Patient size matters: Effect of tube current modulation on size-specific dose estimates (SSDE) and image quality in low-dose lung cancer screening CT

Abstract

Purpose: We compare the effect of tube current modulation (TCM) and fixed tube current (FTC) on size-specific dose estimates (SSDE) and image quality in lung cancer screening with low-dose CT (LDCT) for patients of all sizes.

Methods: Initially, 107 lung screening examinations were performed using FTC, which satisfied the Centers for Medicare & Medicaid Services' volumetric CT dose index (CTDI_vol ) limit of 3.0 mGy for standard-sized patients. Following protocol modification, 287 examinations were performed using TCM. Patient size and examination parameters were collected and water-equivalent diameter (D_w ) and SSDE were determined for each patient. Regression models were used to correlate CTDI_vol and SSDE with D_w . Objective and subjective image quality were measured in 20 patients who had consecutive annual screenings with both FTC and TCM.

Results: CTDI_vol was 2.3 mGy for all FTC scans and increased exponentially with D_w (range = 0.96-4.50 mGy, R² = 0.73) for TCM scans. As patient D_w increased, SSDE decreased for FTC examinations (R² = 1) and increased for TCM examinations (R² = 0.54). Image quality measurements were superior with FTC for smaller sized patients and with TCM for larger sized patients (R² > 0.5, P < 0.005). Radiologist graded all images acceptable for diagnostic evaluation of lung cancer screening.

Conclusion: Although FTC protocol offered a consistently low CTDI_vol for all patients, it yielded unnecessarily high SSDE for small patients and increased image noise for large patients. Lung cancer screening with LDCT using TCM produces radiation doses that are appropriately reduced for small patients and increased for large patients with diagnostic image quality for all patients.

Keywords: image quality; lung cancer screening CT; radiation dose; size-specific dose estimates; tube current modulation.

Fig. 1

(a) CTDI_vol and (b) SSDE as a function of patient D_w for examinations performed with FTC and TCM. CTDI_vol, volumetric CT dose index; D_w, water‐equivalent diameter; FTC, fixed tube current; TCM, tube current modulation.

Fig. 2

(a) Noise and (b) CNR as a function of patient D_w for examinations performed with FTC and TCM. CNR, contrast to noise ratio; D_w, water‐equivalent diameter; FTC, fixed tube current; TCM, tube current modulation.

Fig. 3

Average subjective image quality scores for evaluation of lung detail, image noise, and soft tissue visualization as a function of patient D_{w .} Scores are described in Table 3. D_w, water‐equivalent diameter.

Fig. 4

Axial images of the smallest sized patient who received annual lung cancer screening CT examinations scanned with (a) FTC the first year and (b) TCM the second year. FTC, fixed tube current; TCM, tube current modulation.

Fig. 5

Axial lung images of the largest sized patient who received annual lung cancer screening CT examinations scanned with (a) FTC the first year and (b) TCM the second year. FTC, fixed tube current; TCM, tube current modulation.