Objectives: To evaluate the effect of patient size on radiation dose for standard CT (SD-CT), ultra-low-dose CT (ULD-CT) and two-view digital radiography (DR).
Methods: Dosimeters were distributed within the lungs of chest phantoms representing males of 65 kg and 82 kg (body mass indices 23 and 29). In contrast to SD-CT and DR which include automatic exposure control (AEC), the ULD scan employs a fixed mAs value. The phantoms were exposed to SD, ULD and DR while recording lung doses. Projected dose data were calculated from the phantoms. The resulting exposure settings were used in Monte Carlo programs to determine the effective dose for a standard-sized (BMI 24.2) adult male (170 cm/70 kg) and female (160 cm/59 kg). Patients previously examined by both ULD- and SD-CT were identified to determine post hoc size-specific dose estimates (SSDEs).
Results: ULD-CT dose was inversely related to patient size; average lung doses summarised in terms of patient size BMI are 5.2/8.1 (SD-CT), 0.56/0.35 (ULD-CT) and 0.05/0.13 mGy (DR), while the effective doses for these techniques on a standard-sized male were 2.9, 0.16 and 0.03 mSv and 2.3, 0.247 and 0.024 mSv for a standard-sized female respectively. SSDEs for 15 patients (averages: BMI 26, range 18-37) averaged 5.5 mGy (3.6-10) for SD-CT and 0.35 mGy (0.42-0.27) for ULD-CT.
Conclusions: The effective doses for a standard-sized male and female examined by ULD-CT are (respectively) ~ 6%/~ 11% of SD-CT and ~ 5/~ 10 times higher than DR. ULD-CT gave a lower radiation dosage to larger patients than DR. AEC is warranted in ULD-CT for improved dose consistency.
Key Points: • For standard-sized patients, ULD-CT dose level is ~ 6%/~ 11% of SD-CT, and ~ 5/~ 10 times higher than DR. For larger patients, ULD-CT is currently being used clinically at lower dose levels than DR. • Using ULD-CT should greatly reduce the risk of late effects from ionising radiation. • AEC in ULD-CT is desirable for increased consistency in patient dose.
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