Description: Purpose To develop a 3D adiabatic T 1ρ prepared ultrashort echo time cones (3D AdiabT 1ρ UTE-Cones) sequence for whole knee imaging on a clinical 3T scanner. Methods A train of adiabatic full passage pulses were used for spin locking, followed by time-efficient multispoke UTE acquisition to detect signals from both short and long T 2 tissues in the whole knee joint. A modified signal model was proposed for multispoke UTE data fitting. The feasibility of this 3D AdiabT 1ρ UTE-Cones technique was demonstrated through numerical simulation, phantom, and ex vivo knee sample studies. The 3D AdiabT 1ρ UTE-Cones technique was then applied to 6 in vivo knee joints of healthy volunteers to measure T 1ρ values of quadriceps tendon, patellar tendon, anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), meniscus, patellar cartilage, and muscle. Results Numerical simulation, phantom and ex vivo knee sample studies demonstrated the feasibility of whole knee imaging using the proposed multispoke 3D AdiabT 1ρ UTE-Cones sequence. The healthy volunteer knee study demonstrated an averaged T 1ρ of 13.9 ± 0.7 ms for the quadriceps tendon, 9.7 ± 0.8 ms for the patellar tendon, 34.9 ± 2.8 ms for the ACL, 21.6 ± 1.4 ms for the PCL, 22.5 ± 1.9 ms for the meniscus, 44.5 ± 2.4 ms for the patellar cartilage, and 43.2 ± 1.1 ms for the muscle. Conclusion The 3D AdiabT 1ρ UTE-Cones sequence allows volumetric T 1ρ assessment of both short and long T 2 tissues in the knee joint on a clinical 3T scanner.

Description: Purpose We introduce the quantitative, continuous marker cartilage cavity that quantifies cartilage lesions by the total lesion volume. The aim was to quantify small lesions as well as large, full-depth lesions. Methods We included 315 knees from the Center for Clinical and Basic Research (CCBR), 972 knees from the Osteoarthritis Initiative (OAI), and 791 knees from the Prevention of OA in Overweight Females (PROOF) cohorts. In a subset, we digitally inserted artificial lesions. Each knee MRI was segmented using the knee imaging quantification (KIQ) framework. We quantified cartilage mean thickness and cavity from high-resolution cartilage thickness maps. Finally, we quantified lesion volume by the gradient peak method (GPM). Results Scan–rescan precision for cartilage cavity was 7.1%/3.0%. The cartilage cavity accuracy on the artificial lesions was determined as linear correlation at 0.88 with an average 8% under-estimation of lesion volume. Cavity and degree of radiographic osteoarthritis (ROA) correlated for all compartments (Spearman's rho between 0.14–0.56, P  〈 0.001). Cavity had modest correlations to whole-organ magnetic resonance imaging score (WORMS) cartilage lesion scores but strong correlations with Boston-Leeds osteoarthritis knee score (BLOKS)/MRI osteoarthritis knee score (MOAKS) scores in most compartments (rho between 0.08–0.65, P  〈 0.001). Cavity correlated with WOMAC pain for all tibio-femoral compartments in OAI (rho between 0.19–0.25, P  〈 0.001) and most compartments in PROOF. Comparing with the GPM estimate, cavity was more precise, more accurate, and correlated stronger with ROA, lesion scores, and pain levels. Conclusion The strong correlations with ROA, radiologist lesion scores, and pain demonstrated that cavity captured OA and lesion features. Thereby, it may be appropriate for quantification of cartilage surface irregularity.

Description: Purpose To improve 2D noncontrast-enhanced MRA by using a helical time-of-flight (TOF) acquisition technique and a slice-super-resolution reconstruction. Methods The TOF technique is combined with a helical trajectory with golden-angle–based radial projection reordering. A continuous spatial shift in slice direction is realized by adjusting the frequency of the excitation pulse between the individual projections. The limited resolution along the shift direction is improved by a deconvolution with simulated slice profile. The helical TOF (hTOF) was compared in vivo with a conventional 2D and 3D TOF. Results Results from in vivo experiments on the carotid show that the visual resolution in slice direction can be improved by using hTOF and the slice-super-resolution reconstruction. The vessels appear up to 1.5 times sharper and can be better separated from each other. Compared to 2D TOF images, the stair step artifacts are strongly reduced in reformatted hTOF images, whereas measurement time is decreased by at least 35%. Compared to 3D TOF, the hTOF offers a higher blood-to-background contrast, better visualization of smaller vessels, and reduced measurement time. Conclusion The hTOF benefits from a 2D acquisition and a 3D reconstruction, which makes it a promising technique for the noncontrast-enhanced imaging of the carotid.

Description: Purpose To evaluate the performance of micro-electromechanical systems (MEMS) switches against PIN diodes for switching a dual-tuned RF coil between 19 F and 1 H resonant frequencies for multi-nuclear lung imaging. Methods A four-element fixed-phase and amplitude transmit–receive RF coil was constructed to provide homogeneous excitation across the lungs, and to serve as a test system for various switching methods. The MR imaging and RF performance of the coil when switched between the 19 F and 1 H frequencies using MEMS switches, PIN diodes and hardwired configurations were compared. Results The performance of the coil with MEMS or PIN diode switching was comparable in terms of RF measurements, transmit efficiency and image SNR on both 19 F and 1 H nuclei. When the coil was not switched to the resonance frequency of the respective nucleus being imaged, reductions in the transmit efficiency were observed of 32% at the 19 F frequency and 12% at the 1 H frequency. The coil provides transmit field homogeneity of ±12.9% at the 1 H frequency and ±14.4% at the 19 F frequency in phantoms representing the thorax with the air space of the lungs filled with perfluoropropane gas. Conclusion MEMS and PIN diodes were found to provide comparable performance in on-state configuration, while MEMS were more robust in off-state high-powered operation (〉1 kW), providing higher isolation and requiring a lower DC switching voltage than is needed for reverse biasing of PIN diodes. In addition, clear benefits of switching between the 19 F and 1 H resonances were demonstrated, despite the proximity of their Larmor frequencies.

Description: Purpose To demonstrate a computationally efficient and theoretically artifact-free method to calculate static field (B 0 ) inhomogeneity in a volume of interest induced by an arbitrary voxelated susceptibility distribution. Methods Our method computes B 0 by circular convolution between a zero-filled susceptibility matrix and a shifted, voxel-integrated dipolar field kernel on a grid of size N S +N T – 1 in each dimension, where N S and N T are the sizes of the susceptibility source and B 0 target grids, respectively. The computational resource requirement is independent of source-target separation. The method, called generalized susceptibility voxel convolution, is demonstrated on three susceptibility models: an ellipsoid, MR-compatible screws, and a dynamic human heartbeat model. Results B 0 in an ellipsoid calculated by generalized susceptibility voxel convolution matched an analytical solution nearly exactly. The method also calculated screw-induced B 0 in agreement with experimental data. Dynamic simulation demonstrated its computational efficiency for repeated B 0 calculations on time-varying susceptibility. On the contrary, conventional and alias-subtracted k-space-discretized Fourier convolution methods showed nonnegligible aliasing and Gibbs ringing artifacts in the tested models. Conclusion Generalized susceptibility voxel convolution can be a fast and reliable way to compute susceptibility-induced B 0 when the susceptibility source is not colocated with the B 0 target volume of interest, as in modeling B 0 variations from motion and foreign objects.

Description: Purpose 2D turbo-spin-echo (TSE) is widely used in the clinic for neuroimaging. However, the long refocusing radiofrequency pulse train leads to high specific absorption rate (SAR) and alters the contrast compared to conventional spin-echo. The purpose of this work is to develop a robust 2D spiral TSE technique for fast T 2 -weighted imaging with low SAR and improved contrast. Methods A spiral-in/out readout is incorporated into 2D TSE to fully take advantage of the acquisition efficiency of spiral sampling while avoiding potential off-resonance-related artifacts compared to a typical spiral-out readout. A double encoding strategy and a signal demodulation method are proposed to mitigate the artifacts because of the T 2 -decay-induced signal variation. An adapted prescan phase correction as well as a concomitant phase compensation technique are implemented to minimize the phase errors. Results Phantom data demonstrate the efficacy of the proposed double encoding/signal demodulation, as well as the prescan phase correction and concomitant phase compensation. Volunteer data show that the proposed 2D spiral TSE achieves fast scan speed with high SNR, low SAR, and improved contrast compared to conventional Cartesian TSE. Conclusion A robust 2D spiral TSE technique is feasible and provides a potential alternative to conventional 2D Cartesian TSE for T 2 -weighted neuroimaging.

Description: Purpose Anesthesia is necessary for most animal studies requiring invasive procedures. It is well documented that various types of anesthesia modulate a wide variety of important metabolic and functional processes in the body, and as such, represent a potential limitation in the study design. In the present study, we aimed to investigate the renal functional and metabolic consequences of 3 typical rodent anesthetics used in preclinical MRI: sevoflurane, inaction, and a mixture of fentanyl, fluanisone, and midazolam (FFM). Methods The renal effects of 3 different classes of anesthetics (inactin, servoflurane, and FFM) were investigated using functional and metabolic MRI. The renal glucose metabolism and hemodynamics was characterized with hyperpolarized [1- 13 C]pyruvate MRI and by DCE imaging. Results Rats receiving sevoflurane or FFM had blood glucose levels that were 1.3-fold to 1.4-fold higher than rats receiving inactin. A 2.9-fold and 4.8-fold increased 13 C-lactate/ 13 C-pyruvate ratio was found in the FFM mixture anesthetized group compared with the sevoflurane and the inactin anesthetized groups. The FFM anesthesia resulted in a 50% lower renal plasma flow compared with the sevoflurane and the inactin anesthetized groups. Conclusion This study demonstrates different renal metabolic and hemodynamic changes under 3 different anesthetics, using hyperpolarized MR in rats. Inactin and sevoflurane were found to affect the renal hemodynamic and metabolic status to a lesser degree than FFM. Sevoflurane anesthesia is particularly easy to induce and maintain during the whole anesthesia procedure, and as such, represents a good alternative to inaction, although it alters the blood glucose level.

Description: Purpose Magnetic resonance imaging has been used extensively to track in vivo implanted cells that have been previously labeled with relaxation enhancers. However, this approach is not suitable to track multiple cell populations, as it may lead to confounding results in case the contrast agent is released from the labeled cells. This paper demonstrates how the use of CEST agents can overcome these issues. After encapsulating paramagnetic lanthanide shift reagents, we may shift the absorption frequency of the intracellular water resonance (δ In ), thus generating frequency-encoding CEST responsive cells that can be visualized in the MR image by applying the proper RF irradiation. Methods Eu-HPDO3A, Dy-HPDO3A, and Tm-HPDO3A were used as shift reagents for labeling murine breast cancer cells and murine macrophages by hypotonic swelling and pinocytosis. The CEST-MR images were acquired at 7 T, and the saturation transfer effect was measured. Samples at different dilution of cells were analyzed to quantify the detection threshold. In vitro experiments of cell proliferation were carried out. Finally, murine breast cancer cells were injected subcutaneously in mice, and MR images were acquired to assess the proliferation index in vivo. Results It was found that entrapment of the paramagnetic complexes into endosomes (i.e., using the pinocytosis route) leads to an enhanced shift of the intracellular water resonance. δ In appears to be proportional to the effective magnetic moment (μ eff ) and to the concentration of the loaded lanthanide complex. Moreover, a higher shift is present when the complexes are entrapped in the endosomes. The cell proliferation index was assessed both in vitro and in vivo by evaluating the reduction of δ In value in the days after the cell labeling. Conclusion Cells can be visualized by CEST MRI after loading with paramagnetic shift reagent, by exploiting the large ensemble of the properly shifted intracellular water molecules. A better performance is obtained when the complexes are entrapped inside the endosomes. The observed (δ In ) value is strongly correlated to the chemical nature of the probe, and to its concentration and cellular localization. Two applications of this method are reported in this paper: (1) for in vivo cell visualization and (2) for the monitoring of the cellular proliferation process, as this method is accompanied by a change in δ In that may be exploited as a longitudinal reporter of the proliferation rate.