Biomedical subjects

R L Ehman

Publications and source records attributed to R L Ehman.

At least 73 records · Page 4Linked to original sources

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

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 measurements of mesenteric venous flow: prospective evaluation in healthy volunteers and patients with suspected chronic mesenteric ischemia.

PURPOSE: To quantify portal vein (PV) and superior mesenteric vein (SMV) flow before and after a standardized meal in healthy volunteers and to prospectively evaluate patients with a clinical suspicion of chronic mesenteric ischemia on the basis of magnetic resonance (MR) measurement of flow in the mesenteric venous system in volunteers. MATERIALS AND METHODS: Cine phase-contrast flow measurements were acquired in 10 asymptomatic volunteers and in 10 patients. RESULTS: In volunteers, the difference between the fasting and post-prandial flows in the SMV and PV was significant (P < .001), with a peak flow augmentation of 245% +/- 74 and 70% +/- 29, respectively. Postprandial augmentation of peak flow in the SMV was significantly less in patients with mesenteric ischemia compared with volunteers (64% +/- 28; P = .02). SMV flow augmentation in patients without mesenteric ischemia did not differ significantly from that in volunteers (206% +/- 36; P = .31). CONCLUSION: Measurement of postprandial flow augmentation in the SMV with MR imaging shows promise as a noninvasive screening test for chronic mesenteric ischemia.

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.

Explore the source record for details and available documents.

Echo-Planar 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

Comparison of time-of-flight and phase-contrast MR neuroangiographic techniques.

Magnetic resonance (MR) angiography constitutes a group of evolving MR imaging techniques that can be used to directly image flow in arteries, veins, and cerebrospinal fluid. By using multiple MR angiographic sequences in each of 18 patients with neurologic conditions such as aneurysms, arteriovenous malformations, and carotid atherostenotic disease, the advantages, disadvantages, and unique artifacts of each technique could be investigated. Time-of-flight imaging is susceptible to saturation effects, and short T1 substances may simulate flow. Two-dimensional time-of-flight imaging is useful in cranial venography in assessing the patency of the dural sinuses or venous drainage from an arteriovenous malformation. Three-dimensional time-of-flight images depict small and medium-sized aneurysms. Phase-contrast imaging has excellent background suppression, allows variable velocity encoding, and provides directional flow information. Two-dimensional phase-contrast imaging is useful in the assessment of the patency of major vascular structures. Three-dimensional phase-contrast imaging (with 30-cm/sec velocity encoding) is also useful in depicting small and medium-sized aneurysms. Although some applications of MR angiography are clear, further study and development of the techniques are necessary to determine which imaging sequence or combination of techniques is best suited for answering specific diagnostic questions.

Adolescent↗

PubMed1993

Phase-contrast cine MR angiography in chronic liver disease.

Thirty-one patients with portal venous hypertension underwent phase-contrast cine magnetic resonance (MR) angiography to validate the technique for assessment of main portal vein patency and blood flow direction. Correlative findings from Doppler ultrasonography, conventional angiography, and surgery were acquired in 30, 17, and 16 patients, respectively. Findings from MR angiography correlated well with those of the other methods for determining portal vein patency and flow direction. The main portal vein was patent in 29 patients and was thrombosed in two patients at MR angiography. With the use of a 40-mm slab projection acquisition that required 5 minutes, the splenic and superior mesenteric veins, as well as extensive varices (when present), were seen in the majority of cases. Phase-contrast cine MR angiography is an accurate method of rapidly assessing morphologic characteristics, patency, and flow direction of the portal venous system and thus is a useful method for evaluating the condition of patients with chronic liver disease.

Adult↗

PubMed1993

Evaluation of portal venous hypertension with cine phase-contrast MR flow measurements: high association of hyperdynamic portal flow with variceal hemorrhage.

Thirty-two patients with portal venous hypertension and endoscopically proved esophageal varices who were being evaluated for possible liver transplantation were studied with cine phase-contrast magnetic resonance (MR) imaging. Flow measurements in the main portal vein were obtained and associated with the presence of variceal hemorrhage within 2 years before the MR examination. Low (hypodynamic) flow was present in 22 patients, while high (hyperdynamic) flow was present in 10 patients. The presence of variceal hemorrhage was significantly associated with a high portal venous flow rate (P = .001), high variceal grade (P = .030), and Child class A or B (P = .003); however, only portal venous flow (P = .006) and variceal grade (P = .044) were found to be associated with variceal hemorrhage in a multiple logistic regression analysis. The portal venous flow rate was significantly higher among patients with Child class A or B disease compared with those with class C disease (median, 24.6 vs 8.0 mL/min.kg; P = .004).

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

Volumetric flow rates in the portal venous system: measurement with cine phase-contrast MR imaging.

OBJECTIVE: The purposes of this study were to (1) validate the accuracy of cine phase-contrast MR flow measurements within the portal vein, (2) develop a suitable protocol for using this method to measure volumetric flow rate in the portal venous system, and (3) use this protocol, with Doppler sonography as a reference, to measure portal venous flow in healthy volunteers and in patients with protal venous hypertension. SUBJECTS AND METHODS: Flow determinations were obtained in a model of fluid movement approximating blood-flow conditions in the portal venous system. A suitable protocol was based on consideration of the theoretical effects of (1) spatial resolution, (2) obliquity of the imaging plane to the direction of flow, and (3) signal-to-noise ratio of the signed quantitative velocity images (in three volunteers) on the accuracy and precision of flow measurements. This protocol was used to obtain cine phase-contrast MR images of the portal venous system in five volunteers and six patients. RESULTS: Values obtained with a flow phantom showed good accuracy of cine phase-contrast-measured vs actual volumetric flow rate (r = .995; p = .0001; MR rate = [0.94 x actual rate] + 65.6 ml/min; standard error of the y estimate = 67.3 ml/min). Velocity encoding and section thickness substantially influenced the signal-to-noise ratio of the velocity images, whereas flip angle and matrix size had only minimal effect. In volunteers and patients, portal volumetric flow rates determined by using MR images and Doppler sonography showed good correlation (r = .94; p = .0003). CONCLUSION: Our results indicate that cine phase-contrast MR imaging is a practical noninvasive method for measuring volumetric flow rates in the portal venous system.

Adult↗

PubMed1993

Measurement of normal renal artery blood flow: cine phase-contrast MR imaging vs clearance of p-aminohippurate.

OBJECTIVE: The purpose of this study was to compare cine phase-contrast MR imaging with renal clearance of p-aminohippurate for measuring normal renal blood flow. A reliable technique for evaluation of renal hemodynamics would be useful for studying renal vascular diseases and the effects of treatment. Measurements of renal blood flow based on renal clearance of p-aminohippurate are limited in that the kidneys are not studied separately, temporal resolution is poor, and normal renal function is required. SUBJECTS AND METHODS: Bilateral oblique sagittal cine phase-contrast MR images were obtained simultaneously in 10 healthy adult volunteers, with the imaging planes oriented perpendicular to the left and right renal arteries in order to measure through-plane flow. Velocity encoding was 150 cm/sec (seven of 10 volunteers) and 100 cm/sec (eight of 10 volunteers). Axial cine phase-contrast images of the abdominal aorta above and below the origins of the renal arteries were obtained in six volunteers. In these cases, renal blood flow was determined by measuring the difference between suprarenal and infrarenal aortic flow. For all volunteers, renal clearance of p-aminohippurate was determined immediately after MR measurements. RESULTS: Renal blood flow measurements determined by using cine phase-contrast MR imaging were in close agreement with those determined by using clearance of p-aminohippurate. At a velocity encoding of 150 cm/sec, the mean difference was 69 ml/min (95% confidence interval, -31 to 169 ml/min). At a velocity encoding of 100 cm/sec, the mean difference was 39 ml/min (95% confidence interval, -100 to 177 ml/min). Aortic flow measurements using cine phase-contrast MR imaging appeared to be less reliable for determining renal blood flow than measurements in the individual renal arteries, with a mean difference of -75 ml/min (95% confidence interval, -381 to 231 ml/min) compared with renal blood flow determined by using clearance of p-aminohippurate. CONCLUSION: We conclude that cine phase-contrast MR imaging is a promising technique for noninvasive measurement of renal blood flow. Measurements agreed closely with those obtained by using clearance of p-aminohippurate. In addition, the MR technique is faster, can be used to measure unilateral or bilateral renal blood flow, and does not depend on renal function.

Adult↗