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Biomedical subjects

D Saloner

Publications and source records attributed to D Saloner.

At least 19 recordsLinked to original sources
PubMed2005

MRI in guiding and assessing intramyocardial therapy.

Cardiovascular intervention, using MRI guidance, is challenging for clinical applications. Real-time imaging sequences with high spatial resolution are needed for monitoring intramyocardial delivery of drug, gene, or stem cell therapies. New generation MR scanners make local intramyocardial and vascular wall therapies feasible. Contrast-enhanced MRI is used for assessing myocardial ischemia, infarction, and scar tissue. Active (microcoils) and passive (T1 and T2* mechanisms) tracking methods have been used for visualization of endovascular catheters. Safety issues related to potential heating of endovascular devices is still a major obstacle for MRI-guided interventions. Fabrication of MRI-compatible interventional devices is limited. Noninvasive imaging strategies will be critical in defining spatial and temporal characteristics of angiogenesis and myocardial repair as well as in assessing the efficacy of new therapies in ischemic heart disease. MRI contrast media improve the capability of MRI by delineating the target and vascular tree. Labeling stem cells enables MRI to trace distribution, differentiation, and survival in myocardium and vascular wall. In the long term, MRI in guiding and assessing intramyocardial therapy may circumvent the limitations of peripherally administered cell therapy, X-ray angiography, and nuclear imaging. MRI represents a highly attractive discipline whose systematic development will foster the implementation of new cardiac and vascular therapies.

Cardiomyopathies↗
PubMed2004

Transport of contrast agents in contrast-enhanced magnetic resonance angiography.

Contrast-enhanced magnetic resonance angiography (CEMRA) often appears to display vessels with good resolution, but it does not adequately visualize the throat of a tight stenosis. Image quality in CEMRA is also governed by the timing of contrast injection and data acquisition. We developed a numerical technique to predict the image appearance of a target vessel by taking into account the passage of contrast agent. Experiments were conducted on a phantom with both centric and linear ordering. An effort to prolong the duration in which the contrast passes by employing a double injection technique gives a wider window of opportunity to acquire quality images.

Biological Transport↗
PubMed2003

Biochemical characterization of atherosclerotic plaque constituents using FTIR spectroscopy and histology.

The behavior of vulnerable atherosclerotic plaques is believed to be closely related to plaque composition. There is a need for an effective in vivo technique for examining plaque constituent properties. In this study, Fourier transform infrared spectroscopy using attenuated total reflectance (FTIR-ATR) was used to assess and analyze the biochemical properties of human atherosclerotic plaques. FTIR spectra clearly revealed prominent spectral features corresponding to plaque constituents of interest: the 2930 cm(-1) and 2850 cm(-1) peaks (indicating the presence of lipids), the 1730 cm(-1) peak (lipid esters), the 1550 cm(-1) and 1650 cm(-1) peaks (fibrous tissues), and the 1100-1000 cm(-1) broad phosphate peak (calcification). Spectral data examined on a qualitative basis correlated well with both gross tissue anatomy and histologic features. Gross spatial mappings of tissue sections of both lipidic and calcified plaques were performed. Spectra from various regions of the plaques demonstrated the evolution of lipid peaks, fibrous tissue peaks, and the phosphate calcification band within the plaques. Histologic analysis corroborated the spectral findings in this study.

Arteriosclerosis↗
PubMed2003

Experimental flow studies in exact-replica phantoms of atherosclerotic carotid bifurcations under steady input conditions.

Extensive flow studies are conducted in two carotid bifurcation flow phantoms. These phantoms exactly replicate the lumen of the plaque excised intact from two patients with severe carotid atherosclerosis. The input flow into the phantom's common carotid artery is steady. Novel scanning techniques for flow visualization and particle image velocimetry are used. In addition, a novel boundary treatment technique is employed in velocimetry to extract first order accurate velocity gradients at walls. The data show that the flow fields are highly three-dimensional. Numerous separation and recirculation zones dominate the flow domain, except at the lowest Reynolds numbers. The separation regions are often so severe that highly directed internal jets form. At high Reynolds numbers, the flows become unsteady and chaotic, even though the input flow is steady. Flow fields have large regions of energetic flow and almost stagnant recirculation zones. These recirculation zones range in size from the full size of the arteries to zones within crevasses smaller than 1 mm. Velocity field and streamline patterns conform well to the lumen geometry. The streamlines are highly tortuous. Stagnation points correlate well with the topological features of the stenosis. Vorticity maps confirm the highly complex and three dimensional nature of the flow. Wall shear stresses at the stenoses are estimated to be on the order of 10 Pa. These studies conclusively show that the nature of the flow in the diseased bifurcation is primarily dictated by the lumen geometry.

Arteriosclerosis↗
PubMed2002

Numerical analysis of flow through a severely stenotic carotid artery bifurcation.

The results of computational simulations may supplement MR and other in vivo diagnostic techniques to provide an accurate picture of the hemodynamics in particular vessels, which may help demonstrate the risks of embolism or plaque rupture posed by particular plaque deposits. In this study, a model based on an endarterectomy specimen of the plaque in a carotid bifurcation was examined. The flow conditions include steady flow at Reynolds numbers of 300, 600, and 900 as well as unsteady pulsatile flow. Both dynamic pressure and wall shear stress are very high, with shear values up to 70 N/m2, proximal to the stenosis throat in the internal carotid artery, and both vary significantly through the flow cycle. The wall shear stress gradient is also strong along the throat. Vortex shedding is observed downstream of the most severe occlusion. Two turbulence models, the Chien and Goldberg varieties of k-epsilon, are tested and evaluated for their relevance in this geometry. The Chien model better captures phenomena such as vortex shedding. The flow distal to stenosis is likely transitional, so a model that captures both laminar and turbulent behavior is needed.

Arteriosclerosis↗
PubMed2001

Structure of plaque at carotid bifurcation: high-resolution MRI with histological correlation.

BACKGROUND AND PURPOSE: The composition of carotid atherosclerosis was visualized by using 3D MRI at high resolution with 200-micrometer (3) voxels. Magnetic resonance signal characteristics were correlated with plaque components, including collagenous cap, necrotic core, and calcification, to define resolution and other requirements for future clinical carotid MRI. METHODS: Twenty-one en bloc carotid endarterectomy specimens were imaged ex vivo by 3D gradient-echo MRI by using a 1.5-T clinical scanner with repetition time, echo time, and flip angle of 40 ms, 18 ms, and 20 degrees, respectively. Plaques were placed in Gd-saline and imaged in a solenoid radiofrequency coil. For quantitative tissue-specific signal analysis, techniques were developed to match tissue sections analyzed by MRI and histology. RESULTS: Three-dimensional imaging resolved complex morphological features not visualized by density- or T(2)-weighted 2D spin-echo imaging. The collagenous cap, necrotic core, and areas of focal calcification showed differing signal characteristics: mean contrast-to-noise ratio for cap versus underlying core was 20. The signal distributions for media and necrotic core overlapped but were resolvable in most specimens. The signal from thrombus was variable. CONCLUSIONS: En bloc specimens provide a useful model for studying plaque MRI. By use of isotropic submillimeter resolution, the collagenous cap and underlying necrotic core typically can be distinguished, and calcification can be identified. Thrombus displays a wide variation in signal intensity. The techniques presented could facilitate future clinicohistological correlation studies for atherosclerotic plaque MRI.

Calcinosis↗
PubMed2000

Influence of stenosis morphology on flow through severely stenotic vessels: implications for plaque rupture.

Flow patterns and flow-related stresses contribute to the characterization of health risks, particularly the risk of plaque rupture, posed by a particular atherosclerotic stenosis. Blood flow in the presence of significant plaque deposits is investigated, and the influence of factors such as stenosis morphology and surface irregularity is evaluated. Solutions for three-dimensional, unsteady flow in these stenotic vessels are obtained for an incompressible, Newtonian fluid. The equations of motion are solved numerically using a finite volume formulation. The resulting flow patterns and shear and normal stresses are interpreted with respect to diagnostic implications, including the possibility of plaque rupture. The inadequacy of "percent stenosis" to characterize the risks posed by a particular plaque is demonstrated. Surface irregularity, stenosis aspect ratio, and the shape of the pulsatile waveform all have considerable influence on the flow field and on the stresses on the plaque. A measure of surface irregularity or plaque symmetry, in particular, may complement percent stenosis in diagnosing the risk of plaque rupture.

Arteriosclerosis↗
PubMed2000

Four dimensional bolus tagging imaging of pulsatile flow.

Inversion bolus tagging MR methods were used to provide a graphic depiction of the axial velocity in three spatial dimensions for pulsatile flow through complex geometries. Visualization of the flow field was readily apparent, and a train of tagged boli were depicted providing an immediate overview of the displacement of flowing fluid over the entire pulsatile cycle. Tagging efficiency obtained using adiabatic inversion pulses was improved compared to that with a windowed sinc pulse. Results from phantom experiments on steady flow were correlated with computational fluid dynamic (CFD) simulations. The use of 3D methods reduced spatial partial volume effects, and the displacement of boli in a steady flow experiment correlated well with CFD simulations. The use of adiabatic inversion pulses resulted in sharp edged inversion regions with good retention of longitudinal magnetization. However in order to keep the pulse duration short, of the order of 2-5 ms, a rather large RF amplitude had to be used. The inversion bolus tagging method is useful in visualizing the flow field in multiple levels for pulsatile fluid flowing through complex geometries, and may be useful in fluid dynamic applications.

Blood Flow Velocity↗
PubMed2000

Atheroemboli to the brain: size threshold for causing acute neuronal cell death.

OBJECTIVE: The objective of this study was to investigate the dilemma posed by the observations that carotid angioplasty dislodges significant numbers of plaque fragments but is reported to have a low rate of neurologic consequences. We examined the fragments released by ex vivo carotid angioplasty. The smaller and most numerous were separated by size and injected into rats to determine the tolerance of the brain to microemboli. METHODS: Ex vivo angioplasty was performed on a total of 20 human carotid plaques removed en bloc. Plaques were placed within polytetrafluoroethylene grafts, and three manipulations were performed: guide wire insertion, 3.5- or 4.0-mm balloon angioplasty, and 5-mm angioplasty with or without a Palmaz stent. After each manipulation, the lumen was flushed, effluent was collected, and fragments were counted under 100x magnification. Using 200-microm and 500-microm micropore mesh, we separated fragments by size into two groups: (1) less than 200 microm and (2) 200 to 500 microm. We then injected rats with saline alone (Group A), with 100 fragments less than 200 microm (Group B), or with 100 fragments 200 to 500 microm (Group C). Animals were euthanized at 1, 3, and 7 days, and brain sections were examined for cell viability and expression of HSP- 72. RESULTS: The total number of fragments dislodged from the plaques varied from 30 to 553. The mean number of fragments released with each manipulation was as follows: guide wire passage, 24; initial balloon angioplasty, 97; second balloon angioplasty, 68; and second angioplasty plus stent, 172. Sixteen of the 20 plaques dislodged fragments that were 1 mm or more in greatest dimension. There was no evidence of brain ischemia in Group A at any time. Group B also showed no injury at 1 or 3 days. However, injection of 200- to 500-microm fragments (Group C) caused a scattered pattern of neuronal cell death. At 7 days, brain sections from both Group B and Group C animals had a scattered pattern of ischemic neurons. There were no classic wedge-shaped infarctions. DISCUSSION: The brain appears to have a surprising tolerance for microembolization in the acute setting. Thus, carotid angioplasty may dislodge plaque fragments, but there may still be a low incidence of stroke. However, even small plaque fragments, less than 200 microm, may cause neuronal ischemia at later time points. Periprocedural microemboli could cause subtle neurologic dysfunction in late follow-up.

Angioplasty, Balloon↗
PubMed1999

Flow and motion.

An understanding of the underlying mechanisms that produce motion-related signal artifacts is useful in designing strategies to reduce or eliminate those effects. These strategies also provide the tools necessary to explore qualitatively and quantitatively a broad range of phenomena associated with flow and motion in the body. Advances in postprocessing methods and scanner hardware will help in the development of important new diagnostic tools.

Artifacts↗
PubMed1998

Determinants of image appearance in contrast-enhanced magnetic resonance angiography. A review.

The use of contrast agents in magnetic resonance (MR) studies of vascular pathology has permitted the exploration of regions that were heretofore poorly evaluated with conventional magnetic resonance angiography (MRA). An important feature of contrast-enhanced MRA (CE-MRA) is the very short acquisition times that are possible. The determination of the parameters to be used in a CE-MRA study rests on an understanding of the dynamics of the passage of the injected contrast agent and the response of the magnetization to the parameters of the MR imaging sequence. An overview of this interaction is presented.

Contrast Media↗
PubMed1996

Calculation of the magnetization distribution for fluid flow in curved vessels.

The signal intensity in magnetic resonance angiography (MRA) images reflects both morphological and flow-related features of vascular anatomy. A thorough understanding of MRA, therefore, demands a careful analysis of flow-related effects. Computational fluid dynamics (CFD) methods are very powerful in determining flow patterns in 3D tortuous vessels for both steady and unsteady flow. Previous simulations of MRA images calculated the magnetization of flowing blood by tracking particles as they moved along flow streamlines that had been determined by a CFD calculation. This manuscript describes MRA simulations that use CFD calculations to determine magnetization variation at a fixed point and, therefore, do not require streamline tracking to calculate the distribution of magnetization in flowing fluids. This method inherently accounts for uniform particle density, avoids problems associated with tracking particles close to the wall, and is well-suited to modeling pulsatile flow.

Blood Flow Velocity↗
PubMed1996

Central intraluminal saturation stripe on MR angiograms of curved vessels: simulation, phantom, and clinical analysis.

PURPOSE: To investigate the appearance of reduced signal intensity in the center of blood vessels on magnetic resonance (MR) angiograms that can mimic intraluminal thrombus. MATERIALS AND METHODS: Simulations and phantom studies were performed to analyze MR angiogram appearance distal to a pronounced curve. RESULTS: Saturation effects substantially lower the signal strength in the center of the vessel relative to that at the vessel periphery. These effects appeared even though the flow was well ordered and laminar. In curved geometries, secondary flow patterns produced counter-rotating vortices, which moved the fastest-moving particles to the outside of the curve and folded the slow-moving particles to the center of the vessel. CONCLUSION: Imaging parameter choices that reduce saturation, such as acquisition of a two-dimensional section transverse to the vessel and through the questionable region, effectively eliminate the central hypointensity effect in vivo.

Adult↗
PubMed1995

Assessment of carotid artery stenosis by ultrasonography, conventional angiography, and magnetic resonance angiography: correlation with ex vivo measurement of plaque stenosis.

PURPOSE: Several studies have investigated the correlation between Doppler ultrasonography (DUS), angiography (CA), and magnetic resonance angiography (MRA) in the evaluation of stenosis of the carotid bifurcation. However, these studies suffer from the lack of a true control-the lesion itself-and therefore conclusions about the diagnostic accuracy of each method remain relative. To determine the absolute accuracy of these modalities, we have prospectively studied lesion size with DUS, MRA, and CA in 28 patients undergoing 31 elective carotid endarterectomies and compared the percent of carotid stenosis determined by each technique to the carotid atheroma resected en bloc. METHODS: All patients were evaluated by each modality within 1 month before the thromboendarterectomy. With DUS, stenosis size was determined by standard flow criteria. For angiography and MRA, stenosis was defined as residual lumenal diameter/estimated normal arterial diameter (European Carotid Surgery Trial criteria). At surgery the carotid atheroma was removed en bloc in all patients. Patients in whom the lesion could not be removed successfully without damage were excluded from the study. Stenosis of the atheroma was determined ex vivo with high-resolution (0.03 mm3) magnetic resonance and confirmed by acrylic injection of the specimen under pressure and measurement of the atheroma wall and lumen. RESULTS: The measurements of the ex vivo stenosis by high-resolution magnetic resonance imaging correlated closely with the size of stenosis determined by the acrylic specimen casts (r = 0.92). By ex vivo measurement, the lesions were placed in the following size categories: 40% to 59% stenosis (n = 2), 60% to 79% stenosis (n = 6), 80% to 89% stenosis (n = 7), and 90% to 99% stenosis (n = 16). CONCLUSIONS: In general, the correlation of measurements of ex vivo stenosis with all modalities was good in these severely diseased arteries, although it was better for DUS (r = 0.80; p < 0.001) and MRA (r = 0.76; p < 0.001) than for CA (r = 0.56; p < 0.05).

Aged↗
PubMed1995

The AAPM/RSNA physics tutorial for residents. An introduction to MR angiography.

This article provides an overview of the basic principles of magnetic resonance (MR) angiography. The parameters in MR imaging manipulated to generate high contrast between flowing nuclei and stationary tissue are discussed. Two primary strategies are used: time-of-flight (TOF) MR angiography, which creates differences in magnetization magnitude between flowing and stationary nuclei, and phase-contrast MR angiography, which induces changes in the spatial orientation, or phase, of flowing nuclei relative to stationary nuclei. The end result of an MR angiographic study is typically a three-dimensional data set composed of either sequential two-dimensional sections or true three-dimensional data. Two-dimensional TOF methods are sensitive to slow flow and are valuable for differentiating between slow flow and occlusion. Three-dimensional TOF methods have better resolution and are more useful in imaging tortuous vessels. Phase-contrast MR angiography can be effectively used to avoid problems of magnetization saturation that occur in three-dimensional TOF studies and to eliminate signal from high-intensity stationary material such as blood products, which may appear bright and mimic flow signal in TOF studies. Careful use of postprocessing tools aids in the assessment of vascular abnormalities once the MR angiographic data have been acquired.

Angiography↗
PubMed1994

MR imaging of flow through tortuous vessels: a numerical simulation.

A novel computer simulation technique is presented that allows the calculation of images from Magnetic Resonance Angiography (MRA) studies of blood flow in realistic curving and branching two-dimensional vessel geometries. Fluid dynamic calculations provide flow streamlines through curved or branching vessels. MR simulations generate images for specific MR pulse sequence parameters. Simulations of steady flow in carotid bifurcation and carotid siphon geometries as imaged by a standard, flow-compensated, spoiled gradient echo sequence illustrate the major features seen in clinical time of flight MRA studies. The simulations provide insight into a number of artifacts encountered in MRA such as displacement artifacts, signal pile-up, truncation artifacts, and intravoxel phase dispersion.

Algorithms↗
PubMed1994

MRA studies of arterial stenosis: improvements by diastolic acquisition.

Cardiac-phase-specific data acquisition is used to reduce signal loss in MR Angiography resulting from disturbed flow. RF pulses are delivered continuously throughout the cardiac cycle, but incrementation of phase-encoding gradients and data storage are enabled only during the chosen part of the cycle. Studies in a stenotic pulsatile flow phantom demonstrate that poststenotic signal loss is primarily determined by the mean flow velocity, and is not appreciably affected by acceleration or deceleration of the mean flow rate. The signal loss is least in diastole. In vivo studies in patients with carotid artery disease show that data acquisition in diastole reduces the apparent degree and extent of carotid bifurcation stenosis and provides a crisper definition of the vascular lumen. The additional time required for cardiac-phase-specific acquisition can be reduced by gating only the lower-order phase-encoding lines while retaining acceptable image quality.

Blood Flow Velocity↗