sample or subject for assurance of safety in another should be performed with caution.
2015
Alon, Leeor; Cho, Gene Y; Yang, Xing; Sodickson, Daniel K; Deniz, Cem M
A method for safety testing of radiofrequency/microwave-emitting devices using MRI Journal Article
In: Magnetic Resonance in Medicine, vol. 74, no. 5, pp. 1397–1405, 2015, ISSN: 07403194.
@article{Alon2014a,
title = {A method for safety testing of radiofrequency/microwave-emitting devices using MRI},
author = {Leeor Alon and Gene Y Cho and Xing Yang and Daniel K Sodickson and Cem M Deniz},
url = {http://doi.wiley.com/10.1002/mrm.25521},
doi = {10.1002/mrm.25521},
issn = {07403194},
year = {2015},
date = {2015-11-01},
journal = {Magnetic Resonance in Medicine},
volume = {74},
number = {5},
pages = {1397--1405},
abstract = {Purpose
Strict regulations are imposed on the amount of radiofrequency (RF) energy that devices can emit to prevent excessive deposition of RF energy into the body. In this study, we investigated the application of MR temperature mapping and 10‐g average specific absorption rate (SAR) computation for safety evaluation of RF‐emitting devices.
Methods
Quantification of the RF power deposition was shown for an MRI‐compatible dipole antenna and a non–MRI‐compatible mobile phone via phantom temperature change measurements. Validation of the MR temperature mapping method was demonstrated by comparison with physical temperature measurements and electromagnetic field simulations. MR temperature measurements alongside physical property measurements were used to reconstruct 10‐g average SAR.
Results
The maximum temperature change for a dipole antenna and the maximum 10‐g average SAR were 1.83°C and 12.4 W/kg, respectively, for simulations and 1.73°C and 11.9 W/kg, respectively, for experiments. The difference between MR and probe thermometry was <0.15°C. The maximum temperature change and the maximum 10‐g average SAR for a cell phone radiating at maximum output for 15 min was 1.7°C and 0.54 W/kg, respectively.
Conclusion
Information acquired using MR temperature mapping and thermal property measurements can assess RF/microwave safety with high resolution and fidelity.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Purpose
Strict regulations are imposed on the amount of radiofrequency (RF) energy that devices can emit to prevent excessive deposition of RF energy into the body. In this study, we investigated the application of MR temperature mapping and 10‐g average specific absorption rate (SAR) computation for safety evaluation of RF‐emitting devices.
Methods
Quantification of the RF power deposition was shown for an MRI‐compatible dipole antenna and a non–MRI‐compatible mobile phone via phantom temperature change measurements. Validation of the MR temperature mapping method was demonstrated by comparison with physical temperature measurements and electromagnetic field simulations. MR temperature measurements alongside physical property measurements were used to reconstruct 10‐g average SAR.
Results
The maximum temperature change for a dipole antenna and the maximum 10‐g average SAR were 1.83°C and 12.4 W/kg, respectively, for simulations and 1.73°C and 11.9 W/kg, respectively, for experiments. The difference between MR and probe thermometry was <0.15°C. The maximum temperature change and the maximum 10‐g average SAR for a cell phone radiating at maximum output for 15 min was 1.7°C and 0.54 W/kg, respectively.
Conclusion
Information acquired using MR temperature mapping and thermal property measurements can assess RF/microwave safety with high resolution and fidelity.
Strict regulations are imposed on the amount of radiofrequency (RF) energy that devices can emit to prevent excessive deposition of RF energy into the body. In this study, we investigated the application of MR temperature mapping and 10‐g average specific absorption rate (SAR) computation for safety evaluation of RF‐emitting devices.
Methods
Quantification of the RF power deposition was shown for an MRI‐compatible dipole antenna and a non–MRI‐compatible mobile phone via phantom temperature change measurements. Validation of the MR temperature mapping method was demonstrated by comparison with physical temperature measurements and electromagnetic field simulations. MR temperature measurements alongside physical property measurements were used to reconstruct 10‐g average SAR.
Results
The maximum temperature change for a dipole antenna and the maximum 10‐g average SAR were 1.83°C and 12.4 W/kg, respectively, for simulations and 1.73°C and 11.9 W/kg, respectively, for experiments. The difference between MR and probe thermometry was <0.15°C. The maximum temperature change and the maximum 10‐g average SAR for a cell phone radiating at maximum output for 15 min was 1.7°C and 0.54 W/kg, respectively.
Conclusion
Information acquired using MR temperature mapping and thermal property measurements can assess RF/microwave safety with high resolution and fidelity.
Deniz, Cem M; Vaidya, Manushka V; Sodickson, Daniel K; Lattanzi, Riccardo
In: Magnetic Resonance in Medicine, vol. 75, pp. 423-432, 2015.
@article{Deniz2015d,
title = {Radiofrequency energy deposition and radiofrequency power requirements in parallel transmission with increasing distance from the coil to the sample},
author = {Cem M Deniz and Manushka V Vaidya and Daniel K Sodickson and Riccardo Lattanzi},
doi = {10.1002/mrm.25646},
year = {2015},
date = {2015-03-05},
journal = {Magnetic Resonance in Medicine},
volume = {75},
pages = {423-432},
abstract = {Purpose
We investigated global specific absorption rate (SAR) and radiofrequency (RF) power requirements in parallel transmission as the distance between the transmit coils and the sample was increased.
Methods
We calculated ultimate intrinsic SAR (UISAR), which depends on object geometry and electrical properties but not on coil design, and we used it as the reference to compare the performance of various transmit arrays. We investigated the case of fixing coil size and increasing the number of coils while moving the array away from the sample, as well as the case of fixing coil number and scaling coil dimensions. We also investigated RF power requirements as a function of lift‐off, and tracked local SAR distributions associated with global SAR optima.
Results
In all cases, the target excitation profile was achieved and global SAR (as well as associated maximum local SAR) decreased with lift‐off, approaching UISAR, which was constant for all lift‐offs. We observed a lift‐off value that optimizes the balance between global SAR and power losses in coil conductors. We showed that, using parallel transmission, global SAR can decrease at ultra high fields for finite arrays with a sufficient number of transmit elements.
Conclusion
For parallel transmission, the distance between coils and object can be optimized to reduce SAR and minimize RF power requirements associated with homogeneous excitation},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Purpose
We investigated global specific absorption rate (SAR) and radiofrequency (RF) power requirements in parallel transmission as the distance between the transmit coils and the sample was increased.
Methods
We calculated ultimate intrinsic SAR (UISAR), which depends on object geometry and electrical properties but not on coil design, and we used it as the reference to compare the performance of various transmit arrays. We investigated the case of fixing coil size and increasing the number of coils while moving the array away from the sample, as well as the case of fixing coil number and scaling coil dimensions. We also investigated RF power requirements as a function of lift‐off, and tracked local SAR distributions associated with global SAR optima.
Results
In all cases, the target excitation profile was achieved and global SAR (as well as associated maximum local SAR) decreased with lift‐off, approaching UISAR, which was constant for all lift‐offs. We observed a lift‐off value that optimizes the balance between global SAR and power losses in coil conductors. We showed that, using parallel transmission, global SAR can decrease at ultra high fields for finite arrays with a sufficient number of transmit elements.
Conclusion
For parallel transmission, the distance between coils and object can be optimized to reduce SAR and minimize RF power requirements associated with homogeneous excitation
We investigated global specific absorption rate (SAR) and radiofrequency (RF) power requirements in parallel transmission as the distance between the transmit coils and the sample was increased.
Methods
We calculated ultimate intrinsic SAR (UISAR), which depends on object geometry and electrical properties but not on coil design, and we used it as the reference to compare the performance of various transmit arrays. We investigated the case of fixing coil size and increasing the number of coils while moving the array away from the sample, as well as the case of fixing coil number and scaling coil dimensions. We also investigated RF power requirements as a function of lift‐off, and tracked local SAR distributions associated with global SAR optima.
Results
In all cases, the target excitation profile was achieved and global SAR (as well as associated maximum local SAR) decreased with lift‐off, approaching UISAR, which was constant for all lift‐offs. We observed a lift‐off value that optimizes the balance between global SAR and power losses in coil conductors. We showed that, using parallel transmission, global SAR can decrease at ultra high fields for finite arrays with a sufficient number of transmit elements.
Conclusion
For parallel transmission, the distance between coils and object can be optimized to reduce SAR and minimize RF power requirements associated with homogeneous excitation
2014
Chang, Gregory; Honig, Stephen; Brown, Ryan; Deniz, Cem M; Egol, Kenneth A; Babb, James S; Regatte, Ravinder R; Rajapakse, Chamith S
In: Radiology, vol. 272, no. 2, pp. 464–474, 2014, ISSN: 0033-8419.
@article{Chang2014ab,
title = {Finite Element Analysis Applied to 3-T MR Imaging of Proximal Femur Microarchitecture: Lower Bone Strength in Patients with Fragility Fractures Compared with Control Subjects},
author = {Gregory Chang and Stephen Honig and Ryan Brown and Cem M Deniz and Kenneth A Egol and James S Babb and Ravinder R Regatte and Chamith S Rajapakse},
url = {http://pubs.rsna.org/doi/10.1148/radiol.14131926},
doi = {10.1148/radiol.14131926},
issn = {0033-8419},
year = {2014},
date = {2014-08-01},
journal = {Radiology},
volume = {272},
number = {2},
pages = {464--474},
abstract = {Purpose
To determine the feasibility of using finite element analysis applied to 3-T magnetic resonance (MR) images of proximal femur microarchitecture for detection of lower bone strength in subjects with fragility fractures compared with control subjects without fractures.
Materials and Methods
This prospective study was institutional review board approved and HIPAA compliant. Written informed consent was obtained. Postmenopausal women with (n = 22) and without (n = 22) fragility fractures were matched for age and body mass index. All subjects underwent standard dual-energy x-ray absorptiometry. Images of proximal femur microarchitecture were obtained by using a high-spatial-resolution three-dimensional fast low-angle shot sequence at 3 T. Finite element analysis was applied to compute elastic modulus as a measure of strength in the femoral head and neck, Ward triangle, greater trochanter, and intertrochanteric region. The Mann-Whitney test was used to compare bone mineral density T scores and elastic moduli between the groups. The relationship (R2) between elastic moduli and bone mineral density T scores was assessed.
Results
Patients with fractures showed lower elastic modulus than did control subjects in all proximal femur regions (femoral head, 8.51–8.73 GPa vs 9.32–9.67 GPa; P = .04; femoral neck, 3.11–3.72 GPa vs 4.39–4.82 GPa; P = .04; Ward triangle, 1.85–2.21 GPa vs 3.98–4.13 GPa; P = .04; intertrochanteric region, 1.62–2.18 GPa vs 3.86–4.47 GPa; P = .006–.007; greater trochanter, 0.65–1.21 GPa vs 1.96–2.62 GPa; P = .01–.02), but no differences in bone mineral density T scores. There were weak relationships between elastic moduli and bone mineral density T scores in patients with fractures (R2 = 0.25–0.31, P = .02–.04), but not in control subjects.
Conclusion
Finite element analysis applied to high-spatial-resolution 3-T MR images of proximal femur microarchitecture can allow detection of lower elastic modulus, a marker of bone strength, in subjects with fragility fractures compared with control subjects. MR assessment of proximal femur strength may provide information about bone quality that is not provided by dual-energy x-ray absorptiometry.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Purpose
To determine the feasibility of using finite element analysis applied to 3-T magnetic resonance (MR) images of proximal femur microarchitecture for detection of lower bone strength in subjects with fragility fractures compared with control subjects without fractures.
Materials and Methods
This prospective study was institutional review board approved and HIPAA compliant. Written informed consent was obtained. Postmenopausal women with (n = 22) and without (n = 22) fragility fractures were matched for age and body mass index. All subjects underwent standard dual-energy x-ray absorptiometry. Images of proximal femur microarchitecture were obtained by using a high-spatial-resolution three-dimensional fast low-angle shot sequence at 3 T. Finite element analysis was applied to compute elastic modulus as a measure of strength in the femoral head and neck, Ward triangle, greater trochanter, and intertrochanteric region. The Mann-Whitney test was used to compare bone mineral density T scores and elastic moduli between the groups. The relationship (R2) between elastic moduli and bone mineral density T scores was assessed.
Results
Patients with fractures showed lower elastic modulus than did control subjects in all proximal femur regions (femoral head, 8.51–8.73 GPa vs 9.32–9.67 GPa; P = .04; femoral neck, 3.11–3.72 GPa vs 4.39–4.82 GPa; P = .04; Ward triangle, 1.85–2.21 GPa vs 3.98–4.13 GPa; P = .04; intertrochanteric region, 1.62–2.18 GPa vs 3.86–4.47 GPa; P = .006–.007; greater trochanter, 0.65–1.21 GPa vs 1.96–2.62 GPa; P = .01–.02), but no differences in bone mineral density T scores. There were weak relationships between elastic moduli and bone mineral density T scores in patients with fractures (R2 = 0.25–0.31, P = .02–.04), but not in control subjects.
Conclusion
Finite element analysis applied to high-spatial-resolution 3-T MR images of proximal femur microarchitecture can allow detection of lower elastic modulus, a marker of bone strength, in subjects with fragility fractures compared with control subjects. MR assessment of proximal femur strength may provide information about bone quality that is not provided by dual-energy x-ray absorptiometry.
To determine the feasibility of using finite element analysis applied to 3-T magnetic resonance (MR) images of proximal femur microarchitecture for detection of lower bone strength in subjects with fragility fractures compared with control subjects without fractures.
Materials and Methods
This prospective study was institutional review board approved and HIPAA compliant. Written informed consent was obtained. Postmenopausal women with (n = 22) and without (n = 22) fragility fractures were matched for age and body mass index. All subjects underwent standard dual-energy x-ray absorptiometry. Images of proximal femur microarchitecture were obtained by using a high-spatial-resolution three-dimensional fast low-angle shot sequence at 3 T. Finite element analysis was applied to compute elastic modulus as a measure of strength in the femoral head and neck, Ward triangle, greater trochanter, and intertrochanteric region. The Mann-Whitney test was used to compare bone mineral density T scores and elastic moduli between the groups. The relationship (R2) between elastic moduli and bone mineral density T scores was assessed.
Results
Patients with fractures showed lower elastic modulus than did control subjects in all proximal femur regions (femoral head, 8.51–8.73 GPa vs 9.32–9.67 GPa; P = .04; femoral neck, 3.11–3.72 GPa vs 4.39–4.82 GPa; P = .04; Ward triangle, 1.85–2.21 GPa vs 3.98–4.13 GPa; P = .04; intertrochanteric region, 1.62–2.18 GPa vs 3.86–4.47 GPa; P = .006–.007; greater trochanter, 0.65–1.21 GPa vs 1.96–2.62 GPa; P = .01–.02), but no differences in bone mineral density T scores. There were weak relationships between elastic moduli and bone mineral density T scores in patients with fractures (R2 = 0.25–0.31, P = .02–.04), but not in control subjects.
Conclusion
Finite element analysis applied to high-spatial-resolution 3-T MR images of proximal femur microarchitecture can allow detection of lower elastic modulus, a marker of bone strength, in subjects with fragility fractures compared with control subjects. MR assessment of proximal femur strength may provide information about bone quality that is not provided by dual-energy x-ray absorptiometry.
Chang, Gregory; Deniz, Cem M; Honig, Stephen; Rajapakse, Chamith S; Egol, Kenneth A; Regatte, Ravinder R; Brown, Ryan
Feasibility of three-dimensional MRI of proximal femur microarchitecture at 3 tesla using 26 receive elements without and with parallel imaging Journal Article
In: Journal of Magnetic Resonance Imaging, vol. 40, no. 1, pp. 229–238, 2014, ISSN: 10531807.
@article{Chang2014a,
title = {Feasibility of three-dimensional MRI of proximal femur microarchitecture at 3 tesla using 26 receive elements without and with parallel imaging},
author = {Gregory Chang and Cem M Deniz and Stephen Honig and Chamith S Rajapakse and Kenneth A Egol and Ravinder R Regatte and Ryan Brown},
url = {http://doi.wiley.com/10.1002/jmri.24345},
doi = {10.1002/jmri.24345},
issn = {10531807},
year = {2014},
date = {2014-07-01},
journal = {Journal of Magnetic Resonance Imaging},
volume = {40},
number = {1},
pages = {229--238},
abstract = {PURPOSE: High-resolution imaging of deeper anatomy such as the hip is challenging due to low signal-to-noise ratio (SNR), necessitating long scan times. Multi-element coils can increase SNR and reduce scan time through parallel imaging (PI). We assessed the feasibility of using a 26-element receive coil setup to perform 3 Tesla (T) MRI of proximal femur microarchitecture without and with PI. MATERIALS AND METHODS: This study had institutional review board approval. We scanned 13 subjects on a 3T scanner using 26 receive-elements and a three-dimensional fast low-angle shot (FLASH) sequence without and with PI (acceleration factors [AF] 2, 3, 4). We assessed SNR, depiction of individual trabeculae, PI performance (1/g-factor), and image quality with PI (1 = nonvisualization to 5 = excellent). RESULTS: SNR maps demonstrate higher SNR for the 26-element setup compared with a 12-element setup for hip MRI. Without PI, individual proximal femur trabeculae were well-depicted, including microarchitectural deterioration in osteoporotic subjects. With PI, 1/g values for the 26-element/12-element receive-setup were 0.71/0.45, 0.56/0.25, and 0.44/0.08 at AF2, AF3, and AF4, respectively. Image quality was: AF1, excellent (4.8 ± 0.4); AF2, good (4.2 ± 1.0); AF3, average (3.3 ± 1.0); AF4, nonvisualization (1.4 ± 0.9). CONCLUSION: A 26-element receive-setup permits 3T MRI of proximal femur microarchitecture with good image quality up to PI AF2.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
PURPOSE: High-resolution imaging of deeper anatomy such as the hip is challenging due to low signal-to-noise ratio (SNR), necessitating long scan times. Multi-element coils can increase SNR and reduce scan time through parallel imaging (PI). We assessed the feasibility of using a 26-element receive coil setup to perform 3 Tesla (T) MRI of proximal femur microarchitecture without and with PI. MATERIALS AND METHODS: This study had institutional review board approval. We scanned 13 subjects on a 3T scanner using 26 receive-elements and a three-dimensional fast low-angle shot (FLASH) sequence without and with PI (acceleration factors [AF] 2, 3, 4). We assessed SNR, depiction of individual trabeculae, PI performance (1/g-factor), and image quality with PI (1 = nonvisualization to 5 = excellent). RESULTS: SNR maps demonstrate higher SNR for the 26-element setup compared with a 12-element setup for hip MRI. Without PI, individual proximal femur trabeculae were well-depicted, including microarchitectural deterioration in osteoporotic subjects. With PI, 1/g values for the 26-element/12-element receive-setup were 0.71/0.45, 0.56/0.25, and 0.44/0.08 at AF2, AF3, and AF4, respectively. Image quality was: AF1, excellent (4.8 ± 0.4); AF2, good (4.2 ± 1.0); AF3, average (3.3 ± 1.0); AF4, nonvisualization (1.4 ± 0.9). CONCLUSION: A 26-element receive-setup permits 3T MRI of proximal femur microarchitecture with good image quality up to PI AF2.
Chang, Gregory; Deniz, Cem M; Honig, Stephen; Egol, Kenneth A; Regatte, Ravinder R; Zhu, Yudong; Sodickson, Daniel K; Brown, Ryan
MRI of the hip at 7T: Feasibility of bone microarchitecture, high-resolution cartilage, and clinical imaging Journal Article
In: Journal of Magnetic Resonance Imaging, vol. 39, no. 6, pp. 1384–1393, 2014, ISSN: 10531807.
@article{Chang2014c,
title = {MRI of the hip at 7T: Feasibility of bone microarchitecture, high-resolution cartilage, and clinical imaging},
author = {Gregory Chang and Cem M Deniz and Stephen Honig and Kenneth A Egol and Ravinder R Regatte and Yudong Zhu and Daniel K Sodickson and Ryan Brown},
url = {http://doi.wiley.com/10.1002/jmri.24305},
doi = {10.1002/jmri.24305},
issn = {10531807},
year = {2014},
date = {2014-06-01},
journal = {Journal of Magnetic Resonance Imaging},
volume = {39},
number = {6},
pages = {1384--1393},
abstract = {Purpose To demonstrate the feasibility of performing bone microarchitecture, high-resolution cartilage, and clinical imaging of the hip at 7T. Materials and Methods This study had Institutional Review Board approval. Using an 8-channel coil constructed in-house, we imaged the hips of 15 subjects on a 7T magnetic resonance imaging (MRI) scanner. We applied: 1) a T1-weighted 3D fast low angle shot (3D FLASH) sequence (0.23 ? 0.23 ? 1-1.5 mm3) for bone microarchitecture imaging; 2) T1-weighted 3D FLASH (water excitation) and volumetric interpolated breath-hold examination (VIBE) sequences (0.23 ? 0.23 ? 1.5 mm3) with saturation or inversion recovery-based fat suppression for cartilage imaging; 3) 2D intermediate-weighted fast spin-echo (FSE) sequences without and with fat saturation (0.27 ? 0.27 ? 2 mm) for clinical imaging. Results Bone microarchitecture images allowed visualization of individual trabeculae within the proximal femur. Cartilage was well visualized and fat was well suppressed on FLASH and VIBE sequences. FSE sequences allowed visualization of cartilage, the labrum (including cartilage and labral pathology), joint capsule, and tendons. Conclusion This is the first study to demonstrate the feasibility of performing a clinically comprehensive hip MRI protocol at 7T, including high-resolution imaging of bone microarchitecture and cartilage, as well as clinical imaging. Copyright ? 2013 Wiley Periodicals, Inc.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Purpose To demonstrate the feasibility of performing bone microarchitecture, high-resolution cartilage, and clinical imaging of the hip at 7T. Materials and Methods This study had Institutional Review Board approval. Using an 8-channel coil constructed in-house, we imaged the hips of 15 subjects on a 7T magnetic resonance imaging (MRI) scanner. We applied: 1) a T1-weighted 3D fast low angle shot (3D FLASH) sequence (0.23 ? 0.23 ? 1-1.5 mm3) for bone microarchitecture imaging; 2) T1-weighted 3D FLASH (water excitation) and volumetric interpolated breath-hold examination (VIBE) sequences (0.23 ? 0.23 ? 1.5 mm3) with saturation or inversion recovery-based fat suppression for cartilage imaging; 3) 2D intermediate-weighted fast spin-echo (FSE) sequences without and with fat saturation (0.27 ? 0.27 ? 2 mm) for clinical imaging. Results Bone microarchitecture images allowed visualization of individual trabeculae within the proximal femur. Cartilage was well visualized and fat was well suppressed on FLASH and VIBE sequences. FSE sequences allowed visualization of cartilage, the labrum (including cartilage and labral pathology), joint capsule, and tendons. Conclusion This is the first study to demonstrate the feasibility of performing a clinically comprehensive hip MRI protocol at 7T, including high-resolution imaging of bone microarchitecture and cartilage, as well as clinical imaging. Copyright ? 2013 Wiley Periodicals, Inc.
Brown, Ryan; Deniz, Cem M; Zhang, Bei; Chang, Gregory; Sodickson, Daniel K; Wiggins, Graham C
In: Investigative Radiology, vol. 49, no. 1, pp. 35–47, 2014, ISSN: 0020-9996.
@article{Brown2014,
title = {Design and Application of Combined 8-Channel Transmit and 10-Channel Receive Arrays and Radiofrequency Shimming for 7-T Shoulder Magnetic Resonance Imaging},
author = {Ryan Brown and Cem M Deniz and Bei Zhang and Gregory Chang and Daniel K Sodickson and Graham C Wiggins},
url = {https://insights.ovid.com/crossref?an=00004424-201401000-00006},
doi = {10.1097/RLI.0b013e3182a5662d},
issn = {0020-9996},
year = {2014},
date = {2014-01-01},
journal = {Investigative Radiology},
volume = {49},
number = {1},
pages = {35--47},
abstract = {OBJECTIVE: The objective of the study was to investigate the feasibility of 7-T shoulder magnetic resonance imaging by developing transmit and receive radiofrequency (RF) coil arrays and exploring RF shim methods.$backslash$n$backslash$nMATERIALS AND METHODS: A mechanically flexible 8-channel transmit array and an anatomically conformable 10-channel receive array were designed and implemented. The transmit performance of various RF shim methods was assessed through local flip angle measurements in the right and left shoulders of 6 subjects. The receive performance was assessed through signal-to-noise ratio measurements using the developed 7-T coil and a baseline commercial 3-T coil.$backslash$n$backslash$nRESULTS: The 7-T transmit array driven with phase-coherent RF shim weights provided adequate B₁⁺ efficiency and uniformity for turbo spin echo shoulder imaging. B₁⁺ twisting that is characteristic of high-field loop coils necessitates distinct RF shim weights in the right and left shoulders. The 7-T receive array provided a 2-fold signal-to-noise ratio improvement over the 3-T array in the deep articular shoulder cartilage.$backslash$n$backslash$nCONCLUSIONS: Shoulder imaging at 7-T is feasible with a custom transmit/receive array either in a single-channel transmit mode with a fixed RF shim or in a parallel transmit mode with a subject-specific RF shim.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
OBJECTIVE: The objective of the study was to investigate the feasibility of 7-T shoulder magnetic resonance imaging by developing transmit and receive radiofrequency (RF) coil arrays and exploring RF shim methods.$backslash$n$backslash$nMATERIALS AND METHODS: A mechanically flexible 8-channel transmit array and an anatomically conformable 10-channel receive array were designed and implemented. The transmit performance of various RF shim methods was assessed through local flip angle measurements in the right and left shoulders of 6 subjects. The receive performance was assessed through signal-to-noise ratio measurements using the developed 7-T coil and a baseline commercial 3-T coil.$backslash$n$backslash$nRESULTS: The 7-T transmit array driven with phase-coherent RF shim weights provided adequate B₁⁺ efficiency and uniformity for turbo spin echo shoulder imaging. B₁⁺ twisting that is characteristic of high-field loop coils necessitates distinct RF shim weights in the right and left shoulders. The 7-T receive array provided a 2-fold signal-to-noise ratio improvement over the 3-T array in the deep articular shoulder cartilage.$backslash$n$backslash$nCONCLUSIONS: Shoulder imaging at 7-T is feasible with a custom transmit/receive array either in a single-channel transmit mode with a fixed RF shim or in a parallel transmit mode with a subject-specific RF shim.
2013
Alon, Leeor; Deniz, Cem M; Brown, Ryan; Sodickson, Daniel K; Zhu, Yudong
Method for in situ characterization of radiofrequency heating in parallel transmit MRI Journal Article
In: Magnetic Resonance in Medicine, vol. 69, no. 5, pp. 1457–1465, 2013, ISSN: 07403194.
@article{Alon2013a,
title = {Method for in situ characterization of radiofrequency heating in parallel transmit MRI},
author = {Leeor Alon and Cem M Deniz and Ryan Brown and Daniel K Sodickson and Yudong Zhu},
url = {http://doi.wiley.com/10.1002/mrm.24374},
doi = {10.1002/mrm.24374},
issn = {07403194},
year = {2013},
date = {2013-05-01},
journal = {Magnetic Resonance in Medicine},
volume = {69},
number = {5},
pages = {1457--1465},
abstract = {In ultra-high-field magnetic resonance imaging, parallel radiofrequency (RF) transmission presents both opportunities and challenges for specific absorption rate management. On one hand, parallel transmission provides flexibility in tailoring electric fields in the body while facilitating magnetization profile control. On the other hand, it increases the complexity of energy deposition as well as possibly exacerbating local specific absorption rate by improper design or delivery of RF pulses. This study shows that the information needed to characterize RF heating in parallel transmission is contained within a local power correlation matrix. Building upon a calibration scheme involving a finite number of magnetic resonance thermometry measurements, this work establishes a way of estimating the local power correlation matrix. Determination of this matrix allows prediction of temperature change for an arbitrary parallel transmit RF pulse. In the case of a three transmit coil MR experiment in a phantom, determination and validation of the power correlation matrix were conducted in less than 200 min with induced temperature changes of textless4 degrees C. Further optimization and adaptation are possible, and simulations evaluating potential feasibility for in vivo use are presented. The method allows general characteristics indicative of RF coil/pulse safety determined in situ.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In ultra-high-field magnetic resonance imaging, parallel radiofrequency (RF) transmission presents both opportunities and challenges for specific absorption rate management. On one hand, parallel transmission provides flexibility in tailoring electric fields in the body while facilitating magnetization profile control. On the other hand, it increases the complexity of energy deposition as well as possibly exacerbating local specific absorption rate by improper design or delivery of RF pulses. This study shows that the information needed to characterize RF heating in parallel transmission is contained within a local power correlation matrix. Building upon a calibration scheme involving a finite number of magnetic resonance thermometry measurements, this work establishes a way of estimating the local power correlation matrix. Determination of this matrix allows prediction of temperature change for an arbitrary parallel transmit RF pulse. In the case of a three transmit coil MR experiment in a phantom, determination and validation of the power correlation matrix were conducted in less than 200 min with induced temperature changes of textless4 degrees C. Further optimization and adaptation are possible, and simulations evaluating potential feasibility for in vivo use are presented. The method allows general characteristics indicative of RF coil/pulse safety determined in situ.
Deniz, Cem M; Brown, Ryan; Lattanzi, Riccardo; Alon, Leeor; Sodickson, Daniel K; Zhu, Yudong
Maximum efficiency radiofrequency shimming: Theory and initial application for hip imaging at 7 tesla Journal Article
In: Magnetic Resonance in Medicine, vol. 69, no. 5, pp. 1379–1388, 2013, ISSN: 07403194.
@article{Deniz2013,
title = {Maximum efficiency radiofrequency shimming: Theory and initial application for hip imaging at 7 tesla},
author = {Cem M Deniz and Ryan Brown and Riccardo Lattanzi and Leeor Alon and Daniel K Sodickson and Yudong Zhu},
url = {http://doi.wiley.com/10.1002/mrm.24377},
doi = {10.1002/mrm.24377},
issn = {07403194},
year = {2013},
date = {2013-05-01},
journal = {Magnetic Resonance in Medicine},
volume = {69},
number = {5},
pages = {1379--1388},
abstract = {Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high-frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power-demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal-to-noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high-frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power-demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal-to-noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging.
2012
Zhu, Yudong; Alon, Leeor; Deniz, Cem M; Brown, Ryan; Sodickson, Daniel K
System and SAR characterization in parallel RF transmission Journal Article
In: Magnetic Resonance in Medicine, vol. 67, no. 5, pp. 1367–1378, 2012, ISSN: 07403194.
@article{Zhu2012,
title = {System and SAR characterization in parallel RF transmission},
author = {Yudong Zhu and Leeor Alon and Cem M Deniz and Ryan Brown and Daniel K Sodickson},
url = {http://doi.wiley.com/10.1002/mrm.23126},
doi = {10.1002/mrm.23126},
issn = {07403194},
year = {2012},
date = {2012-05-01},
journal = {Magnetic Resonance in Medicine},
volume = {67},
number = {5},
pages = {1367--1378},
abstract = {The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E-field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject-dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel-by-channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction-guided optimization, proactively managed.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E-field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject-dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel-by-channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction-guided optimization, proactively managed.