Stereotactic body radiation therapy(SBRT) is a potent treatment for early stage primary and limited metastatic disease. Accurate tumor localization is essential to administer SBRT safely and effectively. Tomotherapy combines helical IMRT with onboard megavoltage CT (MVCT) imaging and is well suited for SBRT; however, MVCT results in reduced soft tissue contrast and increased imagenoise compared with kilovoltage CT. The goal of this work was to investigate the use of increased imagingdoses on a clinical tomotherapy machine to improve image quality for SBRTimage guidance.Methods:
Two nonstandard, high-dose imaging modes were created on a tomotherapy machine by increasing the linear accelerator(LINAC) pulse rate from the nominal setting of 80 Hz, to 160 Hz and 300 Hz, respectively. Weighted CTdose indexes (wCTDIs) were measured for the standard, medium, and high-dose modes in a 30 cm solid water phantom using a calibrated A1SL ion chamber. Image quality was assessed from scans of a customized image quality phantom. Metrics evaluated include: contrast-to-noise ratios(CNRs), high-contrast spatial resolution, image uniformity, and percent imagenoise. In addition, two patients receiving SBRT were localized using high-dose MVCT scans. Raw detector data collected after each scan were used to reconstruct standard-dose images for comparison.Results:
MVCT scans acquired using a pitch of 1.0 resulted in wCTDI values of 2.2, 4.7, and 8.5 cGy for the standard, medium, and high-dose modes respectively. CNR values for both low and high-contrast materials were found to increase with the square root of dose. Axial high-contrast spatial resolution was comparable for all imaging modes at 0.5 lp/mm. Image uniformity was improved and percent noise decreased as the imagingdose increased. Similar improvements in image quality were observed in patient images, with decreases in imagenoise being the most notable.Conclusions:
High-dose imaging modes are made possible on a clinical tomotherapy machine by increasing the LINAC pulse rate. Increasing the imagingdose results in increased CNRs; making it easier to distinguish the boundaries of low contrast objects. The imagingdose levels observed in this work are considered acceptable at our institution for SBRT treatments delivered in 3–5 fractions.
This work was partially supported by a research agreement with Accuray, Inc.
II. METHODS AND MATERIALS
II.A. The TomoTherapy imagingsystem
II.B. High-dose imaging modes
II.C. Imagingdose measurements
II.D. Image quality characterization
II.E. Clinical image comparison
III.A. Dose measurements
III.B. Image quality
III.C. Clinical images
- Medical imaging
- Medical image quality
- Image sensors
- Computed tomography
- Medical image noise
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