Alternating Look-Locker for quantitative T1 , T1ρ and B1 3D MRI mapping

PURPOSE

To develop a new MRI method, entitled alternating Look-Locker (aLL), for quantitativeT 1 $$ {T}_1 $$ ,T 1 ρ $$ {T}_{1\uprho} $$ , andB 1 $$ {B}_1 $$ 3D mapping.

METHODS

A Look-Locker scheme that alternates magnetization from +Z and -Z axes of the laboratory frame is utilized in combination with a 3D Multi-Band Sweep Imaging with Fourier Transformation (MB-SWIFT) readout. The analytical solution describing the spin evolution during aLL, as well as the correction required for segmented acquisition were derived. The simultaneousB 1 $$ {B}_1 $$ andT 1 $$ {T}_1 $$ mapping are demonstrated on an agar/saline phantom and on an in-vivo rat head.T 1 ρ $$ {T}_{1\uprho} $$ relaxation was achieved by cyclically applying magnetization preparation (MP) modules consisting of two adiabatic pulses.T 1 ρ $$ {T}_{1\uprho} $$ values in the rat brain in-vivo and in a gadobenate dimeglumine (Gd-DTPA) phantom were compared to those obtained with a previously introduced steady-state (SS) method.

RESULTS

The accuracy and precision of the analytical solution was tested by Bloch simulations. With the application of MP modules, the aLL method provides simultaneousT 1 $$ {T}_1 $$ andT 1 ρ $$ {T}_{1\uprho} $$ maps. Conversely, without it, the method can be used for simultaneousT 1 $$ {T}_1 $$ andB 1 $$ {B}_1 $$ mapping.T 1 ρ $$ {T}_{1\uprho} $$ values were similar with both aLL and SS techniques. However, the aLL method resulted in more robust quantitative mapping compared to the SS method. Unlike the SS method, the aLL method does not require additional scans for generatingT 1 $$ {T}_1 $$ maps.

CONCLUSION

The proposed method offers a new flexible tool for quantitative mapping ofT 1 $$ {T}_1 $$ ,T 1 ρ $$ {T}_{1\uprho} $$ , andB 1 $$ {B}_1 $$ . The aLL method can also be used with readout schemes different from MB-SWIFT.

An improved intraoral transverse loop coil design for high-resolution dental MRI

PURPOSE

To improve intraoral transverse loop coil design for high-resolution dental MRI.

METHODS

The transverse intraoral loop coil (tLoop) was modified (mtLoop) by overlapping the feed port conductors, bending the posterior section, introducing a parallel plate capacitor, optimizing the insulation thickness, and using it in receive-only mode. In addition, an MR-silent insulation was introduced. The performances of the mtLoop and tLoop coils were compared in terms of sensitivity, image SNR, and eddy currents using electromagnetic simulations and MRI measurements at 3T.

RESULTS

The receive-only mode of the mtLoop increases the sensitivity at the apices of the roots, and the overlapped feed port design eliminated signal voids along the incisors. The bent posterior section with the parallel plate capacitor reduced the unwanted signal of the tongue by a factor of 2.3 in the selected region off interest and lowered the eddy currents by 10%. The proposed new coil provided higher SNR by elevenfold and 2.5-fold at the incisors and apices of the molar roots within the selected regions of interest, respectively, in the experiments, as well as improved comfort. Optimal insulation thickness was determined as 1 mm. With the mtLoop, a (250 μm)³ isotropic resolution of the dental arch could be realized using a UTE sequence within 2 min total acquisition time. A T₂ -SPACE protocol with (350 μm)² in-plane resolution was also demonstrated.

CONCLUSION

The proposed new coil offers higher SNR at the incisors and apices of the molar roots, less unwanted signals from tongue, lower eddy currents, and improved patient comfort.

Standalone RF Self-Interference Cancellation System for In-vivo Simultaneous Transmit and Receive (STAR) MRI

Demonstrated is a standalone RF self-interference canceller for simultaneous transmit and receive (STAR) magnetic resonance imaging (MRI) at 1.5T. Standalone STAR cancels the leakage signal directly coupled between transmit and receive RF coils. A cancellation signal, introduced by tapping the input of a transmit coil with a power divider, is manipulated with voltage-controlled attenuators and phase shifters to match the leakage signal in amplitude, 180° out of phase, to exhibit high isolation between the transmitter and receiver. The cancellation signal is initially generated by a voltage-controlled oscillator (VCO); therefore, it does not require any external RF or synchronization signals from the MRI console for calibration. The system employs a field programmable gate array (FPGA) with an on-board analog to digital converter (ADC) to calibrate the cancellation signal by tapping the receive signal, which contains the leakage signal. Once calibrated, the VCO is disabled and the transmit signal path switches to the MRI console for STAR MR imaging. To compensate for the changes of parameters in RF sequences after the automatic calibration and to further improve isolation, a wireless user board that uses an ESP32 microcontroller was built to communicate with the FPGA for final fine-tuning of the output state. The standalone STAR system achieved 74.2 dB of isolation with a 94 second calibration time. With such high isolation, in-vivo MR images were obtained with approximately 40 mW of RF peak power.

Sensitivity and Specificity of MRI Versus CBCT to Detect Vertical Root Fractures Using MicroCT as a Reference Standard

INTRODUCTION

Vertical root fracture (VRF) in root-canal-treated teeth frequently results in tooth loss, partly because VRFs are difficult to diagnose and when detected the fracture is often beyond the point of preservation with surgical intervention. Nonionizing magnetic resonance imaging (MRI) has demonstrated the ability to detect small VRFs, but it is unknown how its diagnostic capabilities compare to the current imaging standard for VRF detection, cone-beam computed tomography (CBCT). This investigation aims to compare the sensitivity and specificity between MRI and CBCT for detecting VRF, using micro-computed tomography (microCT) as a reference.

METHODS

One hundred twenty extracted human tooth roots were root canal treated using common techniques, and VRFs were mechanically induced in a proportion. Samples were imaged using MRI, CBCT, and microCT. Axial MRI and CBCT images were examined by three board-certified endodontists, who evaluated VRF status (yes/no) and gave a confidence assessment for that decision, from which an ROC curve was generated. Intra- and inter-rater reliability were calculated, sensitivity and specificity, and AUC.

RESULTS

Intra-rater reliability was 0.29-0.48 for MRI and 0.30-0.44 for CBCT. Inter-rater reliability for MRI was 0.37 and for CBCT 0.49. Sensitivity was 0.66 (95% CI: 0.53-0.78) and 0.58 (95% CI: 0.45-0.70), and specificity 0.72 (95% CI: 0.58-0.83) and 0.87 (95% CI: 0.75-0.95) for MRI and CBCT, respectively. AUC was 0.74 (95% CI: 0.65-0.83) for MRI and 0.75 (95% CI: 0.66-0.84) for CBCT.

CONCLUSIONS

There was no significant difference in sensitivity or specificity between MRI and CBCT in detecting VRF, despite MRI's early-stage development.

Minimal Detectable Width of Tooth Fractures Using Magnetic Resonance Imaging and Method to Measure

INTRODUCTION

Vertical root fracture (VRF) in root canal-treated (RCT) teeth is a common cause of pain, bone resorption, and tooth loss. VRF is also difficult to diagnose and measure. Magnetic resonance imaging (MRI) has the potential to identify VRF due to beneficial partial volume averaging, without using ionizing radiation. This investigation aimed to describe the narrowest VRFs detectable based on MRI, using micro-computed tomography (microCT) as the reference standard and proposes a method using profile integrals to measure the widths of small VRFs.

METHODS

VRFs were induced in 62 RCT tooth root samples. All samples were imaged in a phantom using MRI and reference imaging was obtained using microCT. The stacks of 3-dimensional axial MRIs were assessed by 3 board-certified endodontists. Evaluators determined the most coronal slice within the stack that was discernible as the extent of the VRF. This slice was measured on correlated microCT sections to determine the minimum VRF width (μm) detectable using a profile integral-based method to measure small fractures and negate the effects of the point spread function.

RESULTS

Using profile integrals to measure VRF width was repeatable and resulted in estimates that were on average 1 μm smaller than known reference widths. Adjusted median VRF width detected using MRI was 45 μm (first quartile: 26 μm, third quartile: 64 μm).

CONCLUSION

Using profile integrals is a valid way to estimate small VRF width. The MRI approach demonstrated ability to repeatedly detect VRFs as small as 26 μm.

Injectable and Repeatable Inductive Heating of Iron Oxide Nanoparticle-Enhanced "PHIL" Embolic toward Tumor Treatment

Deep-seated tumors of the liver, brain, and other organ systems often recur after initial surgical, chemotherapeutic, radiation, or focal treatments. Repeating these treatments is often invasive and traumatic. We propose an iron oxide nanoparticle (IONP)-enhanced precipitating hydrophobic injectable liquid (PHIL, MicroVention inc.) embolic as a localized dual treatment implant for nutrient deprivation and multiple repeatable thermal ablation. Following a single injection, multiple thermal treatments can be repeated as needed, based on monitoring of tumor growth/recurrence. Herein we show the ability to create an injectable stable PHIL-IONP solution, monitor deposition of the PHIL-IONP precipitate dispersion by μCT, and gauge the IONP distribution within the embolic by magnetic resonance imaging. Once precipitated, the implant could be heated to reach therapeutic temperatures >8 °C for thermal ablation (clinical temperature of ∼45 °C), in a model disk and a 3D tumor bed model. Heat output was not affected by physiological conditions, multiple heating sessions, or heating at intervals over a 1 month duration. Further, in ex vivo mice hind-limb tumors, we could noninvasively heat the embolic to an "ablative" temperature elevation of 17 °C (clinically 54 °C) in the first 5 min and maintain the temperature rise over +8 °C (clinically a temperature of 45 °C) for longer than 15 min.

Event-recurring multiband SWIFT functional MRI with 200-ms temporal resolution during deep brain stimulation and isoflurane-induced burst suppression in rat

PURPOSE

To develop a high temporal resolution functional MRI method for tracking repeating events in the brain.

METHODS

We developed a novel functional MRI method using multiband sweep imaging with Fourier transformation (SWIFT), termed event-recurring SWIFT (EVER-SWIFT). The method is able to image similar repeating events with subsecond temporal resolution. Here, we demonstrate the use of EVER-SWIFT for detecting functional MRI responses during deep brain stimulation of the medial septal nucleus and during spontaneous isoflurane-induced burst suppression in the rat brain at 9.4 T with 200-ms temporal resolution.

RESULTS

The EVER-SWIFT approach showed that the shapes and time-to-peak values of the response curves to deep brain stimulation significantly differed between downstream brain regions connected to the medial septal nucleus, resembling findings obtained with traditional 2-second temporal resolution. In contrast, EVER-SWIFT allowed for detailed temporal measurement of a spontaneous isoflurane-induced bursting activity pattern, which was not achieved with traditional temporal resolution.

CONCLUSION

The EVER-SWIFT technique enables subsecond 3D imaging of both stimulated and spontaneously recurring brain activities, and thus holds great potential for studying the mechanisms of neuromodulation and spontaneous brain activity.

Cortical layer-specific differences in stimulus selectivity revealed with high-field fMRI and single-vessel resolution optical imaging of the primary visual cortex

The mammalian neocortex exhibits a stereotypical laminar organization, with feedforward inputs arriving primarily into layer 4, local computations shaping response selectivity in layers 2/3, and outputs to other brain areas emanating via layers 2/3, 5 and 6. It cannot be assumed a priori that these signatures of laminar differences in neuronal circuitry are reflected in hemodynamic signals that form the basis of functional magnetic resonance imaging (fMRI). Indeed, optical imaging of single-vessel functional responses has highlighted the potential limits of using vascular signals as surrogates for mapping the selectivity of neural responses. Therefore, before fMRI can be employed as an effective tool for studying critical aspects of laminar processing, validation with single-vessel resolution is needed. The primary visual cortex (V1) in cats, with its precise neuronal functional micro-architecture, offers an ideal model system to examine laminar differences in stimulus selectivity across imaging modalities. Here we used cerebral blood volume weighted (wCBV) fMRI to examine if layer-specific orientation-selective responses could be detected in cat V1. We found orientation preference maps organized tangential to the cortical surface that typically extended across depth in a columnar fashion. We then examined arterial dilation and blood velocity responses to identical visual stimuli by using two- and three- photon optical imaging at single-vessel resolution-which provides a measure of the hemodynamic signals with the highest spatial resolution. Both fMRI and optical imaging revealed a consistent laminar response pattern in which orientation selectivity in cortical layer 4 was significantly lower compared to layer 2/3. This systematic change in selectivity across cortical layers has a clear underpinning in neural circuitry, particularly when comparing layer 4 to other cortical layers.

Vitrification and Rewarming of Magnetic Nanoparticle-Loaded Rat Hearts

To extend the preservation of donor hearts beyond the current 4-6 h, this paper explores heart cryopreservation by vitrification-cryogenic storage in a glass-like state. While organ vitrification is made possible by using cryoprotective agents (CPA) that inhibit ice during cooling, failure occurs during convective rewarming due to slow and non-uniform rewarming which causes ice crystallization and/or cracking. Here an alternative, "nanowarming", which uses silica-coated iron oxide nanoparticles (sIONPs) perfusion loaded through the vasculature is explored, that allows a radiofrequency coil to rewarm the organ quickly and uniformly to avoid convective failures. Nanowarming has been applied to cells and tissues, and a proof of principle study suggests it is possible in the heart, but proper physical and biological characterization especially in organs is still lacking. Here, using a rat heart model, controlled machine perfusion loading and unloading of CPA and sIONPs, cooling to a vitrified state, and fast and uniform nanowarming without crystallization or cracking is demonstrated. Further, nanowarmed hearts maintain histologic appearance and endothelial integrity superior to convective rewarming and indistinguishable from CPA load/unload control hearts while showing some promising organ-level (electrical) functional activity. This work demonstrates physically successful heart vitrification and nanowarming and that biological outcomes can be expected to improve by reducing or eliminating CPA toxicity during loading and unloading.

Whole-brain studies of spontaneous behavior in head-fixed rats enabled by zero echo time MB-SWIFT fMRI

Understanding the link between the brain activity and behavior is a key challenge in modern neuroscience. Behavioral neuroscience, however, lacks tools to record whole-brain activity in complex behavioral settings. Here we demonstrate that a novel Multi-Band SWeep Imaging with Fourier Transformation (MB-SWIFT) functional magnetic resonance imaging (fMRI) approach enables whole-brain studies in spontaneously behaving head-fixed rats. First, we show anatomically relevant functional parcellation. Second, we show sensory, motor, exploration, and stress-related brain activity in relevant networks during corresponding spontaneous behavior. Third, we show odor-induced activation of olfactory system with high correlation between the fMRI and behavioral responses. We conclude that the applied methodology enables novel behavioral study designs in rodents focusing on tasks, cognition, emotions, physical exercise, and social interaction. Importantly, novel zero echo time and large bandwidth approaches, such as MB-SWIFT, can be applied for human behavioral studies, allowing more freedom as body movement is dramatically less restricting factor.

In vivo quantitative imaging biomarkers of bone quality and mineral density using multi-band-SWIFT magnetic resonance imaging

Bone is a composite biomaterial of mineral crystals, organic matrix, and water. Each contributes to bone quality and strength and may change independently, or together, with disease progression and treatment. Even so, there is a near ubiquitous reliance on ionizing x-ray-based approaches to measure bone mineral density (BMD) which is unable to fully characterize bone strength and may not adequately predict fracture risk. Characterization of treatment efficacy in bone diseases of altered remodeling is complicated by the lack of imaging modality able to safely monitor material-level and biochemical changes in vivo. To improve upon the current state of bone imaging, we tested the efficacy of Multi Band SWeep Imaging with Fourier Transformation (MB-SWIFT) magnetic resonance imaging (MRI) as a readout of bone derangement in an estrogen deficient ovariectomized (OVX) rat model during growth. MB-SWIFT MRI-derived BMD correlated significantly with BMD measured using micro-computed tomography (μCT). In this rodent model, growth appeared to overcome estrogen deficiency as bone mass continued to increase longitudinally over the duration of the study. Nonetheless, after 10 weeks of intervention, MB-SWIFT detected significant changes consistent with estrogen deficiency in cortical water, cortical matrix organization (T₁), and marrow fat. Findings point to MB-SWIFT's ability to quantify BMD in good agreement with μCT while providing additive quantitative outcomes about bone quality in a manner consistent with estrogen deficiency. These results indicate MB-SWIFT as a non-ionizing imaging strategy with value for bone imaging and may be a promising technique to progress to the clinic for monitoring and clinical management of patients with bone diseases such as osteoporosis.

Design of an Intraoral Dipole Antenna for Dental Applications

OBJECTIVE

In dental MRI, intraoral coils provide higher signal-to-noise ratio (SNR) than coils placed outside the mouth. This study aims to design an intraoral dipole antenna and demonstrates the feasibility of combining it with an extraoral coil.

METHODS

Dipole antenna design was chosen over loop design, as it is open toward the distal; therefore, it does not restrain tongue movement. The dipole design offers also an increased depth-of-sensitivity that allows for MRI of dental roots. Different dipole antenna designs were simulated using a finite-difference-time-domain approach. Ribbon, wire, and multi-wire arms were compared. The best design was improved further by covering the ends of the dipole arms with a high-permittivity material. Phantom and in vivo measurements were conducted on a 3T clinical MRI system.

RESULTS

The best transmit efficiency and homogeneity was achieved with a multi-wire curved dipole antenna with 7 wires for each arm. With an additional high-permittivity cap the transmit field inhomogeneity was further reduced from 20% to 5% along the dipole arm. When combined with extraoral flexible surface-coil, the coupling between the coils was less than -32dB and SNR was increased.

CONCLUSION

Using intraoral dipole design instead of loop improves patient comfort. We demonstrated feasibility of the intraoral dipole combined with an extraoral flexible coil-array for dental MRI. Dipole antenna enabled decreasing imaging field-of-view, and reduced the prevalent signal from tongue.

SIGNIFICANCE

This study highlights the advantages and the main challenges of the intraoral RF coils and describes a novel RF coil that addresses those challenges.

Development and validation of 3D MP-SSFP to enable MRI in inhomogeneous magnetic fields

PURPOSE

We demonstrate the feasibility of MRI with missing-pulse steady-state free precession (MP-SSFP) in a 4T magnet with artificially degraded homogeneity.

METHODS

T₁ , T₂ , and diffusion contrast of MP-SSFP was simulated with constant and alternate radiofrequency (RF) phase using an extended phase graph. To validate MP-SSFP performance in human brain imaging, MP-SSFP was tested with two types of artificially introduced inhomogeneous magnetic fields: (1) a pure linear gradient field, and (2) a pseudo-linear gradient field introduced by mounting a head-gradient set at 36 cm from the magnet isocenter. Image distortion induced by the nonlinear inhomogeneous field was corrected using B₀ mapping measured with MP-SSFP.

RESULTS

The maximum flip angle in MP-SSFP was limited to ≤10° because of the large range of resonance frequencies in the inhomogeneous magnetic fields tested in this study. Under this flip-angle limitation, MP-SSFP with constant RF phase provided advantages of higher signal-to-noise ratio and insensitivity to B₁ ⁺ field inhomogeneity as compared with an alternate RF phase. In diffusion simulation, the steady-state magnetization in constant RF phase MP-SSFP increased with an increase of static field gradient up to 8 to 21 mT/m depending on simulation parameters. Experimental results at 4T validated these findings. In human brain imaging, MP-SSFP preserved sufficient signal intensities, but images showed severe image distortion from the pseudo-linear inhomogeneous field. However, following distortion correction, good-quality brain images were achieved.

CONCLUSION

MP-SSFP appears to be a feasible MRI technique for brain imaging in an inhomogeneous magnetic field.

Multi-band SWIFT enables quiet and artefact-free EEG-fMRI and awake fMRI studies in rat

Functional magnetic resonance imaging (fMRI) studies in animal models provide invaluable information regarding normal and abnormal brain function, especially when combined with complementary stimulation and recording techniques. The echo planar imaging (EPI) pulse sequence is the most common choice for fMRI investigations, but it has several shortcomings. EPI is one of the loudest sequences and very prone to movement and susceptibility-induced artefacts, making it suboptimal for awake imaging. Additionally, the fast gradient-switching of EPI induces disrupting currents in simultaneous electrophysiological recordings. Therefore, we investigated whether the unique features of Multi-Band SWeep Imaging with Fourier Transformation (MB-SWIFT) overcome these issues at a high 9.4 T magnetic field, making it a potential alternative to EPI. MB-SWIFT had 32-dB and 20-dB lower peak and average sound pressure levels, respectively, than EPI with typical fMRI parameters. Body movements had little to no effect on MB-SWIFT images or functional connectivity analyses, whereas they severely affected EPI data. The minimal gradient steps of MB-SWIFT induced significantly lower currents in simultaneous electrophysiological recordings than EPI, and there were no electrode-induced distortions in MB-SWIFT images. An independent component analysis of the awake rat functional connectivity data obtained with MB-SWIFT resulted in near whole-brain level functional parcellation, and simultaneous electrophysiological and fMRI measurements in isoflurane-anesthetized rats indicated that MB-SWIFT signal is tightly linked to neuronal resting-state activity. Therefore, we conclude that the MB-SWIFT sequence is a robust preclinical brain mapping tool that can overcome many of the drawbacks of conventional EPI fMRI at high magnetic fields.

Capturing exchange using periodic radiofrequency irradiation

The dynamics of spin system coupled by chemical exchange between two sites with different chemical shifts during periodic radiofrequency (RF) irradiation was here investigated. When the instantaneous π-flip of effective frequency during the course of frequency sweep was applied, a significant increase of exchange-induced relaxation rate constants was observed for small tip angle of magnetization in the laboratory frame of reference. This increase of the rate constants corresponds to the side bands generated by the periodic irradiation during the RF pulses. The exchange - induced relaxation rate constants depend on the exchange conditions, the RF power and the irradiation period. The described phenomenon promises applications for studying protein dynamics and for generating exchange specific relaxation contrasts in MRI.

Imaging of a high concentration of iron labeled cells with positive contrast in a rat knee

PURPOSE

The sweep imaging with Fourier transformation (SWIFT) imaging technique has been shown to provide positive contrast from diluted cell suspensions labeled with super-paramagnetic iron oxide (SPIO) in a tissue, as an alternative to T2*-weighted imaging. Here we demonstrate a variation of the SWIFT technique that yields a hyperintense signal from a concentrated cell suspension. The proposed technique provides minimal background signal from host tissue and facilitates visualization of injected cells.

METHODS

The proton resonance frequency and linewidth were determined for SPIO solutions of different concentrations. The original SWIFT sequence was modified and a dual saturation Gaussian shape RF pulse with ~200 Hz bandwidth was incorporated into the acquisition protocol to suppress host tissue and fat signals. This modification of the original acquisition protocol permits the detection of a hyperintense signal from grafted cells with minimal background signal from the host tissue.

RESULTS

SPIO particles not only induce broadening of NMR line-width but also an initiate proton resonance frequency shift. This shift is linearly proportional to the concentration of the iron oxide particles and induced by the bulk magnetic susceptibility of SPIOs. The shift of the resonance frequency of iron labeled cells allowed us effectively suppress the host tissues with saturation RF pulse to improve MRI detection of grafted cells.

CONCLUSIONS

Iron oxide particles increase the resonance frequency of water proton signal. This shift permitted us to add the tissue/fat saturation RF pulse into the original SWIFT acquisition protocol and detect distinct hyperintense signals from grafted cells with minimal background signal from the host tissue.

Positive contrast from cells labeled with iron oxide nanoparticles: Quantitation of imaging data

PURPOSE

Conventional T₂ -weighted MRI produces a hypointense signal from iron-labeled cells, which renders quantification unfeasible. We tested a SWeep Imaging with Fourier Transformation (SWIFT) MRI pulse sequence to generate a quantifiable hyperintense signal from iron-labeled cells.

METHODS

Mesenchymal stem cells (MSCs) were labeled with different concentrations of iron oxide particles and examined for cell viability, proliferation, and differentiation. The SWIFT sequence was optimized to detect and quantify the amount of iron in the muscle tissue after injection of iron oxide solution and iron-labeled MSCs.

RESULTS

The incubation of MSCs with iron oxide and low concentration of poly-L-lysine mixture resulted in an internalization of up to 22 pg of iron per cell with no adverse effect on MSCs. Phantom experiments showed a dependence of SWIFT signal intensity on the excitation flip angle. The hyperintense signal from iron-labeled cells or solutions was detected, and an amount of the iron oxide in the tissue was quantified with the variable flip angle method.

CONCLUSIONS

The SWIFT sequence can produce a quantifiable hyperintense MRI signal from iron-labeled cells. The graft of 18 x 10⁶ cells was detectable for 19 days after injection and the amount of iron was quantifiable. The proposed protocol simplifies the detection and provides a means to quantify cell numbers. Magn Reson Med 78:1900-1910, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

In vivo MR imaging with simultaneous RF transmission and reception

PURPOSE

To present a practical scheme of a simultaneous radiofrequency (RF) transmit (Tx) and receive (Rx) (STAR) system for MRI, discuss the challenges and solutions, and show preliminary in vivo MR images obtained with this new technique.

METHODS

A remotely controlled STAR system was built and tested with a transverse electromagnetic head coil on a 4T (Oxford, 90 cm-bore) MRI scanner equipped with an Agilent DirectDrive console (Agilent, Santa Clara, CA). In vivo head images have been acquired using continuous sweep excitation and acquisition.

RESULTS

The bench test and MR experimental results show our STAR system to have high isolation (60 dB) between Tx and Rx, with insensitivity to load swings created by head motion. To acquire in vivo head images, ultralow RF peak power of 50 mW was used.

CONCLUSION

A novel motion-insensitive STAR MRI technique was developed and experimentally tested. The first in vivo MR images using this method were acquired. Magn Reson Med 76:1932-1938, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

Quantification and biodistribution of iron oxide nanoparticles in the primary clearance organs of mice using T1 contrast for heating

PURPOSE

To use contrast based on longitudinal relaxation times (T₁ ) or rates (R₁ ) to quantify the biodistribution of iron oxide nanoparticles (IONPs), which are of interest for hyperthermia therapy, cell targeting, and drug delivery, within primary clearance organs.

METHODS

Mesoporous silica-coated IONPs (msIONPs) were intravenously injected into 15 naïve mice. Imaging and mapping of the longitudinal relaxation rate constant at 24 h or 1 week postinjection were performed with an echoless pulse sequence (SWIFT). Alternating magnetic field heating measurements were also performed on ex vivo tissues.

RESULTS

Signal enhancement from positive T₁ contrast caused by IONPs was observed and quantified in vivo in liver, spleen, and kidney at concentrations up to 3.2 mg Fe/(g tissue wt.) (61 mM Fe). In most cases, each organ had a linear correlation between the R₁ and the tissue iron concentration despite variations in intra-organ distribution, degradation, and IONP surface charge. Linear correlation between R₁ and volumetric SAR in hyperthermia therapy was observed.

CONCLUSION

The linear dependence between R₁ and tissue iron concentration in major organs allows quantitative monitoring of IONP biodistribution in a dosage range relevant to magnetic hyperthermia applications, which falls into the concentration gap between CT and conventional MRI techniques. Magn Reson Med 78:702-712, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Imaging human teeth by phosphorus magnetic resonance with nuclear Overhauser enhancement

Three-dimensional phosphorus MR images ((31)P MRI) of teeth are obtained at a nominal resolution of 0.5 mm in less than 15 minutes using acquisition pulse sequences sensitive to ultra-short transversal relaxation times. The images directly reflect the spatially resolved phosphorus content of mineral tissue in dentin and enamel; they show a lack of signal from pulp tissue and reduced signal from de-mineralized carious lesions. We demonstrate for the first time that the signal in (31)P MR images of mineralized tissue is enhanced by a (1)H-(31)P nuclear Overhauser effect (NOE). Using teeth as a model for imaging mineralized human tissue, graded differences in signal enhancement are observed that correlate well with known mineral content. From solid-state NMR experiments we conclude that the NOE is facilitated by spin diffusion and that the NOE difference can be assigned to a higher water content and a different micro-structure of dentin. Thus, a novel method for imaging mineral content without ionizing radiation is proposed. This method has potential use in the assessment of de-mineralization states in humans, such as caries of teeth and osteoporosis of bones.

Role of MRI for detecting micro cracks in teeth

OBJECTIVES

To evaluate the limit of tooth crack width visualization by two MRI pulse sequences in comparison with CBCT.

METHODS

Two extracted human teeth with known crack locations and dimensions, as determined by reference standard microCT, were selected for experimental imaging. Crack location/dimension and the presence of common dental restorative materials such as amalgam were typical of that found clinically. Experimental imaging consisted of conventional CBCT scans and MRI scans with two pulse sequences including Sweep Imaging with Fourier Transformation (SWIFT) and gradient echo (GRE). CBCT and MR images of extracted teeth were acquired using acquisition parameters identical to those used for in vivo imaging. Experimental and reference standard images were registered and the limit of tooth crack visualization was determined.

RESULTS

Collected images indicate that SWIFT could demonstrate cracks with 20-µm width, which is 10 times narrower than the imaging voxel size. Cracks of this size were not visible in GRE images, even with a short echo time of 2.75 ms. The CBCT images were distorted by artefacts owing to close location of metallic restorations.

CONCLUSIONS

The successful visualization of cracks with the SWIFT MRI sequence compared with other clinical modalities suggests that SWIFT MRI can effectively detect microcracks in teeth and therefore may have potential to be a non-invasive method for the in vivo detection of cracks in human teeth.

Capturing fast relaxing spins with SWIFT adiabatic rotating frame spin-lattice relaxation (T1ρ) mapping

Rotating frame spin-lattice relaxation, with the characteristic time constant T1ρ, provides a means to access motion-restricted (slow) spin dynamics in MRI. As a result of their restricted motion, these spins are sometimes characterized by a short transverse relaxation time constant T2 and thus can be difficult to detect directly with conventional image acquisition techniques. Here, we introduce an approach for three-dimensional adiabatic T1ρ mapping based on a magnetization-prepared sweep imaging with Fourier transformation (MP-SWIFT) sequence, which captures signal from almost all water spin populations, including the extremely fast relaxing pool. A semi-analytical procedure for T1ρ mapping is described. Experiments on phantoms and musculoskeletal tissue specimens (tendon, articular and epiphyseal cartilages) were performed at 9.4 T for both the MP-SWIFT and fast spin echo (FSE) read outs. In the phantom with liquids having fast molecular tumbling and a single-valued T1ρ time constant, the measured T1ρ values obtained with MP-SWIFT and FSE were similar. Conversely, in normal musculoskeletal tissues, T1ρ values measured with MP-SWIFT were much shorter than the values obtained with FSE. Studies of biological tissue specimens demonstrated that T1ρ-weighted SWIFT provides higher contrast between normal and diseased tissues relative to conventional acquisitions. Adiabatic T1ρ mapping with SWIFT readout captures contributions from the otherwise undetected fast relaxing spins, allowing more informative T1ρ measurements of normal and diseased states.

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.

Assessment of dysmyelination with RAFFn MRI: application to murine MPS I

Type I mucopolysaccharidosis (MPS I) is an autosomal recessive lysosomal storage disorder with neurological features. Humans and laboratory animals with MPS I exhibit various white matter abnormalities involving the corpus callosum and other regions. In this study, we first validated a novel MRI technique, entitled Relaxation Along a Fictitious Field in the rotating frame of rank n (RAFFn), as a measure of myelination and dysmyelination in mice. We then examined differences between MPS I mice and heterozygotes using RAFF5 and histology. RAFF5 (i.e., RAFFn with n = 5) relaxation time constants were highly correlated with histological myelin density (R2 = 0.68, P<0.001), and RAFF5 clearly distinguished between the hypomyelinated and dysmyelinated shiverer mouse and the wild-type mouse. Bloch-McConnell theoretical analysis revealed slower exchange correlation times and smaller exchange-induced relaxation rate constants for RAFF4 and RAFF5 compared to RAFF1-3, T1ρ, and T2ρ. These data suggest that RAFF5 may assess methylene protons in myelin lipids and proteins, though other mechanisms (e.g. detection of myelin-bound water) may also explain the sensitivity of RAFF5 to myelin. In MPS I mice, mean RAFF5 relaxation time constants were significantly larger for the striatum (P = 0.004) and internal capsule (P = 0.039), and marginally larger for the fornix (P = 0.15). Histological assessment revealed no differences between MPS I mice and heterozygotes in myelin density or corpus callosum thickness. Taken together, these findings support subtle dysmyelination in the brains of mice with MPS I. Dysmyelination may result from myelin lipid abnormalities caused by the absence of α-L-iduronidase. Our findings may help to explain locomotor and cognitive deficits seen in mice with MPS I.

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.

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.

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-DTPA^2- 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.

SWIFT MRI enhances detection of breast cancer metastasis to the lung

PURPOSE

To evaluate the capability of longitudinal MR scans using sweep imaging with Fourier transformation (SWIFT) to detect breast cancer metastasis to the lung in mice.

METHODS

Mice with breast cancer metastatic to the lung were generated by tail vein injection of MDA-MB-231-LM2 cells. Thereafter, MR imaging was performed every week using three different pulse sequences: SWIFT [echo time (TE) ∼3 μs], concurrent dephasing and excitation (CODE; TE ∼300 μs), and three-dimensional (3D) gradient echo (GRE; TE = 2.2 ms). Motion during the long SWIFT MR scans was compensated for by rigid-body motion correction. Maximum intensity projection (MIP) images were generated to visualize changes in lung vascular structures during the development and growth of metastases.

RESULTS

SWIFT MRI was more sensitive to signals from the lung parenchyma than CODE or 3D GRE MRI. Metastatic tumor growth in the lungs induced a progressive increase in intensity of parenchymal signals in SWIFT images. MIP images from SWIFT clearly visualized lung vascular structures and their disruption due to progression of breast cancer metastases in the lung.

CONCLUSION

SWIFT MRI's sensitivity to fast-decaying signals and tolerance of magnetic susceptibility enhances its effectiveness at detecting structural changes in lung parenchyma and vasculature due to breast cancer metastases in the lung.

Quantifying iron-oxide nanoparticles at high concentration based on longitudinal relaxation using a three-dimensional SWIFT Look-Locker sequence

PURPOSE

Iron-oxide nanoparticles (IONPs) have proven utility as contrast agents in many MRI applications. Previous quantitative IONP mapping has been performed using mainly T2 * mapping methods. However, in applications requiring high IONP concentrations, such as magnetic nanoparticles based thermal therapies, conventional pulse sequences are unable to map T2 * because the signal decays too rapidly. In this article, sweep imaging with Fourier transformation (SWIFT) sequence is combined with the Look-Locker method to map T1 of IONPs in high concentrations.

METHODS

T1 values of agar containing IONPs in different concentrations were measured with the SWIFT Look-Locker method and with inversion recovery spectroscopy. Precisions of Look-Locker and variable flip angle (VFA) methods were compared in simulations.

RESULTS

The measured R1 (=1/T1 ) has a linear relationship with IONP concentration up to 53.6 mM of Fe. This concentration exceeds concentrations measured in previous work by almost an order of magnitude. Simulations show SWIFT Look-Locker method is also much less sensitive to B1 inhomogeneity than the VFA method.

CONCLUSION

SWIFT Look-Locker can accurately measure T1 of IONP concentrations ≤53.6 mM. By mapping T1 as a function of IONP concentration, IONP distribution maps might be used in the future to plan effective magnetic nanoparticle hyperthermia therapy.

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.

MRI contrasts in high rank rotating frames

PURPOSE

MRI relaxation measurements are performed in the presence of a fictitious magnetic field in the recently described technique known as RAFF (Relaxation Along a Fictitious Field). This method operates in the 2(nd) rotating frame (rank n = 2) by using a nonadiabatic sweep of the radiofrequency effective field to generate the fictitious magnetic field. In the present study, the RAFF method is extended for generating MRI contrasts in rotating frames of ranks 1 ≤ n ≤ 5. The developed method is entitled RAFF in rotating frame of rank n (RAFFn).

THEORY AND METHODS

RAFFn pulses were designed to generate fictitious fields that allow locking of magnetization in rotating frames of rank n. Contrast generated with RAFFn was studied using Bloch-McConnell formalism together with experiments on human and rat brains.

RESULTS

Tolerance to B0 and B1 inhomogeneities and reduced specific absorption rate with increasing n in RAFFn were demonstrated. Simulations of exchange-induced relaxations revealed enhanced sensitivity of RAFFn to slow exchange. Consistent with such feature, an increased grey/white matter contrast was observed in human and rat brain as n increased.

CONCLUSION

RAFFn is a robust and safe rotating frame relaxation method to access slow molecular motions in vivo.

Abstract P4-01-19: Enhanced sensitivity to detect metastases of breast cancer to the lung and monitor response to inhibition of metastasis by IGF1R inhibitor using SWIFT MRI

Breast cancers metastasize to the bone, lung, brain and liver and mortality from breast cancer is due to metastases. Drugs that inhibit metastasis in particular of triple negative (TN) breast cancers, more sensitive methods to image metastases and imaging biomarkers to monitor response to these drugs at sites of metastasis are needed to reduce mortality. The type I insulin-like growth factor receptor (IGF1R) regulates metastasis and drugs targeting IGF1R and or insulin receptor inhibit metastasis of TN breast cancer cells in mouse models. The initial clinical trials of these drugs have not been very successful and point to the need to develop imaging biomarkers for this therapy. While MRI is a powerful tool for detecting and imaging cancer, its utility in imaging metastasis to the lung is limited due to the challenges of lung MRI with conventional 3D gradient echo (GRE). MRI does not visualize lung well, mainly due to the abundance of air-tissue interfaces, which cause the MR signal to decay too rapidly for conventional MRI pulse sequences to capture. Clinically lung metastases are monitored by CT or PET but exposure of patients to ionizing radiation is a concern and problematic in longitudinal studies monitoring response to a targeted drug. Further MRI can also be useful in measuring metabolite levels using MR spectroscopy (MRS). Therefore, we evaluated the capability of a novel MR sequence called sweep imaging with Fourier transformation (SWIFT), where the data is acquired concurrently with the radiofrequency pulse, to detect breast cancer metastases to lung and test if inhibition of metastases by an IGF1R targeted drug can be monitored by SWIFT. We used the tail vein injection model of breast cancer metastasis with MDA-MB-231-LM2, a lung-seeking metastatic TN breast cancer cell line. Lung metastases were monitored with bioluminescence imaging (BLI) and MRI every week. MRI was done with two pulse sequences: SWIFT (with a short echo time or TE∼3 μs) and GRE (with a longer TE = 2.2 ms). SWIFT showed significantly higher sensitivity in detecting signals from lung parenchyma compared to GRE and structural information was visible in the area of the lungs. Metastatic tumor growth in the lung induced a progressive increase in signal from the lung parenchyma in SWIFT images. MIP images from SWIFT clearly visualized the lung vascular structures and their disruption due to progression of breast cancer metastasis. To test if SWIFT can also be useful in monitoring inhibition of metastases, mice injected with cells were treated with either vehicle or an IGF1R inhibitor (huEM164) twice a week. When inhibition of metastases in the huEM164 treated group compared to the vehicle group was evident by BLI, MR images were acquired. SWIFT was sensitive in detecting the inhibition of metastases by the IGF1R antibody. Our data show that high sensitivity to fast-decaying signals and tolerance to the magnetic susceptibility in SWIFT enhance the capability to detect signal and structural changes in the lung parenchyma and vasculature due to metastasis of breast cancer. Further, SWIFT was also sensitive to monitor inhibition of metastasis in response to IGF1R targeted drugs.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-01-19.

Rapid ex vivo imaging of PAIII prostate to bone tumor with SWIFT-MRI

PURPOSE

The limiting factor for MRI of skeletal/mineralized tissue is fast transverse relaxation. A recent advancement in MRI technology, SWIFT (Sweep Imaging with Fourier Transform), is emerging as a new approach to overcome this difficulty. Among other techniques like UTE, ZTE, and WASPI, the application of SWIFT technology has the strong potential to impact preclinical and clinical imaging, particularly in the context of primary or metastatic bone cancers because it has the added advantage of imaging water in mineralized tissues of bone allowing MRI images to be obtained of tissues previously visible only with modalities such as computed tomography (CT). The goal of the current study is to examine the feasibility of SWIFT for the assessment of the prostate cancer induced changes in bone formation (osteogenesis) and destruction (osteolysis) in ex vivo specimens.

METHODS

A luciferase expressing prostate cancer cell line (PAIII) or saline control was inoculated directly into the tibia of 6-week-old immunocompromised male mice. Tumor growth was assessed weekly for 3 weeks before euthanasia and dissection of the tumor bearing and sham tibias. The ex vivo mouse tibia specimens were imaged with a 9.4 Tesla (T) and 7T MRI systems. SWIFT images are compared with traditional gradient-echo and spin-echo MRI images as well as CT and histological sections.

RESULTS

SWIFT images with nominal resolution of 78 μm are obtained with the tumor and different bone structures identified. Prostate cancer induced changes in the bone microstructure are visible in SWIFT images, which is supported by spin-echo, high resolution CT and histological analysis.

CONCLUSION

SWIFT MRI is capable of high-quality high-resolution ex vivo imaging of bone tumor and surrounding bone and soft tissues. Furthermore, SWIFT MRI shows promise for in vivo bone tumor imaging, with the added benefits of nonexposure to ionizing radiation, quietness, and speed.

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.

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.

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.

Detection of calcifications in vivo and ex vivo after brain injury in rat using SWIFT

Calcifications represent one component of pathology in many brain diseases. With MRI, they are most often detected by exploiting negative contrast in magnitude images. Calcifications are more diamagnetic than tissue, leading to a magnetic field disturbance that can be seen in phase MR images. Most phase imaging studies use gradient recalled echo based pulse sequences. Here, the phase component of SWIFT, a virtually zero acquisition delay sequence, was used to detect calcifications ex vivo and in vivo in rat models of status epilepticus and traumatic brain injury. Calcifications were detected in phase and imaginary SWIFT images based on their dipole like magnetic field disturbances. In magnitude SWIFT images, calcifications were distinguished as hypointense and hyperintense. Hypointense calcifications showed large crystallized granules with few surrounding inflammatory cells, while hyperintense calcifications contained small granules with the presence of more inflammatory cells. The size of the calcifications in SWIFT magnitude images correlated with that in Alizarin stained histological sections. Our data indicate that SWIFT is likely to better preserve signal in the proximity of a calcification or other field perturber in comparison to gradient echo due to its short acquisition delay and broad excitation bandwidth. Furthermore, a quantitative description for the phase contrast near dipole magnetic field inhomogeneities for the SWIFT pulse sequence is given. In vivo detection of calcifications provides a tool to probe the progression of pathology in neurodegenerative diseases. In particular, it appears to provide a surrogate marker for inflammatory cells around the calcifications after brain injury.

Dental magnetic resonance imaging: making the invisible visible

INTRODUCTION

Clinical dentistry is in need of noninvasive and accurate diagnostic methods to better evaluate dental pathosis. The purpose of this work was to assess the feasibility of a recently developed magnetic resonance imaging (MRI) technique, called SWeep Imaging with Fourier Transform (SWIFT), to visualize dental tissues.

METHODS

Three in vitro teeth, representing a limited range of clinical conditions of interest, imaged using a 9.4T system with scanning times ranging from 100 seconds to 25 minutes. In vivo imaging of a subject was performed using a 4T system with a 10-minute scanning time. SWIFT images were compared with traditional two-dimensional radiographs, three-dimensional cone-beam computed tomography (CBCT) scanning, gradient-echo MRI technique, and histological sections.

RESULTS

A resolution of 100 μm was obtained from in vitro teeth. SWIFT also identified the presence and extent of dental caries and fine structures of the teeth, including cracks and accessory canals, which are not visible with existing clinical radiography techniques. Intraoral positioning of the radiofrequency coil produced initial images of multiple adjacent teeth at a resolution of 400 μm.

CONCLUSIONS

SWIFT MRI offers simultaneous three-dimensional hard- and soft-tissue imaging of teeth without the use of ionizing radiation. Furthermore, it has the potential to image minute dental structures within clinically relevant scanning times. This technology has implications for endodontists because it offers a potential method to longitudinally evaluate teeth where pulp and root structures have been regenerated.

Detecting Fleeting MRI Signals with Frequency-Modulated Pulses

We describe a fundamentally different approach to MRI referred to as SWIFT (sweep imaging with Fourier transformation). SWIFT exploits time-shared RF excitation and signal acquisition, allowing capture of signal from spins with extremely short transverse relaxation time, T(2)*. The MR signal is acquired in gaps inserted into a broadband frequency-swept excitation pulse, which results in acquisition delays of only 1 - 2 microseconds. In SWIFT, 3D k-space is sampled in a radial manner, whereby one projection of the object is acquired in the gaps of each frequency-swept pulse, allowing a repetition time (TR) on the order of the pulse length (typically 1 - 3 milliseconds). Since the orientation of consecutive projections varies in a smooth manner (i.e., only small increments in the values of the x, y, z gradients occur from view to view), SWIFT scanning is close to inaudible and is insensitive to gradient timing errors and eddy currents. SWIFT images can be acquired in scan times similar to and sometimes faster than conventional 3D gradient echo techniques. With its ability to capture signals from ultrashort T(2)* spins, SWIFT promises to expand the role of MRI in areas of research where MRI previously played no or negligible role. In this article, we show wood and tooth images obtained with SWIFT as examples of materials with ultrashort T(2)*. Early experience suggests SWIFT can play a role in materials science and porous media research.

SWIFT detection of SPIO-labeled stem cells grafted in the myocardium

We report initial results from studies using sweep imaging with Fourier transformation (SWIFT) to detect superparamagnetic iron oxide (SPIO) particle-labeled stem cells in the rat heart. In experiments performed on phantoms containing titanium balls or SPIO-labeled cells, frequency-shifted signals surrounding the paramagnetic objects produced a pileup artifact visualized by SWIFT. Total signal intensity was retained to a much greater extent by SWIFT as compared to gradient echo imaging. SWIFT imaging of excised and in vivo hearts showed (a) reduced blooming artifact as compared with gradient echo imaging, which helped reduce ambiguity in the detection of SPIO-labeled cells; (b) enhancement of off-resonance signals relative to the background in the imaginary component of images; and (c) detailed myocardial anatomy in magnitude images, which provided anatomic reference. These features suggest SWIFT can facilitate the detection of SPIO-laden cells in the cardiovascular system.

Gapped pulses for frequency-swept MRI

A recently introduced method called SWIFT (SWeep Imaging with Fourier Transform) is a fundamentally different approach to MRI which is particularly well suited to imaging objects with extremely fast spin-spin relaxation rates. The method exploits a frequency-swept excitation pulse and virtually simultaneous signal acquisition in a time-shared mode. Correlation of the spin system response with the excitation pulse function is used to extract the signals of interest. With SWIFT, image quality is highly dependent on producing uniform and broadband spin excitation. These requirements are satisfied by using frequency-modulated pulses belonging to the hyperbolic secant family (HSn pulses). This article describes the experimental steps needed to properly implement HSn pulses in SWIFT. In addition, properties of HSn pulses in the rapid passage, linear region are investigated, followed by an analysis of the pulses after inserting the "gaps" needed for time-shared excitation and acquisition. Finally, compact expressions are presented to estimate the amplitude and flip angle of the HSn pulses, as well as the relative energy deposited by the SWIFT sequence.

Fast and quiet MRI using a swept radiofrequency

A novel fast and quiet method of magnetic resonance imaging (MRI) is introduced which creates new opportunities for imaging in medicine and materials science. The method is called SWIFT, sweep imaging with Fourier transformation. In SWIFT, time-domain signals are acquired in a time-shared manner during a swept radiofrequency excitation of the nuclear spins. With negligible time between excitation and signal acquisition, new possibilities exist for imaging objects consisting of spins with extremely fast transverse relaxation rates, such as macromolecules, semi-solids, and quadrupolar nuclei. The field gradient used for spatial-encoding is not pulsed on and off, but rather is stepped in orientation in an incremental manner, which results in low acoustic noise. This unique acquisition method is expected to be relatively insensitive to sample motion, which is important for imaging live objects. Additionally, the frequency-swept excitation distributes the signal energy in time and thus dynamic range requirements for proper signal digitization are reduced compared with conventional MRI. For demonstration, images of a plastic object and cortical bone are shown.

Product operator analysis of the influence of chemical exchange on relaxation rates

Measurements of chemical-exchange processes by NMR are widely used to obtain valuable information about molecular dynamics and structure. Here, a computational method is introduced to assess the influence of chemical exchange on spin relaxation rates. The method is based on the inclusion of a random exchange process in product operator calculations on a microscopic level. This product operator approach can be applied to estimate exchange contributions when using sophisticated pulse sequences that cannot be easily described analytically. The method applies to the full range of exchange times measurable by NMR and can incorporate interference effects between exchange and other processes such as scalar coupling. To demonstrate its utility, simulated relaxation data were compared with theoretical predictions of spin-locking and Carr-Purcell spin-echo sequences with hard and adiabatic pulses, using different time scales for a two-site chemical-exchange process. Finally, simulations were used to examine a system in which a second random process is superimposed on a simple two-site exchange process. The method was found to provide a simple and robust tool to analyze pulse sequences and equations commonly used to study exchange-induced relaxation.

A simple method to measure 13CH2 heteronuclear dipolar cross-correlation spectral densities

Here, we report a method to simultaneously determine CH2 cross-correlation spectral densities and T1 relaxation times in the laboratory and rotating frames. To accomplish this, we have employed an indirect approach that is based on measurement of differences in relaxation rates acquired with and without cross-correlation terms. The new method, which can be employed using multidimensional NMR and standard relaxation pulse sequences, is validated experimentally by investigation of a selectively 13C-enriched hexadecapeptide and the uniformly 13C-enriched immunoglobulin-binding domain of streptococcal protein G (GB1). Use of this approach makes determination of CH2 cross-correlation spectral densities in uniformly 13C-enriched proteins now routine and provides novel information concerning their internal motions.

Transverse relaxation in the rotating frame induced by chemical exchange

In the presence of radiofrequency irradiation, relaxation of magnetization aligned with the effective magnetic field is characterized by the time constant T1rho. On the other hand, the time constant T2rho characterizes the relaxation of magnetization that is perpendicular to the effective field. Here, it is shown that T2rho can be measured directly with Carr-Purcell sequences composed of a train of adiabatic full-passage (AFP) pulses. During adiabatic rotation, T2rho characterizes the relaxation of the magnetization, which under adiabatic conditions remains approximately perpendicular to the time-dependent effective field. Theory is derived to describe the influence of chemical exchange on T2rho relaxation in the fast-exchange regime, with time constant defined as T2rho,ex. The derived theory predicts the rate constant R2rho,ex (= 1/T2rho,ex) to be dependent on the choice of amplitude- and frequency-modulation functions used in the AFP pulses. Measurements of R2rho,ex of the water/ethanol exchanging system confirm the predicted dependence on modulation functions. The described theoretical framework and adiabatic methods represent new tools to probe exchanging systems.

Measuring protein self-diffusion in protein-protein mixtures using a pulsed gradient spin-echo technique with WATERGATE and isotope filtering

Here we report a modified pulsed gradient spin-echo (PGSTE) pulse sequence to measure diffusion coefficients. This approach incorporates WATERGATE combined with isotopic filtering into a standard PGSTE experiment. Doing this eliminates much of the disadvantages from the combination of diffusion encoding and heteronuclear selection intervals and allows for facile modification of the diffusion pulse sequence with flexibility of the time period between RF pulses. The new diffusion pulse sequence is demonstrated using an 15N-labeled peptide and an 15N-labeled protein in a mixture with a protein of similar size.

Comparison of (13)C(alpha)H and (15)NH backbone dynamics in protein GB1

This study presents a site-resolved experimental view of backbone C(alpha)H and NH internal motions in the 56-residue immunoglobulin-binding domain of streptococcal protein G, GB1. Using (13)C(alpha)H and (15)NH NMR relaxation data [T(1), T(2), and NOE] acquired at three resonance frequencies ((1)H frequencies of 500, 600, and 800 MHz), spectral density functions were calculated as F(omega) = 2omegaJ(omega) to provide a model-independent way to visualize and analyze internal motional correlation time distributions for backbone groups in GB1. Line broadening in F(omega) curves indicates the presence of nanosecond time scale internal motions (0.8 to 5 nsec) for all C(alpha)H and NH groups. Deconvolution of F(omega) curves effectively separates overall tumbling and internal motional correlation time distributions to yield more accurate order parameters than determined by using standard model free approaches. Compared to NH groups, C(alpha)H internal motions are more broadly distributed on the nanosecond time scale, and larger C(alpha)H order parameters are related to correlated bond rotations for C(alpha)H fluctuations. Motional parameters for NH groups are more structurally correlated, with NH order parameters, for example, being larger for residues in more structured regions of beta-sheet and helix and generally smaller for residues in the loop and turns. This is most likely related to the observation that NH order parameters are correlated to hydrogen bonding. This study contributes to the general understanding of protein dynamics and exemplifies an alternative and easier way to analyze NMR relaxation data.

Protein dynamics using frequency-dependent order parameters from analysis of NMR relaxation data

A novel approach is described to analyze NMR relaxation data on proteins. This method introduces the frequency-dependent order parameter, S(2)(omega), in order to estimate contributions to the generalized order parameter S(2) from different motional frequencies occurring on the picosecond to nanosecond time scales. S(2)(omega) is defined as the sum of a specified set of weighting coefficients from the Lorentzian expansion of the spectral density function. 15N NMR relaxation data (500, 600, and 800 MHz) on protein GB1 exemplify the method. Using this approach provides information on motional restrictions over specific frequency or time scale ranges and provides a normalized comparison of motional restrictions between proteins having different overall tumbling correlation times.