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

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436 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.?Magnetic Resonance Imaginghigh spatial resolution (typically 2- × 2- × 5-mm voxel dimensions), high temporalresolution (∼1 s), and the ability to acquire axial images at arbitrary locationsthrough the chest. CT scanners, however, are more expensive and less availablethan ECG imagers. The simultaneous acquisition of multiple 2-D CT images for3-D–image reconstruction was initially demonstrated in 1980, with the dynamicspatial reconstructor [DSR (18)]. The DSR uses multiple X-ray sources with coneshapedbeams and a hemicylindrical fluorescent screen to acquire volumetric datawith high temporal resolution. Whereas the DSR has remained primarily a researchtool, electron-beam CT (EBCT) (19) has become increasingly popular forclinical use. In EBCT, X-rays are produced by scanning a single-source electronbeam onto a tungsten target that is positioned in a semicircle below the patient(20). Mechanical motion in the gantry is eliminated (bypassing the typical 1-sinterscan delay), and exposure times of 50–100 ms are possible. In typical multislicemode, four tungsten target rings and two detector rings are used to acquireup to eight image slices without movement of the patient table, reducing motionartifact. Spiral CT (also known as helical CT) is another emerging techniquefor very rapid acquisition of 3-D image data; with higher spatial but lower temporalresolution than EBCT. Spiral CT imaging uses a conventional fan-shapedbeam source, which rotates continuously about the patient, and a fixed array ofreceivers. Contiguous 2-D axial images are acquired as the sample moves alongthe scanner bore. Multiple receivers are often used to acquire four slices simultaneously.Retrospective gating is commonly used in EBCT and spiral CT to reducemotion artifact (21). With both multislice EBCT and multislice spiral CT, 3-Dimage data sets can be acquired in times on the order of seconds—quickly enoughthat susceptibility to remaining motion artifact is significantly reduced. On-linereconstruction algorithms have been developed to aid in the graphical display of3-D CT cardiac data sets and in simulating views from obliquely oriented crosssections (22).Magnetic resonance (MR) imaging (MRI) both provides high CNRs between softtissues and blood without the injection of contrast agents and enables imaging atarbitrary image angles. MRI scans are relatively expensive and time consuming,and they require a certain amount of patient cooperation. In-plane spatial resolutioncan be high (1 × 1 mm); however, the slice thickness is typically 5–10 mm; thusthe images are highly susceptible to partial-volume artifacts. This is problematicespecially towards the heart apex, where the wall slope is high. The quality andresolution of MR cardiac images have steadily improved with the inclusion ofbreath holding, black-blood saturation (which improves the contrast between themyocardium and blood in the cavity), rapid imaging techniques (i.e. gradient echoand echo-planar imaging for quicker acquisition), k-space segmentation (for improvedtemporal resolution), and dedicated cardiac radiofrequency receiver coils(for improved SNR) (23). With prospective gating and gradient echo sequences,
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 437the image data for 10–14 temporal images at a single slice location in the heartcan be acquired within a 16- to 20-s breath hold. Breath holds of this duration arefeasible in cardiac patients, even those with acute myocardial infarction (3). Theacquisition of cine data for 8–10 short-axis slices thus requires ∼10–15 min, allowingfor patient recovery after each breath hold. Newer sequences, using SMASH(simultaneous acquisition of spatial harmonics) for example (24), can cut this timeby a factor of 2 or 3. MRI can also be used to perform retrospective respiratorygating with navigator echo signals from the diaphragm, although the implementationof this technique is difficult (25). A recent review of MRI applied to cardiacmotion analysis by McVeigh (25) is highly recommended.LV volume, ejection fraction, and wall thickness are perhaps the most frequentlyused indices of cardiac performance and patient prognosis (26, 27). However, thepertinent geometric information must first be extracted from the images before 3-Dsurface reconstruction. The challenge is to detect, within the image, heart contourfeatures that are irregular in shape and position, changing with time, and varying incontrast and whose image intensity gradient profile can change in both magnitudeand sign. Contour segmentation is the most laborious and error-prone part of mostanalysis schemes. Real-time clinical application of modern 3-D cardiac mechanicstechniques is going to require faster and better automated segmentation. We referthe reader to the article in this volume of Annual Review of Biomedical Engineeringthat pertains to image segmentation, by Pham et al (27a).LV wall thickness measurements have been linked to regional ischemia (27) andlead to estimates of mean wall stress (28) and mean wall stiffness. Wall thicknessmeasurements were made as early as 1964, by X-ray angiography (4), but morerecently Jakob et al (29) improved the technique by using injected-contrast anddigital-subtraction angiography. In comparison with the more commonly used M-mode ECG, the Jakob method showed good agreement towards end-diastole butpoorer agreement at end-systole.Although wall thickness changes are often the most apparent geometric indicatorof altered mechanical function (30), the relationship between wall thickness andcardiac health is not as clear. Significant correlations were found by Lawson et al(31) between thallium uptake and systolic wall thickness in ischemic hearts, but notwith diastolic wall thickness and thallium uptake (31, 32). Similarly, Curiel et al(33) found that the magnitude of inotropic reserve is not related to diastolic wallthickness or to basal systolic wall thickening. Heupler et al (34) showed that theLV end-diastolic cavity diameter, not wall thickness, was associated with thalliumperfusion defects and therefore myocardial ischemia. Dong et al (35) concludedANALYSES OF VENTRICULAR SIZE AND GEOMETRYWall Thickness Measurements
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