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S J Riederer

Publications and source records attributed to S J Riederer.

At least 109 records · Page 6Linked to original sources
PubMed1991

Real-time interactive color flow MR imaging.

Real-time interactive color flow magnetic resonance (MR) imaging is a combination of real-time MR imaging and color encoding of velocity-induced phase angle. Flow-compensated (FC) and flow-encoded (FE) images are acquired continuously by using gradient echoes and a 12-msec repetition time. Each image is reconstructed within 200 msec of acquisition, and the FC magnitude image is displayed in gray-scale format. The phase difference between the reconstructed FC and FE images, a difference proportional to velocity along the flow-encoding direction, is encoded in color and superimposed on the gray-scale FC image. Magnitude and phase information are thus presented simultaneously. The viewer may interactively adjust many acquisition parameters during data acquisition. Experimental results of phantom and in vivo human studies validate the method. Characteristics of the color flow MR imaging technique are compared with those of duplex color ultrasound.

Blood Flow Velocity↗
PubMed1990

CNR enhancement in the presence of multiple interfering processes using linear filters.

Given several images of the same slice, a linear filter can produce an image in which the contrast-to-noise ratio (CNR) between pathological and normal tissues is greater than in any of the initial images. To distinguish the pathology from more than one tissue, the filter should optimize the set of CNRs between the pathology and each of the interfering tissues. We define the optimal filter as the one which provides the largest value for the minimum CNR in the set and show how it is selected from a field of only four possibilities. The filter is demonstrated with both experimental phantom studies and clinical cases. Filter performance is compared with that of other techniques for distinguishing a desired feature from more than one interfering process.

Image Enhancement↗
PubMed1990

Analysis of T2 limitations and off-resonance effects on spatial resolution and artifacts in echo-planar imaging.

Several aspects of blipped echo-planar imaging (EPI) are treated mathematically. An expression relating the necessary readout gradient strength and sampling time to the spatial resolution and readout duration is derived. It is shown how the net spatial resolution may be limited by the object's T2 characteristics and B0 field homogeneity, irrespective of the number of sampled points. Additionally, off-resonance effects result in a loss of spatial resolution and image distortion to a considerably greater degree than in conventional two-dimensional Fourier transform imaging. The extent of these effects is directly related to the time required to acquire the data matrix, and is therefore amplified when EPI is implemented on a standard commercial whole-body system which because of limited gradient performance uses necessarily longer sampling durations. Specific hardware modifications to a standard commercial imager are considered to allow successful EPI implementation. EPI image characteristics are compared quantitatively with those of conventional methods.

Magnetic Resonance Imaging↗
PubMed1990

Real-time interactive magnetic resonance imaging.

We describe a system for performing interactive MRI in real time. Using a TR/TE 7.1/3.5 ms sequence, the operator may alter a scan parameter and observe the effects of the alteration on the image within a few hundred milliseconds. With this system, we can interactively control the oblique scan slice orientation and, using inversion pulses, the image contrast.

Fluoroscopy↗
PubMed1990

A spoiling sequence for suppression of residual transverse magnetization.

We present a spoiler sequence which substantially eliminates residual transverse magnetization in rapid magnetic resonance imaging. Such magnetization is due to the repetition time TR being on the order of or shorter than the transverse relaxation time T2. This sequence was designed using computer simulation methods to model the slice profile and signal. The spoiler pulse is applied in the slice-selecting direction and the magnitude of the gradient spoiler is incremented with each repetition as an arithmetic series. A finite number of different spoiler amplitudes is repeated with a cycle time several times greater than the anticipated T2. Each spoiler is shown to nullify the residual transverse magnetization from the just-measured signal of the experiment as well as that from preceding repetitions. It is shown experimentally that the spoiler pulse can be as short as 2 ms. This is considerably shorter than existing methods using constant spoiling or phase-tagged spoiling. This sequence was used to acquire T1-weighted images free of the central line artifact of FLASH imaging. Furthermore, images formed with the new spoiled technique are shown to have contrast-to-noise comparable if not superior to standard T1-weighted spin-echo images while taking only a fifth of the scan time.

Brain↗
PubMed1990

High-speed line scan MR angiography.

We describe a system which forms MR angiographic images at high speed. Multiple axial sections are imaged sequentially using a 2DFT GRASS sequence with TR/TE 50/15 ms, 64 phase encodings per image. Reconstruction and projection of each image are performed immediately (within 220 ms) after data for that image are acquired. The projection angiogram is constructed line by line as the scan progresses, thereby totally eliminating any additional time required for reconstruction and projection.

Blood Vessels↗
PubMed1990

The importance of phase-encoding order in ultra-short TR snapshot MR imaging.

We show the importance of phase-encoding order in ultra-short TR (less than 10 ms) snapshot MR imaging. Contrast is more heavily dictated by the contrast preparation phase early in the measurement phase than later. When the low spatial frequencies are measured first, the resultant image contrast more closely resembles that established by the preparation phase. The transient nature of the data acquisition causes a kappa-space filter to be applied to the data. The effects of this filter can be minimized by appropriate choice of scan parameters.

Filtration↗
PubMed1990

Rapid MR imaging of blood flow with a phase-sensitive, limited-flip-angle, gradient recalled pulse sequence: preliminary experience.

To assess blood flow rapidly, a limited-flip-angle, gradient recalled pulse sequence was modified to acquire two views at the same phase-encoding step in successive repetitions. One view is obtained with first-moment flow compensation, while the second view is obtained with selectable flow encoding (non-zero first moment) along one direction. Blood flowing along the encoded direction acquires a phase difference between the two views, resulting in signal dependent on both direction and speed of flow. Stationary tissues undergo no phase change. Therefore, the phase shift between the two views produces an image that spatially renders flow direction and velocity. With a 24-msec repetition time, a 256 X 128 matrix, and two excitations, data acquisition is completed in 13 seconds per location (both a magnitude image and a flow image are produced at each location). Images generated with flow phantoms confirmed the accuracy of this method. Preliminary clinical evidence in 23 human subjects suggests that this method is useful in evaluating portal hypertension, distinguishing arterial from venous flow, distinguishing between slow flow and clot, and confirming the presence of clot. This method appears to be a fast, easy way to assess blood flow in large vessels.

Adolescent↗
PubMed1990

MR angiography with oblique gradient-recalled echo technique.

Magnetic resonance (MR) angiography with use of an oblique gradient-recalled echo sequence is discussed. The technique was developed for the efficient acquisition of angiographic data when the desired projection direction is already known. The raw data set is acquired directly at the projection angle; thus, a high-resolution projection is created, despite a decrease in the number of phase-encoding views acquired per axial image. Excessive reduction in the number of views acquired causes a loss of contrast in the projection images rather than a loss of resolution. High-resolution, high-contrast MR projection angiograms can be obtained in 2 1/2 minutes, with 50 3-mm sections and 48 phase encodings per section, a repetition time of 50 msec, an echo time of 15 msec, and a flip angle of 45 degrees. This represents one-fifth the time required for a conventional angiographic image. Venous blood is successfully saturated when the saturation band is placed at a fixed distance from the current imaging section. The method is demonstrated in the imaging of the carotid bifurcation in healthy volunteers and of a patient who had undergone carotid endarterectomy.

Carotid Arteries↗
PubMed1990

Adaptive technique for three-dimensional MR imaging of moving structures.

The authors describe an adaptive motion correction method for three-dimensional magnetic resonance (MR) imaging. Three-dimensional imaging offers many advantages over two-dimensional multisection imaging but is susceptible to image corruption due to motion. Thus, it has been of limited use in the imaging of mobile structures, and the relatively long imaging times required have hindered its use in patients who tend to move during imaging. The authors' technique uses interleaved "navigator" echoes to provide a measure of displacement for each image echo in the acquisition and then uses this information to allow correction of the image data. The theory for signal corruption due to motion and the correction scheme that follows from it are presented. This method can produce excellent results when the motion is correctly modeled.

Humans↗
PubMed1989

Flow-compensated limited flip angle MR angiography.

A method of subtraction angiography that has an acquisition time of 8 s per slice is described. Flow-compensated and uncompensated measurements are acquired in an interleaved fashion using limited flip angles and gradient refocusing. Magnitude images are reconstructed and subtracted to generate the angiogram. Results were generated in vivo in the imaging of the carotid bifurcation of several human volunteers. Susceptibility and inhomogeneity induced artifacts are prominent in thick slices, but can be greatly reduced by imaging several thin slices and adding them together. Thin slices do not require dephasing gradients to reduce the dynamic range, and there is no signal cancellation in overlapping vessels. The method is ideal for acquiring scout angiograms, and with averaging may produce images of diagnostic quality.

Blood Flow Velocity↗
PubMed1989

Compensation for effects of linear motion in MR imaging.

Various compensation methods for different types of motion during MR image acquisition have been proposed. Presented here is a postprocessing scheme for eliminating artifacts due to linear, intra-slice motion of known velocity. The data for each phase encoding or "view" acquired from a moving object are shown to differ from those which would be measured from the stationary object by a phase factor which depends on the object's displacement from a reference point. This derivation is then used to propose a correction scheme for linear motion in which all phase encodings measured at different positions of the moving object are collapsed onto the same reference position. After subsequent reconstruction, the object appears perfectly "focused." By selection of different reference positions, the method permits positioning of the compensated object as desired within the field of view of the image. This property allows the method to be extended to create sequences of corrected images with smooth object motion between frames of the sequence. The basic correction scheme and its variations were tested experimentally in phantom studies with velocities as large as 8 cm/s.

Humans↗
PubMed1989

Real-time MR fluoroscopic data acquisition and image reconstruction.

We describe an experimental system for performing high-speed reconstruction of MR image data acquired with a GRASS sequence. System characteristics are an image acquisition time of 627 ms, continuous image reconstruction at a rate of 6 images/s, and an image reconstruction time of 120 ms. The results is a system for performing MR imaging in real time.

Equipment Design↗
PubMed1989

MR fluoroscopy: initial clinical studies.

Magnetic resonance (MR) fluoroscopy is a method for high-speed MR image acquisition with the goals of short acquisition time per image (500 msec or less), high image rate (10 images or more per second), and high-speed image reconstruction (150 msec or less from data acquisition to image display). The authors present their results with the first two goals in volunteers. MR fluoroscopic image data were acquired with a limited flip angle pulse sequence with reduced repetition times (TRs) and fewer phase encodings used per image. The sequence was applied continuously, and images were formed by updating one set of data with data from the most recently taken measurements. Sample head images were generated with TR/echo times as small as 11/5.5 msec and 48 phase encodings for a total acquisition time of about 500 msec. Images were acquired while the volunteer flexed his head. Artifacts from the motion became less evident on images as progressively shorter acquisition times were used.

Brain↗
PubMed1988

MR fluoroscopy: technical feasibility.

A method of magnetic resonance image acquisition and reconstruction is described in which high imaging rates and fast reconstruction times are allowed. The acquisition is a modification of the basic FLASH sequence but with a restricted number N of phase encodings. The encodings are applied sequentially, periodically, and continuously. Images are formed by sliding a window of width N encodings along the acquired data and reconstructing an image for each position of the window. In general the acquisition time per image exceeds the time between successive images, and the method thus has a temporal lag. Experimental studies were performed with a dynamic phantom using 48 phase encodings and a TR of 20 ms, for an image acquisition time of about 1 s. The image display rate in the reconstructed sequence was 12.5 images/s, and the image sequence portrayed the motion of the phantom. Additional studies were done with 24 encodings. It is shown how the sliding window technique lends itself to high-speed reconstruction, with each newly acquired echo used to quickly update the image on display. The combination of the acquisition technique described and a hardware implementation of the reconstruction algorithm can result in realtime MR image acquisition and reconstruction.

Fluoroscopy↗
PubMed1988

Fast limited flip angle MR subtraction angiography.

A fast MR angiography method is introduced that is capable of generating difference images of blood vessels in scan times of 10-20 s. This is an order of magnitude faster than many previous methods. The fundamental concept of this approach is to use cardiac gating and acquire several phase encodings at least twice during each cardiac cycle using limited flip angles (LFAs) and repetition times in the 20 to 50 ms range. The encodings acquired during diastole are subtracted from those acquired during systole to generate the difference image. The contrast in the difference image is due both to the influx of unsaturated spins and to the loss of phase coherence of systolic blood moving at high velocity along a magnetic gradient. The systolic peak of the cardiac cycle is determined during reconstruction by shifting the systolic and diastolic "windows" until the difference signal is maximized. Ghost artifacts due to pulsatile flow are eliminated by a phase reordering technique similar in concept to those developed for suppression of breathing artifacts. Arteries in thick slices are successfully imaged and initial in vivo results are presented.

Angiography↗
PubMed1988

Efficient pulse sequence for multisection dual-repetition time MR image acquisition.

A magnetic resonance imaging pulse sequence was developed in which multisection spin-echo image data are simultaneously acquired for two repetition time (TR) intervals (TR1 and TR2) in one imaging sequence. In a conventional multisection image at a single TR, the number of sections is limited to TR/TS, where TS is the readout time. With this new sequence, the number of sections that can be imaged at both TRs in one acquisition is equal to (TR1 + TR2)/(TS1 + TS2), where TS1 and TS2 may be different for the two TRs. Imaging time is equal to that for a single image at a TR of TR1 + TR2. Clinical images were obtained with the new sequence from 15 patients and compared with images acquired at the same TR/TE by means of standard multisection single-TR methods. Relative image quality was assessed by three radiologists in 37 comparisons. In general, the dual-TR results at the long TR were judged equivalent to those from a single-TR image. Dual-TR results at the short TR had a modest reduction in contrast, but in none of 15 cases were any pathologic features missed.

Magnetic Resonance Imaging↗