Dynamic Keyhole Research Articles

Formation mechanisms of pores and spatters during laser deep penetration welding

During laser deep penetration welding process, defects cannot be completely avoided yet. Due to the dynamic process, especially process pores and spatters occur. Keyhole dynamics are assumed to be responsible for the initiation of both pores and spatters. However, it is not completely clear yet how spatters and pores are formed. A semianalytical model of a keyhole is used to simulate dynamic keyhole properties of laser beam welding of aluminum. These are related to characteristics of spatters and pores occurring during laser deep penetration welding recorded using high-speed-imaging technique and x-ray analysis, respectively. Correlations of keyhole shapes and pore formation show that the volume underneath a keyhole collapse can be sufficient to capture the gas that is necessary for the formation of a pore and a keyhole expansion is not necessarily needed to produce the pores found in the weld seam. Calculated keyhole wall fluctuations are found to be not sufficient to detach spatters from the keyhole wall without additional forces from the melt pool.

A three-dimensional model of coupling dynamics of keyhole and weld pool during electron beam welding

In electron beam welding (EBW), the coupling dynamics between the keyhole and the weld pool play significant roles in joint quality, yet the mechanisms of these dynamics have not been well understood, since the rise of EBW in the 1960s. The present study reported a novel three-dimensional mathematical model of EBW and used it to theoretically understand the coupling behaviors between the keyhole and the weld pool during EBW of a Ti-6-Al-4V alloy. A new phenomenological heat source model, dependent on the dynamic keyhole profile and temperature, was developed. The physical effects, including the beam reflection and absorption of the keyhole wall, scattering of secondary electrons, keyhole evaporation heat loss and beam defocus, were rigorously treated in the heat source model. The transient free surface keyhole dynamics were tracked with a high-resolution level set method (LSM). On the free surface boundary, the majority of the key physical effects, such as the recoil pressure, Marangoni shear stress, the surface tension and hydrodynamic pressure of the molten liquid, were incorporated using a recently developed sharp interface model. The coupling behaviors of the three-dimensional keyhole evolutions, heat transfer, and fluid flow in the weld pool asa function of the process parameters in the EBW of Ti-6-Al-4-V alloys were simulated, of which the results reasonably agreed with independent experimental data. The dynamical keyhole violently exhibited fluctuations within the sub-millisecond characteristic time in the radial direction. The oscillations of the keyhole, which were characterized by its surface area and volume, were theoretically studied asa function of the welding speed and surface tension coefficient. The welding speed has stabilization effects on the keyhole and fully stabilized the keyhole under certain high welding speed conditions, an effect similar to that observed in keyhole mode laser welding. The surface tension governed the keyhole oscillation periods and was proposed asa major source of oscillations. In addition, the complex and violent flow patterns of weld pool were predicated and consistent with the well-accepted X-ray imaging experimental results. The EBW transient keyhole and weld pool dynamics were self-consistent. The present study is to present the successful development of EBW, thereby allowing the establishment of the self-consistent keyhole and weld pool dynamics asa function of the process parameters and material properties, to provide a promising avenue for process optimizations in industrial applications.

Sub-microsecond vapor plume dynamics under different keyhole penetration regimes in deep penetration laser welding

It is well-known that distinct vapor plume dynamics occur during deep penetration laser welding under different keyhole penetration states. However, there is little knowledge about the physical characteristics of vapor plumes (velocity, pressure, flow patterns, etc) located inside transient keyholes of varying penetration regimes in laser welding. This lack of knowledge is primarily because mesoscale vapor plumes are highly dynamic and generally invisible. Based on a well-tested three-dimensional multiphase laser welding model, we conducted a computational study on vapor plume dynamics inside transient keyholes during the fiber laser welding of 304 austenite stainless steel as a function of keyhole penetration regimes. We observed three keyhole regimes of penetration: full penetration, partial penetration and no penetration. We then physically analyzed the vapor plumes in these regimes. We determined that the vapor plume velocities and pressures in all three regimes were uneven and oscillated following the dynamic keyhole with a characteristic timescale in sub-microseconds. Only when the keyhole approached the full penetration regime did vapor plumes begin to violently eject from the bottom of the keyhole opening, whereas in the partial penetration regime, even when the bottom part of the keyhole was open, most of the vapor plume ejected from the upper keyhole opening. This latter observation was similar to that in the no penetration mode. We studied the physical mechanism of this behavior by analyzing the keyhole temperature and vapor plume velocity distributions. We determined that the upward ejection of the vapor plume from the upper keyhole opening was the result of an uneven micro-meter scale boiling phenomenon of the transient keyhole governed by Fresnel absorptions dependent on the local inclination angle of the keyhole wall. Similarly, we determined that the ejection of the vapor plume from the bottom of the keyhole opening resulted from pressure differences between the inside and outside of the keyhole (as long as there was a relatively stable open state at the bottom of the keyhole opening). Additionally, we conducted quantitative studies on the velocity and pressure of vapor plumes in transient keyholes for all three regimes. We observed a decrease in the average velocity of vapor plumes at the upper keyhole opening, and an increase in average velocity at the bottom opening when the penetration regime moved from no penetration to full penetration. Moreover, the pressure distributions of vapor plumes decreased and became more uniform as the penetration regime varied from no penetration to full penetration. For the investigated process parameters used for the fiber laser welding of 1 mm thick 304 stainless steel, the vapor plume pressure decreased approximately 500–1200 Pa inside the millimeter scale keyhole. The findings in this study give the first physical insights into vapor plume dynamics inside transient keyholes as a function of keyhole penetration states during deep penetration laser welding. Moreover, our findings can be used as theoretical references for welding process parameter optimization in industrial applications.

TH‐EF‐BRA‐05: A Method of Near Real‐Time 4D MRI Using Volumetric Dynamic Keyhole (VDK) in the Presence of Respiratory Motion for MR‐Guided Radiotherapy

Purpose:To develop a novel method that enables 4D MR imaging in near real‐time for continuous monitoring of tumor motion in MR‐guided radiotherapy.Methods:This method is mainly based on an idea of expanding dynamic keyhole to full volumetric imaging acquisition. In the VDK approach introduced in this study, a library of peripheral volumetric k‐space data is generated in given number of phases (5 and 10 in this study) in advance. For 4D MRI at any given time, only volumetric central k‐space data are acquired in real‐time and combined with pre‐acquired peripheral volumetric k‐space data in the library corresponding to the respiratory phase (or amplitude). The combined k‐space data are Fourier‐transformed to MR images. For simulation study, an MRXCAT program was used to generate synthetic MR images of the thorax with desired respiratory motion, contrast levels, and spatial and temporal resolution. 20 phases of volumetric MR images, with 200 ms temporal resolution in 4 s respiratory period, were generated using balanced steady‐state free precession MR pulse sequence. The total acquisition time was 21.5s/phase with a voxel size of 3×3×5 mm3 and an image matrix of 128×128×56. Image similarity was evaluated with difference maps between the reference and reconstructed images. The VDK, conventional keyhole, and zero filling methods were compared for this simulation study.Results:Using 80% of the ky data and 70% of the kz data from the library resulted in 12.20% average intensity difference from the reference, and 21.60% and 28.45% difference in threshold pixel difference for conventional keyhole and zero filling, respectively. The imaging time will be reduced from 21.5s to 1.3s per volume using the VDK method.Conclusion:Near real‐time 4D MR imaging can be achieved using the volumetric dynamic keyhole method. That makes the possibility of utilizing 4D MRI during MR‐guided radiotherapy.

Quantifying the accuracy of the tumor motion and area as a function of acceleration factor for the simulation of the dynamic keyhole magnetic resonance imaging method.

The dynamic keyhole is a new MR image reconstruction method for thoracic and abdominal MR imaging. To date, this method has not been investigated with cancer patient magnetic resonance imaging (MRI) data. The goal of this study was to assess the dynamic keyhole method for the task of lung tumor localization using cine-MR images reconstructed in the presence of respiratory motion. The dynamic keyhole method utilizes a previously acquired a library of peripheral k-space datasets at similar displacement and phase (where phase is simply used to determine whether the breathing is inhale to exhale or exhale to inhale) respiratory bins in conjunction with central k-space datasets (keyhole) acquired. External respiratory signals drive the process of sorting, matching, and combining the two k-space streams for each respiratory bin, thereby achieving faster image acquisition without substantial motion artifacts. This study was the first that investigates the impact of k-space undersampling on lung tumor motion and area assessment across clinically available techniques (zero-filling and conventional keyhole). In this study, the dynamic keyhole, conventional keyhole and zero-filling methods were compared to full k-space dataset acquisition by quantifying (1) the keyhole size required for central k-space datasets for constant image quality across sixty four cine-MRI datasets from nine lung cancer patients, (2) the intensity difference between the original and reconstructed images in a constant keyhole size, and (3) the accuracy of tumor motion and area directly measured by tumor autocontouring. For constant image quality, the dynamic keyhole method, conventional keyhole, and zero-filling methods required 22%, 34%, and 49% of the keyhole size (P < 0.0001), respectively, compared to the full k-space image acquisition method. Compared to the conventional keyhole and zero-filling reconstructed images with the keyhole size utilized in the dynamic keyhole method, an average intensity difference of the dynamic keyhole reconstructed images (P < 0.0001) was minimal, and resulted in the accuracy of tumor motion within 99.6% (P < 0.0001) and the accuracy of tumor area within 98.0% (P < 0.0001) for lung tumor monitoring applications. This study demonstrates that the dynamic keyhole method is a promising technique for clinical applications such as image-guided radiation therapy requiring the MR monitoring of thoracic tumors. Based on the results from this study, the dynamic keyhole method could increase the imaging frequency by up to a factor of five compared with full k-space methods for real-time lung tumor MRI.

Formation and influence mechanism of keyhole-induced porosity in deep-penetration laser welding based on 3D transient modeling

Keyhole-induced porosity occurring in deep-penetration laser welding is a large issue in laser manufacturing large-thickness components. In order to improve understanding of the keyhole-induced porosity formation and its influencing factors, a three dimensional (3D) transient model is proposed to provide better insight into the correlations of dynamic keyhole behavior with porosity formation under various parameters. Fluid flow, bubble movement and solidification are coupled in this model. Simulation results indicate that the number of bubbles is mainly determined by the frequency of keyhole collapse. The keyhole tends to collapse with increasing laser power and increasing gap size between plates, as well as decreasing travel speed and decreasing beam size. The depth to width ratio of the keyhole and fluid flow also affect the keyhole behavior. However, porosity number not only depends on the bubbles number, but also relies heavily on the evolution efficiency from bubbles to porosity. A good agreement is obtained between simulation and experimental results under the experimental conditions examined here.

Dynamic keyhole: a novel method to improve MR images in the presence of respiratory motion for real-time MRI.

In this work, the authors present a novel magnetic resonance imaging reconstruction method to improve the quality of MR images in the presence of respiratory motion for real-time thoracic image-guided radiotherapy. This new reconstruction method is called dynamic keyhole and utilizes a library of previously acquired, peripheral k-space datasets from the same (or similar) respiratory state in conjunction with central k-space datasets acquired in real-time. Internal or external respiratory signals are utilized to sort, match, and combine the two separate peripheral and central k-space datasets with respect to respiratory displacement, thereby reducing acquisition time and improving image quality without respiratory-related artifacts. In this study, the dynamic keyhole, conventional keyhole, and zero-filling methods were compared to full k-space acquisition (ground truth) for 60 coronal datasets acquired from 15 healthy human subjects. For the same image-quality difference from the ground-truth image, the dynamic keyhole method reused 79% of the prior peripheral phase-encoding lines, while the conventional keyhole reused 73% and zero-filling 63% (p-value < 0.0001), corresponding to faster acquisition speed of dynamic keyhole for real-time imaging applications. This study demonstrates that the dynamic keyhole method is a promising technique for clinical applications such as image-guided radiotherapy requiring real-time MR monitoring of the thoracic region. Based on the results from this study, the dynamic keyhole method could increase the temporal resolution by a factor of five compared with full k-space methods.

WE‐G‐18C‐08: Real Time Tumor Imaging Using a Novel Dynamic Keyhole MRI Reconstruction Technique

Purpose:To test the hypothesis that the novel Dynamic Keyhole MRI reconstruction technique can accelerate image acquisition whilst maintaining high image quality for lung cancer patients.Methods:18 MRI datasets from 5 lung cancer patients were acquired using a 3T MRI scanner. These datasets were retrospectively reconstructed using (A) The novel Dynamic Keyhole technique, (B) The conventional keyhole technique and (C) the conventional zero filling technique. The dynamic keyhole technique in MRI refers to techniques in which previously acquired k‐space data is used to supplement under sampled data obtained in real time. The novel Dynamic Keyhole technique utilizes a previously acquired a library of kspace datasets in conjunction with central k‐space datasets acquired in realtime. A simultaneously acquired respiratory signal is utilized to sort, match and combine the two k‐space streams with respect to respiratory displacement. Reconstruction performance was quantified by (1) comparing the keyhole size (which corresponds to imaging speed) required to achieve the same image quality, and (2) maintaining a constant keyhole size across the three reconstruction methods to compare the resulting image quality to the ground truth image.Results:(1) The dynamic keyhole method required a mean keyhole size which was 48% smaller than the conventional keyhole technique and 60% smaller than the zero filling technique to achieve the same image quality. This directly corresponds to faster imaging. (2) When a constant keyhole size was utilized, the Dynamic Keyhole technique resulted in the smallest difference of the tumor region compared to the ground truth.Conclusion:The dynamic keyhole is a simple and adaptable technique for clinical applications requiring real‐time imaging and tumor monitoring such as MRI guided radiotherapy. Based on the results from this study, the dynamic keyhole method could increase the imaging frequency by a factor of five compared with full k‐space methods.

Energy propagation in plasma arc welding with keyhole tracking

A three dimensional mathematical model was developed to research the dynamic interaction between energy propagation and keyhole evolution in PAW (Plasma Arc Welding). Particularly, a keyhole-tracking heat source model was proposed to reflect the energy propagation in the wake of keyhole evolution, which was tracked by the VOF (Volume of Fluid) technique. The model consists of a Gaussian heat flux on top surface and a lower developing conical heat source related to the dynamic keyhole growth. In addition, a dynamic energy distribution coefficient was established, bound up with the keyholing process for the first time. The design of this heat transfer model introduces the analogy to actual energy propagation in experiment. Evolution of the dynamic energy density distribution concerning keyhole effect was analyzed in details, and the corresponding temperature field was calculated and displayed to reveal the mechanism of thermal process in the workpiece. Furthermore, the keyholing process and molten metal flow in the weld pool were investigated to exhibit how the keyhole promotes the deep penetration welding. Finally, the experiments were carried out on the stainless steel plate, and the measured weld bead geometry, keyhole size and time for workpiece completely penetrated through were all close to simulation results.

Dynamic keyhole profile during high-power deep-penetration laser welding

Based on a butt-joint configuration assembled from transparent glass and stainless steel, the dynamic keyhole profile was observed during 10-kW fiber laser welding. The “gauffers” and vapor flow on the front keyhole wall moved downwards to the bottom of the keyhole. The downward-flowing vapor met the upward-flowing vapor at the bottom of the keyhole, which resulted in a whirlpool of vapor. The suspended droplets were drove to move in a random fashion by the whirlpool. The fluctuating vapor flow and pressure were the key factors causing the fluctuating keyhole wall and resulting in a vapor-generated wave (VGW) on the rear keyhole wall. The dynamic VGW was a very important factor affecting the dynamic welding process. When the VGW broke at the keyhole outlet, it was accompanied by swellings, spatter, columns, a decrease of the diameter of keyhole inlet, and a change in the direction of plume.

Visualization of dynamic keyhole behavior in waveform-controlled plasma arc welding

To overcome the shortcomings of conventional plasma arc welding (PAW), the “controlled pulse keyholing” strategy is proposed and the waveform-controlled keyhole PAW system is developed. To deeply understand the dynamic behaviors of keyhole in this novel PAW process, it is essential to measure and monitor the dynamic variation information of the keyhole geometry in real time. In this study, a vision system is developed to acquire the images of the keyhole from the underside of the workpiece. When a fully penetrated keyhole (open keyhole) is formed, clear keyhole images are captured. Both CCD camera and an efflux plasma voltage sensor are used to measure and characterize the keyhole shape and size during the waveform-controlled PAW process. The dynamic variation features of keyhole shape in a pulse cycle are visualized. The phenomena of “one keyhole in each pulse” and periodic partial-open keyhole transformation are experimentally sensed. The observation results lay solid foundation for controlling keyhole stability and optimizing the process parameters in keyhole plasma arc welding.

Numerical analysis of the heat and fluid flow in a weld pool with a dynamic keyhole

In keyhole plasma arc welding, the interaction between the keyhole and the weld pool occurs when the keyhole appears inside the weld pool. The change of the keyhole shape and dimensions has direct effect on the heat and fluid flow in the weld pool, and the latter will also influence the keyhole geometry. In this study, the coupled behaviors of weld pool and keyhole are treated to develop a three-dimensional model for analyzing the heat and fluid flow inside a weld pool with a dynamic keyhole. In view of the characteristics of PAW process, a combined volumetric heat source model (double-ellipsoid plus conic body source) is established, and one of its distribution parameters is adjusted dynamically with the variation of the depth of keyhole. The physical phenomena, such as the weld pool development, the keyhole formation, the evolution of fluid flow and thermal field, the full-penetration of the test plates, and the transformation from a blind keyhole to an open keyhole, are quantitatively analyzed. The numerical results reveal the regularity of fluid flow in weld pool with a keyhole. The calculated keyhole shape and the fusion zone of plasma arc welds are compared with the experimental measurements. Both agree with each other generally. It lays foundation for optimizing the welding process parameters and improving the stability of plasma arc welding process.

Measurement of Dynamic Keyhole Behaviors in Controlled-pulse Plasma Arc Welding

The developed novel variant process of controlled-pulse keyholing plasma arc welding is able to effectively extend the suitable process parameters window,and is of great application potential in joining high-performance metals with medium thickness.To fully understand the underlying thermo-physical mechanisms of this welding process,it is critical to measure and monitor the dynamic variation of the keyhole status in each pulse cycle of the welding current.The experimental sensing system and technique of the dynamic keyhole behavior in controlled-pulse keyholing plasma arc welding are introduced.Under the action of controlled-pulse current waveform,both heat input and keyhole's establishment and closure are actively controlled through adjusting the dropping slopes at the trailing edges of the pulse current,so that complete keyhole formation and full penetration are ensured.The sequential images of the keyhole exit are captured by the vision sensor during the controlled-pulse keyholing plasma arc welding.The information on the shape,size and deviation distance of the keyhole exit is extracted based on the image processing and calibration.It presents the transient variation of the keyhole dimension and location relative to the torch axis during the cyclic process of keyhole establishing-expanding-contracting-closing in each pulse.The research results lay solid foundation for the process optimization of the controlled-pulse keyholing plasma arc welding.

WE-G-217A-03: Respiratory-Related External/Internal Motion Based MR Image Reconstruction Using Dynamic Keyhole for Real-Time Tumor Monitoring.

It is aim to test the hypothesis that a dynamic keyhole MRI reconstruction technique using external/internal respiratory surrogate position reduces acquisition time while retaining image quality for real-time tumor monitoring, compared to the conventional keyhole technique. 46 thoracic MRI studies with 13 healthy human subjects have been acquired using a 3T GE MRI. Acquired MR images were reconstructed using zero-filling, conventional keyhole and the proposed respiratory motion based dynamic keyhole techniques; resultant images were then compared for image quality. Undersampled k-space rate in the phase encoding direction was determined based on the difference between the original image and the reconstructed image. The position of abdominal muscles and diaphragm were used to determine any excess data that exists in the overlaid temporal data. In addition, the feasibility of the dynamic keyhole method was applied using lung tumor MR images. The result from dynamic keyhole using respiratory motion demonstrated significant improvement compared with the zero-filling and conventional keyhole methods. Firstly, the dynamic keyhole method using external respiratory motion had an overlaid average of 79.7% (204 lines) of 256 lines from 46 datasets, compared to 63.9% in zero-filling and 74.3% in conventional keyhole. Secondly, dynamic keyhole method using internal diaphragm motion had an overlaid average of 84.5% (216 lines) of 256 lines from all datasets compared to 67.1% and 77.8% or zero-filling and conventional keyhole, respectively. Lastly, dynamic keyhole has been validated with one dataset involving lung tumor MR images. Image blurring artifacts and inferior resolution were not present in the final MR images using dynamic keyhole. Dynamic keyhole method using respiratory external/internal surrogate motion has been proposed to reconstruct MR images without image artifacts and with superior resolution. This method is applicable to MR images targeting lungs and other organs affected by respiratory motion for real-time tumor motion monitoring.

Characteristic Dynamic Enhancement Pattern of Magnetic Resonance Imaging for Malignant Thyroid Tumor: A Preliminary Report

Most thyroid diseases are evaluated by radionuclide scintigraphy, sonography or percutaneous needle biopsy. Sonography is perhaps the most definitive method for evaluation of nodules in a thyroid gland. However, the disadvantages of this technique include limited penetration in some anatomic locations, such as retrotracheal and substernal areas, and a limited field of view for assessing large masses. Further, current diagnostic methods may not enable the accurate detection of the malignant nature of a thyroid nodule, especially in a multinodular goiter. Therefore, new techniques are being investigated to improve the evaluation of thyroid nodules. The magnetic resonance (MR) imaging has a minor role in the evaluation of most patients with thyroid nodules or goiters, because of inadequate spatial and contrast resolution and only a few cases documented. However, technological advances have provided good spatial and contrast resolution in the neck including the thyroid gland, parathyroid gland and lymph node. Also it has made available the dynamic keyhole imaging of the thyroid nodule with spin echo sequences. The aim of our study was to evaluate this new technique in the routine evaluation of possible thyroid cancer, and particularly for the determination of the characteristic MR dynamic Original Article

High speed imaging technique Part 2 – High speed imaging of power beam welding phenomena

The high speed imaging technique is an attractive tool to elucidate welding phenomena. In this paper, applications of this technique are reviewed in order to understand the power beam welding phenomenon. Monochromatic imaging technique, which can take pictures of plasma with any specific spectrum wavelength, is used to analyse state of the laser induced plasma and distribution of some species, such as ions and atoms of gases and metals. During electron beam welding, intermittent melting process and spiking phenomenon can be clearly recorded by a pinhole X‐ray streak camera. The in situ X‐ray transmission imaging system is useful to observe the dynamic keyhole and fluid flow behaviour, which cannot be directly observed by any other method. High speed observations of the solidification front clearly show the solidification cracking process. In the present paper, methods to analyse the solidification cracking using high speed imaging are also described.

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Time-resolved MRA has recently been reported, and high-resolution MRA that does not miss timing is possible. In this examination, CEMRA that used 4D time-resolved angiography using keyhole (4D-TRAK) at 3T for the pelvic region was studied. 4D-TRAK is a method of using keyhole imaging together with sensitivity encoding (SENSE) and contrast-enhanced timing robust angio (CENTRA). The method changed flip angle (FA) and examined relative signal intensity, TR, TE, dynamic keyhole scan duration (DKSD) of the dilution contrast media, and keyhole% (KP). Image quality was examined with a blood vessel phantom. The presence of the partial echo method (PE) was examined in all cases. Relative signal intensity rose when FA increased. It has decreased in the PE method. TR did not show a difference by the PE method in FA15 degrees or more. DKSD was extended by the PE method. In the blood vessel Phantom, the PE method made the ringing artifacts remarkable. The artifacts that originated in keyhole imaging were observed. Shortening TR is difficult because of peculiar SAR to 3T. The PE method is not effective and becomes useless. It is necessary to note a point different from the parameter setting by 1.5T.

Modeling of the Temperature Field during Electron Beam Welding of Aluminum Alloy by a Pre-Defined Keyhole

The transient temperature field of Al alloy during electron beam welding (EBW) process was simulated using a three-dimensional finite element method. Different from the most previous models which were based on the assumption that the welding pool was solid and neglected the existence of keyhole by meshing the solid as a whole, a dynamic three-dimensional keyhole was applied in this model. The profile of the keyhole was ellipse and its size was determined before simulation based on the results of experiments. Following the heat source, the pre-defined keyhole moved along the welding line. A three-dimensional complex heat source model, including a modified Gaussian distribution source and a uniform source, was used in this study. The result shows that the shape of the keyhole had a direct effect on the temperature distribution and contribution to the special shape of the welding pool in EBW.

Control of dynamic keyhole welding process

Weld joint penetration control is a basic research topic in the welding research community. The authors propose using an innovative plasma arc welding process referred to as the quasi-keyhole process to achieve less application-dependent weld joint penetration sensing and control. To control the quasi-keyhole process, the peak current and keyhole sustaining current are selected as the control variables to maintain the keyhole establishment and sustaining periods at desired values. The dynamic quasi-keyhole process is approximated by a linear model with interval parameters. A control algorithm has been developed for the multivariable interval quasi-keyhole process based on a predictive control algorithm for interval SISO models. Experiments have been conducted to test the effectiveness of the control system developed.

Time‐resolved, high‐resolution contrast‐enhanced MR angiography of dialysis shunts using the CENTRA keyhole technique with parallel imaging

To evaluate the use of a dynamic keyhole magnetic resonance angiography (MRA) sequence combined with sensitivity encoding (SENSE) for hemodialysis shunts, because surveillance with conventional contrast-enhanced MRA (CE-MRA) is limited by its low temporal resolution, resulting in arteriovenous overlay. A total of 12 patients with Brescia-Cimino shunts were investigated prospectively using the new technique. During the contrast passage (gadoterate, Gd-DOTA) a series of five to nine dynamic central k-space measurements (10% for upper-arm shunt, 25% for lower-arm shunt) followed by a full reference data set were acquired. The outer k-space data of the single reference scan were used to complete the dynamic data sets. All studies were diagnostic (17 stenoses, three aneurysms) without complications. The acquisition times for a single dynamic scan of the upper- and lower-arm shunts were 2.2 and 3.2 seconds, respectively, while the reference scan needed 13 and 22.4 seconds, respectively. The dynamic angiograms allowed the differentiation of arterial and venous filling despite a mean peak delay time of only 4.2 seconds in the shoulder region. Image quality qualified in consensus by two experienced readers was rated "good" in 19 cases and "intermediate" in five cases with high mean values for signal-to-noise ratios (SNRs) and contrast-to-noise-ratios (CNRs). We have successfully implemented a fast, dynamic, CE-MRA technique with CE timing robust angiography (CENTRA) keyhole and SENSE in clinical routine. High spatial and temporal resolution improve the diagnostics of dialysis shunts and allow the assessment of detailed, dynamic, four-dimensional (4D) information.