Development and validation of a gel wax phantom to evaluate geometric accuracy and measurement of a hyperechoic target diameter in diagnostic ultrasound imaging.

Diagnostic ultrasound (US) scanners are generally evaluated using proprietary quality assurance (QA) phantoms, but their prohibitively high cost may prevent organizations to perform the necessary tests. This study aimed to develop a low-cost gel wax phantom with targets to determine the lateral and axial resolution and diameter of a hyperechoic target in an US scanner. The acoustic property (AP) of gel wax, which includes the speed of sound (cus), acoustic impedance (Z), and attenuation coefficient (µ), were determined for multiple transducers operating at 2.25, 5, 10, 15, and 30 MHz. These results were compared to the AP of soft tissue. Two polytetrafluoroethylene (PTFE) rectangular frames with holes separated by 5, 10, and 20 mm were constructed. Nylon filaments and stainless-steel disc (SS disc) (diameter = 16.8 mm) were threaded through the frames and suitably placed in gel wax to obtain orthogonal targets in the phantom. The target dimensions obtained from computerized tomography (CT) and US images of the phantom were compared for phantom validation. The average cus=1431.4 m/s, mass density ρ = 0.87 g/cm3, Z = 1.24 MRayls, and µ ranged from 0.7 to 0.98 dB/cm/MHz for gel wax at 22 °C. The US image measurement exhibited a maximum error in determining the diameter of the SS disc, resulting in a value of 18 mm instead of its actual value of 16.8 mm. The phantom volume decreased by 1.8% in 62 weeks. The present phantom is affordable, stable, customizable, and can be used to evaluate diagnostic US scanners across multiple centers.

Acoustic and ultrasonographic characterization of polychloroprene, beeswax, and carbomer-gel to mimic soft-tissue for diagnostic ultrasound.

Materials with acoustic properties similar to soft-tissue are essential as tissue-mimicking materials (TMMs) for diagnostic ultrasound (US). The velocity (cus), acoustic impedance (AI) and attenuation coefficient of US (µ) in a material collectively define its acoustic property. In this work, the acoustic properties of polychloroprene rubber, beeswax, and Carbomer-gel are determined. The pulse-echo technique is used to estimate cus and µ. The product of a sample density (ρ) and cus gives its AI. Using a reference based on the International Commission on Radiation Units and Measurements Report-61, Tissue Substitutes, Phantoms and Computational Modelling in Medical Ultrasound, the results are evaluated. The acceptance criteria are 1.043 ± 0.021 g/cm3 (ρ), 1561 ± 31.22 m/s (cus), 1.63 ± 0.065 MRayls (AI) and µ within 0.5-0.7 dB/cm/MHz. Sample computerized tomography (CT) and US scanning are performed to evaluate their similarities (contrast and speckle pattern) with respective images of the human liver (a clinical soft-tissue). The average errors in measuring cus and µ were 0.14% and 1.2% respectively. From the present findings, acoustic properties of polychloroprene and beeswax are unacceptable. However, the results of Carbomer-gel ρ = 1.03 g/cm3, cus = 1567 m/s, AI = 1.61 MRayls are satisfactory and µ = 0.73 dB/cm/MHz, is higher than the reference (4.3%). Carbomer-gel could produce CT and US images, efficiently mimicking the respective liver images. Carbomer-gel containing 95% water is a low-cost material with a simple formulation. Present results suggest, Carbomer- gel mimics soft-tissue and can be used as a TMM for diagnostic US.

Dosimetric comparison of iPlanⓇ Pencil Beam (PB) and Monte Carlo (MC) algorithms in stereotactic radiosurgery/radiotherapy (SRS/SRT) plans of intracranial arteriovenous malformations.

Stereotactic radiosurgery/radiotherapy (SRS/SRT) is a hypofractionated treatment where accurate dose calculation is of prime importance. The accuracy of the dose calculation depends on the treatment planning algorithm. This study is a retrospective dosimetric comparison of iPlanⓇ Monte Carlo (MC) and Pencil Beam (PB) algorithms in SRS/SRT plans of cranial arteriovenous malformations (AVMs). PB plans of 60 AVM patients who were already treated using 6 MV photons from a linear accelerator were selected and divided into 2 groups. Group-I consists of 30 patients who have undergone embolization procedure with high density OnyxⓇ prior to radiosurgery whereas Group-II had 30 patients who did not have embolization. These plans were recalculated with MC algorithm while keeping parameters like beam orientation, multileaf collimator (MLC) positions, MLC margin, prescription dose, and monitor units constant. Several treatment coverage parameters, isodose volumes, plan quality metrics, dose to organs at risk, and integral dose were used for comparing the 2 algorithms. The isodose distribution generated by the 2 algorithms was also compared with gamma analysis using 1%/1 mm criterion. The difference between the 2 groups as well as the differences in dose calculation by PB and MC algorithms were tested for significance using independent t-test and paired t-test respectively at 5% level of significance. The results of the independent t-test showed that there is no significant difference between the Group-I and Group-II patients for PB as well as MC algorithm due to the presence of high density embolization material. However, results of the paired t-test showed that the differences between the PB and MC algorithms were significant for several parameters analyzed in both groups of patients. The gamma analysis results also showed differences in the dose calculated by the 2 algorithms especially in the low dose regions. The significant differences between the 2 algorithms are probably due to the incorrect representation of the loss of lateral charged particle equilibrium and lateral broadening of small photon beams by PB algorithm. MC algorithms are generally considered not essential for dose calculations for target volumes located in the brain. This study demonstrates PB algorithm may not be sufficiently accurate to predict dose distributions for small fields where there is loss of LCPE. The lateral broadening due to the loss of LCPE as predicted by the MC algorithm could be the main reason for significant differences in the parameters compared. Hence, an accurate MC algorithm if available may prove valuable for intracranial SRS treatment planning of such benign lesions where the long life expectancy of patients makes accurate dosimetry critical.

Evaluation of Plan Quality Metrics in Stereotactic Radiosurgery/Radiotherapy in the Treatment Plans of Arteriovenous Malformations.

AIM: Several plan quality metrics are available for the evaluation of stereotactic radiosurgery/radiotherapy plans. This is a retrospective analysis of 60 clinical treatment plans of arteriovenous malformation (AVM) patients to study clinical usefulness of selected plan quality metrics. MATERIALS AND METHODS: The treatment coverage parameters Radiation Therapy Oncology Group (RTOG) Conformity Index (CIRTOG), RTOG Quality of Coverage (QRTOG), RTOG Homogeneity Index (HIRTOG), Lomax Conformity Index (CILomax), Paddick's Conformity Index (CIPaddick), and dose gradient parameters Paddick's Gradient Index (GIPaddick) and Equivalent Fall-off Distance (EFOD) were calculated for the cohort of patients. Before analyzing patient plans, the influence of calculation grid size on selected plan quality metrics was studied on spherical targets. RESULTS: It was found that the plan quality metrics are independent of calculation grid size ≤2 mm. EFOD was found to increase linearly with increase in target volume, and a linear fit equation was obtained. CONCLUSIONS: The analysis shows that RTOG indices and EFOD would suffice for routine clinical radiosurgical treatment plan evaluation if a dose distribution is available for visual inspection.