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Biomedical subjects

R L Ehman

Publications and source records attributed to R L Ehman.

140 records · Page 8Linked to original sources

Cardiovascular MR imaging: current level of clinical activity.

Relative to other clinical magnetic resonance (MR) imaging activities, cardiovascular (CV) MR imaging has been slow to demonstrate a clinical presence. To better understand the present situation in clinical CV MR imaging, a survey of Society for Magnetic Resonance Imaging (SMRI) members was conducted. A large majority (78%) of the 90 sites responding to the survey reported clinical activity in CV MR imaging. Of these 70 sites, 46% restricted such activity to routine clinical work, while 3% restricted it to clinical research. The remaining 51% conducted both. At all clinical sites, the overall frequency of performance of clinical CV MR imaging was variable (mean, 4.2 and 13.8 cases per month at routine-only and combined routine-research sites, respectively). In clinical CV studies, gated static, multi-level spin-echo and dynamic gradient-echo (cine) techniques were most common. At the 68 sites involved in routine clinical CV MR imaging, primarily anatomic studies composed a much higher proportion of the total (mean, 86%) than primarily functional studies. The evaluation of acquired thoracic aortic disease, congenital cardiac malformation, and para- or intracardiac mass were the most prevalent anatomic indications overall. Approximately half of the same sites assessed functional aspects of CV disease. The assessment of ventricular dysfunction and valvular dysfunction were the most common functional objectives. The survey indicated that the level of clinical activities in CV MR imaging was low and that most responding sites were involved in clinical CV MR imaging primarily for detection and delineation of anatomic abnormalities.

Cardiovascular Diseases↗

Altered phase-encoding order for reduced sensitivity to motion in three-dimensional MR imaging.

A method of reordering phase and slab encoding that can be used to address some of the inherent problems due to motion in three dimensional imaging is described and implemented. The method is shown to be more robust with respect to reducing artifacts resulting from several fundamental types of motion. It can be readily implemented on a standard magnetic resonance imager with essentially no increase in total imaging time. Results of simulations and phantom and in vivo experiments are presented.

Artifacts↗

MR imaging of knee hyaline cartilage: evaluation of two- and three-dimensional sequences.

Magnetic resonance (MR) imaging of cadaveric knees was performed to determine optimal sequences for visualization of hyaline cartilage. Full-thickness cartilage lesions ranging in diameter from 1 to 5 mm and a partial-thickness cartilage lesion 15 mm in diameter were created in the femoral articular surfaces of three cadaveric knees. The knees were then imaged with a 1.5-T imager with various two-dimensional and high-resolution three-dimensional (3D) techniques. After imaging, the knee specimens were sectioned for evaluation. Measurements of cartilage thickness in fast spin-echo images correlated best with those in the gross specimen. Diffuse areas of cartilage thinning were also most accurately identified with fast spin-echo images. Small, focal cartilage defects were best delineated in 3D SPGR (spoiled GRASS [gradient-recalled acquisition in the steady state]) images.

Cadaver↗

Abdominal phase-contrast MR angiography: breath-hold versus non-breath-hold techniques.

Breath-hold magnetic resonance (MR) imaging is now replacing many non-breath-hold pulse sequences in the upper abdomen because of faster imaging times and improved image quality. The authors compared non-breath-hold cine phase-contrast (PC) and breath-hold 2D phase-contrast (2DPC) magnetic resonance (MR) angiograms of the main portal vein (MPV) and superior mesenteric artery (SMA) in 12 volunteers. All angiograms were graded in overall image quality, vessel conspicuity, and signal-to-noise ratios (SNR). In the MPV MR angiograms, the breath-hold 2DPC sequence produced better images than the non-breath-hold cine PC sequence as graded by overall image quality (P = .016) and SNR (P = .004). Conversely, in the SMA MR angiograms, the non-breath-hold cine PC sequence produced better images than the breath-hold sequence in terms of overall image quality (P = .008) and SNR (P = .008). By reducing the most significant cause of image artifacts, (ie, using a breath-hold 2DPC sequence to decrease respiratory misregistration of the MPV, and using a cardiac-gated cine PC sequence to minimize pulsatile artifacts of the SMA), one can clearly optimize the quality of MR angiography.

Abdomen↗

Translational molecular self-diffusion in magnetic resonance imaging. I. Effects on observed spin-spin relaxation.

The reduced T2 (spin-spin) relaxation times (T2obs less than 200 ms) measured on pure fluids on our 0.35T magnetic resonance imagers stimulated an investigation into this phenomenon. The cause for the short T2obs of fluids was found to be translational molecular self-diffusion of hydrogen nuclei through the pulsed slice-selective magnetic gradient in the imagers. Similar reductions in biological tissue T2obs were also attributed to molecular self-diffusion.

Animals↗

Translational molecular self-diffusion in magnetic resonance imaging. II. Measurement of the self-diffusion coefficient.

By varying slice-selective gradients in successive data acquisitions, the first in vitro measurements of molecular self-diffusion coefficients were performed in a magnetic resonance imager at 0.35 Tesla. Reasonably accurate measurements were found by the MRI method in comparison with 2.3 T NMR spectrometer measurements on the same samples, and in comparison to reported literature values. Thus, in addition to T1, T2, mobile proton density, flow velocity, magnetic susceptibility, and chemical shift, molecular self-diffusion coefficients are now added to the list of biophysical parameters measurable by magnetic resonance imaging in the noninvasive characterization of biological systems.

Diffusion↗

Relative intensity of abdominal organs in MR images.

Knowledge of the normal relative intensity of organs and tissues is a valuable aid to clinical interpretation of magnetic resonance images. In this study the in vivo spin echo image intensities of normal parenchymal organs and other structures in the upper abdomen were evaluated for eight parameter combinations. The examinations of 40 patients were used. Image intensity and calculated T1, T2, and spin density values were obtained for liver, spleen, pancreas, renal cortex, renal medulla, bone marrow, skeletal muscle, and fat. Repetition times (TR) of 500, 1,000, 1,500, and 2,000 ms and echo times of 28 and 56 ms were used. The T1 and T2 values and relative spin density were calculated using a new algorithm. Liver had the smallest relative standard deviation of T1 of all the tissues studied. For comparison purposes, relative image intensities were calculated by normalizing them to the intensity of liver in the same image. The resulting compiled data show the normal ranks and ranges for relative intensity for the tissues in each of eight types of spin echo images. Although images with short TR and echo time (TE) are known to display the greatest T1 contrast, the mean relative intensities of all tissues except muscle and fat in the TR = 500 and TE = 28 ms images were within 20% of liver. A much larger spread in the normal relative intensities was observed with longer TE and TR.

Abdomen↗

Proton MR chemical shift imaging using double and triple phase contrast acquisition methods.

Conventional chemical shift magnetic resonance (MR) imaging with the phase contrast technique has a number of limitations with respect to quantitative accuracy. The hypothesis of this study is that the accuracy of phase contrast chemical shift MR may be improved by increasing the number of basis images from two to three. Water and fat images were obtained from phantoms and volunteers with a 1.5 T MR system using double and triple acquisition phase contrast chemical shift methods. Longitudinal relaxation time and relative water and fat content were calculated from these basis images. The T1 relaxation times of the aqueous component of composite phantoms were determined more accurately using the triple acquisition method than with the double acquisition method. In vivo studies demonstrate that the triple acquisition method separated fat and water signals more accurately and showed less field inhomogeneity dependence than the modified double acquisition method. The new method also provided a map of static field magnetic inhomogeneity and tissue magnetic susceptibility. The triple acquisition phase contrast chemical shift imaging technique should improve the prospect for quantitative tissue characterization in clinical MR.

Adipose Tissue↗

Sciatic endometriosis: MR appearance.

Endometriosis of the sciatic nerve is a rare but important cause of sciatica. Early on, the symptoms may be cyclic and be treated with local excision of the endometrioma from the sciatic nerve. If left untreated, it can result in marked cicatricial change of the sciatic nerve and require more radical surgical treatment. Due to the hemorrhagic nature of endometriomas and their characteristic location, the MR imaging findings can permit a specific diagnosis of sciatic endometriosis to be suggested. Therefore, MR imaging is recommended for evaluation of sciatica if the symptoms vary with the menstrual cycle or if there is a history of endometriosis. Early recognition of this disorder will facilitate appropriate treatment and minimize permanent damage to the sciatic nerve. The first surgically proven case of sciatic endometriosis with preoperative MR imaging is presented.

Adult↗

MR angiography in portal hypertension: detection of varices and imaging techniques.

This study was performed to compare time-of-flight MR angiography in detecting varices with conventional portography and endoscopy in patients with chronic liver disease and to compare MR tomographic images with projection angiograms. In eight patients findings on conventional arterial portography (considered the gold standard) were compared with the MR findings. Varices were graded by size and extent on a scale of 0-3. Splenic varices were detected in all patients by MR but were scored larger on portography in 6 (55%) of 11 comparisons. All left gastric varices were identified at MR and were rated within one grade of those seen at portography. All esophageal varices were identified at MR. Among 12 patients with upper endoscopy, MR rated varices significantly (p less than 0.05) larger than endoscopy in 8 (67%) of 12 comparisons. Varices were detected in two patients on MR that were not seen endoscopically. Extraperitoneal varices were identified in six (75%) of eight patients on MR and were only visualized in one patient at portography. Varices were equally well detected using either axial or coronal images. No significant difference existed when tomographic images were compared with projection images. Time-of-flight MR angiography is a valid technique for noninvasive imaging of abdominal varices. Tomographic single slice images are generally as good as projection images except possibly in the demonstration of extraperitoneal varices.

Adult↗

Renal vein and inferior vena cava tumor thrombus in renal cell carcinoma: CT, US, MRI and venacavography.

Renal cell carcinoma has a propensity to extend as tumor thrombus into the renal vein and inferior vena cava (IVC). The preoperative assessment for the presence and extent of renal vein and IVC tumor thrombus is important for planning appropriate surgical resection. Imaging procedures [CT, ultrasound (US), MR, venacavography] were correlated with surgical findings and pathology in 431 consecutive patients who had a radical nephrectomy for renal cell carcinoma. Ninety-nine (23%) patients had tumor thrombus extending at least into the main renal vein. Of these, 29 had tumor thrombus extending within the IVC. Patients were classified into two groups based on the surgical extent of tumor thrombus. Group A patients had no tumor thrombus or had tumor thrombus only in the renal vein proximal to the site of surgical ligation. Group B patients had tumor thrombus that extended to or beyond the distal renal vein at the site of surgical ligation. Forty-one patients had Group B tumor thrombus. Group B tumor thrombus was not seen in a renal cell carcinoma that was smaller than 4.5 cm. The sensitivity of CT for detecting Group B tumor thrombus was 79% and that of US was 68%. However, a much higher percentage of US examinations were technically indeterminate. In the patients who had either MR or venacavography, both imaging procedures were 100% sensitive for detecting group B tumor thrombus. Magnetic resonance imaging and venacavography appear to be the most sensitive means of identifying tumor thrombus.

Adult↗

Cine phase-contrast MR flow measurements: improved precision using an automated method of vessel detection.

OBJECTIVE: The purpose of this study was to construct a method of vessel edge detection that correctly identifies vessel pixels and to compare the interuser variability of cine phase contrast MR volumetric flow rates obtained with the conventional manual method and an automated method. MATERIALS AND METHODS: The automated method was developed based on a magnitude image threshold value and compared with the manual method in a flow phantom (three users) and in velocity images of the portal vein (five users). The threshold value determined from the magnitude image was applied to a region of interest surrounding the vessel of interest on the magnitude image to construct a vessel edge detection mask m(x,y). The velocity images were then multiplied by the mask m(x,y) and volumetric flow rates determined using the identified vessel pixels. RESULTS: In the flow phantom, flow measurements with the magnitude threshold method had significantly less interuser variability compared with the manual method (p < 0.01) and were within 10% (mean 6.0%) of the actual flow versus 35% (mean 18.6%) with the manual method. Regarding flow measurements in the portal vein of six volunteers, the magnitude threshold method was significantly more precise (p < 0.01) than the manual method with a mean standard deviation between the five users of 40.4 +/- 12.9 ml/min (range 22-60 ml/min) and 110.4 +/- 32.7 ml/min (range 70-155 ml/min), respectively. CONCLUSION: The magnitude threshold method of vessel edge detection developed in this study yields flow measurements that are accurate in the model system and have significantly less interuser variability than the manual method. This method shows promise for improving the precision of cine phase contrast flow measurements.

Adult↗

Spatial misregistration of vascular flow during MR imaging of the CNS: cause and clinical significance.

Spatial misregistration of signal recovered from flowing spins within vascular structures is a common phenomenon seen in MR imaging of the CNS. The condition is displayed as a bright line or dot offset from the true anatomic location of the lumen of the imaged vessel. Its origin is the time delay between application of the phase- and frequency-encoding gradients used to locate spins within the plane of section. The principal condition necessary for the production of spatial misregistration is flow oblique to the axis of the phase-encoding gradient. Flow-related enhancement (entry slice phenomenon), even-echo rephasing, and gradient-moment nulling contribute to the production of the bright signal of spatial misregistration. Familiarity with the typical appearance of flow-dependent spatial misregistration permits confirmation of a vessel's patency; identification of the direction of flow; estimation of the velocity of flow; and differentiation of this flow artifact from atheromas, dissection, intraluminal clot, and artifacts such as chemical shift.

Brain↗