Home About us Contact
|
Home About us Contact | ||
|
|
||
Gradient Echo Sequence (gradient + echo_sequence)
Selected AbstractsComparison of segmentation methods for MRI measurement of cardiac function in ratsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2010Johannes Riegler MSc Abstract Purpose To establish the accuracy, intra- and inter-observer variabilities of four different segmentation methods for measuring cardiac functional parameters in healthy and infarcted rat hearts. Materials and Methods Six Wistar rats were imaged before and after myocardial infarction using an electrocardiogram and respiratory-gated spoiled gradient echo sequence. Blinded and randomized datasets were analyzed by various semi-automatic and manual segmentation methods to compare their measurement bias and variability. In addition, the accuracy of these methods was assessed by comparison with reference measurements acquired from high-resolution three-dimensional (3D) datasets of a heart phantom. Results Relative inter- and intra-observer variability were found to be similar for all four methods. Semi-automatic segmentation methods reduced analysis time by up to 70%, while yielding similar measurement bias and variability compared with manual segmentation. Semi-automatic methods were found to underestimate the ejection fraction for healthy hearts compared with manual segmentation while overestimating them in infarcted hearts. However, semi-automatic segmentation of short axis slices agreed better with 3D reference scans of a heart phantom compared with manual segmentation. Conclusion Semi-automatic segmentation methods are faster than manual segmentation, while offering a similar intra- and inter-observer variability. However, a potential bias has been observed between healthy and infarcted hearts for different methods, which should also be considered when selecting the most appropriate analysis technique. J. Magn. Reson. Imaging 2010;32:869,877. © 2010 Wiley-Liss, Inc. [source] Sliding multislice MRI for abdominal staging of patients with pelvic malignancies: A pilot studyJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2008Gregor Sommer MS Abstract Purpose To integrate SMS (sliding multislice imaging technique for acquiring axial images during continuous table motion) into a high-resolution pelvic MRI protocol for additional staging of the entire abdomen within one examination. Materials and Methods Axial two-dimensional images were acquired during continuous table motion using a fat-saturated contrast-enhanced T1-weighted gradient echo sequence. Patients held their breath during the first 20 s of the examination and breathed normally afterward while data acquisition continued. Measurement parameters were adjusted to optimize image quality throughout the total field of view. The method was investigated in 22 patients with pelvic malignancies. Two readers independently compared SMS image quality to conventional abdominal MR images, generated by a stationary multi-breath-hold gradient echo sequence. Results Qualitative evaluation yielded high diagnostic value of SMS data in body regions with no or minor breathing motion, and in those acquired during the initial breath-hold. Image quality in the upper abdomen, retroperitoneum, and pelvis is reproducible and equivalent to stationary MRI. Interfering artifacts are related to the intestine in the mid-abdomen. Conclusion SMS is a promising technique that may have the potential for a first-line abdominal staging tool in patients with pelvic malignancies. J. Magn. Reson. Imaging 2008;27:666,672. © 2008 Wiley-Liss, Inc. [source] In vivo MR imaging of pulmonary arteries of normal and experimental emboli in small animalsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2006Mathieu Lederlin MD Abstract Purpose To demonstrate the feasibility of pulmonary MRA in living rodents. Materials and Methods A three-dimensional (3D) gradient echo sequence was adapted to perform a time-of-flight (TOF) angiography of rat lung. Angiogram with a spatial resolution of 195 × 228 × 228 ,m3 was acquired in around 33 minutes. The method was then applied in animals before and after pulmonary embolism (PE) induction. Section of the proximal right pulmonary artery was measured and compared between the two populations. Results Good quality images were obtained with a contrast-to-noise ratio (CNR) of 9 ± 3 in the proximal part of the pulmonary artery. Cross-section areas of the right main artery are statistically different before (3.45 ± 0.69 mm2) and after induction of PE (4.3 ± 0.86 mm2). Conclusion This noninvasive tool permits angiogram acquisition at around 200 ,m spatial resolution and objective distinction between healthy and embolized arteries. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc. [source] Magnetic Resonance Imaging in Patients Diagnosed With Papilledema: A Comparison of 6 Different High-Resolution T1- and T2(*)-Weighted 3-Dimensional and 2-Dimensional SequencesJOURNAL OF NEUROIMAGING, Issue 2 2002Johannes Seitz MD ABSTRACT Purpose. To evaluate visualization and signal characteristics of macroscopic changes in patients with ophthalmologically stated papilledema and to find a suitable high-resolution magnetic resonance imaging (MRI) protocol. Method. Nine consecutive patients with 12 ophthalmologically stated papilledemas underwent MRI of the head and orbits, which consisted of the following high-resolution sequences: 3-dimensional (3D), T2*-weighted (T2*w) constructive interference in steady-state sequence (CISS); 3D, T1-weighted (T1w) magnetization prepared-rapid gradient echo sequence (MP-RAGE) (with and without intravenous contrast medium); transverse 3D and 2-dimensional (2D) (2mm), T2-weighted (T2w) turbo spin echo (TSE); transverse 2D (2mm), contrast-enhanced T1w TSE with fat-suppression technique; and transverse 2D (5mm), T2w TSE. A quantitative and qualitative evaluation of the papilla, optic nerve, optic nerve sheath, optic chiasm, and the brain was performed. The 6 high-resolution sequences were compared. Results. The elevation of the optic disc into the optic globe in ophthalmologically stated papilledema was best visualized in T2w, 3D CISS sequence. The pathological contrast enhancement was best seen in T1w contrast-enhanced 2D TSE sequence with fat-suppression technique. The mean width of the optic nerve sheath directly behind the globe was 7.54 mm (± 1.05 mm) in the pathological eyes, compared to 5.52 mm (± 1.11 mm) in the normal eyes. In all patients, the cerebral indices calculated showed no signs of increased intracranial pressure or other abnormalities changing the volume of the brain or ventricles. The contrast of the orbital fat versus the optic nerve sheath, the optic nerve sheath versus the surrounding cerebrospinal fluid (CSF), the surrounding CSF versus the optic nerve, the optic chiasm versus the CSF, and the optic papilla versus the optic globe were best visualized in the 3D, T2*w CISS sequence. An enhancement of the swollen optic nerve head was best seen in all 12 cases in the T1w contrast-enhanced 2D TSE sequence with fat-suppression technique. Conclusion. An MRI protocol consisting of a 5-mm transverse T2w TSE sequence; a T2*w, 3D CISS sequence; a T1w, 3D MP-RAGE sequence with and without contrast medium; and a transverse T1w, (2-mm) 2D TSE sequence with fat-suppression technique with intravenous contrast medium is suitable to visualize the macroscopic changes in papilledema. In addition, this combination is an excellent technique for the examination of the orbits and the brain. [source] Robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP)MAGNETIC RESONANCE IN MEDICINE, Issue 5 2004L. Martyn Klassen Abstract Quantitative MRI techniques as well as methods such as blood oxygen level-dependent (BOLD) imaging and in vivo spectroscopy require stringent optimization of magnetic field homogeneity, particularly when using high main magnetic fields. Automated shimming approaches require a method of measuring the main magnetic field, B0, followed by adjusting the currents in resistive shim coils to maximize homogeneity. A robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP) using a 3D multiecho gradient echo sequence that measures B0 with high precision was developed. Inherent compensation and postprocessing methods enable removal of artifacts due to hardware timing errors, gradient propagation delays, gradient amplifier asymmetry, and eddy currents. This allows field maps to be generated for any field of view, bandwidth, resolution, or acquisition orientation without custom tuning of sequence parameters. Field maps of an aqueous phantom show ± 1 Hz variation with altered acquisition orientations and bandwidths. Subsequent fitting of measured shim coil field maps allows calculation of shim currents to produce optimum field homogeneity. Magn Reson Med 51:881,887, 2004. © 2004 Wiley-Liss, Inc. [source] Assessment and compensation of susceptibility artifacts in gradient echo MRI of hyperpolarized 3He gasMAGNETIC RESONANCE IN MEDICINE, Issue 2 2003Jim M. Wild Abstract The effects of macroscopic background field gradients upon 2D gradient echo images of inhaled 3He in the human lung were investigated at 1.5 T. Effective compensation of in-slice signal loss in 3He gradient echo images was then demonstrated using a multiple acquisition interleaved single gradient echo sequence. This method restores signal dephasing through a combination of separate images acquired with different slice refocusing gradients. In vivo imaging of volunteers with the sequence shows substantial restoration of signal at the lung periphery and close to blood vessels. The technique presented may be useful when using 3He MRI for volumetric measurements of lung ventilation and in studies using 3He combined with intravenous contrast as a means of assessing lung ventilation/perfusion (V/Q). Magn Reson Med 50:417,422, 2003. © 2003 Wiley-Liss, Inc. [source] Quantitative evaluation of susceptibility and shielding effects of nitinol, platinum, cobalt-alloy, and stainless steel stentsMAGNETIC RESONANCE IN MEDICINE, Issue 5 2003Yi Wang Abstract The purpose of this study is to quantitatively estimate the shielding and susceptibility effects of commonly used metallic stents on MR signal. Two experiments were performed using a 3D gradient echo sequence with short TE to image a stent phantom: 1) short TR and high flip angle (contrast enhanced MRA parameters), and 2) long TR (TR , T1) and low flip angle. The factor characterizing susceptibility effects was estimated from the signal phase of the first experiment, and then the factor characterizing the shielding effects was derived from the second experiment. Susceptibility induced signal loss was negligible (<1%) for nonstainless-steel (nitinol, platinum, and cobalt-alloy) stents and totally destructive (100%) for the stainless steel stent. Signal loss due to RF shielding was 31,62% for nitinol stents, 14,50% for platinum stents, 50,77% for the cobalt-alloy stents (undetermined for the stainless steel stent), varied with stent orientation, diameter, and wall geometry. In summary, stents made of nitinol, platinum, and cobalt-alloy have negligible susceptibility effects but stents made of stainless steel may have complete dephasing. All stents have substantial shielding effects, which vary with composition, geometry, and orientation. Large platinum stents may have the smallest artifacts and are the best suited for postinterventional MR imaging. Magn Reson Med 49:972,976, 2003. © 2003 Wiley-Liss, Inc. [source] Magnetic resonance angiography of collateral vessels in a murine femoral artery ligation modelNMR IN BIOMEDICINE, Issue 1 2004Shawn Wagner Abstract The in vivo detection of growing collateral vessels following arterial occlusion is difficult in small animals. We have addressed the feasibility of performing high resolution time-of-flight angiograms to monitor the growth of collateral vessels after femoral artery occlusion in mice. We will also present a low-pass quadrature birdcage coil construction with a sufficient signal-to-noise ratio to produce high resolution. After a 4-month recovery period a C57BL/6 mouse with a surgical occlusion of the right femoral artery was used to assess the image quality and time requirements to produce magnetic resonance angiograms sufficient to assess collateral artery development using a two-dimensional gradient echo sequence. At a resolution of 100,×,100,×,100,,m and a matrix size of 256,×,128,×,256 for a 2.56,cm isometric volume, three scans were performed with one, two and four repetitions resulting in signal-to-noise ratios for the femoral artery proximal to the ligation site of 58, 126 and 194, respectively. Five C57BL/6 mice were additionally measured 4 weeks after occlusion using two repetitions and the visual collateral vessels were assessed for number and location: 2.0,±,1.2 in quadriceps muscle, 0.6,±,0.5 in adductor (deep adductor vessel), 0.0,±,0.0 in adductor (surface adductor vessels). The results showed a significant difference, two-sided t -test, p,<,0.05, in number of vessels in all the locations. We have shown that this method can be utilized to elucidate the contribution of collateral vessels to arterial flow. Copyright © 2004 John Wiley & Sons, Ltd. [source] T1 corrected B1 mapping using multi-TR gradient echo sequencesMAGNETIC RESONANCE IN MEDICINE, Issue 3 2010Tobias Voigt Abstract This work presents a new approach toward a fast, simultaneous amplitude of radiofrequency field (B1) and T1 mapping technique. The new method is based on the "actual flip angle imaging" (AFI) sequence. However, the single pulse repetition time (TR) pair used in the standard AFI sequence is replaced by multiple pulse repetition time sets. The resulting method was called "multiple TR B1/T1 mapping" (MTM). In this study, MTM was investigated and compared to standard AFI in simulations and experiments. Feasibility and reliability of MTM were proven in phantom and in vivo experiments. Error propagation theory was applied to identify optimal sequence parameters and to facilitate a systematic noise comparison to standard AFI. In terms of accuracy and signal-to-noise ratio, the presented method outperforms standard AFI B1 mapping over a wide range of T1. Finally, the capability of MTM to determine T1 was analyzed qualitatively and quantitatively, yielding good agreement with reference measurements. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source] Measuring the arterial input function with gradient echo sequencesMAGNETIC RESONANCE IN MEDICINE, Issue 6 2003Matthias J.P. van Osch Abstract The measurement of the arterial input function by use of gradient echo sequences was investigated by in vitro and in vivo experiments. First, calibration curves representing the influence of the concentration of Gd-DTPA on both the phase and the amplitude of the MR signal were measured in human blood by means of a slow-infusion experiment. The results showed a linear increase in the phase velocity and a quadratic increase in ,R as a function of the Gd-DTPA concentration. Next, the resultant calibration curves were incorporated in a partial volume correction algorithm for the arterial input function determination. The algorithm was tested in a phantom experiment and was found to substantially improve the accuracy of the concentration measurement. Finally, the reproducibility of the arterial input function measurement was estimated in 16 patients by considering the input function of the left and the right sides as replicate measurements. This in vivo study showed that the reproducibility of the arterial input function determination using gradient echo sequences is improved by employing a partial volume correction algorithm based on the calibration curve for the contrast agent used. Magn Reson Med 49:1067,1076, 2003. © 2003 Wiley-Liss, Inc. [source] Imaging the changes in renal T1 induced by the inhalation of pure oxygen: A feasibility studyMAGNETIC RESONANCE IN MEDICINE, Issue 4 2002Richard A. Jones Abstract The effect of the inhalation of pure oxygen on the kidney was evaluated by measuring monoexponential T1 and T relaxation times in nine volunteers using a multiple-shot turbo spin echo and multiple echo gradient echo sequences, respectively. The T1 of the renal cortex decreased significantly when breathing pure oxygen as compared to normoxia (from 882 ± 59 to 829 ± 70 msec, P < 0.05), while that of the renal medulla was unchanged. No significant changes were seen in the T of either compartment. Dynamic imaging using an inversion recovery sequence with an optimized inversion time typically produced signal changes of 20% in the renal cortex. Studies to assess if oxygen-induced changes in flow contributed to this effect showed that the flow contribution was not significant. Although longer inversion times (880 ms) produced optimal contrast, acceptable contrast was also obtained at shorter inversion times (450 msec) in the renal cortex, spleen, and lung, with the latter being of opposite polarity to the other two tissues, implying a shorter parenchymal T1 than previously reported in the literature. The results are consistent with oxygen acting as an intravascular contrast agent which induces a shortening of T1 in the arterial blood volume. Magn Reson Med 47:728,735, 2002. © 2002 Wiley-Liss, Inc. [source] Relationship of meniscal damage, meniscal extrusion, malalignment, and joint laxity to subsequent cartilage loss in osteoarthritic kneesARTHRITIS & RHEUMATISM, Issue 6 2008Leena Sharma Objective Progressive knee osteoarthritis (OA) is believed to result from local factors acting in a systemic environment. Previous studies have not examined these factors concomitantly or compared quantitative and qualitative cartilage loss outcomes. The aim of this study was to test whether meniscal damage, meniscal extrusion, malalignment, and laxity each predicted tibiofemoral cartilage loss after controlling for the other factors. Methods Laxity and alignment were measured at baseline in individuals with knee OA. Magnetic resonance imaging included spin-echo coronal and sagittal imaging for meniscal scoring and axial and coronal spoiled gradient echo sequences with water excitation for cartilage quantification. Tibial and weight-bearing femoral condylar subchondral bone area and cartilage surface were segmented. Cartilage volume, denuded bone area, and cartilage thickness were quantified in each plate, with progression defined as cartilage loss >2 times the coefficient of variation for each plate. Qualitative outcome was assessed as worsening of the cartilage score. Logistic regression analysis with generalized estimating equations yielded odds ratios for each factor, adjusting for age, sex, body mass index, and the other factors. Results We studied 251 knees in 153 persons. After full adjustment, medial meniscal damage predicted medial tibial cartilage volume loss and tibial and femoral denuded bone increase, while varus malalignment predicted medial tibial cartilage volume and thickness loss and tibial and femoral denuded bone increase. Lateral meniscal damage predicted every lateral outcome. Laxity and meniscal extrusion had inconsistent effects. After full adjustment, no factor except medial laxity predicted qualitative outcome. Conclusion Using quantitative cartilage loss assessment, local factors that independently predicted tibial and femoral loss included medial meniscal damage and varus malalignment (medially) and lateral meniscal damage (laterally). A measurement of quantitative outcome was more sensitive at revealing these relationships than a qualitative approach. [source] | ||||||||||