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Journal of Thoracic Imaging:
doi: 10.1097/RTI.0b013e3181c227b2
Symposium

CT Evaluation of Diffuse Infiltrative Lung Disease: Dose Considerations and Optimal Technique

Mayo, John R. MD

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University of British Columbia, Vancouver General Hospital, Canada

Reprints: John R. Mayo, MD, Professor of Radiology and Cardiology, University of British Columbia, Director of Advanced Cardiac Imaging, Vancouver General Hospital, Canada (e-mail: John.Mayo@vch.ca).

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Abstract

This review provides a comprehensive outline of radiation issues in the high-resolution computed tomography (HRCT) scans obtained for the diagnosis and follow-up of diffuse lung disease. Specifically, the evidence indicating risk from the radiation at the level used in HRCT, recommended dose levels for HRCT, radiation dose reduction strategies, and suggested protocols are discussed.

The introduction of computed tomography (CT) in the 1970s represented a major advance in the diagnostic imaging of the chest. CT provides cross-sectional or slice images compared with the projection images of the plain radiograph. The cross-sectional nature of the image eliminates the line integral limitation of the plain projection radiograph, improving the contrast resolution of the CT image.1 In addition, the geometry of the CT acquisition decreases the contamination of the image by scattered x-ray photons. These improvements in contrast resolution and the cross-sectional viewpoint provide improved detection and characterization of diffuse lung diseases.

CT contains acquisition and reconstruction parameters that optimize the resulting images for either maximum contrast or maximum spatial resolution. The natural high contrast of the air soft tissue matrix of the lung allows CT to be optimized for high-spatial resolution, a technique known as high-resolution CT (HRCT).2 HRCT has been shown to be highly accurate for the diagnosis of diffuse lung diseases3 and has become the primary imaging modality for this clinical indication.

However, using CT as the primary imaging modality for diffuse lung diseases comes with a price, substantially increased radiation dose to the patient in comparison with the plain radiograph. As the CT image is of high quality, image degradation caused by a limited number of detected x-ray beam photons is easily seen. HRCT images with too few image photons contain excessive image noise, which affects the accuracy of the CT numbers or Hounsfield units. To minimize these noise effects, CT images are obtained at relatively high radiation doses compared with plain radiographic images. This, combined with the wide availability and use of CT for medical diagnosis, accounts for the fact that approximately 60% of the medical radiation doses in developed countries can be ascribed to CT examinations.4,5

Excessive image noise caused by too few image x-ray photons decreases accuracy of disease measures based on CT numbers (eg, emphysema, airway wall measurements). The use of longitudinal HRCT scans to measure disease progression requires accurate, low noise images and the standardization of acquisition and reconstruction parameters (eg, patient centering, kVp, mA, scan time, tube current modulation, reconstruction algorithm, and field of view).6 In addition, there may be differences in CT number values between different makes and models of CT scanners, even when acquisitions are performed identically. A careful attention to these technical details is critical if CT is used as a surrogate measure of disease activity, disease progression, or treatment response.

In the absence of standardized validated protocols, it has been shown that radiologists often obtain CT images using excessively high radiation exposure levels4,7 with no detectable difference in patient outcomes. Radiologists, referring clinicians and patients may be unaware of the high level of radiation exposure associated with these examinations.8 A lack of knowledge regarding these radiation dose issues contributes to the overuse of chest CT in low-yield diagnostic situations and in follow-up examinations that have no or minimal impact on patient management.

The purpose of this review is to outline (a) evidence indicating the detrimental effect of radiation dose at the level administered in HRCT examinations, (b) parameters that affect CT radiation dose, (c) advances in dose reduction in HRCT, and (d) suggested clinical protocols for the examination of patients with suspected diffuse interstitial lung disease.

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CLINICAL UTILITY

Multidetector CT (MDCT) scanners can image the entire chest in 2 to 5 seconds, acquiring respiratory motion-free scans in majority of patients. Multidetector HRCT chest protocols produce up to 500 images, each built of submillimeter volume elements (voxels) providing exquisite multiplanar images of the lungs. These high-quality large field of view images provide noninvasive evaluation of the lung parenchyma, airways, and vessels similar to that obtained at gross pathologic examination. Expert radiologic interpretation of these images can differentiate interstitial lung diseases that show structural changes but are indistinguishable at clinical history and physical examination. CT images acquired during the administration of intravenous contrast media or during breathing maneuvers (eg, end expiration) can show regional lung perfusion, gas transport function, or dynamic changes in the airways. Accurate noninvasive measurements of lung density, air volume, and lung mass can be made using standardized and calibrated acquisition protocols.9 Therefore, CT can provide noninvasive measures of disease severity, disease progression, and treatment effects.

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POPULATION DOSE CONTRIBUTION OF CT

It is estimated that in 2006 more than 62 million CT scans were performed in the United States, approximately a 20-fold increase compared with the 3 million studies carried out in 1980.10 A similar increase in utilization has been reported in the United Kingdom7 and Canada.4 In diffuse lung disease, adding CT to the diagnostic imaging algorithm substantially increases both diagnostic accuracy and patient x-ray radiation exposure. For example, a chest CT examination (3 to 6 mSv) delivers 60 to 120 times more radiation dose compared with a posteroanterior chest radiograph acquired using film (approximately 0.05 mSv) or 150 to 300 times that of the same view obtained using digital radiography (approximately 0.02 mSv). In the setting of diffuse lung disease, the increased diagnostic power of chest CT usually justifies the associated increased radiation dose.

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EVIDENCE FOR HARM SECONDARY TO CT RADIATION DOSE

Exposure to ionizing radiation causes 2 major effects in humans. The deterministic effects are seen in days to weeks after very high levels of exposures and the stochastic effects are seen years later (6 to 25 y) after low levels of exposure. The deterministic effects, skin erythema, skin necrosis, and hair loss occur only above a threshold dose, lying well above those deliverable using diagnostic chest CT parameters. In medical imaging, deterministic effects are generally seen only when long duration, high-intensity fluoroscopy has been carried out (eg, complex interventional radiology). There is no debate regarding the link between high-level radiation exposure and these deterministic effects.

By comparison, there is an extensive debate regarding the link between low-dose radiation exposure and stochastic effects.11,12 Mechanistically, stochastic effects are believed to be mediated by chemical damage to the DNA molecule and manifest as an increased risk of cancer and genetic defects. Stochastic effects occur randomly and the risk of their occurrence depends on the type of ionizing radiation administered, the tissue receiving the radiation, and the age of the subject at the time of irradiation. The majority opinion regarding stochastic effects holds that there is no radiation dose threshold. Therefore, there is a risk associated with any level of exposure, including the low radiation dose delivered during chest CT. It is believed that dose fractionation, a substantial modifier of detrimental effect for deterministic radiation doses, does not substantially modify the stochastic risk.13 Stochastic risks are believed to be cumulative with increasing risk over successive exposures. Therefore, repetitive CT imaging is associated with increasing stochastic risk.14

The atomic bomb explosions at Hiroshima and Nagasaki in 1945 are the primary source of data linking low-level radiation exposure to stochastic cancer risk. This group of irradiated individuals is unique because it is large, covers all ages, was not selected on the basis of the underlying disease, and has been extensively studied over the last 65 years. A substantial portion of the 30,000 survivors received less than 50 mSv, an exposure that approximates the cumulative dose commonly seen in chest CT subjects. The major negative effect seen in the low-level exposure group is an increase in the cancer rate, above that seen in a nonradiated population. An earlier presentation of cancers has not been noted. However, calculating the risk from a single chest CT scan requires extrapolation of these data to even lower doses and the nature of this extrapolation has proven controversial.

Disagreement regarding the low-dose extrapolation of nuclear explosion data is based on 3 nonresolvable issues: uncertainty in the actual radiation exposure received as onsite radiation dose measurements were not obtained, differences in the natural cancer risk of the Japanese population compared with the other populations, and the different quality of the radiation imparted by atomic bombs compared with x-ray-based medical imaging. As a result of the differences in the interpretation of the data, learned societies have come to varying conclusions on the risk of low-level radiation exposure. The International Commission on Radiological Protection, used a linear no threshold extrapolation of nuclear explosion data and an estimated 50 additional fatal cancers induced per million people exposed to 1 mSv of medical radiation.15 In contrast, the French Academy of Science concluded that there was not sufficient evidence to support an increased cancer risk associated with radiation exposures less than 20 mSv,16 a level above that delivered in a single chest CT examination (<9 mSv). Further conflicting evidence on the impact of low-level radiation exposure is found in tissue culture experimental studies, showing biochemical evidence supporting induction of free radical detoxification mechanisms after low-level radiation exposure.17 This has led some to suggest that low-level radiation exposure may be beneficial, an effect known as radiation hormesis. Finally, the long-term study of British radiologists found lower cancer mortality than predicted by the atomic bomb data.18 Rationale for these results has focused on 3 issues: the healthy worker effect, the beneficial effects of dose fractionation, and overestimation of the dose received by these physicians.

In 2007, additional data were added to this debate19 when the 15-country study reported on the frequency of cancer in 407,000 radiation workers followed for more than 20 years (5.2 million person-years follow-up). This study is unique as it reports on the largest cohort to date, has accurate dosimetry, and investigated the multiethnic workers. Ninety percent of the subjects received a dose less than 50 mSv and on average each worker received a dose of 19 mSv. Therefore, this study is focused on low-level doses, close to that received during a single chest CT examination (3 to 9 mSv). The authors reported an excess relative risk for all cause mortality of 0.42 per Sievert (0.00042/mSv) and a statistically significant increasing excess relative risk with increasing radiation dose (P<0.02) (dose effect). The increased risk in all cause mortality was mainly because of an increase in mortality from all cancers.

A subanalysis stratified by dose categories (less than 400, 200, 150, and 100 mSv) showed that cancers in the highest dose categories did not drive the risk estimates. Therefore, this study supports the concept that there is a small cancer risk from low-dose radiation delivered in CT examinations. These new data add supportive evidence to the concern over radiation dose delivered in chest CT examinations.

However, there are limitations to these new data. As workers were studied, there is no information on the radiation in children. As 90% of the workers were men and they received over 98% of the cumulative dose, there are insignificant data regarding women. The largest excess mortality from all contributing countries is found in the data from Canada and statistical significance is lost if this cohort is not included. The largest discrepancy between this study and the atomic bomb cohort arises in the lung cancer mortality, suggesting that the confounding effects of smoking may not have been adequately assessed. Finally, the mean energy of the radiation received by workers in the atomic industries is different from that delivered in diagnostic radiology, which may limit the generalization of these data to the radiology exposed population.

The influence of age at exposure and sex has been studied in the nuclear explosion cohort, showing that radiation risk is substantially modified by these subject factors.20,21 The increased radiation sensitivity of children is felt to arise from 2 biologic facts: they have more time to express the cancer-inducing effect of radiation and have more rapidly dividing cells than adults that are inherently more radiation sensitive.

It has been found that women have approximately twice the risk compared with men for the same level of radiation exposure. Increased female risk is heightened in chest CT by the presence of radiosensitive breast tissue in the radiated field. Radiation dose to breast tissue in chest CT examinations has been calculated22 and directly measured23,24 with reports showing wide variation in average values, ranging from 10 to 70 mGy. The variation in values is related to CT parameter settings, differences in size and configuration of breast tissue, and methods to calculate or directly measure radiation dose. There is no debate that all CT-associated breast radiation dose values are substantially greater than the average glandular dose of 3 mGy for standard 2-view screening mammography. It is important to note that there is a strong age at exposure effect for breast tissue, with lower risk for individuals25 above the age of 40 years. These factors must be taken into account in setting chest CT radiation dose parameters in CT chest examinations for younger women (<45 y). Breast shields, thyroid shields,26,27 and x-ray tube current modulation techniques have been used to decrease radiation doses to these superficial and radiosensitive tissues within the chest. These techniques have been shown to decrease breast radiation exposure delivered in chest CT scans. However, these dose-modifying techniques must be used with consideration of their impact on image quality.

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A SIMPLE MEASURE OF RADIATION DOSE

Effective dose is a widely used simple measure of CT radiation exposure designed to estimate the whole body dose that would be required to produce the same stochastic risk as the partial body dose that was actually delivered in medical imaging (eg, chest CT). Effective dose is calculated by summing the absorbed doses to individual organs weighted for their radiation sensitivity.15 The measurement unit is the sievert or milli-sievert. As effective dose requires the determination of absorbed dose to each body organ multiplied by their radiation sensitivity, the distribution of radiation dose in the body must be determined. Chest CT has a markedly asymmetric dose distribution, with higher dose found peripherally and lower dose centrally on account of the shielding effects of body tissue. This makes it difficult to calculate the exact effective dose for each patient. Instead, a simpler calculation is made (Fig. 1). Scanner manufacturers use dose data derived from measurements made in the head and body phantoms to determine a weighted CT dose index (CTDI) for each CT scanner model at all available selections of tube voltage (kVp), tube current (mA), and rotation time. The selected pitch value is then incorporated to produce a CT dose index called the CTDIVOL. Once the scan length is determined from the topogram, the appropriate CTDIVOL is combined with the actual length scanned in the patient to calculate the dose length product (DLP). As the administered radiation dose is linearly related to the length scanned in the patient, technologists should ensure that the scanned volume is confined to the region of interest to avoid excessive radiation dose.

Figure 1
Figure 1
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The DLP is a measure of the radiation dose delivered to that patient during the scan. An estimated effective dose for the specified CT scan can be calculated by multiplying the DLP value by the normalized effective dose coefficient for the scanned body part28 (chest, 0.017 mSv/mGy/cm). This normalized effective dose coefficient accounts for the radiation sensitivity of the body region scanned. The DLP value is displayed on the scanner console once the topogram has been obtained and the scan prescribed. In chest CT, multiplying the DLP by 0.017 allows the radiologist or technologist to calculate the estimated effective dose of the examination before scan acquisition. The DLP value can be archived in the picture archiving and communication system by storing the protocol page.

It is noted that effective dose, although easy to calculate and convenient, is an imperfect dose descriptor. As tissue-weighting factors are averaged over sex and age, effective dose risk assessment is appropriate to a 30-year-old hermaphrodite. An alternative approach has been described29 that uses measured or calculated organ radiation doses, applies them to age-specific and sex-specific organ risk estimates (from the BEIR 7 report) and calculates an effective risk from the examination. Effective risk would attempt to estimate the risk of developing cancer from the partial body irradiation of the examination. It would not consider hereditary effects that are currently embodied in the effective dose calculation. Effective risk could be adjusted for age and sex. Although this is a new approach requiring further evaluation, it has the potential to improve communication of the risk from CT radiation exposure to patients and physicians. Radiation dose surveys have noted wide variations in DLP settings for identical examinations between institutions.4,7,30,31 To decrease this variation and protect the public from inadvertent overexposure, the European Community has published suggested reference dose values32 for chest CT examinations, with a DLP value of 650 mGy/cm. This reference dose value is based on the 75th percentile value for DLP reported in a survey of a large number of European institutions. This recommended DLP serves as a guide to acceptable practice when examining patients of normal size [body mass index (BMI) 18 to 25]. However, the DLP value would need to be reduced in those patients with a BMI less than 18 and increased to provide acceptable image quality in those patients with a BMI greater than 25.

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CT ACQUISITION PARAMETERS

CT scan acquisition parameters (kVp, mA, rotation time, pitch, helical scan, step, and shoot) can be varied providing a multitude of trade-offs with respect to the scan duration, radiation dose, and image quality. Once the acquisition protocol is chosen and the scanning is done, acquisition parameters and associated radiation dose cannot be modified. Millions of projection attenuation measurements are obtained for every 360-degree rotation of the x-ray tube, forming the raw scan data set. The raw scan data are then converted into cross-sectional images using a reconstruction algorithm that has associated reconstruction parameters (slice thickness, slice interval, reconstruction kernel, and field of view). These reconstruction parameters affect the noise and spatial resolution of the image. In contrast to acquisition parameters, so long as the raw scan data are available, images with varying noise, contrast resolution, and spatial resolution can be created by the use of different reconstruction parameters. These images can maximize spatial resolution at the price of increased image noise (lung reconstruction) or minimize image noise at the cost of reduced spatial resolution (mediastinal reconstruction). In general, there are more reconstruction options (slice thickness, slice interval) available with helical acquisitions than with step and shoot acquisitions.

In the step and shoot acquisition technique, the patient remains stationary while a slab of x-ray attenuation measurements are obtained. For contiguous images, the table is then moved along the width of the detector array and the cycle is repeated. If gapped slices are obtained, the table is moved further than the detector width. This acquisition technique provides images free of table motion but the chest is scanned slowly. To cover the entire chest, the patient must either sustain a long breath hold or images must be obtained over a number of breath holds. In the latter case, differences in the level of inspiration between breaths can result in regions being scanned twice or not at all. As discrete slices are obtained, overlapping slices with improved spatial resolution cannot be generated without another scan and associated radiation dose. Gapped slices can be acquired that leave regions of the chest nonirradiated. Gapped slices substantially reduce the radiation dose but provide no information in the gaps. Gapped imaging has most commonly been applied to diffuse interstitial lung disease, in which sampling, only a portion of the lung, does not severely limit diagnostic accuracy. However, the discontinuous views of vessels, nodules, and airways can lead to interpretation errors (Fig. 2).33

Figure 2
Figure 2
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In helical acquisitions, the x-ray tube is energized while the patient moves continuously. A continuous helix of x-ray measurements is obtained that allows images to be taken at any position without additional radiation exposure, improving spatial resolution. By adjusting the length of interpolation along the helical band (z-axis), images of variable thickness can be produced. There is a minor degradation of image quality by the table motion during the image acquisition. Helical acquisitions also suffer from wasted radiation dose equal to the width of the detector array at the top and the bottom of the scanned volume, as 360 degrees of raw scan data are required to reconstruct slices. This effect, known as overrange artifact, is most substantial in helical CT scanners using wide detector arrays that are scanning small subjects (eg, infants, children). Linking the motion of the collimator to that of the table can eliminate this wasted radiation effect.

Cardiac and respiratory-gated acquisitions can be obtained in both step and shoot and also in helical acquisition modes.34 Gated acquisitions reduce motion artifact but increase the scan time and to a variable extent, increase radiation dose. At this time, these modes are not widely used in diffuse interstitial lung disease chest CT scans.

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RADIATION DOSE REDUCTION

In general, reduction in CT radiation dose results in increased image noise and decreased image quality. Studies assessing the subjective evaluation of chest CT scans have shown that holding reconstruction parameters constant, radiologists consistently gave higher image quality scores to images obtained with a higher radiation dose.35,36 Image noise can be measured by placing a region of interest (>100 pixels) in an area of uniform density (eg, the thoracic aorta). The standard deviation of the pixel values represents the image noise and is a measure of the image noise. The reconstruction algorithm used affects image noise with higher noise associated with high-spatial-frequency reconstruction algorithms (eg, bone or lung algorithms) compared with low-spatial-frequency algorithms (eg, standard, soft tissue algorithm). As high-spatial-frequency reconstruction algorithms are most commonly used to assess the high-contrast air soft tissue lung parenchyma, increased image noise is usually not a diagnostic problem. However, increased noise may interfere with the quantitative measures such as emphysema scores and airway wall measurements. New adaptive reconstruction techniques37 are being developed, which modify the raw scan data and use iterative methods for image reconstruction. Images obtained using these techniques show reduced image noise with no decrease in spatial resolution. These images have the potential to provide improved image quality at lower radiation dose, facilitating radiation dose reduction.

Radiation dose can be adjusted at the time of image acquisition by changing the x-ray tube current or voltage. In practice, the tube current is most frequently adjusted to change the radiation dose and image noise. In most CT scanners, the tube current is adjustable in steps from 20 mA to approximately 400 mA. Decreasing the tube voltage also decreases the radiation dose, but also changes subject contrast and to a small extent CT number values. Tube voltage reduction from 120 kVp to 100 or 80 kVp in patients weighing less than 80 or 60 kg, respectively, is increasingly being used to reduce dose in small body habitus.38,39 These reduced tube voltage images also have an improved match between the mean energy of the x-ray beam and the k edge of iodine, improving subject contrast in contrast-enhanced chest CT. It is noted that the radiation exposure delivered at a given tube current and voltage will vary between different models and manufacturers of CT scanners because of the differences in scanner geometry (x-ray tube-to-patient separation) and x-ray tube filtration. Therefore, the transfer of parameters from one CT scanner to another will not result in identical radiation doses or image quality.

In the past, the tube current used in CT was unchanging through out the scan. However, the chest is an elliptical object that has higher attenuation from left to right than from anterior to posterior. Attenuation also varies as the chest is scanned cranial to caudal because of the attenuation of the shoulders and upper abdominal soft tissues. It has been shown that the CT image quality is disproportionately degraded by views with too few photons (photon starvation) compared with the image quality improvement associated with views with high photon counts. To address this issue, manufacturers have introduced programs that adjust the tube current depending on the attenuation of the object in both the transverse (x, y) and longitudinal (z) directions to minimize either photon starved or photon-rich projections, maximizing image quality while minimizing radiation dose. This tube current modulation technique has been shown to produce a substantial reduction in radiation dose40–42 with a minimal impact on the image quality. The routine use of dose modulation systems is recommended as they compensate for asymmetry in the size and density of the body section being scanned, resulting in a signal-to-noise ratio that is adequate for diagnosis but is not excessive.43 Advanced tube current modulation schemes with novel reconstruction algorithms are being developed to reduce radiation dose to superficial radiation-sensitive tissues such as the breast and thyroid. However, dose modulation systems may produce variations in image noise that can interfere with measures of emphysema extent. Therefore, these systems need to be used with caution in longitudinal studies of emphysema. In addition, dose modulation systems may interact with the patient position and x-ray beam filters, potentially increasing radiation dose in off centered patients. Algorithms to automatically center patients within the gantry have been developed44 to address this issue. Finally, repeated scanning of the same region increases the radiation dose in a linear fashion. Therefore, the timing of the follow-up examinations involves a trade-off between additional information and the cumulative effects of the radiation administered.

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DIFFUSE LUNG DISEASE CT SCAN PROTOCOLS

All CT scan protocols reflect a balance between radiation exposure and diagnostic yield. The greatest radiation dose reduction is achieved by eliminating unnecessary examinations. These can be either diagnostic examinations that are not indicated by clinical history, physical examination, or risk factor analysis or follow-up examinations conducted at too short time intervals. Referral guidelines (eg, Royal College of Radiologists,45 American College of Radiology,46 and Canadian Association of Radiologists47) have been developed to assist clinicians in ordering the correct radiology investigation at the correct time. It is acknowledged that the decision to not perform CT may be very difficult. It is rational to more aggressively limit CT examinations in children and young adults compared with older individuals, on account of their greater radiation sensitivity. Once the decision has been made to obtain a CT examination the acquisition protocol and the associated radiation dose must be optimized to the patient size while using all available dose-reduction tools.

Diffuse lung disease can be evaluated using HRCT protocols using either the traditional gapped step and shoot48 or MDCT contiguous helical33 acquisition techniques. In both acquisition techniques, diagnostic accuracy is optimized using HRCT parameters of narrow section collimation reconstructed with high-spatial resolution algorithms [eg, GE Bone (GE Healthcare, Milwaukee, Wisconsin), Philips D (Philips Healthcare, Eindhoven, Netherlands), Siemens B45 or 60 (Siemens Medical Solutions, Erlangen, Germany), Toshiba FC51 (Toshiba Medical Systems Corporation, Otawara, Japan)]. Experience has shown that targeting the reconstruction volume to a single lung to improve spatial resolution, as was recommended earlier, is not commonly required. Using single detector row scanners, gapped thin section images were obtained as the slow acquisition speed made it impractical to acquire contiguous thin section images throughout the chest. These 1-mm to 2-mm thick sections obtained every 10 or 20 mm sampled 5% to 20% of the lung tissue (Table 1). Owing to the diffuse nature of interstitial lung disease, this discontinuous sample provided a reasonably accurate diagnosis of diffuse lung diseases3 with the additional benefit of substantially reduced radiation dose. However, the discontinuous sampling led to substantial problems in the assessment of focal lung abnormalities including bronchiectasis, air trapping, mucous plugging, nodules, and abnormal vascular structures (Fig. 2).

Table 1
Table 1
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The development of multidetector helical CT scanners in the late 1990s allowed the entire chest to be scanned using thin section collimation in a single breath hold. A suggested diffuse interstitial lung disease helical protocol for a MDCT scanner is presented in Table 2. Using the earlier described radiation dose modulation systems and size-based kVp modifications, acceptable image quality can be obtained using DLP values of 300 to 400 mGy/cm for standard dose and 100 to 200 mGy/cm for reduced dose HRCT protocols. If expiratory scans are required for the assessment of regional air trapping, these can be acquired using low-dose techniques to minimize radiation dose build-up effects. These volumetric HRCT data sets give contiguous thin section images of the entire chest with essentially isotropic voxels. Using isotropic voxels the volume can be viewed in any orientation (transverse, sagittal, coronal, and off axis) without the loss of spatial resolution. High-resolution volumetric images can also be created (Fig. 2C), which may improve communication with surgical and medical colleagues. Reformatted and volumetric image manipulations improve both diagnostic accuracy and reading speed through improved detection of abnormalities and recognition of disease distribution. Maximum intensity projection and minimum intensity projection sliding slab reformations are also useful, highlighting nodules,49 emphysema, and regional air trapping. Additional scan using physiologic maneuvers can be added to improve diagnostic sensitivity to; air trapping by scanning in expiration, airway collapse by performing dynamic scanning at a single level during a breathing maneuver. Finally, the location of dependent atelectasis can be modified by scanning in the prone position. This can aid in separating dependent atelectasis from interstitial fibrosis in the posterior basal segments of the lower lobes.

Table 2
Table 2
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In a repeated measures experimental design comparing gapped high-resolution single section CT images with MDCT high-resolution images, MDCT showed improved accuracy for vessels, bronchiectasis, air trapping, and lung nodules.33 This study also found better interobserver agreement and better correlation with spirometric measurements using MDCT.

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SUMMARY

Current helical multidetector row CT scanners can provide contiguous, HRCT images of the chest that allow confident noninvasive diagnosis of many diffuse lung diseases while delivering acceptable radiation exposure. Attention to the justification of diagnostic and follow-up examinations and the use of all available dose-reduction tools will optimize the benefit-to-risk ratio. Given the complexity of current CT scanner acquisition protocols, dose optimization is most effective using the combined efforts of radiologists, medical physicists, and radiology technologists.

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Keywords:

medical physics; radiation dose; high-resolution CT; dose reduction; diffuse lung disease

© 2009 Lippincott Williams & Wilkins, Inc.

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