Image Update Rate Research Articles

Software-Based Phase Control, Video-Rate Imaging, and Real-Time Mosaicing With a Lissajous-Scanned Confocal Microscope.

We present software-based methods for automatic phase control and for mosaicing high-speed, Lissajous-scanned images. To achieve imaging speeds fast enough for mosaicing, we first increase the image update rate tenfold from 3 to 30 Hz, then vertically interpolate each sparse image in real-time to eliminate fixed pattern noise. We validate our methods by imaging fluorescent beads and automatically maintaining phase control over the course of one hour. We then image fixed mouse brain tissues at varying update rates and compare the resulting mosaics. Using reconstructed image data as feedback for phase control eliminates the need for phase sensors and feedback controllers, enabling long-term imaging experiments without additional hardware. Mosaicing subsampled images results in video-rate imaging speeds, nearly fully recovered spatial resolution, and millimeter-scale fields of view.

Solid-state 3D imaging using a 1nJ/100ps laser diode transmitter and a single photon receiver matrix.

A 3D imaging concept based on pulsed time-of-flight focal plane imaging is presented which can be tailored flexibly in terms of performance parameters such as range, image update rate, field-of-view, 2D resolution, depth accuracy, etc. according to the needs of different applications. The transmitter is based on a laser diode operating in enhanced gain-switching mode with a simple MOS/CMOS-switch current driver and capable of producing short (~100ps FWHM) high energy (up to nJ) pulses at a high pulsing rate. The receiver consists of 2D SPAD and TDC arrays placed on the same die, but in separate arrays. Paraxial optics can be used to illuminate the target field-of-view with the receiver placed at the focal plane of the receiver lens. To validate the concept, a prototype system is presented with a bulk laser diode/MOS driver operating at a wavelength of 870nm with a pulsing rate of 100kHz as the transmitter and a single-chip 9x9 SPAD array with 10-channel TDC as the receiver. The possibility of using this method as a solid-state solution to the task of 3D imaging is discussed in the light of the results derived from this prototype.

Dynamic properties of a small-size streak tube

Scannerless (flash) lidar system based on streak camera is able to realize three-dimensional (3D) multi-spectral fluorescence imaging and 3D imaging polarimetry. Compared with conventional lidar system, the flash lidar system overcomes image distortions caused by the motion between the target and the sensor platform. Other advantages of the flash lidar system are higher image update rates and the potential for creating a miniaturized lidar system. To meet the requirements for developing this new technology, a super small-sized, large photocathode area and meshless streak tube with spherical cathode and screen is designed with the aid of computer simulation technology (CST) software. The tube with nearly 28 mm wide photocathode work area contains two electrostatic focusing lens, a pair of deflection plates, and a 50 mm diameter output screen. The external dimension of the tube is merely 50 mm100 mm. And its electromagnetic fields are calculated in the CST Particle Studio based on the finite integration theory. Some dynamic properties of the tube are analyzed via observing different electron trajectories launched from a number of different points on the cathode. The influences of the deflector position on deflection sensitivity and spatial resolution are analyzed. Increasing the distance between the deflector and the anode pin hole leads to a worse deflection sensitivity but a better spatial resolution. As for the temporal resolution, three electron pulses separated by 30 ps can be well resolved by the streak tube in the dynamic mode. Thus, the dynamic temporal resolution of the streak tube is better than 30 ps. And a 10 lp/mm spatial resolution across the 28 mm long slit on the photocathode can be obtained by estimating modulation transfer functions of the electron trajectories. Temporal distortions at the entire photocathode working area are evaluated, and the data reveal that the larger the photocathode working area, the bigger the temporal distortions are. Also, the temporal distortion is present mainly in the photocathode-to-deflection plates region. In addition, the slit image of the streak tube working in the dynamic mode is simulated and presented. The phenomenon that the slit image is curved due to the temporal distortion is analyzed. Two rectangular electron pulses separated by 50 ps are well resolved by the streak tube. Therefore, the temporal resolution of this small-size steak tube is better than 50 ps with a rectangular slit dimension of 30 mm50 m on the photocathode, and its electron-optic magnification is 1.2.

Detection of the interface between two metals by eddy current thermography

The capability of eddy current thermography in detecting the interface between two (different) metals is investigated numerically. The simulations concern the detection of: (a) the boundary between two halves of a plate, each half made of a different material, and (b) the position and shape of a small region, consisting of a different material than the surrounding one. A simple observation of numerical snapshots (isotherms) gives only a vague indication about the interface; however, the numerical experiment results are improved considerably by data processing techniques, like image subtraction, and the depiction of the norm of the spatial derivatives of: (i) the instantaneous temperature and (ii) the mean value of temperature over a time period. The most effective technique proved to be the latter one, which determines the interface quite clearly, even in the case where the electrical and thermal properties of the two materials are similar. The sharpness of the interface depends on how dissimilar the two materials are, on the number of gradations in the temperature field (a function of the sensitivity of the infrared camera as well as the maximum increase in temperature within the work-piece), on the image update rate and on the time period over which the mean value of temperature is calculated. It depends also on the temperature distribution, which is affected by the exciting frequency. Higher exciting frequencies generate higher heating rates, but this does not ensure sharper images for the interface than those obtained by lower frequencies.

Tracking of an interventional catheter with a ferromagnetic tip using dual-echo projections

Interventional devices with ferromagnetic components can be manipulated remotely using forces induced by the MRI gradients. To deflect the tip of an endovascular catheter, large ferromagnetic spheres of 2mm diameter are required to exert sufficiently high magnetic forces; however, tracking of these devices is difficult due to the large image artifacts. In this study, a new dual-echo technique is proposed to improve the stability of localizing and tracking medical devices with ferromagnetic components. MR tracking methods with selective off-resonant excitation and phase compensation with a rephasing gradient can detect ferromagnetic spheres up to a diameter of 1mm only. In this work, a dual-echo technique is used with two rephasing gradients to stabilize the off-set localization. With rephasing being applied in orthogonal directions, an SNR of 5 was achieved in the signal projections. Compared to a single-echo acquisition the dual-echo method reduces the position error in a phantom from 8mm to 1.6mm. In an in vivo study a tracking precision of 4mm was measured without steering gradients at an image update rate of 2 images per second. Steering experiments were successfully performed with a prototype catheter with ferromagnetic sphere in an aorta phantom and in the vena cava of a pig.

Magnetization spoiling in radial FLASH contrast‐enhanced MR digital subtraction angiography

To increase the in-plane spatial resolution and image update rates of 2D magnetic resonance (MR) digital subtraction angiography (DSA) pulse sequences to 0.57 × 0.57 mm and 6 frames/sec, respectively, for intracranial vascular disease applications by developing a radial FLASH protocol and to characterize a new artifact, not previously described in the literature, which arises in the presence of such pulse sequences. The pulse sequence was optimized and artifacts were characterized using simulation and phantom studies. With Institutional Review Board (IRB) approval, the pulse sequence was used to acquire time-resolved images from healthy human volunteers and patients with x-ray DSA-confirmed intracranial vascular disease. Artifacts were shown to derive from inhomogeneous spoiling due to the nature of radial waveforms. Gradient spoiling strategies were proposed to eliminate the observed artifact by balancing gradient moments across TR intervals. The resulting radial 2D MR DSA sequence (2.6 sec temporal footprint, 6 frames/sec with sliding window factor 16, 0.57 × 0.57 mm in-plane) demonstrated small vessel detail and corroborated x-ray DSA findings in intracranial vascular imaging studies. Appropriate gradient spoiling in radial 2D MR DSA pulse sequences improves intracranial vascular depiction by eliminating circular banding artifacts. The proposed pulse sequence may provide a useful addition to clinically applied 2D MR DSA scans.

Accuracy of Bronchoscopic Implantation of an Anchored Beacon Transponder in a Cadaveric Human Lung Tumor Model

Real-time localization of implanted fiducials in the lung is limited by subjective image interpretation, poor fiducial stability, image update rates and non-therapeutic radiation. An anchored transponder (AT) with a 5-legged anchoring feature and novel dedicated delivery catheter were developed for use with an electromagnetic, real-time tracking system (The Calypso System, Calypso Medical Technologies, Inc., Seattle, WA) and has shown excellent fixation rates in healthy canine airways (Mayse, IJROBP, 2008;72(1):S430).

Active microcoil tracking in the lungs using a semisolid rubber as signal source

A new method to localize and track medical devices in air-filled body cavities is proposed that uses active microcoils with a semisolid filling. In air spaces, e.g., the lung, microcoils require an independent signal source, which should be made of a biocompatible, solid and sterilizable material with a long shelf time. In a measurement of the T(1) and T*(2) and the relative spin density of several semisolid materials, latex was identified as a suitable material from which a prototype catheter was constructed with a microcoil at its tip. In a dual-echo tracking pulse sequence, the very short T*(2) of the rubber material allowed suppressing the background signal from surrounding tissue with a subtraction technique and additional dephasing gradients. With a roadmapping reconstruction, the microcoil's trajectory could be visualized on a previously acquired reference image set with a tracking rate of up to 60 Hz at a spatial resolution of better than 2mm. In a real-time tracking implementation, an image update rate of 4 Hz was achieved by combining the tracking with a fast real-time imaging sequence. Both methods were successfully applied in vivo to track the catheter in the lung of a pig.

A data acquisition and control system for high-speed gamma-ray tomography

A data acquisition and control system (DACS) for high-speed gamma-ray tomography based on the USB (Universal Serial Bus) and Ethernet communication protocols has been designed and implemented. The high-speed gamma-ray tomograph comprises five 500 mCi 241Am gamma-ray sources, each at a principal energy of 59.5 keV, which corresponds to five detector modules, each consisting of 17 CdZnTe detectors. The DACS design is based on Microchip's PIC18F4550 and PIC18F4620 microcontrollers, which facilitates an USB 2.0 interface protocol and an Ethernet (IEEE 802.3) interface protocol, respectively. By implementing the USB- and Ethernet-based DACS, a sufficiently high data acquisition rate is obtained and no dedicated hardware installation is required for the data acquisition computer, assuming that it is already equipped with a standard USB and/or Ethernet port. The API (Application Programming Interface) for the DACS is founded on the National Instrument's LabVIEW® graphical development tool, which provides a simple and robust foundation for further application software developments for the tomograph. The data acquisition interval, i.e. the integration time, of the high-speed gamma-ray tomograph is user selectable and is a function of the statistical measurement accuracy required for the specific application. The bandwidth of the DACS is 85 kBytes s−1 for the USB communication protocol and 28 kBytes s−1 for the Ethernet protocol. When using the iterative least square technique reconstruction algorithm with a 1 ms integration time, the USB-based DACS provides an online image update rate of 38 Hz, i.e. 38 frames per second, whereas 31 Hz for the Ethernet-based DACS. The off-line image update rate (storage to disk) for the USB-based DACS is 278 Hz using a 1 ms integration time. Initial characterization of the high-speed gamma-ray tomograph using the DACS on polypropylene phantoms is presented in the paper.

Dynamic magnetic resonance imaging of swallowing and laryngeal motion using parallel imaging at 3 T

To evaluate the feasibility of an optimized MRI protocol based on high field imaging at 3 T in combination with accelerated data acquisition by parallel imaging for the analysis of oropharyngeal and laryngeal function. Fast 2D gradient echo (GRE) MRI with different spatial resolutions (1.7x2.7 and 1.1x1.5 mm2) and image update rates (4 and 10 frames per second) was employed to assess pharyngeal movements and visualize swallowing via tracking of an oral contrast bolus (blueberry juice). In a study with 10 normal volunteers, image quality was semi-quantitatively graded by three independent observers with respect to the delineation of anatomical detail and depiction of oropharynx and larynx function. Additionally, the feasibility of the technique for the visualization of pathological pre- and post-surgical oropharynx and larynx function was evaluated in a patient with inspiratory stridor. Image grading demonstrated the feasibility of dynamic MRI for the assessment of normal oropharynx and larynx anatomy and function. Superior image quality (P<.05) was found for data acquisition with four frames per second and higher spatial resolution. In the patient, dynamic MRI detected pathological hypermobility of the epiglottis resulting in airway obstruction. Additional post-surgical MRI for one clinical case revealed morphological changes of the epiglottis and improved function, i.e., absence of airway obstruction and normal swallowing. Results of the volunteer study demonstrated the feasibility of dynamic MRI at 3 T for the visualization of the oropharynx and larynx function during breathing, movements of the tongue and swallowing. Future studies are necessary to evaluate its clinical value compared to existing modalities based on endoscopy or radiographic techniques.

Rapid passive MR catheter visualization for endovascular therapy using nonsymmetric truncated k-space sampling strategies

Passive catheter tracking guidance by MRI is a promising approach for endovascular therapy that may have several clinical advantages over the more frequently employed active MR approaches. However, real-time MR passive tracking is problematic because it is difficult to have an image update rate >1 Hz and preserve adequate spatial and image contrast resolution. One solution for improving real-time temporal performance is the use of nonsymmetric truncated k-space sampling strategies, which acquire only a fraction of the data in both the readout and phase-encoding directions. This article investigated these acquisition strategies in combination with using (a) multicycle projection dephaser (mcPD) gradients for background suppression and (b) the projection-onto-convex sets (POCS) algorithm to reconstruct the images. The use of mcPD gradients allowed the data sampling strategies to exploit the k-space energy structure of the catheter, and POCS allowed reconstruction of high-quality MR images that were suitable for real-time passive catheter tracking and demonstrated improved geometric representations of catheter width and tip position compared to zero filling. The use of nonsymmetric truncated k-space reduced the total acquisition time.

An Optimal Radial Profile Order Based on the Golden Ratio for Time-Resolved MRI

In dynamic magnetic resonance imaging (MRI) studies, the motion kinetics or the contrast variability are often hard to predict, hampering an appropriate choice of the image update rate or the temporal resolution. A constant azimuthal profile spacing (111.246 degrees), based on the Golden Ratio, is investigated as optimal for image reconstruction from an arbitrary number of profiles in radial MRI. The profile order is evaluated and compared with a uniform profile distribution in terms of signal-to-noise ratio (SNR) and artifact level. The favorable characteristics of such a profile order are exemplified in two applications on healthy volunteers. First, an advanced sliding window reconstruction scheme is applied to dynamic cardiac imaging, with a reconstruction window that can be flexibly adjusted according to the extent of cardiac motion that is acceptable. Second, a contrast-enhancing k-space filter is presented that permits reconstructing an arbitrary number of images at arbitrary time points from one raw data set. The filter was utilized to depict the T1-relaxation in the brain after a single inversion prepulse. While a uniform profile distribution with a constant angle increment is optimal for a fixed and predetermined number of profiles, a profile distribution based on the Golden Ratio proved to be an appropriate solution for an arbitrary number of profiles.

Dynamic coil selection for real‐time imaging in interventional MRI

MR-guided intravascular interventions require image update rates of up to 10 images per second, which can be achieved using parallel imaging. However, parallel imaging requires many coil elements, which increases reconstruction times and thus compromises real-time image reconstruction. In this study a dynamic coil selection (DCS) algorithm is presented that selects a subset of receive coils to reduce image reconstruction times. The center-of-sensitivity coordinates and the relative signal intensities are determined for each coil in a prescan. During the intervention m coils are selected for reconstruction using a coil ranking based on the distance to the current slice or catheter position. In a phantom experiment for m = 6, an optimal signal-to-background ratio (SBR) was achieved and foldover artifacts were avoided. In three animal experiments involving catheter manipulation in the aorta and the right heart chamber, the anatomy was successfully visualized at frame rates of about 5 Hz using active catheter tracking.

Active catheter tracking using parallel MRI and real‐time image reconstruction

In this work active MR catheter tracking with automatic slice alignment was combined with an autocalibrated parallel imaging technique. Using an optimized generalized autocalibrating partially parallel acquisitions (GRAPPA) algorithm with an acceleration factor of 2, we were able to reduce the acquisition time per image by 34%. To accelerate real-time GRAPPA image reconstruction, the coil sensitivities were updated only after slice reorientation. For a 2D trueFISP acquisition (160 x 256 matrix, 80% phase matrix, half Fourier acquisition, TR = 3.7 ms, GRAPPA factor = 2) real-time image reconstruction was achieved with up to six imaging coils. In a single animal experiment the method was used to steer a catheter from the vena cava through the beating heart into the pulmonary vasculature at an image update rate of about five images per second. Under all slice orientations, parallel image reconstruction was accomplished with only minor image artifacts, and the increased temporal resolution provided a sharp delineation of intracardial structures, such as the papillary muscle.

Real‐time modulation of acoustic gradient noise in interventional MRI

AbstractAcoustic noise is inherent to magnet resonance imaging (MRI) and poses safety risks to patients and interventional radiologists. Although a spectrum of noise reduction techniques has been implemented, new developments further increase sound levels. Silent pulse sequences based on derating gradient currents have been proposed but are limited to slow pulse sequences not suitable for interventional MRI (iMRI). However, iMRI does not continuously require fast imaging with high image update rates. Therefore, the aim of this study was to develop a tool that could remotely derate the gradient currents in real time, operated by the interventionalist. The duration of the derated gradients is increased to retain image resolution [this extends imaging time and echo time (TE)]. The experiment setup consisted of a custom‐built remote controller located in the interventional MR room. A fast spoiled gradient recalled echo (FSPGR) sequence was tested and acoustic recordings were made with a microphone in the magnet bore. An optical link sent data from the remote controller to a computer that was connected to a 1.5‐T cardiovascular MR scanner over ethernet. A software server installed on the MR system received the requests and passed these on to the integrated pulse generator in order to control the real‐time imaging. Slew rate adjustments were accomplished by calculating instruction period changes and adjusted waveform amplitudes accordingly. The experimental setup reduced the slew rate up to 16‐fold, which resulted in a considerable acoustic noise reduction of 21.1 dBA. The remote controller performed well and image quality showed no substantial qualitative changes. We made use of the previously described idea of silent pulse sequence designing. Although derating is at the expense of the image refresh rate, our real‐time slew rate reduction is compatible with the fact that iMRI procedures do not continuously require fast image update rates. © 2004 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 20B: 34–39, 2004.

Image parameters for driving with indirect viewing systems

Indirect viewing systems such as tv cameras can potentially support drivers under low visibility conditions or when the driver's field of view (FOV) is restricted. In three experiments, we identified the critical image parameters of such systems on vehicle control. We used a taskbattery that measured lateral and longitudinal vehicle control in both simulated and real world driving. Important parameters are magnification factor (compared to unity, a magnification of 0.5 leads to a lower course stability and overestimation of speed and distance) and FOV (increasing the FOV from 50° to 100° improves performance in lateral control tasks). However, the positive effects of a doubled FOV cannot outweigh the negative effects of magnification 0.5, when both factors are confounded. Less critical is the image resolution (lowering the image resolution leads to distance overestimation and degrasded longitudinal control) and image update rate (rates below 5 – 10 Hz decrease lateral control). Camera viewpoint (i.e., the location of the camera) is not critical for vehicle control. Overall, we can conclude that vehicle control with an indirect viewing system is proficient when the image parameters are adequately chosen. This supports the further development of these kind of driver support systems.

Automatische Schichtverfolgung in der interventionellen Magnetresonanztomographie

Magnetic resonance imaging (MRI) is ideally suited to monitor minimally invasive operations with catheters or needles, since it offers both a superior soft-tissue contrast and the possibility to perform functional tests. In the present study, small radio-frequency coils were attached to the instruments in order to localize the MR-invisible instruments. The implementation of active instrument tracking is described on the basis of the example of active catheter tracking. In this case, the current position information of the instrument is used to automatically position the MRI slice at the catheter location. In combination with a user interface, the interventional radiologist is offered the possibility to perform vascular interventions from within the MR scanner room. At image update rates of approximately 3 Hz, tracking and placement of catheters in vascular structures are possible with interactive switching of slice orientation and image contrast. In an animal model, the technique was successfully used to selectively visualize the abdominal vessels and their branches under MRI guidance.

Simultaneous real-time visualization of the catheter tip and vascular anatomy for MR-guided PTA of iliac arteries in an animal model.

To examine the feasibility of simultaneous MR real-time active tip tracking and near real-time depiction of the vascular anatomy for percutaneous angioplasty of iliac arteries under MR guidance. Nine surgically created stenoses of external iliac arteries in pigs were dilated with MR-compatible balloon catheters (Cordis, Roden, The Netherlands). These catheters were equipped with a microcoil for active tracking of the catheter tip with an in-plane update rate of 10 positions per second. The procedures were performed on an interventional 1.5 T Gyroscan ACS-NT scanner (Philips, Best, The Netherlands). Real-time calculation of images acquired by radial k-space filling was performed on a specially designed backprojector exploiting the sliding window reconstruction technique (Philips Research Laboratories, Hamburg, Germany). The image update rate was 20 frames per second using a radial gradient-echo technique (TR = 12 msec, TE = 3.3 msec, 300 radials). MR angiography and X-ray digital subtraction angiography on the X-ray system positioned in-line next to the interventional MR system served as control for the angioplasty results. Real-time guidance and positioning of the balloon catheters was possible. The actual position of the catheter tip was indicated in the MR images without any time delay for the reconstruction of the anatomical MR images, which were updated with a rate of 20 frames per second. This yielded a combination of a roadmap and fluoroscopy image, in which the catheter position and the anatomical background image both were continuously updated in real time. Six out of nine stenoses were successfully dilated. The effects of the angioplasty could be visualized by the real-time MR technique, as was proven by X-ray digital subtraction angiography. Active tip tracking simultaneous with MRI of the vascular anatomy-both in real time-is possible with the applied technique, enabling MR-guided percutaneous dilatation (PTA) of iliac arteries.

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.

Image quality issues for an enhanced vision head up display

A part task simulation study was performed to determine the ability of pilots to land an aircraft using head up display (HUD) guidance symbology overlaying an emulated millimeter wave imagery. The task was to land in Category IIIa weather at a Category I facility. Three image parameters were varied: image update rate, image processing latency, and the luminance contrast ratio of the runway image to the background noise. ILS beam bending that was representative of a Category I facility was randomly varied across the experimental runs. Nine pilots completed the test matrix. The only variable that made a significant difference was the runway to background contrast ratio. >