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Standard-Dose and 50%—Reduced-Dose Chest CT: Comparing the Effect on Image Quality

¹ All authors: Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Founders 216, 55 Fruit St., Boston, MA 02114.

Received December 24, 2001; accepted after revision February 11, 2002.

 
Supported in part by a grant from General Electric Medical Systems.

Address correspondence to T. McLoud.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. CT accounts for considerable population-based radiation dose from radiographic diagnostic studies. The technical factors for CT examinations are not appropriately adjusted on the basis of patient size and anatomy. We hypothesized that radiation doses for routine chest CT can be reduced by 50% for the evaluation of normal structures without seriously jeopardizing image quality.

SUBJECTS AND METHODS. After receiving institutional review board approval, we prospectively studied 24 patients with cancer who were 65 years old and older on a multidetector CT scanner. Each patient underwent imaging with four slices (centered at the carina) at the standard dose (220-280 mAs) and at 50%—reduced dose (110-140 mAs) at a constant 140 kVp. Single breath-hold scanning was performed with a 2.5-mm detector configuration, a tube rotation time of 0.8 sec, and a pitch of 6:1. Contiguous images were reconstructed at 5-mm intervals. Two subspecialty-trained chest radiologists who were unaware of the CT technique reviewed randomized images for overall image quality and anatomic detail of the structures in the lung, airway, mediastinum, and chest wall using a 5-point scale (1, worst; 2, suboptimal; 3, adequate; 4, very good; 5, excellent). The data were analyzed using the Wilcoxon's signed rank test.

RESULTS. Although overall image quality was better with standard-dose CT, the quality of reduced-dose CT was acceptable. The differences in mean scores were statistically significant. There was a correlation of 0.59 between observers. The mean scores of standard-dose CT were always greater than or equivalent to those of low-dose CT for both observers. The assessment of great vessels and soft tissue of the chest wall contributed mainly to the differences in image quality. Both the central and peripheral lung parenchyma and the airway were adequately visualized on low-dose CT. Radiation doses (based on weighted-CT—dose index) from standard-dose CT and 50%—reduced-dose CT were 15.6-21.4 mSv and 7.8-10.7 mSv, respectively (from the manufacturer's data).

CONCLUSION. Chest CT image quality appears to be acceptable for evaluating normal anatomic structures even with a 50% reduction in radiation dose.

Introduction

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Chest CT is widely used for evaluating a multitude of abnormalities and disorders affecting the airway, mediastinum, and lungs [1,2,3]. In most clinical conditions of the chest, CT has become a necessary adjunct to conventional radiography. Conventional chest CT is usually performed using settings between 220 and 280 mAs. Although low- and ultra low-dose chest CT are used in some institutions primarily for lung cancer screening, this approach has not received widespread acceptance and practice for routine indications [4,5,6].

Increased use of CT in recent times has the potential to increase radiation burden to the general population. According to recent estimates, based on the global use of CT, the medical radiation burden from CT is 34% even though CT constitutes only 5% of radiographic diagnostic studies [7]. In the United States, CT accounts for more than two thirds of the total radiation dose from radiographic diagnostic studies [8]. Because of the heightened awareness of escalating population-based radiation burden from CT, radiologists and the imaging industry have augmented efforts to reduce radiation dose from CT. The principle of ALARA (as low as reasonably achievable) is more relevant in this era of increasing use of CT for diagnosis and interventional procedures.

Several studies have suggested that substantial dose reduction during chest CT is feasible because of the high inherent contrast in the chest and lower pulmonary absorption of radiation [9, 10]. We hypothesized that low-dose chest CT with a 50%—reduced dose can be performed without seriously jeopardizing image quality.

Subjects and Methods

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Twenty-four cancer patients (14 women and 10 men) with an age range of 65-81 years (mean age, 74 years) who were referred for routine chest CT constituted the study group. The patients were classified on the basis of their body weight into three groups: less than 55 kg (n = 3), 55-80 kg (n = 17), and greater than 80 kg (n = 4). The study was approved by the institutional review board.

All scans were obtained on a multidetector CT scanner (LightSpeed, QX/i; General Electric Medical Systems, Milwaukee, WI). After routine chest CT was performed, four additional images were obtained at the level of the carina using the standard tube voltage (140 kVp) and tube current (220-280 mAs). Four low-dose CT images were subsequently obtained by reducing the tube current by one half the standard dose (110-140 mAs) and maintaining the constant peak tube voltage (140 kVp). The scanning parameters common to both techniques included a detector configuration of 2.5 mm, a table speed of 15 mm per rotation, a nonoverlapping and interspersed scanning mode (pitch, 6:1), and a tube rotation time of 0.8 sec. Images were reconstructed at 5-mm intervals.

Qualitative analysis of randomized images was independently performed by two thoracic radiologists who were unaware of the technical parameters. The lung, mediastinum, chest wall, and airway were assessed on a 5-point scale (1, worst; 2, suboptimal; 3, adequate; 4, very good; 5, excellent). Noise, image contrast, sharpness, and overall image quality were qualitatively evaluated (Appendix 1). Radiation doses from the standard-dose and low-dose CT scans were recorded as displayed on the CT monitors (from the manufacturer's data; General Electric Medical Systems).

Results

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The mean scores for standard-dose chest CT across the three weight categories for the first observer were 3.66 (<55 kg), 3.67 (55-80 kg), and 3.75 (>80 kg). The mean scores for standard-dose chest CT across the three weight categories for the second observer were 4.0 (<55 kg), 3.9 (55-80 kg), and 3.75 (>80 kg). The mean scores for 50%—reduced-dose CT in the three weight categories for the first observer were 3.3 (<55 kg), 3.14 (55-80 kg), and 3.5 (>80 kg). The mean scores for 50%—reduced-dose CT in the three weight categories for the second observer were 4.0 (<55 kg), 3.58 (55-80 kg), and 3.12 (>80 kg). The differences between standard dose and 50% reduced dose for each observer at each weight category were statistically significant (p < 0.005) except for the low-weight patients for observer 2 in whom the reduced dosage was not shown to make a difference. There was a correlation of 0.59 between observers.

We found that the perceived difference between high- and low-dose CT scans is greater than the perceived variation among patients on the same dose. Although the image quality of the standard-dose CT was better than that of low-dose CT across all weight categories, the image quality of low-dose CT was acceptable across all weight categories (Figs. 1A,1B,1C,1D,2A,2B,2C,2D,3A,3B,3C,3D). The assessment of great vessels and soft tissue of the chest wall contributed mainly to the differences in image quality. Both the central and peripheral lung parenchyma and the airway were adequately visualized on low-dose CT. Radiation doses (based on weighted-CT—dose index) from standard-dose CT and 50%—reduced-dose CT were 15.6-21.4 mSv and 7.8-10.7 mSv, respectively (from the manufacturer's data displayed on the CT monitor; General Electric Medical Systems).

View larger version (103K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —74-year-old woman with history of breast cancer and weight of 48 kg. Contrast-enhanced CT scans (mediastinal window settings) obtained just inferior to level of carina using 140 kVp and 220 mAs (A) and 140 kVp and 110 mAs (B) show no significant change in image quality.

View larger version (100K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —74-year-old woman with history of breast cancer and weight of 48 kg. Contrast-enhanced CT scans (mediastinal window settings) obtained just inferior to level of carina using 140 kVp and 220 mAs (A) and 140 kVp and 110 mAs (B) show no significant change in image quality.

View larger version (88K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —74-year-old woman with history of breast cancer and weight of 48 kg. Contrast-enhanced CT scans (lung window settings) obtained using 140 kVp and 220 mAs (C) and 140 kVp and 110 mAs (D) show that image quality of low-dose CT is comparable with that of standard-dose CT.

View larger version (95K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D. —74-year-old woman with history of breast cancer and weight of 48 kg. Contrast-enhanced CT scans (lung window settings) obtained using 140 kVp and 220 mAs (C) and 140 kVp and 110 mAs (D) show that image quality of low-dose CT is comparable with that of standard-dose CT.

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —79-year-old man with history of colon cancer and weight of 77 kg. Contrast-enhanced CT images (mediastinal settings) using 140 kVp and 240 mAs (A) 140 kVp and 120 mAs (B) show satisfactory delineation of mediastinal structures.

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —79-year-old man with history of colon cancer and weight of 77 kg. Contrast-enhanced CT images (mediastinal settings) using 140 kVp and 240 mAs (A) 140 kVp and 120 mAs (B) show satisfactory delineation of mediastinal structures.

View larger version (82K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C. —79-year-old man with history of colon cancer and weight of 77 kg. Contrast-enhanced CT images (lung window settings) using 140 kVp and 240 mAs (C) 140 kVp and 120 mAs (D) show satisfactory delineation of lung parenchyma and pulmonary vasculature.

View larger version (82K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2D. —79-year-old man with history of colon cancer and weight of 77 kg. Contrast-enhanced CT images (lung window settings) using 140 kVp and 240 mAs (C) 140 kVp and 120 mAs (D) show satisfactory delineation of lung parenchyma and pulmonary vasculature.

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —77-year-old man with history of colon cancer and weight of 95 kg. Contrast-enhanced CT scans (mediastinal window settings) using 140 kVp and 260 mAs (A) and 140 kVp and 130 mAs (B) show that although chest wall structures appear noisy, mediastinal structures are optimally visualized on low-dose image.

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —77-year-old man with history of colon cancer and weight of 95 kg. Contrast-enhanced CT scans (mediastinal window settings) using 140 kVp and 260 mAs (A) and 140 kVp and 130 mAs (B) show that although chest wall structures appear noisy, mediastinal structures are optimally visualized on low-dose image.

View larger version (88K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —77-year-old man with history of colon cancer and weight of 95 kg. Contrast-enhanced CT scans (lung window settings) using 140 kVp and 260 mAs (C) and 40 kVp and 130 mAs (D) show satisfactory delineation of pulmonary parenchyma.

View larger version (88K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3D. —77-year-old man with history of colon cancer and weight of 95 kg. Contrast-enhanced CT scans (lung window settings) using 140 kVp and 260 mAs (C) and 40 kVp and 130 mAs (D) show satisfactory delineation of pulmonary parenchyma.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Carcinogenesis is one of the late stochastic adverse effects of ionizing radiation. Stochastic effect implies that the biologic event is an all-or-none phenomenon with a no-dose threshold. The probability of cancer occurring as a result of radiation is related to dose, but the severity of cancer is not related to dose [11]. Strong epidemiologic evidence suggests that high-level exposure to radiation leads to increased cancer risk in some select organs. The estimation of lifetime radiation risk based on atomic bomb survivor data and nuclear reactor accidents has shown increased risk of developing cancers that is proportional to radiation exposure [12].

Radiation doses associated with CT scans are comparatively low. Although controversial, linear extrapolation of results from high-dose exposures is favored by some regulatory agencies for calculating radiation-induced cancer risk [13]. The relative-risk model of extrapolation of low-dose risk from high-dose situations assumes that natural cancer prevalence increases by a constant factor because of radiation. Using this hypothesis, Hall [11] estimated that the lifetime cancer risk from low-dose exposure such as that associated with CT is 4% per sievert (3% for fatal cancers and 1% for nonfatal cancers). An increased number of CT examinations performed in all age groups, therefore, may potentially result in an overall increase in population-based radiation dose and hence in increased natural cancer prevalence due to radiographic diagnostic studies.

The lifetime radiation risk of developing cancers because of CT has been a matter of interest recently, especially in regard to pediatric CT. Using the linear extrapolation algorithm, Brenner et al. [14] estimated that 500 children (<15 years old) might develop cancers as a direct result of CT scans obtained in the United States. Although organ radiosensitivity and radiation dose from an examination are higher in children, concern about CT-related cancers in adults is also increasing. Because of the increased radiation burden associated with CT, the lowest possible doses that permit acceptable quality images should be implemented.

Several initiatives from the radiology community and industry are needed to decrease population-based radiation burden from CT. CT must be used judiciously to answer specific clinical questions, and repeated examinations must be avoided by ensuring optimal technical parameters. The routine use of recent technologic innovations such as better collimation of X-ray beams, new filter designs, and online tube-current modulation may help to limit radiation dose from CT [15,16,17].

Several factors influence patient radiation dose from CT including tube voltage, tube current, scanning time, pitch, slice thickness, and scanning volume. Radiation dose is linearly related to tube current, scanning time, and scan volume [7] and inversely related to pitch. Although scanning times are decreased on modern helical CT scanners, the radiation dose associated with helical scanners is greater than that of other imaging procedures because of the increased tube current and volume of tissue irradiated [7, 18].

Radiation dose is directly proportional to tube current at a constant peak tube voltage (kVp), slice width, and pitch. Hence, reduction in tube current decreases radiation dose. The tube current should be tailored to the patient's body weight, body habitus, and type of CT.

Some of the pioneer research in low-dose CT was performed in thoracic imaging. Naidich et al. [19] obtained diagnostic-quality images of the lung using reduced milliamperage (as low as 10 mAs). A study by Zwirewich et al. [20] showed no considerable difference in the diagnostic content of low and standard-dose high-resolution CT images. In addition, no appreciable loss of spatial resolution or increased incidence of streak artifacts was present. Rusinek et al. [21] showed the value of low-dose chest CT for primary screening of lung nodules.

Several studies have shown that radiation dose can be appreciably reduced without seriously compromising image quality. Using tube currents between 40 and 80 mAs, Ambrosino et al. [10] obtained acceptable-quality high-resolution images of the chest in children. In a study that included 203 pediatric patients, Lucaya et al. [22] obtained good-quality high-resolution CT images of the lung using tube currents between 72% and 80% of those of the standard-dose CT. Several studies of chest CT for lung cancer screening have shown no significant difference in nodule detection at low-dose CT performed at 10-30% of the standard tube currents [4, 6]. Cohnen et al. [23] concluded that there was no loss of diagnostic quality in CT images of the head obtained with a dose reduction of 40%. Sohaib et al. [24] obtained acceptable-quality images of the sinonasal region with a dose reduction of 75%.

Increased pitch reduces patient radiation dose by reducing the volume of irradiated tissue and scanning time. When the pitch is doubled, radiation dose is reduced by half [25]. This method is especially valuable in patients undergoing survey or follow-up examinations. However, low-dose CT using either a higher pitch or lower tube current has some potential drawbacks that include increased incidence of artifacts and, more important, the potential danger of missing findings [9, 20, 26].

Our study has several limitations, foremost of which is the modest sample size. A study involving a larger patient population with a wider variation in body weight may also have yielded different results. In addition, we reduced the tube current by one half in all patients across three different weight categories. A weight-based reduction of the tube current may have yielded different results. Our study was targeted at the efficacy of low-dose CT to show normal structures. Thus, at this point we can only show that reduced-dose CT is acceptable for evaluating anatomic structures. Its effect on diagnostic information remains to be evaluated.

In conclusion, the image quality of chest CT performed at 50% of the standard dose is acceptable for evaluating normal structures. The diagnostic accuracy of routine low-dose chest CT involving patient populations of diverse weight categories should be assessed.

References

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