Comparison of Planer Dose Equilibrium and Computed Tomography Dose Index and Implications for Reported Patient Dose Information

Technical developments are ongoing in CT, and there has been a continually increasing trend in patient prescription, resulting in increased exposure. Currently, doses delivered during CT are generally evaluated using computed tomography dose index (CTDI), which is measured with a 10 cm pencil ionization chamber placed in a 14 cm PMMA phantom. However, shortfalls in CTDI have been identified by the American Association of Physicists in Medicine (AAPM) who have proposed a new method, dose equilibrium (DEq). In this paper, the dose equilibrium was used to estimate the dose in two protocols (thoracic and abdominopelvic) and compared to CTDI values. In addition, a retrospective correction was applied to 20 patient CTDI’s by characterizing the specific DEq profile of the system scans. The results indicated the dose equilibrium estimations of two protocols, thoracic and abdominopelvic, were 29% and 30% respectively, higher than those informed by the CT scanner. In addition, a retrospective dose correction estimation of a random sample of twenty patients demonstrated an annual underestimation in absorbed dose by between 26% and 28%. Continued use of the CTDI method in quality assurance of modern CT could result in greater patient risk. AAPM Task Group 111 presents a more accurate, safer method to estimate dose and its adoption is paramount.


Introduction
Computed Tomography (CT) comprises approximately 5% of global medical X-ray procedures [1]. However, the dose from CT accounts for 34% of the yearly dose for the population from all medical X-ray imaging procedures worldwide [1]. This is not unexpected due to the high dose per examination in CT procedures [2].
The absorbed doses result from primary radiation as well as scattered radiation. Moreover, Computed Tomography Dose Index (CTDI) is used to determine CT quality assurance (QA) measurements and dose measurements, being the absorbed dose along the longitudinal axis (z-axis) during a single X-ray source rotation [3]. This measurement is usually conducted in a cylindrical phantom using a 100 mm ionization chamber. However, the chamber is responsible for a significant error in the dose profiles as it does not take into account some of the radiation scattered beyond the relatively short (100 mm) range of integration along the z-axis [4] [5]. This is mostly due to over-beaming in multi-slice CT, where the z-collimation of the source radiation is broadened to achieve umbra-region incidence uniformly across detectors.
Due to the increase of the detection system size along the z-axis, CT beams became larger, and much of the radiation not utilized by the detectors is incident on the patient. The more recent generations of CT scanners provide helical scanning mode or cone-beam irradiation geometries; however, the pencil chambers in these scenarios are too short to measure the radiation completely.
The AAPM Task Group Report No. 111 [6] outlined a new method of measurement derived from CTDI using a small volume ionization chamber in a cylindrical water phantom that is long enough to determine dose equilibrium.
With this dose equilibrium (DEq) measurement, we acquire a value sufficiently equivalent to both the primary and scatter radiation present from the beam [6].
The aim of this study is two-fold. The first is to use the AAPM dose equilibrium (DEq) method [6] [7] to estimate the dose values, and then compare it to CTDI values. The second is to retrospectively correct twenty random anonymous patient records of CTDI with new DEq estimates over the course of a year.

Materials and Methods
This study used a Toshiba CT scanner, Aquilion 16, a third-generation multi-slice helical CT scanner, a 60-kW generator, a 7.5 MHU tube.
The DEq phantom was built in-house [8] using a water-filled phantom  The collection volume of the Farmer chamber, which was calibrated by the National Standards of the German National Laboratory, was 0.6 cm 3 . In addition, a Sun Nuclear PC electrometer, equipped with a cable that allowed it to be placed outside the scatter-radiation field so as to avoid extraneous currents, was used with the chamber to provide a bias voltage of ±300 V.
Thoracic and abdominopelvic clinical protocols were chosen to measure DEq and CTDI. Table 1 provides the scan parameters for kV, mA, rotation time, slice thickness and pitch used in this study, while the scan length for each protocol was 450 mm. Open Journal of Medical Imaging

The Planer Average Equilibrium Dose Measurement and Comparison with CTDI Value Measurement
A detailed DEq phantom characterization and comparison to CTDI has previously been reported [8] The DEq method uses an upper limiting value based on the scanning length (L) and the cumulative dose (D (0)). There is a direct relationship between the scanning length (L) and the cumulative dose at z = 0, along with accumulating contributions from the outlying scan sections, until an upper limiting value is reached and the scatter radiation produces negligible contributions [6] [8] [12]. The equilibrium dose (DEq) is given by [ (where h (L) is approach to equilibrium function, h (L) = 1 when L becomes large enough to yield scatter equilibrium at z = 0) [6]. In this study, the equilibrium dose was determined for the centre, and peripheral axes for the two protocols (Table 1), with the planer average equilibrium dose then determined using Equation (2)

Correction of Past Patient Data to Dose Equilibrium
CTDI vol reports for the procedures and rescans performed throughout the year of a random sample of twenty anonymous patients were extracted. Using the lookup table (Table 2 and Figure 2) characterizing our specific CT system, the patient-specific CTDI vol was retrospectively corrected to DEq.
To apply the correction, we measure the DEq and CTDI vol at varying clinically relevant kV and mA ranges (as shown in Table 2). DEq was plotted against CTDI vol , and a linear regression was fitted to extrapolate a standard fit to estimate DEq measurements for any given CTDI vol within the range (Figure 2).
From this linear regression, the DEq can be estimated from a given CTDI vol within a standard CT range for our specific CT system allowing for the retrospective correction of past patient reported CTDI vol for this specific scanner. Open Journal of Medical Imaging

Comparison between Planer Average Equilibrium Dose and CTDIvol
The DEq measurement was compared to the CTDI vol measurement for both the thoracic and the abdominopelvic protocols, as shown in

Correct Past Patient Data to Dose Equilibrium
The CTDI vol lookup table was used to estimate DEq for the performed procedure (Table 2, Figure 2). This revealed that over the course of several scans, our random patient sample in Table 4 received an absorbed dose anywhere from 4 mGy up to 13 mGy greater than previously estimated by CTDI vol . Therefore, the CTDI vol method significantly underestimated the absorbed dose when compared to the DEq method for all patients.
An example of an individual male patient undergoing prostate imaging is shown in Table 5, broken down by each scan as well as summed over all scans performed throughout the year. This process was performed for all patients among the sample who's scan protocol was torso or pelvic.
Patient 4 received four scans and the system output was estimated for both CTDI vol and DEq. In scan 1, the CTDI vol was estimated at 2.9 mGy while the DEq was measured at 3.8 mGy, a difference of 31%. In scan 2, the CTDI vol was estimated at 6.2 mGy while the DEq was measured at 7.9 mGy, a difference of 27%.

Conclusions
With the new generation of CT scanners, utilizing helical scanning mode and increased beam width and depth with associated increased detector size, the use of CTDI is no longer appropriate. With an average absorbed dose underestimation of 27% [5] [8] for all patients compared to that of DEq. The continued use of CTDI in dose estimation presents a greater risk to patient safety.
From the experimental results, CTDI vol values, as informed by the CT scanner, Open Journal of Medical Imaging were lower than planer average equilibrium doses (DEq) values for both protocols, determined in agreement with AAPM TG111, differences ranged between 29% and 30%.
In addition, the absorbed dose estimated for a sample of patients was underestimated by CTDI when compared to DEq by between 26% and 28%. The systemic underestimation of absorbed dose leaves both patients and staff misinformed and at greater risk.
In conclusion, it can be seen from the results that the characterization of dose through the use of CTDI is insufficient and inaccurate for modern CT machines. The results exhibited significant error in the characterization of dose profiles and implementing the new method outlined by the American Association of Physicists in Medicine Task Group report No. 111 [6] is a key to a more accurate characterization of the dose profiles from modern CT scanners. This method is relatively simple to follow and can be adapted to different phantom designs to determine DEq. The DEq method, therefore, is a simple, standardized measure of the dose output of the CT scanner that can be used for quality assurance.