Effects of Intrafractional Motion on Water Equivalent Pathlength in Respiratory-Gated Heavy Charged Particle Beam Radiotherapy

PURPOSE

To analyze the water equivalent pathlength (WEL) fluctuations resulting from cardiac motion and display these variations on a beam's-eye-view image; the analysis provides insight into the accuracy of lung tumor irradiation with heavy charged particle beams.

MATERIALS AND METHODS

Volumetric cine computed tomography (CT) images were obtained on 7 lung cancer patients under free-breathing conditions with a 256-multislice CT scanner. Cardiac phase was determined by selecting systole and diastole. A WEL difference image (DeltaWEL) was calculated by subtracting the WEL image at end-systole from that at end-diastole at respiratory exhalation phase. Two calculation regions were defined: Region 1 was limited to the volume defined by planes bounding the heart; Region 2 included the entire body thickness for a given beam's-eye-view angle.

RESULTS

The DeltaWEL values observed in Region 1 showed fluctuations at the periphery of the heart that varied from 20.4 (SD, 5.2) mm WEL to -15.6 (3.2) mm WEL. The areas over which these range perturbation values were observed were 36.8 (32.4) mm(2) and 6.0 (2.8) mm(2) for positive and negative WEL, respectively. The WEL fluctuations in Region 2 increased by approximately 3-4 mm WEL, whereas negative WEL fluctuations changed by approximately -4 to -5 mm WEL, compared with WEL for Region 1; areas over 20 mm WEL changes in Region 2 increased by 9 mm(2) for positive DeltaWEL and 2 mm(2) for negative DeltaWEL.

CONCLUSIONS

Cine CT with a 256-multislice CT scanner captures both volumetric cardiac and respiratory motion with a temporal resolution sufficient to estimate range fluctuations by these motions. This information can be used to assess the range perturbations that charged particle beams may experience in irradiation of lung or esophageal tumors adjacent to the heart.

Physical evaluation of the weighted Feldkamp algorithms applied to the 256-detector row CT scanner for volumetric cine imaging

RATIONALE AND OBJECTIVES

To improve effective scan time and image quality in cone-beam computed tomography (CT), Parker's weighting function (half scan [HS]-Feldkamp-Davis-Kress [FDK]) extended to a larger range up to 2pi was proposed as new half-scan algorithm (NHS-FDK). We conducted a practical physical evaluation of NHS-FDK and HS-FDK using 256-detector row CT.

MATERIALS AND METHODS

Three types of weighting function (full-scan [FS-FDK], HS-FDK, and NHS-FDK) were evaluated by using 256-detector row CT for five variables, ie, point spread function, image noise, CT number uniformity, Feldkamp artifact, temporal resolution, and clinical evaluation.

RESULTS

Image noise, Feldkamp artifact, and temporal resolution were dependent on weighting function. Image noise magnitude was independent of projection angle for all regions of interest with FS-FDK, but showed a symmetric pattern with projection angle with HS-FDK and NHS-FDK. With regard to temporal resolution, NHS-FDK did not remove the motion artifact in the heart except in such slower motion organs as the pulmonary vessels, whereas HS-FDK reduced the motion artifact in the heart. HS-FDK had an even more incomplete data region in the Radon space than FS-FDK, suggesting that it would provide poor image quality distant to the midplane in the longitudinal direction. In practical testing in human subjects, HS-FDK showed inferior performance in all variables except temporal resolution.

CONCLUSION

Despite its inferiority to FS-FDK for static objects, HS-FDK may be useful in chest imaging. Contrary to previous findings using static images, NHS-FDK failed to show advantages over HS-FDK or FS-FDK in a moving phantom and human subjects.

Properties of the prototype 256-row (cone beam) CT scanner

We evaluated Feldkamp artifacts, which are specific to cone-beam computed tomography (CT), in phantom and clinical studies using the 256-multidetector-row CT (256MDCT), and compared the reconstruction accuracy of axial and helical scans. Image noise, slice sensitivity profile (SSP) and artifacts with the 256MDCT were evaluated using a phantom, and the results were compared to those of a 64MDCT. We also examined chest and abdomen scans produced with the 256MDCT in volunteers. For the axial scan, Feldkamp artifacts were visualized as high-frequency streak-like artifacts that are oriented horizontally at the edge of the scan region in the phantom study. Similar results were obtained with the volunteers in soft-tissue regions near either bony structures or air pockets. Feldkamp artifacts with the 256MDCT can lead to misdiagnosis if not correctly identified and minimized via helical scanning. Image noise was less for axial than helical scans, while SSP was better with helical than axial scans. Feldkamp artifacts observed in the 256MDCT images, however, did not generally affect the interpretation of images. The 256MDCT promises more accurate diagnosis, and will provide volumetric cine images of wider cranio-caudal coverage, enabling new applications of CT.

Quasi-exact filtered backprojection algorithm for long-object problem in helical cone-beam tomography

Exact reconstruction from axially truncated cone-beam projections acquired with a helical vertex path is a challenging problem for which solutions are currently under investigation by some researchers. This paper deals with a difficult class of this problem called the long-object problem. Its purpose is to reconstruct a central region of interest (ROI) of a long object when the helical path extends only a little bit above and below the ROI. By extending the authors' recent approach based on the triangular decomposition of the Grangeat formula, we derive quasi-exact reconstruction algorithms whose overall structure is of filtered backprojection (FBP) style. Unlike the previous similar approaches to the long-object problem, the proposed FBP algorithms do not require additional two circular scans at the ends of the helical path. Furthermore, the algorithms require a significantly smaller detector area and achieve improved image quality even for a large pitch compared with the approximate Feldkamp algorithms. One drawback of the proposed algorithms is the computational time, which is much longer than for the Feldkamp algorithms. We show some simulation results to demonstrate the performances of the proposed algorithms.

Exact radon rebinning algorithm for the long object problem in helical cone-beam CT

This paper addresses the long object problem in helical cone-beam computed tomography. We present the PHI-method, a new algorithm for the exact reconstruction of a region-of-interest (ROI) of a long object from axially truncated data extending only slightly beyond the ROI. The PHI-method is an extension of the Radon-method, published by Kudo, Noo, and Defrise in issue 43 of journal Physics in Medicine and Biology. The key novelty of the PHI-method is the introduction of a virtual object fpsi(x) for each value of the azimuthal angle psi in the image space, with each virtual object having the property of being equal to the true object f(x) in some ROI omegam. We show that, for each psi, one can calculate exact Radon data corresponding to the two-dimensional (2-D) parallel-beam projection of fpsi(x) onto the meridian plane of angle psi. Given an angular range of length pi of such parallel-beam projections, the ROI omegam can be exactly reconstructed because f(x) is identical to fpsi(x) in Omegam. Simulation results are given for both the Radon-method and the PHI-method indicating that 1) for the case of short objects, the Radon- and PHI-methods produce comparable image quality, 2) for the case of long objects, the PHI-method delivers the same image quality as in the short object case, while the Radon-method fails, and 3) the image quality produced by the PHI-method is similar for a large range of pitch values.