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K F King

Publications and source records attributed to K F King.

18 recordsLinked to original sources
PubMed2000

Spiral scan peripheral nerve stimulation.

Time-varying magnetic fields induce electric fields that can cause physiological stimulation. Stimulation has been empirically characterized as a function of dB/dt and duration based on experiments using trapezoidal and sinusoidal gradient waveforms with constant ramp time, amplitude, and direction. For two-dimensional (2D) spiral scans, the readout gradient waveforms are frequency- and amplitude-modulated sinusoids on two orthogonal axes in quadrature. The readout gradient waveform therefore rotates with amplitude and angular velocity that are generally not constant. It does not automatically follow that spiral stimulation thresholds can be predicted using available stimulation models. We scanned 18 normal volunteers with a 2D spiral scan and measured global thresholds for axial, sagittal, and coronal planes. We concluded that the stimulation model evaluated accurately predicts slew rate-limited spiral mean stimulation thresholds, if the effective ramp time is chosen to be the half-period at the end of the spiral readout.

Electric Stimulation↗
PubMed2000

Investigation of a solid-state detector for advanced computed tomography.

Utilization of solid-state detectors for computed tomography (CT) has been the focus of many studies. Previous phantom and clinical experiments have shown that one of the important performance parameters for the solid-state detector is the primary speed and afterglow. In this paper, we present a detailed investigation on the signal decay characteristics of the HiLight (GE Medical Systems, Milwaukee, WI) scintillating detector. The detector primary speed and afterglow are modeled by a multiexponential function and fully characterized by a set of time constants and relative strengths. The sensitivity of these parameters to X-ray photon energy, detector aging, and radiation exposure is then established and analyzed. No statistically significant variation is observed in these parameters due to changes in the above external variables. The impact of various decay time constants on CT image quality, such as spatial resolution, noise, and artifacts, is subsequently illustrated with computer simulations and phantom experiments. Finally, an algorithmic correction scheme is derived to compensate for detector afterglow. The correction scheme employs a recursive filter to remove adverse effects of the detector decay on image quality. Experimental results have shown that the correction scheme successfully restores system spatial resolution, produces a more homogeneous noise pattern, and eliminates ring-band image artifacts due to detector afterglow. The effectiveness and robustness of the correction scheme are demonstrated by extensive phantom and clinical experiments.

Artifacts↗
PubMed1999

Concomitant gradient field effects in spiral scans.

Maxwell's equations imply that imaging gradients are accompanied by higher order spatially varying fields (concomitant fields) that can cause artifacts in MR imaging. The lowest order concomitant fields depend quadratically on the imaging gradient amplitude and inversely on the static field strength. Time-varying concomitant fields that accompany the readout gradients of spiral scans cause unwanted phase accumulation during the readout, resulting in spatially dependent blurring. Concomitant field phase errors are independent of echo time and, therefore, cannot be detected using Dixon-type field map measurements that are normally used to deblur spiral scan images. Data acquisition methods that reduce concomitant field blurring increase off-resonant spin blurring, and vice versa. Blurring caused by concomitant fields can be removed by variations of image reconstruction methods developed to correct for spatially dependent resonance offsets with nonrectangular k-space trajectories.

Artifacts↗
PubMed1999

A computationally efficient method for tracking reference position displacements for motion compensation in magnetic resonance imaging.

A fast and computationally efficient method for detecting and tracking the displacement of a reference structure within the body using MR imaging is described. This method is used to determine the position of the diaphragm in order to synchronize the data acquisition to the same relative position of the abdominal and thoracic organs, thereby minimizing or eliminating respiratory motion artifacts. The method described uses the time domain linear phase shift of a reference structure to determine its spatial positional displacement as a function of the respiratory cycle. The signal from a two-dimensional rectangular excitation column is first Fourier-transformed to the image domain, apodized, and then transformed back to the time domain. The relative displacement of a target edge in the image domain is determined from an autocorrelation of the resulting time domain information. This technique was found to require between three and eight times less computation than either cross-correlation or least-squares analysis, depending on the navigator parameters. Magn Reson Med 42:548-553, 1999.

Algorithms↗
PubMed1998

Concomitant gradient terms in phase contrast MR: analysis and correction.

Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.

Artifacts↗
PubMed1998

Spiral scanning with anisotropic field of view.

Spiral scanning has been used to achieve much shorter scan times than conventional techniques for a wide range of applications. The major drawback with spiral scans is blurring from off-resonant spins, which is proportional to the readout time. Blurring limits maximal spatial resolution and minimal scan time potentially achievable with spiral scanning. Anisotropic field of view is used in conventional scanning to improve image quality by matching k-space trajectory to object characteristics. Anisotropic field of view improves spatial resolution in spiral scanning without increasing scan time or blurring. The resolution improvement results from increased maximal k-space radius allowed by the lower field of view. A field of view reduction by a factor of 2 in one direction provides up to 60% resolution improvement in that direction. Reduced SNR also results from non-uniform k-space sampling.

Algorithms↗
PubMed1995

Optimized gradient waveforms for spiral scanning.

Spiral scanning gradient waveforms can be optimized with respect to blurring from off-resonance effects by minimizing the readout time. This is achieved by maximizing the gradient amplitude during the scan so that the edge of k-space is reached as quickly as possible. Gradient hardware constraints are incorporated by considering a circuit model for the gradient coil and amplifier. The optimized gradient waveforms are determined by a set of coupled differential equations. The resulting solutions have shorter readout time than solutions that do not consider the circuit model.

Acceleration↗
PubMed1981

Closed intramedullary nailing of femoral shaft fractures. A review of one hundred and twelve cases treated by the Küntscher technique.

Since May 1972, the standard treatment of femoral shaft fractures at the Western General Hospital has been the closed femoral nailing technique of Küntscher. We have found that the use of intraoperative skeletal traction eliminates the need for immediate operation, preoperative skeletal traction, or the use of a distraction apparatus to prevent preoperative shortening. By the use of a cross-pinning technique, the closed femoral nailing method has been extended to include severely comminuted fractures of the femoral shaft and fractures of the distal third of the femur, with effective control of shortening and rotatory deformity. This allows early mobilization and discharge from the hospital for patients with these difficult fractures. One hundred and twelve consecutive traumatic fractures of the femoral shaft treated in this manner united within three to six months. The clinical results in terms of early joint movement, early weight-bearing, and rapid discharge from the hospital have been excellent.

Bone Nails↗
PubMed1978

A recent development in the treatment of severe compound fractures.

The Wagner leg lengthening device has recently been used successfully as an external fixation device in the stabilization of severe compound fractures. Fixation is rigid, yet adjustable, it does not interfere with the fracture site, and it allows clear access to wounds.

Fracture Fixation↗
PubMed1977

Back injury.

Explore the source record for details and available documents.

Back Pain↗
PubMed1976

Periosteal pedicle grafting in dogs.

An experimental model was designed to find a theoretical solution to the clinical problem of congenital tibial pseudarthrosis in children. The osteogenic properties of periosteum having been accepted, paired pedicled flaps of periosteum were raised from the subcutaneous surface of the tibia, tubed and anastomosed in a series of young dogs. In the last twenty-four consecutive cases the rate of complete ossification in these tubes rose to one in three. In the later stages of the work, in addition to the construction of periosteal tube grafts, pseudarthroses were induced in the middle third of each canine tibia in order to see if hypertrophy would occur under the stress of weight-bearing. It was found that in each of twenty-four specimens some hypertrophy occurred in the ossified periosteal tube grafts, and in three of the specimens this hypertrophy was associated with complete ossification of the graft that by-passed the pseudarthrosis.

Animals↗
PubMed1975

Controlled drug release from polymeric delivery devices. III: In vitro-in vivo correlation for intravaginal release of ethynodiol diacetate from silicone devices in rabbits.

Forty female rabbits were implanted with silicone vaginal devices containing ethynodiol diacetate for up to 8 weeks. As predicted from in vitro studies, a Q - t1/2 (matrix-controlled) release profile was observed in vivo. The in vivo drug release profile was compared with in vitro data measured at three hydrodynamic conditions, and the diffusional resistance across the vaginal wall was estimated. Drug released from silicone devices yielded a prolonged plasma level when compared with data following intravaginal or intravenous administration of a solution dose. The rate constant for elimination was unchanged. The plasma concentration of the drug was related to the intravaginal drug release profile both theoretically and experimentally and was above the concentration required to inhibit fertilization.

Animals↗
PubMedDate not supplied

A unified description of NMR imaging, data-collection strategies, and reconstruction.

In nuclear magnetic resonance (NMR) imaging by the zeugmatographic methods, there is a common and unified theoretical description. All forms of two-dimensional and three-dimensional imaging involve NMR data which trace various geometric representations, in reciprocal transform space, of the subject's spatially blurred "effective" spin density. The effective density is proportional to the physical density modulated spatially by the several factors of receiver coil (B/I) ratio, rf pulse excitation terms, T1-relaxation terms, and T2-relaxation terms. These factors depend upon the rf pulse sequence and field-gradient modulation sequence,and they may be calculated according to some model or directly measured. From this viewpoint, all different imaging modes appear as variations in data-collection and image-reconstruction strategies. The results are used here to describe slice-oriented polar and Cartesian strategies, three-dimensional Cartesian and two forms of spherical strategies, and multiecho strategies of the "planar-echo" type.

Image Enhancement↗
PubMedDate not supplied

Computed tomography scanning with simultaneous patient translation.

This paper deals with methods of reducing the total time required to acquire the projection data for a set of contiguous computed tomography (CT) images. Normally during the acquisition of a set of slices, the patient is held stationary during data collection and translated to the next axial location during an interscan delay. We demonstrate using computer simulations and scans of volunteers on a modified scanner how acceptable image quality is achieved if the patient translation time is overlapped with data acquisition. If the concurrent patient translation is ignored, structured artifacts significantly degrade resulting reconstructions. We present a number of weighting schemes for use with the conventional convolution/backprojection algorithm to reduce the structured artifacts through the use of projection modulation using the data from individual and multiple slices. We compare the methods with respect to structured artifacts, noise, resolution and to patient motion. Review of preliminary results by a panel of radiologists indicates that the residual image degradation is tolerable for selected applications when it is critical to acquire more slices in a patient breathing cycle than is possible with conventional scanning.

Algorithms↗