Abstract: An NMR system produces an angiogram using the complex-difference method. Artifacts caused by system phase errors are reduced by calculating a corrected phase angle using the acquired NMR data sets and using the corrected phase angle in calculating the complex difference in the corresponding elements of the image domain NMR data sets.

Abstract: Motion and deformation is assessed and imaged using phase contrast magnetic resonance imaging for determining velocity of a plurality of regions at a plurality of frames during a period of interest such as the cardiac cycle. A region of interest is tracked from frame to frame and based on a known last position and the velocity at the known position. Strain and rotation are determined from the changes in velocity along selected axes.

Abstract: An NMR system measures the velocity of flowing spins in the presence of stationary spins by performing three separate measurement cycles. The first cycle is a reference, the second cycle includes a magnetic field gradient having an incremental, flow sensitizing first moment .DELTA.M.sub.1, and a third measurement cycle includes a magnetic field gradient having an incremental flow sensitizing first moment -.DELTA.M.sub.1. The resulting NMR signals are processed to produce a velocity measurement.

Abstract: In an NMR system a method for measuring the motion of spins moving in an unknown direction includes four separate measurement cycles. The first cycle establishes a reference phase and each of the subsequent three measurement cycles includes motion encoding field gradients for measuring motion along respective pairs of Cartesian coordinates. All four measurements are combined to calculate each of the motion components along x, y and z coordinates.

Abstract: An apparatus for reducing image artifacts in NMR imaging matches elements of a set to values of a substantially periodic function so that the values exhibit a predetermined relationship to the elements. The matching is performed by evaluating the relative probability of the values of the substantially periodic function from the samples in the growing database of the values and assigning the values to the elements by using the evaluated relative probability, so as to maximize the probability that subsequent valves may be assigned the remaining elements according the predetermined relationship. A method for correcting the matching of values with elements freezes the database and acquires additional values which are substituted for earlier matched values based on the frozen database.

Abstract: A method and apparatus for improved digital processing of the analog echo signals in a coherent imaging system is described which simplifies the channel circuitry requirements. The analog echo signals detected with a phased array of transducer elements are first compressed in a non-linear manner then expanded non-linearly with analog-to-digital converter means to provide increased instantaneous dynamic range in the overall system. Representative phased array coherent imaging systems having the improved digital processing means are also disclosed.

Abstract: A method of reducing image artifacts in tomographic, projection imaging systems due to periodic motion of the object being imaged includes the acquisition of a signal indicative of the periodic motion. This signal is used to identify a quiescent period in the periodic motion so that the acquisition of projection data may be coordinated to be centered within the quiescent period. The sequence of the angles at which projection data is acquired is controlled so that the primary axis of periodic motion is perpendicular to the axis of projection at the center of the quiescent period. Multiple periodic signals may be coordinated by awaiting their best alignment over a fixed number of cycles of one of the periodic motions.

Abstract: A receiver processes an NMR signal to produce a baseband image information signal from which two quadrature component signals are derived. An intermediate frequency section mixes the received NMR signal with two reference signals to shift the image information into a frequency band having a bandwidth BW and centered at a frequency that is 1.5 times the bandwidth BW. The resultant signal is filtered to remove extraneous signals outside the image information band. An analog to digital converter samples the filtered signal at a rate that is twice the bandwidth BW and digitizes the samples into a digital signal. A quadrature detector derives I and Q output signals from the digital signal by alternately selecting digital samples and negating every other sample selected for each of the I and Q output signals. The quadature detector also digitally filters the I and Q signals which are then used to construct an NMR image.

Abstract: Method and apparatus for minimizing scatter in an X-ray imaging system of the type including a controllable source of X-ray radiation and means for collimating the X-ray radiation into a fan-shaped beam and effecting scanning of the X-ray beam across a target area. The improved system includes a grid assembly extending in a transverse direction across the target area and having a relatively narrow width in the beam scanning direction. A drive is coupled to the grid removing the grid in synchronism with the X-ray beam and an X-ray impervious belt is coupled to the grid and extends in a transverse direction across the target area and is movable with the grid for covering the target area outside the grid to prevent scatter radiation from impinging on X-ray film located beneath the grid. In one form, the grid includes a plurality of air interspersed septa to allow reduced beam energy.

Abstract: The transceiver of an NRM instrument includes a digital synthesizer that produces an analog carrier signal which can be controlled in both frequency and phase and which can be frequency and phase modulated. The analog carrier signal is mixed with reference signals to boost its frequency to the Larmor frequency of the instrument.

Abstract: A multiecho NMR pulse sequence is modified such that the bandwidth of the encoded NMR echo signal is reduced for the second NMR echo signal. This reduction is achieved by reducing the magnitude of the read-out magnetic field gradient, increasing the duration of the acquisition by a proportional amount and decreasing the sample rate by a proportional amount for the second NMR echo signal.

Abstract: A method is provided for characterizing the spurious, time dependent magnetic field gradient response and for compensating therefor. In accordance with the method, two small sample objects are placed at two positions with respect to the system isocenter where the gradient to be compensated has different values. A magnetic field gradient pulse is applied to the sample, followed by the application after a variable time of a 90.degree. radio frequency pulse. The phase of the resulting free induction decay (FID) signal is monitored as a function of total time since the end of the gradient pulse. These data are related to the impulse response of the gradient and are employed to construct compensation filters for both the gradient coils and the polarizing field shim coil.

Abstract: A pulse sequence of an NMR system is disclosed which produces two spin echo signals which may be acquired. In addition, one or more gradient echos are produced and acquired. The gradient echo data is combined with the spin echo data to improve signal to noise ratio, provide T.sub.2 ' mapping, reduce motion induced artifacts, measure and correct for RF pulse imperfections, and to provide improved T.sub.2 measurement.

Abstract: A method for reducing image artifacts due to signal variations in the course of examining a subject using nuclear magnetic resonance (NMR) techniques includes the acquisition of NMR data for imaging the object. The NMR data is composed of a number of views. The acquisition of each view includes the implementation of a pulse sequence to generate an NMR signal and application of a magnetic gradient along at least one dimensional axis of the object. A gating technique is used to define a window during a portion of each respiratory cycle and NMR data is acquired only during each window. A view ordering technique is employed during the data acquisition by altering the magnitude of a field gradient in a nonmonotonic manner.

Abstract: In a system for reducing artifacts in a desired image due to substantially periodic variations in the imaging signal detected from a portion of an object under examination, a method and apparatus for generating from a signal y(t) related to the variations and whose values are not equally likely, a signal .phi.(t) whose values are substantially equally likely for use in distributing events substantially evenly over the variations. In order to provide the signal .phi.(t), an apparatus is disclosed for executing the following steps: (a) establishing a historical data base derived from values of the signal y(t) for a period of time preceding the present time, (b) measuring parameters related to the present value of the signal y(t); and (c) producing a value for .phi.(t) using the present measured parameters and the historical data base; wherein the values of .phi.(t) are more evenly distributed than the values of y(t).

Abstract: An NMR method for producing a sequence of images throughout the cardiac cycle is disclosed. A fast scan NMR pulse sequence is continuously and asynchronously applied during successive cardiac cycles, and a phase encoding magnetic field gradient is incremented between cardiac cycles to acquire NMR data which is reconstructed using a 2DFT technique. The phase encoding magnetic field gradient is stepped in subincrements, and the odd and even phase encoding views are acquired in separate segments of the procedure in order to eliminate image distortion due to disturbance of the residual transverse magnetization.

Abstract: A method for reducing image artifacts due to periodic motion of a subject undergoing examination by nuclear magnetic resoance (NMR) techniques includes the acquisition of scan data for imaging the object. The scan data is composed of a number of views. The acquisition of each view includes the generation of an NMR signal and the application of a magnetic gradient along at least one dimensional axis of the object. The magnetic field gradient is characterized by a parameter value (e.g., amplitude or direction) adjustable from view to view to encode spatial information into the NMR signal. The method includes the step of selecting the value of the adjustable parameter value for each view. A view order is then selected so as to attempt to obtain a predetermined relationship between the object motion and the adjustable parameter of the magnetic field gradient. The relationship is chosen to minimize artifacts in the reconstructed image.

Abstract: A method is provided for characterizing the spurious, time-dependent magnetic field gradient response and for compensating therefor. In accordance with the method a small sample object is placed away from the system isocenter where the gradient to be compensated is non-zero. A magnetic field gradient pulse is applied to the sample, followed by the application after a variable time of a 90.degree. radio frequency pulse. The phase of the resulting free induction decay (FID) signal is monitored as a function of total time since the end of the gradient pulse. These data are related to the gradient impulse response and are fit to a set of (e.g., two or three) exponentials by regression techniques wherein an initial estimate is improved in one embodiment by chi-squared minimization of a Taylor's series expansion about the initial estimate point.

Abstract: A method for reversing residual transverse magnetization due to spatial encoding magnetic field gradient pulses, used in magnetic resonance imaging to encode spatial information, employs a reversing gradient pulse applied in the same direction as the encoding gradient pulse following the observation of the spin-echo signal. The encoding gradient pulse is applied following the 180.degree. RF pulse to avoid the effects of imperfections associated therewith. In one embodiment, the amplitudes of the encoding and reversing gradient pulses are selected to be approximately the negatives of each other so as to substantially cancel the residual magnetization. In another embodiment, the amplitude of the reversing gradient pulse is selected such that the algebraic sum thereof with the corresponding amplitude of the encoding gradient pulse is a constant. In this case, the residual magnetization is not necessarily cancelled, but rather, is left in the same state after each view of the pulse sequence.

Abstract: A method for reducing image artifacts due to signal variations in the course of examining a subject using nuclear magnetic resonance (NMR) techniques includes the acquisition of scan data for imaging the object. The scan data is composed of a number of views. The acquisition of each view includes the implementation of a pulse sequence to generate an NMR signal and application of a magnetic gradient along at least one dimensional axis of the object. The magnetic field gradient is characterized by a parameter (e.g., amplitude or direction) adjustable from view to view to encode spatial information into the NMR signal. The parameter value is selected just prior to implementation of the pulse sequence for that view and depends on the phase of the signal variation at that point in time. The final view order depends on the measured phase during the scan. In this manner, the view order can be continuously adjusted to obtain optimum artifact reduction.