The Obese Emergency Patient: Imaging Challenges and Solutions 1

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1 Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at TRAUMA/EMERGENCY RADIOLOGY The Obese Emergency Patient: Imaging Challenges and Solutions CME FEATURE See /rg_cme.html LEARNING OBJECTIVES FOR TEST 5 After reading this article and taking the test, the reader will be able to: Explain the principles of physics that affect the quality of US and radiography in the obese emergency patient. Describe the technical challenges encountered when performing CT in the obese emergency patient. Identify the causes of artifacts commonly seen at CT in obese patients and the ways of circumventing them. Michael J. Modica, MD Kalpana M. Kanal, PhD, DABR Martin L. Gunn, MBChB, FRANZCR The dramatic rise in the prevalence of obesity among children and adults in the United States over the last several decades has brought several new challenges to the delivery of healthcare. The increased utilization of and dependence on imaging for accurate and timely diagnosis has placed the radiology department in a unique position in the provision of care for the obese emergency patient. Radiology practices must be cognizant of the imaging challenges presented by the obese patient and adjust their imaging algorithms accordingly to optimize all types of diagnostic studies. The article systematically reviews common pitfalls and offers methods to improve image quality when using radiography, ultrasonography, and computed tomography to image the obese patient population. RSNA, 2011 radiographics.rsna.org Abbreviations: BMI = body mass index, CDC = Centers for Disease Control and Prevention, FOV = field of view, SNR = signal-to-noise ratio RadioGraphics 2011; 31: Published online /rg Content Codes: 1 From the Department of Radiology, University of Washington, 325 Ninth Ave, Box , Seattle, WA Recipient of a Certificate of Merit award for an education exhibit at the 2009 RSNA Annual Meeting. Received May 20, 2010; revision requested June 30 and final revision received September 27; accepted September 28. For this CME activity, the authors, editors, and reviewers have no relevant relationships to disclose. Address correspondence to M.L.G. ( marting@u.washington.edu). RSNA, 2011

2 812 May-June 2011 radiographics.rsna.org Introduction Over the last 2 decades, there has been a rapid increase in the utilization of medical imaging procedures, especially computed tomography (CT) (1 5). This trend has been paralleled by a steady increase in the prevalence of obesity among adults in the United States (6,7). In 2007, 33.8% of adults (8) and 16.9% of children in the United States were obese (9), and 68.0% of U.S. adults were overweight (8). Radiology departments are increasingly challenged in their ability to perform imaging studies with acceptable diagnostic quality in these patients (10,11). Medical imaging device manufacturers have responded by designing new products to accommodate larger patients, and some hospitals are purchasing specialized equipment and remodeling their facilities (12). In order to adequately image the obese patient, knowledge of what equipment is available, the limitations of this equipment, imaging artifacts, and relative benefits of differing modalities in obese patients is necessary. Modifications to standard imaging techniques, or special protocols, are invaluable for optimizing the quality of imaging in obese patients. In this article, we discuss several challenges encountered in acquiring and interpreting diagnostic images specifically, radiographs, sonograms, and CT scans in obese emergency patients and provide practical solutions to improve the diagnostic quality and accuracy of imaging studies in this patient population. Epidemiology of Obesity The Centers for Disease Control and Prevention (CDC), National Institutes of Health, and World Health Organization define obesity as a body mass index (BMI) of 30 or more (13). A BMI of 40 or more represents extreme (morbid) obesity (14,15) (Table 1). The BMI is calculated by dividing the weight (in kilograms) by the square of the height (in meters) (16). Although the BMI is not the most accurate predictor of mortality, it is a standard measure used in epidemiologic studies of obese populations. In addition to the BMI, the body diameter and weight are helpful measures for deciding which obese patients should undergo imaging and which modality should be used. The prevalence of obesity among adults in the United States increased steadily after 1985, when the CDC conducted its first Behavioral Risk Factor Surveillance survey, but has stabilized in recent years (8) (Fig 1). In 2006, more than 72 million Americans aged 20 years or older were obese (17). Obesity is slightly more prevalent among Table 1 World Health Organization Classification of Normal Weight, Overweight, and Obesity BMI (kg/m 2 ) Classification Normal weight Overweight Obesity 40 Extreme ( morbid ) obesity women (35.5%) than among men (33.2%) in the United States (8). Although childhood obesity rates rose rapidly in earlier decades, they too have stabilized in the past 10 years (9) (Fig 2). Among other developed countries, the prevalence of adult obesity varies widely. For example, in Mexico it is 30.0%; in New Zealand, 26.5%; in England, 22.0%; in Australia, 21.7%; in Canada, 18.0%; in Germany, 13.6%; in France, 10.5%; and in Japan, 3.9% (20). As is to be expected with the increased prevalence of obesity in the general population, the number of obese patients requiring complex emergent medical care also has increased. Apart from a higher rate of extremity fractures, the patterns of trauma seen in obese patients do not differ greatly from those seen in patients with normal weight (21,22). The existence of a cushion effect, whereby overweight patients with a BMI of have a lower risk of internal abdominal injuries, has been hypothesized (22). However, morbidity and mortality rates among obese patients who are severely ill are greater than those among nonobese patients, with obese patients in intensive care units having a longer average stay and higher mortality (21 24). Imaging of Obese Patients Ultrasonography Ultrasonography (US) is a rapid, portable, and commonly used modality for imaging emergency patients (25 27). However, the thickness of subcutaneous fat and the sound-attenuating properties of fat present challenges (28). Sound attenuation in fat (in decibels) is defined as the product of the attenuation coefficient (in decibels per centimeter at 1 MHz), the transducer frequency (in megahertz), and the thickness of fat (in centimeters) (29,30). Sound attenuation increases with fat thickness and transducer frequency: The higher the frequency, the greater the attenuation (31). For example, the sound wave produced by a 7-MHz transducer is attenuated 50% after traveling through 1 cm of

3 RG Volume 31 Number 3 Modica et al 813 Figure 1. Maps show an increase in the prevalence of obesity in the U.S. adult population between 1990 and In 1990, no state reported an obesity prevalence of more than 14%. In 2008, only one state (Colorado) had an obesity prevalence of less than 20%. (Source. Reference 7.) Figure 2. Graph shows an increase in the prevalence of obesity among children of all ages in the United States between and However, a comparison of data from with data from shows that the rate of increase has slowed over the past decade among 6 11-year-olds (blue) and yearolds (green). Red = 2 5-year-olds. (Source. References 18, 19.) Figure 3. US evaluation of deep vascular structures in a man with a BMI of 65. (a) Color Doppler image obtained with the use of tissue harmonic imaging and penetration mode (the lowfrequency range of a multifrequency transducer) allows clear visualization of portal vein patency. (b) B-mode image shows a normal caliber of the upper abdominal aorta approximately 25 cm below the skin surface. fat. Hence, in imaging of an obese patient with 8 cm of subcutaneous fat, 94% of the original sound wave is attenuated before it reaches the peritoneal cavity (30). Using a lower-frequency transducer decreases total attenuation so that more of the primary beam can penetrate the subcutaneous fat, reaching peritoneal organs and deep vascular struc-

4 814 May-June 2011 radiographics.rsna.org Figure 4. Comparison of subcostal oblique B-mode US images obtained without (a) and with (b) tissue harmonic imaging in an obese patient shows improved depiction of hepatic and renal detail due to increased spatial resolution in b. tures (Fig 3). In obese patients, the technique known as tissue harmonic imaging may produce images of higher quality than those obtainable with conventional US (Fig 4). In tissue harmonic imaging, harmonic waves are generated within tissue, and the intensity of the ultrasound beam increases with depth, up to a point. The maximum intensity of the harmonic waves generated is directly proportional to the nonlinearity coefficient of the tissue. Fat has the highest nonlinearity coefficient, increasing the harmonic waves created and thus increasing the beam effective frequency, which improves image quality. In conventional US, by contrast, the wave intensity decreases immediately after passing the skin surface. Harmonic imaging is beneficial in obese patients because it reduces the deleterious effects of the subcutaneous tissues, yielding improved lesion visibility (32). Along with the improvement in tissue penetration, resolution is improved, and side-lobe as well as reverberation artifacts in cysts and the gallbladder are reduced (26). US can be valuable despite individual variations in fat distribution and sound attenuation. In addition to selecting the transducer with the lowest frequency, the operator should ensure that tissue harmonic imaging is enabled. Additionally, the use of compound imaging, speckle reduction filters, and some processing filters can improve the signal-to-noise ratio (SNR) (28). The ultrasound machine should be set to penetrate mode to enable greater depth penetration at a lower transducer frequency. Placing the patient in a modified lateral decubitus position displaces the fatty panniculus and aids scanning through the flank. This position decreases the thickness of the adipose tissue that the beam must penetrate to reach abdominal and retroperitoneal organs, resulting in much better image quality (33). In particular, the aorta and kidney may be scanned with a coronal or posterior oblique approach in some obese patients. Radiography Radiography is often the initial imaging modality used in emergency patients. However, in the obese emergency patient, increased patient thickness results in increased photon scatter and reduced contrast resolution (34). In addition, the higher peak tube voltage needed to penetrate excess tissue reduces image contrast, while increased exposure time increases the probability of motion artifact (35,36). The production of scatter radiation degrades subject contrast and is directly influenced by the x-ray field of view (FOV) and patient thickness (35). Scatter radiation is measured as the scatter to primary radiation (S/P) ratio (35). Typically, the S/P ratio for a 20-cm-thick, nonobese patient when an FOV of inches is used is about 3. This corresponds to three scatter photons for each primary photon (or 75% of the photons) striking the detector. For a 30-cm-thick obese patient, the S/P ratio can reach 6. This means that six of every

5 RG Volume 31 Number 3 Modica et al 815 Figure 5. Comparison of anteroposterior computed radiographs obtained with wide (a) and narrow (b) collimations in the left hip of an obese man after fracture repair shows improved depiction with reduced x-ray scatter in b. Figure 6. Comparison of radiographs of the left shoulder of an obese patient without (a) and with (b) the use of a grid. Imaging with a grid (b) improves anatomic detail by reducing x-ray scatter and increasing SNR. seven photons (or 86% of the photons) reaching the detector are scatter and carry no anatomic information. This S/P ratio degrades contrast according to the scatter degradation factor (SDF) defined by (1+ S/P) -1, or by a factor of seven (35,37). Subject contrast is defined by (DP/P) /SDF, where DP is the difference in primary x-ray intensity between the object and its surroundings and P is the intensity of the surroundings. Tight collimation of the beam to the region of interest reduces the FOV, reduces scatter, and is essential to improve image quality (38) (Fig 5). Another way to reduce scatter in obese patients is to use a grid. A typical grid consists of a series of radiopaque strips made of lead, which are separated by a series of radiolucent spacers, usually made of aluminum or fiber. Because scatter photons are not directed perpendicular to the grid, they are preferentially absorbed by the lead spacers, while most of the primary photons pass through to form the image. Typically, 85% 95% of scatter photons are absorbed, in comparison with 40% 50% of primary photons (37). An antiscatter grid with a high grid ratio (8:1 or 10:1) can dramatically reduce scatter and greatly improve image quality (39) (Fig 6). The major compromises include an increased radiation dose and increased likelihood of motion artifact because of longer exposure times (34,39).

6 816 May-June 2011 radiographics.rsna.org Table 2 Key Parameters of Current-Generation CT Scanners Used for Obese Patients Scanner Manufacturer and Model Aperture Diameter (cm) Maximum Reconstruction FOV (cm) Maximum Table Load (lbs) GE Healthcare LightSpeed VCT LightSpeed Xtra Philips Brilliance CT 64-Channel * Brilliance CT Big Bore Siemens Somatom Definition AS Somatom Sensation Open Toshiba Aquilion Aquilion Large Bore *A table with a maximum load limit of 650 lbs is an option available on this model. A table with a maximum load limit of 660 lbs is an option available on this model. Computed Tomography Scanner Limitations. With regard to imaging of obese patients, there are several physical limitations imposed by the design of CT scanners. Parameters that may vary according to the scanner manufacturer and model include the gantry aperture diameter, table load limit, reconstruction FOV, and scanning FOV (Table 2). Gantry Aperture Diameter. The gantry aperture diameters of current-generation CT scanners vary (Fig 7). To determine whether a person will fit inside a gantry, one must not only know the size of the gantry aperture but also calculate the usable portion of its anteroposterior diameter. The usable portion of the gantry aperture diameter is determined by subtracting the portion occupied by the table at its lowest position from the total anteroposterior diameter. A typical 70-cm gantry aperture diameter leaves a usable diameter of only 53 cm, although bariatric and radiation therapy scanners have gantry aperture diameters as large as 90 cm to accommodate obese patients. Table Load Limit. The table load (patient weight) limit is another key consideration in imaging of the obese emergency patient. Observation of the listed weight limit guarantees z-axis accuracy as the patient is conveyed through the scanner and helps ensure the diagnostic quality of the image. If the weight limit is exceeded, the table may bend or break, and the patient may be injured. However, since many tables are strength tested at weights of up to four times the specified limit, such occurrences are thought to be unlikely. Reconstruction FOV. The area of a reconstructed image is referred to as the reconstruction FOV or display FOV. The size of the reconstruction FOV is established to optimize the balance between spatial resolution and coverage. Most standard scanners have a reconstruction FOV of 50 cm, equal to the largest scanning FOV available for torso applications (Fig 8). In situations where the periphery of the patient lies outside the scanning FOV, extrapolation techniques may be applied to reconstruct projection data from the periphery (40).

7 RG Volume 31 Number 3 Modica et al 817 Figure 7. Diagram shows effective CT scanner diameters during scanning. The laterolateral dimension of the gantry aperture (D G ) is wider than the anteroposterior dimension (D AP ) because of the presence of the table. For the same reason, the laterolateral dimension of the scanning FOV (D SFOV ) is greater than the anteroposterior dimension (SFOV AP ), although the FOV is smaller than the aperture. Figure 8. Diagram shows the relation of the scanning FOV (dashed black circle) to the gantry aperture (red circle). The diameter of the scanning FOV is determined by the fan angle of the multiple projections from the x-ray tube and is smaller than the diameter of the gantry aperture. Unless extrapolation algorithms are used to reconstruct incomplete projection data between the scanning FOV and the gantry aperture, the reconstruction FOV cannot exceed the scanning FOV. Figure 9. Axial contrast-enhanced CT image obtained for evaluation of blunt abdominal trauma in a patient with a BMI of 60 provides poor depiction of the anterior abdominal wall. Scanning FOV. The limited scanning FOV can have critical implications in imaging of obese patients. In some situations, the periphery of the patient may lie outside the area of the reconstructed image, excluding relevant anatomy (Fig 9). To circumvent this, special positioning of the patient or region of interest in the center of the scanning FOV can help image the relevant anatomy. Positioning a patient so the anatomy of interest lies directly on the CT table will ensure that it lies within the scanning FOV. If extrapolation algorithms are available, one or more reconstructions may be performed with a wider reconstruction FOV to evaluate the anatomy outside the standard scanning FOV. So-called bariatric CT scanners support FOV extrapolation and truncation artifact correction. CT Artifacts in the Obese Patient. Truncation Artifacts. Truncation artifacts may occur when tissues lie outside the scanning FOV (41). The x-ray beam is attenuated by all tissue it passes through, even tissue outside the scanning FOV. The scanner reconstruction algorithms may assume that all attenuation from these truncated projections comes from within the scanning FOV (42). After reconstruction with filtered back projection, the periphery of obese patients

8 818 May-June 2011 radiographics.rsna.org Figure 10. Truncation artifact. (a) Axial diagram of a CT gantry containing an obese patient (dark green shaded oval at center) shows that the right flank (*) extends beyond the scanning FOV (green circle) and approaches the gantry aperture (red circle). In certain projections, the x-ray beam is attenuated by tissues that lie outside the scanning FOV, but because reconstruction algorithms are predicated on the assumption that all tissues lie within the FOV, the reconstructed images show a high-attenuation artifact adjacent to the body parts that lie outside the scanning FOV. (b) Axial CT image obtained in the pelvis of a morbidly obese patient by using a scanning FOV of 50 cm shows a truncation artifact (arrowheads) caused by x-ray attenuation in soft tissues that extended beyond the scanning FOV. (c) Axial unenhanced CT image obtained in the same patient on a scanner with a reconstruction algorithm allowing extrapolation of the projection data to a 65-cm reconstruction FOV shows an absence of truncation artifact. can appear significantly attenuated adjacent to portions that extend beyond the scanning FOV, giving a bright appearance to the edges of the image (Fig 10). Extrapolation procedures can be used during the convolution step of filtered back projection to reconstruct data that lie outside the central scanning FOV in the original source images, although the image quality in these regions will be reduced because of incomplete projection data (42). Truncation artifact correction can be used to reduce or remove truncation artifact, thus showing the periphery of the patient more clearly (40,43) (Fig 10c). Photon Starvation. Another type of artifact that occurs at CT in obese patients is photon starvation artifact. With increasing patient thickness, photon attenuation increases exponentially (36). In large patients, insufficient photon transmission to detectors may result in excessive quantum mottle (44) (Fig 11). The resultant appearance on CT images has been termed photon starvation artifact (45). Photon starvation artifact results in a reduced contrast-tonoise ratio and overall degradation in diagnostic image quality. Because the human trunk is ovoid in the axial view, attenuation is usually greatest as the beam travels horizontally, and noise is

9 RG Volume 31 Number 3 Modica et al 819 Figure 11. Photon starvation artifact. Axial unenhanced CT image obtained in a morbidly obese patient demonstrates insufficient photon transmission to the detectors, with a resultant reduced SNR. Because of patient asymmetry, the decrease in photon transmission and a resultant beam hardening artifact are greater in lateral projections (arrow). Greater image noise is visible in the flanks than in the anterior and posterior subcutaneous tissues (*). greatest in these projections. Some reconstruction algorithms amplify noise, increasing the horizontal streak artifact (45). Photon starvation artifacts are worse with the use of a low-power (eg, 60-kW) generator tube system, low peak tube voltage (eg, kvp), and fast rotation time (eg, second) (46). The severity of these artifacts can be reduced by using an adaptive filtration (47) or iterative reconstruction technique, increasing the gantry rotation time (eg, from 0.5 to 1 second), reducing the pitch to 1 or less, increasing the reconstructed section thickness, and increasing the peak tube voltage to kvp for all CT studies except CT angiography (44). Modern CT scanner generators have power ratings that range from 60 to 100 kw. The dual-source CT scanner has two 80-kW generators, providing the maximum tube output available (48). Higher generator output and x-ray tube heat capacity, coupled with efficient detector materials and electronics, allow the use of a more rapid gantry rotation time without producing excessive image noise (49). Optimizing CT for the Obese Patient. To minimize common CT artifacts and obtain diagnostic images of high quality in obese patients, adjustments may have to be made in the scanning techniques and reconstruction methods used. The next five sections describe changes that may be appropriate in some settings. Increase the Tube Current and Rotation Time. Increase the number of photons by increasing the tube current and rotation time, and decrease the pitch. Doubling of the tube current rotation time product (milliampere-seconds) results in an increase of 41% in the SNR, greatly improving contrast resolution. With a fixed pitch and tube current, slower gantry rotation increases contrast resolution. Increase the Peak Tube Voltage. The percentage of energy fluence transmitted through a patient varies greatly, depending on the patient s BMI and the peak tube voltage selected at the scanner. Smaller adults (BMI <30) should be scanned with 100 or 120 kvp to optimize iodine-induced attenuation while reducing the radiation dose (50). For nonangiographic CT studies in morbidly obese patients (BMI 40), the use of 140 kvp should be considered so as to improve the contrast-to-noise ratio in soft tissue. The use of 120 kvp should be considered for CT angiography in most obese patients. Increase the Reconstructed Section Thickness. An increase in the reconstructed section thickness increases the number of photons used to produce each image. With peak tube voltage, tube current, and rotation time remaining fixed, doubling of the section thickness increases the SNR by 41%, improving contrast resolution (Fig 12a, 12b). However, as the thickness of reconstructed sections increases, partial volume averaging artifact becomes more prominent (Fig 12c). Partial volume averaging artifact may be reduced by reconstructing overlapping sections and utilizing coronal and sagittal reformations (51).

10 820 May-June 2011 radiographics.rsna.org Figure 12. (a, b) Comparison of axial pulmonary CT angiograms obtained with section thicknesses of mm (a) and 5 mm (b) in a morbidly obese patient shows a significant increase in the SNR in b. (c) Axial pulmonary CT angiogram obtained with a greater section thickness (10 mm) in the same patient shows a significant partial volume averaging artifact that simulates a filling defect in the right pulmonary artery (arrow). New Reconstruction Technologies. Use of iterative reconstruction instead of filtered back projection results in less noisy images (52) (Fig 13). Unfortunately, iterative reconstruction of large datasets obtained on multidetector scanners takes a long time, even with current high levels of computing power. Techniques such as adaptive statistical iterative reconstruction, which blends an iterative reconstruction technique with filtered back projection, are less computationally intensive. Manufacturers now allow users to specify relative proportions of each technique to be used (adaptive statistical iterative reconstruction vs filtered back projection) so as to more closely control the appearance of reconstructed images (53). These techniques may produce less noisy images when photon starvation artifacts are present (Fig 13). Bundling. When obese patients lie flat, subcutaneous fat and large breasts fall to the side. This causes a very asymmetric profile, leading to beam hardening and photon starvation artifacts in lateral CT projections (Fig 14a). Bundling involves wrapping the patient in sheets or commercial devices before scanning. This reduces artifacts by providing a symmetric profile of excess soft tissues (Fig 14b). Radiation Dose in the Obese Patient The effects of obesity on radiation exposure have been little studied. The available data from studies performed in phantoms indicate that obese patients receive higher radiation doses at CT and radiography than nonobese patients do (54,55). The use of automatic exposure control results in a higher tube current rotation time product in larger patients, which sometimes leads to a marked increase in radiation exposure. However, the dose deposited in the abdominal organs does not increase linearly with increasing tube current and rotation time in obese patients, because much of the additional radiation is absorbed by the excess adipose tissue (56). Although there is a significant increase in organ dose, the results of phantom studies have shown a greater percentage of radiation dose deposited in the skin than in the abdominal organs (54,57). This is due to greater photon attenuation by the skin and soft tissues. The skin entrance dose to the breasts, testes, and thyroid gland, in particular, is likely to be significantly higher.

11 RG Volume 31 Number 3 Modica et al 821 Figure 13. Comparison of axial contrast-enhanced CT images of the upper abdomen, obtained with a pure filtered back projection (a) and a 40% adaptive statistical iterative reconstruction combined with a filtered back projection (b), shows decreased noise in b, particularly in the liver (arrow) of this obese patient. Photon starvation and truncation artifacts are visible in both images. Figure 14. (a) Axial contrast-enhanced CT image of the upper chest shows an asymmetric profile and a high level of image noise, with a truncation artifact (arrow) caused by extension of the breast tissue outside the scanning FOV. (b) Axial contrast-enhanced CT image obtained with the patient wearing a bra to make the chest contour more symmetric shows that the entire circumference of the patient is within the scanning FOV, helping reduce image noise and eliminate the truncation artifact. Radiologists perceive higher image quality in abdominal CT scans when patient diameter increases despite constant image noise (58). This perception, which likely is due to increased fat outlining the abdominal organs, suggests that radiologists will accept noisier CT images in obese patients. However, lesion detectability in obese patients with increased image noise, and therefore a lower contrast-to-noise ratio, has not been evaluated. Nevertheless, the use of specific protocols that include automatic exposure control is prudent when scanning obese patients. These protocols should be designed to avoid an excessive increase in the tube current rotation time product with increasing patient size, which could result in very large radiation doses (54,56,57). Summary The prevalence of obesity among children and adults in the United States has increased dramatically over the past several decades. Radiologists, radiologic technologists, and imaging departments must be prepared to face the challenges of examining the increasing numbers of obese patients, especially in emergent settings. For each modality and each imaging system, various steps can be taken to optimize the imaging of obese patients so as to achieve a diagnostic quality study at the lowest dose possible. These steps include optimizing scanning parameters, using antiscatter

12 822 May-June 2011 radiographics.rsna.org grids with tight collimation, bundling patients, and using iterative reconstruction and artifact reduction techniques. Manufacturers are now designing systems that will facilitate high-quality imaging in obese patients, and healthcare institutions should consider purchasing such equipment and reconfiguring emergency departments to accommodate the increasing numbers of these patients. References 1. Mettler FA Jr, Thomadsen BR, Bhargavan M, et al. Medical radiation exposure in the U.S. in 2006: preliminary results. Health Phys 2008;95(5): Broder J, Warshauer DM. Increasing utilization of computed tomography in the adult emergency department, Emerg Radiol 2006;13(1): Weir ID, Drescher F, Cousin D, et al. Trends in use and yield of chest computed tomography with angiography for diagnosis of pulmonary embolism in a Connecticut hospital emergency department. Conn Med 2010;74(1): Coursey CA, Nelson RC, Patel MB, et al. Making the diagnosis of acute appendicitis: do more preoperative CT scans mean fewer negative appendectomies? a 10-year study. Radiology 2010;254(2): Levin DC, Rao VM, Maitino AJ, Parker L, Sunshine JH. Comparative increases in utilization rates of ultrasound examinations among radiologists, cardiologists, and other physicians from 1993 to J Am Coll Radiol 2004;1(8): Centers for Disease Control and Prevention. National health and nutrition examination survey ( and ). Atlanta, Ga: Centers for Disease Control and Prevention, Centers for Disease Control and Prevention. US obesity trends: trends by state Web site of the Centers for Disease Control and Prevention. Accessed February 17, Flegal KM, Carroll MD, Ogden CL, Curtin LR. Prevalence and trends in obesity among US adults, JAMA 2010;303(3): Ogden CL, Carroll MD, Curtin LR, Lamb MM, Flegal KM. Prevalence of high body mass index in US children and adolescents, JAMA 2010;303(3): Uppot RN, Sahani DV, Hahn PF, Kalra MK, Saini SS, Mueller PR. Effect of obesity on image quality: fifteen-year longitudinal study for evaluation of dictated radiology reports. Radiology 2006;240(2): Ginde AA, Foianini A, Renner DM, Valley M, Camargo CA Jr. The challenge of CT and MRI imaging of obese individuals who present to the emergency department: a national survey. Obesity (Silver Spring) 2008;16(11): Diconsiglio J. Hospitals equip to meet the bariatric challenge: rising number of obese patients necessitates specific supplies. Mater Manag Health Care 2006;15(4): Expert Panel on the Identification, Evaluation, and Treatment of Overweight in Adults. Clinical guidelines on the identification, evaluation, and treatment of overweight and obesity in adults: executive summary. Am J Clin Nutr 1998;68(4): Flegal KM, Carroll MD, Ogden CL, Johnson CL. Prevalence and trends in obesity among US adults, JAMA 2002;288(14): National Task Force on the Prevention and Treatment of Obesity. Medical care for obese patients: advice for health care professionals. Am Fam Physician 2002;65(1): Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. J Chronic Dis 1972;25(6): Ogden CL, Carroll MD, McDowell MA, Flegal KM. Obesity among adults in the United States: no change since Hyattsville, Md: National Center for Health Statistics, Ogden CL, Fryar CD, Carroll MD, Flegal KM. Mean body weight, height, and body mass index: United States, Advance data from vital and health statistics, no 347. Hyattsville, Md: National Center for Health Statistics, McDowell MA, Fryar CD, Ogden CL, Flegal KM. Anthropometric reference data for children and adults: United States, National health statistics reports, no 10. Hyattsville, Md: National Center for Health Statistics, Organisation for Economic Co-operation and Development. OECD health data Paris, France: Organisation for Economic Co-operation and Development, Byrnes MC, McDaniel MD, Moore MB, Helmer SD, Smith RS. The effect of obesity on outcomes among injured patients. J Trauma 2005;58(2): Arbabi S, Wahl WL, Hemmila MR, Kohoyda-Inglis C, Taheri PA, Wang SC. The cushion effect. J Trauma 2003;54(6): Bercault N, Boulain T, Kuteifan K, Wolf M, Runge I, Fleury JC. Obesity-related excess mortality rate in an adult intensive care unit: a risk-adjusted matched cohort study. Crit Care Med 2004;32(4): Diaz JJ Jr, Norris PR, Collier BR, et al. Morbid obesity is not a risk factor for mortality in critically ill trauma patients. J Trauma 2009;66(1): Lingawi SS, Buckley AR. Focused abdominal US in patients with trauma. Radiology 2000;217(2): Dolich MO, McKenney MG, Varela JE, Compton RP, McKenney KL, Cohn SM. 2,576 ultrasounds for blunt abdominal trauma. J Trauma 2001;50(1):

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14 Teaching Points May-June Issue 2011 The Obese Emergency Patient: Imaging Challenges and Solutions Michael J. Modica, MD Kalpana M. Kanal, PhD, DABR Martin L. Gunn, MBChB, FRANZCR RadioGraphics 2011; 31: Published online /rg Content Codes: Page 814 (Figure on page 814) In obese patients, the technique known as tissue harmonic imaging may produce images of higher quality than those obtainable with conventional US (Fig 4). Page 815 (Figure on page 815) Tight collimation of the beam to the region of interest reduces the FOV, reduces scatter, and is essential to improve image quality (38) (Fig 5). Another way to reduce scatter in obese patients is to use a grid. Page 815 (Figure on page 815) An antiscatter grid with a high grid ratio (8:1 or 10:1) can dramatically reduce scatter and greatly improve image quality (39) (Fig 6). Page 817 (Figure on page 817) The limited scanning FOV can have critical implications in imaging of obese patients. In some situations, the periphery of the patient may lie outside the area of the reconstructed image, excluding relevant anatomy (Fig 9). To circumvent this, special positioning of the patient or region of interest in the center of the scanning FOV can help image the relevant anatomy. Positioning a patient so the anatomy of interest lies directly on the CT table will ensure that it lies within the scanning FOV. If extrapolation algorithms are available, one or more reconstructions may be performed with a wider reconstruction FOV to evaluate the anatomy outside the standard scanning FOV. Page 819 Photon starvation artifacts are worse with the use of a low-power (eg, 60-kW) generator tube system, low peak tube voltage (eg, kvp), and fast rotation time (eg, second) (46). The severity of these artifacts can be reduced by using an adaptive filtration (47) or iterative reconstruction technique, increasing the gantry rotation time (eg, from 0.5 to 1 second), reducing the pitch to 1 or less, increasing the reconstructed section thickness, and increasing the peak tube voltage to kvp for all CT studies except CT angiography (44).