Himanshu P. Shukla

Partial-volume correction in PET: Validation of an iterative postreconstruction method with phantom and patient data

Partial-volume errors (PVEs) in PET can cause incorrect estimation of radiopharmaceutical uptake in small tumors. An iterative postreconstruction method was evaluated that corrects for PVEs without a priori knowledge of tumor size or background. Methods: Volumes of interest (VOIs) were drawn on uncorrected PET images. PVE-corrected images were produced using an iterative 3-dimensional deconvolution algorithm and a local point spread function. The VOIs were projected on the corrected image to estimate the PVE-corrected mean activity concentration. These corrected mean values were compared with uncorrected maximum and mean values. Simulated data were generated as a first test of the correction algorithm. Phantom measurements were made using 18F-FDG–filled spheres in a scattering medium. Clinical validation used 154 surrogate tumors from 9 patients. The surrogate tumors were blood-pool images of the descending aorta as well as mesenteric and iliac arteries and veins. Surrogate tumors ranged in diameter from 5 to 25 mm. Analysis used 18F-FDG and 11C-CO datasets (both dynamic and static). Values representing “truth” were derived from imaging the blood pool in large structures (e.g., the left ventricle, left atrium, or sections of the aorta) where PVEs were negligible. Surrogate tumor sizes were measured from contrast CT. Results: The PVE-correction technique, when applied to the mean value in spheric phantoms, yielded recovery coefficients of 87% for an 8-mm-diameter sphere and between 100% and 103% for spheres between 13 and 29 mm. For the human studies, PVE-corrected data recovered a large fraction of the true activity concentration (86% ± 7% for an 8-mm-diameter tumor and 98% ± 8% for tumors between 10 and 24 mm). For tumors smaller than 18 mm, the PVE-corrected mean values were less biased (P < 0.05) than the uncorrected maximum or mean values. Conclusion: Iterative postreconstruction PVE correction generated more accurate uptake measurements in subcentimeter tumors for both phantoms and patients than the uncorrected values. The method eliminates the requirement for segmenting anatomic data and estimating tumor metabolic size or tumor background level. This technique applies a PVE correction to the mean voxel value within a VOI, yielding a more accurate estimate of uptake than the maximum voxel value.

Displacement-based binning of time-dependent computed tomography image data sets

Respiration can cause tumors in the thorax or abdomen to move by as much as 3 cm; this movement can adversely affect the planning and delivery of radiation treatment. Several techniques have been used to compensate for respiratory motion, but all have shortcomings. Manufacturers of computed tomography (CT) equipment have recently used a technique developed for cardiac CT imaging to track respiratory-induced anatomical motion and to sort images according to the phase of the respiratory cycle they represent. Here we propose a method of generating CT images that accounts for respiratory-induced anatomical motion on the basis of displacement, i.e., displacement-binned CT image sets. This technique has shown great promise, however, it is not fully supported by currently used CT image reconstruction software. As an interim solution, we have developed a method for extracting displacement-binned CT image data sets from data sets assembled on the basis of a prospectively determined breathing phase acquired on a multislice helical CT scanner. First, the projection data set acquired from the CT scanner was binned at small phase intervals before reconstruction. The manufacturers software then generated image sets identified as belonging to particular phases of the respiratory cycle. All images were then individually correlated to the displacement of an external fiducial marker. Next, CT image data sets were resorted on the basis of the displacement and assigned an appropriate phase. Finally, displacement-binned image data sets were transferred to a treatment-planning system for analysis. Although the technique is currently limited by the phase intervals allowed by the CT software, some improvement in image reconstruction was seen, indicating that this technique is useful at least as an interim measure.

CT Truncation artifact removal using water-equivalent thicknesses derived from truncated projection data

Large patient anatomies and limited imaging fleld-of-view (FOV) lead to truncation of CT projections. Truncation introduces serious artifacts into reconstructed images, including central cupping and bright external rings. FOV may be increased using laterally offset detectors, but this requires sophisticated imaging hardware and full angular scanning. We propose a novel method to complete truncated projections based on the observation that the thickness of the patient may be estimated along the projection rays by calculating water-equivalent thicknesses (WET). These values are not at all affected by truncation and thus constitute valuable auxiliary information. We parameterize pairs of points along each ray that intersects the unknown object boundary. These points are separated by the measured WET value (obtained from projections that have been corrected for scatter and beam-hardening). We assume, for all large body parts, that the patient outline may be roughly approximated as an ellipse. Using a deterministic optimization algorithm, we simultaneously estimate the point positions and ellipse parameters by minimizing the distance between point sets and the ellipse boundary. The optimal ellipse is used to complete the truncated projections. Reconstruction then ensues. We apply the algorithm to a severely truncated CT dataset of a typical abdomen. The RMS error between complete data and truncated reconstructions (corrected using an empirical extrapolation approach) is 20.4% for an abdominal dataset. The new algorithm reduces this error to 1.0%. Even thought the algorithm assumes an elliptical patient cross-section, truly impressive increases in quantitative image quality are observed. The presence of pelvic bone in the image does not appreciably bias the ellipse position even though it does bias the thickness estimates for some rays. The algorithm incurs low computational cost and is suitable for on-line clinical workflows.

A planning and delivery study of a rotational IMRT technique with burst delivery.

PURPOSE A novel rotational IMRT (rIMRT) technique using burst delivery (continuous gantry rotation with beam off during MLC repositioning) is investigated. The authors evaluate the plan quality and delivery efficiency and accuracy of this dynamic technique with a conventional flat 6 MV photon beam. METHODS Burst-delivery rIMRT was implemented in a planning system and delivered with a 160-MLC linac. Ten rIMRT plans were generated for five anonymized patient cases encompassing head and neck, brain, prostate, and prone breast. All plans were analyzed retrospectively and not used for treatment. Among the varied plan parameters were the number of optimization points, number of arcs, gantry speed, and gantry angle range (alpha) over which the beam is turned on at each optimization point. Combined rotational/step-and-shoot rIMRT plans were also created by superimposing multiple-segment static fields at several optimization points. The rIMRT trial plans were compared with each other and with plans generated using helical tomotherapy and VMAT. Burst-mode rotational IMRT plans were delivered and verified using a diode array, ionization chambers, thermoluminescent dosimeters, and film. RESULTS Burst-mode rIMRT can achieve plan quality comparable to helical tomotherapy, while the former may lead to slightly better OAR sparing for certain cases and the latter generally achieves slightly lower hot spots. Few instances were found in which increasing the number of optimization points above 36, or superimposing step-and-shoot IMRT segments, led to statistically significant improvements in OAR sparing. Using an additional rIMRT partial arc yielded substantial OAR dose improvements for the brain case. Measured doses from the rIMRT plan delivery were within 4% of the plan calculation in low dose gradient regions. Delivery time range was 228-375 s for single-arc rIMRT 200-cGy prescription with a 300 MU/min dose rate, comparable to tomotherapy and VMAT. CONCLUSIONS Rotational IMRT with burst delivery, whether combined with static fields or not, yields clinically acceptable and deliverable treatment plans.

Beam-centric algorithm for pretreatment patient position correction in external beam radiation therapy

PURPOSE In current image guided pretreatment patient position adjustment methods, image registration is used to determine alignment parameters. Since most positioning hardware lacks the full six degrees of freedom (DOF), accuracy is compromised. The authors show that such compromises are often unnecessary when one models the planned treatment beams as part of the adjustment calculation process. The authors present a flexible algorithm for determining optimal realizable adjustments for both step-and-shoot and arc delivery methods. METHODS The beam shape model is based on the polygonal intersection of each beam segment with the plane in pretreatment image volume that passes through machine isocenter perpendicular to the central axis of the beam. Under a virtual six-DOF correction, ideal positions of these polygon vertices are computed. The proposed method determines the couch, gantry, and collimator adjustments that minimize the total mismatch of all vertices over all segments with respect to their ideal positions. Using this geometric error metric as a function of the number of available DOF, the user may select the most desirable correction regime. RESULTS For a simulated treatment plan consisting of three equally weighted coplanar fixed beams, the authors achieve a 7% residual geometric error (with respect to the ideal correction, considered 0% error) by applying gantry rotation as well as translation and isocentric rotation of the couch. For a clinical head-and-neck intensity modulated radiotherapy plan with seven beams and five segments per beam, the corresponding error is 6%. Correction involving only couch translation (typical clinical practice) leads to a much larger 18% mismatch. Clinically significant consequences of more accurate adjustment are apparent in the dose volume histograms of target and critical structures. CONCLUSIONS The algorithm achieves improvements in delivery accuracy using standard delivery hardware without significantly increasing total treatment session duration. It encourages parsimonious utilization of all available DOF. Finally, in certain cases, it obviates the need of a robotic couch having six DOF for the correction of patient displacement and rotations.

Automatic coregistration of volumetric images based on implanted fiducial markers.

The accurate delivery of external beam radiation therapy is often facilitated through the implantation of radio-opaque fiducial markers (gold seeds). Before the delivery of each treatment fraction, seed positions can be determined via low dose volumetric imaging. By registering these seed locations with the corresponding locations in the previously acquired treatment planning computed tomographic (CT) scan, it is possible to adjust the patient position so that seed displacement is accommodated. The authors present an unsupervised automatic algorithm that identifies seeds in both planning and pretreatment images and subsequently determines a rigid geometric transformation between the two sets. The algorithm is applied to the imaging series of ten prostate cancer patients. Each test series is comprised of a single multislice planning CT and multiple megavoltage conebeam (MVCB) images. Each MVCB dataset is obtained immediately prior to a subsequent treatment session. Seed locations were determined to within 1 mm with an accuracy of 97 ± 6.1 % for datasets obtained by application of a mean imagingdose of 3.5 cGy per study. False positives occurred in three separate instances, but only when datasets were obtained at imagingdoses too low to enable fiducial resolution by a human operator, or when the prostate gland had undergone large displacement or significant deformation. The registration procedure requires under nine seconds of computation time on a typical contemporary computer workstation.

Automatic co-registration of volumetric images based on implanted fiducial markers

The accurate delivery of external beam radiation therapy is often facilitated through the implantation of radio-opaque fiducial markers (seeds). Before the delivery of each treatment fraction, seed positions can be determined via volumetric imaging. By registering these seed locations with the corresponding locations in the previously acquired treatment planning CT, it is possible to adjust the patient position or the treatment plan so that seed displacement is accommodated. We present an automatic algorithm that identifies seeds in both planning and pretreatment images and subsequently determines the geometric transformation between the two sets. The algorithm is applied to the imaging series of 10 prostate cancer patients. Each series is comprised of a single multislice planning CT and several megavoltage conebeam CT images obtained immediately prior to a subsequent treatment session. Seed locations were determined for 164 images to within 1 mm with an accuracy of 98 plusmn 6.3%.

SU‐FF‐T‐210: Evaluation of CT Extended Field of View Imaging Impact On Radiation Therapy Treatment Planning

Purpose:CTimaging of patients for radiotherapytreatment planning frequently includes anatomy that extends beyond the 50cm nominal field‐of‐view (nFOV): 28.35% of the 575 CT scans we acquired from July–December 2006. The purpose of this study was to evaluate 1) the degradation of Hounsfield units (HU) in the extended field of view (eFOV), and 2) the dosimetric impact of ignoring or correcting this degradation within the treatment planning system (TPS). Method and Materials:CTimages were acquired at maximum FOV (82cm) on a diamond‐shaped 30×30×13cm solid‐water phantom, challenging the reconstruction algorithms ability to model the truncated projection data adequately. A unique dataset was acquired for 19 phantom locations (1cm intervals outside of the nFOV) and each imported into a TPS. Each phantom was contoured using a threshold of −200HU. A single treatment beam with isocenter placed at the center of the phantom was planned to deliver 100cGy to isocenter for each CT data set; dose map comparisons with and without homogenous correction for density (HU>−700 =1gm/cc) were performed relative to the control phantom within the nFOV. Results: Significant variation of HU was observed as a function of phantom displacement outside the nFOV (range +513 to −873HU); most dramatically ∼5cm beyond the nFOV border. If uncorrected, these changes in HU produced significant dose errors. Plans were compared to control plans and dose difference maps were generated; uncorrected images 3cm outside the nFOV demonstrated >5% difference, where overriding HU values above −700HU maintained <5% error for phantom positions ∼10cm beyond nFOV. Conclusion: HU values differ significantly for anatomy 2cm outside the nFOV, can be visualized and should be corrected for dose calculations. These results show <5% dose error can be accomplished for anatomy extending ∼10cm beyond the nFOV (which accounts for 98% of eFOV patients here).

SU‐E‐T‐417: A Planning and Delivery Study of a Rotational IMRT Technique with Burst Delivery

Purpose: A novel rotational IMRT (rIMRT) technique using burst delivery (continuous gantry rotation with beam off during MLC repositioning) is investigated. We evaluate the plan quality and delivery efficiency and accuracy of this dynamic technique with a conventional flat 6MV photon beam. Methods: Burst‐delivery rIMRT was implemented in a planning system (Panther, Prowess) and delivered with a 160‐MLC linac (Artiste, Siemens). Ten rIMRT plans were generated for five anonymized patient cases encompassing head and neck, brain, prostate, and prone breast. All plans were analyzed retrospectively and not used for treatment. Among the varied plan parameters were the number of optimization points, number of arcs, gantry speed, and gantry angle range (alpha) over which the beam is turned on at each optimization point. Combined rotational/step‐and‐shoot rIMRT plans were also created by superimposing multiple‐segment static fields at several optimization points. The rIMRT trial plans were compared with each other and with plans generated using helical tomotherapy. Plans were delivered and verified using a diode array, ionization chambers, and thermoluminescent dosimeters. Results: Burst‐mode rIMRT can achieve plan quality comparable to helical tomotherapy, while the former may lead to slightly better OAR sparing for certain cases and the latter generally achieves slightly lower hot spots. Few instances were found in which increasing the number of optimization points above 36, or superimposing step‐and‐shoot IMRT segments, led to statistically significant improvements in OAR sparing. Using an additional rIMRT partial arc yielded substantial OAR dose improvements for the brain case. Measured doses from the rIMRT plan delivery were within 4% of the plan calculation in low dose gradient regions. Delivery time range was 228–375 seconds for single‐arc rIMRT 200‐cGy prescription with a 300 MU/min dose rate. Conclusions: Rotational IMRT with burst delivery, whether combined with static fields or not, yields clinically acceptable and deliverable treatment plans.

SU‐GG‐T‐130: Rotational IMRT Delivery Using a Siemens Artiste in Flattening‐Filter‐Free, High Dose‐Rate Mode

Purpose: To demonstrate the ability to deliver unflattened (high dose rate), modulated arc plans using the Siemens Artiste linear accelerator using prototype linac firmware developed by Siemens and prototype TPS software developed by Prowess. Method and Materials: Prototype Prowess treatment planning software was used to develop a set of high dose rate (1,000 mu/min) modulated arc plans for four treatment sites ‐ head/neck, lung, prostate and breast. A step‐and‐shoot plan (SS) was also created for delivery efficiency comparison to arc‐based plans. The same planning criteria were used for all plans to ensure their ‘equivalence’. With the linear accelerator converted to the experimental state, all plans were delivered to a solid water phantom. Dose was measured using an ion chamber for point dose measurements and film for evaluation of relative dose distribution with a gamma criteria of 3%/3mm. Delivery times were collected to assess delivery efficiency. Results: Plans developed using all arc modalities were successfully delivered for all cases. The maximum difference between measured and calculated doses was 3.08% for ion chamber measurements and the minimum 3%/3mm gamma agreement was 87% of pixels. All arc treatments were delivered in less time than the corresponding SS plans, with the lungSBRT plans being delivered in 5 minutes for a 20 Gy prescription. Conclusion: Unflattened, modulated arc delivery is possible using prototype firmware on a Siemens Artiste machine and the Prowess treatment planning system accurately predicted doses for such deliveries. Modulated arc delivery in combination with unflat (high dose rate i.e. 1,000+ mu/min) mode may allow for very efficient delivery of highly conformai isodose distributions. Conflict of Interest: This work is funded by a grant from Siemens Oncology.