T1 and T2 temperature dependence of female human breast adipose tissue at 1.5 T: groundwork for monitoring thermal therapies in the breast

The T₁ and T₂ temperature dependence of female breast adipose tissue was investigated at 1.5 T in order to evaluate the applicability of relaxation‐based MR thermometry in fat for the monitoring of thermal therapies in the breast. Relaxation times T₁, T₂ and T_2TSE (the apparent T₂ measured using a turbo spin echo readout sequence) were measured in seven fresh adipose breast samples for temperatures from 25 to 65 °C. Spectral water suppression was used to reduce the influence of the residual water signal. The temperature dependence of the relaxation times was characterized. The expected maximum temperature measurement errors based on average calibration lines were calculated. In addition, the heating–cooling reversibility was investigated for two samples. The T₁ and T_2TSE temperature (T) dependence could be fitted well with an exponential function of 1/T. A linear relationship between T₂ and temperature was found. The temperature coefficients (mean ± inter‐sample standard deviation) of T₁ and T_2TSE increased from 25 °C (dT₁/dT = 5.35 ± 0.08 ms/°C, dT_2TSE/dT = 3.82 ± 0.06 ms/°C) to 65 °C (dT₁/dT = 9.50 ± 0.16 ms/°C, dT_2TSE/dT = 7.99 ± 0.38 ms/°C). The temperature coefficient of T₂ was 0.90 ± 0.03 ms/°C. The temperature‐induced changes in the relaxation times were found to be reversible after heating to 65 °C. Given the small inter‐sample variation of the temperature coefficients, relaxation‐based MR thermometry appears to be feasible in breast adipose tissue, and may be used as an adjunct to proton resonance frequency shift (PRFS) thermometry in aqueous tissue (glandular + tumor). Copyright © 2015 John Wiley & Sons, Ltd.     

Predictive accuracy of single‐ and multi‐slice MRI for the estimation of total visceral adipose tissue in overweight to severely obese patients

The quantification of visceral adipose tissue (VAT) is increasingly being considered for risk assessment and treatment monitoring in obese patients, but is generally time‐consuming. The goals of this work were to semi‐automatically segment and quantify VAT areas of MRI slices at previously proposed anatomical landmarks and to evaluate their predictive power for whole‐abdominal VAT volumes on a relatively large number of patients. One‐hundred and ninety‐seven overweight to severely obese patients (65 males; body mass index, 33.3 ± 3.5 kg/m²; 132 females; body mass index, 34.3 ± 3.2 kg/m²) underwent MRI examination. Total VAT volumes (V_VAT‐T) of the abdominopelvic cavity were quantified by retrospective analysis of two‐point Dixon MRI data (active‐contour segmentation, visual correction and histogram analysis). V_VAT‐T was then compared with VAT areas determined on one or five slices defined at seven anatomical landmarks (lumbar intervertebral spaces, umbilicus and femoral heads) and corresponding conversion factors were determined. Statistical measures were the coefficients of variation and standard deviations σ₁ and σ₅ of the difference between predicted and measured VAT volumes (Bland–Altman analysis). V_VAT‐T was 6.0 ± 2.0 L (2.5–11.2 L) for males and 3.2 ± 1.4 L (0.9–7.7 L) for females. The analysis of five slices yielded a better agreement than the analysis of single slices, required only a little extra time (4 min versus 2 min) and was substantially faster than whole‐abdominal assessment (24 min). Best agreements were found at intervertebral spaces L3–L4 for females (σ_5/1 = 523/608 mL) and L2–L3 for males (σ_5/1 = 613/706 mL). Five‐slice VAT volume estimates at the level of lumbar disc L3–L4 for females and L2–L3 for males can be obtained within 4 min and were a reliable predictor for abdominopelvic VAT volume in overweight to severely adipose patients. One‐slice estimates took only 2 min and were slightly less accurate. These findings may contribute to the implementation of analytical methods for fast and reliable (routine) estimation of VAT volumes in obese patients. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterizing human adipose tissue lipids by long echo time 1H‐MRS in vivo at 1.5 Tesla: validation by gas chromatography

The aim of this study was to investigate the use of ¹H‐MRS with various echo times to characterize subcutaneous human adipose tissue (SAT) triglyceride composition and to validate the findings with fatty acid (FA) analysis of SAT biopsies by gas chromatography (GC). ¹H‐MRS spectra were acquired with a 1.5 Tesla clinical imager from the SAT of 17 healthy volunteers, with 10 undergoing SAT biopsy. Spectra were localized with PRESS and a series of echo times; 30,50,80,135,200,300 and 540 ms were acquired with TR = 3000 ms. Prior knowledge from phantom measurements was used to construct AMARES fitting models for the lipid spectra. SAT FA composition were compared with serum lipid levels and subject characteristics in 17 subjects. Long TE (135,200 ms) spectra corresponded better with the GC data than short TE (30,50 ms) spectra. TE = 135 ms was found optimal for determining diallylic content (R = 0.952, p < 0.001) and TE = 200 ms was optimal for determining olefinic content (R = 0.800, p < 0.01). The improved performance of long TE spectra is a result of an improved baseline and better peak separation, due to J‐modulation and suppression of water. The peak position of the diallylic resonance correlated with the average double bond content of polyunsatured fatty acids with R = 0.899 (p < 0.005). The apparent T₂ of the methylene resonance displayed relatively small inter‐individual variation, 88.1 ± 1.1 ms (mean ± SD). The outer methyl triplet line of ω‐3 PUFA at 1.08 ppm could be readily detected and quantitated from spectra obtained at TE = 540. The ω‐3 resonance correlated with the ω‐3 content determined by GC with R = 0.737 (p < 0.05, n = 8). Age correlated significantly with SAT diallylic content (R = 0.569, p = 0.017, n = 17), but serum lipid levels showed no apparent relation to SAT FA composition. We conclude that long TE ¹H‐MRS provides a robust non‐invasive method for characterizing adipose tissue triglycerides in vivo. Copyright © 2010 John Wiley & Sons, Ltd.     

Fraction of unsaturated fatty acids in visceral adipose tissue (VAT) is lower in subjects with high total VAT volume – a combined 1H MRS and volumetric MRI study in male subjects

Visceral adipose tissue (VAT) is thought to play an important role in the pathogenesis of obesity and insulin resistance. However, little is known about the composition of VAT with regard to the amount of mono‐ (MUFAs) and polyunsaturated fatty acids (PUFAs) in triglycerides. Volume‐selective MRS was performed in addition to MRI for the quantification of VAT. Analysis comprised proton signals from the vinyl‐H group (H–C = C–H), including protons from MUFA + PUFA, and diallylic‐H, i.e. methylene‐interrupted PUFAs. The methyl (?CH₃) resonance, which is the only peak with a defined number of protons/triglyceride (n = 9), served as reference. Twenty male subjects participated in this prospective study and underwent MRS of VAT on a 3‐T whole‐body unit. Spectra were recorded by a single‐voxel stimulated echo acquisition mode (STEAM) technique (TE/TM/TR = 20/10/4000 ms; volume of interest between 20 × 25 × 20 and 30 × 30 × 20 mm³; 48–80 acquisitions depending on the size of the volume of interest; bandwidth, 1200 Hz). Post‐processing was performed by a Java‐based magnetic resonance user interface (jMRUI; AMARES). The volume of VAT was quantified in a separate session on a 1.5‐T imager a few days prior to the MRS session by T₁‐weighted imaging. The relative amount of VAT was calculated as a percentage of body weight (%VAT). Ratios of vinyl‐H to –CH₃ and diallylic‐H to –CH₃ were calculated. All spectra recorded from VAT were of high quality, enabling reliable quantification of the mentioned resonances. %VAT and vinyl‐H/CH₃ varied over a broad range (2.8–8.3% and 0.45–0.64, respectively). A strong negative correlation between %VAT and vinyl‐H/CH₃ was found (r = ?0.92), whereas diallylic‐H/CH₃ alone was clearly less well correlated with %VAT (r = ?0.21). The composition of VAT shows strong interindividual variations. The greater the total amount of VAT, the less unsaturated the fatty acids. This is a preliminary result in mainly obese male subjects, and it remains to be determined whether this correlation holds for other cohorts of different age, gender and body mass index. Furthermore, changes in VAT composition during weight loss or different forms of diet have yet to be examined. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of fat distribution in Prader–Willi syndrome: Relationships with adipocytokines and influence of growth hormone treatment

Marked anthropometric changes are seen in Prader–Willi syndrome (PWS). Emaciation is observed during infancy, whereas severe obesity is found in older children and adults. Growth hormone (GH) treatment modifies the anthropometric changes in PWS patients. In this study, we examined changes in the body composition of 51 PWS patients (age range, 6–54 years; median, 16.5 years), with a focus on the amount of abdominal visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT), VAT/SAT ratio, and serum levels of adipocytokines (adiponectin, leptin, and resistin). The relationships between VAT, SAT, and adipocytokines, and lipid abnormalities and type 2 diabetes in 24 patients with obese PWS were also evaluated. With increasing age, SAT and VAT both increased markedly, but in 18 patients receiving GH treatment, VAT remained low at ≤30 cm². In the GH‐completed patients (n = 19), VAT and SAT increased with age to levels similar to those in non‐GH‐treated patients (n = 14). In the obese group, adiponectin decreased as VAT increased (r = –0.35, P = 0.11). Leptin (r = 0.67, P < 0.001) and resistin (r = 0.45, P = 0.04) showed positive correlations with SAT. Total cholesterol, low‐density lipoprotein, and triglyceride levels correlated negatively with adiponectin (r = –0.59, r = –0.56, r = –0.56, respectively, P < 0.05) and hemoglobin A1c (r = –0.42, P = 0.08). To maintain lower VAT and prevent cardiovascular disease risk factors, GH treatment may be advisable even in adult patients with PWS. © 2012 Wiley Periodicals, Inc.     

39K and 23Na relaxation times and MRI of rat head at 21.1 T

At ultrahigh magnetic field strengths (B₀ ≥ 7.0 T), potassium (³⁹K) MRI might evolve into an interesting tool for biomedical research. However, ³⁹K MRI is still challenging because of the low NMR sensitivity and short relaxation times. In this work, we demonstrated the feasibility of ³⁹K MRI at 21.1 T, determined in vivo relaxation times of the rat head at 21.1 T, and compared ³⁹K and sodium (²³Na) relaxation times of model solutions containing different agarose gel concentrations at 7.0 and 21.1 T. ³⁹K relaxation times were markedly shorter than those of ²³Na. Compared with the lower field strength, ³⁹K relaxation times were up to 1.9‐ (T₁), 1.4‐ (T_2S) and 1.9‐fold (T_2L) longer at 21.1 T. The increase in the ²³Na relaxation times was less pronounced (up to 1.2‐fold). Mono‐exponential fits of the ³⁹K longitudinal relaxation time at 21.1 T revealed T₁ = 14.2 ± 0.1 ms for the healthy rat head. The ³⁹K transverse relaxation times were 1.8 ± 0.2 ms and 14.3 ± 0.3 ms for the short (T_2S) and long (T_2L) components, respectively. ²³Na relaxation times were markedly longer (T₁ = 41.6 ± 0.4 ms; T_2S = 4.9 ± 0.2 ms; T_2L = 33.2 ± 0.2 ms). ³⁹K MRI of the healthy rat head could be performed with a nominal spatial resolution of 1 × 1 × 1 mm³ within an acquisition time of 75 min. The increase in the relaxation times with magnetic field strength is beneficial for ²³Na and ³⁹K MRI at ultrahigh magnetic field strength. Our results demonstrate that ³⁹K MRI at 21.1 T enables acceptable image quality for preclinical research. Copyright © 2016 John Wiley & Sons, Ltd.     

Bi‐exponential 23Na T2* component analysis in the human brain

The purpose of this study was to measure the sodium transverse relaxation time T₂* in the healthy human brain. Five healthy subjects were scanned with 18 echo times (TEs) as short as 0.17 ms. T₂* values were fitted on a voxel‐by‐voxel basis using a bi‐exponential model. Data were also analysed using a continuous distribution fit with a region of interest‐based inverse Laplace transform. Average T₂* values were 3.4 ± 0.2 ms and 23.5 ± 1.8 ms in white matter (WM) for the short and long components, respectively, and 3.9 ± 0.5 ms and 26.3 ± 2.6 ms in grey matter (GM) for the short and long components, respectively, using the bi‐exponential model. Continuous distribution fits yielded results of 3.1 ± 0.3 ms and 18.8 ± 3.2 ms in WM for the short and long components, respectively, and 2.9 ± 0.4 ms and 17.2 ± 2 ms in GM for the short and long components, respectively. ²³Na T₂* values of the brain for the short and long components for various anatomical locations using ultra‐short TEs are presented for the first time.     

Measuring renal tissue relaxation times at 7 T

As developments in RF coils and RF management strategies make performing ultra‐high‐field renal imaging feasible, understanding the relaxation times of the tissue becomes increasingly important for tissue characterization, sequence optimization and quantitative functional renal imaging, such as renal perfusion imaging using arterial spin labeling. By using a magnetization‐prepared single‐breath‐hold fast spin echo imaging method, human renal T₁ and T₂ imaging studies were successfully performed at 7 T with 11 healthy volunteers (eight males, 45 ± 17 years, and three females, 29 ± 7 years, mean ± standard deviation, S.D.) while addressing challenges of B₁⁺ inhomogeneity and short‐term specific absorption rate limits. At 7 T, measured renal T₁ values for the renal cortex and medulla (mean ± S.D.) from five healthy volunteers who participated in both 3 T and two‐session 7 T studies were 1661 ± 68 ms and 2094 ± 67 ms, and T₂ values were 108 ± 7 ms and 126 ± 6 ms. For comparison, similar measurements were made at 3 T, where renal cortex and medulla T₁ values of 1261 ± 86 ms and 1676 ± 94 ms and T₂ values of 121 ± 5 ms and 138 ± 7 ms were obtained. Measurements at 3 T and 7 T were significantly different for both T₁ and T₂ values in both renal tissues. Reproducibility studies at 7 T demonstrated that T₁ and T₂ estimations were robust, with group mean percentage differences of less than 4%. Copyright © 2014 John Wiley & Sons, Ltd.     

T2 measurement of J-coupled metabolites in the human brain at 3T

Proton T₂ relaxation times of metabolites in the human brain were measured using point resolved spectroscopy at 3T in vivo. Four echo times (54, 112, 246 and 374 ms) were selected from numerical and phantom analyses for effective detection of the glutamate multiplet at ~ 2.35 ppm. In vivo data were obtained from medial and left occipital cortices of five healthy volunteers. The cortices contained predominantly gray and white matter, respectively. Spectra were analyzed with LCModel software using volume‐localized calculated spectra of brain metabolites. The estimate of the signal strength vs. TE was fitted to a monoexponential function for estimation of apparent T₂ (T₂^?). T₂^? was estimated to be similar between the brain regions for creatine, choline, glutamate and myo‐inositol, but significantly different for N‐acetylaspartate singlet and multiplet. T₂^?s of glutamate and myo‐inositol were measured as 181 ± 16 and 197 ± 14 ms (mean ± SD, N = 5) for medial occipital cortices, and 180 ± 12 and 196 ± 17 ms for left occipital cortices, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

In vivo GABA T2 determination with J‐refocused echo time extension at 7 T

A method to measure the T₂ relaxation time of GABA with spectral editing techniques is proposed. Spectral editing techniques can be used to unambiguously extract signals of low concentration J‐coupled spins such as γ‐aminobutyric acid (GABA) from overlapping resonances such as creatine and macromolecules. These sequences, however, generally have fixed and relatively long echo times. Therefore, for the absolute quantification of the edited spectrum, the T₂ relaxation time must be taken into account. To measure the T₂ relaxation time, the signal intensity has to be obtained at multiple echo times. However, on a coupled spin system such as GABA this is challenging, since the signal intensity of the target resonances is modulated not only by T₂ decay but also by the J‐coupling, which strongly influences the shapes and amplitudes of the edited signals, depending on the echo time. Here, we propose to refocus the J‐modulation of the edited signal at different echo times by using chemical shift selective refocusing. In this way the echo time can be arbitrarily extended while preserving the shape of the edited signal. The method was applied in combination with the MEGA‐sLASER editing technique to measure the in vivo T₂ relaxation time of GABA (87 ± 11 ms, n = 10) and creatine (109 ± 8 ms, n = 10) at 7 T. The T₁ relaxation time of these metabolites in a single subject was also determined (GABA, 1334 ± 158 ms; Cr, 1753 ± 12 ms). The T₂ decay curve of coupled spin systems can be sampled in an arbitrary fashion without the need for signal shape correction. Furthermore, the method can be applied with any spectral editing technique. The shortest echo time of the method is limited by the echo time of the spectral editing technique. Copyright © 2013 John Wiley & Sons, Ltd.     

Noninvasive quantification of T2 and concentrations of ascorbate and glutathione in the human brain from the same double-edited spectra

The transverse relaxation times (T₂) and concentrations of Ascorbate (Asc) and glutathione (GSH) were measured from a single dataset of double‐edited spectra that were acquired at several TEs at 4 T in the human brain. Six TEs between 102 and 152 ms were utilized to calculate T₂ for the group of 12 subjects scanned five times each. Spectra measured at all six TEs were summed to quantify the concentration in each individual scan. LCModel fitting was optimized for the quantification of the Asc and GSH double‐edited spectra. When the fitted baseline was constrained to be flat, T₂ was found to be 67 ms (95% confidence interval, 50–83 ms) for GSH and ≤115 ms for Asc using the sum of spectra measured over 60 scans. The Asc and GSH concentrations quantified in each of the 60 scans were 0.62 ± 0.08 and 0.81 ± 0.11 µmol/g [mean ± standard deviation (SD), n = 60], respectively, using 10 µmol/g N‐acetylaspartate as an internal reference and assuming a constant influence of N‐acetylaspartate and antioxidant T₂ relaxation in the reference solution and in vivo. The T₂ value of GSH was measured for the first time in the human brain. The data are consistent with short T₂ for both antioxidants. These T₂ values are essential for the absolute quantification of Asc and GSH concentrations measured at long TE, and provide a critical step towards addressing assumptions about T₂, and therefore towards the quantification of concentrations without the possibility of systematic bias. Copyright © 2010 John Wiley & Sons, Ltd.     

Proton and phosphorus magnetic resonance spectroscopy of the healthy human breast at 7 T

In vivo water‐ and fat‐suppressed ¹H magnetic resonance spectroscopy (MRS) and ³¹P magnetic resonance adiabatic multi‐echo spectroscopic imaging were performed at 7 T in duplicate in healthy fibroglandular breast tissue of a group of eight volunteers. The transverse relaxation times of ³¹P metabolites were determined, and the reproducibility of ¹H and ³¹P MRS was investigated. The transverse relaxation times for phosphoethanolamine (PE) and phosphocholine (PC) were fitted bi‐exponentially, with an added short T₂ component of 20 ms for adenosine monophosphate, resulting in values of 199 ± 8 and 239 ± 14 ms, respectively. The transverse relaxation time for glycerophosphocholine (GPC) was also fitted bi‐exponentially, with an added short T₂ component of 20 ms for glycerophosphatidylethanolamine, which resonates at a similar frequency, resulting in a value of 177 ± 6 ms. Transverse relaxation times for inorganic phosphate, γ‐ATP and glycerophosphatidylcholine mobile phospholipid were fitted mono‐exponentially, resulting in values of 180 ± 4, 19 ± 3 and 20 ± 4 ms, respectively. Coefficients of variation for the duplicate determinations of ¹H total choline (tChol) and the ³¹P metabolites were calculated for the group of volunteers. The reproducibility of inorganic phosphate, the sum of phosphomonoesters and the sum of phosphodiesters with ³¹P MRS imaging was superior to the reproducibility of ¹H MRS for tChol. ¹H and ³¹P data were combined to calculate estimates of the absolute concentrations of PC, GPC and PE in healthy fibroglandular tissue, resulting in upper limits of 0.1, 0.1 and 0.2 mmol/kg of tissue, respectively.     

Cardiac cine magnetic resonance fingerprinting for combined ejection fraction,T1 and T2 quantification

This study introduces a technique called cine magnetic resonance fingerprinting (cine‐MRF) for simultaneous T₁, T₂ and ejection fraction (EF) quantification. Data acquired with a free‐running MRF sequence are retrospectively sorted into different cardiac phases using an external electrocardiogram (ECG) signal. A low‐rank reconstruction with a finite difference sparsity constraint along the cardiac motion dimension yields images resolved by cardiac phase. To improve SNR and precision in the parameter maps, these images are nonrigidly registered to the same phase and matched to a dictionary to generate T₁ and T₂ maps. Cine images for computing left ventricular volumes and EF are also derived from the same data. Cine‐MRF was tested in simulations using a numerical relaxation phantom. Phantom and in vivo scans of 19 subjects were performed at 3 T during a 10.9 seconds breath‐hold with an in‐plane resolution of 1.6 x 1.6 mm² and 24 cardiac phases. Left ventricular EF values obtained with cine‐MRF agreed with the conventional cine images (mean bias ?1.0%). Average myocardial T₁ times in diastole/systole were 1398/1391 ms with cine‐MRF, 1394/1378 ms with ECG‐triggered cardiac MRF (cMRF) and 1234/1212 ms with MOLLI; and T₂ values were 30.7/30.3 ms with cine‐MRF, 32.6/32.9 ms with ECG‐triggered cMRF and 37.6/41.0 ms with T₂‐prepared FLASH. Cine‐MRF and ECG‐triggered cMRF relaxation times were in good agreement. Cine‐MRF T₁ values were significantly longer than MOLLI, and cine‐MRF T₂ values were significantly shorter than T₂‐prepared FLASH. In summary, cine‐MRF can potentially streamline cardiac MRI exams by combining left ventricle functional assessment and T₁‐T₂ mapping into one time‐efficient acquisition.     

Simultaneous bilateral T1, T2, and T1ρ relaxation mapping of the hip joint with magnetic resonance fingerprinting

Quantitative MRI can detect early biochemical changes in cartilage, but its bilateral use in clinical routines is challenging. The aim of this prospective study was to demonstrate the feasibility of magnetic resonance fingerprinting for bilateral simultaneous T₁, T₂, and T_1ρ mapping of the hip joint. The study population consisted of six healthy volunteers with no known trauma or pain in the hip. Monoexponential T₁, T₂, and T_1ρ relaxation components were assessed in femoral lateral, superolateral, and superomedial, and inferior, as well as acetabular, superolateral, and superomedial subregions in left and right hip cartilage. Aligned ranked nonparametric factorial analysis was used to assess the side's impact on the subregions. Kruskal–Wallis and Wilcoxon tests were used to compare subregions, and coefficient of variation to assess repeatability. Global averages of T₁ (676.0 ± 45.4 and 687.6 ± 44.5 ms), T₂ (22.5 ± 2.6 and 22.1 ± 2.5 ms), and T_1ρ (38.2 ± 5.5 and 38.2 ± 5.5 ms) were measured in the left and right hip, and articular cartilage, respectively. The Kruskal–Wallis test showed a significant difference between different subregions’ relaxation times regardless of the hip side (p < 0.001 for T₁, p = 0.012 for T₂, and p < 0.001 for T_1ρ). The Wilcoxon test showed that T₁ of femoral layers was significantly (p < 0.003) higher than that for acetabular cartilage. The experiments showed excellent repeatability with CV_rms of 1%, 2%, and 4% for T₁, T₂, and T_1ρ, respectively. It was concluded that bilateral T₁, T₂, and T_1ρ relaxation times, as well as B₁⁺ maps, can be acquired simultaneously from hip joints using the proposed MRF sequence.     

Initial evaluation of hepatic T1 relaxation time as an imaging marker of liver disease associated with autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) is a potentially lethal multi‐organ disease affecting both the kidneys and the liver. Unfortunately, there are currently no non‐invasive methods to monitor liver disease progression in ARPKD patients, limiting the study of potential therapeutic interventions. Herein, we perform an initial investigation of T₁ relaxation time as a potential imaging biomarker to quantitatively assess the two primary pathologic hallmarks of ARPKD liver disease: biliary dilatation and periportal fibrosis in the PCK rat model of ARPKD. T₁ relaxation time results were obtained for five PCK rats at 3 months of age using a Look–Locker acquisition on a Bruker BioSpec 7.0 T MRI scanner. Six three‐month‐old Sprague‐Dawley (SD) rats were also scanned as controls. All animals were euthanized after the three‐month scans for histological and biochemical assessments of bile duct dilatation and hepatic fibrosis for comparison. PCK rats exhibited significantly increased liver T₁ values (mean ± standard deviation = 935 ± 39 ms) compared with age‐matched SD control rats (847 ± 26 ms, p = 0.01). One PCK rat exhibited severe cholangitis (mean T₁ = 1413 ms), which occurs periodically in ARPKD patients. The observed increase in the in vivo liver T₁ relaxation time correlated significantly with three histological and biochemical indicators of biliary dilatation and fibrosis: bile duct area percent (R = 0.85, p = 0.002), periportal fibrosis area percent (R = 0.82, p = 0.004), and hydroxyproline content (R = 0.76, p = 0.01). These results suggest that hepatic T₁ relaxation time may provide a sensitive and non‐invasive imaging biomarker to monitor ARPKD liver disease. Copyright © 2015 John Wiley & Sons, Ltd.     

Intra‐ and interindividual variability of fatty acid unsaturation in six different human adipose tissue compartments assessed by 1H‐MRS in vivo at 3 T

It is generally accepted that the amount and distribution of adipose tissue (AT) in the human body play an important role in the pathogenesis of metabolic diseases. In addition, metabolic effects of released saturated fatty acids (FAs) in blood are known to be more critical than those of unsaturated FAs. However, little is known about the variability in unsaturation of FAs in various AT compartments. The aim of this prospective study was the assessment of mono‐ and polyunsaturated FAs in various AT compartments by localized ¹H‐MRS in order to obtain insight into the intra‐ and interindividual variability. Associations of FA unsaturation with intrahepatic lipids (IHLs), insulin sensitivity and related AT volumes were analyzed. Fifty healthy Caucasians (36 male, 14 female) participated in this study. Spectroscopic examinations were performed in subcutaneous adipose tissue in the neck (SCAT_neck), abdominal deep subcutaneous adipose tissue (DSCAT), abdominal superficial subcutaneous adipose tissue (SSCAT), visceral adipose tissue (VAT), tibial bone marrow (BM) and subcutaneous adipose tissue of the lower leg (SCAT_calf) at 3 T. Unsaturated index (UI) was calculated by the ratio of olefinic and methyl resonances, polyunsaturated index (PUI) by the ratio of diallylic and methyl resonances. Volumes of AT compartments (by T₁‐weighted MRI) and IHL (single‐voxel STEAM) were assessed at 1.5 T, insulin sensitivity by an oral glucose tolerance test. UI was highest for SCAT_calf (0.622) and lowest for BM (0.527). Highest PUI was observed for SSCAT (0.108), lowest for BM (0.093). Significant intraindividual differences between UIs—but not PUIs—are present for most compartments. There is a non‐significant trend for higher UI in males but otherwise no correlation to anthropometric data (age, BMI). A significant negative correlation between UI and AT volume was observed for VAT but for none of the other compartments. Neither UIs nor PUIs show a relation with IHL; insulin sensitivity is significantly correlated to UI in BM (p < 0.01). Unsaturation indices in several distinct AT compartments are location dependent. Our cohort showed only moderate gender‐related differences, with a trend towards less unsaturated FAs (mainly PUI) in females. In BM, insulin resistant subjects are characterized by a higher UI compared with the insulin sensitive ones. Further studies in larger cohorts are necessary to gain further insight into unsaturation of AT.     

A rapid inversion technique for the measurement of longitudinal relaxation times of brain metabolites: application to lactate in high‐grade gliomas at 3 T

The aim of this study was to develop a time‐efficient inversion technique to measure the T₁ relaxation time of the methyl group of lactate (Lac) in the presence of contaminating lipids and to measure T₁ at 3 T in a cohort of primary high‐grade gliomas. Three numerically optimized inversion times (TIs) were chosen to minimize the expected error in T₁ estimates for a given input total scan duration (set to be 30 min). A two‐cycle spectral editing scheme was used to suppress contaminating lipids. The T₁ values were then estimated from least‐squares fitting of signal measurements versus TI. Lac T₁ was estimated as 2000 ± 280 ms. After correcting for T₁ (and T₂ from literature values), the mean absolute Lac concentration was estimated as 4.3 ± 2.6 mm . The technique developed agrees with the results obtained by standard inversion recovery and can be used to provide rapid T₁ estimates of other spectral components as required. Lac T₁ exhibits similar variations to other major metabolites observable by MRS in high‐grade gliomas. The T₁ estimate provided here will be useful for future MRS studies wishing to report relaxation‐corrected estimates of Lac concentration as an objective tumor biomarker. Copyright © 2016 John Wiley & Sons, Ltd.     

Evidence of multiexponential T2 in rat glioblastoma

The aim of this study was to characterize multiexponential T₂ (MET₂) relaxation in a rat C6 glioblastoma tumor model. To do this, rats (n = 11) were inoculated with the C6 cells via stereotaxic injection into the brain. Ten days later, MET₂ measurements were performed in vivo using a single‐slice, multi‐echo spin‐echo sequence at 7.0 T. Tumor signal was biexponential in eight animals with a short‐lived T₂ component (T₂ = 20.7 ± 5.4 ms across samples) representing 6.8 ± 6.2% of the total signal and a long‐lived T₂ component (T₂ = 76.4 ± 9.3 ms) representing the remaining signal fraction. In contrast, signal from contralateral grey matter was consistently monoexponential (T₂ = 48.8 ± 2.3 ms). Additional ex vivo studies (n = 3) and Monte Carlo simulations showed that the in vivo results were not significantly corrupted by partial volume averaging or noise. The underlying physiological origin of the observed MET₂ components is unknown; however, MET₂ analysis may hold promise as a non‐invasive tool for characterizing tumor microenvironment in vivo on a sub‐voxel scale. Copyright © 2009 John Wiley & Sons, Ltd.     

Three‐dimensional ultrashort echo time cones T1ρ (3D UTE‐cones‐T1ρ) imaging

We report a novel three‐dimensional (3D) ultrashort echo time (UTE) sequence employing Cones trajectory and T_1ρ preparation (UTE‐Cones‐T_1ρ) for quantitative T_1ρ assessment of short T₂ tissues in the musculoskeletal system. A basic 3D UTE‐Cones sequence was combined with a spin‐locking preparation pulse for T_1ρ contrast. A relatively short TR was used to decrease the scan time, which required T₁ measurement and compensation using 3D UTE‐Cones data acquisitions with variable TRs. Another strategy to reduce the total scan time was to acquire multiple Cones spokes (N_sp) after each T_1ρ preparation and fat saturation. Four spin‐locking times (TSL = 0–20 ms) were acquired over 12 min, plus another 7 min for T₁ measurement. The 3D UTE‐Cones‐T_1ρ sequence was compared with a two‐dimensional (2D) spiral‐T_1ρ sequence for the imaging of a spherical CuSO₄ phantom and ex vivo meniscus and tendon specimens, as well as the knee and ankle joints of healthy volunteers, using a clinical 3‐T scanner. The CuSO₄ phantom showed a T_1ρ value of 76.5 ± 1.6 ms with the 2D spiral‐T_1ρ sequence, as well as 85.7 ± 3.6 and 89.2 ± 1.4 ms for the 3D UTE‐Cones‐T_1ρ sequences with N_sp of 1 and 5, respectively. The 3D UTE‐Cones‐T_1ρ sequence provided shorter T_1ρ values for the bovine meniscus sample relative to the 2D spiral‐T_1ρ sequence (10–12 ms versus 16 ms, respectively). The cadaveric human Achilles tendon sample could only be imaged with the 3D UTE‐Cones‐T_1ρ sequence (T_1ρ = 4.0 ± 0.9 ms), with the 2D spiral‐T_1ρ sequence demonstrating near‐zero signal intensity. Human studies yielded T_1ρ values of 36.1 ± 2.9, 18.3 ± 3.9 and 3.1 ± 0.4 ms for articular cartilage, meniscus and the Achilles tendon, respectively. The 3D UTE‐Cones‐T_1ρ sequence allows volumetric T_1ρ measurement of short T₂ tissues in vivo.     

A method for rapid in vivo measurement of blood T1

We present a technique to measure the longitudinal relaxation time constant of venous blood (T_1b) in vivo in a few seconds. The MRI sequence consists of a thick‐slab adiabatic inversion, followed by a series of slice‐selective excitations and single‐shot echo planar imaging readouts. The time intervals between excitations were chosen so that blood in macroscopic vessels is fully refreshed between excitations, making the blood signal follow an unperturbed inversion recovery curve. Static tissue, which experiences the inversion and all excitation pulses, quickly reaches a steady state at a low signal as a result of partial saturation. This allows blood‐filled voxels to be discriminated from those containing static tissue, and to be fitted voxel‐by‐voxel to a simple inversion recovery model. The sequence was tested on a flow phantom with the proposed method, yielding T₁ values consistent to within 3% of those obtained using a conventional inversion recovery sequence with a spin‐echo readout. The method was applied to seven adult volunteers and 18 neonates. The blood T₁ of the neonates (1799 ± 206 ms; range, 1393–2035 ms) was found to be more variable than that of adults (1717 ± 39 ms; range, 1662–1779 ms). A linear correlation between the inverse of T_1b and the haematocrit was established in 12 neonates (R² = 0.90). Copyright © 2010 John Wiley & Sons, Ltd.