Index of content:
Volume 37, Issue 7, July 2010
37(2010); http://dx.doi.org/10.1118/1.3377778View Description Hide Description
- MEDICAL PHYSICS LETTERS
37(2010); http://dx.doi.org/10.1118/1.3447726View Description Hide DescriptionPurpose:
Signal encoding in magnetic particle imaging (MPI) is achieved by moving a field-free point (FFP) through the region of interest. One way to increase the sensitivity of the method is to scan the region of interest with a field-free line (FFL) instead of the FFP. Recently, the first feasible FFL coil setup was introduced. The purpose of this article is to improve the efficiency of the FFL coil geometry even further.Methods:
In order to reduce the electrical power loss of the setup, an additional Maxwellcoil pair is introduced that is tailored to generate the static part of the FFL field.Results:
Using the proposed coil assembly, the electrical power loss for the generation of a rotating FFL is considerably reduced compared to previously known coil setups. Furthermore, the quality of the generated FFL is significantly increased.Conclusions:
The proposed coil assembly is almost as efficient as an equivalent FFP scanner. Furthermore, the assembly cannot only be used for FFL imaging but for FFP imaging as well. Hence, the findings of this article denote an important step toward the first practical implementation of the FFL coil geometry.
- RADIATION THERAPY PHYSICS
37(2010); http://dx.doi.org/10.1118/1.3442028View Description Hide DescriptionPurpose:
The aim of this work was to simulate the effect of dose distribution changes on detector arraycalibrations and to explore compensatory methods that are used during calibrationmeasurements.Methods:
The array calibration technique that was investigated is known as wide field (WF) calibration. Using this method, a linear array [-axis (65 detectors) of the IC PROFILER™ (Sun Nuclear Corporation, Melbourne, FL)] is calibrated with three measurements (, , and ); each measurement uses the same radiation field, which is larger than the array. For measurement configuration , the array is rotated by 180° from its position in ; for , the array is shifted by one detector from its position in . The relative detector sensitivities are then determined through ratios of detector readings at the same field locations (using and ). This method results in error propagation that is proportional to the number of detectors in the array. During the procedure, the calibration protocol operates under three postulates, which state that (a) the beam shape does not change between measurements; (b) the relative sensitivities of the detectors do not change; and (c) the scatter to the array does not change as the array is moved. The WF calibration’s sensitivity to a postulate (a) violation was quantified by applying a sine shaped perturbation (of up to 0.1%) to , , or , and then determining the change relative to a baseline calibration. Postulate (a) violations were minimized by using a continuous beam and mechanized array movement during and . A continuously on beam demonstrated more stable beam symmetry as compared to cycling the beam on and off between measurements. Additional side-scatter was also used to satisfy postulate (c).Results:
Simulated symmetry perturbations of 0.1% to or resulted in calibration errors of up to 2%; was relatively immune to perturbation ( error). Wide field calibration error on a linear accelerator with similar symmetry variations was ±1.6%. Using a continuous beam during and with additional side-scatter reduced the calibration error from ±1.6% to ±0.48%.Conclusions:
This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations primarily due to linear accelerator symmetry variations. The same technique would work for any array using WF calibration.
Total skin electron therapy (TSET): A reimplementation using radiochromic films and IAEA TRS-398 code of practice37(2010); http://dx.doi.org/10.1118/1.3442301View Description Hide DescriptionPurpose:
The aim of this work is to present an updated implementation of total skin electron therapy (TSET) using IAEA TRS-398 code of practice for absolute dosimetry and taking advantage of the use of radiochromic films. The optimization of quality control tests is also included.Methods:
A Varian 2100 C/D linear accelerator equipped with the special procedure (high dose rate total skin electron mode, ) was employed to perform TSET irradiations using the modified Stanford technique. The commissioning was performed following the AAPM report 23 recommendations. In particular, for dual-field beams irradiation, the optimal tilt angle was investigated and the dose distribution in the treatment plane was measured. For a complete six dual-field beams irradiation, the treatment skin dose on the surface of a cylindrical phantom was evaluated by radiochromic films and the factor which relates the single dual-field skin dose to the six dual-field skin dose was assessed. Since the TRS-398 reference conditions do not meet the requirements of TSET absolute dosimetry, GafChromic EBT films were also employed to check and validate the application of the protocol. Simplified procedures were studied to verify beam constancy in PMMA phantoms without the more difficult setup of total skin irradiation.Results:
The optimized geometrical setup for dual-field beams was: Tilt, , and the beam degrader placed at 320 cm from the source. As regards to dose homogeneity in the treatment plane, for dual-field beams irradiation, the mean relative dose value was (normalizing to 100% at the calibration point level). For six dual-field beams irradiation, the multiplication factor was 2.63. In addition, beam quality, dose rate, and bremsstrahlung contribution were also suitable for TSET treatments. The TRS-398 code of practice was used for TSET dosimetry, as dose measurements performed by ionization chamber and radiochromic film agreed within 2.5%. Simplified quality control tests and baseline values were presented in order to check flatness, symmetry, and field size with radiochromic films and output and beam quality constancy with ionization chamber. Short-term reproducibility and MU linearity tests were also included.Conclusions:
Commissioning parameters met the requirements of TSET treatments and the matching of AAPM guidelines with the IAEA code of practice was successful. Frequent beam performance controls can be easily performed through the presented quality assurance tests. Radiochromic dosimetry facilitated the TSET commissioning and played a major role to validate the application of TRS-398.
37(2010); http://dx.doi.org/10.1118/1.3451114View Description Hide DescriptionPurpose:
This study characterizes the dosimetric properties of the iBEAM evo carbon fiber couch manufactured by Medical Intelligence and examines the accuracy of the CMS XiO and Nucletron Oncentra Masterplan (OMP) treatment planning systems for calculating beam attenuation due to the presence of the couch.Methods:
To assess the homogeneity of the couch, it was CT scanned at isocentric height and a number of signal intensity profiles were generated and analyzed. To simplify experimental procedures, surfacedose and central axis depth dosemeasurements were performed in a solid water slab phantom using Gafchromic film for 6 and 10 MV photon beams at gantry angles of 0° (normal incidence), 30°, and 60° with an inverted iBEAM couch placed on top of the phantom. Attenuation measurements were performed in a cylindrical solid water phantom with an ionization chamber positioned at the isocenter. Measurements were taken for gantry angles from 0° to 90° in 10° increments for both 6 and 10 MV photon beams. This setup was replicated in the XiO and OMP treatment planning systems. Dose was calculated using the pencil beam, collapsed cone, convolution, and superposition algorithms.Results:
The CT scan of the couch showed that it was uniformly constructed. Surfacedose increased by for a 6 MV beam and for a 10 MV beam passing through the couch at normal incidence. Obliquely incident beams resulted in a higher surfacedose compared to normally incident beams for both open fields and fields with the couch present. Depth dose curves showed that the presence of the couch resulted in an increase in dose in the build up region. For 6 and 10 MV beams incident at 60°, nearly all skin sparing was lost. Attenuation measurements derived using the ionization chamber varied from 2.7% (0°) to a maximum of 4.6% (50°) for a 6 MV beam and from 1.9% (0°) to a maximum of 4.0% (50°) for a 10 MV beam. The pencil beam and convolution algorithms failed to accurately calculate couch attenuation. The collapsed cone and superposition algorithms calculated attenuation within an absolute error of ±1.2% for 6 MV and ±0.8% for 10 MV for gantry angles from 0° to 40°. Some differences in attenuation were observed dependent on how the couch was contoured.Conclusions:
These results demonstrate that the presence of the iBEAM evo carbon fiber couch increases the surfacedose and dose in the build up region. The inclusion of the couch in the planning scan is limited by the field of view employed and the couch height at the time of CT scanning.
Evaluation of two-dimensional bolus effect of immobilization/support devices on skin doses: A radiochromic EBT film dosimetry study in phantom37(2010); http://dx.doi.org/10.1118/1.3439586View Description Hide DescriptionPurpose:
In this study, the authors have quantified the two-dimensional (2D) perspective of skindose increase using EBT filmdosimetry in phantom in the presence of patient immobilization devices during conventional and IMRTtreatments.Methods:
For 6 MV conventional photon field, the authors evaluated and quantified the 2D bolus effect on skindoses for six different common patient immobilization/support devices, including carbon fiber grid with Mylar sheet, Orfit carbon fiber base plate, balsa wood board, Styrofoam, perforated AquaPlast™ sheet, and alpha-cradle. For 6 and 15 MV IMRT fields, a stack of two film layers positioned above a solid phantom was exposed at the air interface or in the presence of a patient alpha-cradle. All the films were scanned and the pixel values were converted to doses based on an established calibration curve. The authors determined the 2D skindose distributions, isodose curves, and cross-sectional profiles at the surface layers with or without the immobilization/support device. The authors also generated and compared the dose area histograms (DAHs) and dose area products from the 2D skindose distributions.Results:
In contrast with 20% relative dose [(RD) dose relative to on central axis] at 0.0153 cm in the film layer for 6 MV open field, the average RDs at the same depth in the film layer were 71%, 69%, 55%, and 57% for Orfit, balsa wood, Styrofoam, and alpha-cradle, respectively. At the same depth, the RDs were 54% under a strut and 26% between neighboring struts of a carbon fiber grid with Mylar sheet, and between 34% and 56% for stretched perforated AquaPlast™ sheet. In the presence of the alpha-cradle for the 6 MV (15 MV) IMRT fields, the hot spot doses at the effective measurement depths of 0.0153 and 0.0459 cm were 140% and 150% (83% and 89%), respectively, of the isocenter dose. The enhancement factor was defined as the ratio of a given DAH parameter (minimum dose received in a given area) with and without the support device. For 6 MV conventional field, the enhancement factor was the highest (3.4) for the Orfit carbon fiber plate. As for the IMRT field, the enhancement factors varied with the size of the area of interest and were as high as 3.8 (4.3) at the hot spot of area in the top film layer (0.0153 cm) for 6 MV (15 MV) beams.Conclusions:
Significant 2D bolus effect on skindose in the presence of patient support and immobilization devices was confirmed and quantified with EBT filmdosimetry. Furthermore, the EBT film has potential application forin vivo monitoring of the 2D skindose distributions during patient treatments.
37(2010); http://dx.doi.org/10.1118/1.3446800View Description Hide DescriptionPurpose:
Varian’s On-Board Imager is a linac-integrated cone-beam CT(CBCT) system used at the authors’ institution to acquire images prior to delivering each fraction of prostate intensity modulated radiotherapy. The images are used to determine a couch shift that realigns the tumor with the position obtained in the planning CT. However, this manual image-guided radiotherapy (IGRT) technique is operator dependent, time consuming, offers limited degrees of freedom, and requires significant imagingdose over the course of treatment. To overcome these problems, the authors propose two fully automatic IGRT techniques that require significantly less imagingdose.Methods:
Dose is reduced by lowering the x-ray tube mA s during CBCT acquisition at the cost of increasing imagenoise. In “forward” IGRT, the CBCTimage is automatically registered to the planning CT to obtain the necessary couch shift. The “reverse” technique offers additional degrees of freedom as it involves nonrigid registration of the planning CT to the CBCT. Both techniques were evaluated using images of an anthropomorphic phantom with simulated motion and by retrospectively analyzing data from ten prostate cancer patients.Results:
IGRT error for the phantom data at 100% relative imagingdose was, , and for setup only, forward, and reverse techniques, respectively. For patient images acquired at 100% relative imagingdose, the errors were , , , and for setup only, manual forward (performed clinically), automatic forward, and reverse IGRT, respectively. Furthermore, imagingdose could be reduced to 20% without a significant loss in image guidance accuracy.Conclusions:
The presented image guidance methods are accurate while requiring only 20% of the standard imagingdose. The combination of low dose, automation, and accuracy enables frequent corrections during treatment, possibly leading to reduced margins and improved treatment outcomes.
Three dimensional intensity modulated brachytherapy (IMBT): Dosimetry algorithm and inverse treatment planning37(2010); http://dx.doi.org/10.1118/1.3456598View Description Hide DescriptionPurpose:
The feasibility of intensity modulated brachytherapy (IMBT) to improve dose conformity for irregularly shaped targets has been previously investigated by researchers by means of using partially shielded sources. However, partial shielding does not fully explore the potential of IMBT. The goal of this study is to introduce the concept of three dimensional (3D) intensity modulated brachytherapy and solve two fundamental issues regarding the application of 3D IMBT treatment planning: The dose calculation algorithm and the inverse treatment planning method.Methods:
A 3D IMBT treatment planning system prototype was developed using theMATLAB platform. This system consists of three major components: (1) A comprehensive IMBT source calibration method with dosimetric inputs from Monte Carlo (EGSnrc) simulations; (2) a “modified TG-43” (mTG-43) dose calculation formalism for IMBT dosimetry; and (3) a physical constraint based inverse IMBT treatment planning platform utilizing a simulated annealingoptimization algorithm. The model S700 Axxent electronic brachytherapy source developed by Xoft, Inc. (Fremont, CA), was simulated in this application. Ten intracavitary accelerated partial breast irradiation (APBI) cases were studied. For each case, an “isotropic plan” with only optimized source dwell time and a fully optimized IMBT plan were generated and compared to the original plan in various dosimetric aspects, such as the plan quality, planning, and delivery time. The issue of the mechanical complexity of the IMBT applicator is not addressed in this study.Results:
IMBT approaches showed superior plan quality compared to the original plans and the isotropic plans to different extents in all studied cases. An extremely difficult case with a small breast and a small distance to the ribs and skin, the IMBT plan minimized the high dose volume by 16.1% and 4.8%, respectively, compared to the original and the isotropic plans. The conformity index for the target was increased by 0.13 and 0.04, respectively. The maximum dose to the skin was reduced by 56 and 28 cGy, respectively, per fraction. Also, the maximum dose to the ribs was reduced by 104 and 96 cGy, respectively, per fraction. The mean dose to the ipsilateral and contralateral breasts and lungs were also slightly reduced by the IMBT plan. The limitations of IMBT are the longer planning and delivery time. The IMBT plan took around 2 h to optimize, while the isotropic plan optimization could reach the global minimum within 5 min. The delivery time for the IMBT plan is typically four to six times longer than the corresponding isotropic plan.Conclusions:
In this study, a dosimetry method for IMBT sources was proposed and an inverse treatment planning system prototype for IMBT was developed. The improvement of plan quality by 3D IMBT was demonstrated using ten APBI case studies. Faster computers and higher output of the source can further reduce plan optimization and delivery time, respectively.
Estimation of microscopic dose enhancement factor around gold nanoparticles by Monte Carlo calculations37(2010); http://dx.doi.org/10.1118/1.3455703View Description Hide DescriptionPurpose:
An approach known as goldnanoparticle-aidedradiation therapy (GNRT) is a recent development in radiation therapy which seeks to make a tumor more susceptible to radiation damage by modifying its photon interaction properties with an infusion of goldnanoparticles (GNPs). The purpose of this study was to quantify the energy deposition due to secondary electrons from GNPs on a nanometer scale and to calculate the corresponding microscopic dose enhancement factor around GNPs.Methods:
The Monte Carlo code EGSnrc was modified to obtain the spectra of secondary electrons from atoms of gold approximating GNPs and molecules of water under photon irradiation of a tumor loaded with GNPs. Six different photon sources were used:, , , , 50 kVp, and 6 MV x rays. Treating the scored electron spectra as point sources within an infinite medium of water, the event-by-event Monte Carlo code NOREC was used to quantify the radial dose distribution, giving rise to gold/water electron dose point kernels and corresponding microscopic dose enhancement factors. These kernels were applied to a test case based on a scanning electron microscope image of a GNP distribution in tissue, enabling the determination of the microscopic dose enhancement at each dose point.Results:
For the lower energy sources, , , and 50 kVp, the secondary electron fluence within a GNP-loaded tumor was increased by as much as two orders of magnitude, leading to two orders of magnitude increase in electron energy deposition over radial distances up to . For the test case considered, the dose was enhanced by factors ranging from 2 to 20 within of GNPs, and by 5% as far away as .Conclusions:
This study demonstrates a remarkable microscopic dose enhancement due to GNPs and low energy photon sources. By quantifying the microscopic dose enhancement factor for a given photon source as a function of distance from GNPs, it also enables the selection of either a passive or an active tumor targeting strategy using GNPs which will maximize the radiobiological benefit from GNRT.
37(2010); http://dx.doi.org/10.1118/1.3453576View Description Hide DescriptionPurpose:
Patient specific IMRT QC is performed routinely in many clinics as a safeguard against errors and inaccuracies which may be introduced during the complex planning, data transfer, and delivery phases of this type of treatment. The purpose of this work is to evaluate the feasibility of detecting systematic errors in MLC leaf bank position with patient specific checks.Methods:
9 head and neck (H&N) and 14 prostate IMRT beams were delivered using MLC files containing systematic offsets (±1 mm in two banks, ±0.5 mm in two banks, and 1 mm in one bank of leaves). The beams were measured using bothMAPCHECK™ (Sun Nuclear Corp., Melbourne, FL) and the aS1000 electronic portal imaging device (Varian Medical Systems, Palo Alto, CA). Comparisons with calculated fields, without offsets, were made using commonly adopted criteria including absolute dose (AD) difference, relative dose difference, distance to agreement (DTA), and the gamma index.Results:
The criteria most sensitive to systematic leaf bank offsets were the 3% AD, 3 mm DTA forMAPCHECK™ and the gamma index with 2% AD and 2 mm DTA for the EPID. The criterion based on the relative dose measurements was the least sensitive to MLC offsets. More highly modulated fields, i.e., H&N, showed greater changes in the percentage of passing points due to systematic MLC inaccuracy than prostate fields.Conclusions:
None of the techniques or criteria tested is sufficiently sensitive, with the population of IMRT fields, to detect a systematic MLC offset at a clinically significant level on an individual field. Patient specific QC cannot, therefore, substitute for routine QC of the MLC itself.
- RADIATION IMAGING PHYSICS
Noise variance analysis using a flat panel x-ray detector: A method for additive noise assessment with application to breast CT applications37(2010); http://dx.doi.org/10.1118/1.3447720View Description Hide DescriptionPurpose:
A simplified linear model approach was proposed to accurately model the response of a flat panel detector used for breast CT (bCT).Methods:
Individual detector pixel mean and variance were measured from bCT projection images acquired both in air and with a polyethylene cylinder, with the detector operating in both fixed low gain and dynamic gain mode. Once the coefficients of the linear model are determined, the fractional additive noise can be used as a quantitative metric to evaluate the system’s efficiency in utilizing x-ray photons, including the performance of different gain modes of the detector.Results:
Fractional additive noise increases as the object thickness increases or as the radiation dose to the detector decreases. For bCT scan techniques on the UC Davis prototype scanner (80 kVp, 500 views total, 30 frames/s), in the low gain mode, additive noise contributes 21% of the total pixel noise variance for a 10 cm object and 44% for a 17 cm object. With the dynamic gain mode, additive noise only represents approximately 2.6% of the total pixel noise variance for a 10 cm object and 7.3% for a 17 cm object.Conclusions:
The existence of the signal-independent additive noise is the primary cause for a quadratic relationship between bCT noise variance and the inverse of radiation dose at the detector. With the knowledge of the additive noise contribution to experimentally acquired images,system modifications can be made to reduce the impact of additive noise and improve the quantum noise efficiency of the bCT system.
Characterization of masses in digital breast tomosynthesis: Comparison of machine learning in projection views and reconstructed slices37(2010); http://dx.doi.org/10.1118/1.3432570View Description Hide DescriptionPurpose:
In digital breast tomosynthesis (DBT), quasi-three-dimensional (3D) structural information is reconstructed from a small number of 2D projection view (PV) mammograms acquired over a limited angular range. The authors developed preliminary computer-aided diagnosis(CADx) methods for classification of malignant and benign masses and compared the effectiveness of analyzing lesion characteristics in the reconstructed DBT slices and in the PVs.Methods:
A data set of MLO view DBT of 99 patients containing 107 masses (56 malignant and 51 benign) was collected at the Massachusetts General Hospital with IRB approval. The DBTs were obtained with a GE prototype system which acquired 11 PVs over a 50° arc. The authors reconstructed the DBTs at 1 mm slice interval using a simultaneous algebraic reconstruction technique. The region of interest (ROI) containing the mass was marked by a radiologist in the DBT volume and the corresponding ROIs on the PVs were derived based on the imaging geometry. The subsequent processes were fully automated. For classification of masses using the DBT-slice approach, the mass on each slice was segmented by an active contour model initialized with adaptive-means clustering. A spiculation likelihood map was generated by analysis of the gradient directions around the mass margin and spiculation features were extracted from the map. The rubber band straightening transform (RBST) was applied to a band of pixels around the segmented mass boundary. The RBST image was enhanced by Sobel filtering in the horizontal and vertical directions, from which run-length statistics texture features were extracted. Morphological features including those from the normalized radial length were designed to describe the mass shape. A feature space composed of the spiculation features, texture features, and morphological features extracted from the central slice alone and seven feature spaces obtained by averaging the corresponding features from three to 19 slices centered at the central slice were compared. For classification of masses using the PV approach, a feature extraction process similar to that described above for the DBT approach was performed on the ROIs from the individual PVs. Six feature spaces obtained from the central PV alone and by averaging the corresponding features from three to 11 PVs were formed. In each feature space for either the DBT-slice or the PV approach, a linear discriminant analysis classifier with stepwise feature selection was trained and tested using a two-loop leave-one-case-out resampling procedure. Simplex optimization was used to guide feature selection automatically within the training set in each leave-one-case-out cycle. The performance of the classifiers was evaluated by the area under the receiver operating characteristic curve.Results:
The test values from the DBT-slice approach ranged from to , while those from the PV approach ranged from to . The highest test of from the nine-DBT-slice feature space was significantly better than the highest test of from the nine-PV feature space.Conclusion:
The features of breast lesions extracted from the DBT slices consistently provided higher classification accuracy than those extracted from the PV images.
37(2010); http://dx.doi.org/10.1118/1.3455287View Description Hide DescriptionPurpose:
Several studies have shown the benefit of an accurate system modeling using Monte Carlo techniques. For state-of-the-art whole-body positron emission tomography(PET)scanners, Monte Carlo-based image reconstruction is associated with a significant computational cost to calculate the system matrix as well as a large memory capacity to store it. In this article, the authors present a simulation-reconstruction framework to solve these problems on the Philips Gemini GS PETscanner.Methods:
A fast, realistic system matrix simulation module was developed using egs_pet, which is an efficient PET simulation code based on EGSnrc. The generated system matrix was then used in a rotator-based ordered subset expectation maximization (OS-EM) algorithm, which exploits the rotational symmetry of a cylindrical PETscanner. The system matrix was further compressed by using sparse storage techniques.Results:
The system matrix simulation took five days on 50 cores of Xeon 2.66 GHz, resulting in a system matrix of 2.01 GB. The entire system matrix could be stored in the main memory of a standard personal computer. The image quality in terms of contrast-noise trade-offs was considerably improved compared to a standard OS-EM algorithm. The image quality was also compared to the clinical software on the scanner using routine parameter settings. The contrast recovery coefficient of small hot spheres and cold spheres was significantly improved.Conclusions:
The results indicated that the proposed framework could be used for this PETscanner with improved image quality. This method could also be applied to other state-of-the-art whole-body PETscanners and preclinical PETscanners with a similar shape.
Light emission efficiency and imaging performance of powder scintillator under x-ray radiography conditions37(2010); http://dx.doi.org/10.1118/1.3451113View Description Hide DescriptionPurpose:
To evaluate powder phosphor as a radiographic image receptor and to compare it to phosphors often used in radiography. is nonhygroscopic, emitting red light with decay time close to that of .Methods:
The light intensity emitted per unit of x-ray exposure rate (absolute luminescence efficiency) was measured for laboratory prepared screens with coating thicknesses of 33.1, 46.4, 63.1, 78.3, and and tube voltages ranging from 50 to 140 kVp. Parameters related to image quality such as the modulation transfer function(MTF) and the detective quantum efficiency (DQE) were also experimentally examined. In addition, a previously validated Monte Carlo code was used to estimate intrinsic x-ray absorption and optical properties, as well as the MTF and the Swank factor of the scintillators.Results:
light intensity was found higher than that of single CsI:Tl crystal for tube voltages up to 100 kVp. The MTF and the DQE were found to be comparable with those of and CsI:Tl screens. MTF estimated by the Monte Carlo code was found very close to the experimental MTF values. showed peak emission in the wavelength range 620–630 nm. Its emission spectrum was excellently matched to various optical detectors (photodiodes, photocathodes, CCDs, and CMOS) employed in flat panel detectors.Conclusions:
is an efficient phosphor potentially well suited to radiography and especially to some digital detectors sensitive to red light.
An attenuation integral digital imaging technique for the treatment portal verification of conventional and intensity-modulated radiotherapy37(2010); http://dx.doi.org/10.1118/1.3447729View Description Hide DescriptionPurpose:
To propose an attenuation integral digital imaging (AIDI) technique for the treatment portal verification of conventional and intensity-modulated radiotherapy(IMRT).Methods:
In AIDI technique, an open in air fluence image and a patient fluence image were acquired under the same exposure. Then after doing the dark field correction for both the and , the AIDI image was simply calculated as , which is the attenuation integral along the ray path from the x-ray source to a detector pixel element. Theoretical analysis for the low contrastdetection and the contrast to noise ratio (CNR) of AIDI was presented and compared to those for the fluence imaging. With AIDI, the variation of x-ray fluence and the variation of individual detector pixel’s response can be automatically compensated without using the flood field correction.Results:
The AIDI image for a contrast detail phantom demonstrated that it can efficiently suppress the background structures such as the couch and generate better visibility for low contrast objects with megavoltage x rays. The AIDI image acquired for a Catphan 500 phantom using a 60° electronic dynamic wedge field also revealed more contrast disks than the fluence imaging did. Finally, AIDI for an IMRT field of a head/neck patient successfully displayed the anatomical structures underneath the treatment portal but not shown in fluence imaging.Conclusions:
For IMRT and high degree wedge beams, direct imaging using them is difficult because their photon fluence is highly nonuniform. But AIDI can be used for the treatment portal verification of these beams.
A symmetric nonrigid registration method to handle large organ deformations in cervical cancer patients37(2010); http://dx.doi.org/10.1118/1.3443436View Description Hide DescriptionPurpose:
Modern radiotherapy requires assessment of patient anatomical changes. By using unidirectional registration methods, the quantified anatomical changes are asymmetric, i.e., depend on the direction of the registration. Moreover, the registration is challenged by the large and complex organdeformations that can occur in, e.g., cervical cancer patients. The aim of this work was to develop, test, and validate a symmetric feature-based nonrigid registration method that can handle organs with large-scale deformations.Methods:
A symmetric version of the unidirectional thin plate spline robust point matching (TPS-RPM) algorithm was developed, implemented, tested, and validated. Tests were performed by using the delineated cervix and uterus and bladder in CT scans of five cervical cancer patients. For each patient, five CT scans with a large variability in organ shape, volume, and deformations were acquired. Both the symmetric and the unidirectional algorithm were employed to calculate the registration geometric accuracy (surface distance and surface coverage errors), the inverse consistency, the residual distances after transforming anatomical landmarks, and the registration time. Additionally, to facilitate the further use of our symmetric method, a large set of input parameters was tested.Results:
The developed symmetric algorithm handled successfully the registration of bladders with extreme volume change for which TPS-RPM failed. Compared to the unidirectional algorithm the symmetric algorithm improved, for the registration of organs with large volume change, the inverse consistency by 78% and the surface coverage by 46%. Similarly, for organs with small volume change, the symmetric algorithm improved the inverse consistency by 69% and the surface coverage by 13%. The method allowed for anatomically coherent registration in only 35 s for cervix-uterus and 151 s for bladder, while keeping the inverse consistency errors around 1 mm and the surface matching errors below 1 mm. Compared to rigid alignment the symmetric method reduced the residual distances between anatomical landmarks from a range of to a range of .Conclusions:
The developed symmetric method could be employed to perform fast, accurate, consistent, and anatomically coherent registration of organs with large and complex deformations. Therefore, the method is a useful tool that could support further developments in high precision image guided radiotherapy.
37(2010); http://dx.doi.org/10.1118/1.3446801View Description Hide DescriptionPurpose:
In this article, the authors propose a multibeam field emissionx-ray (MBFEX) system along with a half-scan fan-beam reconstruction algorithm.Methods:
The proposed system consists of a linear CNT-based MBFEX source array, a single large area detector that is divided into same number of segments as the number of x-ray beams, a multihole collimator that aligns each beam with a corresponding detector segment, and a sample rotation stage. The collimator is placed between the source and the object to restrict the x-ray radiations through the target object only. In this design, all the x-ray beams are activated simultaneously to provide multiple projection views of the object. The detector is virtually segmented and synchronized with the x-ray exposure and the physiological signals when gating is involved. The transmitted x-ray intensity from each beam is collected by the corresponding segment on the detector. After each exposure, the object is rotated by a step angle until sufficient data set is collected. The half-scan reconstruction formula for MBFEX system is derived from the conventional filtered backprojection algorithm. To demonstrate the advantages of the system and method in reducing motion artifacts, the authors performed simulations with both standard and dynamic Shepp–Logan phantoms.Results:
The numerical results indicate that the proposed multibeam system and the associated half-scan algorithm can effectively reduce the scanning time and improve the image quality for a time-varying object.Conclusions:
The MBFEX technique offers an opportunity for the innovation of multisource imaging system.
37(2010); http://dx.doi.org/10.1118/1.3455701View Description Hide DescriptionPurpose:
LungImageDatabase Consortium (LIDC) is the largest public CTimagedatabase of lung nodules. In this study, the authors present a comprehensive and the most updated analysis of this dynamically growing database under the help of a computerized tool, aiming to assist researchers to optimally use this database for lungcancer related investigations.Methods:
The authors developed a computer scheme to automatically match the nodule outlines marked manually by radiologists on CTimages. A large variety of characteristics regarding the annotated nodules in the database including volume, spiculation level, elongation, interobserver variability, as well as the intersection of delineated nodule voxels and overlapping ratio between the same nodules marked by different radiologists are automatically calculated and summarized. The scheme was applied to analyze all 157 examinations with complete annotation data currently available in LIDC dataset.Results:
The scheme summarizes the statistical distributions of the abovementioned geometric and diagnosis features. Among the 391 nodules, (1) 365 (93.35%) have principal axis length; (2) 120, 75, 76, and 120 were marked by one, two, three, and four radiologists, respectively; and (3) 122 (32.48%) have the maximum volume overlapping ratios for the delineations of two radiologists, while 198 (50.64%) have the maximum volume overlapping ratios . The results also showed that 72.89% of the nodules were assessed with malignancy score between 2 and 4, and only 7.93% of these nodules were considered as severely malignant (malignancy).Conclusions:
This study demonstrates that LIDC contains examinations covering a diverse distribution of nodule characteristics and it can be a useful resource to assess the performance of the nodule detection and/or segmentation schemes.
37(2010); http://dx.doi.org/10.1118/1.3447727View Description Hide DescriptionPurpose:
In the past decade, phase-contrast imaging (PCI) has been applied to study different kinds of tissues and human body parts, with an increased improvement of the image quality with respect to simple absorption radiography. A technique closely related to PCI is phase-retrieval imaging (PRI). Indeed, PCI is an imaging modality thought to enhance the total contrast of the images through the phase shift introduced by the object (human body part); PRI is a mathematical technique to extract the quantitative phase-shift map from PCI. A new phase-retrieval algorithm for the in-line phase-contrast x-ray imaging is here proposed.Methods:
The proposed algorithm is based on a mixed transfer-function and transport-of-intensity approach (MA) and it requires, at most, an initial approximate estimate of the average phase shift introduced by the object as prior knowledge. The accuracy in the initial estimate determines the convergence speed of the algorithm. The proposed algorithm retrieves both the object phase and its complex conjugate in a combined MA (CMA).Results:
Although slightly less computationally effective with respect to other mixed-approach algorithms, as two phases have to be retrieved, the results obtained by the CMA on simulated data have shown that the obtained reconstructed phase maps are characterized by particularly low normalized mean square errors. The authors have also tested the CMA on noisy experimental phase-contrast data obtained by a suitable weakly absorbing sample consisting of a grid of submillimetric nylon fibers as well as on a strongly absorbing object made of a 0.03 mm thick lead x-ray resolution star pattern. The CMA has shown a good efficiency in recovering phase information, also in presence of noisy data, characterized by peak-to-peak signal-to-noise ratios down to a few dBs, showing the possibility to enhance with phase radiography the signal-to-noise ratio for features in the submillimetric scale with respect to the attenuation-based imaging.Conclusions:
It has been shown that phase-retrieved radiographies can be used both to have quantitative phase information about soft tissues, complementary to the attenuation information, and to enhance the visibility of details inside soft tissues, with higher efficiency with respect to phase radiography.
A generic geometric calibration method for tomographic imaging systems with flat-panel detectors—A detailed implementation guide37(2010); http://dx.doi.org/10.1118/1.3431996View Description Hide DescriptionPurpose:
To present a generic geometric calibration method for tomographic imagingsystems with flat-panel detectors in a very detailed manner, in the aim to provide a useful tool to the public domain.Methods:
The method is based on a projection matrix which represents a mapping from 3D object coordinate system to 2D projection image plane. The projection matrix can be determined experimentally through the imaging of a phantom of known marker geometry. Accurate implementation was accomplished through direct computation algorithms, including a novel ellipse fitting using singular value decomposition and data normalization. Benefits of the method include: (1) It is capable of being applied to systems of different scan trajectories, source-detector alignments, and detector orientations; (2) projection matrices can be utilized in image reconstructions or in the extraction of explicit geometrical parameters; and (3) the method imposes minimal limits on the design of calibration phantom. C++ programs that calculate projection matrices and extract geometric parameters from them are also provided. For validation, the calibration method was applied to the computer simulation of a cone-beam CTsystem, as well as to three tomosynthesis prototypes of different source-detector movement patterns: Source and detector rotating synchronizedly; source rotating and detector wobbling; and source rotating and detector staying stationary.Results:
Projection matrices were computed on a view by view basis. Geometric parameters extracted from projection matrices were consistent with actual settings. Images were reconstructed by directly using projection matrices, and were compared to virtual Shepp–Logan image for CT simulation and to central projection images of CIRS breast phantoms for tomosynthesis prototypes. They showed no obvious distortion or blurring, indicating the high quality of geometric calibration results. When the computed central ray offsets were perturbed with Gaussian noises of 1 pixel standard deviation, the reconstructed image showed apparent distortion, which further demonstrated the accuracy of the geometric calibration method.Conclusions:
The method is suitable for tomographic imagingsystems with flat-panel detectors.