Fat Protons Research Articles

In vitro mapping of 1H ultrashort T2* and T2 of porcine menisci

In this study, mapping of ultrashort T2 and T2* of acutely isolated porcine menisci at B0 = 9.4 T was investigated. Maps of T2 were measured from a slice through the pars intermedia with a spin echo-prepared two-dimensional ultrashort-TE T2 mapping technique published previously. T2* mapping was performed by two-dimensional ultrashort-TE MRI with variable acquisition delay. The measured signal decays were fitted by monoexponential, biexponential and Gaussian-exponential fitting functions. The occurrence of Gaussian-like signal decays is outlined theoretically. The quality of the curve fits was visualized by mapping the value δ = abs(1 - χ(2) red). For T2 mapping, the Gaussian-exponential fit showed the best performance, whereas the monoexponential and biexponential fits showed regionally high values of δ (δ > 20). Interpretation of the Gaussian-exponential parameter maps was found to be difficult, because a Gaussian signal component can be related to mesoscopic (collagen texture) or macroscopic (slice profile, shim, sample geometry) magnetic field inhomogeneities and/or residual (1) H dipole-dipole couplings. It seems likely that an interplay of these effects yielded the observed signal decays. Modulation of the T2* signal decay caused by chemical shift was observed and addressed to fat protons by means of histology. In the T2 measurements, no modulation of the signal decay was observed and the biexponential and Gaussian-exponential fits showed the best performance with comparable values of δ. Our results suggest that T2 mapping provides the more robust method for the characterization of meniscal tissue by means of MRI relaxometry. However, mapping of ultrashort T2, as performed in this study, is time consuming and provides less signal-to-noise ratio per time than the mapping of T2*. If T2* mapping is used, pixel-wise monitoring of the fitting quality based on reduced χ(2) should be employed and great care should be taken when interpreting the parameter maps of the fits.

Study of the impact of wheat flour type, flour particle size and protein content in a cake-like dough: Proton mobility and rheological properties assessment

The study of food products is always a challenge due to the number of components involved and the interactions that may occur between them. Water is a particular ingredient which interacts with all hydrophilic compounds, although affinities may differ for limiting water amount. During this study, results obtained using 1H NMR on cake dough were compared in terms of the effects of flour type (soft or medium hard), the addition of gluten (5%–20%) and the use of soft flour fractions (flour particle fractions smaller or larger than 50 μm). T2 values and the signal intensities of different proton populations were studied as a function of the wheat protein contents of dough samples. Physicochemical characterization methods were used to better understand how the origin and particle size of flour might impact the hydration properties and mobility of a model system. Increasing the protein content in dough samples was related to an increase of the mobility of fat protons and of the least mobile proton population (relaxation times ranging from 175 to 180 ms and from 5 to 7 ms, respectively).

Fat‐referenced MR thermometry in the breast and prostate using IDEAL

To demonstrate a three-echo fat-referenced MR thermometry technique that estimates and corrects for time-varying phase disturbances in heterogeneous tissues. Fat protons do not exhibit a temperature-dependent frequency shift. Fat-referenced thermometry methods exploit this insensitivity and use the signal from fat to measure and correct for magnetic field disturbances. In this study, we present a fat-referenced method that uses interpolation of the fat signal to correct for phase disturbances in fat free regions. Phantom and ex vivo tissue cool-down experiments were performed to evaluate the accuracy of this method in the absence of motion. Non-heated in vivo imaging of the breast and prostate was performed to demonstrate measurement robustness in the presence of systemic and motion-induced field disturbances. Measurement accuracy of the method was compared to conventional proton resonance frequency shift MR thermometry. In the ex vivo porcine tissue experiment, maximum measurement error of the fat-referenced method was reduced 42% from 3.3 to 1.9°C when compared to conventional MR thermometry. In the breasts, measurement errors were reduced by up to 70% from 6.4 to 1.9°C. Ex vivo and in vivo results show that the proposed method reduces measurement errors in the heterogeneous tissue experiments when compared to conventional MR thermometry.

Correlation between liver histology and novel magnetic resonance imaging in adult patients with non‐alcoholic fatty liver disease – MRI accurately quantifies hepatic steatosis in NAFLD

Conventional magnetic resonance imaging (MRI) techniques that measure hepatic steatosis are limited by T1 bias, T(2)* decay and multi-frequency signal-interference effects of protons in fat. Newer MR techniques such as the proton density-fat fraction (PDFF) that correct for these factors have not been specifically compared to liver biopsy in adult patients with non-alcoholic fatty liver disease (NAFLD). To examine the association between MRI-determined PDFF and histology-determined steatosis grade, and their association with fibrosis. A total of 51 adult patients with biopsy-confirmed NAFLD underwent metabolic-biochemical profiling, MRI-determined PDFF measurement of hepatic steatosis and liver biopsy assessment according to NASH-CRN histological scoring system. The average MRI-determined PDFF increased significantly with increasing histology-determined steatosis grade: 8.9% at grade-1, 16.3% at grade-2, and 25.0% at grade-3 with P ≤ 0.0001 (correlation: r(2) = 0.56, P < 0.0001). Patients with stage-4 fibrosis, when compared with patients with stage 0-3 fibrosis, had significantly lower hepatic steatosis by both MRI-determined PDFF (7.6% vs. 17.8%, P < 0.005) and histology-determined steatosis grade (1.4 vs. 2.2, P < 0.05). NAFLD patients with grade 1 steatosis were more likely to have characteristics of advanced liver disease including higher average AST:ALT (0.87 vs. 0.60, P < 0.02), GGT (140 vs. 67, P < 0.01), and INR (1.06 vs. 0.99, P < 0.01), higher stage of fibrosis and hepatocellular ballooning. MRI-determined proton density-fat fraction correlates with histology-determined steatosis grade in adults with NAFLD. Steatosis is non-linearly related to fibrosis progression. In patients with NAFLD, a low amount of hepatic steatosis on imaging does not necessarily indicate mild disease.

Reducing localized signal fluctuation in fMRI using spectral–spatial fat saturation

Conventional 1D, spatially nonselective fat saturation can generate uncrushed fat signals in areas far outside the imaging slice where crushers are weak because of reduced gradient linearity. These fat signals can corrupt in-slice water signal, and in functional MRI, they can manifest themselves as artifacts such as clouds in image background or localized signal fluctuation over time. In this article, a spectral-spatial radiofrequency pulse is proposed to replace the conventional, spatially nonselective fat saturation pulse. The advantage of the proposed method is that fat protons far outside the image slice would not be excited because of the spatial selectivity, thereby removing the root cause of the fat aliasing artifacts. The proposed method also preserves thin slice capability, pulse duration, and fat suppression performance of the conventional method. Bloch simulation and human volunteer results show that the method is effective in reducing the fat aliasing artifacts seen in functional MRI.

A simple isotropic phantom for diffusional kurtosis imaging

Dairy cream is shown to be a simple, inexpensive, isotropic phantom useful for testing diffusional kurtosis imaging data acquisition and postprocessing. The MR-visible protons of cream exhibit slow and fast diffusion components, attributed to the fat and water protons, respectively, which give rise to a diffusion coefficient of 1.1 μm(2)/ms and a diffusional kurtosis of 1.2. These parameter values are similar to those observed in vivo for human brain. Heating the cream is found to increase the T(2)-relaxation time of the fat protons, which facilitates the evaluation of typical diffusional kurtosis imaging protocols used in clinical settings. The diffusion coefficient and diffusional kurtosis can both be measured directly and predicted based on the corresponding diffusion parameters of the individual water and fat components, which are independently measurable due to chemical shift misregistration, thus providing an important consistency check. This phantom is proposed as a convenient calibration standard for multicenter diffusional kurtosis imaging studies.

Removal of olefinic fat chemical shift artifact in diffusion MRI

Diffusion-weighted (DW) MRI has emerged as a key tool for assessing the microstructure of tissues in healthy and diseased states. Because of its rapid acquisition speed and insensitivity to motion, single-shot echo-planar imaging is the most common DW imaging technique. However, the presence of fat signal can severely affect DW-echo planar imaging acquisitions because of the chemical shift artifact. Fat suppression is usually achieved through some form of chemical shift-based fat saturation. Such methods effectively suppress the signal originating from aliphatic fat protons, but fail to suppress the signal from olefinic protons. Olefinic fat signal may result in significant distortions in the DW images, which bias the subsequently estimated diffusion parameters. This article introduces a method for removing olefinic fat signal from DW images, based on an echo time-shifted acquisition. The method is developed and analyzed specifically in the context of single-shot DW-echo-planar imaging, where image phase is generally unreliable. The proposed method is tested with phantom and in vivo datasets, and compared with a standard acquisition to demonstrate its performance.

Fat-free MRI based on magnetization exchange

An MRI technique is proposed for complete fat signal elimination. This approach exploits the fact that water rapidly exchanges magnetization with protons in protein and membrane phospholipid of tissue and cells but does not exchange magnetization with triglyceride or fat protons in the tissue. Saturation of the proton signal from protein and membrane phospholipid thus results in partial saturation of the water proton signal, allowing acquisition of an image including a portion of the water signal and the full fat signal. Subtraction of this image from the standard image, containing both water and fat signals, results in an image in which all fat signal is cancelled. This fat-free image is sensitive to magnetization transfer and to water density and relaxation time, providing the possibility of additional contrast. Unlike most fat suppression techniques, this method is not compromised by the static or radiofrequency field heterogeneity and is equally efficient for all fat resonances independent of their chemical shift frequency.

Body MRI Using IDEAL

The intrinsic differences of water and fat protons in the MR environment allow selective interrogation of their contribution to the MR signal. Fat-suppression techniques and chemical shift imaging are routinely used in clinical body MRI. Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) is a novel imaging technique for separating fat and water. This article describes the basic principles of IDEAL MRI and illustrates the use of IDEAL imaging as an alternative to fat-suppression techniques and chemical shift imaging for body MRI.

Correction of proton resonance frequency shift temperature maps for magnetic field disturbances using fat signal

To improve the immunity of the proton resonance frequency shift (PRFS) method of MRI temperature mapping against magnetic field disturbances. Since PRFS is a phase-sensitive method, it misinterprets magnetic field disturbances as artifact temperature changes. If not corrected, the resulting temperature artifacts can completely obscure the true temperature estimation, especially if the temperature elevations are small. Since the fat protons experience the same magnetic field disturbances as the water protons, but no temperature-related frequency shift, the fat signal has been used for correcting PRFS temperature maps for the disturbances. A simple correction method is proposed that has either better compensation capability than the phase correction methods previously reported or higher spatial and temporal resolution than the spectroscopic correction methods previously reported. The evaluated method is based on the utilization of several gradient and spin echoes acquired within one repetition interval with water- and fat-selective scans. In a series of phantom experiments, the improved method is shown to enable the reconstruction of accurate temperature maps in spite of interscan motion, suboptimal fat-water separation, and a wide range of magnetic field disturbances. Our approach can be used for the guidance of thermal therapies involving tissues containing fat or surrounded by fat.

NMR assessment of ice cream: Effect of formulation on liquid and solid fat

The capacity of low field NMR spectrometry to characterize the different behaviours of fat protons in unfrozen ice cream mix and in ice cream was investigated. Various formulations comprising different types of fat, protein and emulsifier were tested. The NMR parameters attributed to fat were sensitive both to the type of fat and also to the components located at the fat globule interface. In the unfrozen ice cream mix and to a lesser extent in ice cream, liquid fat was more sensitive to the type of protein, whereas the solid fat was sensitive to the type of emulsifier. The solid to liquid fat ratio calculated from the solid part of the NMR signal in the unfrozen mix was mainly dependent on the fat and emulsifier types. Changes in the states of liquid fat were detected during cold storage and interpreted as deviation from an equilibrium state recoverable after prolonged storage.

Determination of water self-diffusion coefficient in complex food products by low field 1H PFG-NMR: comparison between the standard spin-echo sequence and the T1-weighted spin-echo sequence

In 1990, Van Den Enden et al. proposed a method for the determination of water droplet size distributions in emulsions using a pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) T 1-weighted stimulated-echo technique. This paper describes both the T 1-weighted spin-echo sequence, an improved method based on this earlier work, and, the standard PFG spin-echo sequence. These two methods were compared for water self-diffusion coefficient measurement in the fatty protein concentrate sample used as a ‘cheese model.’ The transversal and longitudinal relaxation parameters T 1 and T 2 were determined according to the temperature and investigated for each sample; fat-free protein concentrate sample, pure anhydrous milk fat, and fatty protein concentrate sample. The water self-diffusion in fat-free protein concentrate samples followed a linear behavior. Consequently, the water self-diffusion coefficient could be easily characterized for fat-free protein concentrate samples. However, it seemed more complicated to obtain accurate water self-diffusion in fatty protein concentrate samples since the diffusion-attenuation data were fitted by a bi-exponential function. This paper demonstrates that the implementation of the T 1-weighted spin-echo sequence, using the different T 1 properties of water and fat phases, allows the accurate determination of water self-diffusion coefficient in a food product. To minimize the contribution of the 1H nuclei in the fat phase on the NMR echo signal, the fat protons were selectively eliminated by an additional 180° pulse. This new method reduces the standard errors of diffusion data obtained with a basic spin-echo technique, by a factor of 10. The effectiveness of the use of the T 1-weighted spin-echo sequence to perform accurate water self-diffusion coefficients measurement in fatty products is thus demonstrated.

Artifacts in musculoskeletal magnetic resonance imaging: identification and correction.

A large number of artifacts occur in magnetic resonance (MR) imaging of the musculoskeletal system. These artifacts may potentially affect the quality of MR images, and may also simulate pathologic conditions and produce pitfalls in interpretation. Motion artifacts may be periodic or random. Protocol-error artifacts include saturation, wraparound, radiofrequency (RF) interference, shading, and partial volume averaging artifacts. Truncation artifacts occur when the number of phase-encoding steps of high spatial frequencies is insufficient (or under-sampled) for faithful reproduction of the true anatomic detail of the original image. Chemical shift artifacts are due to the protons in fat being mismapped relative to water protons. Susceptibility artifacts occur at the interfaces of structures with different magnetic susceptibilities. Artifacts special to the musculoskeletal system include the magnic angle phenomenon and spurious signal induced at very short echo times, both of which affect anisotropic structures such as tendon, ligament, and cartilage. Recognition and, if possible, correction of these artifacts are an important aspect of practical musculoskeletal MR imaging.

Classification and detection of bone marrow lesions with magnetic resonance imaging.

A logical approach to bone marrow alterations is proposed that takes into account the high sensitivity and specificity of MR imaging for the detection of marrow fat. Marrow signal intensity on T1-weighted images is assumed to reflect the balance between fat and nonfat marrow components. Elementary patterns of marrow change include marrow depletion, infiltration, replacement and signal void. These patterns can be observed alone or in combination, and can be distributed in a focal or diffuse manner. Marrow depletion pattern shows high signal intensity reflecting increase in fat content and is frequently irrelevant clinically. Signal intensity is decreased in marrow infiltration, replacement and signal void patterns, which indicates partial or complete disappearance of fat. Focal marrow infiltration is frequently reactive to an adjacent abnormality, whereas focal marrow replacement more frequently indicates a more profound marrow alteration. The T1-weighted spin-echo sequence is satisfactory for lesion detection by virtue of its relatively high sensitivity in the detection of altered fat/nonfat marrow balance. T2-weighted sequences with saturation of the signal of fat protons or out-of-phase gradient-echo sequences improve lesion conspicuity in all situations in which the difference in fat/nonfat marrow balance between the abnormal area and the adjacent normal marrow is reduced.

Slice-selective fat saturation in MR angiography using spatial-spectral selective prepulses.

Presaturation of fat signals by frequency-selective radiofrequency (RF) pulses is often applied in MR angiography to improve the visualization of the blood vessels. Unfortunately, standard fat saturation methods might cause a considerable reduction of the blood signal in the measured slices. This effect is caused by an attenuation of blood magnetization in remote tissue regions with water protons showing a similar Larmor frequency as the fat protons in the recorded slice. The affected blood water protons subsequently flow into the recorded slice and provide low signal intensity. Suitable spatial-spectral selective methods for slice-selective fat saturation were developed to avoid this unwanted effect. A spatial-spectral fat saturation technique was compared with a corresponding only spectrally selective approach. Both saturation techniques were included in a standard two-dimensional (2D) cine sequence and applied in angiographic examinations of the thighs. The results indicate that spatial-spectral saturation (acting slice selectively) leads to a clearly higher blood signal intensity in fat-suppressed MR angiography compared with standard techniques, especially in measurements performed during the systolic phase of the cardiac cycle.

A three‐point dixon method for water and fat separation using 2D and 3D gradient‐echo techniques

A water and fat separation technique based on a three-point Dixon method using two-dimensional (2D) and three-dimensional (3D) gradient-echo sequences and a new phase-unwrapping approach is presented. Using velocity-compensated sequences, three images, with water and fat protons in phase, opposed phase, and in phase, were acquired in an interleaved fashion for each phase-encoding line of the image. A fast 2D scan permitted acquisition of images within a single breath-hold, eliminating respiratory motion artifacts. The 3D sequence allowed coverage of a large region of interest with thin, contiguous slices. To correct field inhomogeneity effects, phase unwrapping was required. This was accomplished by comparing the corresponding pixels in the two water and fat in-phase images on a pixel-by-pixel basis. This phase-unwrapping approach is faster, more reliable, and requires less user interfacing than other methods. The water and fat separation method performed well in various parts of the body.

Imaging of incidentally discovered adrenal masses.

IntroductionIn the last decade the increased use of cross-sectional imaging,especially computed tomography (CT) and magnetic resonanceimaging (MRI), has led to an increase in the number ofincidentally discovered adrenal masses. Even in patients with aknown malignancy, the majority of these will be benign,nonhyperfunctioning adenomas (Oliver et al., 1984). Rapidlyevolving imaging techniques used to evaluate these masses hasmeant that intervention is now indicated in only a minority ofpatients. The approach to their investigation must include aconsideration of several factors including: diagnostic costs,discomfort to the patient, risks from biopsy, consequences offalse-positive results, and disease prevalence.This article reviews the imaging appearances of the mostcommon causes of an incidentally discovered adrenal mass, andsuggests a strategy for the investigation of such masses.PrevalenceAdrenal masses of varying pathology are found in 0·35-4·4% ofpatients imaged with CT for reasons other than suspectedadrenal pathology (Glazer et al., 1982; Prinz et al., 1982;Abeccassis et al., 1985; Belldegrun et al., 1986; Herrera et al.,1991), and are usually referred to as adrenal ‘incidentalomas’.Large autopsy series of more than 1000 patients report aslightly higher incidence of 1·4-5·7% (Russi & Blumenthal,1945; Devenyi, 1967; Kokko et al., 1967; Granger & Genest,1970; Russel et al., 1972; Commons & Callaway, 1984). It islikely that with the increased use of abdominal CT and MRI,improving spatial resolution, and the use of thinner slices, therewill be a further increase in the prevalence of such incidentallydiscovered adrenal masses.Differential diagnosisThe vast majority of truly incidental adrenal masses (i.e. thosediscovered in patients with no other pathology) are adrenaladenomas. The incidence of other adrenal masses such asmyelolipomas, adrenal cysts, phaeochromocytomas, carci-noma and metastases varies considerably in the literaturedepending on the size of the series (Kloos et al., 1995). Evenin patients known to have malignant disease, adrenaladenomas are often more common than metastases (Oliver etal., 1984).The first step in investigating an adrenal mass detectedincidentally is to establish whether it is biochemically active.If shown to be biochemically active the mass is dealt withaccordingly. If the mass is not hyperfunctioning a substantialdifferential diagnosis remains. Some of these masses, such asmyelolipomas and cysts, have specific imaging features whichmay allow characterization of the mass on imaging alone. Inpatients with an adrenal mass which cannot be characterizedon imaging alone, the problem is then to differentiate benignfrom malignant lesions. In patients with a known malignancythe differential diagnosis is essentially between adrenalmetastases and an adrenal adenoma, whilst in patients withouta known malignancy adrenal carcinoma must also beconsidered.Biochemical investigationThe major hyper-secretory syndromes associated with adrenalmasses are Cushing’s syndrome, Conn’s syndrome andphaeochromocytomas. Cushing’s syndrome may be clinicallyobvious, and can be confirmed by grossly elevated urinary freecortisol or, with greater sensitivity, by a failure of serumcortisol at 0900h to suppress to<50nmol/l in the standard low-dose dexamethasone suppression test (Perry & Grossman,1997). However, it may be noted that some, possibly themajority, of ‘nonfunctioning’ adenomas may also show afailure of suppression, although presumably in these cases theoverall secretory rate remains below or within the normal range(Tsagarakis et al., 1998). Patients with Conn’s syndrome willdemonstrate hypokalaemic alkalosis, although this may bemasked by a low sodium diet: further investigation will reveal asuppressed plasma renin and elevated aldosterone:renin ratio(Vallotton, 1996b). Such tumours are usually small and havethe signal characteristics of adenomas. Finally, secretoryphaeochromocytomas can be identified by elevated urinarycatecholamine excretion using either GCMS or HPLC withelectrochemical detection: the sensitivity of this techniquecurrently approaches 100% (Ross et al., 1993; Bouloux &Fakeeh, 1995).

Rapid moisture and fat determination by pulsed NMR for monitoring drying and coating processes of extrudates

SummaryAfter extrusion cooking, extrudates must be dried and are often coated by spraying with fatty materials before cooling and packaging. As a means to control the drying and coating processes of pet foods, a rapid NMR method was developed to determine water and fat contents over a large temperature range without weighing, heating or measuring the temperature of the samples. Weighing was avoided by normalizing all NMR intensities by the solid echo maximum of each sample. Formulae using estimates of the transverse relaxation time and the concentration of liquid fat protons were used to determine total fat and moisture contents without measuring the temperature of the samples. Fat content estimation had a standard deviation better than 0.2% over a large solid fat index range above 20°C. Water content was determined with a standard deviation of about 0.2% at all temperatures studied.

Developing diagnostic techniques: The role of magnetic resonance imaging in tumour staging

AbstractMagnetic resonance (MR) is a state‐of‐the art imaging modality which does not use ionizing radiation. It is now widely available as an imaging technique in the U.K. and is no longer confined to specialist centres. MR has now become part of the clinician's diagnostic armamentarium and is not merely a research utility.A magnetic field and radio‐frequency waves are used to excite protons and produce an image. Protons exist in many different environments but for imaging purposes the hydrogen protons in fat and water are used. The rate at which excited protons relax is described by two characteristic times, T1 and T2, which vary in different tissues. T1‐weighted images show normal anatomy, whilst T2‐weighted images generally highlight abnormal tissue. Injection of a paramagnetic gadolinium‐based contrast agent enhances T1‐weighted images by reducing relaxation times. Initially it was felt that MR would be tissue‐specific,1 enabling correlation with histopathological findings. Unfortunately, research and clinical use has shown that MR does not fulfil these expectations at present.2 Future developments such as spectroscopy may improve tissue specificity and hence diagnostic accuracy.The value of MR for imaging the neuro‐axis and musculo‐skeletal system is well established. New developments are increasing its applications in breast, thorax, and liver imaging. There has been much research into the role of MR in staging malignant tumours, particularly in the areas of bladder and lung carcinomas. The present article concentrates on this specific application.

Quantifizierung der Signalmodulation des blutbildenden Knochenmarkes in Gradienten-Echo-Sequenzen

The signals from spongy bone on gradient echo sequences depend on the fat and water proton content as well as the differences in magnetic susceptibility at the border between the trabeculae and the bone marrow. From the signal intensities for different echo times, signal modulation was quantified by a special algorithm and at the same time the T2 relaxation time was calculated. Using a fat-water phantom, it was shown that the product of the initial fat and water signal intensity (modulation coefficient) is applicable to indicate the extent of signal modulation and thereby the chemical shift and, moreover, to indicate changes in the fat/water ratio sensitively. In 16 individuals of different ages (23 to 64 years, average 42.4 years) there was a tendency for the T2 relaxation time from the spongiosa of lumbar vertebrae to increase with age, this can be explained by a reduction in the trabecular content, causing reduction in the magnetic inhomogeneity of the bone marrow. The results indicate the usefulness of the sequence and the mathematical model for evaluating the spongy bone in cases of osteoporosis and of malignant haematological disorders.