Estimating patient dose from CT exams that use automatic exposure control: Development and validation of methods to accurately estimate tube current values

Abstract

Purpose: The vast majority of body CT exams are performed with automatic exposure control (AEC), which adapts the mean tube current to the patient size and modulates the tube current either angularly, longitudinally or both. However, most radiation dose estimation tools are based on fixed tube current scans. Accurate estimates of patient dose from AEC scans require knowledge of the tube current values, which is usually unavailable. The purpose of this work was to develop and validate methods to accurately estimate the tube current values prescribed by one manufacturer's AEC system to enable accurate estimates of patient dose.

Methods: Methods were developed that took into account available patient attenuation information, user selected image quality reference parameters and x-ray system limits to estimate tube current values for patient scans. Methods consistent with AAPM Report 220 were developed that used patient attenuation data that were: (a) supplied by the manufacturer in the CT localizer radiograph and (b) based on a simulated CT localizer radiograph derived from image data. For comparison, actual tube current values were extracted from the projection data of each patient. Validation of each approach was based on data collected from 40 pediatric and adult patients who received clinically indicated chest (n = 20) and abdomen/pelvis (n = 20) scans on a 64 slice multidetector row CT (Sensation 64, Siemens Healthcare, Forchheim, Germany). For each patient dataset, the following were collected with Institutional Review Board (IRB) approval: (a) projection data containing actual tube current values at each projection view, (b) CT localizer radiograph (topogram) and (c) reconstructed image data. Tube current values were estimated based on the actual topogram (actual-topo) as well as the simulated topogram based on image data (sim-topo). Each of these was compared to the actual tube current values from the patient scan. In addition, to assess the accuracy of each method in estimating patient organ doses, Monte Carlo simulations were performed by creating voxelized models of each patient, identifying key organs and incorporating tube current values into the simulations to estimate dose to the lungs and breasts (females only) for chest scans and the liver, kidney, and spleen for abdomen/pelvis scans. Organ doses from simulations using the actual tube current values were compared to those using each of the estimated tube current values (actual-topo and sim-topo).

Results: When compared to the actual tube current values, the average error for tube current values estimated from the actual topogram (actual-topo) and simulated topogram (sim-topo) was 3.9% and 5.8% respectively. For Monte Carlo simulations of chest CT exams using the actual tube current values and estimated tube current values (based on the actual-topo and sim-topo methods), the average differences for lung and breast doses ranged from 3.4% to 6.6%. For abdomen/pelvis exams, the average differences for liver, kidney, and spleen doses ranged from 4.2% to 5.3%.

Conclusions: Strong agreement between organ doses estimated using actual and estimated tube current values provides validation of both methods for estimating tube current values based on data provided in the topogram or simulated from image data.

Keywords: Monte Carlo simulations; computed tomography; organ dose; radiation dose; tube current modulation.

Figure 1

AP and LAT water‐equivalent estimates of patient size extracted from the DICOM header of a topogram. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

(Left) Air scan and patient scan fluence profiles at a particular table location. (Right) Patient attenuation profile along the detector calculated by dividing the fluence profile from the air scan by the fluence profile from the patient scan. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Diagram of scanner geometry with all components of Eq. (4) labeled.

Figure 4

(Left) Maximum patient attenuation at each table position calculated from patient size data using Eq. (5). (Right) Tube current profile calculated from patient attenuation data using Eq. (6) at all table positions with corresponding axial CT images. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

(Left) Angular attenuation profile determined from patient size data extracted from topogram. (Right) Angular modulation scheme calculated from angular attenuation data using Eq. (8) at all table positions with corresponding axial CT images. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

Actual and estimated tube current values of adult chest patient. Estimated tube current values calculated from control curve and angular modulation scheme from Figs. 4 and 5, respectively. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 7

Workflow of available methods to generate Siemens AEC schemes. This investigation introduces methods to estimate AEC schemes using patient size calculated from either an actual or simulated topogram. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8

(Left) Estimated AP and LAT patient dimensions compared with AP and LAT dimensions of patient size extracted from the measured topogram of a patient who underwent a clinically indicated chest CT exam (Patient Chest Adult 7 from Table 1). (Right) Estimated AP and LAT patient dimensions compared with AP and LAT patient dimensions size extracted from the measured topogram of a patient who underwent a clinically indicated abdomen/pelvis CT exam (Patient Abdomen/Pelvis Adult 4 from Table 1). [Note: The estimated patient attenuation data (solid lines) was derived from the patient's axial image data and therefore does not include the extra anatomy before and after the scan range in the patient's actual topogram (dashed lines).] [Color figure can be viewed at wileyonlinelibrary.com]

Figure 9

(Left) Tube current values from simulated topogram and actual tube current values for chest patient (Patient Chest Adult 7 from Table 1). (Right) Tube current values from simulated topogram and actual tube current values scheme for abdomen/pelvis patient (Patient Abdomen/Pelvis Adult 4 from Table 1). [Color figure can be viewed at wileyonlinelibrary.com]

Figure 10

Estimated and actual tube current values for a patient undergoing a chest scan on a Sensation 16 (left) and a separate patient undergoing a chest scan on a Definition Flash (right). For the Sensation 16 case, the error between the average tube current from the estimated and actual AEC scheme was 0.2%. For the Definition Flash case, the error between the average tube current from the estimated and actual AEC scheme was 0.8%. [Color figure can be viewed at wileyonlinelibrary.com]