RESUMEN
BACKGROUND: Adipose tissue (AT) can be classified into white and brown/beige subtypes. Chemical shift encoding-based water-fat MRI-techniques allowing simultaneous mapping of proton density fat fraction (PDFF) and T2 * result in a lower PDFF and a shorter T2 * in brown compared with white AT. However, AT T2 * values vary widely in the literature and are primarily based on 6-echo data. Increasing the number of echoes in a multiecho gradient-echo acquisition is expected to increase the precision of AT T2 * mapping. PURPOSE: 1) To mitigate issues of current T2 *-measurement techniques through experimental design, and 2) to investigate gluteal and supraclavicular AT T2 * and PDFF and their relationship using a 20-echo gradient-echo acquisition. STUDY TYPE: Prospective. SUBJECTS: Twenty-one healthy subjects. FIELD STRENGTH/SEQUENCE ASSESSMENT: First, a ground truth signal evolution was simulated from a single-T2 * water-fat model. Second, a time-interleaved 20-echo gradient-echo sequence with monopolar gradients of neck and abdomen/pelvis at 3 T was performed in vivo to determine supraclavicular and gluteal PDFF and T2 *. Complex-based water-fat separation was performed for the first 6 echoes and the full 20 echoes. AT depots were segmented. STATISTICAL TESTS: Mann-Whitney test, Wilcoxon signed-rank test and simple linear regression analysis. RESULTS: Both PDFF and T2 * differed significantly between supraclavicular and gluteal AT with 6 and 20 echoes (PDFF: P < 0.0001 each, T2 *: P = 0.03 / P < 0.0001 for 6/20 echoes). 6-echo T2 * demonstrated higher standard deviations and broader ranges than 20-echo T2 *. Regression analyses revealed a strong relationship between PDFF and T2 * values per AT compartment (R2 = 0.63 supraclavicular, R2 = 0.86 gluteal, P < 0.0001 each). DATA CONCLUSION: The present findings suggest that an increase in the number of sampled echoes beyond 6 does not affect AT PDFF quantification, whereas AT T2 * is considerably affected. Thus, a 20-echo gradient-echo acquisition enables a multiparametric analysis of both AT PDFF and T2 * and may therefore improve MR-based differentiation between white and brown fat. LEVEL OF EVIDENCE: 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:424-434.
Asunto(s)
Tejido Adiposo/anatomía & histología , Imagen por Resonancia Magnética/métodos , Adulto , Nalgas/anatomía & histología , Clavícula/anatomía & histología , Femenino , Humanos , Masculino , Persona de Mediana Edad , Estudios Prospectivos , Valores de Referencia , Adulto JovenRESUMEN
PURPOSE: To remove the confounding effect of unsuppressed fat on the imaging-based apparent diffusion coefficient (ADC) of the vertebral bone marrow water component when using spectrally selective fat suppression and to compare and validate the proposed quantification strategy against diffusion-weighted magnetic resonance spectroscopy (DW-MRS). METHODS: Twelve subjects underwent diffusion-weighted imaging (DWI) and DW-MRS of the vertebral bone marrow. A theoretical model was developed to take into account and correct the effects of residual fat on ADC, incorporating additional measurements for proton density fat fraction (PDFF) and water T2 (T2w ). Uncorrected and corrected DWI-based ADC was compared with DW-MRS-based ADC using the Bland-Altman method. RESULTS: There was a systematic bias equal to 0.118 ± 0.116 × 10-3 mm2 /s between DWI and DW-MRS when no correction was performed. Taking into account measured PDFF and constant T2w reduced the bias to 0.006 ± 0.128 × 10-3 mm2 /s. Using the proposed approach with both individually measured PDFF and T2w reduced both the bias and the limits of agreement between DWI and DW-MRS (0.018 ± 0.065 × 10-3 mm2 /s). CONCLUSION: By taking into account the presence of residual fat in a modified signal model that incorporates additional individual measurements of PDFF and T2w , good agreement of imaging-based ADC with MRS-based ADC can be achieved in vertebral bone marrow. Magn Reson Med 78:1432-1441, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
Asunto(s)
Tejido Adiposo/diagnóstico por imagen , Médula Ósea/diagnóstico por imagen , Imagen de Difusión por Resonancia Magnética/métodos , Espectroscopía de Resonancia Magnética/métodos , Columna Vertebral/diagnóstico por imagen , Adulto , Femenino , Humanos , Procesamiento de Imagen Asistido por Computador/métodos , Masculino , Adulto JovenRESUMEN
PURPOSE: To propose a phase error correction scheme for monopolar time-interleaved multi-echo gradient echo water-fat imaging that allows accurate and robust complex-based quantification of the proton density fat fraction (PDFF). METHODS: A three-step phase correction scheme is proposed to address a) a phase term induced by echo misalignments that can be measured with a reference scan using reversed readout polarity, b) a phase term induced by the concomitant gradient field that can be predicted from the gradient waveforms, and c) a phase offset between time-interleaved echo trains. Simulations were carried out to characterize the concomitant gradient field-induced PDFF bias and the performance estimating the phase offset between time-interleaved echo trains. Phantom experiments and in vivo liver and thigh imaging were performed to study the relevance of each of the three phase correction steps on PDFF accuracy and robustness. RESULTS: The simulation, phantom, and in vivo results showed in agreement with the theory an echo time-dependent PDFF bias introduced by the three phase error sources. The proposed phase correction scheme was found to provide accurate PDFF estimation independent of the employed echo time combination. CONCLUSION: Complex-based time-interleaved water-fat imaging was found to give accurate and robust PDFF measurements after applying the proposed phase error correction scheme. Magn Reson Med 78:984-996, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
Asunto(s)
Procesamiento de Imagen Asistido por Computador/métodos , Imagen por Resonancia Magnética/métodos , Tejido Adiposo/diagnóstico por imagen , Simulación por Computador , Humanos , Hígado/diagnóstico por imagen , Fantasmas de ImagenRESUMEN
OBJECTIVES: To investigate the feasibility of employing a 3D time-interleaved multi-echo gradient-echo (TIMGRE) sequence to measure the proton density fat fraction (PDFF) in the vertebral bone marrow (VBM) of children and to examine cross-sectional changes with age and intra-individual variations from the lumbar to the cervical region in the first two decades of life. MATERIALS AND METHODS: Quantitative water-fat imaging of the spine was performed in 93 patients (49 girls; 44 boys; age median 4.5 years; range 0.1-17.6 years). For data acquisition, a six-echo 3D TIMGRE sequence was used with phase correction and complex-based water-fat separation. Additionally, single-voxel MR spectroscopy (MRS) was performed in the L4 vertebrae of 37 patients. VBM was manually segmented in the midsagittal slice of each vertebra. Univariable and multivariable linear regression models were calculated between averaged lumbar, thoracic and cervical bone marrow PDFF and age with adjustments for sex, height, weight, and body mass index percentile. RESULTS: Measured VBM PDFF correlated strongly between imaging and MRS (R 2 = 0.92, slope = 0.94, intercept = -0.72%). Lumbar, thoracic and cervical VBM PDFF correlated significantly (all p < 0.001) with the natural logarithm of age. Differences between female and male patients were not significant (p > 0.05). CONCLUSION: VBM development in children showed a sex-independent cross-sectional increase of PDFF correlating with the natural logarithm of age and an intra-individual decrease of PDFF from the lumbar to the cervical region in all age groups. The present results demonstrate the feasibility of using a 3D TIMGRE sequence for PDFF assessment in VBM of children.
Asunto(s)
Tejido Adiposo/diagnóstico por imagen , Tejido Adiposo/metabolismo , Médula Ósea/diagnóstico por imagen , Médula Ósea/metabolismo , Imagen por Resonancia Magnética/métodos , Columna Vertebral/diagnóstico por imagen , Columna Vertebral/metabolismo , Adolescente , Factores de Edad , Niño , Preescolar , Estudios Transversales , Estudios de Factibilidad , Femenino , Humanos , Imagenología Tridimensional , Lactante , Metabolismo de los Lípidos , Imagen por Resonancia Magnética/estadística & datos numéricos , Masculino , Valores de Referencia , Agua/metabolismoRESUMEN
PURPOSE: To propose a novel combination of robust Dixon fat suppression and motion insensitive PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) MRI. METHODS: Two different echoes were acquired interleaved in each shot enabling water-fat separation on individual blades. Fat, which was blurred in standard PROPELLER because the water-fat shift (WFS) rotated with the blades, was shifted back in each blade. Additionally, field maps obtained from the water-fat separation were used to unwarp off-resonance-induced shifts in each blade. PROPELLER was then applied to the water, corrected fat, or recombined water-fat blades. This approach was compared quantitatively in volunteers with regard to motion estimation and signal-to-noise ratio (SNR) to a standard PROPELLER acquisition with minimal WFS and fat suppression. RESULTS: Shifting the fat back in each blade reduced errors in the translation correction. SNR in the proposed Dixon PROPELLER was 21% higher compared with standard PROPELLER with identical scan time. High image quality was achieved even when the volunteers were moving during data acquisition. Furthermore, sharp water-fat borders and image details were seen in areas where standard PROPELLER suffered from blurring when acquired with a low readout bandwidth. CONCLUSION: The proposed method enables motion-insensitive PROPELLER MRI with robust fat suppression and reduced blurring. Additionally, fat images are available if desired.
Asunto(s)
Encéfalo/anatomía & histología , Aumento de la Imagen/métodos , Hígado/anatomía & histología , Imagen por Resonancia Magnética/métodos , Tejido Adiposo , Adulto , Agua Corporal , Femenino , Voluntarios Sanos , Humanos , Procesamiento de Imagen Asistido por Computador/métodos , Masculino , Movimiento (Física) , Relación Señal-RuidoRESUMEN
Magnetic resonance-based assessment of quadriceps muscle fat has been proposed as surrogate marker in sarcopenia, osteoarthritis, and neuromuscular disorders. We presently investigated the association of quadriceps muscle fat with isometric strength measurements in healthy males using chemical shift encoding-based water-fat magnetic resonance imaging. Intermuscular adipose tissue fraction and intramuscular proton density fat fraction correlated significantly (P < 0.05) with isometric strength (up to r = -0.83 and -0.87, respectively). Reproducibility of intermuscular adipose tissue fraction and intramuscular proton density fat fraction was 1.5% and 5.7%, respectively.
Asunto(s)
Interpretación de Imagen Asistida por Computador , Grasa Intraabdominal/fisiología , Contracción Isométrica/fisiología , Imagen por Resonancia Magnética , Fuerza Muscular/fisiología , Músculo Cuádriceps/diagnóstico por imagen , Músculo Cuádriceps/fisiología , Adiposidad/fisiología , Adulto , Agua Corporal/diagnóstico por imagen , Agua Corporal/fisiología , Humanos , Grasa Intraabdominal/diagnóstico por imagen , Masculino , Valores de Referencia , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
Bone marrow fat fraction mapping using chemical shift encoding-based water-fat separation is becoming a useful tool in investigating the association between bone marrow adiposity and bone health and in assessing cancer treatment-induced bone marrow damage. Vertebral bone marrow is characterized by short T2* relaxation times, which are in general different for the water and fat components and can confound fat quantification. The purpose of the present study is to compare different approaches to T2* correction in chemical shift encoding-based water-fat imaging of vertebral bone marrow using single-voxel MRS as reference. Eight-echo gradient-echo imaging and single-voxel MRS measurements were made on the spine (L3-L5) of 25 healthy volunteers. Different approaches were evaluated for correction of T2* effects: (a) single-T2* correction, (b) dual-T2* correction, (c) T2' correction using the a priori-known T2 from the MRS at each vertebral body and (d) T2' correction using the a priori-known T2 equal to previously measured average values. Dual-T2* correction resulted in noisier imaging fat fraction maps than single-T2* correction or T2' correction using a priori-known T2. Linear regression analysis between imaging and MRS fat fraction showed a slope significantly different from 1 when using single-T2* correction (R(2) = 0.96) or dual-T2* correction (R(2) = 0.87). T2' correction using the a priori-known T2 resulted in a slope not significantly different from 1, an intercept significantly different from 0 (between 2.4% and 3%) and R(2) = 0.96. Therefore, a T2' correction using a priori-known T2 can remove the fat fraction bias induced by the difference in T2* between water and fat components without degrading noise performance in fat fraction mapping of vertebral bone marrow.
Asunto(s)
Tejido Adiposo/fisiología , Adiposidad/fisiología , Médula Ósea/fisiología , Vértebras Lumbares/fisiología , Imagen por Resonancia Magnética/métodos , Modelos Biológicos , Tejido Adiposo/anatomía & histología , Adulto , Médula Ósea/anatomía & histología , Simulación por Computador , Femenino , Humanos , Interpretación de Imagen Asistida por Computador/métodos , Vértebras Lumbares/anatomía & histología , Masculino , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
BACKGROUND: The assessment of bone marrow composition has recently gained significant attention due to its association with bone loss pathophysiology and cancer therapy-induced bone marrow damage. The purpose of our study was to investigate the anatomical variation of the vertebral bone marrow fat using chemical shift-encoding based water-fat MRI and to assess the repeatability of these measurements. METHODS: Chemical shift-encoding based water-fat MRI of the whole spine was performed in 28 young, healthy subjects (17 males, 11 females, 26 ± 4 years). Six subjects were scanned three times with repositioning to assess the repeatability of these measurements. Proton density fat fraction (PDFF) maps were computed and manually segmented to obtain PDFF of C3-L5. RESULTS: Mean PDFF of all subjects significantly increased from C3 to L5 (P < 0.05) with r = 0.88 (P < 0.05). PDFF averaged over C3-7, T1-6, T7-12, and L1-5 of males and females amounted to 31.7 ± 7.9% and 23.0 ± 7.8% (P = 0.002), 33.8 ± 6.8% and 24.6 ± 8.8% (P = 0.005), 33.8 ± 6.4% and 26.1 ± 6.4% (P = 0.023), and 38.8 ± 7.6% and 31.5 ± 12.4% (P = 0.063), respectively. The repeatability for PDFF measurements expressed as absolute precision error was 1.7% averaged over C3-L5. CONCLUSION: Whole spine vertebral bone marrow fat could be reproducibly assessed by using chemical shift-encoding based water-fat MRI and showed anatomical variations.
Asunto(s)
Tejido Adiposo/anatomía & histología , Médula Ósea/anatomía & histología , Interpretación de Imagen Asistida por Computador/métodos , Imagen por Resonancia Magnética/métodos , Espectroscopía de Resonancia Magnética/métodos , Columna Vertebral/anatomía & histología , Tejido Adiposo/fisiología , Adiposidad/fisiología , Adulto , Algoritmos , Médula Ósea/fisiología , Humanos , Masculino , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Columna Vertebral/fisiología , Agua/análisis , Imagen de Cuerpo Entero , Adulto JovenRESUMEN
OBJECTIVES: To compare systematically quantitative MRI, MR spectroscopy (MRS), and different histological methods for liver fat quantification in order to identify possible incongruities. METHODS: Fifty-nine consecutive patients with liver disorders were examined on a 3 T MRI system. Quantitative MRI was performed using a dual- and a six-echo variant of the modified Dixon (mDixon) sequence, calculating proton density fat fraction (PDFF) maps, in addition to single-voxel MRS. Histological fat quantification included estimation of the percentage of hepatocytes containing fat vesicles as well as semi-automatic quantification (qHisto) using tissue quantification software. RESULTS: In 33 of 59 patients, the hepatic fat fraction was >5% as determined by MRS (maximum 45%, mean 17%). Dual-echo mDixon yielded systematically lower PDFF values than six-echo mDixon (mean difference 1.0%; P < 0.001). Six-echo mDixon correlated excellently with MRS, qHisto, and the estimated percentage of hepatocytes containing fat vesicles (R = 0.984, 0.967, 0.941, respectively, all P < 0.001). Mean values obtained by the estimated percentage of hepatocytes containing fat were higher by a factor of 2.5 in comparison to qHisto. Six-echo mDixon and MRS showed the best agreement with values obtained by qHisto. CONCLUSIONS: Six-echo mDixon, MRS, and qHisto provide the most robust and congruent results and are therefore most appropriate for reliable quantification of liver fat. KEY POINTS: ⢠Six-echo mDixon correlates excellently with MRS, qHisto, and the estimated percentage of fat-containing hepatocytes. ⢠Six-echo mDixon, MRS, and qHisto provide the most robust and congruent results. ⢠Dual-echo mDixon yields systematically lower PDFF values than six-echo mDixon. ⢠The percentage of fat-containing hepatocytes is 2.5-fold higher than fat fraction determined by qHisto. ⢠Performance characteristics and systematic differences of the various methods should be considered.
Asunto(s)
Hígado Graso/patología , Adolescente , Adulto , Anciano , Femenino , Hepatocitos/patología , Técnicas Histológicas , Humanos , Imagen por Resonancia Magnética/métodos , Espectroscopía de Resonancia Magnética/métodos , Masculino , Persona de Mediana Edad , Estudios Prospectivos , Programas Informáticos , Adulto JovenRESUMEN
PURPOSE: To improve coronary vessel visualization in whole-heart coronary magnetic resonance angiography (CMRA), fat suppression is typically applied. However, recent studies have shown that cardiac fat can also have diagnostic value. To enhance CMRA image quality by improved fat suppression and to provide additionally fat-only information highly resolved, dual-echo Dixon CMRA approaches have been developed. METHODS: In this pilot study, approved by the institutional review board, 30 patients were investigated comparing whole-heart T1 -weighted dual-echo Dixon CMRA to conventional whole-heart fat-suppressed balanced fast field echo CMRA, integrated into a routine clinical protocol that includes the administration of gadolinium for perfusion and late enhancement measurements. Signal-to-noise-ratio, contrast-to-noise-ratio, and image quality were analyzed. RESULTS: Dual-echo Dixon significantly (P<0.000001) improved image quality compared with conventional fat-suppressed balanced fast field echo CMRA. Signal-to-noise-ratio and contrast-to-noise-ratio were found to be comparable when balanced fast field echo was performed before gadolinium and dual-echo Dixon fast field echo after gadolinium administration. CONCLUSION: Dual-echo Dixon can help to improve whole-heart CMRA image quality significantly. The additional whole-heart fat information delivered by this approach can support a number of new clinical studies addressing the diagnostic and the predictive value of intramyocardial and extramyocardial fatty deposits.
Asunto(s)
Tejido Adiposo/patología , Agua Corporal/citología , Enfermedad de la Arteria Coronaria/diagnóstico , Interpretación de Imagen Asistida por Computador/métodos , Angiografía por Resonancia Magnética/métodos , Imagen de Perfusión Miocárdica/métodos , Técnica de Sustracción , Angiografía Coronaria/métodos , Femenino , Humanos , Aumento de la Imagen/métodos , Masculino , Persona de Mediana Edad , Proyectos Piloto , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
PURPOSE: Breath-holding is an established strategy for reducing motion artifacts in abdominal imaging. However, the breath-holding capabilities of patients are often overstrained by scans with large coverage and high resolution. In this work, a new strategy for coping with resulting incomplete breath-holds in abdominal imaging is suggested. METHODS: A sampling pattern is designed to support image reconstruction from undersampled data acquired up to any point in time using compressed sensing and parallel imaging. In combination with a navigator-based detection of the onset of respiration, it allows scan termination and thus reconstruction only from consistent data, which suppresses motion artifacts. The spatial resolution is restricted by a lower bound of the sampling density and is increased over the scan, to strike a compromise with the signal-to-noise ratio and undersampling artifacts for any breath-hold duration. RESULTS: The sampling pattern is optimized in phantom experiments and is successfully applied in abdominal gradient-echo imaging including water-fat separation on volunteers. CONCLUSIONS: The new strategy provides images in which motion artifacts are minimized independent of the breath-holding capabilities of patients, and which enhance in terms of spatial resolution, signal-to-noise ratio, and undersampling artifacts with the a priori unknown breath-hold duration actually achieved in a particular scan.
Asunto(s)
Abdomen/anatomía & histología , Algoritmos , Artefactos , Contencion de la Respiración , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Mecánica Respiratoria , Adulto , Femenino , Humanos , Imagenología Tridimensional/métodos , Masculino , Persona de Mediana Edad , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
The suppression of signal from fat constitutes a basic requirement in many applications of magnetic resonance imaging. To date, this is predominantly achieved during data acquisition, using fat saturation, inversion recovery, or water excitation methods. Postponing the separation of signal from water and fat until image reconstruction holds the promise of resolving some of the problems associated with these methods, such as failure in the presence of field inhomogeneities or contrast agents. In this article, methods are reviewed that rely on the difference in chemical shift between the hydrogen atoms in water and fat to perform such a retrospective separation. The basic principle underlying these so-called Dixon methods is introduced, and some fundamental implementations of the required chemical shift encoding in the acquisition and the subsequent water-fat separation in the reconstruction are described. Practical issues, such as the selection of key parameters and the appearance of typical artifacts, are illustrated, and a broad range of applications is demonstrated, including abdominal, cardiovascular, and musculoskeletal imaging. Finally, advantages and disadvantages of these Dixon methods are summarized, and emerging opportunities arising from the availability of information on the amount and distribution of fat are discussed.
Asunto(s)
Tejido Adiposo/anatomía & histología , Tejido Adiposo/química , Algoritmos , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Imagen por Resonancia Magnética/métodos , Técnica de Sustracción , Animales , Humanos , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Agua/químicaAsunto(s)
Tejido Adiposo/diagnóstico por imagen , Agua Corporal/diagnóstico por imagen , Campos Magnéticos , Imagen por Resonancia Magnética , Adulto , Algoritmos , Tobillo/diagnóstico por imagen , Vértebras Cervicales/diagnóstico por imagen , Simulación por Computador , Femenino , Voluntarios Sanos , Humanos , Aumento de la Imagen , Interpretación de Imagen Asistida por Computador , Procesamiento de Imagen Asistido por Computador/métodos , Masculino , Modelos Estadísticos , Fantasmas de Imagen , Imagen de Cuerpo EnteroRESUMEN
PURPOSE: To compare different lipid multipeak spectral models to the single-peak model in Dixon-based fat-water separation and to evaluate differences between visually scored magnetic resonance (MR) images and quantitatively assessed fat fractions in muscle of Duchenne muscular dystrophy patients. MATERIALS AND METHODS: T1-weighted and 3-point Dixon imaging of the upper and lower leg was performed in 13 Duchenne patients and six healthy controls. Three-, four-, and five-peak lipid spectrum models were compared to a single-peak model and to each other. T1-weighted images were visually scored by two radiologists and quantitative fat fractions were obtained from Dixon images. RESULTS: Differences between the multipeak spectral models were minimal. The three-peak model was used for subsequent comparisons. Although there was high correlation between quantitative and visual scores, visual scores were consistently higher than quantitative values of the same muscles. CONCLUSION: There are minor differences between the various lipid spectral models in terms of quantifying fat fraction in a large number of skeletal muscles in the legs of Duchenne patients and healthy controls. Quantitative 3-point Dixon MRI is more precise and reliable than visual radiological methods for evaluation of fat fractions for potential longitudinal follow-up or therapy evaluation of Duchenne patients.
Asunto(s)
Tejido Adiposo/patología , Algoritmos , Interpretación de Imagen Asistida por Computador/métodos , Imagen por Resonancia Magnética/métodos , Músculo Esquelético/patología , Distrofia Muscular de Duchenne/patología , Adolescente , Niño , Femenino , Humanos , Aumento de la Imagen/métodos , Masculino , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
OBJECTIVE: To investigate the feasibility of subtractionless first-pass single contrast medium dose (0.1 mmol/kg) peripheral magnetic resonance angiography (MRA) at 1.5 T using two-point Dixon fat suppression and compare it with conventional subtraction MRA in terms of image quality. METHODS: Twenty-eight patients (13 male, 15 female; mean age ± standard deviation, 66 ± 16 years) with known or suspected peripheral arterial disease underwent subtractionless and subtraction first-pass MRA at 1.5 T using two-point Dixon fat suppression. Results were compared with regard to vessel-to-background contrast. A phantom study was performed to assess the signal-to-noise ratio (SNR) of both MRA techniques. Two experienced observers scored subjective image quality. Agreement regarding subjective image quality was expressed in quadratic weighted κ values. RESULTS: Vessel-to-background contrast improved in all anatomical locations with the subtractionless method versus the subtraction method (all P < 0.001). Subjective image quality was uniformly higher with the subtractionless method (all P < 0.03, except for the aorto-iliac arteries for observer 1, P = 0.052). SNR was 15 % higher with the subtractionless method (31.9 vs 27.6). CONCLUSION: This study demonstrates the feasibility of subtractionless first-pass single contrast medium dose lower extremity MRA. Moreover, both objective and subjective image quality are better than with subtraction MRA. KEY POINTS: ⢠MRA is increasingly used for vascular applications. ⢠Dixon imaging offers an alternative to image subtraction for fat suppression. ⢠Subtractionless first-pass peripheral MRA is possible using two-point Dixon fat suppression. ⢠Subtractionless peripheral MRA is possible at 1.5 T a single contrast medium dose. ⢠Subtractionless first-pass peripheral MRA provides good image quality with few non-diagnostic studies.
Asunto(s)
Tejido Adiposo/patología , Medios de Contraste , Angiografía por Resonancia Magnética/métodos , Enfermedad Arterial Periférica/diagnóstico , Enfermedad Arterial Periférica/patología , Adolescente , Adulto , Anciano , Anciano de 80 o más Años , Angiografía de Substracción Digital/métodos , Niño , Medios de Contraste/farmacología , Estudios de Factibilidad , Femenino , Humanos , Procesamiento de Imagen Asistido por Computador/métodos , Masculino , Persona de Mediana Edad , Variaciones Dependientes del Observador , Fantasmas de Imagen , Relación Señal-Ruido , Técnica de Sustracción , Adulto JovenRESUMEN
OBJECT: Parallel transmission facilitates a relatively direct control of the RF transmit field. This is usually applied to improve the RF field homogeneity but might also allow a reduction of the specific absorption rate (SAR) to increase freedom in sequence design for high-field MRI. However, predicting the local SAR is challenging as it depends not only on the multi-channel drive but also on the individual patient. MATERIALS AND METHODS: The potential of RF shimming for SAR management is investigated for a 3 T body coil with eight independent transmit elements, based on Finite-Difference Time-Domain (FDTD) simulations. To address the patient-dependency of the SAR, nine human body models were generated from volunteer MR data and used in the simulations. A novel approach to RF shimming that enforces local SAR constraints is proposed. RESULTS: RF shimming substantially reduced the local SAR, consistently for all volunteers. Using SAR constraints, a further SAR reduction could be achieved with only minor compromises in RF performance. CONCLUSION: Parallel transmission can become an important tool to control and manage the local SAR in the human body. The practical use of local SAR constraints is feasible with consistent results for a variety of body models.
Asunto(s)
Algoritmos , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Imagen por Resonancia Magnética/métodos , Modelos Biológicos , Imagen de Cuerpo Entero/métodos , Simulación por Computador , Humanos , Imagen por Resonancia Magnética/instrumentación , Fantasmas de Imagen , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
In this work, a new two-point method for water-fat imaging is described and explored. It generalizes existing two-point methods by eliminating some of the restrictions that these methods impose on the choice of echo times. Thus, the new two-point method promises to provide more freedom in the selection of protocol parameters and to reach higher scan efficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multigradient-echo sequence. While depending on the choice of echo times, it is generally found to be favorable compared to existing two-point methods. Notably, water images with higher spatial resolution and better signal-to-noise ratio were attained with it in single breathholds at 3.0 T and 1.5 T, respectively. The use of more accurate spectral models of fat is shown to substantially reduce observed variations in the extent of fat suppression. The acquisition of in- and opposed-phase images is demonstrated to be replaceable by a synthesis from water and fat images. The new two-point method is finally also applied to autocalibrate a multidimensional eddy current correction and to enhance the fat suppression achieved with three-point methods in this way, especially toward the edges of larger field of views.
Asunto(s)
Tejido Adiposo/anatomía & histología , Algoritmos , Imagen Eco-Planar/métodos , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Técnica de Sustracción , Agua/análisis , Humanos , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
Multi echo chemical shift-based water-fat separation methods allow for uniform fat suppression in the presence of main field inhomogeneities. However, these methods require additional scan time for chemical shift encoding. This work presents a method for water-fat separation from undersampled data (CS-WF), which combines compressed sensing and chemical shift-based water-fat separation. Undersampling was applied in the k-space and in the chemical shift encoding dimension to reduce the total scanning time. The method can reconstruct high quality water and fat images in 2D and 3D applications from undersampled data. As an extension, multipeak fat spectral models were incorporated into the CS-WF reconstruction to improve the water-fat separation quality. In 3D MRI, reduction factors of above three can be achieved, thus fully compensating the additional time needed in three-echo water-fat imaging. The method is demonstrated on knee and abdominal in vivo data.
Asunto(s)
Abdomen/anatomía & histología , Rodilla/anatomía & histología , Imagen por Resonancia Magnética/métodos , Tejido Adiposo/anatomía & histología , Algoritmos , Agua Corporal , Humanos , Aumento de la Imagen/métodos , Procesamiento de Imagen Asistido por Computador , Imagenología Tridimensional , Análisis de los Mínimos Cuadrados , Distribución de Poisson , Estudios RetrospectivosRESUMEN
Compressed sensing (CS) holds considerable promise to accelerate the data acquisition in magnetic resonance imaging by exploiting signal sparsity. Prior knowledge about the signal can be exploited in some applications to choose an appropriate sparsifying transform. This work presents a CS reconstruction for magnetic resonance (MR) parameter mapping, which applies an overcomplete dictionary, learned from the data model to sparsify the signal. The approach is presented and evaluated in simulations and in in vivo T(1) and T(2) mapping experiments in the brain. Accurate T(1) and T(2) maps are obtained from highly reduced data. This model-based reconstruction could also be applied to other MR parameter mapping applications like diffusion and perfusion imaging.
Asunto(s)
Algoritmos , Encéfalo/anatomía & histología , Compresión de Datos/métodos , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Imagen por Resonancia Magnética/métodos , Humanos , Imagen por Resonancia Magnética/instrumentación , Fantasmas de Imagen , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
PURPOSE: To integrate water-fat-resolved spiral gradient-echo imaging with off-resonance correction into a clinical MR scanner and to evaluate its basic feasibility and performance. MATERIALS AND METHODS: Three-point chemical shift imaging was implemented with forward and strongly T(2)*-weighted reverse spiral sampling and with off-resonance correction after water-fat separation. It was applied in a volunteer study on single breathhold abdominal imaging, which included a brief comparison with Cartesian sampling. RESULTS: Water-fat-resolved, off-resonance-corrected forward and reverse three-dimensional interleaved spiral imaging was found to be feasible on a clinical MR scanner with only minor changes to the existing data acquisition and reconstruction, and to provide good image quality. Three-point chemical shift encoded data thus support both, water-fat separation and off-resonance correction with high accuracy. CONCLUSION: The combination of chemical shift encoding and appropriate postprocessing could pave the way for water-fat-resolved spiral imaging in clinical applications.