Bariatric Procedures in CT
Objectives
After reading this article you should be able to:
- Define the term Bariatrics
- Explain the process of contrast media selection
- Explain the process of technical factor selection
- Understand radiation exposure concerns
Introduction
Obesity is a disease that has reached epidemic proportions in the United States and around the world. Statistics show the percentage of adults aged 20 and over with obesity was 41.9% during the period of 2017-March to 2020. We should also note that during the same time, the percentage of adolescents aged 12-19 years with obesity was about 22.2%. Despite recent evidence that obesity rates are beginning to plateau, current projections estimate that 42%–51% of the U.S. population will be obese by 2030. During the past 2 decades, bariatric surgery has become an increasingly popular form of treatment for morbid obesity.
Contrast Media Considerations
Two specific imaging modalities/examinations are used to evaluate the success of bariatric surgeries- fluoroscopic upper gastrointestinal (UGI) and abdominal CT scans. Fluoroscopic UGI examinations with water-soluble contrast agents and abdominal CT are useful imaging tests for detecting leaks after a Roux-en-Y gastric bypass. “Upper GI barium studies are better for detecting anastomotic strictures, whereas CT optimizes detection of small bowel obstructions, internal hernias, and intussusceptions.” -(Imaging of Bariatric Surgery – RSNA Publications Online).
Both fluoroscopic UGI examinations with water-soluble contrast agents and CT scans are useful imaging tests for detecting leaks after sleeve gastrectomy. Barium studies are also useful for showing strictures or gastric outlet obstruction as a complication of this surgery. -(Imaging of Bariatric Surgery – RSNA Publications Online). Commercially available contrast materials in the United States have iodine concentrations of as much as 370 mg of iodine per milliliter, and administration rates as high as 6 mL/sec may be appropriate for imaging of certain bariatric patients. Contrast-enhanced studies in bariatric patients require intravenous access suitable for high rates of injection, precise scan timing, and a high mass of iodine delivered to the target parenchymal organ.
Contrast-enhanced CT scans can be divided into two broad categories: those requiring vascular enhancement and those in which a high degree of parenchymal enhancement is desired. Two main patient-related factors affect vascular enhancement- the cardiac output and the intravascular volume. Parenchymal enhancement depends primarily on the total mass of iodine delivered to the target parenchymal organ, along with the effect of a secondary dependence on the rate of injection. The dynamics of contrast material in bariatric patients are unique because of the increased body weight, correlating directly with increased intravascular and interstitial volume. If high doses of contrast material cannot be tolerated by the bariatric patient because of renal insufficiency, scanning at lower tube voltages may be appropriate in select cases, maximizing-edging the K-edge effect of iodine and increasing the attenuation value while using a lower volume of contrast material.
The diagram shows a laparoscopic adjustable gastric band surrounding the proximal stomach, producing a small gastric pouch above the band. When saline is introduced into the band via subcutaneous port, luminal narrowing causes early satiety and weight loss. (Reprinted, with permission, from reference 51.) Radiology Vol. 270, No. 2: 315-634
Normal appearance of Roux-en-Y gastric bypass at CT. Axial CT image after oral and intravenous contrast material administration shows a small gastric pouch (P) separated by a staple line from the excluded stomach (ES) laterally. Jejunal Roux limb (J) is anastomosed to the gastric pouch anteriorly. Note oral contrast material opacifying pouch and Roux limb, whereas excluded stomach is not opacified. [LINK]
Roux-en-Y gastric bypass with a postoperative anastomotic leak at CT after oral but not intravenous contrast material administration. Axial CT image shows extravasated contrast material in peri splenic space (L), indicating a postoperative leak. [LINK]
Technical Factor Considerations
One way of improving vascular and parenchymal contrast enhancement without increasing the dose of administered contrast material is to scan at a lower tube voltage. Using voltages of 100 kVp or less results in higher CT attenuation of contrast material because of the photon energies being closer to the K-edge of iodine. For example, an iodine concentration of 5 mg/mL will produce approximately 130 HU of contrast enhancement at 120 kVp and approximately 205 HU of contrast enhancement at 80 kVp. Thus, the amount of contrast material required to achieve similar degrees of vascular and parenchymal enhancement is much lower at 80 kVp, compared with 120 kVp.
Unfortunately, the application of this principle is limited in the bariatric population, because lowering the tube voltage will invariably result in noisy images. It may, however, be possible in select cases when other methods of reducing image noise (i.e., appropriate tube current, pitch, section thickness, and iterative reconstruction) are implemented.
The following values represent the maximum CT scanner parameters from leading scanner manufacturers that are available for bariatric imaging: table load limit, 308 kg (680 lbs.); gantry aperture, 85 cm (90-cm apertures are available on radiation oncology scanners); scan field of view, 65 cm (85-cm scan field of view options are available on radiation oncology scanners); tube current, 835 mA; and tube voltage, 140 kVp. In addition, major scanner manufacturers and some third-party vendors offer iterative reconstruction options to assist in generating the highest-quality images from the noise-limited datasets typically encountered at bariatric CT imaging of bariatric patients.
Iterative reconstruction methods, high tube current, and high tube voltage can reduce the image noise that is frequently seen in bariatric CT images. Truncation artifacts (produced whenever any part of the patient or imaged object is present in some but not all the views obtained for a slice}, cropping artifacts, and ring artifacts frequently complicate the interpretation of CT images of larger patients. If recognized, these artifacts can be easily reduced by using the proper CT equipment, scan acquisition parameters, and postprocessing options.
There are at least four commercial software applications on CT protocols in use today: CT-Expo, NCICT, NCICTX, and Virtual Dose.
Radiation Exposure Concerns
Also concerning to imaging professionals when imaging obese patients is the increased exposure to radiation since it requires more radiation to scan their bodies.
Radiation exposure is cumulative over a patient’s lifetime and the risk associated with a radiation dose from a single CT scan is small when compared with the clinical benefit of this procedure. Radiation dose in computed tomography (CT) has become a topic of high interest due to the increasing numbers of CT examinations performed worldwide with patients increasingly undergoing multiple CT scans and other radiation-based procedures, leading to unnecessary radiation risk. Hence, dose tracking and organ dose calculation software are increasingly used today. Studies have evaluated the organ dose variability associated with the use of different software applications or calculation methods.
A study from Rensselaer Polytechnical Institute was the first to calculate exactly how much additional radiation obese patients receive from a CT scan. The results of this research study showed the internal organs of obese men received 62% more radiation during a CT scan than those of normal-weight men, and for obese women, it was an increase of 59%.
Exposure from a diagnostic CT examination is referred to as having a stochastic effect. An epidemiological study of radiation-induced tumor risk for patients undergoing CT procedures first requires an assessment of the dose delivered to the organs and tissues exposed. The organ dose is defined as the dose received by the specific organ per unit of mass. It depends on the patient’s anatomy, scan region, and scanner’s output. Its estimate is the basis for risk analysis.
If CT technologists use normal technical settings to perform a CT scan on an obese patient, the resulting images are blurry as the X-ray photons must travel further and make their way through layers of fat. As a result, the settings are adjusted to a higher technique, which produces a better image but exposes the obese patient to additional radiation.
Summary
With the increasing prevalence of obesity, bariatric CT imaging is becoming common in day-to-day radiology practice. Bariatric patients present numerous unique challenges, and basic knowledge of scanner characteristics, image reconstruction, and obesity-related radiation exposure concerns, both in nonenhanced and contrast-enhanced studies, is essential for the acquisition of diagnostic-quality CT images in day-to-day radiology practice.
Take Aways
- Verify that the bore length and diameter will accommodate the patient
- Increase the kVp and increase mAs to reduce noise
- To increase the effective mAs, decrease the gantry rotation
- In MDCT scanners with automated tube current modulation, changing the scanner from “Fixed mAs” to “Automatic mAs” will allow the scanner to determine the amount of mAs to deliver per body section. Both solutions increase image quality but subject the patient to a higher dose of radiation
- Dynamics of contrast material in bariatric patients are unique because of the increased body weight, correlating directly with increased intravascular and interstitial volume
References
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- Huda W, Scalzetti EM, Levin G. Technique factors and image quality as functions of patient weight at abdominal CT. Radiology 2000;217(2):430–435. Link, Google Scholar
- Bariatric CT Imaging: Challenges and Solutions, Dzmitry M. Fursevich, Gary M. LiMarzi, Matthew C. O’Dell, Manuel A. Hernandez, and William F. Sensakovic, RadioGraphics 2016 36:4, 1076-1086
- Lehr JL. Truncated-view artifacts: clinical importance on CT. AJR Am J Roentgenol. 1983 Jul;141(1):183-91. doi: 10.2214/ajr.141.1.183. PMID: 6602518.Uppot RN, Sahani DV, Hahn PF, Gervais D, Mueller PR. Impact of obesity on medical imaging and image-guided intervention. AJR Am J Roentgenol 2007;188(2):433–440. Crossref, Medline, Google Scholar
- De Mattia, C., Campanaro, F., Rottoli, F. et al. Patient organ and effective dose estimation in CT: comparison of four software applications. Eur Radiol Exp 4, 14 (2020). [LINK] OPEN ACCESS
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