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Biomedical subjects

J N Oshinski

Publications and source records attributed to J N Oshinski.

At least 19 recordsLinked to original sources
PubMed2001

Imaging time after Gd-DTPA injection is critical in using delayed enhancement to determine infarct size accurately with magnetic resonance imaging.

BACKGROUND: In patients with acute myocardial infarction (MI), delayed enhancement is seen in MRI 5 to 7 minutes after gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) injection, and the enhancement occurs in regions that later show recovery of function. However, in a canine model of acute MI, delayed enhancement 20 to 30 minutes after injection only occurs in necrotic regions and not in surrounding, reversibly injured myocardium. The objective of the present study was to determine (1) if the size of the enhanced region varies with time after Gd-DTPA injection and (2) if and when the size of the enhanced region corresponds to the true infarct size. METHODS AND RESULTS: The left coronary artery was occluded in 15 Lewis rats for 30 minutes (n=9) or 2 hours (n=6); this was followed by reperfusion. MRI scans were performed 48+/-2 hours after-MI. Midventricular short-axis images were obtained continuously for 40 minutes after Gd-DTPA injection (0.3 mmol/kg). The sizes of enhanced regions at each time were determined by threshold analysis and compared with triphenyltetrazolium chloride-stained sections of the excised rat heart. In all animals, the enhanced region overestimated infarct size (28+/-5%) immediately after the injection of Gd-DTPA, although it then gradually receded to match the size of the infarct. The time required for enhancement to accurately determine infarct size was significantly different between 2-hour infarcts (16+/-2 minutes) and 30-minute (26+/-4 minutes) infarcts (P<0.05). CONCLUSIONS: In reperfused acute MI, accurate determination of infarct size by delayed enhancement MRI requires imaging at specific times after Gd-DTPA injection, and this time varies with the duration of occlusion.

Animals↗
PubMed2001

Evaluation of the precision of magnetic resonance phase velocity mapping for blood flow measurements.

Evaluating the in vivo accuracy of magnetic resonance phase velocity mapping (PVM) is not straightforward because of the absence of a validated clinical flow quantification technique. The aim of this study was to evaluate PVM by investigating its precision, both in vitro and in vivo, in a 1.5 Tesla scanner. In the former case, steady and pulsatile flow experiments were conducted using an aortic model under a variety of flow conditions (steady: 0.1-5.5 L/min; pulsatile: 10-75 mL/cycle). In the latter case, PVM measurements were taken in the ascending aorta of ten subjects, seven of which had aortic regurgitation. Each velocity measurement was taken twice, with the slice perpendicular to the long axis of the aorta. Comparison between the measured and true flow rates and volumes confirmed the high accuracy of PVM in measuring flow in vitro (p > 0.85). The in vitro precision of PVM was found to be very high(steady: y = 1.00x + 0.02, r = 0.999; pulsatile: y = 0.98x + 0.72, r = 0.997; x: measurement #1, y: measurement #2) and this was confirmed by Bland-Altman analysis. Of great clinical significance was the high level of the in vivo precision (y = 1.01x - 0.04, r = 0.993), confirmed statistically (p = 1.00). In conclusion, PVM provides repeatable blood flow measurements. The high in vitro accuracy and precision, combined with the high in vivo precision, are key factors for the establishment of PVM as the "gold-standard" to quantify blood flow.

Aorta, Thoracic↗
PubMed2001

Construction of a physical model of the human carotid artery based upon in vivo magnetic resonance images.

A method is described for construction of an in vitro flow model based on in vivo measurements of the lumen geometry of the human carotid bifurcation. A large-scale physical model of the vessel lumen was constructed using fused deposition modeling (a rapid prototyping technique) based on magnetic resonance (MR) images of the carotid bifurcation acquired in a healthy volunteer. The lumen negative was then used to construct a flow model for experimental studies that examined the hemodynamic environment of subject-specific geometry and flow conditions. The physical model also supplements physician insight into the three-dimensional geometry of the arterial segment, complementing the two-dimensional images obtained by MR. Study of the specific geometry and flow conditions in patients with vascular disease may contribute to our understanding of the relationship between their hemodvnamic environment and conditions that lead to the development and progression of arterial disease.

Adult↗
PubMed2001

Quantitative prediction of improvement in cardiac function after revascularization with MR imaging and modeling: initial results.

PURPOSE: To evaluate a model that can be used quantitatively to predict changes in postrevascularization left ventricular function based on classification of myocardial tissue as hibernating, scarred, or normal with cine magnetic resonance (MR) imaging. MATERIALS AND METHODS: Eleven patients with chronic left ventricular dysfunction were studied before and after revascularization with cine MR imaging. Regional myocardial contractility and wall thickness were used in the model to predict postrevascularization ejection fraction (EF). The actual EF from the postrevascularization MR images was compared with the EF from the prerevascularization images predicted with the model by using regression analysis and Bland-Altman analysis. RESULTS: Correlation between the actual EF after revascularization and the EF predicted by using the model yielded an R value of 0.98, with a standard error of 1.3 EF percentage points. Predicting changes in function in a myocardial segment was less successful because only 55% of segments classified as hibernating actually improved resting function after revascularization. In nonimproved segments, 78% were either adjacent to infarcted segments or had nontransmural wall thinning. CONCLUSION: A simple mathematical model combined with functional information provided by MR imaging was used to predict improvements in global EF resulting from revascularization.

Aged↗
PubMed1999

Use of navigator-echo-gated MRI to diagnose a coronary shunt involving an anomalous origin of the right coronary artery from the pulmonary artery.

Origin of the right coronary artery from the main pulmonary artery is an anomaly that can cause formation of a left-to-right coronary shunt, leading to myocardial ischemia and early onset of congestive heart failure. We describe a case in which magnetic resonance imaging was able to show the anomalous origin of the right coronary artery, and magnetic resonance phase velocity mapping was able to demonstrate the presence of a left-to-right shunt through the coronary artery by showing retrograde flow in the right coronary artery.

Adult↗
PubMed1999

MRI techniques for cardiovascular imaging.

Over the last several years, cardiovascular MRI has benefited from a number of technical advances which have improved routine clinical imaging techniques. As a result, MRI is now well positioned to realize its longstanding promise of becoming the comprehensive cardiac imaging test of choice in many clinical settings. This may be achieved using a combination of basic advanced techniques. In this overview, the basic cardiac MRI techniques which are clinically useful are reviewed, and the recent technical advances which are clinically promising are described. These advances include routine black blood and cine bright blood techniques that are high speed (<10s per black blood image or cine slice), multislice whole heart perfusion imaging methods, and recently emerging real-time imaging methodologies. J Magn. Reson. Imaging 1999;10:590-601.

Cardiovascular System↗
PubMed1998

Magnetic resonance coronary angiography using navigator echo gated real-time slice following.

Navigator echo gating allows for the elimination of breath-holding in MR imaging by providing a real-time monitor of respiratory position to gate image acquisition. In this study we examined the advantages and utility of real-time, navigator echo gated slice following technique in 2D magnetic resonance coronary angiography of patients with coronary artery disease. Thirteen patients with coronary artery disease were examined. MR images of the right coronary artery (RCA) were obtained parallel to the atrioventricular groove to image long sections of the RCA in a small number of slices. In-plane resolution was 0.7 x 0.9 mm and 2-6 signals were averaged to support this high spatial resolution. Targeted maximum intensity projection (MIP) images were generated from the slices to present the RCA in a single image. All patients had x-ray angiograms available for comparison with the MR images. Using the navigator echo gated real-time slice following technique, MRI successfully obtained images in 11 of 13 cases. The technique failed in two patients with irregular breathing patterns. The average length of the RCA seen in the 11 successful MR exams was 61 mm and the average length seen in the x-ray angiograms was 80 mm. Eight patients were determined to be without disease in the RCA by x-ray angiography, and all eight were correctly identified as normal on the MRI exam. In the three patients who had a successful MRI exam and were determined to have disease in the RCA by x-ray angiography, MRI identified the lesion in two cases. In the third case MRI indicated a discrete lesion and x-ray angiography indicated diffuse disease without a focal lesion. Navigator echo gating improves patient tolerance, provides aligned sections of coronaries over multiple slices, and allows for improved resolution through signal averaging. This preliminary patient study suggests that navigator echo gated magnetic resonance coronary angiography may play a role in evaluating coronary artery disease.

Coronary Angiography↗
PubMed1998

Quantification of the aortic regurgitant volume with magnetic resonance phase velocity mapping: a clinical investigation of the importance of imaging slice location.

BACKGROUND AND AIMS OF THE STUDY: Current techniques for assessment of aortic regurgitation (AR) are mainly qualitative. Magnetic resonance phase velocity mapping (PVM) provides accurate measurements of arterial blood blow. In AR, the aortic regurgitant volume (ARV) can be quantified with a single imaging slice measurement in the ascending aorta. The aim was to use PVM to: (i) quantify the regurgitant volume in patients with AR using an in vitro validated technique; and (ii) confirm in vivo our previous in vitro findings of the importance of measurement location. METHODS: Four healthy volunteers and 19 patients with AR, varying from mild to severe, were examined in a 1.5 Tesla MRI scanner. In 13 patients, the slice was placed: (i) between the aortic valve and the coronary ostia; (ii) at the sinotubular junction (SJ); and (iii) 2 cm above the SJ. In six patients, one measurement was taken as close as technically possible to the aortic valve. PVM measurements of the ARV were compared with angiographic/echocardiographic AR grading. RESULTS: No ARV was measured in healthy subjects. In patients, PVM results correlated well with angiographic/echocardiographic data. Repeatability of the PVM results was excellent and interobserver variability very small. The measured ARV decreased as the slice distance from the aortic valve increased, due to aortic compliance, in agreement to previous in vitro results. Close to the valve, acceleration did not affect the accuracy of velocity measurements. CONCLUSIONS: PVM has great potential to measure AR in a purely quantitative manner. Measurement location is important and results suggest that the closer the measurement to the valve the more accurate the ARV quantification.

Aortic Valve Insufficiency↗
PubMed1997

Slice location dependence of aortic regurgitation measurements with MR phase velocity mapping.

Although several methods have been used clinically to assess aortic regurgitation (AR), there is no "gold standard" for regurgitant volume measurement. Magnetic resonance phase velocity mapping (PVM) can be used for noninvasive blood flow measurements. To evaluate the accuracy of PVM in quantifying AR with a single imaging slice in the ascending aorta, in vitro experiments were performed by using a compliant aortic model. Attention was focused on determining the slice location that provided the best results. The most accurate measurements were taken between the aortic valve annulus and the coronary ostia where the measured (Y) and actual (X) flow rate had close agreement (Y = 0.954 x + 0.126, r2 = 0.995, standard deviation of error = 0.139 L/min). Beyond the coronary ostia, coronary flow and aortic compliance negatively affected the accuracy of the measurements. In vivo measurements taken on patients with AR showed the same tendency with the in vitro results. In making decisions regarding patient treatment, diagnostic accuracy is very important. The results from this study suggest that higher accuracy is achieved by placing the slice between the aortic valve and the coronary ostia and that this is the region where attention should be focused for further clinical investigation.

Aorta↗
PubMed1997

Computational simulation of turbulent signal loss in 2D time-of-flight magnetic resonance angiograms.

Time-of-flight magnetic resonance (MR) angiography is currently limited in the evaluation of arterial stenoses by flow-induced signal loss. This signal loss has been attributed to phase dispersion and to phase misregistration. We have developed a fluid mechanics model of 2D time-of-flight MR angiograms to study the amount of signal loss caused by random turbulence. The simulations were created by stochastic analysis of particle pathlines determined by computational fluid dynamics for turbulent flow. The images obtained by the model compare well to actual MR images of flow in stenoses. By selectively removing the random turbulent motion in the simulation, it can be seen that random phase dispersion is the dominant mechanism of signal loss. Phase misregistration and mean flow phase dispersion act as secondary effects. The MR simulation model recreates accurately the variation of signal loss over a range of echo times. The model can be used further to explore and design new pulse sequences. For example, the current study showed that high slew rate gradient waveforms can significantly reduce poststenotic signal loss. In conclusion, computational modeling of MR angiography can be a useful approach for the analysis of MRA signal loss and the design of improved pulse sequences.

Blood Flow Velocity↗
PubMed1996

Myocardial suppression in vivo by spin locking with composite pulses.

Improved myocardium-blood contrast by myocardial suppression resulting from T1 rho-weighting in contrast-enhanced, gradient-echo, bright-blood cine images, acquired at 1.5T, is shown. In the standard images, blood has twice the intensity of muscle. In similar T1 rho-weighted images, it has 3-4 times the intensity of muscle. A composite spin-lock pulse before each observation pulse provides T1 rho-weighting. A typical pulse was: 90y-135x-360x-135x-90(-y) with element durations: 0.84, 1.26, 8.12, 1.26, and 0.84 ms. The tolerance of this composite pulse to shimmering and frequency errors allows spin locking with comparatively weak RF and therefore low specific absorption rate (SAR). Initial clinical evaluation on patients with poor ventricular function demonstrates both a qualitative and quantitative improvement in delineation of myocardial borders.

Absorption↗
PubMed1996

Cine-MRI-aided endomyocardectomy in idiopathic hypereosinophilic syndrome.

The idiopathic hypereosinophilic syndrome is a leukoproliferative disorder marked by a predilection to damage specific organs, including the heart. This report describes a patient with extensive endocardial fibrosis accompanying this syndrome. Right ventricular endomyocardectomy with preservation of the tricuspid valve was performed. The procedure was aided by cine-magnetic resonance imaging for preoperative assessment and follow-up of surgical results.

Adult↗
PubMed1996

Improved measurement of pressure gradients in aortic coarctation by magnetic resonance imaging.

OBJECTIVES: This study evaluated whether magnetic resonance imaging (MRI) and magnetic resonance (MR) phase velocity mapping could provide accurate estimates of stenosis severity and pressure gradients in aortic coarctation. BACKGROUND: Clinical management of aortic coarctation requires determination of lesion location and severity and quantification of the pressure gradient across the constricted area. METHODS: Using a series of anatomically accurate models of aortic coarctation, the laboratory portion of this study found that the loss coefficient (K), commonly taken to be 4.0 in the simplified Bernoulli equation delta P = KV2, was a function of stenosis severity. The values of the loss coefficient ranged from 2.8 for a 50% stenosis to 4.9 for a 90% stenosis. Magnetic resonance imaging and MR phase velocity mapping were then used to determine coarctation severity and pressure gradient in 32 patients. RESULTS: Application of the new severity-dependent loss coefficients found that pressure gradients deviated from 1 to 17 mm Hg compared with calculations made with the commonly used value of 4.0. Comparison of MR estimates of pressure gradient with Doppler ultrasound estimates (in 22 of 32 patients) and with catheter pressure measurements (in 6 of 32 patients) supports the conclusion that the severity-based loss coefficient provides improved estimates of pressure gradients. CONCLUSIONS: This study suggests that MRI could be used as a complete diagnostic tool for accurate evaluation of aortic coarctation, by determining stenosis location and severity and by accurately estimating pressure gradients.

Adolescent↗
PubMed1996

Two-dimensional coronary MR angiography without breath holding.

PURPOSE: To determine whether breath holding can be eliminated in two-dimensional magnetic resonance (MR) imaging of the coronary arteries by using real-time respiratory gating. MATERIALS AND METHODS: Thirty-one subjects (20 healthy volunteers, 11 patients) underwent MR imaging. In 13 subjects, a respiratory monitoring belt was used, and in 18 subjects, a navigator echo was used. MR imaging was performed with breath holding, respiratory gating, and respiratory gating with two signals acquired. Three reviewers conducted a blinded review of the images, and overall image quality was rated on a scale from 1 (poor) to 5 (excellent). RESULTS: Respiratory gating with two signals acquired provided image quality superior to that with breath-hold imaging (3.7 vs 3.0, respectively; P < .05). Measurements of signal-to-noise ratio (14.5 for respiratory gating with two signals acquired and 11.9 for breath holding) supported the results of the image review. Navigator-echo gating provided better image quality than the monitoring belt (3.7 vs 3.1, respectively; P < .05). CONCLUSION: Breath holding may be eliminated by gating image acquisition to a real-time monitor of respiratory position. Respiratory gating enables improved resolution by means of acquisition of multiple signals, provides aligned sections of coronary arteries, and improves patient tolerance.

Adult↗
PubMed1996

The accuracy of magnetic resonance phase velocity measurements in stenotic flow.

Nuclear magnetic resonance (MR) can be used to measure velocities in fluid flow using the technique of phase velocity mapping. Advantages of MR velocimetry include the simultaneous mapping of the entire flow field through a non-contacting, magnetic window. The phase velocity mapping technique assumes that velocity is constant over the measurement time (typically around 10 ms). For many fluid flows, this assumption is not valid. The current study showed that MR phase velocity measurements of velocity through stenotic flow can be in error by over 100% immediately upstream and downstream of the stenosis throat and by 20% far downstream of the throat in comparison with laser Doppler anemometer measurements taken at the same location. Highly turbulent flow also led to significant errors in velocity measurement. These errors can be attributed to several sources including low signal-to-noise ratio, additional phase shifts due to non-constant velocities, and non-stationary transit-time effects. Velocity measurement errors could be reduced to under 30% at all measurement locations through the use of MR sequences with high signal-to-noise ratios, low echo times, and thick slices.

Arterial Occlusive Diseases↗
PubMed1995

Turbulent fluctuation velocity: the most significant determinant of signal loss in stenotic vessels.

Studies of flow in a 90%-stenosis phantom were conducted to elucidate the parameters and mechanisms responsible for signal loss in MR angiographic images. The studies independently evaluated the effect of velocity, Reynolds number, turbulent fluctuation velocity, and turbulence intensity on the amount of post-stenotic signal loss. Results suggested that the magnitude of the turbulent fluctuation velocity, not merely the presence of turbulence or the intensity of turbulence, was the parameter that determined the extent of the signal loss. The study suggests that future flow phantom studies should be conducted with fluids having physiologic velocities and viscosities to obtain accurate levels of turbulent fluctuation velocities and hence reproduce results of in-vivo signal-loss patterns. The mechanism for signal loss is that the temporal and spatial variations of the turbulent fluctuation velocity cause a range of phases to be present within a voxel. Examination of the theoretical aspects of fluid turbulence suggest that shortening gradient durations and imaging during diastole may help reduce signal loss.

Blood Flow Velocity↗
PubMedDate not supplied

Effects of acceleration on the accuracy of MR phase velocity measurements.

Acceleration in blood flow can affect the accuracy of phase velocity measurements. Convective acceleration is due to changes in flow geometry and is independent of the time-varying acceleration caused by flow pulsatility. To analyze the effects of convective acceleration on flow velocity measurements, phase velocity measurements were obtained in steady laminar flow in the convergent segment of a 90%, hourglass-shaped stenosis phantom at a Reynolds number of 1,500. Measurements at the stenosis indicated that convective acceleration caused the measured values of average cross-sectional velocity to deviate as much as 37% from the theoretical values. The magnitude of the error could be accounted for by including the convective acceleration term in the phase shift equation. Convective acceleration effects should not be ignored in flow velocity measurements through stenoses, even when time-dependent acceleration due to flow pulsatility can be neglected.

Acceleration↗