Gapped pulses for frequency-swept MRI

Quantitative 1H Magnetic Resonance Imaging on Normal and Pathologic Rat Bones by Solid-State 1H ZTE Sequence with Water and Fat Suppression

BACKGROUND

Osteoporosis (OP) and osteomalacia (OM) are metabolic bone diseases characterized by mineral and matrix density changes. Quantitative bone matrix density differentiates OM from OP. MRI is a noninvasive and nonionizing imaging technique that can measure bone matrix density quantitatively in ex vivo and in vivo.

PURPOSE

To demonstrate water + fat suppressed 1H MRI to compute bone matrix density in ex vivo rat femurs in the preclinical model.

STUDY TYPE

Prospective.

ANIMAL MODEL

Fifteen skeletally mature female Sprague-Dawley rats, five per group (normal, ovariectomized (OVX), partially nephrectomized/vitamin D (Vit-D) deficient), 250-275 g, ∼15 weeks old.

FIELD STRENGTH/SEQUENCE

7T, zero echo time sequence with water + fat (VAPOR) suppression capability, μCT imaging, and gravimetric measurements.

ASSESSMENT

Cortical and trabecular bone segments from normal and disease models were scanned in the same coil along with a dual calibration phantom for quantitative assessment of bone matrix density.

STATISTICAL TESTS

ANOVA and linear regression were used for data analysis, with P-values <0.05 statistically significant.

RESULTS

The MRI-derived three-density PEG pellet densities have a strong linear relationship with physical density measures (r2 = 0.99). The Vit-D group had the lowest bone matrix density for cortical bone (0.47 ± 0.16 g cm-3), whereas the OVX had the lowest bone matrix density for trabecular bone (0.26 ± 0.04 g cm-3). Gravimetry results confirmed these MRI-based observations for Vit-D cortical (0.51 ± 0.07 g cm-3) and OVX trabecular (0.26 ± 0.03 g cm-3) bone groups.

DATA CONCLUSION

Rat femur images were obtained using a modified pulse sequence and a custom-designed double-tuned (1H/31P) transmit-receive solenoid-coil on a 7T preclinical MRI scanner. Phantom experiments confirmed a strong linear relation between MRI-derived and physical density measures and quantitative bone matrix densities in rat femurs from normal, OVX, and Vit-D deficient/partially nephrectomized animals were computed.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 2.

Preparation-based B 1 + mapping in the heart using Bloch-Siegert shifts

PURPOSE

To develop and evaluate a robust cardiacB 1 + $$ {\mathrm{B}}_1^{+} $$ mapping sequence at 3 T, using Bloch-Siegert shift (BSS)-based preparations.

METHODS

A longitudinal magnetization preparation module was designed to encode| B 1 + | $$ \mid {\mathrm{B}}_1^{+}\mid $$ . After magnetization tip-down, off-resonant Fermi pulses, placed symmetrically around two refocusing pulses, induced BSS, followed by tipping back of the magnetization. Bloch simulations were used to optimize refocusing pulse parameters and to assess the mapping sensitivity. Relaxation-inducedB 1 + $$ {\mathrm{B}}_1^{+} $$ error was simulated for variousT 1 $$ {\mathrm{T}}_1 $$ /T 2 $$ {\mathrm{T}}_2 $$ times. The effective mapping range was determined in phantom experiments, and| B 1 + | $$ \mid {\mathrm{B}}_1^{+}\mid $$ maps were compared to the conventional BSS method and subadiabatic hyperbolic-secant 8 (HS8) pulse-sensitized method. CardiacB 1 + $$ {\mathrm{B}}_1^{+} $$ maps were acquired in healthy subjects, and evaluated for repeatability and imaging plane intersection consistency. The technique was modified for three-dimensional (3D) acquisition of the whole heart in a single breath-hold, and compared to two-dimensional (2D) acquisition.

RESULTS

Simulations indicate that the proposed preparation can be tailored to achieve high mapping sensitivity across variousB 1 + $$ {\mathrm{B}}_1^{+} $$ ranges, with maximum sensitivity at the upperB 1 + $$ {\mathrm{B}}_1^{+} $$ range.T 1 $$ {\mathrm{T}}_1 $$ /T 2 $$ {\mathrm{T}}_2 $$ -induced bias did not exceed 5.2% $$ \% $$ . Experimentally reproducedB 1 + $$ {\mathrm{B}}_1^{+} $$ sensitization closely matched simulations forB 1 + ≥ 0 . 3 B 1 , max + $$ {\mathrm{B}}_1^{+}\ge 0.3{\mathrm{B}}_{1,\max}^{+} $$ (mean difference 0.031± $$ \pm $$ 0.022, compared to 0.018± $$ \pm $$ 0.025 in the HS8-sensitized method), and showed 20-fold reduction in the standard deviation of repeated scans, compared with conventional BSSB 1 + $$ {\mathrm{B}}_1^{+} $$ mapping, and an equivalent 2-fold reduction compared with HS8-sensitization. Robust cardiacB 1 + $$ {\mathrm{B}}_1^{+} $$ map quality was obtained, with an average test-retest variability of 0.027± $$ \pm $$ 0.043 relative to normalizedB 1 + $$ {\mathrm{B}}_1^{+} $$ magnitude, and plane intersection bias of 0.052± $$ \pm $$ 0.031. 3D acquisitions showed good agreement with 2D scans (mean absolute deviation 0.055± $$ \pm $$ 0.061).

CONCLUSION

BSS-based preparations enable robust and tailorable 2D/3D cardiacB 1 + $$ {\mathrm{B}}_1^{+} $$ mapping at 3 T in a single breath-hold.

Quantitative 31P magnetic resonance imaging on pathologic rat bones by ZTE at 7T

BACKGROUND

Osteoporosis is characterized by low bone mineral density (BMD), which predisposes individuals to frequent fragility fractures. Quantitative BMD measurements can potentially help distinguish bone pathologies and allow clinicians to provide disease-relieving therapies. Our group has developed non-invasive and non-ionizing magnetic resonance imaging (MRI) techniques to measure bone mineral density quantitatively. Dual-energy X-ray Absorptiometry (DXA) is a clinically approved non-invasive modality to diagnose osteoporosis but has associated disadvantages and limitations.

PURPOSE

Evaluate the clinical feasibility of phosphorus (31P) MRI as a non-invasive and non-ionizing medical diagnostic tool to compute bone mineral density to help differentiate between different metabolic bone diseases.

MATERIALS AND METHODS

Fifteen ex-vivo rat bones in three groups [control, ovariectomized (osteoporosis), and vitamin-D deficient (osteomalacia - hypo-mineralized) were scanned to compute BMD. A double-tuned (1H/31P) transmit-receive single RF coil was custom-designed and in-house-built with a better filling factor and strong radiofrequency (B1) field to acquire solid-state 31P MR images from rat femurs with an optimum signal-to-noise ratio (SNR). Micro-computed tomography (μCT) and gold-standard gravimetric analyses were performed to compare and validate MRI-derived bone mineral densities.

RESULTS

Three-dimensional 31P MR images of rat bones were obtained with a zero-echo-time (ZTE) sequence with 468 μm spatial resolution and 12-17 SNR on a Bruker 7 T Biospec having multinuclear capability. BMD was measured quantitatively on cortical and trabecular bones with a known standard reference. A strong positive correlation (R = 0.99) and a slope close to 1 in phantom measurements indicate that the densities measured by 31P ZTE MRI are close to the physical densities in computing quantitative BMD. The 31P NMR properties (resonance linewidth of 4 kHz and T1 of 67 s) of ex-vivo rat bones were measured, and 31P ZTE imaging parameters were optimized. The BMD results obtained from MRI are in good agreement with μCT and gravimetry results.

CONCLUSION

Quantitative measurements of BMD on ex-vivo rat femurs were successfully conducted on a 7 T preclinical scanner. This study suggests that quantitative measurements of BMD are feasible on humans in clinical MRI with suitable hardware, RF coils, and pulse sequences with optimized parameters within an acceptable scan time since human femurs are approximately ten times larger than rat femurs. As MRI provides quantitative in-vivo data, various systemic musculoskeletal conditions can be diagnosed potentially in humans.

3D T1 relaxation time measurements in an equine model of subtle post-traumatic osteoarthritis using MB-SWIFT

The aim of this study is to assess whether articular cartilage changes in an equine model of post-traumatic osteoarthritis (PTOA), induced by surgical creation of standard (blunt) grooves, and very subtle sharp grooves, could be detected with ex vivo T1 relaxation time mapping utilizing three-dimensional (3D) readout sequence with zero echo time. Grooves were made on the articular surfaces of the middle carpal and radiocarpal joints of nine mature Shetland ponies and osteochondral samples were harvested at 39 weeks after being euthanized under respective ethical permissions. T1 relaxation times of the samples (n = 8 + 8 for experimental and n = 12 for contralateral controls) were measured with a variable flip angle 3D multiband-sweep imaging with Fourier transform sequence. Equilibrium and instantaneous Young's moduli and proteoglycan (PG) content from OD of Safranin-O-stained histological sections were measured and utilized as reference parameters for the T1 relaxation times. T1 relaxation time was significantly (p < 0.05) increased in both groove areas, particularly in the blunt grooves, compared with control samples, with the largest changes observed in the superficial half of the cartilage. T1 relaxation times correlated weakly (Rs  ≈ 0.33) with equilibrium modulus and PG content (Rs  ≈ 0.21). T1 relaxation time in the superficial articular cartilage is sensitive to changes induced by the blunt grooves but not to the much subtler sharp grooves, at the 39-week timepoint post-injury. These findings support that T1 relaxation time has potential in detection of mild PTOA, albeit the most subtle changes could not be detected.

Configuration space representation of MRI sequences

PURPOSE

Local solutions provide little intuition about the contrast, generated by MRI sequences with unbalanced gradients. A configuration space representation of the spin density allows to formalize signal localization and thereby overcome these limitations.

THEORY AND METHODS

The continuous configuration model (CCM) constitutes a Fourier integral decomposition of the spin density, such that intrinsic tissue properties are separated from accumulated effects due to gradients and/or bulk off-resonance. Thereby, any set of local dynamic equations is automatically transformed into a corresponding set of differential equations between configurations.

RESULTS

The CCM generalizes the Fourier-based EPG formalism such that it becomes applicable to arbitrary MRI sequences. It enables a rigorous and concise treatment of signal localization (selective excitation, spatial encoding), inhomogeneous broadening and motion. Applied to frequency swept NMR, a close connection between SWIFT and SSFP sequences can be found.

CONCLUSION

The CCM allows to view arbitrary MRI sequences from a signal processing perspective, which might simplify the development and optimization of novel imaging strategies.

Analysis and mitigation of noise in simultaneous transmission and reception in MRI

PURPOSE

In simultaneous transmission and reception (STAR) MRI, along with the coupling of the excitation pulse to the received signal, noise, and undesired distortions (spurs) coming from the transmit chain also leak into the acquired signal and degrade image quality. Here, properties of this coupled noise and its relationship with the transmit amplifier gain, transmit chain noise density, isolation performance, and imaging bandwidth are analyzed. It is demonstrated that by utilizing a recently proposed STAR technique, the transmit noise can be reduced. The importance of achieving high isolation and careful selection of the corresponding parameters are demonstrated.

THEORY AND METHODS

A cancellation algorithm, together with a vector modulator, is used for transmit-receive isolation. The scanner is modeled as a pipeline of blocks to demonstrate the noise contribution from each block. With higher isolation, coupled transmit noise can be reduced to the point that the dominant noise source becomes acquisition noise, as in the case for pulsed MRI. Amplifiers with different gain and noise properties are used in the experiments to verify the derived noise-transmit parameter relation.

RESULTS

With the proposed technique, more than 80 dB isolation in the analog domain is achieved. The leakage noise and the spurs coupled from the transmit chain, are reduced. It is shown that the transmit gain plays the most critical role in determining sufficient isolation, whereas the amplifier noise figure does not contribute as much.

CONCLUSION

The transmit noise and the spurs in STAR imaging are analyzed and mitigated by using a vector modulator.

Chirped ordered pulses for ultra-broadband ESR spectroscopy

Recently, applications of swept-frequency pulses proved to be a useful approach to circumvent the problem of limited excitation bandwidth in pulsed ESR posed by conventional pulses. Here, we present a chirped excitation sequence, CHirped ORdered pulses for Ultra-broadband Spectroscopy (CHORUS), for ultra-broadband ESR spectroscopy. It will be demonstrated that the application of this sequence can address the problems of excitation non-uniformity and sensitivity to instrumental instabilities to a greater extent compared to the current state of the art. This sequence is highly promising for finding applications beyond single excitation in many ESR experiments. Theoretical and experimental results for the proposed method are presented along with calibration strategies for experimental implementation.

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.

Rapid survey of nuclear quadrupole resonance by broadband excitation with comb modulation and dual-mode acquisition

Nuclear Quadrupole Resonance (NQR) provides spectra carrying information as to the electric-field gradient around nuclei with a spin quantum number I > 1/2 and offers helpful clues toward characterizing the electronic structure of materials of chemical interest. A major challenge in NQR is finding hitherto unknown resonance frequencies, which can scatter over a wide range, requiring time consuming repetitive measurements with stepwise frequency increments. Here, we report on an efficient, two-step NQR protocol by bringing rapid-scan and frequency-comb together. In the first step, wideband excitation and simultaneous signal acquisition, both realized by a non-adiabatic, frequency-swept hyperbolic secant (HS) pulse with comb modulation, offers a clue for the existence/absence of the resonance within the frequency region under investigation. When and only when the sign of the resonance has been detected, the second step is implemented to compensate the limited detection bandwidth of the first and to unambiguously determine the NQR frequency. We also study the spin dynamics under the comb-modulated HS pulse by numerical simulations, and experimentally demonstrate the feasibility of the proposed scheme, which is referred to as RApid-Scan with GApped excitation with Dual-mode Operation (RASGADO) NQR.

MRI exploiting frequency-modulated pulses and their nonlinear phase

Frequency-modulated (FM) pulses can provide several advantages over conventional amplitude-modulated pulses in the field of MRI; however, the manner in which spins are manipulated imprints a quadratic phase on the resulting magnetization. Historically this was considered a hindrance and slowed the widespread adoption of FM pulses. This article seeks to provide a historical perspective of the different techniques that researchers have used to exploit the benefits of FM pulses and to compensate for the nonlinear phase created by this class of pulses in MRI. Expanding on existing techniques, a new method of phase compensation is presented that utilizes nonlinear gradients to mitigate the undesirable phase imparted by this class of pulses.

Selective magnetic resonance signal suppression by colored Frank excitation

Inspired by the growing interest in miniaturized NMR devices and their applications in material science as well as in chemical and biological research, low power rf excitation is explored. ¹H NMR spectra have been measured with low power Frank excitation and are compared to spectra obtained by single-pulse excitation. Frank excitation consists of a large number of phase-modulated, constant-amplitude rf-pulses. A Frank sequence is divided into packages of discrete phase wavelets that correspond to a scan across a spectral frequency range. The largely coherent excitation is found experimentally to require less power than white noise excitation. The package structure suggests that individual wavelets can be omitted to skip individual frequency regions in the excitation, converting the white Frank excitation into colored Frank excitation. This work explores different approaches of colored, selective Frank excitation for spectroscopy and imaging. It is motivated by the aim to eliminate the rf amplifier from the NMR spectrometer so as to enable further miniaturization of NMR instruments. Colored Frank excitation bears promise as a low-power modality for solvent signal suppression in spectroscopy and motion tagging in magnetic resonance imaging.

High-resolution short-T2 MRI using a high-performance gradient

PURPOSE

To achieve high resolution in imaging of short-T₂ materials and tissues by using a high-performance human-sized gradient insert with strength up to 200 mT/m and 100% duty cycle.

METHODS

Dedicated short-T₂ methodology and hardware are used, such as the pointwise encoding time reduction with radial acquisition (PETRA) technique with modulated excitation pulses, optimized radio-frequency hardware, and a high-performance gradient insert. A theoretical analysis of actual spatial resolution and SNR is provided to support the choice of scan parameters and interpretation of the results. Imaging is performed in resolution phantoms, animal specimen, and human volunteers at both conventional and maximum available gradient strengths and compared using image subtraction.

RESULTS

Calculations suggest that increasing gradient strength beyond conventional values considerably improves both actual resolution and SNR efficiency in short-T₂ imaging. Resolution improvements are confirmed in all investigated samples, in particular 2 mm slots were resolved in a hard-plastic plate with T₂ ≈ 10 μs and in vivo musculoskeletal images were acquired at isotropic 200 μm resolution. Expected improvements in signal yield are realized in fine structures benefitting from high resolution but to less extent in regions of low contrast where decay-related blurring leads to signal overlap between neighboring locations.

CONCLUSION

Strong gradients with high duty cycle enable short-T₂ imaging at unprecedentedly high resolution, holding the potential for improving MRI of, eg, bone, tendon, lung, or teeth. Moreover, it allows direct access of tissues with T₂ of tens of microseconds such as myelin or collagen.

Short-T2 MRI: Principles and recent advances

Among current modalities of biomedical and diagnostic imaging, MRI stands out by virtue of its versatile contrast obtained without ionizing radiation. However, in various cases, e.g., water protons in tissues such as bone, tendon, and lung, MRI performance is limited by the rapid decay of resonance signals associated with short transverse relaxation times T₂ or T₂*. Efforts to address this shortcoming have led to a variety of specialized short-T₂ techniques. Recent progress in this field expands the choice of methods and prompts fresh considerations with regard to instrumentation, data acquisition, and signal processing. In this review, the current status of short-T₂ MRI is surveyed. In an attempt to structure the growing range of techniques, the presentation highlights overarching concepts and basic methodological options. The most frequently used approaches are described in detail, including acquisition strategies, image reconstruction, hardware requirements, means of introducing contrast, sources of artifacts, limitations, and applications.

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.

Designing 3D selective adiabatic radiofrequency pulses with single and parallel transmission

PURPOSE

To introduce a method of designing single and parallel transmit (pTx) 3D adiabatic π pulses for inverting and refocusing spins that are insensitive to transmit B1 ( B1+) inhomogeneity.

THEORY AND METHODS

A 3D adiabatic pulse is created by replacing each piece-wise constant element (or sub-pulse) of an adiabatic full passage (AFP) by a 2D selective pulse. In this study, the parent AFP is an HS1 and each sub-pulse is a 2D pulse derived from a jinc function designed using a spiral k-trajectory. Spatial selectivity in the third direction is achieved by blipping the slab-selective gradient between sub-pulses, yielding a rectangular slab profile identical to that of the parent AFP. The slew-rate limited sub-pulse can be undersampled utilizing pTx, thus shortening the overall pulse width. Simulations and experiments demonstrate the quality of spatial selectivity and adiabaticity achievable.

RESULTS

The 3D adiabatic pulse inverts and refocus spins in a sharply demarcated cylindrical volume. When stepping RF amplitude, an adiabatic threshold is observed above which the flip angle remains π. Experimental results demonstrate that pTx is an effective means to significantly improve pulse performance.

CONCLUSION

A method of designing 3D adiabatic pulses insensitive to B1 inhomogeneity has been developed. pTx can shorten these pulses while retaining their adiabatic character. Magn Reson Med 79:701-710, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Murine pulmonary imaging at 7T: T2* and T1 with anisotropic UTE

PURPOSE

To measure the T2* and T₁ of mouse lung at 7T via anisotropic-resolution radial ultrashort echo-time imaging with ellipsoidal k-space coverage.

METHODS

Ellipsoidal field-of-view was created by expanding uniform spherical k-space coverage. The effects of T2* and ellipsoidal sampling on image resolution were investigated by using point-spread-function analysis and resolution phantoms. Finally, this ellipsoidal sampling approach was used to measure the lung T2* and T₁ of healthy C57BL/6 mice at the increasingly common preclinical field strength of 7T.

RESULTS

Lung parenchyma T2* of 17- to 23-week-old mice at 7T was 0.395 ± 0.033 ms. T₁ of lung and left- and right-heart ventricles was 1452.5 ± 87.0 ms, 1810.5 ± 54.6 ms, and 1602.6 ± 120.9 ms, respectively. Ellipsoidal k-space sampling provides enhanced resolution for a fixed scanning time or provides equivalent (although anisotropic) spatial resolution with reduced scanning times, while simultaneously avoiding fold-in artifacts.

CONCLUSION

Using these techniques, the first T2* and T₁ measures of mouse lung at 7T are reported. Ultrashort echo-time imaging with ellipsoidal k-space coverage significantly increases in-plane resolution without increasing scanning time, or equivalently, decreases scanning time while maintaining equivalent in-plane resolution. Magn Reson Med 79:2254-2264, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

MB-SWIFT functional MRI during deep brain stimulation in rats

Recently introduced 3D radial MRI pulse sequence entitled Multi-Band SWeep Imaging with Fourier Transformation (MB-SWIFT) having virtually zero acquisition delay was used to obtain functional MRI (fMRI) contrast in rat's brain at 9.4 T during deep brain stimulation (DBS). The results demonstrate that MB-SWIFT allows functional images free of susceptibility artifacts, and provides an excellent fMRI activation contrast in the brain. Flip angle dependence of the MB-SWIFT fMRI signal and elimination of the fMRI contrast while using saturation bands, indicate a blood flow origin of the observed fMRI contrast. MB-SWIFT fMRI modality permits activation studies in the close proximity to an implanted lead, which is not possible to achieve with conventionally used gradient echo and spin echo - echo planar imaging fMRI techniques. We conclude that MB-SWIFT fMRI is a powerful imaging modality for investigations of functional responses during DBS.

Establishing the overlap of IONP quantification with echo and echoless MR relaxation mapping

PURPOSE

Iron-oxide nanoparticles (IONPs) have shown tremendous utility for enhancing image contrast and delivering targeted therapies. Quantification of IONPs has been demonstrated at low concentrations with gradient echo (GRE) and spin echo (SE), and at high concentrations with echoless sequences such as swept imaging with Fourier transform (SWIFT). This work examines the overlap of IONP quantification with GRE, SE, and SWIFT.

METHODS

The limit of quantification of GRE, SE, inversion-recovery GRE, and SWIFT sequences was assessed using IONPs at a concentration range of 0.02 to 89.29 mM suspended in 1% agarose. Empirically derived limits of quantification were compared with International Union of Pure and Applied Chemistry definitions. Both commercial and experimental IONPs were used.

RESULTS

All three IONPs assessed demonstrated an overlap of concentration quantification with GRE, SE, and SWIFT sequences. The largest dynamic range observed was 0.004 to 35.7 mM with Feraheme.

CONCLUSIONS

The metrics established allow upper and lower quantitative limitations to be estimated given the relaxivity characteristics of the IONP and the concentration range of the material to be assessed. The methods outlined in this paper are applicable to any pulse sequence, IONP formulation, and field strength. Magn Reson Med 79:1420-1428, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Full analytical solution of the bloch equation when using a hyperbolic-secant driving function

PURPOSE

The frequency-swept pulse known as the hyperbolic-secant (HS) pulse is popular in NMR for achieving adiabatic spin inversion. The HS pulse has also shown utility for achieving excitation and refocusing in gradient-echo and spin-echo sequences, including new ultrashort echo-time imaging (e.g., Sweep Imaging with Fourier Transform, SWIFT) and B1 mapping techniques. To facilitate the analysis of these techniques, the complete theoretical solution of the Bloch equation, as driven by the HS pulse, was derived for an arbitrary state of initial magnetization.

METHODS

The solution of the Bloch-Riccati equation for transverse and longitudinal magnetization for an arbitrary initial state was derived analytically in terms of HS pulse parameters. The analytical solution was compared with the solutions using both the Runge-Kutta method and the small-tip approximation.

RESULTS

The analytical solution was demonstrated on different initial states at different frequency offsets with/without a combination of HS pulses. Evolution of the transverse magnetization was influenced significantly by the choice of HS pulse parameters. The deviation of the magnitude of the transverse magnetization, as obtained by comparing the small-tip approximation to the analytical solution, was < 5% for flip angles < 30 °, but > 10% for the flip angles > 40 °.

CONCLUSION

The derived analytical solution provides insights into the influence of HS pulse parameters on the magnetization evolution. Magn Reson Med 77:1630-1638, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Magnetization transfer SWIFT MRI consistently detects histologically verified myelin loss in the thalamocortical pathway after a traumatic brain injury in rat

Traumatic brain injury (TBI) is associated with various neurocognitive deficits, and rapid assessment of the damage is potentially important for the prevention and treatment of these deficits. Imaging assessment of mild or moderate damage outside the primary lesion area after TBI, however, remains challenging. Magnetization transfer (MT) has clearly been underutilized in imaging the damage caused by TBI. Here, we applied the MT ratio (MTR) using sweep imaging with Fourier transformation (SWIFT) to study microstructural tissue damage in the thalamocortical pathway outside the primary lesion in a lateral fluid percussion injury rat model of TBI, 5 months after injury. MTR was decreased in layers VIb-IV of the barrel cortex and related subcortical areas, mainly indicating demyelination, which was verified by histology. The largest MTR change in the cortex was in layer VIb (-8.2%, p_FDR  = 0.01), and the largest MTR change in the subcortical areas was in the caudal-most portion of the internal capsule (-11.0%, p_FDR  < 0.005). These areas exhibited the greatest demyelination and substantial cellularity attributed to gliosis. Correlation analysis of group-averaged results from the subcortical areas revealed an excellent correlation of MTR with myelin (r²  = 0.94, p < 0.001), but no correlation with increased cellularity as detected by Nissl staining. Thus, MTR using SWIFT can be a valuable tool for the assessment of subtle changes after TBI in both cortical and subcortical areas.

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.

Investigation of the Longitudinal Relaxation Time of Rat Tibial Cortical Bone Using SWIFT

Sweep imaging with Fourier transform (SWIFT) method has been developed to image tissues with very short T₂ values, such as cortical bone. The purpose of this study was to measure the T₁ value of the rat cortical bone. It was approximately 120 ms on 7.04T. This result could thus be useful for studying bony tissue according to the SWIFT method in the future.

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.

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.

Sodium MRI of glioma in animal models at ultrahigh magnetic fields

High magnetic fields expand our capability to use sodium MRI for biomedical applications. The central goal of this review is devoted to the unique features of sodium MRI in tumor animal models, mainly in glioma, performed at 9.4 and 21.1 T. The ability of sodium MRI to monitor tumor response to therapy was evaluated. It is noteworthy that sodium MRI can detect glioma response to chemotherapy earlier than diffusion MRI. Especially attractive is the ability of sodium MRI to predict tumor therapeutic resistance before therapy. The non-invasive prediction of tumor chemo-resistance by sodium MRI presents a potential to individualize strategies for cancer treatment. Specifics of sodium MRI and technical aspects of imaging are also presented.

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.

Phase imaging in brain using SWIFT

The majority of MRI phase imaging is based on gradient recalled echo (GRE) sequences. This work studies phase contrast behavior due to small off-resonance frequency offsets in brain using SWIFT, a FID-based sequence with nearly zero acquisition delay. 1D simulations and a phantom study were conducted to describe the behavior of phase accumulation in SWIFT. Imaging experiments of known brain phase contrast properties were conducted in a perfused rat brain comparing GRE and SWIFT. Additionally, a human brain sample was imaged. It is demonstrated how SWIFT phase is orientation dependent and correlates well with GRE, linking SWIFT phase to similar off-resonance sources as GRE. The acquisition time is shown to be analogous to TE for phase accumulation time. Using experiments with and without a magnetization transfer preparation, the likely effect of myelin water pool contribution is seen as a phase increase for all acquisition times. Due to the phase accumulation during acquisition, SWIFT phase contrast can be sensitized to small frequency differences between white and gray matter using low acquisition bandwidths.

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.

A new RF tagging pulse based on the Frank poly-phase perfect sequence

Radio frequency (RF) spectrally selective multiband pulses or tagging pulses, are applicable in a broad range of magnetic resonance methods. We demonstrate through simulations and experiments a new phase-modulation-only RF pulse for RF tagging based on the Frank poly-phase perfect sequence. In addition, we introduce an extended version with a WURST modulation (Frank-WURST). The new pulses exhibit interesting and flexible spin tagging properties and are easily implemented in existing MR sequences, where they can substitute slice-selective pulses with no additional alterations.

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.

Imaging mouse lung allograft rejection with (1)H MRI

PURPOSE

To demonstrate that longitudinal, noninvasive monitoring via MRI can characterize acute cellular rejection in mouse orthotopic lung allografts.

METHODS

Nineteen Balb/c donor to C57BL/6 recipient orthotopic left lung transplants were performed, further divided into control-Ig versus anti-CD4/anti-CD8 treated groups. A two-dimensional multislice gradient-echo pulse sequence synchronized with ventilation was used on a small-animal MR scanner to acquire proton images of lung at postoperative days 3, 7, and 14, just before sacrifice. Lung volume and parenchymal signal were measured, and lung compliance was calculated as volume change per pressure difference between high and low pressures.

RESULTS

Normalized parenchymal signal in the control-Ig allograft increased over time, with statistical significance between day 14 and day 3 posttransplantation (0.046→0.789; P < 0.05), despite large intermouse variations; this was consistent with histopathologic evidence of rejection. Compliance of the control-Ig allograft decreased significantly over time (0.013→0.003; P < 0.05), but remained constant in mice treated with anti-CD4/anti-CD8 antibodies.

CONCLUSION

Lung allograft rejection in individual mice can be monitored by lung parenchymal signal changes and by lung compliance through MRI. Longitudinal imaging can help us better understand the time course of individual lung allograft rejection and response to treatment.

In vivo quantification of SPIO nanoparticles for cell labeling based on MR phase gradient images

Along with the development of modern imaging technologies, contrast agents play increasingly important roles in both clinical applications and scientific research. Super-paramagnetic iron oxide (SPIO) nanoparticles, a negative contrast agent, have been extensively used in magnetic resonance imaging (MRI), such as in vivo labeling and tracking of cells. However, there still remain many challenges, such as in vivo quantification of SPIO nanoparticles. In this work, an MR phase gradient-based method was proposed to quantify the SPIO nanoparticles. As a calibration, a phantom experiment using known concentrations (10, 25, 50, 100, 150 and 250 µg/ml) of SPIO was first conducted to verify the proposed quantification method. In a following in vivo experiment, C6 glioma cells labeled with SPIO nanoparticles were implanted into flanks of four mice, which were scanned 1-3 days post-injection for in vivo quantification of SPIO concentration. The results showed that the concentration of SPIO nanoparticles could be determined in both phantom and in vivo experiments using the developed MR phase gradients approach.

Improving detection specificity of iron oxide nanoparticles (IONPs) using the SWIFT sequence with long T(2) suppression

In order to improve the detection specificity of iron oxide nanoparticles (IONPs) delivered to tumors, we embedded saturation pulses into the sweep imaging using Fourier transformation (SWIFT) sequence to suppress long T(2) tissues and fat. Simulation of the Bloch equation was first conducted to study behavior of the saturation pulses of various lengths under different T(2) and off-resonance conditions. MR experiments were then conducted using in vivo mouse xenografts and a phantom consisting of IONPs, vegetable oil, and explanted tumor specimen, without and with long T(2) suppression under a 7T magnetic field. For the in vivo study, arginine-glycine-aspartate (RGD) coated 10nm IONPs (RGD-IONPs) were delivered to tumors implanted in nude mice through both intra-tumor and intravenous injections. Histological studies confirmed that RGD-IONPs efficiently homed to tumors through RGD-integrin interaction. Compared to conventional SWIFT, the proposed method resulted in sufficient suppression on long T(2) species but less influence on short T(2) species. For both the in vivo and ex vivo studies, significantly improved contrast-to-noise ratio (CNR) was achieved between the IONPs and the long T(2) species.

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.

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.

Hybrid-SPRITE MRI

In a FID based frequency encoding MRI experiment the central part of k-space is not generally accessible due to the probe dead time. This portion of k-space is however crucial for image reconstruction. SPRITE (Single Point Ramped Imaging with T1 Enhancement), SPI with a linearly ramped phase encode gradient, has been employed to image short relaxation time systems for many years with great success. It is a robust imaging method in significant measure because it provides acquisition of high quality k-space origin data. We propose a new sampling scheme, termed hybrid-SPRITE, combining phase and frequency encoding to ensure high quality images with reduced acquisition times, reduced gradient duty cycle and increased sensitivity. In hybrid-SPRITE, numerous time domain points are collected to assist image reconstruction. An Inverse Non-uniform Discrete Fourier Transform (INDFT) is employed in 1D applications. A pseudo-polar grid is exploited in 2D hybrid-SPRITE for rapid and accurate image reconstruction.

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.

Evaluation of chondral repair using quantitative MRI

Various quantitative magnetic resonance imaging (qMRI) biomarkers, including but not limited to parametric MRI mapping, semiquantitative evaluation, and morphological assessment, have been successfully applied to assess cartilage repair in both animal and human studies. Through the interaction between interstitial water and constituent macromolecules the compositional and structural properties of cartilage can be evaluated. In this review a comprehensive view of a variety of quantitative techniques, particularly those involving parametric mapping, and their relationship to the properties of cartilage repair is presented. Some techniques, such as T2 relaxation time mapping and delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), are well established, while the full potential of more recently introduced techniques remain to be demonstrated. A combination of several MRI techniques is necessary for a comprehensive characterization of chondral repair.

Osteochondral repair: evaluation with sweep imaging with fourier transform in an equine model

PURPOSE

To evaluate the status of articular cartilage and bone in an equine model of spontaneous repair by using the sweep imaging with Fourier transform (SWIFT) magnetic resonance (MR) imaging technique.

MATERIALS AND METHODS

Experiments were approved by the Utrecht University Animal Ethics Committee. Six-millimeter-diameter chondral (n = 5) and osteochondral (n = 5, 3-4 mm deep into subchondral bone) defects were created in the intercarpal joints of seven 2-year-old horses and examined with SWIFT at 9.4 T after spontaneous healing for 12 months. Conventional T2 maps and gradient-echo images were obtained for comparison, and histologic assessment of cartilage and micro-computed tomography (CT) of bone were performed for reference. Signal-to-noise ratio (SNR) analysis was performed, and a radiologist evaluated the MR images. Structural bone parameters were derived from SWIFT and micro-CT datasets. Significance of differences was investigated with the Wilcoxon signed rank test and Pearson correlation analysis.

RESULTS

SWIFT was able to depict the different outcomes of spontaneous healing of focal chondral versus osteochondral defects. SWIFT produced constant signal intensity throughout cartilage, whereas T2 mapping showed elevated T2 values (P = .06) in repair tissue (mean T2 in superficial region of interest in an osteochondral lesion = 50.0 msec ± 10.2) in comparison to adjacent intact cartilage (mean T2 = 32.7 msec ± 4.2). The relative SNR in the subchondral plate with SWIFT (0.91) was more than four times higher than that with conventional fast spin-echo (0.12) and gradient-echo (0.19) MR imaging. The correlation between bone volume-to-tissue volume fractions determined with SWIFT and micro-CT was significant (r = 0.83, P < .01).

CONCLUSION

SWIFT enabled assessment of spontaneous osteochondral repair in an equine model.

MRI of fast-relaxing spins

MR imaging of extremely fast-relaxing spins is currently a topic of much interest due to recent technical innovations and groundbreaking studies demonstrating its utility in biomedical research applications. From a technical perspective, this article examines the different classes of pulse sequences currently available to image spins with ultra-short transverse relaxation times (T2 and T2*), with particular attention focused on the newest member of the class, sweep imaging with Fourier transformation (SWIFT).

MRI of fast-relaxing spins

MR imaging of extremely fast-relaxing spins is currently a topic of much interest due to recent technical innovations and groundbreaking studies demonstrating its utility in biomedical research applications. From a technical perspective, this article examines the different classes of pulse sequences currently available to image spins with ultra-short transverse relaxation times (T2 and T2*), with particular attention focused on the newest member of the class, sweep imaging with Fourier transformation (SWIFT).

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.