Imaging methods to assess SKM

Question 1

Explain in brief, how ultrasound works to measure tissue thickness

Answer 1

• The probe emits high-frequency sound waves which reflect back differently depending on the tissue structures, these are then detected by the probe and interpreted into an image
• The probe is placed on the skin in a sagittal plane, where the whole thickness of the muscle can be viewed
• The probe allows provide anatomical focal points to be measured
• Thickness of the muscle can then be measured using the boundaries of the superficial and deep aponeurosis

Question 2

Explain the importance of muscle thickness

Answer 2

• Important for the assessment of muscle mass and changes in hypertrophy/atrophy
• Thicker muscles will have a greater contractile mass and will have a larger force production

Question 3

What is the importance of fascicle length?

Answer 3

-Fascicles = contractile units of the muscle and contract linearly along their length
- Fascicle lengths will increase in response to muscle hypertrophy
- This will result in a greater force production as the length each fibre is able to contract will be greater

Question 4

What is the importance of the pennation angle?

Answer 4

• As fasciccles increase in length and new fibres are added in series, the pennation angle will decrease
• Decreased angle = allows a greater shortening of individual fibres

Question 5

What is aponeurosis?

Answer 5

The boundaries of the muscles

Question 6

Descrone the aspects influencing the quality of muscle between old and young individuals

Answer 6

• Fat infiltration in between and within the muscles will decrease the ‘quality’ of the muscle through reductions in the amount of contractile regions available as the total mass is lost

Question 7

Why is DXA used for body composition analysis to compare bone mineral densities in 60 year old individuals?

Answer 7

• DXA relatively quick and automated bone density analysis

Question 8

What is used to determine if a group of 1000 children were overweight (body composititon analysis)?

Answer 8

• Body weight scales - quick and simple to use

Question 9

How would you quantify levels of myosteatosis (fat infiltration) in 12 older men ?

Answer 9

• MRI - gold standard measurement as it allows accurate quantification of muscle/fat
• May be more time consuming than other imaginh techniques but the only one able to determine these changes

Question 10

How would you compare the muscle mass of elderly intensive care patients compared to an age match group living in a care home to determine risk factors?

Answer 10

• Using an ultrasound - easy non-invasive measures which can accurately quantify muscle mass i.e. thickness

Question 11

What is dual X-ray absorptiometry (DXA)

Answer 11

• 2 different energy level X-rays
• Lean,fat and bone mass each reduce (attenuate) the X-ray signal in unique ways
• Quantifies bone mineral content, total fat mass and FFM
• Computer analyses scan point by point to determine body composition
• Can assess regions
• Method:
• Low dose radiation
• ~20 mins (Applicalble to young and old)
• Applicable to young and old

Question 12

What is MRI?

Answer 12

• Electromagnetic radiation (non ionising)
• Provides quantitative information on muscle volume/CSA & total/ subcutaneous adipose tissue
• Method:
• ~30-90 mins
• Expensive!
• Potentially time-consuminf analysis

Question 13

What is an ultrasound?

Answer 13

• High frequency pulsed sound waves penetrate the skin surface to fat-muscle interface
• Can show structural composition of tissues
• Assess’ thickness of diffferent tissues

Question 14

What are some strengths and limitation of DXA?

Answer 14

Strengths:
- Accurate, reliable, no discomfort for patients, 4-compartement model (minderal, bones ), can be used to look at regional composition
Limitations:
- Expensive, specialist equipment
- Radiation exposure
- Train staff required to operate
- No reference value for certain groups