imaging three-dimensional cardiac function - Walter G. O'Dell, PhD

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432 O’DELL MCCULLOCHAnnu. Rev. Biomed. Eng. 2000.2:431-456. Downloaded from arjournals.annualreviews.orgby UNIVERSITY OF ROCHESTER LIBRARY on 07/27/09. For personal use only.?INTRODUCTIONDEFORMATION ANALYSIS ....................................... 441Implanted Markers .............................................. 443Magnetic Resonance Imaging Tagging ................................ 443Strain Rate Analysis ............................................. 444Slice Motion Correction .......................................... 445Model-Based Deformation Analysis ................................. 446Limitations of the Reconstructions ................................... 446REGIONAL STRAINS IN DISEASE .................................. 447MEASUREMENT OF THREE-DIMENSIONAL TISSUE ARCHITECTURE ..... 448FUTURE DIRECTIONS ........................................... 449The three-dimensional (3-D) nature of heart deformation is well appreciated anda subject of great interest both to cardiovascular scientists and clinical cardiologists.The application of clinical imaging to cardiac function has traditionally beenlimited to global geometric measurements, including left ventricular (LV) wallmass, ventricular volume, stroke volume, ejection fraction, and wall thickness(1). These measures of heart function provide invaluable diagnostic informationto the clinician regarding the severity of disease and the long-term prognosis(2). The ability to measure regional myocardial deformation provides additionalinformation that is valuable for identifying the location and extent of affectedareas, quantifying the degree of mechanical dysfunction, and differentiating betweenfunctionally distinct disease states. Among the investigational tools available,visualization and quantification of regional cardiac mechanical function areperhaps the most direct and reliable indicators of cardiac health (3). For example,although quantification of coronary artery patency is important in directingtreatment of regional myocardial ischemia, the existence of severe vessel occlusionis not necessarily associated with insufficient mechanical function, owingto myocardial revascularization and ventricular wall remodeling. The realm of3-D cardiac mechanical analysis encompasses global and regional myocardialfunctions, 3-D fiber tissue architecture, electrical propagation of activation, thegeneration of internal wall stresses by the contracting myocytes, and the distributionof those stresses by the surrounding tissue matrix. This article describes andcontrasts the prevailing modalities for monitoring 3-D heart function, describesthe current techniques for manipulating image data to model cardiac anatomyand deformation, and outlines what lies ahead for the field of functional cardiacimaging. Length constraints preclude coverage of other areas of cardiac imaging researchand applications, such as metabolic imaging [positron emission tomography(PET) and magnetic resonance spectroscopy (MRS)], electric- and magnetic-fieldimaging, electrical propagation and mechanoelectrical feedback, and perfusionimaging.
Annu. Rev. Biomed. Eng. 2000.2:431-456. Downloaded from arjournals.annualreviews.orgby UNIVERSITY OF ROCHESTER LIBRARY on 07/27/09. For personal use only.?IMAGING 3-D CARDIAC FUNCTION 433Various imaging modalities exist to measure heart wall geometry. These can becompared objectively by the following criteria: signal quality [indicated by thesignal to noise ratio (SNR)], discrimination between myocardial and neighboringtissue [indicated by the contrast to noise ratio (CNR)], temporal and spatial resolution,susceptibility to image blurring and artifact, acquisition and analysis time,ease of use, relative cost, and availability. For accurate quantification of heart wallgeometry and ventricular volumes, CNR is often the more informative measurebecause it governs the ability to discern tissue boundaries. Spatial resolution is typicallygiven by the pixel dimensions in the two-dimensional (2-D) image (voxeldimensions for 3-D); however, for some modalities, such as MR imaging (MRI)and computed tomography (CT), the image slice thickness can be comparativelylarge, leading to partial-volume artifacts that diminish the effective in-plane resolution.Temporal resolution relates to both the time interval between successiveimages (temporal sampling rate) and the time over which the data for each imageare acquired (acquisition window). In X-ray imaging, motion while the shutter isopen leads to blurring and loss of image quality. In MRI, motion during acquisitioncauses a phase dispersion across the image, creating a characteristic artifact.Generally one needs to acquire data on a time scale that is smaller than thefastest motions of interest. For a typical heart rate of 1 Hz, the duration of isovolumicrelaxation or isovolumic contraction is ∼100 ms; hence a temporal samplingrate of
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