Motion-compensated diffusion encoding in multi-shot human brain acquisitions: Insights using high-performance gradients.

PURPOSE: To evaluate the utility of up to second-order motion-compensated diffusion encoding in multi-shot human brain acquisitions. METHODS: Experiments were performed with high-performance gradients using three forms of diffusion encoding motion-compensated through different orders: conventional zeroth-order-compensated pulsed gradients (PG), first-order-compensated gradients (MC1), and second-order-compensated gradients (MC2). Single-shot acquisitions were conducted to correlate the order of motion compensation with resultant phase variability. Then, multi-shot acquisitions were performed at varying interleaving factors. Multi-shot images were reconstructed using three levels of shot-to-shot phase correction: no correction, channel-wise phase correction based on FID navigation, and correction based on explicit phase mapping (MUSE). RESULTS: In single-shot acquisitions, MC2 diffusion encoding most effectively suppressed phase variability and sensitivity to brain pulsation, yielding residual variations of about 10° and of low spatial order. Consequently, multi-shot MC2 images were largely satisfactory without phase correction and consistently improved with the navigator correction, which yielded repeatable high-quality images; contrarily, PG and MC1 images were inadequately corrected using the navigator approach. With respect to MUSE reconstructions, the MC2 navigator-corrected images were in close agreement for a standard interleaving factor and considerably more reliable for higher interleaving factors, for which MUSE images were corrupted. Finally, owing to the advanced gradient hardware, the relative SNR penalty of motion-compensated diffusion sensitization was substantially more tolerable than that faced previously. CONCLUSION: Second-order motion-compensated diffusion encoding mitigates and simplifies shot-to-shot phase variability in the human brain, rendering the multi-shot acquisition strategy an effective means to circumvent limitations of retrospective phase correction methods.

Fourier transform temporal diffusion spectroscopy.

Temporal diffusion spectroscopy (TDS) currently uses the oscillating gradient spin echo (OGSE) experiment to measure the spectral density of translational velocity autocorrelation at single frequencies. Due to timing restrictions imposed by the transverse relaxation, the frequency selectivity and the sampling density of OGSE are limited, especially at low frequencies. We propose to overcome this problem by adopting the principles of Fourier transform spectroscopy. The new method of Fourier transform TDS (FTDS) uses two broadband gradient waveforms with different relative delays to make the spin echo attenuation sensitive to a broad range of diffusion frequencies with different harmonic modulations and calculates the spectrum by discrete Fourier transform. The method was validated by a measurement of diffusion spectra in highly restrictive tissues of a celery stalk and provided results consistent with OGSE, however, on a denser frequency grid.

Evaluating diffusion dispersion across an extended range of b-values and frequencies: Exploiting gap-filled OGSE shapes, strong gradients, and spiral readouts.

PURPOSE: To address the long echo times and relatively weak diffusion sensitization that typically limit oscillating gradient spin-echo (OGSE) experiments, an OGSE implementation combining spiral readouts, gap-filled oscillating gradient shapes providing stronger diffusion encoding, and a high-performance gradient system is developed here and utilized to investigate the tradeoff between b-value and maximum OGSE frequency in measurements of diffusion dispersion (i.e., the frequency dependence of diffusivity) in the in vivo human brain. In addition, to assess the effects of the marginal flow sensitivity introduced by these OGSE waveforms, flow-compensated variants are devised for experimental comparison. METHODS: Using DTI sequences, OGSE acquisitions were performed on three volunteers at b-values of 300, 500, and 1000 s/mm2 and frequencies up to 125, 100, and 75 Hz, respectively; scans were performed for gap-filled oscillating gradient shapes with and without flow sensitivity. Pulsed gradient spin-echo DTI acquisitions were also performed at each b-value. Upon reconstruction, mean diffusivity (MD) maps and maps of the diffusion dispersion rate were computed. RESULTS: The power law diffusion dispersion model was found to fit best to MD measurements acquired at b = 1000 s/mm2 despite the associated reduction of the spectral range; this observation was consistent with Monte Carlo simulations. Furthermore, diffusion dispersion rates without flow sensitivity were slightly higher than flow-sensitive measurements. CONCLUSION: The presented OGSE implementation provided an improved depiction of diffusion dispersion and demonstrated the advantages of measuring dispersion at higher b-values rather than higher frequencies within the regimes employed in this study.

Diffusion-weighted imaging and diffusion tensor imaging of the heart in vivo: major developments.

Diffusion-weighted magnetic resonance imaging (DWI) is a powerful diagnostic tool. Contrast in DWI images is dictated by the differences in diffusion of water in tissues, which depends on the tissue type, hydration and fluid composition. Therefore DWI can differentiate between hard and soft tissues, as well as visualize their condition, such as edema, necrosis or fibrosis. Diffusion tensor imaging (DTI) is a DWI technique which additionally delivers information about the microstructure. In cardiovascular applications DWI/DTI can non-invasively characterize the acute to chronic phase of the area at risk and microstructural dynamics without the need to use contrast agents. However, cardiac DWI/DTI differs from other applications due to serious anatomic and technologic challenges. Over the years, scientists have stepped up overcoming more and more advanced obstacles associated with complex 3D myocardial motions, breathing, blood flow and perfusion. The aim of this article is to review milestone technologic advances in DWI/DTI of the heart in vivo. The discussed development begins with the adjustment of the diffusion imaging block to the electrocardiogram-based most quiescent phase, next considers different pulse sequence designs for first-, second- and higher-order motion compensation and SNR improvement, and ends up with prospects for further developments. Reviewed papers show great progress in this research area, but the gap between the scientific development and common clinical practice is tremendous. Cardiac DWI/DTI has promising clinical relevance and its addition to routine imaging techniques of patients with heart disease may empower clinical diagnosis.

Interoception of breathing and its relationship with anxiety.

Interoception, the perception of internal bodily states, is thought to be inextricably linked to affective qualities such as anxiety. Although interoception spans sensory to metacognitive processing, it is not clear whether anxiety is differentially related to these processing levels. Here we investigated this question in the domain of breathing, using computational modeling and high-field (7 T) fMRI to assess brain activity relating to dynamic changes in inspiratory resistance of varying predictability. Notably, the anterior insula was associated with both breathing-related prediction certainty and prediction errors, suggesting an important role in representing and updating models of the body. Individuals with low versus moderate anxiety traits showed differential anterior insula activity for prediction certainty. Multi-modal analyses of data from fMRI, computational assessments of breathing-related metacognition, and questionnaires demonstrated that anxiety-interoception links span all levels from perceptual sensitivity to metacognition, with strong effects seen at higher levels of interoceptive processes.

Improved gradient waveforms for oscillating gradient spin-echo (OGSE) diffusion tensor imaging.

The dependence of the diffusion tensor on frequency is of great interest in studies of tissue microstructure because it reveals restrictions to the Brownian motion of water molecules caused by cell membranes. Oscillating gradient spin-echo (OGSE) sequences can sample this dependence with gradient shapes for which the power spectrum of the zeroth moment is focused at a target frequency. In order to maintain the total spectral power (ie the b-value), oscillating gradient amplitudes must grow with the frequency squared. For this reason, OGSE applications on clinical MRI scanners are limited to low frequencies, for which it is difficult to obtain a narrow frequency bandwidth of the gradient moment in a useful echo time. In particular, the commonly used pair of single-period trapezoidal-cosine pulses separated by a half-period produces significant side lobes away from the target frequency. To mitigate this effect, improved OGSE waveforms are proposed, which reduce the gap between the two gradient pulses to the minimum duration required for the refocusing RF pulse. Additionally, a slight deviation from the periodicity of the waveforms is proposed in order to permit using the maximum slew rate of the gradient system for all lobes of trapezoidal waveforms while maintaining advantageous spectral properties, which is not the case for the currently used OGSE sequences. Numerical calculations validate these changes, showing that both modifications significantly narrow the gradient moment power spectrum and increase the contribution of its main lobe to the b-value, thus improving the specificity of the measurement. The utility of the new shapes is demonstrated by diffusion tensor measurements of human white matter in vivo over the range of 30-75 Hz with a b-value of nearly 1000 s/mm2 , using a high-performance gradient insert. However, the improvement should increase the sampling precision of OGSE experiments for all gradient systems.

T-Hex: Tilted hexagonal grids for rapid 3D imaging.

PURPOSE: The purpose of this work is to devise and demonstrate an encoding strategy for 3D MRI that reconciles high speed with flexible segmentation, uniform k-space density, and benign T2∗ effects. METHODS: Fast sampling of a 3D k-space is typically accomplished by 2D readouts per shot using EPI trains or spiral readouts. Tilted hexagonal (T-Hex) sampling is a way of acquiring more k-space volume per excitation while maintaining uniform sampling density and a smooth T2∗ filter. The k-space volume covered per shot is controlled by the tilting angle. Image reconstruction is performed with a 3D extension of the iterative SENSE approach, incorporating actual field dynamics and static off-resonance. T-Hex imaging is compared with established 3D schemes in terms of speed and noise performance. RESULTS: Tilted hexagonal acquisition is found to achieve greater imaging speed than known alternatives, particularly in combination with spiral trajectories. The interplay of the proposed 3D trajectories, array detection, and off-resonance is successfully addressed by iterative inversion of the full signal model. Enhanced coverage per shot is of greatest utility for high speed in an intermediate resolution regime of 1 to 4 mm. T-Hex EPI combines the benefits of extended coverage per shot with increased robustness against off-resonance effects. CONCLUSION: Sampling of tilted hexagonal grids is a feasible means of gaining 3D imaging speed with near-optimal SNR efficiency and benign depiction properties. It is a particularly promising technique for time-resolved applications such as fMRI.

Minimizing the echo time in diffusion imaging using spiral readouts and a head gradient system.

PURPOSE: Diffusion weighted imaging (DWI) is commonly limited by low signal-to-noise ratio (SNR) as well as motion artifacts. To address this limitation, a method that allows to maximize the achievable signal yield and increase the resolution in motion robust single-shot DWI is presented. METHODS: DWI was performed on a 3T scanner equipped with a recently developed gradient insert (gradient strength: 200 mT/m, slew rate: 600 T/m/s). To further shorten the echo time, Stejskal-Tanner diffusion encoding with a single-shot spiral readout was implemented. To allow non-Cartesian image reconstruction using such strong and fast gradients, the characterization of eddy current and concomitant field effects was performed based on field-camera measurements. RESULTS: An echo time of only 19 ms was achieved for a b-factor of 1000 s/mm2 . An in-plane resolution of 0.68 mm was encoded by a single-shot spiral readout of 40.5 ms using 3-fold undersampling. The resulting images did not suffer from off-resonance artifacts and T 2 ∗ blurring that are common to single-shot images acquired with regular gradient systems. CONCLUSION: Spiral diffusion imaging using a head gradient system, together with an accurate characterization of the encoding process allows for a substantial reduction of the echo time, and improves the achievable resolution in motion-insensitive single-shot DWI.

A transmit-receive array for brain imaging with a high-performance gradient insert.

PURPOSE: The aim of this work was to provide parallel imaging capability for the human head in a gradient insert of 33-cm inner diameter within the related constraints of space, encoding ambiguity, and eddy current immunity. METHODS: Eddy current behavior of the 8-channel transmit-receive array coil was investigated via heating and field deviation measurements. RF performance was evaluated using S-parameters, noise statistics, B1 maps, and g-factor maps. In vivo images of a human head and knee were acquired with Cartesian readout and TE below 0.45 ms. RESULTS: Under intense gradient use, the shield was heated up to 55°C and other coil structures to 40°C. After standard preemphasis calibration, eddy current-related field distortions caused by the developed RF coil were smaller than for a commercial receive-only coil. In the ambiguous regions of the gradient, B1+ is 20 dB lower than in the center of the FOV. Coupling between elements is below -15 dB, and noise correlation is less than 0.31 when the coil is loaded with a head. Power efficiency was 0.52 ± 0.02 µT/√W, and the SD of the flip angle was below 10% in central slices of the brain. 2D, up to fourfold acceleration causes less than 30% noise amplification. The RF coil can be used during full gradient performance. CONCLUSION: Based on the described features, the presented coil enables parallel imaging in the high-performance gradient insert.

Echo-planar imaging of the human head with 100 mT/m gradients and high-order modeling of eddy current fields.

PURPOSE: To demonstrate the utility of a high-performance gradient insert for ultrafast MRI of the human head. METHODS: EPI was used for the first time with a readout gradient amplitude of 100 mT/m, 1200 T/m/s slew rate, and nearly 1 MHz signal bandwidth for human head scanning. To avoid artefacts due to eddy currents, the magnetic field was dynamically monitored with NMR probes at multiple points, modeled by solid harmonics up to fifth order, and included in the image reconstruction. An approximation of a negligible intra-echo effect of the eddy currents was made to accelerate the high-order reconstruction. The field monitoring-based approach was compared with a recently proposed phase error estimation from separate reconstructions of even and odd echoes. RESULTS: Images obtained with the gradient insert have significantly lower distortions than it is the case with the whole body 30 mT/m, 200 T/m/s gradients of the same system. However, eddy currents of high spatial order must be properly characterized and corrected for in order to avoid a persistent 2D Nyquist ghost. Multi-position monitoring proves to be a robust method to measure the eddy currents and allows higher undersampling rates than the image-based approach. The proposed approximation of the eddy currents effect allows a significant acceleration of the high-order reconstruction by a separate processing of each spatial dimension. CONCLUSION: Strong gradients with adequate switching rates are highly beneficial for the quality of EPI provided that robust measures are taken to include the contribution of eddy currents to the image encoding.

Low-distortion diffusion tensor MRI with improved phaseless encoding.

Due to the motion-related instability of the signal phase, diffusion MRI is usually performed with single-shot techniques such as the echo-planar imaging (EPI), which are resolution-limited and suffer from distortions caused by resonance offsets. Multi-shot methods may improve the images but require time-consuming navigators or a trade-off of the sensitivity encoding to measure shot-dependent phase errors. We have recently introduced an alternative approach to multi-shot MRI called phaseless encoding, which, by analogy to optical super-resolution methods, relies on the magnitude value of images taken in different shots thus discarding the phase error without navigators, and demonstrated its capability to perform diffusion MRI at sub-millimeter scale on a standard 3T scanner. In this work, we apply phaseless encoding in a routine diffusion tensor imaging (DTI) protocol with a moderately high resolution that is still within reach of single-shot EPI with the same hardware, and compare both techniques with respect to image distortions. A qualitative comparison of the phaseless encoding with the established navigator-based readout-segmented EPI is also presented. Several technical improvements are proposed to make phaseless encoding compatible with the routine scanning mode. The tagging radiofrequency pulses used in the encoding sequence are made slice-selective to avoid artefacts caused by saturation effects in multi-slice scans and their flip angle is optimized to reduce the intrinsic SNR loss. The super-resolution reconstruction algorithm is also improved to better suppress Gibbs ringing and to correct for possible signal amplitude fluctuations. Our study shows that the phaseless encoding is a promising approach to diffusion weighted imaging. It can easily be implemented in multi-slice sequences and produces less distorted images than the single-shot EPI at the same resolution and hardware parameters. It provides similar results to readout-segmented EPI but without the need of navigators.

In-plane "superresolution" MRI with phaseless sub-pixel encoding.

PURPOSE: Acquisition of high-resolution imaging data using multiple excitations without the sensitivity to fluctuations of the transverse magnetization phase, which is a major problem of multi-shot MRI. THEORY AND METHODS: The concept of superresolution MRI based on microscopic tagging is analyzed using an analogy with the optical method of structured illumination. Sinusoidal tagging is shown to provide subpixel resolution by mixing of neighboring spatial frequency (k-space) bands. It represents a phaseless modulation added on top of the standard Fourier encoding, which allows the phase fluctuations to be discarded at an intermediate reconstruction step. Improvements are proposed to correct for tag distortions due to magnetic field inhomogeneity and to avoid the propagation of Gibbs ringing from intermediate low-resolution images to the final image. The method was applied to diffusion-weighted EPI. RESULTS: Artifact-free superresolution images can be obtained despite a finite duration of the tagging sequence and related pattern distortions by a field map based phase correction of band-wise reconstructed images. The ringing effect present in the intermediate images can be suppressed by partial overlapping of the mixed k-space bands in combination with an adapted filter. High-resolution diffusion-weighted images of the human head were obtained with a three-shot EPI sequence despite motion-related phase fluctuations between the shots. CONCLUSION: Due to its phaseless character, tagging-based sub-pixel encoding is an alternative to k-space segmenting in the presence of unknown phase fluctuations, in particular those due to motion under strong diffusion gradients. Proposed improvements render the method practicable in realistic conditions.

A high-performance gradient insert for rapid and short-T2 imaging at full duty cycle.

PURPOSE: The goal of this study was to devise a gradient system for MRI in humans that reconciles cutting-edge gradient strength with rapid switching and brings up the duty cycle to 100% at full continuous amplitude. Aiming to advance neuroimaging and short-T2 techniques, the hardware design focused on the head and the extremities as target anatomies. METHODS: A boundary element method with minimization of power dissipation and stored magnetic energy was used to design anatomy-targeted gradient coils with maximally relaxed geometry constraints. The design relies on hollow conductors for high-performance cooling and split coils to enable dual-mode gradient amplifier operation. With this approach, strength and slew rate specifications of either 100 mT/m with 1200 mT/m/ms or 200 mT/m with 600 mT/m/ms were reached at 100% duty cycle, assuming a standard gradient amplifier and cooling unit. RESULTS: After manufacturing, the specified values for maximum gradient strength, maximum switching rate, and field geometry were verified experimentally. In temperature measurements, maximum local values of 63°C were observed, confirming that the device can be operated continuously at full amplitude. Testing for peripheral nerve stimulation showed nearly unrestricted applicability in humans at full gradient performance. In measurements of acoustic noise, a maximum average sound pressure level of 132 dB(A) was determined. In vivo capability was demonstrated by head and knee imaging. Full gradient performance was employed with echo planar and zero echo time readouts. CONCLUSION: Combining extreme gradient strength and switching speed without duty cycle limitations, the described system offers unprecedented options for rapid and short-T2 imaging. Magn Reson Med 79:3256-3266, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

MRI with phaseless encoding.

PURPOSE: Fourier encoded MRI signal is complex and, therefore, sensitive to uncontrolled phase variations caused, e.g., by object motion. An alternative encoding is proposed which leads to phaseless (positive real) signals and allows the phase fluctuations to be removed by simple magnitude calculation before the Fourier transform. THEORY AND METHODS: Phaseless encoding uses harmonic modulation of the longitudinal magnetization with different frequencies and phases before excitation. It can be combined with Fourier encoding of complementary dimensions to produce, e.g., a 3D version of echo planar imaging insensitive to intershot phase variations. It can also be mixed with Fourier encoding of the same dimension allowing a high-resolution image to be obtained from magnitude-reconstructed low-resolution components. The latter is a generalization of the super-resolution MRI with microscopic tagging proposed recently. Improved reconstruction for this technique was adopted from its optical analogue, harmonic excitation light microscopy (HELM). RESULTS: Artifact free images were obtained despite phase fluctuations caused by random receiver reference and object motion during diffusion weighting. Proposed reconstruction of mixed-encoded data reaches higher resolution than the original super-resolution method. CONCLUSION: Spatial information can be encoded in the magnitude of the MR signal rendering the experiment insensitive to phase fluctuations. Magn Reson Med 78:1029-1037, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

SENSE reconstruction for multiband EPI including slice-dependent N/2 ghost correction.

PURPOSE: Sensitivity encoding (SENSE) reconstruction of multiband echo planar imaging (EPI) may cause artifacts when simultaneously excited slices require different phase correction to remove the EPI-specific ghost shifted by half of the matrix size (N). We propose a simplified solution of this problem that combines SENSE unfolding with the EPI phase correction in the image domain. THEORY AND METHODS: Slice-dependent phase correction was included in equations linking folded slice images reconstructed separately from even and odd echoes of all receivers with the true images of each slice. Compared with the previously proposed combination of ghost suppression with SENSE based on a direct image fit to echo data, our method reduces the problem complexity by N(2) /4. It was applied to reconstruct images of phantoms and human brain. RESULTS: The proposed method tolerates high differences of phase correction between slices, which may result, e.g., from anisotropic gradient delay. It suppresses artifacts better than standard SENSE even when the latter is repeated with the ghost correction targeting each of the slices and works significantly faster than the direct fit version of ghost-correcting SENSE. CONCLUSION: With the proposed modification SENSE allows a rapid separation of slices simultaneously acquired with EPI even when the phase correction needed for each slice is different. Magn Reson Med 76:873-879, 2016. © 2015 Wiley Periodicals, Inc.

ZTE imaging with enhanced flip angle using modulated excitation.

PURPOSE: Zero echo time (ZTE) imaging is a fast, robust, and silent three-dimensional technique for direct MRI of tissues with rapid transverse relaxation. It is conventionally performed with hard, block-shaped excitation pulses short enough to excite spins uniformly over a large bandwidth. With this approach, the achievable flip angle (FA) is limited by the available B1 amplitude. The purpose of this work is to accomplish ZTE imaging with larger FAs by combined amplitude and frequency modulation of the excitation pulse while keeping the pulse duration short enough to limit acquisition dead time. METHODS: Quantitative performance criteria for FA yield and uniformity of radio frequency (RF) pulses were developed and used to optimize hyperbolic secant pulse shapes. The RF pulses were implemented on a 4.7 T animal MRI system, included in algebraic image reconstruction, and tested in experiments on phantoms and tissue samples. RESULTS: The optimized modulated pulses provide considerably improved performance with respect to uniformity and mean FA as compared with block-shaped counterparts of the same maximum length. Using these pulses, ZTE images of excellent uniformity were obtained with enhanced FA and thus expanded contrast versatility. CONCLUSION: The performance of ZTE imaging can be significantly improved by employing optimized short amplitude- and frequency-modulated RF pulses.

Radial spectroscopic MRI of hyperpolarized [1-(13) C] pyruvate at 7 tesla.

PURPOSE: The transient and nonrenewable signal from hyperpolarized metabolites necessitates extensive sequence optimization for encoding spatial, spectral, and dynamic information. In this work, we evaluate the utility of radial single-timepoint and cumulative spectroscopic MRI of hyperpolarized [1-(13) C] pyruvate and its metabolic products at 7 Tesla (T). METHODS: Simulations of radial echo planar spectroscopic imaging (EPSI) and multiband frequency encoding (MBFE) acquisitions were performed to confirm feasibility and evaluate performance for HP (13) C imaging. Corresponding sequences were implemented on a 7T small-animal MRI system, tested in phantom, and demonstrated in a murine model of anaplastic thyroid cancer. RESULTS: MBFE provides excellent spectral separation but is susceptible to blurring and T2 * signal loss inherent to using low readout gradients. The higher readout gradients and more flexible spectral encoding for EPSI result in good spatial resolution and spectral separation. Radial acquisition throughout HP signal evolution offers the flexibility for reconstructing spatial maps of mean metabolite distribution and global dynamic time courses of multiple metabolites. CONCLUSION: Radial EPSI and MBFE acquisitions are well-suited for hyperpolarized (13) C MRI over short and long durations. Advantages to this approach include robustness to nonstationary magnetization, insensitivity to precise acquisition timing, and versatility for reconstructing dynamically acquired spectroscopic data.

High-resolution ZTE imaging of human teeth.

MRI with zero echo time (ZTE) is achieved by 3D radial centre-out encoding and hard-pulse RF excitation while the projection gradient is already on. Targeting short-T(2) samples, the efficient, robust and silent ZTE approach was implemented for high-bandwidth high-resolution imaging requiring particularly rapid transmit-receive switching and algebraic image reconstruction. The ZTE technique was applied to image extracted human teeth at 11.7T field strength, yielding detailed depictions with very good delineation of the mineralised dentine and enamel layers. ZTE results are compared with UTE (ultra-short echo time) MRI and micro-computed tomography (µCT), revealing significant differences in SNR and CNR yields. Compared to µCT, ZTE MRI appears to be less susceptible to artefacts caused by dental fillings and to offer superior sensitivity for the detection of early demineralisation and caries lesions.

MRI with zero echo time: hard versus sweep pulse excitation.

Zero echo time can be obtained in MRI by performing radiofrequency (RF) excitation as well as acquisition in the presence of a constant gradient applied for purely frequency-encoded, radial centre-out k-space encoding. In this approach, the spatially nonselective excitation must uniformly cover the full frequency bandwidth spanned by the readout gradient. This can be accomplished either by short, hard RF pulses or by pulses with a frequency sweep as used in the SWIFT (Sweep imaging with Fourier transform) method for improved performance at limited RF amplitudes. In this work, the two options are compared with respect to T2 sensitivity, signal-to-noise ratio (SNR), and SNR efficiency. In particular, the SNR implications of sweep excitation and of initial or periodical acquisition gaps required for transmit-receive switching are investigated. It was found by simulations and experiments that, whereas equivalent in terms of T2 sensitivity, the two techniques differ in SNR performance. With ideal, ungapped simultaneous excitation and acquisition, the sweep approach would yield higher SNR throughout due to larger feasible flip angles. However, acquisition gapping is found to take a significant SNR toll related to a reduced acquisition duty cycle, rendering hard pulse excitation superior for sufficient RF amplitude and also in the short-T2 limit.

Sweep MRI with algebraic reconstruction.

In the recently proposed technique Sweep Imaging with Fourier Transform (SWIFT), frequency-modulated radiofrequency pulses are used in concert with simultaneous acquisition to facilitate MRI of samples with very short transverse relaxation time. In the present work, sweep MRI is reviewed from a reconstruction perspective and several extensions and modifications of the current methodology are proposed. An algorithm for algebraic image reconstruction is derived from a comprehensive description of signal formation, including interleaved radiofrequency transmission and acquisition of arbitrary timing as well as the relevant filtering and decimation steps along the receiver chain. The new reconstruction approach readily permits several measures of optimising the signal sampling strategy as demonstrated in simulations and imaging experiments. Employing a variety of radiofrequency pulse envelopes, water and rubber phantoms as well as bone samples with transverse relaxation time in the order of 500 µsec were imaged at signal bandwidths of up to 96 kHz.