Volume 36, Issue 6, June 2009
Index of content:
- Imaging Scientific Session: Room 304A
- Cone‐beam Computed Tomography
36(2009); http://dx.doi.org/10.1118/1.3182230View Description Hide Description
Purpose: To analyze the stationarity of 3D x‐ray cone‐beam computed tomography(CBCT) through spatial and frequency domain descriptions of noise. Such analysis of the stationarity assumption is an essential component of task‐based assessment of CBCTimaging performance. Method and Materials: Three objects (air, water cylinder + bowtie filter, and water cylinder only) were imaged using a CBCT benchtop and the Feldkamp algorithm for 3D filtered backprojection. Noise‐only 3D images were analyzed from the difference of two CBCT volumes under the three conditions. In the spatial domain, noise stationarity was analyzed in terms of variance maps, and in the frequency domain, in terms of the covariance matrix of the Fourier coefficients — of which the diagonal is the noise‐power spectrum (NPS). For a stationary random process, the Fourier covariance matrix (FCM) is diagonal and the same at every location in the image.Results: The three experimental conditions affected the degree of stationarity: air (highest stationarity), water+bowtie, water cylinder (least stationary). This rank order was consistent in terms of variance maps and the NPS. The off‐diagonal elements of the FCM were non‐negligible in comparison to the NPS and believed to have measurable effect on detectability. Conclusion: The degree of non‐stationarity in CBCT can be controlled experimentally and analyzed through both spatial and frequency domain techniques. From a practical standpoint of system design and evaluation, the findings suggest strategies for meaningful CBCTnoise and NPS analysis. Model observer analysis should assess the validity of the stationarity assumption, recognizing its potential effect on task performance.
36(2009); http://dx.doi.org/10.1118/1.3182231View Description Hide Description
Purpose: To study noiseproperties of low‐dose cone‐beam CT(CBCT) projection data and to improve the performance of statistics‐based image processing algorithms for dose reduction in CBCT with a more accurate projection data noisemodel.Method and Materials: We first performed repeated scan on an anthropomorphic chest phantom using an Acuity simulator (Varian Medical Systems, Palo Alto, CA). The data acquisition protocols included a tube voltage of 125 kVp, tube current 10 mA and duration of tube pulse 10 ms. At a fixed gantry angle, 500 repeated projections were acquired. From the repeated measurements, we first calculate the mean and variance of each detector pixel and then calculated the correlation coefficients of noise among detector pixels. The knowledge of the noiseproperties of CBCT projection was incorporated into a statistical image process algorithm based on the penalized weighted least‐squares (PWLS) criterion. The PWLS criterion with the improved noisemodel was then used to suppress noise in low‐dose CBCT projections. Results: Our analyses on repeated measurements show that the noise correlation coefficients are non‐zero between nearest neighboring pixels of CBCT projection data. An experimental phantom study shows that the PWLS with improved noisemodel can suppress the noise in low‐dose CBCT effectively without noticeable loss of resolution. Conclusion:Noise is correlated among nearest neighbors of CBCT projection data. An accurate noisemodel of CBCT projection data is established for statistics‐based image processing algorithm. CBCT dose can be significantly reduced through using the PWLS‐based projection smoothing algorithm that incorporates the projection noise correlation.
36(2009); http://dx.doi.org/10.1118/1.3182232View Description Hide Description
Purpose: Scatter correction is crucial to the quality of reconstructed images in X‐ray cone‐beam computed tomography(CBCT). Most of existing scatter correction methods assume smooth scatter distributions. The high‐frequency scatter noise remains in the projection images even after a perfect scatter correction. To circumvent the problem and truly gain from scatter correction, an effective scatter noise suppression method must be in place. Method and Materials: We analyze the noiseproperties in the projections after scatter correction, and propose to use a penalized weighted least‐squares (PWLS) algorithm to reduce the noise in the reconstructed images. The method aims to estimate the ideal linear integrals by minimizing a PWLS objective function which models the first and second moments of the projection data. Results: Experimental results on an evaluation phantom (Catphan@600) show that the proposed algorithm further reduces the reconstruction error in a scatter corrected image from 10.6% to 1.7%, and increases the CNR by a factor of 3.6. Significant image quality improvement is also shown in the results on an anthropomorphic phantom, in which the global noise level is reduced and the local streaking artifacts around bones are suppressed. Conclusion: In this paper, using a clinical CBCT system and a measurement‐based scatter correction, we show that a scatter correction alone does not provide satisfactory image quality and the loss of the contrast‐to‐noise ratio (CNR) of the scatter corrected image may overwrite the benefit of scatter removal. A statistics‐based PWLS is proposed to reduce the imagenoise after scatter correction and the algorithm performance is demonstrated using experiments.
36(2009); http://dx.doi.org/10.1118/1.3182233View Description Hide Description
Purpose:Image lag degrades image quality in cone‐beam CT(CBCT). This work investigates the magnitude of lag artifacts and develops an optimized lag coefficient model to correct lag artifacts in CBCTimages to improve guidance of radiotherapy(Elekta Synergy XVI). Method and Materials:Image lag and its relationship with various parameters including signal strength and frame number was investigated for a PerkinElmer (RID1640) flat‐panel imager. A new lag correction model referred to as the “average‐optimized lag coefficients model” (ALCM) is developed to correct CBCTimages. The optimization of lag coefficients was purely based on quantitative improvement in lag corrected CBCTimages. Each projection was corrected for lag effects by subtracting previous projections weighted by the magnitude of image lag. The quantification/measurement of lag coefficients for four different techniques including RESF(Rising‐Edge‐Step‐Response‐Function), IRF(Impulse‐Response‐Function), FESF(Rising‐Edge‐Step‐Response‐Function) and ALCM(Average‐Optimized‐Lag‐Coefficient‐Model) for the same detector. These models are applied/tested in correcting CBCTimages of two customs made phantoms referred to as Ellipse_Lucite (MTF and skinline) and Irregular_Lucite (CNR).Results: Experimental results illustrate that the nth frame lag of the imager for all four model shows different behavior with frame number. The RCTN at 5 mm depth after lag correction was measured in CT♯ as 4.38±1.01, 10.51±1.35, 8.35±1.31 and 2.121±0.81 for RESF, IRF, FESF and ALCM, respectively. Similarly, the spatial frequency/cm for MTF(50%) before and after lag correction for RESF, IRF, FESF and ALCM was measured as 6.3±0.24, 5.7±0.23, 5.8±0.22 and 6.5±0.24, respectively. CNR for ALCMwas almost two times higher than nominal. Conclusion: Lag artifacts can be reduced by correction of the projection images using the ALCM model. Lag correction is most important for high contrast and irregularly shaped objects. The performance metrics suggest a significant improvement for RESF and ALCM and strongly support their use for lag correction in cone‐beam CT.
Research sponsored by Elekta.
MO‐D‐304A‐05: Optimal Material Selection of Primary Modulator for Scatter Correction Based On the K‐Edge Discontinuity36(2009); http://dx.doi.org/10.1118/1.3182234View Description Hide Description
Purpose: To select an optimal material for the primary modulator in order to minimize its effect of beam hardening (BH). A measurement‐based scatter correction method that uses a checkerboard pattern of attenuating material (primary modulator) placed between the X‐ray source and the object has been developed and verified. In the practical implementation, BH is a limiting factor because the signal modulation depends on the object in the field of view (FOV). Methods: The transmission factor for different modulator materials (Be, Al, Cu, Ag, W, Ho, Er and Tm) was calculated for object thicknesses ranging from 0 to 30 cm equivalent water by using a simulated spectrum of our tabletop cone‐beam CT system at 120 kVp. We then generated scatter‐free projection images of a 30‐cm diameter water cylinder, with and without the modulator in place, and reconstructedCT slices after demodulation. Visibility of rings was compared for no noise, and with 2% added Gaussian noise. We also measured the transmission factors of 25 μm of Cu, W and Er for different combinations of Al (0–8 mm) and Cu (0–0.3 mm) filtration. Results: For a transmission factor of ∼0.9, simulations show that Er provides the least amount of variability as a function of added filtration (max variation<1.8%). The ring artifacts in simulated reconstructions were significantly reduced when using the Er, particularly in the presence of noise.Measured variability of transmission factors were 2.5%, 1.0% and 8.6% for 25 μm of Cu, Er and W, respectively. Conclusions: An optimization of modulator material to minimize BH artifact was proposed and validated. The variation of the transmission factor as a function of X‐ray energy reaches a local minimum when the K‐edge of the modulator material is near the mean energy of the spectrum. Design and evaluation of an Erbiummodulator is underway.
MO‐D‐304A‐06: Filtered Region‐Of‐Interest Cone‐Beam CT From Rotational Angiography for Image‐Guided Interventions: Evaluation of Dose Reduction and Image Quality36(2009); http://dx.doi.org/10.1118/1.3182235View Description Hide Description
Purpose: Rotational angiography is widely used to generate 3D data for image‐guided interventions. In standard practice, full‐field acquisitions are obtained, though the surgeon's region of interest (ROI) is considerably smaller, e.g. for stent placement. To reduce dose to the patient while delivering the desired image quality for the intervention, we developed a filtered ROI (FROI) method for reducing the beam intensity outside the ROI by using an x‐ray attenuating filter. We evaluated dose reduction and image quality resulting from this method. Method and Materials: A gadolinium‐based filter with a central circular opening is mounted on the aperture of the x‐ray tube of a rotational angiography system effectively splitting the beam into a high‐intensity ROI and a low‐intensity filtered region. Artifact‐free reconstruction is achieved by equalizing the filtered‐region intensities to those in the ROI using a calibrationimage. Dose reduction in FROI‐CT is assessed theoretically through the dose area product and experimentally using dose measurements. Image quality in unfiltered and equalized filtered images is assessed using the scatter fraction and the contrast‐to‐noise ratio (CNR) in both projection images and reconstructed volumes. Results: Experimental dose measurements for a filter thickness of 1.29 mm and an ROI size of 20% FOV yield a 60% dose reduction, similar to theoretical calculations. Scatter fractions inside the ROI are reduced by 35% compared to conventional images.CNR in projection images in the ROI is improved from 450:1 (conventional RA) to 533:1 (FROI) and is similar in the filtered region. CNR in the reconstructed ROI is 65:1, similar to conventional RA, and lower in the filtered region (45:1 filtered, 75:1 conventional). Conclusion: The results support the potential dose‐reduction capabilities and utility of ROI imaging in rotational angiography while supplying the desired image quality inside the (reconstructed) ROI.
36(2009); http://dx.doi.org/10.1118/1.3182236View Description Hide Description
Purpose. Compare images and spectra using a dual energy diamond x‐ray target for megavoltage cone‐beam CT (MVCBCT) with those of carbon and tungsten targets. Materials and Methods. A Siemens Oncor linac with megavoltage cone‐beam CT was modified to use either a tungsten,carbon, or diamond target for imaging. The carbon target was used with an accelerating potential of 4.2 MV. The diamond target had a higher density than the carbon, thus reducing electron leakage and allowing the linac waveguide to operate at the same accelerating potential as the treatment beam, namely 6 MV. For this study, the diamond target was operated at both 4.2 and 6 MV. With both diamond and carbon targets, the flattening filter was out of the beam path. Images were acquired both of phantoms, to determine contrast‐to‐noise ratio (CNR), and patients. Results. CNR measured with the phantom were nearly the same with the diamond target and the carbon target when both were operated at 4.2 MV. At 6 MV, CNR with the diamond target was degraded, especially for tissue. The highest quality images of head and neck patients were with the diamond target at 4.2 MV. As compared to the carbon target, there were fewer scattering artifacts, and tissue resolution was better. At 6 MV, there were even fewer artifacts but the contrast was reduced. Conclusions. The diamond target is a viable alternative to the carbon target for the IBL. It can be used at either the same energy as the treatment beam, which simplifies commissioning and QA, or at reduced energy for enhanced contrast.
Support from Siemens OCS.
36(2009); http://dx.doi.org/10.1118/1.3182237View Description Hide Description
Purpose: Micro‐CT scanners are used to create high‐resolution images and to measure physical properties (e.g. bone mineral density) of the objects observed. Modern micro‐CT scanners make use of cone beam geometry making the image information suffering from scatter radiation. This work aims to characterize the scatter radiation in the detector plane of a micro‐ct scanner using Monte Carlo methods.Method and Materials: EGSnrc was used to simulate the particle transport through the main components of the XtremeCT (Scanco Medical AG, Switzerland) and through different phantoms. Based on phase space information in the detector plane, the primary and the scatter radiation were analyzed. By implementing a dedicated LATCH method, the scatter radiation was subdivided into several components providing a detailed characterization of the scatter radiation. In order to study the dependence of these scatter components on the object to be scanned, Monte Carlo simulations were performed for different phantoms which vary in size or composition. Results: For typical object sizes the scattered radiation contribution is in the order of 10% of the total radiation in the detector plane. Most of these scattered particles result from interactions in the object. As expected, the relative scatter contribution increases with increasing density and size of the object. On the other hand, the Monte Carlo simulations show that scatter radiation, particularly the component which is due to scattering within the object, contains information about the structure of the object and this information increases with increasing density. Conclusion: In this work, the scatter radiation of a micro‐CT scanner was analyzed using Monte Carlo simulations. It is shown that the scattered radiation is dominated by scattering within the object. The results of this work provide a basis for future scatter correction methods in micro‐CT applications. Conflict of Interest: This work was supported by Scanco Medical AG.
MO‐D‐304A‐09: Multi‐Slice CT Versus Cone Beam CT for Breast Imaging: Radiation Dose Distributions with Monte Carlo Simulation36(2009); http://dx.doi.org/10.1118/1.3182238View Description Hide Description
Purpose: To use Monte Carlo simulation method to investigate and compare the overall radiation dose and dose distribution for breast imaging using multi‐slice CT and cone beam CT(CBCT) techniques. Method and Materials:CBCTimages of a breast specimen have been combined with multi‐slice CTimages of a patient chest to form a 210×206×110 model for a patient lying in prone position with both breasts freely protruding downward. Image segmentation was applied to separate the images into areas for adipose tissue, soft tissue,muscle, air and bone. Rep78 was used to compute the spectra for 120 and 80 kVp x‐rays with a HVL of 4.65 and 4.08 mm Al, respectively, for simulating multi‐slice CT and CBCT, respectively. A collimator was added to simulate the x‐ray fan beam used in multi‐slice CT and a half‐cone beam for CBCT. To simplify the simulation, the table was assumed to shift by one fan beam width after the gantry makes one full rotation. 2×107photons were used to calculate the dose for multi‐slice CT with DOSXYZnrc and 1×109photons for CBCT scan of the same breast phantom. The results of the dose calculations were rescaled for 240 mAs and 25 mAs, respectively. Results: The results of Monte Carlo calculations for multi‐slice CT indicated that dose levels in the heart and bone structures were about twice as high as those in the breasts. The breast dose from multi‐slice CT was from 0.8 to 2 times as high as those in cone beam breast CT.Conclusion: Results from our Monte Carlo calculations indicated that breast doses were found to be higher with a typical multi‐slice CT scan of the chest. An additional dose problem for multi‐slice CT is full exposures to the rest of the chest, incurring high radiation doses in the heart and bone structures.
- Computed Tomography and Radiation Dose
TU‐C‐304A‐01: The Need and Feasibility of a Modern Software for Reporting Patient Doses From CT Scans36(2009); http://dx.doi.org/10.1118/1.3182348View Description Hide Description
Purpose: To demonstrate the need and feasibility to develop a modern software tool for reporting the organ dose and effective dose for patients undergoing CT examinations. Method and Materials: Existing CT dose reporting software do not meet the need because of the simplified anatomical phantoms, updated ICRP data and scanner information. A new software is designed with original dose data derived from Monte Carlo simulations involving CTscanner models from various companies and anatomically realistic phantoms. X‐ray sources and protocols are modeled. The Pregnant Women, Adult Male and Adult Female phantoms are utilized. Organ doses and effective doses are computed using both the ICRP‐60 and the latest ICRP‐103 recommendations. The software is developed using the Visual C♯.NET with a modern graphical user interface (GUI) design to allow a user to specify the patient type, body scan region, and scanner operating parameters. Object‐oriented programming technology allows the phantoms to be displayed in 3D interactively. Results: Compared to values reported by the existing software, the organ dose estimates can be different by a ratio as 0.77 to 1.24 for the organ or tissue covered in the scan range, and 0.13 for the organs out of the scan region between calculations using the anatomically realistic phantoms. In addition to the improved dose accuracy, the new program offers a number pf modern GUI features through which 3D phantoms are vividly inspected for organs that receive a high doses. Based on the user‐specified scanning parameters, organ and effective doses are rapidly reported. Conclusion: Preliminary results have demonstrated the aim of the new software design in addressing the needs for new CTscanners, ICRP recommendations and anatomically realistic phantoms. When fully developed, this new tool is expected to improve both the accuracy and usability in reporting CT doses in the future.
TU‐C‐304A‐02: The Impact of the New ICRP‐103 Recommendations On the Assessment of Effective Doses From CT Procedures36(2009); http://dx.doi.org/10.1118/1.3182349View Description Hide Description
Purpose: To apply a pair of adult phantoms representing ICRP‐89 50th‐percentile adult males and females to the study of impact of the new ICRP‐103 recommendations on the assessment of Effective Dose from CT procedures. Method and Materials: a pair of mesh‐based computational phantoms, RPI Adult Male (RPI‐AM) and RPI Adult Female (RPI‐AF) that were recently developed to represent the ICRP‐89 50th‐percentile adult males and adult females. This pair of phantoms has the detailed bone structures, including the spongiosa which contains the red bone marrow. The detailed RBM distribution was adjusted according to ICRP Publication 70. The CT scanner model used in this study is an MDCT scanner which includes the source geometry and movement, the source energy spectrum, the bow‐tie filter as well and the beam shape. CT scan protocols including whole body scan were carefully modeled in this study, and tube potential of 120 kVp were considered. All simulations were performed using the Monte Carlo code, MCNPX 2.5.0. The three‐correction factor method was used to calculate the RBM dose. Effective Dose results were calculated following the algorithm from ICRP 103. Results: A new set of organ absorbed dose results has been presented using this pair of new developed reference adult phantoms from CT procedures, as well as the effective dose results. Also the new results of red bone marrow dose have been provided. The recently published ICRP 103 updated the radio‐sensitive organ list; also it improved the algorithm of effective dose calculations. Conclusion: Advanced red bone marrow dose calculation method has been used in this study due to the detailed bone structures of this pair of RPI‐AM and RPI‐AF phantoms. This new set of effective dose dataset based on the new ICRP‐103 recommendations could be used to provide latest information for clinical diagnostic dosimetry area.
TU‐C‐304A‐03: Stylized MIRD Phantoms Should Be Replaced by Anatomically Realistic Phantoms: Discrepancies In Red Bone Marrow Doses From CT Scans36(2009); http://dx.doi.org/10.1118/1.3182350View Description Hide Description
Purpose: To test the hypothesis that the stylized MIRD phantoms would cause significant error in the estimated red bone marrow (RBM) dose from CT scans in comparison with anatomically realistic phantoms. Method and Materials: The MC model of the CT scanner include the source geometry, movement, source energy spectrum, bow‐tie filter, as well and the beam shape. MCNPX 2.5.0 was used to simulate the RBM dose from various CT scanning procedures. To calculate the absorbed dose to the RBM as a function of photon fluence in the spongiosa and the photon energy, an F4 tally together with a set of DE/DF cards in MCNPX were used to score the photon fluence in MCNPX. The stylized MIRD phantom and the anatomically realistic RPI Adult Male and Adult Female phantoms were implemented in the MCNPX to determine organ doses using the same dose algorithm. Results: For all the cases studied, the RBM doses calculated using RPI adult phantoms were gearter than those obtained from MIRD‐ORNL phantoms. For the chest CT scan, the RBM dose ratio (RPI‐AM to MIRD‐ORNL) is about 1.50 (1.48–1.51), and RBM dose ratio of female phantoms is about 1.28 (1.24–1.32). For the abdominal‐pelvis CT scan, the RBM dose ratios are 1.30 (1.28–1.31) and 1.32 (1.28–1.38) for male and female phantom, respectively. These differences are mainly from the anatomical differences in the phantoms. Conclusion: As the RBM is not uniformly distributed in the human body, the homogeneous bone mixtures definition by MIRD phantoms underestimated the dose by as much as 50% in certain cases. This test concludes that the simplified MIRD phantoms used in existing CT dose software should and can be replaced by realistic phantoms. This is an opportunity to improve the anatomical realism and therefore the associated dose and risk assessments for patients who undergo CT examinations.
TU‐C‐304A‐04: Monte Carlo Based Multidetector CT Modeling and Dose Calculations for Pregnant Patients36(2009); http://dx.doi.org/10.1118/1.3182352View Description Hide Description
Purpose: To model and validate the multidector CT (MDCT) scanner and to assess radiation dose to the fetus and pregnant patient in three different gestational periods. Method and Materials:Monte Carlo code, MCNPX, was used to simulate the x‐ray source including the energy spectrum, filter, and scan trajectory. Detailed CTscanner components were specified using an iterative trial‐and‐error procedure for a GE LightSpeed CTscanner. The scannermodel was validated by comparing simulated results against measured CTDI values and dose profiles reported in the literature. The source movement along the helical trajectory was simulated using the pitch of 0.9375 and 1.375, respectively. The validated scannermodel was then integrated with phantoms of a pregnant patient in three different gestational periods to calculate organ doses and fetal doses. Results: Comparison between simulated results and reported results in literature shows good agreement in terms of CTDI values as well as dose profiles. It was found that the dose to the fetus of the 3‐month pregnant patient phantom was 0.13 mGy/100mAs and 0.57 mGy/100mAs from the chest and kidney scan, respectively. For the chest scan of the 6‐month patient phantom and the 9‐month patient phantom, the fetal doses were 0.21 mGy/100mAs and 0.26 mGy/100mAs, respectively. All these scans were performed with protocols that did not contain the fetus directly in the x‐ray beam. The paper also discusses how these fetal dose values can be used to evaluate imaging procedures and to assess risk using recommendations of the report from AAPM Task Group 36. Conclusion: This work demonstrates the ability of modeling and validating MDCT scanner by Monte Carlo method, as well as rapidly and accurately assessing fetal dose and organ doses by combining the MDCT scannermodel and pregnant patient phantom.
TU‐C‐304A‐05: Dose Reductions of Bismuth Shields in Diagnostic Radiology: Measurements and Monte Carlo Simulations36(2009); http://dx.doi.org/10.1118/1.3182353View Description Hide Description
Purpose: To assess the dosimetric characteristics of bismuth breast shields for a CT beam with ion chamber measurements and Monte Carlo simulations.Method and Materials: Primary attenuation and backscattereffects of both adult and pediatric bismuth superficial organ (e.g. breast) shields were measured with a 0.18‐cc ion chamber and a rectangular slab phantom made of a tissue equivalent material. Simulated CT beams (120 kVp with 100∼400 mAs) were used to irradiate the ion chamber with and without bismuth shields. Both 2‐ply (pediatric) and 4‐ply (adult) bismuth shields (F&L Medical Products, Vandergrift, PA) were placed at the front of the chamber for primary attenuation measurements and at the back of the chamber to measure backscatterradiation.Radiationdoses were measured free‐in‐air and in the tissue‐equivalent slabs. Monte Carlo simulations for the same measurement settings were performed by using EGSnrc/BEAMnrc code. Results: Mean radiationdose reduction from primary attenuation was about 23% (2‐ply) and 40% (4‐ply) for the free‐in‐air and tissue slab measurements. The radiationdose increase from backscatter was around 2% for both the 2‐ply and 4‐ply shields. The Monte Carlo simulations produced the dose reduction from primary attenuation was about 20% (2‐ply) and 38% (4‐ply), and the dose increase from backscatter was around 6% for both shields.
Conclusion: Primary attenuation is the dominant factor that induces radiationdose changes in bismuth shields in CT examinations. The dose contribution from backscatteredradiation in bismuth shields is very small.
36(2009); http://dx.doi.org/10.1118/1.3182354View Description Hide Description
Purpose: Previous work demonstrated that there are significant dose variations on the peripheral, or surface of either a CTDI 32cm phantom or an anthropomorphic phantom when helical CT scanning is performed. The purpose of this work is to investigate the effectiveness of exploiting these variations to reduce dose to targeted radiosensitive organs solely by varying the tube start angle in CT scans.Method and Materials: Radiation dose to several radiosensitive organs (including breasts, thyroid, uterus, gonads, lens of eyes) from a MDCT CT scanner were estimated using Monte Carlo simulation methods on GSF Baby phantom. Whole body scans were simulated using 120kVp, 300mAs, 28.8 mm nominal collimation, pitch 1.5 under a wide range of start angles (0 to 340 degrees in 20 degree increments). The relationship between tube start angle and organ dose was examined for each organ and the potential dose reduction was calculated. Results: The organ dose shows obvious variation depending on the tube start angle. For small peripheral organs, (e.g. the lens of eyes), the minimum dose can be 35% lower than the maximum dose, depending on tube start angle. For pitch 1.5 scans, the dose is usually lowest when the tube start angle is such that the x‐ray tube is posterior to the patient when it passes the longitudinal location of the organ.Conclusion: Helical MDCT scanning results in “cold spots” and “hot spots” that are created both at surface and even in‐depth locations within patients. If organs have a relatively small longitudinal extent, their dose may be reduced by selecting the tube start angle such that the location of these “cold spots” may be manipulated by appropriately selecting the tube start angle. This dose reduction should not have any implications for image quality as there is no change in mAs or total mAs.
TU‐C‐304A‐07: X‐Ray Tube Current Modulation and Effective Dose Per Unit Dose‐Length Product Conversion Factors in CT Dosimetry36(2009); http://dx.doi.org/10.1118/1.3182355View Description Hide Description
Purpose: To quantify how axial and longitudinal x‐ray tube current modulation influence effective dose per unit dose‐length product (E/DLP) conversion factors in chest CT.Method: We simulated a chest CT examination using a 4 cm beam width with projections obtained at every 15° x‐ray tube position at a constant tube output (120 kV). A radiographic patient dosimetry software package (PCXMC) was used to quantify relative patient effective dose as a function of the angular position and longitudinal location (z) of the x‐ray tube. Typical angular and longitudinal mA modulation schemes were obtained from the scientific literature. E/DLP conversion factors were generated for: (a) no mA modulation; (b) angular modulation alone; and (c) longitudinal modulation alone. Results: As the x‐ray tube rotates around the patient, the highest effective dose was at 285° (AP projection) and the lowest effective dose was at 195° (lateral projection), with the maximum to minimum ratio of 2.2. An angular mA modulation scheme with an AP/PA tube current one third of the lateral tube current reduces the E/DLP conversion factor in chest CT by 4.2%. For x‐ray tube movement along the z‐axis, the maximum to minimum ratio of patient effective dose was 3.3. In chest CTimaging, the longitudinal mA modulation changes the tube current approximately seven fold between the central lung area and the upper thorax region above the patient's lungs. Application of this longitudinal mA modulation scheme reduces the E/DLP conversion factor in chest CT by 9.2%. Conclusions: Use of longitudinal and angular mA modulation schemes in chest CT examinations could reduce E/DLP conversion factors by ∼13%.
36(2009); http://dx.doi.org/10.1118/1.3182356View Description Hide Description
Purpose:CT manufacturers are required to report dose using standardized CTDI phantoms. However, CTDI can be confusing when applied to pediatric patients due to variations in patient size. We propose a simple method to scale standard CTDI to better represent a pediatric patient's dose by providing universal conversion tables that are indexed by patient size ranges. Method and Materials: The Medical Imaging and Technology Alliance (MITA) and Image Gently Pediatric CT Physics Work Group have obtained CTDI ratios between the 32 cm, 16 cm and 10 cm phantoms for current 16 and 64 slice scanners. X‐ray attenuation was measured for the CTDI phantoms and retrospectively determined from pediatric and adult pre‐scan projection images. Regression relationships between patient dimensions, attenuation and dose ratios were used to create tables of CTDI scale factors as a function of patient dimensions. Results: The range of attenuation ratios of the 2nd and 3rd quartiles for pediatric heads compared to the standard CTDI phantoms was 0.85 to 0.95 (16 cm) and 0.46 to 0.51 (32 cm). The range for pediatric bodies was 1.03 to 1.35 (16 cm) and 0.85 to 0.95 (32 cm). Individual measurement errors relative to the mean CTDI ratio due to manufacturer, scanner generation and kVp or bowtie filter selections were minimal. Preliminary results show errors increased from 0 % to 14 % with increasing difference between the attenuation of the pediatric patient and CTDI phantom. Conclusion: Universal dose conversion tables can be provided based on dimensions or can be automatically calculated from a patient's attenuation determined from a pre‐scan projection image with reasonable accuracy to scale CTDI or its future replacement to better represent dose for pediatric patients.
36(2009); http://dx.doi.org/10.1118/1.3182357View Description Hide Description
Purpose: The purpose of this study was to evaluate potential radiation dose reduction for pediatric CT.Materials and Methods: A dose‐reduction simulation tool, which adds synthetic noise to raw projection measurements and reconstructsimages at a simulated lower dose (Massoumzadeh, et al. Med. Phys. Vol. 36, pp. 174–189, 2009), was used to simulate low‐dose CTimages. Simulated low‐dose CTimages are created from full‐dose CTimages of normal and pathological slices (lung nodules and abdominal visceral lesions). The amount of added noise was task dependent, with 10 sets of simulated low dose ranged from 1% to 85% of original dose. Sixteen pediatric cases were selected, including eight normal, three patients with pulmonary nodules, two patients with abdominal visceral organ lesions, and three patients with appendicitis without perforation. All studies performed on a 16‐row scanner, with the effects of tube current modulation and bow tie filters included. Following a short training session, 19 volunteer radiologists, from various clinical centers in the world who were attending the International Pediatric Radiology conference in Montreal in 2006, used a 5‐point scale to rate a sequence of simulated, low‐to‐high exposure images for the presence or absence of lesions. They were total of 176 images with viewing sessions limited to 45 minutes. Diagnostic agreement between full‐dose and reduced‐dose images was assessed with a weighted Kappa statistic. Result: For detection of pulmonary nodules, a decrease in the average intra‐observer agreement (kappa= 0.90) was found at 80% dose reduction, while for the detection of abdominal lesions or appendicitis a 50% reduction was observed. Conclusion: There is potential for dose reduction in CT studies, which is task dependent and greater for pulmonary nodule than for abdominal visceral lesions and appendicitis. The noise simulation methodology is a powerful tool to help understand the relationship among dose,noise, and observer agreement.
36(2009); http://dx.doi.org/10.1118/1.3182358View Description Hide Description
Purpose: The purpose of this research was to systematically explore the change in contrast‐noise ratio (CNR) for a variety of substances as a function of kV in CT examinations for small body habitus under the condition of constant dose. Method and Materials: The head module of a Gammex RMI 461A Head/Body phantom (constructed from solid water) was suspended from the end of the table of a Philips Brilliance 6‐slice CTscanner. An ion chamber introduced into the center of the phantom was used to adjust the mAs at kVp values of 90, 120 and 140 so that the exposure at the center was the same at all kVp values. Fifteen materials were tested at the center of the phantom for each of the kVp values with the CT number mean and standard deviation (SD) measured for each of the resulting 45 scans. A control area in the solid water was also tested for each scan using an identical ROI. Related tests were performed on all samples at once using other scanners.
Results: For all 45 scans, the SD was independent of kVp demonstrating that the noise was a function of dose only and not kV. For high density materials, lowering the kV improved the CNR in a predictable way. For materials with CT numbers not far from water, however, in some cases the CNR became worse with decreasing kVp while in others, the CNR improved. Conclusion: While lowering the kVp vs. lowering the mAs in small body habitus may result in an improved CNR, it should not be assumed that this is always the case. Dose reduction strategies for particular situations may benefit more by reducing the beam intensity and keeping the kV high.
- Computed Tomography and Reconstruction
TU‐D‐304A‐01: Development and Testing of a Novel, 4D Maximum A Posteriori (MAP) Image Reconstruction Algorithm36(2009); http://dx.doi.org/10.1118/1.3182389View Description Hide Description
Purpose: Deformable image registration has been proven to be useful in tracking organ motion for dose calculation using artifact‐free 4D RCCT images. Such methods are challenged in the presence of image artifacts. We present an alternative method which avoids binning artifacts by directly estimating organ deformation during the reconstruction process. Method and Materials: We have developed a maximum a posteriori (MAP) algorithm for tracking organ motion that uses raw time‐stamped data to reconstruct the images and estimate deformations in anatomy simultaneously. Since the algorithm does not rely on a binning process, binning artifacts are avoided. Signal‐to‐noise ratio (SNR) is also increased since the algorithm uses all of the collected data. The increased SNR provides the opportunity to reduce dose to the patient during scanning. This framework also facilitates the incorporation of fundamental physical properties such as the conservation of local tissue volume during the estimation of organ motion. In order to validate the accuracy of the 4D reconstruction algorithm, a phantom study was performed using the CIRS anthropomorphic thorax phantom in a CT scanner. An improvement in image quality was also demonstrated by application of the algorithm to data from a real liver stereotactic body radiation therapy(SBRT) patient. Results: The algorithm accurately estimated the known motion of the anthropomorphic phantom. Additionally, a significant SNR increase was observed when using 4D reconstruction over binning, even for a scan with X‐ray tube current reduced to 10%. Conclusion: A novel method of fully 4D CTreconstruction was presented. The geometric accuracy of the estimated deformation was validated in phantom. A marked improvement in image quality was observed when applying the algorithm to image data from a real liverSBRT patient. The method allows reduction of X‐ray tube current during scanning while simultaneously improving motion estimates for use in dose calculation.