Jae-Seung Lee

Isolating chemical exchange saturation transfer contrast from magnetization transfer asymmetry under two-frequency rf irradiation.

Chemical exchange saturation transfer (CEST), arising from mobile groups, and magnetization transfer (MT) contrast arising from immobile protons, have enjoyed wide popularity recently in MRI applications. It is often difficult to separate genuine CEST signatures from MT effects, which are asymmetric with respect to the water resonance. A two-pool model for magnetization transfer (MT) is established based on Provotorovs theory of saturation, and then extended to the situation of simultaneous two-frequency rf irradiation. Numerical simulations and experimental results demonstrate that two-frequency rf irradiation can flatten out MT asymmetry when both frequency components lie within the spectrum of an MT pool. Based on this result, we propose a strategy to isolate chemical exchange saturation transfer (CEST) contrast from MT asymmetry contrast by using the two-frequency rf irradiation technique.

Sodium inversion recovery MRI of the knee joint in vivo at 7T.

The loss of proteoglycans (PG) in the articular cartilage is an early signature of osteoarthritis (OA). The ensuing changes in the fixed charge density in the cartilage can be directly linked to sodium concentration via charge balance. Sodium ions in the knee joint appear in two pools: in the synovial fluids or joint effusion where the ions are in free motion and bound within the cartilage tissue where the Na(+) ions have a restricted motion. The ions in these two compartments have therefore different T₁ and T₂ relaxation times. The purpose of this study is to demonstrate the feasibility of a fluid-suppressed 3D ultrashort TE radial sodium sequence by implementing an inversion recovery (IR) preparation of the magnetization at 7T. This method could allow a more accurate and more sensitive quantification of loss of PG in patients with OA. It is shown that adiabatic pulses offer significantly improved performance in terms of robustness to B₁ and B₀ inhomogeneities when compared to the hard pulse sequence. Power deposition considerations further pose a limit to the RF inversion power, and we demonstrate in simulations and experiments how a practical compromise can be struck between clean suppression of fluid signals and power deposition levels. Two IR sequences with different types of inversion pulses (a rectangular pulse and an adiabatic pulse) were tested on a liquid phantom, ex vivo on a human knee cadaver and then in vivo on five healthy volunteers, with a (Nyquist) resolution of ∼3.6 mm and a signal-to-noise ratio of ∼30 in cartilage without IR and ∼20 with IR. Due to specific absorption rate limitations, the total acquisition time was ∼17 min for the 3D radial sequence without inversion or with the rectangular IR, and 24:30 min for the adiabatic IR sequence. It is shown that the adiabatic IR sequence generates a more uniform fluid suppression over the whole sample than the rectangular IR sequence.

Sodium MRI: Methods and applications

Sodium NMR spectroscopy and MRI have become popular in recent years through the increased availability of high-field MRI scanners, advanced scanner hardware and improved methodology. Sodium MRI is being evaluated for stroke and tumor detection, for breast cancer studies, and for the assessment of osteoarthritis and muscle and kidney functions, to name just a few. In this article, we aim to present an up-to-date review of the theoretical background, the methodology, the challenges, limitations, and current and potential new applications of sodium MRI.

Ultrafast Scanning of Exchangeable Sites by NMR Spectroscopy

Chemical exchange saturation transfer (CEST) and magnetization transfer (MT) have become widely popular techniques for generating enhanced MRI contrast in vivo and ex vivo.[1] In order to fully characterize these phenomena, and to investigate suitable candidates for contrast agents, the polarization of the reporter signal (typically water) is measured as a function of the frequency offset of the saturating RF irradiation (Z-spectrum).[2] Here, we present an ultrafast method to obtain a Z-spectrum over a large range of frequency offsets from only two signal excitations. This method can be useful for fast screening of imaging phantoms and para- and diamagnetic CEST contrast agents under different experimental conditions (saturation time, power, etc).

Implementing unitary operators in quantum computation

We present a general method which expresses a unitary operator by the product of operators allowed by the Hamiltonian of spin-1/2 systems. In this method, the generator of an operator is found first, and then the generator is expanded by the base operators of the product operator formalism. Finally, the base operators disallowed by the Hamiltonian, including more than two-body interaction operators, are replaced by allowed ones by the axes transformation and coupling order reduction technique. This method directly provides pulse sequences for the nuclear magnetic resonance quantum computer, and can be generally applied to other systems.

Implementation of the refined Deutsch-Jozsa algorithm on a three-bit NMR quantum computer

We implemented the refined Deutsch-Jozsa algorithm on a three-bit nuclear magnetic resonance quantum computer. All of the balanced and constant functions were realized exactly. The results agree well with theoretical predictions and clearly distinguish the balanced functions from constant functions. Efficient refocusing schemes were proposed for the soft z pulse and J coupling, and it is shown that the thermal equilibrium state gives the same answer as the pure state for this algorithm.

Pseudopure state of a twelve-spin system

Pseudopure states of a system of twelve interacting spins are experimentally demonstrated. The system is a cluster of dipolar-coupled nuclear spins of fully labeled (13)C(6)-benzene in a liquid crystalline matrix. At present, this is the largest and the most complex composite system where individual quantum states have been addressed.

Uniform saturation of a strongly coupled spin system by two-frequency irradiation

The theoretical basis of two-frequency saturation is given here in the framework of Provotorov theory. The parameters influencing the saturation efficiency are discussed and studied experimentally using a liquid-crystalline test system. It is shown that double-frequency irradiation can be extremely efficient when the irradiation frequencies are placed at opposite sides of the characteristic frequency of the spin system, and that the frequency separation in the double-frequency irradiation can be varied over a large range. Provotorov theory is also shown to provide good insights into the experimental findings, which would otherwise be difficult to obtain from simulations.

The quantum state tomography on an NMR system

The quantum state tomography on an NMR system is analyzed. The evolution of a given system by its Hamiltonian during the measurement can be understood as the inverse evolution of physical observables. In this way the NMR signal is expressed as a composition of the ensemble averages of observables, which facilitates the build-up of the procedure for estimating the density operator. The widely used procedure in which only single-qubit rotations are used to get information about the ensemble averages of a quorum of observables is devoted and the lower bound for the number of experiments is obtained.

Uniform magnetization transfer in chemical exchange saturation transfer magnetic resonance imaging

The development of chemical exchange saturation transfer (CEST) has led to the establishment of new contrast mechanisms in magnetic resonance imaging, which serve as enablers for advanced molecular imaging strategies. Macromolecules in tissues and organs often give rise to broad and asymmetric exchange effects, called magnetization transfer (MT) effects, which can mask the CEST contrast of interest. We show here that the saturation of these macromolecular pools simultaneously at two distinct frequencies can level out the asymmetric MT effects, thus allowing one to isolate the CEST effects in vivo. For the first time, clean CEST contrast for glycosaminoglycans (gagCEST) in cartilage in the human knee joint is presented. In addition, the method allows one to clearly demarcate glycosaminoglycan measurements from cartilage and synovial fluid regions. This uniform-MT CEST methodology has wide applicability in in vivo molecular imaging (such as brain, skeletal muscle, etc).