Since its introduction in the late 1980s, computed tomography (CT) has revolutionized diagnostic imaging of the chest. Single-detector CT (SDCT) units and, more recently, multidetector CT (MDCT) units have substantially increased the number of indications of CT. As a result, the number of CT examinations performed has increased dramatically, as has the average scanned volume per patient. The subsequent increase in collective radiation dose has been of concern to radiologists, medical physicists, and governmental regulatory authorities, and it has been suggested on multiple occasions that the radiation dose used for CT was excessive.1,2
The radiation dose administered to patients undergoing diagnostic radiologic examinations by CT is generally in the order of 1 to 24 mSv per examination for adults3 and 2 to 6.5 mSv for children.4 These average effective doses can be classified as low, even though they are invariably greater than those resulting from conventional radiography. Typically, a chest radiographic examination with 2 views delivers 0.08 to 0.30 mSv. In contrast, a standard-dose MDCT delivers 8 mSv, which is equal to a 100-fold risk of death by cancer according to current modeling. In other words, 1 death by cancer is expected for every 250,000 chest x-rays and every 2500 MDCT examinations.5 Most importantly, more than one-half of the collective radiation dose delivered for diagnostic imaging procedures is due to CT examinations.6 Although CT is an imaging technique that uses relatively high radiation doses, it should be noted that it has replaced other techniques—such as pulmonary angiography and bronchography—that delivered even higher doses. Nevertheless, a further step in reducing the radiation dose is needed, as CT has become the main source of the radiation delivered by medical procedures. Therefore, particular attention has to be paid to dose optimization and dose reduction, and radiologists and medical physicists should be aware of their responsibility in achieving the appropriate balance between the image quality required for diagnostic purposes and the amount of radiation dose delivered to patients.6 In the rapidly evolving field of MDCT, the quest for the highest possible image quality has often obscured possible issues regarding radiation dose.
In this article, we will first review the technical basis for the assessment of radiation delivered by CT. We will then review the interactions between the image quality, diagnostic performances, and radiation dose. Furthermore, we specifically focus on clinical advances in dose reduction in chest CT.
ASSESSMENT OF RADIATION DOSE
The substantial overall increase in radiation dose delivered by diagnostic CT has been discussed above. The issue of increased delivery of radiation is compounded by the fact that younger and thus more radiation-sensitive patients are being scanned with increasing frequency (eg, for suspected pulmonary embolism), and the trend is to recruit (per definition) asymptomatic individuals for CT screening studies. The resulting public concerns have stimulated the publication of guidelines for maximum dose levels administered by CT. The European Guideline for Quality in Computed Tomography (EUR 16262) defines such dose levels for all organs. Guidelines specifically designed for CT of the chest have been published by the Fleischner Society.7 This brief paragraph will focus on the factors that determine dose delivery in clinical chest CT and on the relationship between radiation dose and image quality.
Measurement of Radiation Dose
Several methods are currently in use to quantify the delivery of ionizing radiation to the patient. The fact that there are several methods attests to the complexity of the issue and may also present an obstacle to the understanding of radiation dose assessment. The most important parameters for radiation dose assessment, together with brief explanations, are summarized in Table 1.8
Technical Factors Influencing Dose Delivery
Dose delivery and image quality are substantially influenced by scanner technology. The following parameters are of practical importance.
To decrease the centrifugal forces of the tube during rotation, manufacturers tend to move the tube closer to the isocenter of the scanner. At fixed mAs settings, this substantially increases the patient dose, notably the skin entry dose.
Focal Spot Tracking
Slightly widening the prepatient collimation (“over-beaming”) was used in early MDCT units to compensate for subtle alterations of the focal spot size during tube rotation. Over-beaming has now been replaced by focal spot tracking that adjusts the collimator setting and is a standard feature in the latest generation scanners.
The geometric efficiency of a detector is determined by the amount of radiation that reaches the detector relative to the amount of radiation that leaves the patient. Geometric efficiency depends on the width, spatial orientation, absorption of the septa-separating detectors, and the width of the dose profile in the z-direction.
Solid-state detectors have an up to 30% higher quantum yield than older xenon gas detectors. Using solid-state detectors, the same image noise can thus be achieved with a considerably lower dose compared with xenon detectors.
The amplifiers of the detector system are responsible for a constant level of noise. The smaller the detector signal, the more important the electronic noise becomes. This is particularly noticeable in obese patients, low-dose protocols, and thin-section imaging.
Noise filtering works on the raw data and averages the signal from neighboring detector elements if the signal from these detectors drops too low. Averaging influences only a small portion of the projectional data.
Tube Current Modulation: Automatic Exposure Control
Tube current modulation, also called automatic exposure control (AEC), is based on the substantial differences in absorption between the anteroposterior and the lateral diameters of the body cross section and widely differing attenuations inherent in thoracic imaging. As attenuation follows an exponential function, small changes in diameter will cause major differences in attenuation. Different technical solutions are currently applied to modulate the mA according to the maximum and minimum patient size as determined by the scanogram. In chest CT, dose modulation allows for dose reduction of up to 30% without loss of image quality, depending on the body habitus of the individual patient. Most importantly, AEC is the only system that adequately adapts the dose to the patient's absorption, providing equalized image quality throughout the acquisition and between patients. Because AEC systems adapt the dose to the patient, reference dose levels can only be given for standard-sized patients (approximately 70 kg and 1m70 tall).
Standard Chest CT
The concept of reducing the radiation dose in chest CT was first introduced by Naidich et al,9 who reduced the tube current on incremental 10-mm collimation CT and showed that with tube current settings as low as 20 mAs, the image quality is sufficient for assessing the lung parenchyma. Although these images were sufficient for diagnostically assessing the lung parenchyma, the increased image noise resulted in marked degradation of the quality of images on mediastinal window settings. Therefore, these investigators recommended that such low-dose techniques would be most suitable for imaging children and for screening purposes. Indeed, these recommendations have been implemented and further investigated in lung cancer screening programs.10–12
Similar dose-reduction strategies have been applied to thin-section CT, in which no significant difference in lung parenchyma structures was detectable between low doses (ie, 40 mAs) and high doses (ie, 400 mAs).13,14 Although the observed differences were not statistically significant, changes in ground-glass opacity were difficult to assess at low-dose CT because of the increased image noise. Therefore, the investigators recommended that 200 mAs should be used for initial thin-section CT and lower doses (ie, 40 to 100 mAs) for follow-up examinations.
The relationship between radiation exposure and image quality at mediastinal and pulmonary window settings has been studied on conventional 10-mm collimation CT images on a single model of CT scanner with mAs settings ranging from 20 to 400 mAs.15 Although this study showed a consistent increase in image quality with radiation dose, no difference in detection of mediastinal and lung abnormalities could be found. These findings were confirmed for MDCT by Dinkel et al,16 who showed that a 90% reduction in dose down to 15 mAs (at 120 kV) compared with standard-dose techniques was not associated with impaired detection of suspicious lesions of malignant lymphoma and extrapulmonary tumors.
To investigate the effect of dose reduction without scanning patients several times at several dose levels, it is now possible to use a computer simulation of dose reduction by adding random noise to the image obtained at standard dose. In a validation trial, it has been shown that experienced chest radiologists were unable to distinguish CT images obtained with simulated reduced doses from those obtained with nonsimulated, real reduced doses.17 This technique of simulated reduced doses thus allows investigators to determine the impact of dose reduction on diagnostic performance without reduction in diagnostic performances without exposing patients to additional radiation.
CT Pulmonary Angiography
The simulated low-dose technique has been used to evaluate the effect of dose reduction on CT pulmonary angiography. A group of 21 individuals scanned at 90 mAs who showed at least 1 filling defect within a pulmonary artery was used to simulate CT pulmonary angiography with reduced radiation doses, at 60, 40, 20, and 10 mAs. This study showed that frequencies of positive and inconclusive results of the branching order of the most distal artery with a filling defect were not changed when the tube current-time product was reduced from 90 to 10 mAs. In contrast, the quality of intravascular contrast enhancement decreased when the tube current-time product setting was lower than 40 mAs. Thus, this study suggests that the reduction of the tube current-time product setting to 40 mAs to achieve a reduced radiation dose at CT pulmonary angiography seems acceptable.18
Sigal-Cinqualbre et al19 have assessed the feasibility of low kilovoltage in CT pulmonary angiography protocols and have evaluated the effect of such protocols on image quality. The investigators simultaneously reduced the tube potential and increased the mAs settings. They showed that in patients weighing less than 75 kg, 80 kV (and 135 or 180 mAs, respectively, in patients weighing <60 or 75 kg, respectively) is sufficient to obtain the same image quality as in patients weighing greater than 75 kg and scanned at 120 kV and 90 mAs. These results need to be confirmed and verified in indications other than CT pulmonary angiography. This study, however, has already suggested that reducing the tube potential could be a valid method of reducing the radiation dose and an alternative to decreasing the mAs settings. The absorption of iodine is higher at 80 kV compared with 120 or 100 kV. This may explain the good image quality as reported by Sigal-Cinqualbre et al19 during CT pulmonary angiography. Another clinical study20 showed that the use of a 100-kVp protocol permits a significant decrease in effective radiation dose compared with that of a standard 120-kVp protocol in patients suspected of having pulmonary embolism and undergoing pulmonary CT angiography. This protocol can be applied to patients weighing up to 100 kg.20
Air Trapping and Expiratory CT
By showing air trapping, expiratory thin-section CT is able to detect a disease earlier than functional tests. This makes this technique an essential part of the diagnosis of bronchiolitis of various origins. As expiratory CT is most often obtained after inspiratory CT, this additional acquisition exposes patients to a supplementary radiation dose. This is of concern in patients with bronchiolitis, because they often can be young, and despite their relatively favorable prognosis, have a high risk of recurrence resulting in repeated follow-up examinations and repeated exposure to CT radiation. To investigate the possible effect of dose reduction on the visual quantification of air trapping, we considered the “bronchiolitis obliterans syndrome” after lung transplantation as a model for bronchiolitis.21 In this model, we applied the simulated low-dose technique on expiratory thin-section CT examinations in patients with possible bronchiolitis obliterans syndrome. In 27 lung transplant recipients, expiratory thin-section CT was performed at 140 kVp and 80 mAs effective. Dose reduction corresponding to 60, 40, and 20 mAs effective was simulated. This study showed that a simulated dose equivalent to 25% of the standard dose, that is, 20 mAs, had no substantial effect on the visual quantification of air trapping. As its radiation dose approximated that of incremental thin-section CT with 10-mm section intervals performed with a standard dose, expiratory low-dose MDCT could thus be used in the assessment of air trapping in patients with suspected bronchiolitis. This model could be extended to other origins of bronchiolitis.
CT Quantification of Pulmonary Emphysema
Pulmonary emphysema is a chronic obstructive pulmonary disease (COPD) defined as a permanent distal airway enlargement with alveolar wall destruction but without fibrosis.22 Worldwide, COPD is the 6th most common cause of mortality and the 12th most common cause of morbidity.23 The severity of COPD can be, at least in part, assessed by pulmonary function tests. These tests are widely available but are unspecific. As CT yields densitometric measurements that are highly reproducible and highly correlated with morphometric measurements of alveolar wall destruction, it can be complementary to pulmonary function testing to assess the extent and/or the severity of pulmonary emphysema. As a result, this technique has been recommended in follow-up studies, particularly in the evaluation of therapeutic interventions.24–27 MDCT is of interest in the quantification of pulmonary emphysema—a heterogeneously distributed disorder—because MDCT is able to image the entire lung parenchyma. In contrast, as this technique increases the radiation dose by an additional 300% per examination compared with incremental SDCT,28 it would be important to reduce the radiation dose as patients with pulmonary emphysema can be young and may have a favorable prognosis. The level of radiation that these patients are exposed to with these examinations is compounded by repeated follow-up examinations throughout their life.
As specific drugs able to stop lung parenchyma destruction or even restore the lung growth have been elaborated and tested in animal models, it is important that individuals included in clinical trials can be imaged with the lowest possible radiation dose that provides valid measurements. We have investigated the effect of radiation dose on quantitative indexes of MDCT in pulmonary emphysema.29 In 70 patients referred for surgical resection of a lung tumor who underwent unenhanced MDCT with 4×1 mm collimation, 120 kVp, and 20 and 120 mAs effective, we compared relative areas of lung with attenuation coefficients lower than 9 thresholds and 8 percentiles of the distribution of attenuation coefficients with the pathologic extent of emphysema. This was measured macroscopically and microscopically. We observed that radiation dose does not substantially influence the strength of the correlation between relative areas (or percentiles) and pathologic references. This suggests that reducing the dose to 20 mAs effective is safe and should be recommended in CT quantification of pulmonary emphysema, especially in patients who face repeated follow-up examinations. Nevertheless, comparisons between examinations, such as in follow-up studies, require that the dose should be kept constant.
With the improved quality of CT examinations and increasing use of MDCT, small pulmonary nodules are frequently detected on CT scans. This is a substantial problem as such nodules can be small and difficult to biopsy using minimally invasive techniques. In the near future, diagnostic and therapeutic issues with small pulmonary nodules are likely to increase quantitatively because of submillimeter slice acquisition in routine MDCT examinations.
Screening projects using CT for early detection of lung cancers found pulmonary nodules in 23% to 74% of populations at risk.10,30–32 In the Mayo Clinic study, almost 70% of the volunteers had noncalcified pulmonary nodules. Only a fraction of these required further invasive follow-up, including resection of benign lesions in 8 patients.30,31 The false-positive rates in that study ranged from 92.9% for nodules larger than 4 mm in diameter to 96% for all nodules.31 By contrast, only 23% of the volunteers at baseline screening in the International Early Lung Cancer Action Project (I-ELCAP) study had noncalcified nodules that needed further evaluation.10 To minimize the number of invasive procedures required to confirm or exclude malignancy and the inherent risk for complications, Libby et al33 created an algorithm based on the ELCAP data and the medical literature from 1993 to 2003, for nodules discovered incidentally on CT. They based it upon the size, number, and density of the nodules and patient characteristics such as age, sex, smoking history, occupational history, and any previous granulomatous disease. An additional advantage of this approach is that it can be used for a wider population than that typically enrolled in CT screening programs.
Lung nodule management remains highly variable among clinicians. A recent survey showed a wide variation of nodule management among radiologists, pulmonologists, and thoracic surgeons.34 To help radiologists and clinicians to assess lung nodules, guidelines for the management of small pulmonary nodules have been published by the Fleischner Society.35
Characterization of Lung Nodule on Baseline CT
If a pulmonary nodule remains indeterminate, the next step is to perform thin-section spiral CT using 1-mm to 3-mm collimation. The length of anatomic coverage should be greater than the nodule size to compensate for slight differences in patient inspiratory volumes and thus ensure inclusion of the entire region of interest within the scanning volume.36
If the thin-selection CT identifies fat or benign calcification, no further workup is needed.37 Eccentric calcifications may occur in malignant nodules and require further evaluation. Recently, small adenocarcinomas of the lung showing ground-glass opacity on CT have been reported.38,39
Small nodules are usually monitored by means of serial CT examinations, with the aim of detecting an increase in size suggestive of malignancy. The ELCAP group10 recommends that follow-up CT be performed 3 months after initial identification of nodules between 5 and 10 mm in diameter. If no growth is detected, CT scanning should be repeated 6, 12, and 24 months later. Biopsy is indicated if growth is detected. Recent review of nodules measuring less than 5 mm on baseline CT screening in the ELCAP study40 showed that noncalcified nodules smaller than 5.0 mm in diameter do not justify immediate workup, but only annual repeat CT screening to determine whether interim growth has occurred. For nodules detected incidentally on CT scans performed outside a lung cancer screening program, the Fleischner Society has edited new guidelines.35
Researchers41 have shown that 2-dimensional (2D) CT measurements were not reliable in the evaluation of small noncalcified pulmonary nodules. They found that both intrareader and interreader agreement for 2D measurement of nodule size on CT scans was poor. The same team of researchers42 showed that 3D volumetric evaluation of nodule growth was more accurate than 2D diameter measurement. Furthermore, Yankelevitz et al,43 in a study of segmentation techniques for assessing the growth rate of pulmonary nodules in 3 dimensions, found that some malignant nodules showed asymmetric growth that was not detected using 2D techniques. The doubling time for most malignant nodules is between 30 and 400 days.44 Recently, many manufacturers have developed computer-assisted detection and 3D measurement tools that allow easy comparison of nodule size on routine CT examinations.
Nodule Enhancement at Dynamic CT
Over the past decade, there has been considerable research interest in the enhancement of indeterminate lung nodules with spiral CT. The hypothesis in most studies was that malignant lung nodules enhance substantially more than benign nodules.45,46 Swensen et al45 initially evaluated patterns of contrast uptake in a total of 163 patients with solitary nodules measuring less than 4 cm in size using an SDCT scanner. After a bolus of 100 mL of intravenous contrast medium injection, injected at a rate of 2 mL/s, 6 serial thin-section 3-mm images were obtained through the nodules at 30-second intervals up to 2 minutes, beginning 60 seconds after the onset of the injection. In each case, a representative CT number was then obtained for user-determined regions of interest to derive a measurement of peak nodule enhancement. Using this technique, the researchers found that malignant neoplasms (median, 40 HU) enhanced to a greater extent than benign lesions (median, 12 HU). Furthermore, using 20 HU as a threshold for identifying a malignant nodule, sensitivity reached 100%, specificity 77%, positive predictive value 90%, negative predictive value 100%, and accuracy 93%. More recently, this same approach has been validated in a larger multi-institutional trial.47 Using 15 HU as a threshold, CT reached a sensitivity of 98%, a specificity of 58%, a positive predictive value of 68%, a negative predictive value of 96%, and an accuracy of 77%. Another team of researchers48,49 has proposed evaluating dynamic enhancement of lung nodules with MDCT (4- and 16-detector row CT). With 30 HU or more of net enhancement as a cutoff value in the differentiation of malignant from benign nodule, Yi et al48 found a sensitivity for malignant nodules of 99%, a specificity of 54%, a positive predictive value of 71%, a negative predictive value of 97%, and an accuracy of 78%.
Infiltrative Lung Diseases
The important question regarding infiltrative lung diseases and dose is: do we scan sequential 1 to 10 mm or do we scan volumetric? With identical CT parameters, sequential CT scanning delivers one-fourth of the dose of volumetric CT only. Typically, sequential CT scanning is associated with dose-length product (DLP) values of 80 to 100 mGy.cm, whereas volumetric DLP values of volumetric MDCT are as high as 400 mGy.cm in a standard patient.50 Volumetric acquisitions have the advantage of enabling postprocessing techniques that bring an added value to axial CT analysis in assessing infiltrative lung diseases. The techniques include multiplanar reconstructions.51,52 Maximum intensity projections are suited for nodule detection or for assessing micronodular patterns such as military or tree-in-bud distribution.53–55 Minimum intensity projections are suited for assessing oligemia, emphysema, or airspace disease, and volume-rendering techniques are suited for central airways analysis.52 The additional radiation risk associated with volumetric MDCT compared with sequential MDCT may be compensated for by the added value of postprocessing techniques. However, this question still needs to be investigated and addressed prospectively.
RECOMMENDATIONS AND PROPOSALS
Automatic modulation of the tube current as a function of the patient's absorption is now available on all modern CT scanners (Fig. 1). Differences still exist between manufacturers regarding the methods used for this modulation and the dose reductions subsequently delivered. Detailed description, limitations, and results of the different AEC devices have been presented and discussed in the literature.56 The most important feature of these devices is that the radiation dose is adapted to the patient's weight and absorption. Consequently, the role of the CT user is now restricted to selecting a tube potential and an image quality appropriate to the clinical indication of the CT examination. A rational approach for selecting this image quality has been presented.57
Recommendations from regulatory authorities such as the European Union are based on a balance between the theoretical radiation risk and the medical benefit expected from the CT examination. In addition, reference values for the upper limits of doses are only based on survey studies. These values are detailed in Tables 2 and 3. The reference value approximates 14 mGy for CTDIvol and ranges between 446 and 580 mGy.cm for DLP. However, lowering these dose values is still possible and, depending on the patient's weight, CTDIvol may be lowered down to 4 to 7 mGy. Using latest generation MDCT scanners and AEC devices, it is now possible to produce CT images of very high standard quality with a dose representing less than one-half of the actual optimized CTDIvol values of 4 to 7 mGy (Figs. 2, 3). The corresponding DLP values are not higher than 120 to 150 mGy.cm in a standard patient. Furthermore, low-dose CT images, as for screening of lung nodules, can be obtained with doses that are 5 to 10 times lower than these doses that are considered as references. Low-dose DLP values can be reduced to 30 to 80 mGy.cm with the latest scanner generation, and in particular with new reconstruction algorithms using iterative reconstruction techniques preferably to filter back projection techniques (Figs. 4–7).58
Optimization Process: Step-by-step Recommendations
* Read the dose reports generated by the CT scanner for each examination you interpret to become familiar with the CTDIvol and DLP values.
* Then take the appropriate time for optimization.
* Always keep your AEC switched “on.”
* Choose a standard-sized patient for optimization.
* Choose an appropriate kV setting as proposed in Table 3.
* Use the reference values expressed in CTDIvol given in Table 3 as the objective of your optimization process.
* On GE scanners, open the mA window (reduce the lowest possible value to 20 mA, increase as high as possible the upper mA value). Before acquisition, check the mA table. mA values must vary between slices in the z axis. If they do not, AEC should be disabled.
* Reduce the index of image quality stepwise, and control the adjusted CTDIvol value on the CT screen. This index of image quality corresponds to the noise index with GE and Toshiba scanners, to the “Reference mAs” with Siemens scanners, and to the mAs with Philips scanners.
* Choose a slice thickness value slightly higher than the detector's nominal thickness, typically 1 mm when acquired with 0.5, 0.6, or 0.625 mm.
* Use smoother reconstruction algorithms if possible. Typically, with a Siemens scanner, use B20 and B60 preferably to B40 and B70.
* Use slightly thickened slices for 3D display to eliminate noise from native images.
* Compare with previous acquisitions in the same patient. Discuss with colleagues. If satisfied, process to a new optimization round. If not satisfied, contact the authors for help and support.
Even if the clinical benefit of MDCT of the chest is expected to be much higher than the potential risks from radiation, reduction and optimization of the radiation dose delivered by MDCT are highly recommended in accordance with the “as low as reasonably achievable” (ALARA) principle. As the chest is composed of organs and structures that are characterized by high differences in attenuation values with a subsequently high spontaneous contrast, it is expected that dose could be dramatically reduced. Numerous clinical scenarios have, indeed, shown that the radiation dose generated by protocols recommended by manufacturers and ranging from 8-14 mSv can be substantially decreased, resulting in protocols with doses of 0.6-2.5 mSv. This is of particular concern in patients with long life expectancy and can be achieved by AEC in conjunction with either reduced tube current-time product or reduced tube potential. Further investigations should be conducted to determine the possible benefit of combined reduction of both tube and current-time product.
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thoracic computed tomography; dose reduction; automatic exposure control; dose; recommendations© 2010 Lippincott Williams & Wilkins, Inc.