Room: Room 202
Purpose: To assess the effects of tube current modulation (TCM) on radiation dose indices and image quality as a function of patient size in lung cancer screening CT with a 16-slice CT scanner.
Methods: Initially, 107 lung screening examinations were performed using a fixed tube current that satisfied the U.S. Centers for Medicare & Medicaid Services (CMS) requirement that volumetric CT dose index (CTDIvol) be 3.0 mGy or less for standard-sized patients. Following protocol modifications, 283 examinations were performed using attenuation-based TCM that accounted for patient size. With IRB approval, patient size and examination parameters were collected and water-equivalent diameter (Dw) and size-specific dose estimates (SSDE) were determined. Regression models were used to correlate CTDIvol and SSDE with Dw. Image quality was evaluated in 20 patients who had consecutive annual screenings with fixed and modulating techniques. CNR was measured and two blinded radiologists graded preference for several features on a scale of -3 to 3 (-3: Strongly prefer Image A, 0: No preference, 3: Strongly prefer Image B) with Image B corresponding to the examination acquired with TCM.
Results: Scans using fixed techniques produced CTDIvol of 2.3 mGy. Scans using TCM produced CTDIvol values (Average=2.14 mGy, Range=0.96â€“4.5 mGy) that increased quadratically with Dw (R2=0.73). As Dw increased, SSDE decreased for fixed exams (R2=1) and increased modulating exams (R2=0.43). All images acceptable for diagnostic evaluation of lung screening. As Dw increased, average image quality scores increased (R2=0.79), and CNR decreased for fixed techniques (R2=0.821) and increased followed by a gradual decrease for TCM (R2=0.824), demonstrating image quality was superior with fixed techniques for small patients and with TCM for large patients.
Conclusion: Patients should be examined with TCM for lung cancer screening CT in order to produce acceptable image quality while utilizing radiation doses As Low As Reasonably Achievable (ALARA).