Fast MRI Research Articles

21 Tesla MRI Microscopy of Mice Kidney, Heart and Skin: Quantitation of MRI Visible Features

AbstractExcised mice kidney, heart and skin MRI visible features were quantitated using ex vivo 21 Tesla MRI microimager. The fast MRI acquisition protocols of FLASH, gradient echo flow compensated (GEFC) techniques were used to visualize microvasculature of mice kidney, mice heart including cardiac arteries, veins, cordate tendons and mice skin epidermis with hair follicle features.

Numerical and in vivo validation of fast cine displacement‐encoded with stimulated echoes (DENSE) MRI for quantification of regional cardiac function

Quantitative assessment of regional cardiac function can improve the accuracy of detecting wall motion abnormalities due to heart disease. While recently developed fast cine displacement-encoded with stimulated echoes (DENSE) MRI is a promising modality for the quantification of regional myocardial function, it has not been validated for clinical applications. The purpose of this study, therefore, was to validate the accuracy of fast cine DENSE MRI with numerical simulation and in vivo experiments. A numerical phantom was generated to model physiologically relevant deformation of the heart, and the accuracy of fast cine DENSE was evaluated against the numerical reference. For in vivo validation, 12 controls and 13 heart-disease patients were imaged using both fast cine DENSE and myocardial tagged MRI. Numerical simulation demonstrated that the echo-combination DENSE reconstruction method is relatively insensitive to clinically relevant resonance frequency offsets. The strain measurements by fast cine DENSE and the numerical reference were strongly correlated and in excellent agreement (mean difference = 0.00; 95% limits of agreement were 0.01 and -0.02). The strain measurements by fast cine DENSE and myocardial tagged MRI were strongly correlated (correlation coefficient = 0.92) and in good agreement (mean difference = 0.01; 95% limits of agreement were 0.07 and -0.04).

General formulation for quantitative G‐factor calculation in GRAPPA reconstructions

In this work a theoretical description for practical quantitative estimation of the noise enhancement in generalized autocalibrating partially parallel acquisitions (GRAPPA) reconstructions, equivalent to the geometry (g)-factor in sensitivity encoding for fast MRI (SENSE) reconstructions, is described. The GRAPPA g-factor is derived directly from the GRAPPA reconstruction weights. The procedure presented here also allows the calculation of quantitative g-factor maps for both the uncombined and combined accelerated GRAPPA images. This enables, for example, a fast comparison between the performances of various GRAPPA reconstruction kernels or SENSE reconstructions. The applicability of this approach is validated on phantom studies and demonstrated using in vivo images for 1D and 2D parallel imaging.

Fast MRI coil analysis based on 3-D electromagnetic and RF circuit co-simulation

To accelerate the analysis of a multi-element MRI coil, a two-way link is used between radiofrequency (RF) circuit and 3-D electromagnetic (EM) simulation tools. In this configuration, only one 3-D EM simulation is required to investigate the coil performance over a range of different tunings, saving considerable computation time. For the purpose of 3-D EM simulation, the coil feed networks and trim capacitors are substituted by 50Ω ports. The entire coil was tuned in the RF circuit domain, and the near-field profiles of the electric and magnetic field components were then calculated, together with the specific energy absorption ratio (SAR) maps in the 3-D EM domain

MRI of cellular layers in mouse brain in vivo

Noninvasive imaging of the brain of animal models demands the detection of increasingly smaller structures by in vivo MRI. The purpose of this work was to elucidate the spatial resolution and structural contrast that can be obtained for studying the brain of C57BL/6J mice by optimized T2-weighted fast spin-echo MRI at 9.4 T. As a prerequisite for high-resolution imaging in vivo, motion artifacts were abolished by combining volatile anesthetics and positive pressure ventilation with a specially designed animal bed for fixation. Multiple substructures in the cortex, olfactory bulb, hippocampus, and cerebellum were resolved at 30 to 40 μm in-plane resolution and 200 to 300 μm section thickness as well as for relatively long echo times of 65 to 82 ms. In particular, the approach resulted in the differentiation of up to five cortical layers. In the olfactory bulb the images unraveled the mitral cell layer which has a thickness of mostly single cells. In the hippocampus at least five substructures could be separated. The molecular layer, Purkinje layer, and granular layer of the cerebellum could be clearly differentiated from the white matter. In conclusion, even without the use of a contrast agent, suitable adjustments of a widely available T2-weighted MRI sequence at high field allow for structural MRI of living mice at near single-cell layer resolution.

Comparison of Two Fast MRI Methods for Perfusion Imaging of the Heart

SummaryTime‐adaptive sensitivity encoding (TSENSE) and generalized autocalibrating partially parallel acquisition (GRAPPA) were applied to a gradient‐echo sequence used for first‐pass myocardial perfusion imaging of 12 patients with coronary artery disease. The two parallel imaging methods were compared in terms of signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR) and image artefacts. Image acquisition was started during the administration of a Gd‐contrast bolus (0.1 mmoL/kg) followed by a 20‐mL saline flush (3 mL/s), and the next perfusion was started at least 15 min thereafter using an identical bolus. The order of perfusion sequences was inverted in every other patient. Both acquisitions had an acceleration rate of 2, and were performed during breath‐holding. The SNR, CNR and image quality of the GRAPPA images were significantly better than were those of the TSENSE images. An exception was the lower CNR of GRAPPA when applied after the second bolus. Differences between subjects were larger with GRAPPA perfusion imaging than with TSENSE. The SNR and CNR also varied relatively much between the GRAPPA images, indicating that the diagnostic value of TSENSE may be superior after all.

SNR enhancement of highly-accelerated real-time cardiac MRI acquisitions based on non-local means algorithm

Real-time cardiac MRI appears as a promising technique to evaluate the mechanical function of the heart. However, ultra-fast MRI acquisitions come with an important signal-to-noise ratio (SNR) penalty, which drastically reduces the image quality. Hence, a real-time denoising approach would be desirable for SNR amelioration. In the clinical context of cardiac dysfunction assessment, long acquisitions are required and for most patients the acquisition takes place with free breathing. Hence, it is necessary to compensate respiratory motion in real-time. In this article, a real-time and interactive method for sequential registration and denoising of real-time MR cardiac images is presented. The method has been experimented on 60 fast MRI acquisitions in five healthy volunteers and five patients. These experiments assessed the feasibility of the method in a real-time context.

Magnetic resonance imaging in the evaluation of the gastric emptying and antral motion: Feasibility and reproducibility of a fast not invasive technique

Purpose The aim of this study was to evaluate reproducibility of a fast MRI protocol to measure gastric emptying and motility of the gastric antrum. Methods and materials Gastric emptying and antral speed were measured in 12 type 1 diabetic patients (mean age 43 years) and 9 healthy volunteers (mean age 31 years). Subjects, fasting from 6 h, were evaluated in supine position using a 1.5 T MR scanner and a eight-channels phased-array body coil after ingestion of 400 ml of a vanillas pudding mixed with 5 ml of Gd-DTPA. Axial 3D T1w sequence at 0 and 30 min for volume evaluation and cine-steady state acquisition every 5 min for a total time of 30 min for gastric wave speed assessing were acquired. Two blinded observers extrapolated T½ from gastric volume assessment and speed of gastric waves. Results All the patients tolerated the examination. The T½ cut-off was of 115 min with an accuracy in differentiate controls from diabetics of 96% (95% CI 0.766–0.992; p < 0.001), while the antral speed cut-off was of 0.15 mm/s with an accuracy of 87% (95% CI 0.628–0.977; p < 0.001). The inter-observer agreement for the volumes at time 0 and 30 min was respectively 0.983 (95% CI 0.9628–0.9929; p < 0.001) and 0.9933 (95% CI 0.9847–0.9971; p < 0.001) with an agreement of 0.9918 (95% CI 0.9853–0.9954; p < 0.001), while for antral speed it was of 0.935 (95% CI 0.9097–0.9528; p < 0.001). Conclusions MRI is a reproducible technique for the evaluation of gastric emptying and antral motility.

Preliminary evaluation of optical CT scanning versus MRI for nPAG gel dosimetry: The Ghent experience

The aim of this study was to evaluate fast laser-scanning optical CT versus MRI for an nPAG gel dosimeter in terms of accuracy and precision. Three small cylindrical volumetric gel phantoms were fabricated and irradiated with photon beams. The gel dosimeters were scanned with an MR scanner and an in house developed laser scanning optical CT scanner. A comparison between MRI and optical CT scanning was performed based on the reconstructed images.Preliminary results show a fair correspondence in the MRI acquired and optical CT acquired dose maps. Still, ringing artifacts contaminate the reconstructed optical CT images. These may be related to sub-pixel misalignments between the blank projection and the acquired transmission projection of the gel phantom. Another artifact may be caused by refraction near the edges of the field. Further optimisation of our optical CT scanner is required to obtain the same accuracy as with MRI. To make a comparison between the two imaging modalities in terms of precision, the intrinsic dose precision on readout (IPD) was calculated which is independent of spatial resolution and acquisition time. It is shown that optical CT has a better intrinsic dose precision.

Der Beitrag neuer MRT-Untersuchungs- und Analysetechniken

Routine MRI is a key element in the diagnosis and differential diagnosis of MS. Conventional MRI contrasts are very commonly used as adjuncts to the clinical and neurophysiological assessment of the patient status in the course of the disease. Background information from MRI frequently offers the best opportunity to provide an individual understanding and concept of the patients disease and provides a basis for a treatment rationale. Beyond the visualisation of the groß pathology on brain MRI imaging of the spinal cord is acknowledged as an area of great importance, however it is technically challenging and therefore its use is more limited. New technical opportunities allow the fast visualisation of the whole cord. We have employed a fast biplanar MRI approach of the whole cord as a routine examination, which can be very helpful to ascertain cord pathology. Another area where MRI can be useful is the detection and quantification of tissue changes that are not easily accessible to the eye. Subtle changes are known to occur in the NAWM of MS patients, including diffuse astrocytic hyperplasia, patchy edema, perivascular infiltration, gliosis, abnormally thin myelin and axonal loss. It is also evident from pathological studies that frequently a slowly progressive axonal loss leeds to brain atrophy. Atrophy assessment becomes increasingly more sophisticated and new quantitative MRI techniques (Q-space diffusion imaging) may open new avenues to also assess the less overt pathological changes in MS. We have recently used Q-space imaging that demonstrates a high sensitivity to detect reductions of the slow diffusion component in the NAWM of patients with different MS types. Color coding of resulting calculated maps of the „Probability of Zero Displacement“ (PZD) show areas of loss of the slow diffusion component quite obviously making it also accessible to a qualitative analysis approach. Such additional information maybe of value for the clinician when the clinical status of the patient is still functionally well compensated in a low EDSS range but more objective information on morphological changes are useful.

Spinal cord lesions and clinical status in multiple sclerosis: A 1.5 T and 3 T MRI study

Objective Assess the relationship between spinal cord T2 hyperintense lesions and clinical status in multiple sclerosis (MS) with 1.5 and 3 T MRI. Methods Whole cord T2-weighted fast spin-echo MRI was performed in 32 MS patients [Expanded Disability Status Scale (EDSS) score (mean ± SD: 2 ± 1.9), range 0–6.5]. Protocols at 1.5 T and 3 T were optimized and matched on voxel size. Results Moderate correlations were found between whole cord lesion volume and EDSS score at 1.5 T ( r s = .36, p = 0.04), but not at 3 T ( r s = 0.13, p = 0.46). Pyramidal Functional System Score (FSS) correlated with thoracic T2 lesion number ( r s = .46, p = 0.01) and total spinal cord lesion number ( r s = 0.37, p = 0.04) and volume ( r s = 0.37, p = 0.04) at 1.5 T. Bowel/bladder FSS correlated with T2 lesion volume and number in the cervical, thoracic, and total spine at 1.5 T ( r s 0.40–0.57, all p < 0.05). These MRI–FSS correlations were non-significant at 3 T. However, these correlation coefficients did not differ significantly between platforms (Choi's test p > 0.05). Correlations between whole cord lesion volume and timed 25-foot walk were non-significant at 1.5 T and 3 T ( p > 0.05). Lesion number and volume did not differ between MRI platforms in the MS group ( p > 0.05). Conclusions Despite the use of higher field MRI strength, the link between spinal lesions and MS disability remains weak. The 1.5 T and 3 T protocols yielded similar results for many comparisons.

Mobile sensor for high resolution NMR spectroscopy and imaging

In this work we describe the construction of a mobile NMR tomograph with a highly homogeneous magnetic field. Fast MRI techniques as well as NMR spectroscopy measurements were carried out. The magnet is based on a Halbach array built from identical permanent magnet blocks generating a magnetic field of 0.22 T. To shim the field inhomogeneities inherent to magnet arrays constructed from these materials, a shim strategy based on the use of movable magnet blocks is employed. With this approach a reduction of the line-width from ∼20 kHz to less than 0.1 kHz was achieved, that is by more than two orders of magnitude, in a volume of 21 cm 3. Implementing a RARE sequence, 3D images of different objects placed in this volume were obtained in short experimental times. Moreover, by reducing the sample size to 1 cm 3, sub ppm resolution is obtained in 1H NMR spectra.

A mass in the fourth ventricle

A 53-year-old man with a history of testicular carcinoma treated by orchidectomy presented with a sudden onset left facial droop and left upper limb weakness that lasted for 4 days. He denied any speech difficulty, lower limb symptoms, seizures, urinary or bowel symptoms. On examination he was alert and orientated, with an occasional head-bobbing tremor. There was mild flattening of the left nasolabial fold, with the remainder of his neurological examination normal. Basic blood investigations were unremarkable. His MRI is shown in Fig. 1, Fig. 2. The tumour was completely removed by a posterior fossa craniotomy. Fig. 2T2-weighted fast spin echo axial MRI showing 1.3 cm hyperintense lobulated lesion in the inferior aspect of the fourth ventricle compressing the posterior aspect of the medulla oblongata. The lesion did not enhance with contrast. View Large Image Figure Viewer Download Hi-res image A mass in the fourth ventricleJournal of Clinical NeuroscienceVol. 16Issue 3PreviewB. Subependymoma. Full-Text PDF

Quantification of vendor-specific relationships between fast gradient echo and steady-state free precession cine MRI for determination of myocardial mass and volumes

Quantification of vendor-specific relationships between fast gradient echo and steady-state free precession cine MRI for determination of myocardial mass and volumes

Quantitative assessment of intramyocardial function using Cine DENSE MRI: a validation study

Non-invasive assessment of myocardial function plays a critical role in the management of heart disease. Conventional imaging modalities, such as echocardiography and cine MRI, are widely used for quantitative assessment of global cardiac function and qualitative assessment of regional cardiac function. Quantitative assessment of regional wall motion may additionally improve the accuracy of detecting wall motion abnormalities due to heart disease. Recently, fast cine displacement-encoded (DENSE) MRI was developed to produce relatively high quality strain maps within clinically acceptable breath-hold duration of 12 cardiac cycles [1]. While this pulse sequence is promising for clinical applications, it has not been validated. Therefore, the purpose of this study was to validate the relative accuracy of fast cine DENSE MRI in controls and patients with heart disease.

Phase labeling using sensitivity encoding (PLUS): Data acquisition and image reconstruction for geometric distortion correction in EPI

Geometric distortion caused by magnetic field inhomogeneity is generally an inevitable tradeoff for fast MRI acquisitions using echo-planar imaging. Most of the existing distortion-correction techniques require separate scans for field maps in order to correct the distortion contained in a measurement. A drawback of these current techniques is that the field map scans and the measurement can capture different patient positions, which invalidates the stationary condition. A new method was developed in this work to correct geometric distortion by using local phase shifts derived directly from the measurement itself, without the need of extra field map scans. This self-sufficient method takes advantage of parallel imaging and k-space trajectory modification to produce multiple images from a single acquisition. The measurement is also used to derive sensitivity maps for parallel imaging reconstruction. The derived phase shifts are retrospectively applied to the measurement for correction of geometric distortion in the measurement itself. The proposed method was successfully demonstrated using experimental data from a phantom and a human brain.

Efficient multiple acquisitions by skipped phase encoding and edge deghosting (SPEED) using shared spatial information

The fast MRI method of Skipped Phase Encoding and Edge Deghosting (SPEED) is further developed to accelerate multiple acquisitions. In a single acquisition, SPEED first acquires three sparse ghosted edge maps with an undersampling factor of N/3, which are modeled with a double-layer structure and described by three equations with two unknown ghosts, each with a unique ghost order index. By minimizing least-square-error, a pair of ghost order indexes is determined. Based on them, the two corresponding ghosts are resolved, leading to a deghosted image. In this case, three equations are needed to determine the ghost order index, while only two equations are required to resolve the two ghosts. This shows both inefficiency and potential. Multiple acquisitions often contain similar spatial information. The similarities can be used to improve efficiency by sharing the ghost order index among different acquisitions, leading to acceleration factors greater than that achievable with single acquisition.

Development of Working Memory Maintenance

The neural circuitry supporting mature visual spatial working memory (VSWM) has been well delineated in nonhuman primates and in human adults. However, we still have limited understanding about developmental change through adolescence in this network. We present results from a fast event-related functional MRI (fMRI) study aimed at characterizing developmental changes in brain mechanisms supporting VSWM across different delay periods. Forty-three healthy subjects (17 adults, 18-30 yr; 13 adolescents, 13-17 yr; 13 children, 8-12 yr) were scanned as they performed an oculomotor delayed response (ODR) task with short (2.5 s) and long (10 s) delay period trials. Results showed that all age groups recruited a common network of regions to support both delay trials, including frontal, parietal, and temporal regions, indicative of a core circuitry needed to perform the task. Several age-related differences were found in the recruitment of regions, supporting short delay trials, including fronto-caudal areas, which could contribute to known differences in initial memory-guided saccade precision. To support extended delay trials, adults primarily recruited additional posterior parietal cortex (PPC), whereas children and adolescents recruited a considerably more extensive distributed circuitry. Our findings indicate that brain processes supporting basic aspects of working memory across cortex are established by childhood. We also find evidence for continued immaturities in systems supporting working memory precision, reflected by differences in the circuitry recruited by children and by continued refinement of fronto-insular-temporal regions recruited by adolescents. Taken together, these results suggest distinct developmental changes in the circuitry supporting visual spatial working memory.

Comparison of Fetal Biometric Values with Sonographic and 3D Reconstruction MRI in Term Gestations

We sought to compare the fetal biometric values head and abdominal circumferences, biparietal and occipital-frontal diameters, and left and right ventricular atrial diameters obtained with contemporaneous sonography and 3D MRI reconstructions in term pregnancies. A total of 107 nulliparous women evaluated as having uncomplicated pregnancies and scheduled for induction at 42 completed weeks gave their informed consent and underwent MRI and sonography within 3 hours of each other. Two single-shot fast spin-echo MRI sequences were performed with 7- and 4-mm slice thicknesses and no gap. A single observer performed MRI postprocessing to obtain biometric values. A single sonographer using a 3- to 5-MHz curvilinear transducer performed transabdominal sonography. Concordance correlation and Bland-Altman analysis of differences were performed. Concordance correlation was poor for both right (0.024) and left (0.005) ventricular atrial diameters. There were moderate concordance correlations for head (0.56) and abdominal (0.53) circumferences and biparietal diameter (0.61). Occipital-frontal diameter had fair correlation (0.27). Comparison between contemporaneous sonographic and 3D reconstructed MR images at late gestational ages shows acceptable correlation between the two techniques for head circumference, abdominal circumference, and biparietal diameter.

SU-GG-J-28: An Investigation of Intra and Inter-Fraction Motion in Cervical Cancer Patients

Purpose: To quantify the intra-fraction motion of the cervix at various time points during a course of external beam radiation therapy. Also to develop patient-specific PTV margins that can account for the observed intra-fraction motion for cervix patients. Method and Materials: Multiple single shot fast spin echo MRI images were acquired through the central sagittal slice of the cervix of 6 patients undergoing external beam radiation therapy for cervical cancer. Images were acquired at 6-second intervals for approximately 20 minutes. MRI scans were performed at 3 time points; pre-treatment, mid-treatment and post-treatment. Cervical contours were delineated on all images using in-house software which also enabled quantification of motion and deformation. Using these contours, the extent of margin expansion to achieve 100% coverage was measured and used to construct customized PTV expansions. Results: During each intra-fraction period a general trend was observed in that the cervix moved superiorly and posteriorly with time (up to 6 mm). This motion was observed to correlate with bladder filling. For scans acquired at different time points, anterior motion increased as a function of treatment progression. Anterior PTV expansion increased from 2.2mm to 6.6 mm while the inferior margin decreased from 7.0mm to 3.1mm. No systematic trends were observed in the posterior or superior directions when comparing scans acquired at different time points through the treatment. Conclusion: This work suggests that as treatment of the cervix progresses, significant changes in intra-fraction motion can occur. Further, each patient's motion is unique and requires individual assessment for custom PTV margin construction. Motion analysis may provide guidance as to when an IMRT boost would be most successful in replacing an interstitial brachytherapy boost.