Proton radiography and fluoroscopy of lung tumors: a Monte Carlo study using patient-specific 4DCT phantoms.

PURPOSE: Monte Carlo methods are used to simulate and optimize a time-resolved proton range telescope (TRRT) in localization of intrafractional and interfractional motions of lung tumor and in quantification of proton range variations. METHODS: The Monte Carlo N-Particle eXtended (MCNPX) code with a particle tracking feature was employed to evaluate the TRRT performance, especially in visualizing and quantifying proton range variations during respiration. Protons of 230 MeV were tracked one by one as they pass through position detectors, patient 4DCT phantom, and finally scintillator detectors that measured residual ranges. The energy response of the scintillator telescope was investigated. Mass density and elemental composition of tissues were defined for 4DCT data. RESULTS: Proton water equivalent length (WEL) was deduced by a reconstruction algorithm that incorporates linear proton track and lateral spatial discrimination to improve the image quality. 4DCT data for three patients were used to visualize and measure tumor motion and WEL variations. The tumor trajectories extracted from the WEL map were found to be within 1 mm agreement with direct 4DCT measurement. Quantitative WEL variation studies showed that the proton radiograph is a good representation of WEL changes from entrance to distal of the target. CONCLUSIONS: MCNPX simulation results showed that TRRT can accurately track the motion of the tumor and detect the WEL variations. Image quality was optimized by choosing proton energy, testing parameters of image reconstruction algorithm, and comparing to ground truth 4DCT. The future study will demonstrate the feasibility of using the time resolved proton radiography as an imaging tool for proton treatments of lung tumors.

Variations in tumor size and position due to irregular breathing in 4D-CT: a simulation study.

PURPOSE: To estimate the position and volume errors in 4D-CT caused by irregular breathing. METHODS: A virtual 4D-CT scanner was designed to reproduce axial mode scans with retrospective resorting. This virtual scanner creates an artificial spherical tumor based on the specifications of the user, and recreates images that might be produced by a 4D-CT scanner using a patient breathing waveform. 155 respiratory waveforms of patients were used to test the variability of 4D-CT scans. Each breathing waveform was normalized and scaled to 1, 2, and 3 cm peak-to-peak motion, and artificial tumors with 2 and 4 cm radius were simulated for each scaled waveform. The center of mass and volume of resorted 4D-CT images were calculated and compared to the expected values of center of mass and volume for the artificial tumor. Intrasubject variability was investigated by running the virtual scanner over different subintervals of each patient's breathing waveform. RESULTS: The average error in the center of mass location of an artificial tumor was less than 2 mm standard deviation for 2 cm motion. The corresponding average error in volume was less than 4%. In the worst-case scenarios, a center of mass error of 1.0 cm standard deviation and volume errors of 30%-60% at inhale were found. Systematic errors were observed in a subset of patients due to irregular breathing, and these errors were more pronounced when the tumor volume is smaller. CONCLUSIONS: Irregular breathing during 4D-CT simulation causes systematic errors in volume and center of mass measurements. These errors are small but depend on the tumor size, motion amplitude, and degree of breathing irregularity.

Evaluation and commissioning of a surface based system for respiratory sensing in 4D CT.

The purpose of this study is to assess the temporal and reconstruction accuracy of a surface imaging system, the GateCT under ideal conditions, and compare the device with a commonly used respiratory surrogate: the Varian RPM. A clinical CT scanner, run in cine mode, was used with two optical devices, GateCT and RPM, to detect respiratory motion. A radiation detector, GM-10, triggers the X-ray on/off to GateCT system, while the RPM is directly synchronized with the CT scanner through an electronic connection. Two phantoms were imaged: the first phantom translated on a rigid plate along the anterior-posterior (AP) direction, and was used to assess the temporal synchronization of each optical system with the CT scanner. The second phantom, consisting of five spheres translating 3 cm peak-to-peak in the superior-inferior direction, was used to assess the quality of rebinned images created by GateCT and RPM. Calibration assessment showed a nearly perfect synchronization with the scanner for both the RPM and GateCT systems, thus demonstrating the good performance of the radiation detector. Results for the volume rebinning test showed discrepancies in volumes for the 3D reconstruction (compared to ground truth) of up to 36% for GateCT and up to 40% for RPM. No statistical difference was proven between the two systems in volume sorting. Errors are mainly due to phase detection inaccuracies and to the large motion of the phantom. This feasibility study assessed the consistency of two optical systems in synchronizing the respiratory signal with the image acquisition. A new patient protocol based on both RPM and GateCT will be soon started.

Accuracy in breast shape alignment with 3D surface fitting algorithms.

Surface imaging is in use in radiotherapy clinical practice for patient setup optimization and monitoring. Breast alignment is accomplished by searching for a tentative spatial correspondence between the reference and daily surface shape models. In this study, the authors quantify whole breast shape alignment by relying on texture features digitized on 3D surface models. Texture feature localization was validated through repeated measurements in a silicone breast phantom, mounted on a high precision mechanical stage. Clinical investigations on breast shape alignment included 133 fractions in 18 patients treated with accelerated partial breast irradiation. The breast shape was detected with a 3D video based surface imaging system so that breathing was compensated. An in-house algorithm for breast alignment, based on surface fitting constrained by nipple matching (constrained surface fitting), was applied. Results were compared with a commercial software where no constraints are utilized (unconstrained surface fitting). Texture feature localization was validated within 2 mm in each anatomical direction. Clinical data show that unconstrained surface fitting achieves adequate accuracy in most cases, though nipple mismatch is considerably higher than residual surface distances (3.9 mm vs 0.6 mm on average). Outliers beyond 1 cm can be experienced as the result of a degenerate surface fit, where unconstrained surface fitting is not sufficient to establish spatial correspondence. In the constrained surface fitting algorithm, average surface mismatch within 1 mm was obtained when nipple position was forced to match in the [1.5; 5] mm range. In conclusion, optimal results can be obtained by trading off the desired overall surface congruence vs matching of selected landmarks (constraint). Constrained surface fitting is put forward to represent an improvement in setup accuracy for those applications where whole breast positional reproducibility is an issue.

Quantification and visualization of charged particle range variations.

PURPOSE: Range variations during respiration affect the penetration of charged particle beams and can result in beam overshoot or undershoot to the target. We have developed analysis tools to quantify the water equivalent pathlength (WEL) variations resulting from respiration (WEL analyzer, Aqualyzer), as well as a data explorer to view WEL variations interactively. METHODS AND MATERIALS: The metrics to characterize and quantify penetration of a charged particle beam during respiration were calculated semiautomatically. The analysis involved the generation of images that (1) encode the radiologic pathlength across a beam's eye view image during the respiratory phase, (2) display the variation of the radiologic pathlength relative to a reference respiratory phase, (3) display isopenetration as a function of breathing, and (4) show range fluctuation images for a compensating bolus when applied to four-dimensional computed tomography. Additional quantities relevant to the analysis of charged particle beams in a breathing patient that are calculated include the beam overshoot volume and beam overshoot distance. These quantities are calculated as a function of time, gantry angle, and position. RESULTS: The software was applied to test cases to illustrate its utility in the analysis of range variations of charged particle beams in the treatment of lung tumors. CONCLUSION: WEL analysis is useful in the rapid assessment of range variations in the treatment of lung tumors and in determining the optimal gantry angle and respiratory gating window. The extension of encoding range fluctuations to a beam's eye view display is helpful in designing plans that are more robust in the presence of motion.

Maximum-intensity volumes for fast contouring of lung tumors including respiratory motion in 4DCT planning.

PURPOSE: To assess the accuracy of maximum-intensity volumes (MIV) for fast contouring of lung tumors including respiratory motion. METHODS AND MATERIALS: Four-dimensional computed tomography (4DCT) data of 10 patients were acquired. Maximum-intensity volumes were constructed by assigning the maximum Hounsfield unit in all CT volumes per geometric voxel to a new, synthetic volume. Gross tumor volumes (GTVs) were contoured on all CT volumes, and their union was constructed. The GTV with all its respiratory motion was contoured on the MIV as well. Union GTVs and GTVs including motion were compared visually. Furthermore, planning target volumes (PTVs) were constructed for the union of GTVs and the GTV on MIV. These PTVs were compared by centroid position, volume, geometric extent, and surface distance. RESULTS: Visual comparison of GTVs demonstrated failure of the MIV technique for 5 of 10 patients. For adequate GTV(MIV)s, differences between PTVs were <1.0 mm in centroid position, 5% in volume, +/-5 mm in geometric extent, and +/-0.5 +/- 2.0 mm in surface distance. These values represent the uncertainties for successful MIV contouring. CONCLUSION: Maximum-intensity volumes are a good first estimate for target volume definition including respiratory motion. However, it seems mandatory to validate each individual MIV by overlaying it on a movie loop displaying the 4DCT data and editing it for possible inadequate coverage of GTVs on additional 4DCT motion states.

Quantitative assessment of range fluctuations in charged particle lung irradiation.

PURPOSE: Water equivalent path length (WEL) variations due to respiration can change the range of a charged particle beam and result in beam overshoot to critical organs or beam undershoot to tumor. We have studied range fluctuations by analyzing four-dimensional computed tomography data and quantitatively assessing potential beam overshoot. METHODS AND MATERIALS: The maximal intensity volume is calculated by combining the gross tumor volume contours at each respiratory phase in the four-dimensional computed tomography study. The first target volume calculates the maximal intensity volume for the entire respiratory cycle (internal target volume [ITV]-radiotherapy [RT]), and the second target volume is the maximal intensity volume corresponding to gated RT (gated-RT, approximately 30% phase window around exhalation). A compensator at each respiratory phase is calculated. Two "composite" compensators for ITV-RT and gated-RT are then designed by selecting the minimal compensator depth at the respective respiratory phase. These compensators are then applied to the four-dimensional computed tomography data to estimate beam penetration. Analysis metrics include range fluctuation and overshoot volume, both as a function of gantry angle. We compared WEL fluctuations observed in treating the ITV-RT versus gated-RT in 11 lung patients. RESULTS: The WEL fluctuations were <21.8 mm-WEL and 9.5 mm-WEL for ITV-RT and gated-RT, respectively for all patients. Gated-RT reduced the beam overshoot volume by approximately a factor of four compared with ITV-RT. Such range fluctuations can affect the efficacy of treatment and result in an excessive dose to a distal critical organ. CONCLUSION: Time varying range fluctuation analysis provides information useful for determining appropriate patient-specific treatment parameters in charged particle RT. This analysis can also be useful for optimizing planning and delivery.

Comparison of target registration errors for multiple image-guided techniques in accelerated partial breast irradiation.

PURPOSE: External beam accelerated partial breast irradiation requires accurate localization of the target volume for each treatment fraction. Using the concept of target registration error (TRE), the performance of several methods of target localization was compared. METHODS AND MATERIALS: Twelve patients who underwent external beam accelerated partial breast irradiation were included in this study. TRE was quantified for four methods of image guidance: standard laser-based setup, kilovoltage imaging of the chest wall, kilovoltage imaging of surgically implanted clips, and three-dimensional surface imaging of the breast. The use of a reference surface created from a free-breathing computed tomography scan and a reference surface directly captured with three-dimensional video imaging were compared. The effects of respiratory motion were also considered, and gating was used for 8 of 12 patients. RESULTS: The median value of the TRE for the laser, chest wall, and clip alignment was 7.1 mm (n=94), 5.4 mm (n=81), and 2.4 mm (n=93), respectively. The median TRE for gated surface imaging based on the first fraction reference surface was 3.2 mm (n=49), and the TRE for gated surface imaging using the computed tomography-based reference surface was 4.9 mm (n=56). The TRE for nongated surface imaging using the first fraction reference surface was 6.2 mm (n=25). CONCLUSIONS: The TRE of surface imaging using a reference surface directly captured with three-dimensional video and the TRE for clip-based setup were within 1 mm. Gated capture is important for surface imaging to reduce the effects of respiratory motion in accelerated partial breast irradiation.

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.

Four-dimensional imaging and treatment planning of moving targets.

Four-dimensional CT acquisition is commercially available, and provides important information on the shape and trajectory of the tumor and normal tissues. The primary advantage of four-dimensional imaging over light breathing helical scans is the reduction of motion artifacts during scanning that can significantly alter tumor appearance. Segmentation, image registration, visualization are new challenges associated with four-dimensional data sets because of the overwhelming increase in the number of images. Four-dimensional dose calculations, while currently laborious, provide insights into dose perturbations due to organ motion. Imaging before treatment (image guidance) improves accuracy of radiation delivery, and recording transmission images can provide a means of verifying gated delivery.

Image interpolation in 4D CT using a BSpline deformable registration model.

PURPOSE: To develop a method for deriving the phase-binned four-dimensional computed tomography (4D CT) image sets through interpolation of the images acquired at some known phases. METHODS AND MATERIALS: Four-dimensional computed tomography data sets for 3 patients were acquired. For each patient, the correlation between inhale and exhale phases was studied and quantified using a BSpline deformable model. Images at an arbitrary phase were deduced by an interpolation of the deformation coefficients. The accuracy of the proposed scheme was assessed by comparing marker trajectories and by checkerboard/difference display of the interpolated and acquired images. RESULTS: The images at intermediate phases could be derived by an interpolation of the deformation field. An analysis of marker movements indicated that 3 mm accuracy is achievable by the interpolation. The subtraction of image analysis indicated a similar level of success. The proposed technique was useful also for automatically mapping the organ contours in a known phase to other phases, and for designing patient-specific margins in the presence of respiratory motion. Finally, the technique led to a 90% reduction in the acquired data, because in the BSpline model, a lattice of only a few thousand values is sufficient to describe a CT data set of 25 million pixels. CONCLUSIONS: Organ deformation can be well modeled by using a BSpline model. The proposed technique may offer useful means for radiation dose reduction, binning artifacts removal, and disk storage improvement in 4D imaging.

Clinical experience with a 3D surface patient setup system for alignment of partial-breast irradiation patients.

PURPOSE: To assess the utility of surface imaging on patient setup for accelerated partial-breast irradiation (APBI). METHODS AND MATERIAL: A photogrammetry system was used in parallel to APBI setup by laser and portal imaging. Surface data were acquired after laser and port-film setup for 9 patients. Surfaces were analyzed in comparison to a reference surface from the first treatment session by use of rigid transformations. The surface model after laser setup was used in a simulated photogrammetry setup procedure. In addition, breathing data were acquired by surface acquisition at a frame rate of 7 Hz. RESULTS: Mean 3D displacement was 7.3 mm (SD, 4.4 mm) and 7.6 mm (SD, 4.2 mm) for laser and port film, respectively. Simulated setup with the photogrammetry system yielded mean displacement of 1 mm (SD, 1.2 mm). Distance analysis resulted in mean distances of 3.7 mm (SD, 4.9 mm), 4.3 mm (SD, 5.6 mm), and 1.6 mm (SD, 2.4 mm) for laser, port film, and photogrammetry, respectively. Breathing motion at isocenter was smaller than 3.7 mm, with a mean of 1.9 mm (SD, 1.1 mm). CONCLUSIONS: Surface imaging for PBI setup appears promising. Alignment of the 3D breast surface achieved by stereo-photogrammetry shows greater breast topology congruence than when patients are set up by laser or portal imaging. A correlation of breast surface and CTV must be quantitatively established.

Design of 4D treatment planning target volumes.

PURPOSE: When using non-patient-specific treatment planning margins, respiratory motion may lead to geometric miss of the target while unnecessarily irradiating normal tissue. Imaging different respiratory states of a patient allows patient-specific target design. We used four-dimensional computed tomography (4DCT) to characterize tumor motion and create treatment volumes in 10 patients with lung cancer. These were compared with standard treatment volumes. METHODS AND MATERIALS: Four-dimensional CT and free breathing helical CT data of 10 patients were acquired. Gross target volumes (GTV) were delineated on the helical scan as well as on each phase of the 4D data. Composite GTVs were defined on 4DCT. Planning target volumes (PTV) including clinical target volume, internal margin (IM), and setup margin were generated. 4DPTVs with different IMs and standard PTVs were compared by computing centroid positions, volumes, volumetric overlap, and bounding boxes. RESULTS: Four-dimensional PTVs and conventional PTVs differed in volume and centroid positions. Overlap between 4DPTVs generated from two extreme tumor positions only compared with 10 respiratory phases was 93.7%. Comparing PTVs with margins of 15 mm (IM 5 mm) on composite 4D target volumes to PTVs with 20 mm (IM 10 mm) on helical CT data resulted in a decrease in target volume sizes by 23% on average. CONCLUSION: With patient-specific characterization of tumor motion, it should be possible to decrease internal margins. Patient-specific treatment volumes can be generated using extreme tumor positions on 4DCT. To date, more than 150 patients have been treated using 4D target design.

Deformable registration of 4D computed tomography data.

Four-dimensional radiotherapy requires deformable registration to track delivered dose across varying anatomical states. Deformable registration based on B-splines was implemented to register 4D computed tomography data to a reference respiratory phase. To assess registration performance, anatomical landmarks were selected across ten respiratory phases in five patients. These point landmarks were transformed according to global registration parameters between different respiratory phases. Registration uncertainties were computed by subtraction of transformed and reference landmark positions. The selection of appropriate registration masks to separate independently moving anatomical subunits is crucial to registration performance. The average registration error for five landmarks for each of five patients was 2.1 mm. This level of accuracy is acceptable for most radiotherapy applications.

Improving retrospective sorting of 4D computed tomography data.

Respiratory correlated CT is commercially available, and we have implemented its routine clinical use in planning lung tumor patients. Its value is determined by the fidelity of the spatiotemporal data set after processing the acquired reconstructed slices. Retrospective sorting of reconstructed slices is based on respiratory phase. However, the existing commercial software inadequately models respiratory phase for about 30% of the patients, mainly due to irregularities in the respiratory cycle. We have developed software that improves phase determination and consequently leads to an improvement of retrospective data sorting to make 4DCT data acquisition feasible for routine clinical use. Peak inhalation and exhalation respiratory states are selected manually; intermediate phases are interpolated. Residual motion artifacts in the resulting 4DCT volumes are reduced and allow use of the 4D imaging studies for treatment planning.

The susceptibility of IMRT dose distributions to intrafraction organ motion: an investigation into smoothing filters derived from four dimensional computed tomography data.

This study investigated the sensitivity of static planning of intensity-modulated beams (IMBs) to intrafraction deformable organ motion and assessed whether smoothing of the IMBs at the treatment-planning stage can reduce this sensitivity. The study was performed with a 4D computed tomography (CT) data set for an IMRT treatment of a patient with liver cancer. Fluence profiles obtained from inverse-planning calculations on a standard reference CT scan were redelivered on a CT scan from the 4D data set at a different part of the breathing cycle. The use of a nonrigid registration model on the 4D data set additionally enabled detailed analysis of the overall intrafraction motion effects on the IMRT delivery during free breathing. Smoothing filters were then applied to the beam profiles within the optimization process to investigate whether this could reduce the sensitivity of IMBs to intrafraction organ motion. In addition, optimal fluence profiles from calculations on each individual phase of the breathing cycle were averaged to mimic the convolution of a static dose distribution with a motion probability kernel and assess its usefulness. Results from nonrigid registrations of the CT scan data showed a maximum liver motion of 7 mm in superior-inferior direction for this patient. Dose-volume histogram (DVH) comparison indicated a systematic shift when planning treatment on a motion-frozen, standard CT scan but delivering over a full breathing cycle. The ratio of the dose to 50% of the normal liver to 50% of the planning target volume (PTV) changed up to 28% between different phases. Smoothing beam profiles with a median-window filter did not overcome the substantial shift in dose due to a difference in breathing phase between planning and delivery of treatment. Averaging of optimal beam profiles at different phases of the breathing cycle mainly resulted in an increase in dose to the organs at risk (OAR) and did not seem beneficial to compensate for organ motion compared with using a large margin. Additionally, the results emphasized the need for 4D CT scans when aiming to reduce the internal margin (IM). Using only a single planning scan introduces a systematic shift in the dose distribution during delivery. Smoothing beam profiles either based on a single scan or over the different breathing phases was not beneficial for reducing this shift.

Four-dimensional image-based treatment planning: Target volume segmentation and dose calculation in the presence of respiratory motion.

PURPOSE: To describe approaches to four-dimensional (4D) treatment planning, including acquisition of 4D-CT scans, target delineation of spatio-temporal image data sets, 4D dose calculations, and their analysis. METHODS AND MATERIALS: The study included patients with thoracic and hepatocellular tumors. Specialized tools were developed to facilitate visualization, segmentation, and analysis of 4D-CT data: maximum intensity volume to define the extent of lung tumor motion, a 4D browser to examine and dynamically assess the 4D data sets, dose calculations, including respiratory motion, and deformable registration to combine the dose distributions at different points. RESULTS: Four-dimensional CT was used to visualize and quantitatively assess respiratory target motion. The gross target volume contours derived from light breathing scans showed significant differences compared with those extracted from 4D-CT. Evaluation of deformable registration using difference images of original and deformed anatomic maps suggested the algorithm is functionally useful. Thus, calculation of effective dose distributions, including respiratory motion, was implemented. CONCLUSION: Tools and methods to use 4D-CT data for treatment planning in the presence of respiratory motion have been developed and applied to several case studies. The process of 4D-CT-based treatment planning has been implemented, and technical barriers for its routine use have been identified.

The correlation between internal and external markers for abdominal tumors: implications for respiratory gating.

PURPOSE: The correlation of the respiratory motion of external patient markers and abdominal tumors was examined. Data of this type are important for image-guided therapy techniques, such as respiratory gating, that monitor the movement of external fiducials. METHODS AND MATERIALS: Fluoroscopy sessions for 4 patients with internal, radiopaque tumor fiducial clips were analyzed by computer vision techniques. The motion of the internal clips and the external markers placed on the patient's abdominal skin surface were quantified and correlated. RESULTS: In general, the motion of the tumor and external markers were well correlated. The maximum amount of peak-to-peak craniocaudal tumor motion was 2.5 cm. The ratio of tumor motion to external-marker motion ranged from 0.85 to 7.1. The variation in tumor position for a given external-marker position ranged from 2 to 9 mm. The period of the breathing cycle ranged from 2.7 to 4.5 seconds, and the frequency patterns for both the tumor and the external markers were similar. CONCLUSIONS: Although tumor motion generally correlated well with external fiducial marker motion, relatively large underlying tumor motion can occur compared with external-marker motion and variations in the tumor position for a given marker position. Treatment margins should be determined on the basis of a detailed understanding of tumor motion, as opposed to relying only on external-marker information.

Four-dimensional computed tomography: image formation and clinical protocol.

Respiratory motion can introduce significant errors in radiotherapy. Conventional CT scans as commonly used for treatment planning can include severe motion artifacts that result from interplay effects between the advancing scan plane and object motion. To explicitly include organ/target motion in treatment planning and delivery, time-resolved CT data acquisition (4D Computed Tomography) is needed. 4DCT can be accomplished by oversampled CT data acquisition at each slice. During several CT tube rotations projection data are collected in axial cine mode for the duration of the patient's respiratory cycle (plus the time needed for a full CT gantry rotation). Multiple images are then reconstructed per slice that are evenly distributed over the acquisition time. Each of these images represents a different anatomical state during a respiratory cycle. After data acquisition at one couch position is completed, x rays are turned off and the couch advances to begin data acquisition again until full coverage of the scan length has been obtained. Concurrent to CT data acquisition the patient's abdominal surface motion is recorded in precise temporal correlation. To obtain CT volumes at different respiratory states, reconstructed images are sorted into different spatio-temporally coherent volumes based on respiratory phase as obtained from the patient's surface motion. During binning, phase tolerances are chosen to obtain complete volumetric information since images at different couch positions are reconstructed at different respiratory phases. We describe 4DCT image formation and associated experiments that characterize the properties of 4DCT. Residual motion artifacts remain due to partial projection effects. Temporal coherence within resorted 4DCT volumes is dominated by the number of reconstructed images per slice. The more images are reconstructed, the smaller phase tolerances can be for retrospective sorting. From phantom studies a precision of about 2.5 mm for quasiregular motion and typical respiratory periods could be concluded. A protocol for 4DCT scanning was evaluated and clinically implemented at the MGH. Patient data are presented to elucidate how additional patient specific parameters can impact 4DCT imaging.

A phantom evaluation of a stereo-vision surface imaging system for radiotherapy patient setup.

External beam irradiation requires precise positioning of the target relative to the treatment planning coordinate system. A three-dimensional (3D) surface imaging system for patient positioning has recently been installed in one of our linear accelerator (linac) rooms. The device utilizes close-range photogrammetry to generate a 3D model of the patient's surface. This geometric model can be made to look like a digital camera image if wrapped with a gray-level image (texture mapping) that shows surface coloration. The system is calibrated to the linac coordinate system and has been designed as a patient setup device. To reproduce patient position in fractionated radiotherapy, the daily patient surface model is registered to a previously recorded reference surface. Using surface registration, the system calculates the rigid-body transformation that minimizes the distance between the treatment and the reference surface models in a region-of-interest (ROI). This transformation is expressed as a set of new couch coordinates at which the patient position best matches with the reference data. If respiratory motion is a concern, the surface can be obtained with a gated acquisition at a specified phase of the respiratory cycle. To analyze the accuracy of the system, we performed several experiments with phantoms to assess stability, alignment accuracy, precision of the gating function, and surface topology. The reproducibility of surface measurements was tested for periods up to 57 h. Each recorded frame was registered to the reference surface to calculate the required couch adjustment. The system stability over this time period was better than 0.5 mm. To measure the accuracy of the system to detect and quantify patient shift relative to a reference image, we compared the shift detected by the surface imaging system with known couch transitions in a phantom study. The maximum standard deviation was 0.75 mm for the three translational degrees of freedom, and less than 0.1 degrees for each rotation. Surface model precision was tested against computed tomography (CT)-derived surface topology. The root-mean-square rms of the distance between the surfaces was 0.65 mm, excluding regions where beam hardening caused artifacts in the CT data. Measurements were made to test the gated acquisition mode. The time-dependent amplitude was measured with the surface imaging system and an established respiratory gating system based on infrared (IR)-marker detection. The measured motion trajectories from both systems were compared to the known trajectory of the stage. The standard deviations of the amplitude differences to the motor trajectory were 0.04 and 0.15 mm for the IR-marker system and the 3D surface imaging system, respectively. A limitation of the surface-imaging device is the frame rate of 6.5 Hz, because rapid changes of the motion trajectory cannot be detected. In conclusion, the system is accurate and sufficiently stable to be used in the clinic. The errors computed when comparing the surface model with CT geometry were submillimeter, and deviations in the alignment and gating-signal tests were of the same magnitude.