PubMed Health⌕ Search

Biomedical subjects

J P Felmlee

Publications and source records attributed to J P Felmlee.

18 recordsLinked to original sources
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↗
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

Hand dose measurements in interventional radiology.

Measurements of radiation dose to the hand were conducted using TLD ring badges for individual interventional radiology cases. Results from over 30 examinations (including transhepatic cholangiograms and biliary and nephrostomy procedures) conducted by four radiologists using identical equipment show an average hand dose of 1.5 mGy (150 mrad) per procedure. Hand dose varied inversely with distance from the patient. Due to variable hand positions during clinical examinations, fluoroscopic time was not found to be a good indicator of hand dose.

Fluoroscopy↗
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↗
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

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

Estimated fetal radiation dose from multislice CT studies.

It is often necessary to estimate the dose of radiation to a fetus from a series of CT scans. To assist in making this process easier and more accurate, we measured fetal doses in an adult anthropomorphic phantom for four CT scanners: Picker 1200, Siemens DRH, GE 9800, and GE 8800. Measurements were made at four kilovoltages (100, 120, 130, and 140 kVp), for 2-, 5-, 8-, and 10-mm thicknesses, for two scanning angles (360 degrees and 398 degrees) and for two patient orientations (prone and supine). The fetal-dose estimates are based on the CT dose index measured by using a pencil ionization chamber at the center position of a 16-cm-diameter cylindrical phantom. Comparison with data from other investigators shows reasonable agreement when phantom differences, X-ray tube output, and equipment calibration are considered.

Female↗
PubMed1989

Magnetic resonance imaging phase encoding: a pictorial essay.

A set of MRI images with various degrees of phase encoding were generated from the same raw data set. The resultant pictorial teaching aid is useful in developing an intuitive understanding of the mechanics and principles of phase encoding in two dimensional, Fourier transform magnetic resonance imaging.

Humans↗
PubMed1989

Adaptive technique for high-definition MR imaging of moving structures.

An adaptive technique for measuring and correcting the effects of patient motion during magnetic resonance image acquisition was developed and tested. A set of algorithms that can reverse the effects of object displacements and phase shifts was used. These algorithms essentially transfer the frame of reference of the image reconstruction from the static frame of the imager couch to the moving "visceral frame." An accurate record of tissue motion during image acquisition is required. To achieve this, the authors used specially encoded "navigator" echoes that are interleaved with the imaging sequence. Postprocessing of the navigator echo data provides a highly detailed record of the displacements and phase shifts that occur during imaging. Phantom studies demonstrated that the technique can directly correct image degradation caused by motion. In contrast to conventional artifact reduction techniques, such as ordered phase encoding and gradient moment nulling, this new method has a unique capacity to reduce motion unsharpness. Preliminary in vivo studies have demonstrated that the technique can markedly improve images degraded by voluntary motion and shows promise for addressing the problem of respiratory motion in thoracoabdominal imaging.

Abdomen↗
PubMed1988

Cerebrospinal fluid-iophendylate contrast on gradient-echo MR images.

The effect on the signal intensities of cerebrospinal fluid (CSF) and iophendylate (Pantopaque) and on CSF-iophendylate contrast was studied in vitro with a small-nutation-angle (alpha) gradient refocused magnetic resonance (MR) imaging technique (GRASS) as alpha, repetition time (TR), and echo time (TE) were varied. CSF signal intensity was consistently greater than that of iophendylate. Therefore, retained intraspinal iophendylate may be considered in the differential diagnosis of focal areas of low signal intensity at the periphery of the spinal canal on GRASS images. At constant TE and TR, an increase in alpha from 6 degrees to 45 degrees increased the signal intensities of CSF and iophendylate but decreased CSF-iophendylate contrast. At constant alpha and TR, an increase in TE from 13 to 28 msec decreased the signal intensities of CSF and iophendylate but increased contrast. At constant alpha and TE, an increase in TR from 50 to 400 msec increased the signal intensities of CSF and iophendylate, as well as contrast. Clinical examples of the contrast behavior of retained intraspinal iophendylate on both spin-echo and GRASS images corroborate the experimental findings. Retained intraspinal iophendylate may mimic the appearance of intra-or extra-dural lesions, magnetic susceptibility artifact, and flow on gradient-echo MR images of the spine.

Cerebrospinal Fluid↗
PubMed1987

Section thickness and contiguity phantom for MR imaging.

Magnetic resonance (MR) imaging systems are uniquely able to acquire data simultaneously for the reconstruction of images from multiple sections, that is, for volumes up to 20 cm or more in length and for axial, coronal, and sagittal planes. A phantom has been developed that allows one to determine if the sections are of the desired thickness throughout the volume, if the sections are contiguous or the section spacing is as specified, if the central section is at the appropriate location, if the profile of the section is as specified by the manufacturer, if the signal strength is uniform for all sections, that the signal strength decreases in a consistent manner for second and subsequent echo images, and that gross spatial distortions are not present. Such measurements are essential for acceptance testing and quality control purposes over reasonably long volumes. The details of this new phantom are described, and results of its use with both low-and high-field-strength MR imaging systems are presented.

Equipment Design↗
PubMed1987

Spatial presaturation: a method for suppressing flow artifacts and improving depiction of vascular anatomy in MR imaging.

In clinical magnetic resonance (MR) imaging, the diagnostic quality of examinations is often degraded by streaklike flow artifacts that obscure anatomic details and reduce contrast. In addition, vascular structures are often not depicted clearly because the desired flow voids are obliterated by spurious intraluminal signals. On the basis of analysis of the physical mechanism of flow artifact formation, the authors developed a new technique for suppressing these artifacts. This applies interleaved, spectrally shaped radio frequency pulses to selectively saturate spins located in regions outside the image volume. In phantom, volunteer, and clinical imaging studies, the technique has proved to be effective by yielding a striking reduction in flow artifacts and markedly improving the reliability with which arterial and venous structures are imaged. The method has few drawbacks: It is applicable to most MR pulse sequences and, in principle, can be implemented on most imagers. It is particularly helpful for high-resolution surface coil studies of the neck, mediastinal imaging, gated cardiac imaging, and for detecting thrombus and other intravascular lesions such as dissections.

Blood Vessels↗
PubMed1986

Influence of physiologic motion on the appearance of tissue in MR images.

Studies were performed to determine the possible influence of physiologic motion on the parenchymal intensity of organs in magnetic resonance (MR) images. It is known that periodic motion associated with respiration and cardiac function causes characteristic artifacts in spin-warp images. The present study shows that bulk motion can also cause striking intensity changes at velocities equivalent to the craniocaudal respiratory excursion of organs in the upper abdomen. The magnitude of the effect depends on the velocity and direction of motion with respect to the three orthogonal axes of the imager and on the technical details of the imager and pulse sequence. Large systematic errors in calculated tissue relaxation times are possible due to this phenomenon. The findings have important implications for clinical imaging because motion can cause artifactual changes in the gray-scale relationships among tissues. Some pulse sequences are much less sensitive to these effects. These results provide guidance for selecting MR techniques that reduce the detrimental effect of respiratory and other physiologic motion on examinations of the upper abdomen and thorax.

Abdomen↗
PubMed1986

Half-value-layer increase owing to tungsten buildup in the x-ray tube: fact or fiction.

The half-value layer (HVL) of an x-ray beam is generally believed to increase with x-ray tube use. This increase in HVL has previously been attributed to the hardening of the x-ray beam as a result of a buildup of tungsten on the x-ray tube glass window. Radiographs and HVL measurements were obtained to determine the effect of tungsten deposited on the x-ray tube windows. This work, along with the HVL data from approximately 200 functioning x-ray tubes used for all applications that were monitored for more than 8 years, indicated there is no significant increase in HVL with diagnostic x-ray tube use.

Glass↗
PubMedDate not supplied

Phantom testing of peripheral artery. Absolute blood flow measurement with digital arteriography.

A technique to measure absolute arterial blood flow in ml/min has been developed utilizing software programs with high-speed digital recording for the off-line analysis of intra-arterial injections of contrast medium. Measurements of pulsatile flow in a phantom for a physical flow model showed that calculations with the final upgrades were within 10% of known flow, using a recording rate of 30 frames/s, a diameter tubing of 5 mm, and flow rates of 300 to 400 ml/min. Quantitative absolute flow of a peripheral artery such as the internal carotid artery may be obtained during routine cerebral arteriography for comparison with anatomic data.

Angiography↗
PubMedDate not supplied

Temporal lobe volume measurement from MR images: accuracy and left-right asymmetry in normal persons.

The effect of several magnetic resonance (MR) variables on the accuracy of volume measurements in phantom objects was investigated by use of an off-line automatic border-outlining and internal area pixel-counting computer program, and an optimal set of imaging variables was identified. Measurements were made of the temporal lobe volumes of a gross fixed brain specimen from MR image data. The range in accuracy was from -2 to +7%, and the standard deviation of the difference in right minus left lobe volume measurements obtained from the MR images and those obtained by use of Archimedes' principle was 1 cm3. This volumetric technique was applied to 25 normal persons, most of whom were right-handed. The median ratio of right to left temporal lobe volume was 1.16 (range 0.99-1.23). The nondominant temporal lobe was significantly larger than the dominant. The mean difference (95% confidence interval) between right and left volumes was 7 cm3 (6-9 cm3). This confidence interval was similar to that obtained when the variability within a subject (estimated from the gross fixed brain specimen) was taken into account. Unilateral temporal lobe atrophy, particularly in patients with temporal lobe epilepsy, should be interpreted from MR images with this range of discrepancy in normal left-right size in mind.

Adult↗
PubMedDate not supplied

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↗