Dose broadening due to target position variability during fractionated breath-held radiation therapy.

Recent advances in Stereotactic Radiosurgery/Conformal Radiotherapy have made it possible to deliver surgically precise radiation therapy to small lesions while preserving the surrounding tissue. However, because of physiologic motion, the application of conformal radiotherapy to extra-cranial tumors is, at present, geared toward slowing the progression of disease rather than obtaining a cure. At the University of Rochester, we are investigating the use of patient breath-holding to reduce respiratory-derived motion in fractional radiotherapy. The primary targeting problem then becomes the small variation in tumor location over repeated breath-holds. This paper describes the effects of residual target position uncertainty on the dose distribution observed by small extra-cranial tumors and their neighboring tissues during fractional radiation treatment using breath holding. We employ two computational methods to study these effects: numerical analysis via Monte Carlo simulation and analytical computation using three-dimensional convolution. These methods are demonstrated on a 2-arc, 10-fraction treatment plan used to treat a representative lung tumor in a human subject. In the same human subject, the variability in position of a representative lung tumor was measured over repeated end-expiration breath-holds using volumetric imaging. For the 7 x 7 x 10 mm margin used to treat this 12 mm diameter tumor and the measured target position variability, we demonstrated that the entire tumor volume was irradiated to at least 48 Gy-well above the tumoricidal threshold. The advantages, in terms of minimizing the volume of surrounding lung tissue that is radiated to high dose during treatment, of using end-expiration breath holding compared with end-inspiration breath-holding are demonstrated using representative tumor size and position variability parameters. It is hoped that these results will ultimately lead to improved, if not curative, treatment for small (5-20 mm diameter) lung, liver, and other extra-cranial lesions.

Imaging three-dimensional cardiac function.

The three-dimensional (3-D) nature of myocardial deformations is dependent on ventricular geometry, muscle fiber architecture, wall stresses, and myocardial-material properties. The imaging modalities of X-ray angiography, echocardiography, computed tomography, and magnetic resonance (MR) imaging (MRI) are described in the context of visualizing and quantifying cardiac mechanical function. The quantification of ventricular anatomy and cavity volumes is then reviewed, and surface reconstructions in three dimensions are demonstrated. The imaging of myocardial wall motion is discussed, with an emphasis on current MRI and tissue Doppler imaging techniques and their potential clinical applications. Calculation of 3-D regional strains from motion maps is reviewed and illustrated with clinical MRI tagging results. We conclude by presenting a promising technique to assess myocardial-fiber architecture, and we outline its potential applications, in conjunction with quantification of anatomy and regional strains, for the determination of myocardial stress and work distributions. The quantification of multiple components of 3-D cardiac function has potential for both fundamental-science and clinical applications.

Three-dimensional residual strain in midanterior canine left ventricle.

All previous studies of residual strain in the ventricular wall have been based on one- or two-dimensional measurements. Transmural distributions of three-dimensional (3-D) residual strains were measured by biplane radiography of columns of lead beads implanted in the midanterior free wall of the canine left ventricle (LV). 3-D bead coordinates were reconstructed with the isolated arrested LV in the zero-pressure state and again after local residual stress had been relieved by excising a transmural block of tissue. Nonhomogeneous 3-D residual strains were computed by finite element analysis. Mean +/- SD (n = 8) circumferential residual strain indicated that the intact unloaded myocardium was prestretched at the epicardium (0.07 +/- 0.06) and compressed in the subendocardium (-0.04 +/- 0.05). Small but significant longitudinal shortening and torsional shear residual strains were also measured. Residual fiber strain was tensile at the epicardium (0.05 +/- 0.06) and compressive in the subendocardium (-0.01 +/- 0.04), with residual extension and shortening, respectively, along structural axes parallel and perpendicular to the laminar myocardial sheets. Relatively small residual shear strains with respect to the myofiber sheets suggest that prestretching in the plane of the myocardial laminae may be a primary mechanism of residual stress in the LV.

Anterior and posterior left ventricular sarcomere lengths behave similarly during ejection.

Previous studies of regional differences in myocardial deformation between the anterior and posterior walls of the canine left ventricle were based on strain, which is not an absolute measure of deformation. We thus compared sarcomere lengths at anterior and posterior sites during ejection in isolated dog hearts. Cineradiographic imaging of regional deformation with radiopaque markers implanted near the midwall in five hearts and just below the epicardium in six hearts, combined with postmortem histology, allowed sarcomere length reconstruction throughout the cardiac cycle. The amount of sarcomere shortening accompanying left ventricular ejection was similar in both walls of the left ventricle for sarcomeres located at epicardial and midwall sites. The mean sarcomere length (taken at the middle of the ejecting range) was also similar between the anterior and posterior sites when averaged over all hearts. The similarity of sarcomere function held not only at end systole but throughout ejection and over wide ranges of ventricular pre- and afterloads. Hence functional measurements of relative myocardial shortening may not be indicative of regional sarcomere length heterogeneity.

A phased array coil for human cardiac imaging.

A prototype cardiac phased array receiver coil was constructed that comprised a cylindrical array and a separate planar array. Both arrays had two coil loops with the same coil dimensions. Data acquisition with the cylindrical array placed on the human chest, and the planar array placed under the back, yielded an overall enhancement of the signal-to-noise ratio (SNR) over the entire heart by a factor of 1.1-2.85 over a commercially available flexible coil and a commercially available four-loop planar phased array coil. This improvement in SNR can be exploited in cardiac imaging to increase the spatial resolution and reduce the image acquisition time.

Three-dimensional myocardial deformations: calculation with displacement field fitting to tagged MR images.

PURPOSE: To reconstruct three-dimensional (3D) myocardial deformations from orthogonal sets of parallel-tagged magnetic resonance (MR) images. MATERIALS AND METHODS: Displacement information in the direction normal to the undeformed tag planes was obtained at points along tag lines. Three independent sets of one-dimensional displacement data were used to fit an analytical series expression to describe 3D displacement as a function of deformed position. The technique was demonstrated with computer-generated models of the deformed left ventricle with data from healthy human volunteers. RESULTS: Model deformations were reconstructed with a 3D tracking error of less than 0.3 mm. Error between estimated and observed one-dimensional displacements along the tags in 10 human subjects was 0.00 mm +/- 0.36 (mean +/- standard deviation). Robustness to noise in the tag displacement data was demonstrated by using a Monte Carlo simulation. CONCLUSION: The combination of rapidly acquired parallel-tagged MR images and field-fitting analysis is a valuable tool in cardiac mechanics research and in the clinical assessment of cardiac mechanical function.

A modified quadrupole gradient set for use in high resolution MRI tagging.

A special purpose gradient set with a Cos(2 theta) current distribution has been constructed for high resolution MR imaging of small samples in a clinical Signa 1.5 T scanner using the existing gradient amplifiers. The X, Y, and Z gradient coils can attain gradient field strengths of 11.3, 4.7, and 15.2 G/cm at 100 amps current, respectively, with a slew rate of 20 G/cm/ms and usable ramp time of 150 microseconds. Field distortions are less than 2% over the central 8 cm of the bore, suitable for high resolution tagging of isolated canine hearts.

Calculation of three-dimensional left ventricular strains from biplanar tagged MR images.

The noninvasive measurement of time-resolved three-dimensional (3D) strains throughout the myocardium could greatly improve the clinical evaluation of cardiac disease and the ability to mathematically model the heart. On the basis of orthogonal arrays of tagged magnetic resonance (MR) images taken at several times during systole, such strains can be determined, but only after heart motion through the image planes is taken into account. An iterative material point-tracking algorithm is presented to solve this problem. It is tested by means of mathematical models of the heart with cylindric and spherical geometries that undergo deformations and bulk motions. Errors introduced by point-tracking interpolation were found to be negligible compared with those due to marker identification on the images. In a human heart studied with this technique, the corrected radial strains at the left ventricular base were approximately 2.5 times the two-dimensional estimates derived from the fixed image planes. The authors conclude that material point tracking allows accurate, time-resolved 3D strains to be calculated from tagged MR images, and that prior correction for motion of the heart through image planes is necessary.