Search PubMed⌕ Search

Biomedical subjects

R L Ehman

Publications and source records attributed to R L Ehman.

At least 91 records · Page 5Linked to original sources
PubMed1993

Correlation of arterial and venous blood flow in the mesenteric system based on MR findings. 1993 ARRS Executive Council Award.

OBJECTIVE: The purpose of this study was to determine if a consistent relationship exists between (1) flow in the portal vein and superior mesenteric vein and (2) flow in the mesenteric arteries. SUBJECTS AND METHODS: Cine phase-contrast methods were used to determine volumetric flow in the celiac axis, superior mesenteric artery, portal vein, and superior mesenteric vein in 11 healthy volunteers. RESULTS: The mean (+/- SD) volumetric flow rates in the celiac axis, superior mesenteric artery, superior mesenteric vein, and portal vein were 9.9 +/- 3.2, 6.0 +/- 1.8, 5.7 +/- 2.0, and 13.7 +/- 1.8 ml/min per kilogram, respectively. A strong correlation was found between measured mesenteric arterial blood flow, defined as the sum of flow in the celiac axis and the superior mesenteric artery, and portal venous flow (r = .97, p = .00002). Good agreement was found between flow in the superior mesenteric artery and that in the superior mesenteric vein (r = .98; p = .00001; bias = 20 ml/min). CONCLUSION: There is a consistent relationship between MR measurements of flow in the portal or superior mesenteric vein and the measured flow in the arteries supplying those veins. Flow in the superior mesenteric vein is an accurate predictor of flow in the superior mesenteric artery.

Adult↗
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

Recursive RF excitation.

We have investigated the properties of a recursive process in which the output signal from a given RF excitation pulse may be used as the input (excitation) pulse of a subsequent iteration. This recursive excitation technique increases contrast and improves feature segmentation for the purpose of motion tracking.

Agar↗
PubMed1992

Clinically occult avascular necrosis of the hip: prevalence in an asymptomatic population at risk.

An abbreviated screening magnetic resonance (MR) imaging examination was used to determine the prevalence of clinically occult avascular necrosis (AVN) of the hip in 100 asymptomatic renal transplantation patients treated with corticosteroids. All patients were more than 18 years old, had been treated with corticosteroids for at least 6 months, and had no symptoms of AVN before MR imaging. Of the 100 patients screened, six were found to have clinically occult AVN at MR imaging (rate: 6%, with a 95% confidence interval of 2.2%-12.6%). No significant differences were found regarding the total dose of corticosteroids and exposure time between patients with and patients without asymptomatic AVN. The authors conclude that there is a significant prevalence of asymptomatic, clinically occult AVN among renal transplantation patients treated with corticosteroids and that a potentially low-cost screening MR imaging examination can be used to detect such disease.

Female↗
PubMed1992

Soft-tissue sarcomas: use of textural patterns in skeletal muscle as a diagnostic feature in postoperative MR imaging.

The authors assessed the value of tissue textural patterns as a diagnostic feature for differentiating nonspecific posttreatment tissue changes from musculoskeletal sarcoma recurrence on magnetic resonance (MR) images. The MR imaging studies of 40 patients who had previously undergone surgery and radiation therapy for soft-tissue sarcomas of the lower extremities were evaluated in a blind fashion. In 31 of the MR imaging studies, T2-weighted images demonstrated diffuse areas of high signal intensity in soft tissues at the operative region. Close examination of the corresponding regions on high-resolution transverse T1-weighted images demonstrated textural features typical of skeletal muscle in 23 patients and the absence of such features in eight. None of the 23 patients with the "texture sign" proved to have macroscopic tumor recurrence at clinical or surgical follow-up. Among the eight patients without recognizable textural features of muscle in the regions suspicious for tumor recurrence, two proved to have recurrent tumor at surgery. Recognition of a texture sign on high-resolution T1-weighted spin-echo images of regions suggestive of tumor recurrence helps improve the diagnostic specificity of follow-up MR examinations in patients who have undergone treatment for soft-tissue sarcomas.

Adolescent↗
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

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↗
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↗
PubMed1991

Intracranial aneurysms and vascular malformations: comparison of time-of-flight and phase-contrast MR angiography.

Twenty-seven patients with 14 aneurysms and 17 vascular malformations were each examined with both time-of-flight (TF) and phase-contrast (PC) magnetic resonance angiography of the head. Three-dimensional (3D) PC imaging depicted the patent lumen of the aneurysms, while 3D TF imaging depicted the patent lumen and a subacute thrombus if present. The 3D PC techniques were superior to 3D TF methods in depicting aneurysms larger than 15 mm. PC angiography allowed velocity resolution of vascular lesions and yielded functional flow information by directly depicting collateral flow to vascular lesions. Artifacts present on TF images were due to substances with short T1, hemosiderin, and surgical clips. Artifacts present on PC images were due to flow-related aliasing. The results showed that PC techniques have specific advantages over TF techniques, including identification of large aneurysms, velocity resolution of vascular lesions, depiction of flow direction about the circle of Willis, and less degradation by artifacts.

Adult↗
PubMed1991

Traumatic trabecular lesions observed on MR imaging of the knee.

Examinations carried out on 302 consecutive patients with MR of the knee between January 1988 and March 1989 were reviewed for detection of trabecular lesion. Twenty-seven patients found presenting trabecular lesion were further reviewed with specific reference to their activity level and need for specific therapy to determine the clinical significance of the trabecular lesion. Twenty-one of the trabecular lesions were in the femur, 5 were in the tibia, and one was in the fibula. Three of them were associated with a direct trauma, 12 with a valgus type injury, 3 with pure rotation mechanism, and 5 with a combination of valgus and rotation. In 17 cases trabecular lesion was a single finding, in 10 cases it was associated with some ligamentous tear. At the follow-up visit, 26 of the 27 patients with trabecular lesion had no symptoms, and the patient with moderate knee symptoms had had similar knee symptoms prior to the accident due to an osteochondral defect. We conclude that a trabecular lesion in an MR image is a benign bone change associated with knee trauma which heals without sequelae.

Adolescent↗
PubMed1990

Flow artifact reduction in MRI: a review of the roles of gradient moment nulling and spatial presaturation.

In the past, flow artifacts and inconsistent depiction of vascular anatomy have represented significant problems in clinical MRI. These difficulties are now generally well addressed by the techniques of gradient moment nulling and spatial presaturation. Gradient moment nulling (GMN) is an effective method for eliminating flow artifacts in gradient echo images, while presaturation is more applicable to the same task in spin echo acquisitions. The GMN technique also has useful applications in spin echo imaging such as combating the effects of tissue and CSF motion in long TE sequences. In contrast to presaturation, however, GMN is not suitable for suppressing artifacts due to pulsatile blood flow in spin echo images.

Blood Circulation↗
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↗
PubMed1990

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.

Blood Flow Velocity↗
PubMed1990

Value of MR imaging in differentiating benign from malignant soft-tissue masses: study of 95 lesions.

MR imaging has largely replaced CT as the technique of choice for preoperative staging of patients with soft-tissue masses. Whether MR imaging can be used to differentiate benign from malignant masses is controversial. Our experience suggests that MR imaging often can characterize soft-tissue masses accurately. To evaluate this question further, we studied 95 consecutive lesions (50 benign and 45 malignant). Consecutive cases were selected to simulate our clinical practice. Surgical proof was available for all masses except hematomas, for which clinical follow-up confirmed the diagnosis. MR images were interpreted twice by three radiologists. The first review was accomplished without any clinical history and the second review with clinical history. Reviewers were asked to classify the lesion as benign or malignant on the basis of their clinical knowledge and analysis of MR image features (size, lesion margin, signal homogeneity, and neurovascular or bone involvement). Although interpretation varied somewhat because of the experience of the reviewers, the specificity and accuracy of diagnosis averaged 90% for both benign and malignant lesions. Negative predictive value for malignancy averaged 94% among the three reviewers. MR imaging is the technique of choice for identification and characterization of soft-tissue masses. The nature of the lesion (benign vs malignant) can be determined in the majority of cases.

Diagnosis, Differential↗