[Estimation of Uncertainty of the VMAT Absolute Dose Measurement Due to the Phantom Setup Error Using a Treatment Planning System].

PURPOSE: When performing single-point dose verification in VMAT, it is necessary to avoid the regions with steep dose gradient. We propose a method to obtain the estimated value ( Uplan) of uncertainty of the absolute dose measurement due to the phantom setup error by using dose gradient calculated from treatment planning system (TPS), for evaluating the appropriate measurement points. METHODS: The dose gradient was calculated from the planned dose values in the vicinity of the isocenter point using TPS. The phantom setup error was estimated. The Uplan was calculated using the proposed formula after estimating the phantom setup error. Then, the dose gradient was calculated from the measured dose values in the vicinity of the isocenter point specified by TPS using the Tough water phantom with ionization chamber (IC), and Umeas was calculated as in Uplan. RESULTS: The correlation coefficient between Uplan and Umeas was 0.984, which indicates a high correlation. The average of the difference between Umeas and Uplan was -0.24%. We considered that this result was caused by the influence of volume averaging effect of IC. CONCLUSION: The Uplan obtained from this proposed method reflects the uncertainty of the absolute dose measurement due to the phantom setup error and is useful for evaluating the appropriate measurement points for absolute dose measurement.

Improvement in visualization of carotid artery uniformity using silent magnetic resonance angiography.

Three-dimensional time-of-flight magnetic resonance angiography (TOF MRA) has been widely used in clinics. TOF MRA can cause dephasing artifacts, which lead to an intraluminal signal decrease. Silent MRA is a novel imaging technique that uses arterial spin labeling to achieve an ultrashort echo time (uTE), which is expected to decrease these effects and allow for accurate assessment of the flow in blood vessels. This study quantified the accuracy of Silent MRA images for visualizing the turbulent flow in flow-phantom and in vivo studies. The vessel contrast and coefficients of variation (CVs) for Silent MRA and TOF MRA were compared using normal and stenosis phantoms. Then, we performed both types of MRA on seven healthy volunteers. In the phantom study, although the contrast in the TOF MRA images was low distal to the stenosis region and at a high flow velocity, the contrast in the Silent MRA images did not change under these conditions. Furthermore, the mean CV for Silent MRA was smaller than that for TOF MRA under stenosis conditions. In the in vivo study, the mean contrast and vessel uniformity were significantly higher for Silent MRA than for TOF MRA. Although Silent MRA has limited spatial resolution and requires additional imaging time, this method may have the potential to improve the image quality of the carotid artery.