1. Impact of Obesity on Medical Imaging and Image-Guided Intervention
Presenter：張敬弘
Supervisor：郭雪梨 醫師, 姚維仁 主任

2. Objective
Discuss the impact
Provide some solutions

3. Obesity The prevalence of obesity is increasing 64% !!
Increased incidence of diabetes, heart disease, and certain types of cancer

4. BMI vs. Weight & Body diameter
Larger hospital beds
Larger wheelchairs
Larger operating room tables
Larger …

5. Several Issues
What is the best technique to image the obese patient for a given indication?
How should the obese patient be scheduled and transported to the radiology department?
How will the patient fit on the imaging equipment available?
How should equipment settings be modified to optimize image quality for the obese patient?

6. Industry Standard

7. Transportation to the Radiology Department

8. Coordinating with the transport department to schedule obese patients only when the larger wheelchairs and stretchers are available
Acquiring portable radiography or sonography equipment for patients who cannot be transported

9. How will the patient fit on the imaging equipment available?

10. Radiography Problems…
For radiography, obese patients’ surface areas may be too large to fit on a 14 × 17 inch (35.56 × cm) cassette
Solutions…
Consider the routine use of multiple cassettes

11. Sonography
Problems…
Patient obesity may limit the ability of technologists to appropriately position patients for quality images
Solutions…
Judicious use of pillows to help support the patient’s body
Use of the spleen or liver window to scan the kidneys can be helpful

12. Nuclear Medicine
Patients who exceed the weight limit of the tables may be imaged on their stretchers with the gamma camera

13. How should equipment settings be modified to optimize image quality for the obese patient?

14. Radiography Problems…
Increased body thickness through which the X-ray beam must travel results in increased exposure time and introduces motion artifact
Solutions…
Using a grid and increasing the kVp and mAs
Increasing the film development speed from 400 speed film to 800 speed film
Adjusting window and level settings

15. Before vs. After

16. Fluoroscopy Problems…
Fluoroscopy is primarily limited by the aperture diameter (45 cm) and table weight (159 kg)
Solutions…
Use serial abdominal radiographs or CT to obtain the answers to the clinical questions

18. Sonography
Problems…
Poor penetration of the ultrasound beam beyond the focal depth
Increased attenuation of the ultrasound beam as it passes through subcutaneous and intraperitoneal fat

19. Sonography Solutions…
Using the lowest frequency transducer available (2 MHz)
Positioning the transducer to image the organ of interest within the range of the focal length of the transducer
Examining the patient’s previous imaging (CT or MRI) to determine the thickness of subcutaneous fat

21. CT Problems… Increased noise due to inadequate beam penetration
Beam-hardening artifact in areas where the patient’s body exceeds the size of the field of view
Image quality limitations because of image cropping

22. Increased noise
A result of inadequate beam penetration

23. Solutions… Increasing the kVp to 140 and increasing the effective mAs
1. decreasing the gantry rotation speed from one rotation in 0.5 second up to one rotation in 1 second to increase the effective mAs
2. changing the scanner settings from “fixed mAs” to “automatic mAs”

25. Limited field of view
Standard 70-cm gantry CT instruments have a field of view of 55–65 cm
In areas where the patient’s body is larger than the field of view but smaller then the gantry diameter, beam-hardening artifact can limit evaluation of internal organs

26. Solutions…
Recognize the artifact during image acquisition and adjust the patient position so that the area of interest does not exceed the field of view

27. Dangers of cropping
Cropping subcutaneous fat can result in the loss of valuable information

28. CT benefits of fat
Patients who have predominantly intraperitoneal or retroperitoneal fat have improved visualization of internal organ structures
Because of the better delineation of internal organ structures by the fat

30. MRI Problems… Radiofrequency penetration and gradient strengths
Limited field of view
Scanning time

31. Radiofrequency penetration and gradient strengths
Larger body habitus also introduces noise and will therefore decrease the contrast-to-noise ratio (CNR)
For receiver coils, the increased distance of inner organs from the coils in obese patients also affects the SNR

32. Limited field of view
Larger fields of view can decrease image resolution
Therefore, in imaging obese patients, the smallest possible field of view is used to image the organ of interest without inducing wraparound artifacts

34. Scanning time
Scanning times are increased because of the larger cross-sectional area and longer craniocaudal dimensions
Increased scanning times can also lead to patient motion, with associated motion artifacts

35. Nuclear Medicine
Obesity degrades image quality by the scatter of photons within the soft tissues, decreasing the SNR

37. Solutions…
Solutions in nuclear medicine imaging of obese patients include using the maximum allowable dose and imaging for a longer time to maximize counts.

38. Mammography
Helpful in mammography, with the improved visibility of lesion relative to the surrounding fat
Increased BMI is associated with decreased geometric sharpness, decreased image contrast, and higher potential for loss of sharpness because of motion

39. Interventional Radiology
The accuracy in targeting lesions decreases the deeper the lesion that is to be biopsied or drained
Proper patient positioning can also be a problem in obese patients

40. Summary
Radiologists and technologists need to be aware of the limitations of imaging equipment and the equipment adjustments that can be made to improve image quality in obese patients