MRI of fast-relaxing spins

Magnetic Resonance Imaging of Water Content and Flow Processes in Natural Soils by Pulse Sequences with Ultrashort Detection

Magnetic resonance imaging is a valuable tool for three-dimensional mapping of soil water processes due to its sensitivity to the substance of interest: water. Since conventional gradient- or spin-echo based pulse sequences do not detect rapidly relaxing fractions of water in natural porous media with transverse relaxation times in the millisecond range, pulse sequences with ultrafast detection open a way out. In this work, we compare a spin-echo multislice pulse sequence with ultrashort (UTE) and zero-TE (ZTE) sequences for their suitability to map water content and its changes in 3D in natural soil materials. Longitudinal and transverse relaxation times were found in the ranges around 80 ms and 1 to 50 ms, respectively, so that the spin echo sequence misses larger fractions of water. In contrast, ZTE and UTE could detect all water, if the excitation and detection bandwidths were set sufficiently broad. More precisely, with ZTE we could map water contents down to 0.1 cm³/cm³. Finally, we employed ZTE to monitor the development of film flow in a natural soil core with high temporal resolution. This opens the route for further quantitative imaging of soil water processes.

Magnetic resonance free induction decay in geological porous materials

Magnetic resonance is an important noninvasive technology across life sciences and industry. Free induction decay is the simplest [Formula: see text]H magnetic resonance measurement method and an important means of probing fast-decaying signals in porous materials such as rocks, lung, and bone. It is commonly assumed that the free induction decay in geological porous materials is single-exponential. We experimentally observed two regimes of free induction decay behavior in geological porous materials: single-exponential and non-exponential decay. Numerical simulations that match experimental data highlight the effect of mass diffusion, especially in the single-exponential behavior. These two regimes of free induction decay in porous materials are associated with a bifurcation point in the solutions of the Bloch-Torrey equation for diffusion of fluids in confined domains in the presence of internal magnetic field gradients. This finding facilitates the extraction of absolute internal magnetic field gradient intensities from simple free induction decay measurements in the laboratory and field. This work also warns against common single-exponential assumptions in surface magnetic resonance methods employed in surveying underground water aquifers.

Comparison Between Ultrasonographic and Standing Magnetic Resonance Imaging Findings in the Podotrochlear Apparatus of Horses With Foot Pain

This prospective study aimed to blindly compare the ultrasonographic and standing magnetic resonance imaging (sMRI) findings in deep digital flexor tendon (DDFT), navicular bone, and navicular bursa in horses with foot pain, positive digital analgesia, and without definitive radiographic diagnosis. Ultrasonography detected more DDFT abnormalities (32/34 feet vs. 27/34 with sMRI) but identified less palmar navicular abnormalities (23/34 feet vs. 30/34 with sMRI). In suprasesamoidean DDFT lesions, which were mainly dorsally located, changes in echogenicity did not correspond to a particular pattern of sMRI signal change. Transcuneal ultrasonography did not allow assessment of morphology and extent of distal DDFT lesions, and sporadically discriminated the affected lobe compared to sMRI. Defects of the palmar compact bone were identified with both modalities except a parasagittal defect, which was only seen at sMRI.

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.

Characterization of hardware-related spatial distortions for IR-PETRA pulse sequence using a brain specific phantom

OBJECTIVE

Inversion recovery-pointwise encoding time reduction with radial acquisition (IR-PETRA) is an effective magnetic resonance (MR) pulse sequence in generating pseudo-CTs. The hardware-related spatial-distortion (HRSD) in MR images potentially deteriorates the accuracy of pseudo-CTs. Thus, we aimed at characterizing HRSD for IR-PETRA.

MATERIALS AND METHODS

gross-HRSD_overall (Euclidean-sum of gross-HRSD_i (i = x, y, z)) for IR-PETRA was assessed using a brain-specific phantom for two MR scanners (1.5 T-Aera and 3.0 T-Prisma). Moreover, hardware imperfections were analyzed by determining gradient-nonlinearity spatial-distortion (GNSD) and B₀-inhomogeneity spatial-distortion (B₀ISD) for magnetization-prepared rapid acquisition gradient-echo (MP-RAGE) which has well-known distortion characteristics.

RESULTS

In 3.0 T, maximum of gross-GNSD_overall (Euclidean-sum of gross-GNSD_i) and gross-B₀ISD for MP-RAGE was 2.77 mm and 0.57 mm, respectively. For this scanner, the mean and maximum of gross-HRSD_overall for IR-PETRA were 0.63 ± 0.38 mm and 1.91 mm, respectively. In 1.5 T, maximum of gross-GNSD_overall and gross-B₀ISD for MP-RAGE was 3.41 mm and 0.78 mm, respectively. The mean and maximum of gross-HRSD_overall for IR-PETRA were 1.02 ± 0.50 mm and 3.12 mm, respectively.

DISCUSSION

The spatial accuracy of MR images, besides being impacted by hardware performance, scanner capabilities, and imaging parameters, is mainly affected by its imaging strategy and data acquisition scheme. In 3.0 T, even without applying vendor correction algorithms, spatial accuracy of IR-PETRA image is sufficient for generating pseudo-CTs. In 1.5 T, distortion-correction is required to provide this accuracy.

Respiratory motion-resolved four-dimensional zero echo time (4D ZTE) lung MRI using retrospective soft gating: feasibility and image quality compared with 3D ZTE

OBJECTIVES

To evaluate the feasibility and image quality of respiratory motion-resolved 4D zero echo time (ZTE) lung MRI compared with that of 3D ZTE.

METHODS

Our institutional review board approved this study. Twenty-one patients underwent lung scans using 3D ZTE and 4D ZTE sequences via prospective and retrospective soft gating techniques, respectively. Image qualities of 3D ZTE and 4D ZTE at end-expiration were compared through objective and subjective assessments. The quality of end-expiratory images of 3D ZTE and 4D ZTE of the two groups with different lung functions was also compared.

RESULTS

Images were successfully acquired in all patients without any adverse events. Signal-to-noise ratios (SNRs) of lung parenchyma and thoracic structures were significantly (all p < 0.001) higher in 4D ZTE. Contrast-to-noise ratios (CNRs) of peripheral bronchi, peripheral pulmonary vessels, and nodules or masses were significantly (all p < 0.001) higher in 4D ZTE. The subjective image quality assessed by two independent radiologists showed that intrapulmonary structures, noise and artifacts, and overall acceptability were superior in 4D ZTE (all p < 0.001). Image qualities of groups with normal and low lung functions differed significantly (all p < 0.05) in 3D ZTE, but not in 4D ZTE. The mean acquisition time was 136 s (127-143 s) in 3D ZTE and 325 s (308-352 s) in 4D ZTE.

CONCLUSIONS

Respiratory motion-resolved 4D ZTE lung imaging was feasible as part of routine chest MRI. The 4D ZTE provides motion-robust lung parenchymal images with better SNR and CNR than the 3D ZTE, regardless of patients' lung function.

KEY POINTS

• ZTE MRI captures rapidly decaying transverse magnetization in the lung parenchyma. • 4D ZTE provides motion-robust lung parenchymal images with better SNR and CNR compared with 3D ZTE. • Compared with 3D ZTE, the image quality of 4D ZTE lung MRI was affected less by patients' lung function and respiratory performance.

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.

Radiofrequency phase encoded half-pulses in simultaneous multislice ultrashort echo time imaging

PURPOSE

To describe a simultaneous multislice (SMS) ultrashort echo time (UTE) imaging method using radiofrequency phase encoded half-pulses in combination with power independent of number of slices (PINS) inversion recovery (IR) pulses to generate multiple-slice images with short T₂ * contrasts in less than 3 min with close to an eightfold acceleration compared with a standard 2D approach.

THEORY AND METHODS

Radiofrequency phase encoding is applied in an SMS (N_SMS = 4) excitation scheme using "sinc" half-pulses. With the use of coil sensitivity encoding (SENSE) and controlled aliasing in parallel imaging (CAIPI) in combination with a gradient echo 2D spiral readout trajectory and IR PINS pulses for contrast enhancement a fast UTE sequence is developed. Images are obtained using a model-based reconstruction method. Sequence details and performance tests on phantoms as well as the heads of healthy volunteers at 3T are presented.

RESULTS

An SMS UTE sequence with an undersampling factor of 4 is developed using radiofrequency phase encoded half-pulses and PINS IR pulses which enables the acquisition of 8 slices at 0.86² mm² resolution in less than 3-min scan time. UTE images of the head are obtained showing highlighted signal of cortical bone. Image quality and T₂ contrast are comparable to the one obtained by corresponding single slice acquisitions with only minor deviations.

CONCLUSIONS

The method combining radiofrequency phase encoded SMS half-pulses and PINS IR pulses presents a suitable approach to SMS UTE imaging. The usage of coil sensitivity information and increased sampling density by means of interleaved slice group acquisitions allows to reduce the total scan time by a factor close to 8.

In vivo MRI with Concurrent Excitation and Acquisition using Automated Active Analog Cancellation

Magnetic resonance imaging (MRI) provides excellent cross-sectional images of the soft tissues in patients. Unfortunately, MRI is intrinsically slow, it exposes patients to severe acoustic noise levels, and is limited in the visualization of certain tissues such as bone. These limitations are partly caused by the timing structure of the MRI exam which first generates the MR signal by a strong radio-frequency excitation and later acquires the weak MRI signal. Concurrent excitation and acquisition (CEA) can overcome these limitations, but is extremely challenging due to the huge intensity difference between transmit and receive signal (up to 100 dB). To suppress the strong transmit signals during signal reception, a fully automated analog cancellation unit was designed. On a 3 Tesla clinical MRI system we achieved an on-resonance analog isolation of 90 dB between the transmit and receive path, so that CEA images of the head and the extremities could be acquired with an acquisition efficiency of higher than 90% at sound pressure levels close to background noise. CEA with analog cancellation might provide new opportunities for MRI in tissues with very short T₂ relaxation times, and it offers a silent and time-efficient MRI acquisition.

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.

Solving the Bloch equation with periodic excitation using harmonic balancing: application to Rabi modulated excitation

In waveform design for magnetic resonance applications, periodic continuous-wave excitation offers potential advantages that remain largely unexplored because of a lack of understanding of the Bloch equation with periodic continuous-wave excitations. Using harmonic balancing techniques the steady state solutions of the Bloch equation with periodic excitation can be effectively solved. Moreover, the convergence speed of the proposed series approximation is such that a few terms in the series expansion suffice to obtain a very accurate description of the steady state solution. The accuracy of the proposed analytic approximate series solution is verified using both a simulation study as well as experimental data derived from a spherical phantom with doped water under continuous-wave excitation. Typically a five term series suffices to achieve a relative error of less than one percent, allowing for a very effective and efficient analytical design process. The opportunities for Rabi frequency modulated continuous-wave form excitation are then explored, based on a comparison with steady state free precession pulse sequences.

Simultaneous evaluation of vascular morphology, blood volume and transvascular permeability using SPION-based, dual-contrast MRI: imaging optimization and feasibility test

Exploiting ultrashort-T(E) (UTE) MRI, T1-weighted positive contrast can be obtained from superparamagnetic iron oxide nanoparticles (SPIONs), which are widely used as a robust T2-weighted, negative contrast agent on conventional MR images. Our study was designed (a) to optimize the dual-contrast MRI method using SPIONs and (b) to validate the feasibility of simultaneously evaluating the vascular morphology, blood volume and transvascular permeability using the dual-contrast effect of SPIONs. All studies were conducted using 3 T MRI. According to numerical simulation, 0.15 mM was the optimal blood SPION concentration for visualizing the positive contrast effect using UTE MRI (T(E) = 0.09 ms), and a flip angle of 40° could provide sufficient SPION-induced enhancement and acceptable measurement noise for UTE MR angiography. A pharmacokinetic study showed that this concentration can be steadily maintained from 30 to 360 min after the injection of 29 mg/kg of SPIONs. An in vivo study using these settings displayed image quality and CNR of SPION-enhanced UTE MR angiography (image quality score 3.5; CNR 146) comparable to those of the conventional, Gd-enhanced method (image quality score 3.8; CNR 148) (p > 0.05). Using dual-contrast MR images obtained from SPION-enhanced UTE and conventional spin- and gradient-echo methods, the transvascular permeability (water exchange index 1.76-1.77), cerebral blood volume (2.58-2.60%) and vessel caliber index (3.06-3.10) could be consistently quantified (coefficient of variation less than 9.6%; Bland-Altman 95% limits of agreement 0.886-1.111) and were similar to the literature values. Therefore, using the optimized setting of combined SPION-based MRI techniques, the vascular morphology, blood volume and transvascular permeability can be comprehensively evaluated during a single session of MR examination.

New opportunities for quantitative and time efficient 3D MRI of liquid and solid electrochemical cell components: Sectoral Fast Spin Echo and SPRITE

The ability to image electrochemical processes in situ using nuclear magnetic resonance imaging (MRI) offers exciting possibilities for understanding and optimizing materials in batteries, fuel cells and supercapacitors. In these applications, however, the quality of the MRI measurement is inherently limited by the presence of conductive elements in the cell or device. To overcome related difficulties, optimal methodologies have to be employed. We show that time-efficient three dimensional (3D) imaging of liquid and solid lithium battery components can be performed by Sectoral Fast Spin Echo and Single Point Imaging with T1 Enhancement (SPRITE), respectively. The former method is based on the generalized phase encoding concept employed in clinical MRI, which we have adapted and optimized for materials science and electrochemistry applications. Hard radio frequency pulses, short echo spacing and centrically ordered sectoral phase encoding ensure accurate and time-efficient full volume imaging. Mapping of density, diffusivity and relaxation time constants in metal-containing liquid electrolytes is demonstrated. 1, 2 and 3D SPRITE approaches show strong potential for rapid high resolution (7)Li MRI of lithium electrode components.

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.