13C imaging—a new diagnostic platform

Adiabatic pulses are a special class of radio frequency (RF) pulses that may be used to achieve uniform flip angles in the presence of a nonuniform B(1) field. In this work, we present a new, systematic method for designing high-bandwidth (BW), low-peak-amplitude adiabatic RF pulses that utilizes the Shinnar-Le Roux (SLR) algorithm for pulse design. Currently, the SLR algorithm is extensively employed to design nonadiabatic pulses for use in magnetic resonance imaging and spectroscopy. We have adapted the SLR algorithm to create RF pulses that also satisfy the adiabatic condition. By overlaying sufficient quadratic phase across the spectral profile before the inverse SLR transform, we generate RF pulses that exhibit the required spectral characteristics and adiabatic behavior. Application of quadratic phase also distributes the RF energy more uniformly, making it possible to obtain the same spectral BW with lower RF peak amplitude. The method enables the pulse designer to specify spectral profile parameters and the degree of quadratic phase before pulse generation. Simulations and phantom experiments demonstrate that RF pulses designed using this new method behave adiabatically.

PASADENA, a chemical method of enhancing nuclear spin polarization has demonstrated 13C polarizations of order unity for the nascent products of molecular addition by parahydrogen. The extreme brevity of signal enhancement obtained by hyperpolarization requires improved 13C MR in vivo imaging techniques for their optimum utility. 13C imaging sequences, including 13C 3D FIESTA, were compiled for a GE LX 1.5 T clinical MR scanner. Two water soluble 13C imaging agents were hyperpolarized utilizing parahydrogen and an automated polarizer. 13C polarization was quantified in flow phantoms and in rats with jugular vein catheters. Fast 3D FIESTA 13C MR imaging technique acquired sequential 3D images (3.66 s/acquisition) with superior SNR. Hyperpolarized 13C solutions and vascular phantoms achieved a maximum signal of 26,624+/-593. In vivo 13C MR images of the cardiopulmonary circulation showed maximum 13C signal of 2,402+/-158. 13C images acquired within 3.66 s showed signal enhancement over 10,000 compared to equilibrium polarization. 3D-FIESTA was effective for sub-second in vivo imaging of hyperpolarized 13C reagents produced in a custom-built parahydrogen polarizer. Application to 13C hyperpolarized by parahydrogen is demonstrated in vitro and in vivo.

Improvements in the quantitative assessment of structure, function, and metabolic activity in the lung, combined with improvements in the spatial resolution of those assessments, enhance the diagnosis and evaluation of pulmonary disorders. Radiologic methods are among the most attractive techniques for the comprehensive assessment of the lung, as they allow quantitative assessment of this organ through measurements of a number of structural, functional, and metabolic parameters. Hyperpolarized nuclei magnetic resonance imaging (MRI) has opened up new territories for the quantitative assessment of lung function and structure with an unprecedented spatial resolution and sensitivity. This review article presents a survey of recent developments in the field of pulmonary imaging using hyperpolarized nuclei MRI for quantitative imaging of different aspects of the lung, as well as preclinical applications of these techniques to diagnose and evaluate specific pulmonary diseases. After presenting a brief overview of various hyperpolarization techniques, this survey divides the research activities of the field into four broad areas: lung microstructure, ventilation, oxygenation, and perfusion. Finally, it discusses the challenges currently faced by researchers in this field to translate this rich body of methodology into wider-scale clinical applications.

Functional magnetic resonance imaging (fMRI) is a unique window to the brain, enabling scientists to follow changes in brain activity in response to hormones, ageing, environment, drugs of abuse and other stimuli. There are two features that make fMRI unique when compared with other imaging modalities used in behavioural neuroscience. First, it can be entirely noninvasive: each animal can serve as its own control over the natural course of its life, vital for following neuroadaptation and other developmental processes critical to understanding behaviour. Second, fMRI has the spatial and temporal resolution to observe patterns of neuronal activity across the entire brain in less than a minute. Although fMRI does not have the cellular spatial resolution of immunostaining, nor the millisecond temporal resolution of electrophysiology, synchronised changes in neuronal activity across multiple brain areas seen with functional MRI can be viewed as functional neuroanatomical circuits coordinating the thoughts, memories and emotions for particular behaviours. Thus, fMRI affords a systems approach to the study of the brain, complementing and building from other neurobiological techniques to understand how behaviour is organised across multiple brain regions. In this review, we present a general background to fMRI and the different imaging modalities that can be used in fMRI studies. Included are examples of the application of fMRI in behavioural neuroscience research, along with discussion of the advantages and disadvantages of this technology.

Compromises in the design of a positron emission tomography (PET) insert for a magnetic resonance imaging (MRI) system should minimize the deterioration of image quality in both modalities, particularly when simultaneous demanding acquisitions are performed. In this work, the advantages of using individually read-out crystals with high-gain silicon photomultipliers (SiPMs) were studied with a small animal PET insert for a 7 T MRI system, in which the SiPM charge was transferred to outside the MRI scanner using coaxial cables. The interferences between the two systems were studied with three radio-frequency (RF) coil configurations. The effects of PET on the static magnetic field, flip angle distribution, RF noise, and image quality of various MRI sequences (gradient echo, spin echo, and echo planar imaging (EPI) at 1H frequency, and chemical shift imaging at 13C frequency) were investigated. The effects of fast-switching gradient fields and RF pulses on PET count rate were studied, while the PET insert and the readout electronics were not shielded. Operating the insert inside a 1H volume coil, used for RF transmission and reception, limited the MRI to T1-weighted imaging, due to coil detuning and RF attenuation, and resulted in significant PET count loss. Using a surface receive coil allowed all tested MR sequences to be used with the insert, with 45–59% signal-to-noise ratio (SNR) degradation, compared to without PET. With a 1H/13C volume coil inside the insert and shielded by a copper tube, the SNR degradation was limited to 23–30% with all tested sequences. The insert did not introduce any discernible distortions into images of two tested EPI sequences. Use of truncated sinc shaped RF excitation pulses and gradient field switching had negligible effects on PET count rate. However, PET count rate was substantially affected by high-power RF block pulses and temperature variations due to high gradient duty cycles.

Hyperpolarized Magnetic Resonance Spectroscopy and Imaging

Application of double spin echo spiral chemical shift imaging to rapid metabolic mapping of hyperpolarized [1 − 13C]-pyruvate

Background: Specific Absorption Rate (SAR) is radiofrequency power delivered to tissue duringa Magnetic Resonance Imaging (MRI) examination, expressed as watts per kg (W/kg). Radiofrequencypower deposition results in increased heating of patient tissues; thus, the use of MRI has to becontrolled to ensure patient safety.
 Objective: The study aimed to evaluate SAR among patients using the 3 Tesla MRI (MRI 3T) and 1.5Tesla MRI (MRI 1.5T) machines.
 Methods: Data were obtained from patients who were examined using MRI 3T (1,159 patients, 8,225series) and MRI 1.5T (1,423 patients, 8,605 series) machines. Age, body weight, SAR, repetition time(TR), type of radiofrequency (RF) pulse and anatomical region exposed were studied.
 Results: Average SAR for all patients using the MRI 3T was lower than that of the MR 1.5T in everypart (p <0.001) = 0.92 ± 0.57 W/Kg, 2.45 ± 1.01 W/Kg, accordingly. The SAR that the patients receivedusing the spin echo technique revealed that T2 weighted image had lower SAR than T1 weighted imagefrom both MRI 3T and MRI 1.5T (p < 0.001), 0.87 and 0.98 W/kg for MRI 3T, 2.20 and 2.83 W/kg forMRI 1.5T, respectively. For underweight patients, the lowest SAR was 0.89 W/Kg (MRI 3T) and 2.40W/Kg (MRI 1.5T), respectively. Whereas, among overweight patients, the SAR was the highest at 0.97W/Kg (MRI 3T) and 2.52 W/Kg (MRI 1.5T). For SAR categorized by the flip angle of the RF pulse,and patients evaluated by the MRI 3T, the study revealed that the group with the flip angle of the RFpulse <75 degrees had lower SAR than the flip angle of the RF pulse >75 degrees, 0.77 W/Kg and 0.94W/Kg, accordingly (p < 0.001) similar to the MRI 1.5T.
 Conclusion: The average SAR of patients evaluated using the MRI 3T was lower than those of patientsevaluated using the MRI 1.5T in every body part examined. SAR was lower when the TR was increasedand flip angle was decreased.

Comparison of Gadoxetic Acid–enhanced MR Imaging versus Four-phase Multi–detector Row Computed Tomography in Assessing Tumor Regression after Radiofrequency Ablation in Subjects with Hepatocellular Carcinomas

Radiofrequency Safety for Interventional MRI Procedures 1

Birdcage coils have been widely used to generate radiofrequency (RF) pulses for magnetic resonance imaging (MRI) because of their capability to produce a highly homogeneous B/sub 1/-field over a large volume within the coil. This is true even if the coil is loaded with a human body or a human head provided that the frequency of the B/sub 1/-field is low (less than 50 MHz). However, when birdcage coils are employed for high frequency MRI, the interaction between the electromagnetic (EM) field and the biological object to be imaged deteriorates the B/sub 1/-field homogeneity. Such a deterioration reduces the quality of the MR images and causes difficulties for advanced MRI studies such as functional MRI and MR spectroscopy. Moreover, the EM interaction at high frequencies also leads to an increased electric field, thus, an increased specific energy absorption rate (SAR) in the biological object, causing concern about the possible adverse effects of MRI on human health. Jin et al. (see Phys. Med. Biol., vol.41, p.2719-38, 1996) proposed a method that was suitable for modelling open MRI coils; however, it has difficulties in modelling coils with an RF shield, which is often used to reduce the unwanted interactions between the EM fields and the exterior structures. We employ the finite-difference time-domain (FDTD) method to study this problem. It is found that the FDTD method can effectively model the complex anatomical structure of the human head, the currents in the MRI coil, and the RF shield.

To improve B1 and B0 inhomogeneity mitigation performance of spatially selective radio-frequency (RF) pulses in parallel transmission while decreasing RF pulse power. Further enhancement of off-resonance correction for rectilinear spoke-trajectory-based RF pulses with known residual geometric distortions after optimization. The appropriate definition of the target magnetization pattern is discussed regarding the maximum physical excitation resolution. Furthermore, a novel variable-density trajectory design is introduced, which subsamples accrued B0 phase error elevations in k-space. A simulation study (echo-planar and spiral 2DRF) at different off-resonance levels and pulse acceleration factors was pursued using data from a whole-body 2-channel parallel transmit 3T MRI system. The new trajectory design for echo-planar 2DRF was validated in human in-vivo experiments. Proper target pattern definition can require spatial filtering, such that RF pulse optimization is prevented from lower excitation performance with significant higher RF power level. The new trajectory design proposed can considerably improve off-resonance compensation, while further reducing the RF power, e.g., 43% less RMSE with 79% less RF power for spoke based pulses. The proposed methods offer significant improvements of the excitation performance (homogeneity and acceleration), while significantly decreasing the RF power. Furthermore, single-channel transmit RF pulse performance can be similarly improved.

Pulmonary magnetic resonance imaging (MRI) offers a variety of radiation-free techniques tailored to assess regional lung ventilation or its surrogates. These techniques encompass direct measurements, exemplified by hyperpolarized gas MRI and fluorinated gas MRI, as well as indirect measurements facilitated by oxygen-enhanced MRI and proton-based Fourier decomposition (FD) MRI. In recent times, there has been substantial progress in the field of FD MRI, which involved improving spatial/temporal resolution, refining sequence design and postprocessing, and developing a comprehensive whole-lung approach. The two-dimensional (2D) phase-resolved functional lung (PREFUL) MRI stands out as an FD-based approach developed for the comprehensive assessment of regional ventilation and perfusion dynamics, all within a single MR acquisition. Recently, a new advancement has been made with the development of 3D PREFUL to assess dynamic ventilation of the entire lung using 8 min exam with a self-gated sequence. The 3D PREFUL acquisition involves employing a stack-of-stars spoiled-gradient-echo sequence with a golden angle increment. Following the compressed sensing image reconstruction of approximately 40 breathing phases, all the reconstructed respiratory-resolved images undergo registration onto a fixed breathing phase. Subsequently, the ventilation parameters are extracted from the registered images. In a study cohort comprising healthy volunteers and patients with chronic obstructive pulmonary disease, the 3D PREFUL ventilation parameters demonstrated strong correlations with measurements obtained from pulmonary function tests. Additionally, the interscan repeatability of the 3D PREFUL technique was deemed to be acceptable, indicating its reliability for repeated assessments of the same individuals. In summary, 3D PREFUL ventilation MRI provides a whole lung coverage and captures ventilation dynamics with enhanced spatial resolution compared to 2D PREFUL. 3D PREFUL technique offers a cost-effective alternative to hyperpolarized 129Xe MRI, making it an attractive option for patient-friendly evaluation of pulmonary ventilation.

A Variable Rate Selective Excitation (VERSE) is a type of Radio Frequency (RF) pulse that reduces the Specific Absorption Rate (SAR) of molecules in a specimen. As high levels of SAR lead to increased patient temperatures during Magnetic Resonance Imaging (MRI) procedures, we develop a selective VERSE pulse that is designed to minimize SAR while preserving its duration and slice profile; called the generalized VERSE (gVERSE). After the formulation of a rigorous mathematical model, the nonlinear gVERSE optimization problem is solved via an optimal control approach. Using the state of the art Sparse Optimal Control Software (SOCS), two separate variations of SAR reducing gVERSE pulses were generated. The Magnetic Resonance (MR) signals produced by numerical simulations were then tested and analyzed by an MRI simulator. Computational experiments involved with the gVERSE model provided constant RF pulse levels and had encouraging results with respect to MR signals. The testing results produced by the gVERSE pulse illustrate the potential advanced optimization techniques have in designing RF sequences.

Spatial Comparison of CT-Based Surrogates of Lung Ventilation With Hyperpolarized Helium-3 and Xenon-129 Gas MRI in Patients Undergoing Radiation Therapy