Gradient-modulated SWIFT.

PURPOSE: Methods designed to image fast-relaxing spins, such as sweep imaging with Fourier transformation (SWIFT), often utilize high excitation bandwidth and duty cycle, and in some applications the optimal flip angle cannot be used without exceeding safe specific absorption rate (SAR) levels. The aim is to reduce SAR and increase the flexibility of SWIFT by applying time-varying gradient-modulation (GM). The modified sequence is called GM-SWIFT. THEORY AND METHODS: The method known as gradient-modulated offset independent adiabaticity was used to modulate the radiofrequency (RF) pulse and gradients. An expanded correlation algorithm was developed for GM-SWIFT to correct the phase and scale effects. Simulations and phantom and in vivo human experiments were performed to verify the correlation algorithm and to evaluate imaging performance. RESULTS: GM-SWIFT reduces SAR, RF amplitude, and acquisition time by up to 90%, 70%, and 45%, respectively, while maintaining image quality. The choice of GM parameter influences the lower limit of short T2 (*) sensitivity, which can be exploited to suppress unwanted image haze from unresolvable ultrashort T2 (*) signals originating from plastic materials in the coil housing and fixatives. CONCLUSIONS: GM-SWIFT reduces peak and total RF power requirements and provides additional flexibility for optimizing SAR, RF amplitude, scan time, and image quality.

High-spatial- and high-temporal-resolution dynamic contrast-enhanced MR breast imaging with sweep imaging with Fourier transformation: a pilot study.

PURPOSE: To report the results of sweep imaging with Fourier transformation (SWIFT) magnetic resonance (MR) imaging for diagnostic breast imaging. MATERIALS AND METHODS: Informed consent was obtained from all participants under one of two institutional review board-approved, HIPAA-compliant protocols. Twelve female patients (age range, 19-54 years; mean age, 41.2 years) and eight normal control subjects (age range, 22-56 years; mean age, 43.2 years) enrolled and completed the study from January 28, 2011, to March 5, 2013. Patients had previous lesions that were Breast Imaging Reporting and Data System 4 and 5 based on mammography and/or ultrasonographic imaging. Contrast-enhanced SWIFT imaging was completed by using a 4-T research MR imaging system. Noncontrast studies were completed in the normal control subjects. One of two sized single-breast SWIFT-compatible transceiver coils was used for nine patients and five controls. Three patients and five control subjects used a SWIFT-compatible dual breast coil. Temporal resolution was 5.9-7.5 seconds. Spatial resolution was 1.00 mm isotropic, with later examinations at 0.67 mm isotropic, and dual breast at 1.00 mm or 0.75 mm isotropic resolution. RESULTS: Two nonblinded breast radiologists reported SWIFT image findings of normal breast tissue, benign fibroadenomas (six of six lesions), and malignant lesions (10 of 12 lesions) concordant with other imaging modalities and pathologic reports. Two lesions in two patients were not visualized because of coil field of view. The images yielded by SWIFT showed the presence and extent of known breast lesions. CONCLUSION: The SWIFT technique could become an important addition to breast imaging modalities because it provides high spatial resolution at all points during the dynamic contrast-enhanced examination.

Gap cycling for SWIFT.

PURPOSE: SWIFT (SWeep Imaging with Fourier Transformation) is a non-Cartesian MRI method with unique features and capabilities. In SWIFT, radiofrequency (RF) excitation and reception are performed nearly simultaneously, by rapidly switching between transmit and receive during a frequency-swept RF pulse. Because both the transmitted pulse and data acquisition are simultaneously amplitude-modulated in SWIFT (in contrast to continuous RF excitation and uninterrupted data acquisition in more familiar MRI sequences), crosstalk between different frequency bands occurs in the data. This crosstalk leads to a "bulls-eye" artifact in SWIFT images. We present a method to cancel this interband crosstalk by cycling the pulse and receive gap positions relative to the un-gapped pulse shape. We call this strategy "gap cycling." THEORY AND METHODS: We carry out theoretical analysis, simulation and experiments to characterize the signal chain, resulting artifacts, and their elimination for SWIFT. RESULTS: Theoretical analysis reveals the mechanism for gap-cycling's effectiveness in canceling interband crosstalk in the received data. We show phantom and in vivo results demonstrating bulls-eye artifact free images. CONCLUSION: Gap cycling is an effective method to remove bulls-eye artifact resulting from interband crosstalk in SWIFT data.

Measurement of T1 relaxation time of osteochondral specimens using VFA-SWIFT.

PURPOSE: To evaluate the feasibility of SWIFT with variable flip angle (VFA) for measurement of T1 relaxation time in Gd-agarose-phantoms and osteochondral specimens, including regions of very short T2 *, and compare with T1 measured using standard methods METHODS: T1 s of agarose phantoms with variable concentration of Gd-DTPA2- and nine pairs of native and trypsin-treated bovine cartilage-bone specimens were measured. For specimens, VFA-SWIFT, inversion recovery (IR) fast spin echo (FSE) and saturation recovery FSE were used. For phantoms, additionally spectroscopic IR was used. Differences and agreement between the methods were assessed using nonparametric Wilcoxon and Kruskal-Wallis tests and intraclass correlation. RESULTS: The different T1 mapping methods agreed well in the phantoms. VFA-SWIFT allowed reliable measurement of T1 in the osteochondral specimens, including regions where FSE-based methods failed. The T1 s measured by VFA-SWIFT were shifted toward shorter values in specimens. However, the measurements correlated significantly (highest correlation VFA-SWIFT versus FSE was r = 0.966). SNR efficiency was generally highest for SWIFT, especially in the subchondral bone. CONCLUSION: Feasibility of measuring T1 relaxation time using VFA-SWIFT in osteochondral specimens and phantoms was demonstrated. A shift toward shorter T1 s was observed for VFA-SWIFT in specimens, reflecting the higher sensitivity of SWIFT to short T2 * spins. Magn Reson Med 74:175-184, 2015. © 2014 Wiley Periodicals, Inc.

Intraoral approach for imaging teeth using the transverse B1 field components of an occlusally oriented loop coil.

PURPOSE: The signal-to-noise ratio and resolution are two competing parameters for dental MRI and are highly dependent on the radiofrequency coil configuration and performance. The purpose of this work is to describe an intraoral approach for imaging teeth with the radiofrequency coil plane oriented orthogonally to the Zeeman field to use the transverse components of the B1 field for transmitting and receiving the NMR signal. METHODS: A single loop coil with shape and size fitted to the average adult maxillary arch was built and tested with a phantom and human subjects in vivo on a whole-body 4 T MRI scanner. Supporting Biot-Savart law simulations were performed with Matlab. RESULTS: In the occlusal position (in bite plane between the upper and lower teeth), the sensitive volume of the coil encompasses the most important dental structures, the teeth and their supporting structures, while uninteresting tissues containing much higher proton density (cheeks, lips, and tongue) are outside the sensitive volume. The presented images and simulated data show the advantages of using a coil in the orthogonal orientation for dental applications. CONCLUSION: The transverse components of the B1 field of a surface coil can effectively be used for imaging of teeth and associated structures.

MRI by steering resonance through space.

PURPOSE: This work introduces a technique to excite MR signals locally and to steer this localized region over the object in a spatiotemporal manner. The purpose is to demonstrate the feasibility of MRI with multidimensional spatiotemporal-encoding in a way that provides the ability to compensate extreme field inhomogeneity. METHODS: The method is called steering resonance over the object (STEREO). A modulated gradient is applied in concert with a frequency-modulated pulse to steer a resonant region through space and thus produce sequential excitation and echo formation. Images are reconstructed using exclusively an inverse problem solution. RESULTS: Images of phantoms and human brain were produced to demonstrate the feasibility of the STEREO sequence and image reconstruction. Simulations support the postulated capability to compensate for extreme field inhomogeneity. CONCLUSION: STEREO represents a substantial departure from conventional MRI in which spins contained in the sample, slab, or slice are excited synchronously. By exciting spins sequentially along a curved spatial trajectory, STEREO in principle affords a unique opportunity to adjust for spatial variations in static and radiofrequency fields. By adjusting field amplitudes and frequencies in a temporal manner in STEREO, in future works it should be possible to perform MRI with highly inhomogeneous fields.

T1 estimation for aqueous iron oxide nanoparticle suspensions using a variable flip angle SWIFT sequence.

PURPOSE: T1 quantification of contrast agents, such as super-paramagnetic iron oxide nanoparticles, is a challenging but important task inherent to many in vivo applications in magnetic resonance imaging. In this work, a sweep imaging with Fourier transformation using variable flip angles (VFAs-SWIFT) method was proposed to measure T1 of aqueous super-paramagnetic iron oxide nanoparticle suspensions. METHODS: T1 values of various iron concentrations (from 1 to 7 mM) were measured using VFA-SWIFT and three-dimensional spoiled gradient-recalled echo with VFAs (VFA-SPGR) sequences on a 7 T MR scanner. For validation, T1 values were also measured using a spectroscopic inversion-recovery sequence on a 7 T spectrometer. RESULTS: VFA-SWIFT demonstrated its advantage for quantifying T1 of highly concentrated aqueous super-paramagnetic iron oxide nanoparticle suspensions, but VFA-SPGR failed at the higher end of iron concentrations. Both VFA-SWIFT and VFA-SPGR yielded linear relationships between the relaxation rate and iron concentrations, with relaxivities of 1.006 and 1.051 s(-1) mM(-1) at 7 T, respectively, in excellent agreement with the spectroscopic measurement of 1.019 s(-1) mM(-1) . CONCLUSION: VFA-SWIFT is able to achieve accurate T1 quantification of aqueous super-paramagnetic iron oxide nanoparticle suspensions up to 7 mM.

Multi-Band-SWIFT.

A useful extension to SWIFT (SWeep Imaging with Fourier Transformation) utilizing sidebands of the excitation pulse is introduced. This MRI method, called Multi-Band-SWIFT, achieves much higher bandwidth than standard SWIFT by using multiple segmented excitations (bands) of the field of view. A description of the general idea and variants of the pulse sequence are presented. From simulations and semi-phenomenological theory, estimations of power deposition and signal-to-noise ratio are made. MB-SWIFT and ZTE (zero-TE) sequences are compared based on images of a phantom and human mandible. Multi-Band-SWIFT provides a bridge between SWIFT and ZTE sequences and allows greatly increased excitation and acquisition bandwidths relative to standard SWIFT for the same hardware switching parameters and requires less peak amplitude of the radiofrequency field (or greater flip angle at same peak amplitude) as compared to ZTE. Multi-Band-SWIFT appears to be an attractive extension of SWIFT for certain musculoskeletal and other medical imaging applications, as well as for imaging materials.

Continuous SWIFT.

This work describes our first efforts to implement SWIFT (SWeep Imaging with Fourier Transformation) in continuous mode for imaging and spectroscopy. We connected a standard quadrature hybrid with a quad coil and acquired NMR signal during continuous radiofrequency excitation. We utilized a chirped radiofrequency pulse to minimize the instantaneous radiofrequency field during excitation of the spin system for the target flip angle and bandwidth. Due to the complete absence of "dead time", continuous SWIFT has the potential to extend applications of MRI and spectroscopy in studies of spin systems having extremely fast relaxation or broad chemical shift distributions beyond the range of existing MRI sequences.

Transformation in mandibular imaging with sweep imaging with fourier transform magnetic resonance imaging.

OBJECTIVE: Current imaging techniques are often suboptimal for the detection of mandibular invasion by squamous cell carcinoma. The aim of this study was to determine the feasibility of a magnetic resonance imaging (MRI)-based technique known as sweep imaging with Fourier transform (SWIFT) to visualize the structural changes of intramandibular anatomy during invasion. DESIGN: Descriptive case study. SETTING: Tertiary academic institution. PATIENTS: Patients with oral carcinoma who underwent segmental mandibulectomy. INTERVENTIONS: Two specimens from each patient were imaged using a 9.4-T Varian MRI system. The SWIFT images were correlated with histologic sections. RESULTS: The SWIFT technique with in vitro specimens produced images with sufficient resolution (156-273 µm) and contrast to allow accurate depiction of tumor invasion of cortical and medullary bone. Both specimens had histopathologic evidence of mandibular invasion with tumor. A high degree of correlation was found between magnetic resonance images and histopathologic findings. CONCLUSIONS: The SWIFT MRI offers 3-dimensional assessment of cortical and medullary bone in fine detail and excellent qualitative agreement with histopathologic findings. Imaging with the SWIFT MRI technique demonstrates great potential to identify mandibular invasion by oral carcinoma.