Longitudinal spin-order-based pulse sequence for lactate editing

Cardiac 1H MR spectroscopy: development of the past five decades and future perspectives

Continued advances in laboratory medicine are required to realize the potential of individualized medicine to impact common cardiovascular diseases. Magnetic resonance imaging (MRI) and spectroscopy (MRS) techniques have advanced over recent years and offer unique, powerful insights into cardiac anatomic and metabolic changes, respectively, occurring in both nascent and advanced heart disease. Although numerous MRI-based in vivo diagnostics are already used in routine clinical practice and more are anticipated, MRS has been less incorporated into routine clinical practice. Given the ability of ¹H MRS to identify and quantify specific molecules with high sensitivity and specificity, its potential utility should be successfully transition from "bench-to-bedside" is tantalizing. The present review will highlight the development of ¹H MRS techniques for cardiac applications, observations in seminal studies with ¹H MRS, and the prospects and challenges for widespread application in patients with cardiovascular disease.

Lactate detection in inducible and orthotopic Her2/neu mammary gland tumours in mouse models

This study compared the steady state concentration of lactate in an inducible Her2/nue transgenic breast cancer mouse model and in tumours from the same Her2/neu model grown orthotopically. In vivo lactate was detected by MRS using the Hadamard encoded selective multiple quantum coherence pulse sequence (HadSelMQC) recently developed by our laboratory. A lower lactate signal was observed in the inducible tumours compared to orthotopic tumours in vivo, while ex vivo analysis of perchloric acid extracts revealed similar amounts of this metabolite in both models. Histological staining of mammary tumour specimens showed a much higher level of fat tissue in inducible tumours compared to the orthotopic model. Phantom studies with [3-(13) C] lactate indicated that a lipid environment could significantly reduce the T2 of lactate and impede its detection. The transgenic inducible model for breast cancer not only better recapitulated the biological aspects of the human disease but also provided additional characteristics related to in vivo detection of lactate that are not available in orthotopic or xenograft models. This study suggests that the level of lactate measured by the HadSelMQC pulse sequence may be underestimated in human patients in the presence of high lipid levels that are typically encountered in the breast.

Noninvasive monitoring of lactate dynamics in human forearm muscle after exhaustive exercise by (1)H-magnetic resonance spectroscopy at 7 tesla

Despite its importance in energy metabolism, lactate in human skeletal muscle has been difficult to detect by noninvasive (1)H-magnetic resonance spectroscopy mainly due to interference from large water and lipid signals. Long echo-time acquisitions at 7 T effectively attenuates the water and lipid signals in forearm muscle allowing direct observation of both lactate resonances, the methine at 4.09 ppm and the methyl at 1.31 ppm. Using this approach, we were able to monitor lactate dynamics at a temporal resolution of 32 s. While lactate was not detectable at rest, immediately after an acute period of exercise to fatigue the forearm muscle, lactate rose to a level comparable to that of creatine (∼30 mmol/kg wet weight). In a typical (1)H-magnetic resonance spectrum collected using a echo-time of 140 ms, the lactate methine and methyl resonances both appear as doublets with an unusually large splitting of ∼20 Hz due to residual dipolar coupling. During muscle recovery following exercise, the lactate signals decay rapidly with a time constant of t½ = 2.0 ± 0.6 min (n = 12 subjects). This fast and simple lactate detection method may prove valuable for monitoring lactate metabolism in cancer and in sports medicine applications.

Slow relaxation of longitudinal multispin orders in weakly and strongly coupled two-spin systems

Longitudinal multispin order (LOMO) corresponds to a nonequilibrium population distribution in spin systems that exhibit scalar (J), dipolar, or quadrupolar coupling. We investigated the relaxation of longitudinal two-spin order (2-LOMO) in systems that had either weakly or strongly J-coupled spins. Our results indicated longer relaxation times for the 2-LOMO state compared with the corresponding longitudinal single-spin state (1-LOMO). Accessing nuclear spin states that have relaxation times longer than T(1), without the use of external contrast agents, is potentially useful for in vivo imaging and also for studying systems using dynamically hyperpolarized nuclear spins where longer life times are sought to increase the time available to study (bio)chemical events.

Analysis of longitudinal multispin orders in strongly coupled spin systems

Longitudinal multispin orders provide an effective way for measurement of scalar couplings and also to probe molecular interactions and dynamics. Analysis of longitudinal orders has been made in strongly coupled AB and ABX spin systems to determine the dependence of strong coupling parameter on these orders. Experimental and simulated spectra of various longitudinal orders are illustrated for these spin systems. This general procedure can be extended to broad range of spin systems to understand the influence of strong coupling on longitudinal orders.

Detection of gamma-aminobutyric acid (GABA) by longitudinal scalar order difference editing

Two novel spectral editing techniques for the in vivo detection of gamma-aminobutyric acid (GABA) are presented. The techniques rely on the generation of longitudinal scalar order (LSO) coherences, which in combination with J-difference editing results in the selective detection of GABA. The utilization of LSO coherences makes the editing sequences insensitive to phase and frequency instabilities. Furthermore, the spectral editing selectivity can be increased independent of the echo time, thereby opening the echo time for state-of-the-art water suppression and/or spatial localization techniques. The performance of the LSO editing techniques is theoretically demonstrated with product operator calculations and density matrix simulations and experimentally evaluated on phantoms in vitro and on human brain in vivo.

Lactate editing and lipid suppression by continuous wavelet transform analysis: application to simulated and (1)H MRS brain tumor time-domain data

Determination of lactate concentrations in vivo is required in the noninvasive diagnosis, staging, and therapeutic monitoring of diseases such as cancer, heart disease, and stroke. An iterative filtering process based on the continuous wavelet transform (CWT) method in the time domain is proposed to isolate the lactate doublet signal from overlapping lipid resonances and estimate the magnetic resonance spectroscopy (MRS) parameters of the lactate methyl signal (signal amplitude, chemical shift, J-coupling and apparent transverse relaxation time (T*(2))). This method offers a number of advantages over the multiple quantum (MQ) and difference spectroscopy approaches, including: 1) full recovery of the lactate methyl signal, whereas the MQ methods usually detect 50% of the signal intensity; 2) in contrast to MQ methods, the lipid signal is retained together with J-coupling data on the lactate peak; 3) the CWT method is much less sensitive to motion artifacts than difference spectroscopy. Application of the method to simulated and real (1)H MRS data collected from human blood plasma and brain tumors demonstrated that this filter provides accurate estimates of the MRS parameters of the lactate doublet and efficiently removes lipid contributions.

In vivo lactate and beta-hydroxybutyrate editing using a pure-phase refocusing pulse train

A refocusing pulse train consisting of a semiselective refocusing pulse and a selective inversion pulse to obtain a pure-phase refocusing at the frequency of maximal excitation of the semiselective refocusing pulse is proposed and applied to in vivo lactate and beta-hydroxybutyrate editing using difference spectroscopy. It is shown, using both rotation matrix theory and phantom experiments, that the soft inversion pulse has to be halved to flank the semiselective pulse to obtain perfect refocusing and cancellation of interfering resonances. The editing method is used to obtain lactate and beta-hydroxybutyrate spectra from the occipital cortex of juvenile epilepsy patients before and after ketogenic diet treatment.

In vivo lactate editing with simultaneous detection of choline, creatine, NAA, and lipid singlets at 1.5 T using PRESS excitation with applications to the study of brain and head and neck tumors

Two T2-independent J-difference lactate editing schemes for the PRESS magnetic resonance spectroscopy localization sequence are introduced. The techniques, which allow for simultaneous acquisition of the lactate doublet (1.3 ppm) and edited singlets upfield of and including choline (3.2 ppm), exploit the dependence of the in-phase intensity of the methyl doublet upon the time interval separating two inversion (BASING) pulses applied to its coupling partner after initial excitation. Editing method 1, which allows for echo times TE = n/J (n = 1, 2, 3, . . . . ), alters the BASING carrier frequency for each of two cycles so that, for one cycle, the quartet is inverted, whereas, for the other cycle, the quartet is unaffected. Method 2, which also provides water suppression, allows for editing for TE > 1/J by alternating, between cycles, the time interval separating the inversion pulses. Experimental results were obtained at 1.5 T using a Shinnar Le-Roux-designed maximum phase inversion pulse with a filter transition bandwidth of 55 Hz. Spectra were acquired from phantoms and in vivo from the human brain and neck. In a neck muscle study, the lipid suppression factor, achieved partly through the use of a novel phase regularization algorithm, was measured to be over 10(3). Spectra acquired from a primary brain and a metastatic neck tumor demonstrated the presence of lactate and choline signals consistent with abnormal spectral patterns. The advantages and limitations of the methods are analyzed theoretically and experimentally, and significance of the results is discussed.

Incorporation of lactate measurement in multi-spin-echo proton spectroscopic imaging

An improved multi-slice, multi-spin-echo proton spectroscopic imaging method for human brain is presented. The technique allows the reconstruction of lactate images, along with choline plus creatine, N-acetylaspartate, and lipid images from one single data set processed in three separate ways. The discrimination between resonances of lipid protons and lactate methyl protons is based on homonuclear spin-spin coupling. The reliability of the separation of the lipid and lactate contribution depends on the T2 of the lipid resonances. Measurements were performed on a standard 1.5 Tesla clinical scanner on healthy volunteers and a patient with high grade CNS lymphoma, demonstrating the ability to obtain high quality metabolite maps within 11 min.

A highly sensitive lactate editing technique for surface coil spectroscopic imaging in vivo

A lactate editing sequence based on the use of spin-locking pulses with surface coils is described. The sequence retains more than 90% of the available lactate magnetization and suppresses at least 95% of the lipid signal in a single scan and ca. 99.5% after a simple two step cycle. The technique allows simultaneous observation of lactate and uncoupled metabolites in the range 2 to 3 ppm. The sequence was combined with two-dimensional spectroscopic imaging to obtain a lactate map of an ischemic rat brain in vivo.