RESUMEN
Diffusion-weighted spin-echo echo-planar imaging was developed and applied for assessment of diffusion coefficients of adipose tissue in human lower leg on a 1.5-T whole-body MR scanner. Because of the higher molecular weight of triglycerides, apparent diffusion coefficients (ADCs) of adipose tissue are approximately two orders of magnitude smaller compared with water, leading to the necessity of using high b-values up to 50,000 sec/mm(2) and an echo time of 240 msec for sufficient diffusion-related signal attenuation. ADC maps of adipose tissue in the human lower leg were derived for diffusion encoding along orthogonal spatial directions in six healthy volunteers. Mean diffusion coefficients in the tibial bone marrow amounted to (1.81 ± 0.10) × 10(-5) mm(2) /sec (left-right), (1.96 ± 0.10) × 10(-5) mm(2) /sec (anterior-posterior), and (1.96 ± 0.20) × 10(-5) mm(2) /sec (head-feet), respectively. Pixel-wise calculated ADC values of subcutaneous adipose tissue showed a distinctly higher variation with the smallest ADC values similar to those measured for tibial bone marrow. Some subcutaneous adipose tissue regions showed increased signal attenuation at higher b-values resulting in ADC coefficients up to 4.2 × 10(-5) mm(2) /sec. It must be noted that diffusion measurements with extremely high b-values in vivo are extremely sensitive to incoherent motion effects in tissue. Nonetheless, it could be shown that in vivo diffusion imaging of adipose tissue in human lower leg is possible at 1.5 T in acceptable measurement time of a few minutes. Potential future applications of fat diffusion imaging are seen in temperature measurements in adipose tissue, detection of free fatty acids in white or brown adipose tissue in case of high lipolysis, differentiation of macro- and microvesicular steatosis, or assessment of the mobility of intramyocellular lipids.
Asunto(s)
Tejido Adiposo/anatomía & histología , Algoritmos , Imagen de Difusión por Resonancia Magnética/métodos , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Pierna/anatomía & histología , Humanos , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
PURPOSE: The feasibility of highly resolved diffusion tensor imaging (DTI) of the human cervical spinal cord was tested on a clinical MR unit operating at 3.0 Tesla. DTI parametrical maps and signal-to-noise ratios (SNRs) were compared to results recorded at 1.5 Tesla. MATERIALS AND METHODS: Eight healthy volunteers and one patient participated in the study. A transverse oriented single-shot ECG-triggered echo-planar imaging (EPI) sequence with double spin-echo diffusion preparation was applied for highly resolved DTI of the spinal cord. The signal yield, fractional anisotropy (FA), and mean diffusivity (MD) were compared for both field strengths. The clinical applicability of the protocol was also tested in one patient with amyotrophic lateral sclerosis (ALS) at 3.0 T. RESULTS: A mean increase in SNR of 95.7 +/- 4.6 % was found at 3.0 Tesla compared to 1.5 Tesla. Improved quality of the DTI parametrical maps was observed at higher field strength (p < 0.02). Comparable FA and MD (reported in units of 10 (-3) mm (2)/s) values were computed in the dorsal white matter at both field strengths (1.5 T: FA = 0.75 +/- 0.08, MD = 0.84 +/- 0.12, 3.0 T: FA = 0.74 +/- 0.04, MD = 0.93 +/- 0.14). The DTI images exhibited diagnostic image quality in the patient. At the site of the diseased corticospinal tract, a decrease of 46.0 +/- 3.8 % in FA (0.40 +/- 0.03) and an increase of 50.3 +/- 5.6 % in MD (1.40 +/- 0.05) were found in the ALS patient. CONCLUSION: The 3.0 Tesla field strength provides higher image quality in DTI of the spinal cord compared to 1.5 T. The proposed DTI protocol seems adequate for the assessment of spinal cord diseases.
Asunto(s)
Vértebras Cervicales/anatomía & histología , Enfermedades de la Médula Espinal/diagnóstico , Médula Espinal/anatomía & histología , Vértebras Cervicales/patología , Electrocardiografía , Humanos , Procesamiento de Imagen Asistido por Computador , Angiografía por Resonancia Magnética , Valores de Referencia , Sensibilidad y Especificidad , Médula Espinal/patologíaRESUMEN
A sequence for echoplanar diffusion tensor imaging of musculature was developed using a stimulated echo preparation. The strategy was optimized in order to obtain reliable diffusion tensor data in a short measuring time. Image distortion problems due to eddy currents arising from long-lasting diffusion sensitizing gradients could be overcome by insertion of additional gradient pulses in the TM interval of the stimulated echo preparation. In contrast to former approaches with similar intention, the proposed strategy does not influence the stimulated echo signal itself and does not lead to prolonged echo time as in the case of spin echo methods. Phantom measurements were performed to compare eddy current induced distortion effects in diffusion weighted images. The diffusion tensor in the musculature of the lower leg was investigated in four healthy subjects and maps of the trace and the three eigenvalues of the diffusion tensor, fractional anisotropy maps, and angle maps were calculated.
Asunto(s)
Imagen de Difusión por Resonancia Magnética/métodos , Imagen Eco-Planar/métodos , Músculo Esquelético/fisiología , Humanos , Pierna , Fantasmas de ImagenRESUMEN
PURPOSE: In-vitro and in-vivo examinations to investigate the influence of one (1D)- and two-dimensional (2D) parallel acquisition techniques (PAT) on contrast-to-noise ratio (CNR) and image quality for bright-lumen 3D MR colonography. MATERIALS AND METHODS: In-vitro measurements were performed on a 1.5 T whole-body MR scanner (MAGNETOM AVANTO(R), Siemens AG, Erlangen) with a standard spoiled 3D gradient-echo (3D GRE) sequence and a volume interpolated 3D GRE (VIBE) sequence using a home-built colon phantom. The relative CNR was determined and image quality evaluated for different acceleration factors (PAT factors). A bright-lumen 3D MR colonography with PAT factors 2, 3, 4 and 6 was performed on a 39-year-old volunteer. The 3D data sets were compared with regard to image quality. RESULTS: The mean relative CNR values for the in-vitro measurements were 1, 0.81, 0.73, 0.52 and 0.4 (3D GRE) and 1, 0.8, 0.65, 0.45 and 0.3 (VIBE) for the PAT factors 1, 2, 3, 4 and 6, respectively. Residual aliasing artifacts not affecting image quality in a distinct manner were visible for 2D PAT, especially for the VIBE sequence. Increasing the PAT factor up to 6 and decreasing the acquisition time down to 10 seconds for the bright-lumen 3D MR colonography could achieve adequate image quality with significantly reduced image artifacts caused by peristalsis and pulsations. DISCUSSION: Even for high PAT factors up to 6, 2D PAT only leads to a moderate CNR loss. For a 3D MR colonography, distinct shorter acquisition times can be achieved with identical resolution.
Asunto(s)
Colon/anatomía & histología , Colon/diagnóstico por imagen , Colonografía Tomográfica Computarizada/métodos , Imagen por Resonancia Magnética/métodos , Colonografía Tomográfica Computarizada/instrumentación , Diseño de Equipo , Humanos , Imagen por Resonancia Magnética/instrumentación , Reproducibilidad de los ResultadosRESUMEN
In skeletal musculature intramyocellular (IMCL) and extramyocellular lipids (EMCL) are stored in compartments of different geometry and experience different magnetic field strengths due to geometrical susceptibility effects. The effect is strong enough to---at least partly---separate IMCL and EMCL contributions in (1)H MR spectroscopy, despite IMCL and EMCL consisting of the same substances. The assessment of intramyocellular lipid stores in skeletal musculature by (1)H MR spectroscopy plays an important role for studying physiological and pathological aspects of lipid metabolism. Therefore, a method using mathematical tools of Fourier analysis is developed to obtain the magnetic field distribution (MFD) from the measured spectra by deconvolution. A reference lipid spectrum is required which was recorded in tibial yellow bone marrow. It is shown that the separation of IMCL contributions can be performed more precisely---compared to other methods---based on the MFD. Examples of deconvolution in model systems elucidate the principle. Applications of the proposed approach on in vivo examinations in m. soleus and m. tibialis anterior are presented. Fitting the IMCL part of the MFD by a Gaussian lineshape with a linewidth kept fixed with respect to the linewidth of creatine and with the assumption of a smooth but not necessarily symmetrical shape for the EMCL part, the only free fit parameter, the amplitude of the IMCL part, is definite and subtraction leads to the EMCL part in the MFD. This procedure is especially justified for the soleus muscle showing a severely asymmetrical distribution which might lead to a marked overestimation of IMCL using common line fitting procedures.
Asunto(s)
Metabolismo de los Lípidos , Espectroscopía de Resonancia Magnética , Músculo Esquelético/metabolismo , Espacio Extracelular/fisiología , Análisis de Fourier , Magnetismo , Protones , Procesamiento de Señales Asistido por ComputadorRESUMEN
BACKGROUND: Current literature indicates tree pollen is less important than grass pollen as the cause of spring allergic rhinitis. The role of fungal spores is secondary and uncertain. OBJECTIVE: To assess the relative allergenicity of tree and grass pollen and fungal spores as the cause of spring allergic rhinitis in central Indiana. METHODS: (1) Chart review of 189 Indiana natives with seasonal allergic rhinitis to ascertain clinical sensitivity to tree or grass pollens. (2) A prospective study conducted in the spring of 1997 in 51 patients, correlating patients' symptom and medication scores with daily pollen and spore counts, obtained with a Burkard spore trap. RESULTS: One hundred thirty of 189 patients (69%) reported symptoms before May with positive prick test to at least one tree pollen. One hundred thirty-four patients (71%) had symptoms in May and June with positive test to grass pollen. The ratio between the two groups is 1:1, in contrast to a ratio of 1:4 derived from the literature. The prospective study revealed a symptom score increase parallel to the rise of tree pollen counts beginning in mid-March and reaching a plateau in early May prior to the onset of grass season. Medication use continued to increase and peaked at the height of grass pollination. Spore counts increased in late May and stayed at high levels throughout June, yet symptom and medication scores steadily declined following the peak of grass pollination in early June. CONCLUSIONS: This study suggests that in central Indiana, tree pollen is more important as the cause of spring allergic rhinitis than that suggested by the literature. This study confirmed previous observations that fungal spores are substantially less allergenic than pollens.