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 (CTDIvol) 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 (Dw) and SSDE were determined for each patient. Regression models were used to correlate CTDIvol and SSDE with Dw. Objective and subjective image quality were measured in 20 patients who had consecutive annual screenings with both FTC and TCM. Results CTDIvol was 2.3 mGy for all FTC scans and increased exponentially with Dw (range = 0.96–4.50 mGy, R2 = 0.73) for TCM scans. As patient Dw 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 CTDIvol 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.


| INTRODUCTION
Lung cancer is the leading cause of cancer-related death in the United States, with an estimated 234,030 new lung cancer cases and 154,050 lung cancer deaths in 2018. 1 This high mortality is due to the asymptomatic nature of lung cancer, where the majority of patients seek medical care after symptoms have developed, often when lung cancer has progressed to more advanced stages. 2 Because lung cancer has a higher survival rate when detected at an early stage, screening in highrisk individuals provides a preventative approach to reduce the number of lung cancer deaths. 3 Launched in 2002, the National lung screening trial (NLST) enrolled 53,454 high-risk smokers to undergo three annual lung cancer screening exams with either standard chest radiography (CXR) or low-dose CT (LDCT). 4 In 2011, the group published that 20% fewer lung cancer deaths were observed in participants screened with LDCT rather than CXR, due to the improved sensitivity of CT and its ability to resolve small nodules. 5 In 2013, the US Preventive Services Task Force (USPSTF) recommended annual lung cancer screening with LDCT, 6,7  Following the recommendations from these groups, our hospital implemented a lung cancer screening program with LDCT in March 2015. However, the initial scan protocol utilized a single fixed tube current (FTC) value for all patients. Over a 1-year period, scan techniques were not modified for different patient sizes, despite statements from the CMS, AAPM, and ACR specifying that radiation doses must be reduced for smaller sized patients and increased for larger sized patients examined for lung cancer screening with LDCT. 8,11,12 Today's CT systems offer automatic exposure control (AEC) with tube current modulation (TCM) to reduce dose to the patient while maintaining image quality. 13,14 Tube current modulation is routinely used in thoracic and abdomen-pelvic CT imaging, where the tube current is increased in higher attenuating regions such as the shoulders, and decreased in lower attenuating regions such as the lungs. TCM also accounts for different patient sizes by delivering sufficient tube current for proper x-ray photon transmission through each patient. [15][16][17] In order to modulate dose as a function of patient size, the original FTC scan protocol was modified to utilize AEC (CARE Dose 4D, Siemens Healthineers). This particular AEC system performs automatic TCM according to the patient's size and attenuation changes measured in the x-ray localizer projection images, together with real-time attenuation measurements measured during each tube rotation. 18,19 The adaptation of the tube current is based on a userdefined image quality parameter called the Quality Reference mAs, expressed as the tube current-time product (milliamperes-second, mAs) divided by the pitch and is selected according to the diagnostic requirements for the study.
Assessment of dose and image quality between both protocols for patients of all sizes is important to provide patient-customized scanning that balances reducing radiation dose while providing acceptable image quality. This is especially important with higher prevalence of obesity in populations where screening is being implemented, and the repetitive exposure to radiation in a screening program. This study evaluates the effect of TCM on patient dose and image quality for 394 lung cancer screening examinations with LDCT, which has unique requirements of balancing detailed visibility of lung parenchyma while performing screening examinations with doses as low as possible.
We first compare Size-Specific Dose Estimates (SSDE) and clinical image quality between patients scanned with FTC and TCM techniques. Second, considering the lack of a formal assessment of radiation dose and image quality for clinical lung cancer screening examinations, this study aims to validate that the recommended TCM-based AAPM scan protocol for a Siemens Sensation 16-slice scanner produces acceptable CT dose indices and diagnostic image quality for a variety of patient sizes.

| MATERIALS AND METHODS
Initially, a standard chest CT protocol was modified to create a lowdose lung cancer screening CT protocol. The tube voltage was reduced from 120 kVp to 100 kVp, and the tube current was modified to utilize a single fixed tube current value of 150 milliamperes (mA) for all patients. In order to modulate dose as a function of patient size, the FTC protocol was modified to utilize 120 kVp and TCM with a Quality Reference mAs of 25, as recommended by the AAPM. 11 Scan protocols utilized before and after the modification are described in Table 1.

2.B | Determination of size-specific dose estimates (SSDE)
CTDI vol is a dose index that provides information about the scanner output for a standard condition. [20][21][22][23] However, the dose received by a patient depends on both patient size and scanner output. AAPM Report 204 introduced a new metric, the SSDE, which can be used to estimate average patient dose based on the CTDI vol and linear patient size measurements. 24 AAPM Report 220 describes an improved method that estimates patient size based on patient attenuation by introducing the water-equivalent diameter (D w ). 25 It is important to consider D w , rather than linear dimensional measurements in regions of the thorax, where attenuation is reduced. 25 Size-specific dose estimates was determined for each patient using [eq. (1)]: where f 32 size is the conversion factor based on the 32-cm diameter PMMA phantom for CTDI vol for specific D w values, determined using [eq. (2)]: within the ROI and area of the ROI were recorded and D w was calculated using equation (3): The two studies for each patient were displayed side by side, with one monitor displaying the image series acquired with FTC, and the other monitor displaying the image series acquired with TCM.

2.C | Image quality evaluation
The order of presentation was often switched on the two monitors to prevent pattern recognition of preferred imaging parameters by the radiologist. First, radiologists evaluated the overall diagnostic image quality acceptance for each study using a 3-point grading scale (Table 2). Next, the radiologists evaluated preference for clinical image quality features specific to lung cancer screening CT including visualization of lung detail, image noise in the lungs, and soft tissue review for incidental findings. Radiologists were asked to provide a verbal response to describe whether they preferred the study on the left monitor or the study on the right monitor, and how strongly they preferred the image. Table 3 describes the verbal response categories they could choose from. The medical physicists conducting the image quality study recorded the verbal response as well as the study accession number displayed on each monitor at the time the question was answered. Using this information, the verbal responses were adapted as scores using a 7-point grading scale (Table 3).

2.D | Statistics
A two-sample t-test was used to determine whether the patients' age, weight, height, BMI, and D w showed statistically significant differences between the two patient groups. A two-sample t-test was also used to compare CTDI vol , SSDE, noise, and CNR between both protocols. Results were considered to be statistically significant for P < 0.05. An interobserver agreement for the two radiologists was

| RESULTS
Over a period of 29 months, 394 lung cancer screening CT examinations were performed on a Siemens Sensation 16-slice CT scanner.
Noise and CNR were dependent on patient size and acquisition method (P < 0.005) and demonstrated two-order polynomial relationships with D w (R 2 > 0.50) (Fig. 2). With the FTC protocol, noise soft tissue review for incidental findings (R 2 = 0.82) (Fig. 3). Considering a score of −3 represents Strongly prefer FTC, and a score of + 3 represents Strongly prefer TCM, the increasing trend in Fig. 3 demonstrates that radiologists preferred images acquired with FTC (score < 0) for smaller sized patients with D w < 28.4 cm and with Patient studies acquired with FTC and TCM were presented either on a left or right display monitor, with order of presentation often changed to prevent radiologist recognition of preferred acquisition parameters. Radiologists were not asked to give numerical scores, but rather present their preference verbally using the seven given response categories. b Medical physicists conducting the observer study recorded which imaging study (FTC or TCM) was presented on which monitor and assigned scores, respectively. screening for high-risk patients ranging from underweight to obese body sizes. It is interesting to note this older 16-slice scanner without iterative reconstruction was able to produce acceptable radiation dose values and image quality evaluation for a variety of patients examined with a low-dose screening protocol, and therefore, hospitals should not assume a dose reduction tool such as iterative reconstruction is necessary to achieve an effective low-dose lung cancer screening CT program.
When FTC techniques were utilized, the CTDI vol and thus radiation output from the scanner were the same for all patients. As a result, smaller patients, having less mass than larger patients, received greater radiation absorbed dose, demonstrated by SSDE decreasing exponentially as the patient size increased [ Fig. 1(b)]. This approach of delivering the same radiation output to all patients independent of their size reflects back to when scan techniques were not modified for small vs large patients and has been widely replaced by recommendations for TCM. [13][14][15][16][17] When the TCM protocol was utilized, smaller patients received less SSDE than larger patients, demonstrated by SSDE increasing exponentially as patient size increased [Fig 1(b)]. Among patients scanned with TCM, the smallest sized patient (weight, 97.0 lbs) had an SSDE of 1.6 mGy, and the largest sized patient (weight 266.0 lbs) had an SSDE of 3.4 mGy. Another study assessing SSDE in CT of the torso reported patient size had an effect on CTDI vol but not on SSDE, concluding that increasing the scanner output for larger patients would not necessarily increase the mean absorbed dose to the patients. 26 Alternatively, our study focusing on the relatively less attenuating chest region observed a statistically significant exponential increase in SSDE as a function of patient size, indicating larger sized patients experienced greater mean absorbed dose than smaller sized patients. These results are also supported by a previous study on patient size and impact of attenuation-based AEC on low-dose lung cancer screening protocols. 27 Our finding that SSDE increases for larger sized patients is critical for other studies estimating longterm stochastic risks in the lung cancer screening population, which requires a multi-vendor investigation of different AEC systems.
Relative to the FTC protocol, objective and subjective image quality decreased for smaller sized patients with the use of TCM. In the smallest patient who received consecutive exams with FTC and TCM, both radiologists preferred images acquired with FTC [ Fig. 4( a)] rather than with TCM [ Fig. 4(b)]. Figure 4 demonstrates less image noise, better low contrast resolution, and reduced artifacts in the FTC image (a) compared to the TCM image (b). CNR was also superior for the FTC images (CNR = 1.67) compared to the TCM images (CNR = 1.21). This was expected, as the study acquired with FTC utilized a lower tube voltage (100 kVp) and a higher CTDI vol (2.25 mGy) than the same patient's study acquired with  Fig. 5(b)] rather than with FTC [ Fig. 5(a)].
It is evident in Fig. 5  Our study has limitations. The initial FTC examinations were acquired with a tube voltage of 100 kVp rather than 120 kVp. Since a lower tube voltage offers improvement in image contrast, the comparison of CNR and subjective image quality is not only due to changes in x-ray output but also increased photon absorption at a lower kVp. However, it is likely that the reduced tube voltage of 100 kVp would offer an improvement in low contrast soft tissue assessment but not as much in high contrast lung detail. 28 Furthermore, if performing an assessment with equivalent tube voltages, we would expect for FTC exams acquired with 120 kVp to produce inferior CNR measurements compared to those measured in our study with 100 kVp, and therefore, the differences in image quality observed in this study would have been even greater. In addition, increasing the tube voltage from 100 kVp to 120 kVp reduces the beam hardening effect and artifacts such as streaking, which would be expected to be greater in the upper thorax and in larger sized patients. However, artifact evaluation was not included in the image quality assessment.  Table 3. D w , water-equivalent diameter.
We also did not investigate lung cancer screening protocols using other vendors. However, all scan protocols should benefit from the use of TCM as it modifies the tube current output accordingly for smaller or larger patients and removes the need for manual technique adjustment by the technologist when scanning small-vs largesized patients. Furthermore, although patient weight is often readily available, it cannot be directly correlated to water-equivalent diameter, as there are variable human somatotypes. This has been shown to be especially true of the thorax where there is higher variation along the z-axis. 25 There remains a need to assess other vendor AEC systems in order to describe the effect of patient size on SSDE for future risk estimates.

| CONCLUSION
In summary, SSDE takes into account the size of the patient and provides a more accurate reflection of patient dose than CTDI vol .
When considering the SSDE for patients of different body sizes, a protocol that balances diagnostic acceptability with dose reduction should be performed. We confirmed there was an unnecessary increase in SSDE for small-sized patients and reduction in SSDE for large-sized patients when scanning with FTC techniques. Scanning with TCM produced more favorable dose output based on patient size and is supported by current AAPM recommendations.
Using TCM to adjust scanner output for lung cancer screening with LDCT resulted in an exponential relationship between patient size, CTDI vol , and SSDE, indicating that increasing scanner output for larger patients also increased the mean absorbed dose to these patients. Furthermore, examinations performed with TCM received superior image quality measurements for larger sized patients (D w >

cm).
In consideration of enforcing the ALARA (As Low As Reasonably Achievable) principle, all patients examined with lung cancer screening CT should be scanned with TCM for optimal clinical practice, considering its ability to automatically modify radiation output as necessary and yield acceptable image quality by interpreting radiologists.

IRB STATEMENT
This work was conducted with permission granted by our Institutional Review Board (IRB) under study number IRB201500809 titled "Lowdose Computed Tomography Screening for Lung Cancer: Initial screening experience at an academic medical center." Full waiver of informed consent was approved, and therefore, subjects were not informed nor consented for retrospective examination data collection.

CONFLI CTS OF INTEREST
The authors have no conflicts of interest.