Tungsten carbide and LMPA electron cutouts: comparison and validation using Monte Carlo modelling and measurement of dose.

Electron applicator cutouts for radiation therapy electron beam shaping are typically cast from Low Melting Point Alloys (LMPA), such as cerrobend. In this work, we describe the Monte Carlo modelling of novel 3D printed cutouts based on tungsten carbide powder and the resulting dose profiles subject to Elekta Agility electron beams. Cerrobend cutouts were also modelled using the Monte Carlo code EGSnrc. Cerrobend and tungsten carbide cutouts were found to have the same dose profiles within model variance. Computed profiles and percentage depth dose (PDDs) curves of the Elekta Agility accelerator model using standard cutouts in water were found to agree with water tank measurements using gamma criteria of 2%/2mm. The Monte Carlo computed dose profiles of the tungsten carbide cutouts in a polystyrene phantom were also found to agree with liquid-filled ionization chamber array measurements using gamma criteria of 2%/2mm. We conclude that the tungsten carbide cutouts are clinically equivalent to LMPA cutouts.

Transducer impulse response correction for a deconvolution derived ultrasound transit time spectrum.

Propagation of ultrasound through a complex composite sample may exhibit phase interference between two or more sonic-rays if differences in transit time are less than the pulse length. The transit time spectrum of a test sample, equivalent to its impulse response, was derived through active-set deconvolution of ultrasound signals with, and without, the test sample. The aim of this study was to test the hypothesis that in cases where only the transmit ultrasound transducer's digitally-coded excitation signal is available, hence not the input ultrasound signal without the test sample, incorporation of the transducer impulse response may increase both accuracy and precision of ultrasound transit time spectroscopy. A digital 1 MHz sinusoid signal was used to create an ultrasound pulse that was propagated through a 5 step-wedge acrylic sample immersed in water. Transit time spectra were obtained through deconvolution utilising an ultrasound input signal, along with a digital input signal, with and without incorporation of the transducer impulse response. Incorporation of the transducer impulse response reduced a quantitative measure of noise-to-signal ratio by a factor of 12. The paper has demonstrated the potential for increased accuracy and precision of transit time spectroscopy when the transducer impulse response is incorporated within active-set deconvolution analysis.

A 3D-printed passive ultrasound phase-interference compensator for reduced wave degradation in cancellous bone - an experimental study in replica models.

The current 'active' solution to overcome the impediment of ultrasound wave degradation associated with transit-time variation in complex tissue structures, such as the skull, is to vary the transmission delay of ultrasound pulses from individual transducer elements. This article considers a novel 'passive' solution in which constant transit time is achieved by propagating through an additional material layer positioned between the ultrasound transducer and the test sample. To test the concept, replica models based on four cancellous bone natural tissue samples and their corresponding passive ultrasound phase-interference compensator were 3D-printed. Normalised broadband ultrasound attenuation was used as a quantitative measure of wave degradation, performed in transmission mode at a frequency of 1 MHz and yielding a reduction ranging from 57% to 74% when the ultrasound phase-interference compensator was incorporated. It is suggested that the passive compensator offers a broad utility and, hence, it may be applied to any ultrasound transducer, of any complexity (single element or array), frequency and dimension.

The dependence of broadband ultrasound attenuation on phase interference in thin plates of variable thickness and curvature: a comparison of experimental measurement and computer simulation.

The measurement of broadband ultrasound attenuation describes the linear increase in ultrasound attenuation with frequency (dB/MHz); this is generally performed at the calcaneus, consisting of a high proportion of metabolically active cancellous bone. Although broadband ultrasound attenuation is not routinely implemented within clinical management since it cannot provide a reliable estimation of bone mineral density and hence clinical definition of osteopenia and osteoporosis, it offers a reliable means to predict osteoporotic fracture risk. One of the potential factors that can influence the accuracy of broadband ultrasound attenuation measurement is the effect of cortical end plates. This study aimed to explore this, performing a comparison of experimental study and computer simulation prediction. A total of three categories of thin discs were three-dimensional (3D) printed to replicate cortical shells of (1) variable constant thickness (planar), (2) variable constant thickness (curved), and (3) variable thickness. A through-transmission technique was used, where two single-element, unfocused, 1 MHz broadband transducers, as utilised clinically, were positioned coaxially in a cylindrical holder and immersed in water. Both quantitative and qualitative analyses demonstrated that broadband ultrasound attenuation measurements of the 'planar' and 'curved' discs were not statistically different (p-values > 0.01). A cyclic relationship between broadband ultrasound attenuation and disc thickness was observed; this was replicated within a computer simulation of phase interference created by a double-reflection echo within each disc (R2 = 97.0%). Variable-thickness discs provided broadband ultrasound attenuation measurements ranging between 31.6 ± 0.1 and 40.60 ± 0.1 dB/MHz. Again applying the double-reflection echo simulation, a high level of agreement between experimental and simulation was recorded (R2 = 93.4%). This study indicates that the cortical end plate can significantly affect the broadband ultrasound attenuation measurement of cancellous bone as a result of phase interference and, therefore, warrants further investigation to minimise its effect on clinical assessment.

Bone volume fraction and structural parameters for estimation of mechanical stiffness and failure load of human cancellous bone samples; in-vitro comparison of ultrasound transit time spectroscopy and X-ray µCT.

Conventional mechanical testing is the 'gold standard' for assessing the stiffness (N mm-1) and strength (MPa) of bone, although it is not applicable in-vivo since it is inherently invasive and destructive. The mechanical integrity of a bone is determined by its quantity and quality; being related primarily to bone density and structure respectively. Several non-destructive, non-invasive, in-vivo techniques have been developed and clinically implemented to estimate bone density, both areal (dual-energy X-ray absorptiometry (DXA)) and volumetric (quantitative computed tomography (QCT)). Quantitative ultrasound (QUS) parameters of velocity and attenuation are dependent upon both bone quantity and bone quality, although it has not been possible to date to transpose one particular QUS parameter into separate estimates of quantity and quality. It has recently been shown that ultrasound transit time spectroscopy (UTTS) may provide an accurate estimate of bone density and hence quantity. We hypothesised that UTTS also has the potential to provide an estimate of bone structure and hence quality. In this in-vitro study, 16 human femoral bone samples were tested utilising three techniques; UTTS, micro computed tomography (µCT), and mechanical testing. UTTS was utilised to estimate bone volume fraction (BV/TV) and two novel structural parameters, inter-quartile range of the derived transit time (UTTS-IQR) and the transit time of maximum proportion of sonic-rays (TTMP). µCT was utilised to derive BV/TV along with several bone structure parameters. A destructive mechanical test was utilised to measure the stiffness and strength (failure load) of the bone samples. BV/TV was calculated from the derived transit time spectrum (TTS); the correlation coefficient (R2) with µCT-BV/TV was 0.885. For predicting mechanical stiffness and strength, BV/TV derived by both µCT and UTTS provided the strongest correlation with mechanical stiffness (R2=0.567 and 0.618 respectively) and mechanical strength (R2=0.747 and 0.736 respectively). When respective structural parameters were incorporated to BV/TV, multiple regression analysis indicated that none of the µCT histomorphometric parameters could improve the prediction of mechanical stiffness and strength, while for UTTS, adding TTMP to BV/TV increased the prediction of mechanical stiffness to R2=0.711 and strength to R2=0.827. It is therefore envisaged that UTTS may have the ability to estimate BV/TV along with providing an improved prediction of osteoporotic fracture risk, within routine clinical practice in the future.

Lower material stiffness in rupture-repaired Achilles tendon during walking: transmission-mode ultrasound for post-surgical tendon evaluation.

PURPOSE: This cross-sectional study used transmission-mode ultrasound to evaluate dynamic tendon properties during walking in surgically repaired and contralateral Achilles tendon (AT), with a median (range) post-operative period of 22 (4-58) months. It was hypothesised that the axial transmission speed of ultrasound (TSOU) during walking would be slower, indicating lower material stiffness in repaired compared with contralateral AT. METHODS: Ten patients [median (range) age 47 (37-69) years; height 180 (170-189) cm; weight 93 (62-119) kg], who had undergone open surgical repair of the AT and were clinically recovered according to their treating clinicians, walked barefoot on a treadmill at self-selected speed (1.0 ± 0.2 m/s). Synchronous measures of TSOU, sagittal ankle motion, vertical ground reaction force (GRF), and spatiotemporal gait parameters were recorded during 20 s of steady-state walking. Paired t tests were used to evaluate potential between-limb differences in TSOU, GRF, ankle motion, and spatiotemporal gait parameters. RESULTS: TSOU was significantly lower (≈175 m/s) in the repaired than in the contralateral AT over the entire gait cycle (P < 0.05). Sagittal ankle motion was significantly greater (≈3°) in the repaired than in the contralateral limb (P = 0.036). There were no significant differences in GRF or spatiotemporal parameters between limbs. CONCLUSIONS: Repaired AT was characterised by a lower TSOU, reflecting a lower material stiffness in the repaired tendon than in the contralateral tendon. A lower material stiffness may underpin greater ankle joint motion of the repaired limb during walking and long-term deficits in the muscle-tendon unit reported with AT repair. Treatment and rehabilitation approaches that focus on increasing the material stiffness of the repaired AT may be clinically beneficial. Transmission-mode ultrasound would seem useful for quantifying tendon properties post AT rupture repair and may have the potential to individually guide rehabilitation programmes, thereby aiding safer return to physical activity. LEVEL OF EVIDENCE: II.

Ultrasound temporal-spatial phase-interference in complex composite media; a comparison of experimental measurement and simulation prediction.

The propagation of ultrasound through solid:liquid complex composite media such as cancellous bone suffers from a lack of a comprehensive understanding of the dependence upon density and structure. Assuming that a propagating ultrasound wave may be considered as an array of parallel sonic rays, we may determine the transit time of each by the relative proportion of the two constituents. A transit time spectrum (TTS) describes the proportion of sonic rays having a particular transit time between the minimum (tmin) and maximum (tmax) values; representing, for example, entire bone tissue and marrow respectively in the case of cancellous bone. Langton has proposed that the primary ultrasound attenuation mechanism in such media is phase-interference. The phase-interference of two or more ultrasound pulses detected at a phase-sensitive transducer has both temporal and spatial components. The temporal component is primarily dependent upon the transit time difference (dt) between the pulses and the propagating pulse-length (PL). The spatial component is primarily dependent upon the lateral separation (ds) of the detectedpulses of differing transit time and the lateral dimension of the ultrasound receive transducer aperture (dL). The aim of the paper was to explore these temporal and spatial dependencies through a comparison of experimental measurement and computer simulation in solid:liquid models of varying temporal and spatial complexity. Transmission measurements at nominal ultrasound frequencies of 1MHz and 5MHz were performed, thereby investigating the dependency upon period. The results demonstrated an overall agreement between experimental measurement and computer simulation of 87±16% and 85±12% for temporal and spatial components respectively. It is envisaged that a comprehensive understanding of ultrasound propagation through complex structures such as cancellous bone could provide an improved non-invasive tool for osteoporosis assessment.

Pulse-echo ultrasound transit time spectroscopy: A comparison of experimental measurement and simulation prediction.

Considering ultrasound propagation through complex composite media as an array of parallel sonic rays, a comparison of computer-simulated prediction with experimental data has previously been reported for transmission mode (where one transducer serves as transmitter, the other as receiver) in a series of 10 acrylic step-wedge samples, immersed in water, exhibiting varying degrees of transit time inhomogeneity. In this study, the same samples were used but in pulse-echo mode, where the same ultrasound transducer served as both transmitter and receiver, detecting both 'primary' (internal sample interface) and 'secondary' (external sample interface) echoes. A transit time spectrum was derived, describing the proportion of sonic rays with a particular transit time. A computer simulation was performed to predict the transit time and amplitude of various echoes created, and compared with experimental data. Applying an amplitude-tolerance analysis, 91.7% ± 3.7% of the simulated data were within ±1 standard deviation of the experimentally measured amplitude-time data. Correlation of predicted and experimental transit time spectra provided coefficients of determination (R(2)%) ranging from 100.0% to 96.8% for the various samples tested. The results acquired from this study provide good evidence for the concept of parallel sonic rays. Furthermore, deconvolution of experimental input and output signals has been shown to provide an effective method to identify echoes otherwise lost due to phase cancellation. Potential applications of pulse-echo ultrasound transit time spectroscopy include improvement of ultrasound image fidelity by improving spatial resolution and reducing phase interference artefacts.

Solid volume fraction estimation of bone:marrow replica models using ultrasound transit time spectroscopy.

The acceptance of broadband ultrasound attenuation (BUA) for the assessment of osteoporosis suffers from a limited understanding of both ultrasound wave propagation through cancellous bone and its exact dependence upon the material and structural properties. It has recently been proposed that ultrasound wave propagation in cancellous bone may be described by a concept of parallel sonic rays; the transit time of each ray defined by the proportion of bone and marrow propagated. A Transit Time Spectrum (TTS) describes the proportion of sonic rays having a particular transit time, effectively describing the lateral inhomogeneity of transit times over the surface aperture of the receive ultrasound transducer. The aim of this study was to test the hypothesis that the solid volume fraction (SVF) of simplified bone:marrow replica models may be reliably estimated from the corresponding ultrasound transit time spectrum. Transit time spectra were derived via digital deconvolution of the experimentally measured input and output ultrasonic signals, and compared to predicted TTS based on the parallel sonic ray concept, demonstrating agreement in both position and amplitude of spectral peaks. Solid volume fraction was calculated from the TTS; agreement between true (geometric calculation) with predicted (computer simulation) and experimentally-derived values were R(2)=99.9% and R(2)=97.3% respectively. It is therefore envisaged that ultrasound transit time spectroscopy (UTTS) offers the potential to reliably estimate bone mineral density and hence the established T-score parameter for clinical osteoporosis assessment.

A deconvolution method for deriving the transit time spectrum for ultrasound propagation through cancellous bone replica models.

The acceptance of broadband ultrasound attenuation for the assessment of osteoporosis suffers from a limited understanding of ultrasound wave propagation through cancellous bone. It has recently been proposed that the ultrasound wave propagation can be described by a concept of parallel sonic rays. This concept approximates the detected transmission signal to be the superposition of all sonic rays that travel directly from transmitting to receiving transducer. The transit time of each ray is defined by the proportion of bone and marrow propagated. An ultrasound transit time spectrum describes the proportion of sonic rays having a particular transit time, effectively describing lateral inhomogeneity of transit times over the surface of the receiving ultrasound transducer. The aim of this study was to provide a proof of concept that a transit time spectrum may be derived from digital deconvolution of input and output ultrasound signals. We have applied the active-set method deconvolution algorithm to determine the ultrasound transit time spectra in the three orthogonal directions of four cancellous bone replica samples and have compared experimental data with the prediction from the computer simulation. The agreement between experimental and predicted ultrasound transit time spectrum analyses derived from Bland-Altman analysis ranged from 92% to 99%, thereby supporting the concept of parallel sonic rays for ultrasound propagation in cancellous bone. In addition to further validation of the parallel sonic ray concept, this technique offers the opportunity to consider quantitative characterisation of the material and structural properties of cancellous bone, not previously available utilising ultrasound.

Experimental and computer simulation validation of ultrasound phase interference created by lateral inhomogeneity of transit time in replica bone: marrow composite models.

The measurement of broadband ultrasound attenuation in cancellous bone for the assessment of osteoporosis follows a parabolic-type dependence with bone volume fraction, having minima values corresponding to both entire bone and entire marrow. Langton has recently proposed that the primary attenuation mechanism is phase interference due to variations in propagation transit time through the test sample as detected over the phase-sensitive surface of the receive ultrasound transducer. This fundamentally simple concept assumes that the propagation may be considered as an array of parallel 'sonic rays'. The transit time of each ray is defined by the proportion of bone and marrow propagated, being a minimum (t(min)) solely through bone and a maximum (t(max)) solely through marrow, from which a transit time spectrum, may be defined describing the proportion of sonic rays having a particular transit time. The aim of this study was to test the hypothesis that there is a dependence of phase interference upon the lateral inhomogeneity of transit time by comparing experimental measurements and computer simulation predictions of ultrasound propagation through a range of relatively simplistic solid:liquid models. From qualitative and quantitative comparison of the experimental and computer simulation results, there is an extremely high degree of agreement of 94.2%-99.0% between the two approaches. This combined experimental and computer simulation study has successfully demonstrated that lateral inhomogeneity of transit time has significant potential for phase interference to occur if a phase-sensitive receive ultrasound transducer is implemented as in most commercial ultrasound bone analysis devices.

Retrospective evaluation of dosimetric quality for prostate carcinomas treated with 3D conformal, intensity modulated and volumetric modulated arc radiotherapy.

INTRODUCTION: This study examines and compares the dosimetric quality of radiotherapy treatment plans for prostate carcinoma across a cohort of 163 patients treated across five centres: 83 treated with three-dimensional conformal radiotherapy (3DCRT), 33 treated with intensity modulated radiotherapy (IMRT) and 47 treated with volumetric modulated arc therapy (VMAT). METHODS: Treatment plan quality was evaluated in terms of target dose homogeneity and organs at risk (OAR), through the use of a set of dose metrics. These included the mean, maximum and minimum doses; the homogeneity and conformity indices for the target volumes; and a selection of dose coverage values that were relevant to each OAR. Statistical significance was evaluated using two-tailed Welch's T-tests. The Monte Carlo DICOM ToolKit software was adapted to permit the evaluation of dose metrics from DICOM data exported from a commercial radiotherapy treatment planning system. RESULTS: The 3DCRT treatment plans offered greater planning target volume dose homogeneity than the other two treatment modalities. The IMRT and VMAT plans offered greater dose reduction in the OAR: with increased compliance with recommended OAR dose constraints, compared to conventional 3DCRT treatments. When compared to each other, IMRT and VMAT did not provide significantly different treatment plan quality for like-sized tumour volumes. CONCLUSIONS: This study indicates that IMRT and VMAT have provided similar dosimetric quality, which is superior to the dosimetric quality achieved with 3DCRT.

Accelerated partial breast irradiation using external beam conformal radiation therapy: a review.

Lumpectomy followed by whole breast radiation therapy (i.e. breast conservation therapy (BCT)) is the standard of care for management of early stage breast cancer. However, its utilization has not been maximized because of a number of reasons including the logistic issues associated with the 5-6 weeks of radiation treatment. Also, pathological and clinical data suggest that most ipsilateral breast cancer recurrences are in the vicinity of the lumpectomy. Accelerated partial breast irradiation (APBI) is an approach that treats only the lumpectomy bed plus a 1-2 cm margin, rather than the whole breast with higher doses of radiation in a shorter period of time. There has been growing interest for APBI and various approaches have been developed and are under phase I-III clinical studies. This paper reviews external beam conformal radiation therapy (EBCRT) as a possible technique to APBI. The various EBCRT approaches such as 3D conformal radiation therapy, IMRT, proton therapy, tomotherapy, and volumetric arc therapy are discussed. Issues with the implementation of these techniques such as target volume delineation and organ motion are also presented. It is evident that EBCRT has potential for APBI of a selected group of early breast cancer patient. However, issues with setup errors and breathing motions need to be adequately addressed.

Accelerated Partial Breast Irradiation (APBI): A review of available techniques.

Breast conservation therapy (BCT) is the procedure of choice for the management of the early stage breast cancer. However, its utilization has not been maximized because of logistics issues associated with the protracted treatment involved with the radiation treatment. Accelerated Partial Breast Irradiation (APBI) is an approach that treats only the lumpectomy bed plus a 1-2 cm margin, rather than the whole breast. Hence because of the small volume of irradiation a higher dose can be delivered in a shorter period of time. There has been growing interest for APBI and various approaches have been developed under phase I-III clinical studies; these include multicatheter interstitial brachytherapy, balloon catheter brachytherapy, conformal external beam radiation therapy and intra-operative radiation therapy (IORT). Balloon-based brachytherapy approaches include Mammosite, Axxent electronic brachytherapy and Contura, Hybrid brachytherapy devices include SAVI and ClearPath. This paper reviews the different techniques, identifying the weaknesses and strength of each approach and proposes a direction for future research and development. It is evident that APBI will play a role in the management of a selected group of early breast cancer. However, the relative role of the different techniques is yet to be clearly identified.

Ultrasonic wave propagation in stereo-lithographical bone replicas.

Predictions of a modified anisotropic Biot-Allard theory are compared with measurements of pulses centered on 100 kHz and 1 MHz transmitted through water-saturated stereo-lithographical bone replicas. The replicas are 13 times larger than the original bone samples. Despite the expected effects of scattering, which is neglected in the theory, at 100 kHz the predicted and measured transmitted waveforms are similar. However, the magnitude of the leading negative edge of the waveform is overpredicted, and the trailing parts of the waveforms are not predicted well. At 1 MHz, although there are differences in amplitudes, the theory predicts that the transmitted waveform is almost a scaled version of that incident in conformity with the data.

Predictions of angle dependent tortuosity and elasticity effects on sound propagation in cancellous bone.

The anisotropic pore structure and elasticity of cancellous bone cause wave speeds and attenuation in cancellous bone to vary with angle. Previously published predictions of the variation in wave speed with angle are reviewed. Predictions that allow tortuosity to be angle dependent but assume isotropic elasticity compare well with available data on wave speeds at large angles but less well for small angles near the normal to the trabeculae. Claims for predictions that only include angle-dependence in elasticity are found to be misleading. Audio-frequency data obtained at audio-frequencies in air-filled bone replicas are used to derive an empirical expression for the angle-and porosity-dependence of tortuosity. Predictions that allow for either angle dependent tortuosity or angle dependent elasticity or both are compared with existing data for all angles and porosities.

Comparison of fat quantification methods: a phantom study at 3.0T.

PURPOSE: To compare different imaging methods with single-voxel MR spectroscopy (MRS) for the quantification of fat content in phantoms at 3.0T. MATERIALS AND METHODS: Imaging and spectroscopy was performed on a GE Signa system. Eleven novel homogeneous fat-water phantoms were constructed with variation in fat content from 0% to 100%. These were imaged using three techniques and compared with single-voxel non-water-suppressed MRS. Pixel-by-pixel maps of fat fraction were produced and mean values compared to MRS-determined measurements. Preliminary in vivo examinations were subsequently performed in the breast and spine to compare the best imaging technique with MRS. RESULTS: All imaging methods significantly correlated with MRS (P < 0.001): IDEAL (r(2) = 0.985), IOP (r(2) = 0.888), WS (r(2) = 0.939), and FS (r(2) = 0.938). In addition, IDEAL provided artifact-free maps of fat fraction with superior uniformity. In vivo results using IDEAL produced values that were between 9% to 16% of the corresponding MRS values. CONCLUSION: This work demonstrates that imaging may be utilized as a high-resolution alternative to MRS for the quantification of fat content. In the future we intend to replace MRS with IDEAL in our clinical studies involving fat measurement.

Measurements of tortuosity in stereolithographical bone replicas using audiofrequency pulses.

The tortuosity of five air-filled stereolithographical cancellous bone replicas has been obtained from measurements using audiofrequency pulses in a rectangular waveguide. The data obtained from the replicas yields information about anisotropy with respect to orthogonal axes of the passages that would be marrow filled in vivo. A strong relationship has been found between the acoustically measured tortuosity and the independently measured porosity. Use of stereolithographical bone replicas has the potential to simulate perforation and thinning of cancellous bone and hence evaluate the dependence of acoustic properties on cancellous bone microstructure. As an "extreme" illustration of such use, "inverses" of the original replicas have been manufactured and acoustic measurements have been made on them. The data reveal significantly greater tortuosity of the passages that are geometrically equivalent to the original solid bone structures.