Lactate editing and lipid suppression by continuous wavelet transform analysis: Application to simulated and 1H MRS brain tumor time‐domain data

Abstract

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 spec- Lactate is a sensitive indicator of glycolysis, cellular oxygenation, and energy status in living tissues. Considerable effort has been devoted to the detection and the quantification of lactate methyl resonances by means of proton ( 1 H) magnetic resonance spectroscopy (MRS) (1-26). Its measurement may play a significant role in diagnosis, staging, and monitoring of therapeutic response of various human diseases. Changes in lactate level have been strongly correlated with several pathologies of muscle (1) and brain (2,3), as well as head and neck tumors (4). Response of tumors to chemotherapy (5) and radiation therapy (6) has been associated with a decrease in MRS detectable lactate. However, the direct measurement of the lactate methyl resonance requires elimination of the interfering lipid resonances. The large coresonant lipid signals at 1.30 ppm may completely obscure the lactate methyl doublet. Numerous spectral editing methods have been designed to selectively separate the lactate and lipid signals. These methods are based on a wide range of approaches, such as difference spectroscopy with J-modulation (7-12), selective polarization transfer (13,14), zeroquantum (15-17), and multiple-quantum coherence transfer (18 -25), as well as longitudinal two-spin order filtering (26,27).The two most popular spectral editing methods are multiple-quantum (MQ)-based sequences and J-modulation difference spectroscopy. Although the spectral methods based on MQ filters offer excellent lipid suppression, they usually fail to provide complete recovery of lactate signal intensity (see Discussion) (28). The MQ methods also do not provide J-coupling information. Methods employing difference spectroscopy detect the lactate doublet with full signal intensity recovery, but the requirement for acquisition of separate signals introduces sensitivity to motion artifacts (13,18), and B 1 field scaling errors (28).The wavelet transform technique is a time-scale method, which can be used to decompose an observation (i.e., a signal), into a set of detail signals the...

Proton T2 measurement and quantification of lactate in brain tumors by MRS at 3 Tesla in vivo

“…One study reported Lac T 2 as 83 ms in brain tumors at 1.5T, which was obtained from two patients using two TEs (135 and 270 ms) . Another study reported brain‐tumor Lac T 2 as 330 ms at 1.5T, obtained using a wavelet transformation method on single short TE data in two patients . In contrast, several prior studies reported Lac T 2 in nontumorous brain diseases, in which Lac was elevated.…”

“…In many types of brain tumors, lipids are substantially increased and consequently in vivo detection of the Lac 1.31 ppm resonance is hampered by the CH 2 ‐chain proton signals of lipids in short‐echo time (TE) MR spectroscopy (MRS). This spectral overlap can be overcome by means of spectral editing or postacquisition data processing . The Lac 1.31 ppm resonance can also be effectively separated from lipids using long TEs of point‐resolved spectroscopy (PRESS), at which the Lac resonance becomes an inverted or positive doublet while the broad lipid signal is extensively attenuated due to the effects of its relatively short T 2 .…”

Compressive Sensing Could Accelerate1H MR Metabolic Imaging in the Clinic

“…Application of compressed sensing for MR spectroscopic imaging is apt because data are sparse in multiple dimensions of frequency and space in transform domains of wavelets and total variation. Wavelet-based analyses including quantification, spectral editing, and denoising have been applied to MR spectroscopy (14)(15)(16)(17). Fast MR spectroscopic imaging has also been accomplished by the use of wavelets to enable compression in the spectral and spatial domains, which exploits the sparsity existing in this transform domain (18,19).…”

Repeatability of edited lactate and other metabolites in astrocytoma at 3T

“…Some discrimination of lactate and lipid signals may be possible in unedited spectra using mathematical modeling (14). However, such techniques remain very difficult to validate in vivo and have not so far found widespread application.…”