Slice Profile Effects Research Articles

The effect of and correction for through-slice dephasing on 2D gradient-echo double angle mapping.

To show that variations through slice and slice profile effects are two major confounders affecting 2D dual angle maps using gradient-echo signals and thus need to be corrected to obtain accurate maps. The 2D gradient-echo transverse complex signal was Bloch-simulated and integrated across the slice dimension including nonlinear variations in inhomogeneities through slice. A nonlinear least squares fit was used to find the factor corresponding to the best match between the two gradient-echo signals experimental ratio and the Bloch-simulated ratio. The correction was validated in phantom and in vivo at 3T. For our RF excitation pulse, the error in the factor scales by approximately 3.8% for every 10 Hz/cm variation in along the slice direction. Higher accuracy phantom maps were obtained after applying the proposed correction; the root mean square error relative to the gold standard decreased from 6.4% to 2.6%. In vivo whole-liver maps using the corrected map registered a significant decrease in gradient through slice. inhomogeneities varying through slice were seen to have an impact on the accuracy of 2D double angle maps using gradient-echo sequences. Consideration of this confounder is crucial for research relying on accurate knowledge of the true excitation flip angles, as is the case of mapping using a spoiled gradient recalled echo sequence.

Free-breathing self-gated continuous-IR spiral T1 mapping: Comparison of dual flip-angle and Bloch-Siegert B1-corrected techniques.

PurposeTo develop a B1‐corrrected single flip‐angle continuous acquisition strategy with free‐breathing and cardiac self‐gating for spiral T1 mapping, and compare it to a previous dual flip‐angle technique.MethodsData were continuously acquired using a spiral‐out trajectory, rotated by the golden angle in time. During the first 2 s, off‐resonance Fermi RF pulses were applied to generate a Bloch‐Siegert shift B1 map, and the subsequent data were acquired with an inversion RF pulse applied every 4 s to create a T1* map. The final T1 map was generated from the B1 and the T1* maps by using a look‐up table that accounted for slice profile effects, yielding more accurate T1 values. T1 values were compared to those from inversion recovery (IR) spin echo (phantom only), MOLLI, SAturation‐recovery single‐SHot Acquisition (SASHA), and previously proposed dual flip‐angle results. This strategy was evaluated in a phantom and 25 human subjects.ResultsThe proposed technique showed good agreement with IR spin‐echo results in the phantom experiment. For in‐vivo studies, the proposed technique and the previously proposed dual flip‐angle method were more similar to SASHA results than to MOLLI results.ConclusionsB1‐corrected single flip‐angle T1 mapping successfully acquired B1 and T1 maps in a free‐breathing, continuous‐IR spiral acquisition, providing a method with improved accuracy to measure T1 using a continuous Look‐Locker acquisition, as compared to the previously proposed dual excitation flip‐angle technique.

A unifying view on extended phase graphs and Bloch simulations for quantitative MRI

Quantitative MRI methods and learning-based algorithms require exact forward simulations. One critical factor to correctly describe magnetization dynamics is the effect of slice-selective RF pulses. While contemporary simulation techniques correctly capture their influence, they only provide final magnetization distributions, require to be run for each parameter set separately, and make it hard to derive general theoretical conclusions and to generate a fundamental understanding of echo formation in the presence of slice-profile effects. This work aims to provide a mathematically exact framework, which is equally intuitive as extended phase graphs (EPGs), but also considers slice-profiles through their natural spatial representation. We show, through an analytical, hybrid Bloch-EPG formalism, that the spatially-resolved EPG approach allows to exactly predict the signal dependency on off-resonance, spoiling moment, microscopic dephasing, and echo time. We also demonstrate that our formalism allows to use the same phase graph to simulate both gradient-spoiled and balanced SSFP-based MR sequences. We present a derivation of the formalism and identify the connection to existing methods, i.e. slice-selective Bloch, slice-selective EPG, and the partitioned EPG. As a use case, the proposed hybrid Bloch-EPG framework is applied to MR Fingerprinting.

Confounding factors in breast magnetic resonance fingerprinting: , slice profile, and diffusion effects.

Magnetic resonance fingerprinting (MRF) offers rapid quantitative imaging but may be subject to confounding effects (CE) if these are not included in the model-based reconstruction. This study characterizes the influence of in-plane , slice profile and diffusion effects on T1 and T2 estimation in the female breast at 1.5T. Simulations were used to predict the influence of each CE on the accuracy of MRF and to investigate the influence of electronic noise and spiral aliasing artefacts. The experimentally observed bias in regions of fibroglandular tissue (FGT) and fatty tissue (FT) was analyzed for undersampled spiral breast MRF data of 6 healthy volunteers by performing MRF reconstruction with and without a CE. Theoretic analysis predicts T1 under-/T2 overestimation if the nominal flip angles are underestimated and inversely, T1 under-/T2 overestimation if omitting slice profile correction, and T1 under-/T2 underestimation if omitting diffusion in the signal model. Averaged over repeated signal simulations, including spiral aliasing artefacts affected precision more than accuracy. Strong in-plane effects occurred in vivo, causing T2 left-right inhomogeneity between both breasts. Their correction decreased the T2 difference from 29 to 5ms in FGT and from 29 to 9ms in FT. Slice profile correction affected FGT T2 most strongly, resulting in -22% smaller values. For the employed spoiler gradient strengths, diffusion did not affect the parameter maps, corresponding well with theoretic predictions. Understanding CEs and their relative significance for an MRF sequence is important when defining an MRF signal model for accurate parameter mapping.

Slice profile effects on quantitative analysis of hyperpolarized pyruvate.

Magnetic resonance imaging of hyperpolarized pyruvate provides a new imaging biomarker for cancer metabolism, based on the dynamic in vivo conversion of hyperpolarized pyruvate to lactate. Methods for quantification of signal evolution need to be robust and reproducible across a range of experimental conditions. Pharmacokinetic analysis of dynamic spectroscopic imaging data from hyperpolarized pyruvate and its metabolites generally assumes that signal arises from ideal rectangular slice excitation profiles. In this study, we examined whether this assumption could lead to bias in kinetic analysis of hyperpolarized pyruvate and, if so, whether such a bias can be corrected. A Bloch-McConnell simulator was used to generate synthetic data using a known set of "ground truth" pharmacokinetic parameter values. Signal evolution was then analyzed using analysis software that either assumed a uniform slice profile, or incorporated information about the slice profile into the analysis. To correct for slice profile effects, the expected slice profile was subdivided into multiple sub-slices to account for variable excitation angles along the slice dimension. An ensemble of sub-slices was then used to fit the measured signal evolution. A mismatch between slice profiles used for data acquisition and those assumed during kinetic analysis was identified as a source of quantification bias. Results indicate that imperfect slice profiles preferentially increase detected lactate signal, leading to an overestimation of the apparent metabolic exchange rate. The slice profile-correction algorithm was tested in simulation, in phantom measurements, and applied to data acquired from a patient with prostate cancer. The results demonstrated that slice profile-induced biases can be minimized by accounting for the slice profile during pharmacokinetic analysis. This algorithm can be used to correct data from either single or multislice acquisitions.

Slice-selective extended phase graphs in gradient-crushed, transient-state free precession sequences: An application to MR fingerprinting.

Slice-selective, gradient-crushed, transient-state sequences such as those used in MR fingerprinting (MRF) relaxometry are sensitive to slice profile effects. Whereas balanced steady-state free precession MRF profile effects have been studied, less attention has been given to gradient-crushed MRF forms. Extensions of the extended phase graph (EPG) formalism, called slice-selective EPG (ssEPG), are proposed that model slice profile effects. The hard-pulse approximation to slice-selective excitation in the spatial domain is reformulated in k-space. Excitation is modeled by standard EPG shift and transition operators. This ssEPG modeling is validated against Bloch simulations and phantom slice profile measurements. ssEPG relaxometry accuracy and variability are compared with other EPG methods in phantoms and human leg in vivo. The role of ∆B0 interactions with slice profile and gradient crushers is investigated. Simulations and slice profile measurements show that ssEPG can model highly dynamic slice profile effects of gradient-crushed sequences. The MRF ssEPG T2 estimates over 0 < T2 < 100 ms improve accuracy by > 10 ms at some values relative to other modeling approaches. Small deviations in B0 can produce substantial bias in T2 estimations from a range of MRF sequence types, and these effects can be modeled and understood by ssEPG. Transient-state, gradient-crushed sequences such as those used in MRF are sensitive to slice profile effects, and these effects depend on RF pulse choice, gradient crusher strength, and ∆B0 . It was found ssEPG was the most accurate EPG-based means to model these effects.

A time domain signal equation for multi-echo spin-echo sequences with arbitrary excitation and refocusing angle and phase.

Accurate T2 mapping using multi-echo spin-echo is usually impaired by non-ideal refocusing due to B1+ inhomogeneities and slice profile effects. Incomplete refocusing gives rise to stimulated echo and so called "T1-mixing" and consequently a non-exponential signal decay. Here we present a time domain formula that incorporates all relaxation and pulse parameters and enables accurate and realistic modelling of the magnetization decay curve. By pulse parameters here we specifically mean the actual refocusing angle and axis, and phase angle of both the excitation and refocusing pulse. The method used for derivation comprises the so called Generating functions approach with subsequent back-transformation to the time domain. The proposed approach was validated by simulations using realistic RF pulse shapes as well as by comparison to phantom measurements. Excellent agreement between simulations and measurements underpin the validity of the presented approach. Conclusively, we here present a complete time domain formula ready to use for accurate T2 mapping with multi-echo spin-echo sequences.

Magnetic Resonance Fingerprinting with short relaxation intervals

PurposeThe aim of this study was to investigate a technique for improving the performance of Magnetic Resonance Fingerprinting (MRF) in repetitive sampling schemes, in particular for 3D MRF acquisition, by shortening relaxation intervals between MRF pulse train repetitions. Material and methodsA calculation method for MRF dictionaries adapted to short relaxation intervals and non-relaxed initial spin states is presented, based on the concept of stationary fingerprints. The method is applicable to many different k-space sampling schemes in 2D and 3D. For accuracy analysis, T1 and T2 values of a phantom are determined by single-slice Cartesian MRF for different relaxation intervals and are compared with quantitative reference measurements. The relevance of slice profile effects is also investigated in this case. To further illustrate the capabilities of the method, an application to in-vivo spiral 3D MRF measurements is demonstrated. ResultsThe proposed computation method enables accurate parameter estimation even for the shortest relaxation intervals, as investigated for different sampling patterns in 2D and 3D. In 2D Cartesian measurements, we achieved a scan acceleration of more than a factor of two, while maintaining acceptable accuracy: The largest T1 values of a sample set deviated from their reference values by 0.3% (longest relaxation interval) and 2.4% (shortest relaxation interval). The largest T2 values showed systematic deviations of up to 10% for all relaxation intervals, which is discussed. The influence of slice profile effects for multislice acquisition is shown to become increasingly relevant for short relaxation intervals. In 3D spiral measurements, a scan time reduction of 36% was achieved, maintaining the quality of in-vivo T1 and T2 maps. ConclusionsReducing the relaxation interval between MRF sequence repetitions using stationary fingerprint dictionaries is a feasible method to improve the scan efficiency of MRF sequences. The method enables fast implementations of 3D spatially resolved MRF.

Effects of RF pulse profile and intra-voxel phase dispersion on MR fingerprinting with balanced SSFP readout

PurposeTo investigate possible errors in T1 and T2 quantification via MR fingerprinting with balanced steady-state free precession readout in the presence of intra-voxel phase dispersion and RF pulse profile imperfections, using computer simulations based on Bloch equations. Materials and methodsA pulse sequence with TR changing in a Perlin noise pattern and a nearly sinusoidal pattern of flip angle following an initial 180-degree inversion pulse was employed. Gaussian distributions of off-resonance frequency were assumed for intra-voxel phase dispersion effects. Slice profiles of sinc-shaped RF pulses were computed to investigate flip angle profile influences. Following identification of the best fit between the acquisition signals and those established in the dictionary based on known parameters, estimation errors were reported. In vivo experiments were performed at 3T to examine the results. ResultsSlight intra-voxel phase dispersion with standard deviations from 1 to 3Hz resulted in prominent T2 under-estimations, particularly at large T2 values. T1 and off-resonance frequencies were relatively unaffected. Slice profile imperfections led to under-estimations of T1, which became greater as regional off-resonance frequencies increased, but could be corrected by including slice profile effects in the dictionary. Results from brain imaging experiments in vivo agreed with the simulation results qualitatively. ConclusionMR fingerprinting using balanced SSFP readout in the presence of intra-voxel phase dispersion and imperfect slice profile leads to inaccuracies in quantitative estimations of the relaxation times.

RF slice profile effects in magnetic resonance fingerprinting

The radio frequency (RF) slice profile effects on T1 and T2 estimation in magnetic resonance fingerprinting (MRF) are investigated with respect to time-bandwidth product (TBW), flip angle (FA) level and field inhomogeneities. Signal evolutions are generated incorporating the non-ideal slice selective excitation process using Bloch simulation and matched to the original dictionary with and without the non-ideal slice profile taken into account. For validation, phantom and in vivo experiments are performed at 3T. Both simulations and experiments results show that T1 and T2 error from non-ideal slice profile increases with increasing FA level, off-resonance, and low TBW values. Therefore, RF slice profile effects should be compensated for accurate determination of the MR parameters.

Slice profile effects on nCPMG SS-FSE.

To determine the effects of the RF refocusing pulse profile on the magnitude of the transverse signal smoothness throughout the echo train in non-Carr-Purcell-Meiboom-Gill (nCPMG) single-shot fast spin echo (SS-FSE) imaging and to design an RF refocusing pulse that provides improved signal stability. THEORY AND METHODS: nCPMG SS-FSE quadratic phase modulation requires sufficiently high and uniform refocusing flip angle to achieve a stable signal. Typically, refocusing pulses used in SS-FSE sequences are designed for minimum duration to minimize echo spacing and as a consequence have poor selectivity. However, delay-insensitive variable rate excitation Shinnar-Le Roux (DV-SLR) refocusing pulses can achieve both improved selectivity as well as a short duration. This class of RF pulse is compared against a traditional low time-bandwidth refocusing pulse in a nCPMG SS-FSE in simulation, phantom, and in vivo. DV-SLR pulses achieve a more stable signal in simulation, phantom, and in vivo cases while maintaining an appropriately short duration as well as not dramatically increasing specific absorption rate (SAR) accumulation. The nCPMG SS-FSE method demonstrates improved robustness when a more selective refocusing pulse is used. Refocusing pulses that use a time-varying excitation gradient can achieve this selectivity while maintaining short echo spacing. Magn Reson Med 79:430-438, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Mis-estimation and bias of hyperpolarized apparent diffusion coefficient measurements due to slice profile effects.

The purpose of this work was to explore the impact of slice profile effects on apparent diffusion coefficient (ADC) mapping of hyperpolarized (HP) substrates. Slice profile effects were simulated using a Gaussian radiofrequency (RF) pulse with a variety of flip angle schedules and b-value ordering schemes. A long T1 water phantom was used to validate the simulation results, and ADC mapping of HP [13 C,15 N2 ]urea was performed on the murine liver to assess these effects in vivo. Slice profile effects result in excess signal after repeated RF pulses, causing bias in HP measurements. The largest error occurs for metabolites with small ADCs, resulting in up to 10-fold overestimation for metabolites that are in more-restricted environments. A mixed b-value scheme substantially reduces this bias, whereas scaling the slice-select gradient can mitigate it completely. In vivo, the liver ADC of hyperpolarized [13 C,15 N2 ]urea is nearly 70% lower (0.99 ± 0.22 vs 1.69 ± 0.21 × 10-3 mm2 /s) when slice-select gradient scaling is used. Slice profile effects can lead to bias in HP ADC measurements. A mixed b-value ordering scheme can reduce this bias compared to sequential b-value ordering. Slice-select gradient scaling can also correct for this deviation, minimizing bias and providing more-precise ADC measurements of HP substrates. Magn Reson Med 78:1087-1092, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Multislice T1 -prepared 2D single-shot EPI: analysis of a clinical T1 mapping method unbiased by B0 or B1 inhomogeneity.

Quantitative MR imaging is as sensitive in detecting lesions as qualitative imaging, but it is potentially more specific in differentiating disease. T1 mapping in particular might help to assess acute ischemic stroke, multiple sclerosis, epilepsy and Alzheimer's disease better. Thus, a rapid and robust clinical technique is vital. In 1990, Ordidge and colleagues developed the multislice T1 -prepared two-dimensional (2D) single-shot echo planar imaging technique. Subsequent studies demonstrated its clinical viability, but none performed an in-depth analysis of the strengths and advantages of this T1 mapping method. Herein, theoretical and experimental evidence shows that the technique accounts for 2D slice profile effects and is unbiased by B0 or B1 inhomogeneity. This is verified explicitly by varying the linear shims, the T1 preparation flip angle and the excitation flip angle. Furthermore, it is shown that the repetition time (and hence scan time) can be reduced without a loss of T1 accuracy. Copyright © 2016 John Wiley & Sons, Ltd.

MR fingerprinting with simultaneous B1 estimation.

PurposeMR fingerprinting (MRF) can be used for quantitative estimation of physical parameters in MRI. Here, we extend the method to incorporate B1 estimation.MethodsThe acquisition is based on steady state free precession MR fingerprinting with a Cartesian trajectory. To increase the sensitivity to the B1 profile, abrupt changes in flip angle were introduced in the sequence. Slice profile and B1 effects were included in the dictionary and the results from two‐ and three‐dimensional (3D) acquisitions were compared. Acceleration was demonstrated using retrospective undersampling in the phase encode directions of 3D data exploiting redundancy between MRF frames at the edges of k‐space.ResultsWithout B1 estimation, T2 and B1 were inaccurate by more than 20%. Abrupt changes in flip angle improved B1 maps. T1 and T2 values obtained with the new MRF methods agree with classical spin echo measurements and are independent of the B1 field profile. When using view sharing reconstruction, results remained accurate (error <10%) when sampling under 10% of k‐space from the 3D data.ConclusionThe methods demonstrated here can successfully measure T1, T2, and B1. Errors due to slice profile can be substantially reduced by including its effect in the dictionary or acquiring data in 3D. Magn Reson Med 76:1127–1135, 2016. © 2015 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.

Closed‐form solution for T2 mapping with nonideal refocusing of slice selective CPMG sequences

T2 quantification with multiecho sequences is typically impaired by the contribution of stimulated echoes to the echo decay due to B1 + inhomogeneity and slice profile effects. In this work, a compact signal model based on the generating functions approach, which accounts for both sources of error, is presented. The generating functions (GF) approach is used to obtain a closed solution to the evolution of the transverse magnetization in an echo train, however, not in the time domain, but in the transformed z-domain. The approach is generalized by the incorporation of flip angle distribution across the refocusing slice profiles. The approach is tested by fitting the model to simulated data as well as to phantom and in vivo measurements, followed by a comparison with the common monoexponential fitting approach. The fitting simulations indicate that T2 errors of up to 30% can be commonplace in a clinical setting using the monoexponential method. Conversely, the GF approach produced accurate results. Phantom and in vivo experiments showed a good agreement of the GF values with spectroscopic measurements and single-echo spin-echo sequences. A correction for stimulated echoes is necessary to compute comparable T2 values. The presented approach provides a solution to this issue.

Optimized in-phase and opposed-phase MR imaging for accurate detection of small fat or water fractions: theoretical considerations and experimental application in emulsions

To optimize strategies and measurement parameters for quantification of small fat and water fractions (<10%) in mixtures of both components by 4-point in-phase and opposed-phase gradient-echo imaging and to compare theoretical results with in-vitro experiments using emulsions. Theoretical analysis was based on steady-state signal equations for spoiled GRE-sequences and on relaxation properties of water and fat components. For quantification, signals were corrected for T2*-decay, T1-decay, and signal contributions from double bonds. Theoretical results were exemplarily compared to measurements at 1.5T on emulsions with either low water or fat fractions (0.5-10%) using spoiled 2D- and 3D-GRE-sequences. Excitation flip angle was varied in order to determine suitable values for sensitive detection of small fat/water fractions. Theoretical results and measurements correlated well, especially for 3D-sequences. Maximal sensitivity to a small signal fraction (S (fat) and S (water), respectively), was provided at the Ernst angle of the lower concentrated component. For 2D-sequences, the nominal flip angle had to be increased for compensation of slice profile effects and B(1) inhomogeneities. IP- and OP-echoes are recommended to be acquired in separate measurements with smallest possible receiver bandwidth to increase SNR/unit-time. Lowest detectable fat/water concentration in emulsions under typical conditions regarding spatial resolution and measuring time was approximately 1%. Using IP/OP-imaging with optimized parameters and post-processing, a sensitive and reliable detection of small fat/water fractions larger than 1% is possible in emulsions.

Slice profile correction for transmit sensitivity mapping using actual flip angle imaging

To enable clinical use of parallel transmission technology, it is necessary to rapidly produce transmit sensitivity (σ) maps. Actual flip angle imaging is an efficient mapping technique, which is accurate when used with 3D encoding and nonselective RF pulses. Mapping single slices is quicker, but 2D encoding leads to systematic errors due to slice profile effects. By simulating steady-state slice profiles, we computed the relationship between σ and the signals received from the actual flip angle imaging sequence for arbitrarily chosen slice selective RF pulses. Pulse specific lookup tables were then used for reconstruction. The resulting σ-maps are sensitive to T(1) in a manner that depends strongly on the specific pulse, for example a precision of ±3% can be achieved by using a 3-lobe sinc pulse. The method is applicable to any RF pulse; simulations must be performed once and thereafter fast reconstruction of σ-maps is possible.

Prediction of myocardial signal during CINE balanced SSFP imaging.

To develop a signal model for accurate prediction of myocardial signal during cine-balanced steady-state free precession (bSSFP) imaging. We present a signal model that takes into account the effects of non-ideal slice profile, off-resonance, and radio-frequency transmit variation on myocardial signal behavior. Each of the three factors was examined over the range of imaging parameters routinely used in cine bSSFP cardiac imaging at 3 Tesla. In five healthy volunteers and over a wide range of prescribed flip angles, the conventional on-resonance signal model exhibited 28.9 +/- 3.9% error, while the proposed model exhibited only 2.9 +/- 1.4% error, and therefore more accurate predictions of myocardial signal behavior. Slice profile effects were found to be significant and accounted for most of the improvement. Off-resonance and RF transmit inhomogeneity effects were less significant but did produce more accurate signal prediction. The proposed signal model produced more accurate predictions of myocardial signal compared to existing models and can be used for the optimization of pulse sequences and protocols.

Slice profile effects in 2D slice-selective MRI of hyperpolarized nuclei

This work explores slice profile effects in 2D slice-selective gradient-echo MRI of hyperpolarized nuclei. Two different sequences were investigated: a Spoiled Gradient Echo sequence with variable flip angle (SPGR-VFA) and a balanced Steady-State Free Precession (SSFP) sequence. It is shown that in SPGR-VFA the distribution of flip angles across the slice present in any realistically shaped radiofrequency (RF) pulse leads to large excess signal from the slice edges in later RF views, which results in an undesired non-constant total transverse magnetization, potentially exceeding the initial value by almost 300% for the last RF pulse. A method to reduce this unwanted effect is demonstrated, based on dynamic scaling of the slice selection gradient. SSFP sequences with small to moderate flip angles (<40 degrees ) are also shown to preserve the slice profile better than the most commonly used SPGR sequence with constant flip angle (SPGR-CFA). For higher flip angles, the slice profile in SSFP evolves in a manner similar to SPGR-CFA, with depletion of polarization in the center of the slice.

Off‐resonance‐dependent slice profile effects in balanced steady‐state free precession imaging

The purpose of this study was the simulation and measurement of balanced steady-state free precession (bSSFP) slice profiles for a detailed analysis of the influence of off-resonance effects on slice profile shape and bSSFP signal intensity. Due to the frequency response function of the bSSFP sequence, measurements that are not on-resonance result in broadened effective slice profiles with different off-resonance-dependent shapes and signal intensities. In this study, bSSFP slice profile effects and their dependence on off-resonance were investigated based on bSSFP signal simulations of phantom data as well as blood and tissue. For a better assessment of the similarity of measured and simulated slice profiles the field map was integrated in the slice profile simulations. The results demonstrate that simulations can accurately predict bSSFP slice profiles. Both measurements and simulations indicate that there is a substantial increase in signal intensity close to the banding artifacts, i.e., at spatial locations with off-resonance frequencies corresponding to a dephasing/TR = +/- pi resulting in signal void (bands). For routine bSSFP imaging, off-resonance-dependent slice broadening may thus result in a substantial difference between nominal and true slice thickness and lead to spatially varying slice thickness and signal intensities across the imaging slice.