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A comparative study of thoracic radiation doses from 64-slice cardiac CT

Department of Radiology, Columbia University and New York Presbyterian Hospital, 177 Fort Washington Avenue, Milstein Bldg Room 3-265B, New York, NY 10032-3784, USA

Correspondence: Professor Edward Lee Nickoloff, DSc, Columbia University, 177 Ft. Washington Avenue, Millstein Hospital Bldg Rm 3-265B, New York, NY 10032-3784, USA. E-mail: eln1{at}columbia.edu

	Abstract

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
The goal of this study was to measure radiation doses for 64-slice cardiac CT angiography studies and to study the dose-savings features of these CT scanners. This was done using various phantoms. These radiation doses were compared with those from typical helical body CT scans, fluoroscopy cardiac catheterization studies and mammography examinations. Radiation measurements were made with a CT ionization detector and a solid state dosimeter. A GE 64-slice Lightspeed VCT and a Siemens Somatom Sensation 64 CT were used to scan a standard 32 cm acrylic phantom and an anthropomorphic phantom. Data were collected in axial and various gated cardiac helical modes. Organ doses and the effective doses were calculated from the measurements. In gated CT cardiac mode with the 32 cm acrylic phantom, the measured radiation doses per study were generally three to seven times greater than those from typical body helical CT examinations; the range depended upon selectable scan parameters. With the anatomical phantom, the surface doses in the anteroposterior (AP) plane were typically 20–60% higher than those measured using the 32 cm phantom. The lateral surface doses were –4% to +15%. These results can be attributed to the shorter AP dimension and the air in the lungs. The CT skin entrance radiation doses were 80–90% less than diagnostic cardiac catheterization studies, and organ doses were similar. Because 64-slice cardiac gated CT uses pitches equal to 0.20–0.27 and high mAs values, the patient radiation doses are appreciably higher than in routine body CT examinations. The female breast, which could receive a radiation dose 10–30 times that received from mammography screening, is an organ of particular concern.

	Introduction

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With the advent of multidetector CT (MDCT), fast rotation times and electrocardiogram (ECG) gating, CT scanners have been able to be used to study cardiac function and image cardiac structures [1–3]. Currently, CT units are capable of simultaneously obtaining 64 thin slices ( 0.6 mm) in a single X-ray tube rotation of 0.3–0.35 s. Modern CT X-ray tubes use tube currents up to 500–800 mA in order to reduce the quantum mottle associated with such thin slices [4, 5]. By using ECG gating with helical (spiral) pitch values of 0.20–0.27, 64-slice CT scanners are capable of producing high quality images of the coronary arteries [6, 7].

However, the high mAs and small pitch values of coronary CT angiography (CTA) studies are likely to result in much higher radiation doses to the patient than are typically obtained from body CT scans [8]. Therefore, it is important to evaluate the potential radiation doses to patients from 64-slice cardiac CT studies and to compare these values with radiation doses received from other types of thoracic imaging studies. It is relevant to compare cardiac CT angiography radiation doses with doses from typical helical body CT examinations of the thorax, fluoroscopic diagnostic cardiac catheterization studies and other routine thoracic X-ray studies. Of particular concern is the radiation dose to the female breasts, lungs and oesophagus/thymus because these organs are sensitive to radiation-induced carcinogenesis [9, 10].

A related concern is the standard 32 cm cylindrical acrylic phantom used by medical physicists to assess body radiation doses to patients [11–13] (Figure 1). This acrylic phantom is used as a standard for the measurement of body CT radiation doses by the American College of Radiology (ACR), the American Association of Physicists in Medicine (AAPM) and many manufacturers of CT scanners. However, this phantom may not accurately represent the attenuation provided by the body of a typical patient, especially in the thoracic region. The human thorax contains bones in the ribs/sternum/spine and air in the lungs. Moreover, the human thorax is elliptical in shape; it is not cylindrical and homogeneous like the standard 32 cm acrylic phantoms routinely used to measure body radiation doses. Although the acrylic phantoms are not anatomically realistic, these phantoms do provide a convenient quality control (QC) tool for the measurement and comparison of CT radiation doses. However, it would be useful to determine the relative differences in CT radiation doses measured with the acrylic phantoms compared with measurements with more realistic anthropomorphic phantoms (Figure 2).

View larger version (145K): [in this window] [in a new window]   Figure 1. The phantom in this CT image is a 32 cm acrylic cylinder used by medical physicists to simulate the human body for radiation dose measurement. The image shows the ionization chamber at the top of the phantom, and solid state radiation detectors are on the surface.

View larger version (94K): [in this window] [in a new window]   Figure 2. This CT image shows the slice of an anthropomorphic phantom used to measure radiation dose in order to compare with the value of the 32 cm acrylic cylinder. The solid state detectors are shown on the surface of this phantom.

	Methods and materials

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
Two different modern 64-slice cardiac CT scanners were evaluated in this study: a GE Lightspeed 64 VCT and a Siemens Somatom Sensation 64 CT. Each scanner has numerous imaging modes by which clinical thoracic images can be gathered. Some of these modes have special features intended to reduce the radiation doses to the patient (e.g. mA modulation, different X-ray beam filters and special ECG gating techniques). In this study, an assortment of the various cardiac ECG-gated CT protocols was investigated under comparable conditions in order to ascertain the range of patient doses (maximum to minimum) that might be expected during clinical studies. The goal of the study was not to compare the two different CT scanners with one another directly, nor was the intent of the study to determine the impact of various scanning protocols on the image quality of coronary CTA. Our goal was to understand relative patient radiation doses from cardiac CT examinations in comparison with other types of thoracic imaging and to understand the relative radiation dose reductions that could be achieved from using dose-saving equipment features available on 64-slice CT scanners.

Standard cardiac collimation and slice thickness values were employed for the measurements. For the GE CT scanner, the cardiac collimation was 40 mm in z-axis length; for the Siemens CT scanner, the collimation was 20 mm in z-axis length. The CT slice thicknesses were about 0.6 mm for both CT scanners.

CT radiation doses were measured using both a MDH^TM 1015C ionization detector system and an Unfors PSD^TM solid state system with three separate detectors.

Both units have a relatively flat energy response (±10%). Three or more measurements were made for each scan mode, and the readings were averaged. For both the ionization chamber and the solid state detectors, one standard deviation (SD) of the readings represented less than ±5% of the mean value. Measurements for both detector systems are given in mGy based upon an "f-factor" of 8.7 mGy R^–1. Radiation dose measurements were made using the approach outlined in the ACR CT Accreditation Program and the standard 32 cm diameter acrylic phantom (which is a uniform cylinder with holes at various positions for the radiation detector) [14–16]. The radiation doses were measured at the periphery (CTDI_p) and at the centre of the phantom (CTDI_c) and expressed as a weighted computed tomography dose index (CTDI_W).

Radiation doses were also measured at the surface of the anthropomorphic phantom, which contained preserved animal lungs, human bones and plastic simulating human tissue (Alderson Laboratories chest/lung phantom); the solid state dosimeters were used for these measurements. Both standard axial and helical body scan techniques and various ECG-gated clinical cardiac angiography scan protocols were used for the radiation dose measurements.

Organ doses for the cardiac CT studies were determined using the radiation measurements and the IMPACT computer program [17]. The IMPACT software uses a CTDI value measured in air at the centre of the CT gantry as input data; however, the program also calculates conventional CTDI_w, which can be compared with standard acrylic phantom measurements. The effective dose was calculated by both the ACR methodology and the IMPACT software; the results from the two methods were averaged to obtain the reported effective dose [14–17]. The ACR methodology divides the measured CTDI_w by the CT scanner pitch in order to obtain the CTDI_vol. The CTDI_vol is then multiplied by the scan length to obtain a dose–length product (DLP). For head scan, the ACR method uses a typical scan length of 17.5 cm; and for abdomen CT calculations, a typical scan length of 25 cm is used to calculate the DLP. For cardiac CTA, a length of 15 cm was used. The DLP values are then multiplied by conversion factors to obtain the effective doses. The ACR methodology uses a conversion factor of 0.0023 mSv/(mGy cm) for head CT scans and a conversion factor of 0.015 mSv/(mGy cm) for abdomen CT scans. The appropriate conversion factor for the chest is 0.017 mSv/(mGy cm).

Breast radiation doses from screening mammography were obtained both from the literature and from the regular MQSA measurements at our facility on various film screen and digital units using the methods described in the ACR Mammography Accreditation Program [18]. As the radiation dose from mammography varies throughout the breast because of exponential attenuation, the breast data were given as the average glandular dose. Because of the high kVp values and greater filtration used in CT, CT breast radiation doses are relatively uniform.

The radiation doses for fluoroscopic cardiac catheterization studies were measured on a new Siemens dBC Axiom Artis^TM flat panel cardiac unit with 16, 20 and 25 cm fields of view (FOV). This unit uses various X-ray beam filters ranging from minimal aluminium filtration up to 0.9 mm of copper. Various fluoroscopic and digital record frame rates are available, but the common pulse/frame rate for the modes is typically 15 pulses s^–1. This is the pulse rate used for exposures made in this study. For diagnostic coronary angiography, typical fluoroscopy times are 4–5 min, and times to acquire images are about 0.7–1.0 min [19]. Accordingly, these exposure times were used in the current study.

Sheets of acrylic plastic ranging in thickness from 15 to 30 cm were used to simulate patient tissue attenuation during coronary angiography. The acrylic plastic was merely used to obtain the range of entrance radiation exposures that would represent variations in patient sizes. The effective dose calculations were based upon Monte Carlo simulations that incorporated inhomogeneities in the patient anatomy and the entrance radiation exposures.

The unit was operated in normal automatic technique selection mode. The entrance radiation dose at the plastic was measured with the same two detector systems described previously using an appropriate ionization chamber for the MDH^TM dosimeter. The organ doses and effective dose for the fluoroscopic cardiac catheterization examinations were then calculated from the measured data using the FDA Handbook for Fluoroscopic and Cineangiographic Examinations [20–21]. Because of variations in patient thickness, various FOV and equipment geometry factors encountered clinically, the data presented in the results section show a range of expected patient entrance doses.

Following the various measurements and calculations, the radiation dose data were compared. The calculations yielded the relative dose from fluoroscopic cardiac catheterization compared with ECG-gated cardiac CT angiography. The relative dose reduction from employing special scan features was also determined. Breast radiation doses from cardiac CT were compared with breast doses from mammography and diagnostic cardiac catheterization studies.

	Results

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
For standard axial CT scans at 120 kVp without overlap and using the 32 cm acrylic phantom, the measured CTDI_W for the two different 64-slice units was essentially the same and ranged between 7 and 9 mGy per 100 mAs. Body CT scans are usually performed at 150–200 mAs with a helical (spiral) pitch of 1.5. For these mAs settings, the range of expected CTDI_vol, which is equal to [(CTDI_W x mAs)/(100 mAs x pitch)], varies between 7 and 12 mGy.

For ECG-gated cardiac CT angiography studies, higher mAs and lower pitch values are typically used. These cardiac scans typically employ 300–800 mA, rotation times of 0.3–0.35 s and a helical (spiral) pitch of 0.20–0.27. For these settings (with the 32 cm acrylic phantom), the measured CTDI_vol was in the range 24–65 mGy. The higher value corresponds to radiation doses for the 32 cm acrylic phantom at 200 mAs without the dose-saving features. The lowest value corresponds to values that result from using patient dose-saving equipment features such as mA modulation. To use mA modulation with the 32 cm uniform acrylic phantom, an ECG pulse generator is used, which produces signals at a rate of 60 beats per minute (bpm). The CT scanner divides the "R–R" interval between pulses into a portion that it assumes is diastole and a portion that it assumes is systole. During diastole, the CT scanner tube current is modulated to the maximum value. During the systolic portion of the interval, the CT scanner tube current is reduced to a small percentage (around 20%) of the maximum value. Thus, the tube current and radiation dose are varied during an acquisition process even though an inanimate object is being imaged. Some of the variation in values is also attributable to differences between the two manufacturers' designs. These data are summarized in Table 1.

To determine the average glandular breast dose for routine screening mammography, the ACR approach was used for the measurements [18]. The actual values varied with breast tissue composition, compressed breast thickness, kVp employed, image receptor type, image processing conditions and the X-ray tube target/filter combination used. The values quoted in this paper assumed a 50% adipose composition with a compressed breast thickness of about 4.2 cm. The average glandular dose to a breast from a two-view mammography study of each breast would be expected to deliver about 3.0–5.0 mGy to each breast, which agrees closely with our measurements [22, 23].

For a diagnostic coronary angiography, the data were based on fluoroscopy times of 5 min and 1 min of digital record mode using a modern flat panel cardiac catheterization room. Pulsed fluoroscopy was employed at 15 pulses per second (pps), and the record frame rate was 15 frames per second (fps). For a thickness of 25 cm of acrylic, the entrance surface dose ranged from 370 to 690 mGy. For a 30 cm thickness, the measured doses ranged from 960 to 1470 mGy. The range of values also results from changes in the image receptor FOV and geometry, which are varied during clinical studies. The data demonstrate that the total patient entrance doses for diagnostic cardiac catheterization can range from about 400 mGy (average patient size with 20 cm FOV) to 1470 mGy (large patient with 16 cm FOV).

From the measured diagnostic cardiac angiography dose rates, typical patient organ doses can be calculated using a Food and Drug Administration (FDA) publication (Table 3) [20]. The highest radiation dose occurs to the skin where the X-ray beam enters the patient. Although some X-ray tube movement may occur during the study, the majority of the radiation is delivered to the lateral and posterior portion of the patient. Because the breasts are located anteriorly during the study, the breast organ doses are relatively low (7–13 mGy). The other organs of concern are the heart and lungs; the heart is relatively insensitive to carcinogenic effects. The lung radiation dose in these studies would be expected to be in the range 35–65 mGy. Table 3 also provides the equivalent doses to 12 organs that received the highest doses during a typical cardiac angiography study. The effective dose is obtained by multiplying each individual organ dose by its appropriate tissue weighting factor, W_T, which expresses the risk factor [27–32]. A comparison of effective doses from various thoracic procedures evaluated in this article is shown in Table 4.

	Discussion

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
The use of cardiac CT is growing rapidly, and it is likely to continue to increase [24, 25]. Thus, it is important that those who prescribe and deliver these studies are fully aware of the associated radiation exposure issues. It has been shown recently that radiologists are often poorly informed about radiation doses from CT, and, not surprisingly, patients are even less well informed [26]. Given the carcinogenic potential of radiation to lung and to the female breast, knowledge of radiation doses to organs in the thorax is especially important.

In routine body scan modes, the radiation doses can be similar to 16-slice MDCT scans. However, when the units are used to do helical (spiral) cardiac CTA with pitch values of 0.20–0.27, the CTDI_vol values are three to seven times greater than routine body scans that use a pitch value of 1.5.

In comparison with screening mammography examinations, cardiac CTA examinations can deliver 10–30 times more radiation to the female breast. As shown in Table 3, the organ dose to the breast from cardiac CTA studies would be expected to be 30–100 mGy. The effective dose values in Table 4 were calculated from the organ doses using tissue weighting factors listed in International Commission on Radiation Protection (ICRP) 60 [27–29]. For the breast, the ICRP 60 tissue weighting factor is 0.05; however, the proposed new ICRP value (suggested in 2005) for the breast tissue weighting factor is even higher at 0.08. The cancer risk factor used is 0.06 Sv^–1 [28–30]. Other publications suggest using an even higher radiation-induced cancer risk factor for the breast of 0.009–0.018 Sv^–1 depending on the age of the female patient [30, 31]. Based on these factors, the cancer risk for the irradiation of the breast during cardiac CTA scans would be between 1 x 10^–4 and 1.8 x 10^–3. These values correspond to a potential cancer risk of between 1 case per 600 and 1 case per 10 000 studies. Fortunately, most female cardiac patients would be expected to be older, which significantly reduces the breast cancer risks [30–32]. Nevertheless, female patients should be made aware of these potential risks.

In comparison with diagnostic fluoroscopic cardiac catheterization examinations, cardiac CTA has similar lung and oesophagus/thymus radiation doses. The effective doses for the two different procedures are also in the same range. Each has associated potential cancer risk in the range 4.8 x 10^–4 to 1.6 x 10^–3. Again, the cancer risks are considerably less for older patients. The overall cancer risks for both cardiac CTA and cardiac catheterization are approximately equal. However, the maximum skin entrance radiation doses are significantly less with CTA than with fluoroscopic cardiac angiography.

There are a number of uncertainties associated with such cancer risk estimates.

Monte Carlo calculations for a standard size person are used to determine organ doses from surface or air dose measurements. Variations in X-ray beam quality, geometry, patient size and tissue composition can lead to inaccuracies in the organ dose measurements. The tissue weighting factors used to calculate effective doses are for dual gender phantoms and are different in the various references. Cancer risk factors are dependent on gender and age of exposure; usually, the cancer risk factor for a 45 year old is quoted. Regardless, it is still instructive to provide estimates of both effective dose and potential cancer risks when assessing new technology.

Cardiac CTA can be acquired using several different scan techniques, which have effects on both the image quality and the patient radiation dose. One method is called ECG-gated acquisition (or "step and shoot"). It uses axial CT images and simultaneously stores the patient ECG information. The image data are post-processed after the scan to reconstruct only the diastolic portion of the scan. Another approach is ECG-gated helical scans, which can be done in two ways. The X-ray beam can be on continuously during the scan or the scan can be divided into sectors with mA modulation. A third approach is helical scanning with ECG modulation of maximum mA. The tube current is pulsed to a higher mA during diastole and a lower value during systole. Modulation of the mA in this fashion reduces the patient radiation dose. A fourth method performs two-dimensional localizer scans prior to the cardiac scans to determine the body attenuation of the X-rays along the z-axis. ECG-gated helical CT scans of the heart can then be performed with attenuation-based mAs modulation.

Patient radiation doses also depend on several other factors. The tube current (mA) can be manually adjusted to lower settings in order to reduce patient radiation doses. However, adjustment to a lower mAs will produce increased CT noise in the images. A clinical decision has to be made about the tolerable amount of CT noise that is acceptable. CT scans with lower mAs settings may use different reconstruction algorithms and/or thicker CT slices to reduce CT noise in the images. Some CT units have different X-ray beam filters that can be used; changes in the filtration yield different patient radiation doses. Additional factors affecting patient radiation doses are patient size, heart rate and the amount of ectopic beats encountered during the study. With the provision that the image quality is not significantly degraded, the various scan options provide flexibility for cardiac CTA. However, because of these different factors and imaging modes, the patient radiation doses have a range of values rather than a single quantity. The data from the current study (Table 1) confirm the dose reduction potential of a number of these scanning options, but do not address the critical issue of how these adjustments affect image quality, i.e. the reliability of coronary CTA obtained using dose reduction strategies. The results of the current study suggest that a clinical trial to investigate the impact of dose reduction on image reliability is warranted. Other dose reduction strategies, such as breast shields, have been used to reduce doses to the female breast from thin slice chest CT [33, 34]. If such approaches could be used without reducing the accuracy of coronary CTA, their usage should also be considered and studied.

The current study also showed that the radiation doses typically measured with a 32 cm acrylic cylinder do not accurately represent true patient radiation doses in the thorax. Acrylic has a greater physical density than human tissue and is uniform. The thorax has air in the lungs, and its geometry resembles an ellipse rather than a circle. In conjunction with the shorter AP dimensions in the chest, the CT radiation doses in patients can be 20–60% greater at the female breast than a typical ACR CT body dose measurement might indicate. This should be considered when evaluating radiation values obtained from studies that used such phantoms.

	Conclusions

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
Technological advances in CT imaging have opened an impressive array of diagnostic tools to physicians. The new 64-slice CT scanners performing cardiac angiography deliver significant radiation doses to the patient. In comparison with screening mammography examinations, the female breast dose from these studies is about 10–30 times greater. In comparison with a routine helical (spiral) body CT study, the radiation doses are three to seven times greater. The CT skin entrance radiation doses were 80–90% less than diagnostic cardiac catheterization studies, and organ doses were similar. For both cardiac CTA and cardiac angiography, potential risks from the relatively high radiation doses to the lungs are another major concern. It is important that the professionals who order and perform these CT examinations take prudent steps to minimize radiation exposures. Patients – especially women – should be informed about the radiation doses and receive advice about relevant risk to benefit ratios. This will allow 64-slice CT technology to be applied in the safest and most effective way possible.

	Acknowledgments

 
The authors would like to acknowledge the assistance with some of the measurements for this study provided by engineers and staff of both GE Healthcare and Siemens Medical Systems. We also wish to thank Ms Rosa Hicks for the typing of the final manuscript.

Received for publication May 1, 2006. Revision received September 18, 2006. Accepted for publication October 6, 2006.

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Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 

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