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Rahim R Rizi

Publications and source records attributed to Rahim R Rizi.

13 recordsLinked to original sources
PubMed2006

Advances in magnetic resonance imaging of lung physiology.

This review presents an overview of some recent magnetic resonance imaging (MRI) techniques for measuring aspects of local physiology in the lung. MRI is noninvasive, relatively high resolution, and does not expose subjects to ionizing radiation. Conventional MRI of the lung suffers from low signal intensity caused by the low proton density and the large degree of microscopic field inhomogeneity that degrades the magnetic resonance signal and interferes with image acquisition. However, in recent years, there have been rapid advances in both hardware and software design, allowing these difficulties to be minimized. This review focuses on some newer techniques that measure regional perfusion, ventilation, gas diffusion, ventilation-to-perfusion ratio, partial pressure of oxygen, and lung water. These techniques include contrast-enhanced and arterial spin-labeling techniques for measuring perfusion, hyperpolarized gas techniques for measuring regional ventilation, and apparent diffusion coefficient and multiecho and gradient echo techniques for measuring proton density and lung water. Some of the major advantages and disadvantages of each technique are discussed. In addition, some of the physiological issues associated with making measurements are discussed, along with strategies for understanding large and complex data sets.

Helium↗
PubMed2005

Measurements of regional alveolar oxygen pressure using hyperpolarized 3He MRI.

RATIONALE AND OBJECTIVES: The aim of this work is to review hyperpolarized (HP) helium-3 (3He) magnetic resonance imaging (MRI) methods to measure regional alveolar oxygen partial pressure (P(A)O2) and oxygen depletion rate (R) in the lung. We point out limitations of the methods and suggest improvements to increase their accuracy. MATERIALS AND METHODS: P(A)O2 and R can be extracted from series of HP gas images acquired during breath hold by making use of the depolarizing effect of oxygen on HP gas. To separate oxygen-induced depolarization from other depolarizing effects, several techniques can be used. We review currently used techniques and point out their advantages and limitations. RESULTS: We show that the precision of oxygen measurements depends on a variety of parameters and can vary within the measurement volume. Accuracy of the measurement also can be influenced by diffusion of oxygen and polarized 3He and generally is different for single-slice and multislice measurements. We present numerical simulations, phantom data, and in vivo data for illustration. CONCLUSION: HP 3He MRI is a noninvasive, nonionizing, and repeatable imaging method that allows for quantitative analysis of lung function. The current techniques for measuring P(A)O2 have the potential to deliver clinically relevant functional images.

Animals↗
PubMed2005

Hyperpolarized 3He MRI in asthma measurements of regional ventilation following allergic sensitization and challenge in mice--preliminary results.

RATIONALE AND OBJECTIVES: Quantitative regional measurement of physiological parameters of lung may improve both early detection of asthma and its response to treatment by elucidating the characteristics of airway obstruction. Recent emergence of hyperpolarized helium-3 magnetic resonance imaging as a sensitive pulmonary imaging tool has shown great potential in capturing important structural and functional aspects of normal and diseased lungs. The objective of this study was to investigate regional ventilation changes in the mouse lung following allergen sensitization and challenge. MATERIALS AND METHODS: A murine model of allergic airway inflammation was created in mice following allergen challenge using Af and IgE-mediated asthma. The creation of model was verified using pulmonary function test and histology. Regional fractional ventilation was then measured in the animals using hyperpolarized 3He MRI on a pixel-by-pixel basis with a planar resolution of 0.24 mm. The sensitized and healthy animals were then compared statistically to assess the potential sensitivity of this technique in detection of such pulmonary abnormalities. RESULTS: In this work, we have demonstrated for the first time the quantitative measurement of regional ventilation in normal and asthmatic mice. Results of this study show significant changes in regional ventilation in murine model of allergic airway sensitization compared with that in normal control animals. CONCLUSION: Further development of this technique can potentially serve as a quantitative marker to investigate the physiology of allergen-induced airway hyperresponsiveness and to assist in disease treatment and prevention.

Animals↗
PubMed2004

Detection of simulated pulmonary embolism in a porcine model using hyperpolarized 3He MRI.

Several radiological imaging modalities are available to assist with the clinical diagnosis of pulmonary embolism (PE). The most frequently used techniques-nuclear medicine ventilation-perfusion (VP) scan, computed tomography (CT), magnetic resonance angiography (MRA), and pulmonary angiography (PA)-all have literature-supported, substantial limitations with respect to timeliness and patient safety. Hyperpolarized 3He magnetic resonance gas distribution imaging (HP 3He MRI) recently has shown potential as a safer and faster alternative. In this study, we performed HP 3He MRI on a porcine model (N = 6) of simulated PE using selective occlusion balloon catheterization (N = 4) and nonselective aged autologous clot injection (N = 1). The technique was also performed on a normal pig and again after the animal was killed. Temporal depletion of regional HP 3He MRI signal intensity provided for a qualitative assessment of simulated PE (N = 4), and regional PAO2 (alveolar partial pressure of oxygen) was calculated in affected airspaces for a quantitative assessment of simulated PE (N = 1). The preliminary results suggest that HP (3)He MRI shows promise as a means of assessing regional pulmonary perfusion abnormalities in the porcine models of simulated PE that were used in this study.

Animals↗
PubMed2004

Determination of regional VA/Q by hyperpolarized 3He MRI.

Alveolar ventilation/perfusion ratio (VA/Q) is a key parameter in functional imaging of the lung. Herein, regional VA/Q was calculated from regional values of alveolar partial pressure of oxygen (PAO2) measured by hyperpolarized 3He gas MRI (HP 3He MRI). Yorkshire pigs (n = 7, mean weight = 25 kg) were paralyzed and maintained under isoflurane anesthesia. Animals were placed into a birdcage coil, then transferred to the bore of a 1.5 T MRI unit. Prior to imaging, animals were manually ventilated with room air for 5 min, then a 3He gas mixture was administered during breathhold and imaging performed. PAO2 was measured based on the decay rate of 3He signal. Subjects' blood gas concentrations were measured and these values and PAO2 values entered into a system of four equations with four unknowns. Calculated VA/Q values were analyzed by preparing frequency distributions for the entire lung and compared to VA/Q frequency distributions previously established in the literature as normal using other diagnostic techniques. Distributions were consistent with those in the literature, indicating that HP 3He MRI may be an accurate, quantitative, noninvasive, and nonradioactive method for acquiring VA/Q for small regions of the lung.

Animals↗
PubMed2003

Co-registration of acquired MR ventilation and perfusion images--validation in a porcine model.

Under controlled imaging conditions, functional, co-registered ventilation and perfusion images were obtained for quantitative MRI assessment of pulmonary function. Experiments were conducted on 25-35 kg Yorkshire pigs (N = 5) under ketamine/isoflurane anesthesia. Ventilation and perfusion MR images were obtained using laser polarized gas and gadolinium (Gd), respectively. The mutual information-based technique was used to quantify the initial misregistration between the acquired ventilation and perfusion images, to cope with different intensity characteristics. Means and standard deviations (SDs) of misregistrations for the five subjects, in terms of absolute translation and absolute rotations around each of the three coordinate axes, were computed to evaluate the hypothesis. For qualitative illustrations, plots of mutual information values under absolute translations and rotations around each of the three coordinate axes are presented. This preliminary study shows the feasibility of acquiring functional image data that are spatially co-registered. This method has potential applications in the quantitative assessment of regional alveolar ventilation and pulmonary perfusion ratio, in both normal and diseased lung.

Animals↗
PubMed2003

An evaluation of pulmonary atelectasis and its re-expansion: hyperpolarized 3He MRI in the Yorkshire pig.

RATIONALE AND OBJECTIVES: Atelectasis, the collapse of small airways, is a significant clinical problem. We use hyperpolarized (HP) 3He magnetic resonance imaging (MRI), or HP 3He MRI, to describe atelectasis in the normal Yorkshire pig, the pig with atelectasis, and the pig with re-expansion of atelectasis. We compare HP 3He MRI findings with depictions of atelectasis by proton MRI. MATERIALS AND METHODS: During end-expiration in the anesthetized and paralyzed Yorkshire pig (n = 6), HP 3He gas produced by the optical pumping spin-exchange method, was delivered via an endotracheal tube. For two separate groups, atelectasis was either induced by Fogarty-catheter occlusion balloon inflation (n = 3), or lateral chest wall administration of sodium hydroxide (NaOH) (n = 3). MRI was performed at time zero, at 5, 9, 13, 15, and 19 minutes after atelectasis production, 30 minutes after balloon deflation, and 10 and 30 minutes after recruitment of atelectatic areas with increased tidal volumes and added positive end-expiratory pressure. High-resolution, cross-sectional MR images were procured, and comparison was made with the traditional proton MRI. RESULTS: Atelectatic areas by HP 3He MRI were easily distinguishable in both subject groups, and correlated with those located by proton MR. HP 3He MR images showed absence of ventilation, whereas proton MR images depicted dense, white areas. Re-expansion of atelectasis was well delineated by HP 3He MRI. CONCLUSION: HP 3He MRI may overcome many of the shortcomings of other well-established radiographic methods. HP 3He MRI is a novel, informative method for describing atelectasis and its re-expansion.

Animals↗
PubMed2003

Operating characteristics of hyperpolarized 3He and arterial spin tagging in MR imaging of ventilation and perfusion in healthy subjects.

RATIONALE AND OBJECTIVES: The authors tested the feasibility of a magnetic resonance (MR) imaging method combining the use of hyperpolarized helium 3 (3He) for ventilation imaging and an arterial spin-tagging sequence for perfusion imaging in six healthy human subjects. MATERIALS AND METHODS: High-resolution sagittal images depicting 3He distribution were acquired after the subjects' inhalation of 500 mL of laser-hyperpolarized 3He produced by spin-exchange optical pumping. Perfusion MR imaging was performed with a steady-state arterial spin-tagging sequence that enabled the acquisition of three-dimensional images of pulmonary perfusion without the need for subject breath holding. RESULTS: The 3He ventilation images display, with high signal intensity and detailed anatomic localization, the airspace of the lung parenchyma. The signal intensity on the perfusion images decreased by 23.2% with the use of arterial spin tagging. Ventilation and perfusion were matched, as is expected in healthy subjects. CONCLUSION: This method may have important applications in the assessment of lung function, enabling the calculation of regional ventilation-perfusion ratios. It may also aid in the selection of candidates for lung volume-reduction surgery.

Adult↗
PubMed2002

Pulmonary ventilation and perfusion scanning using hyperpolarized helium-3 MRI and arterial spin tagging in healthy normal subjects and in pulmonary embolism and orthotopic lung transplant patients.

Conventional nuclear ventilation/perfusion (V/Q) scanning is limited in spatial resolution and requires exposure to radioactivity. The acquisition of pulmonary V/Q images using MRI overcomes these difficulties. When inhaled, hyperpolarized helium-3 ((3)He) permits MRI of gas distribution. Magnetic labeling of blood (arterial spin-tagging (AST)) provides images of pulmonary perfusion. Three normal subjects, two patients who had undergone single lung transplantation for emphysema, and one subject with pulmonary embolism (PE), were imaged. (3)He distribution and blood perfusion appeared uniform in the normal subjects and throughout the lung allografts. Gas distribution and perfusion in the emphysematous lungs were non-uniform and paralleled radiographic abnormalities. AST imaging alone revealed a lower-lobe wedge-shaped perfusion defect in the patient with PE that corresponded to computed tomography (CT) imaging. Hyperpolarized (3)He gas is demonstrated to provide ventilation images of the lung. Blood perfusion information may be obtained during the same examination using the AST technique. The sequential application of these imaging methods provides a novel tool for studying V/Q relationships.

Helium↗
PubMed2002

Magnetization structure contrast based on intermolecular multiple-quantum coherences.

In vivo and ex vivo MRI based on intermolecular multiple-quantum coherences (iMQC) is predicted to provide a fundamentally different source of contrast for MRI. This article investigates the dependence of image contrast upon the choice of correlation distance for a heterogeneous material. A closely packed array of parallel hollow cylinders was used to demonstrate signal intensity variations when the correlation distance becomes comparable to the gap size between the cylinders. The observed effects agree well with three-dimensional calculations of the time evolution of magnetization under the nonlinear Bloch equations.

Contrast Media↗
PubMedDate not supplied

3D airway segmentation via hyperpolarized 3He gas MRI by using scale-based fuzzy connectedness.

Computerized segmentation of 3D tracheobronchial tree is a necessary first step for subsequent registration and analysis of pulmonary airway and vascular magnetic resonance (MR) images obtained by using hyperpolarized 3Helium gas and Gadolinium. The scientific and clinical implications of acquiring these data on the tracheobronchial tree (for studying ventilation, V) and on the coinciding pulmonary arterioles (for studying perfusion, Q), is the next frontier for static and dynamic pulmonary MRI. In this paper, we report an airway segmentation method from 3He MR images based on the scale-based fuzzy connectedness approach. Incorporated in this method are the pre-processing steps of inhomogeneity correction and intensity standardization. The basic sequential steps in the proposed airway segmentation method are: (1) image acquisition, (2) radio frequency field inhomogeneity correction, (3) standardization of MR image intensity scale, (4) seed specification, (5) scale-based fuzzy connected segmentation of airways, and (6) thresholding and binarization. The majority of these steps are automatically executed; others allow interaction through a graphical interface provided in the 3DVIEWNIX software system, in which the algorithms are implemented. The method achieves an overall precision of about 98% in terms of the extent of overlap in repeated segmentations. Its level of accuracy can be described by a true positive volume fraction of about 98% (considering manual delineation as the surrogate of true delineation), and a false negative and positive volume fraction of about 1%. The total operator and computational time required per study are on the average 2 and 20 min.

Bronchi↗