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

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

Computed Tomography (CT) Imaging

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

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

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

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)

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

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

Computed Tomography Muscle Visceral SubQ IntraMusc ular

Computed Tomography Muscle Visceral SubQ IntraMuscular

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

Computed Tomography Concerning features: Thin/wasted muscle volume

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

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

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

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