Spectral resolution amelioration by deconvolution (SPREAD) in MR spectroscopic imaging

Abstract

Purpose: To develop, implement, and evaluate a novel postprocessing method for enhancing the spectral resolution of in vivo MR spectroscopic imaging (MRSI) data. Materials and Methods:Magnetic field inhomogeneity across the imaging volume was determined by acquiring MRI datasets with two differing echo times. The lineshapes of the MRSI spectra were derived from these field maps by simulating an MRSI scan of a virtual sample whose resonance frequencies varied according to the observed variations in the magnetic field. By deconvolving the lineshapes from the measured MRSI spectra, the linebroadening effects of the field inhomogeneities were reduced significantly.Results: Both phantom and in vivo proton MRSI spectra exhibited significantly enhanced spectral resolutions and improved spectral lineshapes following application of our method. Quantitative studies on a phantom show that, on average, the full width at half maximum of water peaks was reduced 42%, the full width at tenth maximum was reduced 38%, and the asymmetries of the peaks were reduced 86%. SPECTRAL RESOLUTION DEFINES the ability to distinguish two closely spaced peaks in a spectrum. It is one of the most important determinants of the quality of MR spectroscopy (MRS) data (1-3). Low spectral resolution can obscure the information available from a spectrum of tissue metabolites, hindering the detection and quantification of some or all of those metabolites. This is especially true for in vivo proton MRS ( 1 H MRS) of the brain, for several reasons. First, the differences in the chemical shifts of brain metabolites are relatively small (i.e., the spectral peaks are close to one another), and the line splitting caused by J-coupling further reduces the separation of the peaks in the spectrum. Consequently, some spectral lines overlap intrinsically and cannot be easily distinguished. Second, inhomogeneities of the magnetic field caused by the spatial variation of the external field, B 0 , and by the local differences in magnetic susceptibility of different tissues, produce line broadening and distortion of the lineshape, thereby reducing spectral resolution. Consequently, some lines that are intrinsically separated may overlap. Third, motion of the tissue being imaged may produce line broadening and reduce spectral resolution. ConclusionMany techniques have been developed to improve the spectral resolution of in vivo MRS. These techniques generally fall into one of two classes, depending on whether they are used during the acquisition or the processing of spectral data. The most commonly used strategy for improving spectral resolution during data acquisition is to improve the homogeneity of the main magnetic field, B 0 . Fast, high-order shimming techniques (4) have been implemented on modern scanners, yet these methods cannot eliminate variations in the local magnetic field across the imaging volume that are caused by differing magnetic susceptibilities of the various tissues that are interposed within the body. Another strategy to increase spectral resolu...

B0-adjusted and sensitivity-encoded spectral localization by imaging (BASE-SLIM) in the human brain in vivo

“…This issue has been partially addressed by various extensions of SLIM, including GSLIM (Liang and Lauterbur, 1991), SLOOP (Vonkienlin and Mejia, 1991), SPLASH (An et al, 2011), BSLIM (Khalidov et al, 2007), NL-CSI (Bashir and Yablonskiy, 2006), and SPREAD (Dong and Peterson, 2009). Among proposed extensions of SLIM, only a few have demonstrated its application to the human brain in a limited scope, with no demonstration of a compartmental separation between GM and WM to date.…”

Proton MRS and MRSI of the brain without water suppression

“…Somewhat sophisticated algorithms are needed when some effects, such as zero-crossing of water signal in the spectral line deconvolution and PSF in the MRSI spectral quantification, are taken into consideration. But these are solved problems and mature methods are readily available [126,130,131,142,143].…”

“…(17) is amplified at points where the magnitude of the water signal is close to zero. The reader is referred to [126,130,131] for possible solutions.…”

Quantification of magnetic resonance spectroscopy signals with lineshape estimation

“…The lineshape problem has been studied using different approaches which require corrections during [5][6][7][8] or subsequent to [9][10][11][12][13][14][15][16][17] MRS acquisition. Methods in the first category are called shimming techniques, and are applied during the MR scanning session in order to try to correct field inhomogeneities and thus improve spectral quality.…”

“…Studies on lineshape correction during signal post-processing aim at enhancing the spectral resolution of in vivo 1 H MRS based on the deconvolution of spectra using as reference the lineshape derived from a reference signal [9,11,13,17], but there exist also several reference-free methods that extract a common lineshape by modelling the signal itself [12,14,20,15,16] or by magnetic field mapping [10].…”