Reference Phantom Research Articles

System-Independent Ultrasound Attenuation Coefficient Estimation Using Spectra Normalization.

Ultrasound attenuation coefficient estimation (ACE) has diagnostic potential for clinical applications such as differentiating tumors and quantifying fat content in the liver. The two commonly used ACE methods in the ultrasound array imaging system are the spectral shift method and the reference-phantom-based methods. The spectra shift method estimates the central frequency downshift along depth, whereas the reference-phantom-based methods use a well-calibrated phantom to cancel system dependent effects in attenuation estimation. In this study, we propose a novel system-independent ACE technique based on spectra normalization of different frequencies. This technique does not require a reference phantom for normalization. The power of each frequency component is normalized by the power of an adjacent frequency component in the spectrum to cancel system-dependent effects, such as focusing and time gain compensation (TGC). This method is referred to as the reference frequency method (RFM), and its performance has been evaluated in phantoms and in vivo liver studies. The RFM technique can be applied to various transducer geometries (e.g., linear or curved arrays) with different beam patterns (e.g., focused or unfocused).

In situ calibration to account for transmission losses in backscatter coefficient estimation

The backscatter coefficient (BSC) has demonstrated the ability to classify disease state and identify the response of cancer to therapy. However, estimating the BSC in vivo using a reference phantom technique does not account for transmission losses due to intervening layers, leading to increase in bias and variance of BSC-based estimates from one sample to the next. To account for transmission losses, an in situ calibration approach is proposed using a titanium sphere that is well-characterized ultrasonically, biocompatible, and embedded inside the sample. Ultrasound scattered from the sphere encounters the same transmission loss and attenuation as the investigated sample and can be used as a reference spectrum. To test the calibration procedures, phantoms were scanned with and without lossy layers on top, and BSCs were estimated using the in situ calibration approach and the reference phantom approach and compared. The differences of the BSCs, using the BSC from the reference phantom without a layer as baseline, were 0.16 ± 2.29 dB, -1.95 ± 2.99 dB and -10.90 ± 3.64 dB using the in situ calibration approach without the layer, with the layer, and using the reference phantom approach with the layer, respectively. The results indicate that an in situ calibration target can account for overlaying tissue losses thereby improving the robustness of BSC-based estimates.The backscatter coefficient (BSC) has demonstrated the ability to classify disease state and identify the response of cancer to therapy. However, estimating the BSC in vivo using a reference phantom technique does not account for transmission losses due to intervening layers, leading to increase in bias and variance of BSC-based estimates from one sample to the next. To account for transmission losses, an in situ calibration approach is proposed using a titanium sphere that is well-characterized ultrasonically, biocompatible, and embedded inside the sample. Ultrasound scattered from the sphere encounters the same transmission loss and attenuation as the investigated sample and can be used as a reference spectrum. To test the calibration procedures, phantoms were scanned with and without lossy layers on top, and BSCs were estimated using the in situ calibration approach and the reference phantom approach and compared. The differences of the BSCs, using the BSC from the reference phantom without a layer as ...

Comparison of two ultrasonic backscatter coefficient methods under nonlinear distortion

The backscatter coefficient (BSC) is a fundamental property of tissues and can be parameterized for tissue characterization. The BSC requires a reference signal, which is estimated through either the planar reflector method (PRM) or the reference phantom method (RPM). In both methods, linear acoustic propagation is assumed. In this work, the BSC estimation methods are evaluated when nonlinear distortion is present. RF data were acquired from two physical phantoms, labeled A and B, with a 5 MHz single-element transducer using low power (1 excitation level) and high power (6 increasing excitation levels) excitation signals. Phantom A contained glass beads with diameters ranging from 75 to 90 μm and phantom B had glass beads with diameters ranging from 9 to 43 μm. The BSCs estimated using the low power setting and high power settings were compared for the both the PRM and RPM through a root mean square error (RMSE). Estimates of the effective scatterer diameter (ESD) were obtained using each method for each high power setting and compared to the low power setting. The RMSE increased as the power setting increased with much higher RMSEs using the PRM compared to RPM, i.e., a maximum of 7.3 times larger for phantom A and 8.6 times larger for phantom B. Estimates of ESD matched the ranges of glass beads sizes for both phantoms except when using the PRM at higher power settings. These findings suggest that the RPM is more robust to nonlinear distortion effects compare to the PRM.

Inter-platform reproducibility of ultrasonic attenuation and backscatter coefficients in assessing NAFLD.

To assess inter-platform reproducibility of ultrasonic attenuation coefficient (AC) and backscatter coefficient (BSC) estimates in adults with known/suspected nonalcoholic fatty liver disease (NAFLD). This HIPAA-compliant prospective study was approved by an institutional review board; informed consent was obtained. Participants with known/suspected NAFLD were recruited and underwent same-day liver examinations with clinical ultrasound scanner platforms from two manufacturers. Each participant was scanned by the same trained sonographer who performed multiple data acquisitions in the right liver lobe using a lateral intercostal approach. Each data acquisition recorded a B-mode image and the underlying radio frequency (RF) data. AC and BSC were calculated using the reference phantom method. Inter-platform reproducibility was evaluated for AC and log-transformed BSC (logBSC = 10log10BSC) by intraclass correlation coefficient (ICC), Pearson's correlation, Bland-Altman analysis with computation of limits of agreement (LOAs), and within-subject coefficient of variation (wCV; applicable to AC). Sixty-four participants were enrolled. Mean AC values measured using the two platforms were 0.90 ± 0.13 and 0.94 ± 0.15dB/cm/MHz while mean logBSC values were - 30.6 ± 5.0 and - 27.9 ± 5.6dB, respectively. Inter-platform ICC was 0.77 for AC and 0.70 for log-transformed BSC in terms of absolute agreement. Pearson's correlation coefficient was 0.81 for AC and 0.80 for logBSC. Ninety-five percent LOAs were - 0.21 to 0.13dB/cm/MHz for AC, and - 9.48 to 3.98dB for logBSC. The wCV was 7% for AC. Hepatic AC and BSC are reproducible across two different ultrasound platforms in adults with known or suspected NAFLD. • Ultrasonic attenuation coefficient and backscatter coefficient are reproducible between two different ultrasound platforms in adults with NAFLD. • This inter-platform reproducibility may qualify quantitative ultrasound biomarkers for generalized clinical application in patients with suspected/known NAFLD.

Increasing organ dose accuracy through voxel phantom organ matching with individual patient anatomy

PurposeGiven the large use of computed tomography (CT) worldwide, the potential harmful effects and the radiation dose delivered to the patient have become high-interest topics among the scientific community. The main objective of this work was to modify existing three-dimensional (3D) voxel phantom models to resemble real patients as much as possible, thus trying to give a contribute towards the concept of a more personalized patient dosimetry. Moreover, comparisons were made of organ dose computed in the standard reference phantom and phantoms modified to match patient body size and organ size. This study focused essentially on one of the biggest and most radiosensitive organs in the thorax, the lungs. MethodsIn order to accomplish this objective a FORTRAN-based program was developed, which is able to semi-automatically modify the volumetric information of organs of interest in a standard voxel phantom (Female ICRP Adult Reference). Monte Carlo (MC) PENELOPE simulation code was used to mimic CT previews and, therefore, generate 2D projections, used for visual organ matching with clinical patient CT images, and calculate organ dose in both phantoms (ICRP standard and ICRP modified). Two CT scan irradiation setups were considered, namely Fan Beam CT (FBCT) and Cone Beam CT (CBCT) geometries. Validation of the implemented MC model (voxel phantom plus X-ray sources in diagnostic energy range) was accomplished through the calculation of lung-absorbed dose per fluence conversion coefficients for the Anterior-Posterior (AP) geometry setup with subsequent comparison with ICRP 116 tabulated values. ResultsThe main results reported that matching the voxel phantom's size and lungs provides organ dose values substantially different from the ones calculated in the ICRP reference phantom. According to the data presented in this study, organ dose can be up to 20% underestimated and 40% overestimated when compared to organ dose in the reference phantom, considering both irradiation setups. ConclusionsThis study demonstrates that voxel phantoms developed using single patient data could provide a better and more precise organ dose assessment by MC methods with respect to a standard phantom.

Attenuation estimation of soft tissue with reference-free minimization of system effects

Estimation of ultrasonic attenuation coefficient (AC) is essential for quantifying and characterizing the features of tissue microstructure. In the conventional AC estimation methods, a well-specified reference phantom is commonly used for minimizing the diffraction and transmit pulse related effects on the ultrasound radio-frequency (RF) signal. In this paper, a novel AC estimation technique is proposed avoiding the need of using any reference data, where the undesired system effects on the RF data are minimized through point spread function (PSF) separation and band-pass filtering of the envelope signal of the tissue reflectivity function (TRF). An improved and computationally efficient non-parametric cepstrum-based technique is used for separating the TRF and PSF from the measured RF signal. The Hilbert transform based temporal envelope is introduced to smooth out the unwanted effects of discontinuity and noise in the TRF and PSF signals. Finally, a band-pass filter based log power approximation of the TRF-envelope is applied to estimate the center frequency component of the attenuating power spectra with reduced diffraction effect. Assuming continuity of AC within a small uniform region, an exponentially weighted-average of logarithmic signal power of the neighboring blocks at the center frequency is measured for different depths, with a view to fit a regression line for obtaining an average AC value from its slope. Comparative results of the proposed reference-free AC estimation method with other conventional reference-based methods are presented for tissue-mimicking (TM) phantoms, in vivo breast and liver data. For the TM phantoms, the AC estimates using the new algorithm are within 10% deviation of the actual values. The obtained results for the breasts, normal livers, and fatty livers are 0.44 ± 0.23 dB/cm-MHz, 0.55 ± 0.21 dB/cm-MHz, and 0.61 ± 0.20 dB/cm-MHz, respectively, which are consistent with the literature-reported AC values. Different from reference-based methods, the novel technique is free from the bias that may result from the dissimilarity between acoustic property of the reference and sample.

Technical challenges in generalizing calibration techniques for breast density measurements.

PurposeWe are developing a calibration methodology for full‐field digital mammography (FFDM). Calibration compensates for image acquisition technique influences on the pixel representation, ideally producing improved inter‐image breast density estimates. This approach relies on establishing references with rigid breast tissue‐equivalent phantoms (BTEs) and requires an accurate estimate of the compressed breast thickness because the system readout is nominal. There is also an attenuation mismatch between adipose breast tissue and the adipose BTE that was noted in our previous work. It is referred to as the “attenuation anomaly” and addressed in this report. The objectives are to evaluate methods to correct for the compressed breast thickness and compensate for the attenuation anomaly.MethodsThickness correction surfaces were established with a deformable phantom (DP) using both image and physical measurements for three direct x‐ray conversion FFDM units. The Cumulative Sum serial quality control procedure was established to ensure the thickness correction measurements were stable over time by imaging and calibrating DPs biweekly in lieu of physical measurements. The attenuation anomaly was addressed by evaluating adipose image regions coupled with an optimization technique to adjust the adipose calibration data. We compared calibration consistency across matched left and right cranial caudal (CC) mammographic views (n = 199) with and without corrections using Bland–Altman plots. These plots were complemented by comparing the right and left breast calibrated average (μa) and population distribution mean (ma) with 95% confidence intervals and difference distribution variances with the F‐test for uncorrected and corrected data.ResultsThickness correction surfaces were well approximated as tilted planes and were dependent upon compression force. A correction was developed for the attenuation anomaly. All paddles (large and small paddles for all units) exhibited similar tilt as a function of force. Without correction, ma = 0.92 (−1.77, 3.62) was not significantly different from zero with many negative μa samples. The thickness correction produced a significant shift in the μa distribution in the positive direction with ma = 13.99 (11.17, 16.80) and reduced the difference distribution variance significantly (P < 0.0001). Applying both corrections in tandem gave ma = 22.83 (20.32, 25.34), representing another significant positive shift in comparison with the thickness correction in isolation. Thickness corrections were stable over approximately a 2‐year timeframe for all units.ConclusionThese correction techniques are valid approaches for addressing technical problems with calibration that relies on reference phantoms. The efficacy of the calibration methodology will require validation with clinical endpoints in future studies.

Patient-Specific Phantomless Estimation of Bone Mineral Density and Its Effects on Finite Element Analysis Results: A Feasibility Study.

Objectives This study proposes a regression model for the phantomless Hounsfield units (HU) to bone mineral density (BMD) conversion including patient physical factors and analyzes the accuracy of the estimated BMD values. Methods The HU values, BMDs, circumferences of the body, and cross-sectional areas of bone were measured from 39 quantitative computed tomography images of L2 vertebrae and hips. Then, the phantomless HU-to-BMD conversion was derived using a multiple linear regression model. For the statistical analysis, the correlation between the estimated BMD values and the reference BMD values was evaluated using Pearson's correlation test. Voxelwise BMD and finite element analysis (FEA) results were analyzed in terms of root-mean-square error (RMSE) and strain energy density, respectively. Results The HU values and circumferences were statistically significant (p < 0.05) for the lumbar spine, whereas only the HU values were statistically significant (p < 0.05) for the proximal femur. The BMD values estimated using the proposed HU-to-BMD conversion were significantly correlated with those measured using the reference phantom: Pearson's correlation coefficients of 0.998 and 0.984 for the lumbar spine and proximal femur, respectively. The RMSEs of the estimated BMD values for the lumbar spine and hip were 4.26 ± 0.60 (mg/cc) and 8.35 ± 0.57 (mg/cc), respectively. The errors of total strain energy were 1.06% and 0.91%, respectively. Conclusions The proposed phantomless HU-to-BMD conversion demonstrates the potential of precisely estimating BMD values from CT images without the reference phantom and being utilized as a viable tool for FEA-based quantitative assessment using routine CT images.

Simulation and analysis of CT examination doses to typical patients based on Chinese reference human phantoms

Objective To simulate and analyze the CT examination doses to typical patients based on Chinese reference human phantom, and to develop an online software for assessing the organ absorbed dose and effective dose to the examinees undergoing CT examination in order to provide a tool for rapid assessment of radiation dose from CT examination. Methods Based on the Monte Carlo simulation, a GE LightSpeed 16 CT scanner was modeled. The organ doses to Chinese adult male, female and 1 year-old child male from single axial scan were calculated based on the Chinese reference phantoms and an organ dose database was established. A web-based CT dose assessment software was developed to read the data from the organ dose database according to the CT scanning parameters entered by the user and to calculate the organ absorbed dose and effective dose rapidly. Results A new CT dose assessment software was developed, which can be used to assess organ doses to Chinese adult male, female and 1-year-old male for different CT scanner, voltage, current, collimator width and other CT parameters. The organ doses calculated in this work were compared with simulation data in two relevant studies. Dose differences for the organs completely included in the scan range were small, with relative differences less than 15% and 25%, respectively. Conclusions The CT dose assessment software provides a powerful tool for the simple and rapid assessment of radiation dose to the examinees undergoing CT examinations. Key words: CT; Monte Carlo; Radiation dose; Human phantom

Dose coefficients of mesh-type ICRP reference computational phantoms for idealized external exposures of photons and electrons

In the present study, we established a comprehensive dataset of dose coefficients (DCs) of the new mesh-type ICRP reference computational phantoms (MRCPs) for idealized external exposures of photons and electrons with the Geant4 code. Subsequently, the DCs for the nine organs/tissues, calculated for their thin radiosensitive target regions, were compared with the values calculated by averaging the absorbed doses over the entire organ/tissue regions to observe the influence of the thin sensitive regions on dose calculations. The result showed that the influences for both photons and electrons were generally insignificant for the majority of organs/tissues, but very large for the skin and eye lens, especially for electrons. Furthermore, the large influence for the skin eventually affected the effective dose calculations for electrons. The DCs of the MRCPs also were compared with the current ICRP-116 values produced with the current ICRP-110 reference phantoms. The result showed that the DCs for the majority of organs/tissues and effective dose were generally similar to the ICRP-116 values for photons, except for very low energies; however, for electrons, significant differences from the ICRP-116 values were found in the DCs, particularly for superficial organs/tissues and skeletal tissues, and also for effective dose.

238. Accuracy and precision of a novel algorithm for T1 and extracellular volume fraction mapping in cardiac magnetic resonance imaging

Purpose Extracellular Volume Fraction (ECV) can diagnose cardiac disorders, and is obtainable using T1 mapping. The sensitivity for ECV quantification is limited by the uncertainty affecting T1 measurements. A new methodology for T1 and ECV mapping was here tested, estimating also its level of uncertainty. Methods A reference phantom containing test objects with known T1 values was used, and 60 subjects (40 diseased and 20 healthy) were examined pre- and post-administration of the contrast agent, using a 1.5 T MR tomograph and MOLLI sequences,. A novel algorithm based on a Fast Non-linear Least Squares (FNLS) [1] method was implemented for modelling MR signals, and compared with other well-established methodologies. Gamma-tool analysis, with tolerances criteria of 3% for T1 difference and 4.21 mm for distance-to-agreement was also used to evaluate differences between measured and reference T1 maps. Results Bland-Altman plots and gamma tool analysis evidenced the good agreement among methods. Differences of mean T1 measured by test objects and FNLS algorithm (with respect to reference values) were in the range −3%/2%. The average and maximal standard deviation were 2.3% and 7.4% for the reproducibility with time, and 1.5% and 2.4%for the T1 spatial reproducibility. The ECV error (in terms of error propagation) ranged between 1.0–3.5%. The inter-patient variability range of the ECV value was 23.4%–29.8% for the healthy controls, and 31.0%–53.9% for the diseased patients, showing significant differences between the two groups (p = 0.012). Conclusion The FNLS algorithm is suitable to achieve accurate T1 and ECV mapping. The values of ECV referred to healthy controls were in very good agreement with previously published results. ECV uncertainty, in terms of propagation of T1 errors, were limited and comparable with the healthy control inter-patient variability, confirming the accurate ECV quantification.

Proton radiography with a commercial range telescope detector using dedicated post processing methods

Proton transmission imaging uses protons with high enough energy to fully traverse the phantom/patient and to be captured in a suitable detector placed behind it. The measured residual energy or residual range provide a direct estimate of the water equivalent thickness (WET) of the image volume. Requirements for proton imaging to be exploitable in clinical practice include: sufficient WET accuracy and integrability into the treatment room and the clinical workflow, as well as an acceptably low dose to the patient and a sufficient spatial resolution. In this work, we report on experiments performed at the Institut Curie—Proton therapy center in Orsay (IC-CPO), France, using a commercial range telescope commonly employed for quality assurance measurements. The purpose was to keep the experimental set-up as simple as possible and to achieve nonetheless high WET accuracy radiographies by developing and applying dedicated post processing methods. We explain these methods in detail and discuss their performance. We assess the WET accuracy based on two different reference phantoms: a CIRS electron density phantom with tissue equivalent inserts and a homogeneous step phantom. We find an agreement between the measured and the reference WET values of 0.2–0.5 mm. The lowest investigated dose was 10 mGy per acquisition. It could be lowered by modifying the irradiation plan and lowering the beam current, though the latter would impose slight optimisations of the detector hardware. Our work suggests that proton radiographies with good WET accuracy can be obtained with a reasonable experimental effort that would facilitate integration into clinical routine.

In-vitro validation of 4D flow MRI measurements with an experimental pulsatile flow model.

The purpose of this study was to assess the precision of four-dimensional (4D) phase-contrast magnetic resonance imaging (PCMRI) to measure mean flow and peak velocity (Vmax) in a pulsatile flow phantom and to test its sensitivity to spatial resolution and Venc. The pulsatile flow phantom consisted of a straight tube connected to the systemic circulation of an experimental mock circulatory system. Four-dimensional-PCMR images were acquired using different spatial resolutions (minimum pixel size: 1.5×1.5×1.5mm3) and velocity encoding sensitivities (up to three times Vmax). Mean flow and Vmax calculated from 4D-PCMRI were compared respectively to the reference phantom flow parameters and to Vmax obtained from two-dimensional (2D)-PCMRI. 4D-PCI measured mean flow with a precision of -0.04% to+5.46%, but slightly underestimated Vmax when compared to 2D-PCMRI (differences ranging from -1.71% to -3.85%). 4D PCMRI mean flow measurement was influenced by spatial resolution (P<0.001) with better results obtained with smaller voxel size. There was no effect of Venc on mean flow measurement. Regarding Vmax, neither spatial resolution nor Venc did influence the precision of the measurement. Using an experimental pulsatile flow model 4D-PCMRI is accurate to measure mean flow and Vmax with better results obtained with higher spatial resolution. We also show that Venc up to 3 times higher than Vmax may be used with no effect on these measurements.

Development of a PMMA phantom as a practical alternative for quality control of gamma knife\xae dosimetry

BackgroundTo measure the absorbed dose rate to water and penumbra of a Gamma Knife® (GK) using a polymethyl metacrylate (PMMA) phantom.MethodsA multi-purpose PMMA phantom was developed to measure the absorbed dose rate to water and the dose distribution of a GK. The phantom consists of a hemispherical outer phantom, one exchangeable cylindrical chamber-hosting inner phantom, and two film-hosting inner phantoms. The radius of the phantom was determined considering the electron density of the PMMA such that it corresponds to 8 g/cm2 water depth, which is the reference depth of the absorbed dose measurement of GK. The absorbed dose rate to water was measured with a PTW TN31010 chamber, and the dose distributions were measured with radiochromic films at the calibration center of a patient positioning system of a GK Perfexion. A spherical water-filled phantom with the same water equivalent depth was constructed as a reference phantom. The dose rate to water and dose distributions at the center of a circular field delimited by a 16-mm collimator were measured with the PMMA phantom at six GK Perfexion sites.ResultsThe radius of the PMMA phantom was determined to be 6.93 cm, corresponding to equivalent water depth of 8 g/cm2. The absorbed dose rate to water was measured with the PMMA phantom, the spherical water-filled phantom and a commercial solid water phantom. The measured dose rate with the PMMA phantom was 1.2% and 1.8% higher than those measured with the spherical water-filled phantom and the solid water phantom, respectively. These differences can be explained by the scattered photon contribution of PMMA off incoming 60Co gamma-rays to the dose rate. The average full width half maximum and penumbra values measured with the PMMA phantom showed reasonable agreement with two calculated values, one at the center of the PMMA phantom (LGP6.93) and other at the center of a water sphere with a radius of 8 cm (LGP8.0) given by Leksell Gamma Plan using the TMR10 algorithm.ConclusionsA PMMA phantom constructed in this study to measure the absorbed dose rates to water and dose distributions of a GK represents an acceptable and practical alternative for GK dosimetry considering its cost-effectiveness and ease of handling.

Power Spectrum Consistency among Systems and Transducers

Use of the reference phantom method for computing acoustic attenuation and backscatter is widespread. However, clinical application of these methods has been limited by the need to acquire reference phantom data. We determined that the data acquired from 11 transducers of the same model and five clinical ultrasound systems of the same model produce equivalent estimates of reference phantom power spectra. We describe that the contribution to power spectral density variance among systems and transducers equals that from speckle variance with 59 uncorrelated echo signals. Thus, when the number of uncorrelated lines of data is small, speckle variance will dominate the power spectral density estimate variance introduced by different systems and transducers. These results suggest that, at least for this particular transducer and imaging system combination, one set of reference phantom calibration data is highly representative of the average among equivalent transducers and systems that are in good working order.

ORGAN AND EFFECTIVE DOSE COEFFICIENTS FOR CRANIAL AND CAUDAL IRRADIATION GEOMETRIES: NEUTRONS.

With the introduction of new recommendations of the International Commission on Radiological Protection (ICRP) in Publication 103, the methodology for determining the protection quantity, effective dose, has been modified. The modifications include changes to the defined organs and tissues, the associated tissue weighting factors, radiation weighting factors and the introduction of reference sex-specific computational phantoms. Computations of equivalent doses in organs and tissues are now performed in both the male and female phantoms and the sex-averaged values used to determine the effective dose. Dose coefficients based on the ICRP 103 recommendations were reported in ICRP Publication 116, the revision of ICRP Publication 74 and ICRU Publication 57. The coefficients were determined for the following irradiation geometries: anterior-posterior (AP), posterior-anterior (PA), right and left lateral (RLAT and LLAT), rotational (ROT) and isotropic (ISO). In this work, the methodology of ICRP Publication 116 was used to compute dose coefficients for photon irradiation of the body with parallel beams directed upward from below the feet (caudal) and directed downward from above the head (cranial). These geometries may be encountered in the workplace from personnel standing on contaminated surfaces or volumes and from overhead sources. Calculations of organ and tissue kerma and absorbed doses for caudal and cranial exposures to photons ranging in energy from 10 keV to 10 GeV have been performed using the MCNP6.1 radiation transport code and the adult reference phantoms of ICRP Publication 110. As with calculations reported in ICRP 116, the effects of charged-particle transport are evident when compared with values obtained by using the kerma approximation. At lower energies the effective dose per particle fluence for cranial and caudal exposures is less than AP orientations while above ∼30 MeV the cranial and caudal values are greater.

P135] Should there be more optimization of cone beam ct imaging used with image guided radiotherapy

Purpose Cone beam CT (CBCT) is used in improving delivery of image guided radiation therapy (IGRT). However, less attention is paid to optimization of imaging for individual patients than in diagnostic applications. Methods are required to evaluate patient doses resulting from imaging procedures in order to facilitate optimization. Methods A modified version of the CT dose index (CTDI) has been proposed by the International Electrotechnical Commission (CTDIIEC) based on measurements with longer CTDI chambers. A dose–width product can be calculated by multiplying the CTDIIEC by the width of the CBCT beam and this is proposed as a quantity that can be used for dose assessment and linked to effective dose for optimization. Monte Carlo simulations based on a Varian On Board Imaging (OBI) system have been used to derive conversion coefficients that enable organ doses and effective dose for ICRP reference phantoms to be determined from the CTDIIEC for different scan protocols. Results Coefficients to allow estimation of effective doses for reference phantoms from the dose-width product have been derived for head (0.0034 mSv(mGy cm)−1), thorax (0.0252 mSv(mGy cm)−1) abdomen, (0.0216 mSv(mGy cm)−1) and pelvis (0.0150 mSv(mGy cm)−1) protocols. These can be helpful in assessing overall optimization of the imaging process. However, organ doses are required for optimization of procedures for individual patients. Since standard exposure factors and field sizes are used frequently for CBCT, more organs are irradiated and dose levels are proportionately higher for smaller patients, than they are in diagnostic procedures. Conclusions Data on effective doses from CBCT imaging can be used in making judgements on the contributions to patient dose from imaging, and thereby assist in optimization of IGRT regimes. More attention is required in selection of exposure factors and field sizes for CBCT used with IGRT.

Removing unwanted background phase with a reference phantom for applications in susceptibility quantification

PurposeA method of removing the background phase with a reference phantom but without overcorrecting the induced phase from objects of interest is proposed. Several factors during the imaging procedure and post-processing are investigated for their accuracies. MethodsA method using a reference phantom to remove eddy currents as well as using the least squares fit to quantify susceptibility and to remove the background phase is proposed. Phase induced from simulated spheroids was fitted and compared to their true magnetic moments, an important concept for the proposed method. A cylindrical phantom and its simulation, a phantom with straws filled with Gd-DTPA, and a simulated head model were used to study systematic errors due to some confounding factors. The feasibility for in vivo applications was demonstrated from an actual human head. Susceptibility and remaining phase after removing the background phase were measured in all cases. ResultsSimulations show that magnetic moments of various spheroids and phantoms can be accurately quantified from images, regardless of the partial volume effect. All measured susceptibility values are within ±0.16 ppm of −9.4 ppm for agarose and 0.05 ppm of 1 ppm for Gd-DTPA. Most residual phase is within ±0.1 rad from the phantom center. Susceptibilities close to −9.4 ppm are also obtained for the simulated and actual head. Correspondingly, the remaining phase has a mean value less than two standard deviations. ConclusionThe proposed method from phantom studies can reliably remove the background phase without overcorrections. The in vivo example demonstrates the feasibility of the method.

Patient-specific phantoms versus reference phantoms: a preliminary comparison on organs dose distribution

Patient-specific phantoms versus reference phantoms: a preliminary comparison on organs dose distribution

Design and application of an MR reference phantom for multicentre lung imaging trials.

IntroductionAs there is an increasing number of multicentre lung imaging studies with MRI in patients, dedicated reference phantoms are required to allow for the assessment and comparison of image quality in multi-vendor and multi-centre environments. However, appropriate phantoms for this purpose are so far not available commercially. It was therefore the purpose of this project to design and apply a cost-effective and simple to use reference phantom which addresses the specific requirements for imaging the lungs with MRI.MethodsThe phantom was designed to simulate 4 compartments (lung, blood, muscle and fat) which reflect the specific conditions in proton-MRI of the chest. Multiple phantom instances were produced and measured at 15 sites using a contemporary proton-MRI protocol designed for an in vivo COPD study at intervals over the course of the study. Measures of signal- and contrast-to-noise ratio, as well as structure and edge depiction were extracted from conventionally acquired images using software written for this purpose.ResultsFor the signal to noise ratio, low intra-scanner variability was found with 4.5% in the lung compartment, 4.0% for blood, 3.3% for muscle and 3.7% for fat. The inter-scanner variability was substantially higher, with 41%, 32%, 27% and 32% for the same order of compartments. In addition, measures of structure and edge depiction were found to both vary significantly among several scanner types and among scanners of the same model which were equipped with different gradient systems.ConclusionThe described reference phantom reproducibly quantified image quality aspects and detected substantial inter-scanner variability in a typical pulmonary multicentre proton MRI study, while variability was greater in lung tissue compared to other tissue types. Accordingly, appropriate reference phantoms can help to detect bias in multicentre in vivo study results and could also be used to harmonize equipment or data.