1. Lean Body Mass Assessment: Interpretations of Computed Tomography
Sarah Peterson MS, RD, CNSC
Clinical Dietitian
Rush University Medical Center
Chicago, Il

2. Learning Objectives
Upon completion of this session, the learner will be able to:
Summarize the principles of measurement for computed tomography
Describe the benefits and limitations of diagnostic abdominal computed tomography to measure body composition
Interpret abdominal computed tomography results

3. Computed Tomography (CT) Imaging

4. Computed Tomography Benefits Limitations
Gold-standard imaging method (along with MRI) for body composition analysis at the tissue-organ level
Diagnostic abdominal CT scans are frequently completed among patients requiring nutrition support
Sequential diagnostic abdominal CT scans can be used to measure change in body composition
Limitations
The radiation dose is significant, exposing a healthy population to this level of radiation for the sole purpose of body composition is unreasonable
Quantification of muscle and fat volume requires appropriate software and training

5. Computed Tomography CT imaging Process: X-ray source rotates
Series of cross-sectional images
Process:
X-ray source rotates
Detectors opposite from the x-ray source measure the strength of the x-rays exiting the section body
Repeated, section by section
Images reconstructed by a computer

6. Computed Tomography
Final CT scan - multiple cross-sectional images or "slices" of anatomy
like the slices in a loaf of bread

7. Computed Tomography CT slice thickness, thin (1 to 10 mm) and uniform
Each slice – composed of 2-dimentional pixels
2-dimentional pixel corresponds to 3-dimentional voxel (cross-sectional area x area between slices)

8. Computed Tomography
Each pixel within the matrix is assigned a number that is related to the linear attenuation coefficient (or loss of intensity) of the x-ray beam
Hounsfield units (HU)
Comparison of x-ray tissue attenuation to equal volume of water
Water is abundant in body and has uniform density
Water is assigned an arbitrary value

9. Computed Tomography HU are converted into a gray scale
High numbers/tissue more dense than water (bone, organs & muscle) = light gray/white
Low numbers/tissue less dense than water (like air and fat) = dark gray/black

10. Computed Tomography
Muscle
Visceral
SubQ
IntraMusc ular

11. Computed Tomography
Muscle
Visceral
SubQ
IntraMuscular

12. Computed Tomography Diagnostic CT scans to examine muscle quality
Typical features:
Muscle volume consistent with normal anatomy
Will appear light gray throughout
Clearly visible demarcation between muscle and fat depots

13. Computed Tomography
Concerning features: Thin/wasted muscle volume

14. Computed Tomography
Concerning features: Difficult to distinguish muscle from visceral and subcutaneous adipose tissue

15. Computed Tomography
Concerning features: High content of intramuscular fat (looks as if areas of muscle are missing)

16. Computed Tomography: Summary
Diagnostic abdominal CT scans are often ordered for patients receiving nutrition support
These images can be used to:
Review muscle volume and quality
Concerning features include:
thin/wasted muscle volume
difficult to distinguish muscle from visceral and subcutaneous adipose tissue
high content of intramuscular fat
Describe rates of sarcopenia
Measure changes in muscle and adipose volume

17. Learning Assessment Question
2. Quantifying muscle pixels at the third lumbar (L3) vertebra region on an abdominal computed tomography represents
L3 muscle cross-sectional area
L3 muscle volume
L3 muscle volume and intra-muscular adipose volume
Total body muscle