This document discusses imaging challenges in obese patients and strategies to address them. Obesity is defined using Body Mass Index (BMI) and has more than doubled worldwide since 1980. Imaging modalities like ultrasound, radiography, fluoroscopy, and CT can have problems penetrating thick layers of fat, resulting in reduced image quality. Solutions include using harmonic imaging in ultrasound, grids in radiography, overhead views in fluoroscopy, and optimized CT protocols with higher kVp and mAs. Automatic tube current modulation can help maintain image quality for different patient sizes. Overall, special techniques are needed to properly image obese individuals across modalities.

1. IMAGING IN LARGE AND OBESE
Dr Syed Arsalan Raza
MBBS, FRCR, FRCPC
Assistant Professor
Dalhousie University
Consultant Radiologist
Nova Scotia Health Authority

2. Outline
• Obesity facts
• Imaging Obesity
• Imaging in Obese with different modalities
– Problems
– Suggested strategies

3. OBESITY FACTS

4. Definitions
Overweight and obesity are defined as abnormal or
excessive fat accumulation that may impair health
Category BMI (kg/m2
)
Very severely underweight less than 15
Severely underweight from 15.0 to 16.0
Underweight from 16.0 to 18.5
Normal (healthy weight) from 18.5 to 25
Overweight from 25 to 30
Obese Class I (Moderately obese) from 30 to 35
Obese Class II (Severely obese) from 35 to 40
Obese Class III (Very severely obese) over 40
Body mass index (BMI)
• Simple index of weight-for-
height that is commonly
used to classify overweight
and obesity in adults
• Defined as a person's
weight in kilograms divided
by the square of his height
in meters (kg/m2)

5. Definitions
• Lean body mass
– The combination of cell solids, extracellular and intracellular water, and
mineral mass of the body
– Comprised of everything in your body besides body fat
– Also called fat-free body mass
• Calculated lean body mass:
– Lean Body Weight (men) = (1.10 x Weight(kg)) – 128 x ( Weight2/(100 x Height(m))2)
Lean Body Weight (women) = (1.07 x Weight(kg)) – 148 x ( Weight2/(100 x Height(m))2)
– Predict the lean body weight "average" of a group of people with
similar height and weight
• Measured Lean Body Mass: TBW(1–BFP/100)
– BFP is body fat percentage as measured by using a commercially available body fat
monitor
James WPT. Research on obesity. London. Her Majesty’s Stationery Office.
Hume R. Prediction of lean body mass from height and weight. J Clin Path. 1966; 19

6. Epidemiology
• Worldwide obesity has more than
doubled since 1980.
• According to the 1985 Canadian
Health Promotion Survey, 6.1% of
Canadian adults were found to be
obese
• 18.1% reported from the 2010
Canadian Community Health Survey
(CCHS)
Current and predicted prevalence of obesity in Canada: a
trend analysis. CMAJ Open. 2014 Mar 3;2(1):E18-26.

7. Current and predicted prevalence of obesity in Canada: a
trend analysis. CMAJ Open. 2014 Mar 3;2(1):E18-26.

8. Consequences
• Mortality attributed to
obesity from 5.1% in 1985 to
9.3% in 2000
• Indirect cost to healthcare
• Economic burden of obesity
4.6 - 7.1 billion annually
http://www.phac-aspc.gc.ca

9. Bariatric Surgery
•Impact on radiology
• Preoperative ultrasound
• Postoperative upper GIs
• Complications
• No optimal imaging modality
• Image quality is severely limited

10. IMAGING OBESITY

11. Adiposity Distribution
• High volumes of Visceral Adipose Tissue (AT)
– Reduced insulin sensitivity
– Dyslipidemia
– Impaired glucose tolerance
• Clinical methods suboptimal to assess different AT
compartments and distribution of AT within the body
– Waist-height-ratio (WHtR) –index for abdominal obesity
• Better differentiation and volumetric assessment of
AT compartments required

12. MRI
Diagnostic imaging in obesity
Best Practice & Research Clinical Endocrinology & Metabolism
(2013) 261–277
High amount of SCAT
(subcutaneous adipose
tissue)
Lower VAT (visceral
adipose tissue)
High amount of VAT
(visceral adipose tissue)
Lower SCAT
(subcutaneous adipose
tissue)

13. Ultrasound
Mesenteric leaves Preperitoneal fat Subcutaneous fat
Mesenteric fat thickness and cardiovascular risk factor.
International Journal of Obesity (2003) 27, 1267–1273

14. IMAGING IN OBESE

15. Individual’s diameter and body weight
is more important than total body fat
or BMI from an imaging perspective

16. Potential Problems
• Ability of patients to fit on existing imaging
equipment
• Ability to acquire and interpret images
adequately
• Increased radiation dose
• Patient positioning (includes access for image-
guided interventional procedures)
• Injury to technologists and transport personnel
• Increased wear and tear on equipment

17. Scheduling/transportation/throughput
• Knowledge of a patient’s weight and body diameter is important
before scheduling a patient for an imaging examination
• Appropriate transportation resources
– larger beds
– larger wheelchairs
– adequate nursing and transport

18. Limitations vary with imaging modality
• For CT, MRI and fluoroscopy
–Can the patient fit on the imaging
equipment?
• For ultrasound, plain radiographs and
nuclear medicine
–Attenuation through excessive fat

19. Habitus-Limited Reports
Modality
Habitus-limited
reports
Ultrasound 2.0%
Chest x-ray 0.5%
Abdominal CT 0.4%
Abdominal x-ray 0.25%
Chest CT 0.25%
MRI 0.1%
Radiology 2006;240(2): 435–439

20. ULTRASOUND

21. Problems
Thickness of
subcutaneous fat
and the sound-
attenuating
properties of fat
present
challenges

22. Solutions
• “Penetrate” mode
• Tissue harmonic imaging
• Compound imaging
• Speckle reduction filters

23. Obstetric scans
• Use all available technical tools improving image
quality in obesity
• Consider approaching fetus through the four major
abdominal areas with least subcutaneous fat:
– Periumbilical area
– Suprapubic area
– Right and left iliac fossae
• Transvaginal approach for CNS in fetus with vertex
presentation
• Color Doppler to check cardiac inflows and outflows
• Report limitations
Ultrasound in Obs & Gyne.
Volume 33, Issue 6, pages 720–729, June 2009

24. Tissue Harmonic Imaging
Ultrasound in Obs & Gyne.
Volume 33, Issue 6, pages 720–729, June 2009
WITH WITHOUT

25. Color Doppler in Cardiac Assessment
D. Paladini. Ultrasound in Obs & Gyne.
Volume 33, Issue 6, pages 720–729, June 2009

26. Abdominal Scans
• Modified lateral decubitus position displaces
the fatty panniculus and aids scanning
through the flank.
• Aorta and kidney with coronal or posterior
oblique approach

27. RADIOGRAPHY

28. Problems

29. Problems
• Increased patient thickness leads to increased
photon scatter and reduced contrast
resolution
• Higher peak tube voltage to penetrate excess
tissue reduces image contrast
• Increased exposure time increases the
probability of motion artifact

30. Solutions
• Tight collimation of the beam to the
region of interest
–Reduces the FOV
–Reduces scatter

31. Solutions
• Use a grid
– Typically, 85%–95% of scatter photons are
absorbed, in comparison with 40%–50% of
primary photons
– An antiscatter grid with a high grid ratio (8:1 or
10:1) can dramatically reduce scatter and greatly
improve image quality

32. Solutions
• Use of a grid
Bariatrics Today 2005; 1:31–33

33. Solutions
• Increasing kVp
• Increasing mAs
• Multiple films to cover the area of interest
(i.e., Segmental/Quadrantic approach)
• Cassette mapping

34. Abdom Imaging (2013) 38:630–646

35. High Radiation Dose
• Effective doses from radiographic
examinations in the extremely obese can
exceed 100 mSv from only a small number of
abdominal examinations
• Positioning the patient so that the thinnest fat
layer (anterior or posterior) is closest to the
image receptor – Substantial dose reduction
• Increasing the tube voltage also reduces the
dose.
Radiology: Volume 252: Number 1—July 2009

36. FLOUROSCOPY

37. Problems
• Table weight capacity
• Width of the opening (table to image tower)
• Table width
• X-ray tube capacity

38. Table weight limit
• Approximately 350 lbs
– 300 to 400 pounds with full table movement
capabilities
– 350 to 550 pounds with a static table (no horizontal or
vertical table movement)
• Newer machines – better profile (our
Multipurpose Room machine: 440 lbs vs old
machine: 330 lbs)
• May be possible to perform the study with the
patient in the upright position only

39. Width of opening and Table width
• The distance from the table top to the digital
image tower
– Standard fluoroscopic equipment: maximum
distance of 45–49 cm
– Newer systems: 76 cm
• Table width
– Standard: 45 cm wide (range 69–80 cm)
– Newer systems: 56 cm (88 cm with extender)

40. X-ray Tube Capacity
• Greater tube capacity will allow for multiple
exposures without tube overload and
decreased delay between exposures

41. Additional Problems
• Suboptimal exposures
– Difficult penetration
– Long-exposure time
– Blurry images
• Small fluoroscopic coverage area may limit
fluoroscopic evaluation
– Use overhead images for diagnosis

42. Overhead Images

43. COMPUTED TOMOGRAPHY

44. Challenges in Large Patients
• Table weight limit
• Gantry and FOV
• Image Quality

45. Table weight limit
• Limitations related to the table motor
• Motor must be able to move the table into the
gantry at a consistent speed to an accuracy of
0.25 mm
• Table weight limit: 450 – 500 lbs

46. Gantry and FOV
Standard gantry diameter: 130 cm
• More clinically relevant measurement is the vertical diameter with the
table in the gantry: 55 cm
Table width: 68 cm
FoV: Standard 50 cm
• Large-bore CT scanners have an extended field of view up to 82 cm
Part of body
outside the
scan FOV;
may yield
artifacts

47. Image Quality

48. Image Quality
Low noise High noise
- Image quality degraded when photons/voxel 
- X-ray beam attenuated by thick layer of
subcutaneous fat: decreased penetrating
capacity of x-ray beam

49. Optimized CT Protocol
Objective:
• Deliver higher x-ray flux
• More photons / voxel
• Improved contrast-to-noise ratio

50. Strategy 1:
–  KVP
–  tube current (mA); ‘‘Fixed mAs’’ to ‘‘Automatic mAs’
Strategy 2:
–  slice thickness (mm)
• Thicker collimation or thicker slices reconstruction
Strategy 3:
–  table speed:
•  beam pitch (mm/rotation)
•  gantry rotation time (seconds)
Strategy 4: Combination
CT parameters and Obesity

51. Automatic tube current modulation
Uppot et al. AJR 2007;188(2):437
xyz-axis modulation according to patient-specific attenuation
High-output X-ray tubes are capable of producing peak tube currents up
to 800 mA to maintain constant image quality over a wide range of patient

52. Automatic tube current modulation

53. Automatic tube current modulation
Automatic tube current modulation yields a significant
increase in effective dose in large patients

54. Inference
• Using the same operator-selected image quality
settings for all-sized patients may results in very
high radiation doses to large patients
• The operator-selected image quality settings (e.g.,
image noise, reference mAs) of automatic tube
current modulation have to be adjusted to large
patients

55. Manual Modification of CT Protocol
Image quality of abdominal-pelvic CT in large patients can be
improved by manually modifying the protocol without
substantially increasing the radiation dose to deep organs.

56. Modified anthropomorphic phantom with one or two
circumferential layers of fat equivalent material (- 80 HU)
4 cm
8 cm
Acad Radiol 2007; 14(4):486-94
8 cm of s/c fat
4 cm of s/c fat

57. CT scanning protocol

58. At 8 cm of subcutaneous fat thickness, the radiation dose
to the liver using the modified protocol B is actually
decreased as compared to standard protocol A at 0 cm of
subcutaneous fat thickness.
Radiation dose to the liver

59. 0
5
10
15
20
25
C/N
RATIO
4 cm 8 cm
Protocol A
Protocol B
Contrast to Noise Ratio
At 4 cm fat thickness, optimized protocol B increased CNR by 86%
At 8 cm fat thickness, optimized protocol B increased CNR by 137 %
Fat thickness

60. Modified High Flux Protocol
The modified, high flux CT protocol for obese patients yields
decreased image noise (40% ↓) and increased CNR (76% ↑).
Schindera et al. RSNA 2006

61. Modified High Flux Protocol
The modified, high flux CT protocol for obese patients
yields increased overall diagnostic acceptancy
High Flux
Regular Flux

62. Inference
• The abdominopelvic CT protocol manually adjusted
for simulated obese patient:
1. Increased mAs
2. Increased section collimation
3. Diminished table speed
• The manually adjusted abdominal-pelvic CT protocol
yields improved image quality in the simulated
obese patient without increasing the radiation dose
to the abdominal organs

63. Linear model may overestimate the amount of
contrast material needed in larger patient
– Metabolically inactive body fat does not disperse
or dilute IV contrast
Contrast administration during MDCT

64. Contrast optimization
Patient’s weight-tailored contrast dose
2.0-2.5 mL/kg of IV contrast material produced
better results than 1.5 mL/kg or a fixed dose
Yamashita et al. Radiology. 2000;216(3):718-723

65. Contrast optimization
Lean Body Weight (LBW) for calculating the
Iodine concenteration
– More precise and consistent hepatic and vascular
enhancement
– Reduced patient-to-patient variability
Ho et al. Radiology. 2007, 243:431-437
Kondo et al. Radiology. 2008;249(3):872-877

66. Summed Enhancement Data for Liver, Portal Vein, Aorta
The measured lean body weight is least variable.
Ho et al. Radiology. 2007, 243:431-437
Contrast optimization

67. Kondo et al. Radiology. 2008;249(3):872-877
No significant correlation between adjusted MHE and BW
positive correlation between adjusted MHE with BMI and BFP
Contrast optimization

68. CT optimization – Summary
• Optimizing abdominal CT protocols by:
– Increased mAs
– Increasing slice thickness
– Slowing table speed
Result in:
– Decreased image noise
– Increased CNR
– Similar or lower organ dose

69. CT optimization – Summary
• Automatic tube current modulation for
abdominal CT can result in very high radiation
doses in large patients
– The quality settings for automatic tube current
modulation have to be adjusted in large patients
• Contrast media dose & rate adjusted based on
lean body weight rather than total body
weight in large patients may deliver better
enhancement

70. Questions or Comments

71. Thank you