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Biomedical subjects

S J Riederer

Publications and source records attributed to S J Riederer.

At least 73 records · Page 4Linked to original sources
PubMed1995

Dual-echo interleaved echo-planar imaging of the brain.

An interleaved echo-planar imaging (EPI) technique is described that provides images from 20 sections of the brain at two echo times (27 and 84 ms) in 1:05. Six echoes per image per repetition are collected in 24 repetitions of the pulse sequence. MR images of the brain obtained from five volunteers using the dual-echo EPI sequence, fast spin-echo (FSE), and conventional dual-echo spin-echo were evaluated qualitatively for diagnostic use and quantitatively for relative signal-to-noise ratio (SNR), contrast, and contrast-to-noise ratios (CNR).

Adipose Tissue↗
PubMed1995

Spatial-frequency-tuned markers and adaptive correction for rotational motion.

A common type of motion present in clinical magnetic resonance imaging examinations is rotational motion, such as that due to voluntary motion during head examinations. The correction scheme presented in this work offers a method for eliminating the effects of rotations within the imaging plane. Integral to the implementation of this technique is the concept and design of spatial-frequency-tuned markers, which are used to track the rotational motion. These studies showed that it is possible to accurately track the motion, measuring both axis and angle of rotation, and use this information to retrospectively correct the acquired images. These markers can also provide information about any translational motion present. The resulting images show a marked decrease in artifacts and improved clarity.

Algorithms↗
PubMed1995

Respiratory motion of the heart: kinematics and the implications for the spatial resolution in coronary imaging.

Respiratory motion is a major limiting factor in improving image resolution and signal-to-noise ratio in MR coronary imaging. In this work the effects of respiration on the cardiac position were studied quantitively by imaging the heart during diastole at various positions of tidal respiration with a breath-hold segmented fast gradient echo technique. It was found that during tidal breathing the movement of the heart due to respiration is dominated by superior-inferior (SI) motion, which is linearly related to the SI motion of the diaphragm. The motion of the heart due to respiration is approximately a global translation. These results provide motivation for employing adaptive motion correction techniques to reduce image blurring in nonbreath-hold coronary MR imaging.

Adult↗
PubMed1995

3D coronary MR angiography in multiple breath-holds using a respiratory feedback monitor.

To reduce respiratory blur and ghosts in 3D coronary imaging, a data acquisition scheme using consistent multiple breath-holds was implemented. A navigator echo was acquired and processed in real time to dynamically measure diaphragm position. This information was provided as a visual prompt to the patient to maintain consistency in breath-hold levels such that the variation range of diastolic heart position was less than 2 mm. Preliminary results indicate that this multiple breath-hold acquisition scheme, compared with acquisition under respiration, can significantly reduce blur and ghost artifacts in 3D coronary imaging.

Adult↗
PubMed1995

Interactive selection of optimal section orientations using real-time MRI.

In applications where precise image section positioning is vital, the interactive section rotation and offset capabilities of interactive MRI should be valuable. However, due to the independent nature of these two adjustments, the desired structure may often not be visible in the image after a rotation. Valuable time is wasted during relocation. An algorithm is presented that automatically alters the section offset after a rotation to provide continuous viewing of a marked structure, greatly improving section orientation efficiency. The technique is illustrated in the determination of double oblique angulation for through-plane imaging of the portal vein. This algorithm is expected to prove useful in applications of interactive MRI requiring precise positioning.

Algorithms↗
PubMed1995

Error in MR volumetric flow measurements due to ordered phase encoding in the presence of flow varying with respiration.

Respiratory ordered phase encoding is often employed in MRI studies to reduce image artifacts due to breathing motion. The purpose of this work was to evaluate error caused by the use of respiratory ordering of phase encoding in MR cine phase-contrast (CPC) volumetric flow measurements when the flow rate is sensitive to respiration. It was hypothesized that this effect is due to the systematic biasing of a respiratory-induced phase modulation function in k-space. A theoretical model for the effects of respiration was developed and then tested in flow phantom studies and in normal volunteer studies. In phantom experiments, the use of respiratory ordering induced an error of as much as 13% in CPC volumetric flow measurements. In preliminary volunteer studies, error was as high as 26% in superior vena cava flow measurements versus less than 1% error in the ascending aorta. It is concluded that a potential for error exists in CPC volumetric flow measurements obtained with the use of respiratory ordering schemes. Volunteer studies with larger numbers are warranted. Clinical applications in which this effect may be important include flow measurements in vessels subject to variations in flow due to respiration, such as the venae cavae, pulmonary vasculature, and portal vein.

Aorta↗
PubMed1995

Orbital navigator echoes for motion measurements in magnetic resonance imaging.

A single "orbital" navigator echo, that has a circular k-space trajectory, is used to simultaneously measure in-plane rotational and multi-axis translational global motion. Rotation is determined from the shift in the magnitude profile of the echo with respect to a reference echo. Displacements are calculated from the phase difference between the current echo and a reference echo. Phantom studies show that this technique can accurately measure rotation and translations. Preliminary results from adaptive motion correction studies on phantom and human subjects indicate that the orbital navigator echo is an effective method for motion measurement in MRI.

Humans↗
PubMed1995

Contrast optimization of fluid-attenuated inversion recovery (FLAIR) imaging.

The optimization of contrast is considered for the fluid-attenuated inversion recovery (FLAIR) MRI pulse sequence, specifically the contrast of multiple sclerosis (MS) to white matter (WM). A performance bound is identified at 1.5 Tesla as that provided using an inversion time (TI) of 2900 ms. It is shown that TR/TI times exceeding 11000/2600 ms provide about 90% of the MS-WM contrast possible theoretically. The commonly reported TR/TI combination of 6000/2000 provides only about 60%. For TR times exceeding 8000 ms, an echo time (TE) of 140 ms is at or near optimum. Use of TR/TI times less than 9000/2400 lacks efficiency in multisection imaging. Predicted relative contrast performance of TR/TI 11000/2600 versus 6000/2000 was evaluated in seven patients with known MS lesions, and measurements closely matched theoretical predictions. It is strongly recommended that for near optimum contrast and high multisection efficiency, FLAIR should be performed with TR/TI times exceeding 10000/2500 ms.

Brain↗
PubMed1995

New technical developments in magnetic resonance imaging of epilepsy.

Within the last several years a number of technical developments have been made in magnetic resonance imaging (MRI) that can potentially impact clinical and research MR imaging application in epilepsy. These include developments in instrumentation and in pulse sequences. Advances in instrumentation include higher capacity gradient systems and multiple receiver coils as directed to brain imaging. Advances in pulse sequence include use of fast or turbo-spin-echo techniques, variants of echo-planar imaging, and sequences such as fluid-attenuation inversion recovery (FLAIR) targeted to specific applications of brain imaging. The purpose of this paper is to review several of these developments.

Brain↗
PubMed1995

Comparison of breath-hold fast spin-echo and conventional spin-echo pulse sequences for T2-weighted MR imaging of liver lesions.

PURPOSE: To evaluate a breath-hold fast spin-echo (SE) technique for T2-weighted magnetic resonance (MR) imaging of liver lesions. MATERIALS AND METHODS: A fast SE technique was developed that enabled six sections to be imaged per 16-second breath hold with a single echo. Resulting images were compared with those obtained with the first echo of a conventional dual-echo T2-weighted SE sequence (16 minutes 55 seconds for 18 sections). Thirty-one patients with malignant focal hepatic lesions were studied prospectively. The images were compared quantitatively and qualitatively. RESULTS: Quantitatively, the contrast and contrast-to-noise ratios for the fast SE images were 20% +/- 5 and 19% +/- 8 greater, respectively, than those for the conventional T2-weighted SE images of the 54 representative lesions. Qualitatively, fast SE images had less image artifact, enabled comparable or better lesion sizing, and greatly improved depiction of extrahepatic structures compared with conventional T2-weighted SE images. CONCLUSION: The fast SE technique with breath holding provides diagnostically useful liver images in a greatly decreased acquisition time.

Adult↗
PubMed1995

MR imaging of the abdomen with a phased-array multicoil: prospective clinical evaluation.

PURPOSE: To prospectively compare use of a phased-array multicoil and a conventional body coil in abdominal MR imaging. MATERIALS AND METHODS: Thirteen patients (seven men, six women; mean age, 55 years) underwent imaging with a phased-array multicoil and with a conventional body coil. Four pulse sequences were used: T2-weighted spin echo (SE), magnetization-prepared gradient-recalled echo (GRE), breath-hold fast SE, and echo planar (EP). RESULTS: Lesion detection improved the most on fast SE, multicoil-acquired images. Signal-to-noise ratio (S/N) increased 64% with fast SE (P = .0005) and EP (P < .0109) sequences. Contrast-to-noise ratio (C/N) doubled (P < .05) with T2-weighted SE sequences. Lesion conspicuity improved on multicoil-acquired images with all fast sequences (magnetization-prepared GRE, P = .015; fast SE, P = .002; EP imaging, P = .013). There was little difference in respiratory and vascular artifact. Depiction of most abdominal structures improved (P < .01). CONCLUSION: Use of the phased-array multicoil provides better MR images of the abdomen than does use of a conventional body coil.

Abdomen↗
PubMed1994

Interleaved echo planar imaging on a standard MRI system.

This work describes an interleaved echo planar imaging (EPI) method for use on a standard whole body scanner. The data acquisition is divided into two to eight repetitions rather than one to two, as implemented by dedicated EPI systems. Interleaving allows the use of a lower sampling bandwidth with a significant increase in signal-to-noise. The method also has the advantages of relative ease of implementation, no need for postprocessing to remove image distortion, and no need for shimming on a case-by-case basis. The interleaved EPI method was applied to two applications ideally suited to EPI: breathhold T2-weighted abdominal imaging and functional imaging. In vivo liver-lesion contrast as measured in a 35-patient study showed increased contrast for the interleaved EPI by an average factor of 1.21 (+/- 0.34) over conventional spin-echo imaging. CNR measurements showed the EPI to be comparable with conventional spin echo with a relative factor of 1.00 (+/- 0.36). Functional imaging with an eight-shot interleaved EPI sequence provided 128 x 128 images of cerebral activation during bilateral finger tapping.

Abdomen↗
PubMed1994

Sensory motor cortex: correlation of presurgical mapping with functional MR imaging and invasive cortical mapping.

PURPOSE: To describe a clinically useful application of functional magnetic resonance (MR) imaging--presurgical mapping of the sensory motor cortex--and to validate the results with established physiologic techniques. MATERIALS AND METHODS: Functional MR mapping of the sensory motor cortex was performed in two women, aged 24 and 38 years. Both had intractable, simple partial motor seizures due to tumors located in or near the sensory motor cortex. They subsequently underwent invasive cortical mapping--direct cortical stimulation and/or sensory-evoked-potential recording--to localize the affected sensory motor area prior to tumor resection. RESULTS: In both patients, the functional MR study demonstrated task activation of the sensory motor cortex. In both cases, results of cortical functional mapping with invasive techniques matched those obtained with functional MR imaging. CONCLUSION: Presurgical mapping of the sensory motor cortex is a potentially useful clinical application of functional MR imaging.

Adult↗
PubMed1994

Imaging of cerebral activation at 1.5 T: optimizing a technique for conventional hardware.

PURPOSE: To empirically optimize a two-dimensional magnetic resonance (MR) imaging technique for detecting changes in signal intensity during cerebral activation with a standard clinical imager. MATERIALS AND METHODS: Visual activation experiments were performed while imaging parameters were manipulated in a serial fashion, to test their effect on the percentage change in signal intensity (PCSI). The parameters tested were section thickness, echo time (TE), field of view (FOV), flip angle, radio-frequency (RF) spoiling, number of readout points, and number of signals averaged. RESULTS: The PCSI for visual activation experiments was typically in the 1%-5% range. The best results were achieved with a small section thickness, long TE, and large FOV. Variations in other acquisition parameters had a negligible influence on the PCSI. CONCLUSION: Good results can be obtained at functional MR imaging with standard 1.5-T hardware. Markedly improved results can be obtained by optimizing several key variables, namely, section thickness, TE, and FOV.

Adult↗
PubMed1994

Magnetic resonance.

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Echo-Planar Imaging↗
PubMed1994

Tongue displacement: visualization with rapid tagged magnetization-prepared MR imaging.

The authors describe a technique to depict tongue muscle displacement with tagged magnetization-prepared gradient-recalled echo magnetic resonance imaging. Three Japanese volunteers (three men, aged 31-36 years) were studied while they articulated specific sounds. Each image was obtained in less than 2 seconds without need for gating. Displacement and alteration of the tags within the tongue muscle during phonation were clearly visible. The technique may prove useful in the study of phoniatric pathophysiology.

Adult↗