NOE Enhancements and T1 Relaxation Times of Phosphorylated Metabolites in Human Calf Muscle at 1.5 Tesla

Nuclear Overhauser effect (NOE) enhancements and relaxation times of ³¹P metabolites in human calf were measured in 12 volunteers (4 men and 8 women) at 1.5 T using a dual tuned four-ring birdcage. The NOE enhancements of inorganic phosphate (P₁), phosphocreatine (PCr), γ-, α-, and β-nucleoside triphosphate (NTP) from 19 measurements were 0.51 ± 0.10, 0.64 ± 0.03, 0.53 ± 0.03, 0.56 ± 0.08, and 0.47 ± 0.05, respectively. The relaxation times were independent of proton irradiation and from 23 measurements were 3.49 ± 0.35, 4.97 ± 0.58, 4.07 ± 0.36, 2.90 ± 0.25, and 3.61 ± 0.25 s for P₁, PCr, γ-, α-, and β-NTP, respectively. No significant differences between gender and age were observed for either NOE enhancements or relaxation times. Also, among nine volunteers, we observed no significant differences in T₁ between the coupled and decoupled cases.     

Measurement of T1 relaxation times of cardiac phosphate metabolites using BIR-4 adiabatic RF pulses and a variable nutation method

T₁relaxation times of PCr and β-ATP in human cardiac and skeletal muscle were evaluated using a variable nutation method. This allows T₁measurements with a constant TR and a significant reduction in acquisition time compared with the partial saturation method. Four 1D CSI datasets were obtained using 30°, 45°, 60°, and 90° BIR-4 adiabatic RF pulses within 40 min. The T₁of the phosphate phantom obtained with this method agreed with values obtained with the partial saturation method. The T₁s of PCr and β-ATP in heart are 3.98 = 0.18 s and 1.86 ± 0.16 s (mean = SE). Our results demonstrated that T₁ values in heart and skeletal muscle are not significantly different.     

Pliosphocreatine T1 measurements with and without exchange in the heart

The intrinsic phosphocreatine (PCr) T₁ values measured by time-dependent magnetization transfer in isolated perfused rat, hamster, and turkey hearts were indistinguishable. The value of 3.5 ± 0.3 s for the rat heart is similar to values measured by other magnetization transfer methods. Irreversibly inhibiting the phosphoryl exchange between PCr and ATP in the rat heart using iodoacetamide changed the apparent T₁ values of the two exchanging species when measured by inversion recovery: The apparent T₁ of PCr increased from 1.92 ± 0.06 s to 3.55 ± 0.06 s, in excellent agreement with the intrinsic T₁, measured by magnetization transfer. The apparent T₁ of [γ-P]ATP decreased from 0.92 ± 0.07 s to 0.44 ± 0.03 s. The value for the T₁ of [γ-P]ATP in hearts with inhibited phosphoryl exchange was similar to T₁ values for [α-P]ATP and [β-P]ATP, which remained unchanged. This illustrates that apparent T₁ values for PCr and [γ-P]ATP measured by inversion recovery in the presence of exchange are average T₁ values in between the intrinsic values. The large differences between the intrinsic T₁ measured by magnetization transfer and the T₁ measured by inversion recovery makes the use of the appropriate value in different applications quantitatively important.     

31P transverse relaxation times of ATP in human brain in vivo

³¹P MRS examinations of the brain of 10 healthy volunteers were performed to determine T₂ of the coupled ATP signals by use of the localized 90° - TE/2 - 2662 - TE/2 - acq frequency selective spin echo sequence for elimination of phase and intensity distortions. The T₂ relaxation times obtained are much longer than usually assumed: γ-ATP: 89 ± 9 ms; α-ATP: 84 ± 6 ms; β-ATP: 62 ± 3 ms.     

Triple repetition time saturation transfer (TRiST) 31P spectroscopy for measuring human creatine kinase reaction kinetics

Human cardiac phosphorus MR saturation transfer experiments to quantify creatine kinase forward rate constants (k_f) have previously been performed at 1.5 T. Such experiments could benefit from increased signal‐to‐noise ratio (SNR) and spectral resolution at 3 T. At 1.5 T, the four‐angle saturation transfer method was applied with low‐angle adiabatic pulses and surface coils. However, low‐angle adiabatic pulses are potentially problematic above 1.5 T due to bandwidth limitations, power requirements, power deposition, and intrapulse spin‐spin relaxation. For localized metabolite spin‐lattice relaxation time (T₁) measurements, a dual repetition time approach with adiabatic half‐passage pulses was recently introduced to solve these problems at 3 T. Because the saturation transfer experiment requires a T₁ measurement performed while one reacting moiety is saturated, we adapt the dual repetition time approach to measure k_f using a triple repetition time saturation transfer (TRiST) method. A new pulsed saturation scheme with reduced sensitivity to static magnetic field inhomogeneity and compatibility with cardiac triggering is also presented. TRiST measurements of k_f are validated in human calf muscle against conventional saturation transfer and found to agree within 3%. The first 3‐T TRiST measurements of creatine kinase k_f in the human calf (n = 6), chest muscle, and heart (n = 8) are 0.26 ± 0.04 s^?1, 0.23 ± 0.03 s^?1, and 0.32 ± 0.07 s^?1, respectively, consistent with prior 1.5 T values. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Assessment of 31P relaxation times in the human calf muscle: A comparison between 3 T and 7 T in vivo

Phosphorus (³¹P) T₁ and T₂ relaxation times in the resting human calf muscle were assessed by interleaved, surface coil localized inversion recovery and frequency‐selective spin‐echo at 3 and 7 T. The obtained T₁ (mean ± SD) decreased significantly (P < 0.05) from 3 to 7 T for phosphomonoesters (PME) (8.1 ± 1.7 s to 3.1 ± 0.9 s), phosphodiesters (PDE) (8.6 ± 1.2 s to 6.0 ± 1.1 s), phosphocreatine (PCr) (6.7 ± 0.4 s to 4.0 ± 0.2 s), γ‐NTP (nucleotide triphosphate) (5.5 ± 0.4 s to 3.3 ± 0.2 s), α‐NTP (3.4 ± 0.3 s to 1.8 ± 0.1 s), and β‐NTP (3.9 ± 0.4 s to 1.8 ± 0.1 s), but not for inorganic phosphate (Pi) (6.9 ± 0.6 s to 6.3 ± 1.0 s). The decrease in T₂ was significant for Pi (153 ± 9 ms to 109 ± 17 ms), PDE (414 ± 128 ms to 314 ± 35 ms), PCr (354 ± 16 ms to 217 ± 14 ms), and γ‐NTP (61.9 ± 8.6 ms to 29.0 ± 3.3 ms). This decrease in T₁ with increasing field strength of up to 62% can be explained by the increasing influence of chemical shift anisotropy on relaxation mechanisms and may allow shorter measurements at higher field strengths or up to 62% additional signal‐to‐noise ratio (SNR) per unit time. The fully relaxed SNR increased by +96%, while the linewidth increased from 6.5 ± 1.2 Hz to 11.2 ± 1.9 Hz or +72%. At 7 T ³¹P‐MRS in the human calf muscle offers more than twice as much SNR per unit time in reduced measurement time compared to 3 T. This will facilitate in vivo ³¹P‐MRS of the human muscle at 7 T. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

The use of dietary loading of 133Cs as a potassium substitute in NMR studies of tissues

¹³³Cs NMR chemical shifts and relaxation times have been measured for tissue samples in vitro and in vivo from rats which have been fed on a high cesium, low potassium diet, which leads to a predominantly intracellular distribution of this ion, similar to that of K ⁺. The high sensitivity, large chemical shift range, and narrow linewidths of ¹³³Cs, compared with ³⁹K, allow chemical shift differences to be observed between tissues, and in subcellular organelles such as mitochondria. For example, in vitro tissue chemical shifts, relative to 150 mM CsCl, are 1.06 ± 0.11 ppm for liver, 0.02 ± 0.05 ppm for brain, 1.76 ± 0.20 ppm for erythrocytes, and ?0.13 ± 0.02 ppm for plasma. T₁ and spin-echo T₂ values range from 1.26 ± 0.05 s (T₂), and 0.028 ± 0.006 s (T₂) for liver, to 6.49 ± 0.19 s and 1.12 ± 0.03 s for plasma. ¹³³Cs relaxation times show the same relative trends between tissues as are observed in ³⁹K tissue Studies.     

A Modified sub-second fast-STEAM sequence incorporating bipolar gradients for in vivo diffusion imaging

A modified high-speed stimulated-echo acquisition mode (STEAM) diffusion sequence (90°-TE/2−90°-TM-[α-TE/2-STE]_n) incorporating bipolar diffusion gradients that are less sensitive to macroscopic motion-induced artifacts is presented. Diffusion encoding was performed only during the first echo interval (TE1) with bipolar gradients that were implemented on all three mutually orthogonal axes. Calibration measurements on phantoms filled with water, isopropanol, and dimethyl sulfoxide yielded apparent diffusion coefficients (ADC) consistent with published values. Non-ECG-triggered in vivo images acquired on rat brain with relatively high b values (˜450 s/mm²) indicated minimal motion artifacts. Evaluated ADCs averaged over the cortex, left mid-brain, right mid-brain, regions yielded (0.91 ± 0.02), (1.06 ± 0.02), (1.01 ± 0.03) × 10⁻³ mm²s⁻¹, respectively.     

31P NMR saturation transfer study of the creatine kinase reaction in human skeletal muscle at rest and during exercise

The creatine kinase reaction has been studied by ³¹P NMR in exercising human calf muscle. Quantitative analysis of high energy phosphates and saturation transfer study of the creatine kinase flux in the direction of ATP synthesis (V_for) were performed at rest and during exercise. As expected, exercise induced a [PCr] decrease (from 28.5 ± 0.9 to 21.9 ± 1.5 mM, P < 0.01) matched by a P₁, increase (from 4.5 ± 0.2 to 8.9 ± 1.8 mM,P = 0.06). pH_i and [ATP] remained unchanged. V_for did not change from rest (12.4 ± 0.9 mM s^?1) to moderate exercise and decreased at the highest exercise level (8.4 ± 1.4 mM s^?1, P = 0.006). This observation differs from the prediction of the creatine kinase rate equation, showing an increase in the flux with exercise intensity. Computations suggest that this discrepancy arises from metabolite compartmentalization and/or from the reaction kinetics of a dead end complex stabilized by planar anions.     

Detection of liver fibrosis with magnetic cross-relaxation

The utility of MRI using magnetization transfer (MT) enhanced pulse sequences to diagnose hepatic cirrhosis in a rat model was investigated. Hepatic T₁ was measured with and without MT off-resonance RF pulses in 17 treated and six control rats. The livers were evaluated histologically, and the hydroxyproline content quantitatively measured. We did not find a statistically significant linear correlation between the MR relaxation times and the degree of tissue injury. However, the MR measurements performed with MT were superior to those without differentiating the treated and control groups. Specifically, the T₁ times were 695 ±76 ms for the treated group, versus 748 ± 61 ms in the controls; P= 0.095. The T_1sat times were also lower in the treated group, with statistical significance: 367 ± 51 ms versus 421 ± 38 ms, P = 0.016. Finally, the change in the relaxation rates (the inverse of the relaxation times) with and without saturation were 1.31 ± 0.22 s^?1 (treated group) versus 1.05 ± 0.12 s^?1 (controls), which differed significantly, P= 0.001.     

Changes of relaxation times (T1, T2) and apparent diffusion coefficient after permanent middle cerebral artery occlusion in the rat: temporal evolution,regional extent,and comparison with histology

The quantitative NMR parameters T₁, T₂, p, and apparent diffusion coefficient (ADC) were determined during the 7 h after middle cerebral artery occlusion in rats. In the normal caudate-putamen (CP), 869 ± 145 ms and 72 ± 2ms for T₁ and for T₂, respectively, were found; the corresponding values for cortex were 928 ± 117 ms and 73 ± 2 ms. The ADC showed significant dependence on gradient direction: diffusion along x resulted in 534 ± 53 μm²/s (CP) and 554 ± 62 μm²/s (cortex), and along y in 697 ± 58 μm²/s (CP) and 675 ± 53 μm²/s (cortex). In the ischemic territory, a continuous increase over time of both relaxation times was observed in the CP, leading to an increase of 29 ± 20% (T₁) and 51 ± 41% (T₂ above control level. ADC dropped to 63 ± 15% of control in the CP and to 74 ± 4% of control in the temporal cortex. No significant change was noted in proton density during the observation period. Strongest ADC reduction was in the center of the ischemic territory (≤ 60% of control) surrounded by a region of lesser reduction (≤ 80% of control). During the early part of the study, the area of reduced ADC was larger than that of elevated relaxation times. Toward the end of the experiment, the area of increased relaxation times approached that of decreased ADC at ≤ 80% of control. Good agreement of histological presentation of infarct with the total area of decreased ADC (≤ 80%) was demonstrated.     

Anomalous Transverse Relaxation in 1H Spectroscopy in Human Brain at 4 Tesla

Longitudinal (T₁) and apparent transverse relaxation times (T₂) of choline-containing compounds (Cho), creatine/phospho-creatine (Cr/PCr), and N-acetyl aspartate (NAA) were measured in vivo in human brain at 4 Tesla. Measurements were performed using a water suppressed stimulated echo pulse sequence with complete outside volume presaturation to improve volume localization at short echo times. T₁-values of Cho (1.2 ± 0.1 s), Cr (1.6 ± 0.3 s), and NAA (1.6 ± 0.2 s) at 4 Tesla in occipital brain were only slightly larger than those reported in the literature at 1.5 Tesla. Thus, TR will not adversely affect the expected enhancement of signal-to-noise at 4 Tesla. Surprisingly, apparent T₂-values of Cho (142 ± 34 ms), Cr (140 ± 13 ms), and NAA (185 ± 24 ms) at 4 Tesla were significantly smaller than those at 1.5 Tesla and further decreased when increasing the mixing interval TM. Potential contributing factors, such as diffusion in local susceptibility related gradients, dipolar relaxation due to intracellular paramagnetic substances and motion effects are discussed. The results suggest that short echo time spectroscopy is advantageous to maintain signal to noise at 4 Tesla.     

4 Tesla gradient recalled echo characteristics of photic stimulation-induced signal changes in the human primary visual cortex

Multi-echo measurements of photic stimulation-induced signal changes in human visual cortex were made at 4 Tesla in order to quantify the nature of the signal change and its vascular origin, and to determine the optimum echo time for detection of the changes. Utilizing high resolution images, two distinct regions (ascribed to be microvasculature and visible venous vessels) were identified as giving rise to the signal increase. The fractional signal changes in gray matter areas depended linearly on echo time (TE) in the range of 10 to 60 ms and extrapolated to virtually zero for TE = 0, indicating that in-flow effects secondary to stimulation-induced blood flow increases were negligible in our functional imaging studies; instead, signal change due to photic stimulation originated from the increase in the apparent transverse relaxation rate, 1/T₂*. This decrease in (1/T₂*, brought about by the alterations in hemodynamic parameters, was 1.3 ± 0.4 s⁻¹ for gray matter and 3.0 ± 0.7 s⁻¹ (averaged over 10 individuals) for venous vessels visible in the images. The optimum choice of echo time was found to be TE ≥ T₂*.     

Gel-entrapment of perfluorocarbons: A fluorine-19 NMR spectroscopic method for monitoring oxygen concentration in cell perfusion systems

Oxygenation is a major determinant of the physiological state of cultured cells. ¹⁹F NMR can be used to determine the oxygen concentration available to cells immobilized in a gel matrix by measuring the relaxation rate (1/T₁) of perfluorocarbons (PFC) incorporated into the gel matrix. In calcium alginate gel beads without cells the relaxation rate (1/T₁) of the trifluoromethyl group of perfluorotripropylamine (FTPA) varies linearly with oxygen concentration, with a slope of 1.26 ± 0.15 × 10^?3 s^?1μM^?1 and an intercept of 0.50 ± 0.04 s^?1. During perfusion with medium equilibrated with 95%/5% O²/CO², changes in PFC T₁s indicate that the average oxygen concentration was reduced from 894 ± 102 μM in the absence of cells to 476 ± 65 μM and 475 ± 50 μM in the presence of 0.7 × 10⁸ EMT6/Ro and RIF-1 murine tumor cells per milliliter of gel, respectively. The presence of 0.2 μl of FTPA/ml of gel had no effect on the energy status of the cells as indicated by ³¹P NMR spectra. To calculate oxygen gradients within the beads from the average PFC T₁ of the sample, a mathematical model was used assuming that oxygen is the limiting nutrient for cell metabolism and that the cellular oxygen consumption rate is independent of oxygen concentration. Data for EMT6/RO cells were fit using experimentally determined perfusion parameters together with literature values for cell volume and oxygen consumption rate. The average PFC 1/T₁s predicted using different literature values for volume and oxygen consumption? 1.10 ± 0.10 and 1.28 ± 0.36 s^?1?agreed well with the experimentally measured value-1.104 ± 0.004 s^?1. Thus, the model is a suitable tool for calculation of oxygen consumption rates from PFC T₁s in well-oxygenated cell perfusion systems.     

Quantitative nmr microscopy of multicellular tumor spheroids and confrontation cultures

In cancer research, tumor spheroids are a well established system to study tumor metabolism resembling the situation in vivo more closely cell monolayers. Spherical aggregates of malignant melanoma cells (MV3) and their invasion into rat brain aggregates have been investigated by quantitative NMR microscopy. Relaxation times (T₁, T₂) and diffusion parameter images were acquired with an in-plane resolution of 14 × 14 μm². The authors were able to demonstrate that the morphology of the spheroids can be visualized on these NMR maps. The contrast was mainly manifested in relaxation maps, where average relaxation times T₁ = 1.94 ± 0.17 s and T₂ = 42.8 ± 6.3 ms were obtained for proliferating cells, and T₁ = 2.49 ± 0.31 s and T₂ = 104.3 ± 29.4 ms for the necrobiotic center. The mean diffusion coefficients were 0.59 ± 0.12 μm²/ms and 0.85 ± 0.14 μm²/ms, respectively. The authors could follow the dynamic process of tumor cell invasion in the investigated co-culture system. Knowledge about tumor cell migration and tumor cell invasion is essential for the understanding of cancer and its therapy. Quantitative NMR microscopy can study this dynamic process noninvasively and therefore may help to assess the influence of therapy on the micromilieu of these spheroids.     

Diffusion-weighted MR imaging including bi-exponential fitting for the detection of recurrent or residual tumour after (chemo)radiotherapy for laryngeal and hypopharyngeal cancers

Objectives

To assess whether diffusion-weighted magnetic resonance imaging (DW-MRI) including bi-exponential fitting helps to detect residual/recurrent tumours after (chemo)radiotherapy of laryngeal and hypopharyngeal carcinoma.

Methods

Forty-six patients with newly-developed/worsening symptoms after (chemo)radiotherapy for laryngeal/hypopharyngeal cancers were prospectively imaged using conventional MRI and axial DW-MRI. Qualitative (visual assessment) and quantitative analysis (mono-exponentially: total apparent diffusion coefficient [ADC_T], and bi-exponentially: perfusion fraction [F_P] and true diffusion coefficient [ADC_D]) were performed. Diffusion parameters of tumour versus post-therapeutic changes were compared, with final diagnosis based on histopathology and follow-up. Mann-Whitney U test was used for statistical analysis.

Results

Qualitative DW-MRI combined with morphological images allowed the detection of tumour with a sensitivity of 94% and specificity 100%. ADC_T and ADC_D values were lower in tumour with values 120 ± 49 × 10⁻⁵ mm²/s and 113 ± 50 × 10⁻⁵ mm²/s, respectively, compared with post-therapeutic changes with values 182 ± 41 × 10⁻⁵ mm²/s (P < 0.0002) and 160 ± 47 × 10⁻⁵ mm²/s (P < 0.003), respectively. F_P values were significantly lower in tumours than in non-tumours (13 ± 9% versus 31 ± 16%, P < 0.0002), with F_P being the best quantitative parameter for differentiation between post-therapeutic changes and recurrence.

Conclusions

DW-MRI in combination with conventional MRI substantially improves detection and exclusion of tumour in patients with laryngeal and hypopharyngeal cancers after treatment with (chemo)radiotherapy on both qualitative and quantitative analysis, with F_P being the best quantitative parameter in this context.

Key Points

• DW-MRI is increasingly used to detect tumour recurrence.

• DW-MRI allows accurate post-treatment recurrence detection in laryngeal or hypopharyngeal cancer

• ADC values in recurrent tumour are lower than in benign tissue alterations

• Both qualitative and quantitative DW-MRI approaches allow detection of recurrence

• DW-MRI can easily be added to daily clinical routine imaging

     

Metabolite proton T2 mapping in the healthy rhesus macaque brain at 3 T

The structure and metabolism of the rhesus macaque brain, an advanced model for neurologic diseases and their treatment response, is often studied noninvasively with MRI and ¹H‐MR spectroscopy. Due to the shorter transverse relaxation time (T₂) at the higher magnetic fields these studies favor, the echo times used in ¹H‐MR spectroscopy subject the metabolites to unknown T₂ weighting, decreasing the accuracy of quantification which is key for inter‐ and intra‐animal comparisons. To establish the “baseline” (healthy animal) T₂ values, we mapped them for the three main metabolites' T₂s at 3 T in four healthy rhesus macaques and tested the hypotheses that their mean values are similar (i) among animals; and (ii) to analogs regions in the human brain. This was done with three‐dimensional multivoxel ¹H‐MR spectroscopy at (0.6 × 0.6 × 0.5 cm)³ = 180 μL spatial resolution over a 4.2 × 3.0 × 2.0 = 25 cm³ (～30%) of the macaque brain in a two‐point protocol that optimizes T₂ precision per unit time. The estimated T₂s in several gray and white matter regions are all within 10% of those reported in the human brain (mean ± standard error of the mean): N‐acetylaspartate = 316 ± 7, creatine = 177 ± 3, and choline = 264 ± 9 ms, with no statistically significant gray versus white matter differences. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Comparison of two ultrashort echo time sequences for the quantification of T(1) within phantom and human Achilles tendon at 3 T

Ultrashort echo time (UTE) techniques enable direct imaging of musculoskeletal tissues with short T₂ allowing measurement of T₁ relaxation times. This article presents comparison of optimized 3D variable flip angle UTE (VFA‐UTE) and 2D saturation recovery UTE (SR‐UTE) sequences to quantify T₁ in agar phantoms and human Achilles tendon. Achilles tendon T₁ values for asymptomatic volunteers were compared to Achilles tendon T₁ values calculated from patients with clinical diagnoses of spondyloarthritis (SpA) and Achilles tendinopathy using an optimized VFA‐UTE sequence. T₁ values from phantom data for VFA‐ and SR‐UTE compare well against calculated T₁ values from an assumed gold standard inversion recovery spin echo sequence. Mean T₁ values in asymptomatic Achilles tendon were found to be 725 ± 42 ms and 698 ± 54 ms for SR‐ and VFA‐UTE, respectively. The patient group mean T₁ value for Achilles tendon was found to be 957 ± 173 ms (P < 0.05) using an optimized VFA‐UTE sequence with pulse repetition time of 6 ms and flip angles 4, 19, and 24°, taking a total 9 min acquisition time. The VFA‐UTE technique appears clinically feasible for quantifying T₁ in Achilles tendon. T₁ measurements offer potential for detecting changes in Achilles tendon due to SpA without need for intravenous contrast agents. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Longitudinal and multi-echo transverse relaxation times of normal breast tissue at 3 Tesla

Purpose

To measure longitudinal (T₁) and multi‐echo transverse (T₂) relaxation times of healthy breast tissue at 3 Tesla (T).

Materials and Methods

High‐resolution relaxation time measurements were made in six healthy female subjects. Inversion recovery images were acquired at 10 inversion times between 100 ms and 4000 ms, and multiple spin echo images were acquired at 16 echo times between 10 ms and 160 ms.

Results

Longitudinal relaxation times T₁ were measured as 423 ± 12 ms for adipose tissue and 1680 ± 180 ms for fibroglandular tissue. Multi‐echo transverse relaxation times T₂ were measured as 154 ± 9 ms for adipose tissue and 71 ± 6 ms for fibroglandular tissue. Histograms of the voxel‐wise relaxation times and quantitative relaxation time maps are also presented.

Conclusion

T₁ and multi‐echo T₂ relaxation times in normal human breast tissue are reported. These values are useful for pulse sequence design and optimization for 3T breast MRI. Compared with the literature, T₁ values are significantly longer at 3T, suggesting that longer repetition time and inversion time values should be used for similar image contrast. J. Magn. Reson. Imaging 2010;32:982–987. © 2010 Wiley‐Liss, Inc.     

Measurement of transverse relaxation times of J‐coupled metabolites in the human visual cortex at 4 T

Accurate quantification of ¹H NMR spectra often requires knowledge of the relaxation times to correct for signal losses due to relaxation and saturation. In human brain, T₂ values for singlets such as N‐acetylaspartate, creatine, and choline have been reported, but few T₂ values are available for J‐coupled spin systems. The purpose of this study was to measure the T₂ relaxation times of J‐coupled metabolites in the human occipital lobe using the LASER sequence. Spectra were acquired at multiple echo times and were analyzed with an LCModel using basis sets simulated at each echo time. Separate basis spectra were used for resonances of protons belonging to the same molecule but having very different T₂ values (e.g., two separate basis spectra were used for the singlet and multiplet signal in N‐acetylaspartate). The T₂ values for the N‐acetylaspartate multiplet (149 ± 12 ms), glutamate (125 ± 10 ms), myo‐inositol (139 ± 20 ms), and taurine (196 ± 28 ms) were successfully measured in the human visual cortex at 4 T. These measured T₂ relaxation times have enabled the accurate and absolute quantification of cerebral metabolites at longer echo times. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.