Measuring Gas Diffusion in the Lungs

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Measuring Gas Diffusion in the Lungs

• John Owers-Bradley

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People

• John Owers-Bradley1 Stan Fichele1, Colin
  McGloin1Abdel Benattayallah1, Richard Bowtell1,
• Alan Moody2, Paul Morgan2.
• 1 The University of Nottingham, School of Physics
  and Astronomy, University Park, Nottingham, UK,
  NG7 2RD.
• 2 The Queens Medical Centre, Nottingham, NG7
  2UH.

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Diffusion

• We have measured apparent diffusion coefficient
  of HP 3He in the lungs using 2 distinct methods
  over a wide range of length scales
• We compare the techniques
• 1) Spin echo attenuation due to transverse
  diffusion in a magnetic field gradient ? DSE
• 2) Decay of sinusoidally modulated longitudinal
  magnetisation (tag) due to transverse
  diffusion ? DTAG

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Hyperpolarised He-3 Gas Production

• 2W fibre laser
• Photons -gt Electrons-gtNuclei
• 3He pressure 5mBar in optical cell
• Time taken to polarise and collect gas 30 mins
• Cell volume 400cm3
• Dilute with 4He to give 1 litre at atmospheric
  pressure
• Transfer to a Tedlar plastic bag

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Administration of HPG

• The hyperpolarised gas is transported in the
  plastic Tedlar bag
• In the bag 3He has a relaxation time of 20mins
• Gas administered simply via an outlet tubevalve
  on the bag
• The volunteer clears his/her lungs with hard
  expiration and then rapidly inhales the helium
  gas
• Breath is held for duration of procedure 5 -
  15s
• Ethical approval for healthy volunteers

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RARE Sequence
TE9-18 ms
TG5 ms
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Transverse Slices 13 mm thick
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3D Rendered Image
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Proton Spin Tagging in the lungs
Strain map - inspiration Chen et al MRM 45 (2001)
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Spin Tagging of HP 3He gas
We can modulate the magnetisation in space -
known as spin tagging. The resulting image has
characteristic stripes with a particular
wavelength and hence k - value. Use to measure
lung movement.
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Strain in the lungs
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Spin Echo in a Field Gradient
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RARE Sequence
TE18 ms
off
TG5 ms
off
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Transverse Spin Diffusion - ADC in lung

• We have measured the relaxation rate (G2DSE) in
  free gas and in the healthy lungs.
• Expected dependence on G2 over wide range - no
  dependence on length scale
• Intrinsic T2 relaxation evident at low G
• Ratio of Ds (free/lung) 10

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Tag Decay and Diffusion
Diffusion of the 3He gas over distances of order
a wavelength causes the decay of the modulation.
The decay rate depends on k (2p/?) and DTAG
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Tag Decay and Diffusion
Hence plot decay of tag profile vs k2 to find
DTAG for lungs. Value of DTAG 0.02 cm2 s-1
- much lower than DSE (0.2 cm2 s-1)
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Three Diffusion Regimes
G
Free Diffusion Bounded Diffusion Rapid
Diffusion (Fukushima, 1994)
Simulation
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Conclusions

• Tagging sequence for 3He can be used to measure
  strain in the lungs.
• Apparent Diffusion Coefficients (ADCs), DSE and
  DTAG have been measured with length scales of
  1mm and 5-30 mm respectively.
• DSE DTAG 2 cm2s-1 for free diffusion in bag.
• ADC values in normal lung DSE 0.2 cm2s-1
  whereas DTAG 0.02 cm2s-1
• ADCs - depend on geometry and technique used.
  Models of the lung are being developed.
• Importance lies in disease diagnosis - DSE is
  much larger in lungs of patients suffering from
  emphysema - typically 0.4 cm2s-1 rather than 0.2
  cm2s-1. Plan to see if DTAG also changes.