@Article{ ChangNESH2017,
title = {Constrained optimization for position calibration of an NMR field camera},
journal = {Magnetic Resonance Imaging},
year = {2018},
month = {8},
volume = {80},
number = {1},
pages = {380-390},
abstract = {Purpose

Knowledge of the positions of field probes in an NMR field camera is necessary for monitoring the B0 field. The typical method of estimating these positions is by switching the gradients with known strengths and calculating the positions using the phases of the FIDs. We investigated improving the accuracy of estimating the probe positions and analyzed the effect of inaccurate estimations on field monitoring.
Methods
The field probe positions were estimated by 1) assuming ideal gradient fields, 2) using measured gradient fields (including nonlinearities), and 3) using measured gradient fields with relative position constraints.
The fields measured with the NMR field camera were compared to fields acquired using a dual-echo gradient recalled echo B0 mapping sequence. Comparisons were done for shim fields from second- to fourth-order shim terms.
Results
The position estimation was the most accurate when relative position constraints were used in conjunction with measured (nonlinear) gradient fields. The effect of more accurate position estimates was seen when compared to fields measured using a B0 mapping sequence (up to 10%–15% more accurate for some shim fields).
The models acquired from the field camera are sensitive to noise due to the low number of spatial sample points.
Conclusion
Position estimation of field probes in an NMR camera can be improved using relative position constraints and nonlinear gradient fields.},
web_url = {https://onlinelibrary.wiley.com/doi/epdf/10.1002/mrm.27010},
state = {published},
DOI = {10.1002/mrm.27010},
author = {Chang P{changp}{Department High-Field Magnetic Resonance}; Nassirpour S{snassirpour}{Department High-Field Magnetic Resonance}; Eschelbach M{meschelbach}{Department High-Field Magnetic Resonance}; Scheffler K{scheffler}{Department High-Field Magnetic Resonance}; Henning A{ahenning}{Department High-Field Magnetic Resonance}}
}

@Article{ ChangNAH2017,
title = {Non-water-suppressed 1H FID-MRSI at 3T and 9.4T},
journal = {Magnetic Resonance in Medicine},
year = {2018},
month = {8},
volume = {80},
number = {2},
pages = {442-451},
abstract = {Purpose
This study investigates metabolite concentrations using metabolite-cycled 1H free induction decay (FID) magnetic resonance spectroscopic imaging (MRSI) at ultra-high fields.
Methods
A non-lipid-suppressed and slice-selective ultra-short echo time (TE) 1H FID MRSI sequence was combined with a low-specific absorption rate (SAR) asymmetric inversion adiabatic pulse to enable non-water-suppressed metabolite mapping using metabolite-cycling at 9.4T. The results were compared to a water-suppressed FID MRSI sequence, and the same study was performed at 3T for comparison. The scan times for performing single-slice metabolite mapping with a nominal voxel size of 0.4 mL were 14 and 17.5 min on 3T and 9.4T, respectively.
Results
The low-SAR asymmetric inversion adiabatic pulse enabled reliable non-water-suppressed metabolite mapping using metabolite cycling at both 3T and 9.4T. The spectra and maps showed good agreement with the water-suppressed FID MRSI ones at both field strengths. A quantitative analysis of metabolite ratios with respect to N-acetyl aspartate (NAA) was performed. The difference in Cre/NAA was statistically significant, ∼0.1 higher for the non-water-suppressed case than for water suppression (from 0.73 to 0.64 at 3T and from 0.69 to 0.59 at 9.4T). The difference is likely because of chemical exchange effects of the water suppression pulses. Small differences in mI/NAA were also statistically significant, however, are they are less reliable because the metabolite peaks are close to the water peak that may be affected by the water suppression pulses or metabolite-cycling inversion pulse.
Conclusion
We showed the first implementation of non-water-suppressed metabolite-cycled 1H FID MRSI at ultra-high fields. An increase in Cre/NAA was seen for the metabolite-cycled case. The same methodology was further applied at 3T and similar results were observed.},
web_url = {https://onlinelibrary.wiley.com/doi/epdf/10.1002/mrm.27049},
state = {published},
DOI = {10.1002/mrm.27049},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Avdievich N{navdievi}; Henning A{ahenning}}
}

@Article{ NassirpourCAH2018,
title = {Compressed sensing for high‐resolution nonlipid suppressed 1H FID MRSI of the human brain at 9.4T},
journal = {Magnetic Resonance in Medicine},
year = {2018},
month = {4},
volume = {Epub ahead},
abstract = {Purpose
The aim of this study was to apply compressed sensing to accelerate the acquisition of high resolution metabolite maps of the human brain using a nonlipid suppressed ultra‐short TR and TE 1H FID MRSI sequence at 9.4T.
Methods
X‐t sparse compressed sensing reconstruction was optimized for nonlipid suppressed 1H FID MRSI data. Coil‐by‐coil x‐t sparse reconstruction was compared with SENSE x‐t sparse and low rank reconstruction. The effect of matrix size and spatial resolution on the achievable acceleration factor was studied. Finally, in vivo metabolite maps with different acceleration factors of 2, 4, 5, and 10 were acquired and compared.
Results
Coil‐by‐coil x‐t sparse compressed sensing reconstruction was not able to reliably recover the nonlipid suppressed data, rather a combination of parallel and sparse reconstruction was necessary (SENSE x‐t sparse). For acceleration factors of up to 5, both the low‐rank and the compressed sensing methods were able to reconstruct the data comparably well (root mean squared errors [RMSEs] ≤ 10.5% for Cre). However, the reconstruction time of the low rank algorithm was drastically longer than compressed sensing. Using the optimized compressed sensing reconstruction, acceleration factors of 4 or 5 could be reached for the MRSI data with a matrix size of 64 × 64. For lower spatial resolutions, an acceleration factor of up to R∼4 was successfully achieved.
Conclusion
By tailoring the reconstruction scheme to the nonlipid suppressed data through parameter optimization and performance evaluation, we present high resolution (97 µL voxel size) accelerated in vivo metabolite maps of the human brain acquired at 9.4T within scan times of 3 to 3.75 min.},
web_url = {https://onlinelibrary.wiley.com/doi/epdf/10.1002/mrm.27225},
state = {published},
DOI = {10.1002/mrm.27225},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Avdievitch N{navdievi}; Henning A{ahenning}}
}

@Article{ NassirpourCH2016,
title = {High and ultra-high resolution metabolite mapping of the human brain using 1H FID MRSI at 9.4T},
journal = {NeuroImage},
year = {2018},
month = {3},
volume = {168},
pages = {211-221},
abstract = {Magnetic resonance spectroscopic imaging (MRSI) is a promising technique for mapping the spatial distribution of multiple metabolites in the human brain. These metabolite maps can be used as a diagnostic tool to gain insight into several biochemical processes and diseases in the brain. In comparison to lower field strengths, MRSI at ultra-high field strengths benefits from a higher signal to noise ratio (SNR) as well as higher chemical shift dispersion, and hence spectral resolution. This study combines the benefits of an ultra-high field magnet with the advantages of an ultra-short TE and TR single-slice FID-MRSI sequence (such as negligible J-evolution and loss of SNR due to T2 relaxation effects) and presents the first metabolite maps acquired at 9.4 T in the healthy human brain at both high (voxel size of 97.6 µL) and ultra-high (voxel size of 24.4 µL) spatial resolutions in a scan time of 11 and 46 min respectively. In comparison to lower field strengths, more anatomically-detailed maps with higher SNR from a larger number of metabolites are shown. A total of 12 metabolites including glutamate (Glu), glutamine (Gln), N-acetyl-aspartyl-glutamate (NAAG), Gamma-aminobutyric acid (GABA) and glutathione (GSH) are reliably mapped. Comprehensive description of the methodology behind these maps is provided.},
web_url = {http://www.sciencedirect.com/science/article/pii/S1053811916307923},
state = {published},
DOI = {10.1016/j.neuroimage.2016.12.065},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Article{ NassirpourCFH2017,
title = {A comparison of optimization algorithms for localized in vivo B0 shimming},
journal = {Magnetic Resonance in Medicine},
year = {2018},
month = {2},
volume = {79},
number = {2},
pages = {1145–1156},
abstract = {PURPOSE:
To compare several different optimization algorithms currently used for localized in vivo B0 shimming, and to introduce a novel, fast, and robust constrained regularized algorithm (ConsTru) for this purpose.
METHODS:
Ten different optimization algorithms (including samples from both generic and dedicated least-squares solvers, and a novel constrained regularized inversion method) were implemented and compared for shimming in five different shimming volumes on 66 in vivo data sets from both 7 T and 9.4 T. The best algorithm was chosen to perform single-voxel spectroscopy at 9.4 T in the frontal cortex of the brain on 10 volunteers.
RESULTS:
The results of the performance tests proved that the shimming algorithm is prone to unstable solutions if it depends on the value of a starting point, and is not regularized to handle ill-conditioned problems. The ConsTru algorithm proved to be the most robust, fast, and efficient algorithm among all of the chosen algorithms. It enabled acquisition of spectra of reproducible high quality in the frontal cortex at 9.4 T.
CONCLUSIONS:
For localized in vivo B0 shimming, the use of a dedicated linear least-squares solver instead of a generic nonlinear one is highly recommended. Among all of the linear solvers, the constrained regularized method (ConsTru) was found to be both fast and most robust.},
web_url = {http://onlinelibrary.wiley.com/doi/10.1002/mrm.26758/epdf},
state = {published},
DOI = {10.1002/mrm.26758},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Fillmer A; Henning A{ahenning}}
}

@Article{ ChangNH2017,
title = {Modeling real shim fields for very high degree (and order) B0 shimming of the human brain at 9.4 T},
journal = {Magnetic Resonance in Medicine},
year = {2018},
month = {1},
volume = {79},
number = {1},
pages = {529–540},
abstract = {Purpose
To describe the process of calibrating a B0 shim system using high-degree (or high order) spherical harmonic models of the measured shim fields, to provide a method that considers amplitude dependency of these models, and to show the advantage of very high-degree B0 shimming for whole-brain and single-slice applications at 9.4 Tesla (T).
Methods
An insert shim with up to fourth and partial fifth/sixth degree (order) spherical harmonics was used with a Siemens 9.4T scanner. Each shim field was measured and modeled as input for the shimming algorithm. Optimal shim currents can therefore be calculated in a single iteration. A range of shim currents was used in the modeling to account for possible amplitude nonlinearities. The modeled shim fields were used to compare different degrees of whole-brain B0 shimming on healthy subjects.
Results
The ideal shim fields did not correctly shim the subject brains. However, using the modeled shim fields improved the B0 homogeneity from 55.1 (second degree) to 44.68 Hz (partial fifth/sixth degree) on the whole brains of 9 healthy volunteers, with a total applied current of 0.77 and 6.8 A, respectively.
Conclusions
The necessity of calibrating the shim system was shown. Better B0 homogeneity drastically reduces signal dropout and distortions for echo-planar imaging, and significantly improves the linewidths of MR spectroscopy imaging.},
web_url = {http://onlinelibrary.wiley.com/doi/10.1002/mrm.26658/epdf},
state = {published},
DOI = {10.1002/mrm.26658},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ AghaeifarCNEHS2018,
title = {A true comparison of B0 shimming with a very high order spherical harmonic based setup and a multi-coil shim array},
year = {2018},
month = {6},
day = {19},
number = {4422},
abstract = {In this work, a true experimental comparison between a very high order spherical harmonic based shim setup and a multi-coil shim array is presented. Each technique has its own cons and pros which are studied through several often-used sequences. All evaluations are performed by shimming of the human brain at 9.4T.},
web_url = {https://www.ismrm.org/18/program_files/EP13.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Aghaeifar A{aaghaeifar}{Department High-Field Magnetic Resonance}; Chang P{changp}{Department High-Field Magnetic Resonance}; Nassirpour S{snassirpour}{Department High-Field Magnetic Resonance}; Eschelbach M{meschelbach}{Department High-Field Magnetic Resonance}; Henning A{ahenning}{Department High-Field Magnetic Resonance}; Scheffler K{scheffler}{Department High-Field Magnetic Resonance}}
}

@Poster{ ChangNH2018,
title = {Measuring Eddy Currents Induced by Switching Gradient/Shim Currents},
year = {2018},
month = {6},
day = {19},
number = {4208},
abstract = {In this work, we measured eddy currents for a very high order B0 shim system. The eddy currents were measured using two methods: a low-resolution B0 mapping sequence and a NMR field camera. The high temporal resolution of the field camera allowed the eddy currents to be corrected using a digital pre-emphasis setup.},
web_url = {https://www.ismrm.org/18/program_files/EP12.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ NassirpourCH2018,
title = {Over-discretized SENSE reconstruction and B0 correction for accelerated non-lipid suppressed 1H FID MRSI of the human brain at 9.4T},
year = {2018},
month = {6},
day = {19},
number = {3840},
abstract = {In this study the acquisition of high resolution (64x64) metabolite maps at 9.4T using a non-lipid suppressed ultra-short TR and TE 1H FID MRSI sequence is accelerated using an improved over-discretized SENSE reconstruction and B0 correction method. The improved reconstruction is compared to conventional SENSE and GRAPPA reconstruction, and reproducible metabolite maps are acquired using this technique.},
web_url = {https://www.ismrm.org/18/program_files/EP09.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Poster{ NassirpourCAH2018_2,
title = {SNR Requirements for Successful Application of Compressed Sensing Acceleration to Non-lipid suppressed 1H MRSI at Ultra-High Fields},
year = {2018},
month = {6},
day = {19},
number = {3841},
abstract = {In this work we systematically investigate the requirements for successful application of compressed sensing for highly accelerating the acquisition of non-lipid suppressed 1H FID MRSI data at ultra-high fields. It is shown that with a combination of parallel imaging and sparse reconstruction, and an RF coil with an even distribution of receive sensitivity, highly accelerated and high resolution metabolite maps can be acquired at 9.4T through compressed sensing.},
web_url = {https://www.ismrm.org/18/program_files/EP09.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Avdievitch N{navdievi}; Henning A{ahenning}}
}

@Poster{ ChangNH2018_2,
title = {Highly Accelerated (R=14) Water Reference Acquisition for High Resolution 1H MRSI using Compressed Sensing},
year = {2018},
month = {6},
day = {18},
number = {1328},
abstract = {In this study, the acquisition of a high resolution (64x64) water reference MRSI data is accelerated by a factor of R=14 using compressed sensing. The results show that this highly accelerated water reference can reliably be used for eddy current and phase correction purposes, as well as internal referencing and quantification. This enables the acquisition of the high resolution water reference MRSI data in 80 seconds at 9.4T.},
web_url = {https://www.ismrm.org/18/program_files/TP01.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ NassirpourCH2018_2,
title = {MultiNet PyGRAPPA: A Novel Method for Highly Accelerated Metabolite Mapping},
year = {2018},
month = {6},
day = {18},
number = {3375},
abstract = {In this work, a novel acceleration method (MultiNet PyGrappa) is introduced which enables high in-plane acceleration factors for non-lipid suppressed 1H MRSI data. By using a variable density undersampling scheme and reconstructing the missing data points with multiple neural networks, this method enables a more robust reconstruction of highly undersampled data. High resolution metabolite maps acquired at 9.4T in the human brain using the proposed method are presented.},
web_url = {https://www.ismrm.org/18/program_files/EP05.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Poster{ WrightNCH2018,
title = {Toward Absolute Quantification Using External Reference Standards at 3T and 9.4T},
year = {2018},
month = {6},
day = {18},
number = {1314},
abstract = {Absolute quantification is a challenge with many paths to reach the final goal of quantifying metabolites in absolute units (e.g. Molarity and molality). Utilizing an external reference standard (ERF) is an attractive method for quantifying in vivo metabolites due to the ability for direct comparison between a known concentration of a metabolite and the in vivo data. A major concern in utilization of an ERF is the differences in coil loading between in vivo and in vitro measurements. To that end, this work describes a method to calibrate and adjust the transmitter voltage in order to maximize signal detection independently of coil load.},
web_url = {https://www.ismrm.org/18/program_files/TP01.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Wright AM{awright}; Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Poster{ Nassirpour2018,
title = {Pushing the limits of MR spectroscopic imaging: benefits and 
limitations of ultra-high field strengths for metabolite mapping of the human brain},
year = {2018},
month = {1},
web_url = {http://www.e-smi.eu/fileadmin/testdir2/ESMI/TOPIM/TOPIM_2018/TOPIM_2018_POSTER_OVERVIEW.pdf},
event_name = {12th ESMI Winter Conference "hot TOPIics in IMaging": Imaging Metabolism (TOPIM 2018)},
event_place = {Les Houches, France},
state = {published},
author = {Nassirpour S{snassirpour}}
}

@Poster{ ChangNH2017_8,
title = {Compressed sensing for 1H MRSI in the human brain at 9.4T},
journal = {Magnetic Resonance Materials in Physics, Biology and Medicine},
year = {2017},
month = {10},
day = {21},
volume = {30},
number = {Supplement 1},
pages = {S485-S486},
abstract = {Purpose/Introduction: Compressed sensing (CS) is an acceleration method that has been used in many MRI applications1, 2. CS randomly undersamples and reconstructs the data using denoising methods. This method allows high acceleration without structured aliasing artifacts. However, the applications in MR spectroscopicimaging are few3, 4.
In this work, we use compressed sensing to accelerate the acquisition of non-lipid suppressed high resolution 1H FID MRSI data of the human brain at 9.4T. Metabolite maps were evaluated for different acceleration factors. This is the first account of applying compressedsensing to accelerate non-lipid suppressed and high-resolution MRSI data.
Subjects and Methods: Fully sampled single-slice MRSI data was acquired with an FID-MRSI sequence5 on a 9.4T whole-body human scanner. The parameters were: FOV = 200 9 200 mm, resolution = 3.125 9 3.125 9 10 mm, TR = 220 ms from the brains of healthy volunteers. The data were retrospectively undersampled with a random variabledensity sampling scheme with effective acceleration factors of: 2, 4, 5, and 10. The reconstruction was performed using a 3D total totalvariation
and a 2D wavelet. Multiple coil channels were combined
during the reconstruction using ESPIRiT6.
Results: Figure 1 shows the spectra of different voxels for different acceleration factors overlaid on the fully sampled spectra. For R = 2, the accelerated data is very consistent with the fully sampled data, while R = 4 results in only slightly higher lipid contamination. However, the higher accelerations result in more lipid contamination. This is
supported by the lipid contamination maps shown in Figure 2 (for two volunteers). These maps show the absolute integral of the spectra between 0.3 and 1.8 ppm. The maps for four major metabolites for two volunteers are shown in Figure 3. No spatial smoothing or filtering was performed. For
R = 2, the maps look almost identical to the fully sampled. The maps get noisier for higher acceleration factors.
Discussion/Conclusion: The metabolite maps get patchier as the acceleration factor increases. This is likely due to the higher levels of lipid contamination that affects the quantification of the spectra. However, note that unlike conventional acceleration schemes such as GRAPPA and SENSE, the lipid artifacts show noise-like contamination as opposed to structured and strong lipid rings overshadowing the metabolite maps. We showed that compressed sensing for non-lipid suppressed MRSI is possible and that high acceleration factors (up to R = 4 or R = 5) is feasible using our proposed reconstruction scheme. Lipid suppressed
data could allow for higher acceleration factors, however lipid suppression at ultra-high fields is very challenging.},
web_url = {https://link.springer.com/content/pdf/10.1007%2Fs10334-017-0634-z.pdf},
event_name = {34th Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2017)},
event_place = {Barcelona, Spain},
state = {published},
DOI = {10.1007/s10334-017-0634-z},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ ChangNH2017_7,
title = {Low SAR lipid suppression for MRSI at ultra-high fields},
journal = {Magnetic Resonance Materials in Physics, Biology and Medicine},
year = {2017},
month = {10},
day = {21},
volume = {30},
number = {Supplement 1},
pages = {S471},
abstract = {Purpose/Introduction: 1H FID MRSI can be performed without
localisation and lipid suppression to acquire metabolite maps from the human brain1, 2. However, voxel bleeding can cause the unsuppressed subcutaneous lipid signal to contaminate the spectra. Furthermore, lipid contamination as a result of residual aliasing artifacts can be caused by acceleration schemes such as GRAPPA3. Outer-volume
saturation can be used to reduce this but this leads to long scan-times and high SAR. An alternative is to use global lipid suppression which has lower SAR4. Lipid suppression schemes that use single- or double-inversion recovery (S/DIR) have a lot of SAR. At ultra high field strengths, the high SAR results in longer scan times. In this work, we present a SIR lipid suppression using a low SAR asymmetric adiabatic pulse for 1H MRSI at 9.4T.
Subjects and Methods: A hypergeometric RF pulse was designed for SIR lipid suppression (Fig. 1) at 9.4T to be spectrally selective over the range 0–1.75 ppm. This is an asymmetric adiabatic pulse and the steep transition was place at 1.75 ppm. This pulse has approximately 40% less SAR than conventional asymmetric adiabatic pulses such as the sech/tanh pulse5. A non-lipid suppressed and a SIR lipid suppressed single-slice MRSI dataset were acquired with a Siemens 9.4T human scanner at high resolution (3.125 9 3.125 mm). The FOV was 200 9 200 9 10 mm. For the lipid suppression, the inversion pulse was placed 130 ms before the excitation pulse. The TR for the nonlipid suppressed and lipid suppressed MRSI was 220 ms and 500 ms, respectively.
Results: Lipid maps were calculated as the absolute integral of the spectra over the range 0–1.7 ppm. Figure 2 shows the normalised lipid maps. The remaining lipid signal was a maximum of 30% of the non-lipid suppressed and the mean was 15.46% of the original lipid content. Spectra from the non-lipid and lipid suppressed data are shown in Figure 3. The spectra show a clear reduction in the lipid range while
the rest of the spectra are largely unaffected.
Discussion/Conclusion: The asymmetric inversion pulse allowed us to reduce the lipid content. Furthermore, the TR could be kept short due to the low SAR of the pulse. The lipid content was greatly reduced to a mean of 15% of the original lipid content. We showed an effective method of lipid suppression while keeping the TR short. This is beneficial for reliable acceleration at ultra-high fields, since conventional lipid suppression schemes prolong the TR
and hence add to the scan time3.},
web_url = {https://link.springer.com/content/pdf/10.1007%2Fs10334-017-0634-z.pdf},
event_name = {34th Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2017)},
event_place = {Barcelona, Spain},
state = {published},
DOI = {10.1007/s10334-017-0634-z},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ NassirpourCH2017_6,
title = {The necessity of parametrizing macromolecules for accurate quantification of ultra-short TE and TR 1H FID MRSI data at 9.4T},
journal = {Magnetic Resonance Materials in Physics, Biology and Medicine},
year = {2017},
month = {10},
day = {21},
volume = {30},
number = {Supplement 1},
pages = {S473-S474},
abstract = {Purpose/Introduction: At ultra-short TE and TR, macromolecule quantification becomes extremely important [1, 2], because of shorter relaxation times and thus lower saturation, these molecules contribute greatly to the spectrum. Measuring the MMs and including them as a single baseline might not be enough because of regional and relaxation time differences between the different macromolecule components [1, 2]. In this work, we measured and modelled individual macromolecule components at 9.4T. We then used our proposed MM model to highlight the necessity of parametrizing the individual macromolecule components for ultra-short TE and TR MRSI measurements.
Subjects and Methods: 8 healthy subjects were scanned on a 9.4T Siemens whole-body human scanner. Metabolite-nulled data using DIR [1] were acquired and used to model the MM baseline. The T1 of these metabolites in gray and white matter were estimated using variable flip angle method on MRSI datasets acquired at 4 different flip angles with a TR of 500 ms. High resolution (3.25mm 9 3.25mm 9 10 mm) ultra-short TE 1H FID MRSI [3–6] data with two different TRs of 300 ms and 500 ms were also collected. These data were fitted using LCMODEL [7] with three different MM baseline models (‘‘no MM’’, ‘‘measured MM’’ as an average single-component MM baseline model, and our proposed multi-component ‘‘modelled MM’’ shown in Figure 1a).
Results: Figure 1 shows our proposed MM model along with the
estimated T1 values. Figure 2 shows the metabolite maps resulting from fitting the datasets acquired at TR = 300 ms with 3 different models for 2 different volunteers. Figure 3 shows a boxplot of how much the metabolite concentrations
are overestimated when different models are used. The
difference between the two different TRs are due to saturation effects that have not been corrected for. However, the figure highlights how using different MM models on each TR can account for the macromolecule saturation effects.
Discussion/Conclusion: Figure 1 shows that there are regional and relaxation time differences between individual macromolecule components that cause metabolite concentrations to differ from the no MM model to various degrees (as shown in Figure 2). Figure 3 further shows that if a single MM baseline is used in fitting, depending on the TR, the concentration of metabolite will be underestimated to varying degrees, since different macromolecule components relax at different rates. Also, due to regional differences,
the overestimation degree varies between white and gray matter. The only way to account for all of this is to use a parametrized MM model like the one we have proposed.},
web_url = {https://link.springer.com/content/pdf/10.1007%2Fs10334-017-0634-z.pdf},
event_name = {34th Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2017)},
event_place = {Barcelona, Spain},
state = {published},
DOI = {10.1007/s10334-017-0634-z},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Poster{ NassirpourCH2017_4,
title = {Comparison of different acceleration methods for high-resolution metabolite mapping using 1H FID MRSI at 9.4T},
year = {2017},
month = {4},
day = {27},
number = {5520},
abstract = {Reliable metabolite mapping of the human brain using ultra-short TE and TR 1H FID-MRSI is possible at ultra-high fields. However, MRSI studies with high spatial resolutions and brain coverage suffer from long scan times. To make these studies clinically relevant, different acceleration methods are used at the price of losing SNR. The aim of this study is to implement and compare different in-plane acceleration methods: SENSE, GRAPPA and compressed sensing for high-resolution metabolite mapping of the human brain at 9.4T without lipid suppression.},
web_url = {http://www.ismrm.org/17/program_files/EP20.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Poster{ ChangNH2017_3,
title = {Decoupling Controller Design for Real-time Feedback of B0 Shim Systems},
year = {2017},
month = {4},
day = {26},
number = {2683},
abstract = {Real-time B0 feedback has been shown to be beneficial for the controlling of B0 fluctuations. However, update rates that have been reported are slow (~100ms) and either use pre-emphasis to correct for the coupling and faster dynamics of the system or only use control the frequency 
We show that for faster update rates (±1ms) pre-emphasis (i.e. dynamic decoupling) is not required and that static decoupling can perform equally well or better.},
web_url = {http://www.ismrm.org/17/program_files/TP10.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ ChangNH2017_4,
title = {Dynamic B0 Shim Controller for Digital Pre-emphasis with Sub-millisecond Update Rate},
year = {2017},
month = {4},
day = {26},
number = {2684},
abstract = {High-order dynamic B0 shimming has been shown to improve the shim quality for multi-slice acquisition schemes. However, for gradient intensive sequences, eddy currents become a major problem and pre-emphasis is required. The pre-emphasis can be done flexibly with the use of digital filters as they can drive arbitrary-shaped waveforms and are scalable to a larger number of channels.
In this work, we design and implement a system to perform dynamic B0 shimming with digital pre-emphasis with a very fast update rate. The setup is then tested for performing pre-emphasis on a 9.4T scanner.},
web_url = {http://www.ismrm.org/17/program_files/TP10.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ ChangNH2017_5,
title = {Flexible General-Purpose Embedded System for Real-time Feedback and Dynamic B0 Shimming},
year = {2017},
month = {4},
day = {26},
number = {2716},
abstract = {B0 shimming methods are becoming more sophisticated. Methods such as multi-coil shimming, dynamic B0 shimming with pre-emphasis and real-time feedback all require additional hardware to drive the shim coils.
In this work, we present a novel general-purpose embedded platform for controlling any of the above mentioned shim systems. Control software can be developed on Linux, while low-level scripts are used for optimal control of hardware interfaces.},
web_url = {http://www.ismrm.org/17/program_files/TP10.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ ChangNH2017_6,
title = {Limitations of NMR Field Cameras for B0 Field Monitoring},
year = {2017},
month = {4},
day = {25},
number = {3902},
abstract = {We compare the fields measured by a field camera to the fields obtained from B0 mapping at 9.4T. The B0 maps have higher spatial resolution and are therefore taken as a benchmark. We analyse the loss of spatial fidelity due to the lower spatial samples of the field camera and compare two different field probe position calibration and optimisation methods that would help alleviate the problem of discrepancies between field monitoring and field mapping.},
web_url = {http://www.ismrm.org/17/program_files/EP07.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ ChangNFH2016,
title = {Correcting Geometric Distortion in B0 Mapping},
year = {2016},
month = {5},
day = {10},
number = {1871},
abstract = {The task of mapping B0 fields to characterise shim fields can be challenging since shim fields generate a highly inhomogeneous field that may be difficult to capture. Furthermore this results in geometric distortion of the B0 map which affects the characterisation of the shim field.
Geometric distortion correction was investigated using a gridded phantom and compared to the effect of using a high bandwidth on the read-out gradient. It was found that using a high bandwidth was more effective in reducing the distortion and that correcting the distortion using a phantom grid was not sufficient.},
web_url = {http://www.ismrm.org/16/program_files/TP06.htm},
event_name = {24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016)},
event_place = {Singapore},
state = {published},
author = {Chang YC{changp}; Nassirpour S{snassirpour}; Fillmer A; Henning A{ahenning}}
}

@Poster{ ChangNH2016_2,
title = {Very high order B0 Shimming of the human brain at 9.4 T considering Real B0 Shim Fields},
year = {2016},
month = {5},
day = {10},
number = {3530},
abstract = {A highly homogeneous B0 field is essential if we are to exploit the advantages of higher field strengths for MR applications. In this work, we model the real field of each shim channel of a 4th order shim system for a 9.4T MR system for in vivo B0 shimming applications. Each shim channel is modelled at a range of frequencies to account for the possibility of amplitude nonlinearities.
By modelling the fields generated by each shim channel, we were able to achieve better shim qualities than if perfect fields were assumed.},
web_url = {http://www.ismrm.org/16/program_files/EP10.htm},
event_name = {24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016)},
event_place = {Singapore},
state = {published},
author = {Chang YC{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ ChangNH2016,
title = {Very High‐order B0 Shimming for Single‐Shot EPI at 9.4 T in the Human Brain},
year = {2016},
month = {3},
web_url = {http://www.ismrm.org/workshops/UHF16/},
event_name = {ISMRM Workshop on Ultra High Field MRI: Technological Advances & Clinical Applications},
event_place = {Heidelberg, Germany},
state = {published},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Poster{ NassirpourKGH2015,
title = {Accelerated Multi-slice 1H FID-MRSI in the human brain at 9.4 T},
year = {2015},
month = {6},
day = {4},
volume = {23},
number = {4711},
abstract = {Magnetic resonance spectroscopic imaging (MRSI) at ultra-high field strengths is a promising technique for mapping of the metabolites over the entire brain volume with a high signal to noise ratio. However, long acquisition time is a major limitation in MRSI. Along with short repetition times (TR) [4] parallel imaging strategies can help accelerate the scan by acquiring only a fraction of the data points in k-space, but an appropriate unfolding reconstruction algorithm is required. To that end, a target driven SENSE [3] reconstruction algorithm has been introduced [1], which minimizes the effects of voxel bleeding. This study represents the first demonstration of short TR twofold SENSE accelerated multi-slice FID MRSI of the human brain at 9.4T.},
web_url = {http://www.ismrm.org/15/program_files/ThuEPS03.htm},
event_name = {23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015)},
event_place = {Toronto, Canada},
state = {published},
author = {Nassirpour S{snassirpour}; Kirchner T; Giapitzakis IA{igiapitzakis}; Henning A{ahenning}}
}

@Poster{ GiapitzakisNAKH2015_2,
title = {Metabolite cycled single voxel 1H spectroscopy at 9.4T},
year = {2015},
month = {6},
day = {4},
volume = {23},
number = {4696},
abstract = {Metabolite cycled proton magnetic resonance spectroscopy (MC 1H-MRS) has been proved to enhance the frequency resolution and the signal to noise ratio (SNR) of the spectrum at static magnetic fields ranging from 1.5 to 7 Tesla. The purposes of this study were to: 1) develop a short duration WS scheme for implementation with a STEAM sequence [5] 2) examine the performance of MC H-MRS compared to a WS STEAM sequence and 3) create spectrum with high frequency resolution at 9.4T enabling the detection of several metabolites.},
web_url = {http://www.ismrm.org/15/program_files/ThuEPS03.htm},
event_name = {23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015)},
event_place = {Toronto, Canada},
state = {published},
author = {Giapitzakis IA{igiapitzakis}; Nassirpour S{snassirpour}; Avdievich N{navdievi}; Kreis R; Henning A{ahenning}}
}

@Poster{ GiapitzakisNKAH2015,
title = {Metabolite cycled proton magnetic resonance spectroscopy at 9.4T},
year = {2015},
month = {3},
day = {19},
volume = {10},
number = {382},
abstract = {Introduction
Non-water suppressed metabolite cycled proton magnetic resonance spectroscopy (MC 1H-MRS) has been proven to enhance the frequency resolution and the signal to noise ratio (SNR) of the spectrum at 3 Tesla [1-2]. Previously the adiabatic inversion pulse for MC 1H-MRS was optimized to exploit these advantages for application in the human brain at 9.4T [3].  In this work, we examine the performance of STEAM [4] based MC 1H-MRS [3,5] compared to water suppressed 1H-MRS using a numerically optimized short water suppression (WS) sequence with respect to spectral resolution and signal-to-noise ratio (SNR) in the human brain at 9.4T.
Methods
All experiments were carried out using a 4 channel transceiver array coil [6] connected to a whole body 9.4Tesla Magnetom SIEMENS scanner. For 1H MRS localization a STEAM sequence (TE/TM/TR: 10/50/3000ms) was used. An optimized water suppression scheme consisting of 7 excitation pulses and orthogonal spoiler gradients was developed. The post processing of the data included in the given order: frequency alignment using the water reference [7], averaging, ECC [8], channel combination  [9] and zero filling using factor of 2. Both sequences were applied on healthy volunteers (fig. 1-2) (NEX: 128-320, voxel size: 15x15x15mm3) placing a grey matter voxel in the occipital lobe.  The optimized MC inversion pulse (IP) for the resulting   B1+ of 22μT had a duration of 23ms and a frequency offset of 350Hz.
Results
 The in vivo data (fig. 1) demonstrated that the simultaneously acquired water reference of MC data allowed for frequency and phase alignment of the different averages (fig. 4) leading to a line width of 25.9Hz and SNR of 38.2. In addition, the MC technique provided a WS factor of 99.8%. On the other hand, the WS spectrum resulted in a line width of 28.9Hz (+3Hz difference), SNR of 33.3 (-5dB difference) and a WS factor of 99.7%. The difference in SNR and linewidth is mainly produced by physiological motion (e.g. breathing, blood flow) as well as motion of the volunteer demonstrating the importance of the simultaneously acquired water reference spectrum both for the ECC and correct averaging of the acquisitions (fig. 2). Finally the MC data enabled the reconstruction of high frequency resolution spectra similar with other studies conducted on 9.4T [10].
Conclusions
1) MC 1H-MRS enables phase and frequency alignment of individual acquisitions as well as ECC of the spectrum at 9.4T 2) MC 1H-MRS performs better in terms of SNR and line width and thus effective spectral resolution compared to WS 1H-MRS and 3) MC 1H-MRS results in a free of gradient modulation sidebands and eddy current artefacts spectrum and excellent WS performance.},
web_url = {http://www.e-smi.eu/index.php?id=emim-2015-tubingen},
event_name = {10th Annual Meeting of the European Society for Molecular Imaging (EMIM 2015)},
event_place = {Tübingen, Germany},
state = {accepted},
author = {Giapitzakis IA{igiapitzakis}; Nassirpour S{snassirpour}; Kreis R; Avdievich NI{navdievi}; Henning A{ahenning}}
}

@Conference{ ChangNASH2018,
title = {Dynamic B0 Shimming for Multi-Slice Metabolite Mapping at Ultra-High Field in the Human Brain: Very High Order Spherical Harmonics vs. Multi-Coil},
year = {2018},
month = {6},
day = {20},
number = {0833},
abstract = {In this work, we evaluated the performance of two B0 shimming approaches (i.e. a very high order spherical harmonic shim system versus a multi-coil shim setup), for dynamic shimming in the context of high resolution and multi-slice metabolite mapping of the human brain at 9.4T.},
web_url = {https://www.ismrm.org/18/program_files/O84.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Chang P{changp}{Department High-Field Magnetic Resonance}; Nassirpour S{snassirpour}{Department High-Field Magnetic Resonance}; Aghaeifar A{aaghaeifar}{Department High-Field Magnetic Resonance}; Scheffler K{scheffler}{Department High-Field Magnetic Resonance}; Henning A{ahenning}{Department High-Field Magnetic Resonance}}
}

@Conference{ NassirpourCH2018_3,
title = {Whole Brain High Resolution Metabolite Mapping Using 1H FID MRSI with Slice-wise B0 Shim Updating at 9.4T},
year = {2018},
month = {6},
day = {19},
number = {0619},
abstract = {In this work, we present high resolution whole-brain metabolite maps acquired at 9.4T for the first time. By combining a robust acceleration method with dynamic B0 slice-wise shim updating, we achieved high quality metabolite maps from 10 slices of the brain with a nominal voxel size of ~80μL in ~25 minutes.},
web_url = {https://www.ismrm.org/18/program_files/O44.htm},
event_name = {Joint Annual Meeting ISMRM-ESMRMB 2018},
event_place = {Paris, France},
state = {published},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Conference{ ChangNH2017_9,
title = {ConsTru: an optimal B0 shimming solution},
journal = {Magnetic Resonance Materials in Physics, Biology and Medicine},
year = {2017},
month = {10},
day = {19},
volume = {30},
number = {Supplement 1},
pages = {S683-S684},
abstract = {Purpose of the software: B0 shimming is a vital pre-scan adjustment that directly affects the quality of the data. However, vendor implemented B0 shimming can be time-consuming and sub-optimal. For smaller regions-of-interest (ROI), such as single voxel spectroscopy,
the B0 shim can be especially poor and exceed the hardware
constraints. We present a software application that can calculate the optimal B0 shims for ROIs of any size, for arbitrary ROIs and also single-voxels. The process is fast, non-iterative and the shim values are always within the hardware constraints. The software package can easily be
installed and used on any MR and shim system, and has the ability to easily be calibrated to account for any imperfections of the system.
Methods and Implementation: The B0 shim values are calculated from B0 maps using an optimized and robust algorithm. We use the ConsTru algorithm1 that has previously been shown to give the optimal shim values for any application: whole brain, single slice, single voxel. The algorithm considers the hardware constraints of the
system and regularizes the problem accordingly so that the shim values are the best for the given shim system.
The software uses B0 maps in DICOM format. The region of interest can be defined by the user or copied from the scan protocol and the B0 shim values are then calculated.
Figure 1 shows the superiority of the ConsTru shimming algorithm over a variety of other B0 shimming algorithms currently used in the field in the case of a challenging shimming application (single voxel located in the pre-frontal cortex at 7 T). The resulting shimmed B0
maps and the standard deviation of the frequency shifts are shown for each algorithm. Figure 2 shows the resulting shim quality using ConsTru at 7 and 9.4 T over 66 volunteers for different ROIs. The software can also be modified to account for non-ideal spherical harmonic fields. Real fields models and reference fields can be used to calculate the optimal shim values2.
Features illustrated at the exhibit: The software is a simple, standalone executable. It can run on Windows 7 and upwards (and Linux binaries can also be compiled).
There is a single-voxel version and an arbitrary ROI version available. The arbitrary ROI version also comes with an autosegmentation feature. The two versions are shown in Figure 3. After the ROI is defined, the shim values are calculated immediately and no further iterations are required. The shim values as well as the defined ROIs
can be saved to file.},
web_url = {https://link.springer.com/content/pdf/10.1007%2Fs10334-017-0635-y.pdf},
event_name = {34th Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2017)},
event_place = {Barcelona, Spain},
state = {published},
DOI = {10.1007/s10334-017-0635-y},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Conference{ NassirpourCH2017_5,
title = {Over-discrete SENSE and B0 correction for accelerated 1H FID MRSI of the human brain at 9.4T},
journal = {Magnetic Resonance Materials in Physics, Biology and Medicine},
year = {2017},
month = {10},
day = {19},
volume = {30},
number = {Supplement 1},
pages = {S112},
abstract = {Purpose/Introduction: Over-discrete SENSE reconstruction with direct control of the shape of the spatial response function (SRF) [1] has been shown to reduce short- and far-reaching voxel bleeding. This makes it a suitable candidate for acceleration of non-lipid suppressed FIDMRSI,
since lipid signals bleeding from the unsuppressed subcutaneous fat into the brain region become very problematic. Additionally, using the B0 map to correct for the frequency shifts on a sub-voxel level has been proven to provide SNR gain and line-shape improvement [2], which is
especially beneficial for highly accelerated data. The aim of this work was to demonstrate the benefits of over-discrete SENSE reconstruction scheme along with the B0 correction on nonlipid suppressed ultra-short TR and TE 1H FID MRSI data at 9.4T.
Subjects and Methods: High resolution (3.125mmx3.125mmx10
mm) single-slice ultra-short TR and TE non-lipid suppressed
1H FID MRSI [3–6] were acquired from the brains of healthy volunteers on a 9.4T Siemens whole-body human scanner with a TR of 300 ms. A high resolution scout anatomical image was used to extract the coil sensitivities using the ESPIRiT method [7], and high resolution B0 map was also acquired using a 2D GRE dual-echo sequence for B0 correction. The MRSI data was retrospectively undersampled by a factor of
R = 2 9 2 and reconstructed with three schemes: (1) conventional SENSE, (2) over-discrete SENSE (OD-SENSE) with an over-discretization factor of 4, along with a Gaussian shape target matrix with the FWHM of the nominal voxel-size without and, (3) with B0 correction on a sub-voxel level (B0 OD-SENSE).
Results: Figure 1 shows lipid contamination maps from the fully sampled along with the R = 4 times accelerated data reconstructed once with a conventional SENSE reconstruction scheme and once with the over-discrete scheme and direct control of the SRF The average SNR of the over-discrete reconstructed R = 2 9 2 SENSE data over the whole slice was 85.9 and 71.2, with and without the B0 correction, respectively.
Discussion/Conclusion: In non-lipid suppressed MRSI, the lipid signals in the subcutaneous region are orders of magnitude larger than the metabolite signals. Therefore, any residual aliasing artifacts resulting from conventional SENSE reconstruction will overshadow the metabolites in the brain. The over-discrete reconstruction scheme greatly reduced this artifact as can be seen in Fig. 1. The
over-discrete B0 correction resulted in 1.2 times gain in SNR which is in accordance with [2].},
web_url = {https://link.springer.com/content/pdf/10.1007%2Fs10334-017-0632-1.pdf},
event_name = {34th Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2017)},
event_place = {Barcelona, Spain},
state = {published},
DOI = {10.1007/s10334-017-0632-1},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Conference{ ChangNH2017_2,
title = {Fast non-water suppressed metabolite cycled 1H FID MRSI at both 3T and 9.4T},
year = {2017},
month = {4},
day = {27},
number = {1250},
pages = {724},
abstract = {The purpose of this study is to use a metabolite cycling scheme combined with FID MRSI, acquire and compare spectra and metabolite maps from both water suppressed and non-water suppressed FID MRSI at 9.4T and 3T.},
web_url = {http://www.ismrm.org/17/program_files/O44.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Chang P{changp}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}

@Conference{ NassirpourCH2017_2,
title = {High resolution maps of individual macromolecule components in the human brain at 9.4T},
year = {2017},
month = {4},
day = {27},
number = {1059},
pages = {619},
abstract = {Although ultra-short TE spectroscopy sequences enhance the information content of the spectrum, they are, in their nature, prone to quantification biases if the macromolecular (MM) components are not taken into account The aim of this study, was to 1) perform macromolecule mapping at 9.4T using an ultra-short TE double-inversion recovery (DIR) MRSI sequence, 2) model and parametrize the individual MM components, and 3) extract high resolution maps of individual MM components using the modelled MM basis set.},
web_url = {http://www.ismrm.org/17/program_files/O46.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Conference{ NassirpourCH2017,
title = {Pushing the limits of ultra-high field MRSI: benefits and limitations of 9.4T for metabolite mapping of the human brain},
year = {2017},
month = {4},
day = {27},
number = {1248},
pages = {723},
abstract = {MRSI can benefit greatly from ultra-high field strengths. Given the higher SNR and higher chemical shift dispersion, metabolite mapping can be done with higher quantification precision and at higher spatial resolution. The aim of this work was to study the competing effects of spatial resolution, SNR, linewidth and higher field strengths by pushing the spatial resolution limits of 3T and 9.4T for metabolite mapping of the human brain.},
web_url = {http://www.ismrm.org/17/program_files/O44.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Conference{ NassirpourCH2017_3,
title = {Multi-slice metabolite mapping with Very-High Degree Dynamic B0 Shim Updating at 9.4T using Accelerated 1H FID MRSI},
year = {2017},
month = {4},
day = {26},
number = {0971},
pages = {571-572},
abstract = {In this work, we address the problem of B0 inhomogeneity in the human brain at 9.4T by using dynamic very high order B0 shimming. This enables multi-slice metabolite mapping in the human brain at this field strength. Furthermore, we investigate the advantage of low (2nd) versus very high (4th+) degree dynamic B0 shimming directly with respect to the quality of the metabolite maps.},
web_url = {http://www.ismrm.org/17/program_files/O83.htm},
event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)},
event_place = {Honolulu, HI, USA},
state = {published},
author = {Nassirpour S{snassirpour}; Chang P{changp}; Henning A{ahenning}}
}

@Conference{ Nassirpour2016_2,
title = {Metabolite Mapping of the Human Brain Using 1H FID MRSI at 9.4T},
year = {2016},
month = {8},
day = {16},
web_url = {http://www.ismrm.org/workshops/Spectroscopy16/program.htm},
event_name = {ISMRM Workshop on MR Spectroscopy: From Current Best Practice to Latest Frontiers},
event_place = {Allensbach-Hegne, Germany},
state = {published},
author = {Nassirpour S{snassirpour}}
}

@Conference{ Nassirpour2016,
title = {A Comparison of Optimization Algorithms for Localized In-Vivo Shimming},
year = {2016},
month = {8},
day = {15},
web_url = {http://www.ismrm.org/workshops/Spectroscopy16/program.htm},
event_name = {ISMRM Workshop on MR Spectroscopy: From Current Best Practice to Latest Frontiers},
event_place = {Allensbach-Hegne, Germany},
state = {published},
author = {Nassirpour S{snassirpour}}
}

@Conference{ NassirpourCFH2016,
title = {A Comparison of Optimization Algorithms for Localized in-vivo B0 Shimming},
year = {2016},
month = {5},
day = {13},
number = {1142},
abstract = {This work presents a study on the performance of several least-squares optimization algorithms used for localized in-vivo B0 shimming. Seven different algorithms were tested in 4 different shim volumes in the brain: global shimming region, single slice, and single voxels in two different positions with 3rd order shimming at 7T. Each algorithm's robustness and convergence were tested against noisy inputs and different starting values. The results give an interesting overview of the properties of each algorithm and their applicability. The regularized iterative inversion algorithm proves to be the best algorithmic approach suited to this problem.},
web_url = {http://www.ismrm.org/16/program_files/O43.htm},
event_name = {24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016)},
event_place = {Singapore},
state = {published},
author = {Nassirpour S{snassirpour}; Chang YC{changp}; Fillmer A; Henning A{ahenning}}
}

@Conference{ GiapitzakisNAKH2015,
title = {1H single voxel spectroscopy at occipital lobe of human brain at 9.4 T},
journal = {Magnetic Resonance Materials in Physics, Biology and Medicine},
year = {2015},
month = {10},
volume = {28},
number = {1 Supplement},
pages = {S208-S209},
abstract = {Purpose/Introduction: Non-water suppressed metabolite cycled
proton magnetic resonance spectroscopy (MC 1H-MRS) enables shotby-shot frequency and phase alignment due to the simultaneous acquisition of water and metabolite spectra [1] enhancing both frequency resolution and signal to noise ratio (SNR). In a previous study the adiabatic inversion pulse for MC 1H-MRS was optimized to exploit these advantages for application in the human brain at 9.4T
[2]. In this work, we examine the performance of STEAM [3] based MC 1H-MRS compared to water suppressed 1H-MRS using a
numerically optimized short water suppression (WS) sequence in human brain at 9.4T.
Subjects and Methods: All experiments were conducted using a 4 channel transceiver array coil connected to a whole body 9.4 Tesla Magnetom SIEMENS scanner. For 1H MRS localization a STEAM sequence (TE/TM/TR: 10/50/3000 ms) was used. An optimized water suppression scheme was developed resulting in a WS scheme with duration of 190 ms. The WS scheme was tested on a spherical phantom filled with an aqueous solution of acetate and lactate (Fig. 1). Finally, both sequences were applied on 4 healthy volunteers
(Figs. 2, 3) placing a grey matter voxel in the occipital lobe.
Results: She simultaneously acquired water reference of MC data allowed frequency and phase alignment for the different averages (Fig. 2) leading to a linewidth of 25.9 Hz and SNR of 38.2. Additionally, the MC technique provided a WS factor of 99.8 %. On the other hand, the WS spectrum resulted in a linewidth of 28.9 Hz (+3 Hz), SNR of 33.3 (-5 dB) and a WS factor of 99.7 %. The difference in SNR and linewidth mainly arises from physiological and volunteer motion demonstrating the importance of the simultaneous
acquisition of water reference spectrum both for the ECC and averaging of the acquisitions (Fig. 2). The MC data enabled the reconstruction of high frequency resolution spectra similar to other studies conducted on 9.4T [4].
Discussion/Conclusion: The conclusions of this study are: (1) MC 1H-MRS enables phase and frequency alignment of individual acquisitions and ECC of the spectrum at 9.4T, (2) MC 1H-MRS performs better in terms of SNR and line width and thus effective spectral resolution compared to WS 1H-MRS and (3) MC 1H-MRS results in a spectrum free of gradient modulation sidebands and eddy current artefacts and excellent WS performance.},
web_url = {http://link.springer.com/content/pdf/10.1007%2Fs10334-015-0488-1.pdf},
event_name = {32nd Annual Scientific Meeting ESMRMB 2015},
event_place = {Edinburgh, UK},
state = {published},
DOI = {10.1007/s10334-015-0488-1},
author = {Giapitzakis IA{igiapitzakis}; Nassirpour S{snassirpour}; Avdievich NI{navdievi}; Kreis R; Henning A{ahenning}}
}

@Conference{ GiapitzakisNH2015,
title = {Short duration water suppresion using optimised flip angles (SODA) at ultra high fields},
journal = {Magnetic Resonance Materials in Physics, Biology and Medicine},
year = {2015},
month = {10},
volume = {28},
number = {1 Supplement},
pages = {S401-S402},
abstract = {Purpose/Introduction: Proton magnetic resonance spectroscopy
(MRS) enables the detection of several metabolites within the human body. The concentration of water is av thousand times higher than those of the metabolites. As a consequence, it introduces baseline distortions, eddy current and gradient modulation artifacts in the final
spectrum [1–2]. Several techniques have been developed trying to suppress the water peak using special water suppression (WS) methods [3–4] etc. At low magnetic fields (i.e.\3 T) the performance of the aforementioned WS techniques can be quite good. However, at higher magnetic fields this is a difficult task due to B1 + inhomogeneity.
A solution to this problem is the development of VAPOR [5].
This method enables very good WS against B1 + ingomogeneity at the expense of time (*700 ms). This duration can prolong significantly the examination time, especially in case of time consuming sequences (e.g. multi slice MRSI). For this reason, the aim of this study was the development of a short duration water suppression using optimised flip angles (SODA) at 9.4 T.
Subjects and Methods: An algorithm was developed trying to
minimize the residual longitudinal magnetization for different number of CHESS [4] pulses, T1 relaxation times, B1 + inhomogeneity and time delays (Fig. 1). After the determination of the desired flip angles and time delays, an algorithm was written in order to find the best
combination of gradient amplitudes for the suppression of the unwanted coherence pathways (Fig. 2; after n pulses, 3n coherence pathways are produced). Afterwards, the final WS sequence was incorporated with a STEAM technique and tested on a phantom and a healthy volunteer. All the experiments were carried out using a 4 channel transceiver array coil connected to a whole body 9.4 Tesla SIEMENS scanner.
Results: The phantom results demonstrated a suppression of 99.7 % of the initial water peak (suppression factor = 449; Fig. 3). In addition, in vivo results showed that SODA scheme allows sufficient WS giving a suppression factor[1300. 
Discussion/Conclusion: In this study we demonstrated a short
duration WS scheme using optimized flip angles (SODA) and gradients showing preliminary results of this study. This method has a lot of room for improvements such as the introduction of phase cycling during SODA and the replacement of the Gaussian pulses with pulses with better profiles.},
web_url = {http://link.springer.com/content/pdf/10.1007%2Fs10334-015-0489-0.pdf},
event_name = {32nd Annual Scientific Meeting ESMRMB 2015},
event_place = {Edinburgh, UK},
state = {published},
DOI = {10.1007/s10334-015-0489-0},
author = {Giapitzakis IA{igiapitzakis}; Nassirpour S{snassirpour}; Henning A{ahenning}}
}