MDCT and Obesity. Introduction. Incidence of Obesity. Definition of Obesity. Raul N. Uppot

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1 7 MDCT and Obesity Raul N. Uppot Introduction Although multi-detector computed tomography (MDCT) is widely available, its use is curtailed in three groups of patients: pediatric patients, pregnant patients, and the morbidly obese. Limitations on imaging pediatric and pregnant patients are self-imposed by radiologists to avoid radiation exposure to these patients. However, limitations for imaging morbidly obese patients are equipment-imposed, including the inability to accommodate the patient on imaging equipment or to obtain adequate-quality images [1, 2]. Thus, the use of MDCT to image the obese patient represents a dichotomy. If the patient fits on imaging equipment and the study is properly planned to take into account the large body habitus, MDCT can be the best and only available imaging tool for such patients. However, if the obese patient exceeds the table weight or gantry diameter limit, he or she cannot be imaged using MDCT and the remaining imaging options are few. The purpose of this chapter is to discuss: (1) the clinical definition of obesity, its increasing incidence, and the factors that impact imaging; (2) the difficulties in CT imaging obese patients and considerations in acquiring images, including proper protocols, intravenous contrast, and radiation exposure; (3) positron emission tomography (PET) PET/CT, CT colonography, and CT-guided interventional procedures in the obese; and (4) pending technologies, such as dual-source CT, which could improve image quality when imaging obese patients. Definition of Obesity Clinically, obesity is defined by body mass index (BMI), defined as a person s weight in kilograms divided by his or her height in meters squared (kg/m 2 ). The definitions of overweight, obese, and morbidly obese are based on BMI (Table 1). The quantification of fat is important, because obesity is the cause of numerous associated health problems, including hypertension, type II diabetes, heart disease, and cancer (endometrial, breast, colon) [1]. Although BMI is used clinically to quantify fat, it is not useful for the purposes of acquiring diagnostic MDCT images. Instead, in MDCT imaging of obese patients, the body dimensions, i.e., body weight and body diameter, are more important. Incidence of Obesity The prevalence of obesity has progressively increased in the USA and throughout the world. Currently, in the USA, approximately 66% of the adult population is considered overweight, obese, or morbidly obese [1]. More than 60 million American adults (> 20 years old) have a BMI > 30 kg/m 2, with 6 million individuals in this group considered morbidly obese (BMI > 40 kg/m 2 ) [1]. Obesity trend maps from the Centers for Disease Control and Prevention (CDC) show a progressive increase in the prevalence of obesity from 1995 to 2006 in all 50 states [3] (Fig. 1). The incidence of obesity has also increased throughout the world. It is estimated that worldwide, approximately 1.7 billion people are considered overweight or obese [4]. Table 1. Clinical weight classification based on body mass index (BMI) Weight classification BMI Underweight <18.5 kg/m 2 Normal weight kg/m 2 Overweight kg/m 2 Obese kg/m 2 Morbidly obese > 40 kg/m 2

2 7 MDCT and Obesity 57 Obesity Trend* Among U.S. Adults BRFSS, 1990, 1998, 2006 (*BMI 30, or about 30 lbs. overweight for 5 4 person) Source CDC Behaveral Risk Factor Survellance Sysem. Additionally, obese patients are entering health care facilities in increasing numbers due to the rise in popularity of gastric bypass or gastric banding surgery. In 2006, it is estimated that 170,000 gastric bypass procedures were performed in the USA [5]. Although not all patients who undergo laparascopic bariatric surgery require MDCT (they all need preoperative ultrasound and postoperative barium swallow), this imaging modality is valuable when the need arises to evaluate for potential postoperative complications. In addition, at many health centers, CT is used to determine total body and intraperitoneal fat. A single-slice CT image acquired at the level of the lumbar spine is used to quantify and monitor the amount of intraperitoneal fat [4]. Fitting Obese Patients on the MDCT Equipment Factors that must be considered when a CT scan is acquired from an obese patient include table weight and gantry diameter limits, and the ability of the patient to remain in the prone position and motionless during image acquisition. However, compared with the design and function of other imaging modalities, such as ultrasound, fluoroscopy, and magnetic resonance imaging (MRI), CT offers a competitive option for imaging large patients (Table 2). In addition, large-bore CT, which supports weight limits up to 390 kg (680 lbs) and accommodates gantry diameters up to 90 cm, is slowly becoming available for diagnostic imaging. Weight Limits Fig. 1. Obesity trends among adults in the USA: 1990, 1998, 2006 for patients with BMI > 30 or 30 lbs overweight for 5 4 person. (From Centers for Disease Control and Prevention. Behavioral risk factor and surveillance. Available at: cdc.gov/nccdphp/dnpa/ obesity/trend/maps. Accessed January 28, 2008) The most important factor in the ability to acquire a CT scan is a patient s body weight. If a patient s weight exceeds the CT table weight limit, the patient cannot be placed on the CT scanner and images cannot be acquired. Table-weight limits are defined by equipment manufacturers. The current industry standard table weight limit for CT is 205 kg (450 lbs).although the Table 2. Industry standard and industry maximum weight limits and aperture diameter limits for various imaging modalities Modality Standard Standard Industry maximum Industry maximum weight limit aperture diameter weight limit aperture diameter Ultrasound None None None None Fluoroscopy 160 kg (350 lbs) 45 cm 318 kg (700 lbs) 47 inches 4- to 64-slice MDCT 204 kg (450 lbs) 70 cm 309 kg (680 lbs) 90 cm Cylindrical-bore MRI 160 kg (350 lbs) 60 cm 250 kg (550 lbs) 70 cm T Vertical field MRI T 250 kg (550 lbs) 55 cm 250 kg (550 lbs) 55 cm MDCT multi-detector row computed tomography, MRI magnetic resonance imaging

3 58 R.N. Uppot CT table can physically sustain weights of more than 205 kg (450 lbs), the limitations are defined by how much weight the table motor can lift vertically and the accuracy with which the table can horizontally move the patient into the gantry. Industry standards require that the table be able to move into the gantry at a constant speed to an accuracy of 0.25 mm while supporting any weight load. Knowing both the maximum allowable CT table weight limit and the patient s weight prior to scheduling CT examinations is important to avoid disruptions. If the patient s weight exceeds the table weight limit, the CT examination should not be scheduled and the radiologists must consult with the referring physician to assess possible alternatives, including standing plain radiographs, ultrasound (which will likely be very limited in image quality but not limited by weight), or MRI [which, for some models, can support weights up to 250 kg (550 lbs)]. Gantry Diameter Limit A patient who meets the CT table weight limit must also meet the gantry diameter limit. The CT gantry diameter is defined by a fixed diameter. The current industry standard for diagnostic CT gantry diameter is 70 cm. This measurement is defined in the horizontal plane. The anteroposterior (AP) diameter of the gantry is approximately cm less due to the table taking up space within the gantry (Fig. 2). Prior to scheduling CT, if a patient s large weight suggests that his or her body diameter may approach the gantry limits, the patient s body diameter must be measured. One unique solution to determine body circumference is to use a hula hoop, with a circumference approximating the gantry diameter [7]. The hula hoop can be taken to the patient s bedside by the technologists and fitted around the patient. If the patient can fit within the hula-hoop, the CT is scheduled. If the patient cannot fit within the hula hoop, the CT examination is canceled, and the disruption incurred by bringing the patient to the department and attempting to fit him or her through the gantry is avoided. The ability to fit into the gantry is even more critical in CT-guided interventional procedures, in which both the patient and the instruments must fit through the gantry. Ability to Remain in the Prone Position and Motionless A third factor in the ability to acquire CT images or perform CT-guided interventional procedures in obese patients is the patient s ability to remain in the prone position and motionless. Although CT technology has advanced to the stage at which an entire head-to-toe CT can be acquired in less than 30 s, the ability to remain prone and motionless during this brief time interval is important. A horizontal position may be a problem in obese patients, who often have associated sleep apnea, respiratory problems, or claustrophobia. Obese patients with respiratory problems, who typically lie inclined on their hospital beds may not tolerate lying flat on a CT table. In addition, obese patients whose body diameter approaches the gantry diameter and who are claustrophobic may not tolerate entering the gantry. One advantage of the CT gantry over closed-bore MRI is that the horizontal length of the CT gantry is typically shorter than that of the MRI bore. The short length of the CT gantry allows portions of the patient (such as the head) to be positioned outside the gantry while other parts of the body are imaged. Other solutions include using a pillow or wedge to allow the patient to be inclined, or to position the patient so that only the area to be imaged passes under the gantry (i.e., feet first to avoid putting the patient s head/chest through the gantry). Acquiring Images Fig. 2. Computed tomography (CT) gantry diameter is 70 cm (black line). Movement of the CT table into the gantry will decrease its vertical diameter to 55 cm (red line). Reprinted from [1], with permission from American Journal of Roentgenology If the patient can be accommodated in the CT scanner then, of all the available imaging modalities, CT is an excellent modality to evaluate the obese patient. MDCT has a resolution as low as cm and therefore can reliably detect small pathologies in a large patient. However, the quality of the CT can vary based on the distribution of fat within the obese patient s body, and equipment factors such as kilovolt peak (kvp), milliampere

4 7 MDCT and Obesity 59 Table 3. Comparison of recommended computed tomography (CT) protocols for the 61-kg (135 lbs) vs. 91-kg ( 200 lbs) patient CT parameters 61-kg patient 91-kg patient (135 lbs) ( 200 lbs) Noise index kilovolt peak (kvp) milliampere-second Manual control Automatic (mas) Gantry rotation 1 rotation/ 0.5 s 1 rotation/1 s Pitch second (mas), pitch, noise index, and field of view (FOV). Specific CT protocols for imaging obese patients are now available and can maximize image quality in these patients (Table 3). In addition, CT artifacts specific to obese patients must be recognized and corrected as necessary. Fat Distribution The distribution of fat, i.e., subcutaneous versus intraperitoneal, is important for image quality in CT. Good-quality images can be obtained from patients with an overabundance of intraperitoneal fat due to the wide separation of central bowel loops and intraperitoneal organs (Fig. 3), which allows for clear visualization of small structures. However, lower-quality images are obtained from patients with fat predominantly in a subcutaneous distribution; this is because not only is there less separation of the intraperitoneal organs, there is also increased attenuation of the penetrating X-ray beams as they pass through the thickness of the subcutaneous fat. Noise Index/kVp/mAs Fig. 3. Axial computed tomography (CT) in 56-year-old man with extensive intra-abdominal mesenteric fat shows separation of the small-bowel mesentery and internal organs, allowing for better visualization. Reprinted from [1], with permission from American Journal of Roentgenology As with other imaging modalities, the limiting factor in obtaining diagnostic-quality images is poor penetration and increased image noise. Adjustments to the noise index, kvp, mas, and pitch can be used to improve image quality in obese patients. At our institution, one of the first adjustments for obese patients made on the CT scanner is the noise index. For patients weighing < 61 kg (135 lbs), the noise index is set at 10; for patients weighing kg ( lbs), it is set at 12.5; and for patients weighing > 91 kg (200 lbs) it is set at 15. The noise index determines the number of X-ray photons that will be used to create an image. In addition, for obese patients, the standard kvp of 120 is increased to 140 kvp to penetrate through the adipose tissue. Studies using 45-cm water phantoms have shown that a kvp of at least 140 is mandatory for photons to be able to penetrate through a morbidly obese patient [8, 9]. Although increasing the kvp increases the overall radiation dose administered, it also has the inverse effect of decreasing the amount of skin dose absorbed, as the stronger photons penetrate through the subcutaneous tissue. Another equipment adjustment to improve image quality in obese patients is an increase in the mas. Most CT scanners have the option of delivering either a fixed mas or an automatic mas. In obese patients, the distribution of fat and soft tissues varies to a greater degree than in normalsized individuals. In a head-to-toe scan, the least amount of fat is encountered in the head and chest; typically, there is more fat in the abdomen. If the CT is allowed to automatically regulate the amount of ma, the automatic mas delivered per slice section results in an overall decrease in radiation dose to areas that do not require large mas and allows for uniformity in the noise of the acquired image (Fig. 4). Another option to increase the effective mas is to decrease the gantry rotation speed. At the standard setting, images are acquired at one rotation in 0.5 s. In obese patients, slowing the gantry rotation to one rotation in 1 s has the same effect as doubling the effective mas. Decreasing the pitch from 1.1 to 0.6 also has the effect of increasing the effective mas. Field of View Standard CT equipment has an FOV of 50 cm; therefore, situations can arise in which patients are able to fit into a 70-cm gantry for imaging but still

5 60 R.N. Uppot a b Fig. 4a, b. A 39-year-old 413-lb female patient. a Axial computed tomography (CT) of the abdomen with fixed milliampere-second (mas), resulted in increased noise. Beam-hardening artifact is visualized where the patient s body exceeds the field of view (arrows). b Repeat axial CT of the abdomen with equipment setting switched to automatic mas allows the CT to increase the mas, thereby decreasing the noise. Reprinted from [1], with permission from American Journal of Roentgenology Large FOV Standard Fig. 5. Image showing gantry diameter, field of view (FOV) and extended field of view. Thin yellow circle represents standard computed tomography (CT) gantry diameter of 70 cm, which can accommodate a patient. However, the standard FOV of 50 cm (orange circle) may not cover the patient s periphery. Larger CTs with extended FOV of up to 70 cm may be able to cover the entire patient (blue checkered area) exceed the 50-cm FOV. Typically, when this occurs, what is seen is similar to a beam-hardening artifact at the peripheral edge of the image. Body parts that lie beyond the 50-cm diameter FOV are not visualized, and the CT computers display them as hyperattenuating streaks, resembling beam-hardening artifacts. Solutions to this problem include adjusting the patient s body position so that the area of interest is re-imaged and placed within the 50-cm FOV. In addition, newer large-bore CTs have an increased FOV up to 70 cm (Fig. 5). Artifacts In addition to beam-hardening type artifacts, another image-quality issue that must be considered in obese patients occurs when images are cropped

6 7 MDCT and Obesity 61 b a Fig. 6a-c. PET/CT in a 52-year-old, 177-lb female with history of carcinoid tumor. a Axial CT in a PET/CT study to look for metastasis. The CT image was cropped to focus on intra-abdominal structures. b PET portion of the study which was not cropped showed an area of FDG uptake (arrow). c Review of the un-cropped axial CT image showed a soft-tissue deposit (arrow) corresponding to the area of FDG uptake seen on PET. Reprinted from [1], with permission from American Journal of Roentgenology c by technologists to focus on internal organ structures. Such cropping in obese patients can result in the exclusion of large amounts of soft tissue and thus of pertinent information, particularly when metastatic or inflammatory processes are the focus of the evaluation (Fig. 6). Intravenous Contrast Obtaining intravenous access in obese patients can be challenging due to the excess subcutaneous tissues obscuring the deep superficial veins. Hospital inpatients will typically present with intravenous access; however, obese outpatients presenting for CT must be scheduled with sufficient time to allow adequate intravenous access to be established. Warm compresses, displacing the adipose tissues, guidance by anatomic landmarks, and the use of multiple tourniquets have been variously reported to aid in obtaining intravenous access in obese patients [10]. If peripheral intravenous access is not possible and intravenous contrast is needed, a central access via a femoral or subclavian approach may be attempted by the radiologist. The standard dose of contrast material administered in CT is weight-based (ml/kg), with the maximum dose typically being 120 ml of iodinated contrast. At our institution, the weight-based dose of Isovue 370 (370 mg iodine/ml; Bracco Diagnostics, Princeton, NJ, USA) is as follows: < 61 kg (135 lbs) = 80 ml, kg ( lbs) = 100 ml, and > 91 kg (200 lbs) = 120 ml. Obese patients (with no renal insufficiency) are typically administered the maximum 120 ml. This amount is not exceeded, as it is adequate for current scanners, which accommodate up to 205 kg (450 lbs). With the newer, larger gantry and table weight CTs, the need for increasing the weight-based contrast dose may need to be addressed. Radiation Increases in the kvp and, more importantly, the mas can incrementally increase the radiation dose

7 62 R.N. Uppot to the obese patients. The typical standard dose of 8 millisievert (msv) for chest CT and 10 msv for abdominal CT can be incrementally greater in obese patients. As noted above, increased kvp can actually decrease the skin dose absorbed as the stronger photons penetrate through the tissues. In any CT examination, the risks of radiation must be balanced with the risks of not obtaining the CT examination. Ultrasound is likely not a viable option in obese patients due to the poor image quality obtained. There are 1.5-Tesla MRIs that can accommodate patients up to 250 kg (550 lbs). PET/CT Modifications to the design of PET/CT scanners to accommodate obese patients will probably only occur after the weight and gantry modifications of MDCT scanners are successfully marketed. Currently, most PET/CTs have a standard weight limit of 205 kg (250 lbs) and gantry-diameter limit of 70 cm. In addition, the co-morbidity of diabetes in obese patient must be taken into account. All diabetics require modifications to their medications prior to a PET/CT study. CT Colonography CT colonography provides an excellent alternative to standard optical colonoscopy in obese patients. Optical colonoscopy is typically performed under conscious sedation and requires passage of a 250-cm transrectally up to the cecum. The limitations in obese patients include the risk of conscious sedation in patients who may have a compromised airway and the possibility of encountering a lengthy tortuous colon that may not be reached with standard colonoscopy. If an obese patient meets the weight- and gantry-limit criteria, CT colonography may be carried out with insufflation of gas via transrectal. Typically, images are acquired with the patient in the supine and prone positions; obese patients may not tolerate remaining prone, and a lateral decubitus position may be used instead. CT-Guided Interventional Procedures CT-guided interventional procedures are affected by the issues discussed above for diagnostic imaging in addition to other issues, including adequate positioning for the instruments to reach the target, adequate space for the instruments to clear the gantry, safe administration of conscious sedation, and the increased risk of post-procedure complications. Fig. 7. Axial computed tomography (CT) in a radiofrequency ablation procedure shows pliability of the subcutaneous fat, which allows the probe to be pushed in further to gain a length advantage (arrows). Reprinted from [2], with permission from Elsevier Adequate Positioning Properly positioning the patient for CT-guided drainage or biopsy is important, as there are limitations in the length of the instruments used for these procedures (Table 3), and some instruments may be too short due the abundance of subcutaneous or intraperitoneal fat. Pre-planning the trajectory is important to find the shortest distance along the safest pathway. Properly positioning the patient can aid in decreasing the distance to the target. Occasionally, pushing the instrument in can help compress the subcutaneous fat and allow the instrument to reach its target (Fig. 7). A second issue regarding positioning obese patients is the difficulty for one technologist alone to move the patient from the stretcher to the CT table and to safely position the patient. Several individuals may be needed to move the obese patient onto the CT table and various splints, pillows, and devices may be needed to position the patient. Clearing the Gantry Once the patient is properly positioned and before starting the procedure, it is necessary to determine whether portions of the instruments outside the patient are able to pass under the gantry for imaging. Positioning of an obese patient must account for the shortest distance to the target and for the ability of the instrument to pass under the gantry. Some equipment manufacturers have redesigned

8 7 MDCT and Obesity 63 b a Fig. 8a, b. Radiofrequency ablation procedure. a Insertion of radiofrequency probe into the liver. Note minimal clearance space for the probe and the obese patient into the computed tomography (CT) gantry. b Radiofrequency probe is flexible and can bend, allowing the probe to clear the gantry for imaging. Reprinted from [2], with permission from Elsevier their instruments to allow them to clear the gantry (Fig. 8). Conscious Sedation Administering conscious sedation to obese patients for a procedure may pose some difficulties, including adequate sedation and pain control and the risk of respiratory compromise. Medications typically used in our institution for CT-guided procedures include lidocaine, midazolam hydrochloride, and fentanyl. Lidocaine administered locally has a maximum limit of 300 mg (30 ml of 1% lidocaine) and may not achieve adequate pain control in a 180-kg (400 lbs) patient. Intravenous medications are weight-based and do not have an absolute maximum limit. They can be administered as long as cardiac and respiratory functions are monitored. In obese patients, the dose of these weight-based drugs can be very large. Post-procedure Complications Obese patients have an increased risk of post-operative and post-procedure complications due to associated co-morbidities such as diabetes. In addition, the larger body mass can strain a healing wound. New Developments Dual-Source CT The higher X-ray tube power of dual-source CT offers the potential for improved image quality in obese patients. Studies using water phantoms show that dual-source MDCT can provide more optimal image quality in large patients due to the higher power of the X-ray tube (160 kw; 80 kw from two sources) and data over-sampling from two detector arrays [11]. Conclusion The increasing prevalence of obesity and the popularity of gastric bypass surgery will continue to challenge radiologists to provide diagnostic quality imaging in the subset of obese and morbidly obese patients. If a patient can be accommodated on a CT scanner, of all the imaging modalities currently available, MDCT remains the best tool for the diagnostic evaluation of obese patients. As large-bore CTs become increasingly available, the issue of accommodating obese patients on MDCT will improve. Properly designed protocols for MDCT studies and pre-planning of CT-guided interventional procedures will allow radiologists to address MDCT-related issues in obese patients.

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