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R L Ehman

Publications and source records attributed to R L Ehman.

At least 55 records · Page 3Linked to original sources
PubMed1996

Real-time adaptive motion correction in functional MRI.

Functional magnetic resonance imaging (fMRI) of the brain is often degraded by bulk head motion. Algorithms that address this by retrospective re-registration of images in an fMRI time series are all fundamentally limited by any motion that occurs through-plane. Here, a technique is described that can account for such motion by prospective correction in real time. A navigator echo is used before every image acquisition to detect superior/inferior displacements of the head. The displacement information is then used to adjust the plane of excitation of the ensuing single-shot echo-planar fMRI axial image. These correction updates can be completed in 100 mm with motion sensitivity at least as small as 0.5 mm. The efficacy of this method is documented in phantom and human studies.

Brain↗
PubMed1996

Dynamic MR digital subtraction angiography using contrast enhancement, fast data acquisition, and complex subtraction.

A dynamic MR angiography technique, MR digital subtraction angiography (MR DSA), is proposed using fast acquisition, contrast enhancement, and complex subtraction. When a bolus of contrast is injected into a patient, data acquisition begins, dynamically acquiring a thick slab using a fast gradient echo sequence for 10-100 s. Similar to x-ray DSA, a mask is selected from the images without contrast enhancement, and later images are subtracted from the mask to generate angiograms. Complex subtraction is used to overcome the partial volume effects related to the phase difference between the flowing and stationary magnetization in a voxel. Vessel signal is the enhancement of flow magnetization resulting from the contrast bolus. MR DSA was performed in 28 patients, including vessels in the lungs, brains, legs, abdomen, and pelvis. All targeted vessels were well depicted with MR DSA. Corresponding dynamic information (contrast arrival time ta and duration of the arterial phase tav) was measured: ta/tav = 3.4/4.7 s for the lung, 10.3/4.9 s for the brain, 12.8/19.3 for the aorta, 15.2/12.6 s for the leg. MR DSA can provide dynamic angiographic images using a very short acquisition time.

Abdomen↗
PubMed1996

3D MR angiography of pulmonary arteries using real-time navigator gating and magnetization preparation.

An ECG-triggered magnetization-prepared segmented 3D fast gradient echo sequence was developed to perform pulmonary arterial MR angiography. A selective inversion recovery pulse was used in the magnetization preparation to suppress venous vasculature. A real-time gating technique based on navigator echoes was implemented to reduce respiration effects. Pencil-beam navigator echoes were acquired immediately before and after the readout train and processed in real-time to dynamically measure the diaphragm position, which was used to control data acquisition with an accept-or-reject-reacquire logic. In a study of 10 volunteers, a gated 3D acquisition with 28 slices required on average approximately 4 min of acquisition time, and six to seven segmental arteries related to the interlobar trunk of the pulmonary artery were depicted. The use of SIR pulse reduced venous signal by 99%. The gated acquisitions were superior to the ungated acquisitions (n = 10, P < 0.005). The real-time navigator gating technique is effective for reduction of respiration effects and thereby makes high resolution 3D MRA of the pulmonary arteries feasible.

Algorithms↗
PubMed1996

Cardiac magnetic resonance fluoroscopy.

A technique is described for high speed interactive imaging of the heart with either white or black blood contrast. Thirty-two views of a segmented, magnetization-prepared gradient echo sequence are acquired during diastole. Using three-quarter partial Fourier sampling, data for a complete 128 x 128 image are acquired in three cardiac cycles. High speed reconstruction provides an image update of each cardiac cycle 159 ms after measurement. An independent graphical user interface facilitates interactive control of section localization and contrast by permitting pulse sequence parameter modification during scanning. The efficiency and image quality of the cardiac MR fluoroscopy technique were evaluated in 11 subjects. Compared with the conventional graphic prescription method, the cardiac fluoroscopy technique provides an approximate eightfold reduction in the time required to obtain subject-specific double oblique sections. Image quality for these scout acquisitions performed during free breathing was sufficient to identify small cardiac structures.

Coronary Angiography↗
PubMed1996

Detection of colorectal polyps by computed tomographic colography: feasibility of a novel technique.

BACKGROUND & AIMS: Computed tomographic colography (CTC) represents a novel technique for colorectal polyp detection. A prospective study was undertaken to determine the optimal CTC scanning parameters based on an artificial colon model and to assess the feasibility of CTC to detect clinically significant colorectal polyps. METHODS: A colon model was scanned by helical computed tomography at multiple parameters. Reformatted two-dimensional and three-dimensional images were then graded for polyp detection and image quality. Subsequently, 10 patients with known colon polyps underwent CTC immediately before colonoscopy. The number of polyps detected by two radiologists using CTC were compared with colonoscopy results that served as the gold standard. RESULTS: The optimal scanning parameters in the colon model were 5-mm collimation, 5 mm/s table speed, and 1-mm reconstruction interval. Ten patients had 30 polyps (range, 0.2-2.0 cm) by colonscopy, and all polyps > or = 0.5 cm were adenomas. Polyp detection by CTC for both observers was 100% (5 of 5) > or = 1 cm, 71% (5 of 7) between 0.5 and 0.9 cm, and 11%-28% (2-5 of 18) < 0.5 cm. CONCLUSIONS: Based on this small, unblinded pilot study, CTC is feasible for colorectal polyp detection > or = 0.5 cm in diameter.

Aged↗
PubMed1996

Navigator-echo-based real-time respiratory gating and triggering for reduction of respiration effects in three-dimensional coronary MR angiography.

PURPOSE: To test the hypothesis that respiration effects in three-dimensional (3D) coronary magnetic resonance (MR) imaging can be reduced with navigator-echo-based gating or triggering according to the superior-inferior position of the diaphragm. MATERIALS AND METHODS: Real-time respiratory gating and respiratory triggering (breath hold with feedback) were implemented with navigator echoes in a magnetization-prepared, segmented, 3D coronary imaging sequence. The two techniques were first tested with a motion phantom. An imaging protocol that compared real-time respiratory-gated acquisition, real-time respiratory-triggered acquisition, and continuous acquisition was then evaluated in six healthy subjects. RESULTS: Real-time respiratory-gated and respiratory-triggered acquisition were superior to continuous acquisition with two signals averaged (P = .025). The performance of the gated acquisition was about the same as that of the triggered acquisition (P = .05). CONCLUSION: Navigator-echo-based, real-time respiratory-gating and respiratory-triggering techniques are practical methods for effective reduction of respiration effects in coronary MR imaging.

Adult↗
PubMed1996

MR imaging of physeal bars.

PURPOSE: To determine if three-dimensional (3D) rendered and projection images derived from magnetic resonance (MR) imaging data are advantageous for the preoperative assessment of physeal bars. MATERIALS AND METHODS: Fifteen patients with suspected physeal bars were examined with MR imaging. Gradient-echo sequences were used with both two-dimensional and 3D MR imaging techniques. The 3D rendered and projection images were then derived from the MR imaging data. Patients were categorized on the basis of the effect of MR images on the decision to perform physeal bar excision. RESULTS: MR findings had either a moderate (n = 7) or a major (n = 6) effect on the surgical treatment of 13 patients (87%). They had no effect in two patients (13%) and were misleading in none. CONCLUSION: MR imaging is an excellent modality for imaging physeal bars. Imaging data can be processed to yield both 3D rendered and projection physeal maps that are particularly useful in preoperative planning.

Adolescent↗
PubMed1996

Colorectal polyp detection with CT colography: two- versus three-dimensional techniques. Work in progress.

PURPOSE: To compare detection of colorectal polyps with two-dimensional (2D) computed tomographic (CT) colography only, three-dimensional (3D) CT colography only, and a combination of 2D and 3D CT colography. MATERIALS AND METHODS: A total of 11 computer-simulated polyps (1-10 mm) were placed randomly in five identical CT data sets for images of a 72-year-old man's polyp-free, rectosigmoid colon. Fifteen CT colographic data sets were produced: five with 2D CT images only, five with 3D CT images only, and five with 2D and 3D CT images. Two radiologists randomly, blindly, and independently evaluated all 15 data sets to detect the simulated polyps. RESULTS: No polyps 2 mm or smaller were detected. No statistically significant differences in the detection of colorectal polyps were found between the three techniques. However, the combination of 2D and 3D CT colography resulted in polyp detection rates that were greater than or equal to those of 2D or 3D CT colography alone. Flat polyps were more difficult to detect than sessile polyps. Five false-positive findings occurred with 2D CT colography. CONCLUSION: A combined display of 2D and 3D CT images likely provides the greatest rate of detection of colorectal polyps.

Aged↗
PubMed1996

Variability of consecutive in vivo MR flow measurements in the main portal vein.

OBJECTIVE: The variability of consecutive cine phase-contrast MR flow measurements could significantly affect their use for clinical decisions, especially during provocative testing. The purposes of this study were to determine the normal variability of flow and consecutive flow measurements in the main portal vein on MR images and to determine how intraobserver variability, interobserver variability, and MR imager variability affect these measurements. SUBJECTS AND METHODS: MR flow measurements were acquired four consecutive times at the same location in the main portal vein of 12 subjects and three consecutive times at the same location in a nonpulsatile vessel model. All acquisitions were completed within 10 min. All main portal vein MR data sets were evaluated manually in a blinded review by two independent observers during three separate sessions spaced a mean of 4.5 weeks apart. Flow model data sets were evaluated during a single session by one observer. Variabilities were subsequently calculated by a components-of-variance analysis and by the coefficient of variation (SD/mean x 100). RESULTS: Of the total variance, 90% was due to flow variability among subjects (intersubject), 6% to flow variability within one subject (intrasubject), 2% to intraobserver variability, and 2% to interobserver variability. The coefficient of variation of consecutive MR portal vein flow measurements within a single subject was 11% +/- 5% (range, 3-23%). Intra- and interobserver variabilities were 5% +/- 2% (range, 1-11%) and 4% +/- 4% (range, 0-17%), respectively. MR imager variability was 1% +/- 1% (range, 0-2%). CONCLUSION: The mean variability of consecutive cine phase-contrast MR flow measurements in the main portal vein is 11% +/- 5% and could affect research and clinical protocols that employ this technique.

Adult↗
PubMed1996

Systemic gadolinium toxicity in patients with renal insufficiency and renal failure: retrospective analysis of an initial experience.

OBJECTIVE: To study the possible deleterious systemic effects of gadolinium in patients with impaired renal function. DESIGN: We retrospectively analyzed the routine laboratory data and clinical course of patients who had undergone a gadolinium-enhanced magnetic resonance imaging (MRI) examination of the brain and spine and had evidence of impaired glomerular filtration. MATERIAL AND METHODS: Between October 1988 and October 1992, 15,830 patients underwent gadolinium-enhanced MRI at our institution, 151 of whom had a serum creatinine value of more than 2 mg/dL. The clinical records of these 151 patients were thoroughly examined for the period from 3 days before to 30 days after the gadolinium-enhanced MRI examination. All data were analyzed in an attempt to detect any adverse events that could be related to free gadolinium as a result of dissociation from the chelating agent due to prolonged elimination times (that is, increased serum creatinine concentrations). In addition, we calculated the 90-day mortality rate for both the study group and a matched control population of 80 patients who had undergone MRI of the brain and spine before gadolinium was available. RESULTS: The overall incidence of adverse events in the study group was 3.6%. No event was severe or life threatening--nausea and rash occurred in two patients each, and seizure and headache occurred in one patient each. These findings were not significantly different from those in previous studies performed in populations with normal elimination times. Moreover, no significant difference was noted in the 90-day mortality rate (14.6% of the study group) in comparison with that in the control group (13.8%). CONCLUSION: On the basis of this initial retrospective analysis, we were unable to detect any clinical deleterious effects of administration of gadolinium for MRI examination in patients with impaired renal function. Further investigation with prospective studies is needed to confirm these initial retrospective findings.

Adult↗
PubMed1995

Medical imaging.

Explore the source record for details and available documents.

Diagnostic Imaging↗
PubMed1995

Magnetic resonance elastography by direct visualization of propagating acoustic strain waves.

A nuclear magnetic resonance imaging (MRI) method is presented for quantitatively mapping the physical response of a material to harmonic mechanical excitation. The resulting images allow calculation of regional mechanical properties. Measurements of shear modulus obtained with the MRI technique in gel materials correlate with independent measurements of static shear modulus. The results indicate that displacement patterns corresponding to cyclic displacements smaller than 200 nanometers can be measured. The findings suggest the feasibility of a medical imaging technique for delineating elasticity and other mechanical properties of tissue.

Acoustic Stimulation↗
PubMed1995

The effect of respiration on the contrast and sharpness of liver lesions in MRI.

This work demonstrates the effects of through-plane motion due to respiration on contrast and sharpness of liver lesions in MRI. The effects of slice coverage with and without such respiratory motion is also reported. This work is comprised of two parts: a theoretical prediction of liver-lesion contrast and blur with and without respiration and an experimental validation using gel phantoms of the predicted results. Both theory and experiment show a loss of contrast, increasing with amplitude of the peak-to-peak motion. The loss of contrast for a 5-mm lesion at normal respiration of 15 mm peak-to-peak superior-inferior motion is approximately 10% with a low order sorted respiratory ordered phase encoding acquisition and approximately 50% for an unsorted acquisition. Lesion blur is greatest for the low order sorted acquisition while the unsorted and high sort acquisitions maintain edge definition. Breath-hold imaging is potentially superior to nonbreath-hold imaging in liver lesion contrast and edge definition, but is more sensitive to inadequate slice coverage.

Artifacts↗
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↗