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Biomedical subjects

S J Riederer

Publications and source records attributed to S J Riederer.

At least 91 records · Page 5Linked to original sources
PubMed1994

Initial clinical experience in MR imaging of the brain with a fast fluid-attenuated inversion-recovery pulse sequence.

PURPOSE: To evaluate fast fluid-attenuated inversion-recovery (FLAIR) technique for imaging brain abnormalities. MATERIALS AND METHODS: A fast FLAIR sequence was developed that provided 36 5-mm contiguous sections in 5 minutes 8 seconds. Resulting images were compared with dual-echo T2-weighted spin-echo images of 41 consecutive patients with brain abnormalities. RESULTS: Contrast and contrast-to-noise ratios (C/Ns) (for contrast between the lesion and background and between the lesion and cerebrospinal fluid) for fast FLAIR exceeded the corresponding values for T2-weighted spin-echo images for all but the second-echo lesion-to-background C/N. Fast FLAIR provided equivalent or greater overall lesion conspicuity and enabled greater lesion detection in 98% and 100%, respectively, of the evaluations. Fast FLAIR images more often had image artifact, but this did not interfere with image interpretation in a significantly (P < or = .05) greater number of evaluations. CONCLUSION: Fast FLAIR provides images that are superior to proton-density- and T2-weighted images for many image quality criteria.

Artifacts↗
PubMed1993

Phase velocity mapping with a real time line scan technique.

A real-time, 20-Hz, one-dimensional MR velocity imaging technique is described. A two-dimensional RF pulse excites a 3-cm diameter column. Velocity maps are formed from the phase difference between successive flow encoded and compensated acquisitions. A three-point subtraction variation provides reduced sensitivity to static spins.

Magnetic Resonance Imaging↗
PubMed1993

Analysis of systematic and random error in MR volumetric flow measurements.

The spatial aspects of error in 2D MR cine phase-velocity mapping are considered in order to define acquisition strategies which will minimize error in measuring volumetric flow. Error was separated into two categories: systematic and random. Potential sources of systematic error examined were intravoxel phase dispersion (IVPD), partial volume effects, misalignment of flow axis and flow-encoding gradients, and improper choice of vessel voxels for flux calculations. Random error was addressed using analysis of propagation of variance. Analytical expressions for sources of error were derived; and computer models were used to test the analytical models. Flow phantom studies examining error in MR volumetric flow measurements were performed and compared with error predicted by the analytical models. Expected error in several clinical situations of interest was then derived to find appropriate acquisition strategies. Spatial resolution, signal to noise ratio, velocity sensitivity and the ratio of the modulus of moving isochromats to that of static isochromats were found to be the most important parameters in controlling error and were found to cause competing effects with respect to systematic and random error.

Artifacts↗
PubMed1993

Magnetization-prepared cardiac imaging using gradient echo acquisition.

Magnetization-prepared 2DFT and 3DFT gradient echo techniques are presented for acquisition of black blood cardiac images. Both methods incorporate RF pulses for blood saturation, centric or centric-like view ordering, data acquisition during late diastole, and breath-holding. In a series of 13 volunteer studies, these methods have consistently provided good visualization of intraventricular structures.

Heart↗
PubMed1993

A monitoring, feedback, and triggering system for reproducible breath-hold MR imaging.

A technique is described that provides improved reproducibility of breath-holding for MR image acquisition by monitoring the superior-inferior (S/I) position of the diaphragm. The method incorporates detection of the level of inspiration using an MR signal, rapid display to the patient of diaphragm position to enable breath-hold adjustment, and triggering of image data acquisition once appropriate position is attained. The response time of the system is short, approximately 10 ms. Studies in six volunteers using this method demonstrate a considerable decrease in the S/I range of diaphragm position over 10 consecutive periods of suspended respiration. The mean range is 1.3 mm with the system, while it is 8.3 mm without using it. It is expected that this method will be of assistance in many abdominal and cardiothoracic studies that use breath-hold techniques.

Feedback↗
PubMed1993

Carotid artery: prospective blinded comparison of two-dimensional time-of-flight MR angiography with conventional angiography and duplex US.

A prospective blinded comparison of two-dimensional (2D) time-of-flight (TF) magnetic resonance (MR) angiography and color duplex flow ultrasound (US) with conventional angiography as a standard of reference was performed in 50 patients with hemispheric ischemic symptoms. The guidelines of the North American Symptomatic Carotid Endarterectomy Trial for measuring stenosis of the internal carotid artery were utilized for maximum-intensity-projection (MIP) images and conventional angiograms. While the 2D TF MIP technique overrepresented the degree of stenosis in the internal carotid arteries, it matched the performance of color duplex flow US as measured by means of receiver operating characteristic curves. A signal void on the MIP images corresponded to a 70% or greater internal carotid artery stenosis in 17 of 20 arteries. At its current state of development, the accuracy of 2D TF MR angiography equals that of US in characterizing the degree of carotid stenosis but cannot be considered a replacement for conventional angiography.

Adult↗
PubMed1993

Echo-planar imaging of the liver with a standard MR imaging system.

PURPOSE: A multisection, whole-body echo-planar imaging (EPI) sequence was developed to obtain T2-weighted images of the liver in one 18-second breath hold with a standard magnetic resonance (MR) imaging system. MATERIALS AND METHODS: This capability was achieved by dividing the data acquisition period into eight interleaved segments rather than one or two as implemented previously with EPI systems having high-power gradient subsystems. RESULTS: The interleaved echo-planar images had excellent depiction of anatomy and no identifiable respiratory artifact. In 26 lesions in 12 patients, the eight-shot echo-planar images (2,000/66 [repetition time msec/echo time msec]) had superior contrast compared with conventional T2-weighted spin-echo (SE) images (2,500/60) by an average factor of 1.22 +/- 0.31 (standard deviation) and an average contrast-to-noise ratio relative to conventional T2-weighted SE images of 0.85 +/- 0.22. CONCLUSION: With a conventional MR imaging system, breath-hold T2-weighted echo-planar images of the liver are comparable in diagnostic quality to conventional T2-weighted SE images.

Adenoma, Islet Cell↗
PubMed1993

Magnetic resonance imaging of pelvic floor relaxation: dynamic analysis and evaluation of patients before and after surgical repair.

OBJECTIVE: To evaluate structures involved in pelvic support using conventional and snapshot magnetic resonance imaging (MRI). METHODS: We used conventional spin-echo MRI and dynamic snapshot GRASS MRI at various levels of the Valsalva maneuver to describe and quantitate the anatomy of pelvic floor relaxation and to assess anatomical changes produced by surgical repair. Ten female volunteers were evaluated to define normal anatomy and reference measurements. Five women with pelvic floor relaxation were evaluated before and after surgical repair. RESULTS: Static and dynamic MRI were more sensitive than clinical pelvic examination in assessing and grading pelvic floor relaxation. Quantitative results showed widening of the levator hiatus and more vertical lie of the levator plate postoperatively. Descent of the pelvic organs on maximal straining postoperatively was the same as that in normal volunteers. The posterior urethrovesical angle on MRI was more than 110 degrees in 14 of 15 continent subjects. CONCLUSIONS: Magnetic resonance imaging may be valuable in analyzing and assessing pelvic floor relaxation and in understanding anatomical changes occurring before and after surgical repair. The increased sensitivity of MRI in grading prolapse may make it useful in evaluating women with symptoms of pelvic floor relaxation but who have negative findings on clinical examination.

Adult↗
PubMed1992

Contrast-to-noise ratios in maximum intensity projection images.

We present a statistical analysis of the maximum intensity projection (MIP) algorithm, which is commonly used for MR angiography (MRA). The analysis explains why MIP projection images display as much as a twofold increase in signal- and contrast-to-noise ratios over those of the source image set. This behavior is demonstrated with simulations and in phantom and MRA image sets.

Algorithms↗
PubMed1992

Respiratory kinematics of the upper abdominal organs: a quantitative study.

Despite the fact that respiratory motion is a major factor limiting the image quality of MR examinations in the upper abdomen, little quantitative information is available about the kinematics of visceral motion during respiration. The objective of this study was to obtain a measure of the relative longitudinal and transverse displacements of the upper abdominal organs during breathing using an MR line scan technique.

Abdomen↗
PubMed1992

Improved efficiency in 2DFT magnetization-prepared rapid gradient echo imaging: application to abdominal imaging.

The incorporation of the phase offset multi planar (POMP) technique into breathheld magnetization-prepared gradient echo imaging is discussed as a means for improving imaging efficiency without sacrificing resolution, contrast, or SNR improvement. The phase encoding order necessary to preserve the centric approach is described. When combined with interleaving, the POMP technique enables four 256 x 256 images to be acquired in a 12-s breathhold, doubling the efficiency of the original technique. This scan efficiency is compared with that of other T1-weighted 2DFT methods.

Abdomen↗
PubMed1992

Rapid T1 estimation using tagged magnetization-prepared gradient-echo MR imaging.

A technique for estimation of the longitudinal relaxation time of a large homogeneous object with an acquisition time of 4 s or less was developed by combining spatially selective rf tagging pulses with a T1-weighted magnetization-prepared gradient-echo sequence. Multiple 5-mm-wide tagged areas are laid orthogonal to the imaging section of interest. The contrast between each tag and the untagged regions differs because each tag is produced at a different time. The T1 value is determined from the nulling time at which tagged and untagged areas have no contrast.

Adipose Tissue↗
PubMed1992

Blood flow imaging through detection of temporal variations in magnetization.

Two hypotheses were tested: (a) that view-to-view variations in bulk phase and modulus of magnetization in vascular volume elements can indicate the presence of disordered blood flow, and (b) that a substantial loss of signal intensity on magnetic resonance (MR) angiograms of poststenotic regions is due to view-to-view changes in magnetization. To test these hypotheses, a technique was developed in which view-to-view variations in transverse magnetization were used to create angiographic projection images, which showed only disordered flow (disordered flow maps) in vitro and in vivo. In phantom studies, this technique recovered signal intensity downstream from stenoses. A combination of disordered flow maps with morphologic images improved visualization of stenotic regions and provided information on characteristics of local flow. These results show that view-to-view variations in transverse magnetization occur in regions of disordered flow and are an important cause of loss of signal intensity. This technique can provide information about dynamic blood flow and improve depiction of anatomic structures on MR angiograms.

Blood Circulation↗
PubMed1991

Adaptive motion compensation in MRI: accuracy of motion measurement.

It has been shown that magnetic resonance image data can be corrected for the effects of motion by using retrospective adaptive techniques which employ navigator (NAV) echoes. We demonstrate the accuracy with which NAV echoes can measure motion, as well as the independent nature of the respective view-to-view and intraview corrections.

Brain↗
PubMed1991

Adaptive motion compensation in MR imaging without use of navigator echoes.

Retrospective correction of magnetic resonance (MR) image data to eliminate the effects of patient motion is possible with use of adaptive correction techniques. These methods require an accurate record of the motion that occurs during imaging. The authors evaluated whether motion information suitable for adaptive correction could be obtained from phase-encoded image data alone rather than from separate navigator echoes. Once such displacements were estimated from the image data, motion correction proceeded with use of the same algorithm used for the navigator echoes. The results show that image data alone can be used to effectively measure view-to-view displacements in phantoms, but external markers are required for accurate measurement during axial head imaging of patients.

Humans↗
PubMed1991

T2-weighted spin-echo pulse sequence with variable repetition and echo times for reduction of MR image acquisition time.

Use of intraacquisition modification of pulse-sequence parameters to reduce acquisition time for conventional T2-weighted spin-echo images was evaluated. With this technique (variable-rate spin-echo pulse sequence), the repetition time and echo time (TR msec/TE msec) were reduced during imaging as a function of the phase-encoding view. To maintain T2-based contrast, TR and TE for the low-spatial-frequency views were left at their prescribed values (eg, 2,000/80). TR and TE for the high-spatial-frequency views were progressively reduced during imaging (eg, to 1,000/20). Acquisition time was reduced by as much as 25%. In one pulse sequence, the duration of multisection imaging nominally performed at TR 2,000 and with 256 phase-encoding views was reduced from 9 minutes 30 seconds to 6 minutes 30 seconds. In all sequences, edges and small structures were enhanced, and T2 contrast was somewhat decreased in high spatial frequencies. Filtering of the raw data before reconstruction can suppress these effects and provide a net increase in contrast-to-noise ratio.

Brain↗
PubMed1991

T1-weighted snapshot gradient-echo MR imaging of the abdomen.

Magnetization-prepared ultrashort-repetition-time (snapshot) gradient-echo imaging is a technique of magnetic resonance (MR) imaging with many potential applications. In the application of this technique to abdominal imaging, the effects on contrast of phase-encoding order, resolution, preparation-phase inversion time, and data-acquisition flip angle were predicted and then demonstrated with images obtained in examinations of 22 patients. In the analysis of 36 liver lesions, snapshot images were compared with corresponding T1-weighted spin-echo images on the basis of signal-to-noise ratio (S/N) of liver and contrast-to-noise ratio (C/N) between liver and lesion. Snapshot MR imaging produced abdominal images with 192 (or 256) x 256 resolution, negligible motion artifact, and C/N 1.29 times (+/- 0.48) higher than that in T1-weighted spin-echo imaging. Acquisition times were 13 seconds or less, short enough for imaging during suspended respiration. Also, use of a phased-array multicoil further improves the S/N in snapshot images without acquisition-time penalty.

Abdomen↗