Fast Magnetic Resonance Imaging Techniques Research Articles

A convergence analysis for projected fast iterative soft-thresholding algorithm under radial sampling MRI

Radial sampling is a fast magnetic resonance imaging technique. Further imaging acceleration can be achieved with undersampling but how to reconstruct a clear image with fast algorithm is still challenging. Previous work has shown the advantage of removing undersampling image artifacts using the tight-frame sparse reconstruction model. This model was further solved with a projected fast iterative soft-thresholding algorithm (pFISTA). However, the convergence of this algorithm under radial sampling has not been clearly set up. In this work, the authors derived a theoretical convergence condition for this algorithm. This condition was approximated by estimating the maximal eigenvalue of reconstruction operators through the power iteration. Based on the condition, an optimal step size was further suggested to allow the fastest convergence. Verifications were made on the prospective in vivo data of static brain imaging and dynamic contrast-enhanced liver imaging, demonstrating that the recommended parameter allowed fast convergence in radial MRI.

Correcting Susceptibility Artifacts of MRI Sensors in Brain Scanning: A 3D Anatomy-Guided Deep Learning Approach

Echo planar imaging (EPI), a fast magnetic resonance imaging technique, is a powerful tool in functional neuroimaging studies. However, susceptibility artifacts, which cause misinterpretations of brain functions, are unavoidable distortions in EPI. This paper proposes an end-to-end deep learning framework, named TS-Net, for susceptibility artifact correction (SAC) in a pair of 3D EPI images with reversed phase-encoding directions. The proposed TS-Net comprises a deep convolutional network to predict a displacement field in three dimensions to overcome the limitation of existing methods, which only estimate the displacement field along the dominant-distortion direction. In the training phase, anatomical T1-weighted images are leveraged to regularize the correction, but they are not required during the inference phase to make TS-Net more flexible for general use. The experimental results show that TS-Net achieves favorable accuracy and speed trade-off when compared with the state-of-the-art SAC methods, i.e., TOPUP, TISAC, and S-Net. The fast inference speed (less than a second) of TS-Net makes real-time SAC during EPI image acquisition feasible and accelerates the medical image-processing pipelines.

Schnelle MRT-Sequenzen für die akute neurologische Abklärung

Neurological symptoms account for approximately 30% of emergency room (ER) visits. Clinical outcome often relies on atimely diagnosis and treatment initiation. Clinical imaging requirements are fast availability and high diagnostic value. Availability and quality of magnetic resonance imaging (MRI) in emergency rooms outside of core hours are limited compared to computed tomography (CT). Common reasons are infrastructural accessibility (hospitals using outpatient radiology centers), alack of experienced and qualified staff and high patient compliance requirements. However, in aneurological emergency setting, MRI may show relevant advantages over CT in certain areas, such as diagnosis of stroke. Advances in MRI technology have led to shorter exam times and robust motion reduction strategies. Common fast sequences and time reduction techniques for imaging of neurological emergencies are presented in this article. Recommendations for specific sequences or techniques depend on the institute's MRI hardware and software components. If available, parallel imaging is highly recommended for imaging of neurological emergencies. Imaging of neurological emergencies requires fast, significant and motion insensitive standard acquisitions. Additional sequences should be acquired dependent on clinical and standard protocol imaging findings. An MRI emergency protocol is introduced for the most common neurologic emergencies including recommendations for fast MRI sequences and techniques for imaging time reduction.

Step adaptive fast iterative shrinkage thresholding algorithm for compressively sampled MR imaging reconstruction

In order to accelerate magnetic resonance imaging (MRI) scanning, fast MRI technique based on compressed sensing (CS) was proposed. The shrinkage thresholding algorithm (STA) is an efficient method in related algorithms to decrease the incoherent artifacts produced by the undersampling in k-space directly. The traditional STA uses the fixed iteration step size during the reconstruction progress, and it is not conducive to accelerate the convergence speed. In order to improve global iteration efficiency, in this paper, step adaptive fast iterative shrinkage thresholding algorithm (SAFISTA) was proposed for MRI reconstruction based on STA. It used a feedback to dynamically adjust the iteration step size. The feedback parameter was calculated from the total variations (TV) of two previous iterations. It can effectively improve the efficiency of iteration. Experiments over three kinds of MR images (human head, blood vessels and knee) under different sample ratios indicated that the proposed algorithm SAFISTA showed better reconstruction performance than iterative shrinkage thresholding algorithm (ISTA), fast iterative shrinkage thresholding algorithm (FISTA) and generalized thresholding iterative algorithm (GTIA) in terms of mean square error (MSE), peak signal to noise ratio (PSNR) and structural similarity index measure (SSIM).

Gradient echo magnetic resonance imaging correlates with clinical measures and allows visualization of veins within multiple sclerosis lesions

Background: Conventional magnetic resonance imaging (MRI) methods do not quantify the severity of multiple sclerosis (MS) white matter lesions or measure pathology within normal-appearing white matter (NAWM). Objective: Gradient Echo Plural Contrast Imaging (GEPCI), a fast MRI technique producing inherently co-registered images for qualitative and quantitative assessment of MS, was used to 1) correlate with disability; 2) distinguish clinical MS subtypes; 3) determine prevalence of veins co-localized within lesions in WM. Methods: Thirty subjects representing relapsing–remitting MS (RRMS), secondary progressive MS (SPMS) and primary progressive MS (PPMS) subtypes were scanned with clinical and GEPCI protocols. Standard measures of physical disability and cognition were correlated with magnetic resonance metrics. Lesions with central veins were counted for RRMS subjects. Results: Tissue damage load (TDL-GEPCI) and lesion load (LL-GEPCI) derived with GEPCI correlated better with MS functional composite (MSFC) measures and most other neurologic measures than lesion load derived with FLAIR (LL-FLAIR). GEPCI correctly classified clinical subtypes in 70% subjects. A central vein could be identified in 76% of WM lesions in RRMS subjects on GEPCI T2*-SWI images. Conclusion: GEPCI lesion metrics correlated better with neurologic disability than lesion load derived using FLAIR imaging, and showed promise in classifying clinical subtypes of MS. These improvements are likely attributable to the ability of GEPCI to quantify tissue damage.

Mapping fetal brain development in utero using magnetic resonance imaging: the Big Bang of brain mapping.

The development of tools to construct and investigate probabilistic maps of the adult human brain from magnetic resonance imaging (MRI) has led to advances in both basic neuroscience and clinical diagnosis. These tools are increasingly being applied to brain development in adolescence and childhood, and even to neonatal and premature neonatal imaging. Even earlier in development, parallel advances in clinical fetal MRI have led to its growing use as a tool in challenging medical conditions. This has motivated new engineering developments encompassing optimal fast MRI scans and techniques derived from computer vision, the combination of which allows full 3D imaging of the moving fetal brain in utero without sedation. These promise to provide a new and unprecedented window into early human brain growth. This article reviews the developments that have led us to this point, examines the current state of the art in the fields of fast fetal imaging and motion correction, and describes the tools to analyze dynamically changing fetal brain structure. New methods to deal with developmental tissue segmentation and the construction of spatiotemporal atlases are examined, together with techniques to map fetal brain growth patterns.

Development of fast magnetic resonance imaging techniques based on k-space accelerated collection

Magnetic resonance imaging(MRI) is one of the most important non-invasive diagnostic tools in routine clinical examination.However,the temporal resolution is still low due to the limitation of Nyquist sampling theorem in k-space signal acquisition.Under the conditions of certain magnetic and gradient field,it takes a long time for signal acquisition to obtain a high resolution image with clinical value.In addition to enhancing the strength of main magnetic field and gradient as well speeding gradient field switch,some mathematical methods have been used to reduce the amount of k-space signal acquisition to shorten MR imaging time.Although under sparse sampling,the final reconstructed image data could be satisfied with Nyquist sampling theorem through these mathematical methods.Furthermore,many fast MRI methods based on data sharing and undersampling of k-space were proposed,such as half-Fourier imaging,keole imaging,parallel imaging,partially separable functions(PSF) and so on.In this review,several typical fast imaging methods were summarized and discussed based on k-space sampling techniques.

Magnetic Resonance Imaging of Pelvic Floor Defects in Women

Contemporary fast magnetic resonance imaging techniques allow dynamic evaluation of the entire female pelvic floor with excellent visualization of pelvic organs and muscular and fascial supportive structures in a single noninvasive study that does not expose the patient to ionizing radiation. This article focuses on the role of magnetic resonance imaging in defining pelvic floor defects that can guide surgical management of women with pelvic organ prolapse, especially those who undergo evaluation for symptoms of multicompartmental involvement before a complex pelvic floor reconstruction or those who have failed previous repairs.

Investigation of biofilm structure, flow patterns and detachment with magnetic resonance imaging

Detachment of biomass from biofilms is still a nearly unknown process which has to be investigated in more detail. Magnetic resonance imaging (MRI) is a promising method which supplies information on the structural data of the biofilm, its surface and the hydrodynamic conditions at the bulk/biofilm interface. Both the structural data and the shear stress are key parameters for understanding biofilm detachment. In this paper a fast quantitative MRI technique was used to investigate the detachment from a heterotrophic biofilm which was grown in a tube reactor. The investigated biofilm was cultivated in a test segment (length 12 cm, diameter 7 mm) of a tube reactor at a constant Reynolds number of 3,000 and a substrate load from 6 g glucose/m2 and day. For the MRI experiments, the test segment with a biofilm of 1,200 μm thickness was connected to the flow loop and placed inside the NMR magnet. During the experiment different hydrodynamic conditions were adjusted for two minutes (Reynolds number Re: 3,000, 4,000, 5,000, 6,000, 7,000, >9,000). Flow velocity and relaxation time were then measured at laminar flow conditions. The MR images show very impressively the increasing detachment of biomass from the biofilm surface with increasing Re. After the last step (Re > 9,000) only a thin biofilm of about 200 μm thickness with a very homogeneous surface remained in the test tube.

CT and MR imaging in Hodgkin's disease – present and future

Initial staging of Hodgkin's disease is crucial to determine the location and extent of disease, and is the hallmark for the choice of treatment. At present, the established radiological technique for staging Hodgkin's disease is computed tomography (CT). Modern multidetector row CT scanners allow fast imaging from the scull base to the groins during a single breath hold with a spatial resolution of approximately 1 mm. Both, nodal and extranodal involvement of Hodgkin's disease can be diagnosed with CT. Magnetic resonance (MR) imaging is another useful cross-sectional imaging modality for staging Hodgkin's disease. The development of fast MR imaging techniques has considerably reduced imaging time without compromising the quality of MR images. As a consequence, MR imaging is now considered to be as diagnostic as CT for staging Hodgkin's disease. The excellent soft-tissue contrast and the lack of exposure to ionizing radiation are the main advantages of MR imaging. For the detection of extranodal Hodgkin's disease, MR imaging is superior to assess involvement of the brain, the spinal cord and bone marrow; while CT allows excellent evaluation of lung disease. Common major problems in staging Hodgkin's disease are still the detection of nodal involvement in normal sized lymph nodes and residual tumor masses after therapy. In the future, newly developed lymphotropic contrast agents for MR imaging might be helpful to answer these questions.

Imaging droplet freezing using MRI

Magnetic resonance imaging (MRI) is used to provide spatially resolved structural and chemical composition characterisation of droplets undergoing freezing. To this end, MRI is applied to a 2 mm diameter sucrose solution droplet, suspended in cold air. During the consequential solidification of the droplet, the spatial location of nucleation and the crystal growth of the droplet are followed using non-invasive two dimensional (2D) images; these are produced using the fast MRI technique, RARE. This is able to both quantify crystal growth rates, as well as the unfrozen liquid mass fraction for the optically opaque freezing droplets. Such information is of major interest in the verification of models describing the freezing of such droplets. The spatial re-distribution of the sucrose solute as a consequence of freezing is monitored using MR 1D chemical shift profiling. The formation of a concentrated sucrose layer at the droplet surface was detected.

Gadolinium-enhanced three-dimensional magnetic resonance angiography of pulmonary and systemic venous anomalies

ObjectivesThe goal of this study was to evaluate the diagnostic value of gadolinium (Gd)-enhanced three-dimensional (3D) magnetic resonance angiography (MRA) in patients with congenital and acquired anomalies of the pulmonary and systemic veins. BackgroundGadolinium-enhanced 3D MRA is a fast magnetic resonance imaging technique that has shown great promise in the evaluation of large and medium-sized arteries. However, its application to venous anomalies has not been studied in detail. MethodsThe study retrospectively analyzed all patients who underwent Gd-enhanced 3D MRA examination from January 1998 through January 2001, were diagnosed with anomalies of the pulmonary or systemic veins and had additional diagnostic data available for comparison with the MRA findings. ResultsSixty-one patients (age 1 day to 60 years) were included. Image acquisition was completed in 29 ± 6.9 s. Pulmonary venous anomalies were found in 37 patients, systemic venous anomalies in 17 patients and both pulmonary and systemic venous anomalies in 7 patients. Compared with available diagnostic information by other modalities, all known or suspected venous anomalies were imaged by 3D MRA. In three patients, catheterization did not detect anomalies of the pulmonary veins that were subsequently diagnosed by MRA. The 3D MRA diagnoses were followed by 10 interventional catheterization procedures and 15 operations. In 74% of patients, 3D MRA either diagnosed previously unsuspected venous anomalies (28%) or added new clinically important information (46%). The mechanism of pulmonary vein compression in eight patients was determined by MRA but not by other imaging modalities. Using a five-level grading system for MRA image quality (1 = nondiagnostic; 5 = excellent), the average grade was 4.6 ± 0.6, with a 0.28 ± 0.6 mean grade difference between two independent observers. ConclusionsGadolinium-enhanced 3D MRA is capable of rapidly and accurately diagnosing a wide spectrum of pulmonary and systemic venous anomalies and is a useful noninvasive alternative to diagnostic catheterization.

A fast magnetic resonance imaging technique for children with mediastinal lymphoma: work in progress.

Mediastinal lymphoma often presents as a large tumor at the time of diagnosis. Usually chest X-ray filming is the first imaging modality and it is used for routine follow-up during the course of the disease. A new and very fast MRI technique has been developed at our center as an alternative. Results in three patients with mediastinal lymphoma during a pilot study are promising. After diagnosis the above patients were additionally investigated in a 0.2 T low-field MR-scanner by a modified true FISP sequence (see text) with an acquisition time of 3.6-4.6 sec. Follow-up was performed by both, X-ray filming and MRI. After diagnosis the patients, again, had both X-ray filming and MRI investigation for follow-up: one patient 2, one patient 3, and one patient 5 times. Images were evaluated and compared by two pediatric radiologists. Total investigation times for radiography and MRI were comparable. The tumor was better visualized by MRI on seven of nine images and gave additional information about the structure and the localization of the tumor as well as concomitant problems such as pericardial effusion. True FISP MRI may prove to be a good alternative to X-ray filming in the diagnosis and follow-up of mediastinal lymphoma.

Optimised real time catheter tracking on a conventional MR Scanner

Commercial high-field magnetic resonance (MR) scanners are generally not equipped with measurement software and hardware to perform interventional studies with active devices that contain small radiofrequency (rf) coils for localisation. In this study, with only minimal modifications to the MR scanner, the active catheter tracking technique is implemented on a clinical MR system. Small rf-coils are attached to a commercial angiography catheter and the movement of the coils is observed with fast MR tracking and imaging techniques. Optimised imaging strategies are explored for both static and moving anatomical regions. Image calculation and display software is implemented using both the MR scanner's fast image reconstruction processor, together with the main console computer and a separate personal computer. With the current implementation, image update rates of up to several Hertz can be achieved, if a roadmap imaging technique is used, where rapid coordinate updates are projected onto previously acquired overview images. To account for cardiac and breathing motion, an interleaved imaging strategy is presented where coordinate and image updates are acquired in rapid succession. With this approach, image update rates of 1–2 Hz are obtained, depending on the details of the image reconstruction process.

Fetal shoulder measurements by fast and ultrafast MRI techniques.

The purpose of this study was to evaluate magnetic resonance imaging (MRI) of fetal shoulder measurements of fetuses with suspected macrosomia. The actual fetal shoulder measurements made immediately after birth were compared with measurements obtained by fast and ultrafast MRI techniques antepartum. Eight singleton diabetic pregnant mothers underwent MRI examination with fast imaging in steady-state precession (TrueFISP) and spin-echo (SE) and gradient-echo (GE) echo-planar (EPI) sequences to show the fetal shoulder width. The actual shoulder width was measured immediately postpartum by a neonatologist. There was a statistically significant correlation between the MRI measurements and the actual shoulder width (P < 0.001 - P < 0.05) for all sequences. TrueFISP (r = 0.98, P < 0.001) was superior to EPI sequences (r = 0.88, P < 0.01 for SE EPI and r = 0.80, P < 0.05 for GE EPI). The images of all three sequences used were free of major motion artifacts. Fast and ultrafast sequences seem to be reliable for fetal shoulder measurements and the TrueFISP was the most accurate sequence compared to SE and GE echo-planar sequences. J. Magn. Reson. Imaging 2001;13:938-942.

Magnetic Resonance Imaging of the Fetal and Neonatal Central Nervous System

After completing this article, readers should be able to: 1. Compare and contrast magnetic resonance technology with computed tomography and ultrasonography. 2. Delineate the primary clinical uses of magnetic resonance spectroscopy, perfusion magnetic resonance, functional magnetic resonance imaging, and diffusion imaging. 3. Describe the primary uses of magnetic resonance imaging in congenital and developmental abnormalities, neurovascular disease, trauma, infections, inflammatory processes, and metabolic and neurodegenerative disorders. Magnetic resonance (MR) is one of several neuroimaging technologies used to diagnose fetal and neonatal central nervous system (CNS) abnormalities. It can aid in differentiating static from progressive encephalopathies and in diagnosing such conditions as congenital and developmental abnormalities, trauma, neurovascular disease, infections and inflammatory processes, and metabolic and neurodegenerative disorders. Although it also is helpful in diagnosing neoplastic processes, which are rare but important causes of progressive encephalopathy, these presentations are not considered in this article. MR, which employs magnetic fields and radiowaves, is a less invasive imaging technology than ultrasonography (US), computed tomography (CT), and nuclear medicine (NM), but it is more expensive. In contrast to the other modalities, which derive their signals from single parameters, the MR signal is derived exponentially from multiple parameters (eg, T1, T2, proton density, T2*, proton flow, proton relaxation enhancement, chemical shift, and molecular diffusion). MR also employs many more basic imaging techniques than other modalities, including spin echo, inversion recovery, gradient echo, and chemical shift imaging methods. Advancing MR capabilities have improved its sensitivity, specificity, and efficiency, particularly compared with US, CT, and NM. Among these advanced capabilities are the flow-attenuated inversion recovery technique (FLAIR), fat suppression short inversion time-inversion recovery imaging, and magnetization transfer imaging for increased structural resolution. Fast and ultrafast MR imaging techniques (fast spin echo, fast gradient echo, echo planar imaging) have reduced imaging times, improved structural resolution, and provided functional imaging. Specific applications …

Obstetric MR imaging.

The surge in the development of fast magnetic resonance (MR) techniques has revolutionized our ability to image the pregnant patient and the fetus. Fast MR imaging techniques provide excellent resolution for imaging the maternal and fetal anatomies without the need for sedation. This article addresses the use of fast MR imaging techniques in the evaluation of the pregnant patient for adnexal masses, pelvimetry, hydroureteronephrosis of pregnancy, and placenta accreta. In addition, fetal anomalies for which MR imaging has proved useful, such as ventriculomegaly, arachnoid cysts, and abdominal masses, are described.

Fast magnetic resonance imaging of liver

Recent magnetic resonance (MR) units with a stronger gradient system have allowed various fast MR imaging techniques to develop. These fast scan techniques have easily realized breath-holding acquisition in the liver and the image quality has been greatly improved without sacrificing spatial resolution. The majority of the fast imaging techniques have been devoted to T2-weighted imaging to obtain useful T2-weighted images in the shortest possible time. Among the fast sequences, fast spin-echo (FSE) sequence is the most promising technique and allows high-quality T2-weighted images with reduced motion artifacts. However, FSE sequences using multiple refocused pulses may essentially realize only poor soft-tissue contrast due to magnetization transfer and T2-filtering effects, and therefore, echo-planar (EP) imaging is expected to provide high image contrast. In addition, single-shot EP imaging allows even diffusion-weighted (DW) and perfusion-weighted (PW) imaging in the liver due to its short scanning time. Recent development of fast gadolinium-enhanced 3D MR angiography has also impacted liver imaging. Combined with such gadolinium-enhanced 3D-MRA sequences and zerofilling image interpolation technique, biphasic gadolinium-enhanced 3D-MRA (whole-liver dynamic MR imaging in the arterial phase and MR portography in the portal phase) can be obtained.

Fast magnetic resonance imaging techniques

This article reviews fast magnetic resonance (MR) techniques currently used for body imaging. Improvements in gradient performance have made very short repetition and echo times on clinical scanners feasible, thus enabling subsecond image acquisition. The article provides a fundamental overview of the technical aspects from the concept of k-space and k-space segmentation technique, fast MR imaging techniques including fast spin echo, fast gradient echo with or without magnetization preparation to echo planar and hybrid techniques. The article also addresses the use of different fat suppression techniques in MR imaging of the body and improvements in coil technology to obtain faster images and higher signal-to-noise.

Will it Be Feasible to Insert Endoprostheses under Interventional MRI?

Purpose: Recent advances in magnetic resonance imaging (MRI) technology may provide a safer and more sensitive monitoring modality than X-ray imaging for endovascular surgical procedures. The purpose of this study was to investigate the feasibility of using MRI to monitor the insertion of endoprostheses. Methods: The endoprostheses we studied were composed of a nitinol stent encased in a polyester sheath. These were characterized with four different MRI techniques: the fast spin-echo; spin-echo; gradient-recalled echo; and the spoiled gradient-recalled echo. The deployment of the endoprosthesis into an artery was simulated in an in vitro model and viewed using a fast spin-echo MRI technique. Results: Image artifacts produced by the nitinol framework in these endoprostheses were minimal when fast spin-echo or spin-echo imaging techniques were used, improving the visibility of the device. In in vitro tests, the catheters and endoprostheses were visualized by MRI with sufficient clarity to guide the placement of a device in the model artery. Conclusions: Insertion of this type of endoprosthesis under interventional MRI guidance is feasible. The convenience and improved safety provided by interventional MR systems and “real-time” imaging capabilities are expected to make this technology an attractive alternative to X-ray imaging techniques.